This is a collection of technical reports on the Parys mountain area.
The original copyright owner’s of these articles is acknowledged.
Anglesey Mining PLC
These extracts are taken from a field guide prepared for a visit on 21 April 98 to Parys Mountain as part of the Symposium on Magmatism and Mineralization in Arcs and Ocean Basins Geoscience 98 Biennial Conference at Keele University, U.K.
The Parys Mountain deposit is located about 3 kilometres from the north coast of Anglesey, in northwestern Wales, within a sequence of Ordovician? to Silurian volcanic rocks and shales (Figure 1). Basement rocks include the Monian Supergroup of late Precambrian age. The deposit was an historic source of copper in Britain over a century (1768-1904), although most production took place during the first 50 years of this period. The ore was recovered from a series of open pits (Figure 2), and also underground workings which extended below the open pits to a maximum of 150 metres below the surface. The originally mined deposits were viewed as vein systems or lodes within Ordovician felsic volcanic and shaley rocks (Greenly, 1919).
No major mining or exploration operations were carried out from 1904 to 1961, when the first drilling programs were begun. From 1961 to 1990, drilling to depths well below the old pits and workings, and in particular to their west, was carried out mainly by CIGOL, Parys Mountain Mines, Intermine, Cominco, Imperial Metals and Anglesey Mining, and an geological picture of the deeper volcanic and sedimentary sequence began to emerge (Hawkins, 1966; Westhead, 1991; Tyler and Charter, 1997). It was during this period of exploration that stratiform lenses of Cu-Zn-Pb-rich massive sulfide mineralization were discovered in the subsurface to the west of the open pits, and also down dip to the north, near the lower contact between a thick sequence of felsic volcanic rocks and underlying shales (e.g. Figures 3 and 4). These zones of sulfide mineralization have been termed the Chapel, Engine, Deep Engine, South Central and North Central Zones.
Various aspects of the massive sulfide deposits and the stratigraphy and structure of their host rocks have been studied over the last two decades (Thanusuthipitak, 1974; Pointon, 1979; Pointon and Ixer, 1980; Southwood, 1982, 1984; Westhead, 1993). During this period, it was generally accepted that the disposition of the main geological units on the property was the result of folding of an initial sequence of mudstone overlain by rhyolite and then shales, into a large anticline-syncline structure with an east-west strike and a moderate dip to the north, with the north limb overturned to the south. The occurrence of Silurian shales flanked by volcanic rocks which were assumed to be Ordovican, based on limited graptolite evidence, was taken as providing support for a major synclinal axis (Westhead, 1991). The age of the Silurian shales, which are exposed in the open pits, has been determined from graptolite studies (Greenly, 1919).
From 1985 to 1990, Anglesey Mining plc carried out further drilling (22 holes totalling over 8000 m) in the western part of the property, and sunk the Morris shaft to 300 m depth in the Engine Zone. Underground drilling in 1990 from the 280 m level intersected several zones of massive sulfides near the lower contact between a thick sequence of felsic volcanic rocks and underlying shales of presumed Ordovician age (e.g. Figures 3 and 4). These are known as the Engine and Chapel Zones (first discovered by Cominco in the 1977-81 period). The White Rock Zone, which is dominantly silica with occurrences of semi-massive sulfides, also is present in this area but its stratigraphic relations with the other Zones are uncertain due to underground faulting. In 1990, minable reserves in the Engine and Chapel Zones were estimated as 1.41 Mt grading 1.99% Cu, 3.42% Pb, 6.65% Zn, 99 g/t Ag and 0.79 g/t Au; with reserves in the White Rock Zone of 0.84 Mt grading 0.49% Cu, 3.43% Pb, 6.72% Zn, 78 g/t Ag and 0.66 g/t Au (Charter, 1995). These form part of an overall estimated geological reserve of 6.45 Mt at similar grades.
In 1995, Anglesey Mining initiated a new phase of work involving relogging of available core, remapping of surface geology, and a major lithogeochemical program. This work has led to a revision in our understanding of the overall stratigraphic and structural setting of the massive sulfide mineralization, and in particular, an abandonment of the idea of strong folding of the sequence, with an emphasis instead on the importance of volcanic facies relationships and their geometry and stratigraphic contacts in the subsurface (Barrett, 1995; Tyler and Charter, 1997; Barrett and MacLean, 1997; Tennant, PhD thesis in progress). These studies support the idea that the distribution of mineralization is determined mainly by its relation to various felsic eruptive centres along a more or less homoclinal sequence which dips to the north. Different volcanic units can be defined and traced, even where altered, using recently developed lithogeochemical methods.
In order to identify different felsic and mafic rock types, particularly in altered sequences, binary plots of immobile element pairs are commonly used, as outlined by MacLean and Kranidiotis (1987), MacLean (1990), and Barrett and MacLean (1991; 1994a,b). The most useful plots involve combinations of the elements Al, Ti and Zr, but Y-Nb-Th-Yb are also helpful. On the Y-axis, a compatible element such as Al2O3 or TiO2 is plotted, whereas on the X-axis an incompatible element such as Zr is used (as a monitor of fractionation). In such plots, primary fractionation trends can be defined using a suite of least altered samples that covers the compositional range involved in the particular volcanic terrane under study. In terranes where several homogeneous, but distinct precursor lithologies are present, rather than a continuous fractionation series, altered samples can be treated by relating them back along one of several alteration lines to the appropriate least-altered precursor rock type. This is the case at Parys Mountain, where several distinct rhyolite types have been identified.
The dispersion of sample points along a given alteration line is due to the effects of gain or loss of mobile elements. Net mass gain (e.g. quartz or carbonate or pyrite addition) causes dilution in the concentration of immobile elements relative to their initial values in a rock, whereas net mass loss (which typically accompanies strong sericitization or chloritization) causes residual concentration of immobile elements. Each altered rock type will have its own alteration line; these lines are generally distinct (without overlap) is most of the immobile element plots. Altered rocks accordingly can be identified using simple immobile element plots. This allows even very altered volcanic units to be readily identified, which in turn greatly improves stratigraphic correlations between drill holes.
The new Parys Mountain lithogeochemical data set currently comprises about 400 drill-core samples and 100 surface samples. Al2O3-Zr relations are shown in Figure 5a, and TiO2-Zr relations in Figure 5b, although for simplicity, only rhyolites A, B and C-1 are shown. Based on these and related plots, five distinct rhyolite units can be identified on the property; these are termed rhyolites A, B, C-1, C-2, and D. In addition, there are two volumetrically very minor mafic groups, probably thin sills, the main occurrence of which is a thin body near the lower contact of the southern rhyolites. Rhyolites A and C-1 are volumetrically dominant in the eastern and central portions of the property, with rhyolite D more aboundant in the eastern portion. Rhyolites B and C-2 occur close to the contact between hangingwall rhyolites and footwall shales in some areas.
Plots of one incompatible and immobile trace element (e.g. Zr, Nb, Th, Y, Yb) against another have also been used to help subdivide the rhyolite and mafic types. In such plots, a linear fractionation trend extending away from the origin is expected for magmas derived from a common source but linked through processes such as fractionation. Alteration effects will move samples along lines which are parallel to, and superimposed upon, any primary fractionation trend. In a plot of Nb versus Zr (not shown), rhyolites A, C-1, C-2 and D lie on essentially the same trend, whereas rhyolite B defines a separate trend (with a higher Zr/Nb ratio). The dispersion of samples along each of these two main trends is due mainly to alteration, although the different slope for rhyolite B reflects a different magma source. Basalts (not shown) plot close to the rhyolite B trend, with Zr contents of 150-200 ppm. A plot of Y versus Zr (not shown) indicates that most rhyolite A samples are of tholeiitic affinity (Zr/Y = 2.0-4.5), whereas the other rhyolite types are mainly of transitional affinity.
Chondrite-normalized REE plots are shown in Figure 6 for representative samples of the main rhyolite and mafic rock types (the rhyolite C samples are C-1 type). The REE patterns also discriminate well between the different rhyolite groups, with the slopes of the patterns (Lan/Ybn) reflecting the differences noted above in other immobile element ratios such as Zr/Y. Samples within a given group retain near-parallel REE patterns, although they show ëapparentí variations in their absolute REE contents. It should be noted, though, that much of the vertical variation within a given group is the result of mass change effects in mobile elements, rather than primary magmatic variations in REE contents.
Downhole Lithogeochemical Variations
Downhole variations in lithological type can be effectively monitored by plotting ratios such as Al2O3/TiO2 or Zr/TiO2, and variations in magmatic affinity by using ratios such as Zr/Nb and Zr/Y. In most VMS hydrothermal systems, these ratios are generally insensitive to alteration, although local Y mobility has been noted in areas of extreme chloritic alteration. Where a change in primary lithology occurs, immobile element ratios such as Al2O3/TiO2 and Zr/TiO2 should change to a new value (although not necessarily by the same absolute factor). At Parys Mountain, Al2O3/TiO2 ratios are generally >50 in rhyolites, about 15-25 in mudstones, and 5-10 in basalts. The corresponding ranges for the Zr/TiO2 ratio are about 800-4000 in rhyolites, 100-200 in mudstones, and 50-100 in mafic rocks.
In hole CZ-4, in the Chapel Zone, footwall mudstones are overlain by rhyolite C, followed by a thin mud interval, and then rhyolite A (Figure 7). In this figure, the Al2O3/TiO2 ratio is shown on the lower X-axis, and the Zr/TiO2 ratio, on the upper X-axis. Within each of the main rhyolite units in hole CZ-4, and in all other holes as well, these ratios remain nearly constant, which indicates that each eruptive unit had a characteristic and near-homogeneous composition from base to top, which in turn forms the basis for defining chemostratigraphic units. It is also worth emphasizing that within a given chemostratigraphic unit, the immobile element ratios remain closely parallel despite wide ranges in downhole alteration effects.
Relative to the main southern rhyolite-shale contact on the western part of the property, the intensity of alteration generally decreases stratigraphically downward (in the shales), and also decreases stratigraphically upward (in the rhyolites). The main alteration minerals are sericite, chlorite, quartz and pyrite. In areas of extreme alteration, as noted earlier, it is not possible to visually identify the original rock types (e.g. rhyolite versus mudstone), let alone distinguish between rhyolite types, although this can be done effectively using the immobile element methods discussed above.
Alteration in the rhyolites includes significant additions of silica in some samples, but losses in others. Samples with silica loss show variable enrichments in Fe, Mg and K, reflecting chloritization and sericitization. Only a few rhyolites are chemically relatively unaltered, and even these have possibly been affected by some alkali exchange, making it difficult to estimate the precursor composition of the rhyolites (this is one reason why subdivision of the felsic lithologies is risky without using immobile element ratios). Many rhyolites display strong to near-total Na depletion, even those far removed from obvious mineralized zones. This raises the possibility that some of the alkali alteration could have taken place prior to the development of mineralizing systems, e.g. during high-temperature emplacement of the rhyolites, or during a period of relatively low-temperature, low intensity interaction with seawater within the volcanic pile.
As a guide to general alteration effects, one can plot mobile components such as SiO2 and MgO against the Zr/TiO2 ratio (Fig. 8). The Zr/TiO2 ratio provides a monitor of primary lithological variation (as discussed above). Examination of Figure 8 indicates that rhyolite B has undergone a wide range of silica addition and depletion, and also moderate to extreme additions of Mg. Rhyolite C commonly shows some silica depletion and Mg addition. Some of the mudstones are extremely altered, with major additions of Mg or K (and silica loss). In order to assess the alteration on an absolute quantitative basis, however, it is necessary to calculate mass changes for each element (e.g. MacLean and Kranidiotis, 1987; MacLean and Barrett, 1993), using a separate precursor composition for each rhyolite type (and for the mudstone). In the next stage of the Parys Mountain lithogeochemical program, calculated mass changes will be used to establish the geometry of the hydrothermal systems, and to define alteration gradients (vectors) in both the rhyolites and mudstones which can help to guide exploration.
The Engine and Chapel Zones, located in the western portion of this sequence, contain reserves plus resources of 3.09 Mt grading 2.1% Cu, 3.6% Pb, 6.8% Zn, 109 g/t Ag, 0.7 g/t Au. The sulfides occur at or near the top of a mudstone footwall, which is overlain by some 200 metres of rhyolites (massive to flow-banded to fiamme-bearing to autobrecciated to rubbly), with a few thin intercalations of mudstone or volcaniclastic sediments. In this area, a widespread upper interval of rhyolite A commonly overlies domes and lenses of rhyolite C and locally thin tongues of rhyolite B, which in turn overlie footwall mudstones. In places, however, rhyolite A rests almost directly on mudstone, or on mineralization. Rhyolite A in the Engine and Chapel Zones contains some orthoclase + albite phenocrysts, whereas Rhyolite C-1 is aphyric and in part medium-grained. The presence of some peperitic contacts with mudstone suggests that rhyolite C-1 was partly injected below the surface, but the occurrence of fiamme-bearing rhyolite C-1 elsewhere in this area suggests that it was extruded as well. Rhyolite B forms a minor volumetric type east of the Pen-y-mynydd fault, where it occurs as both massive units and altered volcaniclastic intervals near the level of mineralization. West of the Pen-y-mynydd fault, in an area which has been little explored, massive rhyolite B is more common. In places, thin basaltic units occur near the rhyolite-mudstone contact, and display peperitic contacts with the mudstone.
Within the Engine and Chapel Zones, abrupt lateral variations in the thickness of rhyolite C-1 over distances of only 50-100 metres probably reflect the presence of local domes of rhyolite C that gradually “inflated” via extrusions and partly subsurface injections of magma. Hydrothermal systems which were discharging through mudstones at the time of rhyolite C emplacement appear to have been largely suffocated, although some alteration and veining penetrated into the lower parts of the domes. Mudstone areas immediately lateral to rhyolite domes appear to be most favorable for hosting sulfide lenses, which in principle could occur either as in-situ accumulations or as sulfide-bearing debris shed by slumping from dome flanks. Eventually, widespread eruptions of rhyolite A buried the accumulations of sulfides, mudstones and volcaniclastic debris in the flanking lows, and also covered the domes of rhyolite C. Work in progress suggests that a center of rhyolite A eruption existed to the north of the rhyolite C domes.
One question which needs to be resolved is whether the rhyolite eruptive centres are related to regional faulting of the basement rocks. Northwestern Wales was the site of rifting of a continental margin in the lower Paleozoic (Kokelaar, 1988; Howells et al., 1991). Syn-mineralization faults are difficult to identify at Parys Mountain because the footwall generally comprises a monotonous shale sequence, while hangingwall rhyolites show lateral thickness variations related at least partly to volcanic constructional processes. With further work, trends in the subsurface disposition of rhyolite eruptive enters may become evident. Most rhyolites are subalkaline with low TiO2 contents and low Zr/Y ratios (2-4) but high Nb and relatively high REE contents, features typical of rifted continental margin rhyolites. Basalts are enriched relative to MORB, with high contents of Ti, Zr, Nb and light REE, and are of transitional-to-alkaline affinity. Together with regional geological data, the affinities of the volcanic rocks and their bimodal distribution suggest that the tectonic setting of the deposit was a rifted continental crustal margin.
Cited and Related References
Barrett, T.J., 1995. Stratigraphic, Lithogeochemical and Petrographic Relations of some Volcanic Rocks at the Parys Mountain Massive Sulfide Deposit, Wales. Internal Report for Anglesey Mining plc, 122 pp.
Barrett, T.J., and MacLean, W.H. 1994a. Mass changes in hydrothermal alteration zones associated with VMS deposits of the Noranda area. Exploration and Mining Geology, 3: 131-160.
Barrett, T.J. and MacLean, W.H., 1994b. Chemostratigraphy and hydrothermal alteration in exploration for VHMS deposits in greenstone and younger volcanic rocks. In: Lentz, D.R., editor, Alteration and Alteration Processes Associated with Ore-Forming Systems. Geological Association of Canada, Short Course Notes, Volume 11, p. 433-467. GAC-MAC Annual Meeting, 1994, Waterloo, Canada.
Barrett, T.J. and MacLean, W.H., 1991. Chemical, mass, and oxygen-isotopic changes during extreme hydrothermal alteration of an Archean rhyolite, Noranda. Economic Geology, 86: 406-414.
Charter, W.J., 1995. Preliminary reassessment of structures and mineralisation, at Parys Mountain, Anglesey, UK, with regard to further exploration. Internal report for Anglesey Mining (by Celtest Geological Services).
Greenly, E., 1919. The geology of Anglesey. Memoir of the Geological Survey of Great Britain, 2 volumes, 980 pp.
Howells, M.F., Reedman, A.J., and Campbell, S.D.G., 1991. Ordovician (Caradoc) marginal basin volcanism in Snowdonia (north-west Wales). London: HMSO for the British Geological Survey. 191 pages.
Kokelaar, P., 1988. Tectonic controls of Ordovician arc and marginal basin volcanism in Wales. Journal of the Geological Society of London, 145: 759-775.
Leat, P.T., Jackson, S.E., Thorpe, R.S. and Stillman, C.J., 1991. Geochemistry of bimodal basalt-subalkaline/peralkaline rhyolite provinces within the Southern British Caledonides. Journal of the Geological Society of London, 143: 259-273.
Lentz, D. and Goodfellow, W., 1992. Re-evaluation of the petrology and depositional environment of the felsic volcanic and related rocks in the vicinity of the Brunswick No. 12 massive sulphide deposit, Bathurst Mining Camp, New Brunswick. In: Current research, Part E; Geological Survey of Canada, Paper 92-1E, 333-342.
Lesher, C.M., Goodwin, A.M., Campbell, I.H., and Gorton, M.P. 1986. Trace-element geochemistry of ore-associated and barren, felsic metavolcanic rocks in the Superior Province, Canada. Canadian Journal of Earth Sciences, 23: 222-237.
MacLean, W.H., 1990. Mass change calculations in altered rock series. Mineralium Deposita, 25: 44-49.
MacLean, W.H. and Barrett, T.J., 1993. Lithogeochemical methods using immobile elements. Journal of Exploration Geochemistry, 48: 109-133.
MacLean, W. H. and Kranidiotis, P., 1987. Immobile elements as monitors of mass transfer in hydrothermal alteration: Phelps Dodge massive sulfide deposit, Matagami, Quebec. Economic Geology, 82: 951-962.
McConnell, B.J., Stillman, C.J. and Hertogen, J., 1991. An Ordovician basalt to peralkaline fractionation series from Avoca, Ireland. Journal of the Geological Society of London, 148: 711-718.
McPhie, J. and Allen, R., 1992. Facies architecture of mineralized submarine volcanic sequences: Cambrian Mount Read volcanics, western Tasmania. Economic Geology, 87: 587-596.
Nobel, D.C. and Parker, D.F., 1975. Peralkaline silicic volcanic rocks of the Western United States. Bulletin volcanologique, 38: 803-827.
Pearce, J.A. and Norry, M.J. 1979. Petrogenetic implications of Ti, Zr, y and Nb variations in volcanic rocks. Contributions to Mineralogy and Petrology, 69: 33-47.
Pointon, C.R. 1979. Palaeozoic volcanogenic mineral deposits at Parys Mountain, Avoca and S.E. Canada – a comparative study. Ph.D. thesis, University of Aston, Birmingham, 265 pp.
Pointon, C.R. and Ixer, R.A., 1980. Parys Mountain mineral deposit, Anglesey, Wales. Transactions of the Institute of Mining and Metallurgy, B89: 143-155.
Reedman, A.J., Colman, T.B., Campbell, S.D.G. and Howells, M.F., 1985. Volcanogenic mineralization related to the Snowdon Volcanic Group (Ordovician), Gwynedd, North Wales. Journal of the Geological Society of London, 142: 875-888.
Sheppard, W.A., 1980. The ores and host rock geology of the Avoca Mines, Co. Wicklow, Ireland. Norges geologiske Undersokelse, 350: 269-283.
Southwood, M.J., 1982. The geological setting of the sulphide deposits at Morfa Du, Parys Mountain, Anglesey. Ph.D. thesis, University of Wales, College of Cardiff, 388 pp.
Southwood, M.J., 1984. Basaltic lavas at Parys Mountain, Anglesey: trace element geochemistry, tectonic setting and exploration implications. Transactions of the Institute of Mining and Metallurgy, B93: 51-54.
Thanasuthipitak, T., 1974. The relationship of mineralization to petrology at Parys Mountain, Anglesey. Ph.D. thesis, University of Aston, Birmingham, 284 pp.
Tennant, S.C., 1997. Spatial Characterisation of Hydrothermal Alteration at Parys Mountain; Some Preliminary Results. Unpublished report for Anglesey Mining plc, 37 pp.
Tennant, S.C., in progress. Stratigraphy and Lithogeochemistry of Volcanic Rocks at the Parys Mountain Massive Sulfide Deposit, Wales, U.K. (PhD study).
Tyler, P.A. and Charter, W.M., 1997. The Parys Mountain Project, Anglesey, Wales, U.K. – Geology, Mineral Resources, Potential and Recommendations. Unpublished report for Anglesey Mining plc McKillen, Tyler & Associates, Toronto, 70 pp.
Westhead, S.J., 1991. Prospects at Parys Mountain. Geology Today, July-August 1991 issue: 130-133.
Westhead, S.J., 1993. The structural controls on mineralization at Parys Mountain, Anglesey, North Wales. Ph.D. thesis, University of Wales, Cardiff, 474 pp.
Timothy J. Barrett
Ore Systems Consulting
2005 – 1323 Homer Street
Canada V6B 5T1 Stephen C. Tennant
Department of Earth Sciences
University of Wales Cardiff
Wales CF1 3YE
United Kingdom Peter A. Tyler
Anglesey Mining plc
Parys Mountain, Amlwch
Wales LL68 9RE
Barrett, T.J., Tennant, S.C. and MacLean, W.H., 1999.
Geology and Mineralization of Parys Mountain
At the Parys Mountain deposit, owned by Anglesey Mining plc, several different styles of base metal mineralization, including polymetallic massive sulfide lenses and Cu-bearing vein systems, are hosted within Ordovician to lower Silurian shales/mudstones and mainly rhyolitic volcanic rocks. The rhyolites extend about 3 km along strike, and almost 1 km across strike. In plan, they form a ‘hairpin’ – the northern and southern rhyolite limbs, which are ˜50-100 m thick, ‘merge’ into a 300 m-thick mass in the western part of the property. A basaltic unit is also present in the latter area. The rhyolite limbs are flanked to the south, west and north by Ordovician shales (Abstract Fig. 1). However, lower Silurian (Llandovery) shales, known as the Central Shales, occur between the rhyolite limbs. A commonly but not universally accepted model for the overall structure invokes an E-W-trending syncline with the northern limb overturned to the south. Lower Paleozoic rocks on Anglesey lie unconformably on a regional metamorphic basement of late Precambrian age.
In the late 1700s to early 1800s, Parys Mountain was one of the world’s leading Cu producers. The Cu was recovered mainly from ‘lodes’ within the Central Shales via shafts and pits, and from the contact between Northern Rhyolites and Northern Shales via underground workings. Information on the mineralization in the old open-pit workings is scanty, but in addition to the Cu-rich ‘lodes’ which were mined, Zn-Pb-rich ‘bluestone’ masses and veins were also present, although the Zn-Pb mineralization generally was discarded as waste at that time. Much of the Cu along the northern contact was in the form of cp-py-qtz veins hosted by silicified shales and an unusual quartz-rich rock. In the 1960s and 1970s, drilling by Canadian and British companies discovered polymetallic massive sulfides in the western part of the property, in an area known as the Engine Zone, at the contact between the Southern Shales and overlying rhyolites, i.e. on the normal-facing limb of the inferred synclinal structure. Massive sulfides have also been located along parts of the southern and northern contacts in the central part of the property. The eastern part of the property remains largely unexplored, as do some of the deeper central parts.
The Engine Zone comprises high-grade Zn-Pb-Cu-sulfide beds and masses within a series of altered and veined shales, thin felsic volcaniclastic beds, and heterolithic mudflows. In 1990, the Robertson Group estimated that the Engine Zone contained probable reserves of 1.41 Mt grading 1.99% Cu, 3.42% Pb, 6.65% Zn, 99 g/t Ag and 0.79 g/t Au; and possible reserves of 2.83 Mt at 3.20% Cu, 1.93% Pb, 4.54% Zn, 22 g/t Ag and 0.14 g/t Au. Probable reserves in the nearby White-Rock Zone were 0.84 Mt grading 0.49% Cu, 3.43% Pb, 6.72% Zn, 78 g/t Ag and 0.66 g/t Au. These areas formed part of an overall estimated geological reserve for the property of 6.45 Mt at 2.34% Cu, 2.60% Pb, 5.35% Zn, 39 g/t Ag and 0.23 g/t Au.
Recently, as part of Anglesey Mining’s exploration and research programme at Parys Mountain, the volcanic rocks have been dated for the first time, by R. Parrish of the British Geological Survey. The results indicate that the northern and southern rhyolite limbs are both of Llandovery age (R. Parrish, pers. comm., 1998) (Abstract Fig. 1). An assessment of existing paleontological data by M. Howe of Leicester University strongly supports overturning, to the south, of the Northern Shales and much of the Central Shales (and thus also the Northern Rhyolites) (M. Howe, pers. comm., 1998). The mineralization of the Engine Zone, although lying on Ordovican shales, is probably also of Llandovery age (i.e. the age of the altered rhyolite immediately above the sulfides).
A large-scale lithogeochemical, relogging and petrographic program was carried out in 1995 and 1997-98 by Ore Systems Consulting. Overall, some 700 samples from 60 drillholes, and 80 outcrop samples, were analysed by XRF techniques. About 80 samples were also studied petrographically. In the basis of immobile element ratios such as Al2O3-TiO2, TiO2-Zr, and Nb-Zr, five distinct rhyolite types can be identified, termed rhyolites A, B, C, D1 and D2, as well as two mafic, and three main mudstone types. Although two rhyolite groups may lie on the same trend in a given plot, they can be separated out on another. Thus, rhyolites A and B lie on almost the same trend in an Al2O3-TiO2 plot, but on much different trends in a Nb-Zr plot, while Rhyolite C is identified by its much higher Zr/Al2O3 ratios relative to the other rhyolite types. In the case of rhyolites D1 and D2, there appears to be a trend from one end-member to the other, which suggests they are chemically related (e.g. through fractionation). The mafic rocks, which are volumetrically minor, fall into two chemical groups (both include basalt to basaltic andesite). One type corresponds to a syn-mineralization sill-like mafic sheet which occurs near the base of the Southern Rhyolites, and the other to ‘late’ mafic sills which intrude the Northern Shales.
All rhyolite types, except B, have elevated Nb (34-56 ppm) but moderate Zr contents (210-370 ppm, which suggests a high-K subalkaline affinity (Leat et al. 1986). They have fairly low Zr/Y ratios (2-5) and relatively flat REE patterns, which suggests a ‘tholeiitic’ affinity. All types except B have similar Zr/Nb ratios, probably due to derivation from a common source, and show little evidence for a subduction influence. Rhyolite B has a transitional affinity. Basalts near the base of the rhyolite sequence are enriched in Ti, Zr, Nb and light REE relative to normal MORB, and have a transitional, within-plate affinity; such features are commonly found in basalts emplaced in continental rift settings. Primary geochemical features of the rhyolites and basalts, as well as the fact that the Ordovician shales regionally lie unconformably on a late Precambrian metamorphic basement, suggest that the Parys Mountain deposit formed during a phase of volcanism (Llandovery) which accompanied intra-plate rifting of submerged continental crust.
Rhyolite A is the volumetrically dominant type in the western part of the property, with rhyolites D1 and D2 dominant in the eastern portion. Rhyolite C outcrops only in the southwestern corner of the property, where it lies above the Southern Shale, but below the main mass of rhyolite A. Rhyolite C thins downdip to the north, and has the overall form of a tapering wedge (maximum thickness of about 80 m). It can be traced downdip about 400 m, by which point it has thinned to a 10-20 m or less, and it can be traced east-west for about 800 m. A thin sheet of basalt, usually 10-20 m thick, generally occurs between rhyolites C and A, although in places it crosses rhyolite C. Where rhyolite C is absent, the basalt is usually absent. The Engine Zone massive sulfides are intimately associated with rhyolite B, which occurs as thin beds of volcaniclastic material or as thin massive lavas. Rhyolite B commonly is either so chloritized that it resembles shale, or so silicified it appears to be ‘quartz-rock’. In the nearby Chapel Zone, massive sulfides occur along the same shale contact, but are overlain by rhyolite C and basalt. In this zone, the sulfides mainly occur marginal to, or beneath rhyolite C. In the Engine and Chapel Zones, rhyolites B and C (and the basalt) are overlain by thick sequences of rhyolite A.
Rhyolite B makes up the surface rhyolite outcrops west of the Penymynydd fault, and is the main volcanic rock at depth in the White-Rock Panel (west of this fault), which hosts massive and vein sulfides. As noted earlier, Rhyolite B also occurs in the deep Engine Zone as massive lavas up to 15 metres thick which lie above the shales and sulfides. These relations suggest that the White-Rock Panel is partly correlative with deep Engine Zone stratigraphy, although the former has probably been inverted and faulted. Finally, rhyolite B also occurs in a separate area between the Northern Rhyolites and the Northern Shales, where it is commonly strongly altered, and hosts an important massive sulfide occurrence. The presence of rhyolite B and sulfides on the northern flank supports the idea that deep Engine Zone stratigraphy is structurally repeated in this area. Rhyolite A extends to the east as a component of both the northern and southern rhyolite limbs. Conversely, rhyolites D1 and D2 become more abundant in this direction. A thick interval (˜200 m) of flow-banded rhyolite A in the western part of the property, and one of rhyolite D1 (˜80 m thick) in the eastern part suggest that these two areas mark the locations of eruptive centres. Although commonly flow-banded and flow-brecciated, rhyolites A, C and D1 also can have a fragmental or pyroclastic appearance. Rhyolite D2 appears to be a pyroclastic interval.
Shales at Parys Mountain have been divided into 3 main chemical types: N, X and C. In addition, there are smaller groups of shale which fall compositionally between the main groups. Most of the Northern Shales are of N-type, while most of the Central Shales are C-type. X-type shales and thin C-type shales locally occur below the first rhyolites (near the massive sulfide horizon).
Mass changes have been calculated for about 600 samples by relating each sample back to its appropriate precursor lithology and using single-precursor mass change methods (MacLean and Kranidiotis, 1987; Barrett and MacLean, 1991) The results have been plotted downhole and contoured on several sections across the property. In the Engine Zone, strong alteration commonly occurs in the upper part of the footwall mudstones, the rhyolite B volcaniclastic beds, and the lowest part of the rhyolite A sequence. On sections across the western and central parts of the property, alteration increases with depth and in the downdip direction, as shown by substantial mass additions of Fe and Mg (mainly as chlorite) to the rhyolites and shales. The most intensely chloritized rocks have also lost K. There is a general correlation between areas of Fe+Mg gain and the known locations of sulfide lenses. Locally, Fe+Mg has been added as ankeritic carbonate, which occurs as clots and veins within shales and rhyolites. Silica shows a wide range of mass changes, with large gains in some of the shales and the rhyolites (e.g. in the White-Rock Panel), but large losses in strongly chloritized or sericitized zones. Areas of Fe+Mg gains and Si gain have likely experienced a phase of chloritization at higher temperatures followed by a phase of silica precipitation at lower temperatures. Mass changes also have been calculated for 65 outcrop samples from across the property. A zone of increased alteration occurs along the northern flank, which may reflect the presence of a deeper hydrothermal system (e.g. the one which formed the massive sulfides and altered rhyolite B and shales in holes H-30 and A-15).
At Parys Mountain, the first volcanism after a long period of Ordovician shale sedimentation produced rhyolite B. Although volumetrically minor, it is important as a marker horizon, as the first polymetallic sulfides were deposited at this time, as high-grade Zn-Pb-Cu sulfide beds and masses (within shales and volcaniclastic rhyolite B beds). There is evidence that the composition of the associated shales was changing at this time, which may reflect the establishment of local grabens. At the west end of the property, rhyolite C was emplaced, probably partly within these shales, and partly above them. Its emplacement may have disrupted some of the sulfide-shale intervals to produce local mud-rich debris flows, some with sulfide clasts. Areas which were marginal to rhyolite C are more likely to host undisturbed sulfides. Where the sulfide beds are not mixed with other material, they are very high in base metals (30-40% Zn+Pb+Cu), with the remaining material consisting of pyrite, quartz and carbonate. An interesting feature of some of the base metal-rich ores in the Engine Zone is their high Ag and Au contents, which respectively are in the 200-1000 g/t and 1-5 g/t ranges (these ores contain (10% iron). Given that some of the high-grade sulfide beds are clastic, it would be important to locate their source area.
The western part of the property apparently was a site of active uprising of magmas, probably along fractures, and of related hydrothermal activity which produced strong alteration of shales and of the lower parts of rhyolite C and A (with local sulfide veining). At more or less the same time as rhyolites C and A were accumulating in the western part of the property, rhyolites D1 and D2 were erupting in the eastern part. The latter area has not been systematically explored to date, although it may represent a second locus of volcanic activity, and thus of seafloor faulting and sulfide mineralization. Mid-Llandovery rhyolite volcanism at Parys Mountain was followed by the deposition of mid-Llandovery Central Shales, which also hosted polymetallic mineralization (in the opencast pits). This suggests that the hydrothermal systems which formed the pre-rhyolite massive sulfides locally became re-established after volcanism and deposited more metals. The Parys Mountain massive sulfide lenses are closely similar to many Kuroko-type deposits in terms of: 1) an association with felsic volcanic and locally basaltic rocks; 2) the occurrence of several laterally separate sulfide lenses along one main time horizon; 3) the generally Zn-Pb-rich nature of the sulfides, which are also Cu-rich in places; 4) the presence of clastic (transported) sulfides; 5) the general absence of pyrrhotite. Parys Mountain differs in terms of the nature of its immediate footwall (shale versus felsic volcanic rocks), the general lack of barite, and the absence of footwall sulfate alteration. The lack of sulfates at Parys Mountain may simply indicate that circulating fluids and local bottom waters were more reducing. The Kuroko deposits probably formed in a volcanic back-arc, whereas the overall tectonic setting of Parys Mountain seems more akin to that of rhyolite-dominated VMS settings in rifted continental crust, e.g. the Iberian Pyrite Belt. Although the deposits in this belt commonly occur as large, single, pyrite-rich lenses, there are also numerous smaller Zn-Pb-rich orebodies. The Iberian deposits generally occur above rhyolite, but locally within shales. At Parys Mountain, precious metal enrichment occurs in parts of the Engine Zone; a well known example of highly Ag-Au-rich clastic sulfides occurs at Eskay Creek, British Columbia, where the ores occur in shales (above rhyolites chemically similar to those at Parys Mountain, and below basalts).
Significant portions of the southern rhyolite-shale contact in the central and eastern part of the Parys Mountain property, from 5000E to about 6500E at Penysarn (AMC grid, in metres) and a depths below about 400 m below mine datum, have not been drilled, although this is the same contact as that hosting massive sulfides in the Engine and Chapel Zones to the west. Several shallower areas of the southern contact are also untested. The downdip mineralization west of the Penymynydd Fault (White-Rock Panel) also has not been drilled off. In addition, the northern contact has the potential to host massive sulfides, as shown by the intersections in H-30 and A-15. The deep northern contact is almost untested east of 5000E, although it may be cut out by the Corwys Fault east of 5800E. Areas of Cu-rich quartz veins in the Northern Shales (i.e. the Northern Copper Zone) are interpreted as stockwork veins which may be potentially related to undiscovered massive sulfides situated along deeper parts of the northern rhyolite-shale contact. Although data are limited, the contents of gold in veined and silicified shales along the northern flanks is commonly anomalous, and should be further investigated. An unexplored and undrilled tract of shales extends east from the Penysarn rhyolite for two km, to the Rhosmynach rhyolite near the coastline. In the past, polymetallic mineralization was locally worked at Rhosmynach, although the area is still undrilled. The Rhosmynach rhyolite is probably correlatable with rhyolites in the eastern part of Parys Mountain, based on chemistry. It is conceivable that rhyolite-shale contacts are present in the subsurface between Penysarn and Rhosmynach. Such contacts could also be present under the shales which extend for at least 1 km to the west of the Engine Zone at Parys Mountain. Drilling is recommended in several areas, firstly to systematically explore the untested known and projected rhyolite-shale contacts on the Parys Mountain property, and secondly to search for further contacts in the areas to the west and east of this.
Department of Geology Leicester University,
Research Studentships for 2001
The inter-relationship of sedimentation, diagenesis, volcanism and mineralization at Parys Mountain, Anglesey
Supervisors: Dr. J.A. Zalasiewicz (University of Leicester); Dr. T.J. Barrett (Geological Consultant to Anglesey Mining plc); and Dr. A.E. Milodowski (British Geological Survey). CASE.
Parys Mountain was once the largest copper mine in the world, and has a long history of exploration and exploitation. But, despite recent advances, the timing and geological context of the mineralization remain unclear, while even the overall geological structure of the mountain remains deeply enigmatic. This project will apply a range of advanced analytical techniques to investigate the relationships between sedimentation, diagenesis, volcanism, fluid migration, tectonism and mineralization that will allow the student to determine to what extent Parys Mountain is, essentially, an ancient submarine ‘black smoker’ or a product of post-tectonic mineralization.
A particular focus will be the mudrocks which host both the ores and the volcanic rocks. Largely neglected to date, they give clues to early Palaeozoic depositional processes and ocean chemistry, and also include important diagenetic phenomena, notably widespread silicification and pyritization, closely associated with the mineralization. We believe that resolution of their complex history is critical to successful mineral exploration.
In this project, these variously altered and mineralized mudrocks will be subject to detailed sedimentological and stratigraphical analysis, complemented by detailed textural examination using back-scattered electron microscopy (BSEM) and allied techniques, centering on the inter-relationship of preserved sedimentary and tectonic fabrics to diagenetic and mineral phases.
The student will receive training in detailed field mapping and sedimentary logging, in a variety of analytical techniques (e.g. SEM, electron probe), and will gain valuable experience of the relationship between multidisciplinary scientific research and the practical development of mineral resources. He or she will then be well placed to seek employment in the field of mineral exploration.
This is planned as a CASE study supported by Anglesey Mining plc, which is currently carrying out broadly-based exploration studies at Parys Mountain.
About the Supervisors: Jan Zalasiewicz has much experience of field-based and multidisciplinary studies and has long-standing interests in the depositional and post-depositional history of mudrocks. Tim Barrett has broad experience of mineralization studies and the commercial sector, and a particular interest in silicification processes. Tony Milodowski has much expertise in analytical techniques, particularly SEM-based, applied to a range of petrographic and geochemical problems.
To apply, please send your CV and a letter of application, complete with the names and addresses of 2 academic referees, to Dr. J.A. Zalasiewicz, Department of Geology, Leicester University, LEICESTER, LE1 7RH. Email: JAZ1@le.ac.uk
Tissue-Level Biomarkers in Sentinel Slugs as Cost-Effective Tools to Assess Metal Pollution in Soils
I. Marigómez, M. Kortabitarte, G. B. J. Dussart
In previous laboratory experiments, slugs were shown to be sensitive to metal pollution. Therefore, they might be invaluable instruments for biological assessment of soil pollution. The present investigation was carried out to validate previous laboratory results in a field study. Slugs were collected from an abandoned copper mine (Parys mountain top, PMT), from a site 7 km away from the mine (Parys mountain bottom, PMB), and from a clean site (Snowdonia Cwm Idwal, SCI) in Wales in early July 1994. Whole soft body and digestive gland Cd, Cu, and Zn concentrations were measured by means of atomic absorption spectrophotometry (AAS). The digestive gland was the main tissue for metal accumulation, with significant differences in tissue metal levels between samples from different sites. PMB presented the highest Cd and Zn levels and the highest Cu levels were found at PMT. In addition, metals were demonstrated in situ by autometallography as black silver deposits (BSD) on histological sections of digestive gland tissue. The extent of BSD within lysosomes of digestive cells was closely related to metal levels determined by AAS. Histochemistry revealed that Ca metabolism and structural and reserve connective tissues might be altered in slugs living in metal-polluted soils. Finally, tissue-level biomarkers of biological effect [mean epithelial thickness (MET), mean diverticular radius (MDR), mean luminal radius (MLR), MET/MDR and MLR/MET] were quantified by image analysis of digestive gland histological sections stained with hematoxylin-eosin. MET and MDR values of slugs collected from SCI were high, while slugs from PMB presented low MLR/MET associated with environmental stress induced by metal exposure. We conclude that exposure and effect biomarkers recorded in sentinel slugs could be sensitive, quick, and cheap indices of metal pollution in soils. A Slug Watch monitoring program could be developed similar to the Mussel Watch program, which is currently applied to assess environmental quality in coastal and estuarine areas.
The evolution of copper tolerance in a terrestrial mollusc
Copper is very toxic to molluscs; so much so that it has often been used as the active ingredient for molluscicides. Yet at Parys Mountain on Angelsey, the site of what was once a huge open-cast copper mine there is a very large population of the snail Helix aspersa. Field work suggests that these snails inhabit non-toxic areas within the mine site and so may avoid toxic areas. However laboratory behaviour studies have failed to show any avoidance, a behaviour which is shown by snails from non-toxic control sites. Further work on behaviour and ecological physiology of mine and control populations is needed. Prospective candidates should have an interest in either evolution or zoology including animal behaviour.
University of Wales Bangor Research Projects
The following examples demonstrate research projects undertaken by the University of Wales, Bangor which have important applications for industry
Environmental Research Projects undertaken by the Department of Chemistry
Control of acid mine drainage on Mynydd Parys
Mynydd Parys is a major site of copper, lead and zinc mineralisation whose exploitation spans 4000 years and once dominated the world’s copper markets. Although active underground mining ceased a century ago, it is currently under consideration by Anglesey Mining plc. The site is of major geochemical and archaeological importance and poses problems both of heavy metal pollution and of conservation.
At present drainage from the mines pollutes local rivers and estuaries and has been investigated in a number of studies. A particular problem is posed by a large body of very acidic water perched in old workings 45m above a drainage adit behind a dam whose condition is now unknown. This is a potential hazard need investigation in terms of its possible effects on local rivers by controlled dewatering as compared to catastrophic failure.
The proposed projects would reassess this situation on the basis of existing information and of specific data collected on the chemistry and flow rates of drainage and would investigate dewatering schemes which would result in minimum pollution.
Bioremediation of Industrial Pollutants
Prof. Williams’ research group works on bacteria which have the capacity to degrade aromatic compounds and in particular hydrocarbon components of petroleum products, as well as the by-products of various industrial processes.
These bacteria can be used for bioremediation of industrial effluents and currently the group has two projects with industry specifically investigating microbes capable of the clean-up of particular classes of industrial pollutants.
As well as their use for bioremediation, many of these bacteria contain interesting enzymes which have the potential for carrying out useful chemical transformations: the technology within the group uses molecular biological techniques to isolate the genes involved in the biotransformation and to over express the relevant enzymes in order to better investigate their properties and their applicability to industry chemical transformations.
INFO: Prof. Peter Williams
Phone: 01248 382363Fax: 01248 370731 e-mail: email@example.com
Bioremediation of acidic waste waters fromderelict mines and on-going industrial operations
Acid Mine Drainage (AMD) characteristically contains high concentrations of iron which, when deposited in receiving streams and rivers coats sediments with a very apparent orange-coloured sediment (ochre) destroying many benthic life forms. In addition the acidity of AMD and presence of toxic heavy metals makes it highly polluting. Whilst it is recognised that this form of pollution is microbial in origin the research has isolated and characterised novel indigenous micro-organisms which can reverse the reactions involved in AMD genesis thereby removing metals and increasing water pH.
Current work involved the development of low-cost bioreactors using these novel micro-organisms for AMD treatment in situ. In a separate project they are (in conjunction with other universities) looking at other novel micro-organisms that can break down recalcitrant organic materials which occur in some acid industrial waste waters, with the aim of developing an holistic approach for remediating waters which contain a mixture of heavy metals and xenobiotics.
INFO: Dr Barrie Johnson Phone: 01248 382358 Fax: 01248 370731
Wetlands Research GroupDr Chris Freeman’s research group has been looking at the potential for using artificial wetlands for treating water pollution problems.The approach is simple and harnesses natural ecological processes in a low cost alternative to conventional treatment. The approach has the added value of creating a valuable wildlife resource and so offers outstanding PR opportunities to companies wishing to improve their ‘green credentials’.
Constructed wetlands can be used for treating wide ranging pollutants from mine drainage to food industry wastes.
The group has recently been working with Anglesey County Council on investigating the potential value of the systems for supporting the expansion of the local food industry capacity.
INFO: Dr. Barrie Johnson Phone: 01248 382358 Fax: 01248 370731 e-mail: firstname.lastname@example.org
Volcanic facies, geochemistry and setting of VMS deposits in the Ambler Range, Alaska, and at Parys Mountain, Wales. Barrett, T.J
In the Ambler district of northwestern Alaska, mid-Paleozoic volcanic sequences are host to the Smucker, Arctic and Sun deposits (each with resources of some 15-30 Mt). In the Dead Creek area (8 km from the Arctic deposit), several sulfide horizons occur within a bimodal but folded volcanic sequence. In this area, quiet accumulation of carbonaceous dacitic volcaniclastic material was followed by a sulfide-barite mineralization event and eruption of low-Zr rhyolite B, then eruptions of dacite. There appears to be a second sulfide horizon associated with high-Zr rhyolite B. These various eruptions took place mainly as pyroclastic flows, which halted further sulfide accumulation in the marine basin. Only when they ceased, and carbonaceous sediments began to slowly accumulate, could further sulfides be deposited. Rhyolites and dacites are of medium-K, transitional to calc-alkaline magmatic affinity, and are interpreted to have been erupted on continental margin, rifting of which allowed emplacement of rather unfractionated, mantle-derived tholeiitic basalts. The nearby Arctic deposit is subjacent to a separate felsic volcanic centre. The sulfide lenses at Arctic are underlain by carbonaceous volcaniclastic sediments, which are commonly chloritized. Eruptions of rhyolite A (and some rhyolite B) eventually buried and terminated the sulfide-depositing system, although hydrothermal alteration affected the hangingwall rhyolites.
At Parys Mountain in Anglesey, Wales, massive sulfide lenses are hosted by a lower Paleozoic volcanic sequence. One horizon occurs at the base of a thick (>200m) sequence of felsic pyroclastic flows and lesser massive rhyolites, at or near the contact with underlying mudstones and volcaniclastic beds. Compositionally, the felsic volcanic rocks are mostly tholeiitic rhyolite A, with a smaller group of transitional rhyolite B. Alteration of rhyolite flows has produced common moderate silica- and K-enrichment, and strong Na-Ca depletion. Some zones of extreme silica addition also occur. Mafic units in part of the footwall are of transitional-to-alkaline E-MORB affinity. Only relatively low-Zr rhyolites (<300 ppm Zr) were sampled in the present study. They lack a subduction signature. Data from other studies indicate that high-Zr (500-1200 ppm Zr) peralkaline rhyolites are also present at Parys Mountain and elsewhere in the Welsh Basin. Based on the general stratigraphic setting and chemical comparisons with modern peralkaline rhyolites and enriched basalts, the Parys Mountain deposits are interpreted to have formed in a marine, ensialic marginal basin in which rifting, foundering and volcanism occurred in response to asthenopheric upwelling.
Volcanic Stratigraphy and Geochemistry, Parys Mountain Massive Sulfide Deposit, Wales.
Barrett, T.J., Tennant, S., Tyler, P.A. and MacLean,
The Parys Mountain deposit occurs in a lower Paleozoic rhyolite and mudstone sequence. The Engine Zone contains a total resource of 3.09 Mt grading 2.1% Cu, 3.6% Pb, 6.8% Zn, 109 g/t Ag, 0.7 g/t Au. The sulfides occur at or near the top of a mudstone footwall, which is overlain by 200-300m of massive to fragmental rhyolites. Immobile element ratios and REE data indicate three main rhyolite groups. Widespread rhyolite A overlies domes of rhyolites C and B, and locally mudstone or sulfide-rich intervals. Thin basalts with peperitic margins occur near the rhyolite-mudstone contact. Massive sulfide lenses are interpreted as in-situ deposits on a mud seafloor; and mud-supported mixed debris containing sulfide clasts as slumps from emerging rhyolite domes (C and B). Lower rhyolites tend to be sericitized and chloritized. Upper mudstones contain sulfide-quartz-carbonate stringer zones and are strongly Mg enriched. Most rhyolites are subalkaline with low Ti and low Zr/Y (2-4) but high Nb contents. Basalts are enriched in Ti, Zr, Nb and REE relative to N-MORB. With regional geological data, this suggests a tectonic setting on a rifted continental margin. The new model relates mineralized and altered zones to early rhyolite volcanism and associated paleotopographic effects, and will help to guide new exploration for sulfide lenses.
Barrett, T.J., 1995b.
Stratigraphic Lithogeochemical and Petrographic relations at Parys Mountain
The Parys Mountain deposit is located in a sequence of Ordovician volcanic rocks and shales in Anglesey, Wales. Although historic Cu production from 1760-1904 was from Cu-rich veins, exploration since 1961 has concentrated on synvolcanic massive sulfides. These appear to lie mainly at the base of a thick (>300m) felsic sequence, at or near the contact with underlying mudstones. In this study, 3 drill holes that intersected volcanic stratigraphy, mudstones and mineralization were examined, described and sampled. The felsic volcanic rocks are mainly pyroclastic flows, with some massive rhyolites. Many primary volcanic textures remain, including glassy fiamme and shards, spherulites and small amygdules. No definite phenocrysts were seen in any felsic samples. Altered glass is common. Fiamme in pyroclastic flows are up to several centimetres long, and are strongly altered to sericite+chlorite; the matrix consists of quartzo-feldspathic ‘silty’ grains with sericite and minor chlorite. In some pyroclastic flows, felsic lithic fragments are also present (up to a few centimetres across). Massive rhyolites are flow-banded with local zones of flow-breccia rubble. Mudstones contains some graded felsic tuff beds.
The felsic volcanic rocks fall into two compositional subgroups termed rhyolites A and B, with most samples belonging to the former group. Based on REE patterns and Zr/Y ratios, rhyolite A is of tholeiitic affinity, whereas rhyolite B is of transitional affinity. Rhyolites A and B are effectively separated, even where altered, using a Nb-Zr plot. Rhyolite A (precursor) has a relatively high Nb content, and is probably not subduction-related. Two mafic samples from the footwall are enriched in Ti, Zr, Nb and P; their overall chemistry suggests that they are intra-plate mafic rocks of transitional-to-alkaline affinity. Two mudstones have REE patterns with strong enrichments in the light to heavy REE, indicating derivation from a different source area (peralkaline or alkaline).
The felsic rocks are commonly moderately silica- and K-enriched, and strongly Na-Ca depleted. This is consistent with the lack of observed feldspar. Some rhyolites contain notable Fe + Mg enrichments (as chlorite or ankerite). Because of the strong downhole variations in alteration of rhyolites A and B, it is necessary to use immobile element ratios such as Al2O3/Zr and Zr/Nb to recognize original volcanic units. Mass changes can also be plotted downhole in order to search for hydrothermal alteration effects.
In the present study, only low-Zr rhyolites were sampled (<300 ppm Zr). However, data in Southwood (1982) indicate that high-Zr peralkaline rhyolites containing 500-800 ppm Zr, and locally up to 1200 ppm, are also present at Parys Mountain. Peralkaline rhyolites are in fact common in both the Welsh Basin and in southeast Ireland; they are typified by high Zr contents (500-1200 ppm) and about 3 to 4% each of Na2O and K2O (Leat et al., 1986; McConnell et al., 1991). These authors interpreted the peralkaline rhyolites as products of ensialic magmatism related to tensional faulting of a continental margin. The rhyolites examined in the present study appear to have been derived from a proximal vent which erupted subalkaline, pyroclastic, fiamme-bearing units and lesser massive flows mostly of one initial composition (rhyolite A). The vent may have been a seafloor synvolcanic fault zone, now represented by the Penymynedd Fault, which also may have been the conduit for some of the VMS-mineralizing fluids. The zones of White Rock, which are locally mineralized, are dominated by hydrothermal silica which may represent a seafloor sinter, or discordant fracture infilling of altered host rocks; some white rock also may have been brecciated by seafloor faulting. It is speculated that the Parys Mountain peralkaline felsics (Southwood’s 1982 data) were derived from a different source area relative to rhyolites A and B, possibly from the Snowdon volcanic region, where peralkaline rhyolites are common. Based on comparisons with modern peralkaline rhyolites, the overall setting for the Parys Mountain massive sulfides is interpreted to be a relatively shallow marine, faulted and foundering ensialic basin behind a volcanic arc. VMS deposits hosted by chemically similar rhyolites have recently been found in the Yukon, where they occur in a tectonic setting similar to Parys Mountain.
Untold mineral wealth
by Richard Bevins, John Mason (both National Museums & Galleries of Wales) and Margaret Wood, Bob Mathews (Countryside Council for Wales)
Photographs by Michael P Cooper for the National Museums & Galleries of Wales
The mineral wealth of Wales has long been recognised. Indeed, the earliest extraction of metals in Wales is known to have taken place during the Bronze Age, with the mining of copper on the Great Orme’s Head in North Wales and at various sites in central Wales, such as Nantyreira and Cwmystwyth. Later, the Romans extracted gold from ores at Pumsaint, near Lampeter. During the Elizabethan period, silver was won from mines in Cardiganshire, which provided ore for the Aberystwyth Mint. From about 1750 onwards, copper was worked at the great opencast pit at Parys Mountain on Anglesey, a mine which was to prove to be the most significant in terms of copper production and world influence of all the mines in Wales. The main period of metal mining in Wales, however, was during the latter half of the 19th and early part of the 20th centuries, when lead, copper and zinc were extracted from hundreds of mines across the whole country.
Perhaps one of the most perplexing aspects of the metal mines in Wales is why the gold deposits of the Dolgellau area were not discovered until the 1840s, whilst the Pumsaint gold was discovered by the Romans. In contrast to the Dolgellau gold, which occurs in very obvious quartz veins, with the gold visible to the eye, Pumsaint gold occurs as microscopic grains only a few microns across, locked in the mineral arsenopyrite, and is only observable using a high power microscope.
The history of the exploitation of metals in Wales is well known, but the character and mineral content of the deposits themselves have been comparatively neglected. In general, the mineral veins and other types of deposits were considered to contain, in mineralogical terms, simple assemblages dominated by the common sulphides of lead, zinc, copper and iron.
Although in some orefields this remains true, recent research in the Central Wales Orefield has revealed significant quantities of a range of rare cobalt-, nickel-, and antimony-bearing minerals. This unmasked a hitherto unknown complexity in a supposedly straightforward vein province.
In addition, the presence elsewhere in Wales of minerals containing a wide range of relatively rare elements, such as bismuth, cadmium, niobium, tantalum and tellurium, suggested that a systematic in-depth study of Welsh mineralisation would be a valuable research programme. Such a study would not only broaden an understanding of the diversity of Welsh mineral species, in terms of their assemblages and genesis, but would also provide a targeted collecting policy for the National Museums and Galleries of Wales by focusing on localities from which the Museum could enhance its collections to make them more representative.
Following the above rationale, in 1996 the Museum embarked on the first phase of the MINESCAN project, a review of the mineralogy of the mine sites of Wales.
Initially, this involved establishing a database of all the mines and principal vein and mineral outcrops across Wales. The information was compiled into a spreadsheet, along with full grid reference data a time consuming exercise given the amount of misleading or even totally erroneous information available in previous literature. At this point, the Countryside Council for Wales sought to co-support the work. In particular, CCW was seeking advice on the relevance of the project’s findings in relation to its Geological Conservation Review (GCR) and RIGS initiatives. CCW fed the database grid references into its Geographic Information System and provided the Museum with maps, at various scales, showing the distribution of all the mines and mineral sites. This allowed refinement of the database, and a further iteration of the maps.
With financial support from CCW, the Museum had, by the end of March 1997, completed a review of the mineralogy of all mines in the counties of Dyfed (as was) and Powys, and assessed their importance for conservation. Appropriate specimens were collected from the sites and, where possible, archive photographs taken. Sometimes the weather hampered photography, at other times the old mine site was nothing more than an overgrown mound in a field or, even worse, completely covered by conifers!
The fieldwork was undertaken by John Mason, working as an independent consultant to the Museum, and the review confirmed some of John’s earlier astute observations on the mines of central Wales made during the course of his recent M. Phil. study.
Nevertheless, there were important findings, in particular in little-studied areas of Dyfed and Powys. For example, there are records which suggest that the little-known St Elvis lead mine, in Pembrokeshire, was worked as early as Elizabethan times for silver; the presence of tetrahedrite (a complex sulphide mineral containing silver) was confirmed during the project, supporting the notion that this was indeed once a silver mine. In addition, Dolyhir Quarry provided some astonishing discoveries, with the identification of mineral veins cutting the limestones dominated by baryte and tennantite (a copper arsenic sulphide), but containing a suite of other rare primary and secondary minerals, many new to Wales, such as proustite, rammelsbergite, greenockite, olivenite and adamite. Finally, the mineral veins of the rather remote mines of the Llangynog area of northern Powys were investigated, and their marked similarities to those of the Central Wales Orefield were noted, leading to the probability that the two areas are metallogenically linked.
After the success of the first phase of the project, CCW is funding a review of the mine and mineral sites in the counties of Gwynedd and Anglesey. Fieldwork is complete, and a report with recommendations has just been presented to CCW.
Like the review of Dyfed and Powys, the Gwynedd and Anglesey work has provided interesting discoveries. The first Welsh occurrence of cosalite, a rare lead bismuth sulphide, has been identified from Braichyroen Mine in Snowdonia, and the presence of bismuth-bearing minerals now appears to be a feature of the volcanic-related mineral veins of central Snowdonia. In addition, the extent of so-called ‘Alpine-type’ veins across Snowdonia has been refined. The mineral veins are important in that they offer a constraint on the timing of metamorphism and rock deformation. A sample from one of these veins exposed at Prenteg has been sent to the British Geological Survey with a view to U-Pb radiometric dating of the monazite crystals. Finally, and perhaps most importantly, field evidence from the cliffs at Friog, near Fairbourne, strongly suggests that the gold-bearing veins of the Dolgellau area are older than has been previously thought.
Funding from CCW has been secured for a third phase of the review during 1998-99, with a study of the mines of the old county of Clwyd. This was once a very important mining area, with important lead mines in the Halkyn and Holywell district, and also to the west of Minera. Unfortunately, there has been extensive land reclamation around Halkyn and Holywell, and consequently the chances of any major discoveries are slight. This a great disappointment considering the quality of pyromorphite specimens that were discovered in this area in the days of Thomas Pennant, the 18th-century naturalist. The Minera district, however, is more promising, as is the area around Moel Famau, where there have been early, speculative reports of gold.
Completion of the third phase of the project will leave only the old counties of Glamorgan and Gwent for study, a task to be undertaken during 1999-2000, drawing to a conclusion this major review of the mineralogy of the mine sites of Wales for the new Millennium.
NRSC Manage UK Hyperspectral and SAR Aerial Campaign
Anthony Denniss, of Exploration Services, has recently had his work cut out for him, managing a large UK airborne campaign for the British Government. The ‘SAR and Hyperspectral Airborne Campaign’ (SHAC), initiated and funded jointly by the British National Space Centre and the Natural Environmental Research Council, involved the mobilisation of two DLR aircraft to the UK during May and June 2000. This is the firstover the UK by current generation SAR and hyperspectral -ever data acquisition systems, giving the UK research community a significant R&D opportunity. (see BNSC’s Web page at www.bnsc.gov.uk/earthobs/shac.htm).
The first aircraft was fitted with the high resolution, multi-frequency, multi-polarisation E-SAR instrument, whilst the second was fitted with the HyMap hyperspectral sensor, from Australia’s HyVista Corporation. In total, SAR data was acquired over 11 research sites, whilst hyperspectral data were acquired over 9 research sites. A number of additional sites were also acquired to take advantage of the excellent flying conditions.
We were particularly fortunate with the weather, given our wet Spring and early Summer. The HyMap scanner arrived, fitted in the Dornier, on the first day of a 4-day hot sunny period, before returning to Germany as the UK weather deteriorated once again – the plague of operating airborne systems in temperate northern Europe.
Figure 1 – HyMap scanner during image acquisition from a DLR Dornier 228-101.
For further information on the BNSC-NERC SHAC project and NRSC’s hyperspectral initiative, please contact Anthony Denniss.
Airborne mapping of surface mineralogy at Parys Mountain, North Wales
One of the research sites flown with the Hymap scanner was Parys Mountain, located on the island of Angelsey, North Wales. This site was proposed by NRSC as being probably the best in the UK for testing imaging spectrometry for mapping surface mineralogy, the proposal being accepted by NERC on scientific grounds as being appropriate for inclusion in SHAC.
Parys Mountain was the world’s largest copper producer at the end of the 18th Century, ceasing in 1904. The orebody is geologically unique in the UK, but displays a range of exposed mineralogy typical of Volcanogenic Massive Sulphide orebodies elsewhere in the world . Although still the subject of modern exploration, an extensive legacy of pits, waste tips, slag, settling ponds, and natural outcrop offer a mineral mapping challenge in both the VNIR and SWIR spectral regions. This is related not only to composition of the source materials but also to major heavy metal contamination in the surrounding district – the subject of several past environmental and geochemical studies.
Under its SHAC obligations, NRSC will be undertaking a program of research. The project will test the efficacy of a current-generation, operational hyperspectral system, HyMap, in building a case history meaningful to the geological and environmental character of the area, a designated Site of Special Scientific Interest. Secondly, we seek an understanding of optimal pixel sizes and issues affecting endmember extraction, unmixing and mosaicking across flight-strip boundaries, pertinent to deploying a methodology in an operational context within the global mining industry.
For further information on the Parys Mountain project, and NRSC’s capability in general in support of the Minerals Industry, contact Alistair Lamb.
Figure 2 – An oblique digital aerial photograph of the Parys Mountain site, collected during the HyMap overflight, showing both the old workings and a major example of acid mine drainage.
Society for Mining, Metallurgy, and Exploration
Metamorphosed volcanogenic sulphides
From “The Atlas of Opaque and Ore Minerals in their Associations”
Copyright by Dr R A Ixer
Pyrite, chalcopyrite, sphalerite, covelline and galena. Parys Mountain, Britain
Euhedral pyrite crystals (pale yellow-white, bottom centre) have relict poorly polished cores (top left), suggesting that they have recrystallized. Chalcopyrite (yellow, centre), which has altered to covelline (blue, centre right and left) about its crystal boundaries, is intergrown with sphalerite (light grey, left). Sphalerite contains abundant chalcopyrite inclusions aligned along crystallographic directions (left) and has suffered chalcopyrite disease. A single grain of galena (white, centre left) is intergrown with sphalerite. Dark grey areas are quartz, black areas are polishing pits.
Polished block, plane polarized light. x80, air
Pyrite, sphalerite, covelline, galena and chalcopyrite. Parys Mountain, Britain
Pyrite aggregates (pale yellow-white, top) have been fractured. These fractures have been infilled by chalcopyrite (yellow, bottom right), galena (blue-white, centre left) and covelline (blue, top left). Sphalerite (light grey, left) carries unoriented chalcopyrite and euhedral pyrite inclusions (left). The central aggregates of covelline have relict chalcopyrite (centre bottom left) within them and show intense bireflectance and reflection pleochroism (light to dark blue). Quartz is dark grey (top right), black areas are polishing pits.
Polished block, plane polarized light, x 80, air
Sphalerite, chalcopyrite, galena and pyrite. Parys Mountain, Britain
Fine-grained and complex intergrowths occur between sphalerite, the main sulphide (light grey), chalcopyrite (yellow, top centre), galena (blue-white, centre bottom) and subhedral pyrite (light yellow-white, bottom centre). Dark grey areas are quartz.
Polished block, plane polarized light, x 80, air
Galena, sphalerite, chalcopyrite and pyrite. Parys Mountain, Britain
Galena (blue-white, centre) shows characteristic triangular cleavage pits (black, centre) due to plucking along the (100) cleavage. It encloses euhedral pyrite (light yellow, high reflectance, centre right) and chalcopyrite (yellow, centre bottom), and is intergrown with sphalerite (light grey, left). Sphalerite carries abundant fine-grained chalcopyrite, partly concentrated along grain boundaries but mainly crystallographically oriented within crystals – this is chalcopyrite disease. The gangue is dark grey, the slightly higher reflectance phases are carbonates (top left, bottom right) which have plucked along their cleavage, the darker phase is quartz showing faint internal reflections (top right). Both pyrite and chalcopyrite show relief against galena. The different orientations of the triangular polishing pits within galena show that it comprises a number of separate crystals.
Sphalerite, chalcopyrite, pyrite and covelline. Parys Mountain, Britain
Euhedral pyrite (light yellow-white, centre right) crystals occur within chalcopyrite (yellow, centre right) and sphalerite (light grey). fractures within chalcopyrite and sphalerite are infilled with covelline (deep blue, centre). Sphalerite shows extensive chalcopyrite disease, with chalcopyrite inclusions oriented along crystal boundaries, twin planes and growth zones (top right). Very fine-grained to submicroscopic chalcopyrite (centre) imparts a yellow-brown surface colour to sphalerite. Black areas are polishing pits.
Sphalerite, chalcopyrite, pyrite, galena and covelline. Parys Mountain, Britain
Chalcopyrite (yellow, top right) is intergrown with sphalerite (light grey, centre). Sphalerite is fractured and partially replaced by covelline (blue, bottom centre). Euhedral pyrite (light yellow, high reflectance, top left) is intergrown with galena (blue-white, top centre). Widespread replacement of sphalerite by chalcopyrite (as chalcopyrite disease) is crystallographically controlled (centre right) or very fine-grained so giving a yellow-brown surface colour to the sphalerite (centre left), black areas are voids.
Polished block. plane polarized light. x80, air
From “The Atlas of Opaque and Ore Minerals in their Associations”
Copyright by Dr R A Ixer
Mineral Reconnaissance Reports
Minerals Group, British Geological Survey
112 Geophysical and geochemical investigations on Anglesey, North Wales
D C Cooper, I F Smith, M J C Nutt and J D Cornwell (1990)
This report describes a number of surveys carried out on Anglesey and not covered by previous reports in the series. A gravity survey of the island identified two large amplitude lows: one is associated with volcanic rocks and granite cropping out southeast of the Menai Strait Fault; the other is centred off the northwest coast and is possibly caused by a concealed granite. If of Caledonian age, such a granite would have influenced the distribution of base-metal mineralisation on the island. Positive anomalies are associated with metabasic rocks in the southeast of the island whilst Carboniferous sedimentary rocks give rise to gravity lows between Malltraeth and Dulas. Geophysical orientation studies of the Ordovician volcanogenic massive sulphide Cu–Pb–Zn–Ag mineralisation at Parys Mountain showed that this style of mineralisation generates strong chargeability anomalies but only weak EM anomalies, prone to interference from artificial sources. VLF(EM) proved useful for detecting steeply dipping conductors, and magnetic anomalies are produced by some basic rocks. A gravity survey detected Bouguer anomalies which two seismic refraction lines showed may be caused by concealed acid volcanic rocks. IP traversing indicated that no substantial mineralisation was associated with the Bouguer anomalies. Ground geophysical surveys confirmed airborne EM and magnetic anomalies at Bodewryd, Rhosbeirio, Treferwydd and Tyntywyn. At Rhosbeirio and Tyntywyn the cause of the EM ground anomalies remains uncertain whilst at Bodewryd and Treferwydd basic dykes are the probable source of magnetic and EM anomalies. Soil sampling was carried out around Cerrigceinwen, City Dulas, Llanbadrig, Llandyfrydog and Lligwy to investigate promising indications of mineralisation arising from earlier regional surveys. In addition, geochemical groundwater surveys were carried out around Cerrigceinwen and Llanbadrig, geophysical traversing at Llanbadrig and City Dulas, and rock sampling at Llandyfrydog. Anomalous results related to mineralisation, possibly of similar style to that found at Parys Mountain or Carmel Head, were recorded at Llanbadrig. Geochemical and geophysical anomalies probably caused by hitherto undiscovered mineralisation were also found at City Dulas. At Llandyfrydog large base-metal anomalies in soils were ascribed to metal-rich water, derived from the Parys Mountain mine, flooding across and percolating into superficial deposits. Some smaller anomalies are probably derived from weak base-metal vein mineralisation. In the Cerrigceinwen area stream sediment and groundwater survey data suggest that mineralisation might be associated with spilitic rocks within the Mona Complex and the basal Carboniferous succession, but limited soil sampling across these lithologies only located a few isolated base-metal anomalies. The single soil traverse sampled across the basal Carboniferous at Lligwy produced similar results.
CAL (SCMRE) ANNUAL REPORT
FY 1994 ANCIENT TECHNOLOGY
Early Mining at the Great Orme
A recent discovery on a headland in northwest Wales has led to the reassessment of the trade in bronze in northern Europe during the Early Bronze Age. A mine of that date has been discovered at Great Orme, and ores from the mine are the subject of CAL research. Its extent underground, limited only by the ancient water table at 250 feet, indicates a very much larger supply of copper in the British Isles than had been supposed and suggests the direction of the bronze trade may have been to, rather than from, the Continent during this period. In a collaborative program with Great Orme Mine Ltd. and the University of Liverpool, CAL is characterizing the ores to test this possibility. An initial selection of small finds and ores has been sectioned and analyzed. Seven ore or bronze metal samples from the mine have been analyzed by lead isotope analyses. The results of the analyses were compared to lead isotope data from the geologic literature on Welsh ore sources. Isotope ratios for two galena and an azurite sample appear to be similar to the copper ores found in Parys Mountain and the Mendips region of Wales but fell outside the normal distribution of those characterized isotopic ore groups. However, the lead isotope composition of the bronze fragments excavated from early levels in the mine show close similarity to the ore samples from the Mendips and southern Wales regions.
Biology and Environment: Proceedings of the Royal Irish Academy Vol 99B, No 1, 67-71 (1999)
LICHENS OF THREE MINE SITES IN CO.WICKLOW, IRELAND
National Botanic Gardens, Glasnevin, Dublin 9, Ireland.
At the inaugural meeting of the Irish Mining History Society in February 1996, after an extensive debate on the archaeological, industrial and cultural heritage of mines in Ireland, the biological heritage of Irish mine sites was highlighted. The conservation case for Irish mines sites ought to include scientific arguments addressing the significance of mine vegetation in each site, the species biodiversity of each site and the potential genetic resources of all organisms living in Irish mine sites.
Research in England and Wales has shown that old mine sites have a specialised lichen flora (Purvis and Halls 1996). Lichen surveys have been carried out at mines such as Coniston, Lake District; Parys Mountain, Anglesey; Stiperstones, Shropshire; Vitifer Tin mines in Dartmoor; Van Lead mines in Llanidloes; Caradon Copper mine, Pensilva in Cornwall; and old mine sites are now routinely visited by lichenologists in general field surveys. Several ascomycete species that had not been found previously in Britain were discovered, including Lecanora handelii, Lecidea atrofulva, Lecidea inops, Psilolechia leprosa, Rhizocarpon furfurosum, Stereocaulon symphycheilum, Vezdaea retigera, etc. (Purvis and James 1985; Coppins 1987; Coppins and Purvis 1987). On the basis of these findings extensive surveys of mine sites were commissioned by statuatory nature conservation agencies in Britain. Alan Fryday and Steve Chambers (pers. comm.) surveyed more than 50 mine sites and quarries in Wales, and they added several more species that had not been seen before in Britain; including Gyalidea subscutellaris and Melaspilea interjecta. Their investigations indicate that the diversity of lichen species at each mine site can vary enormously, depending on the area, age, complexity and pH of the niches available at individual mine sites. Regularly occurring lichens include Arthrorhaphis spp, Baeomyces rufus, Cladonia spp, Dibaeis baeomyces, yellow Lecanora spp, Micarea spp, Peltigera spp, Stereocaulon spp, various Trapeliaceae and Vezdaea spp (A. Fryday, pers. comm.; Purvis and Halls 1996).
Alan Fryday (pers. comm.) and Peggy Cayton (unpublished) observed that one typical group of lichens at mine sites, of the genus Vezdaea, are highly seasonal in their fruiting. The species can be identified only for a short duration in early spring, when the ascospores that form in the fruiting bodies (apothecia) become mature in February and March. Vernal ascospore maturation can be expected in many other lichens of mine sites. In autumn and winter, mine spoil surfaces are almost continuously humid for several months. In the Penrhyn Slate Quarries at Bethesda, North Wales, the peak release of ascospores of Rhizocarpon lecanorinum is in April and May (Clayden 1997). Fruiting is completed before soil surface desiccation begins with the onset of dry weather in late spring and early summer.
Preliminary observations on the flora of three mine sites in County Wicklow indicate that interesting lichens occur (Table 1). Selected voucher specimens are retained in the National Herbarium of Ireland (DBN). Lichen nomenclature largely follows Purvis et al. (1992). The old mines at Avoca (T17), Glendasan (T09) and Glendalough (T09) were examined, and these sites extend over substantial areas. Avoca is surrounded by intensive agricultural grassland and the lower valley is wooded, while Glendalough and Glendasan are open sites set in upland rocky heath in the Wicklow Mountains National Park. Several taxa in Table 1 are additions to the species list for Co. Wicklow (Seaward 1994), including Sarcosagium campestre and Solorina spongiosa. The first published records of Placopsis lambii and Vezdaea leprosa in Wicklow were based on observations at mine sites. In addition to those taxa mentioned in Table 1, several unidentified lichens have been seen. Only two or three full days fieldwork has been undertaken in each site. Intensive study, particularly in Glendasan and Avoca, should reveal some of the other scarce and characteristic mine site species which are known from broadly similar habitats in England and Wales.
Mineralogy of Pb-P grains in the roots of Agrostis capillaris L-by ATEM and EXAFS
CotterHowells JD, Champness PE, Charnock JM
MINERALOGICAL MAGAZINE, 1999, Vol.63, No.6, pp.777-789
Analytical transmission electron microscopy (ATEM) and X-ray absorption spectroscopy (XAS) have been used to determine the mineralogy of Pb-P deposits in the roots of the heavy metal tolerant grass cultivar Agrostis capillaris L. cv. Parys Mountain. The deposits have a pyromorphite (Pb-5(PO4)(3)Cl)type structure and composition although some of the Cl may be substituted by OH. Energy-dispersive mapping under the scanning electron microscope demonstrated that the majority of these deposits are present in the outer cell wall of the epidermis (the outermost layer of root cells). The phosphate composition of these grains contrasts with the phytate (C6H18O24P612-) composition of Zn-P deposits observed in similar electron microscopy studies. The physiological role of heavy metal P deposits is unclear. Heavy metal P precipitates may form actively as a tolerance mechanism to heavy metals or passively, sequestering P in a metabolically inactive form.
Early diagenetic controls on metal mobility in contaminated sediments
Paul Sullivan, Kevin G. Taylor, Adrian Watson.
Department of Environmental & Geographical Sciences, Manchester Metropolitan University, Chester Street, M1 5GD
Dulas Bay situated on the east coast of Anglesey, receives polluted waters from diverse locations. It has a primary point source of pollution in the form of Parys Mountain, a disused copper mine which feeds into the river Afon Goch. The Afon Goch waters are some of the most metal and acid contaminated waters in the UK. In addition the Irish Sea with its local inputs from the Mersey and North West conurbations inpacts the estuary.
The estuary provides an excellent example of a system rich in inorganic pollutants for the investigation of diagenetic controls on metal mobility. Core samples taken from a site situated in the fringing salt marsh were chosen as it provides both porewater and solid phases. This enables a characterisation of redox chemistry within diagenetic zones. This can then be contrasted with the associated solid phase mineralogies.
Reassessing the geological setting of the Parys Mountain polymetallic mineral deposit and the role of Lithogeochemistry in an ongoing exploration program.
S. C. Tennant University of Wales, Cardiff
The polymetallic (Zn-Pb-Cu) mineralisation at Parys Mountain, North Wales, is located in a sequence of bi-modal Ordo-Silurian volcanic rocks and shales within the Southern British Caledonides. Between 1768 and 1904 over 130,000 tonnes of Cu metal were produced. Since the pioneering work of Edward Greenly (1919), the genetic relationship between the host rocks and mineralisation has been much discussed: ideas have ranged from an epigenetic, post-cleavage origin for the Cu-dominated lodes to a synsedimentary-volcanogenic origin for the polymetallic sulphides with remobilisation during the Caledonian orogen. The apparently complex structure and the volcano-stratigraphy are also points of debate. The volcano-stratigraphy has defied previous attempts at correlation, based on lithological criteria, it being limited to the deci-metre scale . Palaentological work lead to an interpretation of the structure as an asymmetric, E-W trending, syncline overturned to the south. This was later refined, the anomalous structure at the western end of the fold being recognised. Since the beginning of modern exploration (1955), through to the most recent phase of activity (1984-1990) exploration programmes have adhered to the synclinal model. Despite over 60Km of drill core being recovered and underground development Parys Mountain has remained non-productive.
In 1995, Anglesey Mining instigated a review of the geology in order to provide a sound geological framework to a more effective exploration programme. This involves the compilation/reinterpretation of previous drilling results, re-logging of available drill core and re-mapping of the surface geology, all in the context of a new exploration model. As an integral part of this review, a research project at Cardiff University (funded by Anglesey Mining), is being carried out. It has the aim of constraining the mineralisation within a chemostratigraphic framework for the volcanic rocks and associated lithologies. This lithogeochemical approach allows correlation of stratigraphy, the determination of deposistional and tectonic environment for the host rocks and their petrogenesis, as well as allowing Hydrothermal alteration to be characterised and quantified (via mass-balance techniques). The ultimate goal is to identify and follow favourable volcanic units/contacts and to recognise areas of increased Hydrothermal alteration to vector into significant mineralisation. The high-field-strength (HFSE) elements, Al, Ga, Ti, Sc, Zr, Y, HREE, Th and possibly Nb are considered to be immobile in Hydrothermal alteration and low-grade metamorphism associated with volcanogenic massive sulphide (VMS) deposits. Spatial analysis of immobile element variations in discrete units is essential in the characterisation and correlation of the host rocks.
X-ray fluorescence and Inductively-Coupled-Plasma Mass-Spectrometry analysis for immobile and rare earth elements has revealed that the felsic volcanic rocks fall into two compositional sub-groups: one is of tholeiitic affinity, with a subordinate transitional affinity group. Within these divisions, a low Zr (<350ppm) and a much sub-ordinate high Zr (500-800ppm), peralkaline, types exist. High Zr, peralkaline rhyolites are common in the Ordovician Snowdon Volcanic Group and at the Avoca VMS deposit. The felsic volcanics are moderately silica-and K-enriched and strongly Na-Ca depleted. Mg-Fe enrichment is observed as chlorite and ankerite. Mafic volcanics occur at the base of the volcanic succession. Their overall chemistry suggests that they are intra-plate basaltic andesites of two affinities: tholeiitic and transitional-to-alkaline. The strong variations in alteration observed mean that immobile element ratios, such as Al/Zr and Zr/Nb, are useful in identifying original volcanic units.
As further data are obtained, a detailed chemostratigraphic framework will emerge. This will be combined with new palaentological data, obtained by the University of Leicester, and radiometric dating of galena from the synsedimentary and epigenetic mineralisation, obtained by the Scottish Reactor Centre at East Kilbride. The accepted view of the structural setting sees Parys Mountain as an overturned syncline. The alternative view presented here is that the volcanic succession was not greatly changed during Caledonian tectonism. The sequence is north-younging and homoclinal. Further, the volcanism is in part lower Silurian (Llandovery) in age. The overall geology is still imperfectly understood; the work described here is ongoing and is being applied to an active exploration programme being carried out at Parys Mountain.
It is hoped an holistic application of this data to the current exploration model will result in Parys Mountain becoming a productive mine in the near future.
3D modelling for assessing complex mineral deposits
3D modelling is also becoming extremely important in assessing the environmental impact of new developments
Virtual-reality and 3D-visualisation models are used increasingly in all areas of life, from architecture to fighter pilot training, and current applications include mineral exploration and mine development. The BGS has the capability for 3D mineral deposit modelling and reserve calculation using VULCAN software. VULCAN is a dynamic, 3D geological modelling and mine-planning system, which has been used in applications as diverse as visual impact assessment for new mines, modelling the distribution of noise pollution around mine sites, and on-going calculation of ore reserves of worked deposits. BGS staff have used the system to create a model of the structurally complex orebody at Foss Mine, Aberfeldy, Scotland on a project under the Technology Access Programme of the Department of Trade and Industry (DTI). Data were provided by the mine personnel in the form of mine plans, mine sections, surface geology, and geophysical, soil and borehole data. These data were integrated into a single model showing the proven extent of the orebody, and the current mine design, with driveages, declines and stopes.
From this model, reserves can be calculated,and the future mine development can be planned. The 3D model helps to optimise the extraction of ore. Under another DTI Technology Access project, BGS staff have worked with Anglesey Mining plc and KRJA Systems Ltd on a model of the geologyand mineralisation at Parys Mountain on Anglesey, North Wales, so that the underground geology can be better understood and new exploration can be targeted. A large number of boreholes have been loaded into the model, so that lithological, geochemical and PIMA (Portable Infrared Mineral Analyser) data can be displayed in 3D together with existing mine shafts, topographic data and structural surfaces.
A preliminary block model of geochemical data, in part of the mine property has been produced to indicate the distribution of lead and copper, mine. 3D modelling is also becoming extremely important in assessing the environmental impact of new developments. For example, it is possible to create a model of an open pit before it is dug, with appropriate landscaping to enable planning authorities to visualize the appearance of the site before, during and after working, from any possible angle. VULCAN software has also been used by the BGS in non-mining contexts.
For example, the geology around the potential nuclear repository site at Sellafield in west Cumbria is complex and a full 3D structural model of the area was required, so that a better understanding of the hydrogeology and movement of groundwater could be gained. The model was constructed by integrating seismic reflection profile interpretations with lithological data from boreholes and surface outcrop mapping. 3D visualisation has always been an essential component of a geologist’s interpretive skills. Now it is possible to develop rigorous computer models to aid the geologist in providing sound advice.
(Courtesy: British Geological Survey)
The evolution of copper tolerance in a terrestrial mollusc
Copper is very toxic to molluscs; so much so that it has often been used as the active ingredient for molluscicides. Yet at Parys Mountain on Angelsey, the site of what was once a huge open-cast copper mine there is a very large population of the snail Helix aspersa. Field work suggests that these snails inhabit non-toxic areas within the mine site and so may avoid toxic areas. However laboratory behaviour studies have failed to show any avoidance, a behaviour which is shown by snails from non-toxic control sites. Further work on behaviour and ecological physiology of mine and control populations is needed. Prospective candidates should have an interest in either evolution or zoology including animal behaviour.
Canterbury Christ Church University College
Seeking the origins of bronze tools
The earliest metal goods probably came to Britain from Ireland. Paul Budd reports
Squeezing through the labyrinth of tortuous passages carved out of the solid rock at depths of 70 metres or more, it is difficult not to spare a thought for Britain’s prehistoric copper miners. Some of the tunnels beneath the Great Orme’s Head, near Llandudno in North Wales, are so small that they could only have accommodated children.
The experience is fascinating and the conditions brought vividly to light. Crawling on hands and knees by the light of a dimly burning taper clenched between the teeth; huddled in the gloom and pounding at the fire-softened rock with a cobblestone hammer; the work must have been excruciating. And yet, 20 years ago this and other astonishing evidence for the earliest copper mining in Britain did not exist and the passages lay undiscovered.
Today, we are entering a new phase of research on Britain’s earliest copper mines. Much of the excavation and recording has taken place, telling us when and how the mining and ore processing was carried out and, to some extent, about the people who did it; but where did all the copper go? A long-standing objective in archaeometallurgy has been to try to link Bronze Age metal tools and weapons to their sources. Now, perhaps for the first time, scientific techniques are beginning to tell us something about this vital key to understanding the organisation of prehistoric metal production and exchange.
Until the early 1980s only one prehistoric copper mining site was known in the British Isles. The site, Mount Gabriel on the Mizen peninsular in the far south-west of Ireland, was simply a cluster of primitive opencast workings and shallow galleries dug into the hillside. The mine was considered unique, perhaps owing its survival to the extremely poor quality of its copper ores and therefore to the lack of subsequent interest in mining them. When it was investigated in the 1960s, it was generally agreed that virtually all the evidence for early copper mining in Britain had been obliterated by later activity. Copper mining peaked in the late 19th century, by which time mechanised techniques were responsible for radical alteration of many mining landscapes. Survival of prehistoric evidence seemed unlikely. Today, this pessimism has been dispelled.
Thanks very largely to the (often unpaid) efforts of a small number of dedicated field workers, some 30 probable or definite prehistoric copper mining sites have now been identified in the British Isles, of which the Great Orme, with its visitors’ centre and guided tours of the Bronze Age mine workings, is the most impressive. Many of these sites survive, despite all the odds, on surface outcrops of copper which, in the historic period, became well known and highly productive. In addition to Mount Gabriel and the Great Orme the best known and best investigated sites to date are at Parys Mountain in North Wales, Cwmystwyth in central Wales, Alderley Edge in Cheshire and Ross Island near Killarney in South-West Ireland.
Over the last decade or so, the antiquity of mining at these sites has been firmly established, mostly by radiocarbon measurements on charcoal and sometimes bone sealed within the mining waste. In addition to the mine, Ross Island also features a `work camp’ area from which radiocarbon dates have been obtained. All of the sites were in use during the Bronze Age. Ross Island appears to be the earliest, with dates clustering in the second half of the 3rd millennium BC. This is just prior to the beginning of the Early Bronze Age in Ireland – the period associated with the introduction of metallurgy in the British Isles. The other sites were all in use at much the same time spanning the Early Bronze Age and earlier Middle Bronze Age from c 1900-1200BC. But what of the evidence for the copper they produced?
Bronze Age metalwork has an enduring fascination and has been the subject of study for two centuries or more. In the latter half of the current century, typological classification of metalwork has given way to a developing interest in its composition in the hope that stylistic or regional metal groups would share characteristic patterns of trace elements which might then be linked to particular ore sources. In the British Isles, significant changes in the impurity pattern of copper and bronze metalwork do occur over time and between different regions, but attempts to relate this to the general pattern of copper mineralisation in the British Isles have always been inconclusive.
Now, with the mines identified, it is becoming possible to develop a clear idea of the impurity patterns likely to have resulted from smelting the ores from particular places. A detailed mineralogical survey by Rob Ixer at the University of Birmingham is now revealing just such information. The work is painstaking and highly skilled. A detailed understanding of metallogenesis and ore geology are required even to select representative mineral samples for further study. Ore petrography is then used to identify the mineral suite and build up a picture of the formation process (or processes) and subsequent geological history of the deposit. Only with this level of understanding is it possible to identify the ore that was actually mined from a particular site.
The results of Ixer’s analyses are fascinating. With one exception, all of the sites investigated can only have produced virtually pure (impurity free) copper. This contrasts strongly with the Bronze Age metalwork for which common impurity patterns have emerged.
The earliest metalwork, with a primary distribution in South-West Ireland, often contains considerable arsenic – sometimes several per cent – with lesser amounts of antimony and silver. Later, at roughly the time that mines such as the Great Orme and Cwmystwyth were in use, these compositions give way to copper with a higher nickel content. By this stage the copper is most often alloyed with tin to form bronze and has a wider distribution across upland areas of Britain.
Of the mines investigated, only Ross Island, the earliest, produced copper with a significant arsenic-antimony-silver impurity pattern. Could it be that Ross Island, perhaps together with as yet undiscovered mines in the region, was the dominant source of the earliest copper before it became mixed and diluted with the pure copper from Wales and northern England? Were Killarney’s Beaker Culture people our first metallurgists? If so, where did the nickel come from in the later metal? Does it represent the growing status of alternative groups of metallurgists with their own as-yet-undiscovered copper supply?
Answers to some of these questions are now emerging from lead isotope analyses of the ores. The isotopic composition of lead within an ore deposit relates to its geological formation process and age, with the result that different deposits can have characteristic values (although they sometimes overlap). Lead isotopes are unchanged by the smelting process so that the signature of the ore is carried by the finished copper.
Brenda Rohl, working at Oxford University’s Isotrace Laboratory, has analysed ores from many of the newly discovered mines as well as numerous Early and Middle Bronze Age copper and copper-alloy artefacts. Some of the earliest,
type A', metal tools do have isotopic signatures which match ores from Ross Island, but the mine is unusual in having two types of ore with quite distinctive isotope signatures. Sometype A’ tools have isotope ratios which suggest they were made by mixing the two.
For the later metalwork analysed by Rohl the picture is more complex with a pattern indicative of the mixing of copper from multiple sources. Only at Ross Island is there evidence of prehistoric metal processing in Britain, and in general we do not know how far ores were transported for smelting. However, the mixing is less likely to have resulted from the original smelting process, and was probably rather the consequence of later melting-down and recycling of artefacts from different sources. This is a relatively simple operation and may have been performed, perhaps from an early date, more commonly than is traditionally thought.
Occasionally, however, very distinctive patterns emerge from which specific conclusions can be drawn. In one case analysis of five of the nickel-rich `type B’ artefacts shows them to have a highly unusual lead isotope composition resulting from uranium associated with the ore. There are only a handful of deposits, all in Cornwall, where such nickel- and uranium-bearing copper ores occur.
The discovery of the copper mines has undoubtedly given a boost to archaeometallurgy in the British Isles, at last allowing us to bridge the gap between artefacts and their sources. Clearly much remains to be done, but interesting results are already emerging which reinforce the suggestion of an early metallurgical focus in South-West Ireland. Their products were soon joined by, and mixed with, those of other miners working the copper deposits of Wales, northern England and, almost certainly, Cornwall, where prehistoric mines may yet be awaiting discovery.
Dr Paul Budd is NERC Advanced Research Fellow in Science-based Archaeology at the University of Bradford
Issue no 56, December 2000
Meet the metal makers
Metal came relatively late to Britain. But it was here that a remarkable new compound was perfected. It was called bronze. Paul Budd reports.
Four thousand years ago, on the gentle slope of a south Wales hillside, a small hole was dug and a precious cargo consigned to the earth. The buried items would have been recognisable to the builders of the medieval castle that came to share the hillside three millennia later. Familiar too to the Victorian nobles who rebuilt those fortifications a thousand years after that, creating a splendid fairytale folly at Castell Coch in the South Glamorgan countryside.
Indeed, the contents of the ancient pit were clear enough to the metal detector user who brought them back to the light of day in 1984.
The buried treasure, known as the Castell Coch hoard, consisted of three weapons, a dagger and two halberds (dagger-like blades hafted as axes), made and deposited in about 2200-1800 BC, not long after the beginning of metal making and use in Britain.
But who made them and where? How did the technology of mining, smelting and weapon making come to these islands in the first place? Today it is not just the form and context of finds like those at Castell Coch that help us to answer these questions, but the very metal itself. The latest archaeological research shows that, although metallurgy came relatively late to Britain, its arrival here sparked a technological revolution whose consequences reached every corner of Europe. It was in Britain that metal workers perfected a new metal. It was called bronze.
For more than half a century archaeologists have grappled with the enigma of Britain’s first use of metals. It appears to have taken the art of metallurgy more than 2,000 years to travel from the ancient Near East and Balkans to Britain, and its dramatic arrival in about 2500 BC prompted early scholars to suggest direct contact between Britain and the great metal-using civilisations of the Mediterranean. The images evoked were of roaming metal prospectors searching savage lands for raw materials. The reality may be more prosaic, but is no less interesting.
In fact, the long history of metallurgy was not just a Mediterranean affair. For its origins we have to look several thousand years before the Castell Coch artefacts were deposited in their shallow sanctuary. The very earliest copper objects come from settlements and graves of the late 8th/early 7th millennium BC in Mesopotamia and Anatolia, and these are thought to be the products of rare outcrops of copper metal (not copper ore) found in some parts of this copper-rich area.
The momentous discovery of smelting came later, in the mid-5th millennium, seemingly independently in Anatolia, Mesopotamia and the Balkans. By this time copper miners were hard at work at places such as Aibunar in Bulgaria and Rudna Glava in Serbia, where rich veins of copper oxide and carbonate minerals were being emptied to make what must have seemed an entirely new kind of material. Hard enough to sharpen to a cutting edge, yet tough enough not to shatter. Infinitely remeltable and reuseable.
This wonder material, copper, could be smelted with relative ease from the weathered and oxidised Balkan ores, simply by heating them in a bed of charcoal. With a little assistance from bellows, this pure carbon fuel could produce a high temperature and maintain the chemically reducing conditions needed to convert the ore to metal. Output was impressive and the Balkan miners were soon possessed of large numbers of massive copper axe-hammers, adzes and chisels. Their success however was not to continue without break.
After perhaps a thousand years of Balkan copper production, the deposition of copper in hoards and graves faded away. The technology was not lost though. As dramatically as it appeared to decline, metallurgy was back, but this time in a different location and with a new sort of metal. In the mid-4th millennium, arsenical copper was now taking centre stage with a new focus on Alpine and sub-Alpine Europe. A similar copper-arsenic alloy was developed in the old copper-producing centres of the Near East, although there the transition took place without the production hiatus apparently experienced in Europe.
Exploitation of the rich Alpine copper required the development of a new technology. Unlike the Balkan ores, the Alpine deposits were mostly of copper sulphide minerals. Unusable as mined, these had to be roasted before smelting to convert the sulphide minerals to the oxides that would have been familiar to the Balkan smelters. In practice, lumps of sulphide ore were placed on a hot wood fire and stirred round, to introduce plenty of oxygen and convert the ore to copper oxide. The oxide ore was then smelted in an enclosed furnace heated by charcoal with as little oxygen as possible to reduce the ore to metal. Such roasting beds and smelting furnaces dating from the later Bronze Age have been found in the Mitterberg region south of Salzburg.
The new focus on arsenic-rich Alpine ores and the widespread occurrence of arsenical copper artefacts in the 4th millennium is something of a `chicken and egg’ conundrum for modern scholars. Did metal-workers deliberately seek out deposits rich in arsenic (a metalloid element) or was the arsenic an unintended inclusion?
Recent research suggests that early metal workers knew exactly what they were doing in using these ores. A significant addition of arsenic to copper produces better mechanical properties, and higher levels produce a metal of striking silvery appearance. Artefacts with higher levels tended to be `high status’ objects such as knives and daggers, while everyday tools, such as the 4th millennium BC Iceman’s axe, contained less. The proportion of arsenic in artefacts ranges from less than 1 to 7 per cent – never more than that – while ores can contain up to 30 per cent, suggesting that arsenic quantities were being controlled.
Such control may have been exercised by mixing arsenic-rich copper with other types of copper, both in pure form and as recycled tools.
The evidence for such mixing comes from slightly later periods, but might apply equally to the 4th millennium. At the mine site at Ross Island in Ireland, for example, dating from the mid-3rd millennium, the ores are varied, containing anything from a few to about 30 per cent arsenic. However, the metal produced was much more consistent, suggesting that the ores were mixed. Later still, in the 2nd millennium, the Great Orme mines in North Wales produced perhaps hundreds of tonnes of copper at a time when most artefacts contained some degree of arsenic, and yet the Great Orme ores contained no arsenic whatsoever. The Great Orme metal was clearly not used without some degree of adaptation.
Whatever the truth of central Europe’s arsenical copper in the 4th millennium, Britain remained literally in the Stone Age. It would be a thousand years before the island periphery of north-western Europe was to experience metallurgy at all. And yet when it came, the metals revolution took off with explosive technological pace. Within a few hundred years not only was a Continental-style arsenical copper industry thriving here, but by about 2000 BC the harder, tougher alloy of copper and tin known as bronze had also been invented. It replaced arsenical copper across Europe and dominated the European metals scene until the coming of iron more than 1,000 years later.
It is perhaps not strictly true to say that bronze was invented in Britain. The very earliest combination of tin and copper is found in Anatolia, but Near Eastern bronze contained less tin, in less standardised quantities, than was found in British bronze. Put simply, it was inferior bronze. In Britain, bronze was produced from the outset with an almost standard composition of 8 to 12 per cent tin, ensuring the optimum mix of qualities. For archaeologists the rapid establishment and spectacular success of metallurgy in the British Early Bronze Age, from 2500-2000 BC, is something of a quandary. How did metallurgy arrive in an apparently advanced state? Who brought it and why did it take off here so well?
If Aegean prospectors could be ruled out as the fathers of British metal making – and there is simply no evidence in Britain of contact with Aegean civilisations – could metal tools and weapons have filtered across the Channel, followed perhaps by those skilled in their manufacture? If this latter scenario were true, we might expect to see a cluster of early metal finds in south-eastern England, but we do not.
In fact, the region where early tools and weapons suddenly appear in large numbers is south-west Ireland, predominantly in the form of simple `flat’ axes. Wherever it was made and traded, more of it was left behind in the rugged Atlantic coastal landscape of Munster than anywhere else. This Irish metal was not inferior stuff either. What was being made and deposited was not the simple copper of the earliest European metallurgy, but arsenical copper, the superior material pioneered in Alpine Europe and, by this time, also commonplace throughout the Mediterranean as far west as Spain and Portugal.
So how did this advanced technology suddenly come to Ireland, and why? Who were these metal makers? To a previous generation of archaeologists such developments could only be explained by the invasion and settlement of new, technologically advanced, people. If not Greeks prospecting for precious ores, perhaps Iberian settlers made their way north along the Atlantic coast seeking out sources of the arsenic-bearing copper ores with which they were familiar.
This notion of a mass movement of people, even an invasion, found support elsewhere in the archaeological record. The arrival of metallurgy was not the only big change taking place in the middle of the 3rd millennium, but the period also saw the appearance of beaker pottery in the British Isles. These highly distinctive vessels, often buried with the dead, were widespread in central Europe and Iberia before they were used in Ireland. Was there a link?
Today, there is some reluctance to see the arrival of a new pottery form, however distinctive and ubiquitous, as necessarily a sign of the arrival of a new population of immigrants. New artefacts and burial practices can, after all, simply reflect changing ideas or trade, which was well-established along the Atlantic seabord in this period. But the idea of a Beaker Culture, with its distinctive pottery, funerary practices, flint arrows and established tradition of metal making, is hard to cast off.
Not many years ago, this would have been about as close as it was possible to get to meeting the metal makers who brought metallurgy to the British Isles and launched the Age of Bronze. But now our understanding is being transformed by a combination of archaeological research on the ground and scientific investigation in the laboratory. When the notion of the Beaker inspired birth of Irish metallurgy was first aired, there was no evidence for prehistoric mining in that country. Proponents of the theory pointed to geological data for the occurrence of `fahlerz’ ores in the region, linked, they said, to the distinctive arsenic-rich Munster axes. However, when prehistoric mines were identified in the 1960s at Mount Gabriel on the Mizen Peninsular on the far south-west of Co Cork, they were a miserable affair indeed – shallow scrapings on the hillside devoted to the recovery of ore which was almost devoid of copper and without arsenic. It seemed an unlikely base for production on the scale suggested by multitudes of axes.
The breakthrough came in the 1990s with the excavation of the earliest prehistoric copper mine yet discovered in the British Isles. At Ross Island, on the eastern edge of Lough Leane in Co Kerry, the archaeologists struck lucky. The site had been worked for copper in the early 19th century and the miners had found older workings. Systematic excavation and radiocarbon dating now show that the earliest of these date to the mid-3rd millennium.
Moreover, Ross Island is unique in preserving not just the mine itself, but also the miners’ work camp in an area of huts and ore processing installations immediately adjacent to the workings. Among the shelters, animal bone food waste and worked flint, were numerous early Beaker sherds confirming the long-suspected link between the users of the distinctive pottery and the mining of metal. Equally striking was the ore itself, not the low-grade copper of Mount Gabriel, but rich arsenic-bearing sulphide ores of the fahlerz type. From a single site all the theories could be confirmed.
Although they do not turn up in the numbers found in Munster, early flat axes have been recovered from all over England, Wales and Scotland. These axes have been chemically analysed and share the distinctive arsenic, antimony and silver impurity pattern of the Munster finds. It seems reasonable to conclude that these earliest British metal artefacts were indeed imports, not from the Continent but from the mines of Munster – of which Ross Island was probably only one of several.
But this Irish dominance was not to last. By the time the Castell Coch weapons were made, axes, daggers and halberds were not just being made in Ireland, but also in the metal-bearing regions of Wales, Scotland and England. Within a few hundred years of the first roasting of Ross Island’s ores, copper mines were opening up at places like the Great Orme and Parys Mountain in North Wales, Cwmystwyth in central Wales and Alderley Edge in Cheshire. The relatively pure copper of the British mines was fed into the pool of arsenic-rich copper in circulation to produce ever-increasing numbers of metal tools and weapons.
But what of the development of bronze? Some of the copper alloy tools made around 2000 BC, including two of the Castell Coch finds, contained significant traces of nickel. We have now tracked down the source of this distinctive ore, and it provides us with the missing link between copper alloys and the development of the new metal made with tin. Different ore sources have distinctive lead isotope ratios which can be used to provenance archaeological artefacts, and the Castell Coch artefacts were highly unusual in having very high lead isotope ratios of a sort that can only occur when uranium is present within the ore. Further analysis of the ratios provided the geological age of the deposit which allowed us to pinpoint the source of the ore even more accurately. Taken together, the data showed that these particular artefacts were made from copper ore that could only have come from one place in north-western Europe – Cornwall.
The Cornish provenance of the Castell Coch hoard and other non-Irish tools and weapons leads us directly to the pioneers of bronze, because it confirms that a mining tradition was established in Cornwall at the time of the invention of bronze, in an area that contains one of the richest tin fields in the world. Along with Afghanistan, Cornwall is one of only two possible major sources of the tin used in bronze throughout Europe after about 2000 BC. No prehistoric mines have yet been found in Cornwall but this is hardly surprising: the landscape has been eaten away by coastal erosion and turned upside down by the vast scale of the post-medieval tin industry. All prehistoric evidence may have been destroyed.
It is unlikely that bronze tools were actually made in Cornwall. Metallurgy probably took place nearer the copper mines of Great Orme and elsewhere, with smelted tin or (more likely) tin ore traded up from Cornwall to be mixed in with molten copper. Strangely enough, as the source of one of Europe’s most valuable commodities, Cornwall contains few signs of conspicuous wealth in the Bronze Age period. There are few great monuments or burials.
This has led some archaeologists to speculate that it was not locals but middlemen who made most of the profit out of this exceptionally lucrative international trade in tin. And who were the middlemen? The most impressive signs of wealth in the Bronze Age are found in the barrows and monuments of Wessex. Were the Wessex chieftains the `barrow boys’ of the Bronze Age economy? It is an intriguing thought, and it may just be true.
Paul Budd is an Honorary Research Fellow at the University of Durham and a specialist in archaeometallurgy
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The British & Irish Graptolite Group
Anna Maria Jones of Leicester University Geology Department is studying the inter-relationship of sedimentation, diagenesis, volcanism, silicification and mineralization at Parys Mountain, Anglesey, North Wales. Uncertainty continues to surround the timing and geological context of the mineralization at Parys Mountain, the largest copper mine in the world in the late 18th century. It is the most extensive volcanogenic massive sulphide deposit known in Britain and although it is currently not being mined, exploration is still going on. A major problem is that the detailed structure and age of the rocks is still unresolved, and the relation of the sequences to the Lower Palaeozoic successions in Wales, the Lake District and Ireland is enigmatic.
My research will aim to set Parys Mountain more firmly in both a local and regional setting, by making a thorough investigation of the mudrocks which host the ores, together with their diagenetic fabrics. These Lower Palaeozoic rocks contain important information bearing on depositional processes and ocean chemistry, and show widespread silicification and pyritization. Decoding the history of events will require knowledge of the stratigraphy, and to this extent the study of graptolites will be invaluable. No other fossil group can offer such reliable, high-resolution of the local biostratigraphy, particularly for the Silurian mudrocks which occur within the core of the Parys Mountain syncline.
There is some superb material here, much of it originally examined by Gertrude Elles some eighty years ago. Sparser graptolite assemblages occur in Early to Mid-Ordovician mudrocks in the limbs of the overturned fold structure. Newly collected specimens from all three mudrock sequences, and graptolites in museum collections, will help to set age constraints on the complex sequence of events which ultimately led to copper ore formation at Parys. Graptolites may also help resolve the problem of an apparent hiatus within the succession. The Middle to Upper Ordovician (late Llanvirn – Ashgill) and the lowermost Silurian (Rhuddanian) strata appear to be missing. Is this really so? Were they never deposited, or deposited but then subsequently thrust out? Underground surveys, by means of disused mineshafts and examination of drilled core material of the relevant contacts, may help to provide an answer, again by using graptolite biozones to determine their sequence in time
Metal Ingots Fron the Wreck Earl of Abergavenny
(P. T. Craddock, E. M. Cumming and D. R. Hook)
(*Department of Scientific Research, The British Museum, London, WC1B 3DG.)
Prior to the excavations of the project team, the wreck attracted the attention of many amateur divers. Quantities of the small cigar-shaped copper bars were recovered at this time, and one was even sent to the British Museum where it was examined and qualitatively analysed by the first author of this note, employing emission spectroscopy. At that time the Museum did not actively collect ingot material and no attempt was made to acquire it for the collection.
Since then, the Earl of Abergavenny team has recovered several hundred ingots of copper, 10 ingots of lead and a single ingot of tin, and the British Museum has begun to collect ingots of metal from dated contexts  , including ingots from the Earl of Abergavenny  . The most important of these contexts are documented shipwrecks. The value of shipwrecks as well documented time capsules is now increasingly appreciated  . This is especially true of artefacts with no distinguishing features to allow them to be categorised, or which usually had only a short existence, such that they are no longer recognised. Many of the ingots fall into this category, such as the small copper bars from the Earl of Abergavenny. Without the context of the wreck they could be of any date from the Bronze Age onwards. This is true for examples of the copper ingots recovered from the seabed off Plymouth, they had no context and thus no date, despite detailed scientific examination  . The same is true of the majority of the ‘ancient’ tin ingots found in the south-west  . Without a date their value is severely compromised.
A bonus with the material from shipwrecks is the potential of documentation giving additional information on where the metal was mined and smelted and where it was destined for. In fact the material that failed to reach its destination in a shipwreck is often far more use for research purposes than that which did arrive only to be preserved in some totally anonymous context, as work  on the copper alloy manillas intended for the West Africa trade has demonstrated. The study of metal ingots from wrecks is shedding new light on international trade in the post medieval-period, through the direct evidence of the ingots themselves.
The British Museum began actively collecting ingot material in 1985  with the purchase of a group of ingots from a number of wrecks, including a single copper ingot  from the Earl of Abergavenny and since then several other ingots of copper have been donated to the Museum by the team, together with two ingots of lead. Studies have been carried out on the single tin ingot to be found so far, together with some copper discs and an iron ballast block.
The ingots also form an invaluable source material for the study of the metallurgy of the post-medieval period. One tends to think that the later the material, the more that is known about it, but this is not necessarily so. Relatively little is known of either the composition or of the metallographic structure of most post-medieval metals. Most modern archaeometric research has focused on the distant past, such that, we now have a fair knowledge of the composition and structure of the metals used from the Bronze Age through to the Romans, but this peters out by the end of the medieval period. There seems to have been a perception that more recent material would have little of interest to tell us, and that anyway the information was likely to be contained in contemporary records. In fact nothing could be further from the truth, the post-medieval period was a time of experimentation, with new sources, new treatments and even new metals coming into use. As for contemporary records of these developments, it is well to remember that this was the great age of technical secrecy and of the industrial spy, also real metallurgical knowledge and investigational techniques were in their infancy. For example, at the time that the Earl of Abergavenny sank, English ironmasters were still not convinced that it was the carbon in the iron which dictated whether it was cast iron, wrought iron or steel, and had even less perception of the presence of other elements in the iron, as exemplified by the phosphorus in the iron blocks from the Earl of Abergavenny (see below). The science of metallography, revealing the structure of metals, only began in the late 19th century, and the analytical chemistry capable of detecting the trace elements which often dictated the properties of the metal as a whole, as exemplified by the bismuth content of the copper bars from the wreck (see below and Table 1), came even later.
The EIC Commercial Journal for 2 January 1805 indicates that apart from the consignment of Broad Cloth at £21,508. 12s. 1d, the copper at £18,344. 6s. 3d, was the next most expensive item of cargo. All this copper, 2000cwt, was destined for Bengal. As mentioned above the copper has been recovered in two forms, a small quantity of round discs of various sizes and several hundred small copper ingots weighing approximately half a pound (200 to 250 g). The ingots were found on the starboard side of the wrecksite about fifteen meters from the keel, scattered over a very large area. They may have been blown away from the wreck during an explosion or, the case they were in, may have broken up near the surface while being salvaged, probably by Braithwaite.
At about £9 per hundred weight the East India Company were determined to avoid theft if at all possible as can be seen from the following extract from the “INSTRUCTIONS TO A COMMANDER”
“That from the time of any part of the Company’s cargo being received on board your ship, your Chief or Second Mate, with other sworn officers, do give constant attention on board.
That if any copper should be laden on board your ship on the Company’s account, you observe the following regulations which the Court of Directors have adopted, for preventing deficiencies in the delivery of that article.
That the copper be weighed at the merchant’s house or wharf, in the presence of the Purser, one of the Company’s officers from the East India Wharf and the merchant’s clerk as hitherto practised; and that an iron hoop be fixed on the inside of each case. That the gross weight and the tare be cut on the case. That an account be taken at the same time, of the number of each package, with these details, and the number of pieces contained therein.
That it is recommended to the owners to cause each package to be re-weighed immediately on its being received on board the ship, in the presence of the Commanding Officer, the master of the craft, the surveyor who had charge of her, and the surveyor on duty on board the ship, and in case the gross weight of any package should differ from the gross weight marked thereon, such package to be returned to the Company’s Wharf by the Commanding Officer, with an account of the number of it and the weight as taken on board the ship, signed by him, and the other persons who saw it weighed. That on such occasions, the Warehouse Keeper at the Wharf do examine the package and take such other steps as may discover the cause of such difference in weight and report all the particulars he shall obtain to the Committee of Shipping unless it shall clearly appear to him to have been owing to a mistake in the original weight, which may easily be discovered by the condition of the chest, and the number of pieces of copper contained in it.
That the cases of copper be weighed again when delivered out of the ship in India, and an account of the weight be taken and that the Commanding Officer and other persons appointed by him be required to attend the weighing. That when it shall be found impracticable to weigh it immediately, it be secured under 2 locks and the Commanding Officer or person appointed by him, have possession of the key of one of the locks, till the whole of the copper shall have been weighed.”
Copper seems to have been traded in three principle forms in the 18th and 19th centuries: the small bars such as those found on the Earl of Abergavenny, rectangular battery plates, so-called because they were of a shape ready to introduce to the water-powered battery trip hammers to be turned into sheet, and granulated copper such as that recovered from the EIC Winterton (sank off Madagascar 1792.)  that was in a convenient form with a large surface area for converting to brass by the cementation process
So far the Earl of Abergavenny has only yielded the small bars (Figures 1&2), the seven ingots donated by the project team to the British Museum have a weight range of from 0.19 to 0.29 kg, and an average weight of 0.23 kg.
Although none of the other recognised ingot forms have been found, there are a number of thin copper discs varying in diameter from 8in. to 28in. (20cm to 71cm). They are of variable thickness, 0.6 to 2.5 grams per square centimetre. These were not a recognised ingot form. It is possible that they are copper preformed into a shape convenient for hammering into vessels, but they are rather thin to allow further hammering and extension, and thus the consensus at present would suggest these were part of the ship’s stores and not cargo. The photograph shows the copper discs, ranging in size from 8 inches (20.32cm) to 28 inches (71.12cm). The thickness of the sheets varied from 0.63 to 2.5 grams per square centimetre.
The composition of the one copper bar to be quantitatively analysed so far from the Earl of Abergavenny (Table 1) is typical of the other late 18th century-early 19th century ingots from English East India Company vessels  , notably in the high arsenic and bismuth contents. Throughout this period, the copper traded by the East India Company came from British sources, predominantly from the south west of England. There was a significant contribution from Parys Mountain, Angelsey, especially in the late 18th century  and again in the 1820’s  , although this died away almost completely during the first decade of the 19th century. The few battery plates with identifiable stamp impressions all seem to be of copper from the south west, and generally have the distinctive arsenic and bismuth contents. The arsenic content, although high, is not that uncommon in fire-refined copper, but the bismuth content, which typically varies between 0.1% and 1.0%, is unparalleled, and was totally unsuspected before the analysis of these ingots began. Even quite small bismuth contents well below 0.1% seriously embrittles the copper by the formation of brittle intermetallic compounds at the boundaries of the copper grains, and no modern copper contains more than a few parts per million of the element. There are hints that the contemporary metalworkers were aware that there was a problem, although of course they could have had no conception that it was bismuth in the copper that was to blame. There were reports of Cornish copper failing when forged into the wrought bolts used to hold copper sheathing to ships’ bottoms  . Similarly analyses of contemporary cast and sheet brass, both made in the Bristol area from Cornish copper, showed that the cast items have a much higher bismuth content than that of the hammered sheet items. The metal workers must have recognised that the copper that was to be made into brass for battery ware had to be carefully refined  .
Of course copper from the south west was being sent all over the world and several ‘native’ artefacts, such as the copper shields of the Indians of the North West coast of America, now in the Department of Ethnography, British Museum, which were previously believed to be of local native copper, have been shown to be probably of Cornish copper by virtue of their high bismuth content  . The unusual and unexpected composition of these copper ingots shows once again the important information to be gained by the scientific study of the relatively late ‘historic metals’, from a period that was previously assumed to hold few metallurgical surprises.
Composition of a copper ingot from the Earl of Abergavenny and other copper ingots dating from the late 18th-early 19th century. Copper to Silver)
METAL FIXATION AND MOBILIZATION IN THE SEDIMENTS OF THE AFON- GOCH ESTUARY – DULAS BAY, ANGLESEY
PARKMAN RH, CURTIS CD, VAUGHAN DJ, CHARNOCK JM
APPLIED GEOCHEMISTRY, 1996, Vol.11, No.1-2, pp.203-210
The surface drainage waters of Parys Mountain, Anglesey (Wales), a site of former mining for base metals, are highly acidic and metal-rich due to the oxidation of sulphide minerals. These acid waters mix with more neutral waters in the Afon Goch, downstream of Parys Mountain, allowing the formation of ochre precipitates which are found throughout the length of the Afon Goch and Dulas Bay. X-ray fluorescence (XRF) analysis of the ochres that settle where the Afon Goch enters Dulas Bay (Fe2O3 – 18 wt%), reveals that they are heavily contaminated with Cu (13,000 mu g/g) and Zn (7700 mu g/g). These sediments are black immediately below the surface and porewater analysis confirms that sulphate reduction is taking place. Samples of both stream ochre and anoxic black mud have been analysed by EXAFS spectroscopy. Data from the EXAFS analysis of the ochre sample reveals that the Fe, Cu and Zn are bonded to oxygen in poorly ordered or amorphous solids. In the anoxic black mud, however, Fe, Cu and Zn are all present as sulphides. The Fe sulphide is either amorphous or poorly ordered whereas Zn forms a discrete sulphide phase similar to sphalerite. The Cu sulphide has short range order with a chalcopyrite-like structure. Sequential Extraction analysis of the same samples was also performed. For the ochre sample, Cu and Zn release is controlled by the Fe hydrated oxides, being recovered primarily in the mildly acid ‘Carbonate’ and more strongly acid and reducing ‘Reducible’ fractions. Fe and Zn are also recovered in these fractions from the black mud, indicating that these metals are present as acid-soluble sulphides. Cu, however, is almost exclusively recovered in the ‘Oxidisable’ fraction, indicating that it is incorporated into a more stable sulphide such as chalcopyrite. Copyright (C) 1996 Elsevier Science Ltd
Radio- isotope dating shed light on ancient massive sulphide deposits
by R R Parrish
Volcanogenic massive sulphide deposits, termed VMS in the trade, host some of the most important sources of the metals copper, lead, zinc and silver on earth.
Black smoker, Main Endeavour Hydrothermal vent field on the Endeavour Segment of the mid-ocean ridge. Copyright Woods Hole Oceangraphic Institution, J.R. Delaney, PI, University of Washington; picture taken by camera mounted on hull of Alvin.
For the most part, they have formed in the vicinity of large submarine centres with associated hydrothermal systems. The discovery and documentation of the spectacular vents, hotsprings and metalliferous ‘black smokers’ with their very unusual associated colonies of life forms, has focussed widespread public attention on these submarine environments. The rate of formation of mineral sulphide deposits on the ocean floor has been studied using these active modern examples.
These types of deposit are found throughout the geological record, with some of the largest systems ever recorded being the oldest. One of the best preserved and largest, a real giant in its class with 150 million tonnes of ore, is the Kidd Creek Deposit, which is found in the Abitibi Greenstone belt of Ontario, Canada. The largest deposit of this type in the United Kingdom is the Parys Mountain deposit in Anglesey, North Wales which has up to 10 million tonnes of copper-lead-silver ore. More often than not, these deposits have been strongly deformed by tectonic processes and consequently they have stratigraphical and structural relationships which obscure their original shape size and form. In the Precambrian deposits such as Kidd Creek, where there are no fossils, determining the ages and duration of hydrothermal and volcanic activity and elucidating many of the complex stratigraphical relationships relies strongly on radioisotope geochronology. This is mainly the very high precision method of uranium -lead dating of the mineral zircon. It is interesting to ask whether such giant deposits like Kidd Creek are unique to the Precambrian and whether the rates of volcanic and hydrothermal processes were different in the distant past.
The Kidd Creek deposit, like may other VMS deposits, is characterised by a bimodal rhyolite- basalt volcanic association, with important breaks in the volcanic activity occupied by the deposition of sedimentary material. Current interpretations of the tectonic setting of this volcanic belt involved a plume origin for many volcanic rocks, which include the world famous spinifex olivine bearing komatiites. Plume related volcanism is though to have occurred in a back arc environment and was certainly accompanied by significant crustal extension and rifting in the submarine environment.
Rhyolitic volcanic rocks contain zircon, a mineral that crystallises from the magma and which is used for high precision geochronology. Because of the unique attributes of the uranium-lead decay scheme which has a coupled radioactive decay scheme (238U-206Pb and 235U -207Pb) it is currently possible to date rocks to ± 500 000 years when the rocks are 2700 million years old.
Analytical precision at this level permits the dating of individual lava flows, enabling geochronology to resolve complex stratigraphical relationships. The technique has only recently been comprehensively applied to deposits of this type but its impact has far reaching consequences for our understanding of the genesis of these ore bodies and the development of strategies which mineral exploration consortia use in locating new reserves.
Several new insights arose from the recent very detailed isotope-dating programme. The main ore bodies, which occur in distinct centres over an original area of at least 2 km2, developed in distinct episodes of time from 2716.0 ± 0.5 to 2711.5 ± 1.2 million years ago. It is possible to calculate that the rate of deposition of massive sulphides was about 10-100 tonnes per year, which is quite similar to the ore deposition rate of the 50 000 year old, five million tonne deposit discovered recently on the Mid Atlantic ridge under several kilometres of sea water. Such conclusions strengthen the notion that similar hydrothermal processes have operated on the Earth for billions of years. Because individual rhyolitic lava flows can be precisely dated several volcanic layers have been identified and in the outlying region which are the same age as the mineralised ones at the ore deposit. The identification of these layers may help to steer future exploration activity to those stratigraphic horizons known to contain ore nearby, a potentially valuable strategic tool for exploration companies. The dating work on Kidd Creek has also resolved two long-standing controversies. Firstly the sedimentary greywackes that underlie the south side of the ore body are younger, not older that the Kidd Creek ore body. Secondly previous dating results (using radioactive decay systems and minerals) which suggested that the VMS ore deposition might be younger than 2690 million years are now known to be wrong.
Similar controversies surround many of the VMS deposits of the world, not the least of which is the Parys Mountain deposit in North Wales. By comparison with the significant cost of drilling new exploration holes, the cost of the programme of work outlined above for the Kidd Creek deposit was quite modest. In surprising ways, the isotopic dating programme solved definitively some very controversial issues that have been the subject of debate for decades.
This article was released in Issue 12 of Earthwise, June 1998
British Geological survey
Mineral Reconnaissance reports 112
Geophysical and geochemical investigations on Anglesey, North Wales
D C Cooper, I F Smith, M J C Nutt and J D Cornwell (1990)
This report describes a number of surveys carried out on Anglesey and not covered by previous reports in the series. A gravity survey of the island identified two large amplitude lows: one is associated with volcanic rocks and granite cropping out southeast of the Menai Strait Fault; the other is centred off the north-west coast and is possibly caused by a concealed granite. If of Caledonian age, such a granite would have influenced the distribution of base metal mineralisation on the island. Positive anomalies are associated with metabasic rocks in the south-east of the island whilst Carboniferous sedimentary rocks give rise to gravity lows between Malltraeth and Dulas. Geophysical orientation studies of the Ordovician volcanogenic massive sulphide Cu–Pb–Zn–Ag mineralisation at Parys Mountain showed that this style of mineralisation generates strong chargeability anomalies but only weak EM anomalies, prone to interference from artificial sources. VLF(EM) proved useful for detecting steeply dipping conductors, and magnetic anomalies are produced by some basic rocks. A gravity survey detected Bouguer anomalies which two seismic refraction lines showed may be caused by concealed acid volcanic rocks. IP traversing indicated that no substantial mineralisation was associated with the Bouguer anomalies. Ground geophysical surveys confirmed airborne EM and magnetic anomalies at Bodewryd, Rhosbeirio, Treferwydd and Tyntywyn. At Rhosbeirio and Tyntywyn the cause of the EM ground anomalies remains uncertain whilst at Bodewryd and Treferwydd basic dykes are the probable source of magnetic and EM anomalies. Soil sampling was carried out around Cerrigceinwen, City Dulas, Llanbadrig, Llandyfrydog and Lligwy to investigate promising indications of mineralisation arising from earlier regional surveys. In addition, geochemical groundwater surveys were carried out around Cerrigceinwen and Llanbadrig, geophysical traversing at Llanbadrig and City Dulas, and rock sampling at Llandyfrydog. Anomalous results related to mineralisation, possibly of similar style to that found at Parys Mountain or Carmel Head, were recorded at Llanbadrig. Geochemical and geophysical anomalies probably caused by hitherto undiscovered mineralisation were also found at City Dulas. At Llandyfrydog large base metal anomalies in soils were ascribed to metal-rich water, derived from the Parys Mountain mine, flooding across and percolating into superficial deposits. Some smaller anomalies are probably derived from weak base metal vein mineralisation. In the Cerrigceinwen area stream sediment and groundwater survey data suggest that mineralisation might be associated with spilitic rocks within the Mona Complex and the basal Carboniferous succession, but limited soil sampling across these lithologies only located a few isolated base-metal anomalies. The single soil traverse sampled across the basal Carboniferous at Lligwy produced similar results.
SAR and HYPERSPECTRAL AIRBORNE CAMPAIGN
Assessing the environmental impact of historical base metal mining at
Parys Mountain, Anglesey, with HyMap data
A.D. Lamb and A.M. Denniss
Infoterra Ltd. (email@example.com)
Parys Mountain mine in Anglesey was the world’s largest copper producer at the end of the 18th Century, ceasing in 1904. Although still the subject of modern exploration, an extensive legacy of pits, waste tips, slag, settling ponds, and natural outcrop offer a mineral mapping challenge in both the VNIR and SWIR spectral regions. This is related not only to composition of the source materials but also to major heavy metal contamination in the surrounding district – the subject of several past environmental and geochemical studies. Two main research objectives were addressed under this proposal:
2.1 Research Theme One – surface composition mapping
The Parys Mountain orebody is geologically unique in the UK, but displays a range of exposed mineralogy typical of sulphide orebodies elsewhere in the world. The combination of historical mining, waste disposal and local copper smelting have given rise to extensive local pollution in soils, waters and coastline. This is through the natural generation of sulphuric acid from the weathering of pyrite, but particularly from treatment of the mined material with sulphuric acid – both giving rise to the leaching of heavy metals.
Therefore expected minerals that should be present include those relating to the pyrite oxidation chain (pyrite->copiapite->jarosite->goethite->hematite). Other minerals, characteristic of the SWIR region, should include species of smectite, illite and chlorite, which would be related to the various host lithologies and the hydrothermal halo’s surrounding the mineralisation. The task was therefore to see if these could be identified and extracted as useful endmember maps over the complex Parys Mountain site.
Summary of results to date:
Several endmember unmixing strategies have been examined including an automated software approach currently in use in the mining industry. Results have been problematic for a number of reasons including: Spectral shapes in the VNIR appear to be corrupted at some wavelengths, making interpretation difficult. This is probably due to an artifact being induced during the atmospheric correction process transforming the data from radiance to reflectance, or incorrect radiance calibration.
SWIR endmembers extracted are not always recognisable due to the spatial mixture of the alteration minerals at surface. However minerals identified included, smectite, chlorite, illite/mica, carbonate, and sulphate-smectite mixtures – probably jarosite.
It was also possible to extract distinct VNIR vegetation endmembers for the fields surrounding the site. The areas of heather around the pit could be easily distinguished as a SWIR 2 endmember.
· The theme of ‘vegetation stress’ has been examined through testing the Red-Edge Vegetation Stress Index
2.2 Research Theme Two – issues constraining operational deployment
Commercial hyperspectral surveys can operationally be challenging and are an interplay between spatial resolution, geographic coverage, time of day versus cost of data acquisition.
Summary of results to date:
Pixel size variation – similar endmember maps could be derived from both the 3m and 5m data, i.e., there was no significant change in the endmembers being extracted despite the reduced signal to noise ratio of the higher resolution 3m data. This indicates that the spectral resolution is more important than the spatial resolution, although there must come a point when pixel resolution can become too coarse to identify small targets. However to prove this you would need to have to very different resolution datasets, for example 3m and 30m.
· Geometric correction – a parametric approach using the supplied IMU-based data is adequate as a first-order correction but not for precision mapping. Industry developments include a digital camera system mounted concurrently with the scanner, to provide a temporally matched ortho-base. Non-parametric approaches, which attempt to auto-fit the aircraft imagery to an orthophoto base through a cross correlation algorithm, have not been successful. One reason is the gap of 8 years between the two data sets (with consequent mismatch of some cultural features), together with noise problems from the digitisation of the photography.
- Ongoing Research Opportunities
· Re-appraisal of radiance – reflectance techniques:
· ACORN FLAASH
· Sun photometer data collected by EPFS
· spectral shifts in vegetation in response to ‘stress’, i.e more comprehensive field spectral surveys
· ground-checking of derived mineral endmembers
· integration with detailed geochemistry
· extension of mapped results to the catchment level
For further details on this project please contact Alistair Lamb (Alistair.firstname.lastname@example.org)
GEODYNAMICS AND CRUSTAL PROCESSES RESEARCH GROUP
A mineralogical study of sulphide oxidation products
Supervisors: Dr Pamela Murphy & Professor Andy Rankin
Oxidation of sulphide minerals in coal and mineral deposits and in mine dumps results in acid mine drainage which leads to acidification and contamination of groundwater and river systems by dissolved metals and sulphate. The rates and mechanisms of pyrite oxidation in solution are well known but the reaction products of other sulphide minerals, and of the non-sulphide minerals present in the mine waste, are less well characterised. Large amounts of material in mine waste dumps is in the unsaturated zone, above the water table, and will therefore undergo a repeated series of wet and dry cycles, with associated sulphide oxidation and formation of secondary minerals (e.g. oxides, hydroxides, sulphates and complex hydrated minerals). The composition of the secondary minerals will depend on the composition of the sulphide and “gangue” minerals along with fluid conditions (themselves controlled by mineral composition through buffering). These mineralogical products are important because of their potential for fixing contaminant metals which might otherwise be released into the water system.
This project will study the mineralogical products of experimentally oxidised sulphide mixtures under conditions representative of mine dump environments, and with a variety of non-sulphide or gangue mineral associations, chosen to represent specific rock types. The principal analytical method will be laser Raman spectroscopy, which allows non-destructive analysis on a microscopic scale and will be used, not only to identify individual grains, but also to study distribution and associations.
The experimental results will be compared with actual mine waste samples, which will be collected from appropriate sites within the UK (e.g., sulphides from Parys Mountain, and coal in South Wales) or elsewhere in Europe.
Samples of sulphide minerals (pyrite, pyrrhotite, chalcopyrite, arsenopyrite, galena, pentlandite, and sphalerite) will be experimentally oxidised, and the progress of the reaction monitored. The sulphide mixtures will be representative of actual mineral deposits (e.g., Pb-Zn, Cu-Ni-Fe, etc) and the “gangue” mineralogy will be prepared to represent specific rock types (shale, granite, limestone).
New mineral species would be expected both within the sulphide mass, (and mineralogy may vary at different levels) and from evaporation from the effluent.
Small portions of the mineral samples will be studied optically and photographed at regular intervals. Raman analysis will identify reaction products of both sulphide and “gangue” minerals. The Eh and pH of the effluent fluid will be measured and a small portion removed and filtered for ICP-MS analysis of dissolved metal content. Dilution of the effluent with water at higher pH will also produce mineral precipitation (as seen when mine effluents drain into river water). The mineralogy and mineral chemistry will also be studied optically and by electron microprobe and/or laser ablation ICP-MS.
In addition, Raman analysis will be applied to metal complexes in the effluent solution.
If you have, or expect to obtain in 2003, a good honours degree (1st or 2:1) in an appropriate subject, you may apply by sending your curriculum vitae, names of two academic referees and a letter explaining your interest to Prof. John Grocott at the School of Earth Sciences & Geography, Kingston University, Penrhyn Road, Kingston upon Thames, Surrey KT1 2EE, UK.
CRYPTIC SPECIATION, EVOLUTION AND MORPHOLOGICAL PLASTICITY IN METALLIFEROUS ACAROSPORA
Supervisors: William Purvis (NHM), Mats Wedin (Umeå University, Sweden)
Anna is a PhD student at Umeå University, Sweden, but she also comes to the NHM a couple of times a year to work with William Purvis. She is funded through a Royal Society Collaborative Joint Project Grant.
Many crustose lichens found in metalliferous habitats are morphologically extremely variable and often appear distinct compared with specimens from non-metalliferous sites. Thus, the traditional taxonomic interpretation is complicated, and has differed considerably between authors. The extent to which metal accumulation is restricted to particular species or merely ecotypes or strains is also unknown.
The overall aims of Anna’s PhD project are to study the evolution, adaptation and speciation of metalliferous lichens in the crustose group, the Acarospora smaragdula and Acarospora rugulosa complexes, using integrative molecular, morphological and ecological techniques. She will test whether metalliferous taxa are distinct species or merely ecotypes, partly by generating multi-gene molecular phylogenies.
During her first year Anna has been gaining an overview of the project. She has reviewed the relevant literature on Acarospora, (Lecanoromycetes), and has reviewed the nomenclatural status of relevant published names. She also initiated a pilot study on the variation of different genes in the study group, utilizing well-known loci where fungal-specific PCR-primers are readily available (the nuclear ITS rDNA and mitochondrial SSU rDNA). This pilot study focused on A. smaragdula (Wahlenb.) Th. Fr. s.lat. and A. sinopica (Wahlenb.) Körb. The results illustrate that specimens belonging to the A. sinopica and A. smaragdula complex form two separate monophyletic groups. However, they do not adequately distinguish putative species within these two complexes. Further investigation is therefore needed on extended phylogenies of the A. smaragdula complex, covering additional metalliferous populations, and utilising new species-delimitation primers.
During this year Anna has also collected various Acarospora samples through periods of fieldwork in both the UK and Sweden: a week at Parys Mountain, Wales with William; a week investigating mines in northern Sweden with Mats and William; and a five day trip in Hemavan, Sweden with Mats. For the various weeks that Anna has been at the NHM this year, she has started microscopical morphological investigations of her recently collected samples. Apart from utilising light microscopy, she has used the JEOL-SEM probe to study the localisation and types of metals that have been accumulated in some samples.
Future investigations will be: to establish whether the chemical environment (and pH) results in quantifiable morphological differentiation within species, and to determine whether speciation occurs on different metal substrata; to assess the variability and plasticity of anatomical traits that have been given taxonomic importance, under different ecological conditions; and to continue studies on the localisation and accumulation of metals in Acarospora.