This forms appendix 2, pp113-114 in Geoarchaeological research into Prehistoric mining for copper in Wales. D.A.Jenkins and
S. Timberlake. Unpublished report to the Leverhulme Trust.
Eight polished thin sections of intensely silicified and carbonate-veined wallrock associated with minor copper and trace lead and zinc mineralization are described.
The wallrocks comprise pale- to dark-coloured, fine-grained metashales interbanded with rarer metasiltstones; the darker metasediments are carbonaceous. The brecciated metasediments have been extensively silicified to give pale-coloured quartz patches enclosing relict, fine-grained TiO2. The clasts are veined by quartz, carbonate or quartz-carbonate and have radiating quartz crystals growing out from them all enclosed within a dolomite-ankerite rich matrix.
The metashales comprise 20-200Ám diameter, rounded porphyroblasts of chlorite or mixed chlorite-white mica. These porphyroblasts lie along the main fabric and some enclose 2-10Ám diameter pyrite or are enclosed within a 5Ám wide organic rim. The phyllite carries abundant, fine-grained, 4-10 x 1Ám, pale, acicular TiO2 accompanied by 20-40Ám long, pale TiO2 laths. In the slightly coarser-grained metasiltstones, 50-200Ám long, tabular, pale TiO2 grains are accompanied by rare, 60Ám diameter leucoxene or orange coloured TiO2, 20Ám diameter zircon and rare muscovite. Thin, 5-20Ám wide mixed organic matter-TiO2-pyrite veinlets lie along the main fabric.
Locally the metashales are carbonaceous with 2-5Ám wide streaks of organic matter or graphite lying along the main fabric or forming 10-20Ám diameter patches of 2-5Ám long graphite. Discrete graphite flakes are 10-20Ám but up to 60Ám in length; longer flecks show kinking. Brown organic matter forms random, up to 60Ám diameter patches accompanied by 2-5Ám diameter pyrite. Minor amounts of pyrite, as 2-20Ám but locally up to 200Ám diameter framboids are present. Although some framboids have escaped recrystallization, the majority has recrystallized to give round aggregates of 2-5Ám, pentagonal dodecahedral crystals that have later oxidised to limonite. Single, 2-20Ám diameter pyrite crystals are scattered throughout the phyllites and some have thin, 2-5Ám wide 'white pyrite' overgrowths. Some fine-grained pyrite is associated with organic-rich veinlets.
Quartz is present within a number of different generations. The earliest quartz comprises the silicification of the phyllite; here the metamorphic rocks progressively lighten in colour to form dirty, cloudy, fine-grained quartz mosaics ('chert') enclosed within clear, coarse-grained quartz. Coarse-grained quartz crystals, often showing a radiating habit, overgrow angular phyllite clasts. Many phyllite clasts are cross-cut by quartz or quartz-carbonate veins; these veins lie at high angles to the main banding of the metashales. Much coarse-grained, 50-200Ám long, euhedral quartz is associated with chalcopyrite, forming rims to the sulphide or discrete, euhedral inclusions within it.
There are a number of carbonate generations. Carbonate, including late-stage sparite, veins cross-cut phyllite clasts and minor amounts of fine-grained rhombic dolomite/ankerite are present within fine-grained quartz mosaics. Most carbonate however is present as medium- to coarse-grained crystals, often cloudy due to the presence of abundant fluid inclusions and with multiple 10-20Ám wide limonite bands concentrated in their outer growth zones. This generation of carbonate has suffered much recrystallization and ?dedolomitization to form very coarse, inclusion-free, clear sparite and numerous void spaces infilled by banded limonite and manganese oxides.
Chalcopyrite is the main sulphide and is present as very, very fine <1Ám diameter grains within the cores of some carbonate crystals but mainly as 5-200Ám diameter, irregular masses, often associated with euhedral, coarse-grained quartz, all within carbonate. Minor amounts of chalcopyrite are present in the phyllite. Chalcopyrite encloses 1-20Ám diameter pyrite and white pyrite; minor amounts of galena and sphalerite are intergrown with chalcopyrite, forming inclusions within, or rims surrounding it.. Most chalcopyrite is unaltered but locally alteration along grain boundaries and cleavage planes forms 2-10Ám wide rims of digenite, yarrowite, limonite or a number of green, secondary copper minerals.
Many phyllite clasts carry a white, high reflectance, cubic ' white pyrite' as thin, 1-2Ám wide rims about pyrite or as 1-20Ám diameter, discrete crystals in quartz mosaics. Elsewhere, 5-40Ám diameter, mixed crystals in coarse quartz or carbonate comprise 1-5Ám diameter, irregular pyrite cores surrounded by a 2-10Ám wide, softer, lower reflectance, blue-white phase (galena/ullmannite) within a hard, high reflectance, cream-white phase (?gersdorffite). Locally clusters of 2-20Ám diameter 'white pyrite' crystals form areas greater than 200Ám in diameter. 'White pyrite' alters to 2Ám wide digenite rims but more often to limonite.
Euhedral to subhedral, 10-60Ám diameter pyrite is present in carbonate or within quartz margins to chalcopyrite and rare, 10-20Ám long, lath-shaped aggregates may be pseudomorphs after pyrrhotite. Very locally, pyrite encloses small, 1-5Ám chalcopyrite or galena inclusions and many grains form the cores to zoned 'white pyrite'. Pyrite alters to pale blue limonite with very strong red internal reflections.
Sphalerite is rare in chalcopyrite forming colourless to brown, up to 50Ám in diameter inclusions, locally accompanied by white pyrite or with very fine-grained chalcopyrite-disease. Discrete, 40-200Ám diameter, twinned sphalerite is present in carbonate. Rare galena forms 20-30Ám diameter, discrete masses in carbonate; the majority of galena is inclusion-free but some carries 2-5Ám diameter white pyrite. Although most galena surrounds chalcopyrite, small, rounded, 10-20Ám diameter grains are enclosed in the copper-iron sulphide. Galena alters to blue copper sulphides, cerussite or green secondary minerals.
Much of the primary sulphide is unaltered and most alteration is superficial. Trace amounts of 5-20Ám diameter cuprite, with intense, red, internal reflections, is associated with 40Ám diameter, blue copper sulphide and 1-2Ám diameter native copper and lies along carbonate cleavage planes. A number of pale to dark green secondary copper minerals, including malachite overgrow chalcopyrite into carbonate or quartz mosaic void spaces or lie along carbonate cleavage planes. Limonite is widespread and replaces earlier sulphides especially pyrite and chalcopyrite. Much limonite forms 10-20Ám wide bands along original growth zones in dolomites/ankerite or lies along their cleavage planes. Botryoidal limonite, up to 45Ám in thickness, infills void spaces. Although trace amounts of MnO are present in the phyllites most MnO is a late-stage infilling of void spaces in carbonate. Here it forms 5-30Ám wide crusts, up to 100Ám long or 10-60Ám diameter spheres comprising fine-grained, very anisotropic ?pyrolusite.Galena-rich ores
One polished block of coarse-grained galena taken from the spoil and one polished thin section taken from in situ mineralization from core VII were made. The in situ mineralization shows galena with minor chalcopyrite, sphalerite and pyrite mineralization associated with quartz and carbonate cementing shattered metamudstone.
The metamudstones carry 1-5Ám long, pale, acicular TiO2 crystals and 1-5Ám diameter, pentagonal dodecahedral pyrite. Locally shale clasts have thin, 1-2Ám wide, discontinuous pyrite rims.
Although much of the galena is inclusion-free some is intergrown with, or encloses, pyrite, white pyrite, chalcopyrite and sphalerite. Galena alters to dark-coloured cerussite and is surrounded by 40-200Ám diameter, pale-coloured, euhedral cerussite. A more extensive alteration is present in the dump material; here galena alters along cleavage planes and grain boundaries to 20-40Ám wide digenite or 10-20Ám wide spionkopite, yarrowite or covelline rims comprising fine-grained, 1-2Ám diameter crystals.
Minor amounts of small, 5-10Ám diameter, peach-coloured pyrite is present as discrete crystals in galena or forms 200 x 2Ám wide veinlets cutting galena. Most pyrite is associated with quartz as 10-30Ám diameter rims about chalcopyrite and galena or as patches, up to 200Ám in diameter, of 2-20Ám diameter crystals. Rare, 20-60Ám diameter pyrite crystals are present in cerussite. Although most pyrite is inclusion-free some carries 1-2Ám diameter galena.
Rare, 5-20Ám diameter, pentagonal dodecahedral, unzoned 'white pyrite' ?ullmannite/gersdorffite is present in quartz, chalcopyrite and galena. Thin veinlets of 1-5Ám 'white pyrite' cross-cut galena. Rare, 1-2Ám diameter, dark coloured sphalerite is enclosed in 'white pyrite'.
Sphalerite is present as small, 5-10Ám diameter, pale yellow to greater than 200Ám diameter, orange-brown masses in carbonate and galena. Small, 10-30Ám diameter sphalerite infills void spaces within chalcopyrite. Much sphalerite shows clear cores surrounded by outer margins showing chalcopyrite-disease. Here chalcopyrite forms very thin, 5mm long, platelets aligned along (111) crystallographic directions.
Chalcopyrite encloses sphalerite within a quartz gangue or is enclosed or intergrown with galena, forming inclusions up to 200Ám in diameter. Chalcopyrite is replaced by 10-20Ám wide rims of white copper sulphide, covelline or spionkopite followed by yarrowite. Very locally chalcopyrite alters to bornite along cleavages planes. The final alteration products of chalcopyrite are a number of green secondary copper minerals including malachite.