The Ore Mineralogy of the Mineralisation at Copa Hill,(Cwmystwyth).

R.A. Ixer. 2003.
This first appeared as 'Ore Mineralogy.' p99 in Excavations on Copa Hill, Cwmystwyth (1986 - 1999). An early Bronze Age copper mine within the uplands of Central Wales. S. Timberlake (ed.) BAR British Series, 348. 2003

Mineralisation in the Central Wales Mining District orefield is associated with polyphase, epigenetic, ENE-WSW-trending lead-zinc-copper sulphide-bearing quartz-ankerite veins that cross-cut Ordovician-Silurian metasediments (Bevins, 1994; Mason, 1994; 1997). The Comet Lode, Copa Hill forms part of the major Ystwyth fault that cuts through the southern margin of the orefield. This lode carries copper-lead-minor zinc mineralisation and was the site of Bronze Age mining activity.

The primary mineralisation at Copa Hill is simple and both the copper-rich, chalcopyrite-dominated and the spatially separated, lead-rich, galena-dominated ore assemblages carry identical mineralogies that only differ in the relative concentrations of their main sulphides. The present description, based upon additional sampling including in situ material taken from all seven of the cores, complements and amends earlier ones (Ixer 1997; Ixer and Budd, 1997).

The wallrocks hosting the mineralisation comprise fine-grained, slightly carbonaceous, metamudstones - metasiltstones with abundant chlorite porphyroblasts. These metasediments are unmineralised but carry trace amounts of pyrite (FeS2) some of which is framboidal and even smaller amounts of a white nickel-cobalt-iron sulpharsenide. The metasediments have been intensely silicified and brecciated so that many specimens comprise angular clasts floating in a medium- to coarse-grained quartz mosaic and later dolomite/ankerite matrix. These matrix minerals are the gangue to the base metal mineralisation.

The copper-rich mineralisation comprises chalcopyrite (CuFeS2) accompanied by pyrite and minor amounts of galena (PbS) and sphalerite (ZnS). Trace amounts of marcasite (FeS2) and a white cubic mineral that is optically identified as being one or more of the nickel-cobalt-iron sulpharsenide minerals belonging to the cobaltite (CoAsS)-gersdorffite (NiAsS) group or possibly ullmannite (NiSbS) are also present. Much of the sphalerite shows fine-grained chalcopyrite-disease and the nickel-cobalt-iron minerals show compositional and/or mineralogical zoning. Chalcopyrite is up to centimetres in size but all other minerals are small, having a diameter less than one millimetre (1000 microns) across for pyrite, sphalerite and galena and less than 100 microns for the white sulpharsenide.

The primary copper-rich sulphide assemblage has altered/weathered to a more complex secondary mineral assemblage comprising iron and copper sulphides, oxides, hydroxides and carbonates. Although the alteration is seen to be widespread and quite intense in the spoil material, it is superficial in the in situ core samples. Chalcopyrite alters to a series of thin, copper sulphide rims including digenite (Cu9S5), covelline (CuS), yarrowite (Cu9S8) and spionkopite (Cu39S28) or to cuprite (Cu2O) accompanied by native copper or to limonite (FeO.OH) and green copper secondary minerals including malachite (Cu2(CO3)(OH2)2). Alteration of the sulphides has produced intense dedolomitization of the main dolomite/ankerite gangue so forming coarse-grained, vuggy calcite crystals. Limonite and black wad (pyrolusite (MnO2)) infill these void spaces to give the dominant brown-black coloration of the specimens.

The lead-rich mineral assemblage comprises centimetre-wide galena masses intergrown with, or enclosing, small, less than one millimetre in diameter, chalcopyrite and sphalerite showing chalcopyrite disease, and trace amounts of pyrite and a white, unzoned, cubic mineral up to 50 microns in size and identified as ullmannite. Galena alters to dark coloured and then pale, euhedral cerussite (PbCO3) or is enclosed within thin digenite, spionkopite, yarrowite or covelline rims.

Although a complete ore-triage (Ixer 2001) has not been performed on the mineralisation from Copa Hill it is possible to make some preliminary comments about the specimens as Bronze Age ores. The coarse-grain size of the chalcopyrite in the copper mineralisation suggests that a high grade, clean, copper separate could be achieved by hand cobbing. If smelted alone this would produce a copper metal with few other metallic impurities. These would include some iron from chalcopyrite, pyrite and limonite, trace amounts of lead and zinc from any admixed galena and sphalerite and perhaps a little manganese from the wad staining. Other elements namely nickel, cobalt, arsenic and antimony would have a negligible content, less than 0.01 wt %. Similarly a clean lead cut could have been collected from the galena-rich mineralisation by simple hand cobbing.

Bevins, R.E. 1994. A Mineralogy of Wales. (Cardiff, National Museum of Wales). Geology Series 16, 146pp

Ixer, R.A. 1997. Report on the Copa Hill mineralisation. In Geoarchaeological research into Prehistoric mining for copper in Wales. D.A.Jenkins and S Timberlake. Unpublished report to the Leverhulme Trust. Appendix 2, 113-114

Ixer, R.A. 2001. An assessment of copper mineralisation from the Great Orme Mine, Llandudno, North Wales, as ore in the Bronze Age. Proceedings of the Yorkshire Geological Society, 53, 213-219.

Ixer, R.A. and Budd, P. 1998. The mineralogy of Bronze Age copper ores from the British Isles: implications for the composition of early metalwork. Oxford Journal of Archaeology, 17, 15-41.

Mason, J.S. 1994. A regional paragenesis for the Central Wales Orefield. Unpublished M.Sc. thesis, University of Wales.

Mason J.S. 1997. Regional polyphase and polymetallic vein mineralisation in the Caledonides of the Central Wales Orefield. Transactions of the Institution of Mining and Metallurgy. (Section B Applied Earth Science) 106, B135-144.

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