Alteration as a vector to copper mineralisation in the Royal Tharsis deposit, Mount Lyell mineral field

Royal Tharsis was mined as an underground operation and as part of the West Lyell Open Cut. The original orebody consisted of steep south-westerly dipping echelon lenses striking 315° and extending to a depth of some 600 metres. The topographical expression would have been steep slopes of CVC alteration assemblages dominated by the Tharsis Ridge, a buttress of Owen Group rock types that separates West Lyell from North Lyell. Sulphide mineralisation is dominantly pyrite and for which at least one generation has been identified, with subordinate chalcopyrite. Bomite, chalcocite-digenite, covellite, molybdenite, sphalerite and galena have also been identified. Volcanic precursors include rhyolitic and dacitic volcanics, volcaniclastics (locally autobrecciated), brecciated lavas and minor porphyries. Intense and selective alteration has resulted in obliteration of primary textures. Feldspar destruction is almost ubiquitous. Rare albitised plagioclase occurs towards the periphery of the alteration system. Broad correlation exists between sulphide mineralisation and alteration patterns. Ten main alteration assemblages have been identified: mixed mica, quartz-mixed mica, quartz-sericite, quartz-pyrite, quartz-chlorite±sericite, chlorite, meta-conglomerate, quartz-haematite, quartz-magnetite and magnetite-apatite. The most common alteration assemblages include: quartz-sericite; quartz-chlorite-sericite and/or quartzsericite-chlorite; and chlorite or quartz-chlorite assemblages. The Ishikawa alteration index increases up stratigraphy and shows a subtle change through the ore zone. The chlorite-pyrite-carbonate alteration index shows a change in gradient through the ore zone relative to hangingwall lithologies. The manganesecarbonate alteration index shows a relative drop at the stratigraphic footwall of copper mineralisation, but otherwise portrays a poorly defined response. The Ti/Zr value falls in the dacite - rhyolite range, with occasional andesite values. Sulphide (pyrite) and carbonate show an almost inverse relationship, with a weak zonation evident between Fe-S-C-±0. Carbonate alteration through the mineralisation is muted and variable, and where distal is probably due to remobilisation rather than primary. K20 shows a subtle response to mineralisation and thus may be a subtle vector to ore. Na20 depletion occurs through the ore halo and into the hangingwall. Barium and Ba/Sr ratio show enrichment through the mineralised halo, identified by a Ba/Sr value that rises above 30. REE show uniform elevated responses through the mineralised halo. Gold, silver, molybdenum, cobalt and ± nickel all correlate reasonably well with copper. Fe2O3 and P2O5 both correlate with copper, indicative of pyrite and apatite relationships respectively, the latter pointing to the influence of magmatic hydrothermal fluids. The abundance of illite is suggestive of the presence of weakly acidic (CO2 -rich) fluids.