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


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Godsall, William J. D 1997 , 'Alteration as a vector to copper mineralisation in the Royal Tharsis deposit, Mount Lyell mineral field', Coursework Master thesis, University of Tasmania.

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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.

Item Type: Thesis - Coursework Master
Authors/Creators:Godsall, William J. D
Keywords: Copper ores
Copyright Holders: The Author
Copyright Information:

Copyright 1997 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (M.Ec.Geol.)--University of Tasmania, 1998. Includes bibliographical references

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