The geochemistry of the Mt. Lyell copper deposits

The Mt. Lyell copper field is located near Queenstown on the west coast of Tasmania. It occurs within Cambrian felsic lavas and pyroclastics which are part of the central lava belt of the Mt. Read Volcanics. The mineralization is probably of Cambrian age and is associated with an extensive zone of hydrothermal alteration. Sulphide deformation and minor remobilisation of silicates and sulphides occurred during subsequent deformation, mainly in the Tabberabberan Orogeny (Devonian). The mineral deposits on the field may be grouped as: (1) Disseminated pyrite-chalcopyrite type Prince Lyell, \A\" Lens Royal Tharsis Cape Horn Crown Lyell No. 1 and 3 Lyell Tharsis (in part) and Western Tharsis. (2) Bornite-chalcopyrite type North Lyell Crown Lyell No. 2 12 West Lyell Tharsis (in part) and Lyell Comstock (in part). (3) Massive pyrite-chalcopyrite type The Blow (Mt. Lyell orebody) (4) Massive pyrite-galena-sphalerite-chalcopyrite type Tasman and Crown Lyell Extended and Lyell Comstock (open cut). Prince Lyell consists of a central core of fragmental felsic volcanics which have been altered to an assemblage of quartz-sericitechlorite-pyrite-chalcopyrite. Minor phases include siderite apatite hematite and some magnetite and barite. Inclusions of pyrrhotite pyrrhotite-chalcopyrite bornite and bornite-chalcopyrite occur in the pyrite. The hangingwall of the orebody consists of chloritised intermediate-mafic volcanics the footwall of massive or fragmental pink rhyolites which have an alteration assemblage of quartzsericite- minor hematite chlorite sulphide siderite and barite. A quartz-sericite-pyrite-minor chalcopyrite alteration assemblage occurs on the northern end of Prince Lyell. Minor lenses of massive magnetite and chlorite occur in the central core and a quartz-chlorite-minor sericite-hematite-magnetite-sulphide assemblage in the footwall adjacent to the central core. The suggested reconstruction of hydrothermal alteration and mineralization is: (1) An early essentially non-sulphide alteration forming the assemblage: quartz-sericite-minor chlorite-hematite-barite-carbonate - possibly clays(?). (2) Initial sulphide deposition which began with pyrrhotite-magnetite-chlorite-siderite formation in reduced conditions and/or at a higher temperature relative to: (3) The mainstage sulphide deposition which probably began in the quartz-sericite-pyrite stable conditions at 250-300¬¨‚àûC (?). Increasing pH and decreasing f0\\(_2\\) as reaction with the host rock proceeded lead to chlorite formation and subsequently to chalcopyrite formation. (4) A late-stage phase of quartz-sericite-pyrite-chalcopyrite mineralization and alteration. Hematite was produced during later metamorphism by the oxidation of magnetite pyrrhotite(?) and pyrite(?). All phases except pyrite appear to have been completely recrystallised and quartz siderite chalcopyrite and chlorite have been locally remobilised. Crown Lyell - North Lyell ores consist of: (1) Disseminated pyrite-chalcopyrite mineralization in chert and silicified volcanics with minor sericite-barite-carbonate (Crown Lyell No. 3). (2) Bornite-chalcopyrite mineralization in chert or chert-breccia with minor sericite and pyrophyllite (North Lyell Crown Lyell No. 2 12 West). (3) Disseminated pyrite-chalcopyrite mineralization in chloritised felsic volcanics; the common assemblage is quartz-chlorite-minor sericite-hematite-magnetite-pyrite-chalcopyrite-barite (Crown Lyell No. 3 footwall Lyell Tharsis Crown Lyell No.l). Bornite-chalcopyrite mineralization suggests a probable lower temperature of deposition relative to Prince Lyell (assuming a similar copper content to the ore fluids of Prince Lyell); the quartz-chlorite-oxide-barite assemblage suggests higher oxidation conditions relative to Prince Lyell. The Cape Horn orebody consists of the assemblages: (1) quartz-sericite-chlorite-pyrite-chalcopyrite-minor barite (2) quartz-chlorite-minor sericite-hematite-magnetite-pyrite-chalcopyrite-barite (3) quartz-sericite-pyrite-chalcopyrite-minor barite. The combination of assemblages common to Prince Lyell and Crown Lyell - North Lyell is consistent with f0\\(_2\\)-T conditions intermediate between the conditions of deposition of Prince Lyell and Crown Lyell-North Lyell. Western Tharsis consists of disseminated pyrite-minor chalcopyrite mineralization in altered felsic volcanics; the common assemblage is quartz-sericite-pyrite-chalcopyrite-minor carbonate and chlorite. Relative to Prince Lyell the mineralization and alteration occurred at lower temperatures(?) or the ore fluid contained a higher initial sulphur content. The ˜í¬•S\\(^{34}\\)\\(_{py}\\) ‚ÄövÑ‚àû and ˜ívÆS\\(^{34}\\)\\(_{py-ccp}\\) ‚ÄövÑ‚àû values for the various deposits are: {The column spacing below is not displaying correctly so the word \"and\" is inserted in each row for ease of interpretation} ˜í¬•S\\(^{34}\\)\\(_{py}\\) ‚ÄövÑ‚àû and ˜ívÆS\\(^{34}\\)\\(_{py-ccp}\\) ‚ÄövÑ‚àû Prince Lyell +10.0 to +5.2 and +1.2 to -0.6 Cape Horn + 6.4 to -0.4 and -1.3 to -1.5 Crown Lyell-North Lyell + 0.8 to -5.3 and +0.3 to +1.2 Western Tharsis + 4.8 to +6.4 and +1.6 to +1.8 The variation in ˜í¬•S\\(^{34}\\)\\(_{py}\\) values from Prince Lyell to Cape Horn to Crown Lyell-North Lyell is consistent with a progressive increase in the oxidation conditions of deposition. Mass transfer calculations indicate the variation in ˜ívÆS\\(^{34}\\)\\(_{py-ccp}\\) is consistent with the oxidation trend; with chalcopyrite precipitating at a higher pH and lower oxidation conditions relative to pyrite and/or precipitating from a solution enriched in S\\(^{34}\\) by the precipitation of a significant amount of the sulphur in solution as pyrite. The estimated initial value of ˜í¬•S\\(^{34}\\)\\(_{‚Äöv†v´S}\\) for the Prince Lyell Cape Horn and Crown Lyell-North ES Lyell ore solutions is +7 per mil. If the Western Tharsis ˜ívÆS\\(^{34}\\)\\(_{py-ccp}\\) data approximates equilibrium partitioning the initial ˜í¬•S\\(^{34}\\)\\(_{‚Äöv†v´S}\\) value for the Western Tharsis solutions must be less than +7 per mil. Cobalt contents of pyrite from the chlorite assemblages of Prince Lyell Cape Horn and Crown Lyell No.3 footwall average 1600 1050 and 1800 ppm respectively with values up to 4000 ppm. In contrast cobalt contents of pyrite from quartz-sericite-pyrite assemblages are less than 1000 ppm although pyrite from the Western Tharsis quartz-sericite-pyrite-chalcopyrite assemblage averages 1400 ppm. Nickel contents are also higher in pyrite from chlorite assemblages particularly Cape Horn (av. 700ppm) and Crown Lyell No.3 footwall. Increasing Co/Fe and Ni/Fe ratios in solution due to the precipitation of chlorite and chalcopyrite replacing pyrite are the likely reasons for cobalt and nickel enrichment in pyrite from chlorite-rich and chalcopyrite-rich assemblages. Mass transfer calculations suggest depletion of Co and Ni in solution is also an important factor controlling the cobalt and nickel content of pyrite."