Geology and mineralisation of the Southern Prince Lyell Deeps, Queenstown, Tasmania

The Prince Lyell copper - gold - silver deposit occurs in the late Cambrian Central Volcanic Complex of the Mt Read Volcanics at Queenstown, Tasmania. The southern portion of the deposit comprises several steeply plunging, broadly conformable lenses truncated at depth by the Owen Conglomerate along the Great Lyell fault. The disseminated and stringer pyrite - chalcopyrite orebody was overturned and deformed during the mid Devonian Tabberabberan orogeny, substantially modifying primary sulphide textures. The orebody host and structural footwall rocks are highly siliceous quartz - sericite ± chlorite rocks derived from alteration of rhyolitic to dacitic lavas and coarse grained volcaniclastic rocks. The structural hangingwall to the mineralisation is formed by chlorite - sericite - quartz altered fine grained intermediate - mafic volcaniclastic rocks. The intense alteration was texturally destructive and imparted a false fragmental appearance on the altered felsic volcanics. Alteration resulted in significant mobilisation of major and trace elements (including so - called "immobile elements") except Ti and, to a lesser extent, Zr. The Ti/ Zr ratio appears to be a largely reliable discriminant between altered felsic and intermediate - mafic rocks, but not between intensely altered rhyolitic and dacitic volcanics. Minor magnetite - apatite mineralisation occurs as largely conformable, but locally transgressive zones of stockwork vein and disseminated mineralisation. Trace elements (Ti, V, REE) and magnetite δ\(^{18}\)Ο ratios (~ 4‰) suggest the mineralisation was derived from a magmatic fluid. This hot, low ΣS, oxidised fluid locally remobilised copper and gold out of magnetite alteration zones. Later hematite ± siderite alteration, precipitated from oxidised fluids probably derived from the hematitic Owen Conglomerate, occurs in the structural footwall volcanics adjacent to the Great Lyell fault. Apart from metal depleted zones attributable to magnetite alteration, significant remobilisation of copper, gold or the Cu/ Au ratio in the orebody has not occurred. The geochemistry of the Great Lyell fault zone and shallowly dipping "flat faults" suggests that while Devonian metamorphic fluids may have been focussed along them, dissolution of adjacent copper mineralisation was of a local scale only. Although extensively fractured, pyrite mineralisation has retained much of its pre-deformation internal structure, revealed by etching and in situ microprobe analysis. Early Co - rich pyrite (Pyrite I), coeval with chalcopyrite mineralisation, was dissolved by later unmineralised fluids and reprecipitated as trace element - poor Pyrite II. Minor amounts of a second Co - rich pyrite (Pyrite III) was precipitated concurrently with Pyrite II. Pyrite sulphur isotope ratios ranged from + 3 to 11‰. Isotopic variation is limited to a poorly defined zoning from higher to lower δ\(^{34}\)S values towards the centre of the ore zone. In situ analyses did not show any consistent δ\(^{34}\)S variation between the various stages in pyrite paragenesis, suggesting recycling of sulphur from a single Cambrian volcanogenic source and deposition of all pyrite generations from reduced volcanogenic fluids. Hematite inclusions in Pyrite II and III indicate that this pyrite mineralisation occurred after deposition of the adjacent Owen Conglomerate, from which the hematite alteration was derived. Pyrite II and III were probably deposited during the last stages of volcanic activity, contemporaneous with initial Owen Conglomerate sedimentation in the early Ordovician. Whole rock oxygen isotope ratios ranged from + 6.6 to 10.6‰ for the altered host rocks. Isotopic equilibrium was approached across the altered sequence. The δ\(^{18}\)Ο values suggest that significant isotopic exchange did not occur with a Devonian low grade metamorphic fluid. The calculated initial fluid δ\(^{18}\)Ο ratio ( + 4.5 - 6.0‰) suggests the fluid may have been derived from Cambrian seawater, but modified by interaction with the volcanic pile prior to mineralisation or by mixing with a relatively isotopically heavy magmatic fluid.