Pyrite composition and ore genesis in the Prince Lyell copper deposit, Mt Lyell mineral field, western Tasmania, Australia |
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| Authors: | Oliver L Raymond |
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| Abstract: | The Prince Lyell copper-gold-silver deposit occurs in the late Cambrian Mt Read Volcanics, at Queenstown, Tasmania. Steeply plunging, broadly conformable lenses of disseminated and stringer pyrite-chalcopyrite mineralisation occur in quartz-sericite-chlorite rocks derived from intense alteration of predominantly felsic lavas and volcaniclastic rocks. Middle Devonian deformation has substantially modified primary sulphide textures.Although extensively fractured, pyrite grains in the ore have retained their original pre-deformation internal structure and chemistry which are revealed by etching and electron microprobe analysis. Earliest sulphide mineralisation produced oscillatory zoned, cobalt-rich pyrite (Pyrite I), coeval with chalcopyrite mineralisation. Cobalt-rich pyrite is commonly associated with Cambrian volcanic rocks in western Tasmania and suggests a volcanogenic origin for the ore fluids at Prince Lyell. Pyrite I was corroded by later hydrothermal fluids and reprecipitated as unzoned, trace element-poor pyrite (Pyrite II), commonly as overgrowths on Pyrite I cores. Minor amounts of a second cobalt-rich pyrite (Pyrite III) occurs with Pyrite II in composite pyrite overgrowths. Sulphur isotope ratios from all pyrite generations fall within a small range (3 to 11‰). In situ isotopic analyses showed no consistent δ34S variation between the various pyrite generations, suggesting recycling of sulphur derived from a single Cambrian volcanogenic source.Hematite alteration, derived from oxidised fluids possibly from the adjacent hematitic Owen Conglomerate, occurs in the structural footwall volcanics and the Great Lyell fault zone. Hematite inclusions in Pyrite II and III indicate that these pyrite generations occurred after or during deposition of the conglomerate. It is postulated that Pyrite II and III were deposited during waning volcanism, contemporaneous with Owen Conglomerate sedimentation in the late Cambrian or early Ordovician. The Great Lyell fault would have acted as a growth fault margin between a terrestrial basin, filling rapidly from the east, and the volcanic terrane to the west. The scenario raises the possibility that the concentration of mineral deposits and hematitic alteration along the Great Lyell fault resulted from the subsurface interaction of reduced volcanogenic fluids and oxidised basin waters along the growth fault contact. |
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