The effect of Devonian metamorphism and metasomatism on the mineralogy and geochemistry of the Cambrian VMS deposits in the Rosebery-Hercules district, Western Tasmania

The Rosebery, Hercules and South Hercules deposits in western Tasmania are composed of polymetallic massive to disseminated sulphide mineralisation hosted in felsic volcanics of the Cambrian Mt Read Volcanic belt. The deposits have been affected by regional metamorphism of upper greenschist facies and associated tectonic deformation related to the Devonian Tabberabberan Orogeny. The Devonian tectonic movement has resulted in folding, shearing and faulting (thrusting) of the ore lenses which caused significant changes in the overall configuration of the original strati.form ores and underlying stringer zones. In addition, the south-end of the Rosebery deposit has undergone metasomatic replacement related to a postorogenic Devonian granite intrusion. The VMS mineralisation at Rosebery consists of three primary sulphide-sulphate zones: lowermost pyrite-chalcopyrite zone (>4% Cu), overlain by a sphalerite-galena ± pyrite ± chalcopyrite zone, and followed by an uppermost massive barite zone. The south-end of the orebody has been overprinted by Devonian transgressive metasomatic mineral assemblages. Three major replacement zones can be distinguished: (1) magnetite-biotite ± chalcopyrite zone, (2) pyrrhotite-pyrite zone and, (3) tourmaline-quartz± magnetite zone. Othermetasomatic minerals such as fluorite, garnet and helvite are present. The main Hercules deposit occurs 10 km south along strike from Rosebery and lies in a similar tuffaceous shale unit The deposit consists of a number of disconnected stratabound Zn-Pb-Cu-Ag-Au ore lenses with ~imilar metal distribution and alteration features to the Rosebery ores. The Hercules deposit shows a zonation from a massiye to disseminated pyritechalcopyrite zone (>4% Cu) at the footwall through a massive sphalerite-galena ±pyrite zone and spotty sphalerite-galena ± pyrite zone to a carbonate ± barite zone at the top. The South Hercules deposit is located about one kilometer south of the main Hercules ore lenses at the same stratigraphic level. The deposit displays comparable mineralogical zonation with Rosebery and Hercules but contains low-grade lead and zinc, and has a low copper content ( <1 wt % Cu) and relatively high (Au+Ag)/(Pb+Zn) ratio. The deposit consists of a disseminated to stringer sulphide zone with variably altered carbonate assemblages. Metal zonation studies indicate that zinc occurs dominantly as blanket-like enrichment in the Rosebery district deposits and silver broadly follows zinc. Gold is largely concentrated at the top of the ore lenses. In the transgressive pyrrhotite-pyrite replacement zone of the Rosebery south-end, zinc is conspicuously depleted but gold values of more than 20 g/t Au are noted. In contrast, the other biotite-, magnetite-, and tourmaline-bearing replacement zones contain low gold grades (generally less than 5.0 g/t Au). Like zinc, silver is also depleted in the replacement zones but a significant amount of copper is present Copper values as high as 2% are observed in the biotite-magnetite zone as well as the pyrrhotite-pyrite zone. This indicates that although zinc and silver were dissolved and removed during the Devonian replacement event, gold and copper appear to have been redistributed and recrystallised. A detailed mineralogical study of gold in the F(J) lens indicates that the gold occurs as electrum inclusions in pyrite, as individual electrum grains in pyrrhotite, and in chalcopyrite veins cutting pyrrhotite. Similar mineragraphic investigations at Hercules and South Hercules show that gold occurs as individual electrum grains locked in the pyrite, and as patches or blebs in the remobilised and recrystallised sphalerite, galena and tetrahedrite. The Devonian metamorphic and metasomatic processes resulted in significant recrystallisation and remobilisation of these gold grains as indicated by their texture, grain size, and fineness variation. The Devonian overprinting processes also resulted in the redistribution of FeS in sphalerites from the Rosebery district deposits. The sphalerite-hexagonal pyrrhotite-pyrite assemblages from the Rosebery south-end were used to apply the sphalerite geobarometer to estimate the pressure conditions during the Devonian replacement process. The estimated pressure is 3.0 ± 0.5 kb for the requilibration of sphalerite in the F(J) lens which would correspond to a depth of 8.0 ± 1.0 km. The correlation of the mole % FeS in sphalerite and the gold grades at the Rosebery, Hercules and South Hercules deposits displays complex patterns owing either to later Devonian metamorphic and metasomatic recrystallistion or to variation of the initial depositional conditions. The VMS carbonates from both north- and south-ends of the Rosebery deposit display tightly constrained and similar isotopic patterns. The isotopic patterns imply that no later isotopic changes have taken place although the south-end of the Rosebery deposit was strongly overprinted by the high temperature (> 300°C), pervasive Devonian replacement process. The 0180 values of biotite and magnetite from the F(J) lens replacement zone were used to calculate the oxygen isotopic composition of the Devonian hydrothermal fluid. The calculated o1 80CH.z0) values for the overprinting Devonian hydrothermal fluid vary from 8.0%0 to 12.0%0 and are consistent with a magmatic fluid. Detailed sulphur isotopic investigations on the hand specimen scale indicate that there are no obvious o34S variations between the lead-zinc lenses of Cambrian volcanogenic origin and the pyrrhotite-pyrite assemblages of Devonian origin. The sulphur isotope values of the sulphide minerals from the South Hercules deposit range from o34S values of 8.2o/oo to 14.1 o/oo and the calculated 034J\\S values in the ore fluids display a permissible correlation with gold grades. This correlation suggests the possibility of using the sulphur isotopic composition as a guide to assist in targeting the gold-rich sections of massive sulphide systems. Fluid inclusion studies indicate that early formed low-temperature (ea 200'C), low-salinity ( <5 NaCl equiv. wt % ) Type I inclusions with no appreciable C02 may be interpreted as primary, Cambrian exhalative fluids that have survived the Devonian recrystallisation. Fluid inclusion characteristics of necking down and healed microfractures together with trapped carbonate and barite daughter minerals identified by laser Raman spectroscopy suggest an extensive remobilisation and recrystallisation of carbonate minerals during the Devonian overprinting processes, which corroborates with the redistribution and recrystallisation of primary Cambrian sulphides (e.g. sphalerite) and gold, during Devonian metamorphic and metasomatic processes. Although the VMS deposits in the Rosebery-Hercules area display no evidence for large-scale chemical remobilisation of ore constituents during metamorphism, the postorogenic Devonian granite intrusion below the south-end of the Rosebery mine resulted in chemical remobilisation of the ore lenses. The early replacement assemblages in the F(J) lens formed from interaction of moderate to high temperature(~ 330°C), saline (~0 NaCl equiv. wt % ) fluid with the original lead-zinc mineralisation. The later stage tourmaline veining and associated replacement assemblages resulted from lower temperature (~ 300°C), less saline (~0 NaCl equiv. wt%) fluid. During the Devonian metasomatic event gold may have been remobilised and recrystallised either as Au(HS)2 - or AuC12 - complexes but the high temperature and salinity conditions favour the AuC1i-complex. Copper also appears to have been recrystallised during the replacement process with minor remobilisation. In comparison to copper and gold, zinc, lead and silver have been dissolved and removed as the very soluble chloride complexes.