Geology and mineralisation of the Endeavour 41 gold deposit, Cowal district, NSW, Australia

Epithermal and porphyry styles of alteration and mineralisation occur at the Endeavour 41 (E41) gold deposit in the Cowal Igneous Complex, New South Wales, Australia. E41 is one of three economically significant gold centres (E46, E42 and E41) in the Cowal district. These deposits formed within the Ordovician Macquarie island arc by subduction-related processes, and are hosted by a subaqueous volcano-sedimentary succession of interbedded sedimentary and resedimented volcaniclastic facies, trachyandesite and porphyritic andesites. The volcanic facies architecture at E41 is consistent with a distal submarine volcanic setting. The host succession at E41 has been intruded by numerous sills, dykes and stocks, which defi ne the E41 intrusive complex. The magmas evolved from mafic to more felsic and then back to mafic compositions with time. There is evidence of both mafic and silicic magmatism of high-K to shoshonitic affinity at the time of gold mineralisation, consistent with an alkalic association for gold mineralisation. The pre-mineralisation Muddy Lake diorite intruded the Cowal district at 461 ± 5.2 Ma. The stratigraphy was then tilted prior to the emplacement of numerous dykes and mineralised veins. A mafic monzonite intrusion emplacement after tilting at 458.5 ± 5.2 Ma provides the upper age constraint on deformation. Magmatic activity culminated in the emplacement of a series of post-mineralisation dioritic dykes around 450 to 447 Ma. Geochronological results have identifi ed two mineralising events in the Cowal district: (a) calc-alkalic Cu-Au porphyry deposits formed in the southeastern part of the district at around 463 Ma, based on Re-Os dating of molybdenite from E43, and (b) epithermal deposits formed in the central western part of the district around 455 Ma (E41, E42 and E46). The earliest fluids that caused hydrothermal alteration at E41 were magmatichydrothermal in origin. They produced potassic alteration (magnetite ± biotite) in clastic units and high temperature propylitic alteration (actinolite – magnetite) in diorite. Rare magnetite- and andradite-bearing veins formed during this early phase of magmatichydrothermal activity. These early fluids were relatively oxidised (hematite- and andradite-stable), hot ~ >400o C (biotite- and actinolite-stable) and had near-neutral to alkaline pH (feldspar-calcite stable). The early high-temperature alteration assemblages and veins have been overprinted by gold-mineralised domains associated with lower-temperature alteration facies. Gold mineralisation at E41 formed during two veining events: (1) quartz – pyrite ± calcite ± adularia veins (stage 3); and (2) carbonate-base metal sulphide veins that contains calcite, ankerite, quartz, pyrite, sphalerite, galena, chalcopyrite, Ag-tellurides, arsenopyrite, hematite, apatite, illite ± muscovite and chlorite (stage 4). Gold occurs principally in the crystal lattice of arsenian pyrite. Stage 4 mineralisation produced Au-Ag-tellurides and Au inclusions in pyrite, sphalerite and chalcopyrite. Hydrothermal alteration halos associated with stage 3 veins evolved from high temperature epidote and K-feldspar – epidote to illite – muscovite – K-feldspar alteration. Stage 4 mineralisation is spatially and temporally associated with illite – muscovite – carbonate alteration assemblages. Late stage gypsum-, calcite-, epidote-, prehnite-, hematite-, and ankerite-bearing veins are unmineralised. Fluid inclusions from actinolite-bearing stage 1 and garnet-bearing stage 2 veins have low (~250oC) homogenisation temperatures, suggesting either that these fl uid inclusions have re-equilibrated, or that significant pressure corrections are required for these temperature estimates. The salinities of stages 1 and 2 were around 11.0 and 7.0 wt. % NaCl, respectively. Main-stage quartz – pyrite veins (stage 3) trapped vapour- and liquid-rich, moderate salinity (~9.0 wt. % NaCl) fluid inclusions under boiling conditions at temperatures around 310oC. Stage 3 veins are estimated to have formed approximately 1 km below the paleosurface at hydrostatic pressure (~90 bars). No fluid inclusions were found in stage 4 veins, but the presence of illite indicates formation temperatures below ~280oC. Sulfur isotope analyses have provided evidence for a magmatic sulfur component prior to and during gold mineralisation. The δ 34Ssulfide values for early vein stages range between -4.9 to -0.5 per mil. The stage 3 has δ34Ssulfide values ranging from -5.2 to +0.8 per mil with the most 34S-enriched sulfides values deposited away from the mineralised centre. Stage 4 sulfides have isotopic compositions from +2.5 to -7.5 per mil. The negative isotopic values are consistent with sulfate-predominant magmatichydrothermal fluids. Sulfur isotopic zonation patterns defined by stage 3 and 4 sulfides at E41 broadly correlate with high-grade domains. Stage 3A-c calcite has δ13C calcite and δ18O calcite values that range from -5.2 to -4.6 and from +11.6 to +12.1 per mil, respectively. Calculated fluids for these mineral values at 300oC (δ13C fluid = -3 per mil; δ18Ofl uid = +6 per mil) are consistent with a magmatic-hydrothermal source of carbon and oxygen during stage 3A-c. A component of meteoric waters is inferred for stage 4, because δ13Ccarbonate and δ18Ocarbonate values range from -6.9 to -0.5 and from +10.9 to +30.1 per mil respectively, corresponding to δ13C fluid and δ18O fluid values of -5 and -2 per mil at 200-250oC. The involvement of external waters during stage 4 is also supported by the δDillite-muscovite and δ18Oillite-muscovite compositions that range from -67.7 to -54.4 and +5.0 to +9.5 per mil, respectively. These values correlate to δ18OH2O and δDH2O values of +2.9 and -85.4 per mil at 250oC, and are consistent with meteoric fluids that have partially equilibrated with volcanic rocks. Gold is inferred to have been transported as a bisulfide complex in stage 3 and 4 in weakly acidic to alkaline aqueous fluids. Gold precipitated due to a combination of boiling and wall rock sulfidation. Some evidence for fl uid mixing is provided by C-O and D-O isotopic data from stage 4, and this process may also have been important for ore formation. E41 records the transition from deep, porphyry-style to shallow-level epithermal style magmatic-hydrothermal activity, and potentially implies unroofi ng of the system synchronous with mineralisation. High-temperature propylitic actinolite and epidote, and potassic assemblages (biotite, orthoclase, magnetite) indicate that E41 is located proximal to an alkalic centre of magmatic – hydrothermal activity. This is the first documented occurrence of low-sulfidation alkalic-style epithermal mineralisation in the Macquarie Arc. Continued exploration around E41 may lead to the discovery of an alkalic porphyry Cu-Au deposit.