The geological evolution of the DeGrussa volcanic-hosted massive sulfide deposit and the Eastern Capricorn Orogen, Western Australia

The Paleoproterozoic DeGrussa copper-gold–silver, volcanic-hosted massive sulfide (VHMS) deposit is located in the eastern part of the Capricorn Orogen of central Western Australia. The DeGrussa deposit, discovered in 2009, occurs within the c. 2000 Ma Karalundi Formation equivalent sedimentary and mafic rocks (DeGrussa Formation), the lowermost stratigraphy of the Bryah Group. DeGrussa is a siliciclastic - mafic style, VHMS deposit located on the northern limb of the Robinson Range syncline and consists of four primary ore lenses (DeGrussa, Conductor 1, Conductor 4 and Conductor 5), with supergene enrichment of copper, providing a total resource of 12.4Mt @ 4.7% Cu and 1.8g/t Au. The host rock sequence is turbiditic quartz-rich sandstone and siltstone to fine-grained mudstone rocks. Peperitic basalt intruded the sedimentary rock sequence. The footwall Magazine Member is hematitic, mudstone-rich turbidites, carbonate-dolomite megabreccia and volcanic conglomeritic rocks. The entire sequence is intruded by dolerite sills which postdate the VHMS mineralisation. The DeGrussa host sedimentary rocks overlie the Yerrida Group (Bubble Well Member and Johnson Cairn Formation), with the mafic rocks of the Johnson Cairn Formation interpreted to have formed in a continental rift setting similar to the Guaymas Basin or Red Sea Rift. Geological and geochemical evidence supports the DeGrussa deposit forming in a submarine continental rift. Dolerites within the host sedimentary rock sequence are tholeiitic, of MORB composition, and have slight REE enrichment suggesting association with seamount and/or hot spot magmatism. The comparable mafic rocks of the Bryah Group, in particular the Narracoota Formation, have a large geochemical variation encompassing tectonic environments from a back arc basin rift to island arc settings. There was a change in basin dynamics from east to west within the Bryah and Yerrida Basins, where an initial continental rift may have progressed into a more open oceanicrift or back arc tectonic setting. Subduction along margins of the Bryah basin is possible given the boninitic affinities of some mafic rocks of the Narracoota Formations. The DeGrussa deposit displays mineral zonation atypical of most VHMS deposits, perhaps evident of its replacement style of formation. Magnetite forms in the hottest, central core of the deposit in association with pyrite and lesser chalcopyrite and pyrrhotite. Sphalerite and galena form in rare horizons throughout the ore lenses. Replacement textures indicate parts of the deposit were formed in the sub seafloor environment, although seafloor textures and chimney structures are present. The Shiraz fault, with reverse sinistral displacement of 500-600m, truncates the southern side of the DeGrussa and Conductor 1 ore lenses, separating them from Conductor 4 and Conductor 5. The Merlot Fault splays off the Shiraz Fault in a south-westerly direction, approximately 500 m north east of the DeGrussa and C1 ore lenses. Alteration proximal to the DeGrussa deposit includes chlorite and talc-carbonate. Chlorite schist surrounds the massive sulfide lenses and is generally strongly sheared, although less deformed areas contain disseminated pyrite and chalcopyrite. Talc-chlorite-carbonate (± pyrite ± chalcopyrite) schist occurs within the ore deposit and as lenses along both the upper or lower contacts of the ore bodies where it is separated from mineralisation by chlorite schist. The pyrite-chalcopyrite- pyrrhotite mineral assemblage and geological evidence suggests that the emplacement of reduced, magmatic-hydrothermal, mineralising fluids, at temperatures of 250-350°C and pH of 5-7, led to replacement of existing sedimentary and basaltic rocks via fault controlled feeder structures. The `δ^(34)S` values of +0.3 to +6.3‰ for DeGrussa sulfide is interpreted to represent leached igneous rock sulfur and/or magmatic-hydrothermal sulfur, mixed with minor seawater sulfate to produce the heavy `δ^(34)S` tail for pyrite up to +9.8‰. The lack of sulfate minerals in the ore, but the presence of peripheral jasper, suggests that DeGrussa formed in a suboxic to oxic, low sulfate oceanic sub-basin from reduced, metal-bearing fluids. Lead isotopes indicate a crustal source of lead. New age dating indicates the DeGrussa VHMS mineralisation formed between 2011 - 2027 ± 7 Ma (Re-O; n=3) and 2060 - 2075 ± 50 Ma (Pb model ages; n=2) constraining the maximum deposition age for the Bryah Group rocks at c. 2075 Ma. Dolerites (2003 to 1991 ± 7 Ma; n=3) post-date the DeGrussa mineralisation, and regional granodiorites provide the first age for the Narracoota Formation of 2014 - 2018 ± 9 Ma (n = 2). A new model age of 2000 ± 35 Ma for the Horseshoe Lights VHMS deposit suggests it may have developed at a similar time to DeGrussa. District mineralisation correlates with the major regional orogenic events: the Glenburgh Orogeny from 2005-1960 Ma (DeGrussa and Horseshoe Lights VHMS and Peak Hill and Fiveways orogenic gold deposits), the Capricorn from 1830-1780 Ma (Labouchere, Mt. Pleasant and Nathans orogenic gold deposits) and the Edmundian Orogeny from 1030-955 Ma (Fortnum-Starlight orogenic gold deposit). These new ages assist in constraining stratigraphy and mineralising events of the Paleoproterozoic Yerrida, Bryah and Padbury Groups and their associations with regional orogenic events. Identification of prospective time periods and stratigraphy associated with major mineralising events will assist in exploration for significant gold and base metal mineralisation in the Capricorn Orogen