The Archean Cu-Zn magnetite-rich Gossan Hill VHMS deposit, Western Australia : evidence of a structurally-focussed, exhalative and sub-seafloor replacement mineralising system

The Archean Cu-Zn Gossan Hill volcanic-hosted massive sulphide deposit is situated on the northeast flank of the Warriedar Fold Belt in the Yilgarn Craton, Western Australia. The deposit is hosted within re-deposited rhyodacitic tuffaceous volcaniclastics of the Golden Grove Formation and is overlain by rhyodacite-dacite lavas and intrusive domes of the Scuddles Formation. The Gossan Hill deposit consists of two discrete subvertical ore zones situated stratigraphically 150 m apart in the middle and upper Golden Grove Formation. The stratigraphically lower Cu-rich ore zone (7.0 Mt @ 3.4% Cu) consists of stratabound, podiform to discordant massive pyrite-chalcopyrite-pyrrhotite-magnetite. In addition to massive sulphides, the lower ore zone also contains discordant to sheet-like zones of massive magnetite-carbonate-chlorite-talc (~12 Mt). The upper Zn-Cu ore zone (2.2 Mt @ 11.3% Zn, 0.3% Cu, 15 g/t Au and 102 g/t Ag) is mound-shaped with sheet-like, stratabound, massive sphalerite-pyrite-chalcopyrite overlying discordant massive pyrite-pyrrhotite-chalcopyrite-magnetite. A sulphide-rich vein stockwork connects the upper and lower ore zones. Metal zonation grades from Cu-Fe (¬¨¬±Au) in the lower ore zone to Zn-Cu-rich sulphides at the base of the upper ore zone. The upper ore zone grades upwards and laterally from Zn-Cu to Zn-Ag-Au (¬¨¬±Cu, ¬¨¬±Pb)-rich sulphides. Regional preservation of primary tuffaceous volcanic textures within the Golden Grove Formation is attributed to an early syndepositional, quartz-chlorite alteration. Induration and differential permeability/porosity reduction of the succession during the early alteration likely promoted more-focussed pathways for successive hydrothermal fluids. Subsequent hydrothermal alteration related to mineralisation at Gossan Hill has a limited lateral extent, and forms a narrow Fe-chlorite-ankerite-siderite envelope to the massive magnetite and sulphide of the lower ore zone, and an intense siliceous envelope surrounding the stockwork and upper ore position. Pervasive calcite-muscovite alteration is recognised in the hangingwall volcanics of the Scuddles Formation. The nature of deformation and metamorphism (greenschist facies: 454 ¬¨¬±4¬¨‚àûC at 1 kbar based on andalusite-chloritoid-quartz equilibrium) is uniform throughout the massive magnetite, massive sulphide and host succession. Sediment-sulphide-magnetite relationships at Gossan Hill suggest the formation of magnetite and sulphide during deposition of the upper Golden Grove Formation. Massive magnetite formed entirely by sub-seafloor replacement processes as inferred from gradational upper and lower contacts and interdigitating volcaniclastics. Replacement occurred along permeable tuffaceous strata outward from a discordant feeder. Massive magnetite was later veined, replaced and cut by massive sulphide. The synchronous formation of both upper and lower sulphide ore zones is indicated by the connecting sulphide stockwork. Both sulphide ore zones formed by sub-seafloor replacement, although stratiform hydrothermal chert-sulphide-sediment layers in, and adjacent to, the upper sulphide zone attest to some exhalation of fluids onto the seafloor. The thickest occurrence of massive magnetite, massive sulphide and stringer stockwork spatially coincide and support a common feeder conduit during massive magnetite and sulphide mineralisation. The asymmetry of hydrothermal alteration envelopes, massive magnetite and massive and veins sulphide zones are consistent with synvolcanic structural controls, with a growth structure occupied and obscured by a younger dacite dome from the Scuddles Formation. A systematic increase in sulphide ˜í¬•\\(^{34}\\)S values (range of -4.0 to 7.8‚ÄövÑ‚àû, average 2.1 ¬¨¬± 1.7‚ÄövÑ‚àû) stratigraphically upwards through massive and vein sulphide is suggestive of progressive mixing of upwelling ore fluids with entrained seawater. Homogeneous ˜í¬•\\(^{34}\\)S values of ~1.5‚ÄövÑ‚àû in the lower ore zone have a consistent homogeneous rock sulphur source with possible magmatic contributions. The ˜í¬•\\(^{18}\\)O\\(_{H20}\\) values of ore fluids responsible for deposition of magnetite in massive magnetite and disseminated magnetite in the sulphide zones range from 6% to 13%. This data is inconsistent with the direct input of Archean seawater, and favours derivation of hydrothermal fluids by rock buffering of circulating fluids, or by direct magmatic contribution. Thermodynamic considerations suggest massive magnetite and sulphide formed from high temperature (300¬¨‚àû to 350¬¨‚àûC), reduced (low `f` O\\(_2\\), slightly acidic hydrothermal fluids. H\\(_2\\)S deficient fluids formed massive magnetite, whilst H\\(_2\\)S-rich fluids formed massive sulphides. Fluid chemistry differences are attributed to magmatic sulphur contributions during sulphide mineralisation. Precipitation of sub-seafloor sulphide in the lower ore zone resulted from chemical entrapment by the interaction of upwelling H\\(_2\\)S-rich fluids with pre-existing massive magnetite. It is suggested that shallow parental magma chambers to the Scuddles Formation drove hydrothermal convection of seawater and may have supplied volatiles and H\\(_2\\)S to the ascending hydrothermal fluids. The Gossan Hill sulphide-magnetite deposit represents an evolving hydrothermal system in an environment characterised by rapid volcaniclastic sedimentation and changing structural and magmatic processes. An important influence on this hydrothermal system was the creation and destruction of porosity and permeability in the host succession. The hydrothermal system initiated as part of a regional seawater convection-alteration system that led to VHMS mineralisation at Gossan Hill by (1) synsedimentary metasomatism and progressive heating of convecting fluids, (2) formation of massive magnetite by host rock replacement above a buried synvolcanic conduit, and (3) structural re-activation and tapping of deeper H\\(_2\\)S-rich and metal-bearing fluids, leading to the sub-seafloor sulphide replacement and local exhalation of hydrothermal fluids forming sulphide and chert. Burial by proximal felsic volcanism led to preservation of the deposit.