Alteration characteristics of the Archean Golden Grove Formation at the Gossan Hill Deposit, Western Australia: Induration as a focusing mechanism for mineralizing hydrothermal fluids

The Archean Golden Grove Formation is a 550-m-thick rhyodacitic tuffaceous volcaniclastic succession that hosts die Cu-Zn-rich Gossan Hill volcanic-ho sted massive sulfide (VHMS) deposit in die Yilgarn craton, Western Australi a. The Golden Grove Formation consists of volcanic quartz and altered pumic e and shards, which were deposited during successive episodes of subaqueous mass flow. Coherent volcanics are absent from the Golden Grove Formation a t Gossan Hill but form the main rock type in the hanging-wall Scuddles Form ation. A massive dacite dome that overlies its volcanic feeder dike is infe rred to occupy, a synvolcanic structure that focused mineralizing fluids du ring the formation of the Gossan Hill deposit. The Gossan Hill deposit consists of two stratigraphically separate ore zone s interconnected by stockwork. The Cu-rich lower ore zone and the Zn-rich u pper ore zone occur in the middle and upper parts of the Golden Grove Forma tion, respectively. Podiform zones of massive magnetite occur in the Cu-ric h ore zone, where the formation of magnetite predates massive sulfide. The asymmetry of massive sulfide, massive magnetite, and alteration zones at th e Gossan Hill deposit attest to synvolcanic structural control during miner alization. The principal lithofacies of the Golden Grove Formation are sandstone and p ebble breccia, which have a regionally, extensive quartz, Fe-rich chlorite, and lesser muscovite alteration. At Gossan Hill, this alteration has resul ted in near-complete replacement of tuffaceous components, causing substant ial chemical modification of the primary lithologies. Quartz-chlorite (+/- muscovite) alteration is characterized by severe K2O, Na2O, and CaO depleti on, with rocks consisting principally of SiO2, FeO, Al2O3, and MgO, along a quartz-chlorite mixing trend. Widespread preservation of pumice and shard volcanic textures within the Golden Grove Formation indicates that quartz-c hlorite (+/- muscovite) alteration occurred soon after, or possibly, during , sedimentation. The absence of diagenetic compaction textures further sugg ests induration of die succession during this early, alteration stage, with the tuffaceous succession largely sealed from the texturally destructive e ffects of subsequent hydrothermal alteration, except where mineralizing flu ids were locally channeled along synvolcanic feeder conduits. Local intense hydrothermal alteration zones surround the Gossan Hill deposi t and overprint earlier quartz-chlorite (muscovite) alteration. These local alteration zones have the same extent as the sulfide vein envelope and rep resent hydrothermal alteration formed during sulfide-magnetite mineralizati on. Intense Fe-rich chlorite (ankerite-siderite) alteration occurs as a str ata-bound envelope around massive magnetite, Cu-rich veins, and massive sul fide in the lower ore zone. This chlorite-rich alteration has strong FeO an d MgO enrichment with minor chloritoid and andalusite that reflect intense acid leaching during hydrothermal alteration. Iron chlorite (ankerite-siderite) alteration grades upward into discordant to strata-bound intense quartz alteration. Intense quartz alteration forms an envelope around Zn-rich veins and massive sulfide in the stockwork and u pper ore zone. The trend from Fe chlorite-ankerite-siderite to quartz alter ation toward the top of the deposit is consistent with the cooling of hydro thermal mineralizing fluids nearing the sea floor, Rhyodacite and dacite vo lcanics of the hanging-wall, Scuddles Formation have a pervasive muscovite- calcite alte ration. Muscovite-calcite alteration led to Na2O depletion and CaO and K2O enrichment associated with burial of the Gossan Hill mineraliz ing system. We propose that the Gossan Hill sulfide-magnetite VHMS deposit formed durin g an evolving Archean hydrothermal system that began as part of a regional- scale, low-temperature seawater convection-alteration system. Initially, th is system caused extensive replacement of the Golden Grove Formation by qua rtz and Fe chlorite ( muscovite); ii process that sealed and indurated the volcaniclastic rocks by infilling of primary porosity and permeability stru ctures. Due to subsequent impermeability of the host-rock succession, later and hotter mineralizing fluids that generated alteration and massive magne tite and sulfide at the Gossan Hill deposit were constrained to, and focuse d upward along, a synvolcanic feeder structure.