Alkali alteration styles and mechanisms, and their implications for a 'brine factory' source of base metals in the rift-related McArthur Group, Australia

McArthur Group alkali metasomatism occurred in several phases, none of which fit a classic model of alkali-mineral zonation towards an evaporitic basin centre as previously proposed. They comprise: (i) trough-margin alteration along the Emu Fault Zone. formed by gravity-driven meteoric-evaporitic brines sourced from adjacent shelves, and focused up to 3.5 km out into the adjacent deeper water elastic succession of the Barney Creek Formation: these formed zoned albite-microcline assemblages, variably overprinted by B-bearing illite: (ii) ore-related alteration at the HYC Zn-Pb-Ag deposit, which overprinted trough-margin alteration in places, adding disseminated ankerite, adularia and base-metal sulfides to some feldspathised beds: (iii) early diagenetic alkali metasomatism that was associated with descending/advecting brines sourced from overlying evaporite sequences, and transmitted through porous elastic sediments (e.g. parts of the Lynott Formation, Mara Dolomite and Tatoolla Sandstone): and (iv) extensive inversion-related K-feldspar metasomatism of volcaniclastic beds towards the basin centre. The descending/advecting brine model (iii) has implications for sedimentary Zn-Pb ore genesis in the basin, because it offers a mechanism to efficiently deliver batches of saline fluid into the deep diagenetic environment, resulting in alkali alteration and metal leaching, and the production of ideal Zn-Pb-transporting ore fluids. A variation in the flux of saline diagenetic water, caused by eustatically and tectonically driven aridity cycles in sedimentary facies, could have imposed strong episodicity on the formation of related Zn-Pb deposits. This speculative model is relevant to metals exploration on the premise that each 'brine factory' potentially gave rise to a related cycle of Zn-Pb ore deposition al the same or an overlying stratigraphic level.