Alkali alteration styles and mechanisms, and their implications for a ‘brine factory’ source of base metals in the rift‐related McArthur group, Australia

Abstract

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 clastic 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 clastic 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 at the same or an overlying stratigraphic level.