K metasomatism and base metal depletion in volcanic rocks from the McArthur Basin, Northern Territory - implications for base metal mineralisation

Evidence for the leaching of base metals by oxidized, saline brines has been recognized from dolerites of the Upper Tawallah Group, southern McArthur basin. In the Mallapunyah dome region, the Upper Tawallah Group consists of a series of shallow-level dolerite sills (Settlement Creek and Gold Creek Volcanics) that have intruded a sequence of fluvial, lacustrine and shallow marine hematitic sandstones and evaporitic siltstones and carbonates. Oxidized sandstones and evaporitic carbonates of the Lower McArthur Group overlie the Tawallah Group. Formation of the Mallapunyah dome occurred due to uplift at end-Tawallah Group times, marked by the emplacement of volcano-sedimentary breccias interpreted as debris flow deposits of the Gold Creek Volcanics. K-metasomatism has resulted in the replacement of dolerites of the Settlement Creek and Gold Creek Volcanics by potassic (orthoclase + quartz ± sericite ± hematite ± dolomite ± anatase ± barite) and chlorite-orthoclase (chlorite-orthoclase-quartz ± dolomite ± sericite ± actinolite ± albite ± anatase) mineral assemblages. Three varieties of potassic alteration have been recognized: intense, texturally destructive potassic alteration halos around quartz ± hematite ± dolomite ± chlorite veins; intense, texturally destructive pervasive potassic alteration; and intense, pervasive, texturally destructive potassic alteration rinds (<1 cm thick) that occur around amygdules. K-metasomatism caused significant chemical changes to dolerites of the Settlement Creek and Gold Creek Volcanics, which now contain up to 11.8 wt percent K 2O, 68.4 wt percent SiO 2 and between 0.4 and 23.5 wt percent Fe 2O 3. Zr, Ti, Al, and Nb remained immobile during the formation of potassic and chlorite-orthoclase alteration, whereas Y and P were mobilized. Mass-balance calculations have shown that significant leaching (~9.8% net mass loss) took place during texturally preserving chlorite-orthoclase alteration at Mallapunyah dome. Additional leaching (3.5-6% net mass loss) at higher water/rock ratios resulted in destruction of primary textures and development of the potassic alteration assemblage. Many chemical components were removed from the dolerites, including an almost 100 percent depletion of Cu, Pb, and Zn. The fluids responsible for potassic alteration were non-gassy, low-temperature (~100°C) saline (>20 wt % NaCl equiv) Na-K-Ca-Mg-rich brines. The high salinities and the presence of hydrocarbons are consistent with brine derivation from the overlying evaporitic carbonates during diagenesis. Carbon-oxygen (carbonate) and oxygen (silicate) isotope analyses are consistent with the formation of dolomite veins and potassic alteration in the Settlement Creek Volcanics during interaction with low-moderate temperature (~100°C) evolved meteoric waters ± seawater (δ 18O((fluid)) ~ -1‰; δ 13C((fluid)) ~ -7‰), which descended through the overlying carbonate horizons, becoming progressively more saline and enriched in 18O as they interacted with the partially lithified sediments. The fluids responsible for metal leaching and potassic alteration of the Tawallah Group dolerites are interpreted to have been low temperature, hematite-stable, saline brines with a high base metal transporting capacity due to their oxidized nature and low H 2S contents. The metals acquired by these brines could have been transported significant distances through a suitable aquifer (e.g., well-sorted hematitic sandstone), and the resultant metalliferous brines were probably involved in the formation of at least some of the stratiform Pb-Zn, MVT, and breccia-hosted Cu deposits in the Upper Tawallah and McArthur Groups.