A genetic model for the HYC deposit, Australia: based on regional sedimentology, geochemistry and sulphide-sediment relationships

The fine-grained, laminated, stratiform zinc-lead ores of the McArthur River (HYC) ore deposit have been the subject of continued debate in regard to the timing of zinc-lead mineralization. Early workers (e.g., Croxford and Jephcott, 1972; Lambert, 1976) considered the ores to be sedimentary-exhalative (sedex), while more recent researchers (e.g., Williams, 1978a; Eldridge et al., 1993; Hinman, 1996) have argued for a syndiagenetic replacement origin. Several lines of evidence, including studies on the regional sedimentology, sediment geochemistry, nature of sulfide layering, paragenesis, and chemistry of base metal transport and deposition, are used here to demonstrate that a synsedimentary origin is likely for the HYC deposit. Studies on the regional sedimentology and geochemistry of the Barney Creek Formation have shown that the host rocks to the ore deposit, the HYC Pyritic Shale, represent a period of water deepening. Water depth was probably in excess of several hundred meters, with the deepest water euxinic facies corresponding to the ore position at the base of the HYC Pyritic Shale. A widespread manganese halo, developed in the W-Fold Shale Member below the ore horizon, is considered to relate to the start of the basin-deepening event, with manganese-bearing connate brines released into the basin along marginal rift faults. Manganese deposition occurred in the transitional dolomitic shale facies, where shallow-water, oxidized basin intermittently mixed with deeper-water, reduced fluids along a redox interface. Sedimentary manganese deposition was immediately followed by zinc-lead mineralization in the deep-water, reduced facies of the HYC Pyritic Shale Member. The intimate relationship between the eight stratiform ore lenses and inter-ore sedimentary breccias indicates that sedimentary breccia deposition and ore formation are genetically linked. It is suggested that deep seismic events led to the following ore cycle: fault movement; sediment breccia deposition sourced from faults; episodic release of hot metalliferous brines along faults accompanied by sedimentary turbidites; and metal sulfide deposition in muds and turbidites adjacent to faults. The three modes of base metal sulfide occurrence in the ores, each considered by Eldridge et al. (1993) to have formed by replacement, are shown to have separate origins. The fine-grained sphalerite-galena bands (Mode 1) are intricately interlayered with pelagic organic-rich beds and thin turbidite beds. They exhibit classic sedimentary textures, such as load casts and flames, indicative of a synsedimentary origin. Coarser-grained Mode 2 base metal sulfides show complex intergrowths with nodular dolomite bands, and are considered to form by replacement during diagenesis. The discrete patches of coarse, Mode 3 base metal sulfides are interpreted to represent clastic input from turbidity currents sourced partly by the erosion of vein-style mineralization from the faulted margins of the basin. The detailed nature of the layering in the ores suggests that the introduction of metalliferous brines into the basin was not continuous but occurred as a series of pulses interspersed with pelagic sedimentation and turbidite deposition. Preliminary calculations suggest that On the order of 10,000 discrete pulses of brine were required to form each ore lens, and it is suggested these pulses were related to microseismic activity following each major seismic event in the basin. Solubility considerations indicate that the mineralized brines were most probably sulfate-rich (SO4 2- > H2S) and capable of transporting up to several thousand parts-per-million zinc and lead at moderate temperatures (150°-250°C), and near-neutral pH. Zinc-lead deposition was the result of two possible processes: (1) thermo-chemical reduction of brine sulfate by organic matter in anoxic basin water, and (2) diffusion of H2S from the anoxic water column into the metalliferous brine layer. Sulfur isotope data are best explained if both processes operated together to deposit galena and sphalerite.