Abstract

The McArthur (H.Y.C.) deposit is an effectively unmetamorphosed, stratiform Pb-Zn-Ag deposit of middle Proterozoic age, located in the Northern Territory of Australia. Stratigraphically, the deposit occurs in the H.Y.C. Pyritic Shale Member of the Barney Creek Formation, near the middle of the dominantly dolomitic McArthur Group. Structurally, the deposit is restricted to the deeper parts of an ancient basin known as the Bulburra Depression. This basin is bounded on the east by the Emu Fault zone, which was active during formation of the deposit and is considered to have been a major feeder zone for metalliferous fluids entering the basin. Sphalerite and galena mineralization occurs principally as delicate, fine-grained, conformable interlaminations within pyrite-rich shales.Within the mineralized sequence, there are several laterally discontinuous layers of fragmented and globular black chert. These cherts are considered to have developed from colloidal silica which precipitated either directly from the water column onto the basin floor or within the sediment during early stages of diagenesis. Characteristically, the chert fragments and blebs are rimmed by sphalerite; less commonly, they are partially or completely rimmed by galena. Sphalerite and galena textures can be related to chert textures, and these relationships suggest that, within the cherty intervals of the deposit, sphalerite and galena deposition and chert formation occurred over a prolonged period prior to final lithification of the sediments. "Colloform" textures in the sphalerite associated with some of these cherts appear to have formed through replacement of radially fibrous, chalcedonic quartz.The black cherts contain a diverse assemblage of microfossils, including filamentous bacteria, unicellular algae, and filamentous algae. Analysis of the preserved biota suggests that the bacteria lived at the bottom of the depositional basin, below the photic zone, and that the algae lived in overlying, near-surface waters. No fossils of sulfate-reducing bacteria have been recognized, although sulfur isotope data from previous studies suggest that such organisms may have been active during formation of the deposit. There is no paleontologic evidence to indicate that any of the preserved organisms were capable of concentrating nonferrous metals, although nearly all of the bacterial fossils are pyritized. It appears that bacteria played an indirect but key role in the formation of the McArthur deposit by consuming oxygen during their heterotrophic decomposition of detrital algal organic matter and, thereby, maintaining the anoxic bottom conditions necessary for accumulation and preservation of fine-grained sulfide minerals. The potential for microfossil biostratigraphy of the McArthur Group and its possible utility in future exploration programs in northern Australia are briefly discussed.

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