Sedimentary Manganese as Precursors to the Supergene Manganese Deposits of the Collier Group; Capricorn Orogen, Western Australia

[摘要] The Capricorn Orogen of Western Australia hosts numerous surficial supergene manganese (Mn) deposits, mostly within the Mesoproterozoic sedimentary stratigraphy of the Edmund and Collier groups, which comprise the Bangemall Supergroup. Among these are the Butcherbird/Yanneri Ridge Mn deposits, the largest undeveloped Mn deposit in Australia, which occurs in the marine Ilgarari Formation of the Collier Group. The Collier Group is a lateral and contemporaneous equivalent of the Manganese Group stratigraphy of the Oakover Basin, which hosts the numerous supergene Mn deposits of the eastern Pilbara, including the Woodie Woodie deposits, which have recently been identified as late-Mesoproterozoic rather than Archean. While some studies have been conducted on the Woodie Woodie supergene deposits in recent years, little research has been conducted on the supergene deposits in the Edmund or Collier groups, and critically, no research has been conducted on the underlying unweathered rocks that are assumed to be the source of the supergene Mn deposits in these basins. Thus the lithological source of the Mn in these deposits is unknown to science, and therefore the genesis of the mineral system remains unknown.Here we present the first study of the unweathered Mesoproterozoic marine stratigraphy that underlies the Collier Group-hosted Butcherbird Mn deposit. Our data show that the shales of the 1.11 Ga Ilgarari Formation hosts bands of anomalously high Mn concentrations (up to ~5% wt [MnO]), characterized by a complex Mn2+ carbonate mineral assemblage, consistent with other older Proterozoic sedimentary Mn deposits such as the massive Kalahari Mn Field in South Africa. Iron speciation data show the redox conditions in the water column varied between anoxic, anoxic-sulfidic and oxic, during a time in which the oceans were assumed to be mostly anoxic. This is implies the Ilgarari Formation was deposited during a time of oxygenation of the atmosphere and oceans, which has implications for the behaviour of redox-sensitive metals such as Mn.We interpret the bands of Mn2+ carbonates to represent cycles of oxidation of soluble Mn2+ in the water column to insoluble Mn4+ oxides, following oxygenation of the upper Mesoproterozoic marine water column. Insoluble Mn4+ oxides were deposited on the anoxic seafloor, which allowed diagenetic reduction to Mn2+ carbonates in the shallow subsurface. Oxidation of the upper water column was then followed by oxidation of the entire water column, accompanied by a cessation of Mn deposition. These sedimentary Mn carbonate bands have since been oxidized during the Mesozoic in surface exposures to form high-grade supergene Mn4+ oxide deposits.These findings demonstrate the link between late-Mesoproterozoic atmosphere-ocean oxygenation and sedimentary Mn deposition across both the 1.11 Ga Collier Group and Manganese Group basins in Australia,. Cumulatively these deposits alone comprise at least 473 million tonnes of supergene-enriched sedimentary Mn, not including the primary unweathered sedimentary Mn. These Mesoproterozoic occurrences, formed during a time previously not associated with sedimentary Mn, are comparable to the periods of massive Mn deposition throughout the Neoproterozoic and Phanerozoic. This has implications for Mn prospectivity in late-Mesoproterozoic basin sequences in Australia. Indeed such a laterally-extensive sedimentary-supergene Mn mineral system between the Pilbara and Capricorn Orogen terranes may be described as a massive ‘manganese field’ in its own right.

[发布日期] 2018-01-12 [发布机构] CSIRO

[学科分类] 地球科学(综合)