EVIDENCE OF HYDROTHERMAL AMMONIUM VENTING INTO THE MID-PROTEROZOIC MCARTHUR BASIN

Some of the largest sedimentary ore deposits formed during the Proterozoic when saline brines rich in base metals vented into the anoxic deep ocean. A number of features indicate that these seeps were biologically active. Microbial sulfate reduction likely played a key role in the precipitation of ore minerals. This observation raises the question how biological activity was sustained in these settings while most of the Proterozoic ocean is considered to have been relatively unproductive. To address this question, we investigated rock samples from the Paleoproterozoic Barney Creek Formation in the McArthur Basin in northern Australia, which hosts a world-class zinc deposit. We analysed ratios of organic carbon to nitrogen as well as nitrogen isotopes in a suite of samples that extends from the fringe of the major ore body to about 60 km distance. Our results show an enrichment in organic-bound nitrogen closer to the ore with a distinct isotopic composition (δ¹⁵N ~ +4 ‰) compared to the more distal settings (δ¹⁵N ~ +7 ‰). These data are most parsimoniously interpreted as a hydrothermal influx of ammonium into the basin during biomass production. The isotopic measurements suggest that the ammonium was sourced from older organic-rich strata in the Wollogorang Formation (1.73 Ga), which has previously been identified as a local source of over-mature hydrocarbons to the McArthur Basin. Hydrothermal venting thus created niches of elevated nitrogen bioavailability by recycling ammonium from the sedimentary record. A similar recycling mechanism has been documented from modern hydrothermal vents that are associated with sediment packages. For the Proterozoic ocean, our results imply that basins with extensive hydrothermal activity may have created oases for microbial life in an otherwise nutrient-starved world. Importantly, high biological productivity and specifically enhanced microbial sulfate reduction around hydrothermal vents may have been crucial for the formation of economic base metal deposits.