Interactions between saline redbed groundwaters and peritidal carbonates, Spencer Gulf, South Australia: significance for models of stratiform copper ore genesis

Brief description

A commonly cited model for the genesis of stratiform Cu deposits that lack either igneous or hydrothermal associations has been tested in modern environments, on the northeastern shore of Spencer Gulf, South Australia. In this area, saline continental redbed groundwaters mobilise Fe and Mn from the aquifer sediments and transport these metals to coastal Holocene carbonate complexes. There they may precipitate in sediments in ephemeral lakes or around springs formed by the emerging ground waters. The continental groundwaters are generally acidic and oxidising, and they discharge mainly in the supratidal zone, remote from the major areas of bacterial sulphate reduction in the intertidal zones of the sedimentary complexes. Conversion of Fe in the groundwaters to sulphides is not widespread, but occurs if the influence of the ground waters is attenuated by seasonal decrease in flow rate or by interaction with the alkaline, reducing marine carbonate environment of the peritidal zones. The Spencer Gulf investigations strongly support the contention that terrestrial groundwaters of suitable pH, Eh, and salinity for the transport of high concentrations of Cu, Pb, and Zn may be generated in semi-arid climates, and that these groundwaters are capable of mobilising large quantities of metals held in Fe-oxide grain coatings. The formation of a stratiform metal deposit zoned in accordance with metal sulphide solubilities is more difficult to envisage. The absence from the supratidal sequence of a thick deposit of organic matter capable of sustaining bacterial sulphate reduction means that the metals are more likely to be removed from solution by adsorption onto the marine carbonates, or by oxidation and precipitation as oxides or hydroxides.

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