Chemical mobilities in a submarine exhalative hydrothermal system

Abstract

The chemical compositions of the texturally contrasting, but spatially related, Cu and PbZnAg host rocks at Mount Isa Mine, Queensland, have been investigated. Identification of a detrital component that was added at a comparatively steady rate to the sediments has allowed assessment of the relative rates of accumulation of the different lithologies. The partially recrystallized sediment that forms the margin of the Cu bodies was deposited approximately 2.3 to 2.5 times more rapidly than the micritic sediments enclosing the PbZnAg layers. This difference in deposition rate is due almost entirely to increased rates of SiO 2 deposition in the recrystallized sediment by the hydrothermal fluids. Owing to its more rapid rate of deposition the recrystallized sediment formed topographic highs through which the hydrothermal solution percolated before flowing out onto the sea floor. Movement of the fluid left the recrystallized sediment enriched in Co, Cu, Fe, Ni, Mn, and deficient in Rb, K 2O, Ba, Pb and Zn compared to the micritic sediment to a greater extent than could be expected from dilution through differing rates of sedimentation. S was added to both facies at approximately the same rate with pyrrhotite forming as the preferred phase with the excess Fe in the recrystallized sediment and pyrite in the micritic sediment. It is argued that the sulphide as iron sulphide originated through inorganic reduction of the seawater sulphate by the reduced hydrothermal fluid and/or was supplied directly by the fluid.