Rare-earth element geochemistry of clastic and chemical metasedimentary rocks associated with hydrothermal sulphide mineralisation in the Olary Block, South Australia

Abstract

Analysis of a variety of rock types from the South Australian portion of the Lower to Middle Proterozoic Willyama Inliers by ICP-MS demonstrate that primary rare-earth element (REE) distributions remained essentially unchanged, despite multiphase deformation and amphibolite grade metamorphism. Variable trends of preferential REE mobilisation are observed only where large amounts of fluid caused the rock to undergo strong retrogression and/or hydrothermal alteration. Except for these conditions, REE can be considered immobile and their distribution is generally independent of mineralogy, thus providing a powerful tool for the characterisation of provenance of protoliths in a high-grade metamorphic terrain. REE signatures and other geochemical constraints of some volcanogenic sediments, for example, point to an andesitic parentage, suggesting bimodal and gradually changing volcanism occurring in a developing intracontinental rift zone. REE distributions in metasediments associated with sulphide mineralisation reflect both physico-chemical characteristics of the ore-bearing fluids and rock compositions. Comparison of their REE profiles allows discrimination between: (a) strata-bound, redox-controlled deposits; (b) exhalative mineralisation; and (c) epigenetic vein-type mineralisation. REE distributions in the analysed rocks are in agreement with other lines of textural and geochemical evidence for that limited syn-tectonic remobilisation (low fluid/rock ratios, unchanged primary REE signatures) but widespread late-stage mobilisation of sulphides (high fluid/rock ratios, selective mobilisation of REE) occurred in the Olary Block. REE signatures of exhalative precipitates associated with sulphide mineralisation do not show a systematic stratigraphic variation but are indicative of fluctuating f O 2 conditions over short distances and display only moderately positive Eu anomalies. This contrasts with findings for the adjacent Broken Hill Block and highlights the dissimilarities of ore formation processes that were active in the Olary and the Broken Hill Block as well as the depositional differences between the two blocks.