RARE-EARTH ELEMENT GEOCHEMISTRY OF ELASTIC AND CHEMICAL METASEDIMENTARY ROCKS ASSOCIATED WITH HYDROTHERMAL SULFIDE MINERALIZATION IN THE OLARY BLOCK, SOUTH AUSTRALIA

Analysis of a variety of rock types from the South Australian portion of the Lower to Middle Proterozoic Willyama Inliers by ICP-MS demonstr ate that primary rare-earth element (REE) distributions remained essen tially unchanged, despite multiphase deformation and amphibolite grade metamorphism. Variable trends of preferential REE mobilisation are ob served only where large amounts of fluid caused the rock to undergo st rong retrogression and/or hydrothermal alteration. Except for these co nditions, REE can be considered immobile and their distribution is gen erally independent of mineralogy, thus providing a powerful tool for t he characterisation of provenance of protoliths in a high-grade metamo rphic terrain. REE signatures and other geochemical constraints of som e volcanogenic sediments, for example, point to an andesitic parentage , suggesting bimodal and gradually changing volcanism occurring in a d eveloping intracontinental rift zone. REE distributions in metasedimen ts associated with sulphide mineralisation reflect both physico-chemic al characteristics of the ore-bearing fluids and rock compositions. Co mparison of their REE profiles allows discrimination between: (a) stra ta-bound, redox-controlled deposits; (b) exhalative mineralisation; an d (c) epigenetic vein-type mineralisation. REE distributions in the an alysed rocks are in agreement with other lines of textural and geochem ical evidence for that limited syn-tectonic remobilisation (low fluid/ rock ratios, unchanged primary REE signatures) but widespread late-sta ge mobilisation of sulphides (high fluid/rock ratios, selective mobili sation of REE) occurred in the Olary Block. REE signatures of exhalati ve precipitates associated with sulphide mineralisation do not show a systematic stratigraphic variation but are indicative of fluctuating f (O2) conditions over short distances and display only moderately posit ive Eu anomalies. This contrasts with findings for the adjacent Broken Hill Block and highlights the dissimilarities of ore formation proces ses that were active in the Olary and the Broken Hill Block as well as the depositional differences between the two blocks.