CLASTIC METASEDIMENTS OF THE EARLY PROTEROZOIC BROKEN-HILL GROUP, NEW-SOUTH-WALES, AUSTRALIA - GEOCHEMISTRY, PROVENANCE, AND METALLOGENIC SIGNIFICANCE

Whole-rock analyses of samples of pelite, psammite, and psammopelite f rom the Early Proterozoic Broken Hill Group (Willyama Supergroup) in t he Broken Hill Block, New South Wales, Australia, reveal distinctive g eochemical signatures. Major-element data show high Al2O3 and K2O, low MgO and Na2O, and relatively high Fe2O3T/MgO ratios, compared to aver age Early Proterozoic elastic metasediments. High field strength eleme nts (HFSE) are especially abundant, including Nb (most 15-27 ppm), Ta (most 1.0-2.2 ppm), Th (17-36 ppm). Hf (4-15 ppm), and Zr (most 170-40 0 ppm); Y (33-74 ppm) is also high. Concentrations of ferromagnesian e lements are generally low (Sc = <20 ppm, Ni = less than or equal to 62 ppm, Co = <26 ppm; Cr = most <100 ppm). Data for rare earth elements (REEs) show high abundances of light REEs (La-CN = 116-250X chondrite; La-CN = 437 in one sample), high La-CN/Yb-CN ratios (5.6-13.9), and l arge negative Eu anomalies (Eu/Eu = 0.32-0.57). The geochemical data indicate derivation of the metasedimentary rocks of the Broken Hill Gr oup by the erosion mainly of felsic igneous (or meta-igneous) rocks. H igh concentrations of HFSE, Y, and REEs in the metasediments suggest a provenance dominanted by anorogenic granites and(or) rhyolites, inclu ding those with A-type chemistry. Likely sources of the metasediments were the rhyolitic to rhyodacitic protoliths of local quartz + feldspa r +/- biotite +/- garnet gneisses (e.g., Potosi-type gneiss) that occu r within the lower part of the Willyama Supergroup, or chemically simi lar basement rocks in the region; alternative sources may have include d Early Proterozoic anorogenic granites and(or) rhyolites in the Mount Isa and(or) Pine Creek Blocks of northern Australia. or in the Gawler craton of South Australia. Metallogenic considerations suggest that t he metasediments of the Broken Hill Block formed enriched source rocks during the generation of pegmatite-hosted deposits and concentrations of La, Ce, Nb, Ta, Th, and Sn in the region. Li, Be, B, W, and U in p egmatite minerals of the district may have been acquired during granul ite-facies metamorphism of the local metasediments.