Abstract

The Middle Proterozoic Willyama Supergroup of the Broken Hill block, Australia, has undergone multiphase coeval deformation and high temperature-low pressure metamorphism. The Willyama Supergroup is interpreted as a sequence of intracontinental rift sediments, volcanics, and hot spring precipitates. Scheelite occurs in regional calc-silicate rocks enriched in F and Zn within a sequence of carbonaceous-sulfidic metapelite, tourmalinite, and amphibolite; it overlies a felsic gneiss-metasediment sequence, part of which is interpreted as meta-evaporites. The distribution of scheelite is unrelated to syntectonic pegmatite or post-tectonic granitic rocks. Finally, scheelite has undergone minor remobilization during tectonism. The calc-silicate rocks are interpreted as dolomitic limey sediment which formed part of a playa-lake sequence into which B-, W-, Zn-, and F-rich alkaline hot springs debouched. Coeval sudden deepening, an increased geothermal gradient, deposition of the Broken Hill Pb-Zn-Ag sulfide orebodies, and siliceous exhalite deposition occurred after deposition of the evaporite sequence. Some components for the highly saline Broken Hill Pb-Zn-Ag ore fluid may have been leached from the underlying evaporites and redeposited higher in the stratigraphy.

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