The early Proterozoic Willyama supergroup: Stratigraphic subdivision and interpretation of high to low‐grade metamorphic rocks in the Broken Hill Block, New South Wales

Abstract

Abstract A coherent stratigraphy has been recognised in the high to low‐grade multiply‐deformed metamorphic rocks of the Broken Hill Block. The sequence is formally subdivided into sixteen formations and four groups, based on regional interpretation of detailed lithological mapping, structure, and younging criteria. This Early Proterozoic sequence is formally termed the Willyama Supergroup. The supergroup can be extended to sequences in the Early‐Middle Proterozoic metamorphic complexes of the Euriowie, Olary and Mount Painter Blocks, and can be broadly compared with Early Proterozoic sequences in the Gawler Craton. In the Broken Hill Block, the lower sequence in the supergroup comprises migmatites (Clevedale Migmatite) overlain by feldspathic metasedimentary composite gneisses (Thorndale Composite Gneiss). The overlying Thackaringa Group is characterised by quartzofeldspathic gneisses and sodic plagioclase‐quartz rocks. A significant change to well‐bedded, dominantly pelitic metasediments occurs within the overlying Broken Hill Group, which comprises a basal metasediment sequence overlain by an interval containing basic gneiss, garnet‐bearing quartzofeldspathic gneiss and ‘lode horizon’ rocks of the Purnamoota Subgroup. The pelitic metasediment facies extends into the overlying Sundown Group, in which basic and quartzofeldspathic gneisses are absent. The upward trend to more mature sedimentation is apparent in the overlying Paragon Group, which comprises carbonaceous pelitic and fine‐grained psammitic metasediments. The Willyama Supergroup is interpreted as a marine sequence of terrigenous and volcanic elastics, with intercalated felsic and basic volcanics, and rare chemical sediments. The supergroup was deposited in a developing rifted zone, relatively proximal to volcanic sources, which progressed from a shallow, unstable environment, to a more stable deepening environment. Stratiform and stratabound Pb‐Zn‐Ag ± W deposits occur in the Broken Hill Group (more specifically the Purnamoota Subgroup). These include the Broken Hill orebodies, which occur in the Hores Gneiss at the top of the sequence. Within the Thackaringa Group, stratiform cobaltiferous pyrite, and Fe‐Cu sulphides associated with quartz‐magnetite, occur in the Himalaya Formation, and minor Broken Hill‐type Pb‐Zn mineralisation occurs in the Cues Formation.