Post‐depositional history of the Willyama Supergroup in the Broken Hill Block, NSW

Abstract

The Broken Hill Block, in far western New South Wales, comprises complexly folded and metamorphosed Proterozoic metasediments and metavolcanics of the Willyama Supergroup, and pre‐ and post‐folding intrusives. The history of the Block is described in four tectonic stages. The precratonic stage commenced with separation of crustal material from the mantle at 2100–2300 Ma. Deposition of the Willyama Supergroup occurred at about 1820 Ma. These rocks were metamorphosed to amphibolite‐granulite facies and deformed twice at about 1660 Ma, in the Olarian Orogeny. A third deformation with accompanying retrograde metamorphism occurred soon after. Before 1570 Ma, retrograde schist zones had formed, some with associated kyanite and staurolite bearing metamorphic assemblages. The transitional tectonic stage commenced at 1490 ± 20 Ma, with the emplacement of Mundi Mundi type granites and minor muscovite pegmatites. Between 1490 Ma and about 1100 Ma, the presently exposed rocks were elevated from 13 to 20 km below the surface, to surface and near surface levels. During the epicratonic stage (1100–500 Ma), continental and shallow marine sediments of the Adelaide System plus minor basalt were deposited in grabens or half‐grabens on Willyama basement. Substantial erosion of the Willyama Supergroup occurred. Alkaline ultramafic plugs and dykes were intruded at 561 ± 7 Ma, and a thermal pulse at 520 ± 20 Ma marks the Delamerian Orogeny. This was accompanied by low grade metamorphism, re‐activation of retrograde schist zones, intrusion of dolerite plugs, dykes, and zoned pegmatites, and gentle folding of the Adelaidean rocks. Peridotite stocks and dykes may have accompanied the ultramafic intrusions at 561 ± 7 Ma, or the dolerite intrusions at 520 ± 20 Ma. During the cratonic stage (500 Ma to present), faulting occurred, fluorite bearing veins and siderite rich Pb‐Ag veins formed. At about 280 Ma, either a thermal event occurred, or substantial erosion dropped the temperature of the presently exposed rocks to less than 100°C. Between 280 Ma and the present there was substantial weathering, and significant uplift and erosion. Silcrete and ferricrete were formed locally, possibly in the Oligocene (36–23 Ma). Specific events of uplift and erosion probably occurred in the Palaeocene to Mid Eocene (65–45 Ma) and Late Miocene to Pliocene (10–2 Ma). These left the Broken Hill Block in its present state, a series of slightly uplifted, tilted and incised sub‐blocks, with adjacent alluvial fans. The Willyama Supergroup contains the Broken Hill Pb‐Zn‐Ag orebody and abundant minor deposits. The top of the orebody was removed by erosion and it is possible that other orebodies have been eroded away. The main period of erosion of potentially ore‐bearing Willyama Supergroup was between 1490 Ma and 1100 Ma. Substantial erosion also occurred between 800 Ma and 600 Ma, between 520 Ma and 280 Ma, or at about 280 Ma, and at intervals during the Cainozoic. It seems likely that the Broken Hill orebody was partly eroded at 280 Ma, and possibly also during the Cainozoic.