The Broken Hill lead-zinc-silver deposit is the largest ever discovered, and is situated within the Curnamona Province, a nucleus of Palaeo – Mesoproterozoic rocks which is traversed by the South Australia and New South Wales border. The deposit is contained by the ~1720 – 1640 Ma, volcano-sedimentary Willyama Supergroup, at a stratigraphic level near the top of the Broken Hill Group, which is characterised by S-type felsic volcanism and mafic magmatism.
The base of the Broken Hill Group is marked by the Ettlewood Calcsilicate Member, the interpreted equivalent of which in South Australia, is the ‘Bimba Sulphide Member’. Underlying this unit are sediments and, in South Australia, A-type felsic volcanics, all of which have been extensively oxidised and sodium metasomatised. The horizon between the lower oxidised part of the sequence and the rest of the sequence, which overlies, represents a redox boundary of regional extent.
The Broken Hill Group and overlying sediments are dominantly fine grained siliciclastics, with calcareous, graphitic, and aluminous variants. In the Olary Domain, sediments and volcanics of the "Bimba Sulphide Member" and below are referred to as the 'Lower Willyama Supergroup' while sediments above this stratigraphic level are referred to as the 'Upper Willyama Supergroup.
Evidence is emerging, especially in the Olary Domain, for several hiati within the sequence.
Evidence for several thermal events is provided within the Curnamona Province, such as:
|~1720 - ~1700 Ma||felsic event, A-type volcanics and high level intrusives.|
|~1690 - ~1680 Ma||mafic intrusives and perhaps extrusives, S-type volcanics and high level granites with which the Broken Hill deposit is associated.|
|~1640 - ~1630 Ma||I-type granites of the Olary Domain.|
|~1600 Ma||peak metamorphism, ranging from granulite facies grade in the south (eg. Broken Hill) to greenschist facies in the north (eg. Danit Bore and Benagerie Ridge).|
|~1590 - ~1580 Ma||major influx of S-type granite, primarily in the Olary Domain.|
|~15?? - ~15??||granite dominated, bimodal magmatism forming the volcanics of the Curnamona Province, and the granites of the Mts Painter and Babbage Blocks in the Northwest.|
The period 1600 Ma – 1575 equates with the magmatic event which on the Gawler Craton was responsible for the Hiltaba Suite granites, the Gawler Range Volcanics and the world class Olympic Dam Cu-Au-U-REE deposit.
The Willyama Supergroup has been subjected to at least five folding events; the first four (OD1-OD3) are ascribed to the early Mesoproterozoic Olarian Orogeny, and the last two to the Ordovician Delamerian Orogeny (DD4 – DD5) which also affected the overlying Adelaidean sediments. OD1-OD3 involved overturning, with the earliest causing a significant part of the Willyama Supergroup to be downward facing. It is not certain whether the earliest layer parallel foliation formed synchronously with OD1, or whether it represents an earlier extensional event.
The lower part of the Willyama Supergroup (quarto-feldspathic and calcsilicate suites of the Olary Domain, Clevedale Migmatite – Thackaringa Group of the Broken Hill Domain) is characterised by intense alkali alteration (mainly sodium), which in the Olary Domain is highly oxidised. Thus albitisation is so prevalent that in places large tracts of country are dominated by layered albitites; these in the Olary Domain generally contain magnetite. The upper part of this albitised sequence contains the Cu-Au prone, sub-redox boundary zone mentioned above. Although the stratigraphy characterised by albitisation terminates at the Bimba Sulphide – Ettlewood Calcsilicate Member horizon, alkali alteration significantly affects, albeit locally, higher parts of the sequence. The source(s) of the alkalis is uncertain with two main contenders being evaporitic or magmatic brines.
At least four stratigraphic metallogenic zones are postulated across the Olary and Broken Hill Domains, these are the:
Mineralisation styles include: