Abstract of Broken Hill Block Synthesis

Record 2001/12

Compiled by Lesley Wyborn

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• Executive summary - geology
• Executive summary - metallogenic
• Future work

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Executive summary - geology

The regional geology of the Broken Hill Block is summarised by Stevens et al. (1988), Stevens (1995) and Willis et al. (1983). The region is dominated by the Willyama Supergroup, which comprises five main groups. The lowermost group can be subdivided into the Redan facies, which is interpreted as a thick lacustrine clasticevaporitic sequence interspersed with non-volcanic pelite, whilst the Thorndale facies is interpreted as a sandy to clayey fluvial deltaic system. The overlying Thackaringa Group contains lacustrine sedimentary rocks, felsic metavolcanic rocks, shallow granite sills and several varieties of iron formation. The overlying Broken Hill Group is most likely shallow-marine and contains the Hores Gneiss which has been interpreted as a series of volcanics or volcanic mass flows and high Fe tholeiite of the Silver King Formation. The Sundown Group represents shallow-marine sedimentation, whilst the youngest group, the Paragon Group, was organic rich in the initial phases and then progressed to fine-grained turbidite (Stevens 1995).

In part because of the inference that felsic magmatism had played a major role in the genesis of the main ore body at Broken Hill, and in part because of its high metamorphic grade, the Broken Hill Block has been treated somewhat differently from other provinces in this project. There is reasonable evidence of felsic magmatism throughout the lower part of the Willyama Supergroup, up to the Sundown Group. All quartzofeldspathic units which had been considered in the literature to have been derived from a felsic igneous rock were assessed as outlined in the Project Proposal. Not one of the suites assessed was considered to have any potential for significant granite-related mineralisation. Those suites assessed were either too restite-rich for fractionation processes to have occurred, or else had too limited a silica range for any significant mineralisation. Most suites are peraluminous (S-type) and the only I-type suite occurred in the lowermost Redan facies.

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Executive summary - metallogenic

The Broken Hill Block contains the world class Broken Hill Pb-Zn-Ag deposit which contains 300 million tonnes averaging greater than 15% combined metal (Wright et al. 1993). The main debate in the genesis of this ore body has focused on whether or not felsic magmatism has played any role in the generation of the rich Pb-Zn-Ag lodes. This review suggests that felsic magmatism (and hence mafic magmatism) is not likely to have played any role in the genesis of Pb, Zn, Ag and Cu mineralisation in the Broken Hill Block.

The only possible relationship of felsic igneous rocks to mineralisation occurs in the rocks labeled BG1 ('Potosi gneiss' rocks) of the Potosi Supersuite. These rocks were found to have analogous chemical trends to crystal-rich arenites of the El Sherana Group of the Coronation Hill Region, Northern Territory (Jagodzinski 1992; Jagodzinski and Cas 1992). The chemical trends within the BG1 were considered to have resulted from a winnowing process whereby the ash component was removed by phreatic eruptions generated as the ignimbrite entered water; the resultant sediment was relatively Fe-enriched due to the loss of the high-SiO2, Fe-poor ash component. The resultant sediment had high porosity and may have subsequently acted as an aquifer for highly reduced metalliferous brines along the lines of the model proposed by Haydon and McConachy (1987) and Wright et al. (1987, 1993). Some support for this is found in the reduced alteration overprint in the BG1 rocks that have the highest Pb and Zn values.

In summary, there is little evidence to disagree with the view of King and Thomson (1953, p. 564) that 'there is no secure basis for a genetic association between the ore (at Broken Hill) and the granite- and pegmatite-producing conditions'.

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Future Work

In terms of felsic igneous rocks, better sampling could be done on rocks of the younger Mundi Mundi Suite to further evaluate their potential relationship to Sn deposits. Using the methodology developed by Jagodzinski (1992) all available analyses of the BG1 rocks could be better assessed to define where the greatest amount of ash winnowing had occurred, as not onlywill this help to better define the palaeogeographic setting, it may also help to define those parts of the BG1 stratigraphy which had the greatest porosity.