Abstract

During the past 15 years there has been increasing acceptance of a volcanic exhalative origin for the giant 300-Mt Broken Hill Pb-Zn orebody. Supposedly, silicic pyroclastic rock types have been identified in the host-rock succession, and because of an association with basic volcanics (amphibolites), comparison is made with Phanerozoic continental rifts. The orebody has also been described as distal and deep water. We report that the so-called silicic volcanic rocks are all normal clastic sediments. The metasedimentary succession (more than 5 km thick) is interpreted as a series of major continental, fluvio-deltaic, progradational wedges interfingering with marine transgressive cycles, deposited in a gradually deepening basin. Pb-Zn mineralization is hosted by shallow-marine sands that later became reservoirs for the metal-bearing brines. Basin formation, stratigraphic architecture, and Pb-Zn mineralization can all be interpreted in terms of concepts understood from studies of the development of younger sedimentary basins. The inference is that Pb-Zn mineralization was generated by compactive expulsion of metal-bearing brines during accumulation of the sedimentary pile.

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