The Geochemistry of the Lower Proterozoic Willyama Complex Volcanics, Broken Hill Block, New South Wales

Abstract

Summary The major and trace element geochemistry of the early to middle Proterozoic meta-igneous rocks of the Willyama Complex in the Broken Hill Block indicates that the basic rocks originated by partial melting of a depleted mantle source, followed by fractional crystallization along a trend of strong iron enrichment coupled with some crustal assimilation. At least some of the intermediate-acid rocks were derived from fractionation of these basic parents, but major crustal involvement may be necessary to explain the large quantities of felsic volcanics in the lower part of the Complex. The volcanic rocks in the Broken Hill mine area show an unusually high degree of primary iron enrichment (up to 25 wt % total iron as Fe 2 O 3 ) and a lesser degree of bimodality than elsewhere. By analogy with Recent ferrobasalt provinces, this extreme iron enrichment may indicate an origin in a propagating rift where small, high-level magma chambers separated from the magma source. If so, the consequent high geothermal gradient and high fracture permeability may have provided the necessary conditions for genesis of the Broken Hill ore deposit.