Magnetite-bearing zones in metasediments at Broken Hill, Australia: signatures of black smokers?

Abstract

High-quality aeromagnetic data show linear magnetic zones within generally non-magnetic metasediments of the Broken Hill and Sundown Groups in the medium- to high-grade metamorphic Willyama Supergroup at Broken Hill, Australia. On-ground investigations show that these anomalies are generated by small amounts of magnetite within the metasediments. The magnetite concentrations are controlled by bedding and transgressed by schistosity. At the outcrop scale, the bedding-parallel nature of the magnetite concentrations is illustrated by decimetre-scale mapping with a magnetic susceptibility meter. At a 1:25 000 scale, there are areas where the relationship between magnetic anomalies and bedding is either unclear or apparently transgressive. Resolution of such situations requires an order of magnitude more detail in both the magnetic data and the geological mapping. This was provided by ground magnetometer surveying, magnetic susceptibility meter surveying and detailed lithological and structural mapping. In the areas studied, no evidence was found for genuinely transgressive relationships between magnetite concentrations and bedding. On a microscopic scale, the magnetite grains are independent of the other main Fe-bearing minerals with no replacement of garnet or biotite by magnetite. Thus, the magnetite did not result from influx of an oxidising fluid during or after high-grade metamorphism. Given that the magnetite concentrations are bedding-parallel and that the magnetite grains are not the result of replacement of ferrous iron-bearing minerals, it is concluded that the magnetite (or its ferric iron precursor) was an original component of the metasediments in these zones. The magnetite does not form clusters as expected with detrital magnetite. The magnetite-bearing zones can be either pelitic or psammitic, so it appears there was not a porosity/permeability control. The most likely origin for the magnetite or its precursor is a rain of particles onto the seafloor from an overlying oxidised water column. Such particles may have been derived from distal plumes resulting from spent orebody-forming fluids. The close relationship in places, between banded iron formation and magnetite-bearing metasediment zones supports this suggestion. Magnetite in metasediments close to the base of Sundown Group may have resulted from spent fluids related to formation of the Broken Hill orebody. However, there are magnetite-bearing metasediment zones within the Broken Hill Group and also stratigraphically higher in the Sundown Group that may be related to as yet undiscovered orebodies. The detailed lithological and structural mapping resulted in some new insights into the structural behaviour of Willyama Supergroup metasediments. In the Yanco Glen southeast area, the distribution of lithologies is largely controlled by D2 shears, producing a series of slices that resemble bedding on aerial photographs. In the Monuments area, D2 shears also control the distribution of lithologies, but extensive melting of psammites and flow parallel to S2 is an added feature.