Geochemistry and Genesis of Manganiferous Silicate-rich Iron Formation Bands in the Broken Hill Deposit, Aggeneys, South Africa

Exploration & Mining Geology, Vol. 3, No. 4, 1994

DENNIS HOFFMANN, Department of Geological Sciences, University of Cape Town, South Africa

Banded iron formation rocks associated with massive sulfide lenses in the Broken Hill Pb-Zn-Cu-Ag deposit in the Namaqualand Metamorpnic Complex, South Africa, were investigated with respect to the geochemistry of manganiferous silicate-rich bands. Two common varieties of these are: amphibole- and garnet-rich iron formation bands. The results reveal that the former are enriched in Fe, Mg, Cu, Co and S. The garnet-rich bands are enriched in Mn, Ca, P, Pb and Ba with significant detrital contamination of Ti, Al, Zr, Cr, Y, V, Ni and Nd. The silicate-rich bands have compositions, in terms of, Si/Al, Si/Fe, Fe/Mn, La/Ce and Co/Zn, comparable to submarine chemical sediments which precipitated from hydrothermal fluids. The garnet-and amphibole-rich bands are interpreted as "evolved" and "immature" hydrothermal sediments, respectively. The latter resulted from rapid precipitation after exhalation, whereas the former had a longer residence within sea water. The silicate-rich bands both exhibit increasing Mn/(Mn + Fe) fractions, which are consistent with a variation expected from proximal to distal depositional settings. The prevalence of garnet-rich bands over amphibole-rich bands is interpreted as the result of injection of hydrothermal fluids from a single vent situated along the northwest margin of the Broken Hill deposit.