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