Abstract

Five massive sulfide lenses and several smaller sulfide occurrences at Gorob, Namibia, are hosted by metaturbidites of the Kuiseb Formation and are associated with amphibolites of the Matchless Member. The strata at Gorob are overturned and regionally metamorphosed at staurolite-kyanite-sillimanite grade. In the stratigraphic footwall of the sulfide lenses, zones of premetamorphic hydrothermal alteration can be identified which are discordant to the lithostratigraphy.Hydrothermal alteration of the original sediments is related to submarine exhalative ore-forming processes. This alteration led to a general enrichment of Mg, Fe, Mn, Cu, Zn, Ba, and P and depletion of Ca, Na, K, Rb, Sr, and Ce as demonstrated by comparison with the geochemistry of unaltered Kuiseb Formation metasediments from Gorob and elsewhere. On this basis, alteration indices can be constructed for major and trace elements. These indices show an increase toward the mineralization and a sharp decrease in the stratigraphic hanging wall of the ore zones. They are independent of the composition of the original sediment as monitored by the TiO 2 /Zr ratio which remained constant during hydrothermal alteration and metamorphism. With the possible exception of Ce, concentrations of rare earth elements were not modified by the alteration process and may provide information on the provenance of the original sediments.

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