IN THE Cu - - Fe - - As - - S SYSTEM ( TSUMEB

Haynes, F.M., 1984. A geochemical model for sulfide paragenesis and zoning in the Cu-Fe--As--S system (Tsumeb, South West Africa/Namibia). Chem. Geol., 47: 183--190. The Cu-bearing hypogene mineralization at Tsumeb, South West Africa/Namibia, is interpreted to have resulted from the interaction of hot circulating Cu-rich saline solutions and host dQlomites. Fluids attending formation of the main sulfide stage were warm (210--280 ° C) and moderately saline (6--12 wt.% NaCl equivalent). Phase relations within the Cu--Fe--As--S system suggest that the observed paragenetic sequence (chalcopyrite ~ bornite ~ chalcocite --* enargite ~ tennantite) is consistent with a model calling for successive introduction of a single Cu-rich solution into a host dolomite. The Cu and As content of the solution decreased as it passed through the permeable dolomite precipitating Cu--As-sulfides and sulfosalts. A concomitant increase in pH created by the dissolution and buffering effect of dolomite produced sufficient changes in the fluid chemistry to shift the stable sulfide mineral assemblage.