Abstract

Gold in Early Proterozoic Birimian greenstone at Prestea in Ghana is associated with base metal sulphides and sulphosalts including arsenopyrite, pyrite, sphalerite, chalcopyrite, pyrrhotite, galena, tetrahedrite, bournonite, boulangerite and jamesonite. The occurrence of the gold is intimately associated with arsenopyrite and the sulphosalts, and to a lesser extent with the other sulphides. The tetrahedrites at Prestea constitute the major component of sulphosalts associated with gold and occurring in two distinct types. Type I show ideal stoichiometric composition. Type II tetrahedrites deviated from the ideal stoichiometry and are represented approximately by the average formula (Cu,Ag) (sub 9.61) (Fe,Zn) (sub 2.39) (Sb,As) 4 S 13 . The tetrahedrites co-precipitated with gold exhibit ideal characteristics indicating an equilibrium state of the mineralizing fluid during precipitation. Three types of pyrites were distinguished by electron-microprobe analyses based on their As, Co and Ni composition. The As content in type I vary from 0.15 to 0.37 wt%, and contain up to 2 wt.% Co. Type II pyrites are As-rich and form the most dominant with As content ranging from 0.2 to 2.69 wt.%. Ni content varies from below-detection to 1000 ppm. Type III pyrites are poor in the trace elements and consistent with the stoichiometric composition. The mineralization occurred in three paragenetic stages from at least a two-phase hydrothermal fluid, with stage II forming a prolonged and main stage of the ore and gold mineralization. Redox changes in ore fluid which were triggered by episodic pressure releases during fissuring and fracturing caused fluctuation of the activity of the As/Ni ratio and subsequent oscillatory zoning of Ni in As-rich ores.

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