Abstract
Secondary Cu and Cu-Ag sulphides are reported from altered metadiabases in the vicinity of a Neoproterozoic ophiolitic melange at the Zalm area in Saudi Arabia. The metadiabases occur in the form of NE-trending dykes forming resistant ridges with occasional chilled margins against a serpentinite host. Microscopic investigations revealed the occurrence of a peculiar assemblage of secondary Cu-bearing minerals down to the nanoscale. In most cases, secondary Cu and Cu-Ag sulphides formed at the expense of disseminated chalcopyrite (presumably argentian) + or - galena as the hypogene ore minerals. The most common textures are exclusively secondary and they are represented by "atoll" structure and rim replacement textures. The identification of ore minerals is based on combined ore microscopy and microchemistry of minerals. An assemblage of anilite-geerite-chalcocite intergrowth is recorded as the Cu sulphides, whereas mckinstryite (Ag (sub 1.18-1.19) Cu (sub 0.80-0.81) Fe (sub 0.01) S) and stromeyerite (AgCu (sub 0.99) Fe (sub 0.01) S) are the extremely minute Cu-Ag sulphides that never exceed 5 mu m in width. Ore fabrics and back-scattered electron (BSE) images clearly indicate that all phases of sulphides are secondary with some limited cationic substitution. Non-sulphides secondary Cu minerals are much more abundant and they are confined to rock fractures with visible hydrated Cu silicate (chrysocolla rimming hydrated Cu sulphate, as chalcanthite) that is either weakly crystalline or amorphous as indicated by the XRD data. Based on the chemical composition, ore textures and comparison with some world examples, it is believed that the Zalm Cu-Ag sulphides formed under oxidizing condition (pH = 5.8-7.2). As the temperature decreases, residual (stoichiometric) Ag (sub 2) S is mobile in meteoric water. Precipitation of residual Ag continued down to 93.3 degrees C to form stromeyerite (i.e. late hydrothermal). During late stages of supergene alteration at the studied Zalm occurrence, loss of CO (sub 2) leads to the formation of Cu-bearing carbonates, sulphates and finally silicates at a very characteristic low S (sub 2) fugacity at shallow depths or at surface (i.e. supergene).