Sulphide Genesis in the Insizwa Intrusion, South Africa: Geochemical Constraints

Introduction The mineralization at Waterfall Gorge was first exploited in 1865 for its copper. In 1908 nickel was reported in the ore, and in 1950s PGEs were reported (Maske and Cawthorn, 1986). Quantitative data on all these metals was presented by Lightfoot et al. (1984). Despite subsequent exploration no further economic resources have been defined, although geochemical modeling suggests the existence of a major hidden resource (Lightfoot et al. 1984). Structure and Stratigraphy Waterfall Gorge occurs at the southern limit of the Insizwa intrusion, which is one of four limbs of the Mount Ayliff Intrusion, where an undulating sheet up to 1000m thick consists of lower olivine-rich cumulates overlain by gabbros. A gravity survey identified major deep fault-bounded structures (Fig. 1), with ultramafic rocks up to 600 m thick (Sander and Cawthorn, 1996). Note that the Waterfall Gorge mineralization may lie in a separate sub-basin from that drilled and described here. Subsequent drilling proved the existence of these structures. A geochemical study of the core (INZ1) is presented here. The bore core revealed a basal olivine-phyric microgabbro that rapidly graded up into olivine-rich cumulates, with very minor homogeneously distributed chromite that contain interstitial orthopyroxene, clinopyroxene and plagioclase, and traces of ilmenite, biotite and quartz. The overlying succession consists of a thin olivine gabbro and thick gabbro.