Abstract

The Emery Lode is one of four hydrothermal, replacement copper orebodies known as the Artonvilla mine, Messina, South Africa. Gangue and ore minerals exhibit a distinct zonal pattern, grading from sericite-quartz-pyrite-magnetite assemblages at the outer edge, through albite-chalcopyrite-bornite-chlorite, albite-epidote-chlorite-specularite-bornite-chalcocite to goethite-prehnite-albite-chlorite-chalcocite-native copper assemblages in the central parts. The body, which is ovoid in plan, is not exposed on surface, and is surrounded on all sides by fresh country rock. A narrow feeder conduit containing rocks of nepheline normative composition extends downward from the orebody. The nature of the mineral zonation and the absence of hydrothermal alteration in the rocks above the Emery Lode are indicative of closed system conditions during mineralization. The ore fluid composition calculated on a basis of observed compositional changes in the host rocks resembles fluids which have been observed in experimental studies on alkaline rocks under hydrous conditions, and contains essentially water, soda, alumina, and silica, with lesser amounts of magnesium, calcium, copper, and sulfur. It is inferred that most of the metals were transported as anionic oxycomplexes in this alkaline fluid. These fluids reacted with country rocks, evolving continuously in consequence, leading ultimately to the formation of a sericitic halo at the outer extremity of the orebody. Subsequent retrograde reaction resulted in zonation of ore and gangue mineral assemblages.The inferred composition of the ore fluid suggests an alkaline, undersaturated source magma.

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