Fluid inclusion studies of the Messina copper deposits, Transvaal, South Africa

Abstract

The Messina copper deposits comprise an important group of breccia pipes, disseminated replacement, and vein deposits emplaced in granulite facies metamorphic rocks of the Limpopo mobile belt. This east-northeast-trending belt was the locus of normal faulting, magmatism, and ore deposition during the period of Karroo igneous activity (210-170 m.y. B.P.).The ore minerals consist primarily of chalcopyrite and bornite with lesser amounts of chalcocite and minor native copper. Pyrite is present only at the outermost margins of the orebodies and forms part of the unusual zonal distribution of ore minerals, which in broad terms comprises concentric zones of chalcopyrite, bornite, and chalcocite occurring successively from the edges toward the centers of the replacement orebodies. Within the breccia-pipe orebodies ore minerals with increasing- copper content occur sequentially with increasing depth in the pipes.Heating and freezing studies on inclusions in quartz and some calcite samples, representing a considerable vertical range within the orebodies, confirm the hydrothermal nature of ore deposition. Fluid inclusion filling temperatures lie in the range 210 degrees C to 120 degrees C for quartz and, coupled with paragenetic studies, indicate that at least the latter stages of copper sulfide deposition occurred at temperatures down to 160 degrees C. Detailed temperature studies on zoned quartz crystals indicate an overall decrease in temperature with time. The deposition of calcite took place in the temperature range 160 degrees C to 60 degrees C.Freezing studies indicate a salinity range for the hydrothermal fluids from approximately 25 to 2 equivalent weight percent sodium chloride, but no halite was seen in any inclusions except one that lost some of its fluid due to leakage during a heating run. Patterns of salinity variation with respect to time appear to be complex but exhibit an overall trend toward decreasing salinity with decreasing filling temperature.Birefringent acicular or tabular daughter minerals are present in many of the inclusions in Messina quartz, and in some occupy up to 20 volume percent or more of the inclusion cavity. However, no clear relations between filling temperature, salinity, and the presence or absence of these minerals are apparent. X-ray studies of a crystal removed from a large inclusion shows it to be layered alumino-silicate mineral, probably sodium montmorillonite.On the basis of the fluid inclusion data and previous alteration and chemical studies, a genetic model involving initial dissolution of host-rock quartz followed by albitization and quartz deposition in open cavities is proposed. Metallization appears to have occurred primarily during the stages of albitization and that of the early quartz deposition. The source of the hydrothermal fluids must remain conjectural at this stage, but the geologic setting and available geochemical data suggest a link between ore deposition, peralkaline magmatism, and active circulation of meteoric water.