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Polymetallic mineralization in the granites of the Bushveld Complex – examples from the central southeastern lobe

School of Geosciences, University of the Witwatersrand, Johannesburg, 2050, South Africa, Current address: Department of Geology, Rand Afrikaans University, P.O. Box 524, Auckland Park, Johannesburg, 2006, South Africa, e-mail: glgy13{at}na.rau.ac.za

L.J. Robb

EGRI-HAL, School of Geosciences, University of the Witwatersrand, Johannesburg, 2050, South Africa, e-mail: robblj{at}geosciences.wits.ac.za

Field mapping of a region in the central southeastern portion of the Bushveld Complex in an area underlain by granites of the Lebowa Granite Suite, granophyres of the Rashoop Granophyre Suite, the Rooiberg Group and Transvaal Supergroup has identified at least four types of polymetallic mineralisation developed in the granites and surrounding country rocks, and of similar affinities to those previously identified in the granites. These are: (1) a cassiterite – tourmaline – sulphide association, (2) a molybdenite –fluorite – REE – sulphide association, (3) a sulphide – carbonate base and precious metal rich association and (4) a late-stage oxidized hematite – fluorite – pitchblende association superimposed on the previous assemblages. Type I occurrences are predominantly endogranitic and syngenetic in nature. Occurrences of this type that are exogranitic have a large component of types III and IV developed. Type II in the area mainly occurs in fine-grained aplitic microgranite, termed the Klipkloof Granite, and is of a syngenetic nature. Types III and IV are preferentially developed in roof rocks and are of an epigenetic nature. Mineralisation is typically developed in red highly fractionated granites, denoted by mineralogical, compositional and geochemical properties and is associated with localized chloritic and sericitic, and in certain areas, hematitic alteration zones. Mineralisation may be either endogranitic or exogranitic but all mineralisation present is derived from the granites. Major lineaments and crustal warps have influenced granite emplacement, structure and subsequent tectonism and the related mineralisation. The work of previous authors suggests that types I and II are derived from high temperature, highly saline fluids whereas type III is derived from low temperature, low salinity fluids derived from mixing between the evolving and cooling magmatic fluids and an externally derived meteoric/connate fluid. Type IV mineralisation developed when the fluids were dominantly of meteoric/connate nature. The present study can offer no intensive substantiation of these findings, but overprinting of earlier assemblages by type IV mineralisation, the mineralogy and associated alteration of the various types of mineralisation and their development in differing rock types with different natures gives some support to these theories.