Bulletin of the Geological Society of Malaysia, Volume 11, Dec, 1979, pp. 239 – 251
R.F. Loxton, Hunting and Associates Canberra, Australia
Abstract: Six tin-fields are recognised within the granitic hub of the Bushveld Igneous Complex. The cassiterite deposits occur as endogranitic pipes and primary disseminations, or as exogranitic fissure veins, fault-breccias or replacement bodies. The Bushveld granites are typically anorogenic and consist, in part, of a crudely stratiform sheet roughly 2800 m thick. Their age is approximately 1950 m.y. They display gross chemical and mineralogical homogeneity and in some localities related stocks and plugs of younger granite intrude into the stratiform phase. In each tin-field the ore is related directly or indirectly to the younger granite(s).
Within the Bushveld Igneous Complex, in particular, and in the Transvaal in general, all known deposits of fluorspar are confined within distinct regional linear zones that demonstrably reflect fundamental Iithostructural and tectonic units in the underlying basement rocks, which are related directly to the tectonic framework of the Kaapvaal Craton. Six fluorine-enriched zones are recognised and it is highly significant that it is within the four of these zones which traverse across the body of the Complex that all the deposits of tin ore in the Bushveld are found. The fluorine-rich zones trend east-north-east or north-north-west. They are individually up to 650 km in length and 30 km wide.
Deposits of sedimentary fluorite in the Lower Proterozoic “Transvaal dolomite” which underlies the Complex, and the setting of the mineralized locality at the western end of a very major palaeotectonic trough which defines the axis of sedimentation of the dolomite group in the Transvaal, and which also corresponds exactly with the most prominent of the recognised zones (Murchison Zone), proves that linear zones of enrichment with fluorine of the present floor-rocks of the Complex existed in these rocks prior to the intrusion of the Bushveld Igneous Complex.
It is generally considered that the main Bushveld granites arose from the partial melting of sialic crustal rocks, that is, the basement rocks below the Complex, after depression of the floor of the Transvaal basin following the deposition of nearly 11 km of sediments. Because fluorine behaves as a flux, it is suggested that following this depression anatexis took place in and around the fluorine-enriched zones in the basement, at low temperatures, selectively, and in localised spots, relative to surrounding areas in which the strong fluxing action of fluorine was lacking. These fluxed-magmas were highly volatile and became enriched with tin and other trace metals by the process of “volatile stripping”. These granites were emplaced within and near their zones of origin on the highly stable craton, and endogranitic and exogranitic tin mineralisation was a consequence of their exceptional volatility and fractionation. This hypothesis explains the undeniable geographic coincidence of the late stage “tin-granites” within the flourine-enriched linear zones and also indicates that the association is a genetic one.
Cassiterite deposits in the Bushveld Igneous Complex are therefore related to late stage granite intrusions derived from anatexis that was brought about by the selective “fluorine-fluxing” of sialic crustal rocks in fluorine-rich loci within ancient, long-lived, linear tectonic zones in the craton. These ancestral zones are today mirrored on the surface by the linear distribution of fluorspar deposits, by the high fluorine content of groundwater, and by other non-granitic intrusions. The knowledge should be applied during future exploration.