Rb-Sr and Nd-Sm isotopes in fluorite related to the granites of the Bushveld Complex

Abstract

The Lebowa Granite Suite of the Bushveld Complex is a differentiated sheeted intrusion, between 1.5 and 3.5km thick, with an areal extent of some 30 000km2. The granite suite and the overlying Rooiberg Group felsic volcanics are near coeval with the mafic Layered Suite all being dated within 2057±3 Ma. Despite this narrow time frame of emplacement ages, a compilation of age dates ranges from ~2.06 to ~1.05 Ga and reported 87Sr/86Sr are often high and with large uncertainties. Because of hydrothermal alteration, a study of chemical and isotopic variations of the whole rocks is equivocal. Since fluorite has a low Rb/Sr ratio, and is stable at low temperatures, the back correction of the 87Sr/86Sr value has minimum uncertainty and the initial 87Sr/86Sr ratio (Ro) of the magma or fluid from which it crystallised can be determined. In this study Rb, Sr, Nd and Sm analyses of fluorite from a number of different types of occurrences are reported. In addition, strontium isotope analyses were made on ‘primitive’ granite from a vein cross-cutting the layered mafic suite in the eastern Bushveld and compared to data obtained for an ‘evolved’ feldspar from the Zaaiplaats granite. We recognise two distinct processes and events: a primary magmatic process that occurred at 2057±3 Ma and took place in a very short time span (<1my). Hydrothermal fluid was not retained and a much later secondary hydrothermal process possibly related to the marginal tectonism of the Kaapvaal craton in the Limpopo belt, which may have been a longer-lived event. The primary fluorite in stockworks, volcanic pipes and disseminated in some granites originated from late magmatic fluid that also infiltrated and engulfed country rock ( eg Buffalo). These fluorites have variable but low 87Rb/86Sr, low Ro and moderate Sm and Nd (>50 and >100ppm respectively). The samples from the Vergenoeg pipe have accessory fluorite with low Sr concentrations (32 to 70ppm) and Ro in the range 0.7166 to 0.7225. Fluorites from Buffalo, Zwartkloof and Spoedwel show a wide range of Sr content and Ro, from 39 to 231ppm and 0.727 to 0.794 respectively, resulting from mixing of a granitic fluid with assorted country rocks. These late magmatic fluids generated large fluorite bodies such as Vergenoeg and had a significant autometasomatic imprint as indicated by chloritisation of biotite such as in the Steelpoort Park granite veins. For the primary fluorite, most samples fall on an array indicating an age of ~2.05 Ga. but with a large scatter. A different and later hydrothermal fluid produced a totally different generation of fluorite and calcite in open vugs and pegmatites at Zaaiplaats and Buffalo. These secondary fluorites display very high Ro, have low abundances of Sm and Nd with Sm varying from 30.97 to 0.17ppm and Nd ranging from 69.59 to 1.22ppm. In particular, different coloured fluorite from one pegmatitic pod show an extreme range of Ro varying from ~ 0.7199 to ~0.8518, with even higher Ro in the calcite.