The geology of the Kruidfontein Volcanic Complex, Transvaal, S. Africa
The Proterozoic Kruidfontein Volcanic Complex (KVC) is a collapsed carbonatitic caldera structure, preserved as a high-level feature within Transvaal Sequence sediments. An outer ring of hills contains silicate pyroclastic rocks composed of lithic and pumice fragments, crystals and recrystallized matrix. These rocks are the products of co-ignimbrite lithic breccias and partially welded ignimbrite flows. An inner caldera was filled with recrystallized carbonatitic bedded volcaniclastic rocks. Relic pyroclastic carbonate fragments, such as droplet and armoured lapilli, containing juvenile calcite laths, are present. Well preserved primary structure sequences indicate emplacement by pyroclastic flow, surge and air-fall. Together with some reworking and debris flow deposits. The volcanism spans from early eruption of phonolitic material, from ring vents associated with caldera collapse, to smaller volume carbonatitic eruptions, producing intracaldera deposits. The processes operating during emplacement of carbonatitic pyroclastic material are essentially the same as those of silicate tuffs. As well as numerous fragments of phonolitic pumice in the silicate tuffs, there are unusual banded fragments composed of alternating silicate and carbonate compositions which appear to have been originally magmas separated by liquid immiscibility. The fragments show replacement of Al by Fe, and have also been K-feldspathized. Sovite and alvikite carbonatite dykes show that variation between CaO, MgO and FeO is consistant with fractionation from sovite to Fe-rich alvikites. All the carbonatites are strongly enriched in REE. The alvikites are enriched in the incompatible elements La, Ce, Nd, Y, Th, compared with the sovites, but are depleted in Sr, P, Ti, because of early fractionation of Sr-rich calcite, apatite and Ti-Fe oxides. The alvikites also have more positive δ18O and less negative δl3C compositions compared with the sovites, with values trending away from "mantle" compositions. This interpretation is consistant with a carbonatite magma chamber beneath the KVC which fractionated to produce the carbonatites seen at the present day surface. The few, highly altered, KVC nephelinitic rocks have trace-element distributions suggesting that they are parental to the phonolites. Fractionation from nephelinites, to phonolites, to trachytes satisfactorilly accounts for the incompatible trace element distributions. Some of the rocks have suffered secondary alteration, but have retained their trace element signatures. Zr and Nd are residual, whilst crystal fractionation involving feldspar, magnetite, and apatite have depleted some rocks in P, REE, and Sr. The fractionation from phonolite to trachyte, which is the reverse of normally observed trends, is ascribed to increasingly high F contents in the fractionating KVC magma. Three types of fluorite mineralization are recognised at KVC: 1) Replacement and disseminated deposits, 2) Fluorite veins and fracture fillings, 3) Fluorite-rich carbonatite and related dykes. Only Type 1) deposits are of economic importance at Kruidfontein. Fluorite selectively replaces calcite rather than ankerite in the KVC rocks, with ankeritization preceeding and inhibiting fluorite mineralization. Shallow dipping ankeritic tuffs form the host rock for a large (Tilde with hyphen below 5xl06 tonnes) sub-economic horizontal stratiform fluorite orebody, emplaced after inward sag of the bedding.