Rare metal mineralization at the Salpeterkop carbonatite complex, Western Cape Province, South Africa

Abstract

The Salpeterkop complex is an eroded central volcano of Late Cretaceous (66 Ma) age with a sediment-filled crater remnant 1 km in diameter associated with radial and concentric carbonatite/K-trachyte dykes as well as K-trachyte, breccia and olivine melilitite plugs. The sedimentary country rocks have been domed. Geochemical anomalies for Ti, Y, Zr, Nb, Th, V, Zn and Mo, based on a core-drilling programme, were found to outline a mineralized zone coinciding with the crater rim, especially on the western and southern sides. The rim rocks are volcanic breccias intimately mixed and veined by decomposed, silicified and ferruginized carbonatite and carbonatitic tuff. A large variety of very fine-grained minerals have been identified in a groundmass consisting mainly of goethite and chalcedony. The principal Nb ore mineral is niobian brookite occurring as microscopic disseminated grains of several morphological types. High-Nb (12% Nb 2O 5) and low-Nb (8% Nb 2O 5) brookites have been distinguished. Some of them contain inclusions of ilmenorutile. Delicate brookite crystal aggregates crystallized in situ. No discrete V and Zn minerals could be identified but Th occurs as thorianite and thorite, Zr occurs in eudialyte and an unidentified KNb silicate and Y occurs as xenotime. Nepheline and eudialyte are probably xenocrysts from an unexposed body of nepheline syenite. It is concluded that mineralization took place in three overlapping stages: 1. i)primary magmatic, with concentration of Nb, Zr, Th, V in both carbonatite and associated silicate rocks; 2. ii)mesothermal, with oxidation and introduction of Fe, Ti, REE, Mo and F; 3. iii)epithermal, with hydration and dissolution of carbonates, removal of Ca and Mg, redistribution of Y, Ti, Nb, Zn and deposition of Si, Ba, Mn and other mobile elements. The suggestion is made that the preservation and extent of the third stage, which may be linked to fumarolic activity, distinguishes the multi-element mineralization at Salpeterkop from other carbonatite complexes. Secondary alteration in the weathered zone merged with the third stage, but is less important.