RARE-METAL MINERALIZATION AT THE SALPETERKOP CARBONATITE COMPLEX, WESTERN CAPE PROVINCE, SOUTH-AFRICA

The Salpeterkop complex is an eroded central volcano of Late Cretaceou s (66 Ma) age with a sediment-filled crater remnant 1 km in diameter a ssociated with radial and concentric carbonatite/K-trachyte dykes as w ell as K-trachyte, breccia and olivine melilitite plugs. The sedimenta ry 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 t o outline a mineralized zone coinciding with the crater rim, especiall y on the western and southern sides. The rim rocks are volcanic brecci as intimately mixed and veined by decomposed, silicified and ferrugini zed carbonatite and carbonatitic tuff. A large variety of very fine-gr ained minerals have been identified in a groundmass consisting mainly of goethite and chalcedony. The principal Nb ore mineral is niobian br ookite occurring as microscopic disseminated grains of several morphol ogical types. High-Nb (12% Nb2O5) and low-Nb (8% Nb2O5) 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 mine ralization took place in three overlapping stages: i) primary magmatic , with concentration of Nb, Zr, Th, V in both carbonatite and associat ed silicate rocks; ii) mesothermal, with oxidation and introduction of Fe, Ti, REE, Mo and F; iii) epithermal, with hydration and dissolutio n of carbonates, removal of Ca and Mg, redistribution of Y, Ti, Nb, Zn and deposition of Si, Ba, Mn and other mobile elements. The suggestio n is made that the preservation and extent of the third stage, which m ay be linked to fumarolic activity, distinguishes the multi-element mi neralization at Salpeterkop from other carbonatite complexes. Secondar y alteration in the weathered zone merged with the third stage, but is less important.