THE MESOPROTEROZOIC STEENKAMPSKRAAL RARE-EARTH ELEMENT DEPOSIT IN NAMAQUALAND, SOUTH AFRICA

Steenkampskraal is a thin, lenticular-shaped ore body of monazite located in the Mesoproterozoic Namaqua crustal province of South Africa. The ore body is exposed for over 400 meters on the surface as an east-west striking band of massive fine-grained ore, with an average thickness of 0.5 meter. The ore body dips 40°–60° to the south, extending down-dip for at least 450 meters in Namaquan granitic gneiss with a zircon age of 1088 Ma. Based on underground exposures of the ore body and on samples from mine dumps, the ore is characterized by a phosphate-rich assemblage of monazite + apatite + chalcopyrite, and to a lesser extent by an oxide-rich assemblage of magnetite + apatite + monazite ± hercynite ± chalcopyrite. Intruding the Namaquan granitic gneiss in the immediate vicinity of the ore body are monazite-bearing alkali-granite dikes (locally containing pyroxene), which are also traceable into the ore body itself. The ore body is postulated to have originated by igneous processes during emplacement of the alkali-granite dikes at about 1045 Ma, based on U-Pb ages of zircon and monazite from both the ore body and the dikes. Emplacement occurred at or near the peak of high-T/low-P granulite-grade conditions (860º C and 5.5 kbar), in a structural setting consistent with constrictional extension. Post-emplacement deformation is largely restricted to ductile shearing, resulting in duplication and folding of the ore body, and to late normal faults that offset the ore body. Although Steenkampskraal does not contain appreciable ilmenite, the closest analog to the Steenkampskraal monazite ore is considered to be nelsonite.