Geochemistry and PGE of the lower mineralized Zone of the Waterberg Project, South Africa

Abstract

The Waterberg deposit is located north of the Northern Lobe of the Bushveld Complex in South Africa and represents a large, high-grade, new Platinum-Group-Element (PGE) discovery. The first comprehensive study of the lower ultramafic section of the area has been completed, and whole-rock chemical analyses and PGE data are presented.The section studied comprises mineralized harzburgite and marginal orthopyroxenite, overlain by troctolite grading into gabbroic rocks. Whole-rock analyses show geochemical variations typical of differentiated assemblages of cumulus olivine, plagioclase, and pyroxene. Normalized trace element data display HREE depletion, strong positive Eu and Sr anomalies and LREE enrichment. The negative anomalies for Th, Rb, Nb, and Ta are typical for rocks of the Bushveld Complex. Normalized PGE distributions are fractionated (Pd/Ir 177), Pd-enriched, and Au-poor.Emplacement of the magmas is believed to have commenced with west-east trending, finger-like intrusions, followed by lateral dilation and emplacement of sulfide droplet-bearing, ultramafic magmas. This was followed by a second phase of intrusions, characterized by sheet-like bodies of troctolite. Fractionation of these magmas led to the development of gabbroic rocks that make up the top of the succession. The Waterberg Project is located in the Southern Marginal Zone of the Limpopo Belt. This position in a structurally active area may have facilitated the creation of space for initial magmas.It is argued, that the mafic to ultramafic succession of the Waterberg Project does not represent a simple marginal extension of the Northern Lobe, nor does it directly correlate with the Platreef. It shares geological features but represents a separate magmatic basin.