Geochemistry and mineralogy of platinum-group elements at Hartley Platinum Mine, Zimbabwe -: Part 1.: Primary distribution patterns in pristine ores of the Main Suffide Zone of the Great Dyke

The Main Sulfide Zone (MSZ) is the most prominent layer of the Great Dyke layered intrusion carrying economic platinum-group element (PGE) mineralization. At the Hartley Platinum Mine, the MSZ is up to some meters thick and is composed of a basal PGE-rich subzone which overlaps slightly with a base metal, sulfide-rich subzone on top. Based on the degree of sulfide mineralization and PGE ratios, the subzones can be further divided into geochemically distinct layers. Stratigraphically upwards, the MSZ displays metal profiles characterized by increasing Cu/Ni, Pt/Pd and PPGE/IPGE ratios, accompanied by a general element decoupling in the order Pd --> Pt --> base metals, more specifically IPGE --> Pd --> Pt --> (Ni,Cu,Co,S (sulfides), Au,Te,Bi. The fine structure of the MSZ is regarded to reflect primary magmatic features of consecutive batches of sulfide accumulation, concomitant scavenging of PGE, and fractionation. Sulfide deposition was followed by a second, limited subsolidus stage of PGE redistribution. Mineralogically, most of the Pd and Rh is hosted in pentlandite.. whereas Pt is dominantly present in the form of discrete platinum-group minerals (PGM). Within the MSZ sequence, sperrylite is present throughout the PGE subzone of the MSZ, cooperite/braggite occur mainly in its basal part, and the (Pt,Pd)-bismuthotellurides concentrate at the top. These findings indicate that a large proportion of the PGE, primarily concentrated in sulfide under magmatic conditions, was redistributed in the subsolidus stage and formed discrete PGM with available reactant partners. Chemical gradients and magmatic-hydrothermal fluids probably led to small-scale redistribution of PGE within the MSZ.