PT-PD REEFS IN MAGNETITITES OF THE STELLA LAYERED INTRUSION, SOUTH AFRICA: A WORLD OF NEW PGE EXPLORATION OPPORTUNITIES

The major PGE reef-type deposits presently exploited around the world are located in the lower to central portions of large layered intrusions, e.g. the Bushveld, Stillwater and Great Dyke. Subeconomic PGE-Au reefs have additionally been found in the upper, magnetite-rich portions of some layered intrusions, notably the 2054 Ma Bushveld Complex, the 2640 Ma Rio Jacare Complex of NE Brazil, the 2435 Ma Koitelainen intrusion of northern Finland, the Triple Group of the 55 Ma Skaergaard intrusion of Greenland, the Birch Lake deposit of the 1107 Ma Duluth Complex, the 992 Ma Rincon del Tigre intrusion of Bolivia, the 1108 Ma Coldwell Complex of Ontario, and the 1096 Ma Sonju Lake intrusion within the Duluth Complex of Minnesota. In the early 1990s, high-grade PGE mineralization has been found in magnetite gabbros and magnetitites of the 3033 Ma Stella layered intrusion, within the Kraaipan greenstone belt of South Africa, indicating for the first time that PGE reefs in the differentiated portions of layered intrusions can be economic. The mineralization is found in a 100 m thick interval that includes a number of laterally consistent PGE reefs. The richest reef is hosted by magnetitite and contains between 10 and 15 ppm Pt+Pd over ca 1 m. The PGE are interpreted to have been concentrated by sulphide melt that segregated from a MORB-type magma, after S saturation had been reached in the advanced stages of differentiation. Late magmatic S-loss caused a paucity of sulfides, but the formation of abundant PGM in most of the mineralized interval. As a result, the reefs cannot be distinguished macroscopically from their unmineralized hostrocks, and we suggest that similar mineralization may have been overlooked in the upper portions of certain other tholeiitic intrusions elsewhere.