New exploration methods for platinum and rhodium deposits poor in base-metal sulphides-NEXTPRIM

Platinum-group elements (PGE) are typically associated with mafic and ultramafic intrusive rocks and the main exploration targets are layers and zones rich in PGE-bearing sulphides. Some PGE occurrences, however, are in sulphide-poor situations and this raises the possibility that PGE deposits may be present in parts of mafic and ultramafic intrusives currently considered to have low exploration potential. A multidisciplinary study was undertaken on four subeconomic deposits of platinum-group metals to develop a model of formation for PGE deposits lacking significant base-metal sulphides. Two of the deposits occur in Albania, in the Tropoja and Bulqiza massifs, and are part of an ophiolitic belt created in an oceanic environment during the Upper Jurassic. The other two deposits occur in Madagascar, in the Andohankiranomena and Lavatrafo ultramafic massifs, and are within a Pan-African rifted zone. A Pt-rich chromitite style of mineralization was identified in the Andohankiranomena and Tropoja deposits, where the PGE are mostly included in chromite. A Pt- and Pd-rich silicate (dunite) style of mineralization was identified in the Lavatrafo and Bulqiza massifs, where PGE mineralization is associated with interstitial material between olivine grains. The four deposits have contrasting patterns of PGE distribution and individual element ratios, suggesting that different mineral species (alloys, arsenides and sulphides) host the PGE. No primary geochemical halos were detected around any of the deposits and weathering has little effect on the distribution of the PGE. The study showed that alloys and arsenides are the main carriers for platinum in all the deposits. A comparison of experimental results with natural phases in the deposits suggests that fluid-assisted exsolution of Pt, Ir and other elements from original higher-temperature solid solutions could be widespread. This supports the fluid-driven multistage mineralization concept suggested by field data. From the proposed genetic model it follows that the presence of chromite layers in an basic-ultrabasic complex increases the prospecting potential for PGE, that silicate rocks above the upper chromite reef are an exploration target and that the absence of significant base-metal sulphides does not preclude the presence of Pt and Pd concentrations.