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

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Abstract

Platinum-group elements (PGE) are typically associated with mafic and ultramafic intrusive rocks and the main exploration targets are layers and zones rich in POP-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. Pt-Fe alloys, in particular, are often present in PGP deposits poor in base-metal sulphides and two phase systems were investigated experimentally: Pt-Ir-Fe-S (and the related subsystem Ir-Fe-S) and Fe(Cu)-Pt-Rh-S. 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. The experimental work also indicated that Pt-Rh-Fe alloys can coexist with two types of immiscible sulphide melt (one low in Rh and high in Cu, the other Rh-rich); both types of sulphide melt may have been present in the Tropoja deposit, where Rh-rich minerals occur. Within all the deposits locally high sulphur fugacity conditions may have developed even in low-sulphur assemblages during the final stages of PGE deposition. In-situ sulphur isotope data from both the Madagascar and Albanian intrusions are compatible with a mantle origin, as are oxygen isotope data on silicates and oxides. Radiogenic isotope data indicate strongly the influence of a crustal component on the Madagascar intrusions. The existence of two distinct radiogenic components at the time of the Albanian PGE mineralization is suggested by some Re-Os isotopes, and this is best interpreted as mantle-derived heterogeneity in the melt from which they formed. The mineralization was subjected to retrograde hydrothermal metamorphism and recrystallization during the cooling of the intrusions. in Madagascar re-equilibration was first to granulite-facies and later to greenschist-facies conditions. A chloride-rich mineralogy resulted from greenschist hydrothermal metamorphism and the fluid movement could explain the absence of sulphides (possibly attacked by a secondary C1-rich fluid), the substantial change of the metal/S ratio and local PGE remobilization. In all the deposits, whether chromitite-style or silicate-style, the PGP metals are concentrated at distinct horizons within the basic-ultrabasic complexes. The field and laboratory studies suggest that both styles of mineralization were produced by a fluid-driven, multistage process within a fractionating silicate magma chamber. Crystallization of Pt-barren massive chromitite was followed by crystallization of Pt-rich chromitite and, finally, by deposition of Pt- and Pt-rich silicate rocks associated locally with gold and disseminated sulphides. The later stages of Pt and Au concentration were due to metal-rich fluids issuing from an ascending intercumulus melt. Once formed, the mineralized layers cooled progressively and subsolidus reactions occurred. The deposits were then to some extent recrystallized and remobilized during regional metamorphism, but from the late magmatic stage behaved essentially as closed systems with respect to PGE. 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. The project was coordinated by the BRGM (France) and the partners were CNRS (France), the University of Copenhagen (Denmark) and the University of Manchester (United Kingdom).

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