Four platiniferous deposits were studied during the EU project so as to propose a new model for the genesis of Pt- and Rh-rich platinum - group element (PGE) concentrations in the absence of significant base metal sulphide (BMS) mineralization. Based on mineralogical and petrological evidence, two types of deposits were recognized, those associated with chromitites (Tropoja, Albania and Andohankiranomena, Madagascar targets) and those occurring in silicate rocks, mostly dunites, devoid of significant BMS and chromite concentration (Bulqiza, Albania and Lavatrafo, Madagascar targets).
The genesis of both types of PGE-mineralization is related to a fluid-driven multi-stage mineralizing process involving a fractionating silicate magma in a magma chamber. The mineralizing process leads first to crystallization of Pt-barren massive chromitite, followed by crystallization of Pt-enriched chromitite, and finally by deposition of Pt-silicate rocks. In the final case, Pt concentrations derive from mineralizing fluids issuing from an ascending intercumulus silicate melt.
Gold mineralization was discovered during the EU project in Pt- and Pd-rich silicate rocks. Gold and (Pt+Pd) anomalies are often decoupled because noble metals are fractionated in the mineralizing system. The range of temperatures and sulphur fugacities for alloy-type mineralization associated with chromitites were determined from experimental studies during the project. From the early emplacement of chromitites to the late stage Pt and Pd deposition in silicate rocks, the temperature of formation decreases from magmatic (approximately 1100 C) to hydrothermal and even lower than the stability temperature of pentlandite (< 560 C). The origin of the mineralizing system was determined from a multidisciplinary approach, integrating the mineralogical composition of the PGE reef, the distribution of PGE, incompatible and volatile elements, and data from stable and radiogenic isotopes. All of the data are compatible with a magmatic origin of mineralizing fluids. However, no geochemical halo was recognized around the mineralized bodies in Madagascar.
The metallogenic process proposed for basic-ultrabasic complexes devoid of significant BMS, studied during the EU project, may be applied to those complexes with PGE mineralized reefs, associated either with chromitite or with disseminated BMS. More especially, the mineralizing system may be applied to the Bushveld complex, which controls about 60% of the total world production of PGE. The origin of these deposits is distinct from those showing sulphides at the margin of the intrusions, as in the Duluth complex, the Muskox intrusion, the Insizwa complex and the Cape Smith belt (Barnes and Francis, 1995). In such cases, the role of crustal mineralizing fluids is more significant.
Prospecting guides for Pt- and Pd- deposits are deduced from the fluid-driven multi-stage mineralizing system developed to model the origin of mineralization (Cr, Ni-Cu, EGP and Au) hosted by basic-ultrabasic complexes. High metallogenic potential of a basic-ultrabasic complex is suggested by the presence of chromite deposits. Pt- and Pd-mineralization has to be sought for in silicate rocks above the upper chromite reef. The presence of BMS in such rocks is dependent on the S content of the whole system.
From the present study, it may be emphasised that the scarcity of base metal sulphides (BMS) in a basic-ultrabasic complex does not imply that no Pt-Pd mineralization exists.
The proposed research is directed at developing a model for the genesis of Pt- and Rh- rich PGE concentration in the absence of significant sulphide mineralization. The existing magmatic PGE concentration model is based on the collection of PGE by sulphide melts and is clearly not applicable to this new type of PGE deposit.
The development of a new model will involve (1) the characterization of geochemical halos around selected targets discovered during previous EEC and national program and (2) the development of appropriate experimental and analytical procedures.
The major research tasks are:
1) Development and/or perfecting of analytical techniques (improvment of detection limits) for PGE analyses (potential volatile mettaloids, relevant stable and radiogenic isotopes).
2) Acquisition of experimental data on Pt- and Rh-rich, base matal ulphide-poor, PGE parageneses for geothermometry and mineral equilibria as well as on PGE solubilities in magmas.
3) Development of a genetic model for Pt- and Rh-rich minelization poor in base metal sulphides.
In short, the proposed research aims at:
- Multidisciplinary characterization of the nature of the geochemical halo around PGE mineralization in order to find effective prospection tools.
- Determination of mineralogical characteristics of the ore and the host rocks for the development of new ore dressing techniques.
Successful completion will enable the identification of new targets for field exploration even in Europe and new ore-processing methods, the development of which should reduce manufacturing costs.
