Role des liquides sulfures dans la formation des mineralisations riches en elements du groupe du platine: Applications au complexe de Bushveld (Afrique du Sud) et au complexe de Stillwater (Etats-Unis).

摘要

Major platinum-group element (PGE) deposits are found associated with large layered intrusions such as the Bushveld Complex (South Africa) and the Stillwater Complex (Montana, U.S.A.). These PGE deposits are referred to as "reef". The Merensky Reef (Bushveld Complex) and the J-M Reef (Stillwater Complex) are two examples of, PGE reefs associated with base-metal sulphides that are located about one third from the base of the layered intrusions.; The goal of this thesis is to test the models for collection of the PGE by a magmatic base-metal sulphide liquid and the role of compaction on the sulphide liquid migration, using different approaches based on microstructural, geochemical and mineral analyses. Possible constraints on PGE remobilization by fluids will also be evaluated.; The major aims were: (1) to determine the distribution of the base-metal sulphides (BMS) in 3-D (using X-ray Computed Tomography) and in 2-D (from thin sections) in the samples; (2) to determine the role of compaction and/or lithologies on BMS distribution (or sulphide liquid migration); (3) to determine the hosts of the PGE (BMS minerals, platinum-group minerals or others phases); (4) to consider the role of a BMS liquid and of fluid migration on PGE enrichment; (5) to synthesize all the results in order to propose a model for the formation of this type of PGE mineralization.; Microtomographical (X-ray computed tomography) analyses indicate that: (1) the BMS in the silicate rocks of both the Merensky and the J-M Reefs are located along millimetre- to centimetre-scale vertical network. In the Merensky Reef, the network terminates in the underlying chromitite where the sulphides occur as small droplets; (2) in the Merensky Reef, the BMS content decreased up-section and the BMS are more abundant just above each chromitite layer.; Microstructural analyses of the samples from both Merensky and J-M Reefs show that: (1) all lithologies exhibit high temperature crystal deformation features such as indented contacts between orthopyroxene, deformation twins in plagioclase, undulose extinction, and the development of sub-grains; (2) the BMS are located along pyroxene, olivine and/or plagioclase grain boundaries and form a vertical network along faces of these crystals.; All the observed features are consistent with a paleovertical maximum principal compressive stress (61) that is responsible for compaction as the cumulate pile formed. During the compaction vertical dilatancies formed and may have been filled by the base metal sulphide liquid.; Geochemical and mineralogical analyses and modelling calculations indicate that: (1) in both reefs, most of the PGE are found in pentlandite and pyrrhotite or in platinum-group mineral (PGM) associated with them; (2) exceptions to this are Os-Ir-Ru in the chromitites of the Merensky Reef which are found in laurite grains with chromites and silicate minerals and Pd-Cu alloy found in secondary magnetite in one J-M Reef sample; (3) in both the Merensky Reef and the J-M Reef, Pt and Au are not hosted by BMS, but are found as platinum-group minerals (mainly Pt +/- Pd sulphides and tellurides, and Pt-Fe alloys) and electrum within the BMS; (4) in the J-M Reef the footwall cumulates have Pd/Pt ratios of 0.3 to 0.7 that are lower than the reef samples averaging Pd/Pt ratio of 3.3.; These observations suggest that:(1) all the PGEs found in the silicate rocks of the Merensky Reef and the J-M Reef were initially collected by a sulphide liquid; (2) the PGE observed in the chromitite layers of the Merensky Reef are the result of the combination of two processes: primary crystallisation of laurite from the silicate magma (before sulphide liquid segregated); and the later percolation of PGE-rich immiscible sulphide liquid; (3) the Pt-Fe alloys observed in both mineralized zones are produced by the partial desulfurization of MSS; (4) the others PGMs (telluride and sulphide essentially) are exsolved from the BMS upon cooling; (5) in the J-M