Platinum-Group Minerals in Chromitites of the Niquelândia Layered Intrusion (Central Goias, Brazil): Their Magmatic Origin and Low-Temperature Reworking during Serpentinization and Lateritic Weathering

摘要: A variety of platinum-group-minerals (PGM) have been found to occur associated with the chromitite and dunite layers in Niquelândia igneous complex. Two genetically distinct populations PGM identified corresponding phases crystallized at high temperatures (primary), others formed or modified during post-magmatic serpentinization lateritic weathering (secondary). Primary moderately serpentinized dunite, usually included fresh chromite grains partially oxidized interstitial sulfides. Due topographically controlled weathering, silicate rocks are totally transformed a smectite-kaolinite-garnierite-amorphous silica assemblage, while is changed into massive aggregate spinel phase having low-Mg low Fe3+/Fe2+ ratio, intimately Ti-minerals, amorphous Fe-hydroxides, goethite, hematite magnetite. The part survive alteration, corroded as result deep chemical weathering. Laurite altered Ru-oxides re-crystallizes together secondary Mg-ilmenite. Other PGM, especially Pt-Fe alloys, re-precipitate within kaolinite Fe-hydroxides. Textural evidence suggests that re-deposition took place by variation redox conditions on microscopic scale.