Abstract
A variety of platinum-group-minerals (PGM) have been found to occur associated with the chromitite and dunite layers in the Niquelândia igneous complex. Two genetically distinct populations of PGM have been identified corresponding to phases crystallized at high temperatures (primary), and others formed or modified during post-magmatic serpentinization and lateritic weathering (secondary). Primary PGM have been found in moderately serpentinized chromitite and dunite, usually included in fresh chromite grains or partially oxidized interstitial sulfides. Due to topographically controlled lateritic weathering, the silicate rocks are totally transformed to a smectite-kaolinite-garnierite-amorphous silica assemblage, while the chromite is changed into a massive aggregate of a spinel phase having low-Mg and a low Fe3+/Fe2+ ratio, intimately associated with Ti-minerals, amorphous Fe-hydroxides, goethite, hematite and magnetite. The PGM in part survive alteration, and in part are corroded as a result of deep chemical weathering. Laurite is altered to Ru-oxides or re-crystallizes together with secondary Mg-ilmenite. Other PGM, especially the Pt-Fe alloys, re-precipitate within the altered chromite together with kaolinite and Fe-hydroxides. Textural evidence suggests that re-deposition of secondary PGM took place during chromite alteration, controlled by variation of the redox conditions on a microscopic scale.
Highlights
The Niquelândia igneous complex, exposed in Central Goias (Figure 1A), was recognized as a layered intrusion [1]
We summarize data from previous works, and present new results of a detailed investigation of the chromite-platinum-group minerals (PGM) paragenesis showing that the primary PGM were deposited at high temperature under relatively high sulfur fugacity, and that they underwent mineralogical reworking at low-temperature under variable redox conditions
A summary of the PGM recovered from the Niquelândia chromitite and dunite is given in Table 1, while Tables 2 and 3 provide a list of representative compositions obtained with an electron microprobe
Summary
The Niquelândia igneous complex, exposed in Central Goias (Figure 1A), was recognized as a layered intrusion [1]. The Niquelândia complex is similar to many other layered intrusions (i.e., Bushveld, Stillwater, Great Dyke, Campo Formoso), and it contains chromite deposits (Figure 1B) with associated platinum-group element (PGE) geochemical anomalies [6]. We summarize data from previous works, and present new results of a detailed investigation of the chromite-PGM paragenesis showing that the primary PGM were deposited at high temperature under relatively high sulfur fugacity, and that they underwent mineralogical reworking at low-temperature under variable redox conditions. This was a result of serpentinization (hydrothermal?). Weathering alteration (lateritization) of the ultramafic rocks under favorable morphologic and climatic conditions
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