Robert Campbell

The Rio Jacaré intrusion is located in north eastern Brazil, inside the state of Bahia. Primarily explored for its abundant titaniferous-vanadium rich magnetite occurrences, the intrusion also contains elevated levels of platinum (Pt) and palladium (Pd) ("Gulcari A" magnetite deposit researched in this study). The layered mafic intrusion is composed predominantly of gabbroic rocks, containing rhythmic cycles of magnetite-pyroxenite-gabbro-anorthosite. Fine grained, magnetite-rich rocks initiate the cycle followed by pyroxenite, generally capped by coarser gabbro or thin lenses of anorthosite. Unlike the common association of Platinum Group Mineral (PGM) mineralization with chromite layers within Layered Mafic Intrusions (LMI), the Rio Jacaré is a rare example of PGM mineralization within magnetite lenses. Processes regarding the enrichment of PGM mineralization remain unclear. A detailed mineralogical investigation was conducted on PGM mineralization within the main magnetite deposit, Gulcari A, aiming to understand the nature of PGM crystallization.

Thirteen samples with elevated Platinum Group Element (PGE) concentrations (1050-5026 ppm Pt, 25-1106 ppm Pd) were selected for this study. Reflected light microscopy and Energy Dispersive Spectroscopy (EDS) analyses were used for mineral identification. The chemical composition of PGMs was determined using electron microprobe (EMP) analyses. The ore consists of a magnetite-ilmenite aggregate, annealed and re-crystallized during metamorphism. Sulphide phases fill the interstitial space and are associated with gangue minerals. The most common Pt-phases found in these slides are: sperrylite (PtAs2), tetraferroplatinum (PtFe), niggliite (PtSn), PtNiFe, PtNi, PtSbSnNiCoS, and PtAsFeNi. The most common Pd-phases found are; PdPtSnCu, PdBiTe, PdBiSb, and PdSn. Pt-phases are found as inclusions within magnetite, ilmenite and occasionally late-forming gangue minerals. Often, PGMs form at the boundary between early magnetite-ilmenite grains and interstitial gangue minerals. Additionally, sperrylite grains have been found as particle inclusions within larger arsenide minerals. Pd-phases are found as inclusions within late gangue minerals, particle inclusions within arsenides, growths along grain boundaries between gangue minerals and primary magnetite-ilmenite grains, and as small subhedral grains within interstitial space. Overall, PGM mineralization varies from small 2µm, to larger 100 µm anhedral grains. The average grain size of PGM mineralization is between 2-10 µm.

Our results suggest two different events responsible for the etrainment and remobilization of PGE mineralization. In our study we tested both the proposed earlier mechanism of exsolution of PGMs from late sulphide phases, and remobilization of PGE's by late hydrothermal fluids.

Pages: 94
Supervisor: Yana Fedortchuk