|
Preliminary data of platinum group minerals/elements (PGM/PGE) have been obtained from different Brazilian chromitites, namely Monte Alegre do Sul, Luanga, Niquelndia, Campo Formoso, Cromínia and Morro Feio. Monte Alegre do Sul, Luanga, Niquelndia and Campo Formoso chromitites are stratiform being associated with Lower Proterozoic layered Complexes. Crominia and Morro Feio are ophiolitic chromitites (Upper Proterozoic). The studied chromitites are strongly altered. All the investigated chromitites contain PGM, however with some differences as it regards the PGM variety. The following PGM have been recognized, Monte Alegre do Sul=laurite, erlichmanite, irarsite, platarsite, hollingworthite, cuprorhodsite, malanite, Rh-pentlandite, Ir-Ni-S, Pd-Ir-Te, Luanga=sperrylite, laurite, irarsite, hollingworthite, platarsite, Pt-Fe alloys, native Pt, tulameenite, Pt-Fe oxide, rustenburgite, cooperite, Pt-Mn-Pb-Fe, Pd-Sb, Pt-Sb-Bi-As, Pd-Pb, Niquelndia=laurite, erlichmanite, irarsite, tetraferroplatinum, isoferroplatinum, Os-Ir alloys, geversite, moncheite, merenskyite, sobolevskyite, stumpflite, Campo Formoso=laurite, erlichmanite, irarsite, ruarsite, hollingworthite, osmium, Pd-Sb, Pt-Te, Crominia=laurite, Ru-oxide, irarsite, and Morro Feio=laurite, sperrylite, irarsite, hollingworthite. Based on their composition, texture and morphology, the PGM have been classified as primary, i.e. magmatic in origin and secondary, i.e. formed or altered during some post magmatic stage (metamorphism/hydrothermalism). The samples of Luanga, which contain abundant Ni-Cu-Co sulfides, contain also the highest number of PGM characterized by a great mineralogical variation. Most of them occur in the altered matrix, mainly associated with chlorite, of a rock composed of a Cr-rich spinel. Also the stratiform chromitites of Niquelndia, Campo Formoso and Monte Alegre do Sul contain specific minerals of all the six PGE, classified as primary and secondary. However, the chromitites of Monte Alegre do Sul are the only one that contain primary inclusions of thiospinel of the cuprorhodsite-cuproirdisite-malanite serie. The chromitite of Cromínia are characterized by the presence of Ru, Ir, Os bearing PGM, classified as secondary. The samples of Morro Feio contain also Pt and Rh specific phases. These preliminary data on PGM found in different Brazilian chromitites indicate that the stratiform are rich in PGM, showing a great mineralogical variation that comprises the presence of minerals of all the six PGE, including the most valuable Pt and Pd. On the contrary, in the ophiolitic chromitites the PGM are less abundant and, with the exception of Morro Feio, they mainly consist of Ru, Ir and Os minerals, as typical of other ophiolitic chromitites worldwide. The presence of primary and secondary PGM in all the studied samples suggests that the metamorphism/hydrothermalism, that affected the chromitites, being stratiform or ophiolitic, produced some effects also on the PGE mineralogy.
|
