Madagascar is rich in chromite deposits scattered through the Archean terranes. The main ore districts are, from North to South, Befandriana, Andriamena and Tamatave. Platinum Group Minerals (PGMs) were detected as inclusions in chromite grains as well as in the chromitite matrix in the Ranomena deposit, belonging to the Tamatave chromite district. The Ranomena deposit is characterised by a series of highly deformed and metamorphosed, subparallel metric chromitite lenses, hosted within basic-ultrabasic rocks. Chromitite is characterized by variable textures from disseminated to massive. The chromitite mineral assemblage is mainly composed of low Cr2O3 chromite (with compositional variation up to ferritchromite and magnetite), orthopyroxene, olivine and secondary phases like chlorite, amphibole, talc and serpentine. Rutile and ilmenite as well as minor sulphides, like chalcopyrite, pentlandite and hazlewoodite, occur in the matrix. Several small (10-30 µm) PGM grains were observed in almost all samples, regardless of the chromitite texture. The most abundant phase is euhedral laurite (RuS2) included in chromite grains. X-ray atomic maps confirmed the polyphase nature of very fine-grained PGM aggregates, in which at least the occurrence of irarsite ((Ir, Ru, Rh, Pt,)AsS) and stillwaterite (Pd8As3) was ascertained by micro-analysis. Some alloys containing Pt, Ru, Fe and traces of As were also detected within the matrix. Platinum Group Elements (PGEs) were analysed on whole rock samples by Nickel Suphide Fire Assay- Instrumental Neutron Activation Analysis (INAA). All analysed samples show similar trends characterized by a general depletion in Pt, Pd and a high Os/Ir ratio. The general PGE trends are in accordance with the occurrence and the composition of PGMs detected, therefore suggesting that they are representative of the chromitite PGM assemblage. According to the model proposed by Maier and Barnes [1] for the UG1-UG2 Bushveld chromitites, the PGE geochemical signatures (except for Pd) suggest a Bushveld-type genesis for the Ranomena Cr-PGE ores. The low abundance of Pd in Ranomena ores might be related to post-ore metasomatic processes. The relationships between the Ranomena chromitite and the other main chromitite ores of Madagascar are still unclear. Chromitite ores display significant compositional differences, in spite of their similar geological settings. These differences might be related either to different depositional processes that influenced also the distribution of PGMs (less abundant in the Andriamena and Befandriana chromitites than in Ranomena), or to different post-ore processes (e.g., metamorphic imprints). [1] Maier, W.D. & Barnes, S.J. (2008) South Afr. J. Geol., 111, 159-176.
Platinum Group Minerals of Ranomena chromitite, Tamatave district, Madagascar / G. Grieco, A. Merlini, A. Cazzaniga, M. Moroni. - In: ACTA MINERALOGICA-PETROGRAPHICA. - ISSN 0365-8066. - (2010). ((Intervento presentato al 20. convegno General Meeting of the International Mineralogiacl Association : Bonds and bridges tenutosi a Budapest nel 2010.
Platinum Group Minerals of Ranomena chromitite, Tamatave district, Madagascar
G. Grieco;A. Merlini;M. Moroni
2010
Abstract
Madagascar is rich in chromite deposits scattered through the Archean terranes. The main ore districts are, from North to South, Befandriana, Andriamena and Tamatave. Platinum Group Minerals (PGMs) were detected as inclusions in chromite grains as well as in the chromitite matrix in the Ranomena deposit, belonging to the Tamatave chromite district. The Ranomena deposit is characterised by a series of highly deformed and metamorphosed, subparallel metric chromitite lenses, hosted within basic-ultrabasic rocks. Chromitite is characterized by variable textures from disseminated to massive. The chromitite mineral assemblage is mainly composed of low Cr2O3 chromite (with compositional variation up to ferritchromite and magnetite), orthopyroxene, olivine and secondary phases like chlorite, amphibole, talc and serpentine. Rutile and ilmenite as well as minor sulphides, like chalcopyrite, pentlandite and hazlewoodite, occur in the matrix. Several small (10-30 µm) PGM grains were observed in almost all samples, regardless of the chromitite texture. The most abundant phase is euhedral laurite (RuS2) included in chromite grains. X-ray atomic maps confirmed the polyphase nature of very fine-grained PGM aggregates, in which at least the occurrence of irarsite ((Ir, Ru, Rh, Pt,)AsS) and stillwaterite (Pd8As3) was ascertained by micro-analysis. Some alloys containing Pt, Ru, Fe and traces of As were also detected within the matrix. Platinum Group Elements (PGEs) were analysed on whole rock samples by Nickel Suphide Fire Assay- Instrumental Neutron Activation Analysis (INAA). All analysed samples show similar trends characterized by a general depletion in Pt, Pd and a high Os/Ir ratio. The general PGE trends are in accordance with the occurrence and the composition of PGMs detected, therefore suggesting that they are representative of the chromitite PGM assemblage. According to the model proposed by Maier and Barnes [1] for the UG1-UG2 Bushveld chromitites, the PGE geochemical signatures (except for Pd) suggest a Bushveld-type genesis for the Ranomena Cr-PGE ores. The low abundance of Pd in Ranomena ores might be related to post-ore metasomatic processes. The relationships between the Ranomena chromitite and the other main chromitite ores of Madagascar are still unclear. Chromitite ores display significant compositional differences, in spite of their similar geological settings. These differences might be related either to different depositional processes that influenced also the distribution of PGMs (less abundant in the Andriamena and Befandriana chromitites than in Ranomena), or to different post-ore processes (e.g., metamorphic imprints). [1] Maier, W.D. & Barnes, S.J. (2008) South Afr. J. Geol., 111, 159-176.
Pubblicazioni consigliate
I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
