Composition and textures of chromite and platinum-group minerals in chromitites of the western ophiolitic belt from Pampean Ranges of Cordoba, Argentina

摘要

Chromitite bodies hosted in the Neoproterozoic western ophiolitic belt of Pampean Ranges of Cordoba (Argentina) were studied at Los Congos and Los Guanacos ultramafic bodies, with regard to the composition and textures of the chromite and platinum group minerals. Primary chromite composition is only preserved in some massive chromitites from the Los Guanacos ultramafic body, and is similar to Al-rich ophiolitic chromitites, suggesting that they crystallized from melts with back arc basin basalts (BABB) affinity in the suprasubduction mantle. Subsequently, these chromitites underwent a prograde metamorphism. Chromites from chromitites and associated metamorphosed ultramafic rocks show complex replacement and exsolution textures. Mineral chemistry and texture indicate that the chromite composition records two main metamorphic trends. A first trend defined by chromite from massive chromitite, in which there is an enrichment in Fe~(3+) and Fe~(2+), Cr remain relatively constant, and slightly depleted in Al, Mg. A second trend is defined by chromite from disseminated chromitite and metamorphosed dunite and harzburgite, in which a Fe-rich phase is replacing the Al-rich chromite. This alteration trend is characterized by enrichment in the total iron content (Fe~(3+)+Fe~(2+)) and a strong depletion in Al and Mg. The chemical composition of all analyzed spinels from Los Guanacos and Los Congos, as plotted on the ternary Fe~(3+)-Cr-Al diagram, correlates well with the Cr-spinels from the upper amphibolite to granulite-facies metamorphism. Platinum group minerals (PGM) identified include native osmium, laurite, erlichmanite, irarsite, platinum and a number of inadequately identified phases such as an oxide or hydroxide of Ru, Pt and Ir-Ru, Pt telluride, Ir-Ru-As-Se and Ir-Ru-Ti compounds. Native osmium was the only PGM which remained unaltered; other PGM underwent mineralogical reworking during metamorphism. Although it is difficult to establish the extent of platinum group element mobilization based on mineralogical observation, our results suggest that the Ru-Os-Ir PGM in the Los Guanacos and Los Congos chromitites were modified in situ, producing re-distribution of these PGE on a smaD scale. The presence of rare Pt and PGE-As-Se minerals was possibly related to remobilization of Pt, As and Se by fluids during the alteration processes.