Minerals (Sep 2016)
The Cedrolina Chromitite, Goiás State, Brazil: A Metamorphic Puzzle
Abstract
The Cedrolina chromitite body (Goiás-Brazil) is concordantly emplaced within talc-chlorite schists that correspond to the poly-metamorphic product of ultramafic rocks inserted in the Pilar de Goiás Greenstone Belt (Central Brazil). The chromite ore displays a nodular structure consisting of rounded and ellipsoidal orbs (up to 1.5 cm in size), often strongly deformed and fractured, immersed in a matrix of silicates (mainly chlorite and talc). Chromite is characterized by high Cr# (0.80–0.86), high Fe2+# (0.70–0.94), and low TiO2 (av. = 0.18 wt %) consistent with variation trends of spinels from metamorphic rocks. The chromitite contains a large suite of accessory phases, but only irarsite and laurite are believed to be relicts of the original igneous assemblage, whereas most accessory minerals are thought to be related to hydrothermal fluids that emanated from a nearby felsic intrusion, metamorphism and weathering. Rutile is one of the most abundant accessory minerals described, showing an unusually high Cr2O3 content (up to 39,200 ppm of Cr) and commonly forming large anhedral grains (>100 µm) that fill fractures (within chromite nodules and in the matrix) or contain micro-inclusions of chromite. Using a trace element geothermometer, the rutile crystallization temperature is estimated at 550–600 °C (at 0.4–0.6 GPa), which is in agreement with P and T conditions proposed for the regional greenschist to low amphibolite facies metamorphic peak of the area. Textural, morphological, and compositional evidence confirm that rutile did not crystallize at high temperatures simultaneously with the host chromitite, but as a secondary metamorphic mineral. Rutile may have been formed as a metamorphic overgrowth product following deformation and regional metamorphic events, filling fractures and incorporating chromite fragments. High Cr contents in rutile very likely are due to Cr remobilization from Cr-spinel during metamorphism and suggest that Ti was remobilized to form rutile. This would imply that the magmatic composition of chromite had originally higher Ti content, pointing to a stratiform origin. Another possible interpretation is that the Ti-enrichment was caused by external metasomatic fluids which lead to crystallization of rutile. If this was the case, the Cedrolina chromitites could be classified as podiform, possibly representing a sliver of tectonically dismembered Paleoproterozoic upper mantle. However, the strong metamorphic overprint that affected the studied chromitites makes it extremely difficult to establish which of the above processes were active, if not both (and to what extent), and, therefore, the chromitite’s original geodynamic setting.
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