Microscopic study of hydrothermal alteration in chromites from the ophiolites of the Havana-Matanzas region. Cuba
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Abstract
This study characterizes the hydrothermal alteration processes that affected chromites in the ophiolitic units of the Havana-Matanzas region, using optical microscopy techniques such as petrography and mineragraphy. Samples of both chromitites and serpentinized ultramafic rocks with dispersed accessory chromite mineralization were analyzed. The analysis was based on fundamental criteria such as the determination of the optical constants of minerals under transmitted and reflected light, and the relationships between mineral grains. The main results of the work are as follows: the hydrothermal alteration of the analyzed chromites developed in two main phases, both under pressure and temperature conditions equivalent to the greenschist facies and with a low fluid-rock ratio. The first phase, associated with an increase in oxygen fugacity during advanced serpentinization, favored the precipitation of ferritchromite along the edges and fractures of chromite crystals. In the second phase, chromite reacted with the surrounding serpentine, losing Al and Mg, resulting in a residual metallurgical-grade chromite surrounded by a chloritic halo. The presence of uvarovite microcrystals indicates that the mineralizing fluids contained, in addition to Al and Cr, low concentrations of Ca. The results of the study highlight the close link between chromite alteration processes and its composition, which allows these alterations to be evaluated as potential mineralogical indicators applicable in analogous ophiolitic contexts.
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