Genesis of Cr-bearing hydrogrossular-rich veins in a chromitite boulder from Ayios Stefanos, West Othris, Greece : a paradigm of micro-rodingites formation at the late stages of oceanic slab emplacement

The retired Ayios Stefanos chrome mine is hosted by deformed and serpentinized mantle peridotites of the western Othris ophiolite of central Greece. Chrome ores consist of m-scale lenticular to podiform chromitite bodies exposed within a tectonically attenuated dunite. We studied a mylonitized ore boulder (∼2 m3 in volume) that consists of massive-textured chromitite intruded by several altered gabbroic veins (≤15–20 cm thick). This chromitite boulder includes previously undescribed hydrogarnet-bearing microveins barely visible to the naked eye. The veinlets are isolated from each other and composed of undeformed Cr-rich hydrogrossular (spherules), clinochlore, serpentine, wollastonite, vesuvianite, brucite and subordinate amounts of other phases. Textural observations of the intruding gabbros indicate that as tremolite substituted for diopside there was production of an oxidizing Ca-rich aqueous medium. This was capable of leaching Cr, Al and Fe2+ from Cr-spinel fragments dispersed within the veinlets. The resultant hydrothermal fluids transported components required to precipitate a mineral assemblage resembling that of rodingite. Hydrogrossular globules in the veinlets display chemical zoning characterized by increasing abundances of Cr2O3 and Fe2O3 and decreasing H2O concentration toward their boundaries. The en echelon structure of the micro-rodingite “arteries” combined with the unstrained nature of the veinlet minerals suggests that they filled late extensional cooling fractures in a mylonitized chromitite body. Overall, the structural-minerochemical characteristics of the veinlets can be adequately explained by general T decrease in the obducting slab concurrent to transport away from the spreading center.