Las cromititas ofiolíticas del yacimiento Mercedita (Cuba). Un ejemplo de cromitas ricas en Al en la zona de transición manto-corteza

Abstract

The Mercedita deposit is located in the ophiolitic Massif of Moa-Baracoa (NE of Cuba) and is considered the most important podiform chromite deposit of America. Chromitite bodies, enclosed in hazbu rgite and residual dunites (mantle-crust transition zone). The chromite ore bodies are concordant with the main structures shown by the enclosing peridotites and also display pull-apart fractures. Chromite lenses enclose and substitute grabbro bodies (sills), that are concordant with the orientation of the host chromitite. Intergranular minerals are olivine, serpentine, and chlorite. Chromite has abundant, distributed solid inclusions of olivine and Na-rich pargasite (up 4 wt % Na2O), and minor laurite and millerite. Toward the contact with the included gabbro sills, abundant clinopy r oxene, plagioclase and rutile occur as inclusions in the chromite. The ores from Mercedita deposit are composed by refractary - grade chromite (Al-rich chromite), where A l 2 O 3 ranges between 25 and 33wt.%. The TiO2 values are relative ly high compared to the most common ophiolitic chromite, TiO2 content varies from 0.05 to 0.52 wt. %. Chromitites of the Mercedita deposit are poor in platinum-group elements (PGE), with total PGE ranging between 55.8 and 165.9 ppb and an average value of 90 ppb. From textural and geochemical data we propose a genetic model from the reaction of a back arc basin basalt, formed by melt-rock reactions, percolated through subhorizontal, porous dunitic channels and mixed with oxidized melts in suprasubduction zone mantle. Mixing of these two melts generated a hybrid melt whose bulk composition fell within the chromite liquidus field in the P-T- fO2 space (Hill and Roeder, 1974). Percolation of the hybrid melt through the dunitic channels promoted dissolution of preexisting silicate minerals and chromite crystallization.