Al- and Cr-rich chromitites from the Mayari-Baracoa ophiolitic belt (Eastern Cuba): Consequence of interaction between volatile-rich melts and peridotites in suprasubduction mantle

The Mayari-Baracoa belt occupies the easternmost part of the east-west-trending Cuban ophiolitic belt. It comprises two large, chromite-rich massifs: Mayari-Cristal and Moa-Baracoa. Neither of these massifs show a complete ophiolite sequence, but they consist of a part of an ideal section made up of (1) harzburgites grading upward into interlayered harzburgtes and dunites, (2) interlayered harzburgites (with minor dunites) and gabbros, (3) gabbros, microgabbros, dolerites, and diabase dikes, and (4) pillowed basalt, cherts, and radiolarites. Chromite deposits can be grouped into three mining districts according to the chemistry of chromite ore: the Mayari district and the Sagua de Tanamo district, both in the Mayari-Cristal massif, and the Moa-Baracoa district in the Moa-Baracoa massif. The latter is the most important as it contains more than 5.5 million tons of ore. All chromitites mainly exhibit massive texture, show a pseudotabular, lenticular shape, and are concordant with the foliation of the enclosing harzburgites. In Moa-Baracoa they tend to occur in the mantle-crust transition zone, commonly contain dunite and gabbro bodies oriented parallel to the elongation of the lenses, and are cut by late pegmatitic gabbro dikes. By contrast, in Mayari, and to some extent in Sagua de Tanamo, chromitites occur deeper in the mantle tectonites and are cut by websterite dikes. Intergranular minerals are olivine, serpentine, and chlorite. Chromite has abundant, randomly distributed solid inclusions of olivine and pargasite, and minor pyroxene, laurite, and millerite. Toward the contact with the included gabbros, chromitite from Moa-Baracoa. shows increasing amounts of gabbro-related alteration products. Abundant clinopyroxene, partly altered plagioclase, and rutile occur as inclusions in the chromite. The composition of the chromite ore varies from typical refractory grade (Al rich) at Moa-Baracoa to metallurgical grade (Cr rich) at Mayari, where the Cr no. ranges between 0.41 and 0.75, the Mg no. between 0.57 and 0.81, and the TiO2 content between 0.09 and 0.52 wt percent. At Moa-Baracoa, the Cr no, of chromite decreases and Toe content increases from harzburgite to dunite and massive chromitite, positively correlated with the forsterite content of coexisting olivine. At Mayari, both the Cr no. and TiO2 content of chromite, and the forsterite content of olivine increase from harzburgite to dunite and chromitite. Bulk platinum-group element abundances in chromitite vary from 20 to 538 ppb and show a broad positive correlation with Cr2O3 percent of chromite. The latter correlation is strongest in the Sagua de Tanamo district. Structural, textural, mineralogical, and chemical characteristics of the studied chromite deposits, as well as the lithophile trace element geochemistry of their host rocks, support a genetic model based on the crystallization of chromite from different types of melts (from back are basin basalts to boninitic andesites) at around 1,200 degrees C, at variable fo(2). Chromite formed when calc-alkaline melts, formed by melt-rock reactions at increasing melt volume, percolated through subhorizontal, porous dunitic channels and mixed with oxidized melts formed by low degrees of hydrous melting and low-temperature melt-rock reactions 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-fo(2) space (Hill and Roeder, 1974). Percolation of the hybrid melt through the dunitic channels promoted dissolution of preexisting silicate minerals and chromite crystallization. The Al-rich chromitites formed at the mantle crust transition zone at high fo(2) (approximate to log fo(2) = -7), whereas Cr-rich chromitites formed deeper in mantle tectonites under more reducing conditions, at log fo(2) approximate to -10, depending on Cr contents of the parental magma.