High-Cr and high-Al chromitites from the Sagua de Tánamo district, Mayarí-Cristal ophiolitic massif (eastern Cuba): constraints on their origin from mineralogy and geochemistry of chromian spinel and platinum-group elements

Several chromitite bodies of variable sizes are hosted in dunite-harzburgite of the small (< 3. km) mining district of Sagua de Tánamo, in the Mayarí-Cristal ophiolitic massif, eastern of Cuba. The chromian spinel in these bodies displays a large range in Cr# (Cr/Cr + Al) atomic ratio from 0.74 to 0.45. In high-Cr chromitites, Cr# and Ti contents increase from harzburgite to dunite, to chromitite. A roughly opposite trend of variation in Cr# is observed in high-Al chromitites, although in this case Ti distributes randomly. The differences in the type of chromian spinel (i.e., high-Cr or high-Al) in the chromitite coincide with a different behavior of the platinum-group elements (PGE). Whereas high-Cr chromitites are rich in PGE and contain abundant grains of platinum-group minerals (PGM), high-Al chromitites are systematically poor in PGE and in PGM. The calculated melts in equilibrium with chromian spinel of high-Cr chromitite are island arc thoeliites (IAT) with boninitic affinity whereas those in equilibrium with chromian spinel of high-Al chromitites are back-arc basin basalts (BABB). The formation of high-Cr chromitites is interpreted as a result of the extensive reaction of harzburgite with migrating island arc tholeiite melts of boninitic affinity. Melt-rock reaction produces boninitic melts with variable composition and porous dunitic channels in which the mixing/mingling of melts promotes crystallization of mononomineralic high-Cr chromian spinel. In contrast, high-Al chromitites formed by the mixing/mingling of BABB melts within conduits not in equilibrium with dunite. Percolation of primitive BABB melts through pre-existing dunite dissolved olivine, producing melt conduits in which BABB melts mixed and formed high-Al chromian spinel. The higher PGE and PGM in high-Cr chromitites is not only a function of the degree of partial melting but is linked to interaction between the migrating melt and mantle harzburgite during the formation of the chromitite. The coexistence of both types of chromitites in one small mining district reflects the temporal and/or spatial variations of separate melt intrusions emplaced in different subarc mantle domains during the opening of a back-arc basin in a supra-subduction zone environment.