Al- and Cr-rich chromitites from the Eastern Havana-Matanzas ophiolites (Western Cuba)

Chromite deposits in the Havana-Matanzas ophiolites of Western Cuba include both Al- ([Cr/(Cr+Al)=0.44- 0.46]) and Cr-rich ([Cr/(Cr+Al) = 0.65-0.74] chromitites that commonly occur in the upper part of mantle ectonites near the mantle-crust transition zone. These chromitites generally form small lenticular bodies spatially associated with strongly serpentinized dunite. Platinum group minerals (PGM) are found only in the Cr-rich chromitites, as euhedral or subeuhedral, very small (less than 10 μm long) crystals. The most abundant PGM is laurite, accompanied by minor osmium, cuproiridsite, iridium and an unnamed phase composed of Ir-Ni-Fe-S, all of which are magmatic in origin. These magmatic PGMs and amphiboles crystallized at high temperatures (~1300 °C) and at relatively high sulphur fugacities from fluid-rich magmas. The chromite mineral chemistry suggests that melts that were in equilibrium with Cr-rich and Al-rich chromitites had island-arc tholeiite – boninite and backarc basin basalt (BABB) affinities, respectively.