Genesis of sulfide-rich chromite ores by the interaction between chromitite and pegmatitic olivine-norite dikes in the Potosi Mine (Moa-Baracoa ophiolitic massif, eastern Cuba)

The Potosi Mine is located in the Moa-Baracoa massif in the easternmost part of the Cuban Ophiolitic Belt. Chromite mineralization occurs within the mantle-crust transition zone. Two events of magma intrusion overprint the chromitite bodies: one gave rise to the crystallization of pegmatitic olivine- norite dikes. and the other produced pegmatitic gabbro dikes. Sulfide-poor chromite ores, brecciated chromite ores, and sulfide-rich chromite ores can be distinguished in the different chromitite bodies. Sulfide-poor ores represent more than 80 vol% of the chromitites. This type occurs far from the zones intruded by pegmatitic gabbro dikes and shows petrographic and chemical features similar to other chromitite bodies described in the Moa-Baracoa massif. Brecciated chromite ores occur within pegmatitic gabbro dikes. In this type, chromite crystals occur included within chromian diopside and plagioclase. These silicates often contain droplet-like sulfide aggregates. Sulfide-rich ores are spatially associated to the contacts between sulfide-poor chromite and pegmatitic olivine- norite dikes. These ores mainly consist of recrystallized (coarse) chromite with interstitial pyrrhotite. pentlandite, cubanite. and chalcopyrite. Chromite from sulfiderich ores exhibits TiO<sub>2</sub>, FeO, V<sub>2</sub>O<sub>3</sub>, MnO, and especially, Fe<sub>2</sub>O<sub>3</sub> contents, higher than those of chromite from brecciated ores and much higher than those of chromite from sulfide-poor ores. The sulfide-rich ores are PGE-rich (up to 1,113 ppb of total PGE), and show nearly flat chondrite-normalized PGE patterns, slightly above 0.1 times chondritic values. Mineralogical and chemical data indicate that the chromite ores of the Potosi Mine were modified by the intrusions of olivine-norite and gabbro dikes. The interaction between preexisting sulfide-poor chromite ores and the intruding volatile-rich silicate melts produced strong brecciation, partial dissolution, and recrystallization (coarsening) of chromite. The sulfide assemblage formed by fractionation of the immiscible sulfide melt segregated from the volatile-rich silicate melt that generated the pegmatitic olivine-norite. The segregation of the sulfide melt can be interpreted as the consequence of chemical interaction between intruding melts and the host chromite. The variable extent of this interaction produced chromite ores with variable sulfide ratios. The magmatic nature of the sulfide mineralization is supported by sulfur isotope data, which range from -0.4 to +0.9 parts per thousand. Sulfide melt collected incompatible PGE (Rh, Pt, Pd) to produce the typical flat chondrite-normalized pattern of sulfide-rich chromite ores