Banded pyrite deposits of Minas Carlota, Cuba

The orebodies of Minas Carlota are steeply dipping pyrite lenses arranged en echelon in isoclinally folded limestones. The lenses occur at faulted contacts with serpentinite or in tensional fold areas.Mineralization was in the approximate order: (1) sphene, picotite; (2) actinolite, albite; (3) quartz, calcite, chlorite; (4) anomalous quartz, rutile, sericite; (5) fine-grained pyrite; (6) coarse pyrite; (7) chalco-pyrite; (8) sphalerite; (9) galena. (1) represents a magmatic or contact-metamorphic phase, (2) dynamic metamorphism, (3) a phase immediately preceding pyritization, while (4) overlapped with (5) and (6). Early pyrite (5) originated by replacement of limestone along graphitic laminae concurrently with rock folding, at a suggested temperature of 600 degrees C. Later pyrite (6) formed at lower temperatures as a space-filling. (7), (8) and (9) comprise less than 10 percent of the ore. Post-ore deformation is limited to marginal ore and represents differential adjustment to late faulting.Chemical studies suggest that low gold, cobalt and nickel values are contained in the fine-grained pyrite.Of pyrite crystals the cube is the only euhedral form in gangue (?), whereas in the late pyrite (6) cubes and cubes modified by a possible octahedral zone are present. A search of the literature suggests that: i) the cube is the commonest form of pyrite, ii) the cube is dominant in veins containing sericite, chlorite and carbonate, iii) the cube where associated with anomalous fringing quartz has grown contemporaneously with rotation and rock movement, iv) the pyritohedron develops at higher temperatures than the cube, v) pyrite octahedrons from meosthermal deposits invariably contain slight manganese values.The ores are regarded as hydrothermal. Their occurrence adjacent to serpentinite is considered an indication of structural control rather than of contact metamorphism. Crystallization of the upper portion of the magma may have led to sulfide enrichment at depth. Renewed earth movement forced the ore fluids upwards along the flanks of the intrusions.Serpentinization preceded pyritization. A decrease in volume on serpentinization created tensional areas in the limestone which were infiltrated by the ascending ore fluids. Mineralization and serpentinization are believed closely related in time and may be post-Jurassic, pre-Maestrichtian.The Carlota orebodies are parallel to regional structure. An investigation into the geotectonic implications of such orebodies is long overdue.