SEDIMENTARY MANGANESE AND SEDEX DEPOSITS IN THE JURASSIC RIFT OF HUAYACOCOTLA, MEXICO-PINAR DEL RÍO, CUBA

The Jurassic system of the Pinar del Río Province, western Cuba, closely resembles that of the Huayacocotla anticlinorium of eastern Mexico. In Cuba, the lower part (shale and sandstone) of the San Cayetano Fm. is similar to the Huayacocotla Fm. of the Lower Jurassic, and the upper part (fossiliferous black limestone) to the Santiago and Chipoco Fms. of the Upper Jurassic. Moreover, the upper part of the San Cayetano Fm. hosts the SEDEX deposits of Matahambre (Cu), dated at 150 Ma (lower Tithonian), and Santa Lucía (Pb, Zn), while in Huayacocotla the deposits of sedimentary Mn of Molango are hosted by the Chipoco Fm. (Kimmeridgian-lower Tithonian).

In the Huayacocotla area, the Jurassic period began with an extensional tectonic event that developed a system of horsts and grabens. The horsts show outcrops of the Proterozoic Huiznopala Gneiss, and of a fossiliferous Permian volcanic-sedimentary sequence (Guacamaya Fm.). In the grabens, the Lower Jurassic Huayacocotla Fm. was deposited in continental conditions at base, marine in the middle and again continental at top.

Middle Jurassic time began with deposition, in angular unconformity over the Huayacocotla Fm., of mixed-facies red beds (Cahuasas Fm.) in an intracratonic rift system. This unit is conformably covered by the Tepéxic Fm. (calcarenite, marl and siltstone) of marine shallow-water facies.

In Late Jurassic, a deepening of the rift system occurred under anoxic conditions. The Santiago Fm. conformably overlies the Tepéxic Fm. and consists of calcareous black shale, rich in organic matter and with pyrite. This unit is conformably covered by the Chipoco Fm. (rhodochrosite-bearing limestone and shale, all of them black).

The faults that gave origin to the horsts and grabens show pyrite, pyrrhotite, and Mn oxides; so, they are interpreted as feeders of the hydrothermal solutions that conveyed Mn to the basin. Similar faults may have been related to the origin of the Pinar del Río SEDEX deposits.

By their similarity in age and in the lithology of their host rocks, it is presumed that the Mn mineralization of Huayacocotla may be a distal product of the SEDEX mineralizing event that took place in the rift, e.g. in Pinar del Río, that developed the Gulf of Mexico.