Abstract

The manganese ores in the Santa Rosalía region, western Mexico, are mainly stratiform horizons or mantos, constrained to the initial stages of sedimentary cycles of the Miocene Boléo Formation. The manganese mineralization is generally restricted to isolated paleo-basins and related to NW-SE faults formed during the early stages of the opening of the Gulf of California. Jasper, Fe, and Mn oxides associated to the NW-SE structures may represent feeder zones for the mineralized system. The manganese oxide minerals include pyrolusite, cryptomelane, todorokite, hollandite, jacobsite, and pyrochroite. Trace elements in the manganese ores indicate a hydrothermal origin for the deposits of the Santa Rosalía area. Rare earth elements (REE) patterns obtained for manganese minerals from the Lucifer and El Gavilán deposits also support a hydrothermal origin, whereas the middle REE enrichment observed in samples from the Boléo district indicates mixing between hydrothermal and hydrogenous sources. Osmium and rhenium concentrations of the manganese minerals range between 33-173 ppt and 0.14-89 ppb, respectively. The initial 187Os/ 188Os ratios in the manganese oxides from Lucifer and the Boléo district range between 0.43 to 0.51 and 0.70 to 0.74, respectively. These ratios are different from seawater at 7 Ma (0.84-0.89), which suggests important contributions of osmium from underlying rocks such as the Miocene volcanic rocks and the Cretaceous quartz-monzonite basement. Field evidence, manganese oxide mineralogy coupled with major and trace element geochemistry and Re-Os systematics support a hydrothermal origin for the manganese deposits from the Santa Rosalía region. The ore deposition style indicates an exhalative-intraformational environment restricted to isolated basins in a diagenetic stage related to the initial evolution of the Gulf of California. © Springer-Verlag 2008.