Abstract

Apart from its large reserves of Cu-Co-Zn mineralization contained within stratiform ore beds or mantos, the Boléo district of central Baja, Mexico, also contains significant concentrations of Mn oxides (typically 2–3 wt % Mn) interspersed with hydrothermal sulfides and both hydrothermal and weathering-related metal oxide minerals. Within the mantos, Mn oxides occur as concentric nodules, disk-shaped concretions, laminae, granules or aggregates, veins, dendrites, and various replacements. The oxides are composed of varying amounts of hollandite, cryptomelane, todorkite, romanèchite, pyrolusite, and X-ray amorphous Mn oxide. Chemically, the manto oxides are unique owing to their extraordinarily high abundances of Co, Cu, Pb, and Zn (typically 2–5 wt %, but up to 23 wt %).

Existing mineralogical and geochemical classifications are ambiguous regarding the genetic classification of Mn oxides within the mantos. A new MnO2-Fe2O3t-MgO diagram is proposed in order to discriminate among highly metalliferous Mn oxides. Based on the new classification scheme, manto Mn oxides record a complex interaction between hydrothermal and diagenetic processes. To better understand the mechanism of oxide deposition, radiogenic isotopes (Sr and Pb) and REE geochemistry were used to delineate fluid and metal sources. Isotopic and REE constraints indicate that the Boléo Mn oxides are largely hydrothermal in origin; however, they show varying degrees of modification resulting from diagenetic processes. Hydrothermal Mn oxides are syngenetic (synsedimentary to syndiagenetic) in origin and were precipitated in association with sulfides, reflecting localized variations in redox conditions within the manto sequence, due to the downward infiltration of metal-bearing brines. Apart from morphological and stratigraphic evidence (e.g., occurrence of nodular crusts along the upper contact of the manto), a syngenetic origin is supported by a single K-Ar date of 7.0 ± 0.2 Ma, which is within the estimated age range for deposition of Boléo Formation clastic sediments. Interaction between brine and seawater or brine and pore fluid, as it infiltrates the sedimentary pile, produced hydrothermal precipitates with certain hydrogenetic attributes (i.e., positive Ce anomaly, LREE enrichment). On the other hand, oxides with hydrogenetic and with mixed hydrothermal-hydrogenetic affinities represent diagenetic alteration and remobilization of earlier formed hydrothermal oxides by meteoric fluids. By far the greatest degree of remobilization occurred in response to compactional dewatering of the manto, where the dewatering pore waters enriched the uppermost manto strata in Mn, Fe, Zn, Co, and to lesser degrees, Cu. Later diagenetic modifications coincided with the development of a supergene overprint, which typifies Boléo Formation strata at elevations above the present-day water table.

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