STRATIGRAPHIC AND MINERALOGICAL CHARACTERISTICS OF CU-ZN-CO-MN MANTOS AT MINERA BOLEO, SANTA ROSALÍA, BCS, MEXICO

Mantos at the Boleo Cu-Zn-Co-Mn deposit near Santa Rosalía, BCS, Mexico possess similar sedimentary and structural features that obscure their distinction. This study employs both stratigraphic and mineral analysis to characterize the mantos, permitting identification of individual mantos in the Boleo mine. Exposures of four of the mantos and their enclosing sediments near the Texcoco Mine adit were measured and described in detail. Bulk mineralogy of the samples was determined by X-ray diffraction analysis and SEM/EDS.

The manganese and iron oxide ores were deposited in clayey horizons overlaying polymitic clay breccias, tuffaceous claystones, and conglomerates. The host claystone is predominantly smectite clay. The individual mantos possess variations that may be useful distinguishing features. In Manto 1, sepiolite is abundant, and a lesser amount occurs in the base of Manto 2 and Manto 3a, which also contains pyrophyllite as detected by XRD. Mantos 1, 2 and 3a possess a feldspathic (sodic anorthite) clastic component. Manto 4 differs, being calcareous due to its replacement of and association with a basal limestone of the Boleo formation. It contains marine bivalves and gastropods replaced by Mn oxides and silica. Manto 1 is distinguished by the presence of the zeolite clinoptilolite in the upper sandy layers. Manto 2 possesses silica-rich minerals including quartz and cristobalite, and metal oxides including maghemite (Fe₂O₃) and minor possible cochromite (Co(Cr,Fe)₂O₄). Manto 3a possesses a thick layer of halloysite (Al₂Si₂O₅(OH)₄) in a bed in the middle of the sequence.

The EDS identification of Mn nodules found within the mantos indicates they are predominantly cryptomelane (K-Mn oxyhdroxide) and hollandite (Ba-Mn oxyhydroxide), with pyrolusite, ramsdellite (both MnO₂), and possibly bixbyite (Mn₂O₃), and hausmannite (Mn₃O₄). A crystallized cavity from Manto 1 exhibits a paragenetic sequence of Mn₃O₄, Mn₂O₃, MnO₂ from oldest to youngest, showing an increase in oxidation state during deposition.

Though these features might be useful for identifying individual mantos in the Texcoco mine area, lateral variation and depositional facies changes might limit their use over the entire area of Boleo mineralization.