GSA Annual Meeting in Seattle, Washington, USA - 2017

Paper No. 153-42
Presentation Time: 9:00 AM-6:30 PM


HELPRIN, Olivia L.1, FLORES, Gabriella Vianne2, TREPP, Kaya3, MUROWCHICK, James B.4 and SALGADO MUNOZ, Valente O.4, (1)Department of Geology, Humboldt State University, 1 Harpst St, Arcata, CA 95521, (2)Department of Geology, Sonoma State University, 1801 E Cotati Ave, Rohnert Park, CA 94928, (3)Department of Geosciences, Oregon State University, 1500 SW Jefferson St., Corvallis, OR 97331, (4)Geosciences, University of Missouri - Kansas City, 5100 Rockhill Road, Room 420 Flarsheim Hall, Kansas City, MO 64110,

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.