GEOCHEMICAL EVOLUTION OF ORE FLUIDS, BOLEO CU-MN-ZN-CO DISTRICT, BCS, MEXICO

The Boleo Cu-Mn-Zn-Co District, located in the transtensional Santa Rosalía Basin, Baja California Sur, Mexico, contains 8 mineralized, copper-rich mantos within the late Miocene sedimentary Boleo Formation. The goal of this study, part of the Baja Basins Research Experience for Undergraduates, is to identify mineral assemblages within the mantos that indicate the geochemical characteristics and evolution of the mineralizing fluids. Samples collected from outcrops at the Boleo Mine were analyzed using SEM/EDS and XRD.

The mantos are dominated by Mn oxide dendrites, laminae, and dense layers up to several meters thick. While much of the Mn mineralization is amorphous, hollandite group minerals and pyrolusite were identified using XRD and SEM analyses. A siliceous sinter surrounding Mn oxide dendrites also contained low levels of all the metals found in the hydrothermal deposits.

Cross-cutting relationships observed in the field and by SEM showed successive mineralization of Mn oxides, goethite, chrysocolla and silica. XRD analyses identified pyrolusite, hollandite/cryptomelane, paratacamite, atacamite, barite, malachite and calcite in Mantos 3 and 4 at the western end of Arroyo Boleo. Gangue minerals include smectites altered from volcanic glass, intermediate sodic anorthite, goethite, and quartz, mainly as reprecipitated silica.

The paragenesis of the manto mineralization began with the weathering or hydrothermal alteration of volcanic glass to smectites. Mn oxides precipitated from hydrothermal fluids during ascent, followed by goethite and silica mineralization. Chrysocolla appears to have formed by reaction of Cu-bearing fluids with existing silica after the deposition of Mn oxides. Late calcite, malachite, barite and rarely atacamite were also observed on the Mn oxides and silica deposits.

Eh and pH constraints on the fluids were defined by the stability fields of the mineral assemblages. Fluid inclusion geothermometry is currently underway. The mineralizing fluids were low temperature, neutral to slightly alkaline pH hydrothermal fluids that oxidized as they ascended, leading to the sequential manto mineralization.