MINERALOGICAL EVIDENCE OF AN EPITHERMAL EXHALATIVE ORIGIN OF THE BOLEO CU-ZN-MN-CO MANTOS, SANTA ROSALIA, BAJA CALIFORNIA SUR, MEXICO

The Boleo Cu-Zn-Mn-Co manto deposits near Santa Rosalia, Baja California Sur, Mexico, are hosted in a series of Pliocene conglomerates, altered tuffs, limestones, and evaporates in the Santa Rosalia Basin. The mantos formed above coarse conglomerates at the bottom of the overlying tuffaceous sandstones of the Boleo Formation, with five main mantos and at least eight mineralized beds in detail. The ores have been mined since the mid-19^th century, and advances in extractive solution chemistry have resulted in the current major mining activity in the district.

Two models of formation of the manto mineralization have been presented. Wilson and Rocha (1955) concluded that hydrothermal fluid rose along faults from the underlying Comondu volcanics into the overlying Boleo Formation. The fluids spread laterally along conglomerate beds, trapped beneath the less permeable tuffaceous sandstones, and deposited the ore mineralization. A more recent model proposed by Conly et al. (2006, 2011) calls on venting of hydrothermal fluids at the surface with accompanying deposition of the ore minerals both as chemical sediments and as cements and replacements of underlying sediments by infiltrating fluids in fluvial to near-shore marine environments. Our investigation of two of the mantos provides evidence supporting Conly’s exhalative model.

Detailed examination of Manto 1 reveals sedimentary features such as syneresis cracks, small ripples in laminated siltstones, and evidence of a highly oxidized evaporitic depositional environment (hematitic halite-bearing siltstone). In Manto 2, below Manto 1, we found sedimentary laminations, and deposits of laminated tufa with ore mineralization. In the rhythmically banded gypsum beds at the eastern edge of the district, several thin heavy mineral lag deposits of Cu-Co-Fe-Zn sulfides (possibly villimaninite and an unidentified Co-rich sulfide with Cu and Fe) were found during SEM/EDS examination.

Based on the findings of the current investigation and those of previous studies, we conclude that an epithermal exhalative process is a better model for the formation of the manto mineralization than the dominantly subsurface lateral flow of fluids.