PETROGRAPHIC CHARACTERIZATION AND INTERPRETATION OF THE CINTA COLORADA MARKER BED, BOLEO DISTRICT, SANTA ROSALIA, BCS, MEXICO

The Cinta Colorada is a prominent red marker bed found across much of the upper Miocene sequence of conglomerates and tuffaceous sandstones and mudstones hosting the Cu-Mn-Zn-Co ore depositions (mantos) of the Boleo mining area. The Boleo Formation contains four main mantos and several minor ones for a total of nine mineralized beds. The Cinta Colorada lies above Manto 3 but below Manto 2, and is an excellent stratigraphic marker used for exploration in the sequence of ore horizons. A petrographic investigation was conducted to better characterize the Cinta Colorada unit. X-ray diffraction analyses indicate the bulk mineralogy of the Cinta Colorada is uniform over its range. The bulk mineralogy includes intermediate sodian anorthite (andesine), augite, hypersthene, hematite (providing the red color of the Cinta Colorada), and smectite (altered glass). The mineralogy is consistent with a volcanic source. Polarized light microscopy and SEM/EDS confirm the XRD results, showing the smectite has replaced volcanic glass, and that the rock is made of sand- to silt-sized crystal and lithic fragments of andesitic to basaltic volcanic rocks. Crystal fragments of andesine and clinopyroxene are broken, but extremely fresh with no observed weathering or alteration. All of the lithic and crystal fragments have a thin, rough coat of very fine ash that appears to be sintered to the larger clasts. That rind likely accumulated on the clasts as they were erupted through the hot ash cloud. The rock is grain-supported, and the interstices are empty—other than the sintered rinds, no very fine-grained material is present. Late Mn oxides cement some of the grains, occasionally filling interstices. The Cinta Colorada appears primarily to be an air-fall tuff at its base with a possible density debris flow at the top. Later introduction of hydrothermal fluids (probably associated with ore deposition in the basin) and oxidation led to replacement of volcanic glass with smectite clays and deposition of the Mn oxide cement, producing the Cinta Colorada in its present state. This research was conducted as part of the ongoing National Science Foundation funded Baja Basins REU project.