| Boléo is a stratiform Cu-Co-Zn deposit that surrounds the town of Santa Rosalía on the eastern side of the Baja Peninsula. Mineralization consists of finely disseminated sulfides within marine claystones and tectonically triggered debris flow sediments that occur at the base of episodic fan-delta cycles of the Miocene Boléo Formation, which comprise the initial rift-fill sediments within the Santa Rosalía basin. Rifting and hydrothermal activity is related to the formation of the Proto-Gulf of California. Mineralization occurs as eight mineralized ore beds, or mantos, that are recognized over 90 km2 and yield a 464 Mt resource grading 0.70% Cu, 0.06% Co, and 0.71% Zn.; A red-bed model has been used previously to explain ore genesis. However, this research presents an exhalative origin, in which low temperature metalliferous brines (<120°C as constrained by oxygen isotope geothermometry) discharged onto the basin floor and infiltrated down into the sedimentary pile. The model is supported by the following evidence: (i) the abundant replacement of diagenetic pyrite; (ii) occurrence of mineralized rip-up clasts in the breccias; (iii) the concentric decrease in Cu/Zn ratio towards the basin margins; (iv) the vertical decrease in Cu/Zn within the mantos; (v) K-Ar ages of Mn oxide mineralization, where the most reliable age of 7.0 ± 0.2 Ma is within the range of published ages for the Boléo Formation; (vi) sulfur isotopes, which range from ∼−30‰ to 0‰ and indicate sulfur from diagenetic sulfides and from high temperature bacterial sulfate reduction; and, (vii) covariations in δ 13C and δ18O of carbonate alteration, which are consistent with the replacement of marine calcite by hydrothermal calcite-siderite. C, O and Sr isotopes also show that brine interacted with pore fluids/seawater that was highly depleted in 13C due to a component of fluvial waters introduced during conglomerate and debris flow deposition.; Potential metal sources were also investigated. Leaching of red-bed conglomerates or basement plutonic and volcanic rocks would have provided an insufficient quantity of metals, although the Pb isotopic compositions of the sulfides indicate that some leaching of basement did occur. Unique trace element relationships have led to the suggestion that a magmatic fluid derived from Cerro San Lucas magmas could have supplied the metals. |
