Cerro de Oro Mining District, Sonora, Mexico: Geology, igneous petrology, and mineral deposits

The Cerro de Oro Mining District is an area of base metal and precious mineralization associated with a regional volcano-plutonic magmatic event that occurred from the Middle to Upper Tertiary. Porphyry intrusions related to the mineralization range in composition from monzonite porphyry to quartz porphyry, which were emplaced as stocks and dike-sill complexes, preferentially following contacts between layers of a sequence of calcareous and siliciclastic sedimentary rocks, which trend N45°W to N55°W and dip at a moderately low angle to the southeast.; Zinc-lead skarn type mineralization occurs at Cerro de Oro along lithologic contacts, along faults, and as bedding replacement in calcareous units. Contact metamorphic rocks include hornfels and minor marble, which are the common hosts for skarn mineralization. The principal ore mineral is sphalerite, accompanied by pyrite, galena, and chalcopyrite. The typical skarn minerals are garnet, clinopyroxene, amphibole, epidote, calcite, chlorite and quartz.; Besides their Zn-Pb-Cu, skarns from Cerro de Oro have significant concentrations of Ag and Au. These Zn-skarn type deposits lack of the common characteristic of manganese-rich mineralogy. However, match the occurrence of ore bodies adjacent to structural and/or lithologic contacts between sedimentary strata. Middle to upper Tertiary volcanic rocks at Cerro de Oro includes basalts and basaltic andesites to rhyodacites, and rhyolites. Major and trace element compositions are typical of calc-alkaline igneous rocks associated with subduction. In general, Cerro de Oro volcanic rocks are enriched in LIL elements, and depleted in HFS elements, which consequently gives high ratios of LIL to HFS elements, and high ratios of LREE to HFS elements, suggesting a high degree of crustal contamination. The Zr/Hf ratios range from 25 to 38, which are consistent with those ratios of potassic igneous rocks from similar tectonic environments.; Cerro de Oro volcanic rocks range from high-K calc-alkali originated from a hybrid and metasomatized mantle with later crustal contamination either during subduction or extension. The petrogenesis of Cerro de Oro volcanic rocks are then related to three components: (1) hydrous slab-derived fluids, (2) depleted and hybrid mantle wedge, and (3) contamination from lower continental crust.