The evolution of Laramide igneous rocks and porphyry copper mineralization in the Cananea district, Sonora, Mexico.

Abstract

This study investigates the relationship between the evolution of the igneous and hydrothermal systems in the Cananea mining district located in northern Sonora, Mexico. The Cananea district was chosen for this study because post-mineral uplift and erosion has tilted the Cananea Range ∼15° to the east and exposed an oblique section through approximately 6 vertical kilometers of a mineralized volcano-plutonic system and because porphyry-related stockwork, breccia, and pegmatitic silicate-sulfide mineralization are all well developed and well exposed. Major, trace element, and isotopic data suggest that the Laramide (∼64-56 Ma) igneous rocks represent a cogenetic calc-alkaline magmatic series ranging in composition from gabbro to granite. Neodymium isotope (εNd = +0.7 to -5.7) and strontium isotope (⁸⁷Sr/⁸⁶Sr(initial) = 0.70570 to 0.71037) values show a smooth inverse correlation and combined with the major and trace element data suggest that the Laramide rocks evolved from a mantle-derived parent melt by coupled assimilation and fractional crystallization. Phase petrology, mineral compositions, whole rock geochemistry, and alteration mineralogy indicate water, metal, chlorine, and sulfur content of the magmas increased with increasing differentiation and reached a maximum in late stage differentiates. The mineralized quartz-feldspar porphyries represent the rapid upward emplacement of this enriched differentiate. Porphyry-related mineralization in the Cananea district includes pegmatitic silicate-sulfide mineralization represented by the La Colorada and Maria deposits. Detailed field and geochemical work on the Maria deposit suggests that the fluids, alteration assemblages and paragenetic relations in silicate-sulfide pegmatites are similar to those seen in stockwork- and breccia pipe-hosted porphyry Cu systems, but that silicate-sulfide mineralization represents transitional conditions in which magmatic fluids are concentrated in a small volume and undergo efficient cooling and decompression leading to formation of high-grade telescoped mineralization.