Petrogenesis and Assembly of the Don Manuel Igneous Complex, Miocene–Pliocene Porphyry Copper Belt, Central Chile

The 4.0-3.6 Ma Don Manuel igneous complex (DMIC), central Chile, provides a window into igneous processes involved in magma genesis associated with porphyry copper-style mineralization. This study uses petrographic, petrologic, geochemical and isotopic data to examine the evolution of magmas from the mid- to lower-crustal source region to shallow emplacement. The data provide evidence of progressive oxidation of magma during differentiation and ascent, fractionation of Cl from S through degassing, and the late-stage, near-solidus removal of Cl from the system. Magmas of basaltic andesite to rhyolite composition were produced by polybaric differentiation of hydrous parental mafic magmas. Variations in crustal differentiation depths led to variable suppression of plagioclase saturation that is recorded in distinctive strontium-anorthite evolution patterns. Hydrous, derivative magmas generated over a wide range of pressures were episodically emplaced into the shallow crust at depths between 3.5 and 5 km. Intermediate porphyry dikes closely associated with copper mineralization contain diverse crystal cargoes indicating significant igneous mixing. These crystal cargoes represent samples of crystal mush entrained from different depths, as well as crystals originating in different magmas and crystals grown in-situ from hybridized magmas. Mafic enclaves, containing plagioclase and amphibole compositions that match those of the basaltic andesites, occur within biotite tonalite testifying to magma mingling during ascent. Sulfur and chlorine contents of apatite within the different DMIC units record variable degassing and decoupling of volatile components with sulfur showing variations of three orders of magnitude compared with one order of magnitude for chlorine. The hypabyssal nature of the DMIC affords a detailed, integrated record of magmatic differentiation processes occurring within trans-crustal magmatic systems of the sort thought to characterize many crustal arc settings and play a fundamental role in driving porphyry-style copper mineralization.