Abstract

Anhydrite occurs along with biotite, feldspars, quartz, Fe-oxides, and sulfides in fine-grained igneous rocks that form the small “Porphyry A” stock at El Teniente. These minerals crystallized together in the space generated by the intrusion of this stock, and they do not replace preexisting phases. Planar crystal boundaries between anhydrite and other phases, as well as the fact that plagioclase and biotite are fresh and unaltered, also suggest that anhydrite is an igneous phase, and not the product of secondary hydrothermal alteration. Plagioclase is unusually Na rich, ranging from oligoclase in mafic varieties of these rocks with >20 percent biotite, to albite in more felsic samples with <10 percent biotite, consistent with its co-crystallization with anhydrite, which sequestered calcium from the melt. Anhydrite has textures that vary from interstitial to poikilitic as its modal abundance increases from <10 to >20 percent. These unusual anhydrite-bearing plutonic rocks, which contain up to >3 wt percent S and >0.5 wt percent Cu, are isotopically similar to all other igneous rocks related to the deposit. These rocks formed during a late Miocene period of regional compressive deformation with no coeval volcanic activity in the area of the deposit, which prevented devolitalization of the oxidized parent magma in the large productive magma chamber inferred to have underlain El Teniente at >4 km below the paleosurface. This allowed igneous fractionation, involving the recharge of mantle-derived mafic magmas into the base of the chamber as well as volatile transfer and concentration near its roof, to produce the Cu- and S-rich magmas that formed the anhydrite-bearing intrusive rocks.

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