PLUTONIC ROCKS CONTAINING PRIMARY IGNEOUS ANHYDRITE IN THE EL TENIENTE CU DEPOSIT, CHILE

El Teniente, located in the Andes of central Chile (34°S), is the world's largest Cu deposit. Primary igneous anhydrite occurs in plutonic rocks described by mine geologists as 'igneous breccia' and/or 'micro-diorite'. These rocks also contain biotite, quartz, plagioclase and alkali feldspars, apatite, rutile and sulfides (chalcopyrite and pyrite), but not amphibole. Modal abundance of biotite varies from >50% in mafic anhydrite-bearing rocks (<52 wt % SiO₂ and >5 wt % each of FeO, MgO and CaO) to <10% in more felsic samples. Plagioclase in both types is Na-rich, ranging from oligoclase to albite, suggesting that it had crystallized from a magma depleted in Ca by the contemporaneous crystallization of anhydrite. Planar crystal boundaries between anhydrite and other phases, as well as inclusions of silicate minerals within anhydrite, are other lines of evidence for anhydrite being a primary igneous phase, and not the product of secondary hydrothermal alteration. Furthemore, samples analyzed are unaltered, containing neither chlorite nor sericite. Primary igneous anhydrite has textures which vary from interstitial to poikilitic as its modal abundance increases from <10% to >30%. These anhydrite-bearing plutonic rocks may contain >0.5% Cu and >6% S, more than previously reported for any extrusive rocks containing primary anhydrite. Spatially, they intrude into and form the core of a late Miocene and Pliocene complex of Cu-rich (>2% Cu) biotite- and anhydrite-bearing magmatic-hydrothermal breccia pipes. There is no evidence of coeval volcanic activity in the area of the deposit during this time period, which may have prevented devolitalization of the magma chamber underlying the deposit and allowed the SO₂ and H₂O contents to increase to the point that hydrothermal breccias were generated above the chamber, followed by intrusion of the SO₂-rich magma that crystallized to form the anhydrite-bearing igneous rocks. The high Cu and S content of these igneous rocks reflects both the depth (>4 km) and confining pressure (>1.3 kbar) on the source magma chamber, as well as its dominantly mafic composition as indicated by the predominance of mafic rocks in the El Teniente deposit.