ROLE OF MANTLE-DERIVED MAFIC MAGMAS IN THE GENERATION OF THE GIANT MIOCENE AND PLIOCENE COPPER DEPOSITS OF CENTRAL CHILE

Lead and osmium isotopic values, measured in galena, chalcopyrite, bornite and sphalerite from the giant Miocene and Pliocene Rio Blanco-Los Bronces and El Teniente copper deposits of central Chile, imply derivation of these elements, and by implication copper, from mantle-derived igneous rocks associated with these deposits. Mantle-derived igneous rocks in these deposits include tholeiitic and calc-alkaline basalts and basaltic andesites. In contrast, late felsic porphyry intrusions in these deposits, which in general intruded after copper mineralization was emplaced, formed by crustal MASH processes and thus are unlike to have been the source of this ore. The high iron, magnesium, calcium and sulfur contents of ore-bearing hydrothermal breccias and veins also is consistent with mafic rather than felsic sources for the metal-rich aqueous fluids that transported the copper ore. Also the strontium and neodynium isotopic composition of silicate minerals in ore-bearing breccias and veins is within the range of the mantle-derived mafic igneous rocks associated with these deposits. For these mafic igneous rocks, strontium isotopic ratios range from 0.7037 to 0.7048, and epsilon values for neodynium range from +4 to -0.5, indicating, along with lead and osmium isotopic compositions, derivation by melting in a subarc mantle-wedge source contaminated by dehydrating subducted oceanic crust, including pelagic and terrigenous sediments as well as continental crust tectonically eroded off the continental margin by subduction processes. Experimental studies, and direct measurements of water in glass inclusions within igneous minerals, indicate that such magmas may contain between 4.5 and 6.5 weight percent water at shallow crustal levels. Magma production rates of between 5 to 25 km³ per million years in the dominantly mafic volcanic arc of the southern Andes have been shown to be sufficient to provide, in multiple events over a 1 to 3 million year period, the enourmous quantity of copper in the giant deposits of central Chile, while the volume of late felsic porphyry intrusions in these deposits is clearly too small for these to have been the source of the copper ore.