Abstract

Re-Os isotopes in pyrite and chalcopyrite from early high-temperature hypogene alteration assemblages in Chilean porphyry copper deposits identify the source of Os and, by inference, Cu in these ore systems. Typical concentrations for Os in both pyrite and chalcopyrite are between 7 and 30 ppt (10−12 g/g), and for Re between 0.200 and 10 ppb (10−9 g/g). Re-Os isochrons yield ages and initial 187Os/188Os ratios for the sulfides. The isotopic data reflect the relative contributions of copper from the mantle and crust in Chilean porphyry copper deposits. Seven ore deposits that reside in different tectonic terranes and represent distinct epochs of mineralization in Chile were studied. The initial osmium ratios of the first stage of mineralization at each of the deposits range from 0.15 to 5. These values are more radiogenic than the present chondritic mantle (∼0.13), and indicate significant crustal contributions of Os to the magmatic and/or hydrothermal systems. There is a strong correspondence between the total copper content and initial Os isotopic ratios in base-metal porphyry deposits. The larger deposits have lower initial Os ratios than the smaller, less significant deposits. This relationship implies that larger deposits acquire a greater proportion of Os from the mantle. The initial Os ratio of samples in the central segment of porphyry copper deposits of northern Chile also decreases with decreasing age of the deposit. A plausible interpretation of the Re-Os data is that the later and larger deposits use regional tectonic and structural features that allow sampling of deeper more primitive magmatic sources.

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