Metallogenesis in Continental Margins: Re-Os Evidence from Porphyry Copper Deposits in Chile
Joaquin Ruiz, Ryan Mathur, 1999. "Metallogenesis in Continental Margins: Re-Os Evidence from Porphyry Copper Deposits in Chile", Application of Radiogenic Isotopes to Ore Deposit Research and Exploration
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Re-Os isotope studies of molybdenite and other more common sulfides have yielded new geochronological and tracer information on the origin of the ore-forming metals for base metal porphyry mineralization. Because the new geochronological data are on the sulfides and not the associated alteration minerals or magmatic rocks, it is possible to constrain the mineralizing events within the context of the magmatic history of the mineralized district or region. Re-Os studies of base metal porphyry mineralization in Chile and Arizona, the two premier base metal porphyry regions in the world, indicate that the ore formation is episodic within the overall life of the magmatic arc. In Arizona, mineralization occurs in two distinct periods: ca. 75 Ma in the northwest part of the state, and ca. 55 Ma in the southeast. In Chile, the new Re-Os data on sulfides support previous K-Ar work that date mineralization at ca. 35 to 40 Ma in the northern part of the country and ca. 3 to 5 Ma in the south. The age data on sulfides typically place mineralization in the later stages of the magmatic history of the studied districts.
Pyrite, chalcopyrite, and bornite from the base metal porphyry deposits have Re concentrations between 0.01 and 4,500 ppt and Os concentrations from 0.01 to 200 ppt. There is no obvious correlation between geographical setting or age and the concentration of Re and Os. There is also no obvious correlation between the Re content of the ore and its Pb isotope signature.