2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 6
Presentation Time: 9:15 AM


MAHER, Kierran C.1, WOOD, Scott2 and LARSON, Peter1, (1)Department of Geology, Washington State University, Pullman, WA 99164, (2)Geological Sciences, Univ of Idaho, P.O. Box 443022, Moscow, ID 83844-3022, kmaher@wsu.edu

Copper isotope data suggest that isotopic fractionation mechanisms operating in high temperature hydrothermal environments can produce significant ranges in d65Cu values. Mineralization from copper porphyry and related skarn deposits tends to have larger d65Cu ranges in primary, hypogene mineralization compared to ranges from other deposits types. To substantiate the significance of fluid-mineral copper isotope fractionation at high temperature, a preliminary fluid-chalcopyrite copper isotope fractionation has been experimentally determined at 300°C. Under the experimental conditions, this fractionation may be up to 0.7‰ (d65CuNIST SRM 976), negative, and appears to vary depending on the fluid salinity. The direction of fractionation suggests that during a single mineralizing event the continued step-wise equilibrium precipitation of chalcopyrite from an ore forming fluid could produce a large range in d65Cu, and chalcopyrite would become isotopically lighter as precipitation proceeded. Massive mineralization produces more limited isotopic ranges with less extreme d65Cu values. Modeling isotopically zoned copper mineralization from the porphyry related skarn at Coroccohuayco, Perú, using this experimentally determined fluid-mineral fractionation and the geologic context of the mineralization constrains the initial d65Cu value of the ore forming fluid to 0.2-0.3‰ d65Cu. However, this value may represent an already isotopically evolved fluid produced from various hydrothermal processes, such as leaching and/or remobilization of copper from metal sources. Other preliminary copper isotope data from high-sulfidation veins superimposed on porphyry systems suggest that some of the copper in these veins may indeed be derived from earlier porphyry ores. Isotopic fractionation of copper during the remobilization process appears to be demonstrated in these late veins, where bornite is isotopically heavy, relative to primary precipitated hypogene bornite.