CAUSES OF VARIATIONS IN COPPER ISOTOPE RATIOS IN THE PORPHYRY/SKARN ORE ENVIRONMENT: SOURCES OR PROCESSES?

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‰ (d⁶⁵Cu_{NIST 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.