PATTERNS IN THE COPPER ISOTOPIC COMPOSITION OF MINERALS IN PORPHYRY COPPER DEPOSITS IN SOUTHWESTERN UNITED STATES

Cu isotope patterns are seen in the American Southwest porphyry deposits. We examined approximately forty drill core and hand specimen samples from each of the three deposits (Silver Bell Mine, Ray Mine and Morenci Mine). The leach cap hematite isotopic values range from -12.29‰ to 6.80‰. The enrichment blanket chalcocite vary from -0.45‰ to 8.12‰. The chalcopyrite form tight clusters from -4.72‰ to 2.45‰. The leach cap is isotopically lighter than the enrichment blanket below. The averages show three distinct isotopic reservoirs that exist. Furthermore, patterns can be seen with grade of Cu, depth and horizontal map locations. Within the leach cap, isotopic values can be modeled by using the Rayleigh Distillation Model. Combined with the grades in the ores, the model can show Cu isotope fractionation. Each of the three deposits has a distinct signature fractionation that correlates with the amount of Cu at each location. This idea shows that the fractionation of Cu could be used as an exploration tool in supergene environments.