COPPER MINERALIZATION AND ISOTOPE FRACTIONATION IN PORPHYRY COPPER DEPOSITS

In order to observe the distribution of Cu isotope ratios in a supergene environment, we collected forty samples (metal oxides and sulfides) from the porphyry copper deposit at Morenci, Arizona for isotope analysis. The different mineral phases of Cu were characterized using a Scanning Electron Microscope with EDS analysis at Juniata College. There are patterns in the mineral phases relating to depth in the supergene environment. In the leach cap there are more oxides, hematite, jarosite, and goethite which are the products of leached and oxidized chalcopyrite, pyrite, and chalcocite. In the enrichment blanket there are increased amounts of chalcocite, covellite, pyrite, and chalcopyrite. The top of the blanket tends to have a greater Cu:S ratio, meaning more chalcocite is present at top and more covellite is present at bottom of the blanket. In the hypogene there is pyrite, chalcopyrite, bornite, magnetite, with trace amount of chalcocite and covellite.

Measured Cu isotopes ratios from the Metcalf pit at Morenci were attained from a multicollector ICP-MS at Washington State University. The data from this pit shows that there is fractionation in the Cu isotope ratios (reported as d65Cu‰= (65Cu/63Cusample)/(65Cu/63CuNIST std 976)) -1]*1000). In chalcocite from the leach cap the d65Cu was -1.05‰, and the average in the enrichment blanket was 2.03‰. Similar patterns were observed in hematite, -0.65‰ in the leach cap and 0.86‰ in enrichment, and Cu oxides averaging 1.34‰ in the leach cap and 3.57‰ in enrichment. Generally the leach cap displayed isotopically light signatures, the enrichment isotopically heavy signatures, and the hypogene signatures around 0‰. The average signature for covellite in the enrichment blanket was 0.47‰, and tended to be lighter than chalcocite at 2.03‰ and Cu oxides at 3.57‰ in the blanket. These relationships display the preferred mobility of heavy Cu due to leaching into the enrichment blanket. Therefore, ratios of Cu isotopes may indicate the amount of leaching that has occurred and also the resulting amount of Cu enrichment that exists beneath.