STABLE ISOTOPE GEOCHEMISTRY OF COPPER CARBONATES AT THE NORTHWEST EXTENSION DEPOSIT, MORENCI DISTRICT, ARIZONA: IMPLICATIONS FOR CONDITIONS OF SUPERGENE OXIDATION AND RELATED MINERALIZATION

Stable isotope analyses were performed on 39 copper carbonate samples from the Las Terrazas fault zone of the Northwest Extension copper oxide deposit, Morenci district, Arizona. The d¹⁸O values of azurite (+23.3 to +25.5 per mil, VSMOW) and malachite (+21.0 to +23.5 per mil, VSMOW) indicate supergene formation from meteoric water. Oxygen isotope thermometry suggests oxidation occurred at temperatures of 20-34°C, with higher temperatures reflecting heat production by exothermic oxidation of sulfides. For malachite, a relation between higher temperatures and higher local oxide copper grade suggest in-situ formation from oxidizing copper sulfides; whereas for azurite the lack of such a relationship and field evidence suggest formation from copper transported along structures. Azurite d¹³C values range from –4.2 to –10.7 per mil and exhibit isotopic enrichment with increasing height above the supergene enrichment blanket (paleo-water table). This suggests that azurite received significant amounts of organic carbon resulting from the oxidation of bacteria known to be instrumental in supergene sulfide enrichment processes adjoining the water table, with shallow mixing with dissolved carbonates producing the observed enrichment in ¹³C with height above the water table. Malachite d¹³C values range from –9.6 to –11.1 per mil, and suggest a carbon source from oxidation of surface vegetation and/or dissolution of primary or secondary carbonate by a rising water table. Evidence indicate that the Northwest Extension deposit formed when a shallow, early-stage, copper-sulfide enrichment blanket was stranded above the water table. Relatively low temperature oxidation of these sulfides produced copper-rich solutions that were transported along major structures, and azurite formed when the fluids intersected pockets of high PCO₂ possibly produced by seasonal oxidation of supergene-related bacteria above the water table. Over time, oxidation and related mineralization reduced permeability along structures, and bacterial populations waned as deeper sulfide enrichment processes slowed. Lower PCO₂ and reduced transport potential produced localized oxidation centered on high-grade zones of the early sulfide blanket remaining in the oxidation zone, with malachite as a dominant phase.