Petrographic, geochemical, X-ray, and stable isotope studies at the Morenci porphyry copper deposit, Greenlee County, Arizona

Abstract

Morenci is located in southeast Arizona. Copper grade hypogene mineralization averages 0.15%; one or more generations of supergene enrichment have increased mineralization to 0.96%. Intense supergene alteration has made it difficult to determine phyllosilicate paragenesis. The purpose of this study is to differentiate hypogene from supergene phyllosilicates. Forty-nine samples from five drill holes were obtained for petrographic, geochemical, mineralogical, and stable isotope data. Petrographic examination, X-ray diffraction and X-ray powder camera techniques established alteration of feldspars to kaolinite, montmorillonite, and sericite. Whole rock analyses using XRF determined cationic gains and losses that correspond with kaolinite and sericite. Stable isotopes of oxygen and hydrogen using kaolinite and sericite from 14 samples were tested to determine provenance of water. Stable isotope data show a trimodal distribution of phyllosilicates consistent with early hypogene alteration of magmatic origin, and two generations of supergene enrichment due to meteoric waters.