SILICATE MELT INCLUSIONS IN PORPHYRY COPPER DEPOSITS: IDENTIFICATION AND HOMOGENIZATION BEHAVIOR

Abstract

Silicate melt inclusions are ubiquitous in phenocrysts in porphyry copper deposits. Compared to melt inclusions in other igneous environments, those in porphyry copper deposits are more difficult to recognize and analyze, for a variety of reasons. The inclusions are usually partially to completely crystallized or altered by hydrothermal fluids, resulting in a dark, granular appearance. In this study, a protocol for identifying and homogenizing crystallized melt-inclusions from porphyry copper deposits is described. The protocol has been used to obtain preliminary data from melt inclusions in the Red Mountain, Arizona, and the Tyrone, New Mexico, porphyry copper deposits. Many melt inclusions in syn- and post-mineralization samples show evidence of alteration by magmatic-hydrothermal fluids that produced alteration and mineralization in the plutons, and many inclusions trapped crystals or fluid (or both) along with the melt. Elevated concentrations of metal in pre-mineralization (H2O-undersaturated) melt inclusions, and lower concentrations of metal in later syn- and post-mineralization (H2O-saturated) melt inclusions are consistent with models that invoke quantitative transfer of copper and other metals from the melt into the magmatic aqueous phase when the melt reaches H2O saturation. Results from Red Mountain and Tyrone are compared to melt inclusions from the White Island, New Zealand, active volcano.