Abstract White Island, New Zealand, is an active andesitic-dacitic volcano that is located near the southern end of the Tonga-Kermadec-Taupo Volcanic Arc at the convergent plate boundary where the Pacific plate is being subducted beneath the Indian-Australian plate. The plate tectonic setting, volcanic features, and petrology of White Island are characteristic of the environment associated with formation of porphyry copper deposits. White Island has been active for at least 10 ka and, as such, is an ideal location to study early magmatic processes associated with formation of porphyry copper deposits. In this study, the geochemistry of the magma chamber at White Island has been characterized through analyses of silicate melt inclusions, phenocrysts, and matrix glass contained in recent ejecta (1977–1991). Most melt inclusions in samples from the 1977, 1988, and 1989 eruptions contained only glass and occasional vapor bubbles and/or trapped solids. The 1991 sample contained daughter minerals, suggesting a different P-T history compared to the other samples. Data obtained from White Island are compared to various major, trace element, and volatile composition trends reported for both economic and noneconomic (or barren) porphyry deposits. Magmas associated with economic porphyry copper deposits are generally peraluminous with Al 2 O 3 /(Na 2 O + K 2 O + CaO) ratios greater than or equal to 1.3, and compositions of melt inclusions from White Island equal or exceed this value. Glass in unhomogenized 1991 melt inclusions is corundum normative, with Si/(Si + Ca + Mg + Fe total ) >0.91, and K/(K + Ca + Mg + Fe total ) >0.36. Melt inclusions from White Island show a positive Eu anomaly. All of these features are characteristic of productive systems. Trends in high field strength elements versus Y and in Mn versus Y are more consistent with barren intrusions than with productive plutons. Analyses of five melt inclusions from White Island indicate Cu concentrations sufficiently high (up to several hundred ppm) to generate an economic porphyry copper deposit, based on theoretical models. Moreover, high Cl/H 2 O ratios (0.15) in melt inclusions favor the efficient extraction of copper from melt by the magmatic aqueous phase. Mineral phases, such as pyrrhotite, biotite, or amphibole, which might scavenge copper from the melt before it could be partitioned into the magmatic vapor phase, are absent. Concentrations of S in the melt are low, which further inhibits pyrrhotite crystallization. The oxidation state of the magma at depth, based on the presence of SO 2 in the magmatic gas, is consistent with that predicted for porphyry copper magmas. Combined geochronologic, tectonic, petrologic, and geochemical data suggest that White Island may represent an embryonic porphyry copper system that has not yet reached the productive stages of copper mineralization.

Abstract Fluid-inclusion analyses have provided some of the most useful information for determining the physical and chemical environments of mineral formation. The purpose of this chapter is to describe those fluid-inclusion characteristics which serve to distinguish relatively near-surface, epithermal formation conditions from deeper and, potentially, higher temperature formation conditions, and to discuss several techniques and problems which are specific to fluid inclusions trapped in the epithermal environment. A detailed summary and critique of fluid-inclusion literature related to epithermal systems has not been attempted. For this information the reader is referred to the recent compilations of Buchanan (1981), Heald-Wetlaufer et al. (1983), Roedder (1984), and Hedenquist and Henley (1985). Moreover, we have not attempted to relate any particular fluid-inclusion characteristic to a specific type or stage of mineralization, because an adequate data base to do so does not presently exist. This presentation is limited to two subjects--the petrography and petrology of fluid inclusions from the epithermal environment--and is intended to provide the explorationist with a basic understanding of the criteria for recognizing and interpreting inclusions trapped in this environment. Two important topics will be discussed in detail: (1) the identification and interpretation of fluid inclusions trapped from boiling fluids, and (2) the identification of gases (mainly CO 2 ) in fluid inclusions and the effect of volatiles on calculated pressures and depths of trapping. We will not, however, discuss the important chemical consequences of boiling and dissolved volatiles, as these subjects are covered in detail in other chapters in this volume (see Henley)