Melt Inclusion Study of the Embryonic Porphyry Copper System at White Island, New Zealand
M. H. Rapien, R. J. Bodnar, S. F. Simmons, C. S. Szabo, C. P. Wood, S. R. Sutton, 2005. "Melt Inclusion Study of the Embryonic Porphyry Copper System at White Island, New Zealand", Volcanic, Geothermal, and Ore-Forming Fluids: Rulers and Witnesses of Processes within the Earth, Stuart F. Simmons, Ian Graham
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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 Al2O3/(Na2O + K2O + 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 + Fetotal) >0.91, and K/(K + Ca + Mg + Fetotal) >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/H2O 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 SO2 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.
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To be honest, I am surprised to find myself addressing a meeting of the Society of Economic Geologists—being neither a geologist nor economic. And looking at the title of my paper, I wouldn’t be offended if people told me that I may be going to talk about something I know nothing about. After listening to some of this afternoon’s talks, however, it is clear to me that I wouldn’t be the only one. With this I don’t mean that the previous speakers were inept but that there are still quite a few basic problems which have to be solved before we may safely say, we know what’s going on in hydrothermal systems. And by basic, I mean basic.
The title of my talk links two processes: magma degassing, something I have been studying now, from the gases’ point of view, for more than 20 years, and mineral deposition, something I had my nose rubbed into by living in close vicinity to some of the biggest gold freaks like Kevin Brown, Jeff Hedenquist, Dick Henley, and Terry Seward. I myself had, quite early on, declared gold a four letter word and had vowed never to use it in any of my papers, together with other uncouthities, such as zinc or lead. Now that the above have dispersed, each into his corner of the globe, I think myself free to reconsider my earlier pledge.