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Book Chapter

Melt Inclusion Study of the Embryonic Porphyry Copper System at White Island, New Zealand

By
M. H. Rapien
M. H. Rapien
Department of Geological Sciences, Virginia Polytechnic Institute and Stage University, Blacksburg, Virginia 24061-0420
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R. J. Bodnar
R. J. Bodnar
Department of Geological Sciences, Virginia Polytechnic Institute and Stage University, Blacksburg, Virginia 24061-0420
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S. F. Simmons
S. F. Simmons
Geothermal Institute, University of Auckland, Private Bag 92019, Auckland, New Zealand
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C. S. Szabo
C. S. Szabo
Department of Petrology and Geochemistry, Institute of Geology, Eotvos University, Budapest, Hungary H-1088
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C. P. Wood
C. P. Wood
Institute of Geological and Nuclear Research, Wairakei Research Center, Private Bag 2000, Taupo, New Zealand
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S. R. Sutton
S. R. Sutton
Department of Geophysical Sciences and Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois 60637
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Published:
January 01, 2005

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 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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Contents

Special Publications of the Society of Economic Geologists

Volcanic, Geothermal, and Ore-Forming Fluids: Rulers and Witnesses of Processes within the Earth

Stuart F. Simmons
Stuart F. Simmons
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Ian Graham
Ian Graham
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Society of Economic Geologists
Volume
10
ISBN electronic:
9781629490342
Publication date:
January 01, 2005

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