Geochemical Evolution at White Island, New Zealand
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Samples studied are vesicular porphyritic andesitic dacites containing phenocrysts of plagioclase, orthopyroxene, and clinopyroxene. A glassy matrix containing crystallites surrounds the phenocrysts. Both mineral and silicate melt inclusions occur in all three phenocryst phases. Inclusions of plagioclase occur in pyroxenes and inclusions of orthopyroxene and clinopyroxene occur in plagioclase. Compositions of minerals are independent of mode of occurrence - that is, plagioclase (and orthopyroxene and clinopyroxene) compositions are the same regardless of whether they occur as phenocrysts or as inclusions in another mineral. Moreover, compositions of mineral inclusions and phenocrysts show no systematic variation within individual samples or in samples representing different eruptive events, indicating that the magma chamber is chemically homogenous over the time-space scale being sampled.
Various major, trace element and volatile compositional features of economic and non-economic (or barren) porphyry copper systems were compared to the White Island data. The Al2O3/(Na2O+K2O+CaO) ratio observed in economic porphyry copper deposits is always greater than or equal to 1.3, and glass in one phase melt inclusions, as well as glass in unhomogenized (1991) inclusions from White Island equal or exceed this value. The glass in the unhomogenized 1991 melt inclusions is corundum normative, with Si/(Si+Ca+Mg+Fet)>0.91, and K/(K+Ca+Mg+Fet)>0.36, all of which are characteristic of productive systems. Melt inclusions from White Island also show a positive Eu anomaly similar to that found in productive porphyry deposits, whereas non-productive systems show a negative Eu anomaly.
Copper concentrations (170-230 ppm) in melt inclusions from White Island are sufficiently high to generate an economic porphyry copper deposit based on theoretical models. High Cl/H2O ratios (0.15) in melt inclusions furthermore indicate that copper will be efficiently partitioned from the melt into the magmatic aqueous phase. The inferred pressure in the magma chamber at depth (1 kbar) is ideal for extracting copper from the melt, and mineral phases (pyrrhotite, biotite or amphibole) which could scavenge copper before it could be partitioned into the magmatic vapor phase are absent. Concentrations of S in the melt are also low, which would prevent pyrrhotite from crystallizing.
The tectonic setting and geochemical characteristics of the magma body at White Island are similar to features observed in economic porphyry systems elsewhere. These data suggest that development of economic porphyry copper mineralization at White Island is likely.
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