The volatile record of volcanic apatite and its implications for the formation of porphyry copper deposits

Author:

Stonadge George1ORCID,Miles Andrew1,Smith Daniel1,Large Simon2,Knott Thomas1

Affiliation:

1. 1School of Geography Geology and the Environment, University of Leicester, University Road, Leicester LE1 7RH, UK

2. 2Department of Earth Sciences, Natural History Museum, Cromwell Road, London SW7 5BD, UK

Abstract

Abstract Volatile saturation influences the physicochemical behavior of magmas and is essential for the sequestration of metals in porphyry copper deposits. Tracking the evolution of volatile components (F, Cl, H2O, S) in arc systems is complicated by their mobility and tendency to rapidly re-equilibrate with late-stage melts. We demonstrate that accurate measurements of volatile concentrations in apatite offer a reliable method for identifying the occurrence of volatile saturation. Fluorine, Cl, S, and calculated OH concentrations in apatite obtained by scanning electron microscope–energy-dispersive X-ray spectroscopy and electron microprobe analysis were used to compare two end-member volcanic systems in the West Luzon Arc (Philippines): Pinatubo (a fluid-saturated analogue for porphyry copper deposits) and Taal (a barren and fluid-undersaturated comparator). Apatites from Pinatubo are S-rich (0.04–0.64 wt%) and show a progressive decrease in XCl/XOH (0.6–0.25) and an increase in XF/XCl (1.5–8) and XF/XOH (0.75–1.2) during crystallization. Modeling indicates that these changes result from efficient partitioning of Cl into a continuously saturated H2O-rich fluid, while high regions of S in apatite reflect episodic flushing by a separate S-rich flux. Little S is evident in apatites from Taal (<300 ppm), which show increasing XCl/XOH and XF/XOH together with constant XF/XCl during crystallization. This cannot be explained using an H2O-saturated model, and instead reflects fluid-undersaturated crystallization and cooling in a reduced and/or S-depleted system. Measured volatiles in apatite therefore effectively discriminate volatile-saturated and undersaturated magmatic systems, providing an important ‘fertility’ filter for porphyry exploration.

Publisher

Geological Society of America

Subject

Geology

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