Using platinum group elements to identify sulfide saturation in a porphyry Cu system: the El Abra porphyry Cu deposit, northern Chile

Cocker, Helen A., Valente, Dianne L., Park, Jung-Woo, and Campbell, Ian H. (2016) Using platinum group elements to identify sulfide saturation in a porphyry Cu system: the El Abra porphyry Cu deposit, northern Chile. Journal of Petrology, 56 (12). pp. 2491-2514.

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Abstract

Geochronological and geochemical studies, including platinum group element (PGE) analyses, were undertaken on samples from the El Abra–Pajonal igneous complex, northern Chile, to investigate the magmatic evolution of the suite. Special attention was paid to identifying the onset of sulfide saturation and to documenting how it influenced the geochemistry of the chalcophile elements and the formation of the El Abra porphyry Cu deposit. The PGE have extreme sulfide melt–silicate melt partition coefficients, making them sensitive indicators of the timing of sulfide saturation in an evolving magmatic system. In arc-related intermediate to felsic magmatic systems, which have the potential to produce porphyry deposits, the timing and extent of sulfide saturation relative to ore-fluid saturation may control the capacity of these systems to produce economic mineralization and, if they do, whether the deposits are Cu-only or Cu–Au. This study incorporates the first comprehensive analysis of PGE in a felsic magmatic suite associated with an economic porphyry system. The suite comprises a series of quartz monzodiorite to granite intrusions with U–Pb zircon ages between 43 and 35 Ma. Their petrography and major element chemistry, including increasing Sr/Y ratios and rare earth element patterns, suggest that crystal fractionation and crustal assimilation were the key magmatic processes governing the evolution of the El Abra–Pajonal suite. Plagioclase fractionation dominated the oldest intrusions, and their associated granites and aplites. Following the injection of a more primitive, wetter, mafic magma at 41–40 Ma, plagioclase fractionation became suppressed and amphibole became the dominant fractionating phase, leading to the formation of the El Abra porphyry intrusion and Cu deposit. Abundances of Pt and Pd in felsic rocks from the El Abra–Pajonal intrusive complex drop rapidly in samples with MgO values below 2·5 wt %, following sulfide saturation of the magmas, which occurred slightly before ore-fluid saturation and formation of the Cu deposit. Modeling suggests that the amount of sulfide formed was very small, enough to strip the PGE and Au from the magma but not Cu, because of the lower partition coefficient of Cu relative to the precious metals, which explains why the mineralization at El Abra is a Cu-only porphyry deposit, rather than a Cu–Au deposit.

Item ID: 64318
Item Type: Article (Research - C1)
ISSN: 1460-2415
Keywords: El Abra; platinum group elements; porphyry copper deposits; sulfide saturation; U–Pbdating
Copyright Information: © The Author 2016. Published by Oxford University Press. All rights reserved.
Date Deposited: 08 Sep 2020 05:01
FoR Codes: 04 EARTH SCIENCES > 0402 Geochemistry > 040299 Geochemistry not elsewhere classified @ 50%
04 EARTH SCIENCES > 0403 Geology > 040304 Igneous and Metamorphic Petrology @ 15%
04 EARTH SCIENCES > 0403 Geology > 040303 Geochronology @ 35%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970104 Expanding Knowledge in the Earth Sciences @ 80%
84 MINERAL RESOURCES (excl. Energy Resources) > 8499 Other Mineral Resources (excl. Energy Resources) > 849999 Mineral Resources (excl. Energy Resources) not elsewhere classified @ 20%
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