Abstract

Uranium-lead ages and trace element compositions of zircon from a series of shallow porphyry intrusions document the temporal, chemical, and thermal magmatic evolution of magmatic-hydrothermal porphyry Cu (Mo-Au) ores in the El Salvador district, Chile. Zircons (n = 240) from 15 Eocene age diorite, granodiorite, and granite porphyry intrusions were analyzed by SHRIMP-RG ion microprobe. The weighted means of 207Pb-corrected 206Pb/238U zircon ages span 3 m.y. from about 44 to 41 Ma, with peak magmatic flux at 44 to 43 Ma. The granodiorite porphyries at the Turquoise Gulch copper deposit record waning stages of magmatism at 42.5 to 42.0 Ma and were followed by postmineral latite dikes at about 41.6 Ma. Porphyry copper ores formed contemporaneously with porphyry intrusion centers that progressed temporally from north to south, from the small deposits at Cerro Pelado (~44.2 Ma), Old Camp (~43.6 Ma), and at M Gulch-Copper Hill (~43.5–43.1 Ma) to the main ore deposit at Turquoise Gulch (~42 Ma). The Eocene porphyry intrusions contain a few Mesozoic (n = 9) inherited zircons and numerous (n ≥19) antecrystic zircons about 1 to 2 m.y. older than the host intrusion that provide evidence of extensive Eocene magmatic recycling.

The Ti-in-zircon geothermometer provides estimates of 890° to 620°C for zircon crystallization and records both core to rim cooling and locally high-temperature rim overgrowths. Most zircon in ore-related K, L, and R porphyries yields near-solidus temperatures of 750° to 650°C and crystallized from compositionally diverse granodiorite porphyries that are a product of crystal fractionation of hornblende, apatite, and titanite with lesser crustal contamination and mixing with high-temperature deep-sourced mafic magma. During a 3-m.y. period, porphyry intrusions tapped an evolving granodioritic magma chamber that was periodically heated, locally remelted, and mixed with mafic magma during recharge events but cooled between recharge events to evolve ore fluids.

Europium anomalies (chondrite-normalized EuN/EuN) in zircons become more pronounced with increased Hf content and cooling but display two distinct evolutionary paths: EuN/EuN of early quartz porphyry evolves from 0.8 to 0.3, whereas the late synmineralization porphyries evolve from 0.8 to 0.65. The EuN/EuN ratio of zircon reflects the Eu3+/Eu2+ ratio of the melt, and therefore the granodiorite porphyries at Turquoise Gulch were the most strongly oxidized of the El Salvador magmas. The strongly oxidized trend porphyry magmas at Turquoise Gulch are apparently directly linked to magmatic degassing at ~700°C to produce large amounts of ore-forming copper, sulfur, and chlorine-enriched magmatic-hydrothermal aqueous fluids.

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