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Abstract
Bridging the gap between the plutonic and volcanic realms is essential for understanding a variety of magmatic processes from caldera-forming eruptions to the formation of magmatic-hydrothermal ore deposits. Porphyry copper deposits are commonly associated with large and long-lived volcanic centers, but the temporal and dynamic link between mineralized intrusions and volcanic eruptions has remained controversial. Based on the combination of (1) high-precision zircon U-Pb geochronology and trace element geochemistry with (2) plagioclase textures, we discovered an intimate connection between an ignimbrite eruption and a nearby world-class porphyry deposit (Bajo de la Alumbrera in the late Miocene Farallon Negro Volcanic Complex of Argentina). Our results indicate that the magmatic-hydrothermal deposit and explosive volcanism were derived from a common magma reservoir that evolved over a minimum duration of 217 ± 25 k.y. before the final eruption. We show that the volcanic pile represents the inverted magma reservoir, recording systematic differences in plagioclase textures and juvenile clast content from bottom to top. This tight temporal and geochemical link suggests that deposit formation and volcanic eruption were both triggered by the same injection of a volatile-saturated primitive magma into the base of the magma chamber. A time gap of 19 ± 12 k.y. between porphyry mineralization and the onset of explosive volcanism indicates a minimum duration of magma reservoir rejuvenation that led to the explosive eruptive event. Catastrophic loss of volatiles by explosive volcanism terminated the ore-forming capacity of the upper-crustal magma chamber, as evidenced by the intrusion of a syn-eruptive barren quartz-feldspar porphyry. Our results demonstrate that porphyry copper deposits provide critical information to understand how volatiles control the fate of hydrous magmas between pluton formation and explosive volcanism.
Highlights
Large andesitic stratovolcanoes are the source of hazardous eruptions, but the dissected edifices of ancient volcanic centers host some of the most valuable economic ore deposits
Hydrothermal copper mineralization commonly occurs as porphyry copper deposits (PCDs), which form through superposition of several small magmatic intrusions and associated hydrothermal fluid pulses within the base of volcanoes (Sillitoe, 1973, 2010; Proffett, 2003)
This contradicts an increasing body of evidence that upper-crustal magma chambers in arc settings are stored at near-solidus temperatures as highly crystalline bodies that remain rheologically immobile throughout most of their lifetime before rejuvenation through the injection of more-primitive magma into the base of the reservoir (Bachmann et al, 2002; Wotzlaw et al, 2013; Cooper and Kent, 2014). These findings are consistent with recent studies suggesting that much of the ore-forming volatiles is derived from mafic magma that is injected into a cooler felsic magma chamber (Tapster et al, 2016; Buret et al, 2016) and with geochemical evidence from explosive volcanism in ancient and modern settings (Hattori and Keith, 2001; Nadeau et al, 2016)
Summary
Large andesitic stratovolcanoes are the source of hazardous eruptions, but the dissected edifices of ancient volcanic centers host some of the most valuable economic ore deposits. U-Pb dates were obtained from nine samples of the FNVC, including six pumice clasts sampled at various stratigraphic levels of the Agua de Dionisio tuff (Fig. 2); two dacitic intrusions (the P2 and P4 porphyries) from Bajo de la Alumbrera PCD, which bracket the Cu mineralization based on cross-cutting relationships
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