Life-times and scales of Cu-Au-mineralizing magmatic-hydrothermal processes: Farallón Negro (Argentina)

AUTHORS: Christoph A. Heinrich, Dimitri Meier, Michaela Erni, Albrecht von Quadt, Florencia Márquez - Zavalía

ABSTRACT: The Farallón Negro Volcanic Complex (Argentina) hosts one of the best-studied porphyry copper – gold deposits, Bajo de la Alumbrera. Three million years of extrusive and subvolcanic intrusive activity prepared the ground for a climactic event of porphyry Cu-Au mineralization, followed later by epithermal base-metal – carbonate – gold ore deposition in major veins cutting through the porphyry deposit and the central neck of the volcano. High-precision single-zircon age dating indicates that multiple pulses of porphyry intrusion and Cu-Au veining occurred within less than ̴0.1 m.y., based on the youngest grains in populations spanning ̴1 million years of zircon crystallization. This study shows that critical interpretation of high-precision zircon dates can resolve the geologically instantaneous emplacement and solidification of a porphyry from the much longer lifetime of the subjacent, crustal-scale magmatic system. Discovery of high-temperature intermediate-density vapor inclusions of low salinity and the new geochonology confirm earlier interpretations that porphyry-Cu-Au ore formation was driven by different parts of an inferred, composite mafic – felsic magma chamber. The combined data imply that the dacite porphyries and associated brines were derived from the upper part of a layered magma chamber, while the low-salinity Cu-mineralizing ore fluid was sourced in the deeper, mafic or mixed parts of the subjacent magma reservoir. The last magmatic fluids, of similar low salinity, probably cooled to form the epithermal veins by mixing with meteoric water.

KEYWORDS: Porphyry copper, epithermal gold, magmatichydrothermal, fluid inclusions, zircon, geochronology, Bajo de la Alumbrera, Farallón Negro, Argentina

EAN: SGA_2011_A001
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