Abstract
Strong differences exist between the helium isotopic compositions of mantle-derived magmas1–8, axial vent hotspring fluids9–14, subduction-related magmas15–20 and many associated hot springs21–24, and the atmospheric and crustal values (Fig. 1). Here we report helium isotope ratios for two late Tertiary fossil hydrothermal systems responsible for precious-metal, base-metal and tungsten mineralization, and compare them to those of circum-Pacific hot-spring emanations. The 3He/4He ratios of inclusion fluids hosted by gangue and sulphide minerals from Casapalca and Pasto Bueno, Peru, all lie between two and three times the atmospheric value, indicating the presence of a dilute component of mantle helium consistent with previously reported stable isotope evidence for the presence of magmatic fluids during mineralization25–27. Although post-trapping modification of the helium isotope composition is possible, we suggest that these apparently low values represent the composition of helium inherited from contributing magmas. As a tracer of magmatic fluids in hydrothermal systems, helium isotopes are decoupled from the factors that govern other stable isotope signatures.
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