Isotope systematics in vein gold from Brusson, Val d'Ayas (NW Italy): 2. (U + Th) /He and K/Ar in native Au and its fluid inclusions

(U + Th)/He and K/Ar isotope systematics in Oligocene hydrothermal vein gold from the Monte Rosa Gold District (NW Italy) are presented. Free gold yielded significant amounts of He and Ar, composed of a major trapped component and a minor in-situ one. Accurate determination of the in-situ ⁴He (⁴He(Black star)) component is possible by degassing the sample in a silica glass tube from which the U lost from the gold on melting can be recovered. Assuming complete retentivity of ⁴He(Black star), our native gold contains at least 98% of trapped He and Ar. Step-wise degassing the gold revealed a major noble gas release below 390°C. In one case this yielded all measurable ³He (44 ± 10 × 10^-17 1) and He-Ar isotopic signatures devoid of any atmospheric contribution (³He/⁴He ≈ 24 ± 6 × 10^-8, ⁴⁰Ar/³⁶Ar ≈ 5800. ³He/³⁶Ar ≈ 0.012). The unexpectedly low degassing temperature for most of the trapped gas indirectly proves the presence of fluid inclusions, which may also have hosted the volatile U fraction. One sample, degassed in 5 steps, yielded disequilibrium step He-Ar abundance patterns. ⁴He/⁴⁰Ar(Black star) ratios range between ∼ 50 (steps at 330° and 390°C) and ∼ 13 (steps at 750° and 1070°C). This differential release can be interpreted on the basis of He-Ar disproportionation during boiling in a thermal system. At low temperatures, high-X_CO2 vapour inclusions are preferentially released from the gold, followed by low-X_CO2 liquid inclusions at higher temperature. This interpretation is in full accordance with previous evidence of fluid unmixing via retrograde boiling coeval with free gold deposition. Thus, step-wise heating may have the potential to resolve different trapped components within a single native gold sample. High and variable amounts of trapped high-X_CO2 vapour and low-X_CO2 liquid preclude (U + Th)/He isochron and K/Ar dating of our gold. The He-Ar isotope composition of the auriferous hydrothermal fluid can be explained as a binary mixture of a dominant crustal component enriched in He, indicated by ⁴He/⁴⁰Ar(Black star) of ≥ 50. and a subordinate mantle component. The latter may originate either from meta-ophiolites at depth, or, less likely, from Oligocene plutons such as are known to exist ∼ 25 km to the southeast. The He-Ar isotope signatures and the low ³⁶Ar abundance suggest that no water with an atmospheric exchange history was ever present in the gold depositing hydrothermal fluid. Therefore, the metamorphic model of vein genesis with metamorphic dehydration of large rock volumes at depth as the fluid source appears to be most plausible. Noble gas isotope systematics of vein gold and possibly other native metals appear to be poor as a dating tool but promising as a fluid-geochemical tracer, and step-wise degassing may resolve different trapped components, hence may reveal possible processes such as boiling which can have triggered gold deposition.