Isotope systematics in vein gold from Brusson, Val dʹAyas (NW Italy) 3. (U + Th)He and KAr in native Au and its fluid inclusions

(U + Th)He and KAr 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★) 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★. our native gold contains at least 98% of trapped He and Ar. ise degassing the gold revealed a major noble gas release below 390°C. In one case this yielded all measurable 3He (44 ± 10 × 10−17 l) and HeAr isotopic signatures devoid of any atmospheric contribution 3He4He ≈ 24 ± 6 × 10−8, 40Ar36Ar ≈ 5800, 3He36Ar ≈ 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. mple,, degassed in 5 steps, yielded disequilibrium step HeAr abundance patterns. 4He40Ar★ 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-XCO2 vapour inclusions are preferentially eased from the gold, followed by low-XCO2 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-XCO2 vapour and low-XCO2 liquid preclude (U + Th)He isochron and KAr dating of our gold. 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 4He40Ar★ 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. 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.