ISOTOPE SYSTEMATICS IN VEIN GOLD FROM BRUSSON, VAL DAYAS (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 vei n gold from the Monte Rosa Gold District (NW Italy) are presented. Fre e gold yielded significant amounts of He and Ar, composed of a major t rapped component and a minor in-situ one. Accurate determination of th e in-situ He-4 (He-4) component is possible by degassing the sample i n a silica glass tube from which the U lost from the gold on melting c an be recovered, Assuming complete retentivity of He-4 our native gol d contains at least 98% of trapped He and Ar. Step-wise degassing the gold revealed a major noble gas release below 390 degrees C. In one ca se this yielded all measurable He-3 (44 +/- 10 x 10(-17) l) and He-Ar isotopic signatures devoid of any atmospheric contribution (He-3/He-4 approximate to 24 +/- 6 X 10(-8), Ar-40/Ar-36 approximate to 5800, He- 3/Ar-36 approximate to 0.012). Th, unexpectedly low degassing temperat ure for most of the trapped gas indirectly proves the presence of flui d inclusions, which may also have hosted the volatile U fraction. One sample, degassed in 5 steps, yielded disequilibrium step He-Ar abundan ce patterns. He-4/Ar-40 ratios range between similar to 50 (steps at 330 degrees and 390 degrees C) and similar to 13 (steps at 750 degrees and 1070 degrees C), This differential release can be interpreted on the basis of He-Ar disproportionation during boiling in a thermal syst em. At low temperatures, high-X(CO2) vapour inclusions are preferentia lly released from the gold, followed by low-X(CO2) liquid inclusions a t higher temperature. This interpretation is in full accordance with p revious evidence of fluid unmixing via retrograde boiling coeval with free gold deposition. Thus, step-wise heating may have the potential t o resolve different trapped components within a single native gold sam ple. 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 gol d. The He-Ar isotope composition of the auriferous hydrothermal fluid can be explained as a binary mixture of a dominant crustal component e nriched in He, indicated by He-4/Ar-40 of greater than or equal to 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 similar to 25 km to the southeast. The He- Ar isotope signatures and the low Ar-36 abundance suggest that no wate r with an atmospheric exchange history was ever present in the gold de positing 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 syst ematics of vein gold and possibly other native metals appear to be poo r as a dating tool but promising as a fluid-geochemical tracer, and st ep-wise degassing may resolve different trapped components, hence may reveal possible processes such as boiling which can have triggered gol d deposition.