ISOTOPE SYSTEMATICS IN VEIN GOLD FROM BRUSSON, VAL DAYAS (NW ITALY) .1. PB PB EVIDENCE FOR A PIEMONTE METAOPHIOLITE AU SOURCE/

The Monte Rosa Gold District (NW Italian Alps) comprises hundreds of l ate Alpine, epigenetic, gold-bearing quartz veins. We report a lead is otope study on an important one of these veins, Fenilia (Brusson, Val d'Ayas), which reveals a complex isotope geochemistry of petrographica lly cogenetic vein minerals. Petrographic observations allow to distin guish a quartz-dominated main-stage and a quartz-poor final-stage mine ral assemblage. Most of the free gold crystallized in the beginning of the final stage, whereas sulphides were deposited during its waning. Three distinct lead isotope fields result for the paragenetic sequence of vein minerals, The variable lead isotope signature (15.517<Pb-207/ Pb-204<15.623) trapped in final-stage free gold, and minor quartz can be interpreted as a variable mixture between a primitive MORB-type and a radiogenic Pb component. It differs distinctly from the radiogenic lead signature of the main-stage mineral assemblage (Pb-207/Pb-204>15. 706), as well as from the uniform radiogenic lead (15.649<207Pb/Pb-204 <15.691) characteristic of the waning final stage. We favour a model w here the deeply circulating final-stage fluid, the ultimate source of which remains obscure, acquired lead and thus possibly its gold in Pie monte metaophiolitic rocks beneath the Monte Rosa nappe. During ascent , the primitive gold-bearing fluid was variably contaminated by an evo lved crustal lead (15.706<Pb-207/Pb-204<15.743) compatible with deriva tion from Monte Rosa nappe rocks. During deposition of final-stage min erals the Pb isotope composition of the fluid changed to a constant si gnature becoming again dominated by a radiogenic lead source. Thus wit hin an assemblage defined as cogenetic on petrographic grounds the Pb isotopic signature is not necessarily uniform and source modelling bas ed on lead isotopes of associated minerals may be inappropriate. It re mains open at present, whether the inferred Piemonte metaophiolite gol d source is confined to the Fenilia vein in Val d'Ayas only, or whethe r it might be more widespread in the Monte Rosa Gold District.