Genesis of orogenic gold veins in the NW Alps

Research project P 14481 Genesis of Orogenic Gold Veins in the NW Alps Larryn W. DIAMOND 26.6.2000 Gold lodes in orogenic belts are widely acknowledged to be of epigenetic, hydrothermal origin, but the source of the ore-bearing fluids is vigorously debated. Our previous geochemical and radiogenic-isotope studies on gold veins in the NW Alps have suggested that the ore-bearing fluids emanated from specific units of metasedimentary rocks (calcschists) in the NW-Alpine nappe stack, in response to mid- to late-Tertiary continental collision and attendant metamorphism. Although the published evidence is qualitatively consistent with the proposed model, quantitative proof is lacking. In fact, the observations could be alternatively interpreted to indicate contamination of the ore-bearing fluids by the calcschists, rather than fluid generation within the calcschists. If this alternative is true, then the fluid source must lie elsewhere. This project alms to discriminate between these interpretations by (1) determining the quantities and ,chemical- and stable-isotopic (H, C, S) compositions of metamorphic fluids that emanated from the calcschists as a function of metamorphic time; and by (2) acquiring additional geochemical and stable-isotopic data on the gold deposits of NW Alps, in order to calculate the fluxes of fluid involved in their formation. Comparison of the results should provide a rigorous test of the proposed model for the gold veins, and thereby yield insight into the genesis of this world-wide class of ore deposits. The methodology will include field sampling, petrography, analyses of fluid inclusions, bulk-rock compositions, and stable isotopes, plus thermodynamic modelling.

Gold deposits of the "orogenic" type are found in mountain belts throughout the world, including several small deposits in the Western Alps. It is well established that the gold precipitated from hot, gas-rich aqueous solutions that ascended through faults and fractures during uplift of the Alps between 30 and 10 Million years ago. The source of the aqueous solutions is not known, and this hinders development of conceptual models to guide exploration for undiscovered gold deposits, both in the Alps and elsewhere around the world. The purpose of this project is to test two of the most popular hypotheses of the origin of the solutions: on the one hand, fluid production from crystallizing magmas, and on the other, fluid production from sediments undergoing high- temperature metamorphism. The study involved sampling of mineral veins related to alpine granites (the magmatic hypothesis) and other veins within calcareous mica-schists (the metamorphic hypothesis) in the Western Alps. Analysis of droplets of the gold- bearing solution trapped in these veins, and chemical and isotopic analysis of the various host rocks, suggests that both hypotheses are permissible for the Alpine gold deposits. Our ongoing computer modelling, based on the data collected in this project, is aimed at quantifying the amount of fluid liberated by Alpine magmatism and metamorphism. The results of these calculations may provide the missing criterion we are searching for to discriminate between the hypotheses.