Abstract

A long-standing debate surrounds the source of hydrothermal fluids that form gold lodes late in the metamorphic history of orogenic belts. 40Ar/39Ar dating of hydrothermal muscovite shows that gold lodes in the northwestern Alps formed from 31.6 to 10.6 Ma. The deposits young progressively along the orogen, parallel to field gradients showing decreasing age of peak metamorphism, increasing metamorphic grade, and increasing extent of exhumation of the host rocks. These correlations suggest that the ore-bearing fluid was produced by a 20 m.y. history of prograde metamorphic devolatilization of calc-schists at depth. The age trend of mineralization reflects continuous ascent of fluid and vein formation in retrograding rocks during extreme differential uplift and denudation of the Alpine orogenic wedge. The intimate correlations between collisional orogenesis, metamorphism, and gold mineralization in the Alps suggest that similar genetic scenarios may apply to late orogenic gold lodes elsewhere.

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