Mesothermal gold lodes in the north-western Alps: A review of genetic constraints from radiogenic isotopes

Identifying the source of the hydrothermal fluid responsible for mesotherma l gold lodes in orogenic belts has proven to be a formidable hurdle. As a c onsequence, several key aspects of the genesis of this worldwide class of d eposits remain poorly understood. This article reviews a wide spectrum of p ublished data on the Monte Rosa Gold District, a belt of mesothermal gold l odes in the Alpine orogen. The data include Sr-, Pb- and He-Ar isotopes, Ar -40/Ar-39 chronology, fluid-inclusion compositions, mineralogy, and the geo logical framework of the deposits. It is demonstrated that simultaneous con sideration of several radiogenic isotope systems is highly valuable in reco nstructing the source characteristics and processes of ore deposition in op en hydrothermal systems. The genetic model suggested by these data involves prograde metamorphic dev olatilization of Mesozoic calcschists during mid-Tertiary continental colli sion and orogenic uplift. The liberated fluids scavenged gold from metabasi tes interlayered with the calcschists, then ascended to form auriferous qua rtz-carbonate-sulphide veins in rocks undergoing retrograde metamorphism. M etamorphic hydrothermal systems of this type recurred along the district ov er a period of at least 20 Ma, their location and timing being controlled b y the progress of differential uplift of the north-western Alps. The nature of this "temporal continuum" of mineralisation contrasts with that reporte d for Archean gold-lode deposits. As well as clarifying aspects of gold-lode genesis, the isotopic approaches discussed have great potential to constrain scenarios of large-scale fluid flow - with or without mineralising potential - in orogenic belts of all a ges.