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 mesothermal gold lodes in orogenic belts has proven to be a formidable hurdle. As a consequence, several key aspects of the genesis of this worldwide class of deposits remain poorly understood. This article reviews a wide spectrum of published data on the Monte Rosa Gold District, a belt of mesothermal gold lodes in the Alpine orogen. The data include Sr-, Pb- and He-Ar isotopes, 40 Ar/ 39 Ar chronology, fluid-inclusion compositions, mineralogy, and the geological framework of the deposits. It is demonstrated that simultaneous consideration of several radiogenic isotope systems is highly valuable in reconstructing the source characteristics and processes of ore deposition in open hydrothermal systems. The genetic model suggested by these data involves prograde metamorphic devolatilization of Mesozoic calcschists during mid-Tertiary continental collision and orogenic uplift. The liberated fluids scavenged gold from metabasites interlayered with the calcschists, then ascended to form auriferous quartz-carbonate-sulphide veins in rocks undergoing retrograde metamorphism. Metamorphic hydrothermal systems of this type recurred along the district over a period of at least 20 Ma, their location and timing being controlled by the progress of differential uplift of the north-western Alps. The nature of this “temporal continuum” of mineralisation contrasts with that reported 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 ages.