Sulfide-associated hydrothermal dolomite and calcite reveal a shallow burial depth of Alpine-type Pb-Zn deposits

Difficulties in dating Mississippi Valley-type (MVT) mineralization and the often closely-associated regional dolomitization events have led to considerable controversy regarding the typical tectonic contexts in which they form. This paper presents the first radiometric datings for the Gorno district in Lombardy, Italy, a classic example of the Alpine subclass of MVT deposits. U-Pb dating of hydrothermal carbonate minerals pre- and post-dating the main ore-forming event shows that both regional dolomitization and base-metal mineralization must have occurred within <10 Ma of the deposition of the Carnian host rocks. This implies that the ore deposits of the Gorno district, and probably other Alpine-type deposits, formed at shallow burial, prior to the western Tethys rifting phase. Concurrent Triassic magmatism and extensional tectonics likely contributed to the high geothermal heat fluxes necessary to drive the associated large-scale hydrothermal system. These findings contradict the general paradigm for MVT genesis, i.e., a preferred occurrence in deep-burial settings associated with compressional tectonics.