Petrographic and trace element characterisation of diagenetic carbonates associated with Pb-Zn sulfide and fluorite mineralisation (Upper Triassic, Lombardy Basin, Northern Italy)

Abstract The petrographic and geochemical characterisation of the diagenetic calcite and dolomite associated with carbonate-hosted ore deposits can contribute to a better understanding of mineralisation mechanisms. The lower Carnian Breno Formation and Calcare Metallifero Bergamasco in the Dossena area (Gorno mining district, Lombardy Basin, Southern Alps) are two shallow marine carbonate lithostratigraphic units hosting Alpine-type Pb-Zn-F-Ba ores. Several interpretations have been proposed for the genesis of mineralisation in these Triassic carbonates: syngenetic from submarine exhalations linked to the Ladinian–Carnian volcanism; or Alpine-type syngenetic to epigenetic in burial settings by hydrothermal fluids, during Early–Middle Jurassic rifting. Recent studies propose that ores formed in shallow burial settings (tens to few hundreds of metres), precipitated from hydrothermal fluids related to Triassic magmatism, prior to the Early Jurassic rifting. Petrographic analysis, δ 18 O data, REEY N normalised patterns, calculated element ratios and lanthanum, cerium and europium normalised anomalies suggest precipitation of the saddle dolomite and calcite cements, filling-fractures preceding and post-dating the mineralisation, from low temperature (<200°C) hydrothermal fluids. The geochemistry of these fluids was influenced by seawater and burial basinal brines. REEY N patterns and Y/Ho ratios suggest the interaction of these fluids with Permian and Ladinian–Carnian siliciclastic and volcaniclastic deposits and/or with the Variscan metamorphic basement, which are possible sources for metals. The identified paragenesis supports a hydrothermal epigenetic origin for the mineralisation, at burial depths not shallower than 300 m. In fact, pre-mineralisation fractures filled by saddle dolomite and calcite cements cross-cut voids occluded by burial sparite cements and stylolites in an already lithified host rock, affected by burial compaction and pressure solution. These conclusions may contribute to the understanding of other global occurrences of carbonate-hosted Pb-Zn mineralisation, providing new insights into complex dolomite and calcite paragenesis preceding and following the ore formation.