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Petrographic and trace element characterisation of diagenetic carbonates associated with Pb-Zn sulfide and fluorite mineralisation (Upper Triassic; Lombardy Basin; Northern Italy)
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DOI：10.1180/mgm.2025.10124.png)
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Abstract
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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; δ18O data; REEYN 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. REEYN 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.
Keywords:
fracture-filling carbonates
saddle dolomite
Pb-Zn mineralisation
fluorite
Southern Alps
Triassic
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