Petrographic and trace elements characterization of diagenetic carbonates associated with MVT mineralization (lower Carnian, Lombardy Basin, N Italy).

The petrographic, geochemical and trace elements characterization of the diagenetic calcite and dolomite phases associated with Mississippi Valley Type (MVT) deposits can contribute to a better understanding of the fluids responsible for the mineralization. The lower Carnian Breno Formation (BRE) and Calcare Metallifero Bergamasco (CMB) in Dossena (Lombardy Basin, Southern Alps) are two shallow-water peritidal carbonate lithostratigraphic units hosting sphalerite (ZnS), galena (PbS) and fluorite (CaF2) MVT mineralization. The identified diagenetic phases in the BRE paragenetic sequence are: 1) Cal0 intra-bioclastic packstonewackestone constituting the host rock (δ¹³C=0.9‰ V-PDB; δ¹⁸O=-6.3‰ V-PDB), 2) Dol1 fracture filling saddle dolomite cement (δ¹³C=1.1‰; δ¹⁸O=-10.9‰), 3) Cal2 fracture and void filling blocky calcite cement (δ¹³C=1.1‰; δ¹⁸O=-7.5‰), 4) CaF2, 5) Cal3 sparite cement filling intercrystalline voids between Dol1 crystals. The CMB paragenetic sequence includes: 1) Cal0 bio-intraclastic packstone-grainstone (δ¹³C=0.6‰; δ¹⁸O=-11.9‰), 2) Dol2 fracture filling saddle dolomite cement (δ¹³C=0.7‰; δ¹⁸O=-11.3‰), 3) Cal4 fracture and void filling blocky sparite cement (δ¹³C=0.6‰; δ¹⁸O=-8.3‰), 4) ZnS-PbS-CaF2, 5) Cal5 sparite cement filling intercrystalline voids between Dol2 crystals. δ¹³C values of both the BRE and CMB diagenetic phases fit the range of expected values for Middle-Upper Triassic pristine marine carbonates, while the low δ¹⁸O values suggest diagenetic alteration by high temperature fluids. Shalenormalized REE+Y(SN) concentrations of the BRE diagenetic phases reveal: 1) flat pattern for Cal0 host rock indicative of siliciclastic contamination, 2) convex-concave pattern for Dol1 with positive Y anomaly, likely from hydrothermal fluids, 3) bell-shaped pattern for Cal2 enriched in MREEs (coeval with CaF2) with positive Eu and Y and negative Ce anomalies, suggesting precipitation by low temperature (<200°C) hydrothermal fluids, 4) depleted pattern in HREEs of Cal3 (post CaF2) with positive Eu, Y and Gd anomalies possibly because of precipitation from hydrothermal fluids. REE+Y(SN) patterns of the CMB diagenetic phases are: 1) flat for Cal0 (host rock) because of siliciclastic contamination, 2) convex-concave for Dol2 with Eu and Y anomalies from hydrothermal fluids, 3) depletion in HREEs for Cal4 (coeval with ZnS-PbS-CaF2) with a weak positive Eu anomaly, possibly precipitated by hydrothermal fluids, 4) weakly depleted HREEs for Cal5 (post ZnS-PbS-CaF2) with positive Eu and Y anomalies, interpretable as precipitated by low temperature hydrothermal fluids, which have interacted with the host rock. In the CMB, Cal4 cement crosscut stylolites, allowing to set the emplacement of the mineralization in burial conditions following pressure solution. This study provides some preliminary information about the features of the fluids involved in the precipitation of carbonates associated with MVT deposits: both host rocks result contaminated by siliciclastic input, while the diagenetic phases preceding and following the MVT mineralization seem to be precipitated by low temperature hydrothermal fluids.