From Alpine-type sulfides to nonsulfides in the Gorno Zn project (Bergamo, Italy)

Nicola Mondillo,Maria Boni,Simone Zanin,Giuseppina Balassone,Fabio Granitzio,Federica Lupone,Marcello De Angelis,Michael Joachimski

doi:10.1007/s00126-019-00912-5

Abstract

Recent exploration of the Gorno Zn-Pb-Ag deposit in northern Italy identified 3.3 Mt of sulfides at 4.9% Zn, 1.3% Pb, and 27.2 g/t Ag (indicated+inferred resources), and a further mineralized nucleus of mixed sulfides-nonsulfides in the Val Vedra area, currently under evaluation. The ores are hosted in Triassic limestone and shale. Sulfides (sphalerite, Ag-bearing galena, minor pyrite, and chalcopyrite) paragenetically follow Mn-Fe-bearing saddle dolomite and sparry calcite. The mineral association, and the carbon and oxygen isotope ratios of the sparry calcite (avg. δ13C = 1.0 ± 0.6‰ V-PDB; avg. δ18O = 19.63 ± 1.25‰ V-SMOW), are in agreement with precipitation from hydrothermal fluids in a deep burial setting. Sulfide emplacement occurred before the Alpine orogeny, likely during the Early-Middle Jurassic, in analogy to other Alpine-type deposits. The nonsulfide ore formed at the expense of sulfides, and mainly consists of smithsonite, hydrozincite, hemimorphite, and cerussite. The C-O-isotope values of the early generations of Zn-carbonates are characterized by δ18O between 24.1 and 26.8‰ V-SMOW and δ13C ratios between − 3.1 and 1.7‰ V-PDB. The later generations have lower δ18O (21.9 to 23.9‰) and lower δ13C (− 6.2 to − 3.9‰). These compositions, as those measured on cerussite (δ13C = −6.3 and − 7.7‰; δ18O = 14.0 and 15.3‰), agree with the formation of the nonsulfides in a supergene environment, under climatic conditions warmer than today. The δ18O decrease from early to late generations suggests progressive involvement of meteoric water sourced from higher altitudes. These characteristics indicate that the nonsulfides formed during the exhumation of the Gorno area from Miocene to Pliocene.

Highlights

The Gorno mining district, located in the central part of the Southern Alps, a few kilometers north of Milan and Bergamo (Lombardy, Italy) (Fig. 1), is characterized by several ZnPb-(Ag) and fluorite-barite orebodies (Omenetto 1966; Vailati 1966)
The aim of the present paper is to provide a petrographic, mineralogical, geochemical, and C-O stable isotope characterization of the Zn-Pb nonsulfides occurring in the areas under exploration, with the purpose of shedding new light on their genesis, and on their relationships with the primary sulfide mineralization
The mineralogical assemblage of the Gorno Zn project documented in this study partly agrees with the data from previous investigations (e.g., Omenetto 1966; Rodeghiero and Vailati 1978)

Summary

Introduction

The Gorno mining district, located in the central part of the Southern Alps, a few kilometers north of Milan and Bergamo (Lombardy, Italy) (Fig. 1), is characterized by several ZnPb-(Ag) and fluorite-barite orebodies (Omenetto 1966; Vailati 1966). The Gorno deposit represents one of the numerous carbonate-hosted base-metal ore concentrations of the Alpine chain (Brigo et al 1977), known as “Alpine-type” Zn-Pb mineralizations (Leach et al 2003). The economically most significant deposits were located on both sides of the regional strike-slip fault zone of the “Insubric Line” (Schmid et al 1989): Raibl, Salafossa, and Gorno (Italy) in the Southern Alps, and Bleiberg (Austria) and Mežica-Topla (Slovenia) in the Northern Alps (Leach et al 2003; Schroll 2005, 2006). The host rocks of the Alpine-type Zn-Pb ores are mainly Triassic shallow water carbonates (Brigo et al 1977).

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