A metallogenic study of the carbonate-hosted Piani Resinelli and Gorno sulphide deposits and of the basement-hosted Primaluna deposit

The Central Southern Alps (CSA) are well-known for the diffuse presence of a wide range of ore deposits hosted both in the basement and in the overlying stratigraphic succession. However the knowledge of the genesis of these deposits as well as their meaning in the context of the geodynamical evolution from the late Variscan orogenic stages and rift-related thinning is still limited and incomplete, and often the existing literature is dated. The main aim of the present study is to provide a characterization of the ores and the ore-forming processes for the deposits involved in the study area (Piani Resinelli, Primaluna, and Vedra Valley deposits), and to investigate if the basement-hosted Primaluna deposit could be related to the same metallogenic event which led to the genesis of the Alpine-type deposits hosted in the Triassic stratigraphic succession of the Lombardian Basin (e.g., Piani Resinelli). Moreover, a detailed structural-geochemical study has also been performed in the Vedra Valley deposit (Gorno district) to investigate the possible presence of Triassic syndepositional faults spatially associated with the mineralizations which may have acted as conduits for the mineralizing fluids during Upper Triassic. To achieve these results both Piani Resinelli and Primaluna deposits have been dated by U-Pb radiometric techniques to constrain the age of their genesis. Stratigraphic, petrographic, and geochemical analyses provided constraints for the age of the mineralization, burial setting, characteristics, and nature of the ore-forming fluids, and finally ore-forming conditions. The resulting metallogenic model for the Piani Resinelli deposit involves the presence of high salinity, modified seawater leaching metals from the Southalpine basement and/or the overlying volcanic-sedimentary Permian cover and then flowing upward during seismic events along extensional and strike-slip faults. The precipitation occurred mainly below a thick tuff layer in the lower part of the Breno Fm., which acted as a permeability barrier for the upflowing fluids. The most likely precipitation mechanism is the mixing between the hydrothermal metal-rich fluid and a connate seawater which had enriched in reduced sulfur by interaction with the organic-rich shaly levels of the Gorno Fm. and then percolated down at the precipitation site during inter-seismic periods. Based on the poorly constrained radiometric age, ranging from late Permian to earliest Carnian, two possible scenarios have been proposed for the genesis of the Primaluna deposit: a i) late Permian to early Triassic scenario with a possible link with the siderite deposits hosted in the Verrucano Lombardo and Servino fms. in the surrounding areas or a ii) Middle to late Triassic scenario which involves a possible metallogenic link with the Alpine-type deposits hosted in the Anisian-Carnian stratigraphic succession of the Lombardian Basin. The structural-geochemical study performed on the Vedra Valley deposit highlights that the ore genesis preceded the Alpine deformation giving further support to the latest metallogenic model proposed for the deposit. The geochemical investigation highlights the occurrence of highly dispersed distribution of Cu and As (which represent key elements to detect possibly feeder structures). This dispersed distribution could be related to different reasons: i) the Vedra Valley deposit is located in a peripheral portion with respect to the main feeder faults and thus no metal zoning can be recognized at the deposit scale; ii) the metal-bearing fluids upflowed diffusely through minor faults and the highly fractured host rock (or the still permeable host rock) rather than being focused around the major faults; iii) the Alpine orogenic event deformation could have determined a remobilization and redistribution of the trace elements. Overall, this research is likely to contribute to a better understanding of the genesis of the ore deposits hosted in the CSA