Formation and age of sphalerite mineralization in carbonate rocks of Bajocian age in the Swiss Jura Mountains: evidence of Mesozoic hydrothermal activity

A combination of petrographic and geochemical techniques was applied to better constrain the origin and evolution of the fluid systems responsible for the formation of disseminated, Cd-rich (up to 0.6 wt%), sphalerite (ZnS) mineralization in the northeastern part of the Jura Mountains, Switzerland. The Rb–Sr ages of sphalerite samples indicate that a main phase of sphalerite formation occurred near the boundary between the late Middle and early Late Jurassic, at around 162 Ma. The negative δ34S values (−22.3 to −5.3 ‰) suggest that biogenic sulfide sulfur was involved in ZnS precipitation. The strontium isotope composition is more radiogenic than that of contemporaneous seawater, reflecting the interaction of mineralizing fluids with silicate rocks. Lead isotope signatures are very uniform (206Pb/204Pb = 18.63–18.67, 207Pb/204Pb = 15.63–15.64, 208Pb/204Pb = 38.51–38.63), indicating an isotopically well-homogenized fluid system. The basement rocks underlying the Jurassic strata are considered to be the main source of metals for the sphalerite mineralization. The migration of deep-sourced hydrothermal saline metal-bearing fluids into the Bajocian host carbonates containing sedimentary reduced sulfur resulted in the precipitation of sulfides. The period of sphalerite formation near the Middle–Late Jurassic boundary is characterized by enhanced tectonic and hydrothermal activity in Europe, related to the opening of the Central Atlantic and tectonic/thermal subsidence during spreading of the Alpine Tethys. Our study provides evidence that the Bajocian carbonate rocks in the Jura Mountains area were affected by the circulation of deep-sourced metal-bearing hydrothermal fluids in response to these continent-wide tectonothermal events. The presence of sphalerite mineralization and associated geochemical anomalies in Zn and Cd contents in carbonate rocks may also be used to trace basement features.