Dissecting the Sadisdorf Sn-W-Li vein and greisen system

The Sadisdorf Sn-W-Li-Cu prospect, Germany, comprises magmatic-hydrothermal greisen and vein-style mineralization associated to a small alkali-feldspar granite porphyry. Petrographic and fluid inclusion data across the full extent of the deposit is used to understand the temporal and spatial evolution of the mineral system and the key factors controlling ore formation.

Four mineral associations have been recognized from proximal to distal to the intrusive stock. These are hosted within quartz-mica veins or greisen bodies: 1) oxide-dominant cassiterite-wolframite-molybdenite association, 2) cassiterite-(stannite) associated with Cu-Zn-Pb-As sulfides mixed association, 3) sulfide-dominant chalcopyrite-sphalerite-galena-arsenopyrite-tennantite-pyrite association, and 4) late-stage fluorite veins and cavity infill.

Fluid inclusions (FI) in quartz, topaz, cassiterite and sphalerite consist of H2O-NaCl or H2O-NaCl-CO2±CH4 fluids. Irrespective of association most FI show the same range in homogenization temperatures (Th; 202-420°C) and salinities (0-13 wt.% NaCl eq.). FI hosted by late-fluorite, however, are marked by lower Th (<269°C) and salinities (0-5 wt.% NaCl eq.).

Results imply that the ore fluid was a low-salinity and high-temperature vapor-rich fluid. Despite the distinct mineralogical zoning within the system, significant spatial variations of temperature or salinity could not be identified. Hence, we propose that zoning is a result of systematic variations in oxygen fugacity and/or availability of reduced sulfur along the fluid flow path, either attributed to consumption of oxygen by early oxides, fluid-rock interaction and/or potential volatile loss. Conversely, the lower salinity and temperatures of the late fluorite association reflects the dilution and cooling of fluids with time and likely the ingress of meteoric water into the system.