Garnet mineral chemistry as proxy for skarn-forming processes in the Schwarzenberg District, Erzgebirge, Germany

Polymetallic W-(Sn), Sn-Zn±(In), and Zn-Pb±(Sn) skarns in the Schwarzenberg District of the western Erzgebirge are expressions of a polyphase mineral system that formed between >330 Ma and ~295 Ma. Due to the polyphase nature of the skarns, the physicochemical conditions of skarn formation and the actual timing of ore formation have remained poorly constrained. To better understand skarn-forming processes in the Erzgebirge, we obtained new mineral chemical data of prograde garnet from all major skarns across the Schwarzenberg District by electron microprobe and laser ablation-inductively coupled plasma-mass spectrometry. Results illustrate that the oldest generation of skarns formed under relatively fluid-buffered conditions and high fluid/rock ratios. Associated garnet has a pronounced andradite component, contains low concentrations of HFSE, Mn, and Ga and is variably enriched in Sn, W, As, and Li. Elevated concentrations of HFSE, Mn, and Ga in garnet from skarns with skarnoid textures indicate that these formed under mainly rock-buffered conditions (low fluid/rock ratios). Associated garnet is dominantly grossular and invariably low in Sn and W concentrations. The mineral chemistry of garnet from the youngest skarn bodies indicates renewed ingress of magmatic-hydrothermal fluids marked by an intermediate grandite composition, low HFSE, Al, and Ga concentrations as well as a variable enrichment of Sn and W. Although these general patterns are well defined, our data also clearly reveal considerable variability of garnet compositions on the local scale. The same is true for Sn and W concentrations in garnet of all stages. These are found to be too variable as to constitute meaningful exploration vectors. However, they are well suited to place general constraints on the physicochemical conditions of polyphase skarn formation in the Schwarzenberg District.