Gold mineralisation in the Surselva region, Canton Grisons, Switzerland

The Tavetsch Zwischenmassif and neighbouring Gotthard Massif in the Surselva region host 18 gold-bearing sulphide occurrences which have been investigated for the present study. In the Surselva region, the main rock constituting the Tavetsch Zwischenmassif (TZM) is a polymetamorphic sericite schist, which is accompanied by subordinate muscovite-sericite gneiss. The entire tectonic unit is affected by a strong vertical schistosity, which parallels its NE-SW elongation. The main ore minerals in these gold occurrences are pyrite, pyrrhotite and arsenopyrite. The mineralisation occurs in millimetric stringers and veinlets, everywhere concordant with the schistosity. Native gold is present as small particles measuring 2–50 μm, and generally associated with pyrite. Average grades are variable, but approximate 4–7 g/t Au, with several occurrences attaining 14 g/t Au. Silver contents of the gold are on the order of 20 wt%. A “bonanza” occurrence consists of a quartz vein coated by 1.4 kg of native gold. The origin of the gold is unknown. On the assumption that the sericite schists are derived from original felsic volcanic tuffs, the gold-bearing sulphides may have been introduced during pre-Alpine magmatism.