A reexamination of the Mount Bischoff cassiterite sulfide skarn, western Tasmania

Abstract

The Mount Bischoff tin deposit occurs in a dolomite unit within structurally complex, late Precambrian rocks which were intruded by a swarm of narrow, vertical to steeply dipping, quartz-feldspar porphyry dikes of Devonian age. The dikes are distributed in a crudely radial pattern and gravity data indicate that a shallow granite ridge beneath Mount Bischoff is connected to the Meredith Granite. The dike rocks have undergone an early stage of potassic alteration which was overprinted to a variable degree by later greisenization. The alteration shows a zonation along and also across the dikes consistent with suggestions that the dikes were major fluid conduits. Within the dolomite, replacement occurred in two separate and distinct stages. During stage 1, serpentine and chondrodite assemblages formed at 400 degrees to 460 degrees C from hydrothermal fluids with 30 to 36 wt percent NaCl. A zonation from serpentine to chondrodite to magnesite reflects the lowering of the silica activity in the fluid as it reacted with the dolomite. During stage 2, quartz, talc, phlogopite, and carbonate assemblages formed at 320 degrees to 360 degrees C from hydrothermal fluids with about 2 rn NaCl, 1.5 m CO 2 , and 0.3 m CH 4 . The zonation in these assemblages from quartz to talc to ferroan magnesite also reflects decreasing silica activity in the fluid. The stage 2 assemblages extensively overprinted the earlier formed assemblages. Pyrrhotite is the dominant sulfide and textural studies suggest that it was formed early in stages 1 and 2 and was overprinted by the later silicates. Cassiterite was precipitated predominantly within the quartz and talc assemblages during stage 2. Fluid inclusions and textures related to the stage 2 mineralization indicate that the fluid was boiling. Loss of H 2 in the vapor, combined with wall-rock reactions, precipitated tin from the fluid...