Fig. 1: A grey muddy sediment infilling a sinkhole in karst topography, with large dolomite-replaced shells of the brachiopod Stringocephalus burtini. Dolomitized brachiopod fragments and a pale yellow sphalerite are supported by a muddy matrix in the sinkhole, within the Dunedin Formation limestones on Tennessee Mountain, North Face showing, Robb Lake, B.C. There are a few black clasts of the host Dunedin Formation. As the sedimentary succession was buried, episodic growth of the dolomite crystals at increasing pressure resulted in curved crystal faces (so-called saddle dolomite). The material here would be described as high-grade (zinc-rich, iron-poor) zinc ore, were this remote deposit ever put into production. The system is Fe-poor, thus the sphalerite is Fe-poor and pale yellow, as opposed to deep red and Fe-rich. At depth in the deposit there are both early red and late yellow generations of sphalerite.
Specimen is roughly 12x12x15 cm in size. This sample was collected by Francis T. Manns during exploration drilling on the Robb Lake deposit, and remains in his collection. Fran was instrumental in the creation of this interesting Rock of the Month" feature.