A comprehensive program of indicator mineral and geochemical sampling of both oxidized and unoxidized till at the glaciated Blackwater Au-Ag deposit in the interior of south-central British Columbia, Canada has provided key insights into glacial and post-glacial dispersal patterns that can be used to explore other glaciated regions more effectively.
The Blackwater deposit is located on the northeastern slope of Mount Davidson and is completely covered by till that was deposited by NE-flowing ice. Only the sulphide-depleted supergene cap of the deposit was exposed to the ice. Indicator mineral sampling of C-horizon till on the mountain slope identified a 1.3 km wide gold grain dispersal train that extends c. 3 km glacially down-ice and 200 vertical metres down-slope from the deposit. The till within this train contains from 10 to 641 gold grains, dominantly of 25 µm size, per 10 kg of its <2 mm matrix. As well, the 0.25 – 0.5 mm fraction contains up to 25 000 grains per sample of spessartine garnet derived from the hypogene alteration envelope of the Blackwater deposit, increasing the probable detectable length of the train to 10 km. Geochemically, in the <0.063 mm fraction of the till, the detectable length of the train is reduced to <1 km, with the best indicator elements being Au and Zn.
In drill holes, the host till is oxidized to a depth of 2 – 5 m. It thickens markedly downslope from a few metres to c. 100 m. The gold-spessartine dispersal train thickens sympathetically to nearly 30 m and also ascends progressively within the till section. Gold grain and spessartine levels in the unoxidized till at depth are similar to those in the oxidized till at surface but the unoxidized till also contains up to 60 000 grains per sample of metal-rich pyrolusite derived from supergene cap of the Blackwater deposit.