The morphology, mineralogy, and behavior of u "fine-grained" gold from placer deposits of Alberta: sampling and implications for mineral exploration

Abstract

The morphology of a large number of gold grains from stream sediments of the North Saskatchewan River and the Athabasca River was studied, together with the partitioning of the placer gold between various size fractions.Two major morphological varieties of gold were observed: (1) flaky, scaly gold, with folded and hammered edges, and with crystals or crystal faces still visible on the surface; and (2) "sandwiched," droplike particles, sometimes toroidal. About 5% of both types of gold were found to be coated with "new," secondary gold.The size of the majority of the gold grains studied varies between 0.500 and 0.010 mm. The most frequent size range in the North Saskatchewan River is 0.125–0.250 mm, whereas in the Athabasca River it is 0.063–0.125 mm. An important fraction of the gold from both rivers is smaller than 0.063 mm.The average Corey shape factor (CSF) of the gravity-recovered gold increases as the particle size decreases, whereas the gold grains lost to the tailings indicate very low average shape factors (i.e., high degree of flattening). This apparent increase in CSF for the smallest fractions recovered when using mechanical techniques is due to the fact that the flaky gold particles are more difficult to recover than the more spherical ones. Moreover, the presence of a gold-rich rim on all the gold grains increases their hydrophobicity.The sampling procedure to be adopted in Alberta and, more generally, in glaciated terrains is discussed. In particular, the use of the −63 μm fraction for routine sampling of stream sediments is suggested.