The MacMillian placer workings are located along the banks of the Cottonwood River near the Boyd Creek junction. The river bars have been worked historically.

There are several placer gold deposits along the Cottonwood River. The deposits are located in benches ranging in height from about a metre to several metres above the present course of the river. The gold is derived from the concentration of pre-existing glacial or pre-glacial deposits, some may have been of Miocene age. The ultimate source of the gold may have been the auriferous veins of the Barkerville terrane from which the Cottonwood River drains. The pre-Tertiary geology of this area consists of mafic volcanic and sedimentary rocks of the Upper Triassic to Lower Jurassic Nicola Group of the Quesnellia Terrane.

The Cottonwood workings produced alluvial platinum and gold. This is a common feature of many of the placer deposits in this region.

The No. 2 shaft, the only shaft that reached true bedrock, contained a small concentration of gold at bedrock. The gold was fine-grained with a substantial quantity of black sand. Shallow drilling revealed only low grade or local pockets of concentrations.

"Data from the Cariboo mining district indicate that supergene leaching of gold dispersed within massive sulphides by Tertiary deep weathering followed by Cenozoic erosion is the most likely explanation for the occurrence of coarse gold nuggets in Quaternary sediments" (Exploration in British Columbia 1989, page 147).