Westerly derived Early Cretaceous gold paleoplacers in the Western Canada foreland basin, southwestern Alberta: tectonic and economic implications

Abstract

Albian-aged (Early Cretaceous) igneous pebble to cobble conglomerates fill multiple, northeast–southwest-oriented, subparallel channels in the upper Blairmore Group (upper Beaver Mines and Mill Creek formations) of the Rocky Mountain foreland basin, southwestern Alberta and southeastern British Columbia. Paleocurrent data show that the conglomerate was derived from the west. Clast petrography implies a provenance that includes granitoids, mafic volcanics, low-grade metamorphic rocks, and shallow-level (ca. 7 km depth) postmetamorphic quartz veins formed from meteoric fluids. The conglomerate was probably derived from the southern Omineca Belt of British Columbia prior to the rise of the Rocky Mountains. The conglomerate contains detrital gold grains up to 150 μm in diameter, and chemical analyses indicate widespread anomalous gold concentrations (up to 910 ppb Au) in conglomerate matrix. Gold content in these igneous-clast conglomerates excèdes that reported from the richest modern placers in Alberta. Less pronounced but persistent As anomalies (up to 260 ppm) occur also. Postdepositional alteration of conglomerate matrix chemically mobilized Au and As from their detrital source grains and redistributed these elements. Gold enrichment in the igneous-clast conglomerate contrasts strongly with background gold concentrations in the underlying conglomeratic Cadomin Formation. The gold concentrations in the igneous-clast conglomerate demonstrate that paleoplacers derived from the Canadian Cordillera have accumulated in the Western Canada foreland basin, a sedimentary succession previously dismissed as a host for detrital gold.