Metal-rich coatings on residual sand deposits in the St. Lawrence upper estuary

Abstract

Reddish-brown, medium to coarse immature sands of fluvioglacial origin cover large areas of the St. Lawrence Upper Estuary. Light and dark particles in these deposits are found to be coated with Fe and Mn hydroxide precipitates enriched in trace elements (Zn, Co, Cu, Ni, P). The Fe ratios for these elements in the coatings are considerably higher than for the sands and for the abundant (> 20% by weight) rock fragments in this material. Laboratory experiments have shown that the coating material can be removed by simple resuspension and abrasion procedures mimicking, but not exceeding, the bottom stresses to which the sands are exposed under natural conditions. Geochemical and sedimentological factors acting respectively as sink and source of trace elements from/to the bottom waters exposed to the sands are discussed. It is proposed that the development and migration of sand waves, very abundant in the estuary, provide a cycle of successively protected and exposed environments where coatings are alternatively formed and destroyed.