Geological features of alluvial placers

Abstract

Alluvial placer deposits may be divided into autochthonous and allochthonous subtypes. Autochthonous placers contain large heavy mineral grains that are practically immovable by streams and thus occur adjacent to primary ore deposits. Rich autochthonous placers accumulate during many stages of river development and are concentrated at the base of the alluvium or in the crevices of its bedrock. Smaller and poorer autochthonous placers are composed of grains received by rivers during the last stages of equilibrium and/or aggradation. These grains cannot get to the bedrock, which is protected from the river action by a layer of a substrative alluvium, but mineral grains may be concentrated in the higher parts of alluvial strata. During later downcutting stages, however, above-bottom placers may be reworked to join and enlarge bottom placers. Allochthonous placers, containing finer heavy mineral grains moved by a river as a part of its alluvial load, form far downstream from primary ore deposits or autochthonous placers, and they occur in surficial parts of channel alluvium. Rivers in an equilibrium state build point-bar placers, and in aggrading rivers several point-bar concentrations may be superimposed to form thick allochthonous placers.As bottom and above-bottom autochthonous placers, and allochthonous placers, require different methods for locating, prospecting, and sampling, the recognition of these types is of a practical importance.