Sedimentology of a Paleoplacer: The Gold-bearning Tarkwaian of Ghana

Abstract

The Precambrian conglomerates of the Tarkwaian Series are alluvial fan deposits with braided stream channels. The variations of thickness, facies and crossbedding direction in the conglomerates indicate four coalescing fans with dispersal from the east and southeast. The conglomerate layers tend to decrease in number and become thinner downcurrent, and it can be shown that erosion and reworking were active in the upstream parts of the fans. The distribution of gold is clearly related to sedimentary features; its grain size distribution, for instance, is very similar to that of placer gold. In general, gold is more abundant in conglomerates that are thinner, well packed, and contain larger pebbles, and a matrix rich in hematite. In these conglomerates the quartz and hematite, and possibly also gold grains of the matrix are in approximate hydraulic equilibrium with each other, but not with the pebbles. One mechanism of gold concentration was, therefore, the trapping into open-work gravel; this gravel was formed in channels and at the margins of point-bars. The distribution of ore-shoots and borehole values in relation to dispersal shows that gold was deposited mainly in two situations: one - two main channels in each fan, and in relatively up-fan areas where reworking was active.