Vertical grade profiles of offshore and coastal placers

Abstract

Accumulation and lag concentration, together with dispersion, are important processes in offshore and coastal placer development. Mineral grains and sediment particles are sorted by flowing water according to their hydraulic equivalence. Distinct placer types are developed. Marine regression and transgression result in reworking leading to modification or destruction. Some placers are stranded on the coast, others are drowned. A vertical grade profile (VGP) graphically depicts the grade variations in the pay section of a placer. A VGP may be plotted for all heavy minerals and every placer type, and two basic patterns are evident. An upward or downward decline in grade indicates that accumulation and lag processes, respectively, were dominant. A reversal in decline direction marks a scour horizon or stillstand. Combinations of the basic patterns exist and some are complex with three grade maxima, revealing multiple mineralisation units. The history of a placer is revealed by its VGP(s) unless part is erased by reworking. Plotting the grade to a common logarithmic scale may assist interpretation. Different placer types have been developed in favoured locations visible on model cross-sections through the coast to the offshore. With minor variations, the VGP pattern displayed by a specific placer type is common to all minerals. Sampling protocol and resource estimation procedures should be guided by the VGP.