Prediction of gold grade potential in erratic vein-hosted deposits using a geochemical discrimination index

Abstract

The erratic and localized occurrence of gold is a feature of many gold-bearing veins. The term ‘ nuggety’ is often used to describe such deposits, where it refers to the erratic high gold values that may accompany low grades (low relative to the mean). The effect is a consequence of localized geologically controlled enrichment and the inclusion or exclusion of sparse gold grains because of the nature of the sampling process. 8 Nuggety deposits do not always contain visible or coarse gold (e.g. >100 µ m) and may be characterized by erratically distributed fine-grained gold. High-grade regions are generally extremely erratic with a low spatial continuity, but may make mineralization economic. Economic grades are generally contained within discrete oreshoots, which are surrounded by barren to low-grade material. A measure of ‘ gold grade potential’ based on trace-element ratios has been developed that allows the user to discriminate between barren, low-, medium- and high-grade mineralization. A preliminary report on the development of this discrimination index, the Gold Proxy Index (GPI), is presented here and the discussion of gold sampling in previous