General considerations of sampling and assaying in a coarse gold environment

Abstract

In many vein-gold systems grade complexity is created by the erratic, localized occurrence of coarse gold. High-grade regions are generally erratic and have little spatial continuity, but they may make a resource or stope block economic. Economic grades are generally contained within discrete ore shoots that are surrounded by barren to low-grade material. The presence of coarse gold makes sampling and assaying of this style of mineralization especially challenging. Comparisons between surface and underground drilling, linear/panel sampling and bulk sampling indicate that drilling and linear/panel samples generally understate bulk-sample grades. Traditional fire assays that use small charges consistently understate the assays that are gained from large charge sizes and from such techniques as screen fire assay and bulk-leachable extractable gold assay. These findings, though not unexpected, demonstrate the importance of bulk sampling in the grade evaluation process for coarse gold-bearing veins. Diamond drilling provides an effective measure of geological continuity, but grade distribution can be assessed reliably only from underground development. In general, it is unlikely that anything above an Inferred Mineral Resource can be estimated from surface drilling alone. Geologically controlled, closely spaced underground development and bulk sampling are likely to be the best way to determine Ore Reserves.