Resource evaluation of narrow gold-bearing veins: problems and methods of grade estimation

Narrow veins-defined as having a true width of less than 3 m-represent a si gnificant source of gold, which is often exploited by junior or small- to m edium-size companies. Their inherent geological and grade complexities crea te particular problems of evaluation. Veins display grade anisotropy within their volume and two populations are generally observed-a low background g rade and high grades associated with discrete ore shoots. Gold may be distr ibuted within the veins and wallrocks. Erratic high grades typify this styl e of mineralization and require special consideration during estimation. St atistical analysis must be undertaken to characterize the grade population and to determine whether high-grade cutting is warranted and at what level. Deposits that display a very nuggety distribution are the most difficult t o evaluate. The estimation of vein width and metal accumulation is best undertaken on e ither vertical longitudinal projections or inclined planes; two-dimensional approaches offer the best representation of narrow-vein systems. Tradition al estimation methods have mostly been used, but geostatistical techniques are sometimes applicable. The geological and grade complexities generally m ake variograms difficult to construct. Unless drill spacings are extremely close, it is usually impossible to define resources beyond the inferred and indicated categories with any confidence on the basis of data from drill-h oles. Underground development is generally required to define measured reso urces and reserves. Bulk sampling may be used to test estimated grades and the proposed mining method. Grade control is essential in ensuring that material being fed to the mill is of economic grade. Where possible, reconciliation should be undertaken b etween the estimated stope grades, grades indicated from stope or truck sam pling and grades realized in the mill so as to determine whether correct ev aluation strategies are being applied. Grade-control activities provide an important framework in which to develop geological understanding of the dep osit: the critical feature of narrow gold veins is their strong geological controls.