Resource evaluation of narrow gold-bearing veins: problems and methods of grade estimation / Оценка ресурсов маломощных золоторудных жил: проблемы и методы оценки ресурсов

Narrow veins—defined as having a true width of less than 3 m—represent a significant source of gold, which is often exploited by junior or small- to medium-size companies. Their inherent geological and grade complexities create particular problems of evaluation. Veins display grade anisotropy within their volume and two populations are generally observed—a low background grade and high grades associated with discrete ore shoots. Gold may be distributed within the veins and wallrocks. Erratic high grades typify this style of mineralization and require special consideration during estimation. Statistical 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 to evaluate.
The estimation of vein width and metal accumulation is best undertaken on either vertical longitudinal projections or inclined planes; two-dimensional approaches offer the best representation of narrow-vein systems. Traditional estimation methods have mostly been used, but geostatistical techniques are sometimes applicable. The geological and grade complexities generally make 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-holes. Underground development is generally required to define measured resources and reserves. Bulk sampling may be used to test estimated grades and the proposed mining method. <...>