S.C. Dominy

Errors and Uncertainty in Mineral Resource and Ore Reserve Estimation: The Importance of Getting it Right

Mineral Resources and their subsequent conversion to Ore Reserves are of key importance to mining companies. Their reliable estimation is critical to both the confidence in a feasibility study, and also to the day-to-day operation of a mine. Together with sampling, assaying, geological and other errors introduced during interpretation and estimation, additional errors are likely to be introduced during the application of technical and economic parameters used for conversion of resources to reserves. There is thus a requirement for high-quality interpretation and estimation to be supported by high-quality data. Any company expecting to make sound investment or operational decisions must base this on both relevant and reliable information. An Ore Reserve statement generally contains a single set of grade and tonnage figures without a discussion of the potential inherent errors in these estimates. Some sensitivity studies may be run, but confidence limits are rarely quoted and, if they are, often do not take into account many of the factors that cause uncertainty in the grade and tonnage estimates. Mineral Resource and Ore Reserve estimates thus carry certain errors leading to uncertainty and risk; some of these are unquantifiable for various reasons, for which the operator should be aware. This paper presents a review of the potential sources of error that might occur during a resource estimation program, which are carried through into the reserve estimate. A number of methods are discussed that allow the estimator to be more transparent about the inherent risks in his/her estimate. Emphasis is placed on data quality, and the requirement for strong quality management to be linked to continuous improvement.

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.

Estimation and Reporting of Mineral Resources for Coarse Gold-bearing Veins

Coarse gold-bearing veins are characterized by high grades that are localized and erratic. Effective sampling of coarse gold-bearing veins is difficult because of the low concentration and erratic nature of the gold particles. Diamond drilling is an effective measure of geological continuity, however, grade distribution can only be reliably obtained from underground development (including close-spaced sampling, bulk sampling, and trial mining). Comparison between surface and underground drilling, underground linear/panel and bulk sampling indicate that drilling and linear/panel samples generally understate bulk sample grades. Bulk samples are likely to be the closest estimators of true grade. It is unlikely that anything above an Inferred Resource category can be estimated from surface drilling alone, and at best the grade will only be a global estimate. Underground development, in-fill drilling and bulk sampling/trial mining will be required to delineate Indicated and Measured Resources. Closely spaced development and bulk sampling is likely to be the only way to determine Reserves. The resource estimation process must driven by a clear geological model that should attempt to understand both geological and grade continuity. Traditionally, grade interpolation has been undertaken using classical methods, though more recently, computer-based block modelling techniques have been used with some success. Three case histories are presented, documenting the problems of estimating resources in coarse gold-bearing veins and the practical ways in which they were approached.

Gold particle clustering: a new consideration in sampling applications

Abstract Recent research indicates that an enhanced coarse gold problem may be present in some gold deposits. This can be due to the presence of gold particle clusters that give the effect of individual coarse-gold particles and leads to a high constitution heterogeneity (CH). These clusters, which may be distributed on the centimetre-scale or greater, give rise to high assay variability in field samples. However, once a sub-sample is pulverised, the effect of the clusters may be removed if the individual gold particles making up the clusters are liberated. Thus any resulting pulp will tend to have a low variability, unless true coarse-gold particles exist. Clusters are readily recognised in coarse gold deposits where they accentuate existing high nugget effect and CH. Clusters may however also occur in fine gold deposits where they produce an apparent coarse gold style. This is signaled by an unexpectedly high nugget effect from variography and poor correlation between field sample duplicates. The Gy sampling equation is applied to model coarse and fine gold deposits with clustered and non-clustered gold grains. Case studies illustrating observed effects of clustering are presented. If clustered particles exist, then proper protocol design at the field and early laboratory stage is paramount. The sample characterisation stage should include an assessment of possible gold particle clustering, in addition to the standard descriptions of the gold particle size distribution.

Core recovery and quality: important factors in mineral resource estimation

The estimation of mineral resources is critical to all mining operations irrespective of size or commodity.1,12 The risks associated with mining are varied and complex, where the dominant source of risk is the orebody itself. Reverse circulation (RC) and diamond core drilling methods are used extensively for the collection of samples from depth. These data generally form the critical base for both geological and grade modelling, leading to the mineral resource estimate and ultimately the ore reserve estimate. It is well known that diamond drilling generally provides a higher quality sample, better suited to resource estimation than RC drilling, especially for gold deposits.1,6,7,11 RC methods are sometimes applicable to the resource evaluation of alluvial/unconsolidated deposits, though can be highly problematic when applied to gold deposits.7 Current methods of resource classification relate to the geological, economic and technical confidence in the resource.14–16 Geological confidence is largely related to the level of drilling and sampling in the orebody, to the geologist’s perceived level of confidence in his or her work, and to the continuity of the mineralisation. This contribution reviews the geological and technical factors that affect core recovery, how core recovery is measured, the impact of poor recovery on the resource estimate, and how to deal with lost core during estimation.

The Occurrence of High-grade Gold Pockets in Quartz Reefs at the Gwynfynydd Mine, Wales, United Kingdom: A Geological Explanation for the Nugget Effect

Gold-bearing quartz reefs commonly show extremely erratic and unpredictable grade variation, although gross geological continuity may be good. This type of variation is often described as being nuggety or having a high nugget effect and can be measured quantitatively using the semi-variogram. Understanding of geological features such as reef texture and structure will provide improved models for the interpretation of assay data, drill core descriptions, etc. In this paper, a case study from the quartz-hosted gold occurrences in the Gwynfynydd mine, United Kingdom, is described. The work provides a starting point for models of lateral variation and demonstrates the types of structural and textural features that may be sought to give clues to the prediction of gold distribution in similar deposits. Gold deposition at Gwynfynydd is primarily associated with the reef footwall. The host veins form a discrete group of structures that may have been emplaced early in the sequence of quartz veins forming the reef. This potentially exposes them to a maximum number of dissection events during repeat dilations of the lode. Gold was deposited in the local porosity at extreme concentrations, equivalent to kg/t grades, when the host fissure had become filled with a porous framework of crystalline quartz and sulfides. Distribution of porosity varied laterally along the vein as the result of the formation of clump-like growths of minerals from the vein walls. The growth style is relevant to the distribution of gold although it may not be relevant to the occurrence of gold in a particular vein. The preservation of pores with some connectivity late in the vein fill sequence may be important in permitting continued but slow fluid flows. This facilitates effective fluid reaction with wall rock-derived methane, thus changing the relative rates of gold and quartz deposition in favor of gold. The textural studies explain the first-order control of nugget distribution at Gwynfynydd but do not allow for prediction and optimization of the resource estimation process. This is principally because of the very low geological continuity and poor predictability of the high-grade pockets. Nonetheless, the work does provide a clear geological explanation for the erratic nature of the gold. It indicates textures and structures that can be used to determine geological continuity of the gold-bearing elements within the gross reef envelope.

Classification and reporting of mineral resources for high-nugget effect gold vein deposits

The often complex, erratic, and localized nature of gold is a common feature of many vein-style gold deposits. This style of mineralization is often referred to as being nuggety or possessing a high-nugget effect. As a result of these complexities resource estimation is difficult and in general, only Exploration Results can be provided or an Inferred Mineral Resource estimated from surface drilling data alone. Underground development, further drilling, and probably bulk sampling will be required to delineate Indicated and Measured Resources. Tonnages can generally be estimated from diamond drill and development information with a reasonable degree of confidence. Grade is much more difficult to define with confidence because it is commonly highly erratic and discontinuous in nature. The dependency of higher confidence Resource categories on development information may create a Catch 22 situation, with funding for such development often depending on the prior definition of at least Indicated Resources. There are no easy solutions to these challenges posed by high-nugget effect deposits, and it is important when classifying and reporting not to downplay the uncertainties often associated with Mineral Resource and Ore Reserve estimates for such deposits. However, in common with all deposit types, if the principles that underpin the estimation, classification, and reporting procedures are borne in mind and common sense applied, most issues can be satisfactorily resolved. This paper discusses the classification and reporting of Mineral Resources for high-nugget effect gold vein deposits within the framework of the JORC Code (JORC, 1999).

Effects of sample mass on gravity recoverable gold test results in low-grade ores

Abstract Gravity recoverable gold characterisation methodologies are the single- or three-stage tests, which generally use a standard feed sample mass. A case study of low-grade coarse gold-dominated mineralisation is presented that demonstrates the high variability of test results using different sample masses. Samples were processed using the single-stage test and subsequently entire development rounds were batched through a plant for comparison. Unsurprisingly the results indicate that the small (≤50 kg) test samples grossly understate plant gravity recoverable gold and display poor precision. Larger samples display improved precision, but still understate plant gravity recoverable gold. The small mass samples are unrepresentative as they do not contain the full gold particle size distribution. Poor representivity is enhanced by gold particle clustering. Small samples generally capture finer more abundant and disseminated gold particles, but rarely contain clustered gold. The use of standard GRG test sample masses is challenged. Test work should be based on spatially distributed representative field samples, that if required are split to representative sub-samples for testing. An early stage gold particle size characterisation programme is required to optimise sample mass and improve representivity.

Optimising sampling protocols via the heterogeneity test: challenges in coarse gold mineralisation

Determination of the sampling constant is an important step when applying the Fundamental Sampling Error equation to optimise sampling protocols. The classic method for its determination is based on the heterogeneity test (HT). In coarse gold-dominated mineralisation, the HT sometimes provides an evaluation of the fine-gold background grade population heterogeneity, but understates that of the important high-grade coarse-gold component. This is because the total mass of fragments is too small to represent the full gold particle size distribution of the mineralisation. Despite this, single heterogeneity tests (SHT) are often undertaken on deposits where the presence of coarse gold is ignored, not realised or understated. Resultant sampling and assaying protocols are frequently of poor quality and do not match the mineralisation characteristics. For coarse gold-dominated mineralisation, an empirical approach for sampling constant determination is recommended via direct estimation of the liberation diameter.

Bulk sampling as a tool for the grade evaluation of gold-quartz reefs

Abstract Gold–quartz reefs are often characterized by localized, erratic high grades and, in many cases, coarse free gold. They typically display a high to extreme nugget effect, and the sampling of these deposits for resource evaluation presents a corresponding challenge. Core drilling is generally an effective measure of geological continuity, but reliable grade data and confirmation of its continuity can only be obtained from underground development. Generally it will be possible to estimate only an inferred mineral resource from drill data alone; underground development with closely spaced infill drilling and bulk sampling will be required to delineate indicated and measured mineral resources. Three case studies are presented that show that bulk sampling provides an effective—though costly—way of assessing likely minable grades.