Geology in the resource and reserve estimation of narrow vein deposits

Narrow veins are an important world-wide source of silver, tin, uranium and particularly gold. To potential financiers, this style of mineralization i s viewed as high risk because of the often relatively small resource involv ed and high cost of estimation. In many cases diamond core drilling will no t enable resource estimation beyond the inferred and indicated categories. Exploratory underground development is required to define measured resource s. The definition of geological and grade continuity are major factors in n arrow vein assessment. A measured resource must be based on strong geologic al and grade continuities. Narrow veins, generally less than 3 m wide, are complex geological phenomena, which commonly display unpredictable geometry and grade distribution. Variations in structural continuity, dip, strike, width, mineralogy and specific gravity are common. Veins may be composite, with ore-grade mineralization restricted to a specific structural domain. B ranches, intersections and braided zones are common features. Potentially h igh-grade zones are often localized within ore shoots, which are surrounded by barren/low-grade regions. Resources must be determined as closely as po ssible to reality and reflect the distribution of the commodity in the rock . Estimation has three main requirements: (1) geological study to determine orebody nature; (2) estimation of tonnage and grade, and (3) the presentat ion of this information using a classification that is clear, unambiguous a nd bankable. Estimation methods employed are either classical or geostatist ical; however, classical methods are often used because of the difficulties in applying geostatistics. Grades generally show a strong positively skewe d distribution testifying the presence of high-grade values. Problems exist in the applicability of cutting outlier values risking over- or underestim ation of grade. It is generally believed that where resource estimates are in error, poor geological control is usually the cause. Overestimation can lead to the closure of a mine through lost revenue and capital. An understa ted estimate can result in an opportunity missed for profit and/or greater economies of scale. Definition of reserves involves the consideration of ec onomic and engineering data, to decide which part of the resource is econom ically mineable. Geology plays a similarly important role at this stage in the assessment of likely mining method, dilution, recovery etc. Throughout resource/reserve estimation, it is crucial that the amount and quality of t he data and the geologist's experience are relevant. The impact of geologic al understandings on estimation is often undervalued or assumed to be of le sser importance than finding grades and tonnages. This paper discusses the geology of narrow vein deposits and the importance of geology in their eval uation. (C) 1999 Canadian Institute of Mining, Metallurgy and Petroleum. Pu blished by Elsevier Science Ltd. All rights reserved.