Geology in the Resource and Reserve Estimation of Narrow Vein Deposits

Exploration & Mining Geology, Vol. 6, No. 4, 1997




Narrow veins are an important world-wide source of silver, tin, uranium
and particularly gold. To potential financiers, this style of mineralization is
viewed as high risk because of the often relatively small resource involved and
high cost of estimation. In many cases diamond core drilling will not enable
resource estimation beyond the inferred and indicated categories. Exploratory
underground development is required to define measured resources. The definition
of geological and grade continuity are major factors in narrow vein assessment.
A measured resource must be based on strong geological 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. Branches, intersections and braided zones are
common features. Potentially high-grade zones are often localized within ore
shoots, which are surrounded by barren/low-grade regions. Resources must be
determined as closely as possible 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 presentation of this information using a classification that is clear,
unambiguous and bankable. Estimation methods employed are either classical or
geostatistical; however, classical methods are often used because of the
difficulties in applying geostatistics. Grades generally show a strong
positively skewed distribution testifying the presence of high-grade values.
Problems exist in the applicability of cutting outlier values risking over- or
underestimation 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 understated
estimate can result in an opportunity missed for profit and/or greater economies
of scale. Definition of reserves involves the consideration of economic and
engineering data, to decide which part of the resource is economically 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 the data and the
geologist’s experience are relevant. The impact of geological understanding on
estimation is often undervalued or assumed to be of lesser importance than
finding grades and tonnages. This paper discusses the geology of narrow vein
deposits and the importance of geology in their evaluation.
Keywords: Geology, Narrow veins, Silver, Tin, Uranium, Gold, Mineralization, Geological study
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