(Un) Certain Exploration Facts From Figures

CIM Bulletin, Vol. 70, No. 781, 1977

J. Boldy, Chief Geologist, Gulf Minerals Canada Limited, Toronto, Ontario, At time of writing — Exploration Advisor, Minerals Division, Shell Canada Limited, Toronto, Ontario

This paper is an attempt to place Canadian Precambrian volcanogenic deposits in perspective, by examining and compiling data regarding certain neglected fundamentals of size, grade, value, frequency distribution and spatial density. The discovery roles played by various exploration methods are compared and contrasted, and, using empirical methods, the probabilities of discovery are discussed, combined with the use of an effective 'cost-per-target' program. From these basic observations, realistic exploration judgments may be made. Using similar methodology, this format may be applied to other types of deposits. Precambrian volcanogenic deposits occur in a natural geometric progression in size. They number 110, of which 70% are pre-producers, producers or past-producers. These deposits are bonanzas and generally have a 'net pre-smelter" value of between $43 and $62 per ton. The statistically typical deposit is that which occupies the modal position, has 400,000 tons of ore, a value of $20 million and occurs with a frequency of 40%. A median deposit has 1.4 million tons of ore, a value of $70 million and occurs with a frequency of 15%. An upper-decile deposit has 13.3 million tons of ore, a value of $750 million and occurs with a frequency of 6.5%. The major, mid-upper-decile deposits have a minimum of 50 million tons of ore, a value of $3 billion and occur with a frequency of 4%. Five major volcanogenic deposits occur in the Precambrian and account for 54% of production. This contrasts with 55 deposits (all below the median) which account for 3.9% of production. The deposits tend to occur in clusters, and well-developed, prolific districts, such as Noranda, have a density of 1 deposit per S3 square miles. This contrasts with the newer Sturgeon Lake district, which has a density of 1 deposit per 125 square miles. Relatively 'low-risk' opportunities occur in areas of high density and offer a better chance for discovery of modal or median-type deposits. On the other hand, 'high-risk' opportunities occur in areas with a low density, and these areas are more likely to yield a deposit in the upper-decile range. During the past 20 years, AEM surveys have discovered 17% of the deposits, which contain 41% of the total tonnage. In contrast, 75 years of prospecting have discovered 44% of the deposits, which contain 48% of the total tonnage. Modal, median and upper-decile AEM discoveries tend to be between 3 and 4 times larger than similar discoveries made by other methods. Empirically, realistic odds for discovery occur in the range between 100:1 and 500:1. This is substantiated by the results of several successful exploration companies. Due to the role played by random chance, there is no control over the ultimate size and value of the discovery. However, an effective 'cost-per-target' program will insure that funds are available to test a sufficient number of meaningful targets over an (un) certain period of time. This has been shown to be the case with an outlay of between $500,000 and $15 million, extending up to 10 years in some instances.
Mots Clés: Mineral exploration, Exploration techniques, Precambrian deposits, Volcanogenic deposits, Discovery rates, Frequency distribution, Electromagnetic exploration, Copper, Zinc, Data analysis.