Prospecting for deep volcanogenic ore

CIM Bulletin, Vol. 74, No. 834, 1981

JULIAN BOLDY, Placer Development Limited, Toronto, Ontario

A review of 115 significant volcanogenic massive sulphide discoveries in the Canadian Precambrian during the past 65 years reveals that only 7% resulted from surface exploration methods aimed at depths below the effective reach of current geotechnical systems. This has been due to the relative ease of shallow discovery, resulting in the harvesting of ore deposits within 100 m of surface. This surficial layer has been more or less effectively supersaturated by routine exploration. In view of the fact that a mining district's metal content is not likely to be confined to a surficial layer, the time has come to explore the vertical dimension, prospecting for ore at depths in excess of 150 m in productive districts.Essential to this task is an understanding of the Precambrian volcanogenic ore deposit model. The deposits generally have a tendency to be associated with the terminal phases of felsic volcanism, and to be located along certain time-stratigraphic horizons, marked by the products of volcanic exhalation. The significant massive sulphide deposits are discrete entities, ranging from the top 5% of deposits greater than 45 million tonnes in size, with a surface area of 0.30 km2, to median-sized deposits greater than 1.2 million tonnes in size with a surface area of 0.02 km2. These significant economic ore deposits range in value from $5.5 billion for the top 5% of deposits to $125 million for median-sized deposits. In terms of their frequency of occurrence, the former group has a 4% distribution and the median-sized deposits have a 15% distribution.An historical review of many of the principal Precambrian mining districts, such as Noranda and Flin Flon, shows that major tonnages are usually discovered during the initial phase of application of each new geotechnical search system, until carried to its ultimate limit of effective depth penetration. With the exception of the Noranda district, the mean depth of ore discovery in the other districts is still generally confined to within 50 m of surface.Targeting deep ore is difficult. One effective prospecting method is the detection of fracture-controlled mercury leakage haloes in outcrop (or core) above and peripheral to blind deposits. This technique has proved to be a useful pathfinder, and focuses attention on deposits buried at depths in excess of 700 m in some instances. Down-hole EM systems also promise to be effective in locating deposits previously bypassed by drilling. In addition, trend surface analysis programs of trace-element geochemistry should be used as an aid to targeting ore by directing exploratory drilling along the plane of ore-hosting stratigraphy.By definition, mining districts have a proven "favour-ability" for the occurrence of ore. Discovery probabilities in these districts will be considerably enhanced if future search efforts are concentrated below a depth of 150 m, and more use is made of "deep" ore search systems coupled with sound geological reasoning. Deep drilling programs to a depth of 800 m should initially be designed to search for the top 25% of deposits greater than 3.3 million tonnes in size, with a surface area of 0.05 km2, a value of $310 million and a 12% frequency of occurrence. This can be accomplished for an over-all cost approximating $40,000 per drill hole. If the search depth is limited to 400 m, the over-all cost of each drill hole is $20,000, equivalent to expenditures routinely allocated to grassroots exploration drilling in the remote greenstone belts of the Precambrian. Depending on a company's objective, prospecting for deep ore offers a stimulating challange — though risky, it has its rewards.

Keywords: Prospecting, Mineral exploration, Deep deposits, Volcanogenic ore, Sulphide deposits, Precambrian Shield, Massive sulphides, Ore deposit models, Noranda district, Flin Flon district, Mattagami district, Timmins district, Snow-Reed Lake district.