西オーストラリアにおけるニッケル・金鉱床への物理探査法の適用について Applying Geophysical Techniques in Nickel and Gold Mines in Western Australia

物理探査法による地表探査法は古くより活用されてきたが,採掘活動への応用がされるようになったのは近年のことである。採掘活動における物理探査法は,鉱床探査から岩種識別まで多岐にわたる。これによるコスト削減効果の極めて大きことは各国で報告されている。物理探査法の効果的活用には,岩種による物理的性質の違いを正確に把握することが重要である。例えばKambaldaにおける鉱床の特徴は高導電性であるため,鉱床探査は電気・電磁法が主体である。これに対して,金鉱床では一般に顕著な物理的性質の違いが現れないが,多くの成功例が報告されている。本論文では,西オーストラリアにおけるニッケル・金鉱床における物理探査法の適用について具体的な例を通じて,有効性・経済効果などについて紹介する。

Whilst geophysical mapping has been long accepted as an integral part of surface prospecting, its use in mining operations is only recently gaining acceptance. In-mine geophysical applications include in-mine exploration, grade control, ore delineation, blast hole and stope design, and in-situ geotechnical rock mass characterization. It is reported in several recent case studies that the cost savings can be substantial. In the INCO nickel mines of Sudbury a saving of the order of C 20 million was estimated for one year. A similar order of magnitude savings is inferred in some of the mines in the Witwatersrand basin, in South Africa. Savings of the order of millions are envisaged at Nickel mines in Kambalda, Western Australia. The key to effective use of geophysics in mines, or anywhere for that matter, is knowledge of the relationship between local petrophysical properties (and petrophysical property contrasts) and the different geological or engineering entities (be they ore, alteration, structure or stress) to be mapped. The most outstanding physical property of the matrix/massive nickel sulfide ores at Kambalda is their extremely high conductivity. Hence the use of geophysics is biased strongly to electrical and electromagnetic methods and is focused on direct detection of ore. In contrast, application of geophysics in gold mines is rarely aimed directly at the ore because of lack of physical property/contrast associated with the ore waste rock. However some applications are described, where geophysical techniques have been successfully used in gold mining. The reported order of magnitude cost savings by using geophysics, the long term decrease in commodity prices, the depletion of open pittable mining resources (forcing higher cost and lower safety underground mining) and the increasing international competitiveness of the mining industry indicate that the prudent use of geophysics in underground mines will continue, if not increase. However it is realized that geophysics is not a panacea, but rather a useful tool in the continuous drive towards lower cost and greater safety mining practices.