MT法による菱刈鉱山本鉱床地域の深部構造調査

抄録

The Hishikari mine, one of the richest epithermal gold-silver deposits, had been wholly concealed except small ruins of old mining. Geophysical surveys have playèd an important role in the exploration throughout the discovery in 1981 and the subsequent mine development. Besides the exploration activity, we also tested various geophysical methods in this area to examine their application to the exploration for this type of gold deposits, including several methods that had never been applied to mineral exploration in Japan, such as the MT (magnetotelluric ) method that is discussed in this paper.
In the MT method, natural electromagnetic fields set up by lightning and solar activity, are used to investigate the electrical substructure of the earth. The advantages of the method are its unique capability for exploration from shallow depth to very great depth without artificial sources. In the first survey carried out in 1983 using BEICIP MT 5C/7 system, an extremely low resistivity zone was detected at the depth of more than 10 km beneath the major mineralized area. To check the results from the previous survey, we made another survey on the same profile line using the latest PHOENIX V5-16 system in 1991.
Prior to the analysis, we removed statics effects from the MT data. The statics effects are defined as vertical displacements of apparent resistivity curves, which are considered as results of near-surface resistivity features. The extremely low resistivity block derived from the previous analysis was nullified by this procedure. Two-dimensional analysis provided better understanding of deep resistivity structure that would be useful for discussion about the tectonic model of the deposit.