Geophysical exploration for epithermal gold deposits: Case studies from the Hishikari Gold Mine, Kagoshima, Japan

Abstract

Various geophysical techniques were applied experimentally to exploration for epithermal gold deposits. The Hishikari gold?silver deposit, located in northeastern Kagoshima, Kyushu, Japan, was selected for this study. This paper presents the results from these case studies and discusses the applicability of each method in several stages of the exploration; the reconnaissance stage, the detailed follow-up stage and the final stage where the surveys are directed at detecting veins. The results are summarized as follows; 1. A high-gravity anomaly was interpreted as the swollen basement. This subsurface structure was considered to be local doming of basement associated with volcanic activity. Local gravity highs are worthy of notice because the doming of basement might cause fractures; 2. A resistivity cross-section was characterized by three layers; an uppermost layer with high resistivity (100?1000 Wm) corresponding to unaltered volcanic rocks, an intermediate layer with low resistivity (3?8 Wm) corresponding to altered volcanic rocks and a bottom layer with relatively high resistivity (80?200 Wm) corresponding to the basement. Resistivity mapping and sounding are useful to identify lithologies and structures that may control gold mineralization; 3. IP anomalies (high chargeability) within the area of low resistivity are the most important indicators of vein zones. For this reason, in the detailed follow-up stage and the final stage of exploration, IP surveys play the most important role.