Magnetic-Gravity Models for the Au-Ag-Mineralized Hokusatsu District of Southern Kyushu, Japan

Abstract

The aeromagnetic and gravity data were recently collected and integrated with the existing data to create nationwide maps. Using these data, magnetic and gravity analyses were performed in the Hokusatsu district, and the relation between the geophysical models and epithermal gold-silver mineralization is discussed.
The gold-silver deposits frequently occur on the edge of magnetic bodies which are inferred to lie to the northwest of the Sakurajima volcano and to the northwest of the Kirishima volcano. Also, they occur around the small closures of gravity high. A poor correspondence between the observed magnetic anomaly, and the magnetic anomaly due to the topographic model and surface geology leads us to conclude that magnetic anomalies are not resulted from outcropping bodies of the Pliocene to early Pleistocene volcanics but concealed intrusive rocks. Small closures of gravity high are interpreted due to concealed bodies having stock-like geometries.
Intrusion magmas would accelerate development of fracture systems and the hydrothermal convection systems. Accordingly, the localization of the gold-silver deposits on the margin of the inferred magnetic bodies is genetic. We estimate that the inferred igneous intrusions are closely related to the volcanisms from Pliocene to early Pleistocene time, taking account of those intrusive rocks being below the Curie point temperature, the present distribution of hot springs in and around the intrusive rocks, and the Curie point depth map in southern Kyushu.