A proposal of caldera-related genesis for the Roseki deposits in the Mitsuishi mining area, southwest Japan

Geology of the largest Roseki (pyrophyllite-clay)-mineralized, Mitsuishi mi ning area was re-examined, and a composite caldera model (Wake caldera) was proposed. Evidence for the caldera formation is (I) Existence of huge volu me of rhyolitic tuffs in the mining area, (2) A basin structure in rhyoliti c tuffs surrounded by the Permo-Triassic basement rocks, (3) Are-like distr ibution of the Roseki deposits along the eastern edge of the proposed calde ra, (4) Tectonic disturbance and intrusive bodies along the caldera wall, a nd (5) Presence of meso- and mega-breccias at the bottom of the caldera wal l. The proposed caldera, sizing NNW-SSE 23 km by ENE-WSW 15 km, may have yo unger, Inner caldera (15 by 15 km), defined by Lower Member of the late Cre taceous rhyolitic tuffs, thus composite in character. The Roseki deposits w ere formed after the Inner caldera collapse by hydrothermal fluids ascended through the caldera wall, then spread into the permeable horizon. This cal dera proposal bears a significant change in the regional exploration strate gy for the Roseki deposits.