1987 Volume 37 Issue 201 Pages 15-28
Miocene granitic stocks intruding into Mesozoic sedimentary rocks and having available outcrop from 700 to 1400 meters above sea level show various vertical variations. The texture is hollocrystalline granitic in the deeper part but porphyritic in the shallower part, and many ore deposits are located in the shallower part. The magnetic susceptibility and arsenic content are lowest in the deeper part and increase generally upward. Pyrrhotite is seen only in the deeper part, while pyrite prevails in most other parts. Iron oxides indicate that oxygen fugacity of the granitoids is lowestin the deeper part and becomes higher in the shallower part. The total sulfur content is higher in the deeper part but is lower in the shallower part, and is highest in the top of the altered stock and also in the ore deposits.
It is suggested that quartz diorite-tonalitic I-type magma of the Green Tuff activity intruded forcibly into Mesozoic sandstone and shale, and received sedimentary sulfur and carbon into the magma chamber from the wall rocks as fluid phase by hydrothermal convection around the intrusive bodies. This unique mechanism of the crustal contamination would be the main cause for the hetrogeneous distribution of magnetic susceptibility and variety of iron oxides and sulfides assemblages in the granitoids and ore deposits. Evolutional trends of two representative ore deposits are shown in Figure 10. Intensive mineralizations around the Chichibu mine stocks could be explained by addition of ore-forming components to the ore deposits from the sedimentary wall rocks.
