1985 Volume 35 Issue 193 Pages 331-344
The geology of southwestern Hokkaido is characterized by intensive volcanisms of Miocene to Plio-Pleistocene age. The studied area, eastern margin of Lake Toya, is one of the centers of the volcanic field where Miocene andesitic and dacitic lavas and pyroclastic rocks (Osarugawa formation), Pliocene dacitic tuff (Kitayuzawa formation) and plagioryolite and Plio-Pleistocene andesite lavas are distributed. The volcanisms subsequently caused a concealed alteration zone, and mineralization such as Date Au-Ag-Cu-Pb-Zn deposits, Kamaya iron-roseki deposits and many other sulfur-iron deposits of Plio-Pleistocene age. The concealed alteration zone and alteration halos associated with the mineralization were examined in terms of field observations, mineralogical investigations and K-Ar ages determination. The results obtained are as follows:
(1) The, concealed alteration zone, Kamaya iron-roseki deposits and sulfur-iron deposits were generated by Pliocene plagioryolite and Plio-Pleistocene andesite volcanisms. The Au-Ag-Cu-Pb-Zn mineralization of the Date deposits was genetically associated with quartz porphyry intrusive to Miocene pyroclastic rocks.
(2) The concealed alteration zone is divided into five zones: mordenite zone (mordenite+clinopt-ilolite+mont-morillonite+mica/montmorillonite mixed layer minerals+kaolinite), laumontite zone (laumontite+Ib chlorite+mica/montmorillonite mixed layer minerals+1M mica+calcite), wairakite zone (wairakite+laumontite+IIb chlorite+1M·2M mica+anhydrite), mica zone (1M·2M mica+IIb chlorite+epidote+anhydrite) and pyrophyllite zone (monoclinic pyrophyllite+mica/montmorillonite mixed layer minerals+kaolin).
(3) In the Date Au-Ag-Cu-Pb-Zn deposits, the alteration characterized by 1M mica+nacrite+ dickite+mica/montmorillonite mixed layer minerals+dioctahedral chlorite at the center, while by montmorillonite+chlorite/smectite mixed layer minerals at the margin. K-Ar age of a mica-bearing altered rock was determined to be 5.2 Ma.
(4) The alteration zone of the Kamaya iron-roseki deposits is observed in plagioryolite and underlying Miocene pyroclastic rocks, The mineralogy changes from the center outwards as follow: triclinic pyrophyllite→monoclinic pyrophyllite→kaolin+dioctahedral chlorite→mica.
(5) The alteration zones of sulfur-iron deposits occur mainly in Pliocene dacitic tuff and overlying andesite lavas. The alteration minerals are dominated by kaolinite, dickite, nacrite, montmorillonite, dioctahedral chlorite am chlorite/smectite mixed layer minerals with disseminated pyrite and marcasite.
