1983 Volume 33 Issue 178 Pages 115-129
In order to disclose the formation processes of veins of the Motoyama deposit, Toyoha mine, an attempt has been made to examine collectively their geologic and tectonic setting, structural controls and mode of distribution of ore metals. Some of our conclusions are summarized as follows:
(1) Fissure systems responsible for the mineralizations are due primarily to the formation of a caldera-like tectonic depression with 5km×4km in horizontal dimension, which was probably followed by the emplacement of the dacite lava dome extruded at a certain period of late Koyanagizawa stage.
(2) The mineralizations can be divided chronologically into two groups, i.e., early stage mineralizations and later stage mineralizations. Between the two groups of mineralizations is a striking contrast in mode of distribution of ore elements, especially as to Pb/Zn ratio and total sulfides content.
(3) Fissures of Tajima and Izumo veins are suggested to have played principal roles as source feeders of ore fluids responsible for the two groups of mineralizations in the Motoyama deposit.
