Sapporo-Iwanai district
is located at the intersection of the northeast Japan arc and
Kuril arc, associated with the subduction zone. This area is characterized by
Miocene submarine volcanism followed by Plio-Pleistocene subaerial
volcanism.

Koryu, a small
epithermal Au-Ag deposit, occurred in Pleistocene (1.4 - 0.85 Ma) within Miocene
sedimentary rocks near a subaerial andesitic volcano, while Toyoha, a large
epithermal Pb-Zn-Ag (-Au) deposit, also occurred near a subaerial andesitic
volcano. Although most of the base metal deposits formed in middle-late Miocene,
Toyoha formed in Plio-Pleistocene (2.93 - 1.60 Ma) within Miocene volcanic and
sedimentary rocks. This epithermal mineralization was partly overprinted by
xenothermal Cu-Sn-In(-Au) mineralization (2.68 - 0.49 Ma).

Hydrothermal alteration
differs vastly between Koryu and Toyoha: host rocks at Koryu were not
intensively altered, although wall rocks adjacent to veins were silicified,
while those at Toyoha show intensive and extensive neutral-pH alterations.
Geological and chronological studies show that middle~late Miocene (12.5 - 8.4
Ma) alteration was pervasive and overprinted by Plio-Pleistocene (2.93 - 0.49
Ma) alteration. The base metal veins in the middle-late Miocene were not well
developed, but were large in number with great depth.

These studies show that
the hydrothermal activity at Koryu is rather small and short-lived, while that
at Toyoha is large, long-lived and characterized by overprinting of different
hydrothermal systems. The subaerial magmatic activities yielded not only the
andesite lavas, but also generated hydrothermal solutions for the formation of
both Koryu and Toyoha. The metals were possibly derived from such magma. At
Toyoha, the hydrothermal solutions could also remobilize metals from old
ores.

The
size, life span of magmatic-hydrothermal system, and overprinting on the old
mineralizing systems could constrain the type of metal deposits (precious and
polymetallic deposits) nearby subaerial andesitic volcanoes in
Plio-Pleistocene.