Tectonic Setting, Geology, And Gold And Copper Mineralization In Cenozoic Magmatic Arcs Of Southeast Asia And The West Pacific

Geologic setting: The Miocene to Recent Kuril magmatic

arc extends approximately 2,200 km from the northeastern

Kamchatka peninsula to southwestern Hokkaido, where it

connects to the Aleutian and northeastern Japan arcs, respectively

(Fig. 11; Table 1). The southwestern portion of the

Kuril arc is associated with the Kuril backarc basin, which

formed before the middle Miocene, due to northeast-southwest

rifting (Baranov et al., 2002). The basement rocks of the

southwestern Kuril arc consist of a Mesozoic accretionary

complex with a cover of Cretaceous and Paleogene sedimentary

rocks. Eocene to middle Miocene ilmenite-series granitoids

intrude the basement rocks (Ishihara et al., 1998). The

volcanism of the southwestern Kuril arc has changed from

middle Miocene andesitic activity to middle to late Miocene

bimodal basalt and rhyolite, including a period from 12 to 8

Ma with basalt-only volcanism. The andesitic and bimodal

volcanic activity migrated trenchward during the middle

Miocene (Watanabe, 1995). The middle to late Miocene bimodal

and basalt-only volcanism occurred mainly in a northsouth–

trending graben perpendicular to the arc trend

(Watanabe, 1995). The basalts of the Miocene bimodal assemblage

changed from island-arc type at 13 to 11 Ma to

backarc basin basalt at 9 to 7 Ma and again changed into island-

arc type at 5 to 4 Ma (Ikeda et al., 2000). Since the

Pliocene, bimodal volcanism in the backarc has disappeared

and andesitic volcanic activity at the volcanic front has become

dominant. This Plio-Pleistocene activity was associated

with formation of calderas several to ten kilometers in diameter,

which erupted large amounts of felsic ignimbrite (Ikeda,

1991).

East-northeasterly trending right-lateral strike-slip faults

were active during the late middle Miocene nearby the volcanic

front of the southwestern Kuril arc due to oblique subduction

of the Pacific plate (Watanabe, 1995). This fault

movement led to the westward migration and collision of the

Kuril forearc sliver with the northeastern Japan arc at southern

Hokkaido, forming the present concave joint between the

Kuril and northeastern Japan arcs (Kimura et al., 1983).

Mineral deposit styles: More than 40 low-sulfidation epithermal

gold and mercury deposits and prospects are distributed

in northeast Hokkaido at the southwestern Kuril arc

(Fig. 11, App. 2). They are associated mainly with rhyolitic

intrusions and domes of the Miocene bimodal assemblage.

The host rocks of these deposits are Cretaceous to Paleogene

sedimentary rocks and Miocene sedimentary and volcanic

rocks. These

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