Petrochemistry of Late Miocene to Quaternary Igneous Rocks and Metallogenesis in Southwest Hokkaido, Japan |
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| Authors: | Eijun OHTA Yoshinobu KAWANO Mitsuru NAKAGAWA Hiroo KAGAMI |
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| Institution: | Hokkaido Branch, Geological Survey of Japan, Kita–8 Nishi–2, Kita–ku, Sapporo 060–0808, Japan [e-mail: ];Department of Geology, Saga University, Honjo–1, Saga 840–0027, Japan;Department of Geology, Niigata University, Igarashi 2–8050, Niigata 950–2102, Japan |
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| Abstract: | Abstract: Southwest Hokkaido is largely covered by Late Miocene to Quaternary igneous rocks, and has a large number of gold veins and base-metal veins of the same age. Investigation of the silica-normalized concentration of elements has revealed regional petrochemical zoning; large ion lithophile elements (LILE) and K2O/(Na2O+K2O) of the rocks increase toward Japan Sea, whereas total FeO, CaO, and 87Sr/86Sr decrease. Mapped concentration isoplethes of these elements are not ideally parallel to the volcanic front, but protrude to the west at Funka Bay, and to the northwest at Matsumae Peninsula. Isoplethes of 87Sr/86Sr show similar patterns and two more northwestward protrusions in the northeast (Jozankei block) of southwest Hokkaido. Contrary to the general petrochemical trend, both high– and low-LILE volcanic rocks occur in the Jozankei block. The ore deposits are distributed in four metallogenic zones; manganese–base–metal zone on the Japan Sea side, pyrite-limonite zone mainly along the volcanic front, gold zone in the middle, and two units of gold–base–metal zone. The northern unit of this zone is in the Jozankei block, and seems a part of the gold zone overlapped by the manganese–base–metal zone. Thus, as a rule, pyrite–limonite, gold, and base-metal deposits accompany low–, intermediate–, and high-LILE igneous rocks, respectively. Individual deposits and volcanic rocks make chains oblique to the zones and the volcanic front. The majority of the ore deposits are distributed along ridges of Bouguer anomalies overlapped by the volcanic chains, which apparently control the patterns of the petrochemical isoplethes. This is typical for two volcanic chains to the north and south of Funka Bay, where the petrochemical isoplethes protrude to the west. This indicates that both the igneous activity and the mineralization have been under the control of tectonic fractures at the roots of the volcanic chains. The geological, petrochemical and metallogenic data support the idea that the chemical characteristics of the deposits are correlated mainly with the chemistry of the associated magmas, and partly with that of the host rocks. |
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| Keywords: | zone chain vein gold manganese lead zinc Sr isotope LILE |
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