K-Ar ages of some vein-type and kuroko-type deposits in the southwestern Hokkaido

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K-Ar measurements on mica-bearing clay samples collected from Toyoha, Chitose, Horobetsu, Kagenosawa, Toya Takarada and Minamishiraoi deposits were made in an attempt to establish the metallogenic epochs of the southwestern Hokkaido, Japan.<BR>Clays collected from alteration halos of the Toyoha Pb-Zn-Ag vein-type deposits and the Chitose Au-Ag vein-type deposits give K-Ar ages of 2.2±0.6 Ma and 3.6±0.3 Ma, respectively. These values indicate that the ages of the vein-type mineralizations of the Toyoha and Chitose deposits were later than those of the kuroko mineralizations in Hokuroku area, northeast Honshu, Japan.<BR>Samples from the Horobetsu and Kagenosawa Au-Ag-Cu-Pb-Zn disseminated and vein-type deposits together with the Toya Takarada and Minamishiraoi kuroko-type deposits give K-Ar age values generally in the range of 12.3-14.2 Ma indicating that the ages of the mineralizations of these deposits were similar to those of the kuroko deposits in Hokuroku area.<BR>A clay sample which contains mica-montmorillonite mixed layer minerals from the foot wall clay zone of the Minamishiraoi deposit presents a little younger K-Ar age (11.6±0.6 Ma) than the mica-bearing clay of the clay zone whose K-Ar age is 12.5±0.6 Ma. This difference in age values can be explained by the existance of exchangeable potassium ions in mixed layer minerals.
資源地質学会の論文