Silver Mineralization at the Toyoha Mine, Hokkaido

Abstract

Two stages of vein-type mineralization, earlier and later, are recognized at the Toyoha mine which is the largest silver producer in Japan. The later stage is classified into five successive substages A, B, C, D and E. Cutting relations of the veins and veinlets indicate that the later-stage ore solution penetrated the earlier veins, and modified the earlier-stage mineral assemblage. Silver occurs in native silver, argentite (acanthite) and electrum in the earlier veins, while it exists in Ag(-Pb)-Sb and Ag-Pb-Bi sulfosalts, a tetrahedrite-tennantite-annivite solid solution, Ag-Sn sulfides, an unnamed Ag-In sulfide (AgInS2), and electrum in the later veins.
Underground and microscopic observations of the ores followed by electron microprobe analyses reveal that argentite and electrum in the earlier veins are attributed to the later-stage mineralization, and that majority of silver in this deposit precipitated during the substages B and C. That is, a redox reaction between silver-sulfide complexes in the ore solution and hematite-containing ore of the earlier stage caused deposition of argentite in the earlier veins, while mixing of the solution with ambient geothermal water caused dissociation of silver-chloride complexes and deposition of the silver minerals in the later veins.