1984 Volume 34 Issue 185 Pages 173-186
The Toyoha zinc-lead-silver ore deposits are typical vein-type ones in the Green Tuff Region of Southwest Hokkaido. It is well known that the veins of the Motoyama deposits are divided into two different groups, the earlier stage veins (Tajima, Harima etc.) and the later ones (Soya, Izumo, Sorachi etc.). The wall rocks of the deposits composed of andesite, dacite, basalt, pyroclastic and sedimentary rocks of Miocene age are hydrothermally altered. The wall rock alteration around the Tajima, Soya, Izumo and Sorachi veins which have different miner-alogical features is summarized as follows:
(1) Hydrothermal alteration around the Motoyama deposits is divided into the following five alteration zones.
A Chlorite-calcite
B Quartz-chlorite-calcite-albite K-feldspar
C Quartz-chlorite-sericite-pyrite
D Quartz-sericite-pyrite
E Quartz-sericite-kaoline-pyrite
(2) Chemistry of chlorite shows that Mg-Fe chlorite ((Fe+Mn)/(Fe+Mn)+Mg=0.3) was formed by alteration A, Fe-Mg chlorite (0.5) by B, and Fe chlorite (0.9) by C. Sericites from alteration zone of B, C and E scarcely contain any expandable layer, while sericite from alteration zone of D contains a small amount of expandable layer.
(3) As to the alteration process: 1. Regional alteration of A occurred widely around the Toyoha mine;
2. Alteration of B occurred locally in the mineralized area; 3. Alteration of C and D enveloped the veins; 4. Geothermal alteration of E took place at the Sorachi vein only.
(4) The hydrothermal alteration observed in the Tajima, Soya, Izumo and Sorachi veins is almost the same. The chemistry of chlorite, however, from the alteration zones of the Toyoha mine differs from that of the hydrothermally produced chlorite in the Chitose (Au-Ag) and the Ohe (Mn-Zn-Pb) mines of the same metallogenic province as the Toyoha mine.
