1983 Volume 78 Issue 1 Pages 21-37
The strata-bound sulfide deposits of the Taro mine are located in the outer belt of the Kitakami massif. On the basis of lead isotope data, it was proposed that the ore deposits of the Taro mine is regarded as the Kuroko-type deposits of the Mesozoic age (Sato and Sasaki, 1978, 1980).
The Honko and the Shinkabu ore deposits at the Taro mine occur in the pelitic and psammytic sediments in the Harachiyama Formation consisting mainly of intermediate to acidic calc-alkaline lavas and pyroclastics. In this paper, a great deal is written about the features of ores of both stratiform ore deposits. The main minerals constituting the sulfide ores are pyrite, sphalerite, galena, chalcopyrite, and a small amount of pyrrhotite with quartz, calcite, sericite and chlorite as gangue minerals. Arsenopyrite occurs as accessory amounts. Tennantite and electrum are rarely observable. Barite, one of the main constituent minerals of the Cenozoic Kuroko-type deposit, is absent in the ore deposits at the Taro mine.
FeS and MnS contents of sphalerites show large variations ranging from 3 to 21 and 0.2 to 12.5 mole percent, respectively. However, CdS content of sphalerites shows the limited variation ranging from 0.2 to 1.0 mole percent. FeS contents of sphalerites in the ores which has pyrite, are less than about 15 mole percent, and those in ores pyrrhotite is present, are more than about 20 mole percent. Ranging from 15 to 20 mole percent of FeS in sphalerites are common with regard to coexisting pyrite and pyrrhotite. Alabandite rarely occurs at the hanging wall side of the Honko deposit, and is associated with pyrrhotite, chalcopyrite and Mn-rich sphalerite containing about 12 mole percent of Mos.
It is worthy of note that colloform texture, such as concentrically zoned, framboidal, pellet-like etc., locally exists in the ores from the marginal parts of the Honko and the Shinkabu stratiform ore deposits.
In the Taro mining district, the Harachiyama Formation is intruded by granodioritic rock of the Neocomian age and by diabase sills. owever, it is probably certain that some of pyrrhotite from the Honko and the Shinkabu ore deposits are formed regardless of thermal effects of these intrusive rocks. Both ore deposits at the Taro mine are presumed to be formed under the lower fs2 and fo2 conditions as compared with the Cenozoic Kuroko depos-its, and the ore of pyrrhotite-sphalerite-chalcopyrite-alabandite assemblage is considered to be formed under the environmental condition of oxidation-reduction boundary and its vicinities.