Pyrometasomatic Copper-Iron Sulphide Ore Deposits of the Yoshiwara

Mine, with Special Reference to the Structural Control of Ore Localization

Abstract

The Yoshiwara Mine is located in the well known copper zone of the Hiraodai-Sannotake District belonging to the late Mesozoic, metallogenetic province of West Chugoku ane North Kyushu. The prevailing rocks in the environs of the mine are Paleozoic sediments, such as clayslate, sandstone and limestone, and basic lava and tuff. These strata are intruded by discordant granodiorite batholiths and porphyrite dikes of Cretaceous age. The pyrometasomatic deposit of the mine is genetically related to the grano diorite.
The recognized sequence of igneous activity and mineralization is as follows : (1) intrusion of porphyrite, (2) intrusion of granodiorite, (3) formation of garnet-pyroxene skarn with deposition of copper-gold minerals, (4) deposition of iron minerals such as pyrite, hematite and magnetite, and (5) deposition of lead-antimony minerals. After the mineralization, the deposits were cut by some high-angle strike-slip faults trending N10°-40°E.
The most important factor in the localization of the main ore bodies is the fold structure of the limestone bed, and also, porphyrite dikes are a contributing factor in some pyrite ore bodies.
(1) Copper Ore Deposit No. 1 consists of some skarn ore bodies in a saddle-reef form. They lie along the crest of an east-plunging anticlinorium between clayslate (hanging wall) and limestone (foot wall). From this the writers consider that replacement has taken place along the anticlinorium apex of the folded limestone bed and that the clayslate of the hanging wall forms a cap rock for ascending mineralizing solutions.
(2) The upper part of the Copper Ore Deposit No. 2 shows a lenticular form in cross-section along the strike. It lies in the north limb of the anticline, where the limestone bed is irregularly intercalated with other sedimentaries.
(3) The lower part of the Ore Deposit No. 2 occurs as a pipe-like mass. It is enclosed within the folded thick limestone bed, and further, is associated with porphyrite dikes. The boundary between the ore body and the surrounding limestone is not sharp.
(4) Iron-sulphide deposits, such as the Sugitani, the Koguchi, the East, and the West Ore Bodies are lenticular or pipe-like bodies of massive pyrite which replace skarn-copper ore, porphyrite, limestone, etc. Some of them are controlled by the pre-existing porphyrite dikes.
The granite contact is not observed in the mining area. Ore may have come from hidden intrusives of granodiorite.