Hematite-Ilmenite Exsolution Intergrowth in Quartz Schist Closely Associated with the Cupriferous Pyritic Deposits of "Kieslager" Type at the Shirataki Mine, Kochi Prefecture
1960 Volume 10 Issue 42 Pages 189-196
Details
1960 Volume 10 Issue 42 Pages 189-196
The ore-deposits of the Shirataki mine are of the "Kieslager" type. The ores consist mainly of pyrite and chalcopyrite with small amounts of magnetite and hematite. Magnetite and hematite occur in layers of quartz schist inter-bedded conformably with bedded sulfide ore-deposits. The paragenesis of this quartz schist are as follows:
magnetite—pyralspite—calcite—quartz and hematite—pyralspite—hornblende
Hematite containing a seriate arrangement of very fine ilmenite is commonly found only along pyral-spite-quartz laminae of quartz schist. The texture between hematite and ilumenite is concluded to be an exsolution intergrowth, which was once contained in unmetamorphosed country rock, on the following grounds:
1) The unmixing temperature of hematite-ilmenite solid solution is estimated at above 900°C. The country rock of the Shirataki ore-deposits belongs to the Sanbagawa glaucophanic metamorphic zone, which is characterized by high pressure and low temperature. It seems that the country rock has. never reached a temperature higher than 300°C during metamorphism.
2) The exolution texture is not found in contact with hornblende. Furthermore, the seriate arrangement of ilmenite in hematite runs rarely into sphene accompanied by hornblende. These matters seems to show that iron was spent to form hornblende only in exsolution hematite bodies.
3) Such a mineral assemblage as hematite-ilmenite-quartz may be stable in the Sanbagawa metamorphic zone.
The quartz schist is conformable to the bedding of the country rock and layered sulfide ore-deposits, and seems to have been derived from such an original rock as chert. The writer reported ferruginous. chert with hematite-ilmenite exsolution grains in the Nebutoyama mine, which belongs to the "Kieslager" type in a weakly metamorphosed area. These exsolution grains between hematite-ilmenite probably were supplied from the effusive matter of submarine volcanism within the Chichibu geosyncline.
This conclusion agrees well with the recent theory that the "Kieslager" resulted syngenetically from submarine volcanism.
