A Consideration on the Metamorphism of Cupriferous Pyritic Deposit at Sekizen, Ehime Prefecture

Abstract

The Sekizen ore deposit is situated at about 7 km east from the Besshi main ore deposit, both pyritic deposits occurring in the Sambagawa crystalline schist region.
The geological setting of the district in question is composed of spotted black schist, spotted quartz schist, spotted green schist and spotted epidote-hornblende schist, These crystalline schists are characterized by the remarkable presence of albite porphyroblasts, and the metamorphic grade of the rocks seems to correspond to the epidote-amphibolite facies. These rocks are intruded by small lenticular serpentinites which. are often found near the lenticular ore bodies.
Stratigraphically the horizon of the deposit belongs to the upper part of the Minawa formation in the Yoshinogawa group, and is found apparently 500m above the horizon of the Besshi main ore deposit.
The following remarkable features of the ore deposit, which must have been caused by the metamorphic action, are well observed:
(1) The structure of the ore bodies is concordant with that of country rocks, and intense intraformational folding is well observed in many parts of ore shoots.
(2) In some parts of the ore bodies fracture cleavage is often found in compact ores; the fracture cleavage may have been formed by the differential movement which acted upon the country rocks and ores of different competency.
(3) Under the microscope, biotite and green hornblende around ore minerals seem to have been formed in equilibrium relation with the ore minerals during metamorphic processes, because biotite and green hornblende with some ore minerals are not replaced by chlorite, epidote and sericite. In some ores stilpnomelane is associated with biotite as gangue minerals.
(4) In some ores quartz and chlorite occur as the "pressure shadow" around pyrite crystals.
(5) Oriented. structure of lamellar pyrrhotite which is observed under the ore microscope may have been caused by the formation and growth under stress.
(6) Ordinary ores of the deposit consist of pyrite, chalcopyrite, and considerable amounts of sphalerite, pyrrhotite, magnetite and hematite, associated with small amounts of valleriite, bornite, chalcocite, tetrahedrite (?), galena and native gold.
Valleriite occurs in exsolution-like form in chalcopyrite grains of pyrrhotite-rich parts, while blebs of chalcopyrite are found in some sphalerite crystals. These microscopic features may suggest the temperature condition which prevailed in the ore bodies. in question during the metamorphic processes.
The lenticular ore bodies are often accompanied by schistose serpentinites and actinolitic rocks, and such association is said to be useful as a guide to discovery of new ores.
Field evidences and microscopic features of the deposit may thus suggest the syngenetic origin of the deposit which was modified later by the regional metamorphism.