김해지역 상동광상산 견운모의 광물화학 및 안정동위원소 조성

Mineral chemistry and stable isotope composition of sericite from the Sangdong sericite mine in the Kimhae area.

Mineral chemistry and stable isotope compositions of sericites from the Sangdong mine in the Kimhae area, Kyungsangnamdo, were studied. The Sangdong sericite deposit occurs in rhyolitic tuff of late Cretaceous age and considers to have been formed by the hydrothermal alteration. The sericites are classified as 2M₁ polytype and are characterized by less celadonite substitution indicating muscovite-phengite series. Their compositions are very close to that of the ideal muscovite but net layer charge ranges 1.71~1.91 which is less than 2per formula unit of ideal muscovite. Predominant interlayer cation is K and K/(K+Na) ratio ranges 0.91 and 0.93. δ¹⁸O values of sericites and quartz separated from the ore range 7.70~9.07 and 8.20~10.87‰, respectively. The formation temperature of sericite can be estimated as 315~340℃ based on δ¹⁸O value of sericite and δD value of of Cretaceous meteoric water. Their formation temperature discrepancy between coexisting sericite and quartz indicates that they are in isotopically inequilibrium. Two types of quartz, coarse grained phenocrysts and micrcrystalline aggregates are observed and the former must have been formed during volcanic eruption and remained isotopically unexchanged during hydrothermal alteration period. δ³⁴S values of pyrites range 1.9~4.5‰ which is within a range of volcanogenic sulfur, indicating magmatic source.