全南 玉埋山鑛床의 熱水變質作用 및 鑛石鑛物에 대한 鑛物學的 연구

Hydrothermal Alteration and Mineralogy of Alunite and Kaolinite in the Ogmae Deposit, Southwest Jeonnam

The Ogmae alunite-kaolinite deposit occurs in acidic tuff, the Hwangsan Tuff, of upper Crataceous age in the Haenam volcanic field, SW Jeonnam. This deposit characterized by advanced argillic alteration formed 71.8±2.8{\sim}73.9±2.8 Ma ago in very shallow depth environment with acid-sulfate solution. Wallrock alteration can be classified into four zones from the center to the margin of the deposit: alunite, kaolinite, illite, and silicified zone. The mineral assemblage in the alunite zone, ore zone, is alunite-quartz-pyritekaolinite. Consideration of stability relation of these minerals suggests that the maximum alteration temperature is estimated at about 250℃ with solution pH of 3 or below assuming that pressure does not exceed 0.3 Kb. Alunite occurs as two different types; replacement and vein-type deposit. The former one consists of fine grained alunite and the later one coarse grained and relatively pure alunite that formed by open space filling. Isomorphous substitution of Na for K in these two types of alunites range 0 to 40 %, indicating that Na/K ratio in the solution is spontaneously changed during the alteration process. Alunite which has higher Na substitution probably formed in an earlier stage while the solution sustain high Na/K ratio. K-Ar age of alunites indicate that the replacement alunite formed earlier(73.9±2.8Ma) than the vein-type alunite(71.8±2.8Ma). The δ³⁴S value of pyrite and alunite indicate that those minerals formed at isotopically nonequillibrium state. The δ¹⁶O and δD values, of kaolintics 5.0 to 9.0‰ and -54 to -99‰, respectively, indicate that those are formed by hydrothermal solution having magmatic origin which have been diluted by low δD meteoric water.