Compositional Variation of Arsenopyrites in Arsenic and Polymetallic Ores from the Ulsan Mine, Republic of Korea, and their Application to a Geothermometer

蔚山의 철ㆍ중석 스카른광상에서 산출되는 硫砒鐵石은 그의 産出狀態ㆍ鑛物共生關係ㆍ化學組成을 근거로 세 가지 유형으로 구분된다. 硫砒鐵石 I 은 多金屬鑛化作用 초기에 정출된 것으로 주로 스카른대 내에서 산점상으로 분포하며, Ni-Fe-Co계 유화물과 밀접한 공생관계를 보여준다. 硫砒鐵石 I 의 화학조성은 Ni, Co의 함량이 현저하게 높고 As/S(原子比)>1으로 過剩의 비소를 함유한다. 硫砒鐵石 II는 Cu 또는 As 광석중에서 산출되며, 비독사석 휘창연석 비스무스 황동석 섬아연석과 밀접한 공생관계를 보여준다. 硫砒鐵石 II의 화학조성은 Ni, Co의 함량이 극히 미량이며, As/S>1으로 과잉(過剩)의 비소를 함유한다. 硫砒鐵石 III은 최후기 열수광맥 형성시기에 정출되었으며, 황철석 방연석 섬아연석 자류철석과 밀접한 共生關係를 보여준다. 硫砒鐵石 III의 化學組成은 As/S≦1로 過剩의 S를 함유한다. 硫砒鐵石 I 은 Ni, Co의 함유량이 1%이상이므로 地質溫度計로 사용할 수 없지만, 硫砒鐵石 II 는 비스무스-휘창연석의 共生關係를 보여 주고 있으므로, 이를 Kretschmar and Scott (1976)에 의한 1/T-f(S₂)도에 적용시켜보면 硫砒鐵石 II의 정출환경은 T=460~470℃, log f(S₂)=-7.4~7.0이고, 硫砒鐵石 III의 정출환경은 T=320~440℃, log f(S₂)=-9.0~7.0으로 추정된다.

Arsenopyrite in arsenic and polymetallic ores from calcic Fe-W skarn deposit of the Ulsan mine, Republic of Korea, has been investigated by means of electron microprobe analysis and X-ray diffractometry. As a result, it is revealed that the Ulsan arsenopyrite may be classified into the following three species with different generation on the basis of its mode of occurrence, chronological order during polymetallic mineralization and chemical composition; arsenopyrites I, II and III. 1) Arsenopyrite I-(Ni, Co)-bearing species belonging to the oldest generation, which has crystallized together with (Ni, Co)-arsenides and -sulpharsenides in the early stage of polymetallic mineralization. In rare cases, it contains a negligible amount of antimony. It occurs usually as discrete grains with irregular outline, showing rarely subhedral form, and is diffused in skarn zone. The maximum contents of nickel and cobalt are 10.04 Ni and 2.45 Co (in weight percent). Occasionally, it shows compositional zoning with narrow rim of lower (Ni+Co) content. 2) Arsenopyrite II-arsenian species, in which (Ni+Co) content is almost negligible, may occur widely in arsenic ores, and its crystallization has followed that of arsenopyrite I. It usually shows subhedral to euhedral form and is closely associated with löllingite, bismuth, bismuthinite, chalcopyrite, sphalerite, bismuthian tennantite, etc. It is worthy of note that arsenopyrite II occasionally contains particles consisting of both bismuth and bismuthinite. 3) Arsenopyrite III-(Ni, Co)-free, S-excess and As-deficient species is close to the stoichiometric composition, FeAsS. It occurs in late hydrothermal veins, which cut clearly the Fe-W ore pipe and the surrounding skarn zone. It shows euhedral to subhedral form, being extremely coarse-grained, and is closely associated with pyrite, “primary” monoclinic pyrrhotite, galena, sphalerite, etc. Among three species of the Ulsan arsenopyrite, arsenopyrite I does not serve as a geothermometer, because (Ni+Co) content always exceeds 1 weight percent. In spite of the absence of Fe-S minerals as sulphur-buffer assemblage, the presence of Bi(l)-Bi₂S₃ sulphur-buffer enables arsenopyrite II to apply successfully to the estimation of either temperature and sulphur fugacity, the results are, T=460~470℃, and log f(S₂)=-7.4~7.0. With reference to arsenopyrite III, only arsenopyrite coexisting with pyrite and “primary” monoclinic pyrrhotite may serve to restrict the range of both temperature and sulphur fugacity, T=320~440℃, log f(S₂)=-9.0~7.0. These temperature data are consistent with those obtained by fluid inclusion geothermometry on late grandite garnet somewhat earlier than arsenopyrite II. At the beginning of this paper, the geological environments of the ore formation at Ulsan are considered from regional and local geologic settings, and physicochemical conditions are suspected, in particular the formation pressure (lithostatic pressure) is assumed to be 0.5kb (50MPa). The present study on arsenopyrite geothermometry, however, does not bring about any contradictions against the above premises. Thus, the following genetical view on the Ulsan ore deposit previously advocated by two of the present authors (Choi and Imai) becomes more evident; the ore deposit was formed at shallow depth and relatively high-temperature with steep geothermal gradient-xenothermal conditions.