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Gold Abundance and Stable Isotopes (H, O, S) in Hydrothermal Alteration Zones of Gasa Island (Gasado), Korea
Author(s) -
Yoon ChungHan,
Jung HyoungSu
Publication year - 2008
Publication title -
resource geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.597
H-Index - 43
eISSN - 1751-3928
pISSN - 1344-1698
DOI - 10.1111/j.1751-3928.2007.00046.x
Subject(s) - alunite , argillic alteration , geology , geochemistry , pyrite , sericite , andesite , pyrophyllite , meteoric water , hydrothermal circulation , mineralogy , rhyolite , vein , volcanic rock , volcano , psychology , psychiatry , seismology
Geology of the Gasa Island (Gasado), Korea, consists mainly of tuffaceous rocks, rhyolite and andesitic rocks related to Cretaceous volcanic activity. These rocks are hydrothermally altered, and are classified into the following four alteration zones based on the alteration mineral assemblages: advanced argillic alteration (alunite‐pyrophyllite‐kaolinite‐pyrite); sericitic alteration (sericite‐kaolinite‐quartz); propylitic alteration (quartz‐chlorite‐carbonate‐pyrite); and silicified zones. Alunite in the advanced argillic zone occurred in two types; a massive or disseminated type and a vein type. Most of the massive or disseminated alunites are ≥50 μm in size, whereas the size of vein alunites is <20–30 μm. Alunite grain size is greater in the central part of disseminated or massive alunite, while it is smaller toward the margins. The gold content of each alteration zone is 21–2900 ppb, 15–88 ppb, 57–1730 ppb, and 2–231 ppb, respectively. The gold content of quartz veins developed in the alteration zones is 39–715 ppb. Gold is enriched in the minerals and rocks around faults and fissures, and is strongly concentrated in the advanced argillic alteration zone around faults. Hydrothermal solutions traveling along the fracture systems might be responsible for the comparatively high gold content in the study area. δ 34 S of alunites occurring in the advanced argillic alteration zone range from +16.5 to +3.9‰, although most are in a comparatively narrow range from +8.6 to +5.2‰. There is no difference between disseminated or massive and vein alunites. The δ 34 S of pyrites in the advanced argillic alteration zone are from +4.8 to −2.9‰. Oxygen and hydrogen isotope values of alunites are from +8.5 to 0‰ and from −59.6 to −97.3‰, respectively. With an assumed temperature of 200°C, δD and δ 18 O of hydrothermal solutions calculated for alunites are from −53.6 to −91.3‰, and from −2.4 to −8.1 for massive or disseminated alunites and from −6.6 to −10.9‰ for vein alunites, respectively. These data suggest that meteoric water dominated during the alunite formation. Isotopic data, geological setting, mineralogy, size of alunite and pure alunite composition (K end member) indicate that alunites of the study area were formed in the steam‐heated environment of acid sulfate alteration.