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Compositional Variation of Hydrothermally Altered Volcanic Rocks in Hishikari Gold Epithermal System: A Useful Geochemical Indicator of Gold–Silver Epithermal Mineralization
Author(s) -
Shikazono Naotatsu,
Takahashi Hiroyuki
Publication year - 2010
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.1751-3928.2010.00120.x
Subject(s) - hydrothermal circulation , geology , andesite , mineralization (soil science) , geochemistry , mineralogy , chemical composition , volcanic rock , volcano , chemistry , soil water , soil science , organic chemistry , seismology
The hydrothermally altered andesite hosting the Hishikari gold‐silver vein deposits in southern Kyushu, Japan, is analyzed with respect to the spatial variation in chemical composition. The (CaO + Na 2 O) content is found to be inversely correlated with the K 2 O content as it progresses away from the site of mineralization. It was found that analytical data plotted on a (CaO + Na 2 O) − K 2 O diagram cannot be explained only by addition of K + from the hydrothermal solution to the original rock and release of Ca 2+ and Na + from the original rock (K‐ alteration). Addition of Ca 2+ and Na + from the hydrothermal solution to the rock and release of K + from the rock but release of K + , Ca 2+ , and Na + to the hydrothermal solution (advanced argillic alteration) is important for causing the wide variations in K 2 O, CaO, and Na 2 O contents on the (CaO + Na 2 O) − K 2 O diagram. These variations can be explained by superimposed potassic, advanced argillic and calcium alterations. The altered rocks in the Honko‐Sanjin area, Yamada area, and Masaki area analyzed by this study are characterized by their intermediate K 2 O content and variable CaO content, high K 2 O content and low CaO content, and low K 2 O content and low CaO content, respectively. The K 2 O, Na 2 O and CaO contents and oxygen isotopic composition of altered andesite, in conjunction with the solubility of gold as a thio complex, suggest that both gold deposition and the observed compositional variation of altered andesite are the result of mixing between acidic groundwater and neutral gold‐bearing hydrothermal solution. The present results indicate that the compositional variation of hydrothermally altered rocks may represent a useful geochemical indicator of epithermal gold–silver mineralization.