Hydrothermal Alteration of Limestone and Mineral Exploration of Zn‐Pb Skarn Deposits in the Sako‐nishi Area of the Kamioka Mine, Central Japan

Abstract

Abstract: Crystalline limestone of the Sako‐nishi area in the Kamioka Zn‐Pb mine, central Japan, is depleted in 18O and 13C toward the center of mineralization due to interaction with hydrothermal fluids with a dominant meteoric water component. The relationship between isotopic composition and mineral assemblage, texture, the chemical composition of the minerals, and the bulk chemical composition in the limestone was examined. A decrease in the δ18OSMOW value correlated with: (1) increase of fine‐grained calcite which is enriched in Mn and exhibits a bright cathodoluminescence, (2) progressive hy‐drothermal alteration of clinopyroxene in the original limestone into tremolite within the weakly‐altered zone, and into chlorite and actinolite within the strongly‐altered zone, (3) dominance of hydrothermal chlorite in altered limestone having δ18O values of less than 10%. This chlorite was enriched in Fe compared to mafic minerals in the unaltered limestone. The enrichment of Fe and Mn was more conspicuous in calcite and chlorite in skarn deposits. The occurrence and chemical composition of hydrothermal minerals in the limestone, skarn, and ore indicate that the 18O–depleted zones were formed in the later stage from fluids, which were responsible for mineralization and skarnization, and for Fe and Mn enrichment.The Al, Mn, and Fe contents, and the ratios of Mg/(Mg+Mn+Fe), Al/Mg, and Mn/Sr in the hydrochloric acid leachate of limestone varied with decreasing δ18O and δ13C values, reflecting increases in high‐Mn calcite and high‐Fe chlorite. These indexes were useful for the identification of hydrothermally altered limestone. Furthermore, the potential score weighted by each index was more effective and accurate means of detecting promising mineralization zones. An anomalous potential score due to the presence of hydrothermal minerals in the outcropping limestone occurred along the Atotsu–1GO fault. This structure indicates that the skarn deposits of the Sako‐nishi area belong to Mozumi‐type Zn–Pb skarn deposits, in which fissures and faults served as major passages for the hydrothermal fluid. High‐Mn carbonate and high‐Fe chlorite widely occur in base‐metal vein deposits and Zn‐Pb type skarn deposits. Leaching of altered rock with hydrochloric acid in addition to stable isotope composition and cathodoluminescence imaging is effective for geochemical exploration for hydrothermal deposits because it makes possible the detection of the elemental composition of hydrothermal minerals such as chlorite and carbonate and because of the rapidity and convenience of analysis.