D/H ratios, origins, and evolution of the ore-forming fluids for the Neogene veins and kuroko deposits of Japan

Abstract

The delta D values of inclusion fluids from quartz and calcite of the Au-Ag vein deposits range from -48 to -72 per mil, showing a strong correlation with values of the local meteoric waters. The 18 O/ 16 O ratios of the ore fluids of quartz, the filling temperatures of the fluid inclusions, and the isotopic fractionation factor between quartz and water show only slight shifts from these values in meteoric water.The delta D values of ore fluids for the Cu-Pb-Zn vein deposits vary from -42 to -74, showing a wider scatter within a single orebody than for the Au-Ag vein deposits. A weak but definite correlation exists between the D/H ratios of local meteoric waters and ore fluids for late-stage ore minerals. The ore fluids for the Cu-Pb-Zn veins have a 2 to 9 per mil positive oxygen isotopic shift from the meteoric values. These results and calculations based on a new model of rock-water interaction indicate that ore fluids for the Au-Ag vein deposits and for the later stage mineralization of the Cu-Pb-Zn deposits evolved from local meteoric waters under low rock/water ratios. Conversely, the major stage of mineralization of the Cu-Pb-Zn vein deposits precipitated from meteoric hydrothermal solutions that could have evolved under high rock/water ratios; this stage has a much higher salinity than fluids of the later stage. The contribution of magmatic fluids could have ranged from 0 to 20 volume percent in the major stage.The delta D and delta 18 O values for the Kuroko ore solutions were -10 to -30 and -1.0 to +1.0, respectively, for deposits in the Hokuroku basins, northeast Honshu, and -30 to -50 and about -2, respectively, for the Iwami mine, southwest Honshu. These isotopic results, the salinity data, and the calculations on the effect of rock-water interactions indicate that the Kuroko ore solutions of the Hokuroku basins could have evolved from mixtures of sea water and meteoric water. A small amount of magmatic water may have been present. The close association of sulfates and Mg-chlorites in the Kuroko deposits is the direct result of the incorporation of sea water into submarine hydrothermal systems.This study shows that, contrary to the prevailing thought, the vein mineralization occurred under terrestrial environments and, thus, is younger (upper Miocene to Pliocene) than the Kuroko deposits. All the constituents of the vein ores were leached from the Tertiary country rocks by the meteoric hydrothermal circulation. Sulfur isotopic studies indicate that the sulfur came from Miocene sea-water sulfate trapped in the Tertiary sediments as dissolved sulfates in pore water or as anhydrite and gypsum. The lead isotope abundances, mineralogy, and chemistry of both the veins and the Kuroko deposits also are in accord with the present results. The close association of sulfates and Mg-chlorites in the Kuroko deposits is the direct result of the incorporation of sea water into submarine hydrothermal systems.