Fluid origins and ages of Au mineralization of the Hosen 8-2 vein, Hishikari deposit, Kagoshima, Japan

The oxygen and hydrogen isotope ratios of ore-forming fluids and formation ages were determined using quartz, adularia, and clay minerals in the Hosen 8-2 vein at −5 ML of the Hishikari deposit. Electrum, the dominant ore mineral, is found exclusively in the early band composed of microcrystalline quartz, granular adularia, and smectite. A relatively low Au-grade zone with Au grades below 100 ppm is identified in the southwestern part of the vein, while a ∼25-meter-wide bonanza zone with Au grades up to 10,800 ppm was identified in the central part. The northeastern part of the vein is referred to as the high Au-grade zone, with Au grades between 100 and 1000 ppm.

The calculated fluid δ¹⁸O values of the bonanza and high Au-grade zones calculated from quartz in the Hosen 8-2 vein are heterogenous (−6.3 to −0.8 ‰, and −6.8 to −2.9 ‰, respectively). This wide variation indicates the association of the mineralized zones with the boiling zone of upwelling deep fluids and the subsequent mixing with shallow meteoric water, which can be a useful vector of highly mineralized zones in the Hishikari deposit. Conversely, the low Au-grade zone is characterized by lower and more homogenous calculated fluid δ¹⁸O values (−5.2 to −4.0 ‰) that are closer to the isotopic composition of meteoric water.

The fluids that precipitated electrum and clay minerals have high δ¹⁸O values (+3.0 to +7.6 ‰) and low δD values (−132 to −100 ‰), which can be caused by the interaction between deeply circulating meteoric water and the sedimentary rocks of the Shimanto Supergroup at temperatures about 200–350 °C. It is also possible that δD values below −100 ‰ reflect contributions from waters released from Fe-poor hydrous sheet silicates in the host rocks.

The δ¹⁸O values of fluids calculated from adularia that formed after the early clay band range from −2.9 to +1.2 ‰. The progressive decrease of the fluid δ¹⁸O values across the paragenetic sequence can be attributed to the influx of ¹⁸O-depleted meteoric waters that mixed with Au-bearing ¹⁸O-enriched deep fluids.

The calculated fluid δD values of mixed smectite, illite, and interstratified chlorite-smectite from the altered host rock are much higher (−87 to −57 ‰) than those of the clays from the vein, although their fluid δ¹⁸O values are relatively similar (−0.3 to +10.2 ‰). These values indicate that the clays from the altered host rock may have originated from the water-rock interaction between shallow fluids and the volcanic rocks of the Hishikari Lower Andesites.

⁴⁰Ar/³⁹Ar dating of four adularia separates from the Hosen 8-2 vein yielded plateau ages from 0.930 ± 0.002 to 0.945 ± 0.001 Ma. The crystallization age of the low Au-grade zone (0.930 ± 0.002 Ma) overlaps within error with that of the bonanza zone (0.932 ± 0.002 Ma), suggesting roughly similar ages that is in contrast with their distinctly different Au grades. The high Au-grade zone formed from 0.945 ± 0.001 to 0.938 ± 0.003 Ma.