Importance of trans-crustal geological processes in understanding the genesis of giant epithermal gold deposits: The case of Hishikari deposit, Japan

Epithermal deposits are formed in shallow (< 1 km) crustal environments by precipitation of ore minerals from hydrothermal fluids, associated with magmatic activities. Epithermal deposits are important sources of gold and associated critical metals (e.g., Ag, Bi, Te) used in high-tech industry (e.g., [1]). However, little is known about the controls on their deposit sizes, which range more than two orders of magnitude (<1 to >100 t of contained Au). Since deposit size is a major determining parameter for mine profitability, estimating the governing processes for metal endowment should have important implications for exploration. Here we show that the giant (> 200 t Au) Hishikari epithermal gold deposit in south Kyushu, Japan, is likely to have formed by combination of the following three trans-crustal geological processes: (i) oxidized source mantle of primary magmas, (ii) differentiation of ore-forming magmas without voluminous sulfide saturation and (iii) structural focusing of exsolved ore-forming fluids. These geological processes were recognized by (i) high V/Yb ratio (> 160) of high MgO regional volcanic rocks which reflect the oxidizing [2], high sulfur and gold solubility condition during mantle melting, (ii) minor contribution from reducing crustal rocks [3] and late magnetite saturation which maintained a relatively high fO₂ as well as high sulfur solubility until later stage of differentiation, and (iii) basement uplift or depression detected by geophysical surveys [4] which created effective conduits for hydrothermal fluids. In contrast, representative districts in Japan with previous extensive exploration campaigns (Toyoha, Nansatsu, Iriki, Masaki) failed to meet more than one of the above criteria, which likely explain the smaller or lack of gold mineralization in these districts. Our findings demonstrate the importance of optimally aligned trans-crustal geological processes in the formation of giant gold deposits and the potential utility of the geochemistry of regional volcanic rocks in early stages of epithermal gold exploration.

[1] Goldfarb, R. J., et al. (2016). Reviews in Economic Geology 18: 217-244.
[2] Laubier, M., et al. (2014). Earth and Planetary Science Letters 392: 265-278.
[3] Hosono, T., et al. (2003). Chemical Geology 201: 19-36.
[4] Izawa, E., et al. (1990). Journal of Geochemical Exploration 36: 1-56.