Auriferous pyrite formed by episodic fluid inputs in the Akeshi and Kasuga high-sulfidation deposits, Southern Kyushu, Japan

Mizuki Ishida,Mathieu Leisen,Fernando Barra,Martin Reich,Kentaro Nakamura,Rurik Romero,Kazutaka Yasukawa,Yasuhiro Kato

doi:10.1007/s00126-021-01053-4

Abstract

Pyrite geochemistry has proven useful for tracking changes in the composition and physico-chemical conditions of hydrothermal fluids in ore-forming environments. Here, we investigated the microtextural features and chemical composition of pyrite, a main Au-bearing phase in the Akeshi and Kasuga deposits (Southern Kyushu, Japan), to better constrain the ore-forming processes in these high-sulfidation epithermal Au deposits. Despite the widespread distribution of Au-bearing pyrite in both deposits, no visible Au minerals coexist with pyrite. However, in situ laser ablation inductively coupled plasma mass spectrometry results show that Au concentrations in pyrite vary from below the detection limit to 41 ppm and are positively correlated with Cu (r = 0.4; up to 7400 ppm) and Bi concentrations (r = 0.44; up to 640 ppm). In both deposits, high Cu and Au concentrations occur in small (< 25 μm) anhedral grains of pyrite, which are interpreted to have rapidly crystallized from the ore-forming hydrothermal fluid. In addition, dissolution–reprecipitation textures and thin, concentric, Cu-rich overgrowths were identified in a number of larger (> 25 μm) pyrite grains and aggregates. These abrupt changes in the trace element compositions of pyrite grains likely record episodic metal-rich fluid inputs. We also propose that gold adsorption onto growing pyrite surfaces played a key role in the mineralization of these deposits.

Highlights

Southern Kyushu, Japan, is known for its intense volcanic– hydrothermal activities related to the subduction of the Philippine Sea plate over the past 5 million years (Watanabe 2005; Mahony et al 2011)
Since pyrite grains are finely disseminated within the vuggy quartz matrix, it was not possible to discriminate pyrite associated with the initial leaching stage from pyrite formed during secondary quartz deposition
Two common characteristics of coupled dissolution–reprecipitation (CDR) were recognized in the studied pyrite grains: (i) a porous phase that preserves the external morphology of the original grain and (ii) sharp reaction fronts between the primary grain and product phases (Fig. 5c)

Summary

Introduction

Southern Kyushu, Japan, is known for its intense volcanic– hydrothermal activities related to the subduction of the Philippine Sea plate over the past 5 million years (Watanabe 2005; Mahony et al 2011). Abundant epithermal Au mineralization is associated with this volcanism, including all four currently mined epithermal Au deposits in Japan (Watanabe 2005; Garwin et al 2005). Other studies have focused on the Pliocene high-sulfidation Au deposits in the Nansatsu district, southern-most Kyushu. Earlier studies have reported ubiquitous pyrite in these deposits (Tokunaga 1954; Nakamura et al 1994; Hedenquist et al 1994b) as an important host of primary Au (Imai et al 1996; Morishita et al 2019). There has been few studies on the microtextural characteristics and the trace element composition of pyrite in these deposits

Results

Discussion

Conclusion