ABSTRACT

The Hishikari Mine, the only gold mine operating in Japan, consists of epithermal gold-silver veins. In 2007, the mine produced 183,000 tonnes of ore, with gold grade of 46 g/t. The veins are extracted mainly by drifting and bench stoping with backfill. Blasted waste rocks are generally used as backfill materials and crushed waste rocks with cement are used for larger stopes. In order to extract one of the veins with wider mineralization and lower grade, closely located to the narrower vein with higher grade of gold, we have studied the applicability of the sub-level open stoping with backfill through in-situ measurements and numerical analyses. Backfilling controls the displacement of excavated surfaces and increases the stope stability, but practical evaluation of a larger open stope created by the sub-level open stoping than the conventional drifting and bench stoping in conjunction with supporting effects of backfilling has not been established yet. The paper presents an approach to estimate supporting effects of backfilling by using FLAC3D numerical analyses and design the backfill quality and mining sequence for more stable and steady operations.

1 INTRODUCTION

The Hishikari Mine, the only gold mine operating in Japan (Figure 1), consists of epithermal gold-silver veins. In 2007, the mine produced 183,000 tonnes of ore, with gold grade of 46 g/t. The Hishikari gold deposit was discovered by the Metal Mining Agency of Japan in 1981. Subsequent exploration and development by Sumitomo Metal Mining Co., Ltd. (SMM), the property owner, have proved Hishikari to be one of the outstanding gold deposits in the Japanese mining history (Ueno 1993). The Hishikari Mine consists of three deposits, namely Honzan, Yamada and Sanjin (Figure 2). The veins are extracted mainly by drifting and bench stoping with backfill. Blasted waste rocks are generally used as backfill materials and crushed waste rocks with cement are used for larger stopes.

In the earliest years, bench stoping of smaller dimension with the height of 11 m was adopted. Then the engineering evaluation of the rock mass was employed, and as the mining operators have advanced in skill and experience, the stope dimension has been getting larger (Sato et al. 2007). Today the height of stope dimension is 19 m in the Honzan district and 24 m in the Yamada district of the mine.

Large stope dimensions in bench stoping and sublevel open stoping make it possible to achieve higher productivity, but have a risk of inducing instability of the stope. Backfilling controls the displacement of excavation surface and increases the stope stability, but practical evaluation for stope dimension in conjunction with backfilling effects has not been established yet. In this paper, we have proposed an approach to evaluate supporting effects of backfilling by using numerical analyses and design the backfill quality and mining sequence for more stable and steady operations.

(Figure in full paper)

2 KE VEIN IN THE HISHIKARI MINE

The KE vein, located in the sanjin deposit, is divided the KE-2 vein and KE-3 vein (Figure 3).

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