Kasuga, Nansatsu District High Sulphidation Gold-Silica Deposit: On the third day of the Tour in Japan, the group visited the Kasuga deposit, which is located 5 km west of the city of Makurazaki on the southern coast of Kyushu. On arrival, the group was given a geological briefing by Mr Eiichi Naruii, CEO and President of Kasuga Mines, a geologist, supported by Dr Toshihiko Hayashi, Director General and Chief Geologist of the Metals Exploration Department of JOGMEC, the Japan Oil, Gas and Metals National Corporation, who wrote some of the definitive papers on the Nansatsu deposits. Following the presentation, Mr Naruii (blue coat in the left image) and Dr Hayashi (on the right in both the left and central images) took the group into the pit to study and discuss exposures of the hosts, ore and alteration styles.
The Kasuga deposit contains ~2 g/t Au in a siliceous host-rock that is sold as a smelter flux, with the gold being recovered from the resultant smelting process. A cluster of Nansutsu deposits form an ~8 km diameter semi-circular string of exposures around Makurazaki harbour, with Kasuga's sister mine, Iwado, being one of those to the east of Makurazaki. The country rocks comprise Jurassic sandstones and shales of the Shimanto Group, intruded by minor Mid-Miocene quartz porphyry, overlain by Late Miocene to Pliocene andesites and tuffs of the Nansatsu Group, covered by welded tuffs of the unconformably overlying Quaternary Ata and Aira Pyroclastics. The Nansatsu deposits formed at ~4 to 5 Ma, and occur as irregular structurally and lithologically controlled hydrothermal silica-gold replacement bodies within the Nansatsu Group, exposed as basement highs and erosion windows through the Quaternary pyroclastics. The silica core of each occurrence is enveloped by a narrow halo of argillic alteration, zoned from an inner alunite-kaolinite, through to an outer smectite and/or interstratified illite/smectite interval, into fresh country-rock. It is underlain by deep alteration characterised by abundant pyrite and typically Na-rich alunite, with dickite dominating over kaolinite. At depth, the alunite-dickite grades into a sericite-chlorite assemblage. A single deep (1200 m) drill hole beneath Kasuga encountered disseminated and veinlet sphalerite and galena mineralisation with quartz and epidote veining, from a depth of ~800 m, consistent with the lateral periphery of a porphyry system.
The pyritic silica core is evident as the light zone in the background image above, fringed by the mottled 'yellow' to 'red-brown' to 'dark-grey' argillic zone, overlain by the brown Ata Pyroclastics in the upper parts of the benches. Photographs by Mike Porter.