Hishikari is an adularia-sericite low sulphidation bonanza vein epithermal gold orebody, with one of the highest average grades of gold recorded for a large deposits. It is located approximately 30 km north of the city of Kagoshima on the island of Kyushu in southern Japan.
In 2004 the total contained gold, both mined and in reserve, totalled 264 tonnes (8.5 Moz) comprising 3.5 Mt @ 60 to 70 g/t Au and 2 Mt @ 20 to 25 g/t Au. Annual production in 2011 was 0.205 Mt of ore averaging 37 g/t Au. Production to the end of 2010 had been ~193 t (6.2 Moz) of gold from 3.9 Mt of ore with an average grade of 49 g/t Au, with a remaining reserve of 150 t (4.8 Moz) of gold at similar grades (~3.5 Mt).
The geology of the deposit comprises a basement sequence of Cretaceous sediments and unconformably overlying Quaternary volcanics. The concealed vein system of the ore deposit is localised by this unconformity. The Cretaceous basement is made up of shale and sandstone, with minor tuffaceous shale and chert of the Shimanto Super Group. These sediments are heavily hematite stained for up to 20 m below the unconformity surface with the overlying volcanics, suggesting a palaeo-weathering surface, and form a restricted basement high in the vicinity of the orebody. The Quaternary volcanics, comprise sub-aerial deposited alternating andesites and dacites, both lavas and pyroclastics of the Hishikari Group, which are calc-alkaline in composition and have magnetic susceptibilities typical of the magnetite series. They are mostly Pleistocene in age (1.6 to 0.55 Ma) and comprise 5 units, with a basal andesite and an upper-most andesite, separated by two dacite and one andesite unit. They are in turn overlain by younger pyroclastics and alluvial accumulations.
The ore veins have been dated at between 1.04 and 0.78 Ma, in the Pleistocene, emplaced soon after the deposition of the Lower Hishikari Andesite, closely correlated with the overlying dacite unit.
The Hishikari deposit is composed of more than 125 veins (generally 1 to 3 m thick, to a maximum of 8 m), which strike at between 30 and 70°, distributed within an altered and mineralised corridor with a width of 500 to 1000 m and strike interval of ~3 km, and dip at from 70°NW to 70°SE. The individual veins are 300 to 400 m long, composed of crustiform banded quartz-adularia with subordinate smectite. The top of the vein system is generally >100 m below the surface, straddling the unconformity between the Shimanto Super Group and the mainly Pleistocene Hishikari Group. Some 60% of the ore is in the Cretaceous basement, the remainder being in the andesitic lowermost unit of the Hishikari Group.
The vein system forms two groups, the Sanjin-Honko section to the northeast, separated by a gap of ~300 m from the southwestern Yamada section. The Sanjin veins are a smaller set 100 to 200 m to the southeast of the main Honko vein swarm. The Honko-Sanjin deposit is predominantly within Cretaceous host rocks, where these form a basement rise, while a larger proportion of the Yamada section is in the Lower Andesite where this unit is thicker on the margin of the rise.
Hydrothermal alteration within the volcanic hosts can be grouped into four zones, namely a: i) cristobalite-smectite zone (cristobalite and/tridymite with smectite), ii) quartz-smectite zone (quartz with smectite and/or kaolin minerals, with pyrite), iii) a mixed-layer clay zone (chlorite/smectite and/or illite/smectite together with quartz, adularia, calcite, and laumontite) and iv) chlorite-illite (quartz-chlorite-adularia-calcite). Chlorite and illite/sericite are the principal alteration minerals surrounding the high grade Honko-Sanjin veins, while the lower grade Yamada vein extends into a smectite-mixed layer clay alteration phase. Mixed layer clays (zone iii) and quartz-smectite (zone ii) form a near horizontal layer of intense argillisation 50 to 100 m above the main mineralisation centre, with an argillic zone at the top consisting of quartz, kaolinite, and pyrite. These are followed by the shallowest and outermost alteration characterised by cristobalite-smectite (zone i), commonly with hydrothermal K feldspar, which in turn grades into unaltered andesite or dacite. The veins are mainly gold-silver bearing quartz-adularia containing electrum, naumannite and other silver sulphides, precipitated at temperatures of 215 to 175°C, and only contain a few percent sulphides, including prominent chalcopyrite, sulphosalts and selenides.
The original discovery is credited to the Metal Mining Agency of Japan in 1981, with the intersection of 15 cm @ 290 g/t Au, 200 m below surface. Subsequently the Sumitomo Metal Mining Co Ltd enlarged, proved and developed the deposit.
(Source: Porter GeoConsultancy, www.portergeo.com.au, 2011)
