Akeshi is a small acid sulphate alteration associated gold deposit in the Nansatsu District of southern Kyushu, Japan, some 13 km north-east of the Kasuga gold deposit.
The deposit is developed within the 4.6 to 3.4 Ma Nansatsu Middle Volcanics and is younger than the ore at both Kasuga and Iwato which are similar gold deposits 6 to 13 km to the south-west. These volcanics are composed of 300 m of hornblende andesite flows and pyroclastics, and tuffaceous siltstone. Mineralisation has a much stronger structural control than at Iwato, reflecting the dominant fracture permeability in the host pyroxene andesite lavas.
The visible alteration consists of a quartz, varying from vuggy residual silica (with vugs often filled with sulphides and sulphur), to massive silica. Younger (<1 Ma) tuffs overlie most of the silica body, although the margin with argillised and weak silicification is observed on the northern margin.
There are three steeply dipping pipe like orebodies within the residual silica alteration. These have dimensions of: 1). 100 m NE-SW and 40 m wide dipping at 60° with a vertical extent of 150 m, 2). 50 m in diameter to a depth of 50 m below surface where it changes to a 40° plunge, and 3). a smaller isolated body. Underground exposures are more sulphide rich (up to 10%) compared to the lower sulphide contact in the oxidised zone of the open pit. Pyrite is the most common sulphide, followed by luzonite and then enargite, with covellite in late vugs. Primary gold is associated with luzonite and pyrite and varies from 1 to 100, averaging 15 microns.
(Source: Porter GeoConsultancy, www.portergeo.com.au, 2005)
