The Hisatsu volcanic province is a northward curved belt of about 100 km in length from northwest to southeast and 25 km wide in the northern part of Southern Kyushu. In this province, the volcanic rocks (Hisatsu volcanic rocks) are continuously distributed throughout most areas with an average thickness of more than 400 m. The volcanic activity in this province occurred in the period from late Miocene to middle Pleistocene. It is divided into three stages based on stratigraphy, lithology, age and mode of occurrence of rocks. In the first stage, the Hisatsu volcanotectonic depression was formed and a large amount of andesitic tuff breccias and some lavas erupted in the depression from 7.6 Ma to 2.5 Ma. In the second stage, andesite lavas formed vast plateaus (flood andesite) during the late Pliocene (2.5 Ma to 2 Ma), and in the third stage from 2 Ma to 0.4 Ma, there were continuous eruptions of the flood andesites. The volcanic rocks of these stages were distributed in all areas of the Hisatsu volcanic province.
     The Hisatsu volcanic rocks are divided into low-potassium (low-K) and high-potassium (high-K) andesites based on chemistry and mode of occurrence. In the first stage, the low-K andesites occurred in the west side of the province and the high-K andesites in the east side. In the second stage, the high-K andesites widely erupted throughout the area. In the third stage, the low-K andesites erupted in the west side again, and the high-K andesites in the east side. In this stage, the low-K andesites also occurred at the easternmost area of the Hisatsu volcanic province. It is thus deduced that the centers of volcanism of the low-K andesites and the high-K andesites existed at west and east, respectively. The volumes of ejecta in the first, the second and the third stages were estimated at about 390, 110 and 60 km³, respectively. The gross volumes of low-K andesite and high-K andesite over the three stages were estimated at about 336 and 224 km³, respectively. Three kinds of parental magmas (high-K high-magnesian andesite, low-K high-magnesian andesite and tholeiite magmas) and four trends of fractional crystallization are recognized in the Hisatsu volcanic rocks. The Hisatsu volcanic activities have some characteristics differing from those of subduction zone magmatism, so it is assumed that the Hisatsu volcanic rocks originated from an ascending heterogeneous mantle. The Hisatsu volcanism represents a main part of the activity involving the upwelling of a 100 km diameter mantle in Southern Kyushu.

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The Hoshino gold area is located in the western part of the Hohi volcanic zone, northern Kyushu. Volcanic rocks in this area vary from andesitic rocks in the north to dacite and rhyolite in the south. The basement is constituted by metamorphic rocks of pre-Cretaceous age. The volcanic rocks of Pliocene age were subdivided into eight volcanic units.
     Seven fission track ages of zircons from five volcanic units have been determined, using the external detector method. The age data obtained, combined with some previously reported ages, show that two main volcanic activities have occurred in the area. The first volcanic activity took place around 4.3 Ma, and resulted into the deposition of the Hoshino Andesite and the Ikenoyama Conglomerate. The second main volcanism started around 3.5 Ma, and was characterized by the eruption of the Shakadake Andesite and the Reiganji Andesite at the early stage, then, by more acidic rocks of the Takeyama Andesite, the Hyugami Dacite and the Kuroki Rhyolite at the later stage. The main volcanism in the area ceased around 2.6 Ma.

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