Petrogenesis of Andesitic Rocks In Sumalata, North Gorontalo

Abstract

Sumalata area locates in North Gorontalo or northern coast of North Arm Sulawesi. This area is rich in natural resources. One of those is a gold mine, which is managed by society. The area is covered by Miocene to Pliocene andesite and tuffs. Andesitic rocks are members of Diorite Boliohuto Formation. This study was conducted to determine petrogenesis of andesitic rocks. The research method used is field observations. The samples were analyzed by petrographic analysis. Field observations shows andesitic rocks consisted of andesite and quartz andesite. Most of the mineral are presence as opaque minerals such as pyrite and chalcopyrite. Petrographic analyses of 20 samples indicte four types of rocks namely andesite, trachyte, latite and quartz latite. Those rocks are formed by alkaline-intermediate magma. The differences of these rocks are their compositions of plagioclase, K-feldspar and quartz. In addition, the secondary minerals are chlorite, sericite, carbonates and opaque minerals. The presence of minerals supposedly due to the alteration of primary minerals such as pyroxene and plagioclase. This can occur if there is a process of alteration and metal mineralization.