Elemental Distribution of Nickel Laterite Deposit related to Weathering Process in Southeastern Sulawesi, Indonesia

Abstract:

Indonesia has plenty of ore deposits with diversity due to its complex tectonic settings. Sulawesi island is made by collision between three tectonic plates, Pacific, Eurasian and Indian-Australian plate. From this tectonic events, the East Sulawesi ophiolite had started to occur in the Late Oligocene. Peridotite is the most common bedrock comprising the East Sulawesi ophiolite and it is composed of lherzolite, harzburgite and minor amount of dunite. However, the dominant rock type is slightly different by areas.

The study area, Morowali, is located at the eastern part of the East Sulawesi ophiolite and next to Matano fault. Nickel laterite deposit occurring in Morowali has been distinguished into four different lateritic horizons from bottom to top: bedrock, saprolite, limonite and ferruginous cap. This lateritic sequences are the result of weathering process of harzburgite and lherzolite. Orthopyroxenes are dominantly observed in bedrock and all pyroxenes are serpentinized in saprolite and limonite horizons. In the lower saprolite horizon, nickel hydrous Mg silicate mineral called garnierite can be found. In general, garnierite is the main ore mineral in this deposit because of its high concentration of nickel. In this study we conducted XRF and EMPA analysis for major (Si, Al, Fe, Mg, Ca, Mn, Ni, Cr and Na) and minor (K, P and Ti) elements concentration on each horizons and minerals. Result shows that relatively immobile elements like Fe, Al, Cr and Ti are enriched in limonite horizon. This result is due to variations in pH conditions changing from acidic to alkaline environment as water, which dissolves elements above, flows downwards. The mobility of elements has a close relationship to the weathering process of peridotite.