PROCEEDINGS PIT IAGI LOMBOK 2010 The 39 th IAGI Annual Convention and Exhibition GEOCHEMISTRY CHARACTERISTICS OF NICKEL LATERITE DEPOSIT OF BAHODOPI AREA, CENTRAL SULAWESI, INDONESIA Didi Melkybudiantoro, Guntur Suryaning Hadi, Sigit Purnomo 1 1 Mine and Exploration Dept. PT International Nickel Indonesia ABSTRACT Laterite is a residual soil formation which is rich in iron and alumina, develops by weathering of the underlying mafic/ultramafic rocks. Ultramafic rocks have widely distribution in Bahodopi, one of concession area of PT INCO in Central Sulawesi Province. Based on physical and chemical properties, laterite in Bahodopi could be classified into three zones, un-weathered zone (fresh rock), semi-weathered zone (saprolite) and high weathered zone (limonite). It is slightly difficult to differentiate them by “naked eye” in the transition zone, limonite to saprolite and saprolite to fresh rock; therefore chemical properties of the material have to be assayed. PT INCO assayed data uses XRF analysis which produces 9 elements; Fe, MgO, Ni, Co, Al, Si, Cr, Ca, Mn. Limonite as high weathered zone generally has high Fe, low MgO, Ni and SiO2. It is a little bit easier to define limonite since it has homogeny material (fine material) while saprolite, due to its incomplete weathering, has heterogenic material such as; fine and coarse material which has high difference chemical properties, especially Fe and MgO. Coarser fraction results in higher MgO and lower Fe and conversely, finer material results in lower MgO and higher Fe. Keywords: Geochemistry, nickel laterite, profile weathering, Central Sulawesi. INTRODUCTION Laterite comes from Latin word “later”, which means a brick, introduced by Buchanan in southern India in 1807. The term laterite has developed and used to define a residual soil formation that is rich in iron and alumina and develops by weathering of the underlying rocks. Nickel laterite, one of laterite type deposit, is formed by specific underlying rock such as mafic/ultramafic rocks. It is required to produce nickel abundant in the laterite zone. Bahodopi is one of the Block of PT Inco concession which lies in Central Sulawesi Province. It has wide distribution of laterite deposit and extensively explored by PT Inco’s exploration team since 2004. SAMPLING METHODS AND LIMITATION Drilling full coring with HQ3 (diameter = 61.10 mm) type and regular drilled pattern was used in sampling techniques in Bahodopi (Figure 1). The coring data was prepared and then controlled by several quality control samples. The sample is analysed by using XRF Spectrometer and resulted 9 elements, such as; Ni, Co, Fe, SiO2, MgO, Cr, Al, Mn, and Ca. LOI (lost on Ignition) analysis also conducted to control the reliability of samples. This paper emphasises to discuss Ni element, since nickel laterite is the type of deposit discussed, and major elements (> 10% element percentage) such as Fe, SiO2 and MgO due to their sensitivities to layer changes in laterite zone. LOCAL GEOLOGY SETTING Central Sulawesi and parts of the SE arm of Sulawesi are composed of sheared metamorphic rocks and in the east a highly tectonised melange complex is present, together comprising the Central Sulawesi metamorphic. Similarities between the pre-Cenozoic rocks and some potassium–argon dates from the metamorphic rocks have been used to suggest that these regions, which include microcontinental
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PROCEEDINGS PIT IAGI LOMBOK 2010
The 39th IAGI Annual Convention and Exhibition
GEOCHEMISTRY CHARACTERISTICS OF NICKEL LATERITE DEPOSIT OF