Analysis of Ultramafic Rocks Weathering Level in Konawe Regency , Southeast Sulawesi , Indonesia Using the Magnetic Susceptibility

* Corresponding author : jahidin_geofisika@uho.ac.id Tel.:+62­81­388­3535­48; fax: ­ Received: Dec 13, 2019. ; Accepted:May 20, 2020. DOI 10.25299/jgeet.2020.5.2.4247  Abstract The Konawe region is part of the Sulawesi Southeast Arm ophiolite belt where ultramafic rocks are dunite and peridotite. The formation of nickel deposits is closely related to the weathering process of ultramafic rocks as a source rock. Ultramafic rocks exposed to the surface will experience weathering which is influenced by many in the form of climate change, topography, and existing geological structures The weathering process in the source rock can influence variations in chemical elements and magnetic properties in laterite soil profiles. For example, the c could affect granulometry as well as the quantity of magnetic minerals in the soil. Condition of weathering of ultramafic roc (initial, moderate and advanced) can affect nick mineral is an indication of the lateralization process that occurs in ultramafic rocks and is carried out through petrographi analysis of thin cuts and polish cuts. Determination mineral serpentine. In this study, the determination of the weathering level of ultramafic rocks (initial, moderate, and continued) uses magnetic susceptibility parameter. A total o from different places and weathered levels were analyzed. The results of the research have shown that the magnetic susceptibility of ultramafic rocks in the study area varies, from susceptibility, magnetic minerals contained in ultramafic rock samples are hematite and geotite minerals. This means that the weathering level of ultramafic rock samples is the continued wea ultramafic rocks in the study area produces nickel laterite deposits with a nickel content of 1.65 0.42% in the saprock zone, and 0.20 ­ 0.51% in the basic rock


Background of Research
The Konawe area is part of the Sulawesi Southeast Arm ophiolite belt. In this section, ultramafic rocks are formed in the form of dunite and peridotite ( 2010).
The formation of nickel deposits is closely related to the weathering process of ultramafic rocks as a source rock. Ultramafic rocks exposed to the surface will experience weathering which is influenced by many factors, including in the form of climate change, topography, and existing geological structures. The weathering process in the source rock can influence variations in chemical elements ( The Konawe region is part of the Sulawesi Southeast Arm ophiolite belt where ultramafic rocks are dunite and peridotite. The formation of nickel deposits is closely related to the weathering process of ultramafic rocks as a source rock. Ultramafic rocks exposed to the surface will experience weathering which is influenced by many in the form of climate change, topography, and existing geological structures. The weathering process in the source rock can influence variations in chemical elements and magnetic properties in laterite soil profiles. For example, the chemical weathering might affect magnetic mineralogy and the physical weathering could affect granulometry as well as the quantity of magnetic minerals in the soil. Condition of weathering of ultramafic roc (initial, moderate and advanced) can affect nickel content in laterite sediments. The weathering profile study of serpentine mineral is an indication of the lateralization process that occurs in ultramafic rocks and is carried out through petrographi analysis of thin cuts and polish cuts. Determination of weathering level like this is based on the level of weathering of the In this study, the determination of the weathering level of ultramafic rocks (initial, moderate, and continued) uses magnetic susceptibility parameter. A total of 232 ultramafic rock core samples obtained from 34 hand samples were taken from different places and weathered levels were analyzed. The results of the research have shown that the magnetic susceptibility of ultramafic rocks in the study area varies, from 580 x 106 SI to 4.724 x 106 SI. Based on the value of magnetic susceptibility, magnetic minerals contained in ultramafic rock samples are hematite and geotite minerals. This means that the weathering level of ultramafic rock samples is the continued weathering level. The level of continued weathering that occurs in ultramafic rocks in the study area produces nickel laterite deposits with a nickel content of 1.65 0.51% in the basic rock zone (bedrock).
The Konawe area is part of the Sulawesi Southeast Arm ophiolite belt. In this section, ultramafic rocks are formed in the form of dunite and peridotite (Surono, formation of nickel deposits is closely related to the weathering process of ultramafic rocks as a source rock. Ultramafic rocks exposed to the surface will experience weathering which is influenced by many factors, including in the form of climate change, topography, and existing geological structures. The weathering process in the source rock can influence variations in chemical elements  and magnetic properties in laterite soil profiles . For example, the chemical weathering might affect magnetic mineralogy and the physical weathering couldaffect granulometry as well as the quantity of magnetic minerals in the soil. The process of soil formation is divided into several zones with varying thickness and mineral element content  soil deposits (Sundari, 2012).
Laterite soils or commonly called laterite or red soil isa type of infertile soil that is fertile and rich in nutrientrich soil, but it is lost because it is dissolved by high rainfall. This type of soil has a low cation exchange capacity (which causes the metabolic The Konawe region is part of the Sulawesi Southeast Arm ophiolite belt where ultramafic rocks are exposed in the form of dunite and peridotite. The formation of nickel deposits is closely related to the weathering process of ultramafic rocks as a source rock. Ultramafic rocks exposed to the surface will experience weathering which is influenced by many factors, including The weathering process in the source rock can influence variations in chemical elements and magnetic properties in hemical weathering might affect magnetic mineralogy and the physical weathering could affect granulometry as well as the quantity of magnetic minerals in the soil. Condition of weathering of ultramafic rocks el content in laterite sediments. The weathering profile study of serpentine mineral is an indication of the lateralization process that occurs in ultramafic rocks and is carried out through petrographic of weathering level like this is based on the level of weathering of the In this study, the determination of the weathering level of ultramafic rocks (initial, moderate, and continued) uses f 232 ultramafic rock core samples obtained from 34 hand samples were taken from different places and weathered levels were analyzed. The results of the research have shown that the magnetic 6 SI. Based on the value of magnetic susceptibility, magnetic minerals contained in ultramafic rock samples are hematite and geotite minerals. This means that the thering level. The level of continued weathering that occurs in ultramafic rocks in the study area produces nickel laterite deposits with a nickel content of 1.65 2.40% in the saprolite zone, ) and magnetic properties in laterite . For example, the chemical weathering might affect magnetic mineralogy and the physical affect granulometry as well as the quantity of magnetic minerals in the soil. The process of soil formation is divided into several zones with rying thickness and mineral element content , for example in laterite . Laterite soils or commonly called laterite or red soil isa type of infertile soil that is fertile and rich in il, but it is lost because it is dissolved by high rainfall. This type of soil has a low cation exchange capacity (which causes the metabolic process of plants to be disrupted (Sudarningsih, 2008). According to Sembiring (2008) the land of exlaterite nickel mining actually shows the condition of the soil that has damaged structure and compaction so that it has a negative effect on the water system and aeration which can directly affect the function and development of roots. This causes the plants to grow normally, dwarf, wither, and die. The deterioration of the soil structure also affects the soil that is unable to store and absorb water during the rainy season resulting in soil erosion. Conversely, in the dry season the soil becomes hard and dense, so the soil becomes difficult to cultivate. Therefore, efforts are needed to increase soil fertility.
Exploration of laterite nickel deposits is thought to be related to weathering of ultramafic rocks in the formation of laterite nickel deposits and the presence of erosion material spread over the surface. In humid tropical climatic conditions, ultramafic rocks decay very quickly and produce ore residues containing nickel, chromium, or iron (Sudarningsih, 2008). The weathering profile study of serpentine mineral is an indication of the lateralization process that occurs in ultramafic rocks and is carried out through petrographic analysis and polish cuts . Determination of weathering levels like this is based on the level of weathering of the mineral serpentine.
In this study, the determination of the weathering level of ultramafic rocks (early, moderate, and continued) uses magnetic susceptibility parameter. This is new and is expected to be an inexpensive and environmentally friendly alternative method for assessing weathering of ultramafic rocks and their relationship to nickel content. An illustration of the relationship of weathering levels of ultramafic rocks with nickel content can be used to support the exploration of the presence of laterite nickel.

Basic Theory
According to regional geology, Sulawesi is located at the confluence of 3 large plates, which causes very complex tectonic conditions, where a collection of rocks from the archipelago, ophiolite, and chunks of microcontinent are carried along with subduction, collision and other tectonic processes (Surono,2010). For the Southeast Sulawesi region which is in the East Sulawesi Ophiolite Lane group, the rocks consist of mafic and ultramafic rocks accompanied by pelagic and melange sedimentary rocks in several places. Ultramafic rocks are dominant in the Southeastern Arm, but the mafic rocks are dominant further north, especially along the North coast of the Southeast Arm of Sulawesi (Fig. 1).
The weathering rock is a process of physical disintegration and chemical decomposition of rock material that is on the surface or near the surface of the earth (Parker, 1997. Ultramafic rocks that undergo chemical weathering will change the composition of the mineral, as illustrated in Table  1.

Research Method
Samples in the form of ultramafic rocks and soil analyzed in this study were taken at Pondidaha District and Puriala District Konawe Regency. The samples were taken at the nickel mining site, rock mining, and post nickel mining. For rock samples, they are taken in the form of hand samples and made in the core for magnetic susceptibility measurement purposes and made in powder form for the measurement of mineral / elemental content.
Measurement of the magnetic susceptibility of ultramafic and soil rock was carried out on 232 ultramafic rock core samples obtained from 34 hand samples and 20 soil samples taken around ultramafic rocks. Magnetic susceptibility values for each rock and soil sample site were measured using the MS2B susceptibilitymeter.
Measurement of mineral content/sample elements was performed using XRay Difraction and XRay Fluorescence.

Magnetic Susceptibility of Samples
Magnetic susceptibility is a function of the concentration, grain size and type of magnetic minerals. Variable magnetic susceptibility values indicate the concentration of magnetic minerals, grain size, and types of magnetic minerals that vary . The greater the value of magnetic susceptibility means the more concentration of magnetic minerals. High magnetic susceptibility also shows that magnetic mineral typesare dominated by ferrimagnetic and ferromagnetic magnetic minerals, magnetic susceptibility in the medium category is dominated by paramagnetic and antiferromagnetic magnetic minerals, whereas magnetic susceptibility is very low (negative) including nonmagnetic (diamagnetic) minerals.
The magnetic susceptibility values for each rock and soil sample site measured using the MS2B susceptibility instrument can be seen in Table 2.

SI)
Based on the Table 2, it can be seen that the magnetic susceptibility value in ultra samples and soil samples in the study area varies. Ultramafic rock samples have magnetic susceptibility values ranging from580 x 10 6 SI to 4,724 x 10 soilsamples have magnetic susceptibility values of 37.9 x 10 8 m3/kg to 991.3 x 10 8 m3/kg. Ultrabasic rock samples such as P3, ST2, and ST3 sites in Puriala District show a greater magnetic susceptibility than other sites. The magnitude of the magnetic susceptibility value indicates that the concentration of magnetic minerals in the sample is higher and is suspected to have a different type of magnetic mineral than the others. In soil samples taken from around the area of the presence of ultramafic rocks, the magnetic susceptibility value per unit of mass indicates that the soil samples originated from weathering of host rock(ultramafic rocks). Based on the value of magnetic susceptibility and field observations, it is suspected that soil samples contain the same magnetic minerals as ultramafic rocks.
From the results of the measurement of magnetic susceptibility in some ultramafic rock samples that  , it can be seen that the value in ultramafic rock samples and soil samples in the study area varies. rock samples have magnetic susceptibility SI to 4,724 x 10 6 SI and samples have magnetic susceptibility values of m3/kg. Ultrabasic rock samples such as P3, ST2, and ST3 sites in Puriala District show a greater magnetic susceptibility than magnitude of the magnetic value indicates that the concentration of magnetic minerals in the sample is higher and is suspected to have a different type of magnetic mineral than the others. In soil samples taken from around the resence of ultramafic rocks, the magnetic susceptibility value per unit of mass indicates that the soil samples originated from weathering of host rock(ultramafic rocks). Based on the value of magnetic susceptibility and field observations, it is suspected that soil samples contain the same magnetic minerals From the results of the measurement of magnetic susceptibility in some ultramafic rock samples that produce two or more core samples from hand sample drilling, it is found that the magnetic susceptibility values for the cores above tend to be greater than the cores below. The above cores are samples taken from rock drilling in the uppermost structure, while the next cores are in the lower structure. The existence of the upper core has a high magnetic susceptibility compared to the bottom core due to weathering ultramafic rock always starts at the top of the structure towards the bottom structure. This allows a greater concentration of magnetic minerals in the upper core than the bottom core. The presence of magnetic susceptibility values of the lower core is higher than the upper core may be caused by the structure in the form of fractures at the bottom so that it will facilitate the entry of water and means the weathering process will be more intensive. As a result, the concentration of magnetic minerals at the bottom becomes greater and the value of magnetic susceptibility becomes higher. Next, the magnetic susceptibility values for the cores at each site are shown in Fig.2, Fig. 3, Fig. 4, Fig. 5 8.
Magnetic susceptibility values of upper core (core 1) and bottom core (core 2) ultramafic rock sample P1 Puriala District Magnetic susceptibility values of the upper core (core 1) and bottom core ( produce two or more core samples from hand sample magnetic susceptibility values for the cores above tend to be greater than the cores below. The above cores are samples taken from rock drilling in the uppermost structure, while the next cores are in the lower structure. The existence of s a high magnetic susceptibility compared to the bottom core due to weathering ultramafic rock always starts at the top of the structure towards the bottom structure. This allows a greater concentration of magnetic minerals in the m core. The presence of magnetic susceptibility values of the lower core is higher than the upper core may be caused by the structure in the form of fractures at the bottom so that it will facilitate the entry of water and means the be more intensive. As a result, the concentration of magnetic minerals at the bottom becomes greater and the value of magnetic susceptibility becomes higher. Next, the magnetic susceptibility values for the cores at each site are . 5, Fig. 6, Fig.7

Mineral Content of Samples
The magnetic mineral content in the sample can be determined based on the magnetic susceptibility value. By referring to the classification of magnetic mineral types based on magnetic susceptibility prices according to Hunt et al. (1995), the magnetic mineral content of ultramafic and soil rock samples can be seen in Table 3. The presence of hematite and geotite magnetic minerals in the ultramafic rock samples was also confirmed by XRD (XRay Difraction) analysis. In addition to the magnetic minerals in the form of hematite and geotite, in the ultramafic rock samples there are other minerals in the form of olivine minerals, cristabolite, wuestite, calcite, and nickel. The presence of hematite and ilmenite magnetic minerals in soil samples is also confirmed by the results of the XRF (XRay Fluorescence) analysis. Based on the results of the XRF analysis, obtained elemental content in soil samples in the form of Fe and Ti as the forming elements of hematite and ilmenite minerals.

Weathering Level of Ultramafic Rock Samples
To find out the level of weathering of ultramafic rocks in the study area, an analysis was made of the presence of magnetic minerals in rock samples based on the magnetic susceptibility of the sample. With reference to the mineral content in the rock weathering level as contained in Table 1, the presence of magnetic minerals in the form of hematite and geotite in the sample shows that the weathering level of ultramafic rocks in the study area is the continued weathering level. Overall, a description of the weathering level of rock samples by site and sub district in the study area can be seen in the following Table 4. In the study area, determination of weathering levels of ultramafic rock samples and nickel content contained in laterite sediments was carried out on several samples representing different sites and sub district areas. The analysis results of these samples are presented in Table 5.

Conclusion
Based on the finding in this study, some conclusions can be summarized, as the following : 1. The magnetic susceptibility value of ultramafic rocks in the study area varies from 580 x 10 6 SI to 4,724 x 10 6 SI. The magnetic susceptibility of ultramafic rock samples shows different values in the upper and lower cores where the magnetic upper core susceptibility values tend to be greater. This relates to the weathering process which always starts in the upper structure so that it allows greater concentrations of magnetic minerals. The difference in value indicates that the magnetic susceptibility parameter can explain the weathering conditions of ultramafic rocks. 2. The magnetic minerals present in the ultramafic rock samples are hematite and geotite minerals. In addition to these two magnetic minerals other minerals in the form of olivine minerals, cristabolite, wuestite, calcite, and nickel. 3. Based on the value of magnetic susceptibility of ultramafic rock samples which shows the magnetic susceptibility of hematite and geotite minerals, the weathering level of ultramafic rock samples in the study area includes continued weathering level (magnetic susceptibility value 10 6 SI to 4,724 x 10 6 SI). 4. The level of weathering of ultramafic rocks can affect nickel content in lateritic nickel The level of continued weathering that occurs in ultramafic rocks in the study area produces nickel laterite deposits with a nickel content of 1.65 % in the saprolite zone, 0.42 % in the saprock zone, and 0.20 0.51 % in the basic rock z 5. Acknowledgment magnetic susceptibility parameter can explain the weathering conditions of ultramafic rocks. he magnetic minerals present in the ultramafic rock samples are hematite and geotite minerals. In addition to these two magnetic minerals, there are other minerals in the form of olivine minerals, cristabolite, wuestite, calcite, and nickel. Based on the value of magnetic susceptibility of ultramafic rock samples which shows the magnetic susceptibility of hematite and geotite minerals, the evel of ultramafic rock samples in the study area includes continued weathering level of samplesis 580 x The level of weathering of ultramafic rocks can affect nickel content in lateritic nickel sediments. The level of continued weathering that occurs in ultramafic rocks in the study area produces nickel laterite deposits with a nickel content of 1.65 2.40 % in the saprolite zone, 0.42 % in the saprock zone, 0.51 % in the basic rock zone (bedrock).