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Abstrak :
ABSTRAK
Pulau Sebuku terletak di Kabupaten Kotabaru, Kalimantan Selatan dan diketahui sebagai salah satu sumber utama besi laterit di Indonesia. Berdasarkan tatanan tektoniknya, Pulau Sebuku terletak di zona sutur yang menghubungkan Southwest Borneo Block (SWB) and East Java West Sulawesi Block (EJWB). Pulau Sebuku terdiri dari variasi batuan yang berasosiasi dengan zona sutur, seperti batuan ofiolit. Tujuan dari penelitian ini adalah untuk menentukan karakteristik dan komposisi mineral penyusun batuan, mineralisasi kromitit serta kaitannya dengan tatanan tektonik di daerah penelitian. Penelitian ini dilakukan melalui analisis petrologi, petrografi, mineragrafi dan geokimia (x-ray fluorescence). Sampel batuan dari Pulau Sebuku telah diperoleh dan hasil dari penelitian ini menunjukkan bahwa sampel batuan terdiri dari dunit terserpentinisasi, harzburgit terserpentinisasi, serpentinit, kromitit, dan basalt yang termasuk batuan penyusun ofiolit dengan afinitas magma toleiitik, mikrodiorit mewakili intrusi kalk-alkalin, andesit, tuf mewakili batuan piroklastik yang terbentuk akibat peristiwa vulkanisme pada Kapur Akhir bagian awal, batulempung, konglomerat, breksi basalt dan breksi karbonat. Batuan induk mineral kromit terdiri dari dunit terserpentinisasi dan serpentinit. Mineral kromit (FeCr2O4) ditemukan dalam bentuk masif dengan tekstur kataklastik dan brecciated serta terdiseminasi dengan tekstur pull-apart. Data kimia kromitit menunjukkan komposisi kromitit Pulau Sebuku adalah Al-rich karena memiliki nilai Cr# = Cr/(Cr + Al) sebesar 0.6 dan diklasifikasikan sebagai endapan kromitit podiform. Endapan kromitit di Pulau Sebuku terbentuk di Supra Subduction Zone yang diduga berasal dari magma berkomposisi boninit (high-Cr) lalu mengalami kristalisasi fraksinasi serta magma mixing membentuk kromitit dengan komposisi high-Al di dekat atau di atas zona supra-Moho.

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ABSTRACT
Sebuku Island is located in Kotabaru Regency, South Kalimantan and is known as one of the main sources of laterite iron in Indonesia. Based on its tectonic setting, Sebuku Island is located within the suture zone that connects Southwest Borneo Block (SWB) and East Java West Sulawesi Block (EJWB). Sebuku Island is composed of various rocks associated with suture zone, such as ophiolite rocks. The aim of this study was to determine the characteristics and composition of rock samples, characteristics of chromite mineralization and its relation to the tectonic setting in Sebuku Island. This study was carried out through petrological, petrographic, ore microscopy analysis, and geochemical analysis (x-ray fluorescence). Rock samples from Sebuku Island were collected and the results of this study indicate that the rock samples consist of serpentinized dunite, serpentinized harzburgite, serpentinite, chromitite, and basalt which are part of an ophiolite sequence with tholeiitic magma affinity, microdiorite represents calc-alkaline intrusion, andesite, tuff represents pyroclastic rock formed by the volcanism events in Late Cretaceous, claystone, conglomerate, basalt breccia and carbonate breccia. Chromite minerals (FeCr2O4) were found in massive forms with cataclastic, brecciated texture and disseminated with pull-apart texture. Chromitite chemical data shows the chromite composition is Al-rich because it has Cr# = Cr/(Cr + Al) value of 0.6 and classified as podiform chromitite formed by fractional crystallization. Chromitite deposits in Sebuku Island are formed in the Supra Subduction Zone which is interpreted to originate from magma with boninite composition (high-Cr) then undergoes fractionation crystallization and magma mixing process formed chromitite with a high-Al composition near or above the supra-Moho zone.

Abstrak :
Penelitian telah dilakukan di Lapangan "SY", Pulau Sebuku, Kotabaru, Kalimantan Selatan. Tujuan dari penelitian ini mengidentifikasi karakteristik laterit berdasarkan hasil domaining geologi dan mengestimasi sumber daya endapan laterit pada daerah penelitian. Data yang digunakan merupakan basis data hasil pengeboran meliputi collar, survey, dan geoassay. Metode pengolahan dan analisis data meliputi analisis statistik univarian dan bivarian serta metode estimasi ordinary kriging. Unsur yang diestimasi meliputi Ni dan unsur lain Co, SiO₂, dan MgO dengan variogram model yang digunakan yaitu Ni sebagai variabel utama. Karakteristik berdasarkan ciri fisiknya domain limonit tersusun atas limonit kuning dan limonit merah sedangkan domain saprolit tersusun earthy saprolit dan rocky saprolit. Domain limonit cenderung mengalami pengayaan secara bertahap unsur Fe dari permukaan hingga zona transisi bersamaan dengan kenaikan unsur Ni, SiO₂, dan MgO secara gradual hingga domain saprolit. Domain bedrock cenderung kaya akan unsur SiO₂ dan MgO. Unsur Co terkonsentrasi di zona transisi antara domain limonit dan saprolit. Diketahui hasil estimasi sumber daya Ni domain limonit berada pada rentang 0-2 % sedangkan domain saprolit 0-2.5 %. Klasifikasi sumber daya berdasarkan parameter geostatistik slope of regression dengan cut off grade Ni 1 % menunjukan daerah penelitian tersusun atas kelas tertunjuk dan terukur. Domain limonit menghasilkan volume 15,938 m³ dan tonase 38,091 ton dengan kandungan rata-rata Ni 1.123 % serta Co 0.126 %, Fe 44.405 %, SiO₂ 8.357 %, dan MgO 4.243 % kelas tertunjuk dan volume 605,313 m³ dan tonase 1,446,697 ton dengan kandungan rata-rata Ni 1.175 % serta Co 0.118 %, Fe 44.629 %, SiO₂ 8.334 %, dan MgO 3.928 % kelas terukur. Domain saprolit menghasilkan volume 5,469 m³ dan tonase 10,938 ton dengan kandungan rata-rata Ni 1.199 % serta Co 0.035 %, Fe 14.195 %, SiO₂ 33.865 %, dan MgO 26.979 % kelas tertunjuk dan volume 389,141 m³ dan tonase 773,125 ton dengan kandungan rata-rata Ni 1.22 % serta Co 0.034 %, Fe 14.535 %, SiO₂ 33.912 %, dan MgO 26.119 % kelas terukur.

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Research has been carried out in the "SY" field, Sebuku Island, Kotabaru, South Kalimantan. The purpose of this study is to identify the characteristics of laterite based on the results of geological domaining and estimating the resources of laterite deposits in the study area. The data used is the database of drilling results including collars, surveys and geoassay. Data processing and analysis methods include univariate and bivariate statistical analysis and ordinary kriging estimation methods. The estimated elements include Ni and other elements Co, SiO2, and MgO with the variogram model used is Ni as the main variable. Characteristics based on physical characteristics limonite domain is composed of yellow limonite and red limonite while the saprolite domain is composed of earthy saprolite and rocky saprolite. The limonite domain tends to gradually enrichment of the Fe element from the surface to the transition zone together with the gradual increase of the Ni, SiO2, and MgO elements to the saprolite domain. The bedrock domain is rich of SiO2 and MgO elements. The Co element is concentrated in the transition zone between the limonite and saprolite domains. The result of resource estimation of Ni for limonite domain in the range of 0-2 % while the saprolite domain is 0-2.5 %. Classification of resources based on the slope of regression geostatistical parameters with a cut-off grade of Ni 1 % showed the study area is composed of indicated and measured classes. The limonite domain produces volume 15,938 m³ and tonnage of 38,091 tons with an average content of Ni 1.123 % and Co 0.126 %, Fe 44.405 %, SiO₂ 8.357 %, and MgO 4.243 % for indicated class and volume of 605,313 m³ and tonnage of 1,446,697 tons with an average content Ni 1.175 % and Co 0.118 %, Fe 44.629 %, SiO₂ 8.334 %, and MgO 3.928 % measured class. The saprolite domain produces a volume of 5,469 m³ and tonnage of 10,938 tons with an average content of Ni 1.199 % and Co 0.035 %, Fe 14.195 %, SiO₂ 33.865 %, and MgO 26.979 % indicated class and volume 389,141 m³ and tonnage 773,125 tons with an average content Ni 1.22 % and Co 0.034%, Fe 14.535 %, SiO₂ 33.912 %, and MgO 26.119 % measured class.