Determination of Laterite Bauxite Deposit Data Domains in The Mempawah Prospect of PT ANTAM Tbk Through Geochemical Characteristics of Each Source Rocks Arif Hindarto (a,b), Guntur Ghiffari (a,b), Dedi Sunjaya (b), Mirzam Abdurrachman (a)
(a) Faculty of Earth Sciences and Technology, Bandung Institute of Technology, Jalan Ganesha 10, Bandung 40132, Indonesia
(b) Unit Geomin, PT ANTAM Tbk, Jalan TB Simatupang 1 Jakarta
Abstract
The regional geology of the study area is included as part of the Singkawang Batolith with Mensibau Granodiorite and Raya Volcanic rocks from the lower cretaceous as potential bauxite-forming rock. PT Antam Tbk was began the investigation of laterite bauxite deposits in Mempawah Regency and its surroundings in 2006. This investigation resulted in an expansion of the laterite belt area in West Kalimantan from previous exploration results discovered by the Japanese in 1943 and Alcomin in 1969-1974. The presence of bauxite deposits investigated by digging testpit exploration, instead of drilling methods, to obtain undisturbed samples of lateritic bauxite. The results of the testpits excavation were then recorded as overburden interval and bauxite interval, samples from the bauxite interval were then washed to remove clay impurities before being analyzed in the laboratory using XRF and Wet Analysis. From the exploration results in the research area, it was found that washed bauxite with an average grade of Al2O3 40,69% with a Coefficient of Variation (CV) of 0,18, Total Silica (T-SiO2) 24,58% with CV of 0,61, Reactive Silica (R-SiO2) 3,88% with CV of 0,78, Fe2O3 12,93% with CV of 0,54, and TiO2 0,90% with CV of 0,38. The average grade and CV values obtained indicate that this deposit is quite homogeneous, but statistical analysis showed that the histogram of T-SiO2, Fe2O3, and TiO2 grades has a distribution with two histogram peaks indicating the presence of multiple data populations that were still mixed in the bauxite laterite intervals of the exploration data. Laterite domain division needs to be done to separate the data population so that it can be used in the process of resource estimation according to the proper lateralization domain to obtain a more reliable and robust resource model. The lithology of the bauxite-forming rocks from the results of detailed geological mapping is evaluated against their geochemical characteristics to separate the bauxite domain laterally. The lithology of granodiorite, porphyritic granodiorite, and diorite from the Mensibau Granodiorite rocks and the lithology of tuff, andesitic lava, and intrusive andesite from the Raya Volcanic rocks as the source rock of the laterite bauxite were analyzed for variations of T-SiO2, Fe2O3, and TiO2 to identify the lateral domain of bauxite in the area. Laterally, two domains were obtained with TSiO2 as the most dominant distinguishing geochemical content. Dom 1 is a domain of low silica bauxite deposited from the source rocks with the lithology of diorite, intrusive andesite, and andesite lava. Dom 2 is a domain of medium to high silica bauxites deposited from source rocks with the lithology of granodiorite, porphyritic granodiorite, and tuff. While vertically, based on the amount of free silica content that tends to increase at the bottom of the bauxite layer, it can be divided into the bauxite (Bx) subdomain at the top and the quartz bauxite (QBx) subdomain at the bottom with a higher silica content. Dom 1 consists only of the Bx subdomain, while Dom 2 consists of the Bx and QBx subdomains. The results of the statistical grade analysis after the separation of bauxite domains in the study area showed that Dom 1 Bx had an average grades of Al2O3 45,56% with CV 0,15, T-SiO2 9,20% with CV 0,91, R-SiO2 2,58% with CV 0,76, Fe2O3 18,67% with CV 0,32, and TiO2 1,22% with CV 0,26. Dom2 Bx showed an average grades of Al2O3 41,30% with CV 0,10, T-SiO2 29,07% with CV 0,25, R-SiO2 3,47% with CV 0,37, Fe2O3 8,85% with CV 0,36, and TiO2 0,72% with CV 0,28. Dom2 QBx showed an average grades of Al2O3 31,39% with CV 0,11, T-SiO2 43,96% with CV 0,14, R-SiO2 5,19% with CV 0,46, Fe2O3 9,27% with CV 0,61, and TiO2 0,71% with CV 0,36. From the separation of bauxite domains and subdomains laterally and vertically, three bauxite data populations were obtained in the form of Dom 1 Bx, Dom 2 Bx, and Dom 2 QBx with the initial bauxite assay sample number of 4,052 samples divided into 1.445 Dom 1 Bx samples, 1.851 Dom 2 Bx samples, 476 Dom 2 QBx samples, and 280 non-bauxite samples that logged as bauxite samples from the exploration results were excluded. Dom 1 Bx and Dom 2 Bx are the two main domains in the research area that have good quality to be developed into resource areas for mine economic evaluation.
