Journal of Applied Geology
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Low-Sulfidation Epithermal Carbonate-Base metal-Gold Mineralization Hosted by Tertiary Sedimentary Rocks in Bastem Prospect, Luwu District, Sulawesi Island, Indonesia: A Preliminary Study
Get PDFIn Indonesia, gold is typically mined out from epithermal, porphyry and skarn deposit types occurred within volcanic belts along magmatic arc or active continental margin setting. Numerous gold prospects, however, are recently discovered in association with metamorphic and sedimentary rocks. This paper is aimed to discuss a preliminary study on the occurrences and characteristics of the sedimentary rock-hosted low-sulfidation (LS) epithermal gold mineralization in Bastem (Bessengan Tempe) prospect, Luwu district, Sulawesi Island, Indonesia. Detailed geological and hydrothermal alteration mapping was performed to understand the distribution and characteristics of ore mineralization. Representative ore samples taken were analysed for ore chemistry by means of Fire Assay – Atomic Absorption Spectrometry (FA-AAS) and Acid Geochemical Digest - Atomic Absorption Spectrometry (GA-AAS). The results from this work that the Bastem prospect is stratigrapically occupied by Tertiary sedimentary rocks of Toraja formation, which is adjacent to volcanic rocks of Lamasi formation. Gold-bearing quartz±carbonate veins are hosted by mudstone and siltstone of Toraja formation. The quartz-carbonate veins show a typical LS epithermal open space filling texture containing erratic gold grade of up to 7.16 g/t with relatively high besemetals (Pb and Zn) grades of up to >0.4 and >1%, respectively. Based on those various features, the LS epithermal deposit is categorized as “carbonate-basemetal-gold mineralization type”, which might be originated in back arc rift/basin setting. This ‘unconventional’ sedimentary rock hosted-gold mineralization type would be the new target of gold exploration in Indonesia
Numerical Evaluation of Tunnel Portal Slope Stability at Bagong Dam Site, East Java, Indonesia
Get PDFGeometries of excavated tunnel portal slopes at Bagong Dam site was initially designed without taking into account earthquake load. The excavated slope designs also assumed the rocks consisting the slopes were homogenous. The purpose of this research was to evaluate stability of the excavated tunnel inlet and outlet slopes at the Bagong Dam site under static and earthquake loads using finite element method. Stability of the natural slopes was also analyzed for comparison. The numerical static and pseudostatic analyses of slope stability were carried out using RS2 software (Rocscience, Inc.). Input data used in the numerical analyses were obtained from engineering geological mapping, rock core analyses, and laboratory tests. Seismic coefficient applied in the pseudostatic slope stability analyses was determined following guideline described in Indonesian National Standard. The engineering geological mapping and evaluation of rock cores indicated that the inlet tunnel slope consisted of four types of materials, namely residual soil, poor quality of volcanic breccia, very poor quality of volcanic breccia, and good quality of volcanic breccia. The outlet portal slope consisted of six types of materials, namely residual soil, very poor quality of limestone, poor quality of limestone, very poor quality of volcanic breccia, poor quality breccia, and good quality breccia. Based on the secondary elastic wave velocity (Vs) values, the rock masses in the research area were classified as hard rock (SA). Seismic analyses based on the earthquake hazard source map with 10% probability of exceedance in 50 years provided by the National Earthquake Center (2017) indicated that the PGA and the corresponding amplification factor FPGA in the research area were 0.3 and 0.8, respectively. The calculated seismic coefficient for the pseudostatic slope stability analyses was 0.12. The numerical analysis results showed that, in general, earthquake load reduced critical Strength Reduction Factor (SRF) values of the slopes. However, the natural and excavated tunnel portal slopes were relatively stable under static and earthquake loads. The natural slope at the tunnel inlet with a 40° inclination had critical SRF value of 4.0, while that of at the tunnel outlet with a 51° inclination had critical SRF value of 2.6. Under static load, the excavated slopes at the tunnel inlet and outlet having a 45° inclination had critical SRF values of 2.4 and 5.0, respectively. Under earthquake load, the excavated slopes at the tunnel inlet and outlet had critical SRF values of 2.3 and 3.5, respectively
Determination of Groundwater Recharge Area by Using Hydroisotope Technic of Sei Bingei Area and Surrounding Areas, Langkat Regency, North Sumatra
No full textDetermination of Groundwater Recharge Area by Using Hydroisotope Technic of Sei Bingei Area and Surrounding Areas, Langkat Regency, North Sumatr
Erosion and its Implication on Hydrocarbon Generation in ‘ARD’ Block, Akimeugah Basin,West Papua
Get PDFAkimeugah Basin in the western part of Aru Trough is included as a Paleozoic Basin which is one of the potential hydrocarbon-producing basins in Eastern Indonesia. Tectonic evolution in Akimeugah Basin during Cambrian to present has produced a very significant erosion that affected the hydrocarbon generation process. ‘ARD’ Block study uses three exploratory well data including well report and 26 lines of 2D seismic data with a total length of 5,812.55 kilometers and the distance between seismic lines ranging from 10 to 15 kilometers. Seismic data is processed with IHS Kingdom software for tectonostratigraphy analysis, while calculation and erosion analysis are performed by combining well data consisting of sonic, vitrinite reflectance and seismic. To get a burial history model and generation & expulsion period, this study utilizes Petromod software. Five phases of the tectonic evolution led to four times of erosional period with a sediment thickness of 290 – 3,370 feet were loss. The erosion of the sedimentary rocks causes the maturation process delayed more than 200 million years. Burial history in the study area with the erosion absence assumption results a hydrocarbon generation starting from around 210 million years ago. Meanwhile, by considering the loss of eroded sedimentary rocks during four tectonic phases, hydrocarbon generation time just occurred 3.1 million years ago
Coastal groundwater aquifer characterization from geoelectrical measurements- A case study at Kalapara, Patuakhali, Bangladesh.
Get PDFVertical electrical sounding has been carried out in a coastal area in the southern part of Bangladesh to locate the groundwater aquifers containing fresh water. The Interpex1X1Dv3 computer program was used to process the field apparent resistivity data sets obtained from the vertical electrical sounding. Geoelectric layers were identified in the context of resistivity and thickness from the vertical electrical sounding data. From the initial parameters layered model was achieved using the inversion technique. Correlation of the obtained layer model with a nearby lithologic log concludes the groundwater aquifer system of the area. From the electrical properties of the subsurface layers, water bearing layers were detected and characterized. Very fine sand geoelectric layer with a thickness varying from 20 to 143 meters is an upper aquifer and has 0.66–14.02 Ωm apparent resistivity value. Fine sand geoelectric layer with 0.21-5.99 Ωm apparent resistivity value is lower aquifer with maximum thickness ~250 meters. From the resistivity value, it is observed that the upper aquifer contains saline to brackish-fresh water while the resistivity value of the lower aquifer indicates that it contains saline water. The water quality of the upper zone varies geographically from the southern to the northern part in the investigated area. The water quality of the upper aquifer is fresh in the northern part of the study while lower aquifer contains saline water there
Total Sulfur and Ash Yield of Tanjung Formation Coal in Sekako, Barito Basin, Central Kalimantan: Implication of Depositional Process
Get PDFTanjung Formation is one of the major coal-bearing deposit in the Barito Basin, Central Kalimantan. The distribution of total sulfur and ash yield in coal is closely related to the depositional environment. This study was to determine the total sulfur and ash yield and the interpretation of the dynamics of depositional process. Coal seam A and B generally have low to medium ash yield 2.82 to 9.23 (wt.%, db) and low total sulfur content of <1 (wt.%, db), except for the 6PLY1 coal sample which has total sulfur content that relatively high at 1.55 (wt.%, db). Coal samples 5PLY1A, 5PLY1B, 5PLY3, 5PLY5, 6PLY2, 6PLY4, 6PLY5, 6PLY7, and 6PLY9 which have low to medium ash yield and low total sulfur content <1% (wt.%, db) are formed in the topogeneous mire (freshwater swamp) in a fluvial environment. The total sulfur content was interpreted to be derived mainly from the parent plant materials. Meanwhile, the 6PLY1 coal sample which has an ash yield of 5.83 (wt.%, db) and total sulfur content of 1.55 (wt.%, db) formed in topogeneous mire in an environment that is invaded by sea water, and the total sulfur content were interpreted coming from the parent plant materials and the effect of seawater invasion which is rich in sulfate (SO4) compounds. It is also supported by the occurrence of syngenetic mineral content (framboidal pyrite) and epigenetic pyrite of 1.23 (vol.%)
Groundwater Flow Modeling at Sejorong Watershed, Sumbawa,West Nusa Tenggara, Indonesia
Get PDFABSTRACT. Batu Hijau mine pit is known as one of the largest copper pit mine in Nusa Tenggara Barat, Indonesia. Similiar as other copper mine pits in Indonesia, This site also faces acid mine water (AMD) problem. Based on the mine management plan, the AMD generated from this pit is being collected into Santong ponds in the southwest of the pit located in the upstrean area if Sejorong watershed. By the next decade, Batu Hijau mine will be in the closure mine period and it is important to understand the probable move- ment of AMD under the Santong ponds whether the AMD leaked to groundwater or not. This research aims to develop a numerical model of groundwater flow and predict the movement of AMD by applying particle tracking method. Secondary and primary data of geology, hydrology, and hydrogeology. Also, groundwater pumpage discharge infor- mation was collected and analysed to develop hydrogeological conceptual model and the numerical model. The conceptual model of hydrogeological system in the research area is known to be built on unconfined aquifer system from a combination of weathered and fractured volcanic rocks in the upstream to middle part of watershed and majorly by allu- vial and coastal deposits in the downstream area. The thickness of aquifer is vary between 20 to about 300 m and divide into 5 layers in the numerical model, with hydraulic conduc- tivity ranges between 5 to 100 m/day and groundwater recharge is vary between 180 to 700 mm/year from downstream to upstream of watershed. Groundwater flow boundaries in the Sejorong watershed are mainly controlled by topographical feature as water divide boundaries and the existing of Sejorong parennial rivers in the middle of this watershed. Steady state particle tracking results from the numerical groundwater flow model show AMD from Santong ponds may migrate in groundwater only to a distance of about 500 m to the downstream and therefore it is unlikely that AMD seepages from Santong pond may contaminate water in the production wells
Mineralogical, Geochemical, and Mechanical Characteristics of Intrusive Andesite Rock Slope in Sangon 2 Area, Kulon Progo Regency, Indonesia
Get PDFThis study was carried out to investigate mineralogical, geochemical, and mechanical characteristics of intrusive igneous rock andesite consisting of Sangon 2 Slope located in Kulon Progo Mountains. Mineralogical characteristics of the rocks consisting of the slope were identified by visual observation of hand specimens in the field and petrographic and X-ray Diffraction (XRD) analyses of the rock samples in the laboratory, which rock genetic classification was determined. Types of rock hydrothermal alterations were determined by the presence of key minerals identified in the petrographic and XRD analyses. Hydrothermal alteration intensity was determined by comparing the number of secondary minerals to the primary minerals observed in the petrographic analyses of thin sections by point counting method. Rock weathering degrees were determined in the field based on the visual characteristics and in the laboratory-based on the Chemical Index of Alteration (CIA) of the rock samples. The rock uniaxial compressive strength (UCS) was determined by point load and compression tests. The results showed that the rocks typically consisted of 6.37 to 74.3% plagioclase, 1.52 to 6.82% quartz, 7.81 to 17.2% hornblende, and 4.81% pyroxene minerals and, therefore, were classified as andesite. The presence of chlorite and calcite as secondary minerals in the andesite rock samples indicated that the parent andesite rocks had undergone prophylactic hydrothermal alteration. Meanwhile, the percentages of primary minerals to secondary minerals indicated that the andesite rocks had undergone weak to high alteration intensity. Based on the visual characteristics, the research area consisted of fresh, slightly, moderately, highly, and completely weathered andesite rocks, and residual soil. The CIA values of the rock samples increased with the increase in rock weathering degree. Rock weathering degrees identified by the visual characteristics appeared to be in good agreement with those classified based on the CIA. The lower part of the mine slope profile consisted of fresh and slightly and moderately weathered andesite rocks, which had relatively high strength and were classified as very strong rocks. Meanwhile, the upper part of the slope profile consisted of highly and completely weathered andesite rocks, which had low compressive strength.Keywords: andesite, hydrothermal alteration, kulon progo, prophylactic alteration, weatherin
Numerical Evaluation of Earthquake Effect on Cisumdawu Tunnel Stability
Get PDFCisumdawu Tunnel is located approximately 3.95 km southeast of the activeLembang Fault. Earthquakes induced by movement of the the active the strike-slip fault may influence stability of the twin tunnel. This paper presents results of numerical analyses carried out to demonstrate effect of a worst-case scenario of earthquake load potentially induced by the Lembang Fault on the stability of the Cisumdawu Tunnel. Static and pseudo-static tunnel stability analyses were carried out at 11 observation stations of tunnel face mapping using RS2 finite element package (Rocscience, Inc.). In the pseudo-static analyses, a 0.48 horizontal seismic load coefficient, which was obtained from a deterministic seismic hazard analysis (DSHA) based on a 6.8 maximum magnitude of estimated earthquake sourced from the Lembang Fault using Campbell-Bozorgnia (2014) attenuation relationship, was applied to the finite element models. The numerical analysis results showed that strength factors of the rock masses around the twin tunnel were greater than unity, both under the static and earthquake loads. The forepolling zones, however, appeared to be under an overstressed condition. Reduction of rock strength factor around the tunnel roof due to application of the earthquake load occurred at all observation stations. Total displacement contours of rock masses around the twin tunnel indicated an increased rock mass displacement due to the earthquake load, as compared to that due to the static load. Under the static load, the largest displacement occurred at the tunnel inverts. The predicted roof displacements obtained from this study were in a reasonably good agreement with those obtained from the field measurements. Number of yielded elements and extend of shear and tension failure zones in the rock masses around the twin tunnel also appeared to increase due to the earthquake load. Despite slight tunnel displacement as predicted in the numerical analyses, the worst-case scenario of earthquake load potentially induced by the Lembang Fault may only cause failures of the rock masses around the Cisumdawu Tunnel. To prevent the twin tunnel displacement caused by such relatively severe earthquake loads, however, stabilizing surrounding the relatively poor ground condition may be necessary
The 22 February 2018 landslide mechanism in Pasir Panjang Village, Brebes Regency, Central Java, Indonesia
Get PDFOn 22 February 2018 landslide occurred in Pasir Panjang Village, Salem District, Brebes Regency of Central Java Province, Indonesia. About 8 people were died, 4 people were injured and several infrastructures were damaged due to this landslide. This research is carried out to understand geological-geotechnical condition and to study the initiation mechanism of the landslide. Field investigation and UAV mapping are carried out to detect slip surface and define slope geometry. The rainfall-induced pore-water pressure is estimated by using the Slope Infiltration Distributed Equilibrium (SLIDE) model. Then, limit equilibrium method is used to estimate the safety factor of the slope, while the shear strength parameters are determined by applying back analysis approach that compared with data from laboratory tests. The results show that landslide occurred in permeable layer of silty sand overlaid above impermeable andesitic breccia. Results from back analysis indicate that the shear strength parameters and rainfall intensity are strongly influence the stability of slope against landslide