The presence of hummocky hills as a typical product of debris avalanche deposits is prominently visible in the northeastern flank of G. Sundoro and G. Sumbing, Temanggung, Central Java. In an attempt to better understand the past behavior of both G. Sundoro and G. Sumbing, we identify the source of the debris avalanche deposit. Interpretation is performed on the basis of the assumption of two possible sector collapse sources, i.e., G. Sundoro and G. Sumbing. The Sumbing source scenario is assumed as freely spreading type considering 1) distribution of the hummocky hills are relatively on the northeastern flank of the volcano, and 2) the present crater structure on the summit of the volcano which is opening to the northeast. The Sundoro source scenario is assumed as valley-filling type considering the distribution of the hummocky hills are relatively on the eastern flank of Sundoro extended to the far distal area and bounded by older high topography of G. Sumbing and North Serayu Mountains. The source identification was done on the basis of field observation of the deposit lithological characteristics combined with image analysis, including hummocky hills morphometry, displacement angle, and spatial distribution. Image analysis identifies approximately 645 hummocky hills ranging from 1,851 mz to 623,828 mz and average of 23,482 m2. Petrographic analysis of 5 representative block lava samples shows variation of olivine basalt, pyroxene andesite, to hornblende andesite. The results show that big size hummocky hills dominate the western side, while small size on the east. Displacement angle varied following the valley orientation with typical downslope topography. These suggested that the hummocky hills were originated from G. Sundoro as a valley-filling debris avalanche deposit.
Characterization of hydrothermal alteration in theCijulang area (West Java, Indonesia) was carriedout using shortwave infrared spectroscopy. Hydrothermal alteration in the Cijulang area occurs in the calc-alkaline volcanic and volcaniclastic rocks. Shortwave infrared spectroscopic measurements of reflectance for altered rocks and minerals were carried out by ASD-FieldSpec and the laboratory spectra acquired were then analysed with “The Spectral Geologist” software program. Shortwave infrared spectroscopy is capable of detecting most finegrained alteration minerals from different hydrothermal alteration zones. Characteristic alteration minerals identified from the SWIR technique include pyrophyllite, alunite, kaolinite, dickite, illite, montmorillonite, polygorskite, gypsum, epidote, paragonite, and muscovite. Most of the spectra show mixture ofalteration minerals and only a few display pure spectra of single mineral. The crystallinity of kaolinite from the samples was also determined from the reflectance spectra and show moderately to high crystallinity. Alteration system of the Cijulang prospectis similar to others documented high-sulfidation epithermal deposits, such as Rodalquilar (Spain), Summitville (Colorado), and Lepanto (Philippines). A characteristic alteration sequence and zonation of advanced argillic, argillic and propylitic alterationoutward from the silica core has resulted from the progressive cooling and neutralization of hot acidic magmatic fluid with the host rocks.Keywords: Cijulang, High-sulfidation, Alteration minerals, Shortwave Infrared Spectroscopy
SARI
Gunungapi Dieng bagian barat memiliki beberapa kawah aktif meliputi Kawah Timbang, Kawah Sinila, dan Kawah Candradimuka. Karakteristik dari kawah ini memiliki kandungan gas CO2, H2S, dan SO2 yang tinggi. Gunungapi Dieng merupakan jenis gunungapi kompleks yang memiliki karakteristik kandungan geokimia yang berbeda-beda. Sehingga dilakukan penyelidikan detail mengenai persebaran unsur merkuri, suhu udara tanah, dan kandungan gas CO2 dalam tanah , serta alterasi permukaan di daerah Kawah Candradimuka dan sekitarnya untuk mengetahui zona panas, zona permeabel, dan sistem panas bumi di daerah tersebut. Pengukuran kandungan CO2 dan suhu udara tanah dilakukan secara langsung di sekitar Kawah Candradimuka. Sementara untuk data merkuri tanah dan alterasi permukaan diperoleh dalam analisa dilaboratorium, yakni analisa dengan alat Zeemaan Mercury Spektometer RA915+ untuk merkuri tanah dan analisa X-Ray Diffraction (XRD) untuk alterasi permukaan. Berdasarkan hasil penelitian didapati bahwa anomali kandungan CO2 dalam tanah tertinggi berada pada kebun dekat Kawah Sinila (62%) dan Lembah Kalisat (38%). Hal ini berkaitan dengan kelanjutan zona patahan yang memiliki kelurusan utara-selatan sehingga meningkatkan permeabilitas di daerah tersebut. Sementara anomali suhu udara tanah paling tinggi terdapat di Kawah Candradimuka yang memiliki suhu sekitar 85o C. Hal ini berkaitan dengan aktivitas kawah dan pergerakan fluida panas bumi yang menyebabkan kenaikan temperatur. Kemudian berdasarkan analisa merkuri menunjukan lokasi anomali merkuri ditemukan di Wanapriya dan Sibanger. Sementara berdasarkan hasil analisa X-Ray Diffraction (XRD) menunjukan alterasi terjadi pada litologi andesit. Mineral primer pada litologi andesit berupa plagioklas dan hornblenda. Sedangkan mineral hasil alterasi berupa mineral kaolinit, alunit, montmorilonit, haloisit, smektit, sulfur, kuarsa, dan kristobalit.
Kata kunci : Dieng, Candradimuka, zona panas, CO2, merkuri, suhu, alterasi
ABSTRACT
In the western part of Dieng Volcano lies several active craters including Timbang Crater, Sinila crater, and Candradimuka crater. This crater contains of which CO2, H2S and SO2 concentration are high. Dieng Volcano is a complex volcanic that contains different geochemical characteristics. Therefore, it needs a detailed investigation of the distribution of the mercury elements, ground air temperature and CO2 concentration in soil, as well as the surface alteration in the area of Candradimuka crater and its surroundings to determine the hot zones, permeable zones, and geothermal systems. The measurement of CO2 concentration in soil and ground air temperature was performed directly around the Candradimuka crater. The data of soil mercury level and its surface alteration will be obtained in laboratory analysis which is using Zeemaan Mercury Spectrometer RA915+ for measuring soil mercury level and analysis of X-Ray Diffraction (XRD) for surface alteration. The result displayed an anomaly of CO2 high concentration in soil in a garden near Sinila crater (62%) and Kalisat Valley (38%). This might relate to the continuation of the fault zone which expands north-south lineament that increases the permeability of the area. On the other hand, the air temperature anomaly was also found in the soil of Candradimuka crater that with a temperature of 85o C. This is related to the crater activity and the geothermal fluid movement which causes the rising temperature. Then from mercury analysis, mercury anomaly was found in Wanapriya and Sibanger. Also, the analysis of X-Ray Diffraction (XRD) showed alteration occurring in andesite. The primer mineral on andesite contains plagioclase and hornblende. The mineral from alteration results in kaolinite, alunite, montmorillonite, halloysite, smectite, sulfur, quartz, and cristobalite.
Keywords: Dieng, Candradimuka, hot zone, CO2, mercury, temperature, alteration
Kompleks Gunung api Arjuno Welirang (KGAW) terletak di Provinsi Jawa Timur merupakan
kompleks gunung api Kuarter yang tersusun oleh beberapa tubuh gunung api, yaitu Arjuno, Welirang,
Kembar, Bulak, Bakal dan Pundak. Kerucut atau tubuh gunung api tersebut membentuk kelurusan
berarah utara – selatan menunjukkan adanya kontrol struktur geologi dalam pembentukan KGAW.
Daerah ini merupakan daerah prospek panas bumi ditandai dengan kemunculan manifestasi panas
bumi seperti mata air panas Cangar dan mata air panas Padusan dengan variasi suhu 39.50C -
46.50C, pH netral serta fumarol di puncak Gunung Welirang.
Untuk melengkapi data awal eksplorasi panas bumi perlu pemahaman mengenai vulkanisme dan
struktur geologi. Oleh karena itu dalam studi ini dilakukan dengan survei geologi lapangan yang
didukung dengan studi penginderaan jauh dan pekerjaan laboratorium berupa pengamatan sayatan
tipis petrografi untuk memahami petrologi batuan yang sangat berguna untuk studi vulkanisme.
Kondisi geologi yang merupakan batuan berumur Kuarter terdiri dari lava andesit, lava andesit
basaltik, breksi vulkanik dan batuan piroklastik bertipe aliran yang bersumber dari hasil erupsi
KGAW. Struktur geologi berupa sesar disekitar mata air panas Padusan mengindikasikan adanya
hubungan erat antara keberadaan manifestasi panas bumi terhadap vulkanisme dan struktur geologi.
Kata kunci : vulkanisme, struktur geologi, panas bumi, Arjuno-Welirang
Abstract Guci Hot Spring is one of the tourist destinations in Central Java Province renowned for its beautiful scenery as well as its health-healing benefit. In 2016, this touristic destination attracted more than 650.000 visitors, this number is growing each year. However, this touristic spot has its hidden hazard and risk of the Slamet Volcano eruption. The historical eruptions of this volcano, recorded since the 18th century, have shown that this volcano is known for its explosive type of eruption. Despite its potential hazard and risk, this hot spring area is growing continuously as a touristic spot. Therefore, this study aims to understand tourists’ perception on volcanic hazard and risk in this area, by interviewing 31 respondents using questionnaires. The results show that 74% of respondents are aware that Slamet Volcano has different potential hazards such as volcanic ashfall, lahars, lava and pyroclastic flows. Although Guci Hot Spring is located in Slamet volcanic hazard area, only a few tourists know this information (42%). Majorities of the respondents did not have any experience on volcanic disaster (68%). Most of the respondents did not get any information on Slamet volcanic hazard (58%), while others only get information through television and/or social media. Nevertheless, almost all of the respondents claimed that they will follow any instructions given by government or related institutions if any volcanic eruption occurs (97%). Therefore, awareness program towards tourists in the volcanic touristic spot should be strengthened through different means to give information on volcanic hazard, its risk as well as mitigation and crisis management.
The existence of nuclear installations in the Yogyakarta Nuclear Area is vulnerable to the eruption of Mount Merapi, the most active volcano in Indonesia. Tephra hazard has the potential to threaten the operational activities of nuclear installations in the Yogyakarta Nuclear Area; thus, it is necessary to analyze the distribution and potential hazard of volcanic ash from Mount Merapi for future eruptions. Numerical modelling is used in analyzing tephra distribution using TEPHRA2 software with parameters of the 2010 Mount Merapi eruption, which is then visualized to isomass and isopach maps of tephra distribution. The analysis resulted in the ash dispersion leading to the Yogyakarta Nuclear Area in April, May, June, and August with an accumulated mass of 20-50 kg/m3 with a thickness of 0.2-12 cm. It is necessary to deal with volcanic ash hazards such as roof strength, secondary cooling system, filtering system, and electrical system for several installations in the Yogyakarta Nuclear Area.
The presence of geothermal manifestation in Bangka Island (Southeast Sumatra, Indonesia) with the absence of Quaternary volcanic activity and also relatively low seismicity events has raised intriguing questions on the control of the geothermal system in this area. As the regional tectonic setting of Indonesia volcanic geothermal systems has been known, that of non-volcanic geothermal systems such as radiogenic system become an issue to be investigated. This study reports the geochemistry and petrography analysis of Triassic granite related to radiogenic production at the vicinity of hot springs in Bangka Island. Surface temperatures of the Bangka hot springs range from 37 to 70.7 °C and pH values vary between 5.6 and 7.5. These hot springs are discharging either in close to massive granite bodies or occur in between two major NE-SW striking faults zones, i.e., Pemali fault and Payung fault. Our results indicate the average radiogenic heat production of Late Triassic Klabat granite in the northern area ranges from 28.5 to 38.34 μWm−3 and the southern area ranges from 28.3 to 49.5 μWm−3. In comparison to similar granite belt located in Malaysia, heat production of granitoid in Bangka hot springs is four times higher, possibly due to their different granite origins.
Kompleks batuan gunung api Adang di daerah Kabupaten Mamuju, Sulawesi Barat secara lebih detail dapat dikelompokkan menjadi tujuh, yaitu kompleks Tapalang, Ampalas, Adang, Malunda, Karampuang, Sumare, dan Labuan Rano. Komplek Adang merupakan salah satu komplek gunungapi utama yang masih dapat diidentifikasi bentukan morfologinya dengan baik. Komplek ini tersusunatas batuan gunung api basa hingga intermediet yang memiliki nilai laju dosis radiasi cukup tinggi yang disebabkan oleh kandungan mineral radioaktif di dalamnya. Keterdapatan mineral radioaktif pada batuan basaltik-andesitik belum pernah dijumpai di Indonesia sehingga hal ini menjadi sangat menarik untuk dilakukan penelitian terutama tataan tektonika pembentukan batuan komplek gunung api tersebut. Tujuan penelitian ini adalah untuk menentukan tipologi magmatik yang terkait dengantataan tektonikanya dengan pendekatan geokimia batuan gunung api menggunakan analisis X-Ray Fluorescence (XRF). Batuan gunung api Adang merupakan hasil dari proses vulkanisme suatu komplekgunung api yang memiliki pusat erupsi dan beberapa kubah lava. Batuan tersebut tersusun atas batuan trachyte-phonolite, dengan afinitas magmatiknya ultrapotasik, Dari data tersebut dapat diinterpretasi bahwa tataan tektonika magmatologinya adalah active continental margin(ACM). Magma asal yang membentuknya dari aktivitas gunung apinya dipengaruhi oleh kerak benua mikro barat daya (South West/SW) Sulawesi. Adang volcanic complexlocated in Mamuju Region, West Sulawesi can be grouped more detail into seven complexes that are Tapalang, Ampalas, Adang, Malunda, Karampuang, Sumare, and Labuan Rano. Adang complex is one of the main volcanic complexes that still can be identified with good morphological formations. This complex is composed of alkaline volcanic rocks with basic to intermediates composition that have high value of radiation dose rate caused by their radioactive mineral content. Radioactive mineral occurrences on the basaltic-andesitic rocks has never been found in Indonesia, so it becomes very interesting to do research mainly tectonic settings of the volcanic rock complex formation. The purpose of this study is to determine magmatiic typology related with the tectonic setting based on volcanic rock geochemistry using X-Ray Fluorences (XRF) analysis. Adang volcanic rock is the result of a complex process of volcanism having a volcanic center and several lava domes. They are composed of phonolite to dacite rock, with ultrapotassic affinity, interpretation of data concluded that tectonic setting of magmatism formed in active continental margin (ACM). Magmatism source from vulcanic activities influenced by South WestSulawesi micro-continental crust.
Abstract Middle Miocene (11.18–10.65 Ma) low sulfidation‐type epithermal gold mineralization occurred in the Cibaliung area, southwestern part of Java Island, Indonesia. It is hosted by andesitic to basaltic andesitic lavas of the Middle Miocene Honje Formation (11.4 Ma) and is covered by Pliocene Cibaliung tuff (4.9 Ma). The exploration estimates mineral resource of approximately 1.3 million tonnes at 10.42 g/t gold and 60.7 g/t silver at a 3 g/t Au cut‐off. This equates to approximately 435,000 ounces of gold and 2.54 million ounces of silver. That resource resulted from two ore shoots: Cibitung and Cikoneng. Studies on ore mineralogy, hydrothermal alteration, geology, fluid inclusion, stable isotopes and age dating were conducted in order to characterize the deposit and to understand a possible mechanism of preservation of the deposit. The ore mineral assemblage of the deposit consists of electrum, naumannite, Ag‐Se‐Te sulfide minerals, chalcopyrite, pyrite, sphalerite and galena. Those ore minerals occur in quartz veins showing colloform–crustiform texture. They are enveloped by mixed layer clay illite/smectite zone, which grades into smectite zone outward. The temperature of mineralization revealed by fluid inclusion study on quartz in the veins ranges from 170 and 220°C at shallow and deep level, respectively. The temperature range is in agreement with the temperature deduced from the hydrothermal alteration mineral assemblage including mixed layered illite/smectite and laumontite. The mineralizing fluid is dilute, with a salinity <1 wt% NaCl equivalent and has stable isotopes of oxygen and hydrogen composition indicating a meteoric water origin. Although the deposit is old enough that it would have been eroded in a tropical island arc setting, the coverage by younger volcanic deposits such as the Citeluk tuff and the Cibaliung tuff most probably prevented this erosion.
This study examines the behavior of trace- and rare-earth elements (REE) in different hydrothermal alteration facies (silicic, advanced argillic and argillic) of Cijulang high-sulfidation epithermal gold deposit, West Java, Indonesia. The results of the study indicate that remarkable differences in the behavior of trace elements and REE are observed in the studied alteration facies. All REE in the silicic facies are strongly depleted. In advanced argillic facies, Heavy rare-earth elements (HREE) are strongly depleted whereas light rare earth elements (LREE) are quite enriched. REE concentrations in the argillic facies show little or no variation with respect to fresh rock counterparts. A strong depletion of REE in the silicic facies is likely to be favored by the highly acidic nature of the hydrothermal fluids, the abundance of complexing ions such as Cl ˉ, F ˉ, and in the hydrothermal solutions and the absence of the secondary minerals that can fix the REE in their crystal structures. An apparent immobility of LREE in advanced argillic facies is possibly due to the presence of alunite. The immobility of REE in the argillic facies suggests the higher pH of the fluids, the lower water/rock ratios and the presence of the phyllosilicates minerals. -
