A geological study was conducted in Kulon Progo and its surrounding areas (Kulon Progo and Purworejo Regency). It focused on regional geology, tectonic configuration, geodynamics and magmatism, lithology and volcanology, and mineralization. Although there has been considerable research of mineralization in the area—particularly in Kokap (Kulon Progo Regency), Bagelen (Purworejo Regency) and Gupit (Magelang Regency), the potential of precious metals has not been determined due to data limitations. The study combined qualitative and semi-quantitative methods using a desktop geologic analysis, which facilitates lithology interpretation, volcanic boundary system, and lineament density assessment. The geology of the region is composed of an ancient volcanic complex of the Old Andesite Formation formed during the Late Oligocene-Miocene, and the mineralization in Kokap, Bagelen, and Gupit is epithermal. Based on the analysis results, the mineralization occurs in the central to proximal facies of the paleo-volcano, and the system ranges from 2.2 to 3.8 km in diameter. The manual analysis of the lineament density showed that the main direction of the lineaments was SE-NW with a maximum density of 2025.9 m/km2 and an anomaly limit of >1800 m/km2. In the combined semi-automatic analysis, the maximum density was 8.3 km/km2. The target area of mineralization included four anomalous areas, namely Bagelen-Kokap, Salaman, Kaligesing, and Loano, associated with the central and proximal facies of each small paleo-volcano.
Sualan prospect is located at Talegong Sub-district of Garut Regency, West Java, Indonesia. The area constitutes calc-alkaline volcanic and volcaniclastic rocks of Tertiary age. The rocks have experienced regional propylitic and argillic alteration. Fluid inclusions from quartz veins were studied in order to constrain the nature, characteristics and evolution of fluids. Microthermometric measurements on fluid inclusion were carried out by freezing and heating experiment. Temperatures of homogenization (Th) and final melting of ice (Tm) were measured for primary, liquid-dominated, two-phase inclusions. The values of Th range from 160°C to 210°C and salinities range from 0.35 to 4.96 wt.% NaCl equiv. Formation temperature of the quartz veins are estimated at 180°C and 190°C and paleo-depth of formation are at 80m and 140m, respectively. Microthermometric data indicates that fluid mixing and dilution were important processes during the evolution of hydrothermal system. Based on fluid inclusion types, microthermometric data, trapping temperature, paleo-depth, texture of quartz and hydrothermal alteration types, quartz veins from prospect were developed under epithermal environment. Keywords: Quartz vein, fluid inclusions, microthermometry, salinities, formation temperature, paleo-depth, epithermal, Sualan prospect.
Godean hills is located approximately 10 km westward from the Yogyakarta City. The landscape of Godean hills and plains is affected by various factors, such as lithology, geological structure, and sub-aerial process. The purpose of this study was to reveal the landscape of Godean. The method consisted of field study, morphological variables assessment, rock sampling, and laboratory analysis. The results of field mapping indicated that the landscape of Godean is an isolated hill with a steep slope of >40° and an elevation of +231 m a.s.l, passed by the rivers flows from northeast to southwest that disembogue into the west part of Kulon Progo. The morphologhy of Godean hills varies including G. (Gunung/Mountain) So (+173 m amsl), G. Gede (+218 m a.s.l), G. Wungkal (+187 m a.s.l), G. Butak (+154 m a.s.l), and G. Berjo ( + 175 m a.s.l), dominated by the lithology of igneous rock, which is composed of porphyry andesite-microdiorite, pumice lapilli, and quartz rich lapilli-tuff. In addition, most of the igneous rocks have weathered and have been altered to clays, while the deposition from Merapi volcano formed a landscape with an altitude between +100–+150 m a.s.l surrounding Godean hills. Sentolo Formation was found in Kembang, Bantul, which is located approximately ±5km in the south of the study area N93ºE/12º, while the distribution in the southwest and northeast relatively covers the Godean hills in curve shape. The results of the analysis provide information related with Godean landscape that it is the remains of the volcanic caldera, with various igneous rock types and volcaniclastics deposits, as well as endured the occurrences of hydrothermal alteration and mineralization. Further geophysical research is required to determine the configuration of igneous rocks under the earth's surface.
Cijulang is a high-sulfidation epithermal system hosted in the calc-alkaline rocks of andesite lava and lapilli tuff. Mineralization in the prospect is characterized by pyrite-enargite-gold and associated acid sulfate alteration. Studies on ore and gangue mineral assemblages and their mutual textural relationships were carried out in order to explore the paragenetic sequence of mineralization. Hypogene mineralization primarily occurs in the silicic core and the advanced argillic zone in the form of massive replacement, fracture-filling veinlets, vug-filling, patches and dissemination. Mineralization is apparently controlled by both lithology and structures. Common ore minerals include pyrite, enargite, luzonite, tennantite, chalcopyrite, covellite, galena, emplectite and Te-bearing minerals. The paragenetic study indicates that the epithermal prospect evolved from an early stage of intense acid leaching resulting in the formation of vuggy silica and advantage argillic mineral assemblage which was followed by the sulfides deposition. Two metal stages were identified during ore deposition: an early Fe-As-S stage and the later Cu-Fe-As-S stage. The former stage is char- acterized high-sulfidation state sulfides such as enargite/ luzonite+covellite whereas a later stage of Cu- Fe-As–S is represented by intermediate sulfidation state sulfides assemblage of tennantite+chalcopyrite. Gold is probably introduced in the early stage within the ore system and more abundant in the late stage. Keywords: Cijulang, high-sulfidation, acid sulfate, mineralization, enargite, paragenetic, metal stages
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