MINERALISASI HIDROTERMAL DI DAERAH PACITAN DA SEKITARNYA, KABUPATEN PACITAN JAWA TIMUR

• Main Author: Setiawan, Iwan
• Format: Theses
• Language: Indonesia
• Subjects: Geologi, hidrologi & meteorologi
• Online Access: https://digilib.itb.ac.id/gdl/view/72906
• Institution: Institut Teknologi Bandung

Observation on hydrothermal mineralization at Pacitan and its surrounding area had been conducted to arrange mineralization model, base on mineralogy and volcanic rocks geochemistry. Pacitan and its surrounding area are situated at the Southern Mountain Range of East Java, tectonically included within Sunda Magmatic Arcs, whiched in experienced at least three events of magmatic activities, resulted in various compositions of volcanic rocks. These complex magmatic and tectonic activities will form complex and various mineralization. Thus, the study of hydrothermal mineralization of Pacitan and its surrounding area become very important. Studied area consists of volcanic and sedimentary rock units which are equal to the Mandalika and the Arjosari Formations. Both rock units are interfingered and had been formed at the Oligo Miocene age. Three groups of rocks ages had been identified by the K-Ar and fission track methods, they are between 29-34 Ma, 12-17 Ma, and less than 3 Ma, resulting in dacite, diorite, basalt flow, the andecite to basalt, and andecite. Some structural features such as the NW-SE, and the NE-SW strike slip faults are related to mineralizations, resulted in mineralized vein, disseminated ore within the wallrock, at Tegalombo and Kasihan area for examples. Petrographically, Pacitan area consists of basaltic-andesitic igneous rocks, crystal tuff, quartz arenite, lithic arenite, altered rock and quartz vein. Alteration processes, formed prophylitic, sub prophylitic, phyllic, argillic, advance argillic, and potassic alteration zones, which are associated with pyrite, magnetite, chalcopyrite, sphalerite, and covelite ore mineralization. The Prophylitic alteration zone is widely distributed, and in some places had been overprinted by the argillic, phyllic, and potassic alteration zones, which are outcropped within prophylitic area as windows. These facts indicate the influences of more than one hydrothermal activities. Hydrothermal activities occurred within the studied area characterized by water rich hydrothermal fluids with low to moderate salinity, equivalent to (0,1-14,6)%wt NaCl equivalent, at Pacitan and Arjosari, whilst at Tegalombo and Tulakan (Kasihan) characterized by moderate to high salinity, equivalent to (6,9-25,8)%wt NaCl equivalent. These suggest that mineralization type is low sulphidation epithermal system for Pacitan and Arjosari with homogenization temperatures range from 225 to 250°C; The transition between epithermal and mesothermal systems, at Kedung Grombyang and kali Grindulu – Tegalombo, with homogenization temperatures range from 200 to 290°C and >350°C; and epithermal related to porhyry systems, with homogenization temperatures range from 210 to 340ºC, at Tulakan (Kasihan). Pyrite, and chalcopyrite mineralization, are also characterized by Cu-Mo enrichments (in case of kali Grindulu-Tegalombo). According to previous study, these facts indicated the influenced of continental rocks origin. The result of major elements analyses shows that almost all of samples are altered, because of high value of LoI or H2O+ (more than 2 wt %). Therefore plotting of SiO2 with major oxide elements by Harker Diagram shows no correlation pattern. Plotting of trace elements and rare earth elements shows a trend of magma fractionation. However, all the analyzed samples show similar pattern. The scientific aspect of this study, gives a picture on geological situation, mineralogy of altered rock, and geochemical characters, and mineralization types occured in the studied area. That is indication of porphyry related mineralization located at Kasihan/Tulakan, Kedung Grombyang and kali Grindulu-Tegalombo; epithermal-mesothermal are around Pacitan area; and epithermal in Banyuanget area which indicated by hotsprings as surficial feature. This study will also benefit to mining industry, in case of helping in arranging mineral exploration planning or strategy, especially to localized the area of mineralized zones.