Comparative Study of Gajah and Ijo Volcano Mineralization in Kulon Progo Dome based on Textural and Mineralogical Characteristics

Gajah Volcano and Ijo Volcano are two tertiary volcanoes located in the Kulon Progo Dome area, Yogyakarta. Gajah Volcano is located in the middle of the Kulon Progo Dome which is the oldest in the complex and belongs to Early Oligocene volcanism period (± 29 mya). On the other hand, Ijo Volcano is a product of younger volcanism period, occurred in the Late Oligocene (± 25 mya). The tectonic deformation occurred during the Late Oligocene-Early Miocene led to the formation of geological structures like faults and joints, which also serve as pathway for acid-intermediate intrusion rocks. The intrusions are associated with hydrothermal alteration and ore mineralization in both volcanoes. There has been no research comparing the characteristics of hydrothermal deposits that formed on Gajah Volcano and Ijo Volcano. This will be the main objective of this research. The research was carried out at two mineralization prospect locations representing each volcano, namely the Kaligono area (Gajah Volcano) and the Hargorojo area (Ijo Volcano). The results were obtained from geological and alteration mapping as well as representative rocks/veins sampling...Petrology,..petrography, mineragra- phy, and XRD analyzes conducted on altered rock and vein samples from the two prospects indicated some differences. Kaligono prospect area (Gajah Volcano) consists various of alteration types ie. phylic (quartz-sericite-illite-pyrite),..propylitic..(chlorite-calcite-pyrite±epi- dote±actinolite), and argillic (illite-smectite-kaolinite±quartz). The mineralized veins found on Gajah Volcano show vein swarm, brecciated, stockwork, and massive vein structure with massive vein textures. The veins in Kaligono show NE-SW and NW-SE trends and hosted by Andesite, Dacite, and Andesite Lava. Gangue minerals that are found in the vein samples are quartz, illite, iron oxide, pyrite, and carbonate minerals. The ore minerals consist of magnetite, chalcopyrite, and sphalerite. Meanwhile in Hargorojo prospect area (Ijo Volcano), the types of alteration found including phylic (quartz-sericite-pyrite),..propylitic..(pyrite-calcite±ch- lorite), and argillic (illite-smectite-kaolinite-quartz). The mineralized veins found on Ijo Volcano have a massive vein structure, brecciation, and stockwork with comb, drussy, and massive vein textures. The veins have NNE-SSW and E-W trend and hosted by Andesite and Dacite. The gangue minerals are carbonate minerals, oxide minerals, pyrite, barite, quartz and chalcedony. The ore minerals include chalcopyrite, silver, galena, and sphalerite. Based on the vein characteristics of Kaligono prospect, such as a complex stockwork structure, hydrothermal breccia, and massive vein texture, which contain high temperature hydrothermal minerals, ie. epidote, actinolite, and magnetite, maybe indicate this deposit is controlled by deep..structure..related..to..the..porphyry miner alization. Whereas in the Hargorojo prospect shows the typical textures of shallow epithermal system (open space filling), such as comb and drussy, which contain lower temperature hydrothermal minerals, such as chalcedony, silver, and galena. Based on textural and mineralogical characteristics, Kaligono prospect suggests that the alteration and mineralization takes place deeper or closer to the magmatic source. On the other hand, Hargorojo prospect suggests the alteration and mineralization process relatively far from the source.