Geology and Characteristics of Hydrothermal Ore Mineralization at Ngerjo Area, Pacitan Regency, East Java, Indonesia
Vilachit, Dethvanhxay, Dr. Arifudin Idrus
2014 | Tesis | S2 Teknik GeologiDesa Ngerjo adalah area penelitian, berada di Kabupaten Pacitan, Jawa Timur, Indonesia. Tujuan dari penelitian ini adalah untuk mengetahui kondisi geologi dan faktor-faktor yang mengontrol pembentukan deposit, untuk memetakan perubahan hidrotermal, dan karakteristik mineralisasi bijih, menyediakan panduan untuk eksplorasi selanjutya di Desa Ngerjo. Ada 70 sampel yang diambil dari daerah investigasi untuk petrografi, pemeriksaan mikroskopis, difraksi X-Ray, dan analisis Geokimia (XRF dan AAS) yang digunakan dalam penelitian ini. Tipe-tipe batuan yang mendefinisikan geologi meliputi batuan vulkanik, batuan piroklastik, dan batuan sedimen. Peta geologi telah dibuat berdasarkan interpretasi petrografi dan bulk-rock geochemistry (XRF) yang dikombinasi dengan invesigasi lapangan. Ada 2 tipe perubahan hidrotermal yakni zona perubahan propilitik dan argilik yang ditunjang dengan petrografi, geokimia keseluruhan batuan, analisis difraksi X-ray dan observasi lapangan. Mineralisasi dari area penelitian terutama dikontrol oleh structur geologi dan urat kuarsa terutama dari timur laut ke barat daya. Urat kuarsa dan logam dasar umumnya dicirikan oleh Komposisi kimia urat didasarkan pada hasil geokimia AAS, ia memiliki potensi logam dasar yang tinggi (Pb, Zn, Cu) dari pada logam berharga (Au, Ag), potensi tembaga (Cu) rata-rata 625 sampai 54520 ppm, timbal (Pb) rata-rata 154 sampai 99000 ppm, Seng (Zn) rata-rata 674 sampai 128500 ppm, perak (Ag) 9,5 sampai 395 ppm, dan potensi emas rata-rata 12 sampai 3850 ppb. Protolit dari mineralisasi logam dasar adalah porpiritik yang dihuni oleh andesit lava, di mana urat kuarsa menampilkan ruang terbuka yang diisi sebagian kemungkinan oleh coarse-grained quartz within comb, dog-teeth, cockade, crustiform banded and tekstur massif. Berdasarkan ciri-ciri tersebut, urat tersebut kemungkinan dikategorikan sebagai sistem epitermal sulpidasi rendah.
The Ngerjo area is the location of study area, situated in Pacitan Regency, East Java, Indonesia. The purposes of this research are to understand the geological condition and factors controlling the formation of deposit, to map the hydrothermal alteration, and characterize the ore mineralization, provide guidelines for further exploration in Ngerjo area. There were seventy samples taken from the field investigation, which Petrography, Ore microscopy, X-ray diffraction, and Geochemical analysis (XRF and AAS) are applied for this research. The rock types that define the geology include volcanic rock and sedimentary rocks. The geological map was drawn base on interpretation of petrography and bulk-rock geochemistry (XRF) combination with field investigation. There are two types of hydrothermal alteration such as propylitic and argillic alteration zones which are supported by petrography, whole- rock geochemistry, x-ray diffraction analysis and field observation. The mineralization of the research area is mainly controlled by geological structures and the trend direction of the quartz veins is mainly North-East to South-West direction. The quartz+base metal veins are commonly characterized by the presence of pyrite, galena, sphalerite, chalcopyrite, and covellite. Chemical compositions of veins based on geochemical AAS results show high grade a potential base-metal ores (Pb, Zn, Cu) and quite low precious metals (Au, Ag), copper (Cu) potential range from 625 to 54520 ppm, lead (Pb) potential range from 154 to 99000 ppm, Zinc (Zn) potential range from 674 to 128500 ppm, silver (Ag) potential 9.5 to 395 ppm and gold potential range from 12 to 3850 ppb. The protolith of the base metal mineralization is porphyritic host by andesite lava, which quartz veins showing open-space filling partly indicated by coarse-grained quartz within comb, dog-teeth, cockade, crustiform banded and massive texture. Based on these characteristics, the veins are probably categorized as lowsulphidation epithermal system.
Kata Kunci : Geologi, alterasi hidrotermal, mineral bijih, Prospek Ngerjo, Pacitan, Indonesia.
