Geology, Mineralogy and Geochemistry of Base-metal Epithermal Vein at Rahtawu Mountain in the Northern Karang Tengah District, Wonogiri Regency, Central Java, Indonesia

KEUTHPHOUNSA, SAKDA, Dr. Arifudin Idrus

2015 | Tesis | S2 Teknik Geologi

Area penelitian terletak pada daerah Gunung Rahtawu di Utara Distrik Karang Tengah, Kabupaten Wonogiri, Jawa Tengah, Indonesia. Tujuan dari penelitian ini untuk memahami control geologi terhadap endapan mineral dan juga untuk mengetahui karakteristik serta jenis proses mineralisasi endapan logam dasar di daerah ini. Metode analisa yang digunakan untuk penelitian berupa analisa petrografi, analisa XRD (bulk batuan dan lempung), dan analisis geokimia XRF dan AAS. Peta geologi digambarkan berdasarkan interpretasi penganatan petrography dan analisis geokimia (XRF) dan investigasi di lapangan. Satuan batuan yang ditemukan di area penelitian berupa batuan andesite, breksi, mikrodiorit dan tuf. Pemetaan zona alterasi ditentukan melalui analisa XRD dan analisa sayatan tipis yang mengidentifikasi alterasi hydrothermal. Terdiri dari alterasi propylitic dan argillic. Berdasarkan analisa mikroskop biji menunjukkan kehadiran mineral berupa logam dasar (Pb-Cu-Zn) mineralisasi terjadi pada urat kuarsa menjilid dibentuk oleh andesite. Urat kuarsa hadir sebagai pembawa mineralisasi secara utama dengan kandungan mineral logam yang beragam antara lain pyrite, sphalerite, galena, chalcopyrite, dan covellite. Pemodelan mineralisasi menggunakan model setelah Buchanan (1981) dalam penelitian ini dan dari hasil analisis menunjukkan bahwa mineralisasi pada area ini merupakan mineralisasi yang terjadi pada zona Epithermal pembawa logam dasar pada system sulpidasi rendah, dengan kedalaman antara 300-400 m dan temperatur antara 200-250oC”. hasil analisa geokimia AAS, menunjukkan kandungan Lead yang tinggi (Pb) dengan kisaran nilai antara 110 ppm sampai 54,910 ppm, tembaga (Cu) dengan kisaran 74 ppm sampai 3,563 ppm, zinc (Zn) antara 146 ppm sampai 39,400 ppm, Perak (Ag) antara 24.2 ppm sampai 48.8 ppm dan kandungan emas minor antara 0.026 ppm sampai 0.186 ppm.

The research area is located at Rahtawu Mountain in the Northern Karang Tengah District, Wonogiri Regency, Central Java, Indonesia. The purpose of this research is to understand the geological control of the deposit, and as well as to know the characteristics and style of mineralization under/within the characteristics of base-metal in research area. The analysis methods consist of Petrographic analysis, XRD analysis, and geochemical analysis as XRF and AAS. The geological map was drawn based on interpretation of petrography and bulk-rocks geochemistry (XRF) and also field investigation. The rock units that defined the geology of the research area include andesite, breccia, microdiorite and tuff. The alteration zone mapping was described by determination of alteration mineral based on Petrography as thin section, and XRD analysis that identified hydrothermal alteration with two types of alteration consist of propylitic and argillic alteration. Based on ore microscopy study indicates of base metals (Pb-Cu-Zn) mineralization occurs in banded quartz veins hosted by andesite. Quartz vein was the maintly for occurrence of mineralization; and mineralization is commonly characterized by the presence of quartz vein with variable amount of pyrite, sphalerite, galena, chalcopyrite, and covellite. The model after Buchanan (1981) is used in this research and the result of mineral styles in research area was within “Base-metal Epithermal Vein in low-sulphidation with the depth is between 300-400 m and temperature is between 200-250oC”. The result of geochemical analysis as AAS, it has high grade lead (Pb) range from 110 ppm to 54,910 ppm, copper (Cu) range from 74 ppm to 3,563 ppm, zinc (Zn) range from 146 ppm to 39,400 ppm, silver (Ag) range from 24.2 ppm to 48.8 ppm and minor gold range from 0.026 ppm to 0.186 ppm.

Kata Kunci : geologi, mineral bijih, alterasi hidrothermal, mineralisasi sulfidasi rendah dasar epitermal, Gunug Rahtawu di Utara Distrik Karang Tengah, Kabupaten Wonogiri, Jawa Tengah, Indonesia