GEOLOGY, HYDROTHERMAL ALTERATION AND MINERALIZATION AT KADEMANGAN DISTRICT, BLITAR REGENCY, EAST JAVA PROVINCE, INDONESIA

KIMHOUY, OY, Dr. Arifudin Idrus, S.T., M.T;Ir.A. Dewi Titisari, M.T. Ph.D

2020 | Tesis | MAGISTER TEKNIK GEOLOGI

Daerah penelitian terletak di kabupaten Kademangan, Kabupaten Blitar, Jawa Timur, Indonesia. Penelitian ini bertujuan untuk mengetahui alterasi, mineralisasi dan karakteristik geokimia batuan. Metode yang digunakan terdiri dari pengamatan kondisi geologi permukaan, dan pemetaan alterasi yang dikombinasikan dengan laboratorium analisis. Analisis laboratorium yang dikerjakan dalam penenlitian ini berupa analisis petrografi, Difraksi Sinar X (XRD), analisis geokimia (senyawa oksida dan unsur-unsur jejak), mikroskop bijih dan Spektrometri Serapan Atom-Api (FA-AAS). Daerah penelitian geologi terdiri dari Formasi Mandalika, Formasi Nampol, Formasi Wuni dan Formasi Wonosari. Berdasarkan hasil dari pengamatan lapangan dan analisis laboratorium, litologi daerah penelitian adalah andesit, dasit, rhyodacite, breksi dan basaltik-andesit. Struktur geologi yang mengontrol daerah penelitian mempunyai tren timur laut-barat daya dan barat laut-tenggara, dimana struktur ini berupa stockwork, normal fault, dextral strike slip fault, sinistral strike slip fault dan urat. Zona alterasi dibagi menjadi argillic advance (alunite, diaspore, pyrophyllite, quartz, kaolinite, illite, illite-smectite, montmorillonite, chlorite, hematite dan jarosite), argillic (illite, smectite, montmorillonite, kuarsa, kaolinit, klorit, kalsit dan natrojarosite) dan zona alterasi profilaksis (klorit, kalsit, serisit, kuarsa). Karakterisasi mineralisasi pirit, kalkopirit, emas, galena, arsenopirit, sphalerit, dan perunggu terjadi pada tekstur kuarsa vuggy, stockwork pada andesit, batuan inang dacite dan kuarsa adularia kecil dengan diseminasi, exsolution, penggantian, dan tekstur pengisian ruang terbuka. Selanjutnya, kisaran kadar emas dari <0,01 ppm hingga 0,04 ppm, sedangkan rata-rata analisis sampel ini memperoleh kadar emas 0,017 ppm. Afinitas magma ditunjukkan dalam Calc-alkaline dan juga dalam Tholeiitic. Selain itu, pengaturan tektonik mengidentifikasi di zona subduksi, busur benua dan busur lautan. Menurut karakteristik di atas, endapan dapat dikategorikan dalam epitermal sulfidasi tinggi, dimana perubahan hidrotermal dan mineralisasi dikendalikan oleh parameter litologi dan geologi struktural.

The research area is located in Kademangan district, Blitar Regency, East Java, Indonesia. This research is aimed to understand the alteration, mineralization and rock geochemistry characteristics. The methods used comprise surficial geology and alteration mapping combined with analysis laboratory. The laboratory analyzed using petrography, X-ray diffraction, Ore microscope, Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and Fire Assay-Atomic Absorption Spectrometry (FA-AAS). The geological research area is composed of Mandalika Formation, Nampol Formation, Wuni Formation and Wonosari Formation. Based on the results from field observation and laboratory analysis, the Kademangan, the lithology is hosted the rock that its formation ranges from older to younger of volcanic breccia, dacite tuff, basaltic-andesite, andesite, intrusive dacite and limestone, respectively. The geological structure controlling direction from northeast-southwest and northwest-southeast, which this structure is formed stockwork, normal fault, dextral strike slip fault, sinistral strike slip fault and small vein. Alteration zone is divided into advance argillic (alunite, diaspore, pyrophyllite, quartz, kaolinite, illite, illite-smectite, montmorillonite, chlorite, hematite and jarosite), argillic (illite, smectite, montmorillonite, quartz, kaolinite, chlorite, calcite and natrojarosite) and prophylitic alteration zone (chlorite, calcite, sericite, quartz). Mineralization characterize of pyrite, chalcopyrite, gold, galena, arsenopyrite, sphalerite and malachite occurs from vuggy quartz texture, stockwork on andesite, dacite host rocks and minor adularia quartz with disseminated, exsolution, replacement and open space filling texture. Furthermore, the gold grade range from <0.01 ppm to 0.04 ppm, while the average of these sample analysis obtained the gold content of 0.017 ppm. The magma affinity is indicated in Calc-alkaline and also in Tholeiitic. Moreover, tectonic setting identifies in subduction zone, continental arc and in oceanic arc. According to the characteristics above, the deposit can be categorized in high-sulfidation epithermal, which hydrothermal alteration and mineralization are controlling by parameters of lithology and structural geology.

Kata Kunci : Kademangan district, Hydrothermal alteration, high-sul����¯&At