Zonasi Alterasi Bawah Permukaan pada Prospek Cijulang, Garut, Jawa Barat dengan Menggunakan Difraksi Sinar-X

NADIA SEKARLANGIT, Dr. Ir. I Wayan Warmada

2018 | Skripsi | S1 TEKNIK GEOLOGI

Prospek Cijulang merupakan salah satu prospek yang dikelola oleh PT. Antam. Tbk. Prospek ini berada di Zona Pegunungan Selatan bagian Barat yang terususun oleh batuan sedimen berumur tersier (Formasi Koloberes) dan batuan vulkanik berumur teriser hingga kuarter (endapan piroklastik tidak terpisahkan) yang telah mengalami alterasi. Penelitian dilakukan untuk mengetahui mineral sekunder yang terbentuk akibat proses alterasi hidrotermal dan membuat zonasi tipe alterasi pada daerah penelitian. Analisis dilakukan meliputi analisis petrografi pada 8 sampel batuan, difraksi sinar X sampel bulk sebanyak 16 sampel, dan difraksi sinar X sampel lempung sebanyak 25 sampel. Litologi daerah penelitian tersusun atas batuan vulkanik berupa tuf lapili, lava andesit, breksi volkanik, dan intrusi diorit. Hasil analisis difraksi sinar X sampel bulk menunjukkan hadirnya mineral-mineral sekunder hasil alterasi pada prospek Cijulang berupa kuarsa, pirofilit, pirit, alunit, diaspor, dan pirolusit. Hasil analisis difraksi sinar X sampel lempung menunjukkan keterdapatan mineral smektit, klorit, ilit, dickit, dan kaolinit. Asosiasi mineral pada batuan pada daerah penelitian dapat dibagi menjadi 5 zona alterasi berdasarkan klasifikasi Hayashi (1973) yaitu IIb-d (alunit-kuarsa; kaolinit, dickit, pirofilit, pirit), IIIb (kaolinit-kuarsa; pirit), IIIc (dickit-pirofilit-kuarsa; kaolinit, diaspor, pirit), IVb (smektit; kuarsa), IVc (klorit-smektit; ilit, pirit). Fluida hidrotermal yang mengalterasi memiliki suhu 100 C hingga 250 C Tekanan total 150-250 atm dan tekanan fluida 30-60 atm. Fluida memiliki pH asam hingga asam lemah.

Cijulang Prospect is one of the prospects managed by PT. Antam, Tbk. This prospect is locaed in the west side of Southern Mountain Zone consist of Tertiary sedimentary rock (Koloberes Formation) and Tertiary-Quartenary volcanic rock (unseparated pyroclastic sediment) that have undergone alteration. This study aims to determine the secondary minerals formed by hydrothermal alteration process and to devide the minerals into several zone base on alteration type. The analysis includes petrography analysis on 8 samples of rock, X-ray diffraction bulk on 16 samples, and X-ray diffraction clay on 25 samples. The litology of the study area consists of volcanic rock. There are lapili tuff, andesite lava, volcanic breccia, and diorite. The result of X-ray diffraction analysis of the bulk samples show the presence of secondary minerals of alteration on Prospect Cijulang in the form of quartz, pyrophillite, pyrite, alunite, diaspore, and pyrolusite. The result of X-ray diffraction analysis od clay sample show that there are smectite, chlorite, illite, dickite, and kaolinite. Assosiation of minerals in the rocks in the study area can be devided into 5 alteration zone based on Hayashi classification (1973) namely IIb-d (alunite-quartz; kaolinite, dickite, pirofilit, pyrite), IIIb (kaolinite-quartz; pyrite), IIIc (dickite-pirofilit-quartz; kaolinite, diaspore, pyrite), IVb (smectite; quartz), IVc (chlorite-smectite; illite, pyrite). Hydrotermal fluid that alterate has a temperature of 100 C to 250 C, total pressure of 150-250 atm, and the fluid pressure of 30-60 atm, fluid has pH from acidic to weak acid.

Kata Kunci : alterasi hidrotermal, difraksi sinar x, zonasi alterasi