Analisis Kualitas Massa Batuan Pada Bukaan Terowongan di Area Undercut (DZ2600L), DMLZ, Distrik Ertsberg, Papua

WINNER JANIS KAMBU , I Gde Budi Indrawan, M.Eng. Ph.D. ; Dr. Ir. I Wayan Warmada

2016 | Skripsi | S1 TEKNIK GEOLOGI

Lokasi penelitian berada di area undercut, DMLZ yang merupakan salah satu tipe endapan skarn di Distrik Ertsberg, Tembagapura, Kabupaten Mimika, Provinsi Papua yang termasuk dalam konsesi PT. Freeport (CoW A). Penelitian ini bertujuan untuk menentukan kondisi geologi berupa litologi dan struktur geologi (sesar dan kekar) yang mengontrol kualitas massa batuan, menentukan karakteristik material pengisi kekar, dan menentukan kualitas massa batuan berdasarkan klasifikas Q-system dan RMR di lokasi penelitian. Penelitian ini dilakukan dengan dua tahap, yaitu tahap lapangan yang meliputi pemetaan geologi bawah permukaan dengan luas daerah 1 x 1,5 km dan analisis inti batuan berupa deskripsi inti batuan dan core orienting, sedangkan tahap laboratorium meliputi analisis near infrared menggunakan ASD (Analytical Spectral Devices) dan XRD (X-Ray Diffraction). Litologi penyusun daerah penelitian tersusun atas dua satuan, yaitu satuan batugamping (Formasi Ekmai) yang tersebar di bagian selatan, sisipan satuan serpih sebagai lapisan penanda, dan satuan dolostone (Formasi Waripi) yang tersebar di bagian utara lokasi penelitian. Struktur geologi yang berkembang berupa kekar dan sesar. Terdapat empat sesar, yaitu sesar NS 2 dan NW 8 dengan tren utara-selatan. Karakteristik sesar NS 2 berupa slickenside, pitch sebesar 20 derajat, dan gouge dengan tren N 162 derajat E/58 derajat  sedangkan sesar NW 8 dengan karakteristik berupa zona gerusan mencapai 2 meter dengan tren utara 137-145 derajat timur/55-60 derajat. Kedua sesar lainnya, yaitu sesar NE 5 dan sesar NE 4 dengan tren tenggara-baratlaut diinterpretasikan berdasarkan korelasi data deskripsi inti batuan pada tujuh lubang pemboran dan data orientasi inti batuan. Kekar memiliki arah dominan utara 175 derajat timur/65 derajat, utara 190 derajat timur/80 derajat, dan utara 158 derajat timur/ 54 derajat. Presentase mineral berdasarkan analisis near infrared yaitu dolomit sebesar 28.5% kemudian diikuti oleh mineral lainya seperti talk, serpentin, Mg klorit, phengit, illit dan muskovit dengan presentase yang sama yaitu sebesar 14.3%. Analisis XRD menunjukan kehadiran beberapa mineral penting, seperti: alunit, klorit, illit, kaolinit, serisit, smektit, talk. Berdasarkan klasifikasi Q-system, nilai kualitas massa batuan di daerah penelitian memiliki dua kelas, yaitu kelas massa batuan dengan kualitas sangat jelek (very poor) dengan interval nilai Q dari 0,41 hingga 0,98 dan kelas massa batuan dengan kualitas jelek (poor) dengan interval nilai Q dari 1,07 hingga 2,46 sedangkan berdasarkan klasifikasi RMR, nilai kualitas massa batuan dibagi menjadi tiga kelas, yaitu: kelas III (sedang), kelas II (baik), dan kelas I (sangat baik).

Location of the study are in the area of undercut, DMLZ which is one of the skarn type deposit in Ertsberg District, Tembagapura, Mimika District, Papua Province which is included in the concession Freeport (CoW A). This study aims to determine the geological conditions such as lithology and geological structures (faults and joints), which controls the quality of the rock mass, determine the characteristics of the materials filling, and determine the quality of the rock mass based on the Q-system and RMR. This research was conducted in two stages: first, field work that includes geological mapping of the subsurface area of 1 x 1.5 km and analysis of lithology from direct mapping and core orienting, while the laboratory stage includes analysis of near infrared using the ASD (Analytical Spectral Devices) and XRD (X-Ray Diffraction). Lithology consist of two units of the study area is composed of limestone (Ekmai Formation) spread in the southern part, the insertion unit flakes as a layer marker and dolostone unit (Waripi Formation) spread in the northern part of the study area. The geological structure that develops in the form of joints and faults. There are four fault, which is fault NS 2 and NW 8 with a north-south trend. Characteristics of fault NS 2 form slickenside, a pitch of of 20 degree, and gouge trends north 162 degree east / 58 degree while fault NW 8 with the characteristics of a sheared zone of 2 meters with the trend north 137-145 degree east / 55-60 degree. The other faults are NE 5 fault and NE 4 fault with a southeast-northwest trend is interpreted based on the correlation of data descriptions from core drilling in seven hole core orientation data. The joints has a dominant direction are north 175 degree east / 65, north 190 degree east/ 80 degree and north 158 degree east/ 54 degree. The present of minerals based on the analysis of near infrared dolomite of 28.5% followed by other minerals such as talc, serpentine, Mg chlorite, phengite, illite and muscovite with the same percentage that is equal to 14.3%. XRD analysis showed the presence of several important minerals, such as: alunite, chlorite, illite, kaolinite, sericite, smectite, talc. Based on the classification of Q-system, the value of the quality of the rock mass in the study area devided into two classes, which is very poor rock mass quality with the interval Q value of 0,41 to 0.98 and poor class rock mass quality with the interval of values Q from 1,07 to 2,46 while based on RMR classification, the value of the quality of the rock mass are divided into three classes, which is class III (moderate), class II (good), and class I (very good).

Kata Kunci : undercut, skarn, DMLZ, Ertsberg, core orienting, Orientasi Intibatuan, near infrared, Q-system, RMR