INDICATION OF HYDROTHERMAL ALTERATION ACTIVITIES BASED ON PETROGRAPHY OF VOLCANIC ROCKS IN ABANG KOMBA SUBMARINE VOLCANO, EAST FLORES SEA

The presence of mineral alteration or secondary processes to rocks on submarine volcano of Abang Komba was caused by an introduction of hydrothermal solutions. Those are indicated by the presence of a resembly of minerals alteration seen in their petrographic analyses. They are characterized by replacement partially surrounding of plagioclase phenocrysts, partially replacing plagioclase by sericite, carbonate and clay minerals. The replacement of pyroxene partly by chlorite, and the presence of albitisation (secondary albite) contained in fine rectangular plagioclase sized. Other fitures occasionally observed by the presence of partial oxidation of ore minerals and the presence of quartz, and epidote as an alteration from plagioclase and pyroxene. Keywords : alteration, resembly of minerals alteration, oxidation, submarine vulcano of Abang Komba. Gejala alterasi atau proses-proses sekunder yang terjadi pada batuan di gunung bawah laut Abang Komba adalah disebabkan oleh introduksi larutan hidrotermal. Semua ini ditunjukkan dengan kehadiran kumpulan mineral ubahan yang terlihat dalam sayatan batuan. Kumpulan mineral ini dicirikan dengan adanya penggantian sebagian yang mengelilingi fenokris plagioklas, penggantian sebagian plagioklas oleh serisit, karbonat dan mineral lempung. Penggantian sebagian piroksen oleh klorit, dan adanya gejala albitisasi (albit sekunder) yang terdapat pada plagioklas berbentuk balokan yang berukuran halus. Gejala lainnya yang kadang-kadang teramati adanya oksidasi sebagian dari mineral bijih dan hadirnya kuarsa, serta epidot sebagai hasil ubahan plagioklas dan piroksen. Kata kunci : alterasi, kumpulan mineral ubahan, oksidasi, gunung bawahlaut Abang Komba

Indication Of Hydrothermal Alteration Activities Based On Petrography Of Volcanic Rocks In Abang Komba Submarine Volcano, East Flores Sea

The presence of mineral alteration or secondary processes to rocks on submarine volcano of Abang Komba was caused by an introduction of hydrothermal solutions. Those are indicated by the presence of a resembly of minerals alteration seen in their petrographic analyses. They are characterized by replacement partially surrounding of plagioclase phenocrysts, partially replacing plagioclase by sericite, carbonate and clay minerals. The replacement of pyroxene partly by chlorite, and the presence of albitisation (secondary albite) contained in fine rectangular plagioclase sized. Other fitures occasionally observed by the presence of partial oxidation of ore minerals and the presence of quartz, and epidote as an alteration from plagioclase and pyroxene. Keywords : alteration, resembly of minerals alteration, oxidation, submarine vulcano of Abang Komba. Gejala alterasi atau proses-proses sekunder yang terjadi pada batuan di gunung bawah laut Abang Komba adalah disebabkan oleh introduksi larutan hidrotermal. Semua ini ditunjukkan dengan kehadiran kumpulan mineral ubahan yang terlihat dalam sayatan batuan. Kumpulan mineral ini dicirikan dengan adanya penggantian sebagian yang mengelilingi fenokris plagioklas, penggantian sebagian plagioklas oleh serisit, karbonat dan mineral lempung. Penggantian sebagian piroksen oleh klorit, dan adanya gejala albitisasi (albit sekunder) yang terdapat pada plagioklas berbentuk balokan yang berukuran halus. Gejala lainnya yang kadang-kadang teramati adanya oksidasi sebagian dari mineral bijih dan hadirnya kuarsa, serta epidot sebagai hasil ubahan plagioklas dan piroksen. Kata kunci : alterasi, kumpulan mineral ubahan, oksidasi, gunung bawahlaut Abang Komba

Geochemistry of plutons in central Sumatra and their correlation to Southeast Asia tectonic history

Previous investigations of plutons in Sumatra were focused on age dating with minimum geochemistry composition analysis. The purpose of this study is to define the geochemistry classification of the intrusions in central Sumatra on describing the emplacement mechanism associated with Southeast Asia tectonics. The rocks composed of quartz, K-feldspar, and plagioclase with amphibole, biotite, epidote, and zircon as accessory minerals. Six of seven studied plutons range from monzodiorite to granite with metaluminous-slight peraluminous, medium to very high-K calc-alkaline, magnesian, calcic to calc-alkalic affinities. The studied samples depict a wide range of total REE composition of 39-1,369ppm. Most of the rocks denote Sr, Ti, Y, Ce, and Eu anomalies on the primitive-mantle and chondrite normalized diagram. LREE are more enriched in comparison to HREE with (La/Sm)N= 1.71-18.75 and (Gd/Lu)N= 0.15-2.59. Most of the studied plutons are classified in the I-type according to the A/CNK value, negative SiO2 to P2O5 correlation with magnesian and arc-associated character. A-type nature of Sijunjung Granite is displayed on its high silica and REE content with ferroan, calc-alkalic, and within-plate affinities. The existence of A-type intrusion implies an extensional setting during long time subduction episodes, which triggered I-type magmatism since Late Permian to Neogene in Sumatra

PENCITRAAN TAHANAN JENIS BAWAH PERMUKAAN DI AREA PROSPEK PANAS BUMI GUNUNG SLAMET BERDASARKAN DATA MAGNETOTELURIK

Magnetotelluric (MT) method has been applied to identify subsurface structures in Guci Geothermal Area, Mount Slamet. The objective of this research is to analyze the subsurface configuration based on the resistivity value beneath the Bojong district, Tegal regency, Central Java province. Stages of data processing started with transforming the data from the time domain to the frequency domain, and then graphing resistivity apparent to frequency and graph phase versus frequency, smoothing the graph and the last, inversion modeling with final result of 2D resistivity cross section. The results of data processing magnetotelluric (MT) revealed three groups of rock resistivity value. The high resistivity value (> 1000 Ω.m) represented the basement as a heat source. Low resistivity value (1000 Ω.m) berkaitan dengan batuan dasar sebagai sumber panas. Nilai tahanan jenis rendah (<10 Ω.m) ditafsirkan sebagai batuan ubahan yang menjadi batuan penudung. Nilai tahanan jenis antara 10-225 Ωm berasosiasi dengan lapisan batuan yang bersifat poros dan permeabel yang menyimpan fluida panas, dapat berperan sebagai reservoir panas bumi. Lapisan lain dengan nilai tahanan jenis antara 225-1000 Ωm kemungkinan sebagai batuan sedimen Tersier. Dari kajian penampang tahanan jenis MT, lokasi yang kemungkinan mempunyai prospek panas bumi, yaitu di daerah depresi Guci, dengan ketebalan reservoir 600- 1000 m pada kedalaman 750-1600 m yang ditutupi oleh lapisan penudung berupa batuan ubahan

Imobilitas Unsur Tanah Jarang (UTJ) selama Mineralisasi Cu pada Granitoid Sulit Air, Provinsi Sumatra Barat

Transfer massa terkait perubahan komposisi geokimia batuan induk akibat alterasi hidrotermal, metasomatisme, maupun pelapukan menjadi topik untuk mempelajari proses geologi terkait. Perubahan massa tersebut dapat dijelaskan dan divisualisasikan melalui metode Isocon. Mineralisasi tembaga teridentifikasi pada salah satu bagian dari Granitoid Sulit Air di Kecamatan X Koto Diatas, Kabupaten Solok. Tulisan ini bertujuan untuk menjelaskan transfer massa akibat mineralisasi Cu pada Granitoid Sulit Air dengan diagram Isocon. XRF dan ICP-MS di Laboratorium Pusat Survey Geologi, Kementerian ESDM (2015) digunakan sebagai perangkat pengukuran kadar oksida utama, unsur jejak, dan unsur tanah jarang. Berdasarkan korelasi antara kandidatnya, Al2O3 dianggap sebagai oksida immobile. K2O, Rb, Sr, dan Ba terkayakan sedangkan oksida utama lain maupun unsur jejak diketahui terkurangkan akibat mineralisasi Cu. UTJ  terdeteksi immobile akibat mineralisasi Cu dengan karakter yang relatif sama antara batuan segar dan teralterasi. Meski demikian, sebagian Ce teroksidasi akibat proses mineralisasi sehingga menurunkan anomali positif Ce. Penurunan nilai anomali negatif Eu pada sampel teralterasi dapat mengakibatkan plagioklas semakin terkurangkan. Karakter tipe-I Granitoid Sulit Air diperjelas melalui nilai perbandingan A/CNK, perbandingan N2O terhadap K2O, perbandingan Rb/Sr, dan perbandingan Rb/Ba. Afinitas granitoid busur kepulauan menunjukkan bahwa pembentukan Granitoid Sulit Air terkait dengan vulkanisme di bagian barat Sumatra.Mass transfer related changes in the geochemical composition of the host rock due to hydrothermal alteration, metamorphism, and weathering is an interesting topic for studying related geological processes. The transfer can be explained and visualized through the Isocon method. Copper mineralization was identified in an area of Sulit Air Suite at X Koto Diatas District, Solok Regency. This paper aims to explain mass transfer due to Cu mineralization on Sulit Air Suite with Isocon diagrams. XRF and ICP-MS of the Center for Geological Survey Laboratory were applied to measure the major oxides, trace elements, and rare earth elements contents of the samples. Based on the correlation between candidates, Al2O3 is considered as the immobile species. K2O, Rb, Sr, and Ba appear to be enriched while other major oxides and rare elements are reduced due to Cu mineralization. REEs are immobile due to Cu mineralization with relatively the same character between fresh and altered rocks. However, some Ce was probably oxidized due to the mineralization process thus reducing the positive anomaly Ce. Moreover, the more negative Eu anomaly means that plagioclase might have been replaced by K-feldspar due to this alteration. The I-type characters of Sulit Air Suite are clarified by  A/CNK value, N2O to K2O comparison, Rb/Sr ratio, and Rb/Ba ratio. The affinity to the volcanic arc granitoid implies that the Sulit Air Suite is related to volcanism in the western part of Sumatra.