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

Ronaldo Irzon; Ildrem Syafri; Iwan Setiawan; Johanes Hutabarat; Purnama Sendjaja; Agus Didit Haryanto

doi:10.14203/risetgeotam2019.v29.1019

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

Ronaldo Irzon, Ildrem Syafri, Iwan Setiawan, Johanes Hutabarat, Purnama Sendjaja, Agus Didit Haryanto

Abstract

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.

Keywords

Cu mineralization, geochemistry, Isocon, Sulit Air Suite

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DOI: http://dx.doi.org/10.14203/risetgeotam2019.v29.1019