STUDY ON EPITHERMAL GOLD MINERALIZATION SYSTEM AT THE MONYWA DISTRICT, CENTRAL MYANMAR WITH SPECIAL REFERENCE TO KYAUKMYET AND SHWEBONTHA GOLD PROSPECTS

TOE NAING OO, Dr. Agung Harijoko, S.T., M.Eng; Dr. Lucas Donny S.

2021 | Disertasi | DOKTOR TEKNIK GEOLOGI

Wilayah prospek Kyaukmyet dan Shwebontha terletak di sebelah barat Sungai Chindwin dan Kota Monywa di Myanmar Tengah. Ini adalah bagian dari Sabuk Vulkanik Tengah yang umumnya berarah utara-selatan. Mineralisasi emas dan logam dasar terkait terutama ditampung oleh batuan vulkanik dan vulkaniklastik kalk-alkali Tersier yang terdiri dari riolit, tufa, tufa lapili, dan breksi tufa serta breksi hidrotermal. Batuan vulkanik dan vulkaniklastik yang mengelilingi urat termineralisasi biasanya mengalami perubahan besar. Kumpulan mineral menunjukkan bahwa jenis alterasi meliputi silisifikasi serta alterasi argilik dan propilitik. Kumpulan mineral alterasi meliputi kuarsa, adularia, kalsit, klorit, epidot, kaolinit, serisit, ilit dan ilit / smektit. Berbagai tekstur urat kuarsa diamati pada urat kuarsa termineralisasi. Mereka dicirikan oleh tekstur primer dan rekristalisasi serta tekstur pengganti. Tekstur vena yang paling umum adalah tekstur masif, berkerak, koloid, berkerak-kerak, berpita, berbilah kisi, kalsit berbilah kisi, kalsit berbilah kisi diganti dengan tekstur kuarsa, mosaik, sisir, simpul pita, zonal, plumose, berbulu/flameboyant, dan sakaroid. Sebagian besar mineralisasi bijih berada di urat pengisi ruang terbuka dengan jumlah yang lebih sedikit sebagai pengganti dan mineral bijih yang tersebar di batuan induk vulkanik dan vulkaniklastik. Batuan termineralisasi mengandung emas- dan logam dasar-bantalan crustiform, crustiform-colloform, atau urat kuarsa masif dari prospek Kyaukmyet dan urat kuarsa breksi yang mengandung emas dan urat kuarsa kalsedonik dari prospek Shwebontha. Tren urat mineralisasi di wilayah penelitian menunjukkan arah ENE-WSW dan NE-ENE. Mineral bijih primer yang paling umum dalam urat termineralisasi di prospek Kyaukmyet dan Shwebontha termasuk pirit, galena, sfalerit, kalkopirit, dan emas/elektrum. Mineral gangue yang dominan adalah pirit, kuarsa, adularia, serisit, dan sejumlah kecil kalsit. Kovelit, goetit, dan hematit terjadi sebagai mineral supergen akhir di bagian dangkal vena. Mineral bijih terjadi sebagai pengganti, diseminasi, dan akumulasi masif di pembuluh darah yang termineralisasi. Batuan vulkanik dan vulkaniklastik yang menjadi inang Kyaukmyet dan Shwebontha diklasifikasikan menggunakan elemen utama (TiO2) dan elemen jejak (Zr, Nb, dan Y), dan mereka menunjukkan bahwa semua batuan inang jatuh di bidang riolit. Selain itu, diagram plot SiO2 vs Na2O + K2O serta diagram AFM menunjukkan bahwa sebagian besar sampel batuan tergolong dalam bidang deret kalk-alkali. Berdasarkan diagram laba-laba dinormalisasi kondrit REE, kandungan REE batuan dari daerah penelitian umumnya mirip dengan kerak kontinensial atas. Dua fase inklusi cairan diamati pada urat kuarsa termineralisasi sebagai inklusi kaya cairan dan kaya uap serta inklusi kaya cairan dan kaya uap yang berdampingan juga ditemukan. Inklusi cairan dalam urat kuarsa termineralisasi memiliki suhu homogenisasi mulai dari 148 celcus hingga 310 celcius dan salinitas dari 0.35% berat hingga 2.75% berat setara NaCl. Kuarsa di urat kuarsa termineralisasi kemungkinan besar diendapkan pada kedalaman 165-540 m di bawah paleosurface. Tekanan hidrostatis pada kedalaman tersebut masing-masing sekitar 15.6 bar (1.56 MPa) dan 54.4 bar (5.42 MPa). Perebusan fluida memberikan kontribusi yang lebih signifikan terhadap pengendapan emas dan logam dasar dalam sistem ini. Selain itu, pencampuran cairan antara cairan bijih bantalan panas, asam dan garam dan air meteorik yang lebih dingin mungkin juga bertanggung jawab atas pengendapan emas dan logam dasar. Berdasarkan analisis SEM-EDX, kandungan FeS dalam sfalerit diperkirakan dalam pirit + sfalerit ± kalkopirit prospek Kyaukmyet dengan hasil 11-11.5 mol% dan prospek Shwebontha dengan hasil masing-masing 5.2-5.6 mol%. Dengan kombinasi suhu formasi dan hasil SEM-EDX, fugacity sulfur dari setiap urat kuarsa dapat dibatasi dan menurut diagram suhu-fS2 log untuk semua urat kuarsa di area prospek Kyaukmyet dan Shwebontha menyarankan agar mereka dapat diklasifikasikan sebagai keadaan sulfidasi menengah. Berdasarkan semua data yang tersedia termasuk perubahan hidrotermal, tekstur urat kuarsa, mineralogi bijih, dan data inklusi cairan serta data geologi relevan lainnya, disarankan bahwa daerah prospek Kyaukmyet dan Shwebontha mungkin terbentuk di bawah kondisi epitermal sulfidasi rendah hingga menengah. Sehubungan dengan deposit Cu-Au sulfidasi tinggi di Letpadaung, model genetik ditetapkan bahwa sistem epitermal sulfidasi rendah hingga menengah di prospek Kyaukmyet dan Shwebontha dibentuk di bagian distal dengan penempatan dari pH yang berkurang, mendekati netral, encer cairan air meteorik yang dominan dengan masukan kecil dari komponen magmatik yang biasanya terkait dengan magmatisme kalk-alkali hingga basa, di lingkungan busur vulkanik. Endapan Letpadaung Cu-Au jenis sulfidasi tinggi terbentuk di proksimal sumber magmatik dan didominasi oleh fluida ber-pH rendah, asam dan teroksidasi.

The Kyaukmyet and Shwebontha prospect areas are located west of the Chindwin River and Monywa City in Central Myanmar. It is a part of generally north-south trending Central Volcanic Belt. Gold and associated base metals mineralizations are mainly hosted by Tertiary calc-alkaline volcanic and volcaniclastic rocks which are composed of rhyolite, tuff, lapilli tuff and tuff brecccia as well as hydrothermal breccias. The volcanic and volcaniclastic rocks surrounding the mineralized veins are commonly extensively altered. The mineral assemblages show that the types of alteration include silicification as well as argillic and propylitic alteration. The alteration mineral assemblages include quartz, adularia, calcite, chlorite, epidote, kaolinite, sericite, illite and illite/smectite. A variety of quartz vein textures are observed in mineralized quartz veins. They are characterized by primary and recrystallization as well as replacement texures. Most common vein textures are massive, crustiform, colloform, crustiform-colloform, banded, lattice bladed, lattice bladed calcite, lattice bladed calcite replaced by quartz, mosaic, comb, cockade, zonal, plumose, feathery/flameboyant and saccharoidal textures. Most of the ore mineralization is in open-space filling veins with lesser amounts in replacement and disseminated ore minerals in the volcanic and volcaniclastic host rock. The mineralized rocks contain gold- and base metal-bearing crustiform, crustiform-colloform, or massive quartz veins from the Kyaukmyet prospect and gold-bearing brecciated quartz vein and chalcedonic quartz vein from the Shwebontha prospect. Mineralization vein trends of research areas show ENE-WSW and NE-ENE directions. The most common primary ore minerals in the mineralized veins at the Kyaukmyet and Shwebontha prospects include pyrite, galena, sphalerite, chalcopyrite, and gold/electrum. The predominant gangue minerals are pyrite, quartz, adularia, sericite, and minor amounts of calcite. Covellite, goethite, and hematite occur as late supergene minerals in the shallow portions of the veins. The ore minerals occur as replacements, disseminations, and massive accumulations in the mineralized veins. The Kyaukmyet and Shwebontha host volcanic and volcaniclastic rocks are classified using major element (TiO2) and trace elements (Zr, Nb, and Y), and they show that all host rocks fall in the fields of rhyolite. In addition, SiO2 vs Na2O+K2O plot diagram as well as AFM diagram indicate that most of rock samples are classified in the field of the calc-alkaline series. Based on the REE chondrite-normalized spider diagrams, the REE contents of rocks from the research areas are generally similar to those of the upper continential crust. Two phase of fluid inclusions are observed in mineralized quartz veins as liquidrich and vapor-rich and coexisting liquid-rich and vapor-rich inclusions are also found. Fluid inclusions in the mineralized quartz veins have homogenization temperatures ranging from 148 celcius to 310 celcius and salinities from 0.35 wt% to 2.75 wt% NaCl equiv. The quartz in the mineralized quartz veins was most likely precipitated at a depth of m below the paleosurface. Hydrostatic pressures at those depths are approximately 15.6 bars (1.56 MPa) and 54.4 bars (5.42 MPa), respectively. Fluid boiling made a more significant contribution to gold and base metals precipitation in this system. In addition, fluid mixing between hot, acidic and saline bearing ore fluid and cooler meteoric water might be also responsible for gold and base-metals deposition. Based on SEM-EDX analysis, FeS contents in sphalerite were estimated in pyrite + sphalerite ± chalcopyrite of Kyaukmyet prospect with the result of 11-11.5 mol% and Shwebontha prospect with the result of 5.2-5.6 mol% respectively. By a combination of formation temperatures and SEM-EDX results, sulfur fugacity of each quartz vein can be constrained and according to log fS2temperaturediagram for all quartz veins in the Kyaukmyet and Shwebontha prospects area suggested that they can be classified as intermediate sulfidation state. On the basis of all available data including hydrothermal alteration, quartz vein textures, ore mineralogy, and fluid inclusion data as well as other relevant geological data, it is suggested that the Kyaukmyet and Shwebontha prospect areas were probably formed under low to intermediate sulfidation epithermal conditions. In relation to the high sulfidation Cu-Au deposit at Letpadaung, a genetic model is established that the low to intermediate sulfidation epithermal systems at the Kyaukmyet and Shwebontha prospects were formed at a distal part by the emplacement of a reduced, near neutral pH, dilute fluids of dominantly meteoric water with minor input of magmatic components typically associated with calcalkaline to alkaline magmatism, in anvolcanic arc environment. Thehigh-sulfidation type Letpadaung Cu-Au deposit was formed proximal to the magmatic source and dominated by low pH, acidic and oxidized fluids.

Kata Kunci : Monywa district, Gold mineralization, hydrothermal fluid, fluid inclusions