Vein Mineralogy and Hydrothermal Gold Mineralization at the Kyaw Soe Thu Prospect in the Wabo Deposit, Mandalay Region, Central Myanmar

The Kyaw Soe Thu Prospect is a part of the Wabo deposit which is situated along Mogok Metamorphic Belt (MMB) in Central Myanmar. Gold bearing veins are hosted in the banded garnet-biotite gneiss, calc-silicate, marble and granitoid of the Wabo area. The granitic rocks intruded into banded gar-net-biotite gneiss. The MMB was intensely deformed and it is a highly fractured metamorphic terrain in which metamorphic facies ranges from amphibolite to granulite facies. Gold mineralization at the Kyaw Soe Thu Prospect in the Wabo deposit occurred in the forms of gold-quartz veins. The quartz veins are a few centimeters in width. The color of the quartz vein at the Kyaw Soe Thu Prospect is commonly smoky and milky. The quartz vein exhibits chalcedonic, saccharoidal and granular textures. Three mineralizations were indicated. Stage I is the thin layered crustiform chalcedonic quartz. Stage II is chalcedonic quartz vein and stage III is Ankerite-quartz-sericite vein. Stage II and stage III are gold rich hydrothermal stages, and Ankerite-quartz-sericite veins were formed with a small amount of sericites in the Kyaw Soe Thu Prospect. The Ankerite-quartz-sericite veinlets range from a centimeter up to three centimeter in width within the stage II chalcedonic quartz vein. Textures of mineralized quartz vein samples were petrographically examined. of very fine-grained chalcopyrite. On the basis of textures of the chalcedonic quartz veins, the Kyaw Soe Thu Prospect is ascribed to be a low-sulfidation epithermal deposit.


Introduction
Gold and copper occurrences are widely distributed in Myanmar. They are 1) Mesothermal gold mineralization, porphyry-type gold and copper and its related  (Figure 1) that is located about 50 km to the north of Mandalay, Central Myanmar. The area is situated at southern continuation of the (MMB). The MMB was initially named by [4] and also has been known as (MMMB) which is one of the most significant metallogenic provinces of Myanmar [3] (Figure 1). The MMB also hosts gems deposits such as Ruby and Sapphire [5] [6] together with some major gold and base metal deposits [7] (Figure 2). Most gold mineralization in the MMB is orogenic gold type and low-sulfidation epithermal gold type, as well as skarn gold-copper type.
The breccia and vein type gold deposits are also hosted by the MMB. The shear related fracture zone-type gold deposits are also found in the MMB. Along the MMB, there are several gold occurrences such as Thayetsu (orogenic gold), Thabeikkyin gold field (mesothermal to epithermal with some skarn type gold), Phayaung Taung (orogenic gold), Modi Taung (orogenic gold), and Shwekyin and Meyon (orogenic gold) ( Figure 2) [3] [7] [8] [9] [10]. It is noted that gold mineralization of the MMB is preferentially confined to the shear zones and fracture systems. The host rocks vary from relatively undeformed and unmetamorphosed carbonaceous and calcareous sedimentary rocks to highly deformed metamorphic rocks of mostly Phanerozoic [10]. Especially, gold mineralization in the Wabo area occurred as gold-quartz veins and mineralized breccia, it is hosted by the garnet-biotite gneiss. This study aims to understand vein texture, characteristics of ore minerals, and mineral paragenesis of the Kyaw Soe Thu Prospect in the Wabo area.

Geological Setting
Myanmar is positioned at the junction of the Alpine-Himalayan Orogenic Belt and the Indonesian Island Arc System [3] (Figure 1). It is a tectonically complex ( Figure 1 and Figure 2). The Eastern Province belongs to the Shan-Thai Block (Shan Plateau) [12], also known as the Sibumasu Block, together with Western Malaysia and Eastern Sumatra [13], whereas the Western Province, which belongs to the West Burma Terrane, comprising the Indo-Myanmar Ranges and the Wuntho-Popa Magmatic Arc [3] [4] (Figure 1 and Figure 2) system overlying the Cretaceous-Eocene strata [14] [15].
The Western Myanmar Province which is also known as the Burma Microplate was originated from Gondwana, and later drifted, accreted and collided to Martaban [3] (Figure 1). The formation of the MMB is considered to be related to the Indian-Asian continental collision events [4]. The oblique convergence and collision between the Indian and Eurasian plates generated strike-slip faults, such as the Nujiang Fault [3] (Figure 1), which traverses western Yunnan, China and the Sagaing Fault that truncates the Myanmar Central Basin [17].
Such Faults extensively disrupted the original orogenic architecture of mainland SE Asia, developing secondary orogenic collages or terranes, such as the MMB [18], and the Shan-Thai Block [17].
The MMB comprises a sequence of high-grade meta-sedimentary and meta-intrusive rocks, representing a foliated amphibolite-grade regionally metamorphosed belt [18] [19], and the MMB is considered as a metamorphic core complex, tectonically unroofed during crustal extension on low-angle detachment faults [14] [20].
The gem-mining in the Mogok area is characterized by a sequence of undifferentiated high-grade metamorphic rocks. The abundant unit is banded gneiss with biotite, garnet, sillimanite and oligoclase. Marble mainly consists of calcite, while dolomitic marbles are limited. Some marbles contain ruby, whereas others have spinel, forsterite or diopside, or phlogopite-graphite. Rutile, graphite, calcite, paragasite, zircon, titanite, apatite, scapolite, spinel and zircon are also present as inclusions in Mogok rubies [6]. Biotite granites are widely exposed at Kabaing and Thabeikkyin [4]  Early Oligocene (43.8 ± 1.0 -27 ± 0.8 Ma), except for older ages that might be derived from detrital and /or inherited grains [22]. On the other hand, Ar-Ar ages, which probably represent the cooling after peak metamorphism, indicated Late Oligocene-Early Miocene ages (24.0 ± 0.8 -16.0 ± 1.0 Ma) [22]. Metamorphic ages by CHIME reported the high temperature-type metamorphism in Central Myanmar, probably reached peak temperature conditions in the Late Eocene, then subsequently shifted to the exhumation and/or cooling stage [22].

Local Geology of the Wabo Deposit
The geology around the Wabo gold deposit is shown in Figure 3. The rock units

Lithology and Nature of Outcrops
In this section, the nature and lithological characteristics of the different rock units of the Wabo deposit are described below.
Garnet biotite gneiss: The gneiss unit in the area exhibits numerous boudinage and augens with varying size of garnet porphyroblasts. The segregation layers are prominent in this rock unit. Most of biotite crystals are larger than 0.5 mm in length. Curved biotite flakes observed around garnet grains seem to be caused by the development of garnet porphyroblasts. Banded gneiss is also exposed in the mine workings. Garnet grains in the rock unit are euhedral to anhedral ranging in size from mm to cm. The banded gneiss is medium-to coarse-grained with feldspar, quartz, biotite, garnet and cordierite. This unit commonly exhibits a banded structure reflecting compositional and/or structural variations. Augen texture is prominent in this unit both in foliation plane and within the band. Rotation of augens during growth, due to shearing, leads to sigmoidal inclusion trails. Drag folding is often observed in the garnet-cordierite biotite gneiss. On the foliation margin, garnets form as aggregated grains.
Marble and calc-silicate rocks: The marble unit is well exposed in the eastern part of Wabo. It is a massive rock unit with phlogopite and graphite. This unit is medium-to coarse-grained, hard and compact and generally exhibits criss-crossed structure. It also exhibits granulose texture. The calc-silicate rock is well charac- It is dominantly composed of feldspar, quartz, and biotite. The exfoliation also occurs in the Wabo area that indicates a rounded hill configuration of the granites. Some aplite dikes occur within in garnet-biotite gneiss body at Myo Lwin and Zaw Min Prospects. Pegmatites are exposed as dykes and veins near the Wabo Village.

Samples and Methods
In this study, the following methods were undertaken: field investigation and sample collection, detailed petrography and textural studies, and X-ray fluorescence (XRF) spectroscopy and X-ray diffractometry (XRD). A total of twen-

Quartz Texture
Quartz is a generally ubiquitous gangue mineral in the Kyaw Soe Thu Prospect. The hydrothermal quartz in the stage I thin crustiform quartz layer, in the stage II chalcedonic quartz vein, and stage III Ankerite-quartz-sericite veins displays distinctive textures associated with gold mineralization. Quartz commonly exhibits chalcedonic texture as well as saccharoidal and granular textures ( Figure   5(a) and Figure 5(b)). Some of quartz crystals are elongated with subhedral to

Ankerite-Quartz-Sericite Mineralized Veins
The Prospect in hand specimen is light pink, milky, and pale brown in color. The Ankerite-quartz-sericite veins range in width from a centimeter to two centimeters.
All ankerite grains at the Kyaw Soe Thu Prospect are rhombohedral ( Figure   5(i) and Figure 5(j)). Ankerite crystals reach over 400 µm in size at most. Ankerite crystals occur together with the very thin calcite veinlet or cross-cutting veinlets. Zoning of the ankerite are observed in well-crystallized euhedral ankerite grains.

Mineralization at the Kyaw Soe Thu Prospect
Stage I mineralization: the earliest stage I consists of quartz, pyrite, and minor chalcopyrite. It occurs as chalcedonic quartz veins. Quartz crystals are fine-grained.
The sulfide minerals appear to have formed as cementing the rock fragments in the veins. Sericite alteration occurred in this stage.
Stage II mineralization: Stage II represents the formation of quartz with sulfide and electrum. Quartz in the stage II veins is smoky and milky associated with pyrite, chalcopyrite and sphalerite. The quartz veins are composed of finegrained randomly distributed quartz crystals. The cluster of electrums ranges up to 1 cm size and mostly coarse grain (Figure 6(a)). The electrum grains are closely associated with pyrite ( Figure 6   closely related with the gold mineralization. The irregular thin layers of calcite and Ankerite-quartz-sericite veinlets are also cross-cutting each other. It appears to be stage III mineralization events as calcite and Ankerite-quartz-sericite vein are cross-cutting the early chalcedonic quartz veins.

Ore Mineralogy and Paragenesis
The gold mineralization stages are classified into three stages (Figure 8) Figure 9(a) and Figure 9(b)) to identify Au -Ag ratio by semiquantitative analysis. The Ag/(Au + Ag) of electrum in the stage II chalcedonic quartz veins of the Kyaw Soe Thu Prospect ranges from 35 to 50 at% ( Figure 10). The Ag/(Au + Ag) of electrum from the stage III Ankerite-quartz-sericite vein ranges from 30 to 50 at% ( Figure 10).

Discussion and Conclusions
The     [26]. Quartz with radiating texture shows a plumose texture. Some plumose textures are interpreted to reflect the crystallization of amorphous silica to quartz [27]. Textures formed by recrystallization of amorphous silica to chalcedony, such as the plumose (radiating) texture, are restricted to near-surface hydrothermal condition [28]. [26] indicated that these textures are loosely packed vitreous to milky fine-grained quartz cluster having the appearance of saccharoidal in hand specimens. [29] indicated that intermediate silica supersaturation with respect to quartz is needed for chalcedony to precipitate directly from solution. [29] also showed that chalcedony forms persistently only at temperature below about 180˚C.
Gold-bearing quartz veins are hosted by the metasedimentary rocks in the Wabo area. The nature of vein margins is generally sharp, and the veins are mostly steeply dipping. In the Kyaw Soe Thu Prospect, slickenside and striation are also observed along the mineralized quartz vein on the vein surfaces. The mineralization occurred in stage II chalcedonic quartz veins and stage III Ankerite-quartz-sericite vein.
We also highlight the evidence and nature of the stage II chalcedonic quartz vein crosscut by stage III Ankerite-quartz-sericite vein in the Wabo deposit.
Zoning ankerite minerals is a good indicator of gold mineralization in the Kyaw Soe Thu Prospect. Based on the texture of the chalcedonic quartz veins, the mineralization at the Kyaw Soe Thu Prospect in the Wabo the gold deposit can be classified as a low-sulfidation epithermal deposit.