Gold Mineralization in the Kyaikhto District, Mon State, Southern Myanmar

The Kyaikhto gold district is located within the Slate Belt and Mogok Metamorphic Belt of Southern Myanmar. The study area is covered by Carboniferous to Lower Permian metasedimentary rocks consisting of slate, phyllite, and schist of the Mergui Group, intruded by later igneous rocks. Four gold occurrences have been identified in the Kyaikhto district: the Kunzeik in the north, Zibyaung, and Thae Phyu Chaung in the center and Meyon in the south. Gold mineralization in the Kyaikhto district is associated with sheeted, stockwork, dissemination, and sulfide-bearing quartz veins. Ore minerals recognized include sphalerite, galena, chalcopyrite, molybdenite and pyrite with minor native gold and electrum. Two types of fluid inclusions were examined in the quartz samples of the Kunzeik and Zibyaung—Type A: aqueous carbonic fluid inclusions and Type B: aqueous fluid inclusions. At the Kunzeik, Type A fluid inclusions homogenize at temperatures from 296 ̊C to 376 ̊C with low salinities (1.6 4.6 wt% NaCl equivalent). The homogenization temperatures of Type B fluid inclusions in vein quartz range from 246 ̊C to 312 ̊C, with salinities of between 1.2 and 10.7 wt% NaCl equivalent. In the Zibyaung, the homogenization temperatures of Type A inclusions vary from 305 ̊C to 378 ̊C, with salinities from 4.6 to 9.6 wt% NaCl equivalent. The homogenization temperatures of Type B fluid inclusions mainly range from 242 ̊C to 298 ̊C, with salinities from 0.9 to 11.8 wt% NaCl equivalent. These characteristics of fluid inclusions are similar to those of orogenic gold mineralization systems.

This belt of gold mineralization was first reported by [1], as gold deposits of Slate-hosted mesothermal quartz-gold veins (orogenic gold deposit) ( Figure 1).
The Slate Belt was intruded by numerous granitic rocks that host tin and  [5]. This belt contains narrow but high-grade orogenic quartz-gold veins discovered almost 20 years ago [6]. [7] suggested that gold and sulfide-bearing veins in the Kyaikhto district were "mesothermal vein deposits".
However, there is no detailed research work about gold mineralization in the Kyaikhto district and ore genesis is poorly understood. In this paper, we present the results from geological, mineralogical and fluid inclusion studies of gold mineralization in the Kyaikhto district. The main purpose of this study is to determine the genesis of ore-formingfluid and to constrain the formation of gold mineralization in the Kyaikhto district.

Regional Geologic Setting
The gold mineralization in the Kyaikhto district is located between the southeastern part of the Indo-Burma Ranges (Burma Plate) and the southwestern part of the Indo-China Block (the southern part the Eastern Highlands) ( Figure   1 and Figure 2). Geological structures at the southwestern margin of the Eastern Highlands are complicated due to the presence of older and some younger active strike-slip faults [9] [10] [11]. The gold mineralization in the Kyaikhto district is located close to the Sagaing Fault, a major young active strike-slip fault trending Figure 2. Regional tectonic setting of Myanmar and its surrounding area (Modified after [6] (Figure 3). The Mergui Group consists of low-grade metamorphic rocks, principally slate, phyllite, and schist. The metamorphic grade of these rocks is greenschist facies, characterized by a mineral assemblage of epidote, biotite, muscovite, chlorite and sericite. Locally, fine-grained pyrite crystals are disseminated in the slate, phyllite, and schist. Some 10 km to the southwest, several primary and alluvial gold workings are operated in serpentinite, exposed beneath the lateritic cover.  [15]. Total salinities of NaCl-H 2 O inclusions were calculated from the final melting temperatures of ice using the equation by [16]. The salinities of CO 2 -bearing fluid inclusions were calculated using the melting temperatures of clathrate [17]. All analyses were undertaken at the Department of Earth Resources Engineering, Kyushu University.

Zibyaung
In the Zibyaung, the quartz-sulfide vein strikes northwest and dips steeply to the north (Figure 4

Thae Phyu Chaung
Gold mineralization in the Thae Phyu Chaung is characterized by white to milky sheeted quartz veins that contain Au-Ag-Te-Bi minerals (Figure 4(g)). Sphalerite-rich ores are the most common in this deposit, associated with massive chalcopyrite and galena. Pyrite ranges from medium to coarse-grained, anhedral aggregates replacing and infilling grain boundaries of chalcopyrite and, to a lesser extent, sphalerite. Native gold is mainly associated with chalcopyrite and galena or located at the boundary between chalcopyrite grains (Figure 4(h) and Figure 4(i)).

Microthermometry
The results of fluid inclusion microthermometry are summarized in Table 1.

Kunzeik
The CO 2 phases of Type A inclusions were totally homogenized to a liquid phase at temperatures (T h-CO2 ) between 28.2˚C and 31.3˚C, indicating the presence of NaCl. The CO 2 melting temperatures T m-CO2 of fluid inclusions range from −58.2˚C to −56.8˚C similar to, or slightly lower than, the triple point of pure CO 2 (−56.6˚C) [15], indicating they are mostly pure CO 2 , with minor other volatile components [15]. The melting temperatures of clathrates (T m-cla = 7.6˚C -9.2˚C) indicates that the salinity of these inclusions is low, about 1.6 to 4.6 wt% NaCl equivalent. Total homogenization temperatures (T h ) of Type A fluid inclusions range from 296˚C to 376˚C (N = 31) (Figure 6(a)). Homogenization  Notes: T m-CO2 -melting temperature of CO 2 ; T m-cla -melting temperature of CO 2 clathrate; T h-CO2 -partial homogenization temperature of CO 2 inclusions; T h -total homogenization temperature of inclusions; T m-ice -final ice melting temperature; wt% NaCl eq., weight percent NaCl equivalent; N = total number of fluid inclusions. temperatures (T h ) of Type B fluid inclusions in vein quartz range from 246˚C to 312˚C (N = 53) (Figure 6(a)). Their final ice melting temperatures (T m-ice ) vary from −2.1˚C to −7.2˚C, which corresponds to salinities between 1.2 and 10.7 wt% NaCl equivalent (Figure 6(b)).

Zibyaung
The CO 2 melting temperatures (T m-CO2 ) of Type A fluid inclusions range from −57.3˚C to −56.8˚C similar to, or slightly lower than, the triple point of pure  (Figure 6(c)). Their final ice melting temperatures (T m-ice ) range from −0.5˚C to −8.1˚C and correspond to salinities from 0.9 to 11.8 wt% NaCl equivalent (Figure 6(d)).

Discussion and Conclusion
The to be the most common types in orogenic gold deposits worldwide [18]- [23].
The factors leading to the formation of orogenic gold deposits are numerous and therefore it is difficult to identify a single responsible case for ore precipitation.
According to [22] [24], metamorphic devolatilization produces low-salinity fluids that transport gold as reduced sulfur complexes. Ore deposition mechanisms in orogenic gold deposits are usually either fluid-rock reactions, fluid phase separation or fluid mixing with meteoric waters [22] [25]. Moreover, several studies on orogenic gold deposits have perceived ore-mineralizing fluids which were low to moderate temperature, with low salinity, and fluid which was rich in