Geochemical Characteristics of Metamorphic Rock-Hosted Gold Deposit At Onzon-Kanbani Area , Central Myanmar

Onzon-Kabani area is located on the western border of generally N-S trending Mogoke Metamorphic Belt. Many artisanal and small-scale gold mines were noted sincelast three decades. Gold mineralization is mainly hosted in marble and occasionally observed in gneiss.Host rocks chemistry is mainly controlled by hydrothermal activities and regional metamorphism. The compatible elements of host rocks are relatively more mobile than incompatible elements during alteration and deformation. Variety of plutonic rocks such as leucogranite, syenite and biotite granite are intruded into older metamorphic rocks. In ACF diagram, leucogranite and syenite are placed in S-type granite field and biotite granite occupied in I-type granite field. Mineralization is observed as fracture filling vein and less amount of disseminated nature in marble unit. Mineralogically, gold areassociated with other base metal such as pyrite, galena, sphalerite, and chalcopyrite. Hydrothermal alteration halos are developed as a narrow zone in the peripheral of hydrothermal conduits from proximal to distal; 1) silicic, 2) sericite-illite, and 3) propylitic alteration. In place, silicic altered zone is rich in SiO2, Al2O3, and K2O. Moreover, concentrations of S, Cu, Zn, and Pb are more than any other alteration zones. The CaO is gradually increase to least altered zone where wt% of loss of ignition (LOI)also increase significantly to least-altered samples (marble). Some fluid boiling characteristics of vein textures and fluid inclusion petrography are also observed in hydrothermal system of research area. The homogenization temperature (Th) of quartz vein samples are 159°C to 315°C and salinity are 0.88 to 12.51 wt% of NaCl equivalent. This research focused to describe the geochemical characteristics of metamorphic rock-hosted gold deposit in Onzon-Kanbani area.


Introduction
Gold and associated base metal mineralization at Onzon-Kanbani areais located in Thabeikkyin Township, Mandalay region, central Myanmar.This area is west 85km faraway from well known Mogok Township (ruby land), Myanmar.Small scale local gold rush and abundant artisanal working were started since last thirty years ago.This area is alsoa part of Mogok Metamorphic Belt (Searle and Haq, 1964)which famous for their precious and semiprecious gem stones production and distinct geological feature.Mineralization is mostly hosted in marble unit and occasionally in gneiss.It is observed as fracture filling veins and minor disseminated nature in marble.Generally, gold mineralization in Mogok Metamorphic Belt is inferred as orogenic gold (Mitchell et al., 2004) but locally epithermal and skarn gold mineralization are also spread along this belt (Myint et al., 2014).The main objective of this paper writing is not onlyto describethe geochemical characteristics of metamorphic rock-hosted gold deposit but also to understand the deposit genesis of research area.

Methods of study
Totally, fifty ofthin and polished sections of altered rocks and ore sampleswere conducted for mineralogical study by optical microscope and confirmed again by scanning electron microscopy with energy-dispersive X-ray (SEM-EDX).Additionally, X-ray Diffraction (XRD) analysis for the hydrothermal alteration minerals that were done for both bulk rock (random powder) and clay fraction.Major and trace elements analyses were carried out on representative samples using XRF and inductively coupled plasma optical emission spectroscopy (ICP-OES).Moreover, quartz samples were collected from mineralization veins to conduct fluid inclusion study.Double polished quartz wafers were prepared for these quartz veins where the thickness of wafers varied between 150 and 300µm.Microthermometric measurements were done using Linkam THMSG600 combined heating and freezing stage, attached to Nikon petrographic microscope with Axiovision software.All of these laboratory analyses were made in Department of Earth Resource Engineering, Mineral Resource lab, Kyushu University, Japan.

Regional geologic setting
Myanmar is tectonically complex region and composed with micro plates such as Sibumasu, West Burma, and India.Along a tectonic history is built by repeatedly occurring subduction, accretion and collision events.These events chiseled the geological landform and metallogenic provinces in Myanmar.Traditionally, the territory of Myanmar is subdivided into (1) the India plate to the west, (2) Burma microplate (West Burma) in the central part and (3) Shan-Mogok Metamorphic Belt is one of the distinct geological and metallogenic province in Myanmar and located between Central low-land (West Burma) and Shan-Thai block (Sibumasu).It is believed that southern continuation of Himalaya (Searle and Haq, 1964)and formed by either collision (Mitchell, 1979) or strike-slip movement (Metcalfe, 2009).In this place, well-known right lateral strike-slip fault, Sagaing Fault (Swe, 1972)is served as awestern boundary.Alternatively, thestructural configuration of this area is closely related to this fault.Mogok Metamorphic Belt is composed of Paleozoic to Mesozoic meta-carbonate rocks andmetapelite where avariety of Cretaceous-Palaeogene plutonic igneous rocks (Barley et al., 2003;Mitchell, 1979;Mitchell et al., 2012)are intruded to older rocks.East of this belt, Shan Plateau (Sibumasu) is a topographic high with anaverage elevation of about 1000m and mainly composed of a series of Ordovician-Triassic carbonate rocks and continental sedimentary rocks (Jurassic?).The western margin of Mogok Metamorphic Belt is juxtaposed with Central lowland (West Burma) which is filled by Eocene to Plio-Quaternary sediments.

Geology of Onzon-Kanbani area
Onzon-Kanbani area is a part of Mogok Metamorphic Belt.Therefore, mainly composed with metamorphic and igneous rocks which are exposed with differing areal coverage (Fig. 2).Metamorphic rocks of research area such as avariety of marbles, calc-silicate rocks and gneiss are widely distributed in eastern and western part.The age of the protolith of metasedimentary was originally considered Precambrian (Bender, 1983), but Permian fossils in marble near Kyaukse (Thein and Win, 1969)indicate that Carboniferous to Triassic ageis more likely.Alternatively, igneous units are generally covered the middle part of the research area and found as fitful.The older metamorphic rocks are intruded by granite and syenite.The main igneous intrusion of the research area is biotite granite Metamorphic Belt ranged that magmatism of leucogranite and syenite melt took place in Eocene time during the regional metamorphism and then Kabaing granite intruded the country rocks of the study area in Miocene after regional metamorphism (Barley et al., 2003).(Barley et al., 2003)

Petrography
The dominant metamorphic rocks in Onzon-Kanbani area which is composed of meta-carbonate rocks (marbles and calc-silicate rock) and biotite gneiss.Heterogeneous igneous intrusions are observed the middle part of the area where diopside calc-silicate rock occur along the margin of igneous intrusions (Fig. 2).
Marble unit appears white (fresh) and dark gray to pale greenish (weathered) in color.The foliated nature also occurs in some marbles (thin foliated marble and diopside marble) and calc-silicate rock.Gneiss unit is medium-crystalline, banded and gneissose with finer and coarser portion.Weathered color light grayand dark gray of fresh surface indicate that abundance of biotite and hornblende in gneiss.In igneous rocks, leucogranite and syenite are observed as small intrusive bodies intruded to marble units.Light to pinkish color appearance of outcrops arecharacteristic feature.Biotite granite is a one of important plutonic rock in research area.It has dark grey to yellowish color weathered surface and light grey offresh surface.The exfoliation nature is common textural feature.
Microscopic observation shows a variety of minerals in marble and gneiss.In marble unit, diopside, phlogopite, calcite are main constituents whereas biotite, plagioclase and quartz are dominant in gneiss.Some of the minerals are believed that the products of regional metamorphism related with India-Asia collision.In syenite and leucogranite, felsic color minerals are mainly observed such as quartz, plagioclase, muscovite and nepheline.The common minerals of biotite granite are quartz, biotite, plagioclase and orthoclase.

Petrochemistry of host rocks
The major and trace elements composition of host rocks (marble and gneiss) are given in Table-1.The chemical characteristics of host rocks are mainly controlled by regional metamorphismas well ashydrothermal activities.Basically, host rock marble is rich in CaO and MgO whereas thenotable amount of Al 2 O 3 content showed that their mica content in marble.Alternatively, gneiss samples are a high content of SiO 2 range from 45.983 to 49.672wt% where CaO is not too much between 9.254 to 11.301wt.%.In order to evaluate the compositional change which accompanies deformation and hydrothermal alteration, the concentrations of elements of interest are compared to those of immobile such as Zirconium.Typically, Zr is used as animmobile and incompatible element during hydrothermal alteration due to thevery high ionic radius where SiO 2 and some of themajor oxide elements cannot use because of alteration effect.In the case of chemically incompatible element Zr versus compatible elements (Zn and Cr) regression lines which are considered to be mobile with each other in the marble and gneiss units during alteration and deformation.The compatible elements (Zn and Cr) are more mobile relative to Zr (incompatible) during deformation and hydrothermal alteration as shown by their poor correlations in Figure (Fig. 3).But in thecorrelation between both of incompatible elements, Zr vs (Sr and Ba) are considered to be immobile with each other in the marbles and gneiss unit while impacting alteration and deformation.The interelement correlation among the trace elements is significant which is consistent with their similar geochemical behavior.

Petrochemistry of intrusive igneous rocks
The chemical composition of plutonic igneous rocks at Onzon-Kanbani area was observed to identify the type of igneous rocks and their tectonic setting.The care during sampling was on fresh rocks, it intended to avoid metasomatic changes during hydrothermal alteration.Results of major and trace elements of plutonic igneous rocks from Onzon-Kabani area are summarized in Table -2.Major element data and converted normative data (CIPW Norm) of plutonic rocks are used to plot on total alkalis-silica (TAS) diagram as well as albiteanorthite-orthoclase triangular diagram to classify these plutonic rocks.According to total alkalis-silica (TAS) discrimination diagram, the plutonic rocks of study area belong to acidic to intermediate rocks of granite and syenite (Fig. 4).The Ab-An-Or discrimination diagram is intended to avoid the criticism that wt.% oxide data do not faithfully and to confirm plutonic rock classification.This triangular diagram also showed that plutonic rocks of research area are granite, syenite, and tonalite (Fig. 4) respectively.All of these samples are pointed out as calc-alkaline type by AFM diagram (Fig. 4).Moreover, ACFdiagram also showed that most plutonic rocks of Onzon-peraluminous except two samples of biotite granite which fall in -metaluminous.Y versus Nb and Rb versus Y+Nb diagrams (Pearce et al., 1984)were used to discriminate the tectonic environment of plutonic rocks in Onzon-Kanbani area.These diagrams showed that most of plutonic rocks are in syn-collisional granite (syn-COLG) and within plate granite (WPG) except some samples which are placed in volcanic arc granite (VGA) (Fig. 5).(Pearce et al., 1984).Abbreviations: ORG=oceanic ridge granite, VAG=volcanic granite, WPG=within plate granite and syn-COLG=syn-collision granite

Mineralization and associated hydrothermal alterations
Gold and base metal mineralization hosted by marble is closely associated with intense hydrothermal alteration.Generally, mineralization is controlled by NE-SW trending faults within fracture and shear zones.Open space fracture filling veins are common characteristicswith minor disseminated mineralization in marble.Many kinds of vein textures are observed in mineralization vein such as massive vein, crustiform, banded, lattice, bladed carbonate, comb, and cockade.Some of the texturesare diagnostic textures of boiling (lattice, bladed, banded, crustiform).They are indicated to boiling fluids of near neutral to alkaline pH condition (Gregg and Jaireth, 1995;Simmons and Christenson, 1994) (Fig. 7).Many local worksites are working on the narrow mineralization zones where the width of veins are generally 0.5 to 5 meters.
Wall rock alterations related with hydrothermal fluid are observed in Onzon-Kanbani area but not wide area scale.Mostly, hydrothermal alteration halos are developed around mineralization veins as narrow zones.Basically, hydrothermal alteration is overlapped to regional metamorphism.Microscopic observation and X-ray diffraction patterns are used to identify the type of alterations which happen by hydrothermal impact.There are three distinct hydrothermal alteration zones from proximal to distal of hydrothermal conduct.
Silicic alteration zone or inner core of alteration halos, it is mainly composed of quartz, calcite, adularia and minor amount of illite and chlorite (Fig. 6).Generally, this alteration is overlapped with mineralization vein but groundmass is strongly silicified.Gold and base metal mineralization are observed in massive and banded quartz veins.Hydrothermal breccia is also frequently observed near this alteration zone.These breccia are characterized by a milled matrix of pyrite.Some of the sulphides are oxidized because of deep weathering.
Sericite-illite alteration is observed as narrow zone next to silicic alteration zone.But this zone is not well developed in some alteration halos.The main constituents of minerals from this altered zone are sericite, illite, quartz, calcite and a minor amount of pyrite.Sericite occurs as fine grained and spread out like dusty (Fig. 6).
Propylitic alteration is an outer most zone of alteration and wild area coverage.This zone is characterized by the presence of chlorite, epidote,actinolite, smectite, and illite, (Fig. 6).It is still preserve the primary texture of the original rock.It is believed that some part of propylitic alteration is not related to ore forming hydrothermal system (Evans, 1987), overlapped to regional metamorphism.Most of hydrothermal minerals from each altered zones showed that near neutral condition of pH (e.g.adularia, calcite, illite, sericite and chlorite).

Geochemistry of altered rocks
The major and trace elements concentration of the altered rocks are varied depend on alteration intensity as well as the presence of a chemical ion exchange between wall rocks and hydrothermal fluids.The altered zonation reflects changes in the fluid composition with time and interaction of hydrothermal fluid and wall rocks (Meyer and Hemley, 1967).In Onzon-Kabani area,alteration zones are basicallyhosted in marble unit as narrow zonation.The major and trace elements of the least-altered rocks and altered rocks of each zones are shown in Table 1 and 3.In this case each of the alteration zones has measured enrichment and depletion of major and trace elements relative to the leastaltered rock (Fig. 8).The petrochemistry of zone 1 showed that silicic altered rocks are enriched in SiO 2 , Al 2 O 3 , K 2 O, and S indicating occurrences of abundant quartz, aluminium-bearing clay (illite), ± adularia and sulphides whereas any other elements such as FeO, MgO, MnO and CaO are depleted.But in zone 2 of sericite-illite alteration zone, SiO 2 is quite reduced compared with silicic alteration zone.Instead of SiO 2 , CaO is increased in remarkable amountbut still lowrelatively to least-altered rocks.Moreover, Al 2 O 3 , K 2 O, FeO and MgO are also high because of abundant illite (sericite) and chlorite.
propylitic alteration zone , CaO shows high amount near least-altered rocks whereas Al 2 O 3 and K 2 O are depleted.Locally enrichments of MgO, Al 2 O 3 and FeO are indicated that the development of chloritization in propylitic alteration.The wt.% of loss of ignition (LOI) increase significantly to least altered samples (marbles).This is due to dissociation of CaCO 3 and dolomite CaMg (CO 3 ) 2 that partially transform themselves into carbon dioxide gas at temperatures higher than 750° C. Generally, high field strength elements of Al, Ti, and Zr were assumed to be immobile elements during the alteration and deformation process.When plotting these minerals from least-altered and altered rocks in TiO 2 vs Zr and TiO 2 vs Al 2 O 3 binary/bivariate diagrams, a general single trend of alteration is observed in both of diagrams but some of silicic and sericite-illite alteration samples are a little bit deficient from these lines (Fig. 9 ).It mean hydrothermal alteration of silicic and sericite-illite alteration is quite high intensity.Note: Total Fe content expressed as FeO(tot); n.d= below detection limit; major element and LOI are weight percent; trace elements are in ppm 9. Ore mineralogy and paragenetic sequence Mineralogically, gold occurs as free grains or locked within pyrite, sphalerite, galena and gangue of Au.Electrum (<80 at %) is mostly observed as large grained in gold bearing quartz vein but native gold (>80 at %)is observed as fine grained in base metal quartz-carbonate vein and carbonate base metal sulphides vein (Fig. 11).Other common ore minerals in research area are pyrite, marcasite, chalcopyrite, sphalerite, galena, arsenopyrite, chalcocite, covellite, hematite, and goethite.Pyrite, galena, sphalerite, chalcopyrite, marcasite, and arsenopyrite are primary ore minerals whereas chalcocite, covellite, hematite, and goethite occur as late minerals, their occurrences reflect the oxidation of primary sulphides such as pyrite and chalcopyrite by circulation of surficial water.Pyrite is the most abundant sulphide mineral in the ore body.It occurs over the entire period of mineralization as fine-grained massive aggregates to large euhedral grains.
According to thevein and mineral structures and textures, there are three main paragenetic stages; 1) mineralization stage, 2) barren stage and 3) oxidation stage .In mineralization stage, the early formed quartz, pyrite, and minor calcite minerals are observed away from late phase sulphides and quartz core vein.And also early formed pyrite and quartz minerals showed their euhedral crystal forms whereas anhedral sphalerite is younger than quartz and pyrite in order of deposition.Some of theanhedral sphalerites are replaced along the boundary by later formed galena.Moreover, disseminated chalcopyrite rods and small grainsoccur in sphalerite as exsolution texture (Fig. 10).Gold is mostly observed in quartz gangue, pyrite, sphalerite, galena and chalcopyrite ground mass as disseminated specks (Fig. 10).Actually, quartz, calcite, and pyrite are observed the entire period of mineralization as more or less amount.Sericite and aminor amount of adularia are also found together with quartz at mineralization vein.After mineralization stage, veins are barren like a calcite or quartz vein with very minor amount of pyrite.Subsequently, oxidation stage is going on by circulated meteoric water whereas hematite, goethite,and chalcocite are formed from primary sulphides.12).Salinities of the fluid inclusions were temperature Tm (Bodnar, 1993).The salinity range is low to moderate 0.88 to 12.51 wt% NaCl equivalent.The plot diagram of homogenization temperature and salinity are showed negative trend (Fig. 13).-of granitoid rocks are observed as intrusive bodies where most of the rocks are placed in the field of syn-collisional granite and within plate granite.Hydrothermal alteration halos are developed as narrow zones beside of hydrothermal conduit or mineralization vein.They are silicic, sericite-illite and propylitic alteration zones from proximal to distal of hydrothermal conduits.Hydrothermal alteration in Onzon-Kanbani area is regard as lowsulphidation epithermal system as indicated by the characteristic of alteration style and alteration mineral assemblages such as quartz, calcite, ±adularia, sericite, illite and smectite.These alteration minerals are showed that the temperature decreasing to the outer zone of alteration as well as indicated that the higher pH, near neutral condition of hydrothermal system.Generally, hydrothermal alteration is overlapped to regional metamorphism.The geochemistry of each altered zone reflects changing of fluid composition and interaction of hydrothermal fluid and wall rock.According to immobile elements characteristics of least-altered and altered rocks, alteration intensity is higher to silicic and sericite-illite alteration.SiO 2 and CaO content are inversely proportional in each altered zone.Enrichments of S, Pb, Zn, and Cu from silicic altered zone indicate that fracture filling vein mineralization is more common than disseminated mineralization.In ore mineralogy, hypogene ore mineral such as pyrite, sphalerite, galena, chalcopyrite and minor native gold and electrum are precipitated in mineralization stage.After this veins are barren by deposition of calcite or quartz at the end of mineralization stage.Some of hypogenesulphide minerals are oxidized to supergene minerals during oxidation stage.According to fluid inclusions petrography whereas coexisting of liquid rich and vapor rich fluid inclusions as well as some of vein textures such as bladed, banded and lattice are strongly advocated that fluid boiling, iseffective on such a precipitation of ore minerals.Moreover, homogenization temperature (159°C-315°C) and salinity (0.88 to 12.51 wt.% NaCl equiv.)range are acceptable to say it formed in the epithermal environment.

Fig.. 3
Fig..3 Binary variation diagrams in Cr, Zn, Sr and Ba (all in ppm) for host rocks of Onzon-Kabani area whereas Zn and Cr are compatible elements, and Zr, Srand Ba are incompatible elements

Fig. 8
Fig.8 Enrichment depletion diagram showing major oxides (wt%) and trace element (ppm) during alteration in different zones of hydrothermal alteration of Onzon-Kanbani area based on mean data of least-altered samples as a reference for calculations, (a) and (b) for silicic alteration zone, (c) and (d) for sericite-illite alteration zone, and (e) and (f) for propylitic alteration zone

Fig. 11
Fig.11 Back scatter image of electrum gold and native gold in mineralization vein (Elt=electrum, Au=native gold, Sp=sphalerite, Py=pyrite, Qtz=quartz) 10.Fluid inclusion study Double polished quartz wafers were prepared for fluid inclusion study.From petrographic study, two-phase (liquid, vapor) fluid inclusions are dominant type of primary fluid inclusion.They are observed as two-phase liquid rich, two-phase coexisting of liquid-rich and vapor-rich and two phase vapor-rich fluid inclusions.Moreover,

Fig. 12
Fig.12 Homogenization temperatures of fluid inclusion in quartz from gold and base metal mineralization vein

Table 1 .
Major and trace element composition of least-altered -Kanbani area Note: Total Fe content expressed as FeO(tot); n.d= below detection limit; major element and LOI are weight percent; trace elements are in ppm

Table 2 .
Major and trace element composition of plutonic rocks from Onzon-Kanbani area