Geochemical Characteristics of the Fluid Inclusion of

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Procedia Environmental Sciences 12 (2012) 366 – 371

2011 International Conference on Environmental Science and Engineering (ICESE 2011)

Geochemical Characteristics of the Fluid Inclusion of the Dafulou Deposits in Guangxi, China Cheng Yongsheng1,2 1

School of Geosciences and Info-Physics, CSU Changsha, China, 410083 2 Guangxi Huaxi Co., Ltd.,Liuzhou, China, 545006 [email protected]

Abstract In this paper, the fluid inclusions study was performed on quartz, sphalerite and calcite from the Sn–polymetallic orebodies of the Dafulou deposit. On the basis of optical observations and microthermometric data, six types of fluid inclusions have been recognized, such as single-phase gaseous fluid inclusions, single-phase salt solution fluid inclusions, two-phase vapour-rich fluid inclusions, two-phase liquid-rich fluid inclusions, three-phase CO2-rich fluid inclusions, solid(s)-bearing fluid inclusions, etc. And it showed that water is the main component, for example the water content of quartz more than 1700ȝg/g, ranges from 1717 to 2320 ȝg/g. The sphalerite has lower content than quartz, only 978ȝg/g. The calcite has lowest water content, i.e. 242ȝg/g and 324ȝg/g respectively. The gas component of inclusions mainly consists of CO2 and H2O. The main cationic consists of Ca2+, secondly Mg2+, K+ and Na+, only little Li+ and NH4+. © 2011 by Elsevier B.V. Selection and/or peer-review under responsibility of National University of Singapore. © 2011Published Published by Elsevier Ltd. Selection and/or peer-review under responsibility of [name organizer] Keywords: Fluid inclusion, Homogenization temperature, Dafulou deposit, Danchi mineralization belt, China

1.

Introduction

The Dachang tin-polymetallic ore field, which is one of the largest Sn ore field in this world, is situated in Nandan county of Guangxi Zhang Autonomous Region, south China, and is also situated at the joining part between the Guangxi platform and the Jiangnan uplift in northwest Guangxi[1-2]. The Dafulou deposit, which belongs to the eastern mineralization belt of the Dachang tin ore field and has been mined for some degree before the foundation of the people's republic of China, is located in the eastern flank of the NNW– SSE-trending Danchi anticlinorium. Besides the Dafulou ore deposit in the eastern mineralization belt, there are some small-scale Sn ore deposits, such as Huile, Tongkan, Hunaglaqiao, Maopingchong and

1878-0296 © 2011 Published by Elsevier B.V. Selection and/or peer-review under responsibility of National University of Singapore. doi:10.1016/j.proenv.2012.01.291

Cheng Yongsheng / Procedia Environmental Sciences 12 (2012) 366 – 371

Dawan, Lanichong-maomaochongding W deposit[3-4]. The most important structures in the Dafulou ore district are the NW-trending faults, such as overthrust fault, which developed parallel with the axis of the Dachang anticlinorium[5-8]. In addition, the NE-trending and SN-trending structures are also very important to the Dafulou Sn-polymetallic mineralization[9-10], especially at the intersection of the NWtrending and the NE-trending structures. 2.

Fluid inclusion petrography

The fluid inclusions have a variety of shapes (mainly round, elongate or square and, less commonly, irregular shapes) and typically are of large size (up to 140 Am). From the view of the petrographic study, six different types of fluid inclusions have been identified on the basis of the number of phases and liquid to vapour ratios at room temperature and the optical observations, microthermometric data (Fig.1).

a

b

c

d

e

f

Fig.1. Photomicrographs of fluid inclusions in the Dafulou deposit

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Type 1 (single-phase gaseous fluid inclusions): This type consists predominantly of vapour, with little vapour CH4 and vapour H2S. The inclusion size varies from 3 to 15 ȝm. These inclusions are of various shapes, such as rice-shaped, ellipse, polygon, irregular, etc. Type 2 (single-phase salt solution fluid inclusions): Fluid inclusions belonging to this type consist of pure saline. This is by far the content of 15 to 65%. The size of fluid inclusions ranges from 1 to 15 ȝm. These inclusions are of various shapes, such as ellipse, rice-shaped, polygon, irregular, etc, with the distribution of freedom mostly, but little orientation distribution along the microfissure of quartz. Type 3 (two-phase vapour-rich fluid inclusions): This type contains two phases: a vapour bubble VH2O and a liquid water LH2O, with various shapes, i.e. ellipse, polygon, rectangle, irregular, etc. The volume of this type in the fluid inclusions varies accounting for 30 to 55%. Fluid inclusions belonging to this type consist of pure saline and vapour water, which distributed dominantly with the form of freedom and small groups. Type 4 (two-phase liquid-rich fluid inclusions): Fluid inclusions belonging to this type consist of pure saline and vapour water. The percentage of this type fluid inclusion is about 15% to 50%, which show various shapes, such as polygon, rectangular, oval, subhedral and little irregular. The size of fluid inclusions ranges from 2 to 25 ȝm. Most of the fluid inclusions belong to negative crystal. Type 5 (three-phase CO2-rich fluid inclusions): This type fluid inclusion consist of three phase: liquid water (LH2O), liquid CO2 (LCO2), and vapour CO2 (VCO2). The percentage of this type fluid inclusion is only 5%. The size of fluid inclusion ranges from 5 to 15 ȝm. The shape of fluid inclusions is diversity, but oval dominantly, polygon and negative crystal secondly, which distribute freely or mixture distribution with other type. Type 6 (solid(s)-bearing fluid inclusions): The typical features of this type is the content of daughter mineral NaCl. This type consists of three phase: liquid water (LH2O), vapour water (VH2O), and solid NaCl (SNaCl). The size of fluid inclusions is about 5 to 30 ȝm, with various shapes, such as oval, polygon, rectangle, etc. The form of distuibution is free or mixed with other type. Table 1. Gas composition testing of fluid inclusions by gas chromatography

3.

Content˄ȝg/g˅ CH4 C2H2 11.241

CO2 623.019

H2O 1822

1.934 ü

10.439 9.753 30.977

ü

569.148 260.270 630.496

1717 978 1925

4.884

ü

28.279

ü

525.138

2110

Y05-4-1

Quartz

9.538

ü

32.138

ü

787.240

1863

Y05-4-2

Quartz

8.689

ü

30.291

ü

659.415

1914

Y07-1

Quartz

7.037

ü

23.844

ü

536.037

2103

Y19-1

Quartz

3.331

ü

14.117

552.245

2320

Y19-2

Quartz

3.519

ü

12.513

ü

569.312

1930

Y11-1

Calcite

1.763

ü

0.678

ü

96.802

242

Y12-1

Calcite

1.921

ü

0.256

ü

98.201

324

Sample

Mineral

Y19-3

Quartz

H2 3.728

N2 ü

Y19-4 Y03-1 Y03-2

Quartz Sphalerite Quartz

3.567 1.990 5.459

Y02-1

Quartz

ü

Fluid inclusion Component

The experiment of ion chromatography and gas chromatography was conducted by the geology laboratory of School of Geosciences and Info-Physics, Central South University, China[11-12]. Both the


ion chromatograph (type DX-120) and the gas chromatography (type Varian-3400) were imported from American. It is suggested that water is the main component, for example the water content of quartz more than 1700ȝg/g, ranges from 1717 to 2320 ȝg/g (Table 1). The sphalerite has lower content than quartz, only 978ȝg/g. The calcite has lowest water content, i.e. 242ȝg/g and 324ȝg/g respectively. The gas component of inclusions mainly consists of CO2 and H2O. The H2O content of inclusion in quartz, sphalerite, calcite ranges from 1717 to 2320, 978, 242 to 342, respectively. The CO2 content of quartz exceeds 500ȝg/g, ranging from 520 to 660. To sphalerite, the content of vapour-CO2 is only 260.270ȝg/g. And the vapourCO2 content of calcite is 96.802ȝg/g and 98.201ȝg/g. The other gas content is very low, such as H2, CH4, O2, CO and C2H6, etc. By the ion chromatography composition testing, it is suggest that that the main cationic consists of Ca2+, secondly Mg2+, K+ and Na+, only little Li+ and NH4+. The ration of Ca2+/Mg2+ has a large scale, from 1.281 to 87.906. The Ca2+ content of fluid inclusions in quartz exceeds 15ȝg/g (18.321, 16.109, 20.658, 18.765, 22.912, 20.123, 18.239, 15.291, 17.627, respectively), with averaging value 18.672. The Ca2+ content of two calcite sample fluid inclusions is 7.326ȝg/g and 8.474ȝg/g. In the sphalerite, the Ca2+ content is only 6.241ȝg/g. Yet anion content consists of SO42-, with the range from 5.269 to 56.115ȝg/g, higher in sphalerite (56.115ȝg/g). Calcite has lower SO42- composition, only 5.269ȝg/g and 5.926ȝg/g. The fluid inclusions in quartz have the medium SO42- composition, ranging from 8.926 to 24.862, with the averaging value 16.072ȝg/g. Besides SO42-, there are other anions, such as Cl-, NO3-, and little F- and PO4. 4.

Discussion

Temperature and salinity of three samples from the Dafulou ore district was tested[13-15]. Two-phase vapour-rich fluid inclusions of sample Y05-3 consist of three types of temperature and salinity (Fig.2): (1) Homogenization temperature 120ć to 145ć, salinity 21.9 to 22.44 wt.% NaCl, and (2) Homogenization temperature 240ć to 265ć, salinity 19.8 to 20.7wt.% NaCl, and (3) Homogenization temperature 350ć to 395ć, salinity 18.9 to 20.0 wt.% NaCl. The CO2-rich fluid inclusion exist two kinds of homogenization temperature and salinity: (1) Homogenization temperature 380ć to 420ć, salinity 18.11 to 18.7wt.% NaCl, and (2) Homogenization temperature 390ć to 410ć, salinity 17.3 to 18.1wt.% NaCl. The homogenization temperature of NaCl-bearing fluid inclusion ranges from 280ć to 310ć. The homogenization temperature of NaCl mineral is greater or equal to 400ć, with the salinity 46wt.% NaCl, accordingly. The homogenization temperature of sample Y05-3 mainly distributes in three scale (Fig.2): 120ć to 150ć, 240ć to 270ć and 350ć to 430ć.

Fig.2. Homogenization temperature of fluid inclusions, sample Y05-3

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Homogenization temperature of sample Y09-2-1 (Two-phase liquid-rich fluid inclusions) ranges from 120ć to 145ć, with the salinity 17.3 to 17.6 wt.% NaCl. The two-phase liquid-rich fluid inclusions of sample Y29-4 has three homogenization temperature and salinity (Fig.3): (1) Homogenization temperature 230ć to 260ć, salinity 20.0 to 20.3wt.% NaCl, and (2) Homogenization temperature 390ć to 420ć, salinity 19.2 to 19.5wt.% NaCl, and (3) Homogenization temperature 380ć to 430ć, salinity 17.2 to 17.6wt.% NaCl. The three-phase Homogenization temperature of CO2-rich fluid inclusions ( LH 2O + LCO2 + VCO2 ) of sample Y29-4 ranges from 390 to 410ć, with the salinity 17.0 to 17.4wt.% NaCl. The NaCl-bearing fluid inclusions ( LH 2O + VH 2O +SNaCl) has two kinds of homogenization temperature, 270 to 290ć, 130 to 160ć, respectively, and with the salinity 28.95 to 30.08wt.% NaCl. So, it is suggest that there are three temperature scale, 130 to 160ć, 230 to 290ć and 380 to 450ć, to sample Y29-4.

Fig.3. Scatter diagram of homogenization temperature of fluid inclusions, sample Y29-4

5.

Acknowledgment

This work was supported by China postdoctoral science foundation (No. 20090451105), the special funds of central college basic scientific research operating expenses (No. 201012200210), and the postdoctoral workstation of China Tin Group Co., Ltd. (No. 043010100). References [1] Ye XS, Yan YX, He HZ. The mineralization factors and tectonic evolution of Dachang super large tin deposit, Guangxi, China[J]. Geochim, vol. 28, 1999, pp. 213–221. [2] Fu M, Changkakoti A, Krouse HR, Gray J, Kwak TAP. An oxygen, hydrogen, sulfur, and carbon isotope study of carbonate-replacement (skarn) tin deposits of the Dachang tin field, China[J]. Econ Geol, vol. 86, 1991, pp. 1683–1703. [3] Wang DH, Chen YC, Chen W, Sang HQ, Li HQ, Lu YF, Chen KL, Lin ZM. Dating the Dachang giant tin-polymetallic deposit in Nandan, Guangxi[J]. Acta Geol Sinica, vol. 78, 2004, pp. 132–138. [4]

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