Geochemistry and petrology of nephrite from Alamas, Xinjiang, NW China

Journal of Asian Earth Sciences 42 (2011) 440–451

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Geochemistry and petrology of nephrite from Alamas, Xinjiang, NW China Yan Liu a,b, Jun Deng a,⇑, Guanghai Shi a, Tzen-Fu Yui c, Guibin Zhang d, Maituohuti Abuduwayiti e, Liqiang Yang a, Xiang Sun a a

State Key Laboratory of Geological Processes and Mineral Resources, Beijing 100083, PR China Institute of Geology, Chinese Academy of Geological Science, Beijing 100037, PR China c Institute of Earth Sciences, Academia Sinica, Taipei 11529, Taiwan, ROC d MOE Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing 100871, PR China e Hetian Bureau of Quality and Technical Supervision, Hetian, 848000 Xinjiang, PR China b

a r t i c l e

i n f o

Article history: Received 24 November 2009 Received in revised form 1 March 2011 Accepted 17 May 2011 Available online 14 June 2011 Keywords: Nephrite Tremolite Geochemistry Petrology Metasomatism Xinjiang NW China

a b s t r a c t The Hetian nephrite deposit is one of the largest nephrite deposits worldwide. Located within the Hetian deposit, Xinjiang, Northwest China, the Alamas ore body occurs as veins on the contact between the Late Variscan granodiorite and Precambrian dolomitic marble. Petrographic studies and electronic microprobe analyses reveal two possible nephrite formation processes: (1) dolomitic marble ? tremolite and (2) dolomitic marble ? diopside ? tremolite. Nephrite veins show continuous coloration zones, ranging from white and white-green zones to a green zone towards the granodiorite, with increasing concentrations of Cr, FeO and TFe2O3. All these nephrite samples have quite low Cr and Ni contents (8.95– 178.7 ppm and 0.05–3.95 ppm, respectively) relative to serpentinite-related nephrite (Cr2O3, 0.07– 0.43 wt.%; NiO, 0.08–0.36 wt.%). Their bulk-rock REE patterns exhibit strong negative Eu anomalies (0.03–0.21) with declined LREE, flat HREE and low RREE concentration (2.84–84.81 ppm), similar to those of host dolomitic marble samples with negative Eu anomalies (0.15–0.23), declined LREE, flat HREE and lower RREE concentration (8.48–11.1 ppm), indicating a close genetic relationship between them. Homogenization temperatures analyses on the fluid inclusions in tremolite yield a minimum temperature 293 °C for the nephrite formation. Nephrites have oxygen and hydrogen isotope compositions in the range from +3.2 to +6.2 per mil and 83.0 to 94.7 per mil, respectively. The corresponding equilibrated fluids have isotope compositions of d18O = +3.1 to +6.1 (293 °C). Combined with field observation, there are at least three possible fluid sources: meteoric water, magmatic water and CO2 derived from decarbonation of dolomite, and a metasomatic and dolomite-related origin is suggested for the Alamas ore body in the Hetian nephrite deposit, Xinjiang, NW China. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Valued for ornamental carvings, ‘‘Nephrite’’ represents a type of nearly monomineralic rock, mainly consisting of tremolite (Ca2Mg5Si8O22(OH)2) and actinolite (Ca2(Mg, Fe2+)5Si8O22(OH)2). The occurrences of nephrites are rare; a dozen main nephrite deposits in the world, including those occurring in the Kunlun Mountains, Xinjiang, China (Tsien et al., 1996; Liu et al., 2010), the East Sayan Mountains, Siberia, Russia (Prokhor, 1991), Chuncheon, South Korea (Yui and Kwon, 2002), South Westland Fields on South Island, New Zealand (Wilkins et al., 2003) and Cowell, South Australia (Flint and Dubowski, 1990). There is an average world production of about 780 tons per year in 1970s (Flint and Dubowski, 1990), and about 20% of that production came from Hetian deposit (Tang et al., 1994). As a world-class nephrite ⇑ Corresponding author. E-mail address: [email protected] (J. Deng). 1367-9120/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2011.05.012

deposit worthy of broad scientific interest, it remains not fully understood. Nephrite petrogenesis has been interpreted to be either metamorphic or metasomatic (Leaming, 1978; Harlow and Sorensen, 2000, 2005). Leaming (1978) assumed that serpentinite-related nephrite has a metasomatic origin and that dolomite-related nephrite has a metamorphic origin. Recently, Harlow and Sorensen (2000) suggested that dolomite-related nephrite deposits might have resulted from contact metasomatic processes caused by magmatic fluids. Yui and Kwon (2002) reached similar conclusions according to isotopic studies. Recently, a few nephrite deposits have been documented petrologically, geochemically and isotopically (Coleman, 1966; Leaming, 1978; Tan et al., 1978; Prokhor, 1991; Cooper, 1995; Wenner, 1979; Yui et al., 1988; Yui and Kwon, 2002; Liu et al., 2010). However, most of these studies failed to provide detailed petrographic observations and geochemical data, leading to limited understanding of how dolomite-related nephrite deposits were formed. Furthermore, there is an increasing need for

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deposit knowledge in source-tracing studies of the Chinese archaic jades. At present, archaeologists reach their conclusions by comparing geochemical data of jades found in museums (Wen and Jing, 1993, 1996; Wan et al., 2002; Sax et al., 2004; Chen et al., 2004; Casadio et al., 2007) to in-sit those samples which remain poorly understood (Zhang, 2002; Cui et al., 2002). For the Hetian nephrite deposit, which has been believed to be a dolomite-related deposit (Yui and Kwon, 2002; Harlow and Sorensen, 2005; Liu et al., 2010), it lacks geochemical or petrographic data for the genesis interpretations, as well as no temperature constraints or fluid compositions, which is key data for understanding roles in hydrothermal water during nephrite or ore mineralization (e.g., Yui and Kwon, 2002; Deng et al., 2009; Yang et al., 2009), establishing the nephrite genesis, and understanding of nephrite formation conditions. To address these points, we present petrographic and geochemical data of nephrite samples from the Alamas orebody in the Hetian nephrite deposit, and attempt to discuss its petrogenesis.

2. Geological background The Hetian nephrite deposit, located in the southern part of the Tarim Basin, occurs in the Kunlun Mountains of Xinjiang, NW China. It includes three main ore belts located at Shache-Yecheng, Hetian-Yutian and Qiemo-Ruoqiang (Fig. 1A), and it is composed of more than twenty nephrite orebodies with similar geological setting. These ores occur at contacts between Precambrian dolomitic marble and intermediate-acidic granite emplaced during the Hercynian Orogeny. The geological setting and common vein-like texture of the Hetian nephrite deposit implies a dolomite-related metasomatic origin (Jiang, 1986). Secondary alluvial nephrites are only found in terraces of the Yurungkash River (White Jade River) and Karakash River (Black Jade River) in the Hetian area (Fig. 1A). Both the primary and secondary deposits in Hetian area are assumed to have dolomite-related metasomatic origin. Within the Hetian nephrite area, the Alamas ore body (Fig. 1B) is situated 40 miles southeast of Liushen Village in Yutian County (E81°550 , N36°120 ) and about 4500 m above sea level. The Alamas orebody is highly valued not only for producing ‘‘mutton-fat’’ luster and varied colors, including white, green-white and green, but also for its long history of about 250 utility years since Qing Dynasty. More than 100 tons of nephrite has been mined since 1957 in this orebody. Granite and granodiorite are widely distributed around nephrite orebodies in Alamas (Fig. 1B). Granites around the deposit have three main periods of emplacement: the Late Variscan, Middle Variscan and Caledonian periods (Jiang, 1986). The Late Variscan granite and granodiorite occur as stocks and were assumed to be closely associated with the formation of nephrite (Jiang, 1986). Wallrock for the nephrite orebodies in Alamas is Precambrian dolomitic marble. About 11 nephrite veins, up to 20 m long (10 m on average) and about 0.1–0.5 m wide, have been found in Alamas (Fig. 1B). Most veins are distributed along faults and fissures within dolomitic marble, possibly resulting from post-magmatic fluid metasomatism. Among mineral zones, white (Fig. 2A), white-green (Fig. 2C) and green nephrite (Fig. 2E) ore bodies also occurred as zones (Fig. 1C), with their polished pieces having great ornamental values (Fig. 2B, D and F). White nephrite and white-green nephrite can sometimes be found in one sample with an obvious boundary line (Fig. 2G and H). The widths of white, white-green and green nephrite zones vary from 30 cm to nearly 50 cm (Fig. 1C). The white nephrite zone, with a width of about 30 cm is the narrowest among all zones. White nephrite samples (ABY1 and ABY2), white-green nephrite samples (AQB1, AQB2, AQB3, AQB4 and AQB6), green nephrite

Fig. 1. A. Geological sketch map showing three main primary nephrite deposit belts along the Kunlun Mountains in the Hetian nephrite deposit: Shache-Yecheng, Hetian-Yutian, Qiemo-Ruoqiang; ‘‘N’’ represents nephrite orebodies; Liushen Village shown as o and the Alamas nephrite orebody as N shown in rectangle; and two secondary nephrite deposits of the Yurungkash River (White Jade River) and Karakash River (Black Jade River) in Hetian nephrite deposit, Xinjiang, China (modified after Tang et al., 1994). B. Y11-1 and Y11-2, the two main nephrite veins in Alamas. Samples are from A–A’ real cross section (modified after Jiang, 1986). C. A cross section shown as A–A’ in Fig. 1B with locations of wallrocks and nephrite samples (from Liu et al., 2010). (Abbreviations: DM: dolomitic marble; DI: diorite; GN: granodiorite; QD: quartz-bearing diorite; SN: syenite; KAD: K (potassium) altered diorite).

samples (AQY1, AQY2, AQY3, AQY4 and AQY5) and host dolomitic marble samples (ACa1, ACa2, ACa3, ACa4, ACa5, ACa6, ACa7, ACa8 and ACa9) were selected for the study from one cross section (Fig. 1C) shown as A–A’ in Fig. 1B in the Y11-2 nephrite vein, which is one of the two main veins in Alamas.

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3. Analytical methods The chemical compositions of minerals in nephrite was determined using a CAMECA-SX-51 electron microprobe (EMP) at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS). Analyses were performed under conditions of an operating voltage of 15 kV and a beam current of 12-nA. The microprobe standards involve synthetic and natural jadeite, amphiboles, feldspar and clinopyroxenes. The results are listed in Tables 1 and 2. All samples were ground to c.200 mesh size in an agate mortar. Major oxides were determined by X-ray fluorescence spectroscopy (XRF) using a Phillips PW2400 system (Phillips, Amsterdam, the Netherlands) at the IGGCAS, Beijing, China. Fused glass disks were

used, and the analytical precision is better than 5%, as estimated from repeat analyses of GSR-3 (basalt, Chinese standard reference materials; see Shi et al., 2008). In addition, TFe2O3 was analyzed by XRF at IGGCAS. Results are presented in Tables 3 and 4. Trace element abundances were obtained by inductively coupled plasma mass spectrometry (ICP-MS) using a VG-PQII system, also at the IGGCAS. Samples were dissolved in distilled HF + HNO3 in 15 ml Savillex Teflon screw-cap beakers at 120 °C for 6 days, dried and then diluted to 50 ml for analysis. A blank solution was prepared and the total procedural blank was <50 ng for all trace elements. Precision for all trace elements is estimated to be 5% (precisions for some elements with lower abundances such as Tb, Ho, Tm and Lu are estimated to be about or more than 10%) and

Fig. 2. A. Picture of handspecimens of white nephrite: ABY1 and ABY2; B. Picture of handspecimens of polished white nephrite sections: ABY1 and ABY2; C. Picture of handspecimens of white-green nephrite: AQB2, AQB3 and AQB4; D. Picture of handspecimens of polished white-green nephrite sections: AQB2, AQB3, AQB4 and AQB6; E. Picture of handspecimens of green nephrite: AQY2, AQY3 and AQY4; F. Picture of handspecimens of polished green nephrite sections: AQY1, AQY2, AQY3, AQY4 and AQY5 sections.

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Y. Liu et al. / Journal of Asian Earth Sciences 42 (2011) 440–451 Table 1 Representative chemical compositions of tremolite (Tr-II and Tr-I) and actinolite in nephrite from Alamas, Xinjiang, China. Minerals

Tremolite

Actinolite

Tr-II

Tr-I

Sample

ABY1-5

ABY 4-2

ABY 4-3

ABY 4-4

AQB1-5

AQB4-2

AQB4-3

AQB4-6

AQY1-5

AQY1-6

AQY5-1

AQY5-2

AQY5-3

Act

SiO2 TiO2 Al2O3 FeO Cr2O3 MnO MgO CaO Na2O K2O Total TSi TAl TFe3 TTi SumT CAl CCr CFe3+ CTi CMg CFe2+ CMn CCa SumC BMg BFe2 BMn BCa BNa SumB ACa ANa AK SumA Sum cat SumOXY Mg/Mg + Fe2+

58.24 0.01 0.40 0.01 0.38 0.02 24.02 13.21 0.04 0.14 96.47 8.01 0.00 0.00 0.00 8.01 0.07 0.04 0.00 0.00 4.89 0.00 0.00 0.00 5.00 0.04 0.00 0.00 1.95 0.01 2.00 0.00 0.00 0.03 0.03 15.04 23.09 1.00

58.14 0.01 0.39 0.02 0.87 0.08 23.80 13.37 0.05 0.07 96.80 7.99 0.01 0.00 0.00 8.00 0.05 0.09 0.00 0.00 4.85 0.00 0.00 0.00 5.00 0.02 0.00 0.01 1.97 0.00 2.00 0.00 0.01 0.01 0.03 15.03 23.08 1.00

58.20 0.02 0.33 0.00 0.83 0.07 23.70 12.98 0.03 0.04 96.20 8.04 0.00 0.00 0.00 8.04 0.05 0.09 0.00 0.00 4.85 0.00 0.00 0.00 5.00 0.03 0.00 0.01 1.92 0.01 1.97 0.00 0.00 0.01 0.01 15.02 23.12 1.00

58.07 0.01 0.35 0.02 0.72 0.04 23.92 12.84 0.00 0.04 96.01 8.03 0.00 0.00 0.00 8.03 0.06 0.08 0.00 0.00 4.86 0.00 0.00 0.00 5.00 0.07 0.00 0.01 1.90 0.00 1.97 0.00 0.00 0.01 0.01 15.01 23.10 1.00

58.25 0.02 0.42 0.03 0.36 0.01 24.41 12.79 0.05 0.05 96.39 8.00 0.00 0.00 0.00 8.00 0.07 0.04 0.00 0.00 4.89 0.00 0.00 0.00 5.00 0.11 0.00 0.00 1.88 0.00 2.00 0.00 0.01 0.01 0.02 15.02 23.07 1.00

58.34 0.00 0.58 0.07 0.25 0.09 24.21 12.63 0.08 0.10 96.35 8.03 0.00 0.00 0.00 8.03 0.09 0.03 0.00 0.00 4.88 0.00 0.00 0.00 5.00 0.09 0.01 0.01 1.86 0.02 1.99 0.00 0.00 0.02 0.02 15.04 23.11 1.00

58.85 0.01 0.49 0.04 0.29 0.06 24.65 12.65 0.03 0.06 97.13 8.02 0.00 0.00 0.00 8.02 0.08 0.03 0.00 0.00 4.89 0.00 0.00 0.00 5.00 0.12 0.01 0.01 1.85 0.01 1.99 0.00 0.00 0.01 0.01 15.02 23.09 1.00

58.03 0.03 0.60 0.05 0.24 0.05 24.33 12.44 0.04 0.08 95.89 8.01 0.00 0.00 0.00 8.01 0.10 0.03 0.00 0.00 4.87 0.00 0.00 0.00 5.00 0.14 0.01 0.01 1.84 0.01 2.00 0.00 0.00 0.01 0.01 15.03 23.09 1.00

57.31 0.00 0.47 0.08 1.18 0.08 22.86 13.18 0.06 0.10 95.32 8.02 0.00 0.00 0.00 8.02 0.08 0.13 0.00 0.00 4.77 0.01 0.01 0.00 5.00 0.00 0.00 0.00 1.98 0.02 1.99 0.00 0.00 0.02 0.02 15.03 23.15 1.00

57.28 0.00 0.52 0.04 1.17 0.05 23.16 12.74 0.00 0.22 95.18 8.03 0.00 0.00 0.00 8.03 0.09 0.13 0.00 0.00 4.79 0.00 0.00 0.00 5.00 0.05 0.01 0.01 1.91 0.00 1.98 0.00 0.00 0.04 0.04 15.04 23.15 1.00

57.91 0.02 0.19 1.34 0.03 0.05 23.34 13.3 0.12 0.07 96.37 8.02 0.00 0.00 0.00 8.02 0.03 0.00 0.00 0.00 4.82 0.15 0.00 0.00 5.00 0.00 0.01 0.01 1.97 0.02 2.00 0.00 0.02 0.01 0.03 15.05 23.06 0.97

57.15 0.04 0.8 3.5 0.07 0.11 21.59 12.87 0.11 0.05 96.29 8 0.00 0.00 0.00 8.00 0.13 0.01 0.00 0.00 4.51 0.35 0.00 0.00 5.00 0.00 0.06 0.01 1.93 0.00 2.00 0.00 0.03 0.01 0.04 15.04 23.09 0.92

57.57 0.01 0.87 3.59 0.08 0.15 21.56 12.8 0.09 0.05 96.77 8.02 0.00 0.00 0.00 8.02 0.14 0.01 0.00 0.00 4.48 0.37 0.00 0.00 5.00 0.00 0.05 0.02 1.91 0.02 2.00 0.00 0.00 0.01 0.01 15.03 23.12 0.91

55.78 0.00 0.23 11.52 0.03 0.28 16.4 12.5 0.05 0.01 96.8 8.06 0.00 0.00 0.00 8.06 0.04 0.00 0.00 0.00 3.53 1.39 0.03 0.00 5.00 0.00 0.00 0.00 1.94 0.01 1.95 0.00 0.00 0.00 0.00 15.02 23.09 0.72

Amphibole formulate were calculated on the basis of 23 oxygens and 15 cations.

Table 2 Representative chemical compositions of diopside in nephrite from Alamas, Xinjiang, China. Phenocryst

SiO2 TiO2 Al2O3 FeO Cr2O3 MnO MgO CaO Na2O K2O Total Si Ti Al Cr Fe3+ Fe2+ Mn Mg Ca Na K Sum

1

2

3

4

Core

Rim

Core

Rim

Core

Rim

Core

Rim

53.80 0.11 1.48 1.52 0.01 0.05 17.22 25.41 0.03 0.00 99.63 1.96 0.00 0.06 0.00 0.02 0.04 0.00 0.93 0.99 0.00 0.00 4.00

53.85 0.01 0.30 3.27 0.01 0.13 16.46 25.07 0.12 0.00 99.22 1.98 0.00 0.01 0.00 0.04 0.07 0.00 0.90 0.99 0.01 0.00 4.00

54.28 0.00 0.23 3.23 0.03 0.14 16.63 25.10 0.12 0.00 99.76 1.99 0.00 0.01 0.00 0.02 0.07 0.00 0.91 0.98 0.01 0.00 4.00

54.60 0.04 0.24 3.63 0.00 0.10 16.58 25.16 0.07 0.00 100.42 1.99 0.00 0.01 0.00 0.01 0.09 0.00 0.90 0.98 0.01 0.00 4.00

53.99 0.14 1.26 1.36 0.01 0.10 17.57 25.25 0.06 0.00 99.74 1.96 0.00 0.05 0.00 0.02 0.02 0.00 0.95 0.98 0.00 0.00 4.00

54.46 0.07 0.57 1.78 0.00 0.05 17.19 25.76 0.09 0.00 99.97 1.98 0.00 0.02 0.00 0.02 0.03 0.00 0.93 1.00 0.01 0.00 4.00

53.16 0.13 1.80 1.54 0.03 0.12 16.93 25.08 0.08 0.00 98.87 1.95 0.00 0.08 0.00 0.02 0.03 0.00 0.93 0.99 0.01 0.00 4.00

53.22 0.09 1.26 2.08 0.01 0.03 17.11 25.15 0.22 0.06 99.23 1.94 0.00 0.05 0.00 0.00 0.06 0.00 0.93 0.98 0.02 0.00 4.00

Diopside formulate were calculated on the basis of 6 oxygens and 4 cations.

accuracy is better than 5% for most elements, as determined by analyses of the GSR-3 standard (Shi et al., 2008). Results are presented in Tables 3 and 4. Oxygen and hydrogen isotope analyses were measured on a MAT-252 mass spectrometer at IGGCAS. Thirteen nephrite samples were ground to 60 to 100 mesh. Oxygen was liberated from samples by reaction with BrF5 (Clayton and Mayeda, 1963) and converted to CO2 on a platinum-coated carbon rod. Reproducibility for isotopically homogeneous pure quartz is about ±0.1‰. For hydrogen isotopic compositions of the samples, water was released by heating the samples to approximately 1000 °C in an induction furnace. Samples were first degassed of labile volatiles by heating under vacuum to 120 °C for 3 h. Water was converted to hydrogen by passage over heated zinc powder at 410 °C (Friedman, 1953). Stable isotope data for hydrogen and oxygen are expressed in the standard d notation as per mil (‰) relative to standard mean ocean water (SMOW). Analyses of standard water samples suggest a precision for dD of ±0.5‰. Results are presented in Table 5.

4. Petrography All nephrite samples consist predominately of tremolite (Tr), with minor accessory minerals, including diopside (Di), actinolite (Act) and calcite (Cal).

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Y. Liu et al. / Journal of Asian Earth Sciences 42 (2011) 440–451

Table 3 Bulk-rock chemical compositions of nephrites from Alamas, Xinjiang, China. Sample

ABY1

ABY2

AQB1

AQB2

AQB3

AQB4

AQB6

AQY1

AQY2

AQY3

AQY4

AQY5

Major element SiO2 TiO2 Al2O3 Fe2O3T FeO MnO MgO CaO Na2O K2O P2O5 LOI Total

oxide (wt.%) 57.09 0.02 1.04 0.50 0.42 0.05 25.97 11.85 0.05 0.28 0.02 2.72 99.57

57.48 0.02 0.83 0.41 0.41 0.05 25.88 12.27 0.01 0.32 0.09 2.02 99.37

53.25 0.04 4.13 0.71 0.66 0.03 26.55 10.20 0.07 0.18 0.05 4.16 99.37

43.59 0.01 0.23 0.96 0.60 0.13 18.89 23.21 0.01 0.03 0.04 12.97 100.08

57.64 0.01 0.25 0.80 0.66 0.05 25.86 12.21 0.01 0.03 0.01 2.48 99.35

56.92 0.02 1.33 0.43 0.42 0.08 26.25 10.97 0.04 0.55 0.07 2.74 99.38

58.58 0.01 0.41 0.54 0.42 0.11 25.49 12.67 0.03 0.05 0.02 1.48 99.40

57.33 0.01 0.66 1.96 1.49 0.13 24.86 12.10 0.05 0.12 0.01 2.94 100.16

57.54 0.01 0.48 1.61 1.33 0.08 24.95 12.21 0.06 0.14 0.03 2.28 99.39

57.62 0.02 0.25 1.03 0.83 0.09 25.46 12.41 0.01 0.04 0.06 2.42 99.40

56.74 0.01 0.85 1.20 1.00 0.10 25.58 12.08 0.01 0.04 0.03 2.76 99.40

57.83 0.01 0.40 1.35 1.00 0.11 25.02 12.26 0.00 0.05 0.05 2.46 99.56

Trace elements Cr Ni Rb Ba Th U Nb Ta La Ce Pr Sr Nd Zr Hf Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu Eu⁄ [Gd/Yb]N [La/Sm]N [La/Yb]N RREE RREE + Y

(ppm) 13.02 0.05 38.42 14.34 0.22 0.64 0.74 0.06 0.59 1.47 0.19 8.36 0.92 2.96 0.08 0.29 0.02 0.33 0.06 0.47 4.91 0.11 0.34 0.05 0.35 0.06 0.06 0.74 1.24 1.09 5.23 10.14

27.72 1.27 46.29 17.44 0.30 1.05 0.27 0.06 0.58 1.09 0.17 7.91 0.77 4.77 0.14 0.21 0.02 0.25 0.05 0.36 3.73 0.08 0.23 0.04 0.23 0.04 0.07 0.90 1.67 1.68 4.10 7.83

45.53 2.41 21.29 19.96 0.10 0.12 0.54 0.09 2.85 6.00 0.75 4.87 2.82 10.29 0.31 0.60 0.07 0.51 0.08 0.56 4.42 0.12 0.34 0.05 0.35 0.05 0.13 1.17 2.92 5.41 15.13 19.55

25.84 3.95 0.62 1.09 0.08 1.32 1.81 0.13 0.38 0.55 0.06 43.07 0.25 0.37 0.01 0.10 0.01 0.18 0.05 0.46 6.22 0.13 0.42 0.07 0.42 0.06 0.05 0.35 2.40 0.59 3.12 9.34

8.95 0.95 1.59 0.90 0.05 0.60 2.02 0.11 0.90 2.50 0.38 3.75 1.85 0.34 0.01 0.62 0.03 0.74 0.16 1.26 10.86 0.31 0.96 0.16 1.10 0.17 0.05 0.54 0.88 0.54 11.16 22.02

33.72 0.49 105.14 13.21 0.36 1.44 1.55 0.07 1.47 2.68 0.37 11.71 1.41 3.14 0.07 0.37 0.01 0.38 0.07 0.51 5.47 0.12 0.34 0.05 0.31 0.05 0.04 1.01 2.42 3.16 8.13 13.6

11.00 0.25 4.43 1.31 0.07 1.11 1.95 0.06 0.20 0.39 0.06 5.88 0.34 0.48 0.02 0.17 0.01 0.30 0.07 0.54 5.56 0.12 0.32 0.04 0.26 0.04 0.04 0.93 0.70 0.50 2.84 8.4

123.72 2.98 26.27 4.20 0.23 0.09 1.74 0.08 26.40 38.72 3.44 12.83 10.45 0.92 0.03 1.36 0.28 1.35 0.17 1.01 8.40 0.21 0.63 0.09 0.61 0.10 0.21 1.79 11.92 28.77 84.81 93.21

178.70 3.40 24.99 5.47 0.12 0.99 0.27 0.05 0.45 0.94 0.12 9.01 0.55 1.89 0.05 0.18 0.02 0.28 0.06 0.53 6.70 0.13 0.38 0.05 0.29 0.04 0.07 0.78 1.55 1.05 4.02 10.72

92.77 1.15 3.02 2.93 0.13 1.15 1.26 0.08 0.25 0.67 0.12 9.47 0.71 1.83 0.05 0.32 0.04 0.46 0.09 0.69 6.94 0.16 0.47 0.07 0.37 0.05 0.11 0.99 0.47 0.44 4.45 11.39

55.38 0.34 3.62 2.15 0.12 0.76 0.99 0.08 0.17 0.44 0.08 7.26 0.44 1.49 0.04 0.22 0.03 0.35 0.08 0.57 5.98 0.13 0.37 0.05 0.31 0.04 0.10 0.92 0.48 0.37 3.28 9.26

151.05 2.26 9.32 1.72 0.58 2.09 7.65 0.30 13.57 19.90 1.64 7.55 4.36 3.45 0.11 0.66 0.02 0.73 0.11 0.75 7.39 0.18 0.54 0.09 0.72 0.13 0.03 0.82 12.62 12.45 43.41 50.8

Major elements from Liu et al. (2010).

4.1. Tremolite

4.2. Actinolite

White nephrite is composed mainly of tremolite, while some samples exhibiting white-green and green color have only a minor amount of actinolite. Tremolite crystals are either undeformed (TrI) or deformed (Tr-II). Tr-I crystals are very rare and occurs as idiomorphic porphyroblasts of about 200 lm long and 100 lm wide. The porphyroblasts commonly have rims of actinolite (Fig. 3A). In some cases, actinolite was cut by tiny crosscutting, needle-like barite crystals, suggesting that these nephrites had experienced latestage replacement by actinolite, as well as infiltration of Ba-rich fluids, similar to that of jadeite jade (e.g., Shi et al., 2010). Tr-II crystals are characterized by fine-grained tremolite aggregates. Most of these aggregates exhibit fine-grained foliated textures (Fig. 3E–G) (Liu et al., 2010), consisting of micro- to cryptocrystalline tremolites. These tremolite crystals have big aspect ratios, some even occurring as long fibrils, similar to the observations of Dorling and Zussman (1985), Germine and Puffer (1989) and O’Hanley (1996). In some domains, crystal preferred orientation is obvious, indicative of recrystallization driven by ductile deformation (Fig. 3H).

Actinolite is very rare in Alamas nephrite, occasionally occurring in a small domain. It has irregular shape and cuts across Tr-I crystal (Fig. 3A).

4.3. Calcite Calcite, which occurs as isolated fragments less than 40 lm in size with irregular outline (Fig. 3B), is disseminated in nephrite. The backscattered electron (BSE) images and corresponding EMP analyses reveal that calcite is homogeneous and nearly pure to calcite end-member.

4.4. Diopside Diopside aggregate in nephrite was partially replaced by tremolite along its grain boundary and through its cracks (Fig. 3C and D).

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Y. Liu et al. / Journal of Asian Earth Sciences 42 (2011) 440–451 Table 4 Bulk-rock chemical compositions of dolomitic marble from Alamas, Xinjiang, China. Samples

ACa1

ACa2

ACa3

ACa4

ACa5

ACa6

ACa7

ACa8

ACa9

SiO2 TiO2 Al2O3 TFe2O3 FeO MnO MgO CaO Na2O K2O P2O5 LOI Total Cr Ni Rb Ba Th U Nb Ta La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Y Lu Sc V Ti Cu Zn Ga Sr Zr Sb Hf Pb Eu⁄ [Gd/Yb]N [La/Sm]N [La/Yb]N RREE RREE + Y

1.11 0.02 0.22 0.15 0.17 0.02 22.08 30.51 0.01 0.01 0.05 45.82 100.01 8.69 17.3 0.15 1.15 0.26 0.25 0.22 <0.05 1.68 2.92 0.38 1.62 0.36 0.09 0.44 0.07 0.42 0.1 0.29 <0.05 0.26 3.76 <0.05 1.44 2.02 87.6 2.54 <0.05 0.35 67.5 3.15 0.38 0.1 1.27 0.23 1.36 2.86 4.27 8.63 12.39

1.40 0.03 0.45 0.19 0.33 0.02 22.45 30.19 0.01 0.01 0.08 45.18 100.01 11.4 15.9 0.15 1.94 0.92 1.22 0.49 0.05 1.75 3.11 0.4 1.68 0.42 0.07 0.53 0.07 0.56 0.13 0.36 <0.05 0.31 4.6 <0.05 1.99 3.96 157 3.31 5.7 0.62 61.6 8.71 3.37 0.28 1.05 0.15 1.37 2.55 3.73 9.39 13.99

1.91 0.02 0.17 0.14 0.36 0.01 21.99 30.76 0.01 0.01 0.16 44.94 100.13 16.1 22.4 0.14 1.48 0.23 0.36 0.21 <0.05 1.57 2.83 0.38 1.65 0.43 0.1 0.52 0.08 0.51 0.11 0.34 <0.05 0.29 4.22 <0.05 1.53 2.55 88.4 5.02 4.3 0.34 67.6 2.81 0.19 0.09 1.85 0.21 1.44 2.24 3.57 8.81 13.03

2.40 0.04 0.39 0.19 0.16 0.02 22.52 29.61 0.01 0.01 0.05 44.31 99.55 107 63 0.2 2.76 0.62 0.62 0.47 <0.05 2.08 3.74 0.49 2.04 0.45 0.08 0.53 0.09 0.59 0.14 0.42 0.06 0.33 5.09 0.06 2.33 4.33 169 7.43 7.9 0.65 62.8 6.97 1.41 0.24 1.46 0.16 1.29 2.83 4.16 11.10 16.19

1.58 0.02 0.20 0.14 0.13 0.02 22.46 29.98 0.01 0.01 0.05 45.11 99.57 79 53 0.12 1.65 0.25 0.22 0.22 <0.05 1.64 2.81 0.39 1.6 0.32 0.08 0.42 0.07 0.46 0.1 0.32 <0.05 0.27 3.89 <0.05 2.16 2.37 78.5 5.84 3.7 0.44 59.4 3.15 1.39 0.1 1.44 0.22 1.25 3.14 4.01 8.48 12.37

1.33 0.06 0.55 0.22 0.23 0.03 22.34 30.08 0.01 0.01 0.47 43.50 98.60 31.6 27.4 0.12 1.57 1.53 2.65 0.94 0.09 1.97 3.32 0.44 1.92 0.49 0.12 0.7 0.1 0.66 0.15 0.42 0.05 0.35 5.66 <0.05 2.5 8.55 334 4.19 14.4 0.78 56.6 13.3 1.02 0.39 1.56 0.2 1.61 2.46 3.72 10.69 16.35

1.58 0.03 0.40 0.17 0.12 0.02 22.44 29.94 0.01 0.01 0.09 45.29 99.98 61.9 41.1 0.13 2.04 0.56 0.63 0.37 <0.05 1.67 2.96 0.4 1.71 0.41 0.09 0.45 0.07 0.51 0.11 0.33 <0.05 0.25 4.06 <0.05 2.23 3.39 128 4.51 25 0.53 61.3 5.67 4.52 0.18 1.27 0.21 1.45 2.5 4.41 8.96 13.02

0.79 0.01 0.16 0.15 0.11 0.02 22.36 30.66 0.01 0.01 0.06 46.12 100.36 8.84 19.3 0.12 1.55 0.13 0.15 0.18 <0.05 1.61 2.79 0.37 1.54 0.37 0.09 0.54 0.08 0.52 0.12 0.37 <0.05 0.31 5.1 <0.05 1.71 1.98 95.2 2.89 2.9 0.4 62.2 2.32 0.32 0.08 1.83 0.2 1.4 2.67 3.43 8.71 13.81

2.38 0.04 0.45 0.22 0.13 0.01 22.33 29.72 0.01 0.01 0.05 44.40 99.62 46.3 32.6 0.23 2.03 0.43 0.41 0.47 <0.05 1.89 3.34 0.45 1.84 0.44 0.08 0.47 0.07 0.54 0.13 0.41 0.05 0.34 4.82 0.05 2.13 3.93 211 4.42 8.2 0.64 60.7 5.55 <0.05 0.19 1.32 0.18 1.11 2.63 3.67 10.10 14.92

5. Mineral chemistry

5.2. Diopside

5.1. Amphibole

Diopside is higher in Fe and lower in Mg from rim to core (Table 2).

According to the classification scheme of Leake et al. (1997), all amphiboles in nephrite belong to the calcic group. Actinolite shows the following chemistries: 8.06 a.p.f.u. of Si and 1.94 a.p.f.u of Ca on the B site, with Mg/ (Mg + Fe2+) being 0.72 (Fig. 4). Actinolite has no Na and K contents on the A site (0 a.p.f.u. Na + K), while FeO concentration in Act is up to 11.52% (Table 1). Tremolite has Si near 8.00 a.p.f.u, Ca on B site more than 1.50 a.p.f.u, (Na + K) on A site less than 0.07 a.p.f.u. (with the exception of one with 0.21 a.p.f.u.), and Mg/(Mg + Fe2+) ratio ranging from 0.9 to 1.0. Tr-I has crystals have higher FeO than Tr-II crystals (Table 1 and Fig. 4).

6. Geochemistry All nephrite samples have pronounced negative Eu anomalies (dEu = 0.03–0.21), declined light REE (LREE: La-Nd), flat middle (MREE: Sm-Ho) and heavy REE (HREE: Er-Lu) (Fig. 5A). They contain very low abundances of RREE ranging from 2.84 to 84.81 ppm (average RREE = 15.81 ppm, approximately 1–5 times those of chondrite, with the lowest at 1  chondrite) (Table 3). They have strong enrichment of Rb, U and Ti and depletion of Ba

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Table 5 Hydrogen and oxygen isotope compositions in nephrite from Alamas and other nephrite deposits. Sample

dD

d18O

d18OH2O 293 °C

ABY-1 ABY-2 AQB-1 AQB-2 AQB-3 AQB-4 AQB-6 AQY-1 AQY-2 AQY-3 AQY-4 AQY-5

86.7 83.0 93.1 89.0 85.1 85.9 94.7 90.2 85.0 91.6 90.4 86.2

3.8 3.2 6.1 4.6 3.5 3.6 6.2 4.1 3.6 4.9 4.8 3.8

3.7 3.1 6.0 4.5 3.4 3.5 6.1 4.0 3.5 4.8 4.7 3.7

Types

Dolomite-related

Nephrite localizations

dD

d18O

Turkestan, China*

108 to 124 57

0.5–2.3

56

1.5

118 to 105

9.9 to 7.9

Cowell, Australia* Wyoming, USA* *

Chuncheon, Korea *

3.4

Serpentinite-related Nephrite localizations Chara Jelgra River, Siberia* Red Mountain, New Zealand* Mt. Ogden, Canada* Shulaps Range, Canada*

dD

d18O

39

6.9

54

7.5

52

9.6

49

8.4

From (Yui and Kwon, 2002).

and Th, whereas the high field strength elements (Zr, Hf and Nb) are more or less depleted (Fig. 5B). In general, contents of trace element in these nephrite samples are very low. All dolomitic marble samples display negative Eu anomalies (dEu = 0.15–0.23), declined light REE (LREE: La-Nd), flat middle (MREE: Sm-Ho) and heavy REE (HREE: Er-Lu) (Fig. 6A). They have even lower content of RREE, ranging from 8.48 to 11.1 ppm (average RREE = 9.43 ppm) (Table 4). They have strong enrichment of U and Sr and depletion of Rb, Ti and Zr, whereas the high field strength elements (Nb and Hf) are more or less depleted (Fig. 6B). The content of FeO in green nephrite is higher than that of white nephrite by EMPA (Table 1). Bulk-rock chemical composition shows that green nephrite samples have more TFe2O3, FeO and Cr concentration than either white or white-green nephrite samples (Table 3 and Fig. 7). Elemems of Fe and Cr are believed to be the color-causing elements of nephrite (Douglas, 1996, 2003). The chemical features suggest a dolomite-related origin, which is consistent with ancient Chinese jades from dolomite-related deposit with low concentration of Cr2O3 and NiO (Cr2O3, 0.07–0.43 wt.%; NiO, 0.08–0.36 wt.%) relative to serpentinite-related type (Cr2O3, 0.07–0.43 wt.%; NiO, 0.08–0.36 wt.%) (Douglas, 1996, 2003). Chemical characteristics of dolomitic marble indicate that it is relatively pure with low content of Al2O3 (0.16–0.55 wt.%), TFe2O3 (0.14–0.22 wt.%), MnO (0.01–0.03 wt.%), Na2O (0.01 wt.%), K2O (0.01 wt.%) and P2O5 (0.05–0.47 wt.%). Based on bulk-rock chemical composition, the content of Fe2O3T and MnO in dolomitic marble is lower than that of nephrite samples (Tables 3 and 4). Nephrite has oxygen and hydrogen isotope compositions ranging from +3.2 to +6.2 per mil and 95 to 75 per mil (Table 5), respectively, showing limited variation. 7. Discussion 7.1. The mechanism underlying zone structure of nephrite ore bodies and formation temperature Rare among Hetian or worldwide nephrite deposits, Alamas nephrite has a color-defined zone structure at the contact,

Fig. 3. Backscattered electron (BSE) images and photomicrographs of nephrites. A. overview of idiomorphic Tr-I crystal and Act. B isolated calcite disseminated in the nephrite. C. view of relationship between diopside and tremolite. D. close-up view of area marked in Fig. 3C showing diopside replaced by tremolite. E. fine-grained textures of white-green nephrite. F. fine-grained textures of green nephrite. G. finegrained foliated textures. H. obvious shape-preferred orientation in nephrite texture formed by means of recrystallization. (B, E, F, and G from Liu et al., 2010).

including white, white-green and green nephrite zones. According to the bulk-rock chemical composition of both dolomitic marble and nephrite samples, dolomitic marble has less content of Fe2O3T. Also, bulk-rock chemical composition shows that green nephrite samples have more TFe2O3, FeO and Cr concentration than white and white-green nephrite samples (Table 3 and Fig. 7). Based on the location of igneous rocks adjacent to the nephrite zone (Fig. 1C), it seems that Fe (Tables 3 and 6, Fig. 7), which are the major color-causing elements of nephrite (e.g., Douglas, 1996, 2003; Leaming, 1978; Wilkins et al., 2003; Casadio et al., 2007), originate from granodiorite. It appears that nephrite zones are caused by the gradient of color-causing elements from igneous rocks to dolomitic marble. Moreover, both chemical and color zone structure are likely to be related to metasomatism. Tremolite formed under low-temperature (330–420 °C) and moderate-low pressure (100–200 MPa) conditions based on studies of both nephrite (tremolite) from both sepentinite-related and dolomite-related nephrite from the Fengtian, Taiwan (Yui et al., 1988), Chuncheon, Korea and the East Sayan, Russia (e.g., Noh et al., 1993; Sekerin et al., 1997). Based on the similarity of dolomite-related type occurrence, it is inferred that nephrite in Alamas also occurred under low temperature between 330 °C and 550 °C at low to moderate pressures of 100–200 MPa. To further constrain the nephrite formation temperature, the fluid inclusions in tremolite mineral assemblages were carried

Y. Liu et al. / Journal of Asian Earth Sciences 42 (2011) 440–451

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components in the large fluid inclusions range from 5 to 20 vol.%, as shown in Table 7. The investigations were carried out only on fluid inclusions, having a size large enough to permit accurate measurements. The fluid inclusions were brought to temperatures of 180 °C and subsequently heated up. Most fluid inclusions show melting temperatures ranging from 4.5 °C to 1.4 °C, indicating H2O-rich fluid inclusions. Homogenization temperatures (L + V = L) of the fluid inclusions range from 228 to 425 °C with an average of 293 °C. Thus, the temperature of 293 °C can be regarded as the minimum temperature for the nephrite formation, which is close to the formation temperature for other dolomite-related nephrite deposits (e.g., Noh et al., 1993; Sekerin et al., 1997). Our data are consistent with those reported those reported for dolomite-related nephrite of Chuncheon, Korea (350–550 °C) and the East Sayan, Russia (e.g. Yui and Kwon, 2002; Noh et al., 1993; Sekerin et al., 1997). Fig. 4. Classification diagram for amphibole in nephrite following Leake et al. (1997).

out. Two-phase, gas/liquid fluid inclusions were found in coarsegrained tremolite assemblages. Most fluid inclusions are not large, ranging from 2 to 5 lm. These fluid inclusions appear to be of primary origin, since they show a random distribution throughout the tremolite, and no occurrence of inclusions along healed fractures or cleavages can be found. The volumetric contents of gaseous

7.2. Petrogenesis of nephrite and derivation of nephrite-forming material Previous genesis models of dolomite-related nephrite include metasomatism and metamorphism (Leaming, 1978; Harlow and Sorensen, 2000, 2005; Yui and Kwon, 2002). However, except for stable isotope studies (Yui and Kwon, 2002), no clear evidence points to a metasomatic origin for dolomite-related nephrite deposit, especially, for those nephrites from various zones in the Alamas. As the REE patterns have been successfully applied to

Fig. 5. A. Chondrite-normalized REE patterns of the Alamas nephrites. B. Primitive Mantle-normalized trace element diagrams for the Alamas nephrites. Normalizing values are after Sun and McDonough (1989).

Fig. 6. A. Chondrite-normalized REE patterns of the dolomitic marble. B. Primitive Mantle-normalized trace element diagrams for the dolomitic marble. Normalizing values are after Sun and McDonough (1989).

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Fig. 7. A. Content of TiO2, TFe2O3, MnO, FeO of bulk-rock chemical compositions determined by XRF in white, white-green and green nephrite (from Liu et al., 2010). B. Concentration of Cr and Ni of bulk-rock chemical compositions determined by ICP-Ms. for white, white-green and green nephrite. C. Concentration of Cr2O3, MnO determined by EMPA of representative white, white-green and green nephrite samples by EMPA from Alamas, Xinjiang, China.

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Y. Liu et al. / Journal of Asian Earth Sciences 42 (2011) 440–451 Table 6 Chemical compositions of wallrocks for nephrite from Alamas, Xinjiang, China. Sample

Dolomitic marble

Dolomitic marble

Granodiorite

Diorite

Quartz diorite

Granodiorite

Syenite

SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O Total

6.04 0.00 0.15 0.12 0.16 0.00 20.62 30.85 0.26 0.14 58.34

1.30 0.06 0.34 0.30 0.00 0.03 20.79 30.21 0.09 0.01 53.13

58.62 0.92 16.77 2.38 0.30 0.10 2.27 5.67 4.54 4.90 96.95

50.57 0.73 19.86 3.60 2.56 0.10 4.30 7.88 2.50 5.82 98.2

58.93 0.90 17.22 2.00 3.98 0.11 3.14 5.51 3.18 2.80 98.21

57.71 0.03 15.27 0.26 0.22 0.00 3.37 4.96 4.54 6.94 93.32

65.52 0.00 19.13 0.39 0.39 0.06 0.15 0.86 6.68 6.12 99.22

From (Tang et al., 1994).

Table 7 Microthermometric results of individual fluid inclusions. Gas content (vol.%)

Th (°C)

Tm (°C)

5 10 20 10 10 10 5 5 5 5 5 10 5 20 20 10 20 10 10

337 316 278 286 292 321 244 349 314 331 362 414 425 280 357 276 278 286 274

3.5 3.3 2.0 2.2 1.4 1.6 2.2 4.5 3.5 4.2 4.0 4.3 4.3 2.1 3.2 2.8 2.2 3.4 2.7

Tm = melting temperature, Th = homogenization temperature.

determine the origin of the ore material or Myanmar jadeite (Jiang et al., 2004; Gu et al., 2006; Shi et al., 2008), the total REE abundance and REE patterns can be utilized to deduce the origin of uncommon rocks. All twelve nephrites samples analyzed yield strong negative Eu anomalies with declined LREE and flat HREE (Fig. 5A), which are characteristics common in most types of igneous rocks. Of all igneous rocks, only some anorthosites display patterns with strong positive Eu anomalies and LREE enrichments (Bhattacharya et al., 1998; James et al., 2002; Charlier et al., 2005). Normalized REE patterns of nephrite is dissimilar to other rocks, such as fluid-dominated serpentinized abyssal peridotities at the Mid-Atlantic Ridge (Paulick et al., 2006) and the jadeitites from Myanmar (Shi et al., 2008). However, nephrite samples have less amount of REE content compared with these rocks. Since both nephrite and dolomitic marble samples have similar REE patterns and since their REE contents are overlapped (Figs. 5 and 6, Tables 3 and 4), it can be inferred that the origin of the Alamas nephrite has a close genetic relationship with dolomitic marble. Calcite found in nephrite might be the reaction product during the metasomatic process (Fig. 3B), and, as such, it is very likely consistent with the following genetic model of nephrite by Harlow and Sorensen (2005):

5CaðMg; FeÞðCO3 Þ2 þ 8SiO2 aq þ H2 O ! Ca2 ðMg; FeÞ5 Si8 O22 þ 3CaCO3 þ 7CO2 aq:

Based on the petrography, geochemical analysis, two possible formations can be proposed for our Alamas nephrite: (1) The dolomitic marble was replaced by tremolite and the relict calcite was left as isolated grain. This is a common explanation for the formation of nephrite: Dolomitic marble ? Tremolite (Liu et al., 2010) aq

5CaðMgÞðCO3 Þ2 þ 8SiO2 þ H2 O ! Ca2 ðMgÞ5 Si8 O22 þ 3CaCO3 þ 7COaq 2 : dolomitic marble

tremolite

calcite

(2) At first, the dolomitic marble was replaced by diopside at first, which was, in turn, replaced by the tremolite. During the second replacement, the content of Fe2+ in the rims is higher than that found in the corresponding cores in diopside relict aggregates. In this situation, Dolomitic marble ? Diopside ? Tremolite occurs. aq

CaðMgÞðCO3 Þ2 þ 2SiO2 ! Ca MgðSi2 O6 Þ þ 2COaq 2 : dolomitic marble

diopside

5CaMgðSi2 O6 Þ þ H2 O ! Ca2 Mg5 ðSi4 O11 Þ2 ðOHÞ2 þ 3Ca2þ þ 6SiO2 diopside

tremolite

Data of fluid inclusions in tremolite-diopside mineral assemblages, nephrite in Alamas yield temperatures between 280 °C and 425 °C, with an average of 293 °C. Assuming that hydrothermal water in equilibrium with nephrite (tremolite) from Alamas occurred at temperature of 293 °C, oxygen isotopic compositions of hydrothermal waters were calculated using the fractionation formula (103lna = 3.95  106/T2  8.28  103/T + 2.38) from Zheng (1993a,b). Oxygen isotope composition of the fluid varies between 3.1 and 6.1 per mil at 293 °C with hydrogen isotopic composition ranging from 94.7 to 83 per mil. These results are close to the isotopic composition range of magmatic water or metamorphic water suggested by Ohmoto (1986) and Sheppard (1986) (Fig. 8A, Table 5). Even though H-isotope composition may change very little, the O-isotope composition of the source fluid would have generally been modified through fluid-rock metasomatic reactions, causing the O-isotope composition of fluids in equilibrium with nephrite to deviate significantly from that of the fluid (Yui and Kwon, 2002). On the basis of field observation, at least three possible fluid sources should be considered in the present case: meteoric water, magmatic water and CO2 derived from decarbonation of dolomite marble. That metamorphic water played a role in nephrite formation is possible, but less likely. Among the three possible candidates, meteoric water might have O-isotope composition in the range of 11 to 9 per mil (assuming H-isotope composition of the meteoric water is 75 to 63 per mil), magmatic water from granitic rocks, around +10 per mil, and CO2 from decarbonation process, >+23 per mil (assuming dolomite has O-isotope

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Fig. 8. A. Diagram of dD and d18OH2O data of nephrites from Alamas, Xinjiang, China (this study) indicating the evolution of hydrothermal fluids (Taylor, 1997). B. Isotopic compositions of nephrite deposits from 1. Turkestan, 2. Cowell, 3. Wyoming and those of serpentinite-related nephrite deposits from, 4. Chara Jelgra River, 5. Red Mountain, 6. Mount Ogden, 7. Shulaps Range, 8. Chuncheon, 9. Fengtien. (1–9 from Yui and Kwon, 2002).

composition >+15 per mil) (Sheppard and Schwarcz, 1970; Chacko et al., 1991). If the source fluid was mainly meteoric water, then the fluid/rock ratio (atomic ratio in terms of oxygen) during nephrite formation would be around 0.4–0.7, assuming an open system at 293 °C. If the source fluid was mainly magmatic water, then it must mix with about 17–39% meteoric water to form nephrite with O-isotope composition in the range of +3.1 to +6.1 at 293 °C. It is noted that pure magmatic water in equilibrium with granitic rocks cannot account for the O-isotope composition of nephrite observed. If the formation temperature is above 293 °C, the calculated results would be slightly higher. If the fluid contains a CO2 component, it would also lead to higher values for the above calculations. In any case, the CO2 content should not be higher than 40% if mixed with meteoric water. These are certainly preliminary estimations by far, with more information on the isotopic compositions of dolomite and granite, quantitative estimations could be made. The isotope compositions of some nephrite samples from other deposits around the world are included in Table 5 (Yui and Kwon, 2002). Nephrite samples from Turkestan (Kunlun Mountains, China), Wyoming (United States) and Cowell (Australia) are all dolomite-related nephrite deposits spatially associated with igneous intrusions and dolomitic marble (Leaming, 1978; Flint and Dubowski, 1990). The difference of oxygen and hydrogen isotope compositions of the Alamas nephrite from those of Turkestan, Wyoming and Cowell (Fig. 8B) clearly indicate differences in formation temperature, isotope compositions of the source rock, and/or fluid/rock ratio during nephrite formation.

8. Conclusions The Alamas nephrite ore body is a typical dolomite-related product occurring at the contact between the Late Variscan granodiorite and Precambrian dolomitic marble. The nephrites have quite low Cr (8.95–178.7 ppm) and Ni contents (0.05–3.95 ppm) relative to serpentinite-related nephrite (Cr2O3, 0.07–0.43 wt.%; NiO, 0.08–0.36 wt.%). Both nephrite and dolomitic marble samples have the same REE patterns with negative Eu anomalies, declined LREE, flat HREE and overlapped REE content, indicating the occurrence of the nephrite has a close relationship with host dolomitic marble.

The temperature for nephrite formation is above 293 °C, which resulted from homogeneous temperature analysis. There are at least three possible fluid sources candidates in the present case, meteoric water, magmatic water and CO2 derived from decarbonation of dolomite. Two lines of evidence suggest that the nephrite in Alamas has a metasomatism origin: (1) gradient concentrations of Cr and Fe elements obtained from white and white-green to green nephrite; (2) petrographic observation and related mineral compositions, revealing two possible formation processes: dolomitic marble ? tremolite and dolomitic marble ? diopside ? tremolite. Acknowledgements We thank Dr. J.G. Liou for his kind and insightful comments. The constructive reviews from Dr J.G. Douglas and Dr. Taijin Lu of an earlier draft of this manuscript are greatly appreciated. Dr. F.S. Zhang and X.T. Jin (IGGCAS) are grateful for their help in obtaining mineralogical and geochemical data. This study was funded by the Major State Basic Research Program of China (2009CB421008), the Program for the New Century Excellent Talents in China (NCET-070771) and the Program for Changjiang Scholars and Innovative Research Team in University, ‘‘111’’ Project (No. B07011). References Bhattacharya, A., Raith, M., Hoernes, S., Banerjee, D., 1998. Geochemical evolution of the massif-type anorthosite complex at Bolangir in the Eastern Ghats belt of India. Journal of Petrology 39, 1169–1195. Casadio, F., Douglas, J.G., Faber, K.T., 2007. Noninvasive methods for the investigation of ancient Chinese jades: an integrated analytical approach. Analytical and Bioanalytical Chemistry 387, 791–801. Chacko, T., Mayeda, T.K., Clayton, R.N., oldsmith, J.R., 1991. Oxygen and carbon isotope fractionation between CO2 and calcite. Geochimica et Cosmochimica Acta 55, 2867–2882. Charlier, B., Auwera, J.V., Duchesne, J.C., 2005. Geochemistry of cumulates from the Bjerkreim–Sokndal layered intrusion (S. Norway): Part II. REE and the trapped liquid fraction. Lithos 83, 255–276. Chen, T.H., Calligaro, T., Pages-Camagna, S., Menu, M., 2004. Investigation of Chinese archaic jade by PIXE and mu Raman spectrometry. Applied Physics. A-Materials Science & Processing 79, 177–180. Clayton, R.N., Mayeda, T.K., 1963. The use of bromine pentafluoride in the extraction of oxygen from oxides and silicates for isotopic analysis. Geochimica et Cosmochimica Acta 27, 43–52. Coleman, R.G., 1966. New Zealand serpentinite and associated metasomatic rocks. Bulletin, New Zealand Geological Survey 76, 102.

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