Behavior of major and trace elements during weathering of sericite–quartz schist

Journal of Asian Earth Sciences 42 (2011) 1–13

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Behavior of major and trace elements during weathering of sericite–quartz schist Qingjie Gong, Jun Deng ⇑, Liqiang Yang, Jing Zhang, Qingfei Wang, Gaixia Zhang State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China

a r t i c l e

i n f o

Article history: Received 6 July 2010 Received in revised form 15 December 2010 Accepted 2 March 2011 Available online 17 March 2011 Keywords: Weathering Schist Immobile plateau REE fractionation Trace provenance

a b s t r a c t Two regolith profiles developed on the sericite–quartz schist in subtropical humid environment were selected to investigate behaviors of major and trace elements during weathering in Mengman gold deposit of Yunnan province, China. One profile located in the mining district sheared by a fault and the other was outside the mining area which represented the normal weathering profile on the schist. Regolith samples were collected in both profiles sequentially. Thirteen major oxides and 23 trace elements (including REE) were analyzed and their behaviors were compared in these two profiles. Based on the idea that immobile element is just a relative notion, we presented a method of immobile plateau to determine immobile elements during each stage in a progressive geochemical process and used mass ratio (MR) to calculate the percentage of gain or loss (Xgp) of each element during the whole process. In both profiles, only TiO2 was immobile during the whole weathering. The regolith profile formed on the mineralized schist recorded the weathering process more sensitively than the regolith profile on the normal schist. REE was mobile and fractionated during the schist weathering. LREE was loss in mass during the soil development stage which resulted from the chemical leaching, but was gain in mass during the pedogenesis stage because of the preferential absorption of soil to LREE. The LREE depletion near the fault during weathering was the collective effects of chemical leaching and physical accumulation. HFSE were all mobile in the mineralized regolith profile especially near the fault. But Nb–Ta and Zr–Hf were covariant in both profiles during the schist weathering. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Behaviors of major and trace elements during weathering are of most interest recently, and various weathering profiles on different parent rocks have been studied intensively (Condie et al., 1995; Aubert et al., 2001; Duzgoren-Aydin and Aydin, 2003; Krishnaswami et al., 2004; Caspari et al., 2006; Braun et al., 2009). The parent rocks are mainly granitoid rocks (Zheng and Lin, 1996; Nesbitt and Markovics, 1997; Panahi et al., 2000; Harlavan and Erel, 2002; Bao and Zhao, 2008) and volcanic rocks (Irfan, 1999; Patino et al., 2003; Pokrovsky et al., 2005; Little and Aeolus Lee, 2006; Di Figlia et al., 2007), secondly carbonate rocks (Walter et al., 1995; Wang et al., 1999; Ji et al., 2004a,b; Bourdon et al., 2009), as well as a few metamorphic rocks (Gardner and Walsh, 1996; Price and Velbel, 2003; Ndjigui et al., 2008; Rajamani et al., 2009). Because weathering is an important geochemical process for the formation of lateritic gold deposits, the lateritic profiles have been interested by many researchers (Braun et al., 1998; Viers and Wasserburg, 2004; Ma et al., 2007; Kamgang Kabeyene Beyala et al., 2009).

⇑ Corresponding author. Address: Faculty of Earth Sciences and Resources, China University of Geosciences, No. 29 Xueyuan Road, Haidian District, Beijing 100083, China. Tel./fax: +86 10 82322301. E-mail address: [email protected] (J. Deng). 1367-9120/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2011.03.003

The aim of this study is to document and explain the behaviors of major and trace elements during weathering of late proterozoic sericite–quartz schist in Mengman lateritic gold deposit of Yunnan province, China. Two weathering profiles were selected in this study. One located in the mining district of Mengman gold deposit which parent rock was affected by the hydrothermal ore-forming process, and the other located outside the mining area which represented the normal weathering profile. The behaviors of major and trace elements in these two profiles during weathering were discussed and a plateau method was presented to determine immobile elements for a progressive geochemical process. 2. Materials and methods 2.1. Site setting Mengman lateritic gold deposit, in Menghai County in southwest Yunnan province of China (Fig. 1a), is located in the west contact zone of Menghai granitic intrusive, in which many medium gold deposits were found in succession in the last two decades such as Manna, Xiding and Jiliang gold deposits (Fig. 1b). Mengman deposit is a large gold deposit which is divided into two areas: Guanghe district in the northeast area and Reshuitang district in the southwest area (Fig. 1c). In Guanghe district the lateritic gold

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Fig. 1. Location of the Mengman lateritic gold deposit (a, b), mining area of Mengman gold deposit and the Pin and Pout regolith profile sites (c). 1-Quaternary; 2-Cretaceous; 3-Jurassic; 4-Carboniferous; 5-Neoproterozoic; 6-Yanshanian Granite; 7-Angular unconformity; 8-Normal fault; 9-Reverse fault; 10-Speculating fault; 11-Sampling profile site; 12-Gold deposit; 13-Iron 875 deposit; 14-Copper deposit; 15-Study area.

deposit is hosted by sericite–quartz schist of the Manlai formation of the late Proterozoic Lancang group, while in Reshuitang district hosted by quartz sandstone of the middle Jurassic Huakaizuo formation. The Huakaizuo formation contacts with Manlai formation in angular unconformity. Mengman fault crosses the deposit area, which strike is northwest and dip is southwest. Some secondary faults parallel the Menghai fault and stretch from bedrock to lateritic layer in vertical profile. The highest elevation is 1211 m near the south deposit region, and lowest is 904 m at a river bed near the northeast deposit region. But the Mengman gold deposit is characterized by low gradient hills. Soils are thickly developed and controlled by the relief. The thickness of the regolith, which consists of two parts as saprolite and soil (Braun et al., 2009), may vary from 3 to 40 m depending on the relief. The gold ore body is hosted by regolith and the thickness usually varies from 5 to 25 m with an average of 17 m. The land near deposit area is covered by tea and rubber plant. The deposit area experiences a sub-tropical humid climate. The annual temperature varies from 0 °C in winter to 37 °C in summer and the mean annual temperature is 18.5 °C. This region is characterized by alternating wetter and drier periods and receives an annual rainfall of about 142 cm, about 87% between May and October. The yearly evaporation is around 170 cm and the relative moisture is about 84%.

2.2. Sampling Regolith samples were collected from two profiles formed on the sericite–quartz schist of late Proterozoic Lancang group. One

profile called Pin located in the Guanghe mining district of Mengman gold deposit (Fig. 1c), the other called Pout located on a riverside bluff outside the mining area (Fig. 1c). Samples numbered Pin1–7 were collected sequentially from topsoil downward to the saprolite in Pin profile. Pin5 and Pin6 sample sites were near a fault struck northwest and located above and below the fault, respectively (Fig. 2 left). Samples of Pout1–6 were collected in Pout profile sequentially from topsoil downward to the saprolite (Fig. 2 right).

2.3. Analytical methods Samples (about 1 kg each) were coarse ground and ball milled smaller than 250 lm for further analysis. Most major oxides (SiO2, Al2O3, Fe2O3, MgO, CaO, Na2O, K2O, MnO, P2O5 and TiO2) were analyzed by X-ray fluorescence spectrometry (XRF) on melted-powder pellets, FeO was determined by volumetric analysis, CO2 by potentiometry, and H2O+ by gravimetry. Trace elements of Mn and Zr were analyzed by XRF on pressed-powder pellets, and Hf, Nb, Sc, Ta, Th, U, Y, and rare earth elements (REE) are analyzed by inductively coupled plasma mass spectrometry (ICP-MS). The analysis results of each sample were listed in Table 1 with their detection limits. All analyses were performed at Quality Supervision and Detection Center Lab in Exploration Geochemistry of the Ministry of Land and Resources of China. The accuracy of analyses for major and trace elements including REE were monitored using GSR1, GSR2, GSD9 and GSD10, and most are to be better than 5% for major and 10% for trace elements. The relative errors of repeated sample analyses are found to be less than 10% and most are less than 5%.

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Mining piled soils

Soils

Soils

Fault Saprolite

Saprolite

Fig. 2. Sample locations in Pin (left) and Pout (right) profiles. The gray circles indicate the locations at which samples were collected, and the arrows indicate deeper sample locations of Pin 7 (left) and Pout 6 (right).

3. Results The bulk analysis of major oxides and trace elements for regolith samples in Pin and Pout profiles were listed in Table 1. 3.1. Major oxides Thirteen oxides were analyzed in this study (Table 1). Contents of MnO, P2O5, and CaO in these regolith profiles on the sericite– quartz schist were within 0.002–0.132%, 0.01–0.1% and 0.065– 0.084%, respectively, which were below or close to their detection limits in Table 1. This phenomenon was also reported by other authors. Kamgang Kabeyene Beyala et al. (2009) reported a lateritic profile formed on chlorite schist in South Cameroon. The contents of MnO, P2O5, CaO and Na2O were within 0.02–0.08%, 0.03–0.13%, 0–0.06% and 0–0.11%, respectively (close to their detection limits), which were clearly depleted relative to their parent rock with contents of 0.1%, 4.89%, 2.06% and 3.48%, respectively. But the contents of Na2O in this study are within 0.16–1.09%, which were higher than those in the regolith on chlorite schist in South Cameroon. Mn was also analyzed by XRF on pressed-powder pellets, which was better than that on melted-powder pellets. In the following discussion, P2O5 and CaO were excluded and Mn was viewed as a trace element rather than MnO. Major oxide contents of the Pin and Pout profile samples were plotted with depth on Fig. 3. The y-axes are different for Pin and Pout profile to compare the variation trends with depth due to the different soil and saprolite thicknesses in these two profiles. Fig. 3 indicates: (1) The SiO2 contents in Pin profile were higher than those in Pout, while Al2O3, TiO2, Fe2O3 MgO and K2O were lower. This may be caused by the dilution of SiO2 in Pin profile which was affected by the hydrothermal ore-forming process. (2) Variation trends of the above six oxides (SiO2, Al2O3, TiO2, Fe2O3 MgO and K2O) with depth were very similar between Pin and Pout profiles, respectively, which indicates the weathering processes in these two regolith profiles were consistent. (3) The variation trend of SiO2 with depth was opposite to that of Al2O3 in both profiles, while variation trends of Al2O3, TiO2 and H2O+ were similar. Variation trends of MgO and K2O were also very similar in both regolith profiles.

These similar variation trends of oxides with depth in both profiles were only illustrated qualitatively in Fig. 3. To describe the relations of oxides quantitatively, correlation coefficients of ten oxides were calculated and oxides with significant correlations were presented in Fig. 4. Except sample Pin1, two dominant components of SiO2 and Al2O3 were negative correlated significantly, which may be resulted from the sum effect of analysis with the intercept of 99.50 in the fitting line. Significant correlations (p = 0.01 level) also occurred between Al2O3, TiO2 and H2O+ with near zero intercepts, but their linear relations were worse than those of Al2O3–SiO2 and K2O–MgO. 3.2. Trace elements 3.2.1. High field strength elements (HFSE) Eight high field strength elements (HFSE) such as Nb, Ta, Zr, Hf, Th, U, Sc and Y were described here. Except contents of Nb and Ta in Pin7 sample close to their detection limits, all data of the HFSE in both regolith profiles were reliable. Contents of these elements in profile Pout were higher than those in the Pin profile, while their variation trends with depth were similar between two profiles by and large (Fig. 5). Elements with significant correlations after correlation analysis of elements in the two regolith profiles were presented in Fig. 6. Nb and Ta were correlated significantly in both profiles and the intercept of the fitting line was near zero (or detection limit), which indicates Nb and Ta were covariant elements during weathering in these profiles. So Zr–Hf and Y–Yb were also covariant element pairs too. These results indicate that all samples in these profiles were in situ weathered products of the underlying schist and the schist was homogeneous in this study area (Wang et al., 1999). The ratios of these covariant element pairs would deviate from their lines, respectively if other source matter was added clearly into these profiles (Gong et al., 2010). 3.2.2. Rare earth elements (REE) The content of REE in profile Pout was higher than that in profile Pin as the above HFSE, but variation trends of La and Yb with depth were different in each profile (Fig. 5). Although linear correlation of La–Yb was significant statistically at the level of 0.01, its fitting quality was worse than the others in Fig. 6, which indicated that REE were fractionated during the weathering.

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Table 1 Major and trace element data of regolith samples in profiles Pin and Pout in Mengman gold deposit. Layer

Soil

Samples Depth/m

Pin1 0.3

Pin2 1.4

Pin3 2.2

Pin4 3.9

Pin5 5.9

Pin6 8

Saprolite Pin7 11

Pout1 0.7

Pout2 1.7

Pout3 2.5

Pout4 3.8

Pout5 5.8

Pout6 11

Major oxides SiO2 Al2O3 Fe2O3 FeO MgO K2O TiO2 CO2 H2O+ Na2O CaO P2O5 MnO Mn⁄ Total REE (lg/g) La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

(%) 63.32 13.88 3.80 1.46 0.26 0.43 0.730 8.71 7.05 0.29 0.084 0.06 0.007 67 100.08

69.42 13.75 4.53 0.74 0.26 0.45 0.712 3.09 6.58 0.32 0.081 0.05 0.005 61 99.97

89.41 5.32 1.16 0.54 0.12 0.22 0.263 0.44 2.32 0.31 0.074 0.02 0.003 49 100.19

80.96 9.94 2.11 0.21 0.35 0.99 0.510 0.46 3.71 0.36 0.070 0.03 0.004 49 99.71

82.93 10.04 0.78 0.26 0.23 0.72 0.521 0.40 3.55 0.27 0.072 0.01 0.003 45 99.79

77.80 12.12 0.39 2.86 0.24 0.87 0.392 0.65 4.20 0.23 0.065 0.01 0.003 51 99.83

88.22 5.11 2.06 0.69 0.32 1.04 0.264 0.64 1.35 0.16 0.075 0.02 0.002 41 99.96

65.41 16.52 3.85 0.75 0.82 2.91 0.736 2.89 6.04 0.28 0.083 0.05 0.026 202 100.36

68.51 14.94 4.64 0.42 0.74 2.73 0.652 1.43 5.01 0.24 0.083 0.04 0.018 160 99.45

66.84 16.31 3.98 0.77 0.86 3.20 0.731 1.26 5.59 0.16 0.071 0.03 0.008 70 99.80

67.76 16.09 3.78 0.52 0.84 3.01 0.724 1.42 5.70 0.18 0.071 0.03 0.012 96 100.14

62.83 18.30 2.58 3.06 2.33 4.81 0.710 0.22 4.07 0.73 0.068 0.07 0.019 143 99.79

74.86 11.92 3.16 0.76 0.96 3.06 0.518 0.33 2.86 1.09 0.073 0.10 0.132 968 99.83

28.5 58.7 6.27 22.93 4.11 0.76 3.17 0.50 2.88 0.58 1.66 0.28 1.69 0.259

23.1 48.9 4.98 18.12 3.22 0.59 2.53 0.41 2.33 0.48 1.40 0.23 1.41 0.224

13.7 24.7 3.06 11.63 1.98 0.36 1.48 0.23 1.21 0.23 0.68 0.13 0.62 0.103

25.3 44.3 5.84 21.99 3.81 0.72 3.24 0.53 3.18 0.64 1.79 0.29 1.70 0.267

11.9 19.0 2.47 8.99 1.57 0.32 1.53 0.31 2.29 0.55 1.75 0.32 1.93 0.297

8.0 12.2 1.47 5.17 0.92 0.20 1.01 0.24 1.80 0.41 1.25 0.23 1.34 0.207

15.5 29.5 3.52 12.52 2.01 0.39 1.50 0.23 1.43 0.31 0.94 0.17 1.04 0.160

42.9 94.2 10.34 37.53 6.69 1.21 5.38 0.80 4.34 0.85 2.42 0.39 2.51 0.384

38.2 76.4 8.52 30.83 5.32 0.95 4.13 0.62 3.46 0.70 2.00 0.33 1.98 0.317

53.0 86.2 12.67 45.73 7.80 1.30 5.73 0.78 4.11 0.81 2.32 0.37 2.25 0.357

50.3 93.9 11.94 44.11 7.57 1.26 5.27 0.73 3.78 0.73 2.05 0.32 2.00 0.313

41.7 84.1 10.05 36.95 6.97 1.25 5.71 0.86 4.48 0.87 2.40 0.38 2.47 0.390

30.8 62.8 7.86 28.85 5.33 1.13 5.09 0.82 4.56 0.91 2.59 0.41 2.47 0.398

1 1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

5.4 0.321 84 2.42 6.75 0.97 4.2 6.5

8.2 0.547 148 4.79 10.15 1.77 8.7 17.8

8.3 0.581 222 6.83 5.02 1.86 5.6 16.5

6.7 0.445 126 4.24 4.34 1.39 5.4 12.4

3.6 0.205 122 3.76 5.61 1.13 3.8 8.9

15.0 1.116 218 6.49 18.77 2.52 14.6 23.8

13.5 0.999 192 5.81 17.13 2.36 13.7 19.4

15.3 1.136 215 6.57 19.28 2.40 14.8 22.7

15.2 1.096 228 6.85 17.86 2.15 13.6 19.8

17.0 1.248 116 3.38 17.82 2.42 17.2 23.8

12.5 0.895 188 5.81 13.43 2.40 9.3 26.2

2 0.2 2 0.2 1 0.2 1 1

High field strength elements (lg/g) Nb 14.2 12.8 Ta 1.085 0.944 Zr 199 203 Hf 6.30 6.34 Th 18.99 16.48 U 2.62 2.36 Sc 13.1 11.8 Y 15.3 13.2 Note:

⁄

Soil

Saprolite

Detection limit

0.1 0.1 0.1 0.1 0.05 0.05 0.05

0.05 0.05 0.05 0.05 10

Mn was analyzed by XRF on melted-powder pellets and on pressed-powder pellets, respectively the latter in lg/g was better and used in the text.

REE data were commonly presented as REE patterns and REE ratios. REE patterns of both regolith samples normalized by Upper Continental Crust (UCC) (Taylor and McLennan, 1995) were shown in Fig. 7. REE patterns of all samples in Pout profile were very similar. Although these patterns showed a very slight enrichment in LREE with respect to UCC and values of (La/Yb)N, (La/Sm)N and (Gd/ Lu)N varied from 0.91–1.84, 0.87–1.08 and 1.08–1.42, respectively, REE patterns of Pout samples were all flat. dCe and dEu varied from 0.76–1.02 and 0.91–1.02, respectively, which indicated there were no clear Ce and Eu anomalies. In profile Pin, REE patterns were also very similar except those of sample Pin5 and Pin6 located near a fault. (La/Yb)N, (La/Sm)N and (Gd/Lu)N varied from 1.09–1.64, 0.99–1.16 and 0.79–1.21, respectively. dCe and dEu varied from 0.83–1.04 and 0.97–1.06, respectively without clear Ce and Eu anomalies. So their patterns were also flat compared to UCC. REE patterns of sample Pin5 and Pin6 showed a significant fractionation of REE relative to other samples. (La/Yb)N were 0.45 and 0.43, (La/Sm)N 1.14 and 1.30, (Gd/Lu)N 0.43 and 0.41, respectively. These REE patterns of Pin5 and Pin6 samples were left-dip patterns without clear Ce and Eu anomalies (dCe were 0.80 and 0.81, respectively, and dEu was 0.98 for both samples). Such REE

fractionation in Pin5 and Pin6 samples could be resulted from heavily weathering or hydrothermal alternation near the fault, which was discussed in the following section.

4. Discussion 4.1. Immobile elements Only SiO2 contents in Pin samples are higher than those in Pout samples, the other components may be diluted by SiO2 (Fig. 3). REE were fractionated significantly in Pin5 and Pin6 samples, which may be due to the loss of LREE or gain of HREE (Fig. 7). In order to investigate the behaviors of major and trace elements during weathering and hydrothermal ore-forming process, the theory of mass transport calculation was used. Mass transport calculation was presented by Gresens (1967) to investigate element transfer during metasomatism, and popularized by Brimhall and Dietrich (1987) and Maclean and Kranidiotis (1987) in the field of weathering and hydrothermal alteration. Many other methods to calculate mass transport were also presented such as enrichment factor (EF) (Zoller et al., 1974; Gong et al., 2008), % change in ratio (Nesbitt and Markovics, 1997),

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Al 2O3

SiO 2 70

80

4

90

Depth (m) for Pout

Depth (m) for Pin

60 0

4

8

8

12

0.6

Fe 2O3 0.8

0

2

4

FeO 6

0

1

2

3

4

6

8

4

K 2O

0

0.6 1.2 1.8 2.4

0

Depth (m) for Pout

Depth (m) for Pin

8

0.4

0

MgO

4

0.2

8

12

0

TiO 2

12 16 20

1

2

3

4

5

0

2

4

6

H 2O +

Na 2O

CO 2 8 10

0

0.4

0.8

1.2

0

2

4

0

4

8 Fig. 3. Contents of major oxides in Pin (circle) and Pout (triangle) profiles were plotted with depth.

Fig. 4. Linear relations of oxides in regolith samples were fitted in scatter plots. Labeled dots were excluded for fitting.

Fig. 5. Contents of trace elements in Pin (circle) and Pout (triangle) profiles were plotted with depth.

mobility index (MI) (Ng et al., 2001) and chemical depletion fraction (CDF or CDFx) (Riebe et al., 2003), which are similar to the mass transport calculation in principle. In these methods, determination of immobile elements is the key to mass transport calculation.

4.1.1. Method of Immobile plateau Several elements have been considered as immobile elements during weathering and hydrothermal alteration such as Al2O3 (Ji et al., 2000; Duzgoren-Aydin et al., 2002; Pokrovsky et al., 2006;

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Fig. 6. Linear relations of trace elements in both profiles were fitted in scatter plots.

Guatemala, and the notion of element immobility cannot be applied to all the corestone-shell complexes (Patino et al., 2003). Therefore, determination of immobile elements is the key to mass transport calculation in the Pin and Pout regolith profiles in this study. Simulation experiment (Hodson, 2002) is a good method to determine immobile elements, but it was limited because of lacking similar simulation data. Isocon diagram presented by Grant (1986) is a simple solution to determine immobile elements, which was improved by Baumgartner and Olsen (1995) and Coelho (2006), but the linear fitting with zero intercept was dominated by the largest content value (unit ignored) of the selected immobile elements. Most authors only selected one element as immobile element, but Coelho (2006) identified three elements as ‘perfectly inert’ elements using Geoiso software and Little and Aeolus Lee (2006) determined Nb and Ta as immobile elements in a weathering profile and calculated chemical depletion fraction (CDF) using average value of Nb and Ta. According these ideas especially by Little and Aeolus Lee (2006), we present a ‘immobile plateau’ method to determine immobile elements for a progressive geochemical process as follows:

Fig. 7. REE patterns of regolith samples in Pout and Pin profiles. Data were normalized by UCC.

Das and Krishnaswami, 2007), TiO2 (Nesbitt and Markovics, 1997; Panahi et al., 2000; Braun et al., 2009), Nb (and Ta) (Brimhall and Dietrich, 1987; Little and Aeolus Lee, 2006), Zr (and Hf) (Öhlander et al., 1996; Hodson, 2002; Riebe et al., 2003; Hastie et al., 2008), Th (Braun et al., 1998; Ma et al., 2007; Ndjigui et al., 2008) and Sc (Shotyk et al., 2001). What all of these elements have in common is that they are relatively high field strength elements with limited solubilities in water (Little and Aeolus Lee, 2006). Immobile element is just a relative notion, and the mobility of element often changed during different geochemical processes. For example, Braun et al. (1998) have studied the Th behavior in a lateritic soil cover in East Cameroon and indicated that Th was immobile in the lateritic cover of the hills but its mobility was enhanced in the swampy organic-rich zones under colloidal form. In the Toorongo granodiorite weathering profile in Australia, Th was mobilized during the weathering (Nesbitt and Markovics, 1997). In the Ville Marie granite weathering profile in Canada, Zr, Hf, Nb, Ta, Ti and Th all remained immobile during chemical weathering and all subsequent alteration (Panahi et al., 2000). Among the above elements Al is actually the most mobile compared with Zr, Ti, Nb and Ta, while Ti has also been shown to be mobile (Little and Aeolus Lee, 2006). Furthermore, Zr, Ti, Al were mobilized during spheroidal weathering of basalts and andesites in Hawaii and

(1) Select Al, Ti and high field strength elements empirically as candidates for immobile elements firstly. In this study, twelve elements of Ta, Nb, U, La, Zr, Hf, Al, Th, Sc, Ti, Yb and Y were selected. (2) Select two contiguous samples during the progressive process (such as Pout1 and Pout2) and one was normalized by the other (or called reference sample) on the condition that most normalized values were lower than 1 for a better illustration in logarithm scale (such as Pout2 normalized by Pout1). (3) Draw spider diagram of candidates. In this study, spider diagrams were ordered by ascending sequence of North American Shale Composite (NASC) values (Condie, 1993) normalized by Upper Continental Crust (UCC) (Taylor and McLennan, 1995). Examples were illustrated in Fig. 8. (4) Because immobile elements or covariance elements have the same normalized values, respectively in theory, they will be depicted as plateaus in the diagram. The probably immobile elements or covariance elements can be selected from the candidates according to the plateau (or flat) points in the diagram. For example, two plateaus occurred in the diagram of Pout2 normalized by Pout1 (Fig. 8). One plateau consisted of ten elements from Ta to Ti, which were immobile elements during the weathering. The other plateau consisted of two elements of Yb and Y, which were covariant elements. A hypothesis in this method is that there must be two or more immobile elements in the weathering (or alteration), and they were also selected as candidates for immobile elements. (5) The mass ratio (MR) of two contiguous samples can be calculate as

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Pout 2/Pout 1

3 2

NASC/UCC

Pin /Pin

Pout /Pout

3 2 1

1

Ta Nb U La Zr Hf Al Th Sc Ti Yb Y

Ta Nb U La Zr Hf Al Th Sc Ti Yb Y 3 2

Pout 4/Pout 3

Pout 2/Pout 3

Pin /Pin

Pout /Pout

3 2 1

Pin 3/Pin 4

3 2

Pin /Pin

Pout /Pout

Ta Nb U La Zr Hf Al Th Sc Ti Yb Y

Pout 6/Pout 5

Pout 4/Pout 5

Pin 4/Pin 5

1

Ta Nb U La Zr Hf Al Th Sc Ti Yb Y 3 2

Pin 3/Pin 2

Pin 2/Pin 1

1

Pin 6/Pin 5

Pin 7/Pin 6

1

Ta Nb U La Zr Hf Al Th Sc Ti Yb Y

Ta Nb U La Zr Hf Al Th Sc Ti Yb Y

Fig. 8. Diagrams of candidates for immobile elements in Pout profile (left) and Pin profile (right).

Table 2 MRi and MR for probably immobile elements selected in profiles Pout and Pin.

Pout2/Pout1 Pout2/Pout3 Pout4/Pout3 Pout4/Pout5 Pout6/Pout5 Pin2/Pin1 Pin3/Pin2 Pin3/Pin4 Pin4/Pin5 Pin6/Pin5 Pin7/Pin6

Ta

Nb

U

La

Zr

Hf

Al

Th

Sc

Ti

Yb

Y

Minimum

Maximum

Average

MR

Sample

MR⁄

0.90 0.88 0.96 0.88 0.72 0.87 0.34 0.59 0.94 0.77 0.46

0.90 0.89 1.00 0.90 0.74 0.90 0.42 0.66 1.00 0.81 0.54

0.94 0.98 0.89 0.89 0.99 0.90 0.41 0.55 0.95 0.75 0.81

0.89 0.72 0.95 1.21 0.74 0.81 0.59 0.54 2.12 0.67 1.95

0.88 0.89 1.06 1.96 1.62 1.02 0.41 0.57 0.67 0.57 0.97

0.89 0.88 1.04 2.03 1.72 1.01 0.38 0.50 0.70 0.62 0.89

0.90 0.92 0.99 0.88 0.65 0.99 0.39 0.53 0.99 1.21 0.42

0.91 0.89 0.93 1.00 0.75 0.87 0.41 0.66 2.02 0.86 1.29

0.94 0.93 0.92 0.79 0.54 0.90 0.35 0.48 1.57 0.97 0.70

0.89 0.89 0.99 1.02 0.73 0.98 0.37 0.52 0.98 0.75 0.68

0.79 0.88 0.89 0.81 1.00 0.84 0.44 0.36 0.88 0.70 0.77

0.81 0.85 0.87 0.83 1.10 0.87 0.49 0.36 1.08 0.75 0.72

0.88 0.85 0.87 0.79 0.72 0.81 0.34 0.48 0.88 0.67 0.68

0.94 0.98 1.06 1.02 0.75 1.02 0.49 0.66 1.08 0.81 0.81

0.94 0.90 0.96 0.89 0.74 0.91 0.40 0.56 0.97 0.74 0.74

0.90 0.89 0.96 0.88 0.74 0.90 0.41 0.55 0.98 0.75 0.72

Pout1 Pout2 Pout3 Pout4 Pout5 Pin1 Pin2 Pin3 Pin4 Pin5 Pin6

1.22 1.10 1.24 1.19 1.35 2.71 2.44 1.00 1.81 1.85 1.39

Note: MRi in italic was excluded to calculate minimum, maximum, average and MR (or median). MR⁄ was adjusted to 1 for the initial sample (Pout6 and Pin7) in each profile.

n

n

MR ¼ Medianð MRi Þ ¼ MedianðRW =RP Þ i¼1

i¼1

ð1Þ

where Rp is the percentage by weight of the immobile element i in the reference sample, Rw is the percentage by weight of the immobile element i in the other sample, n is the count of probably immobile elements selected. The median is used for MR because of its statistical steady than the average. Because the reference sample was varied in each pair samples, the MR can be adjusted at last by the parent sample, that is, the MR of parent sample is adjusted to 1. (6) The mobility index (MI) and the percentage of gain or loss (Xgp) of element i presented by Ng et al. (2001) can be calculated as

MIi ¼ ðRP RiW Þ=ðRW RiP Þ ¼ ðRiW =RiP Þ=MR

ð2Þ

X gpi ¼ 100ðMIi  1Þ

ð3Þ

Rip

where is the percentage by weight of the mobile element i in the parent sample, Riw is the percentage by weight of the

mobile element i that remains in the weathered (or altered) product by Ng et al. (2001). In this study the method of immobile plateau was used to determine immobile elements in profile Pout and Pin. 4.1.2. Determination of MR during weathering in both profiles According to immobile plateau method, twelve elements were selected as candidates for immobile elements firstly, and their spider diagrams in each profile were illustrated in Fig. 8. In spider diagram of Pout2 normalized by Pout1 in profile Pout, ten elements were selected as immobile elements and the mass ratio (MR) was calculated by Equation 1 (Table 2). In spider diagram of Pout2/Pout3, La was omitted from candidates for immobile elements clearly. In diagram of Pout4/Pout3, all elements were used to calculate MR. In diagram of Pout4/Pout5, Zr and Hf formed a plateau which were covariant elements. La was omitted from candidates clearly. The other nine elements were used to calculate MR. In diagram of Pout6/Pout5, Zr and Hf were excluded firstly. Then Sc, U, Yb, Y and Al were omitted in further steps. Finally the other five elements were used to calculate MR.

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Table 3 Qualitative descriptions of element mass transport during weathering. Qualitative descriptions

Mass loss (mobile) Extreme Intense Moderate Present mass in parent mass

Weak

Slight loss

Perfect constant

Slight gain

Weak Moderate Intense Parent mass in present mass

Extreme

Mass percentage Range of Xgp

<20 <80

60–80 40 to 20

80–90 20 to 10

90–100 10 to 11

90–80 11–25

80–60 25–67

<20 >400

20–40 80 to 60

Mass constant (immobile)

40–60 60 to 40

In profile Pin, MR was calculated for each pair of samples according to the above steps. The results were listed in Table 2. It is the diagram of Pin7/Pin6 that was needed to be explained. Firstly Ta and Nb were excluded because their contents were close to their detection limits. Secondly La, Al and Th were omitted in the diagram. Zr and Hf were also excluded in a further step. Therefore Sc, Ti, Yb, Y and U were selected to calculate MR. In Table 2, we can find that Ti was the only immobile elements during each progressive process of weathering and Ta, Nb were also immobile elements except the Pin7 sample in which their contents were close to their detection limits. So Ti, Ta and Nb could be immobile elements during not only the weathering but also the hydrothermal ore-forming process. Here we used the median value (Equation 1) to calculate MR in each stage. According to the calculated MR in Table 2, we selected Pout6 and Pin7 as initial (or parent) sample for each profile, respectively and adjusted the MR values (Table 2). 4.1.3. Qualitative description of Xgp The percentage of gain or loss (Xgp) of element i can be calculated by Equation 3. In order to discuss the behavior of elements conveniently, we presented the qualitative classification in Table 3. Elements can be mainly divided into three groups as loss, constant and gain in mass. Four mobile degrees were classified according to the percentage of present mass in parent mass for the loss group, and corresponding four mobile degrees were also presented according to the percentage of parent mass in present mass for the gain group. The 80% was selected to confine mass constant or immobile elements because of the analysis and sampling errors, and the 90% was further selected to divide the mass constant or immobile elements into three types as slight loss, perfect constant and slight gain. 4.2. Behavior of major oxides The percentage of gain or loss (Xgp) for mass transport was used to describe the behavior of major oxides during weathering. The percentage of gain or loss (Xgp) of major oxides were calculated by Equation 3 using the adjusted MR in Table 2, shown in Fig. 9. 4.2.2. Profile Pout During the saprolite development stage in Pout profile (from Pout6 to Pout5), SiO2, Fe2O3, Na2O, CO2 were loss in weak to moderate degree, while MgO, FeO were gain in moderate to intense degree. TiO2 and H2O+ were perfect constant, but Al2O3, K2O were slight gain in mass. These slight gains of Al2O3, K2O and clear gains of MgO, FeO were consistent with their higher contents of MgO, K2O, FeO, Al2O3 in Pout5 sample (Fig. 3), which may be resulted from more sericite in Pout5. During the transition stage from saprolite to soil (from Pout5 to Pout4), K2O, MgO, Na2O, FeO were loss from weak to moderate, intense and extreme degree, respectively. H2O+ and Fe2O3 were weak gain, and CO2 was extreme gain. Al2O3 was perfect constant and TiO2, SiO2 were slight gain in mass. When the reference sample changed such as from Pout6 to Pout5, the new Xgp of Pout4 sample

Mass gain (mobile)

60–40 67–150

40–20 150–400

(Xgp4New) relative to the new reference sample, Pout5, was calculated as

X gp4New ¼ 100½ðX gp4 þ 100Þ=ðX gp5 þ 100Þ  1

ð4Þ

where Xgp4 and Xgp5 were the old Xgp values of Pout4 and Pout5 samples relative to the old reference sample such as Pout6. During the soil development period (from Pout4 to Pout1), SiO2, K2O, MgO, Al2O3, TiO2 and H2O+ behaved constant in mass, while Fe2O3, FeO, Na2O and CO2 seemed mobilized. 4.2.3. Profile Pin During the saprolite development stage in profile Pin (from Pin7 to Pin6), SiO2, Fe2O3, CO2 were mass loss like their behaviors in profile Pout, while Na2O was perfect constant rather than moderate loss in Pout profile. MgO, K2O were loss unlike their gain in Pout profile. TiO2 was perfect constant like in Pout profile. H2O+, Al2O3, FeO were gain in moderate to intense degree. During the transition stage from saprolite to soil (from Pin6 to Pin5) in Pin profile, K2O, MgO, Na2O, FeO were loss like their behaviors in Pout profile. Al2O3, H2O+ and CO2 were clear loss in mass rather than clear gain in profile Pout. Fe2O3 was weak gain and TiO2 was perfect constant in mass like their behaviors in profile Pout. SiO2 was weak loss rather than slight gain in Pout profile. Because the Pin5 sample near the fault in profile Pin, the soil development stage was described by soil samples from Pin4 to Pin1. In this stage, K2O and MgO were progressive loss. SiO2 and Na2O were also progressive loss except the Pin3 sample. Al2O3 and TiO2 were perfect constant. H2O+, Fe2O3, FeO and CO2 were progressive gain in mass. 4.2.4. Comparison with two profiles During weathering, only TiO2 was perfectly constant in mass in profile Pin, while both TiO2 and Al2O3 were mass constant in profile Pout. In profile Pin, the Xgp of SiO2 and Na2O were higher in sample Pin3 than other soil samples. In profile Pout, this occurred in sample Pout2 for SiO2 and Fe2O3. These phenomena may be resulted from more quartz gravel (or nodules) in this layer. In sample Pin6, Al2O3, H2O+, FeO were gain in moderate to intense degree, but K2O, MgO were moderate loss in mass, and Na2O was constant. In sample Pout5 which was the counterpart of Pin6 in Pin profile, Al2O3, H2O+, K2O were constant, but FeO, MgO were intense to moderate loss in mass, and Na2O was moderate loss. In this layer more sericite occurred in both profiles. If we set sample Pout5 as the criterion in this layer, K2O and MgO must be heavily loss in sample Pin6 which can be verified by the fact that sample Pin6 located near a fault (Fig. 2), while Al2O3, H2O+, Na2O (or SiO2) must be input during the weathering. During the soil development stage of weathering in profile Pin, TiO2, Al2O3 and CO2 behaved similarly as in profile Pout. While SiO2, MgO, K2O were loss progressively and H2O+ was gain progressively unlike their perfect constant behavior in profile Pout. If we selected the Pout profile as the weathering criterion (or normal weathering) in Mengman gold deposit area which hadn’t been affected by ore-forming progress, the component behaviors of progressive loss or gain in mass during soil development stage of profile Pin must be resulted from the ore-forming process, which

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Q. Gong et al. / Journal of Asian Earth Sciences 42 (2011) 1–13

X pg

Depth (m) for P out

-40 0

X pg

0

40

80

-80 -40

0

X pg 40

0

80

160

320

480

640

4

8

MgO TiO 2

K2O Al2O3

Fe2O3 SiO2 Na2O H2O

X pg

Depth (m) for Pin

0

-80 -40

0

FeO

CO2

X pg 40

80

-80 -40

0

X pg 40

80

-80

0

80 160 240 320 400

4

8

12

MgO TiO 2

K2O Al2O3

Fe2O3 SiO2 Na2O

FeO H2O

CO2

P

Fig. 9. Xgp of major oxides in Pout and Pin profiles were plotted with depth.

Fig. 10. REE patterns normalized by initial samples in Pout and Pin profiles.

had input soluble components such as SiO2, K2O, MgO, Na2O etc. into the system. These soluble components were leached progressively during the soil development stage in profile Pin. While soils in profile Pout weren’t affected by ore-forming progress and no soluble components were input. Furthermore, their sedimentary-metamorphic parent rock, schist, had been weathered before formation. Therefore, soils of profile Pout were less sensitive to weathering.

4.3. Behavior of REE 4.3.1. Fractionation of REE during weathering Fractionation of REE was commonly described by the REE patterns and REE ratios. In order to investigate REE fractionation during weathering, the initial sample during the whole weathering or the stage weathering in each profile was used

to normalize the REE data in REE patterns, which were plotted in Fig. 10. When the initial sample Pout6 was selected to normalize the other samples in profile Pout, REE patterns were almost flat with a slight enrichment of LREE. Values of (La/Yb)N, (La/Sm)N and (Gd/Lu)N varied from 1.35–2.02, 1.03–1.24 and 1.02–1.31, respectively. dCe and dEu varied from 0.83–1.11 and 0.89–0.94, respectively. During the pedogenesis stage of weathering (from Pout6 to Pout4), enrichment of LREE in soil sample (Pout4) with (La/ Yb)N = 2.02 was clearer than that in saprolite sample (Pout5) with (La/Yb)N = 1.35, which indicated pedogenesis must be the main cause for the REE fractionation with LREE enrichment. During the soil development stage of weathering (from Pout4 to Pout1), REE patterns normalized by Pout4 sample were a slight left-dip. Values of (La/Yb)N were 0.94, 0.77 and 0.68 for Pout3, Pout2

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and Pout1, respectively, which indicated the left-dip pattern was clearer with heavier soil development. Therefore, soil development must be the main cause for the REE fractionation with LREE depletion. In profile Pin, REE patterns of Pin6 and Pin5 normalized by Pin7 were left-dip clearly and values of (La/Yb)N were 0.40 and 0.41, respectively. These REE fractionations may be caused by the fault weathering effect. REE pattern of Pin4 was left-dip slightly and pattern of Pin3 was a little right-dip. During the subsequent soil development stage of weathering, LREE was depleted clearly like those in profile Pout. Comparing REE fractionations in both profiles during weathering, we inferred LREE were enriched during the pedogenesis stage and then depleted during the following soil development stage. The above REE fractionations were also found in other regolith profiles. Kamgang Kabeyene Beyala et al. (2009) reported a lateritic profile developed on chlorite schist in South Cameroon. If REE data were normalized by their parent rock, chlorite schist, the REE pattern of sample ON01 which was the only massive saprolite sample was a little LREE enrichment, and REE patterns of the other regolith samples were all LREE depletion (Fig. 11a). Oliva et al. (1999) reported two pit soil profiles in a small tropical watershed in Cameroon. REE patterns of the PZS soil profile were illustrated in Fig. 11b normalized by the deepest sample called 230s and REE patterns of all soils were LREE enrichment. REE patterns of the upper soils were LREE depletion when the sample in 150 cm depth called 150s was used to normalize the upper soils (Fig. 11c). The other profile HG5L6 was discussed by Oliva et al. (1999, in their Fig. 5) and Viers and Wasserburg (2004, in their Fig. 10) and the similar results were presented. These results indicated that REE were fractionated during weathering. LREE were enriched during the saprolite stage of weathering and then were depleted during the soil development stage. Hodson (2002) presented a soil leaching experiment finding that LREE was leached out preferentially. Harlavan et al. (2009) reported leaching experiments of soil with different ages finding extracts of young soils such as profile 29 with age 2.0 ka was enriched in LREE (Fig. 11d). These experiments suggested LREE was leached preferentially and the residual soil would be depleted in LREE, and this dissolved phase leaching was called chemical leaching in this study.

However, Harlavan et al. (2009) reported extracts of old soils from profile 5 with age > 300 ka was depleted in LREE (Fig. 11e), leading the LREE enrichment in the residual soils. Aubert et al. (2001) reported REE fractionation in granite (as parent rock), soils (only soil 5 cm and 30 cm listed here), suspended load (4108/ 33Susp) of the stream water (4108/33Leach) and spring water (CR) (Fig. 11f). Normalized by the granite, soils were enriched in LREE especially in the finest fraction of soils, but spring water (CR), stream water (4108/33Leach) and its suspended load (4108/ 33Susp) which separated from the soils were clearly depleted in LREE. These results could be attributed to the preferential absorption of soil to LREE. According to the above analysis, we proposed LREE enrichment during the pedogenesis stage in these regolith profiles could be resulted from the preferential absorption of soil to LREE and LREE depletion during the soil development stage of weathering could be attribute to the preferential chemical leaching of solution to LREE. If the suspended particles or top soils, which depleted in LREE by heavily chemical leaching, moved and accumulated near the fault in profile Pin during weathering, samples Pin6 and Pin5 which located near the fault would be depleted in LREE but were gain in mass on components of Al2O3, H2O+, Na2O (or SiO2). This coincided with the facts observed in samples Pin6 and Pin5. This mechanism of LREE depletion, which resulted from the physical accumulation of suspended particles or top soils, was called physical accumulation in this study. Therefore, LREE depletion of samples Pin6 and Pin5 in profile Pin were the collective effects of chemical leaching and physical accumulation.

4.3.2. Mass transport of REE during weathering The percentage of gain or loss (Xgp) of REE was calculated by Equation 3 using the adjusted MR in Table 2 and illustrated in Fig. 12. In profile Pout, 14 rare earth elements were divided into three groups according to their behaviors. The first group consisted of La to Sm, the second group included Eu and Gd, and the third was composed of Tb to Lu. In profile Pin three groups were divided too. The first group consisted of La to Eu, the second Gd and Tb, and the third Dy to Lu, respectively. In fact, the second group was only the transition elements from LREE to HREE. Therefore, only LREE

d a e b f c Fig. 11. REE patterns in different regolith profiles. REE data were from Kamgang Kabeyene Beyala et al. (2009) for a, Oliva et al. (1999) for b and c, Harlavan et al. (2009) for d and e, and from Aubert et al. (2001) for f.

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X pg -20

0

20

4

-60

-40

-20

0

20

40

4

8

12

8

Tb Er

-80 0

40

Depth (m) for Pin

Depth (m) for Pout

-40 0

X pg

Dy Ho Tm Yb Lu

Eu Gd

La Ce Pr Nd Sm

La Ce Pr Nd Sm Eu

Gd Tb

Dy Ho Er Tm Yb Lu

Fig. 12. Xgp of REE in profile Pout and Pin were plotted with depth.

and HREE were discussed here, the former was represented by La to Sm and the latter was Ho to Lu typically. During the saprolite development stage of weathering in profile Pout, LREE was perfect mass constant but HREE was weak loss in mass. This result can be explained using the preferential absorption of soil to LREE. Then HREE was perfect constant in mass during the following weathering. During the transition stage from saprolite to soil (from Pout5 to Pout4), LREE was weak gain in mass, which may be attributed to the accumulation of LREE leached from the upper soils. LREE was progressive loss in mass by and large during the upper soils resulted from the chemical leaching. In profile Pin, during the soil development stage of weathering (from Pin4 to Pin1), REE behaved similarly and was progressive loss in mass like in profile Pout. But REE fractionation was not clear in Pin profile which may be attributed to the superimposition of mineralization. In sample Pin6 and Pin5, LREE were intense loss in mass and HREE were perfect constant relative to the initial sample (Pin7), which resulted from the preferential chemical leaching of LREE. If we set profile Pout as the weathering criterion in this area, HREE was weak gain in mass and LREE was intense loss in samples Pin6 and Pin5. Because the chemical leaching process couldn’t result in the gain of HREE, Al2O3, H2O+, Na2O (or SiO2) in samples Pin6 and Pin5, the physical accumulation must occur near the fault during the weathering, which resulted in the depletion of LREE.

4.4. Behavior of HFSE The percentage of gain or loss (Xgp) of HFSE were calculated by Equation 3 using the adjusted MR in Table 2 and illustrated in Fig. 13.

4.4.2. Nb and Ta In profile Pout, Xgp of Nb and Ta ranged from 1.4% to 2.3% and from 0 to 3.3%, respectively and kept perfectly constant in mass during weathering. However, in profile Pin, Nb and Ta were mobile. Contents of Nb and Ta in sample Pin7 were close to their detection limits, which indicated Nb and Ta were depleted relative to other samples. If we ignored samples Pin6 and Pin5 located near the fault, Nb and Ta gained progressively in mass. Therefore, Nb and Ta were overmuch gain in mass in samples Pin6 and Pin5.

4.4.3. Zr and Hf In profile Pout, Xgp of Zr and Hf ranged from 8% to 2% and from 9% to 0%, respectively and were perfectly constant during weathering except sample Pout5, in which Xgp were 54% and 57%, respectively (indicating a moderately depletion). Except sample Pin5 in profile Pin, Zr and Hf lose progressively in mass during weathering.

Fig. 13. Xgp of HFSE in profile Pout and Pin were plotted with depth.

4.4.4. Th In profile Pout Th was constant in mass during the whole weathering although slight gain in mass occurred during the transition of saprolite to soil. Th was also immobile in profile Pin except samples Pin6 and Pin5, in which Th was overmuch lose in mass. 4.4.5. U In profile Pout U depleted weakly during the saprolite development and then kept constant in mass during the following weathering. In profile Pin U was constant in mass during the whole weathering. 4.4.6. Sc In profile Pout Sc was weak gain during the saprolite development and then kept constant in mass during the following weathering. In profile Pin Sc also behaved like in profile Pout during weathering except samples Pin6 and Pin5, in which Sc depleted overmuch in mass relative to the corresponding layer in profile Pout. 4.4.7. Y In profile Pout, behavior of Y was similar to that of U during the whole weathering. In profile Pin, Y was constant in mass from Pin7 to Pin4, and then lost weakly in mass. If we set profile Pout as a weathering criterion in this area, Y must gain in mass

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in samples Pin6 and Pin5 relative to their corresponding layers in profile Pout. In a word, Nb, Ta, Th were constant in mass or immobile in profile Pout during the whole weathering, but they were all mobile in profile Pin. The difference must be resulted from the superimposition of hydrothermal ore-forming process on weathering. If profile Pout was set as a criterion during weathering, Th, Sc were lose and Nb, Ta, Zr, Hf, U, Y were gain in mass in samples Pin6 and Pin5 which located near the fault. The gain in mass of these insoluble elements can’t be explained by the chemical leaching mechanism, but the physical accumulation of suspended particles in soil solution near the fault could lead to these gains in mass. 5. Conclusion Determination of immobile elements is the key to mass transport calculations during weathering and hydrothermal alteration. But immobile element is just a relative notion, and the mobility of element often changed in different environments. Immobile plateau method was presented to determine immobile elements in each stage of a progressive geochemical process rather than during the whole process. During the whole weathering of both sericite–quartz schist regolith profiles located inside and outside of the mining district area, respectively, only TiO2 was immobile among the major oxides. REE was mobile and fractionated during weathering. LREE lose in mass during the soil development resulted from the chemical leaching mechanism. While LREE enriched in the pedogenesis stage attributed to the preferential absorption of soil to LREE. The LREE depletion near the fault during weathering was the collective effects of chemical leaching and physical accumulation. Although Nb and Ta were perfectly mass constant during the whole weathering in the normal schist regolith profile, HFSE were all mobile in the regolith profile on mineralized schist especially near the fault. Nb–Ta and Zr–Hf were covariant mobile elements, respectively during weathering in the subtropical humid environment. Acknowledgments This research was supported by the National Basic Research Program of China (973 Program) (2009CB421006) and the State Key Laboratory of Geological Processes and Mineral Resources (GPMR200843). References Aubert, B., Stille, P., Probst, A., 2001. REE fractionation during granite weathering and removal by waters and suspended loads: Sr and Nd isotopic evidence. Geochimica et Cosmochimica Acta 65, 387–406. Bao, Z., Zhao, Z., 2008. Geochemistry of mineralization with exchangeable REY in the weathering crusts of granitic rocks in South China. Ore Geology Reviews 33, 519–535. Baumgartner, L.P., Olsen, S.N., 1995. A least–squares approach to mass transport calculations using the isocon method. Economic Geology 90, 1261–1270. Bourdon, B., Bureau, S., Andersen, M.B., Pili, E., Hubert, A., 2009. Weathering rates from top to bottom in a carbonate environment. Chemical Geology 258, 275– 287. Braun, J.-J., Descloitres, M., Riotte, J., Fleury, S., Barbiéro, L., Boeglin, J.-L., Violette, A., Lacarce, E., Ruiz, L., Sekhar, M., Mohan Kumar, M.S., Subramanian, S., Dupré, B., 2009. Regolith mass balance inferred from combined mineralogical, geochemical and geophysical studies: Mule Hole gneissic watershed, South India. Geochimica et Cosmochimica Acta 73, 935–961. Braun, J.-J., Viers, J., Dupré, B., Polve, M., Ndam, J., Muller, J.-P., 1998. Solid/liquid REE fractionation in the lateric system of Goyoum, East Cameroon: the implication for the present dynamics of the soil covers of the humid tropical regions. Geochimica et Cosmochimica Acta 62, 273–299. Brimhall, G.H., Dietrich, W.F., 1987. Constitutive mass balance relations between chemical composition, volume, density, porosity and strain in metasomatic

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