tracing the source of ‘metals in soil gas’ with pb isotope ......4 x. yang et al. geological...

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Geochemistry: Exploration, Environment, Analysis

Published online October 8, 2014 doi:10.1144/geochem2013-239 | Vol. 15 | 2015 | pp. 3 –11

Most of the world’s shallow or outcropping mineral deposits have already been discovered. It has become increasingly common for geologists to drill through 100 m or more of exotic overburden in search for buried mineral deposits. Although common gases such as carbon dioxide, methane, sulphur dioxide, hydrocarbons and mer-cury are known to be more abundant in soils above some deposits and the collection and measurements of these gases can be used as one tool for mineral exploration under exotic overburden (McCa-rthy et al. 1986; Lovell et al. 1983), the migration mechanism(s) of halogens and volatile metal compounds from deeply buried ore bodies to the surface has been controversial (Clark et al.1997). The ‘Metals in Soil Gas’ technique (MSG), studied by Chinese geo-chemists, has been used for detecting concealed mineralization of Au, Cu, Zn, Pb, Mo and Ni-Cu sulphides in the past twenty years (Ren et al. 1995; Wu et al. 1996; Liu et al. 1997; Tong et al. 1998; Wang et al. 1999, 2008; Xie et al. 1999; Gao et al. 2011).

Chinese researchers also paid much attention to the formation mechanism by which metals migrate in gaseous form from deeply buried ore deposits to near-surface soil. Ren et al. (1995) first put forward that metal and nano-metal elements from ore deposits might enter the ‘Geogas’ flows in the form of nanoparticles. Yin et al. (1997) thought that Geogas might migrate as an aerosol on N2, O2, Ar, CO2, SO2, Hg and small amounts of hydrocarbons. Tong et al. (1998) and Liu & Tong (2009) provided evidence that Geogas migrates as nanoparticles using simulation experiments. However, there is still much debate regarding the source of the metallogenic materials and the formation of these MSG anomalies. Therefore, there is a critical need to continue studying this technique.

In recent years, with the use of a clean laboratory, liquid collec-tors and ICP-MS (Inductively Coupled Plasma-Mass Spectrometry), the elemental concentrations of Cu, Pb, Ni, Ag and Bi in blank liq-uid collectors can be lower than 1 ppb and good responses can be observed over concealed deposits (Liu et al. 2003; Wang et al.

2008; Gao et al. 2011). In particular, the Geogas survey over the Jiaolongzhang deposit clearly highlighted the deeply buried miner-alization (Liu et al. 2003; Wang et al. 2008; Gao et al. 2011) and demonstrated the superiority of this approach to both total (no response) and partial extraction (mixed responses) of near-surface soils (Wang et al. 2008). Thus, Jiaolongzhang is a reasonable study area to trace the source of materials in MSG samples with an iso-tope tracing technique.

Lead is an important metallogenic element, and is an effective indicator for sulphide deposits. Three of the four Pb isotopes except 204Pb (stable) are radiogenic and have increased in abundance as a function of the decay rate of their parent isotopes since the forma-tion of the Earth; 206Pb, 207Pb and 208Pb are derived from 238U, 235U and 232Th, respectively. The use of Pb isotopes as geochemical trac-ers is based on the principles: (1) for the three radiogenic isotopes of Pb, the half-lives of the parent isotopes are relatively short and are very different from each other; (2) the chemistry of Pb is quite different from that of its parents, U and Th which leads to large variations in 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios; (3) the Pb isotope ratios of an ore body do not change during the transition to a secondary weathering environment; and (4) different types of ore deposits have distinct Pb isotope ratios or signatures (Gulson et al. 1992; Rosman et al. 1997). An example of using Pb isotopes as tracers is the Pb isotopic study of selective extractions of surficial soils over a VMS site as reported by Hall et al. (2004, 2005), which strongly suggests that Pb was transported into the exotic glacial overburden from the concealed sulphides beneath.

In this work, the authors applied the Pb isotope technique to trace the possible sources of Pb in MSG anomalies samples over the Jiaolongzhang deposit and to build a relationship between sur-face MSG anomalies and concealed mineralization by determining the Pb isotope ratios in MSG samples and different media includ-ing surface soils, wall rocks and sulphide ores.

Tracing the source of ‘Metals in Soil Gas’ with Pb isotope ratios at the Jiaolongzhang base metal deposit, north-western China

Xu Yang1, Wang Mingqi1*, Gao Yuyan2 & Zhang He1

1 China University of Geosciences, Beijing 100083, China2 China Railway Resources Exploration Co., Ltd, Beijing 100074, China*Correspondence: [email protected]

Abstract: The ‘Metals in Soil Gas’ (MSG) survey has proven to be a useful tool for mineral exploration under exotic overburden. Tracing the source of these metals with Pb isotopes is helpful to understand the formation of MSG. Lead isotope ratios in MSG samples were determined by ICP-MS (Model HP4500); the Pb isotope ratios in loess, red soil, wall rocks and ores were measured following decomposition and separation using a VG-354 thermal ionization mass spec-trometer (TIMS) for comparison. The results of the study of samples collected over the Jiaolongzhang base metal deposit show that the Pb isotope ratios of MSG background samples are markedly distinct from those ratios of any medium (loess, red soil layer, wall-rocks and ores) in the vicinity of the deposit. The Pb isotope ratios in the MSG anomalous samples (206Pb/204Pb = 18.34–18.56, 207Pb/204Pb = 15.622–15.809 and 208Pb/204Pb = 38.184–38.691) are totally different from those in the samples of background areas (206Pb/204Pb = 16.46–17.68, 207Pb/204Pb = 13.985–14.945 and 208Pb/204Pb = 34.199–36.884). The Pb isotope ratios of MSG anomalous samples scatter near the ratios of the mineralized wall-rocks (206Pb/204Pb = 18.554–18.874, 207Pb/204Pb = 15.618–15.755 and 208Pb/204Pb = 38.629–39.126) and sulphides (206Pb/204Pb = 18.130–18.251, 207Pb/204Pb = 15.671–15.767 and 208Pb/204Pb = 38.350–38.582). It can be concluded that some of the Pb in MSG anomalous samples originates from deep sulphide mineralization and Pb isotope ratios of MSG anomalous samples indicate that an MSG survey can detect the deeply concealed mineral deposits under exotic cover.

Keywords: Metals in soil gas, Pb isotope ratios, source of MSG

Received 8 October 2013; revised 3 January 2014; accepted 5 January 2014

2013-239research-articleResearch ArticleXXX10.1144/geochem2013-239X. Yang et al.Tracing the source of ‘MSG’ with Pb isotope ratios2014

Research article

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X. Yang et al.4

Geological Setting

Aeolian loess covers over 600 000 km2 in China. Conventional surface geochemical techniques have not been successfully employed in these areas. The Jiaolongzhang base metal deposit is located in a loess-covered area of eastern Gansu, north-western China (Fig. 1).

The polymetallic mineralization zones, which are c. 3600 m in length and 200 m in width, are 300–500 m in depth and are totally covered by loess (50–80 m) and the half-solidified red soil layer (20–50 m, deposited as lake sediment below the loess) (Ren et al. 1995; Wang et al. 2008; Gao et al. 2011). The overburden, espe-cially the loess, is quite geochemically homogeneous with only minor variability in concentration for most of the elements (Gao et al. 2011).

The buried ore bodies contain mainly Cu, Pb-Zn and pyrite. The bedrock is composed mostly of intermediate and acidic marine volcanics, pyroclastics and felsic sandstone, and is exposed only at the bottom of gullies. Intrusive rocks are mostly granodiorite and plagiogranite porphyry. The host rock is chlorite-quart sandstone and limestone and the main sulphide minerals are pyrite, sphaler-ite, galena and chalcopyrite.

To understand the distribution of metallogenic elements in the bedrock and select the indicators for the MSG survey, composite samples of the drill-holes (ZK22, ZK20, ZK33 and ZK28) along Line 48 were systematically sampled at intervals of 5–10 m in bar-ren wall-rock and 2–5 m around the ore bodies (Ren & Zhang 1990). Lead, Zn, Ag, As, Sb, Hg, Mo and Cu exhibit marked anomalies around the ore bodies on Line 48 (Fig. 2). The lithogeo-chemical results show that the orebodies were deeply eroded before the Neogene and anomalous concentrations of Pb, Zn, Ag, As, Sb Cu, Mo and Hg in the bedrock surface are helpful for them to migrate up through the overburden and form strong MSG responses in the top soil.

Methods

Sampling

Active sampling with liquid collectors was used for MSG sample collection; the procedures were introduced in detail by Wang et al. (2008). When sampling, a cone-shaped sampler was pushed 40–60 cm into the soil, and the 'soil gas' was pumped by a battery-operated pump to enter the liquid collector (15 ml of 3% nitric

acid) through a silica gel tube and the Millipore filter (0.45 μm). To obtain enough Pb for isotopic analysis, the pumping lasted 2 min-utes with a total of 5 l of gas pumped through at each hole. At each sampling point, composite samples were collected from three holes at an interval of 2–3 m to accumulate more metals and improve the sampling reproducibility. The samples were collected at 20-m intervals over mineralized bodies and 40-m intervals over non-mineralized bodies (Fig. 3). According to previous research, a total of 14 samples including background samples (9) and anoma-lous samples (5) were collected along a 700-m long line in a cross-section (Line 48) through the Jiaolongzhang deposit for the measurement of Pb isotope ratios.

The samples of host rocks, ores and red soil were collected from boreholes and the loess was sampled at a depth of 30–40 cm cor-responding with the MSG sampling sites. Average contents of major metallogenic elements of Cu, Pb and Zn in different media are shown in Table 1.

Analysis

MSG samplesThe concentrations of the blank (Table 2) and MSG samples (Table 3) were determined using ICP-MS in the Central Laboratory of the IGGE (Institute of Geophysical and Geochemical Exploration) for more than 40 elements including Cu, Ag, Pb, Zn, Bi, and the REE.

The Pb isotope ratios in the MSG samples were determined using the HP 4500 ICP-MS at the KLAS (the Key Laboratory of Analytical Sciences) of the Ministry of Education of China, Xiamen University. Numerous experiments had to be done to determine the Pb isotope ratios of MSG samples with ultra-low Pb contents; the Pb content in MSG background samples is normally only 0.n-n ng ml−1 and the content in MSG anomalous samples is just n-10n ng ml−1 (few samples can reach up to 100n–1000n ng ml−1).

Optimization studies have been performed to yield a sufficient precision (RSD < 0.5%) in combination with a reasonable measure-ment time for the high concentration of Pb samples. This study was carried out using the NIST standard reference material SRM981 with certified Pb isotope ratios. In ICP-MS, the precision of isotope ratio measurements is controlled by the counting statistics. The number of integrated counts of a measurement depends on the actual count rate per time unit and the integration time. Within certain lim-its, low counting rates (low concentrations) can be compensated for

Fig. 1. Geological sketch map and location of the profile at the Jiaolongzhang base metal deposit, Gansu Province, China.



Tracing the source of ‘MSG’ with Pb isotope ratios 5

by longer integration times. A compromise had to be found between a reasonable measurement time (and consumption of sample solu-tion) and sufficient precision of the measurement.

For the instrumental optimization to attain good precision of Pb isotope ratios, the standard reference material SRM 981 was meas-ured under various conditions. As a compromise between preci-sion and further improvement using a longer measurement time, the following parameters were chosen: 20 ms dwell time, 16 ns dead time, and 1000 sweeps. To evaluate the precision, SRM981 at 5 ng ml−1 was analysed for Pb isotope ratios under the above opti-mized conditions; the results are shown in Table 4. It can be seen that the mean RSD for 207Pb/206Pb under optimized conditions was found to be c. 0.2% at a solution concentration of 5 ng ml−1 Pb.

After optimization, different concentrations of MSG samples were run with the chosen operating conditions. The ratio of 205Tl /203Tl was measured and the Pb isotope ratios were corrected for possible mass discrimination using the invariant ratio 205Tl/203Tl =2.3871.

Mineral samplesThe samples of loess, red soil and host rocks were digested in HF and the sulphide ores were dissolved in aqua regia and reduced to dryness. The residues were re-dissolved in 0.5M HBr and centri-fuged to yield supernatant. The total procedural blank of Pb is less than 2 ng.

Anion exchange techniques (Kraus & Moore 1953; Strelow 1978) were applied in separating Pb from the sample matrix.

Fig. 2. Distribution of elements in a cross-section (Line 48) through the Jiaolongzhang base metal deposit, Gansu Province, China.



X. Yang et al.6

Fig. 3. Geogas signatures of Bi, Zn, Pb, Cu along Line 48 over ore bodies in the Jiaolongzhang base metal deposit, Gansu Province, China.




AG-1 X8 (hydrobromic acid form, 200- to 400-mesh) anion exchange resin was utilized for Pb separation in an ultra-clean laboratory. Quantitative Pb recoveries were observed from the column separation. After enrichment and purification, Pb isotope ratios were determined using the VG Elemental Model 354 ther-mal ionization mass spectrometer (TIMS) at the Institute of Geophysics, Chinese Academy of Sciences.

Results

The concentrations of indicators in MSG samples

Marked responses are spatially associated with the known mineral-ized zone covered by thick transported loess for the elements including Cu, Pb, Zn, Cd, Ag, Bi, Ni, Sb, Tb, Tl and Yb. Elemental distributions for Cu, Pb, Zn and Bi in this MSG survey along Line 48 are shown in Figure 3. Responses for Cu, Pb and Bi are particu-larly strong over mineralization; that for Zn is a one-point anomaly over a noisy background.

It is evident that Pb contents in MSG background samples vary from 0.8 to 3.4 ng ml−1 and those in most of the MSG anom-alous samples are greater than 10 ng ml−1 (the highest content is 1480 ng ml−1) (Fig. 3). Thus, Pb is a good indicator for the buried base metal mineralization and the study of Pb isotopes could decipher the source of the increasing Pb concentration in MSG samples over the concealed ore bodies.

Lead isotope ratios in different media

Loess, red soil, host rocks and oresLead isotope ratios in different media over the Jiaolongzhang deposit are shown in Table 5. It can be seen that the Pb isotope ratios in loess and red soil vary very slightly (Fig. 4). Ratios of 208Pb/204Pb in loess (39.084–39.167) are slightly higher than those in red soil (38.854–38.893); the ratios of 206Pb/204Pb and 207Pb/204Pb in loess (18.864–18.877, 15.761–15.763, respectively) are a little lower than those in red soil (18.718–18.766, 15.660–15.679, respectively). The features of Pb isotopes indicate that loess and red soil are homogenous.

Table 1. Average contents (µg/g) of metallogenetic elements (Cu, Pb, Zn) in different media from the Jinglongzhang base metal deposit, Gansu province, China

Description of sample Number of samples Cu Pb Zn

Loess 13 18.2 19.3 56.8Red soil 9 22.3 23.4 64.9Sandstone 3 42.0 25.0 52.0Volcanic rock 7 24.0 40.0 85.0Mineralized wall rock 6 187 401 1581Sulphide ore 5 318 18159 54385

Table 2. Elemental contents in the blank liquid collector for the Jiaolongzhang base metal deposit sampling (ng ml−1)

Element Content Element Content Element Content Element Content Oxide Content

Ag 0.002 Cu 0.263 Sc 0.472 Ce 0.010 Al2O3 0.07Au 0.003 Li 1.190 Sr 0.080 Dy 0.004 CaO 0.48Ba 0.159 Mn 0.741 Th 0.007 La 0.008 Fe2O3 0.08Be 0.010 Mo 0.027 Tl 0.001 Lu 0.001 K2O 5.12Bi 0.001 Nb 0.054 Zn 2.326 Nd 0.007 MgO 0.03Cd 0.023 Ni 1.105 As 0.145 Pr 0.002 Na2O 4.48Co 0.079 Pb 0.360 Hg 0.006 Sm 0.005 Cr 2.102 Rb 0.056 U 0.001 Y 0.006 Cs 0.003 Sb 0.012 P 49.90 Yb 0.005

Table 3. Elemental contents in MSG samples (ng ml−1) from the Jiaolongzhang base metal deposit

Element Content (Min)

Content (Max)

Ratios (Max/Min)

Element Content (Min)

Content (Max)

Ratios (Max/Min)

Ag 0.005 0.014 3 U 0.010 0.038 4Ba 1.092 5.017 5 V 22.41 50.51 2Bi 0.012 8.687 755 W 0.029 0.081 3Cd 0.020 0.457 23 Zn 52.19 12392 237Co 0.148 0.270 2 Ce 0.070 0.361 5Cr 1.846 2.771 2 Dy 0.011 0.046 4Cs 0.011 0.035 3 Er 0.005 0.023 4Cu 2.262 6203 2742 Eu 0.004 0.012 3Mo 0.081 0.136 2 Gd 0.010 0.042 4Li 0.059 0.187 3 La 0.045 0.213 5Nb 0.018 0.036 2 Lu 0.001 0.034 28Ni 1.112 8.896 8 Nd 0.030 0.172 6Pb 0.796 1480 1860 Pr 0.009 0.049 5Rb 0.269 0.932 3 Sm 0.009 0.042 4Sb 0.020 0.222 11 Y 0.030 0.192 6Th 0.009 0.132 14 Yb 0.008 0.129 16



X. Yang et al.8

The Pb isotope ratios show much greater variation in different types of host rocks, especially for 206Pb/204Pb. The ratios of 206Pb/204Pb in volcanic rocks are in the range of 18.618–18.781 and the ratios in sandstone are19.451–19.489. The ratios of 206Pb/204Pb and 208Pb/204Pb in mineralized wall-rocks are close to those in the volcanic rock and range from 18.554 to 18.874 and 38.629 to 39.126, respectively. The 206Pb/204Pb and 208Pb/204Pb ratios in sul-phide ores are in the range of 18.130–18.251 and 38.350–38.582, respectively, and show less 238U and 232Th radiogenic Pb than in

loess, red soil and host rocks. The large difference in Pb isotope ratios between sulphide ores and host media is helpful to distinguish whether the increasing Pb content in anomalous MSG samples comes from the ore bodies.

MSG samplesLead isotope ratios in MSG samples are different from those in the loess and red soil. The ratios of 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb in MSG are in the range of 16.46–18.56, 13.985–15.809

Table 4. Precision of Pb isotope ratios determined by ICP-MS for SRM981 (5 ng Pb ml−1)

Measurement frequency 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb 208Pb/206Pb 207Pb/206Pb

1 16.80 15.39 36.08 2.149 0.916072 16.79 15.39 36.08 2.150 0.916623 16.81 15.38 36.09 2.147 0.914934 16.82 15.38 36.13 2.147 0.914395 16.79 15.38 36.12 2.148 0.91602Average 16.802 15.384 36.100 2.1482 0.915612SD 0.026 0.011 0.047 0.0026 0.00183%RSD 0.155% 0.071% 0.130% 0.1214% 0.19992%Reference value* 16.937 15.491 36.721 2.1681 0.91464

*After Catanzaro et al. (1968).

Table 5. Pb isotope ratios in different media from the Jiaolongzhang base metal deposit, Gansu province, China

Description of Sample 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb 207Pb/206Pb

Loess (n = 3) Min. 18.864 15.761 39.084 0.835Max. 18.877 15.763 39.167 0.836Average 18.871 15.762 39.118 0.8352SD 0.013 0.002 0.087 0.001%RSD 0.069% 0.013% 0.223% 0.082%

Red soil (n = 3) Min. 18.718 15.660 38.854 0.835Max. 18.766 15.679 38.893 0.837Average 18.745 15.670 38.872 0.8362SD 0.049 0.019 0.040 0.002%RSD 0.261% 0.121% 0.102% 0.253%

Volcanic rock (n = 3) Min. 18.618 15.580 38.554 0.832Max. 18.781 15.642 39.865 0.839Average 18.684 15.614 39.416 0.8362SD 0.172 0.063 1.493 0.007%RSD 0.919% 0.404% 3.788% 0.849%

Sandstone (n = 2) Min. 19.451 15.724 39.697 0.807Max. 19.489 15.787 39.734 0.812Average 19.470 15.756 39.716 0.8092SD 0.054 0.089 0.052 0.007%RSD 0.276% 0.565% 0.132% 0.842%

Mineralized wall rock (n = 4) Min. 18.554 15.618 38.629 0.835Max. 18.874 15.755 39.126 0.842Average 18.684 15.673 38.856 0.8392SD 0.287 0.119 0.448 0.007%RSD 1.534% 0.756% 1.152% 0.793%

Sulphide ore (n = 3) Min. 18.130 15.671 38.350 0.864Max. 18.251 15.767 38.582 0.865Average 18.176 15.712 38.440 0.8642SD 0.131 0.099 0.248 0.001%RSD 0.721% 0.628% 0.646% 0.140%




and 34.199–38.691, respectively. The 206Pb/204Pb ratio in the MSG background samples are much lower than those in anoma-lous samples and have greater variability (Table 6). The ratios of 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb in the MSG background samples are 16.46–17.68, 13.985–14.945 and 34.199–36.884, respectively. The wide range of Pb isotopic ratios in the MSG background samples are possibly caused by the mixing of Pb sources, including liquid blank reagent, lab environment, soil, host rocks, and ores.

The ratios of 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb in anoma-lous MSG samples above deposits are very similar (in the range of 18.34–18.56 for 206Pb/204Pb, 15.622–15.809 for 207Pb/204Pb, and 38.184–38.691 for 208Pb/204Pb) and totally different from those in the background samples. It can be concluded that some of the Pb in the MSG samples is related to the sulphide mineralization. Therefore, the increased Pb in anomalous MSG samples greatly reduces the effects from both the Pb content in the blank liquid collector and other geological backgrounds.

Discussion

The scatter plot of 207Pb/206Pb vs. 206Pb/204Pb was drawn to better understand the relationships of MSG samples with different media. Figure 5 shows that the 207Pb/206Pb and 206Pb/204Pb ratios in loess are rather similar to those in red soil, with small variations. However, MSG samples with higher 207Pb/206Pb ratios (0.843–0.853) and

lower 206Pb/204Pb (16.46–18.56) ratios are greatly different from those of loess (0.835–0.836 for 207Pb/206Pb and 18.864–18.877 for 206Pb/204Pb) and red soil (0.835–0.837 for 207Pb/206Pb and 18.718–18.766 for 206Pb/204Pb), indicating that Pb in the MSG samples did not originate from the topsoil.

It is also noteworthy that the anomalous MSG samples (0.852–0.853 for 207Pb/206Pb and 18.34–18.56 for 206Pb/204Pb) markedly differ from those of MSG background samples (0.843–0.853 for 207Pb/206Pb and 16.46–17.68 for 206Pb/204Pb). The 206Pb/204Pb ratios of MSG background samples are very low, with large vari-ations, and deviate from the main geologic bodies, which demon-strates that some Pb in MSG background samples is contributed from the blank liquid collector and the lab environment rather than from soil, host rocks and ores (no analytical result of the blank is available as it was thought to be impossible to obtain meaningful results of Pb isotope ratios at such low Pb concentra-tions in the blank by using the HP 4500 ICP-MS). The 206Pb/204Pb ratios of anomalous MSG samples over the deposit, however, are much higher than those of background samples. The anomalous samples are located in a narrow range and are distributed along the curve of loess and red soil.

Figure 6 shows the plot of 207Pb/206Pb vs. 206Pb/204Pb of host rocks, ores and MSG samples. It can be seen that the different media spread along the curve of the main geologic bodies and the 207Pb/206Pb ratios increase and the 206Pb/204Pb ratios decrease from old host rocks (sandstone, volcanic rocks) to young sulphide ores.

Fig. 4. Box-plots for Pb isotope ratios and sample media (main box depicts 25th to 75th percentiles, the 50th percentile is shown as a bar within the box).



X. Yang et al.10

Table 6. Lead isotope ratios in MSG samples from the Jiaolongzhang base metal deposit, Gansu province, China

Description of sample

Sample 206Pb/204Pb 2σ 207Pb/204Pb 2σ 208Pb/204Pb 2σ 207Pb/206Pb 2σ

Samples in J2 17.42 ±0.188 14.750 ±0.128 36.059 ±0.232 0.847 ±0.001background area(n=9)

J3 17.12 ±0.172 14.570 ±0.129 35.674 ±0.189 0.851 ±0.002

J4 16.46 ±0.140 13.985 ±0.107 34.199 ±0.168 0.850 ±0.002 J5 17.62 ±0.129 14.945 ±0.087 36.594 ±0.266 0.848 ±0.001 J6 16.70 ±0.204 14.225 ±0.148 34.633 ±0.225 0.852 ±0.001 J7 17.68 ±0.176 14.911 ±0.115 36.884 ±0.261 0.848 ±0.001 J8 17.16 ±0.172 14.518 ±0.112 35.572 ±0.157 0.846 ±0.001 J9 17.21 ±0.115 14.672 ±0.094 36.027 ±0.326 0.853 ±0.002 J14 17.49 ±0.106 14.750 ±0.109 36.513 ±0.192 0.843 ±0.002Min. 16.46 13.985 34.199 0.843 Max. 17.68 14.945 36.884 0.853 Average 17.207 14.592 35.795 0.849 2SD 0.820 0.629 1.792 0.006 %RSD 4.765% 4.312% 5.007% 0.745% Samples in J1 18.44 ±0.099 15.734 ±0.026 38.505 ±0.037 0.853 ±0.001anomaly area (n=5)

J10 18.34 ±0.084 15.622 ±0.039 38.184 ±0.042 0.852 ±0.001

J11 18.56 ±0.086 15.809 ±0.023 38.691 ±0.021 0.852 ±0.002 J12 18.36 ±0.094 15.649 ±0.034 38.248 ±0.054 0.852 ±0.001 J13 18.53 ±0.118 15.784 ±0.045 38.620 ±0.078 0.852 ±0.001Min. 18.34 15.622 38.184 0.852 Max. 18.56 15.809 38.691 0.853 Average 18.446 15.720 38.450 0.852 2SD 0.197 0.164 0.449 0.001 %RSD 1.067% 1.043% 1.168% 0.105%

Fig. 5. Plots of 207Pb/206Pb against 206Pb/204Pb for MSG samples, loess and red soil at the Jiaolongzhang base metal deposit, Gansu province, China.

The sulphide ores with the lowest 206Pb/204Pb ratios (18.130–18.251) and highest 207Pb/206Pb ratios (0.864–0.865) are located in a limited range. Although MSG samples in background areas deviate from those of the main geologic bodies and their Pb iso-tope ratios vary greatly, Pb isotope ratios of MSG samples in anomalous areas are close to each other (0.852–0.853 for 207Pb/206Pb and 18.34–18.56 for 206Pb/204Pb) and scatter between the mineralized host rocks and sulphide ores.

Conclusion

In this study, Pb isotope ratios of MSG samples were compared with those of soil, wall-rocks and ores at the Jiaolongzhang base metal deposit using optimized analytical methods based on ICP-MS for the MSG samples. Improvement in precision and accuracy allowed bet-ter differentiation of isotopically distinct sources of Pb in the MSG samples (RSD of 207Pb/206Pb ratios of c. 0.2% at 5 ng ml−1 of Pb).

Fig. 6. Plots of 207Pb/206Pb against 206Pb/204Pb for MSG samples, wall-rocks and sulphide ores at the Jiaolongzhang base metal deposit, Gansu province, China.




The results show that the Pb isotope ratios of MSG samples in the anomalous areas (206Pb/204Pb=18.34–18.56, 207Pb/204Pb=15.622– 15.809 and 208Pb/204Pb=38.184–38.691) are markedly different from those of the background areas (206Pb/204Pb=16.46–17.68, 207Pb/204 Pb=13.985–14.945 and 208Pb/204Pb=34.199–36.884), but close to the Pb isotope ratios of mineralized wall-rocks (206Pb/20

4Pb=18.554–18.874, 207Pb/204Pb=15.618–15.755 and 208Pb/20

4Pb=38.629–39.126) and sulphide ores (206Pb/204Pb=18.130–18.251, 207Pb/204Pb=15.671–15.767 and 208Pb/204Pb=38.350–38.582), and different from those of host rocks and overburden.

It is suggested that the anomalous Pb in the MSG derives mainly from the sulphide mineralized bodies and the Pb isotope tracing technique can be used to trace the Pb sources in MSG samples. The MSG technique can be used to detect deeply concealed mineral deposits under exotic cover, where conventional geochemical and geophysical exploration methods are invalid. However, more study is required to improve Pb isotope ratio precision and accu-racy in low Pb-content MSG background and blank samples by high resolution and multi-collector ICP-MS.

Acknowledgements and FundingThe authors thank the National Science Foundation Committee and the Program 863, Ministry of Science and Technology, China for the financial support of the research project. We are grateful to Prof. Xu Ronghua and Prof. Zhuang Zhixia for their support of this study on determining the Pb isotope ratios in different samples. Thanks should be given to Central Lab, IGGE for the analysis of the elemental concentrations in MSG samples. Special thanks go to G. E.M. Hall for the time spent in rewriting this paper and to the reviewers whose valuable suggestions improved this manuscript.

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tracing the source of ‘metals in soil gas’ with pb isotope ......4 x. yang et al. geological...

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