Progress in the collection of Geogas in China

Transcription

1 Progress in the collection of Geogas in China Ming-qi Wang 1, Yu-yan Gao 1 & Ying-han Liu 2 1 China University of Geosciences, Beijing , China ( mingqi@cugb.edu.cn) 2 Institute of Geophysical and Geochemical Exploration, Langfang, Hebei , China ABSTRACT: Geogas surveys have been carried out in China for more than 15 years. Although much valuable experience has been obtained from these studies, little progress (until recently) has been made in understanding the mechanisms involved and in testing procedures. One of the major challenges is to design a collector for the metals in Geogas effectively, using the right medium for complete sorption, without knowing their actual forms. Several absorbers have been tested in China. Almost all solid collectors used in Geogas testing contain variable quantities of metals which are difficult to remove chemically or physically. Thus some of the published data related to Geogas surveys should be re-evaluated. Newly developed liquid collectors, used since 1999, improve the accuracy of the Geogas data because they are low in blank levels, and their use negates the need for time-consuming digestion, the aqueous medium being easily analysed by inductively coupled plasma -mass spectrometry. With this refinement in collectors, the recent Geogas anomalies obtained in soils over buried mineralization attest to the presence of metals within soil gas and confirm the validity of this technique. KEYWORDS: China Geogas survey, solid collectors, liquid collectors, geochemical exploration, overburden, Geochemistry: Exploration, Environment, Analysis, Vol , pp INTRODUCTION Geogas surveys in mineral resource exploration were first introduced and applied by Swedish scientists (Kristiansson & Malmqvist 1982; 1987; Malmqvist & Kristiansson 1984; Kristiansson et al. 1990). The technique was later also referred to as metal-in-soil-gas, NAMEG (Xie et al. 1999) and nanoscale material survey (Liu et al. 1997). Since 1990, researchers in China have carried out extensive investigations into the potential applications of the Geogas survey in mineral exploration with the support of the former Ministry of Geology and Mineral Resources, the Chinese Geological Survey and the National Science Foundation Committee (NSFC). In the initial work of Malmqvist & Kristiansson (1984), elements in Geogas (i.e. soil gas) were collected on aerosol-type filters from the natural flux of Geogas, a method known as passive sampling. Because both the flux rate of Geogas and its element contents are very low, the success of the passive method requires integrated sampling over long periods of time (weeks to months). More recently, Wang et al. (1995) developed an active sampling method, in which Geogas was pumped through polyurethane foam treated to collect metals (Liu et al. 2003). Other types of solid collectors have also been investigated, such as a silver-nitrate-impregnated membrane (Pedersen 1988), glass wool impregnated with hydrogen peroxide, ion exchange resins (Maguire 1987) and activated carbon (Pauwels et al. 1999; Cameron 2001). One of the main concerns with solid collectors, however, is the potential contamination from the collectors themselves. All types of solid collectors inevitably contain certain levels of elements that are measured in a Geogas survey. However, few data are available on the blank element levels of various solid collectors and the means by which to eliminate these contamination effects. Since 1999, an active sampling method using a liquid collector has been tested in China. One of the major advantages of a liquid collector is their low inherent concentration of elements. Because the liquid collector can be prepared with ultra-pure reagents in a clean room environment, the potential contamination is minimized. With reduced blanks in a liquid collector, the contrast of anomalies in Geogas above mineralization covered by overburden may be enhanced. In the past few years, we have conducted systematic field experiments to investigate the blank levels of various types of collectors used in Geogas surveys in China, including solid collectors such as polystyrene, polyurethane foam and activated carbon, as well as liquid collectors. The results are presented in this contribution. The impacts of blank levels on the signals in Geogas anomalies over deeply buried mineralization are also discussed. METHODS Sampling methods There are two approaches to sampling Geogas in China: passive and active accumulation. The passive method, first described by Malmqvist & Kristiansson (1984), has been used by several researchers at the Institute of Geophysical and Geochemical Exploration (IGGE) in China (Ren et al. 1995; Liu et al. 1995; Wu et al. 1995) and by Tong et al. (1990, 1991, 1992, 1997, 1998, 2002); Tong & Li (1999). Instead of using a polyethylene membrane, Chinese researchers used polyurethane foam and activated carbon. The passive collection, however, requires integrated sampling over long periods of time, rendering the deployment and recovery of the sampling devices difficult. Wang et al. (1995) developed a dynamic sampling method ( active sampling) /08/$ AAG/ Geological Society of London

2 184 M. Wang et al. Fig. 1. Schematic of the sampling device for collecting Geogas. The active method was modified to use a liquid collector. The schematic of the system is presented in Figure 1. The sampling device consists of a cone-shaped sampler, a Millipore filter (0.45 µm), a liquid collector, and a battery-operated pump. The liquid collector consists of a high density polyethylene (HDPE) bottle containing 15 ml of 3% HNO 3 or aqua regia prepared in a clean room with ultra-pure HNO 3 and deionized water (Milli-Q water). When sampling, the cone-shaped sampler is pushed cm deep into the soils, and the gas is pumped through a silica gel tube and the Millipore filter to prevent coarse particles entering the liquid collector. The pumping lasts 2 3 minutes at a rate of 1.5 l/min with a total of l of gas pumped through at each hole. At each sampling site, a composite of samples is collected from three holes at a spacing of 2 3 m to improve sampling reproducibility. Analytical methods The analytical methods to determine a large element suite in Geogas collectors in China comprise instrumental neutron activation analysis (INAA), graphite furnace atomic absorption spectrometry (GFAAS) and inductively coupled plasma mass spectrometry (ICP-MS). INAA was used to determine more than 20 elements including Au, Ag, Sb, Bi, Hg, Zn, Fe, Mn, Br, Cr, Ni and Co by Wang et al. (1995, 1997a, b, 1999); Tong et al. (1990, 1991, 1992, 1997, 1998, 2002); Tong & Li (1999). Although INAA avoids the sample digestion procedure and provides low detection limits for numerous elements simultaneously, it is time-consuming, expensive, and unsuitable for the determination of some important indicator elements such as Cu and Pb. Table 1. Blank levels of Ag, Cu, Pb and Zn in activated carbon (in ppb (ng/g)). Sample Analytical method Ag Cu Pb Zn B68 AAS B69 AAS B70 AAS B71 AAS INAA The activated carbon was eluted with (1+1) HNO 3 for 24 hours and air-dried before use. The treated activated carbon was analysed by AAS for Ag, Cu, Pb, Zn after ashing and digestion in (1+1) HNO 3 and by INAA directly for Ag, Zn, Au, As, Sb, Br and Ni. The resulting value was reported in terms of ng/g per weight of activated carbon. Fig. 2. The weight distribution of the polyurethane foam columns specially treated in 3% HNO 3 for 24 hours and air-dried (1 cm in diameter and 3 cm in height). Analysis by GFAAS, which is low-cost and suitable for elements such as Cu, Pb, Zn, Ag, Au, Ni and Co, was used mostly to determine elements in foam collectors by Liu et al. (1995, 1997); Ren et al. (1995); Wu (1995, 1996). The specially treated foams (mostly 1 cm in diameter and 3 cm in height) were first carbonized at 200 C, and then ashed at 600 C ina crucible for c. 2 hours. The residue was digested in 5 ml of 1+1 aqua regia+water at 200 C, evaporated to near-dryness and extracted with 1 ml of 10% HCl. The resulting value is reported in terms of ng/g (ppb) weight of activated carbon or foam. Drawbacks of GFAAS include high detection limits for Au and Ag and the complex sample digestion procedure. Table 2. Blank levels of Cu, Pb and Zn in polyurethane foam (in ng). Sample Weight (g) Cu Pb Zn PM PM PM PM PM PM PM PM PM PM PM The specially treated foam columns (1 cm in diameter and 3 cm in height) were analysed by GFAAS after carbonization at 200 C and ashing at 600 C in a crucible, and digestion in (1+1) aqua regia. Table 3. Values of Ni, Cu, Zn and Pb in both blank and sample polyurethane foams, in a Geogas survey in 1996 over the Jiaolongzhang Pb-Zn-Cu-Ag deposit, Eastern Gansu (in ng). Polyurethane foam Ni Cu Zn Pb Blank Min Max Mean (n=10) Samples Min Max Mean (n=53) The same analytical method (GFAAS) was used as in Table 2

3 Collection of Geogas in China 185 Table 4. Element concentrations in both blank and sample polyurethane foams in a Geogas survey in 1996 over the Tangshang Carlin-type gold deposit, Yunnan (in ng). Foam Weight (g) As Au Ba Co Fe Hg Sb Sc Th Zn Blanks Blank Blank Blank Blank Mean (n=4) SD Samples Min Max Mean (n=23) SD Table 5. Element concentrations in blank liquid collectors used in Geogas surveys (ng/ml). Elements 3% HNO 3 * in 2001 (n=5) 3% HNO 3 ** in 2003 (n=19) 3% HNO 3 *** in 2004 (n=13) Mean SD Mean SD Mean SD Ag Ba Co Cu Mn Pb Sb Sr Zn *Reagent grade; **semiconductor or electronic grade; ***repurified electronic grade. Fig. 3. Location and schematic geological map of the Jiaolongzhang area.

4 186 M. Wang et al. ICP-MS was used to determine the elements in liquid Geogas collectors by Liu et al. (1999 unpublished data) and has shown great advantages over the other methods in Geogas surveys after several years of testing. The results presented here are mostly from the IGGE Central Laboratory. PROBLEMS WITH SOLID COLLECTORS Activated carbon Activated carbon/charcoal has a strong ability to absorb elements and was used as a collector of Geogas in China in the early investigations. It was also tested as a collection medium by Pauwels et al. (1999). Activated carbon, however, contains high and variable contents of metals (Table 1) which are difficult to elute chemically. As elements in soil gas are at extremely low concentrations (ppb or ppt), activated carbon was subsequently discarded as a collector of Geogas in China. Fig. 4. Distribution of Pb, Zn and Cu in Geogas, using solid collectors, along Line 48 over the Jiaolongzhang Pb-Zn-Cu-Ag deposit, Eastern Gansu (after Liu et al. 1995). Polyurethane foam As stated, polyurethane foam of various shapes and sizes was widely used as a Geogas collector in China (Tong et al. 1990, 1991, 1992, 1997, 1998, 2002; Tong & Li (1999); Wang et al. 1995, 1997a, b, 1999; Xie et al. 1999; Liu et al. 1995, 1997, 2003; Wu et al. 1995, 1996). However, these publications lack detailed investigation of the blank levels associated with the polyurethane foams used in Geogas surveys. The blank element level obtained in the polyurethane foam procedure is controlled by many factors including its density, the raw material, the purity of acid and water used, and the errors intrinsic in the treatment prior to analysis. The weight of foam columns shows a normal distribution (Fig. 2), the largest deviation in weight being c. 10%. The blank level of polyurethane foam, analysed by GFAAS, is variable and not related to weight (Table 2). Further investigation found this variability to be caused by metal Table 6. Elemental concentrations in liquid blank and sample collectors from the Geogas survey over the Jiaolongzhang Pb-Zn-Cu-Ag deposit, Eastern Gansu. Elements Blank (n=5) (ng/ml, in liquid volume) Samples (n=14) (ng/ml, in gas volume) Min. Max. Mean SD Min. Max. Mean SD Ag Au Ba Bi Cd Ce Co Cs Cu Dy La Lu Mo Nd Ni Pb Pr Rb Sb Sc Sm Th Tl Yb Zn U

5 Collection of Geogas in China 187 Fig. 5. Distribution of Cu, Pb, Zn, Cd, Bi, Sb, Lu and Tb in Geogas, using liquid HNO 3 collectors, along Line 48 over the Jiaolongzhang Pb-Zn-Cu-Ag deposit, Eastern Gansu (2004). heterogeneity in the raw material of the foam itself, the acid and water blank levels, and the elements added from the laboratory environment during sample preparation including ashing, digestion and analysis. Although element results in real samples in Geogas surveys using polyurethane foam were usually significantly higher than those in the blanks, this was not always the case (Table 3). These data cast doubt upon the validity of the Geogas technique. INAA, used in a Geogas survey over the Tangshang Carlintype gold deposit, Yunnan, also demonstrated a sometimes high and variable metal value for blank polyurethane foam (Table 4). Although blank values for As, Hg and Sb are reasonably low and precise, thus facilitating their use in this exploration technique for such a deposit type, those for the other elements such as Au and Zn, are too high compared to the results for the real samples. PROGRESS IN LIQUID COLLECTORS To eliminate the high contamination effects caused by solid collectors, different acidic (e.g. HCl, aqua regia, HNO 3 ) and alkaline liquid media were investigated. Table 5 demonstrates the decrease in blank levels obtained in Geogas surveys from 2001 to 2004 when 3% HNO 3 was used as the liquid collector, progressing from reagent-grade acid to a laboratory-purified acid in Nitric acid was favoured over other liquid media as it collected the elements well, is easy to purify and is an ideal medium for analysis by ICP-MS. Other improvements made

6 188 M. Wang et al. Fig. 6. Distribution of Tl, Ni and Rb in Geogas, using liquid HNO 3 collectors, along Line 48 over the Jiaolongzhang Pb-Zn-Cu-Ag deposit, Eastern Gansu (2004). included the use of Milli-Q water, better control of the environment surrounding the preparation and analytical facilities and more thorough cleaning of all equipment. APPLICATION AT THE JIAOLONGZHANG PB-ZN DEPOSIT Setting Loess is widely distributed in northern China and covers c km 2. The thickness of the Chinese loess varies from <50 m to >200 m. A red soil layer (half-solidified), greater than 100 m in thickness, was deposited as lake sediment and lies below the loess; with the loess, it forms a double layer of exotic cover. The thick loess plateau of China has limited the use of routine geochemical exploration techniques. The Jiaolongzhang polymetallic Pb-Zn-Cu-Ag deposit is located in eastern Gansu and was discovered in 1974 by an aeromagnetic survey (Fig. 3). The study area is covered by transported loess (50 80 m) and red soil (20 50 m). The groundwater table usually lies at the bottom of loess. Average annual precipitation is 420 mm. The bedrock, composed mostly of intermediate and acidic marine volcanics, pyroclastics and felsic sandstone, is exposed Fig. 7. Distribution of Pb, Zn, Cu and Ag in soil (30 cm depth), using a total digestion (hot HCl-HNO 3 -HF attack) along Line 48 over the Jiaolongzhang Pb-Zn-Cu-Ag deposit, Eastern Gansu (1998). only at the bottom of gullies. Intrusive rocks are mostly granodiorite and plagiogranite porphyry. The polymetallic mineralization zones, which are c m in length and 200 m in width, are 300 to 500 m in depth and are totally covered by loess and the red soil layer. The blind orebodies contain mainly Zn, Pb-Zn and pyrite. The host-rock is chlorite-quartz sandstone and limestone, and the main ore minerals are pyrite, sphalerite, galena and chalcopyrite. Mineralization contains up to 1% Pb, 0.8% Zn and 0.6% Ag. Geogas surveys Initial surveys were based on the foam solid collectors. The responses for Pb, Zn and Cu are shown in Figure 4. Lead does not show any response whereas Zn displays anomalies over mineralization with a noisy background. The concentrations for Zn are up to c.3000 ng, well above the blank values for the foam shown in Tables 2 and 3. Copper shows an unconvincing one-point anomaly. Table 6 lists the elemental concentrations in liquid collectors of blanks and samples from a subsequent Geogas survey. The

7 Collection of Geogas in China 189 Fig. 8. Distribution of Pb, Zn, Cu and Ag in soil (30 cm depth), using a partial 1M NH 4 Cl digestion, along Line 48 over the Jiaolongzhang Pb-Zn-Cu-Ag deposit, Eastern Gansu (1998). Fig. 9. Distribution of Pb, Zn, Cu and Ag in soil (30 cm depth), using a partial 5% acetic acid digestion, along Line 48 over the Jiaolongzhang Pb-Zn-Cu-Ag deposit, Eastern Gansu (1998). blank bottles were actually taken to the field with the sampling bottles and underwent identical operations (e.g. lids of the bottles were opened and gas tubes inserted but without pumping). Therefore the blank results in Table 6 represent real field blanks, unlike those shown in Table 5. The data show that element concentrations of the blanks are low and reasonably consistent whereas the concentrations of deposit elements in the sample collectors are much higher, with Pb, for example, reaching c.1500 ng/ml (cf. maximum in the blank of 0.2 ng/ml), Zn up to 12 µg/ml (cf. maximum blank of 3 ng/ml) and Cu up to 6 µg/ml (cf. maximum blank of 0.3 ng/ml). Other elements such as Ba, Bi, Cd, Cs and several REEs are also significantly elevated over blank levels. Elemental distributions for Cd, Zn, Cu, Pb, Tb, Lu, Sb, Bi in this Geogas survey along Line 48 are shown in Figure 5. Responses for Cu, Pb and Bi are particularly strong over mineralization; those for Zn and Cd are one-point anomalies over a noisy background. Profiles for Tl, Ni and Rb, elements not enriched in the mineralization, are shown in Figure 6. Nickel appears to show a one-point anomaly, as yet unexplained. The success of this Geogas survey contrasts sharply with the results obtained in the earlier survey using solid collectors, especially for Cu and Pb (Fig. 4). A soil survey along Line 48, sampling at a depth of 30 cm, and employing a total digestion, resulted in no response for these deposit elements (Pb, Zn, Cu, Ag), as demonstrated in Figure 7. However, a one-point anomaly over mineralization for Pb and Zn was evident when the weak partial leach, 1M NH 4 Cl, was applied to these soils (Fig. 8). Another weak leach, 5% acetic acid, failed to highlight mineralization (Fig. 9). CONCLUSIONS More than 15 years of continuous improvement to Geogas testing in China have produced much valuable experience using this Deep Penetrating Geochemistry technique. Most published data from Geogas surveys in China prior to 1999, however, could be suspect because of the high and variable blank element concentrations encountered. Almost all solid collectors used in Geogas testing contained variable quantities

8 190 M. Wang et al. of elements which are difficult to remove chemically or physically; additional contamination was introduced in the preparation procedures used prior to analysis. Newly developed liquid collectors, employed with the active soil gas sampling procedure, have made the Geogas approach far more reliable. Blank element concentrations are now very low and the medium can be analysed directly, negating the need for element elution or digestion. The Geogas survey over the Jiaolongzhang polymetallic Pb-Zn-Cu-Ag deposit clearly highlighted the deeply buried mineralization and demonstrated the superiority of this refined approach to both total (no response) and partial extraction (mixed response) of near-surface soils. The findings also show that gaseous forms of elements do exist in soil gas and can be used as a powerful mineral exploration technique. However, numerous unknowns surrounding Geogas still remain; for example the mechanism(s) responsible for creating such gaseous forms, the exact identity of these forms, and the factors controlling their behaviour and stability in different environments. Further study is being supported by NSFC is to aid in solving these questions. The authors thank the National Science Foundation Committee for funding the research. We gratefully acknowledge the assistance from Ren Tianxiang, Yang Zhongfang, Zhang Deen and Wu he. Thanks are also given to Lu Yinxiu, Sun Xiaoling and Fan Hui, Zhao Yanqing, Tian Weizhi and He Hongliao for their analytical work. We are also very grateful to Gwendy Hall for the time spent in editing this paper and for encouraging us to publish it. REFERENCES CAMERON, E.M Testing of BRGM Gaz Collectors. CAMIRO Deep- Penetrating Geochemistry, Phase II Report. 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