Secondary geochemical dispersion in the Precambrian auriferous Hutti-Maski schist belt, Raichur district, Karnataka, India. Part I: anomalies of As, Sb, Hg and Bi in soil and groundwater

Journal of Geochemical Exploration 71 (2000) 269–289 www.elsevier.nl/locate/jgeoexp

Secondary geochemical dispersion in the Precambrian auriferous Hutti-Maski schist belt, Raichur district, Karnataka, India. Part I: anomalies of As, Sb, Hg and Bi in soil and groundwater N.R. Sahoo 1, H.S. Pandalai* Department of Earth Sciences, Indian Institute of Technology — Bombay, Powai, Mumbai 400076, India Received 12 January 1999; revised 19 September 2000

Abstract The nature of secondary geochemical dispersion of As, Sb, Hg and Bi in soil and ground water of the semi-arid, tropical, Archaean, auriferous, Hutti-Maski greenstone belt has been investigated for identification of appropriate geochemical techniques for Au exploration in similar terrains. Results indicate that the ⬍180 mm size-fraction of C-horizon soil is an appropriate sampling medium for delineating pedogeochemical anomalies of As, Sb, Hg and Bi related to gold mineralisation. These pedogeochemical anomalies along with anomalous values of alkalinity, chloride, sulphate, As and Sb in groundwater are controlled significantly by primary mineralisation located along shear zones in the greenstone belt. Arsenic anomalies in soil are broad, whereas, those of Sb and Bi are restricted to narrow zones directly over mineralised areas. In contrast, Hg anomalies around known mineralised areas are irregular and do not clearly demarcate the mineralised areas. The study indicates that anomalies of As, Sb and Hg in soil are principally hydromorphic, whereas those of Bi are clastic. The study recommends use of groundwater sampling at 2–3 km spacing with routine analysis of chloride, sulphate and alkalinity along with As and Sb in the first phase. This may be followed up with sampling of C-horizon of soils on a 1 km square grid for As-anomalies. Arsenic-anomalous areas may be sampled for As, Sb, Hg and Bi on a 500 m square grid for detailed exploration. 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: hydrogeochemical anomalies; pedogeochemical anomalies; gold prospecting; arsenic speciation; leachate analysis

1. Introduction Geochemical dispersion in the secondary environment is a result of physical and chemical processes. These processes are linked to weathering, soil formation and landscape development (Fortescue, 1975; Budel,

* Corresponding author. E-mail addresses: [email protected] (N.R. Sahoo), [email protected] (H.S. Pandalai). 1 Geoinformatics, Applied Technology Group, Tata Infotech Limited, SDF-5, SEEPZ, Mumbai, India 400 096.

1982). Since geological, geomorphological, and environmental conditions differ from area to area, geochemical landscapes developed over mineralised bodies vary from locality to locality depending on geology, climate and topography. Nevertheless, many similarities in dispersion patterns can be present over terrains of approximately similar characteristics. Dispersion patterns are surficial expressions of mineralisation and studies on secondary dispersion patterns help in selecting appropriate sampling media and in evaluating the nature, extent and significance of anomalies in geochemical surveys (Butt and Zeegers, 1989).

0375-6742/00/$ - see front matter 䉷 2000 Elsevier Science B.V. All rights reserved. PII: S0375-674 2(00)00158-8

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In the search for gold, special consideration is essential for identifying and applying appropriate geochemical exploration methodologies such as choice of medium of sampling, sampling density and size-fraction of the soil samples to be studied. Since gold has a high specific gravity and normally occurs in native state in very low concentrations (ppb range) even in significant gold deposits, it is difficult to obtain a representative sample from drainage sediment, soil or rock (Clifton et al., 1969; Harris, 1982; Boyle, 1987; Nichol et al., 1989). The problem of poor sample representativity and poor analytical reproducibility of gold in geochemical samples may be overcome by taking large samples and looking at finer size fractions or by using multi-element geochemical studies (Boyle, 1987; Xuejing and Xueqiu, 1991). Studies on multi-element geochemical studies for gold in varied terrains (Hale, 1981; Carr et al., 1984, 1986; Boyle, 1987; Chaffee and Hill, 1989; Smith et al., 1989; Nuchanong et al., 1991) have shown that such an approach permits broad sample spacing and overcomes the problem of nugget effect which occurs often when using Au itself as the target element. The choice of the pathfinder elements is based on the type of deposit, conditions of oxidation and soil formation, relative mobility of the elements and the suitability of the available analytical techniques. Dispersion of indicator elements in soil and groundwater depends on their primary concentration in gold bearing zones and their mobility in groundwater. The geochemistry of As, Sb, Hg and Bi and their potential application in gold exploration has been discussed in Boyle (1979, 1987) and Hale (1981). The Hutti-Maski schist belt is a region of low rainfall (⬍25 inches a year). Annual temperature varies between 10⬚C in winter and 42⬚C in summer. As a result of the low rainfall and a relatively flat topography, catchment areas of drainage systems are small. This results in non-availability of stream sediments for effective drainage survey. Although weathering is deep in places, the soil profile is, in general, poorly developed and often consists, partly or wholly, of transported overburden. It is a semi-arid tropical terrain, which is common in many of the tropical Precambrian greenstone terrains in the world. Groundwater and weathered bedrock are the most suitable media for detecting dispersion halos related

to mineralisation in such terrains (Butt and Smith, 1980). The present study was thus carried out on soil and groundwater samples. In this paper, analytical results of the typical chalcophile association: As, Sb, Hg and Bi, common to most of the gold mineralised areas has been examined. The method of exploration presented here is based on selection of a favourable sampling medium and the best chemical analytical technique for these elements. Hydrogeochemical anomalies of alkalinity, sulphate and chloride are also analysed. Speciation of As in groundwater under observed natural conditions has been studied to understand mechanism of transport. Fixation of the pathfinder elements of gold in soil has been studied using the sequential partial extraction technique.

2. Bedrock geology and mineralisation The Hutti-Maski greenstone terrain is a narrow, elongated, hook shaped, NNW trending belt consisting of predominant meta-volcanic rocks and subordinate meta-sediments. The volcanics are represented by pillowed tholeiitic metabasalts, minor rhyolites, acid tuffs and pyroclastic rocks. Metasediments include banded ferrugenous chert, quartzite, carbonaceous-phyllite and garnetiferous mica-schist (Roy, 1991; Srikantia, 1995). The metasediments occur as thin impersistent bands in metabasalts (Giritharan and Rajamani, 1998). The rocks of the greenstone belt are surrounded by multiple phases of intrusive diapiric granitoids. Fig. 1 shows the detailed geological map of the greenstone belt. Vesicular metabasalt is the host rock for the auriferous lodes. These lodes can be grouped into gold–quartz–sulphide lodes and gold–quartz– ankerite–sulphide lodes. The auriferous quartz reefs are mylonitised, brecciated and mainly localised in the major shear zone (Fig. 1) in the western segment of the greenstone belt. The geometry and orientation of the auriferous lodes are affected by the structural fabric of the shear zone (Roy, 1979). Gold occurs in native state in quartz and in association with sulphides in the alteration zones. Arsenopyrite, pyrrhotite, pyrite, chalcopyrite, scheelite and sphalerite are the commonly associated sulphide minerals (Roy, 1991; Srikantia, 1995; Biswas, 1990). The known deposits

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Fig. 1. Geological map of Hutti-Maski schist belt (after Sundaram et al., 1995).

271

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Fig. 2. Location of gold occurrences in the Hutti-Maski schist belt and soil sampling points.

over the belt are located at Uti, Wandalli, Hutti, HiraBuddinni, Tupadhur, Buddinni and Sanbal (Fig. 1). The shear zone and the granophyre–metabasalt contacts are the loci of the known gold deposits. The wallrock alteration zone around mineralised zones is extensive and characterised by chloritisation, biotitisation, sericitisation, silicification and carbonatisation. Quartz, ankerite, plagioclase, chlorite, biotite and sericite are the most common minerals in the wall rock alteration zone.

3. Physiography The Hutti-Maski schist belt has an undulating topography with occasional residual hills. The drainage pattern is controlled by structure and lithology. A

dendritic drainage pattern is developed over most flat-lying areas of the schist belt whereas a parallel drainage pattern is observed in the hilly north-western part of the schist belt. Lineaments in the schist belt include granite–metabasalt contacts in the north-eastern part of the schist belt and the NW–SE trending shear zone in the south. Three different soil-types have been identified on the basis of their colour and physico-chemical characteristics. Distribution of the soil-types is controlled mainly by topography and bedrock type. In general, weathered metabasalts, which underlay areas of level to gentle slope, are overlain by black soils, whereas granites and gneisses, which occur in areas of steeper slope, are overlain by red soils. Mixed, intermediatecoloured soils occur along slopes from high relief areas towards flat-lying areas.

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273

Fig. 3. Groundwater sampling locations.

On slopes, the soil profile is usually ⬍5–10 cm thick. In contrast to this, the soil profile in flat areas (with black soil) is usually about 1 m thick. The typical black soil profile has an A-horizon about 1 m thick underlain by a thin stony C-horizon. These soils can be classified as vertisols as per the FAO classification given by FitzPatrick (1983). In these soils, accumulation of carbonate as concretions results in the formation of a dark-coloured, massive, A-horizon.

4. Methodology 4.1. Selection of sampling medium and sampling technique A square grid was used for soil sampling because of the diverse orientation of geological features that control localisation of mineralisation. The sampling interval was selected on the basis of the anticipated size of anomalies. Sampling was carried out from in

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Table 1 Physico-chemical characterisation of typical black (H5, H8 and W14) and red (U5, U20 and W3) soils Sample

H5 H8 U5 U20 W3 W14

Size fraction (%)

pH (1:1) Suspensoid

Sand

Slit

Clay

11 4 7 6 10 20

17 22 22 26 23 15

72 74 71 68 67 65

7.8 7.9 8.1 7.7 7.9 8.1

CEC (meq/100 g) Ca

Mg

Na

K

40.4 38.5 42.3 46.7 38.1 34.5

25.1 20.2 14.1 10.3 17.4 22.4

8.7 9.7 10.5 7.2 10.2 11.9

1.1 0.7 0.9 0.5 0.8 0.7

situ soil just above the C-horizon (i.e. bottom of the Ahorizon). Pitting was chosen as the soil-sampling technique. Samples of more than 1 kg were collected from 370 locations. Composite samples at 1m intervals were collected from a few sampling sites. Regionally, samples were collected on a square grid of 1 km. A smaller square grid of 0.5 km was used in mineralised areas. The soil samples were packed in pre-cleaned air-tight polythene bags. The soil sample locations are given in Fig. 2. Groundwater samples were collected during the autumn season of 1997 and 1998 (Fig. 3). Sampling sites were chosen on the basis of availability of open wells and in a manner such that the whole area was uniformly covered. An approximate sample spacing of 2–3 km was maintained. Acid-cleaned polythene bottles were used for collection of water samples as recommended by Laxen and Harrison (1981) and Annon (1981). At each station the bottles were rinsed Table 2 Major element chemistry of typical samples of black (H5 and H8) and red (U5 and U20) soils. Major oxides

H5

H8

U5

U20

SiO2 Al2O3 Fe2O3 FeO TiO2 CaO MgO Na2O K2O P2O5 Loss on Ignition Total

43.58 18.36 11.23 0.53 11.78 6.37 0.34 0.04 0.17 0.26 7.06 99.72

44.07 16.37 13.74 0.72 10.53 4.83 0.61 0.09 0.08 0.43 8.13 99.60

47.56 17.87 10.32 0.86 9.72 4.73 0.09 0.08 0.03 0.54 7.83 99.63

45.76 15.15 13.04 0.97 12.93 4.23 0.14 0.06 0.05 0.19 7.46 99.98

Organic matter (%)

Organic carbon (%)

2.23 1.97 2.14 1.86 1.79 1.93

0.93 0.83 0.64 0.79 0.63 0.75

four times with the surface water and the last rinse was left for a few minutes to block the active sites in the container wall. Two samples, of one litre each, were collected from each site. One of the two was filtered through Whatman-42 paper and then through 0.45 mm millipore filter paper. The filtered sample was acidified with conc. HNO3 to bring the pH down to 2. This was done to inhibit adsorption of trace metal ions on to the container surface and to prevent metal precipitation and bacterial activity (Stoeppler, 1992). The pH and TDS were measured in the field. 4.2. Physico-chemical analysis of soil Six typical soil samples, two from each of three soil types, were selected for determination of physicochemical parameters and mineralogy of soil. The physico-chemical parameters studied include particle size distribution, pH, organic matter content, organic carbon content and cation exchange capacity (CEC). Particle size analysis, pH, organic matter and organic carbon were determined by the methods described in Jackson (1960). The CEC of soil was determined by NH4OAc method (Thomas, 1982). The major element analysis was carried out using the method of Shappiro and Brannock (1964). Mineralogical analysis was carried out using the method of Jackson (1960). Identification of mineral phases was done by comparing the X-ray diffractograms with standard peaks given in the Joint Committee on Powder Diffraction studies (JCPDS) software. Major element chemistry of the black, intermediate and red soils is similar. Mineralogically the soils consist of clay minerals (illite, kaolinite, montmorillonite and halloysite), quartz, calcite, dolomite, chlorite, goethite, hematite, magnetite and includes

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Table 3 Mineralogical composition of typical red and black soils Black soil

Red soil

H5

H8

U5

U20

Calcite Goethite Halloysite Hematite Illite Kaolinite Montmorillonite Magnetite Scorodite Quartz

Calcite Dolomite Goethite Hematite Illite Kaolinite Montmorillonite Quartz

Calcite Chlorite Dolomite Goethite Halloysite Hematite Kaolinite Montmorillonite Magnetite Scorodite

Calcite Goethite Halloysite Hematite Illite Montmorillonite Magnetite Scorodite Quartz

scorodite (an as mineral) as a minor phase. The content of organic carbon and organic matter are significantly different in the different soil types. Black soil, however, has a higher proportion of clays and also a higher CEC. The pH of all soils lies in the neutral to slightly alkaline range. Results of the analysis of physico-chemical parameters and mineralogy of soil samples are presented in Tables 1–3. 4.3. Analytical technique for trace element concentrations in soil The ⬍180 mm size fraction of soil samples were digested in a microwave system, with a closed pressure relief container using HF–HClO4 –HNO3 for total analysis as described in Chao and Sanzolone (1992). This has the advantage of vigorous digestion at elevated temperature and pressure, retention of volatile elements, reduced possibility of contamination, and safety (Van Eanbergen and Bruninx, 1978; Neas and Collins, 1988; Rantala and Loring, 1989). The Hydride Generation–Inductively Coupled Plasma– Optical Emission Spectroscopy (HG–ICP–OES) technique was adopted because of its high sensitivity. This allowed working with a smaller sample size and greater dilution of the sample digest, thereby minimizing elemental interference (Smith, 1975). The digested samples were acidified with concentrated HCl to give a 25% (v/v) (approximately 5 M) solution. Arsenic and Sb were analysed by reduction of these from their V states to their III states using KI

followed by on-line HG–ICP–OES using NaBHO4 and NaOH solutions (Trafford, 1986). International USGS-soil standards (GXR-2 and GXR-6) were used for calibration. A few samples were re-run in different batches with the same standard calibration curves, to check the accuracy of the results.

4.4. Analytical technique for trace element concentration in groundwater The non-acidified fraction of groundwater samples were analysed for alkalinity, chloride, sulphate, silica, hardness, and major cations. Alkalinity, chloride, hardness, Ca and Mg were estimated by titration methods; Na, K, silica and sulphate were estimated photometrically (Greenberg et al., 1992). The acidified portions of the groundwater samples were preconcentrated by heating at a temperature less than 80⬚C. The HG–ICP–OES technique, as discussed in the earlier section was used for the analysis of As, Sb, Hg and Bi. Detection limits of 0.2 ppb for As, 0.2 ppb for Bi, 0.1 ppb for Hg, 0.1 ppb for Sb and 0.5 ppb for Se were achieved. In soil samples, Sb, Bi and Hg were detected only where the samples were located close to mineralised areas. While As and Sb were detected in all water samples, Hg and Bi were below their respective detection limits in all samples. Selenium, Ag and Mo were also analysed in soil and water samples, but were found to be below their respective detection limits.

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5. Analysis of the anomalies 5.1. Pedogeochemical anomalies Arsenic values in soil were contoured using an inverse distance weighting algorithm. The contour maps (Fig. 4a and b) highlight both regional variations and anomalies defined by concentrations exceeding 100 ppm. The maps show good spatial association between the anomalous values and known gold mineralised areas (Fig. 4b). Mineralised areas are contained within 150 ppm contours of As. On the other hand, As in areas distant from mineralised zones varies between 2 and 79 ppm. The average value of As concentration in such areas is 40 ppm. On this basis, a threshold of 150 ppm and a regional background of 40 ppm was identified for As. Arsenic-profiles from a suite of samples collected along transects crossing various bedrock lithologies and topography are shown in Fig. 5. Anomalous values of As are observed over shear zones. Metabasalts have relatively higher concentrations of As than other lithologies. Close inspection of the Fig. 5a–c shows that higher values of As occur in topographically flat areas as compared to neighbouring areas of higher slope. The control of topography on As dispersion indicates that As is being transported downslope, either mechanically in clastic form, or hydromorphically in dissolved form. In soil, Hg, Sb and Bi were present in detectable concentrations (greater than 125, 125 and 25 ppb, respectively) only in As-anomalous zones. The concentrations of these elements within the anomalous zones were contoured with an inverse distance weighting algorithm (Fig. 6). It is seen that Hg anomalies are less informative than those of Sb and Bi in describing mineralised areas. High values of Sb and Bi are restricted to areas almost directly above mineralisation. A sharp variation in concentration of Sb, Hg and Bi away from the lode zones is observed in samples collected in few traverses across lode zones (Fig. 7). 5.2. Hydrogeochemical anomalies Fig. 4. Contours (in ppm) of As in soil: (a) shows contours below 100 ppm and highlights the regional variations; (b) shows contours above 100 ppm in the same area. The anomalous values overlie known gold occurrences.

Water chemistry data (Table 4) shows that at all locations, pH is near-neutral to alkaline (7.12–8.27). Conductivity values of all groundwater samples range between 307 and 2820 mS/cm. This indicates

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277

Fig. 5. Profiles of As values (ppm) along traverses IJ, KL and MN, respectively across the mineralised shear zones at Gajalgatta (A), across the Uti deposit (B) and in an unmineralised area (C). The dashed line shows the topography and indicates that higher values of As occur in lowlying areas in all profiles. Locations of traverses are shown in Fig. 2.

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Fig. 6. Contours (in ppm) of Sb, Hg and Bi in soil of the Uti block (I) and the Hira-Buddinni block (II). Contours show characteristic anomalies in both the blocks which are known to be mineralised. Location of blocks I and II is given in Fig. 2.

that the ionic strength of water in these areas is appreciably high. Dissolved silica ranges from 23 ppm in barren areas to as high as 142 ppm near areas of mineralisation. Total alkalinity, which is the and HCO3⫺ concentrations, sum of OH ⫺, CO2⫺ 3 ranges from 1411 ppm in barren areas to 4717 ppm in mineralised areas. The contribution of (OH) ⫺ to alkalinity is low in all samples and thus indicates that the high values in mineralised areas are the result of weathering of carbonates and bicarbonates from alteration zones close to the deposits. Sulphate concentration ranges between 24 and 400 ppm and the chloride concentration between 12 and 80 ppm. The sulphate concentration is due primarily to weathering of sulphides associated with the gold mineralisation.

The values of alkalinity, sulphate and chloride in groundwater were contoured with the inverse distance weighting algorithm. Fig. 8 shows contours of sulphate, chloride and alkalinity measured in groundwater. Wide anomalies of alkalinity, chloride and sulphate are observed around mineralised areas. Values of alkalinity, sulphate and chloride greater than 2000, 50 and 20 ppm, respectively, differentiate mineralised areas from barren areas. Flat areas have high background values of sulphate and chloride. In areas far from mineralisation, there is a clear relationship between topography and values of sulphate, chloride and total alkalinity. Groundwater in flat terrains show higher values in comparison to groundwater in steeper terrain. Total As concentration in groundwater varies

N.R. Sahoo, H.S. Pandalai / Journal of Geochemical Exploration 71 (2000) 269–289 Fig. 7. Antimony, mercury and bismuth concentration (ppm) in soils along traverses AB and CD in Uti and along EF and GH in Hira-Buddinni areas. The mineralised areas are shaded black. Location of the traverses is given in Fig. 2. 279

PH

Conductivity (mS/cm)

Alkalinity (ppm)

Sulphate (ppm)

Chloride (ppm)

7.38 7.96 7.72 8.18 8.08 8.25 7.89 7.92 7.48 7.57 7.33 8.25 7.84 8.15 8.13 7.39 7.69 7.54 7.66 7.46 7.36 7.43 7.74 7.63 7.38 7.39 7.49 7.82 7.64 7.87 7.23 7.54 7.76 7.78 7.34 7.32 7.54 7.23 7.89 7.65 7.12

428 583 486 307 952 988 647 887 892 590 1102 508 697 508 1240 466 671 1914 621 847 684 609 685 764 627 671 753 840 693 827 686 708 660 690 874 1364 858 727 1140 883 764

2411 2467 2096 1572 3669 4717 3354 4193 3354 3145 2830 2935 2620 2096 4193 4330 3145 1572 2935 3193 3564 3669 4088 4193 2620 2096 3402 2483 2830 2459 2450 4026 2708 2409 3510 3705 3110 2506 3510 3482 3009

56.9 74.5 46.8 37.9 165.9 106.5 59.4 93.6 220.7 42.3 224.8 53.7 87.9 44.5 337.7 68.5 400.1 49.8 24.4 104.6 65.8 34.3 39.5 121.6 358.1 234.7 257.5 187.1 161.6 190.6 37.8 102.3 103.5 36.9 27.5 105.4 62.5 41.4 107.3 259.5 208.6

22 23 20 19 42 26 20 32 52 21 70 21 20 19 69 19 26 52 21 27 20 23 80 33 26 34 30 29 29 31 21 36 12 13 33 29 13 24 33 23 20

Ca (ppm)

Mg (ppm)

Silica (ppm)

Na (ppm)

K (ppm)

As (ppb) Sb (ppb)

35.3 27.2 12.8 16.0 24.0 24.9 28.9 32.1 32.1 22.4 59.3 40.1 40.1 27.2 24.0 28.9 33.7 46.5 35.3 33.7 40.1 35.3 30.5 40.1 48.1 33.7 38.5 33.1 41.7 48.1 54.3 85.8 88.6 60.0 34.3 108.6 57.2 60.0 80.0 91.5 108.6

23.3 52.5 39.8 40.8 28.2 28.2 32.1 29.2 48.6 42.8 58.3 53.5 38.9 45.4 53.5 45.7 63.7 94.3 58.3 124.4 38.9 17.5 39.8 45.6 42.8 52.5 52.5 52.5 44.7 59.3 16.0 86.1 86.3 58.3 76.2 52.3 81.4 63.2 70.0 79.7 59.6

95.4 93.7 65.6 96.1 66.9 141.9 85.4 112.0 80.4 23.5 92.1 68.8 97.1 126.9 63.7 91.9 123.6 116.9 128.6 95.5 70.6 62.2 72.1 105.4 94.5 25.6 75.6 83.8 63.8 93.6 74.4 40.5 67.5 103.3 66.7 63.9 61.4 56.2 74.9 61.5 67.1

2.4 5.1 2.5 4.6 11.1 15.0 6.8 12.5 5.9 4.6 5.8 3.4 5.9 3.6 15.0 3.8 6.4 8.9 3.2 7.8 5.1 5.4 6.8 7.4 4.8 5.1 5.1 8.5 4.6 6.3 1.4 2.5 2.3 1.7 1.6 4.5 2.8 1.2 5.5 8.1 10.2

0.2 0.1 0.1 0.1 0.8 1.6 0.1 0.1 2.6 0.1 5.8 0.1 0.1 0.1 0.1 0.1 0.1 8.2 0.6 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.3 0.1 0.1 0.2 0.1 0.1 0.1 0.1 0.2 0.1 0.2

6.2 15.0 6.4 0.1 0.1 41.5 0.1 0.6 0.2 0.2 7.1 0.3 0.7 0.1 0.1 26.9 0.5 0.4 0.4 0.3 0.2 0.3 12.8 0.3 1.1 0.7 0.6 0.1 0.1 0.1 0.9 0.9 0.7 0.9 0.6 5.4 1.2 0.9 1.3 4.7 2.8

3.1 6.1 2.3 0.3 0.3 9.6 0.5 3.7 4.3 0.8 8.4 0.6 0.2 0.4 0.3 5.6 0.2 0.3 0.1 0.1 0.2 0.1 3.5 0.1 2.3 0.3 0.3 0.2 0.5 0.4 0.4 0.2 0.3 0.4 0.2 1.1 0.5 0.5 4.2 1.6 0.3

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

280

Table 4 Analyses of the hydrogeochemical parameters

Table 4 (continued) Conductivity (mS/cm)

Alkalinity (ppm)

Sulphate (ppm)

Chloride (ppm)

7.43 7.56 7.32 7.65 7.45 7.87 7.81 7.34 7.91 7.72 7.28 7.36 7.49 7.18 7.28 7.34 7.45 8.12 8.27 8.07 7.98 7.56 7.34 7.27 7.37 7.79 8.09

674 701 646 2820 745 978 930 1069 1039 1052 742 680 580 771 672 842 802 810 745 685 715 915 907 980 840 842 1087

2680 3003 2808 3081 2809 3087 2380 2743 3508 2960 2765 3124 3641 3143 2675 2641 3381 2786 2943 3675 3749 3564 3343 3007 2782 3672 3789

50.4 120.4 207.7 260.3 210.6 120.5 79.7 124.9 140.9 256.7 205.8 107.9 240.9 253.6 250.9 112.6 278.5 247.4 287.3 256.7 108.9 99.8 105.5 109.5 123.6 254.9 267.5

12 14 21 26 31 61 26 32 60 23 22 31 36 31 23 31 23 21 24 26 61 32 33 30 23 50 48

Ca (ppm)

Mg (ppm)

Silica (ppm)

Na (ppm)

K (ppm)

As (ppb) Sb (ppb)

40.0 105.8 88.6 82.9 40.0 105.8 88.6 82.9 40.0 20.0 37.2 45.7 54.3 60.0 45.7 31.4 25.7 17.2 22.9 28.6 31.4 25.7 45.7 17.1 11.4 54.3 14.3

69.9 53.4 65.2 192.3 44.7 90.9 96.0 48.9 47.6 26.2 59.0 67.6 57.7 35.9 10.3 47.8 35.5 43.4 42.1 62.1 77.9 56.9 37.5 71.6 42.9 34.4 51.9

57.1 116.7 91.2 108.7 64.3 45.2 44.1 33.3 90.8 97.7 66.4 31.9 62.3 95.4 86.7 91.2 92.4 82.1 72.9 108.9 25.6 32.3 38.9 35.6 38.7 91.2 103.6

5.2 0.6 4.4 14.7 16.4 4.8 0.8 3.2 10.5 16.4 16.2 1.4 1.2 5.0 7.6 4.4 3.6 6.3 5.4 3.3 4.8 2.8 5.7 4.4 2.7 2.4 4.8

1.6 0.1 5.9 1.5 2.3 2.8 0.5 0.4 0.6 1.0 1.0 1.5 1.7 2.5 1.6 5.5 8.8 1.0 7.6 2.7 1.4 0.6 0.1 5.4 3.4 5.1 6.1

2.4 0.4 4.2 6.2 5.9 4.7 0.9 1.2 4.8 6.4 3.2 1.4 4.3 5.6 5.1 7.4 21.2 12.3 16.5 19.9 8.3 3.8 1.0 26.6 5.8 36.5 34.4

0.9 0.2 1.4 1.2 2.5 1.1 0.4 0.4 5.0 1.1 0.7 0.4 3.2 1.9 4.1 2.5 18.1 1.2 8.3 6.3 1.8 2.1 0.5 8.9 3.3 16.1 15.9

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42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68

PH

281

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Fig. 8. Contours for alkalinity, sulphate and chloride in ppm and Sb and As in ppb in groundwater. Location of known mineralised areas is shown in A. Hydrogeochemical anomalies of As, Sb, alkalinity, sulphate and chloride delineate the areas of gold mineralisation.

between 0.1 ppb in unmineralised areas to 41 ppb in mineralised areas and that of Sb ranges from 0.1 ppb in unmineralised areas to 16 ppb in mineralised areas. Background values in groundwater are in the range of 0.1–2.0 ppb for As and 0.1–1.0 ppb for Sb. All known mineralised zones in the belt are contained within the 5 ppb contours of As and 2.5 ppb contours of Sb. The threshold values are thus taken to be 5 ppb for As and 2.5 ppb for Sb. The contours of As and Sb is shown in Fig. 8b and c.

6. Nature of mobility and precipitation The major controls on the dispersion of ore and

pathfinding elements in soil is the availability of reaction sites. Reaction sites include oxides of Fe–Mn, oxides of Al and Si, organic matter, carbonates and secondary sulphides (Fortescue, 1975). Effective ways of determining the mode of transport and distribution of trace elements in the soil profile is by the use of leachate analysis and speciation studies. In the present study, sequential extraction and speciation studies are used to determine the principal phases responsible for fixation of the pathfinder elements. The appropriateness of sequential extraction schemes to study the phase-wise distribution of elements is discussed by Fonseca and da Silva (1998) and Hall (1998). In practice, the success of a sequential extraction scheme depends on several

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Fig. 9. Relative enrichment of As, Sb and Hg in different chemical phases as obtained from partial extraction studies on soil. Three size fractions have been studied from two samples each from red and black soils.

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factors such as chemical proportion of extractants, experimental condition (time, nature of contact, sample to extractant–volume ratio), choice and position of the reagent or mixtures in the extraction scheme and specific matrix effects (Fonseca and da Silva, 1998). In the present work, the sequential extraction scheme of Tessier et al. (1979) was used without modification. The results are discussed on the assumption that this scheme is valid for the soils being studied. 6.1. Studies on partial extraction of As, Sb, Hg and Bi. Sequential extraction of As, Sb, Hg and Bi was done on two samples each of red and black soils. Each of these samples were divided into three size fractions (⬍212 to ⬎106 mm, ⬍106 to ⬎53 mm, and ⬍53 mm). The study was carried out to assess the variation in content of these trace elements in different reaction sites (carbonate, exchangeable sites, Fe–Mn oxides, organic matter, sulphides and residual phase). The step-wise scheme of analysis of the sequential partial extraction technique is given below. Step 1: removal of adsorbed metals (exchangeable phase). Step 2: removal of carbonate bound metals. Step 3: removal of metals bound to amorphous Fe– Mn hydroxides. Step 4: removal of organic and sulphide bound metals. Step 5: removal of the resistates. Solutions prepared in each step were brought to pH 2 with the use of HNO3 and stored for HG–ICP analyses. Step 5 was carried out with HF–HClO4 – HNO3 mixture in closed relief-pressure teflon bombs in a microwave system. The analysis for As, Sb, Hg and Bi have been carried out using the HG–ICP–OES technique, as described earlier. The concentration of the elements in each reaction site was determined. To check the result, the sum of concentrations obtained by partial extraction studies was compared with the total concentration obtained by direct analysis of the whole sample. In each case the error was found to be low (⬍10%), indicating analytical reliability. The percentage distribution of

As, Sb and Hg with respect to its total content was computed from the data to quantify the relative importance of various chemical phases, soil type and size fractions of soil on the fixation of the trace elements. Fig. 9 shows the partitioning of As, Sb and Hg into different phases in three size-fractions of four samples from the two soil-types. Arsenic shows an almost similar distribution with all the reaction sites in all size-fractions of the two soil-types. Among the different reaction sites, As is found to be principally associated with the organic-sulphides, amorphous Fe–Mn hydroxides and resistates. The low concentration of total organic matter (1.79–2.23%) in soil samples indicates that the sulphide phase has a dominant control on the partitioning of As. Antimony is principally associated with carbonates, organic-sulphides and resistates. Mercury is partitioned preferentially into amorphous Fe–Mn hydroxides, organicsulphides, exchangeable sites and resistates. The type of soil does not significantly affect the preferential fixation of Hg into different reaction sites. Bismuth was detected only in the resistates, irrespective of size-fractions and type of soil. The anomalies of Bi observed in the soil medium may thus be of primary or clastic dispersion. 6.2. As speciation in groundwater Speciation of As in groundwater under prevailing conditions of Eh and pH helps in understanding the possible mineral phases of As which may precipitate from groundwater and exist in equilibrium with it. Speciation calculations involve the study of equilibrium relationships between various aqueous species and co-existing mineral phases through a set of mass action, mass balance and charge balance equations. Simultaneous solution of this set of non-linear equations results in determination of species activities and mineral saturation indices (Andersen and Crerar, 1993; Glynn and Brown, 1997). The PHREEQC geochemical code of Parkhrust (1995) was used for speciation studies. The software has a built-in set of specific reactions and equilibrium constants. It provides the concentration and activity of all species considered, the ion activity product, solubility product and saturation index of minerals. Physico-chemical parameters of four groundwater samples, two (W1 and W6) from mineralised areas

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Table 5 Analysis of W1, W5, W6 and W8 of groundwater used as input for speciation studies. Concentrations are in ppm (except for pH and pe) Parameters

W1

W5

W6

W8

pH Pe P PCa PMg PNa K P PFe PMn Al P PSi PSO4 Cl Total Alkalinity P As

7.38 11.67 35.27 23.33 2.40 0.20 0.15 0.01 0.06 95.36 56.90 21.99 2410.90 0.00625

8.08 11.75 24.05 28.19 11.10 0.84 0.27 0.02 0.04 66.95 165.90 41.99 3668.76 0.00287

8.25 11.92 24.98 28.19 15.01 1.60 2.19 0.004 0.14 141.90 106.49 25.99 4716.98 0.0411

7.92 11.87 32.06 29.16 12.50 0.10 0.17 0.02 0.05 112.005 93.69 31.99 4192.87 0.00069

and two (W5 and W8) from barren areas were used in the study. The input data contained total dissolved concentrations of all species and elements (Ca, Mg, ⫺ Na, K, Fe, Mn, Al, As, Cl ⫺, SO2⫺ 4 and HCO3 †; pH, pe and temperature. The data used as input for the four samples is given in Table 5. Significant concentration of As is observed in the groundwater samples from mineralised areas (W1 and W6), whereas the two samples from barren areas (W5 and W8) showed very low concentrations of As. The output of speciation calculations is given in Table 6. These results show the concentrations of various species of major and trace cations in groundwater. Arsenate species constitutes more than half of all the dissolved arsenic in all samples. This is attributed to the high oxidation potential of the system. ⫺ Among all the arsenate species, HAsO2⫺ 4 ; H2 AsO4 3⫹ and H3AsO4 (aq) are predominant. Arsenite (As ) species constitute only 2.4–6.7% of the total

dissolved As. Among the arsenites, H3AsO3 is the dominant species. Saturation index (SI) calculations for a range of As phases including Ca3(AsO4)2, AlAsO4:2H2O, Mn3(AsO4):8H2O, As2O5, arsenolite and clauditite show that precipitation of As in the form of these minerals is unlikely, as the SI of these minerals is negative. Scorodite is the only As-phase in equilibrium with the water samples. This mineral phase was also identified from XRD of selected soil samples (Table 3). All the studied samples of groundwater show supersaturation with respect to chlorite, goethite, hematite, magnetite, calcite, dolomite and the clay minerals (montmorillonite, illite, halloysite and kaolinite). Fig. 10 shows the diffractograms of minerals identified from the typical soil samples. The minerals actually identified from XRD studies of soil samples (Table 3) correspond very well with

Table 6 Concentrations (in ppm) of predicted complexes of As in samples from mineralised (W1 and W6) and barren (W5 and W8) areas

Arsenites (As(III))

Arsenates (As(V))

Species

W1

W5

W6

W8

H3AsO3 H2 AsO⫺ 3 HAsO⫺2 3 H4 AsO⫹ 3 HAsO2⫺ 4 ⫺ H2 AsO4 AsO3⫺ 4 H3 AsO4

4:179 × 10⫺6 7:001 × 10⫺35 2:412 × 10⫺39 1:019 × 10⫺40 7:285 × 10⫺8 1:078 × 10⫺8 9:831 × 10⫺12 6:663 × 10⫺14

2:112 × 10⫺36 1:826 × 10⫺37 0.0000 0.0000 3:741 × 10⫺6 1:021 × 10⫺9 2:886 × 10⫺11 1:222 × 10⫺15

6:067 × 10⫺36 7:903 × 10⫺37 2:034 × 10⫺40 0.0000 1:201 × 10⫺6 2:114 × 10⫺8 1:483 × 10⫺9 1:68 × 10⫺14

1:223 × 10⫺36 7:380 × 10⫺38 0.0000 0.0000 8:903 × 10⫺9 3:343 × 10⫺10 4:941 × 10⫺12 5:883 × 10⫺16

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Fig. 10. X-ray diffractograms of coarse, medium and fine (I, II and III, respectively) size fractions of black (H5 and H8) and red (U5 and U20) soils.

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the minerals which have SI greater than 1.0 in groundwater. Boyle and Jonasson (1973) have shown that pH is a major factor that controls mobility of As. In groundwater samples of the Hutti-Maski schist belt, pH ranges between 7.3 and 8.25. In this pH range, As is likely to be associated with organic-sulphide phase, clay minerals of high CEC and Fe–Mn hydroxides. From the presence of high proportion of these phases in the soil samples of Hutti-Maski schist belt, it is inferred that association of As in these phases may also be important.

7. Summary The results of studies on secondary dispersion of chalcophile elements in the Hutti-Maski schist belt may be summarised as follows. Concentrations of As and Sb in groundwater is sufficient to produce detectable anomalies. Topography has a control on concentrations of these elements in groundwater and soil. Arsenic is carried predominantly as arsenate species and fixed as scorodite and may also be adsorbed on clay minerals, amorphous Fe–Mn hydroxides, organic-sulphides and resistates. Antimony is mainly associated with carbonates, organic-sulphides and resistates. It is inferred from the above that As and Sb anomalies in soil may principally be hydromorphic. Mercury was not detected in groundwater samples but Hg anomalies related to gold mineralisations were observed in soil samples. These anomalies were located over known deposits but the anomaly patterns were somewhat irregular. Although Hg also occurs in the resistate fractions of soil, the association of Hg with clay minerals, Fe–Mn hydroxides and organicsulphides indicates that Hg anomalies in soil may also be hydromorphic. Bismuth is concentrated in the resistate fraction of soils of the Hutti-Maski schist belt. This observation, in addition to the fact that Bi is below detection limits in ground water and that its anomalies in soil are restricted to narrow zones almost overlying known deposits indicate that secondary dispersion of Bi is minor and that anomalies may be classified as clastic anomalies. Primary concentration of pathfinder elements are

287

controlled by deposits located along shear zones and granite–metabasalt contacts. All gold mineralised areas were found to be delineated by anomalous values of alkalinity, sulphate, chloride, As and Sb in groundwater. Since hydrogeochemical anomalies are clearly associated with mineralisation, and because sampling and analysis of water samples is relatively rapid and easy, it is recommended that groundwater sampling at 2–3 km spacing be taken up first during regional studies. Routine analysis of chloride, sulphate and alkalinity along with the analyses of As and Sb are sufficient for delineating prospective areas. The C-horizon soils may be sampled in hydrogeochemically anomalous areas on a 1 km grid for Asanomalies. For detailed work, C-horizon of soils in As-anomalous areas may be sampled for As, Sb, Hg and Bi on a 500 m grid to pinpoint location of deposits. Since anomalies of As and Sb in soil are principally hydromorphic, factors such as depth of water table, drainage texture and depth of weathering zone will control secondary dispersion of these elements in soil.

Acknowledgements The authors acknowledge the Department of Earth Sciences, Indian Institute of Technology, Bombay for providing the necessary facilities for carrying out this work. The hospitality provided by the Hutti Gold Mines Co. Ltd and the guidance given by Dr K. K. Raju, DGM (Exploration) for carrying out field work in and around Hutti is gratefully acknowledged. The analytical work would not have been possible without the cooperation and dedication of Mr L. S. Mombasawala, Research Engineer and the Analysts, Deepa and Bakul, of the ICP-AES Laboratory, Regional Sophisticated Instrumentation Centre, IIT, Bombay. The authors are also grateful to the USGS for providing them with GXR-2 and GXR-6 soil standards.

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