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Pathfinder applications of arsenic, antimony and bismuth in geochemical exploration
Journal of Geochemical Exploration, 15 (1981) 307--323
307
Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands Drainage Geochemistry PATHFINDER APPLICATIONS OF ARSENIC, ANTIMONY BISMUTH IN GEOCHEMICAL EXPLORATION
AND
M. HALE
Applied Geochemistry Research Group, Imperial College, London (England) (Received January 15, 1981)
ABSTRACT
Hale, M., 1981. Pathfinder applications of arsenic, antimony and bismuth in geochemical exploration. In: A.W. Rose and H. Gundlach (Editors), Geochemical Exploration 1980. J. Geochem. Explor., 15: 307--323. The potential applications of As, Sb and Bi as pathfinder elements in geochemical exploration have been researched using a new, rapid technique for the simultaneous determination of the three elements. Following a warm hydrochloric acid sample leach, the volatile hydrides of the elements are generated and flushed into an inductivelycoupled plasma linked to an emission spectrometer. The technique offers a combination of good analytical precision and detection limits of 100 ppb for each of the elements. The principal sulphide ore minerals commonly contain traces of As, Sb and Bi, and concentrations of more than 1% o f any one of these have been found in some sulphide specimens. During sub-aerial oxidation of sulphides, any As, Sb and Bi present is released and forms dispersion patterns in the surficial environment. Geochemical surveys of localities in the United Kingdom have demonstrated that anomalous dispersion trains of these elements can be detected in the sediments of streams draining the mineralized localities. In a geochemical mapping programme covering 16,000 km 2 of central Nepal, over 3500 stream sediment samples were analyzed for As, Sb and Bi, and many known occurrences of Cu, Pb and Zn mineralization are reflected by As, Sb and Bi anomalies. However, bedrock lithology appears to be an important factor influencing Sb and Bi dispersion patterns. In the areas studied, some or all of the elements As, Sb and Bi produce stream sediment anomalies that compare favourably in terms of contrast and extent with the heavy metal expressions, even though none of the three elements have been reported as important constituents of the mineralization with which they occur.
INTRODUCTION
T h i s w o r k d e s c r i b e s s e v e r a l s t r e a m s e d i m e n t s u r v e y s in w h i c h t h e m o r e m e t a l l i c e l e m e n t s o f p e r i o d i c g r o u p V - B , A s , S b a n d Bi, h a v e b e e n u s e d as pathfinders for base metal mineralization. A comprehensive review of the g e o c h e m i s t r y o f A s a n d i t s p r o s p e c t i v e v a l u e as a p a t h f i n d e r h a s a l r e a d y b e e n given by Boyle and Jo.nasson (1973). Some common features in the chemist r y o f A s , S b a n d B i p r o v o k e d t h e i r s i m u l t a n e o u s s t u d y in t h i s p r e s e n t w o r k .
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© 1981 Elsevier Scientific Publishing Company
308 Although the natural abundances of these elements are low, they each exhibit strong siderophile to chalcophile characteristics, which suggest a marked geochemical contrast between concentrations in sulphide minerals as compared to silicate or carbonate rocks. The abundance of As, Sb and Bi in average crustal rocks is estimated at 2 ppm As, 0.1 ppm Sb and 0.1 ppm Bi (Rose et al., 1979). In general, igneous rocks contain the lowest concentrations of these elements (Table I). C o m m o n sedimentary and metamorphic rocks tend to contain only slightly more As, Sb and Bi, and values at the upper end of the ranges quoted in Table I probably represent analyses of pyritic sediments and metasediments. The principal sulphide minerals of economic significance typically have a substantially higher content of As, Sb and Bi. Pyrite and chalcopyrite host the highest concentrations of As, while galena and sphalerite more readily accommodate Sb. Bismuth is concentrated in galena in particular and is also enriched in chalcopyrite and sphalerite. The As, Sb and Bi concentrations reported may occur as substitutions in the lattice of the host sulphides, or as inclusions of As, Sb and Bi minerals in solid solution. As rocks weather and sulphide minerals oxidize, As, Sb and Bi are likely to be carried into drainage systems as clastic fragments and in solution. Therefore, in a geochemical exploration drainage reconnaissance programme, these elements can assume the role of pathfinder elements for potentiallyeconomic sulphide mineral deposits. Their value in this role depends upon m a n y factors, but in particular upon the consistency of their association with the type of deposits sought, upon their geochemical dispersion TABLE I Range of As, Sb and Bi content of common rock types and common sulphide minerals (summarized from Boyle and Jonasson, 1973; Rose et al., 1979; and Wedepohl, 1974) Range (ppm) As
Sb
Bi
Rock type
Crustal average Mafic igneous (mainly dolerite, basalt) Acid igneous (mainly granite) Shale and argillite Schist and phyllite
2
0.1
0.1
0.18--113 0.18--15 0.3--500 0.5--143
0.1--0.9 0.1--0.3 0.2--2.4 1--7
<0.01--0.8 0.02--0.9 0.1--2.1 0.5--10
5--1000 <25 500--10,000 5--5000
<10--1000 < 10--5000 10--50,000 <10--5000
Mineral
Pyrite Chaleopyrite Galena Sphalerite
5--5600 10--1000 5--60 5--400
309
characteristics and u p o n the ease with which geochemical analysis can be performed. Following the development of a suitable analytical procedure, studies on the pathfinder applications of these elements were carried o u t at t w o localities in the United Kingdom. Determination of As, Sb and Bi in stream sediments was also included in a programme of geochemical mapping in central Nepal. Data from these projects allow an evaluation of the scope and limitations for As, Sb and Bi in mineral exploration geochemistry. ANALYTICAL TECHNIQUE
Stream sediment samples were air-dried, or oven-dried at 100°C, and sieved with retention of the minus 200 ~m fraction. This was pulverized to a fine p o w d e r in a ball mill. For determination of As, Sb and Bi, samples were digested by a pressurized acid leach (Pahlavanpour et al., 1980). In this procedure, 250 mg of powdered sample was weighed into a t y p e of borosilicate glass test-tube which could be temporarily sealed. Samples were treated with 5 ml hydrochloric acid, and any evolution of gas from carbonate minerals was allowed to cease. The test-tubes were then sealed with a disc of Teflon FEP film held in place b y a screw-on or crimp-on cap (according to tube design) lined with a disc of silicone rubber. The sealed tubes were placed in an aluminium heating block at 150°C for 2 hours. This attack is known to destroy magnetite, hematite, goethite, montmorillonite, apatite, and biotite, b u t n o t pyroxene, hornblende, kaolinite, muscovite, albite and orthoclase. The minerals destroyed include those most likely to adsorb chemically dispersed As, Sb and Bi, while many silicates in which these elements may undergo clastic dispersion are unaffected. After removal from the block, the tubes were cooled, unsealed, and 5 ml of 0.2% w/v potassium iodide solution was added to each t u b e to reduce As and Sb in solution to the trivalent state, and suppress possible Cu interference with the generation of bismuth hydride. The contents of each tube were homogenized and the sediment allowed to settle. From the supernatant solutions, simultaneous instrumental determination of As, Sb and Bi was performed as follows (Thompson et al., 1978). Sample solution and sodium tetrahydroborate solution were p u m p e d at a constant rate into the hydride generator, where they were mixed and reacted. The reaction products were carried into a liquid/gas separation cell so that the hydrides of the analytes were released and flushed in a stream of argon into an inductively-coupled radio-frequency argon plasma. The intensities of emissions in the plasma at characteristic wavelengths for As, Sb and Bi were recorded by an emission spectrometer and converted automatically to concentrations in the parent solution on this basis of calibration solutions. Solutions containing one or more analytes at a concentration above the limit of linear calibration (800 ng mL -1 As, 1500 ng mL -1 Sb and 500 ng
310
mL - i Bi) were diluted tenfold and rerun; this procedure was repeated if necessary. The lower limit of detection was a b o u t 2 ng mL -1 for each of the three elements, equivalent to less than 0.1 ppm in the sample. Approximately 200 samples could be analyzed in an eight-hour day. Determination of Cu, Pb and Zn was by atomic absorption spectrophotometry following sample digestion in 4 : 1 nitric/perchloric acid. D E T A I L E D S U R V E Y S IN T H E U N I T E D K I N G D O M
Studies comparing the dispersion characteristics of As, Sb and Bi with heavy metals in drainage systems were carried out at t w o localities in the UK where known or suspected base metal mineralization is associated with granitic intrusions. At Maudlin, south of Bodmin, Cornwall, copper mineralization occurs in a series of sub-parallel veins striking approximately east-west (Fig. 1). The host rocks are Devonian metasediments, mainly slates and shales. The mineralizing fluids are thought to be hydrothermal solutions associated with the intrusion of the St. Austell granite, which outcrops a few kilometres to the west. Maudlin is an area of gently rolling pasture land, with freely-drained residual softs on the low hills and slopes, and alluvium in valleys. Sediment samples were collected at intervals of 50 to 200 m along a stream draining the vicinity of the known mineralized veins and tributary draining a non-mineralized locality. Samples were analyzed by the techniques already described {Fig. 2). Local geochemical thresholds, set at the maximum values found in the sediments of the tributary draining the non-mineralized locality, are 60 ppm Cu, 80 p p m As, 3 p p m Sb and 5 ppm Bi. Along the stream draining the
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l__l Fig. 1. Stream sediment sample sites, Maudlin, and l o c a t i o n of area in southwest England.
311
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Fig. 2. Geochemical profiles of stream sediment data, Maudlin. Upper segment, main stream; lower segment, unmineralized tributary.
312
mineralized veins, concentrations below or near threshold characterize the stream course above the vicinity of the veins. Immediately below this part of the stream, where mineralization occurs in the valley slopes to the south, roughly coincident anomalies of up to 900 ppm Cu, 2100 ppm As, 9 ppm Sb and 65 ppm Bi occur in the stream sediments. These anomalies represent a geochemical contrast of 25 times threshold for As and 13 times threshold for Bi b u t only 3 times threshold for Sb, compared with 15 times threshold for Cu. Further downstream, the pattern of coincident anomalies persists, punctuated by zones where Cu, As, Sb and Bi levels fall close to or below threshold. This trace element distribution along the stream course is attributed to the introduction of material derived from the mineralization into the stream channel at several points, consistent with the known occurrence of mineralized veins in the valley slopes. The data show that As, Sb and Bi act as pathfinder elements for the Maudlin copper mineralization and illustrate improved anomaly contrast for As compared with Cu in stream sediments. Further detailed work was conducted in the Cairngorm mountains of Invernessshire, Scotland. There is no identified mineralization in this area, b u t the Cairngorm granite, which occupies much of the region, was found to be relatively uraniferous during a geochemical reconnaissance carried o u t by the Institute of Geological Sciences (J. Plant, pers. commun., 1980). During the present work, base metal anomalies were found in sediments of the one stream draining the northern sector of the granite. The Cairngorm mountains comprise rugged terrain rising to over 1200 m. Within the area sampled there is little development of soft. A blanket of
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313
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314 peat covers much of the area, but granite outcrop is widespread. Stream sediment samples were collected at 200 m intervals along two tributaries which rise in corries near the summit of Cairngorm and converge several kilometres downstream from their sources (Fig. 3). Analysis by the techniques described above disclosed a subdued geochemical relief in one tributary and the stream beyond the confluence. From the maximum trace element concentrations f o u n d in the sediments of these drainages, local thresholds were set at 5 ppm Cu, 30 ppm Zn, 2 ppm As and 1 ppm Bi. In the second tributary, anomalous concentrations of these elements are f o u n d several hundred metres downstream from the headwaters (Fig. 4). The Cu anomaly reaches 16 ppm, representing a geochemical contrast of just over 3 times threshold. The highest Zn, As and Bi concentrations are 150 ppm Zn, 10 ppm As, and 5 ppm Bi, or 5 times threshold. Although both the levels of trace elements in the sediments and anomaly contrast are much lower in the Cairngorm area than in the Maudlin area, As shows a consistent anomaly contrast advantage over Cu in the two areas. The Cairngorm anomalies appear to be derived from a single source or, at most, two nearby sources. Consequently, it is possible to compare the lengths of dispersion trains of the anomalous elements in the stream sediments. From the peak of Cu and Bi anomalies, concentrations above the threshold of these elements persist for about 1000 m downstream. Using the same criteria, the dispersion train for As can be recognized over approximately 1500 m, and that for Zn about 2500 m. Therefore, the UK detailed surveys indicate that in terms of geochemical contrast and length of dispersion train, As is superior to Cu, and similar to or inferior to Zn, and Bi is similar to Cu. A n t i m o n y exhibited poor contrast in the area in which it was studied. REGIONAL GEOCHEMICALMAPPING IN CENTRAL NEPAL A more comprehensive assessment of the role of As, Sb and Bi in geochemical exploration is brought out by the results of a regional geochemical mapping program in central Nepal. Central Nepal, as referred to here, is an almost triangular belt of the Lesser Himalayas, bounded in the north by the 28°N parallel of longitude, in the east by the 86°45'E meridian of latitude and to the southwest by the line joining the points 28°N/84°E and 27°N/86°45'E. The area is strongly dissected with high relief. Elevations range from 150 to 4500 m. The Lesser Himalayas are made up mainly of clastic sediments with subordinate but locally important carbonates and granites (StScklin, 1980}. The Nawakot Complex, which occupies the east and west of the area, consists almost exclusively of low-grade metasediments, particularly schists, quartzites and marbles, followed by unmetamorphosed or weakly metamorphosed sediments such as slates, quartzites and limestones. The Kathmandu Complex is superimposed on the Nawakot Complex by large-scale thrusting.
315 Intense tectonic deformation characterizes the outer zones of the Lesser Himalayas, b u t broad open folds are found in the centre of the area. The Kathmandu Complex contains gneisses and granites, which appear to be genetically related. Field evidence suggests a very y o u n g age for the granites and a clear intrusive relationship with many pre-existing structures. Gneisses and granites are absent from the N a w a k o t Complex. Known mineralization in central Nepal comprises minor occurrences of copper, lead and zinc sulphides (Jnavali, 1980). Copper mineralization is mainly associated with schists and quartzites and is c o m m o n l y tectonically controlled. Lead and zinc mineralization is clearly associated with carbonate formations and its distribution seems to have sedimentary control. Most of the known mineral occurrences are clustered in the west-central part of the area, although isolated occurrences are found elsewhere (Fig. 5). Regional geochemical mapping of central Nepal has been undertaken by the Mineral Exploration Development Board of Nepal, supported by His Majesty's Government of Nepal and the United Nations Development Program (Tooms and Shrestha, 1978). Over 20,000 stream sediment samples were collected t h r o u g h o u t the area and analyzed for heavy metals by various techniques. Of this sample suite, 3542 samples, representing coverage of the entire area at a density of approximately one sample per 4 km 2, were analyzed for As, Sb and Bi b y the m e t h o d described above. Analytical results for As, Sb and Bi range from values below the limits of detection to 273 p p m As, 39 p p m Sb and 41 p p m Bi. Each of the data sets appears to be uni-modal and conforms approximately to a lognormal distribution, l~egional geochemical thresholds for each element were set at the 99th percentile of the element data ranked by increasing concentration. This procedure defines 35 anomalies for each element, above thresholds of 64 p p m As, 8 p p m Sb and 4.5 p p m Bi. The spatial distributions of As, Sb and Bi are mapped in Figs. 6--8, in which sample sites with concentrations below threshold are shown as dots and sites with concentrations above threshold are marked as open squares. On a regional scale, it is apparent that many of the As, Sb and Bi anomalies occur in the vicinity of known mineralization. Of the remainder, some may be related to as y e t undiscovered mineralization, and others may be non-significant for the purposes of mineral exploration. Arsenic anomalies are scattered t h r o u g h o u t the area, while Sb anomalies are largely restricted to the west, and Bi anomalies are almost confined to the centre and southeast. For the full data set of 3542 samples, there is a good correlation of +0.65 between As and Sb, and this is reflected in the occurrence of many coincident As and $b anomalies in the west of the area. There is a poorer correlation of +0.30 b e t w e e n As and Bi, and there are a number of coincident As and Bi anomalies in the centre and southeast. The correlation between Sb and Bi is insignificant at +0.15, and there is an evident antipathy of Sb and Bi anomalies.
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The sample suite was divided into four components, representing the dominant lithologies of the drainage basins from which samples were taken. Most samples belong to either pelite or psammite populations, and the remainder come from smaller carbonate or acid rock l.~eiss and granite} populations. Data distributions of As, Sb and Bi within these populations again conform to uni-modal a n d approximately log-normal distributions. Plotted as cumulative frequency curves on a probability scale, the lines representing the four As populations group close together (Fig. 9) and a Kolmogorov-Smirnov test shows no significant differences exist between them. However, the data for the Sb and Bi populations do not cluster to the same degree. In the case of Sb, the population drawn from dominantly
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319
carbonate rocks exhibits the highest mean and the population associated with acid rocks the lowest mean (Fig. 10). A Kolmogorov-Smirnov test points to the difference between these populations being significant (i.e. real). For the Bi data, the population reflecting the acid rocks has the highest mean and that from carbonates the lowest (Fig. 11); again the difference is significant. Further inspection of Figs. 9 11 shows that whereas As anomalies are equally likely to be associated with any of the four principal rock types, Sb anomalies tend to occur in carbonate environments and Bi anomalies tend to be found in areas of gneiss and granite. The geological information presented here as Fig. 5 is sufficient to show that the spatial distribution of anomalies is likely to be consistent with the statistical interpretation of the data. While As anomalies are associated with all of the major geological groups, Sb anomalies are confined to the Nawakot Complex, in which carbonate formations are best represented, and are absent from the granite areas. Conversely, many Bi anomalies occur near the margins of granites. Lithology therefore appears to play an important role in the antipathy between Sb and Bi anomalies. 10'
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DISCUSSION
At least t w o factors contribute to the development of the As, Sb and Bi dispersion patterns recorded in the United Kingdom and Nepal. These are the extent o f occurrence of these elements in the rocks and mineralization of each area, and the constraints on geochemical dispersion of the elements in the surficial environment. Background levels of As, Sb and Bi in stream sediments are probably partly derived by clastic dispersion and therefore do not differ greatly from the
320
concentrations of these elements in the parent rock. Using as a basis for comparison the (scant) information available a b o u t the As, Sb and Bi contents of c o m m o n rock types (Table I), background levels of these elements in stream sediments are similar in central Nepal, low at Cairngorm and high at Maudlin. The large data sets from central Nepal represent low density coverage of an extensive region with a diversity of rock types, and regional background ranges consistent with formerly published generalizations are to be expected. The small data sets from the United Kingdom represent high density coverage of limited areas, and therefore reflect local background ranges characteristic of their areas of origin. A low background at Cairngorm is ascribed to clastic dispersion of mainly non-mineralized granite. The Maudlin area is located within the metallogenic province of southwest England, with which is associated a broad zone of As enrichment in bedrock (Webb et al., 1978); comparable Sb and Bi enrichment has n o t been reported, b u t may be expected to some degree. Above the selected geochemical thresholds, As, Sb and Bi anomalies appear in general to be related to known or suspected mineralization. However, n o t all known occurrences of mineralization within the areas studied are represented by such anomalies. The prerequisite for As, Sb and Bi anomalies is an association of one or several of these elements with the parent mineralization. Analysis of c o m m o n sulphide minerals suggests that As is most widely distributed in pyrite and Sb and Bi tend to be found in galena (Table II). The occurrence of significant quantities of these minerals, either in ore minerals or their associates, is therefore an important source of As, Sb and Bi. These three elements can also form their own minerals, which occur in minor quantities associated with some deposits. The work carried o u t here has not extended to the determination of As, Sb and Bi in the sulphide minerals of the known deposits. However, the stream sediment data suggests that at least one of these elements, most usually As, appears to be present in the mineralization in the quantities required for the application of pathfinder geochemistry in drainage surveys. Significant anomalies of As, Sb and Bi, related to oxidizing sulphide mineralization, are likely to include a large hydromorphic c o m p o n e n t compared with background levels of these elements. Mobility of the elements in the aqueous phase therefore greatly influences the occurrence of anomalies. Many factors contribute to controlling element mobility in the natural environment, b u t a much simplified picture can be gleaned from examination of the constraints imposed by redox potential (Eh) and pH on aqueous systems. In Figs. 12--14 the principal soluble species (light) and insoluble species (shaded) of As, Sb and Bi in aqueous solutions of a wide Eh-pH configuration are illustrated. Aqueous solutions in the natural environment are likely to fail within the field outlined by dashed lines. Conditions portrayed in the t o p left of this field are approximately those of the aqueous phase in oxidizing sulphide mineral deposits. The lower half of the field illustrates groundwater environments and surface waters plot in the t o p right.
321 TABLE
II
Frequency of occurrence of detectable concentrations of As, Sb, and Bi in c o m m o n sulphide minerals (from Fleischer, 1955). Data are percentage frequency of occurrence in the stated number of specimens analysed (shown in parentheses). Analytical sensitivity ranges from 1 to 100 ppm
Mineral
As
Sb
67 (99)
Pyrite
23 (35)
22
Galena Sphalerite
Bi
35 (17)
84
62
(229)
(224)
(327)
25 (235)
24 (197)
25 (186)
1.2
1.2
1,0
-7
25 C 1 otto
1,0
• 0.8
(18
0.0
0.6
OA
0.4
0.2
Eh
Eh 0~0
Sb.tO
-~
|
-0.2
-0.4
-0.4
- OJS
- o~
- 0.8
- 1.o
-- 110
1
2
3
4
5
6
7
8
9
10
11
12
pH
Fig. 12. Eh-pH diagram for As - H20.
13
14
1
2
3
4
5
6
?
8
0
10
11
12
13
14
pH
Fig. 13. E h - p H diagram for Sb - H20.
All three of the elements As, Sb and Bi are soluble and mobile in the vicinity of oxidizing sulphides, provided their host minerals are undergoing oxidation. Arsenic (Fig. 12) remains mobile under all conditions likely in the natural environment, leading to the development of relatively long As dispersion trains and As anomalies in a variety of host rock environments. This is
322
¢X8
0.6
04
a2 Eh Q~
-o.2
-0.4
- 0.8
1Q 1
2
3
4
5
6
7
8
9
10
11
12
f3
~4
pH
Fig. 14. Eh-Ph diagram for Bi - H20. consistent with patterns recorded in the detailed studies at Maudlin and Cairngorm and regional geochemical mapping in central Nepal. A n t i m o n y (Fig. 13) is least mobile in acidic groundwaters, but more mobile in groundwaters with a higher pH. It is therefore not surprising that in central Nepal, Sb levels are highest and Sb anomalies are best developed in carbonate environments. The absence of carbonate rocks from the United Kingdom study areas may contribute to the poor Sb anomalies there. Finally, Bi (Fig. 14) is mobile in acidic groundwaters, but becomes progressively less mobile with increasing pH. Thus in detailed studies in the United Kingdom, Bi is seen to exhibit shorter dispersion trains than As. In central Nepal Bi levels are highest and most Bi anomalies occur in areas of granite and gneiss, which are likely to have neutral to slightly alkaline groundwaters, and Bi anomalies seem to be absent from the higher pH carbonate environments. However, it should n o t be forgotten that Fe and Mn oxide colloids, clay mineral particles and organic matter also affect the dispersion of As, Sb and Bi, especially when soluble species of the elements enter the surface drainage systems. CONCLUSIONS This work shows that As, Sb and Bi can be determined rapidly and economically in stream sediment samples, and that anomalies of one or more of these elements are associated with a number of known mineral occurrences in the UK and central Nepal. The exploration significance of some anom-
323
alies has not been established, but a number of these may be related to undiscovered mineralization while others may be of no economic significance. Such anomalies are not unique in exploration geoc]aemistry, and can usually be discarded during skillful data interpretation. These elements therefore constitute viable pathfinders in exploration geochemistry, and their potential value in proposed exploration programs could be usefully assessed by inclusion in orientation surveys. Groundwater pH influences the hydromorphic dispersion patterns of As, Sb and Bi in different ways, and this requires consideratio n during data interpretation. ACKNOWLEDGEMENTS
The research described here is funded by the Natural Environment Research Council of the United Kingdom, and the Primary Raw Materials Research and Development Programme of the EEC. Permission to use data from central Nepal was kindly granted by the Mineral Exploration Development Board of Nepal.
REFERENCES
Boyle, R.W. and Jonasson, I.R., 1973. The geochemistry of arsenic and its use as an indicator element in geochemical prospecting. J. Geochem. Explor., 2: 251--296. Fleischer, M., 1955. Minor elements in some sulphide minerals. Econ. Geol., 50th Anniv. vol. : 970--1024. Hem, J.D., 1977. Reactions of metal ions at surfaces of hydrous iron oxide. Geochem. Cosmochim. Acta, 41: 527--538. Jnavali, B.M., 1980. Regional Geochemical Mapping and Data Interpretation in the Lesser Himalayas of Central Nepal. DIC thesis, Imperial College, London (unpubl.). Pahlavanpour, B., Thompson, M. and Thorne, L., 1980. Simultaneous determination of trace concentrations of arsenic, antimony and bismuth in soils and sediments by volatile hydride generation and inductively coupled plasma spectrometry. Analyst, 105: 756--761. Pourbaix, M., 1963. Atlas d'~quilibres ~lectrochemique ~ 25°C. Gauthier Villars, Paris. Rose, A.W., Hawkes, H.E. and Webb, J.S., 1979. Geochemistry in Mineral Exploration (2nd Ed.). Academic Press, London, 657 pp. St~cklin, J., 1980. Geology of Nepal and its regional frame. J. Geol. Soc. London, 137: 1--34. Thompson, M., Pahlavanpour, B., Walton, S.J. and K_/rkbright, G.F., 1978. Simultaneous determination of trace concentrations of arsenic, antimony, bismuth, selenium and tellurium in aqueous solution by introduction of the gaseous hydrides into an inductively-coupled plasma source for emission spectrometry. The Analyst, 1 0 3 : 5 6 8 - - 5 7 9 and 705--713. Tooms, J.S. and Shrestha, P.L., 1978. Mineralization and exploration methods in the H i m a l a y ~ of Nepal. Proc. l l t h Commonwealth Min. and Metall. Congr., pp. 249-260. Wedepohl, R.H. (Editor), 1969--1978. Handbook of Geochemistry. Springer-Verlag, Berlin. Webb, J.S., Thornton, I., Thompson, M., Howarth, R.J. and Lowenstein, P.L., 1978. The Wolfson Geochemical Atlas of England and Wales. Clarendon Press, Oxford.
307
Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands Drainage Geochemistry PATHFINDER APPLICATIONS OF ARSENIC, ANTIMONY BISMUTH IN GEOCHEMICAL EXPLORATION
AND
M. HALE
Applied Geochemistry Research Group, Imperial College, London (England) (Received January 15, 1981)
ABSTRACT
Hale, M., 1981. Pathfinder applications of arsenic, antimony and bismuth in geochemical exploration. In: A.W. Rose and H. Gundlach (Editors), Geochemical Exploration 1980. J. Geochem. Explor., 15: 307--323. The potential applications of As, Sb and Bi as pathfinder elements in geochemical exploration have been researched using a new, rapid technique for the simultaneous determination of the three elements. Following a warm hydrochloric acid sample leach, the volatile hydrides of the elements are generated and flushed into an inductivelycoupled plasma linked to an emission spectrometer. The technique offers a combination of good analytical precision and detection limits of 100 ppb for each of the elements. The principal sulphide ore minerals commonly contain traces of As, Sb and Bi, and concentrations of more than 1% o f any one of these have been found in some sulphide specimens. During sub-aerial oxidation of sulphides, any As, Sb and Bi present is released and forms dispersion patterns in the surficial environment. Geochemical surveys of localities in the United Kingdom have demonstrated that anomalous dispersion trains of these elements can be detected in the sediments of streams draining the mineralized localities. In a geochemical mapping programme covering 16,000 km 2 of central Nepal, over 3500 stream sediment samples were analyzed for As, Sb and Bi, and many known occurrences of Cu, Pb and Zn mineralization are reflected by As, Sb and Bi anomalies. However, bedrock lithology appears to be an important factor influencing Sb and Bi dispersion patterns. In the areas studied, some or all of the elements As, Sb and Bi produce stream sediment anomalies that compare favourably in terms of contrast and extent with the heavy metal expressions, even though none of the three elements have been reported as important constituents of the mineralization with which they occur.
INTRODUCTION
T h i s w o r k d e s c r i b e s s e v e r a l s t r e a m s e d i m e n t s u r v e y s in w h i c h t h e m o r e m e t a l l i c e l e m e n t s o f p e r i o d i c g r o u p V - B , A s , S b a n d Bi, h a v e b e e n u s e d as pathfinders for base metal mineralization. A comprehensive review of the g e o c h e m i s t r y o f A s a n d i t s p r o s p e c t i v e v a l u e as a p a t h f i n d e r h a s a l r e a d y b e e n given by Boyle and Jo.nasson (1973). Some common features in the chemist r y o f A s , S b a n d B i p r o v o k e d t h e i r s i m u l t a n e o u s s t u d y in t h i s p r e s e n t w o r k .
0375-6742/81/0000--0000/$02.50
© 1981 Elsevier Scientific Publishing Company
308 Although the natural abundances of these elements are low, they each exhibit strong siderophile to chalcophile characteristics, which suggest a marked geochemical contrast between concentrations in sulphide minerals as compared to silicate or carbonate rocks. The abundance of As, Sb and Bi in average crustal rocks is estimated at 2 ppm As, 0.1 ppm Sb and 0.1 ppm Bi (Rose et al., 1979). In general, igneous rocks contain the lowest concentrations of these elements (Table I). C o m m o n sedimentary and metamorphic rocks tend to contain only slightly more As, Sb and Bi, and values at the upper end of the ranges quoted in Table I probably represent analyses of pyritic sediments and metasediments. The principal sulphide minerals of economic significance typically have a substantially higher content of As, Sb and Bi. Pyrite and chalcopyrite host the highest concentrations of As, while galena and sphalerite more readily accommodate Sb. Bismuth is concentrated in galena in particular and is also enriched in chalcopyrite and sphalerite. The As, Sb and Bi concentrations reported may occur as substitutions in the lattice of the host sulphides, or as inclusions of As, Sb and Bi minerals in solid solution. As rocks weather and sulphide minerals oxidize, As, Sb and Bi are likely to be carried into drainage systems as clastic fragments and in solution. Therefore, in a geochemical exploration drainage reconnaissance programme, these elements can assume the role of pathfinder elements for potentiallyeconomic sulphide mineral deposits. Their value in this role depends upon m a n y factors, but in particular upon the consistency of their association with the type of deposits sought, upon their geochemical dispersion TABLE I Range of As, Sb and Bi content of common rock types and common sulphide minerals (summarized from Boyle and Jonasson, 1973; Rose et al., 1979; and Wedepohl, 1974) Range (ppm) As
Sb
Bi
Rock type
Crustal average Mafic igneous (mainly dolerite, basalt) Acid igneous (mainly granite) Shale and argillite Schist and phyllite
2
0.1
0.1
0.18--113 0.18--15 0.3--500 0.5--143
0.1--0.9 0.1--0.3 0.2--2.4 1--7
<0.01--0.8 0.02--0.9 0.1--2.1 0.5--10
5--1000 <25 500--10,000 5--5000
<10--1000 < 10--5000 10--50,000 <10--5000
Mineral
Pyrite Chaleopyrite Galena Sphalerite
5--5600 10--1000 5--60 5--400
309
characteristics and u p o n the ease with which geochemical analysis can be performed. Following the development of a suitable analytical procedure, studies on the pathfinder applications of these elements were carried o u t at t w o localities in the United Kingdom. Determination of As, Sb and Bi in stream sediments was also included in a programme of geochemical mapping in central Nepal. Data from these projects allow an evaluation of the scope and limitations for As, Sb and Bi in mineral exploration geochemistry. ANALYTICAL TECHNIQUE
Stream sediment samples were air-dried, or oven-dried at 100°C, and sieved with retention of the minus 200 ~m fraction. This was pulverized to a fine p o w d e r in a ball mill. For determination of As, Sb and Bi, samples were digested by a pressurized acid leach (Pahlavanpour et al., 1980). In this procedure, 250 mg of powdered sample was weighed into a t y p e of borosilicate glass test-tube which could be temporarily sealed. Samples were treated with 5 ml hydrochloric acid, and any evolution of gas from carbonate minerals was allowed to cease. The test-tubes were then sealed with a disc of Teflon FEP film held in place b y a screw-on or crimp-on cap (according to tube design) lined with a disc of silicone rubber. The sealed tubes were placed in an aluminium heating block at 150°C for 2 hours. This attack is known to destroy magnetite, hematite, goethite, montmorillonite, apatite, and biotite, b u t n o t pyroxene, hornblende, kaolinite, muscovite, albite and orthoclase. The minerals destroyed include those most likely to adsorb chemically dispersed As, Sb and Bi, while many silicates in which these elements may undergo clastic dispersion are unaffected. After removal from the block, the tubes were cooled, unsealed, and 5 ml of 0.2% w/v potassium iodide solution was added to each t u b e to reduce As and Sb in solution to the trivalent state, and suppress possible Cu interference with the generation of bismuth hydride. The contents of each tube were homogenized and the sediment allowed to settle. From the supernatant solutions, simultaneous instrumental determination of As, Sb and Bi was performed as follows (Thompson et al., 1978). Sample solution and sodium tetrahydroborate solution were p u m p e d at a constant rate into the hydride generator, where they were mixed and reacted. The reaction products were carried into a liquid/gas separation cell so that the hydrides of the analytes were released and flushed in a stream of argon into an inductively-coupled radio-frequency argon plasma. The intensities of emissions in the plasma at characteristic wavelengths for As, Sb and Bi were recorded by an emission spectrometer and converted automatically to concentrations in the parent solution on this basis of calibration solutions. Solutions containing one or more analytes at a concentration above the limit of linear calibration (800 ng mL -1 As, 1500 ng mL -1 Sb and 500 ng
310
mL - i Bi) were diluted tenfold and rerun; this procedure was repeated if necessary. The lower limit of detection was a b o u t 2 ng mL -1 for each of the three elements, equivalent to less than 0.1 ppm in the sample. Approximately 200 samples could be analyzed in an eight-hour day. Determination of Cu, Pb and Zn was by atomic absorption spectrophotometry following sample digestion in 4 : 1 nitric/perchloric acid. D E T A I L E D S U R V E Y S IN T H E U N I T E D K I N G D O M
Studies comparing the dispersion characteristics of As, Sb and Bi with heavy metals in drainage systems were carried out at t w o localities in the UK where known or suspected base metal mineralization is associated with granitic intrusions. At Maudlin, south of Bodmin, Cornwall, copper mineralization occurs in a series of sub-parallel veins striking approximately east-west (Fig. 1). The host rocks are Devonian metasediments, mainly slates and shales. The mineralizing fluids are thought to be hydrothermal solutions associated with the intrusion of the St. Austell granite, which outcrops a few kilometres to the west. Maudlin is an area of gently rolling pasture land, with freely-drained residual softs on the low hills and slopes, and alluvium in valleys. Sediment samples were collected at intervals of 50 to 200 m along a stream draining the vicinity of the known mineralized veins and tributary draining a non-mineralized locality. Samples were analyzed by the techniques already described {Fig. 2). Local geochemical thresholds, set at the maximum values found in the sediments of the tributary draining the non-mineralized locality, are 60 ppm Cu, 80 p p m As, 3 p p m Sb and 5 ppm Bi. Along the stream draining the
R Fo ey
Alluvium
Devonian
•
Sample
m e t asedJment s
sites
Lodes
0
200m
l__l Fig. 1. Stream sediment sample sites, Maudlin, and l o c a t i o n of area in southwest England.
311
A lO°Ot 5001 PDm |i Cu B
J i
5001
A
C
ppmi°°~t
,
B
v
,
p
v
As lOOt A
C
is-
1
0
ppm Sb B
-
,
"'
i
r
,
i
~
i
v
I
I
!
A 100t
ppmso ]
Bi
B
i
I
I
50"t 9
4.oom
Fig. 2. Geochemical profiles of stream sediment data, Maudlin. Upper segment, main stream; lower segment, unmineralized tributary.
312
mineralized veins, concentrations below or near threshold characterize the stream course above the vicinity of the veins. Immediately below this part of the stream, where mineralization occurs in the valley slopes to the south, roughly coincident anomalies of up to 900 ppm Cu, 2100 ppm As, 9 ppm Sb and 65 ppm Bi occur in the stream sediments. These anomalies represent a geochemical contrast of 25 times threshold for As and 13 times threshold for Bi b u t only 3 times threshold for Sb, compared with 15 times threshold for Cu. Further downstream, the pattern of coincident anomalies persists, punctuated by zones where Cu, As, Sb and Bi levels fall close to or below threshold. This trace element distribution along the stream course is attributed to the introduction of material derived from the mineralization into the stream channel at several points, consistent with the known occurrence of mineralized veins in the valley slopes. The data show that As, Sb and Bi act as pathfinder elements for the Maudlin copper mineralization and illustrate improved anomaly contrast for As compared with Cu in stream sediments. Further detailed work was conducted in the Cairngorm mountains of Invernessshire, Scotland. There is no identified mineralization in this area, b u t the Cairngorm granite, which occupies much of the region, was found to be relatively uraniferous during a geochemical reconnaissance carried o u t by the Institute of Geological Sciences (J. Plant, pers. commun., 1980). During the present work, base metal anomalies were found in sediments of the one stream draining the northern sector of the granite. The Cairngorm mountains comprise rugged terrain rising to over 1200 m. Within the area sampled there is little development of soft. A blanket of
'!!!iiiiil .........i:i iii i!i!iiiiiiiiiiiiii!iiiiii
Pre-
~- Cairn
Cambrian mataNdiments
Granite
Z
iii]!
!;':?!;ii:?::
:
. . . . . . .
':i:i:
Fig. 3. Stream sediment sample sites, Cairngorm, and location of area in northern Scotland.
313
A
C
Ppm B "Cu
~
,
A
C
100 50.
ppm
,
Zn
~
~
B
A
ppm As
10-~ 5B
5-'~,
, "'t
6A
C
2
ppm Bi
~
~
B
Ol
2(~OOm
Fig. 4. G e o c h e m i c a l p r o f i l e s o f s t r e a m s e d i m e n t d a t a , Cairngorm. U p p e r s e g m e n t , m a i n stream; lower segment, unmineralized tributary.
314 peat covers much of the area, but granite outcrop is widespread. Stream sediment samples were collected at 200 m intervals along two tributaries which rise in corries near the summit of Cairngorm and converge several kilometres downstream from their sources (Fig. 3). Analysis by the techniques described above disclosed a subdued geochemical relief in one tributary and the stream beyond the confluence. From the maximum trace element concentrations f o u n d in the sediments of these drainages, local thresholds were set at 5 ppm Cu, 30 ppm Zn, 2 ppm As and 1 ppm Bi. In the second tributary, anomalous concentrations of these elements are f o u n d several hundred metres downstream from the headwaters (Fig. 4). The Cu anomaly reaches 16 ppm, representing a geochemical contrast of just over 3 times threshold. The highest Zn, As and Bi concentrations are 150 ppm Zn, 10 ppm As, and 5 ppm Bi, or 5 times threshold. Although both the levels of trace elements in the sediments and anomaly contrast are much lower in the Cairngorm area than in the Maudlin area, As shows a consistent anomaly contrast advantage over Cu in the two areas. The Cairngorm anomalies appear to be derived from a single source or, at most, two nearby sources. Consequently, it is possible to compare the lengths of dispersion trains of the anomalous elements in the stream sediments. From the peak of Cu and Bi anomalies, concentrations above the threshold of these elements persist for about 1000 m downstream. Using the same criteria, the dispersion train for As can be recognized over approximately 1500 m, and that for Zn about 2500 m. Therefore, the UK detailed surveys indicate that in terms of geochemical contrast and length of dispersion train, As is superior to Cu, and similar to or inferior to Zn, and Bi is similar to Cu. A n t i m o n y exhibited poor contrast in the area in which it was studied. REGIONAL GEOCHEMICALMAPPING IN CENTRAL NEPAL A more comprehensive assessment of the role of As, Sb and Bi in geochemical exploration is brought out by the results of a regional geochemical mapping program in central Nepal. Central Nepal, as referred to here, is an almost triangular belt of the Lesser Himalayas, bounded in the north by the 28°N parallel of longitude, in the east by the 86°45'E meridian of latitude and to the southwest by the line joining the points 28°N/84°E and 27°N/86°45'E. The area is strongly dissected with high relief. Elevations range from 150 to 4500 m. The Lesser Himalayas are made up mainly of clastic sediments with subordinate but locally important carbonates and granites (StScklin, 1980}. The Nawakot Complex, which occupies the east and west of the area, consists almost exclusively of low-grade metasediments, particularly schists, quartzites and marbles, followed by unmetamorphosed or weakly metamorphosed sediments such as slates, quartzites and limestones. The Kathmandu Complex is superimposed on the Nawakot Complex by large-scale thrusting.
315 Intense tectonic deformation characterizes the outer zones of the Lesser Himalayas, b u t broad open folds are found in the centre of the area. The Kathmandu Complex contains gneisses and granites, which appear to be genetically related. Field evidence suggests a very y o u n g age for the granites and a clear intrusive relationship with many pre-existing structures. Gneisses and granites are absent from the N a w a k o t Complex. Known mineralization in central Nepal comprises minor occurrences of copper, lead and zinc sulphides (Jnavali, 1980). Copper mineralization is mainly associated with schists and quartzites and is c o m m o n l y tectonically controlled. Lead and zinc mineralization is clearly associated with carbonate formations and its distribution seems to have sedimentary control. Most of the known mineral occurrences are clustered in the west-central part of the area, although isolated occurrences are found elsewhere (Fig. 5). Regional geochemical mapping of central Nepal has been undertaken by the Mineral Exploration Development Board of Nepal, supported by His Majesty's Government of Nepal and the United Nations Development Program (Tooms and Shrestha, 1978). Over 20,000 stream sediment samples were collected t h r o u g h o u t the area and analyzed for heavy metals by various techniques. Of this sample suite, 3542 samples, representing coverage of the entire area at a density of approximately one sample per 4 km 2, were analyzed for As, Sb and Bi b y the m e t h o d described above. Analytical results for As, Sb and Bi range from values below the limits of detection to 273 p p m As, 39 p p m Sb and 41 p p m Bi. Each of the data sets appears to be uni-modal and conforms approximately to a lognormal distribution, l~egional geochemical thresholds for each element were set at the 99th percentile of the element data ranked by increasing concentration. This procedure defines 35 anomalies for each element, above thresholds of 64 p p m As, 8 p p m Sb and 4.5 p p m Bi. The spatial distributions of As, Sb and Bi are mapped in Figs. 6--8, in which sample sites with concentrations below threshold are shown as dots and sites with concentrations above threshold are marked as open squares. On a regional scale, it is apparent that many of the As, Sb and Bi anomalies occur in the vicinity of known mineralization. Of the remainder, some may be related to as y e t undiscovered mineralization, and others may be non-significant for the purposes of mineral exploration. Arsenic anomalies are scattered t h r o u g h o u t the area, while Sb anomalies are largely restricted to the west, and Bi anomalies are almost confined to the centre and southeast. For the full data set of 3542 samples, there is a good correlation of +0.65 between As and Sb, and this is reflected in the occurrence of many coincident As and $b anomalies in the west of the area. There is a poorer correlation of +0.30 b e t w e e n As and Bi, and there are a number of coincident As and Bi anomalies in the centre and southeast. The correlation between Sb and Bi is insignificant at +0.15, and there is an evident antipathy of Sb and Bi anomalies.
27 ° N
QUATERNARY
Siwalik sandstone
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~
Pb-Zn-lCu)
v
Fe
Mirteraliztd Outcrop • c. (~ Pt)- Zn- (Cu) Fe Mineralized Float • Cu © Pb-Zn-(Cu)
(~
c.
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•
Fig. 5. Generalized geology and mineralization, central Nepal.
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F i g . 7. A n t i m o n y
B's'm
distribution
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in s t r e a m s e d i m e n t s , c e n t r a l N e p a l .
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Fig. 8. Bismuth distribution in stream sediments, central Nepal.
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318
The sample suite was divided into four components, representing the dominant lithologies of the drainage basins from which samples were taken. Most samples belong to either pelite or psammite populations, and the remainder come from smaller carbonate or acid rock l.~eiss and granite} populations. Data distributions of As, Sb and Bi within these populations again conform to uni-modal a n d approximately log-normal distributions. Plotted as cumulative frequency curves on a probability scale, the lines representing the four As populations group close together (Fig. 9) and a Kolmogorov-Smirnov test shows no significant differences exist between them. However, the data for the Sb and Bi populations do not cluster to the same degree. In the case of Sb, the population drawn from dominantly
100
oA
(~' A []
o
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es
~'
Carbonates
I
50 frequency
10
0.1
°/o
A ~ ch C - q
°l
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Fig. 9. Cumulative probability distributions of central Nepal As data.
10 /
5
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A
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n
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Fig. 10. Cumulative probability distributions of central Nepal Sb data.
O1
319
carbonate rocks exhibits the highest mean and the population associated with acid rocks the lowest mean (Fig. 10). A Kolmogorov-Smirnov test points to the difference between these populations being significant (i.e. real). For the Bi data, the population reflecting the acid rocks has the highest mean and that from carbonates the lowest (Fig. 11); again the difference is significant. Further inspection of Figs. 9 11 shows that whereas As anomalies are equally likely to be associated with any of the four principal rock types, Sb anomalies tend to occur in carbonate environments and Bi anomalies tend to be found in areas of gneiss and granite. The geological information presented here as Fig. 5 is sufficient to show that the spatial distribution of anomalies is likely to be consistent with the statistical interpretation of the data. While As anomalies are associated with all of the major geological groups, Sb anomalies are confined to the Nawakot Complex, in which carbonate formations are best represented, and are absent from the granite areas. Conversely, many Bi anomalies occur near the margins of granites. Lithology therefore appears to play an important role in the antipathy between Sb and Bi anomalies. 10'
A
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o
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z,
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o
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.~
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lo
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Fig. 11. Cumulative probability distributions of central Nepal Bi data.
DISCUSSION
At least t w o factors contribute to the development of the As, Sb and Bi dispersion patterns recorded in the United Kingdom and Nepal. These are the extent o f occurrence of these elements in the rocks and mineralization of each area, and the constraints on geochemical dispersion of the elements in the surficial environment. Background levels of As, Sb and Bi in stream sediments are probably partly derived by clastic dispersion and therefore do not differ greatly from the
320
concentrations of these elements in the parent rock. Using as a basis for comparison the (scant) information available a b o u t the As, Sb and Bi contents of c o m m o n rock types (Table I), background levels of these elements in stream sediments are similar in central Nepal, low at Cairngorm and high at Maudlin. The large data sets from central Nepal represent low density coverage of an extensive region with a diversity of rock types, and regional background ranges consistent with formerly published generalizations are to be expected. The small data sets from the United Kingdom represent high density coverage of limited areas, and therefore reflect local background ranges characteristic of their areas of origin. A low background at Cairngorm is ascribed to clastic dispersion of mainly non-mineralized granite. The Maudlin area is located within the metallogenic province of southwest England, with which is associated a broad zone of As enrichment in bedrock (Webb et al., 1978); comparable Sb and Bi enrichment has n o t been reported, b u t may be expected to some degree. Above the selected geochemical thresholds, As, Sb and Bi anomalies appear in general to be related to known or suspected mineralization. However, n o t all known occurrences of mineralization within the areas studied are represented by such anomalies. The prerequisite for As, Sb and Bi anomalies is an association of one or several of these elements with the parent mineralization. Analysis of c o m m o n sulphide minerals suggests that As is most widely distributed in pyrite and Sb and Bi tend to be found in galena (Table II). The occurrence of significant quantities of these minerals, either in ore minerals or their associates, is therefore an important source of As, Sb and Bi. These three elements can also form their own minerals, which occur in minor quantities associated with some deposits. The work carried o u t here has not extended to the determination of As, Sb and Bi in the sulphide minerals of the known deposits. However, the stream sediment data suggests that at least one of these elements, most usually As, appears to be present in the mineralization in the quantities required for the application of pathfinder geochemistry in drainage surveys. Significant anomalies of As, Sb and Bi, related to oxidizing sulphide mineralization, are likely to include a large hydromorphic c o m p o n e n t compared with background levels of these elements. Mobility of the elements in the aqueous phase therefore greatly influences the occurrence of anomalies. Many factors contribute to controlling element mobility in the natural environment, b u t a much simplified picture can be gleaned from examination of the constraints imposed by redox potential (Eh) and pH on aqueous systems. In Figs. 12--14 the principal soluble species (light) and insoluble species (shaded) of As, Sb and Bi in aqueous solutions of a wide Eh-pH configuration are illustrated. Aqueous solutions in the natural environment are likely to fail within the field outlined by dashed lines. Conditions portrayed in the t o p left of this field are approximately those of the aqueous phase in oxidizing sulphide mineral deposits. The lower half of the field illustrates groundwater environments and surface waters plot in the t o p right.
321 TABLE
II
Frequency of occurrence of detectable concentrations of As, Sb, and Bi in c o m m o n sulphide minerals (from Fleischer, 1955). Data are percentage frequency of occurrence in the stated number of specimens analysed (shown in parentheses). Analytical sensitivity ranges from 1 to 100 ppm
Mineral
As
Sb
67 (99)
Pyrite
23 (35)
22
Galena Sphalerite
Bi
35 (17)
84
62
(229)
(224)
(327)
25 (235)
24 (197)
25 (186)
1.2
1.2
1,0
-7
25 C 1 otto
1,0
• 0.8
(18
0.0
0.6
OA
0.4
0.2
Eh
Eh 0~0
Sb.tO
-~
|
-0.2
-0.4
-0.4
- OJS
- o~
- 0.8
- 1.o
-- 110
1
2
3
4
5
6
7
8
9
10
11
12
pH
Fig. 12. Eh-pH diagram for As - H20.
13
14
1
2
3
4
5
6
?
8
0
10
11
12
13
14
pH
Fig. 13. E h - p H diagram for Sb - H20.
All three of the elements As, Sb and Bi are soluble and mobile in the vicinity of oxidizing sulphides, provided their host minerals are undergoing oxidation. Arsenic (Fig. 12) remains mobile under all conditions likely in the natural environment, leading to the development of relatively long As dispersion trains and As anomalies in a variety of host rock environments. This is
322
¢X8
0.6
04
a2 Eh Q~
-o.2
-0.4
- 0.8
1Q 1
2
3
4
5
6
7
8
9
10
11
12
f3
~4
pH
Fig. 14. Eh-Ph diagram for Bi - H20. consistent with patterns recorded in the detailed studies at Maudlin and Cairngorm and regional geochemical mapping in central Nepal. A n t i m o n y (Fig. 13) is least mobile in acidic groundwaters, but more mobile in groundwaters with a higher pH. It is therefore not surprising that in central Nepal, Sb levels are highest and Sb anomalies are best developed in carbonate environments. The absence of carbonate rocks from the United Kingdom study areas may contribute to the poor Sb anomalies there. Finally, Bi (Fig. 14) is mobile in acidic groundwaters, but becomes progressively less mobile with increasing pH. Thus in detailed studies in the United Kingdom, Bi is seen to exhibit shorter dispersion trains than As. In central Nepal Bi levels are highest and most Bi anomalies occur in areas of granite and gneiss, which are likely to have neutral to slightly alkaline groundwaters, and Bi anomalies seem to be absent from the higher pH carbonate environments. However, it should n o t be forgotten that Fe and Mn oxide colloids, clay mineral particles and organic matter also affect the dispersion of As, Sb and Bi, especially when soluble species of the elements enter the surface drainage systems. CONCLUSIONS This work shows that As, Sb and Bi can be determined rapidly and economically in stream sediment samples, and that anomalies of one or more of these elements are associated with a number of known mineral occurrences in the UK and central Nepal. The exploration significance of some anom-
323
alies has not been established, but a number of these may be related to undiscovered mineralization while others may be of no economic significance. Such anomalies are not unique in exploration geoc]aemistry, and can usually be discarded during skillful data interpretation. These elements therefore constitute viable pathfinders in exploration geochemistry, and their potential value in proposed exploration programs could be usefully assessed by inclusion in orientation surveys. Groundwater pH influences the hydromorphic dispersion patterns of As, Sb and Bi in different ways, and this requires consideratio n during data interpretation. ACKNOWLEDGEMENTS
The research described here is funded by the Natural Environment Research Council of the United Kingdom, and the Primary Raw Materials Research and Development Programme of the EEC. Permission to use data from central Nepal was kindly granted by the Mineral Exploration Development Board of Nepal.
REFERENCES
Boyle, R.W. and Jonasson, I.R., 1973. The geochemistry of arsenic and its use as an indicator element in geochemical prospecting. J. Geochem. Explor., 2: 251--296. Fleischer, M., 1955. Minor elements in some sulphide minerals. Econ. Geol., 50th Anniv. vol. : 970--1024. Hem, J.D., 1977. Reactions of metal ions at surfaces of hydrous iron oxide. Geochem. Cosmochim. Acta, 41: 527--538. Jnavali, B.M., 1980. Regional Geochemical Mapping and Data Interpretation in the Lesser Himalayas of Central Nepal. DIC thesis, Imperial College, London (unpubl.). Pahlavanpour, B., Thompson, M. and Thorne, L., 1980. Simultaneous determination of trace concentrations of arsenic, antimony and bismuth in soils and sediments by volatile hydride generation and inductively coupled plasma spectrometry. Analyst, 105: 756--761. Pourbaix, M., 1963. Atlas d'~quilibres ~lectrochemique ~ 25°C. Gauthier Villars, Paris. Rose, A.W., Hawkes, H.E. and Webb, J.S., 1979. Geochemistry in Mineral Exploration (2nd Ed.). Academic Press, London, 657 pp. St~cklin, J., 1980. Geology of Nepal and its regional frame. J. Geol. Soc. London, 137: 1--34. Thompson, M., Pahlavanpour, B., Walton, S.J. and K_/rkbright, G.F., 1978. Simultaneous determination of trace concentrations of arsenic, antimony, bismuth, selenium and tellurium in aqueous solution by introduction of the gaseous hydrides into an inductively-coupled plasma source for emission spectrometry. The Analyst, 1 0 3 : 5 6 8 - - 5 7 9 and 705--713. Tooms, J.S. and Shrestha, P.L., 1978. Mineralization and exploration methods in the H i m a l a y ~ of Nepal. Proc. l l t h Commonwealth Min. and Metall. Congr., pp. 249-260. Wedepohl, R.H. (Editor), 1969--1978. Handbook of Geochemistry. Springer-Verlag, Berlin. Webb, J.S., Thornton, I., Thompson, M., Howarth, R.J. and Lowenstein, P.L., 1978. The Wolfson Geochemical Atlas of England and Wales. Clarendon Press, Oxford.