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GEOLOGIAN TUTKIMUSKESKUS GEOLOGICAL SURVEY OF FINLA Tutkimusraportti 114 Report of Investigation 114

Esko Kontas (ed.)

ANALYTICAL METHODS FOR DETERMINING GOLD IN GEOLOGICAL SAMPLES

Espoo 1993

Kontas, Esko (ed.), 1993. Analytical methods for determining gold in geological samples. Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey of Finland, Report of bwestigation 114, 41 pages, 16 tables, 1 appendix.

Key words (GeoRef Thesaurus, AGI): chemical analysis, gold, sample preparation, methods, spectroscopy, atomic absorption, techniques, neutron activation analysis, accuracy

Esko Kontas, Geological Survey of Finlmd, P.O.Box 77, SF-96101 Rovmtiemi, Finland

ISBN 95 1-690-476-9 ISSN 0781-4240

CONTENTS

Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . EskoKontas 4

Analyttcal methods for determining gold in geological samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . EslwKontas 5

Determination of gold by atomic absorption after lead fire assay separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . Riitta Juvonen and P. J. Vaananen 13

Determination of gold by aqua regia-potassium bromate digestion, methyl isobutyl-ketone extraction and flame atomic absorption

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . P.Noras 17 Determination of gold and palladium by aqua regia digestion, dibutylsulphide-di-isobutyl-ketone extraction and flame atomic absorption

U. Penttinen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Determination of gold, palladium and platinum by aqua regia digestion, dibutylsulphide-di-isobutyl-ketone extraction and flameless atomic absorption

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . E.Ojaniemi 25 Determination of gold and palladium by aqua regia digestion, stannous chloride-mercury coprecipitation and flameless atomic absorption

E.Kontas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Neutron activation analysis of gold in geological samples

R. Rosenberg, Maija Lipponen and Riitta Zilliacus . . . . . . . . . . . . . . . . . 33 Gold concentrations of some reference samples - discussion

E.Kontas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

APPENDIX: The effect of sample weight and digestion and separation method on the results of gold determinations.

FOREWORD

This collection of papers documents the analytical methods on which geological gold studies in Finland are mainly based. It includes a general review of the problems associated with the determination of gold, and details of six procedures used by the laboratories of the Geological Survey of Finland, the Geoanalflcal Laboratory of Outokumpu Metals & Resources, the Research Centre of Rautaruukki Co. and the Reactor Laboratory of Technical Research Centre of Finland. Being very similar to those for gold, the analytical methods for palladium and platinum are also described. Finally, the gold contents of five reference samples analysed by each laboratory are presented, and their applicability for different purposes is discussed. As the procedures have all been reported previously or are otherwise well known, no new scientific features are presented. However, many of the methods have been developed and improved over the years, and thus a close look at the procedures may reveal useful information for chemists confronted by problems when analysing noble metals. Because the correct selection of analytical methods is crusial in gold studies, the topic is worth a document of its own.

I thank all chemists and all the other persons who have contributed to this work in one way or another and so made publication possible. In particular I thank Dr. Pekka A. Nurmi and Olli Lehto, Geological Survey of Finland, who reviewed the report and made many useful suggestions, and Pentti Noras, Geological Survey of Finland, and Pertti Hautala, Outokumpu Oy, for critical discussions and valuable comments. The English of the manuscript was corrected by Gillian Hakli.

Rovaniemi 25.2.1992 Esko Kontns Geological Survey of Finland

Analytical methods for determining gold in geological samples Edited by Esko Kontas Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey of Finland, Report of Investigation 114, 5-11, 1993

ODS FOR DE GOLD IN GEOLOGICAL SA

by Esko Kontas

Kontas E. 1993. Analytical methods for determining gold in geological samples. Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey of Firzland, Report of Investigation 114, 5-1 1, 2 tables,

Because of the heterogenous mode of occurrence, or "nugget effect", the preparation and representativeness of analytical samples pose special problems in assaying gold. Field samples of gold ores and their host rocks should be crushed to a grain size of at least 95% below 2 mm or preferably below 1 mm before splitting and subsequent milling to analysis grain size. About 100 g of powder is needed for analyses and possible control analyses.

The six analytical procedures presented below are those most often used for gold studies in Finland. The principles of the procedures, including sample weight, digestion, separation, determination, detection limits and capacities, are presented in Table 1.

Table 1. The principles of the analytical procedures.

Sample Digestion Separation Determi- Detect. Capacity weight nation limit g ppm sampleslw

25-50 lead fire assay FAAS 0.1 150 20 aqua regia-KBrO, MIBK FAAS 0.05 200 10 aqua regia DBS-DIBK FAAS 0.05 300 5 aqua regia DBS-DIBK GAAS 0.02 400 1 aqua regia SnC1,-Hg GAAS 0.001 500 0.6 EN A A 0.003 570

(Abbreviations: AAS = atomic absorption spectrometry, FAAS = flame AAS, GAAS = graphite furnace AAS, MIBK = methyl isobutyl ketone, DBS = dibutyl sulphide, DIBK = di-isobutyl ketone, ENAA = instrumental epithermal neutron activation analysis).

For the evaluation of methods, five internal reference samples prepared by the Geological Survey were analyzed several times with the above procedures. The relative standard deviations varied from below 10% up to 90%, being, as expected, largest for the methods that used the smallest sample weights.

The classical lead fire assay method (with a sample weight of 50 g) is suited best for inventories of gold deposits and for assaying gold concentrates. The methods with sample weight of 5-20 g and aqua regia digestion are appropriate for exploration analyses of gold ores and for geochemical prospecting. Owing to their low detection limits and high capacities, the last two methods in the above table are suitable for the geochemical mapping and prospecting on a regional scale and for basic research.

Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey of Finland, Report of Investigation 114, 1993 Esko Kontas

Key words (GeoRef Thesaurus, AGI): chemical analysis, gold, sample preparation, methods, spectroscopy, atomic absorption, techniques, neutron activation analysis, accuracy

Esko Kontas, Geological Survey of Finland, P.O.Box 77, SF-96101 Rovanierni, Finland

INTRODUCTION

There are three factors due to which exploration for gold may be viewed almost as its own branch of science. The first is the rarity of gold, the second is its heterogenous, nugget like mode of occurrence of the metal in the nature, and the third is its high economic value. As a result sampling, sample preparation and analytical methods have to meet more strin- gent requirements than other element.

The average crustal gold content is estimated 0.0035 ppm (Li & Yio 1966). The lower limit of the abundances in gold ores is about a thousand times higher. For geochemical exploration and for studies of the richest ores, the analybcal methods of gold should cover a concentration range of 104 to 102 PPm.

The gold concentrations in common geological materials are as follows:

1) Igneous rocks (median): ultramafic 0.0032, mafic 0.0032 and granites 0.0023 ppm (Wedepohl 1969-1978), 2) sedimentary rocks (median): limestones 0.005, sandstones and quartzites 0.005 and shales 0.004 ppm (Wedepohl 1969-1978), 3) soils (average): 0.002 ppm (Brooks 1972) and 4) fresh water (median): 0.002 ppb (Turekian 1977).

The mineralogical mode of occurrence of gold in nature varies widely but native gold is predominant (Boyle 1979). Even in very low concentrations it usually appears as discrete particles of considerable size, and is rarely, if ever, uniformly distributed. The discrete particles tend to segregate easily due to their high density relative to other minerals (Brown & Hilchey 1974). Being soft and malleable, gold grains are not easily reduced in milling; moreover in in- tense milling the grains may adhere to the walls of milling vessels and thus cause losses of gold from

the sample (Harris 1982, Riddle 1983, Burn 1984). Because of this uneven distribution, the main problem in the gold analyse and one which is often very difficult to resolve, is the representativeness of samples. Owing to its high economic value, gold should be determined with high accuracy and reliability. For instance, the gold in many base metal concentrates has a major effect on their price, and even small errors in assays may have marked economical signif- icance when translated into masses of material.

In practice, the analytxal capacities, detection limits and operating costs of procedures also have to be taken into account. Therefore no single procedure can effectively and economically meet all these requirements. For one procedure, the representativeness of an analybcal sample may be good enough but the detection limit may be too high and capacity too low; for another, the representativeness may be poor but the capacity high, and so on. Furthermore, there are very many procedures that are very similar in principle and chemistry but very different in practice. The difference is particularly marked in the analytical apparatus and other facilities. The classical f ~ e assay with lead collection requires only a simple fusion furnace and a microbalance. Two advanced instruments often used are the rather simple and cheap flame atomic absorption spectrometer (FAAS) and the more complex and expensive graphite furnace atomic absorption spectrometer (GFAAS). A tool that is rapidly gaining popularity is the very powerful plasma mass spectrometer (ICP-MS) (Date et al. 1987, Jackson et al. 1990). Most powerful of all is the nuclear reactor, which is used for the neutron activation analysis (NAA) (Hoffman et al. 1978, Hoffman and Brooker 1982). The following takes a look at all these procedures except the plasma-based techniques.

SAMPLE REPRESENTATIVENESS

Gold occurs in nature most commonly as native ver, copper or platinum-group metals. Some gold grains or as the main component of alloys with sil- and gold-silver tellurides, stibnites, selenides and

Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey of Finland, Report of I~zvestigation 114, 1993 Analytical methods for determining gold in geological samples

bismuthides are also known. But the predominant ore minerals are native gold, aurostibnite and various tellurides (Boyle 1979).

Gold tellurides are brittle and readily grindable minerals (Burn 1984). However, gold deposits also contain native gold as grains, which are not pulverized in milling. These grains may be extremely variable in size. Furthermore, gold can be incorporated in common sulphide minerals. Discrete inclusions, smaller than 0.1 pm, are termed "invisible gold", as they are not detectable by optical and scanning elec- tron microscopy. The concentrations of invisible gold range from less than 0.5 ppm to more than 1000 ppm in sulphide grains from 12 different ore deposits, as determined by ion-probe microanalysis (Cook & Chryssoulis, 1990). The most important gold carriers are arsenopyrite and arsenic-rich varietes of pyrite. In the extended gold province of Wit- watersrand, the average grain size of gold is 80 pm (Boyle 1979). In placer deposits gold grains may weigh thousands of grams although weighths of a few milligrams are more common.

The representativeness of samples containing gold particles has been theoretically and experimentally studied by Clifton et al. (1969). According to them: "It can be shown mathematically that the number of gold particles in the sample is the only factor cont- rolling the precision of the chemical analysis. If the following assumptions are valid:

1) gold particles are of uniform mass, 2) gold particles comprise less than 0.1 % of all particles, 3) the sample contains a total of over 1000 particles, 4) analyt~cal errors are disregarded 5) gold particles are randomly distributed within the sample,

a precision of S O % at the 95% confidence level will be achieved when the sample taken for analysis contains twenty particles of gold. For reconnaissance studies, a smaller sample containing fewer particles of gold may suffice. It is important, however, to note that as the expected number of particles per sample falls below five, the chance of having no gold particles in a given sample greatly increases."

In fact the above assumptions are never valid, especially those concerning uniform grain sizes and analytical errors. Nevertheless, they provide a practical starting point for estimating the representativeness of a sample, as long as something is known about the grain size. If the size distribution of the gold particles is unknnown and analyses of splits are not available, an adequate sample size can be determined by assuming that all the gold in the sample

occurs uniformly in grains as large as the maximum size which has an effect on the total gold content (Clifton et al. 1969).

The analytical representativeness of samples with discrete particles has been studied at lenght by Gy (1982). The standard deviation of the fundamental sampling error for particulate materials can be estimated theoretically from some basic properties of the material to be sampled. Gy (1982) derived the following equation for making the estimation:

where b is the relative standard deviation of the fundamental sampling error, M, the mass of the sample, M, the mass of the lot to be sampled, Z and C the sampling constants and d the dimension of the largest pieces in the lot to be sampled. Because M, is large related to MS the equation can be simplified:

where Z is defined as Cd3. The sampling constant C for the particular material to be sampled contains four parameters characteristic of the material: C = f g l c, where f is the particle shape factor, g the size range factor, 1 the liberation factor and c the composite factor. Approximations or calculations for these factors has been briefly and clearly presented by Minkkinen (1987). The sampling constants C or Z can also be estimated empirically by determining the relative standard deviation of the sampling error and calculating it from the original equation. This requires a number of analyses of subsamples having the same size for fine materials and analyses of indi- vidual fragments for coarse materials (Gy 1982).

On the basis of Gy's theory of sampling, Minkkinen (1987) developed a computer program (SAMPEX) for solving practical sampling problems. The method involves estimating the sampling constant C. For well-characterized materials, C can be estimated from the material properties; for unknown materials it can be evaluated experimentally. The program can be used to solve the following problems: the minimum sample size for a tolerated relative standard deviation of the fundamental sampling error; relative standard deviation for a given sample size; the maximum particle size of the material for a specified standard deviation and sample size; the balanced design of a multi-stage sampling and sample-reduction process; and sampling for particle size determination.

When the grade is low and the gold particles are large the sample size required increases very rapidly. However, normal laboratory instruments and other

Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey of Finland, Report of I?zvestigatio~t 114, 1993 Esko Korltas

facilities often restrict the use of large analytical samples. In many production geared, commercial laboratories sample weights range from 10 g to 30 g. For very inhomogeneous materials it may be best to use physical methods of concentration for the determination of coarse-grained gold and the chemical analysis of fine-grained gold (Nichol 1985). Howev- er, the more complex procedure the higher are the operation costs.

Mathematical determination of the representative sample weight always calls for some information about the mode of occurrence of gold and its mine- ralogy.

In practice, when gold explorations begin in a new area, laboratories have to rely on assumptions. No data are available, not even on the possible existence of gold.

Since the precision and accuracy of analyses are highly dependent on the mode of occurrence of gold in samples, chemists need all the information geologists can give them about the samples. With good cooperation between geologists and chemists, it is possible to choose the methods that are the most ad- vantageous and economically viable for each pur- pose.

SAMPLE PREPARATION

The physical preparation of rock samples involves drying, crushing, splitting and pulverizing. Because of the heterogeneous distribution of gold, the normal rules for safely reducing sample weight do not apply (Gy, 1979). The laboratory should therefore work with the complete sample submitted as far as possible (Riddle 1983). The Geological Survey of Finland obtained acceptable accuracies and precisions for gold in different types of ore and their host rocks using the following procedure: Bulk samples (2-5 kg) were crushed in two steps in a jaw crusher to a grain size of 95% less than 2 mm. The samples were then split in a rotatory tube divider to obtain a subsample of about 100 g, which was then milled for analysis in a swing mill. There were 137 samples in all and their concentration range was from background to ore grade (Nurmi et al. 1991). Samples weighing 20 g were analysed for gold both by fire assay and by GFAAS after aqua regia digestion. The correlation coefficient obtained between these determination~ was 0.93 (Appendix).

Geoanalytical Laboratory of Outokumpu Metals & Resources (Olli Lehto, P.O. Box 74, SF-83501 Outokumpu, Finland, pers. commun.) uses the following procedure for preparing rock and drill core samples: Samples are crushed in a jaw chrusher to a grain size of 90% less than 4 mm. All coarse material is then crushed in a roller mill to a grain size of 95% less than 1 mm. The sample is split in a rotatory tube divider into eight fractions. Depending on the amount of the sample one or more fractions are pulverized completely in a swing mill to obtain about 60 g powder.

Based on the above methods of preparation, a recommended procedure might be as follows: Before the first splitting a whole field sample is crushed to a grain size of 95% less than 1 mm; a portion of

about 100 g of the crushed sample is then pulverized for analysis.

A jaw crusher is a cost-effective apparatus univer- sally used for crushing rocks. It is quick and easy to use and contaminates the sample only very slightly. However, the grain size achieved is at most 95% less than 2 mm. Nevertheless, a jaw crusher is fully adequate for precrushing. A grain size of 95% less than 1 mm is easily obtained by precrushing material in a roller mill, but then contamination of samples by rollers is a considerable problem. Samples, particularly those containing metallic gold, need frequent and very careful cleaning because the gold grains can smear and stick to the surface of the rollers. A disk mill, which is very effective for fine crushing down to grain sizes of approximately 0.15 mm and is frequently employed at laboratories specializing in the appraisal of gold ores. The disks, however, wear easily when hard rocks are milled and produce contamination by iron and its alloys, which is harmful if the same powder is used for other geochemical studies (Olli Lehto, pers. commun.).

Grinding ore-grade samples may smear rather than grind the gold grains. Thus the grinding vessels contaminated with gold whereas the sample is slightly depleted in gold (Riddle, 1983, Burn, 1984). E. Ojaniemi (Rautaruukki CO, Research Centre, Raahe, Finland) performed quantitative experiments to test the smearing of gold on the surfaces of grinding vessels in a swing mill as follows: Three samples with varied gold concentrations were milled for different length of time. After each milling the grinding pots were washed with water before milling sterile quartz. The gold concentrations of the milled quartz were determined to check whether or not any gold had smeared (Table 2).

Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey of Firrland, Report of brvestigatiorz 114, 1993

The relative depletion of gold during the usual milling time of about 60 S is insignificant. But the contamination of subsequent samples may become significant if samples of ore grade and of background contents are crushed and milled in the same batch and with the same equipment.

Soil samples are usually dried and sieved to specific fractions. The "nugget effect" of sediment samples is much more problematic than that of ore samples because the grade is generally low whereas the gold grains are often relatively coarse (Clifton et al., 1969). Plant and other organic samples are usually ashed before digestion. Warren and Horsky (1986) have successfully determined gold and thallium directly on pulverized plant samples after nitric acid and aqua regia digestion.

~ n a l ~ t i c a l methods for determining gold in samples

Table 2. Gold concentrations in 20 g portions of cleaning quartz (milling time 60 S) in relation to the milling times of three samples.

Milling Amount Au i n quartz after cleaning time of of quartz sample Sample 1 Sample 2 Sample 3 S g PPm PPm PPm

30 20 0.016 0.003 0.024 60 20 0.014 0.007 0.020 120 20 0.030 0.073 0.087 240 20 0.056 0.119 0.127

Sample 1 = copper concentrate with 32.0 ppm Au, sample 2 = albitisized schist with 1.2 ppm Au and sample 3 = albitized schist with 13.5 ppm Au.

GOLD RING CONTAMINATION

Experiences has shown that a gold ring may be a source of heavy contamination, especially during sampling but also during sample preparation. Gold ring contamination becomes significant when the gold concentration of samples is at the ppb level. For example, gold concentrations of 500-600 ppb were found in a fine fraction of till when samples were handled after sampling by a person wearing a gold ring. Also rather soft materials such as plant samples

are easily contaminated by a ring. The plastic capsules used in neutron activation analysis may pick up significant quantities of gold from a ring. After slight contamination, Au contents of 15-170 ng were found on capsules (Kontas 1990). It should be re- membered that the neutron activation method measures all the gold in a capsule regardless of whether the gold is outside or inside the capsule.

REVIEW ON ANALYTICAL METHODS

The physical preparation of samples is usually Neutron activation analysis is the only method that followed three stages: allows gold to be determined directly on samples

with a relatively low detection limit without any 1) decomposition of the sample, preconcentration. However, in most activation labo- 2) separation and preconcentration of the gold and ratories, the direct method restricts the size of the 3) measurement. sample, making it too small and unrepresentative.

Decompositon of samples

Fusion

Fire assay for gold uses reductive fusion and the classic collection by lead in a procedure that has been used since ancient times to concentrate andiso- late the noble metals (Beamish & Van Loon 1977, Van Loon 1984). Lead oxide is added to the fusion pot. The oxide is reduced to lead metal in the fusion process, which then quantitatively extracts and collects any gold or silver in the sample. More re- cently, nickel sulphide has also been used as a col-

lector in the fire assay of noble metals (Robert et al. 1971).

Acid treatment

Mixtures of hydrochloric and nitric acids or aqua regia are most commonly used in acid treatment procedures. A normal mixture is HCl+HNO, = 3+1 but others can also be used. Even if aqua regia dis- solves silicate minerals only to some degree, partial attack with aqua regia usually results almost total

Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey of Finlmzd, Report of htvestigation 114, 1993 Esko Kontas

dissolution of gold from geological materials when regia (Rubeska et al. 1977). Hydrobromic acid with samples are roasted (Signiholfi et al. 1984). Silicate bromium is also useful in dissolving gold from roast- samples can also be digested with hydrofluoric and ed samples (Thompson et al. 1968). perchloric acids before gold is dissolved in aqua

Separation and preconcentration

In the conventional fire assay, lead is separated from the fusion slag, hammered into a cube or button and placed on a heated cupel of bone ash or magne- sia. This in turn is placed in a cupellation furnace where the lead is oxidated into lead oxide, which is absorbed by the cupel leaving behind a small gold- silver alloy bead that contains all the gold to be determined. Gold is determined either gravimetrically or instrumentally. In the ~zickel sulphide fire assay procedure the NiS button obtained by fusion is milled and dissolved in hydrochloric acid. Gold and the other noble metals precipitate as sulphides or metals.

The precipitate is filtered, rinsed and dissolved for measurement (AAS, ICP, ICP-MS) or the noble metals are measured directly from the precipitate by neutron activation (Hoffman et al. 1978).

Solvent extraction

The gold in a solution may be separated and concentrated with liquid-liquid extraction. Numerous solvents and complexing agents are in use and sum- maries of the methods have been given by Beamish and Van Loon (1977). The most common method involves extraction of gold(II1) from a hydrochloric or hydrobromic acid solution with methyl isobutyl ketone (MIBK). In Rubeska et al. (1977), acid attack is followed by extraction into dibutylsulphide in to- luene and in Parkes and Murray-Smith (1979) into dibutylsulphide in di-isobutyl ketone.

Coprecipitation

In accordance with its character, metallic gold is easily precipitated by reductants. Since solutions are highly diluted in relation to gold, a coprecipitant is necessary. Tellurium is the usual coprecipitant and stannous chloride (Fryer and Kerrich 1978) or hypophosphorous acid the most common reductant (McHugh 1983). Stannous chloride-mercury coprecipitation is another attractive method for separating gold (Kontas 1981, Kontas et al. 1986) and some other readily reduced elements such as the platinum-

group metals, silver, tellurium and selenium (Niska- vaara and Kontas 1990). An advantage of coprecipitation is that the analyses can be performed in water solutions, which are more stable and easier to handle than the organic solutions and solvents.

Measurement

In the classical lead cupellation method gold is determined gravimetrically. The silver-noble metal bead or prill obtained by fusion and cupellation is treated with dilute nitric acid to separate the gold as pure metal, which is then weighed. The detection limit is generally 0.1 ppm. Fire assay separation has not changed much down the ages, but gravimetry has, being first replaced by optical emission spectrometry and then by atomic absorption spectrometry. The method is easy to apply and is used for gold ores and concentrates. Various separation and pre-concentration methods can be combined with different instrumental measuring methods. Nowadays atomic absorption instruments are very popular (Van Loon 1985). If a detection limit of 0.05 ppm is sufficient, flame atomic absorption is a cost-effective method. Graphite furnace atomization readily permits a ppb level, but measuring is slower and the equipment is more expensive than in flame atomization. Emission spectrometry had became almost obsolete until the new plasma emission and plasma mass instruments gave it a new lease of life. The most sensitive measuring method, plasma mass spectrometry, permits ultra trace contents to be determined. Neutron activation analysis is nowadays also widely applied, and if it is combined with a suitable preconcentration method, a small sample size does not impair its representativeness. The sensitivity of X-ray analysis is very low. However, based on X-ray analysis an interesting method has been developed for reconnaissance determination of gold in the field: gold is dissolved from a sample into a cyanide solution and adsorbed and concentrated on the surface of a carbon disk. Gold is then determined from the disk with a portable X-ray instrument (ASOMA Instruments, Austin, Texas).

Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey of Finland, Report of Investigation 114, 1993 Analytical methods for determining gold in geological samples

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Nurmi P. A., Lestinen P. & Niskavaara H. 1991. Geo- chemical characteristics of mesothermal gold deposits in the Fennoscandian shield and a comperison with selected Canadian and Australian deposits. Geol. Surv. Finland, Bulletin 351, 101 p.

Parkes A. & Murray-Smith R. 1979. A rapid method for the determination of gold and palladium in soils and rocks. Atomic Abs. Newsl. Vol. 18, No 2, 57-58.

Riddle C. 1983. Analytical methods for gold. 272-278 in The Geology of Gold in Ontario, ed. by A. C. Colvine. Ont. Geol. Surv. Miscellaneous Paper 110, 450 p.

Robert R. V. D., Van Wyck E. & Palmer R. 1971. The collection and determination of the noble metals in ores and concentrates by the fusion technique using nickel sulphide as a collector. National Institute for Metallurgy (South Africa) Report 1371.

Rubeska I., Koreckova J. & Weiss D. 1977. The determination of gold and palladium in geological materials by atomic absorption after extraction with dibutylsulfide. Atomic Abs. Newsl. Vol. 16, No 1, 1-3.

Signiholfi G. P., Gorgoni C. & Mohamed A. H. 1984. Comprehensive analysis of precious metals in some geological standards by flameless A.A. spectrocopy. Geostandards Newsletter, Vol. 8, No 1, 25-29.

Thompson C. E., Nagakava H. M. & Vansickle G. H. 1968. Rapid analysis for gold in geologic materials. U. S. Geol. Surv. Prof. 600-B, 130-132.

Turekian K. K. 1977. Geochemical distribution of elements. 111 Encyclopedia of Science and Technology, 4th edn. 627-630. McGraw-Hill, New York.

Van Loon J. C. 1984. Accurace determination of the noble metals. I . Sample decomposition and methods for separation. Trends Anal. Chem. 3, 10, 272.

Van Loon J. C. 1985. Accurace determination of the noble metals. 11. Determination methods. Trends Anal. Chem. 4, 1, 24.

Warren V. H. & Horsky S. J. 1986. Thallium, a biogeochemical prospecting tool for gold. J. Geochem. Expl. 26, 215-221

Wedepohl K. H. Editor, 1969-1978. Handbook of Geo- chemistry, Vol. 11-4. Springer Verlag, Berlin.

Analytical methods for determining gold in geological samples Edited by Esko Kontas Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey of Finland, Report of Investigatioti 114, 13-16, 1993

D IN GEOLOG MATERIALS EAD FIRE ASSAY SE ARATION

by Riitta Juvonen and Paavo J. Vaananen

Juvonen, R. & Vaananen, P.J. 1993. Determination of gold in geological materials by atomic absorption after lead fire assay separation. Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey of Finland, Report of Investigation 114, 13-16, 3 tables.

Key words (GeoRef Thesaurus, AGI): chemical analysis, techniques, gold, atomic absorption, reagents, accuracy

Riitta Juvonen and Paavo J. VaanZinen, Geological Survey of Finland, SF-02150 Espoo, Finland

INTRODUCTION

Analysis of gold and silver with the classical fire assay technique has been in use at the Geo- logical Survey of Finland for decades. The method with its many variations is well docu- mented in numerous publications and monog- raphes (e.g. Haffty et al. 1977, Beamish et al. 1977, Moloughney 1986). The method is widely used all over the world not only for analysing gold but also for silver, platinum and palladium. It permits use of a large sample, which is neces-

sary because of the uneven distribution of gold in geological material (Moloughney 1986).

The demand for gold analyses in our laboratory has increased. We have therefore studied the method carefully, seeking to make it faster and easier to perform without sacrificing the quality of the results. The following describes the method as it is currently performed at the laboratory of the Geological Survey of Finland.

REAGENTS AND APPARATUS

1) Naber N-41 H, muffle furnace with a vent on 4) Atlantic-Schmelztiegel GmbH, fusion cruci- top of the furnace, bles, roasting dishes and cupels, 2) Perkin-Elmer Model 5000, atomic absorption 5) Merck, fusion flux reagents and lead oxide spectrophotometer equipped with AS 50, (PbO), extra pure grade, autosampler 6) Merck, HNO, (G%), HCI (37%) and AgNO,, 3) Perkin-Elmer Model 2380 with HGA-500 reagent grade, graphite furnace equipped with AS 40 autosampler, 7) BDH Chemicals Ltd, gold, platinum and

Geologiau tutkimuskeskus, Tutkimusraportti - Geological Survey of Finland, Report of I~westigation 114, 1993 Riitta Juvonen and Pnavo J. ViiZi~zlinen

palladium standard solutions and tartrate and 0.6 kg of glass powder are mixed 8) Fusion flux: 0.8 g Na2C03, 1.3 kg K2C03, well in a ball mill. 1.0 kg Na2B,0,, 1.3 kg of potassium hydrogen

EXPERIMENTAL

A 25 g sample of ground rock is mixed well with 50 g of lead oxide (litharge) in a plastic bag. About 130 g of the fusion flux is added and the contents of the plastic bag are mixed well. A known amount of silver is added in the form of a silver nitrate solution. The plastic bag is placed into a fusion pot and the pot is transferred to a preheated furnace at 1100°C. The sample is left in the furnace for 1 hour, after which the melt is poured into an iron mould to cool. Most of the slag is hammered off, and the lead regulus is further cleaned by soaking in 10% HCI. The regulus is hammered into the form of a cube, brushed clean and cupelled. The lead is oxidized by air to lead oxide at a furnace temperature of 940°C. The liquid lead oxide is absorbed by the magnesite cupel and a small bead of silver is left behind. It contains the gold and silver and the platinum-group metals to varying degrees. Twelve samples are fused and cupelled at a time, using the same furnace type

for both operations. The silver bead obtained by fire assay is flat-

tened and placed in a 10-m1 graduated test tube. A small glass bead is added to avoid overboiling the solution. HNO, (65%), 0.5 ml, is added and the test tube is warmed carefully in a water bath for about one hour. The tube is cooled and 1.5 m1 of HCl (37%) is added. The solution is agi- tated, allowed to stand overnight at room temperature, and warmed very carefully, avoiding overboiling, on a water bath for about 30 minutes. When the dissolution is complete, the tube is cooled and filled to the mark with 6 M HCl. Gold standards are prepared to contain the same amount of acids as the samples. Silver need not be added to the standard solutions. Gold is determined by flame atomic absorption. The solution obtained is also used for determining platinum and palladium by graphite furnace atomic absorption.

THE LEAD FIRE ASSAY METHOD

Extraction of noble metals into lead

The well ground sample is melted together with lead oxide and a reducing fusion flux mixture, in which gold and the platinum-group metals together with added silver are reduced and dissolved in the simultaneously reduced metallic lead. Quantitative extraction of the noble metals into lead requires a complete reac- tion between the sample and the fusion flux. The sample must be finely ground and well mixed with the reagents before fusion. The viscosity of the flux should be such that the noble metals are dissolved by the metallic lead as it sinks to the bottom of the fusion pot. If the flux is too fluid, the lead will sink too fast to bring the noble metals down with it. If the flux is too viscous,

lead globules will remain in the slag, resulting in low metal values.

The best possible fusion flux has been devised for each rock type (Haffty et al. 1977). Howev- er, in routine work it is not generally possible to prepare the optimum fusion mixture for each sample. The flux, mentioned above, in routine use at the laboratory of the Geological Survey of Finland, generally works well. To control the fusion process, a check is kept on the weight and the appearance of the lead regulus. Reference samples are frequently analysed along with the sample series. Samples with more than 2% sulphur are roasted at 600°C in special roasting dishes before fusion.

Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey of Fiidmrd, Reporr of Iirvestigntiora 114. 1993 Determination of gold in geological materials by atomic absorption after lead fire assay separation

SCORIFICATION OF THE LEAD REGULUS

The lead regulus may be further purified by a process called scorification if the sample contains high concentrations of interfering elements, mainly copper and nickel. In the fusion process copper, cobalt and nickel are partly reduced along with the lead and noble metals. Cobalt will not dissolve in the lead, but copper and nickel will.

If the sample contains appreciable amounts of copper or nickel, further purification of the lead regulus is necessary. The regulus along with

about 1 g of borax is put into a shallow scorification crucible, which is transferred to a furnace with ample air flow, at 1000°C. In the furnace, part of the lead is oxidized and simultaneously copper and nickel are oxidized and dissolved in the slag. The scorification is re- peated, and metallic lead is added until a clear, smooth and malleable lead regulus is obtained. For example, the scorification of one gramme of copper requires 50 g of lead.

CUPELLATION

The lead is oxidized by a process called cu- placed on the cupels. The regulus melts almost pellation: the lead regulus is heated in the furna- instantly, the lead begins to oxidize and is ab- ce on a cupel, made of magnesite, which absorbs sorbed by the cupel. In the last stages of the the forming lead oxide. A small bead of silver cupellation, only a small spot remains in the containing the noble metals is left. centre of the cupel. This finally disappears, giv-

In practice, the cupels are first heated in the ing off a bright flash of light at the end. Pro- furnace at 940"C, after which the lead reguli are longed heating causes losses of silver.

DETERMINATION OF NOBLE METALS

In the conventional method of analysis, the silver bead is flattened by hammer and anvil, the flattened bead is weighed, silver is dissolved in dilute nitric acid, and the resulting flake of gold is weighed. The difference gives the total amount of silver. Along with the gold, any rhodium and iridium together with small amounts of platinum and palladium remain undissolved in nitric acid treatment (Beamish et al. 1977). The samples should contain about three times more silver than gold for a good partition of the two. The addition of silver also increases the size of bead, making it easier to handle.

Analysis with atomic absorption has replaced gravimetry in many laboratories (Kallmann et al. 1970, Moloughney 1986). At the laboratory of the Geological Survey of Finland, the flattened silver bead is dissolved in aqua regia and the gold is analysed by flame absorption and

Table 1. Graphite furnace programme for palladium and platinum.

Step Furnace Time Internal No. temperature Ramp Hold gas f low

"C ml/min

Palladium

1 2 3 4 5 Platinum

1 2 3 4 5

platinum and palladium by graphite furnace atomic absorption.

Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey of Finland, Report of Investigation 114, 1993 Riittn Juvonen and P a m o J. Viiiiniinen

Table 2. Gold concentrations of five Canadian reference samples.

Sample Weight F:

Number of This work Recommended determinations AU P Q ~ values ppm

CH-1 10 CH-2 10 MA- l a 5 MA-3 5 GTS-1 10

Data collected over three years.

Table 3. Platinum and palladium concentrations of reference samples.

S ample Weight Number of det's This work Recommended values

Pt Pd Pt Pd Pt Pd g PPn' PPm PP"' PPm

SARM-7 25 9 3 3.4550.22 1.4950.21 3.740 1 S 3 0 SU-la 5 4 0.3850.02 - 0.37

Data collected over three years.

PRECISION AND ACCURACY

Five gold ores of the Canadian Reference palladium were determined from the reference Materials Project were analysed with the present samples SARM-7 (South African Committee for method but with reduced sample weights. Corn- Certified Reference Materials) and SU-l a (Cana- parison of the results with the recommended dian Certified Reference Materials Project). The values is presented in Table 2. Platinum and results are presented in Table 3.

REFERENCES

Beamish P. E. & Van Loon J. C. 1977. Analysis of Noble Kallmann S. & Wobart E. W. 1970. Determination of Metals. Overview and Selected Methods. Academic silver gold and palladium by combined fire assay atomic Press, New York, 327 p. absorption. Talanta 17, 845-850.

Hafty J., Riley L. B. & Goss W. D. 1977. A Manual on Moloughney P. E. 1986. Assay methods used in CANMET Fire Assaying and Determination of the Noble Metals in for the determination of precious metals. CANMET SP Geological Materials. U. S. Geol. Surv. Bull. 1445, 58 p. 86-lE, 33 p.

Analytical methods for determining gold in geological samples Edited by Esko Kontas Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey of Finlmzd, Repori of Investigation 114, 17-19, 1993

by P. Noras

Noras, P. 1993. Determination of gold by aqua regia-potassium bromate digestion, methyl isobutyl ketone extraction and flame atomic absorption. Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey of Finlmzd, Report of bwestigation 114, 17-19, 1 table.

Key words (GeoRef Thesaurus, AGI): chemical analysis, gold, techniques, atomic absorption, sample preparation, reagents, accuracy

P. Noras, Geological Survey of Finland, SF-02150 Espoo, Finland

INTRODUCTION

In the late 1960s the need arouse for a rapid method to determinate gold, particularly owing to the increase in geochemical exploration, where both speed and cost are of paramount importance. Thompson et al. (1968) introduced a method based on Br,-HBr digestion, methyl isobutyl ketone (MIBK) extraction of AuBr, and AAS determination of Au. The method, described here uses aqua regia and potassium bromate for digestion before extraction of Au- halogenide complexes and determination of Au in very much the same way as the method of Thompson et al. The present method was developed and used extensively by the Mineral Pro- ject of the U.N. Development Programme in

Colombia on which the author was employed in 1974-77. Practical improvements were made to the procedure in the Geological Survey of Finland.

The method, which allows sample weights of up to 20 g to be used, offers a detection limit of 0.05 ppm Au in the sample, and accepts most types of material encountered in geochemical exploration. MIBK extraction has the advantage over the conventional fire assay method that it saves labour and materials expence, and can be operated by less trained technicians. The disadvantage is that it cannot be used to extract other precious metals. Moreover some interference is caused by high contents of iron in the MIBK phase.


1) Reagents: HCI 37%, HNO, 65%, KBrO,, grade, methyl isobutyl ketone (MIBK), all reagent 2) apparatus: Furnace for calcination (maximum

Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey of Firrlmld, Report of Investigation 114, 1993 P. Noras

temperature preferably 1000°C and Elmer Model 460 equipped with three-slot bur- 3) atomic absorption spectrophotometer Perkin- ner head.

SAMPLE PRETREATMENT AND DIGESTION

If samples are to be pulverized, avoid grin- to a 250-m1 Erlenmeyer flask, add 25 m1 of ding them too fine to prevent the formation of concentrated HC1, 5 m1 of concentrated HNO, emulsions during the extraction stage. and 0.5 g of KBrO,, shake the flask, stopper it

Weigh 20 g of dry sample powder into a wide lightly and allow it to stand at room temperature roasting dish. To destroy organic matter, calcine overnight. Then add 0.5 g of KBrO,, shake the sediment samples at 600°C for 2-5 hours, and flask again and place it in a water bath at 90°C rock samples at 700°C for one hour to decom- for one hour. pose sulphides. Transfer the calcined sample

EXTRACTION

Cool and dilute the sample solution by adding of the flask helps to separate the two phases. 170 m1 of water and shaking the flask. Then add Difficulty in breaking up the emulsion in the 20.0 m1 of MIBK, stopper the flask and shake it acid phase may be encountered with very fine vigorously for 2 min. Let it stand until the clear grain sizes. It is recommended that such sample organic phase has settled out. Rapping the neck solutions be filtered before the MIBK is added.

INSTRUMENTAL CONDITIONS AND DETERMINATION OF GOLD

Gold is determined by AAS in an air-acetyle- water is aspirated. The spectrometer is calibrated ne flame at a wavelength of 242.8 nm. A three- using standard MIBK solutions of gold prepared slot burner head is employed, and the flame is in the same way as the unknown samples. The adjusted to lean blue when aspirating MIBK. working curve is linear, typically up to 2 pg Au The flame is just beginning "jump up" when the in 1 m1 of MIBK.

DISCUSSION

The precision and accuracy of the method were studied by determining Au in some international standard reference samples and in-house reference samples. Due to the high gold value of the international materials, sample weights were limited to 2 g (Table 1).

At higher concentration levels (more than 0.15 ppm) the method shows good agreement with recommended values and acceptable precision. Unfortunately, for assessing the precision of the method at lower levels, the data mainly derive from the 2-g samples. The detection limit (3xb) of the method is estimated to be 0.05 ppm

Au in sample. The only serious interference is due to the

high contents of acid soluble iron in the sample. Iron extracted from an acid solution to MIBK gives "false" absorption at the measuring wavelength. It has been shown that 15% acid soluble iron in a sample is critical. Fortunately, the iron content is not that high in most sample types, although the critical concentration may be reached in pyrite ores. To overcome the possible interference, the MIBK phase should be washed with 0.1 M HCl solution until free of yellow colour.

Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey of Finland, Report of Investigation 114, 1993 Determination of gold by aqua regia-potassium bromate digestion, methyl isobutyl ketone extraction and flame atomic absorption

Table 1. Gold concentrations of some reference samples determined by the above method and the recommended values (in ppm).

S ample Weigh Number Au Stand. Recommended number of det's dev. value1

a P P ~

USBM G2 10 USGS 2.6 10 GXR- 1 2

Gladney and Burns (1984).

The determination limit of this method is fairly acceptable for rock samples, alluvial sedi- ments and residual soil samples. However, the method is certainly not sensitive enough for studying regional geochemical patterns e. g. till samples. For lower levels of Au and Ag, Bratzel et al. (1972) have described a method in which digestion and extraction are similar to the above but determination of Au is by carbon-rod AAS.

The daily rate of the method run by two technicians is 30-50 effective determinations. The method does not need highly trained staff or sophisticated instruments and thus is easily applicable in developing countries. Since MIBK is a flammable chemical and harmful for health, proper fume cupboards and ventilation are essential.

REFERENCES

Bratzel M. P. Jr., Chakrabarti C. L., Sturgeon R. E., elemental concentration data for the United States Geo- MacIntyre M. V. & Agemian H. 1972. Determination of logical Survey's geochemical exploration reference sam- gold and silver in parts per billion or lower levels in ples GXR-1 to GXR-6. Geostandards Newsletter Vol. 8, geological and metallurgical samples by atomic absorp- No. 2, 119-154. tion spectrometry with carbon rod atomizer. Anal Chem Thompson C. E., Nagakava H. M. 82 Vansickle G . H. 44, 372. 1968. Rapid analysis for gold in geological matrials. U.

Gladney E. S. & Burns C. E. 1984. 1982 Compilation of S. Geol. Surv. Prof. P. 600-B, 130-132.


DETERMINA A DIGESTION, EXTRACTION

by U. Penttinen

Penttinen, U. 1993. Determination of gold by aqua regia digestion, dibutylsulphide-di-isobutyl ketone extraction and flame atomic absorption. Geologian tutkimuskeskus, Tutkimusraportti - Geological Srmey of Finland, Report of Investigation 114, 21-23, 1 table.

Key words (GeoRef Thesaurus, AGI): chemical analysis, techniques, gold, palladium, atomic absorption, sample preparation, reagents, accuracy

U. Penttinen, retired Outokurnpu Metals & Resources Geoanalytical Laboratory P.O. Box 74, SF-83501 Outokunnpu, Finland

INTRODUCTION

Prospecting for the gold and platinum metals Outokurnpu Oy. The use of dibutylsulphide in has increased greatly, giving rise to a need for a gold determination was first described by fast and sufficiently accurate analytical method. Yudelevich et al. in 1970. The method has since Introduction of the method described below has been refined by Rubeska et al. (1977) and definitely improved the effectiveness of the Parkes and Murray-Smith (1979). We developed prospecting for gold and platiniferous deposits our gold and palladium determinations from undertaken by the Exploration Department of these methods in 1980.

THE PRINCIPLES OF THE METHOD

A sample should be well-homogenized and or, alternatively, the sample should be submitted preferably large. Organic material and sulphide to total digestion with hydrofluoric and nitric sulphur are eliminated by roasting at 600°C acids. The gold is dissolved in aqua regia. This (Strong and Murray-Smith 1974). The graphite, is preceeded by hydrochloric acid treatment to if present, should then be burned in the oxygen dissolve the iron precipitates on the surface of stream at 600°C. If gold occurs as inclusions in the gold particles. The gold and the palladium silicate particles, very fine grinding is needed are extracted from 2M hydrochloric acid solu-

Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey of Finland, Report of I?n~estigation 114, 1993 U. Penttinen

tion with dibutylsulphide dissolved in di-iso- from the washed and centrifugued organic ex- butyl ketone. The gold and palladium concentra- tract. The other elements do not interfere with tions are determined by flame atomic absorption the determination.


1) HCl 37%, HNO 65%, reagent grade, 3) Atomic absorption spectrophotometer: Perkin- 2) Dibutylsulphide solution: Dissolve 17.4 m1 Elmer Model 5000 and dibutylsulphide in 500 m1 of di-isobutyl ketone, 4) Shaking machine: Buhler Model B.

Procedure

Weigh 10 g of fine ground and well mixed sample into a porcelain crucible and roast it at 600°C for 2-3 hours. Mix as needed. If the sample contains arsenic, roast it at first for 2 hours at 450°C and then for 2 hours at 600°C to avoid partial evaporation of gold. If graphite is to be eliminated, burn it afterwards in a tube furnace in oxygen stream at 600°C.

Transfer the roasted sample to a 250 m1 beaker, add 80 m1 of diluted HCl (1+1), cover it with a watch glass and keep it at 80-100°C for two hours. Add 20 m1 of concentrated HNO,, and keep it covered at the same temperature for one hour, stirring from time to time. Evaporate to "wet salts". If evaporated to dryness, gold may be reduced to a lower valence and cannot be extracted.

Add 15 m1 of concentrated HCl and 20 m1 of water. Heat to dissolve the salts. Filter through

a Whatman GFIB glass fibre filter and wash with water. Rinse into a 100-m1 measuring flask, cool, fill to the mark with water and stir.

Transfer a 50-m1 aliquot to a 100-m1 separatory funnel. Add 5 m1 of dibutylsulphide solution and shake using the full effect and shaking amplitude of the machine for two minutes. Let the phases separate. Wash the organic extract with 25 m1 of 2M HCI. Centrifuge for 10 minutes at 3500 rpm.

Flame atomic absorption of gold is measured at a wavelenght of 242.8 nm using background correction. The intergation time is 0.5 S and the reading an average of ten measurements. Palla- dium is measured using a wavelength of 244.8 nm and background correction.

For calibration, the extraction is done using 1, 2 and 3 m1 of solution containing 10 pglml of gold and palladium in 2M HCl.

ACCURACY AND PRECISION

The following samples were analysed to check Mine (Sample B) and the accuracy and precision of the method: 3) gold ore MA-2, Canmet (Sample C).

1) Cu-Bi-CO matte, Falconbridge (Sample A), The results are presented in Table 1. 2) Cu concentrate, Outokumpu Oy, Pyhasalmi

DETECTION LIMIT AND CAPACITY

If a 10 g sample, a 100 m1 measuring bottle, a palladium is about 0.05 ppm. A team of three 50 m1 aliquot and 5 m1 of dibutylsulphide solu- laboratory workers can make 60 determinations tion are used, the detection limit for gold and a day.

Geologian tutkimuskeskus, Tutkimusraportti - Geologicnl Survey of Finlrn~d, Report of bwestigation 114, 1993 Determination of gold by aqua regia digestion, dibutylsulphide-di-isobutyl ketone extraction and flame atomic absorption

Table 1. Gold and palladium concentrations in some reference samples.

S ample Number of This Stand. Coeff. of Recommended det's work dev. var. % value

REFERENCES

Parkes A. & Murray-Smith R. 1979. A rapid method for Strong B. & Murray-Smith R. 1974. Determination of gold the determination of gold and palladium in soils and in copper-bearing sulphide ores and metallurgical flota- rocks. At. Abs. Newsl. Vol. 18, No 2, 57-58. tion products by atomic absorption spectrometry. Talanta

Rubeska I., Koreckova J. & Weiss D. 1977. The determi- 21, 1253. nation of gold and palladium in geological materials by Yudelevich I. G., Wall G. A., Torgov V. G. & Korda T. atomic absorption after extraction with dibutylsulphide. M. 1970. Extraction-atomic absorption determination of At. Abs. Newsl. Vol. 16, No 1, 1-3. gold in solutions. Zh. Anal. Khim. 25, 870.

Analytical methods for determining gold in geological samples Edited by Esko Koutas Geologian tutkirnuskeskus, Tutkimusraportti - Geological Survey of Finland, Report of Irzvestigation 114, 25-27, 1993

b Y E. Ojaniemi

Ojaniemi, E. 1993. Determination of gold, palladium and platinum by aqua regia digestion, dibutylsulphide-di-isobutyl ketone extraction and flameless atomic absorption. Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey of Fidand, Report of Investigation 114, 25-27, 2 tables.

Key words (GeoRef Thesaurus, AGI): chemical analysis, techniques, gold, palladium, platinum, atomic absorption, sample preparation, reagents

E. Ojanienzi Rautaruukki CO, Research Centre SF-92170 Raahe Finland

INTRODUCTION

The method was developed for the exploration tractions. First gold and palladium are extracted of platinum-group metal deposits containing from 2M hydrochloric acid solution into dibutyl gold as well as for the determination of gold. sulphide in di-isobutyl ketone (Parkes and The preparation and dissolution of a sample also Murray-Smith 1979). After reduction with stan- allow the determination of palladium and plati- nous chloride, platinum can be extracted into the num. The method is based on hot aqua regia same reagent (Simonsen 1970). Measurements digestion and two successive liquid-liquid ex- are made by flameless atomic absorption.


Reagents: tone. The solution is kept out of the daylight in a refrigerator,

1) HCl 37%, HNO, 65%, reagent grade, 3) Stannous chloride solution: 100 g of SnCl, 2) 0.2M DBS in DIBK: 14.6 g of dibutyl sul- dissolved in 1000 m1 of 2M HCl. New solution phide dissolved into 500 m1 of di-isobutyl ke- is required daily,

Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey of Finland, Report of Investigation 114, 1993 E. Ojanietni

4) Ferric chloride solution: 25 mg Felml in spectrophotometer equipped with an HGA 500 2 M HCI. graphite furnace and an AS 40 autosampler,

2) Shaking machine: Desaga, tube mixer: Instruments: Heidoplh.

1) Perkin-Elmer Model 5000 atomic absorption


Before digestion, roast the samples at 800°C for 1.5 h to decompose sulphides and organic matter.

Weigh 5.00 g of sample into a porcelain crucible. After roasting, transfer it to a 50-m1 test tube having a plug and a magnetic stirrer on the bottom. Add 20 m1 of 6M HCl and shake well. Heat the tube on an aluminium block for 1 h at 90°C. Remove the tube from the block and allow it to cool for about 15 rnin at room temperature.

Add 5 m1 of concentrated HNO, and shake well. Put the tube back on the aluminium block and leave the solution to evaporate overnight. Re- move the tube from the block in the morning. The residue must be pulpy; very dry or roasted residue does not give correct results. Dissolve the residue in 35 m1 of 2M HCl, shake the tube well and make sure that all the particles have disintegrated. Warm the tube again at 90°C for 30 min.

EXTRACTION OF GOLD AND PALLADIUM

Remove the solution from the test tube into a 100-m1 measuring bottle. Rinse the tube and fill the bottle up to the mark with 2M HC1, and shake. Let the silicates settle overnight, or centrifuge the required aliquot of the solution (3 min, 3500 rpm). Pipette an aliquot of 50 m1 into a 100-m1 separatory funnel and add 5.00 m1 of DBS-DIBK-solution. Extract Au and Pd into

the organic phase by shaking the funnel for 2 rnin manually or for 10 rnin mechanically. Al- low the phases to separate. Transfer the water phase to another 100-m1 separatory funnel for the extraction of platinum. Remove the organic phase to a 10-m1 test tube for the determination of gold and palladium. If necessary, clear the phases by centrifuging (3 min, 2000 rpm).

EXTRACTION OF PLATINUM

Add 2 m1 of stannous chloride solution to the separatory funnel with the water phase, and, while stirring, add smaller amounts (about 0.5 ml) until the yellowish brown colour of the ferric iron has disappeared. Add another 5 ml, stir and let the solution stand for 10 min. Extract Pt into 5.00 m1 of DBS-DIBK solution by shaking manually for 2 rnin or mechanically for

10 min. When the phases have separated, transfer the aliquot needed for the measurement into a 10-m1 test tube. If necessary, clear the phases by centrifuging. If platinum need not be determined gold and palladium can be extracted directly from the 2M HCl solution in the test tube that contains the sample residue.

STANDARD SOLUTIONS AND CALIBRATION

The standard stock solutions of gold and them in small volumes of aqua regia. After palladium, with concentrations of 1000 pmlml, careful evaporation, the residues are dissolved in are prepared from pure metals by dissolving hydrochloric acid to yield a concentration of

Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey of Finland, Report of Investigation 114, 1993 Determination of gold, palladium and platinum by ...

HCl of 0.6 M in the final solutions. The solutions are kept in dark bottles. The standard stock solution of platinum, with a concentration of 100 pglml, is prepared in the same way by dissolving pure metal in aqua regia. Before evaporation some potassium chloride is added. The final solution should be 1.2M in respect of HCl.

Standards are calibrated by preparing a working solution from the stock solutions. It contains Au 0.5 yglml, Pd 1.0 ~ g l r n l and Pt 2.5 pglml. Aliquots of 0.0, 0.25, 0.50, 1 .O, 1.50, 2.00, 3.00 and 4.00 m1 are pipetted into separatory funnels. Then 5 m1 of FeC1, solution is added and the solution is diluted to 50 m1 with 2M HCl. Gold and palladium are extracted into 5.00 m1 of the DBS-DIBK solution. The water phases are separated from the organic phases, and after the adding of stannous chloride solution platinum is extracted into 5.00 m1 of

DBS-DIBK solution. If only Au and Pd have to be determined the

standard solutions are prepared in the same way as the samples but without adding of FeC1,. The sample extracts remains fresh in closed test tubes for a month and the pure standard solutions for at least two months.

Table 1. Instrument parameters for Au in DBS-DIBK solution

STEP TEMP. RAMP HOLD READ REC INT.FLOW "C S S

Tube: pyrocoated graphite tube, sample volume 10 p1. Purge gas: argon.

DISCUSSION

The accuracy of the method was controlled by od described are 0.02 ppm for Au and Pd and analysing reference samples SARM-7 and Kont- 0.1 ppm for Pt. tijarvi over several months. The results are pre- Two technicians working together are able to sented in Table 2. prepare 40 samples and to make 120 determina-

The detection limits achieved with the meth- tions a day.

Table 2. Gold, palladium and platinum concentrations (ppm) in reference samples SARM-7 and Konttijarvi.

Element Au Pd Pt

Sample SARM-7 Konttijarvi SARM-7 Konttijarvi SARM. Konttijarvi

Numb.det's 15 15 15 15 15 15 Average 0.27 0.16 1.41 5.98 3.87 1.78 Stand.dev. 0.05 0.017 0.08 0.38 0.57 0.16 C. V. % 19.0 10.6 5.7 6.4 14.7 9.0 Recomm. val. 0.31 0.17 1.53 6.3 3.74 1.80

REFERENCES

Parkes A. & Murray-Smith R. 1979. A rapid method for Simonsen A. 1970. Determinaton of platinum in basic rocks the determination of gold and paIIadium in soils and by solvent extraction and atomic absorption spectroscopy. rocks. At. Abs. Newsl. Vol. 18, No 2. 25-27, Anal. Chim. Acta 49, 368-370.


DETERMINATION 0 REG DIGESTION

by E. Kontas

Kontas, E. 1993. Determination of gold and palladium by aqua regia digestion, stannous chloride-mercury coprecipitation and flameless atomic absorption. Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey of Finland, Report of Investigation 114, 29-32, 2 tables.

Key words (GeoRef Thesaurus, AGI): chemical analysis, techniques, gold, palladium, atomic absorption, sample preparation, reagents, accuracy

E. Kontas Geological Survey of Finland P. 0. Box 77 SF-96101 Rovanierni, Finland

INTRODUCTION

Geochemical prospecting for gold and palladium requires a rapid method of analysis with sensitivity reaching the ppb level and sometimes even sub ppb level. This method, which was earlier reported for gold analyses (Kontas 1981), has now been refined to apply to both gold and palladium (Kontas et al. 1986). In the original method the detection limit for gold was about 2 ppb, but it is now about 0.2 ppb for Au and Pd. The method is based on a sample weighing 1 g and overnight aqua regia digesting at the room temperature. Heating is partly compensated for the long digestion time. Gold and palladium are separated from analyte solution by re-

ductive precipitation using stannous chloride as a reductant and mercury as a coprecipitant (Barnard and Zeeman 1958). For the determina- t i on~ , the Hg(Au-Pd) precipitate is dissolved in aqua regia and diluted with water. Mercury does not interfere with the determination of gold or palladium by graphite furnace AAS.

Centrifugation is used to separate a clear sample solution after digestion and Hg(Au-Pd) precipitation from the bulk of the analyte solution. Centrifugation makes the method faster and more suitable for handling large batches of samples than conventional filtering.

Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey of Finland, Report of Investigation 114, 1993 E. Ko~ttns

Reagents, all reagent grade:


1) HCI 37%, HNO, 65%, 2) stannous chloride solution: 20% SnC12.2H,0 in 1M HC1, 3) mercurous nitrate solution: 1 mg Hg/ml in 0. I M HNO, and 4) mercury chloride solution: 0.5 mg Hg/ml in 5 0 vol-% aqua regia.

Standard stock solutions:

Merck Titrisol

Apparatus :

1) Retsch Rhetoterm roasting apparatus, 2) Beckman centrifuge with receptables for 56 test, tubes (100X16mm), 3) Perkin-Elmer Model 2280 atomic absorption spectrophotometer equipped with a D, background corrector and an HGA 500 graphite furnace equipped with an AS 40 autosampler and 4) Perkin-Elmer Model 3030 Zeeman atomic absorption spectrophotometer equipped with an HGA 600 graphite furnace and an AS 60 autosampler.


Samples with carbon, graphite or large amounts of sulphides are roasted at 750°C for 20 min. If sulphides exist only in moderate abundaces, roasting is not necessary.

Weigh l g of samples into plastic tubes (100x16 mm) or into porcelain crucibles if roasting is necessary. After roasting transfer them into tubes. Add 2.5 m1 of conc. HCI and shake; then add 0.5 m1 of conc. HNO, and shake. Let them stand overnight at room tem-

perature. In the morning shake them again and add 4 m1 of water; centrifuge for 10 min at 2000 rpm. Pour the clear, supernatant solutions into conical glass tubes (100x16 mm) or take aliquots for further separation of Au and Pd.

If the samples are ashes of organic materials, reduce the weight of samples 29-32, g), replace HNO, with 0.5 m1 of H202, and the water in the morning with 4 m1 of 3M HCI.

Separation of gold and palladium

Add 2 m1 of SnCl, solution and 1 m1 of trifugation is not generally required in rinsing, Hg2(NO,), solution to the sample solutions. Wait since the Hg precipitate remains on the bottom for about five minutes and centrifuge the soh- or the walls of the tubes. Make sure that this has tions for 10 min at 2200 rpm. Pour the solutions happened. away; rinse the Hg(Au-Pd) precipitates and the For organic ashes, use only 0.5 m1 of tubes by filling them with water and then pour- Hg,(NO,), solution. ing it away. One rinsing is often enough. Cen-

Dissolving the Hg(Au-Pd) precipitate and calibration

Add 0.7 m1 of conc. HCI and 0.3 m1 of conc. by graphite furnace AAS. The final volume of HNO, to the tubes containing the Hg(Au-Pd) the sample solutions is 2 ml, because about 0.1 precipitate. Shake the tubes and let them stand m1 of water is left in the tubes after rinsing. for about one hour. Then add 0.9 m1 of 3M From the stock solutions, the calibration stan- HNO,, shake the tubes once more and let them dards are diluted with mercury chloride solution. stand overnight. The analysis can then be done

Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey of Finland, Report of Investigation 114, 1993 Determination of gold and palladium by ...

Table 1. Instrument parameters for gold' and palladium2.

Au Pd

STEP 1 2 3 4 5 1 2 3 4 5

TEMP. "C 200 500 1900 2500 100 RAMP s 15 15 0 1 5 HOLD s 15 15 4 5 5 READ 0 REC 2 BASELINE - 25 1NT.FLOW 300 300 0 300 300

' perkin-~lmer Model 30302 AAS and HGA 600 graphite furnace. Au wavelength 242.8 nm, slit 0.7 nm. Pyrocoated graphite tube with L'vov's platform, peak area integration, purge gas argon. 'Perkin-Elmer Model 2280 AAS with HGA 500 graphite furnace. Pd wavelength 244.8 nm, slit 0.2 nm. Pyrocoated graphite tube, peak height integration, purge gas argon.

The concentration range of the standards, which depend on the detection limits required. The in- varies from 5 to 100 nglml of Au or Pd, and the strument parameters are presented in Table 1. solution volumes injected into a graphite furnace

DISCUSSION

The precision and accuracy of the method was tested for gold by analysing USGS reference samples GXR 1-6 (Table 2). Except for the sample GXR 1, the recommended values (Gladney and Burns 1984), were not precise enough for accuracy to be assessed. Therefore the results of Meier (1980) are shown. Meier also used graphite furnace AAS but after HBr-Br2 digestion and methyl isobutyl ketone (MIBK) extraction. Recommended values for palladium were not found in the litterature for GXR's and so reliability could not be evaluated. However, the precision for Pd determinations observed from replicate weighings was very good.

Despite good agreement with the values of Meier (1980), the small size of the analytical sample is a disadvantage in the above method and thus the poor representativeness of samples with coarse gold grains. On the other hand, the use of small samples is an advantage in analysing ashes of organic materials. As we can assume that gold exists homogeniously in organic material, there is no need to collect and burn big samples.

The method seems to be very suitable for determining the background contents of gold and palladium in silicate rocks (Kontas et al., 1986) and other geological materials. The detection limits are low enough for many purposes. Con- tamination by reagents is improbable, because their amounts are small and they are generally very pure. Except for platinum-rich samples, partial attack with aqua regia after roasting

usually ensures almost total dissolution of gold and palladium from geological materials of different composition (Signiholfi et al. 1984).

This method is mainly used for the geochemical exploration of gold and palladium, of which the latter is a good pathfinder element for all platinum-group metals. The main sample materials are fine till fraction (-0.064 mm) and bedrock. Stream sediment, peat and vegetable samples have also been analysed.

Carbon and graphite are the matrix substanc- es, that cause the worst interference as they adsorb the gold from solutions quantitatively (Lakin et al. 1974). Fortunately, even small a- mounts are easily detected at the digestion stage, and new portions can be taken and roasted before the digestion. The ashes of organic samples often contain finely divided carbon which can be destroyed with hydrogen peroxide. Therefore hydrogen peroxide is used instead of nitric acid as the oxidizing agent for ashes.

Samples exceedingly rich in iron oxides or base metal sulphides are somewhat troublesome since sample solutions become concentrated in iron or other metals, which severely interfere with the separation. This difficulty can be avoided by taking a fifth or tenth part of the sample solution for follow-up analysis. The detection limits are then five or ten times higher, respectively. In favourable cases the detection limit is 0.1 ppb for gold and 0.5 ppb for palladium. The capacity is about 2000 samples per one month and two persons.

Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey of Finland, Report of hrvestigation 114, 1993 E. Kontas

Table 2. Gold and palladium concentrations of GXR reference samples (aritmetic meanrtstandard deviation). Recommended values after Gladney and Burns (1984) and the results obtained by Meier (1980).

Au P P ~ Pd P P ~

S ample No. of Recommended Meier This work This work det's values

GXR l 5 3 1005 200 2950 3 1005200 0.1 GXR 2 3 46f 19 22 10.0rt0.3 0.1 GXR 3 3 3-600 3 2.4rt1.7 0.2 GXR 4 3 440rt160 353 419rt14 0.2 GXR 5 3 80560 7 7.0rt0.9 0.4 GXR 6 5 70rt10 63 8 6+6 2.2

The method is easy to modify. If necessary method is run on the basis of a 20 g sample. digestion can be intensified by heating; the Results are in good agreement with those ob- sample weight can be increased and an aliquot tained by fire assay method (Appendix). taken for follow-up analysis. Nowadays the

REFERENCES

Barnard E. & Zeeman P. B. 1958. Die Konsentrering en Spektrgrafiese Bepaling van Edelmetale Gesteendes en Rotse. Tegnikon 1 1, 2, 63-69.

Gladney E. S. & Burns C. E. 1984. 1982 Compilation of elemental concentration data for the United States Geo- logical Survey's geochemical exploration reference samples GXR-1 to GXR-6. Geostandards Newsletter Vol. 8, No 2, 119-154.

Kontas E. 1981. Rapid determination of gold by flameless atomic absorption spectrometry in the ppb and ppm ranges without organic solvent extraction. At. Spectr. Vol. 2, NO 2, 59-61.

Kontas E., Niskavaara H. & Virtasalo J. 1986. Flameless

atomic absorption determination of gold and palladium in geological reference samples. Geostandards Newsletter Vol. 10. NO 2, 169-171.

Lakin H. W., Curtin C. G. & Hubert A. E. 1974. Geo- chemistry of Gold in the Weathering Cycle. U. S. geol. Surv. Bull. 1330, 80 p.

Meier A. L. 1980. Flameless atomic absorption determination of gold in geological materials. J. Geochem. Expl. 13, 77-85.

Signiholfi G. P., Gorgoni C. & Mohamed A. H. 1984. Comprehensive analysis of precious metals in some geological standards by flameless A. A. spectroscopy. Geostandards Newsletter, Vol. 8, No 1, 25-29.

Analytical methods for determining gold in geological samples Edited by Esko Kontas Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey of Finland, Report of bzvestigation 114, 33-37, 1993

GOLD

b Y R. J. Rosenberg, Riitta Zilliacus and Maija Lipponen

R. J. Rosenberg, Riitta Zilliacus and Maija Lipponen. 1993. Neutron activation analysis of gold in geochemical samples. Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey of Fiiinlatzd, Report of Irwestigation 114, 33-37, 2 tables.

The analytical techniques used for the analysis of gold in geochemical materials at the Reactor Laboratory of the Technical Research Centre of Finland are described. These are epithermal instrumental neutron activation analysis of solid samples, thermal neutron activation analysis of freeze- dried water and snow samples and radiochemical neutron activation analysis of inorganic and organic samples. The detection limits are 3 ppb, 5 ng/l and 0.1 ppb, respectively. The accuracy and capacity of the methods are discussed.

Key words (GeoRef Thesaurus, AGI): chemical analysis, gold, till, methods, reliability

R. J. Rosenberg, Riitta Zilliac~is and Maija Lipponen Technical Research Centre of Firilnnd Reactor Laboratory SF-021 50 Espoo, Finland

INTRODUCTION

At the Reactor Laboratory gold is determined from geological samples on an analytical service basis. The annual number of samples has been about 13,000 since 1982. The main customers are mining companies, geological surveys and universities in Finland and Sweden.

Three methods are in use: Instrumental epithermal neutron activation analysis (ENAA) for pulverized samples. With this technique 23 other elements are determined simultaneously with gold (Rosenberg et al. 1982). Thermal neutron activation analysis (NAA) for water sam-

ples evaporated by freeze-drying. Radiochemical neutron activation analysis (RNAA) for very low gold concentrations, especially in biological materials (Zilliacus 1983). Because the number of samples needed in geochemical exploration is fairly large the cost must be reasonable. The low limit of detection is also critical if all anomalies are to be detected. Therefore special emphasis was laid on detection limit and cost in addition to accuracy. The analytical techniques and the organization of the work are described in the following.

Geologiau tutkimuskeskus, Tutkimusraportti - Geological Survey of Finland, Report of btvestigatiort 114, 1993 R. J. Rosenberg, Riitta Zilliacus and Maija Lipporterr

EXPERIMENTAL

Standards

Two different kinds of standard are used. solution of gold is dissolved in ethylene silicate Solutions of gold are used for the analysis of all with the aid of ethanol. This mixture is hydro- except pulverized geological samples. Stock lyzed with ammonia, dried and ignited so as to solutions of 1000 ppm are prepared by dissolv- yield a dry silicate powder homogeneously ing metallic gold in aqua regia. Suitable dilu- doped with gold. tions are made with deionized water before use. A composite standard containing several other Powdered standards prepared according to the elements is usually prepared. procedure of Date (1977) are used for ENAA. A

Equipment

A Triga Mk I1 research reactor is used for irradiations. The reactor is run for about 7 h per day from Monday to Friday. Sometimes Monday is needed for service work. The rotary specimen rack contains 40 irradiation positions. The thermal flux is 1.2 xlO1' cm-'s-' and the Cd ratio for gold is 2. Twenty of the positions are used for thermal irradiations and twenty for epithermal irradiations. The epithermal flux is obtained by using containers of aluminium 30 cm X 25 cm in size, lined with 1 mm of cadmium and again with 0.2 mm of aluminium. These containers are permanently located in the reactor and are only taken up to change samples. One container holds 32 capsules of 0.5 ml, and thus 640 sam-

ples can be irradiated simultaneously in an epithermal flux. For RNAA the samples are irradiated in the central thimble, where the thermal neutron flux is 1013 cm-'s-l.

Measurements are performed with automatic gamma-spectrometers comprising a Ge(Li) or Ge detector with auxiliary electronics, a sample changer, a multichannel analyzer, a microcom- puter and inputloutput devices (Vanska et al. 1983, Rosenberg et al. 1985). Such a system measures a series of samples automatically and simultaneously calculates the elemental concentrations which are printed on paper and cassette. Five gamma-spectrometers of this kind are available for activation analysis.

Procedure for ENAA

The standards and powdered samples are weighed into polyethylene capsules with an inner volume of 0.5 ml. Thus the sample size is 0.5-1 g, depending on the density. The samples are transported and stored in styrox boxes hold- ing 100 capsules each. The samples are often weighed by the customers, and a list of the weights accompanies the box on its arrival at the laboratory. 32 capsules, four in a plane, are wrapped in aluminium foil. One irradiation series comprises four standards, 12 control samples and 144 samples. These are inserted into cadmium containers. The samples are irradiated for one week (25-35 h) and measured after a decay time of 4-5 days with an automatic gamma-spectrometer. The measurement time is 20 min per sample. Thus measurement of one series takes almost two and a half days.

The 41 1.8 keV of l g 8 ~ u is used for calculating the results. The half-life is 2.7 days. A peak with a statistical error of 30% or less is accept- ed. If a peak is not found, the upper level is cal- culated according to the principles of Currie (1968). The detection limit for most sample types is 3 ppb.

The work is organized in such a way that two series are inserted in the reactor on Monday morning and two on Thursday morning. The two series taken out of the reactor are allowed to decay and thenloaded onto the sample changer on Wednesday and Monday, respectively. Thus two gamma-spectrometers are needed for analysing 576 net samples per week. During week-ends the analyzers are free for other measurements. Ex- cluding the weighing, one person can handle these samples and wrap them in aluminium foil,

Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey of Finland, Report of Investigation 114, 1993 Neutron activation analysis of gold in geochemical samples

load and unload them into and out of the reactor on floppy discs from which they are copied onto and sample changers, and the prepare the data a cassette for input into the memory of the com- files. The sample codes and weights are printed puter at the start of a measurement series.

Procedure for NAA of water samples

Melted snow and ice as well as water samples are preconcentrated by evaporation before irradiation. This is performed by freeze drying. The samples are collected into polyethylene bottles and then preserved with the addition of 1 m1 of suprapure nitric acid (Merck) per litre of water. 200 m1 of sample is dropped onto a sheet of polyethylene foil in a Petri beaker and freeze dried with a WKF L 0 5 lyophilizer. This takes about 40 h. Standards and blanks are treated in the same way. The polyethylene sheets are

wrapped and inserted in polyethylene capsules for irradiation. Then the samples are irradiated for 25 h in a thermal flux of 1 . 2 ~ 1 0 ~ ~ c m - ~ s - ~ . After a decay time of five days the samples are measured with an automatic gamma-spectrometer. The measurement time is 1 h per sample. The average blank is 1 ng and thus an effective detection limit using 200 m1 samples is 5 ngll.

The number of samples is limited by the low capacity of the lyophilizer. Only 15 samples a week can be dried.

Procedure for RNAA

This method is intended for solid geochemical samples when concentrations lower than 3 ppb have to be determined. Because the method is more expensive than the instrumental ENAA, it is used more seldom. The method is also suitable for plants and other organic materials. The method has been described in detail by Zilliacus (1983) and in this context will be described only briefly. 2 0 0 4 0 0 mg samples are weighed into ampoules of quartz and irradiated for 25 h in a thermal neutron flux of 10i3 ~ m - ~ s - ' . After a decay time of 2 - 4 days the samples are sub- jected to chemical separation. 1 mg of gold carrier is added. Mineral samples are dissolved in hydrochloric acid followed by aqua regia. The sample is evaporated to dryness three times with aqua regia. Organic samples are first wet-ashed with nitric acid and hydrogen peroxide and then treated with aqua regia. After this 2 m1 of satu-

rated boric acid solution are added to the dry sample followed by 3 m1 of hydrochloric acid. The sample is diluted to 20 m1 with water and filtered through 50 mg of activated carbon through a chimney. The carbon is washed with Cr-carrier solution and water. The filter papers are inserted into polyethylene capsules and measured with an automatic gamma-spectrometer. The measurement time is 2 h.

The yield of the chemical separation is determined by reactivation. It varies between 60 and 90%. The detection limit for gold is 0.04 ng corresponding to 0.1 ppb in a sample when a 400 mg weighing is used.

The method is rather fast. One person can make 12 separations per day and the same number of samples can be measured with one counting system.

Evaluation of the method

There are a number of possible sources of cause this is the principal technique in use. The error in activation analysis. Some of these can specific problems of the other techniques are be avoided almost completely and others only at discussed at the end of this chapter. The subjects unreasonable expense. In the following these discussed here are: sources are discussed in some detail, and the overall accuracy of the techniques is evaluated. 1) representativeness of the sample,

The discussion applies mainly to ENAA be- 2) contamination,

Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey of Finlnnd, Report of Investigation 114, 1993 R. J. Rosenberg, Riitta Zilliacris and Maijn Lipponen

Table 1. The gold concentrations in the US Geological Survey geochemical standards (GXR 1-6) according to this work (ENAA and RNAA, RNAA from Zilliacus 1983) and other authors (in ppm). Allcott a is the arithmetic mean of 50 determiuations and b the arithmetic mean of 76 determinations. Maier a was made by flame and b by graphite furnace atomic absorption spectrometry.

GXR l GXR 2 GXR 4 GXR 5 GXR 6

EN A A 3.6k0.2 0.03 3kO.004 0.52k0.02 0.01 l+ 0.002 RNAA 3.65 0.031 0.52 0.008 Allcott 1974 a 2.8 0.038 0.58 0.12 - " - 1974 b 3.7 0.056 0.74 0.058

Motooka 1979 3.4 0.65 Kontas in this issue 3.1 0.010 0.419 0.007 Maier 1980 a 3.0 0.070 0.35 - " - 1980 b 2.95 0.022 0.353 0.007

Gladney 1984 3.1k0.2 0.046+0.019 0.44k0.16 0.080~0.060

3) neutron flux distribution, 4) neutron absorption, 5) competitive reactions, 6) counting geometry, 7) peak evaluation and 8) counting statistics.

The small sample size, 0.5-1 g, causes a problem in some cases because of the inhomogeneous distribution of gold in many samples. But, according to investigations by the authors, increasing the sample size from 1 g to 10 g does not significantly alter the situation.

Contamination is a special problem in analysing gold. The reasons for this are the low concentrations dealt with and the fact that so many people wear gold rings. Metallic gold is soft and contaminates easily. Contamitions of different kind have been encountered during the work. In some cases the samples themselves have been contaminated during preparation and sometimes the surfaces of the polyethylene capsules have been contaminated during weighing. The only way to avoid this is to control rigorously that people handling the samples are not wearing gold rings.

The production of the radionuclide on which the analysis is based depends on the neutron flux reaching the gold atoms. Variations in this flux can be caused by the natural variation of the flux in the reactor and neutron absorption in the sample itself. The neutron flux varies by tens of percent depending on irradiation positions. Most of this can be compensated by mapping the flux and inserting flux monitors. But it would be too expensive to measure the flux separately for every sample. The horizontal flux variation inside a cadmium container is little more than 5% and because the positions of the samples in the reactor cannot be controlled the error caused by this variation is not compen-

sated. Another problem is neutron absorption in the

sample. This is a problem specific to gold. Gold is usually present in the form of small grains. Therefore the resonance neutron neutron absorption in these grains may be significant, even if the average concentration is small. Because the grain size cannot be controlled, this error cannot be controlled either. In average gold concentrations of less than 1 ppm it has not been of sig- nificance. But in some cases intercomparisons with other analytical techniques have shown a 10% negative error in concentrations of 10 ppm or higher.

Competitive reactions are no problem for gold. Errors in the counting geometry may be caused by variations in the sample position during measurement and variations in sample size. Sample positioning by the sample changer is precise, and customers are requested to B11 the capsules to avoid variations in sample size.

The only possible spectral interference is caused by 1 5 2 ~ ~ , but a europium concentration of 50 ppm is needed to give the number of counts corresponding to the detection limit, 3 ppb of gold. Such europium to gold ratios are highly unlikely. Because the peak of gold is situated on an undisturbed smooth part of the Compton continuum in geological samples, evaluation of the peak area causes no significant errors apart from that caused by the counting statistics, which depends on the sample type and gold concentration.

When water samples are analysed, two additional problems arise: the possible loss of gold during handling and the high risk of contamination because of the extremely low concentrations. Investigations with tracers have shown that no loss occurs through absorption on the walls of the collection vessel or during evaporation. The polyethylene foils onto which the

Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey of Finland, Report of Investigation 114, 1993 Neutron activation analysis of gold in geochemical samples

samples are evaporated contain gold and therefore a blank cannot be avoided. The blank varies between 0.6 and 1.2 ng per 200 m1 sample, lead- ing to a corresponding uncertainity in the analysis.

In RNAA the special problem is the variable yield of the separation. This is corrected as described and does not cause an additional error.

The results of control samples were monitored to investigate the average precision of the method. 26 samples were chosen at random from runs during two months. The result was as follows. The nominal concentration of the sample is 0.59 ppm. The mean of 26 determinations was 0.586 ppm, and the standard deviation 0.028 ppm. Consequently, the average relative precision at the 95% confidence level is +9.8%. Because of the random character of this error

Table 2. Results of an intercomparison conducted by IAEA (1985). The sample is marine sediment SD-N-112.

This work Other laboratories

the error caused by counting statistics starts influencing the total precision significantly from 60 ppb downwards. Theoretically a total precision of +1.7% is reached at 30 ppb and +22% at 10 ppb at the 95 % confidence level.

The overall reliability of the method was investigated by analysing standard reference samples and by making intercomparisons.

REFERENCES

Allcott G. H. & Lakin H. W. 1975. The homogeinity of six geochemical exploration reference samples. Pp 659-681 in Geochemical Exploration 1974, ed. by I. L. Elliott and W. K. Fletcher. Elsevier Scientific Publishing Co. Am- sterdam, 720 p.

Currie L. A. 1968. Limits for quantitative detection and quantitative determination. Anal. Chem. 40, 586-593.

Date A. R. 1977. Preparation of trace element reference materials by a CO-precipitated gel technique. Institute of Geological Sciences, Geochemical Division, Analytical et Ceramics Unit, Report No 101. 16 p.

Gladney E. S. & Burns C. E. 1984. 1982 Compilation of elemental concentration data for the United States Geo- logical Survey's geochemical exploration reference samples GXR-1 to GXR-6. Geostandards Newsletter Vol. 8, NO. 2, 119-154.

Intercomparison of trace element measurements in marine sediment sample SD-N-112. International Atomic Energy Agency, Report No 24, 18 p.

Meier A. L. 1980. Flameless atomic absorption determination of gold in geological materials. J. Geochem. Expl.

13, 77-85. Motooka J. M., Mosier E. L., Sutley S. J. & Viets J. G.

1979. Induction-coupled plasma determination of Ag, Au, Bi, Cd, Cu, Pb and Zn in geological materials using a se- lective extraction technique - preliminary investigation. Appl. Spectr. 33, 456-460.

Rosenberg R. J., Kaistila M. & Zilliacus R. 1982. Inst- rumental epithermal neutron activation analysis of solid geochemical samples. J. Radioanal. Chemistry 71, 419-428.

Rosenberg R. J. & Vanska L. 1985. STOAV84, a computer program for an automatic gamma spectrometer used for activation analysis. Technical Research Centre of Fin- land, Research reports No 415, 49 p.

Vanska L., Rosenberg R. J. & Pitkanen V. 1983. An automatic gamma spectrometer for activation analysis. Nu- clear Instruments and Methods 213, 343-347.

Zilliacus R. 1983. Radiochemical neutron activation analysis of gold. Radiochemical and Radioanalytical Letters 57, 137-144.

Analytical methods for determining gold in geological samples Edited by Esko Kontas Geologian tutkimuskeskus. Tutkimusraportti - Geological Survey of Fidand, Report of I~westigation 114, 39-41, 1993

GOLD CONCENTRATIONS OF SOME REFERENCE SAMPLES - DISCUSSION

b Y E. Kontas

Kontas, E. 1993. Gold concentrations of some reference samples - discussion. Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey of Finland, Report of bzvestigation 114, 39-41, 3 tables.

The gold concentrations of five reference samples, AAV-1, AAV-2, AAV-3, SUK-1 and SUK-2 were determined in six laboratories using the methods described in this volume. The sample material was the fine fraction of till (-0.064 mm); all samples contained metallic gold grains. The sample weights ranged from 0.6 to 50 g, depending on the procedure. Several determinations were made in each laboratory. When the highest and lowest outlier values for each sample were omitted, the following ranges of gold concentrations were obtained: AAV-1, 24-120 ppb; AAV- 2, 198-1100 ppb; AAV-3, 127-1000 ppb; SUK-1, 19-83 ppb; and SUK-2, 28-95 ppb. The variations were greatest for the samples with the lowest weights. However, anomalously high gold values were found in every determination at all laboratories.

From the capacities, detection limits and representativeness of the analytical samples, the best applications of the methods are as follows: fire assay with sample weights of 25-50 g for the assessment of gold deposits, analysis of concentrates and quality control when cheap methods are called for; methods based on aqua regia digestion and solvent extraction with sample weights of 5-20 g for the analysis of ores and local geochemical prospecting; the method based on aqua regia digestion and separation by coprecipitation with a l-g sample weight for regional geochemical mapping, preliminary prospecting and some basic research; instrumental neutron activation analysis (NAA) with a sample weight of about 0.6 g for regional geochemical prospecting and some special analyses.

Key words (GeoRef Thesaurus, AGI): chemical analysis, gold, till, methods, reliability

E. Kontns. Geological Survey of Finland P . 0 Box 77 SF-96101 Rovaniemi. Finland

INTRODUCTION

To help readers and clients compare the dif- (Rovaniemi), and subsequently submitted to ferent analytical methods described in this vol- analysis at the six participating laboratories. ume, five reference samples were prepared at the Since laboratories employ several methods for laboratory of the Geological Survey of Finland analysing gold, the methods were divided be-

Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey of Finland, Report of bnlestigation 114, 1993 E. Kontas

tween the laboratories to cover as many as possible of the procedures required by this study. The samples were known to contain varying amounts of gold, at least partly in native grains. The analysed sample material consisted of the fine fraction of till (-0.064 mm) commonly used in prospecting for gold and base metals in all Fennoscandian countries, Canada and parts of the United States.

Gold typically occurs in till as native grains varying greatly in size. The maximum size is defined by the mesh number of a sieve, but not

absolutely, because the oblong or threadlike shape of the grains enables even larger grains to pass the sieve. The representativeness of the analytical portion is a problem, as it is dependent not only on the grain size but also on the grade of gold in a sample (Clifton et al. 1969). In addition, it is very difficult to homogenize such samples, because the gold grains separate gravitatively in the sample powder. The fine fraction of till consists mainly of minerals of very low density; gold, however, has the highest density of all minerals.

SAMPLE PREPARATION

The samples, each weighing about 40 kg, ses. Two 250 g aliquots of the fine fractions were collected from Outokumpu Oy prospects in were sent to each laboratory for analyses. The Kittila district, Finnish Lapland. The material is homogeneity of aliquots was studied by till, much of it of local origin. analysing them for base metals (Cu, Mn, CO

Samples AAV-1, AAV-2, AAV-3, SUK-l and etc.). These elements turned out to be very SUK-2 were dried at 70°C, after which the homogeneously distributed in the different bat- -0.064 fraction was sieved for chemical analy- ches.

RESULTS AND DISCUSSION

The precisions are clearly poorest with the methods which used the smallest sample weights. In some the variations are even significant with greater sample weights. The "real" gold contents of all aliquots were probably not exactly the same.

The results suggest (Table 1) that the analytical portions of the samples studied in this work should be about 10 g or more, if relative standard deviations of f50% or below are aimed at. The methods of Kontas used both l-g and 9-g portions of samples. The results (Table 1) show that precision improved dramatically when larger analytical samples were used. The results of the two fire assay procedures, with 30-g and 50-g samples, showed almost ideal agreement. Generally, when subsampling is deficient, analytical results tend to be too low rather than too high (Ingamells and Switzer 1973, Ingamells 1981).

Very important characteristics of analytical methods for gold are detection limit, capacity, the representativeness of the analytical portion and the costs per analysis. The costs are difficult to estimate, since the prices of instruments and

other equipment vary markedly. Generally, large capacity and little need for attendance mean low costs per analysis.

The general trend is that the higher the capacity, the lower the weight (Table 2). At the same time, the representativeness of the analytical portion declines, which is a serious drawback of these effective methods. But different methods are applicable for different purposes. When gold ores or gold bearing concentrates are analysed, representativeness, precision and accuracy are the most important criteria. A detection limit of 0.05-0.1 ppm is usually low enough and a lower capacity is acceptable. A very high capacity and a low detection limit are particularly important in geochemical exploration. The fea- sibilities of different procedures are evaluated in Table 3.

In general, a low detection limit is necessary in basic research because the background values of gold in geological materials are usually very low. A small analytical portion is not always a disadvantage, because the gold at background levels is often homogeneuously distributed.

Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey of Finland, Report of Investigation 114, 1993 Gold concentrations of some reference samples - discussion

Table 1. Gold concentrations (Au ppb) in reference samples AAV-I. AAV-2, AAV-3, SUK-1 and SUK-2 determined by different laboratories. (arithmetic mean 5 standard deviation).

Procedure 1 2 2A

Sample weight g 0.6 1 9 Numb. of det's 8 6 3

AAV- 1 49518 4 852 8 3956 AAV-2 398rt262 7455435 699577 AAV-3 4595362 6035522 636567 SUK-1 2757 44rt22 3 059 SUK-2 33rt10 3 858 4551 l

Procedures: 1 Rosenberg et al., 2 and 2A Kontas, 3 Ojaniemi, 3A Ojaniemi, lead fire assay, 4 Penttinen, 5 Noras, 6 Juvonen and Vaananen.

Table 2. Some important characteristics of the analytical methods for gold described above.

Procedure Size of Capacity Sample Det. of team samples1 weight limit

week g P P ~

Juvonen and Vaananen 4 150 25-50 0.1 Noras 2 200 20 0.05 Penttinen 3 3 00 10 0.05 Ojaniemi 2 400 5 0.02 Kontas 2 5 00 1 0.0001 Rosenberg et al. 2 570 0.5-1 0.003

Table 3. Evaluation of the best applications of different analytical methods for gold.

Method Applications

Juvonen and Vaananen Assessment of gold deposits and analysis of concentrates, quality control

Noras Analysis of ores, local geochemical prospecting

Penttinen Analysis of ores, local geochemical prospecting

Ojaniemi Geochemical prospecting, analyses of gold and platinum ores

Kontas Regional geochemical mapping and preliminary prospecting, basic research

Rosenberg et al. Regional geochemical prospecting and mapping, special analyses

REFERENCES

Clifton E. H., Hunter R. E., Swanson F. J. & Phillips R. 547-568. L. 1969. Sample size and meaningful gold analysis. U. S. Ingamells C. 0. 1981. Evaluation of skewed exploration Geol. Surv. Prof. Paper 625-C, 17 p. data - the nugget effect. Geochim. Cosmochim. Acta,

Ingamells C. 0. & Switzer P. 1973. A proposed sampling 45, 1209-1216. constant for use in geochemical analysis. Talanta, 20,

APPENDIX. The effect of sample weight and digestion and separation method on the results of gold determinations.

Effects of sample weights and digestion and separation methods on the detection of gold were studied by analysing 137 samples collected from 53 gold deposits and their host rocks (Nurmi et al., 1991). With some exceptions, the samples were taken from drill core profiles: one sample from the orebody itself and one sample from either side of it. Gold contents were determined from samples weighing 1 and 20 g, using aqua regia digestion and stannous chloride-mercury coprecipitation as a separation method (Kontas, this volume). Sample weights of 20 and 25 g were used in the lead fire assay method (XRAL, Canada, Juvonen and Vaananen, this volume).

The results show that gold ores and their closest haloes in the bedrock can generally be detected with all the sample weights used. Therefore a quick and effective method with a sample weight of 1 g would seem to be very useful in reconnaissance prospecting for gold. However, even the sample weight of 20 g does not always seem to be reliable for the inventories of gold deposits.

Table 1. Gold concentrations (in ppb) in 53 gold deposits and their closest haloes in bedrock. Aqua regia digestion method with sample weights of 1 and 20 g (Kontas, this volume). Lead fire assay with sample weight of 20 g (XRAL, Canada) and 25 g when the gold concentration exceeds 10000 ppb (Juvonen and Vaananen, this volume).

Method: Aqua regia, GFAAS Aqua regia, GFAAS Lead fire assay Sample: 1 g 20 g 20 or 25 g

Gold deposit

Mont Charlotta

New Celebration

MA-la, CANMET #

Hoyle Pond Owl Creek

Kerr Addison

Renabie

Ferderber

Page Williams Lokkiluoto

_ " _

Muurinsuo - ' l -

- -

Korvilansuo

Ramepuro - -

Kuittila _ " _

- -

Lalleanvuoma -'l-

- - Sukseton

- - - -

n. d. = not determined # = Reference sample, CCRM, MA-la, recomended value for gold 21400+400 ppb.

Table 1. (continued.) Gold concentrations (in ppb) in 53 gold deposits and their closest haloes in bedrock. Aqua regia digestion method with sample weights of 1 and 20 g (Kontas, this volume). Lead fire assay with sample weight of 20 g (XRAL, Canada) and 2 5 g when the gold concentration exceeds 10000 ppb (Juvonen and Vaananen, this volume).


Gold deposit Kivimaa

_'l_

_ " _

Suurikuusikko

Hirvilavanmaa _ " _

_'I_

Rovaselka - - -'l-

Saattopora N _ " _

_ " _

Saattopora S - - - -

Isokuotko - - - -

Soretiavuoma - - - -

Sirkka W _'I_

Bidjovagge Juomasuo

- -

Saynajavaara - -

Sivakkaharju - -

Konttiaho _ "_

Makararova _ " _

Laivakangas N - - - -

Jokisivu - - - -

Isovesi _ "_

- -

Antinoja -

- -

Vesipera -

_ " _

~ n ~ e s u e v a _"_

- -

n. d. = not determined # = Reference sample, CCRM, MA-la, recomended value for gold 21400!~400 ppb.

Table 1. (continued.) Gold concentrations (in ppb) in 53 gold deposits and their closest haloes in bedrock. Aqua regia digestion method with sample weights of 1 and 20 g (Kontas, this volume). Lead fire assay with sample weight of 20 g (XRAL, Canada) and 25 g when the gold concentration exceeds 10000 ppb (Juvonen and Vaananen, this volume).


Gold deposit Kiimala 100 160 170

_ l 1 _ 1600 2650 2600 - - 3 0 60 62

Pirila S 40 100 60 _rl_ 12000 32000 9500 _ t p _ 10 60 56

Kalliosalo 210 280 3 40 _I(_ 5000 6700 7500 -l1- 3 0 120 90

Kurula 40 8 0 72 - - 1000 1280 1100 - - 100 300 240

Pirila N 100 240 260 - - 1700 3100 2800 - - 3 0 40 64

Kaapeliukulma 100 210 240 - I I - 1700 3250 3200 - - 70 260 140

Laivakangas S 200 450 3 60 - - 3900 3500 5500 - - 10 120 100

Pohlola 840 410 320 _l1_ 8800 12800 12500 -lt- 40 60 49

Kopsa 890 410 310 - - 2400 3 800 3500

Kangaskyla 290 3 40 280 - 2100 2300 2000

- - 250 520 3 40 Osikonmaki E 350 540 550

_ q v _ 5800 7000 7000 - - 2060 1280 l200 Osikonmaki W 350 620 680

- - 3400 5400 5000 - - 100 270 430

Bjorkdahl 1000 2850 3600

n. d. = not determined # = Reference sample, CCRM, MA-la, recomended value for gold 21400rt400 ppb.

REFERENCE

Nurmi P. A., Lestinen P. & Niskavaara H. 1991. Geo- Canadian and Australian deposits. Geol. Surv. Finland, chemical characteristics of mesothermal gold deposits in Bulletin 351, 101 p. the Fennoscandian shield, and a comparison with selected

geologian - gtktupa.gtk.fi/julkaisu/tutkimusraportti/tr_114.pdf · tion and analytical methods have...

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