w~mwW~}~;\\h}~!/\\H!:W.i The Determ1 nat1on of-Gold 

In Geologic Materials by 

Neutron-Activation Analysis 

Using Fire Assay for the 

Radiochemical Separations 

The Determination of Gold 

In Geologic Materials by 

Neutron-Activation Analysis 

Using Fire Assay for the 

Radiochemical Separations 

By J. J. Rowe and F. 0. Simon 

G E 0 l 0 G I C A l S U R V E Y C I R C U l A R 599 

Woshingfon 1968 
United States Department of the Interior 

STEWART L. UDALL, Secretary 

Geological Survey 

William T. Pecora, Director 

Free on application to the U.S. Geological Survey, Washington, D.C. 20242 
CONTENTS 

Page 

Abstract- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1 

Introduction - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1 

Acknowledgments - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1 

Reagents and apparatus- - - - - - - - - - - - - - - - - - - - - - - - - - - 1 

Procedure- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1 

Results and discussion - - - - - - - - - - - - - - - - - - - - - - - - - - - 2 

Irradiation and decay time - - - - - - - - - - - - - - - - - - - - - - - - 2 

Fire assay - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 2 

Interferences - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 2 

Precision and accuracy - - - - - - - - - - - - - - - - - - - - - - - - - 2 

Sampling - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 4 

Application - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 4 

References cited - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 4 

TABLES 

Table 1. Recovery of radioactive gold tracer in the fire assay 

using gold carrier - - - - - - - - - - - - - - - - - - - - - - - 2 

2. Eight replicate determinations of gold in Hawaiian 

basalts, G-1, and W -1 - - - - - - - - - - - - - - - - - - - - - 3 

3. Comparison of gold values obtained by this method with 

those from other methods - - - - - - - - - - - - - - - - - - - 3 

III 

The Determination of Gold in Geologic Materials by 

Neutron-Activation Analysis Using Fire Assay for 

the Radiochemical Separations 

By J. J. Rowe ond F. 0. Simon 

Abstract 

In this rapid technique for determining gold at the parts-perbillion concentration level, 1-gram samples are irradiated for 10 

hours in a flux of Sx1Ql2 neutrons per square centimeter per 

second and are allowed todecayfor lOdays, The samples are then 

carried through classical fire-assay separations in the presence 

of 20 to 50 milligrams of gold carrier, and the 2.7-day Aul98 is 

counted in a gamma-ray spectrometer. Forty sam p 1 e s can be 

processed in 8 hours, and a detection limit of 0.05 part per billion 

gold in a 1-gram sample is attainable. 

INTRODUCTION 

Neutron -activation methods provide exception a 1 

sensitivity for the determination of gold and generally 

eliminate the problem of reagent blanks. Methods 

developed by Vincent and Smales (1956), Vincent and 

Crockett (1960), Shcherbakov and Perezhogin (1964), 

DeGrazia and Haskin (1964), and Baedeker and 

Ehmann (1965) and those summarized by Emery and 

Leddicotte (1961) provide for the irradiation of 0.1 

to 0.5 g (gram) of rock sample. After a short cooling 

period, the gold is radiochemically s epa r at e d for 

gamma counting of the 0.413 Mev photopeak for Au 198 

or for beta counting. Generally, the accepted 

activation-analysis methods involve extensive chemical manipulations to achieve the necessary radiochemical purity. 

Washington and Holman (1966) applied activation 

analysis to the silver bead obtained after the conventional fire-assay procedure. Their detection limit 

was 10 ppb (parts per billion) gold on a 15-g sample 

because of the Ag 110m activity and the comparatively 

large reagent blank. Simon and Millard (1968) improved on this method by irradiating the lead button 

prior to cupellation with silver. Although this method 

has the advantage of using a 10- to 20-g sample for 

analysis, it is not applicable to routine analysis because only a few samples can be irradiated simultaneously and the fire-assay flux must be purified before assay to maintain a low blank. 

In the method described here, activation analysis 

was combined with the classical fire assay for the 

radiochemical separation of gold. The detection limit 

for gold (defined as 3a, where a is the square root of 

the background) is 0.05 ppb if the sample contains 

less than 10 ppm (parts per million) silver. Irradiation and decay times were adjusted to provide low 

1 

radioactivity less than 1 me (millicurie) for 20 1-g 

samples). The fire-assay procedure provides for the 

rapid radiochemical separation of gold with a minimum 

of operator attention, so that as many as 40 samples 

can be processed in 8 hours. 

ACKNOWLEDGMENTS 

The authors wish to express their thanks to L. P. 

Greenland, U.S. Geological Survey, for hismanyhelpful suggestions and review of the manuscript. The 

assistance of L. P. Harris of the Naval Research Laboratory reactor staff and F. W. Brownofthe U.S. Geological Survey is also gratefully acknowledged. 

REAGENTS AND APPARATUS 

Standard gold-stock solutions: Solution A (1,000 ppm 

Au)-Dissolve 1.00 g of gold metal in aqua regia and 

dilute to 1 liter with 2 M HCl. Solution B (10 ppm 

Au)-dilute 5.00 ml (milll'iiter) of solution A to 500 

ml with 2 M HCl. Solution B is stable for several 

months. Solution C (0.5 ppm Au)-dilute 5.00 ml of 

solution B to 100 ml with 2 N HCl. Solution C should 

be prepared each time monitors are prepared. 

Fire-assay flux: Thoroughly mix 1,000 g PbO, 100 g 

Na2C03, 20 g Na2B407, and 50 g flour (r e·d u c in g 

power:lO g Pb per g flour). 

Gold carrier: For each sample, place a 20- to 50-mg 

(milligram) piece of gold metal in a heat-resistant 

dish, and heat with a torch until the gold fuses into a 

small bead. Weigh the bead (.±0.1-mg accuracy), and 

place it in a numbered vial. 

Counting equipment: A pulse-height-analyzer system, 

using a four -input mixer -router coupled to four 3- by 

3-inch Nai(Tl) well-type crystals, was used to collect 

four spectra simultaneously. 

PROCEDURE 

1. Prepare each standard (flux monitor) by placing 

approximately 0.2 g of gold-free spectrographically pure silica in a small polyethylene vi a 1. 

Weigh the silica and vial(:tO.Ol-mgaccuracy); 

add two drops of solution C, and weigh again. 

From the weight of solution and its specific 

gravity, calculate the amount of gold added (approx 50 ng (nanograms)). Dry overnight and seal 

the vial with an electric soldering iron. 
2. Grind each sample to less than 100 mesh and 

carefully mix and quarter. Weigh and seal 1 g of 

each sample in a 215-dram polyethylene vial. Irradiate 16 samples plus four monitors for 10 

hours in a neutron flux of 5xl012 neutrons per 

square centimeter per second. Store samples for 

10 days to allow decay oftheshort-livedisotopes 

(Prim a r i 1 y 15-hour Na24 and 2.6-hour Mn56). 

3. For each sample, place 80 g of fire-assay f 1 u x 

into a fire-assay crucible and make a depression 

in the flux to receive the sample. 

4. Transfer the irradiated sample to the depression 

in the flux and mix. 

5. Add one bead of gold carrier; cover the mixture 

with a layer of borax glass and cover the crucible with a scorifying dish. 

6. Place the crucible in a fire-assay furnace preheated to 900°-1, 000°C and fuse for 1 hour. 

7. Pour each melt into a steel mold; cover with a 

piece of asbestos and allow to cool. 

8. Break the slag away from each lead button in a 

plastic bag to prevent scattering of glass. Each 

button should weigh 2 8±8 g. 

9. Cupel each lead button at 800°-900°C. 

10. Weigh each final gold bead to determine the chemical yield and place in a plastic vial. 

11. Collect spectra. After several days, collect another set of spectra to verify the decay of 2. 7 -day 

Aul98. 

12. Standards (monitors) are treated the same as 

samples. 

130 Calculate the area of each 0.413 Mev photopeak 

for Aul98 (Covell, 1959) and adjust data to zero 

decay time. 

14. Determine the amount of gold in each sa mp I e 

using the equation: 

.A§ I Y§ 

W.§.=Wm ~m I Ym.. 

where Vi is the weight of gold, A is the activity, 

and Y is the carrier yield; the subscripts s and 

m refer to sample and monitor, respectiVely. 

RESULTS AND DISCUSSION 

Irradiation and decay time 

After samples are irradiated for 10 hours and then 

are permitted to decay for 10 days, the residual radioactivity is generally less than 1 me. The Au198 activity is sufficient to provide 300 to 400 counts per 

minute per ng of gold in the 0.413 Mev photopeak using 

Covell's (1959) method for calculating areas. 

Fire Assay 

Bugbee (1940), Beamish (1966), and Chow and 

Beamish ( 1967) have published comprehensive studies 

of the fire-assay procedures. They found that the fireassay fusion will recover at least 98 percent of the 

gold in the samples. From radiotracer experiments 

2 

Simon and Millard ( 1968) found that the slag retained 

from 0.3 to 1.5 percent of the gold and that the silver 

bead recovered between 97 and 99 percent ofthe gold. 

A study was made to de.termine the fire-assay recovery 

of gold tracer when gold carrier was added to give a 

gold bead after cupellation. Better than 98 percent of 

the gold tracer was recovered in the final gold bead 

(table 1). As the fusion is virtually quantitative in collecting gold into the lead button, exchange between 

carrier and active gold during the fusion step is not 

critical. However, losses may occur during cupellation. The carrier yield will indicate any losses during 

cupellation, provided that carrier and radioactive gold 

have exchanged in the molten lead. A brief tracer experiment was made to test for exchange: 1 ml of 

tracer-gold solution was dried on 1 g Si02 in a fusion 

crucible; flux was added, along with gold carrier in 

the form of a bead; after fusion, the lead button was cut 

into two parts and cupelled separately; the final gold 

beads were weighed and counted. It was found that the 

specific activity of gold in each portion was identical. 

Hence, the carrier yield was used to correct for radioactive gold which may have been lost during cupellation. 

Although yields we r e usually 97 to 99 percent, an occasional run was as low as 90 percent. 

Slag--------------- 0.1 

Crucible----------- .3 

Cupel-------------- 1.0 

Gold bead----------98.6 

Interferences 

Generally the method is specific for gold, in that the 

0.413 Mev photopeak of Aul98 is used for the calculations. Most silver originally in the sam p 1 e will be 

found in the final bead. AgllOm has gammaphotopeaks 

at 0.445, 0.619, 0.665, and 0.884 Mev; thephotopeak at 

0.445 Mev is very small compared with that at 0,665 

Mev. The silver contribution to the gold photopeak is 

negligible unless the silver in the original sample is 

greater than 10 ppm. Small amounts of antimony (60-

day Sbl24) and cerium (33-hour Cel44) may also be 

found in the gold bead; however, these do not interfere 

with the 0.413 Mev photopeak of Aul98. 

DeGrazia and Haskin ( 1964) reported that the formation of Aul98 from the fast n,p (neutron, proton) reaction of Hg 198 is negligible. The formation of Au 199, 

the 3.15-day daughter of 30-minute Ptl99, is not an 

interference, as it has photopeaks at 0.208 and 0.158 

Mev which do not interfere with the 0.413 Mev photopeak for Aul98. 

Precision and accuracy 

The neutron flux is monitored by using four standards 

in each irradiation, placed at different levels in the 

irradiation bucket. The specific activity of the monitors 

varies by less than 5 percent from the mean, which can 

be presumed to be the flux variation. 
Table 2. -Eight reElicate determinations of gold, in Earts Eer billion, 

in Hawaiian basalts, G-1, and W-1 

TLW-67-17 HW-6 HK 1955 

Cone Crater, Basaltic glass, Kilauea, G-1 W-1 

Hawaii Hawaii Hawaii 

4.28 1. 56 1. 03 3.69 4.94 

4.50 1. 66 .83 2.65 5.59 

4.08 1. 67 .98 2.93 4.62 

4.98 1. 63 .93 2.72 3.87 

4.18 1. 63 .97 2.43 3.74 

4.38 1. 23 . 85 2.99 4.00 

4.16 1. 38 .81 2.85 4.87 

4.15 1. 46 1. 00 2.88 5.35 

Mean------------- 4.33 1. 53 . 93 2.89 4.62 

Standard 

deviation------ .27 .16 . 08 .37 .69 

Table 3. -Com b 

Sample Rock type This method Other methods 

W-1 Diabase------ 1

4.62 2 

3. 4' 3 3. 4' If 4. 9' If 5 4.8, 8. 5' 5

8.4' 6 4. 9' 7 

4. 8' 8 3. 6' 9 5.3 

G-1 Granite------ 12.89 21.9' 3 

2. 6' If 2. 0' 7

4.6, 5

4.3' 9

11. 2 

G-2 ----do------- 1.1' 1. 4' 1. 3' 

2 3 

0. 8' If 

0. 8' 1.1 

1.1 

GSP-1 Granodiorite- 1. 5' 1. 6' 1. 4' 

2 31.2' If 

1.1, 1.3 

1.1' 1.3 

PCC-1 Peridotite--- 0.63, 0.73 2

0.6, 3 

0. 7' '~o.7 

AGV-1 Andesite----- 0.46, 0.42 2 

0. 4' 3 0. 5' '~o.6 

DTS-1 Dunite------- 0.98, 0.58, 2 

0. 5' 21.0' 3 

0. 7' '~o.7 

0.45 

2 2 3 If BCR-1 Basalt------- 0.50, 0.51, 0.4, 0. 7' 0. 6' 0.9 

ZAG-3 Granite from 0.68 

Kershaw, 3 N.C-------- 2.5 2. 4 

ZAG-S ----do------- 4.1' 3. 7' 3.6 3

4.0 

ZAG-10 ----do------- 2. 6' 2. 7' 3.0 3 3. 0 

1

See table 2. 2

Millard (unpub. data, 1968), neutron-activation analysis with separation of gold on ion-exchange paper. 3

Millard, Rowe, and Brown (unpub. data, 1967), neutron-activation analysis with radiochemical separation of gold. '~Baedeker (1967), neutron-activation analysis with radiochemical separation of gold, 5

Vincent and Crockett (1960), neutron-activation analysis with radio- 6 chemical separation of gold. 

Hamaguchi and others (1961), neutron-activation analysis, method not 

7 described. 

Shcherbakov and Perezhogin (1964), neutron-activation analysis with 

8 radiochemical separation of gold. 

Simon and Millard (1968), neutron-activation analysis after fire-assay 

9 preconcentration. 

Sarma, Sen, and Chowdhury (1965), emission spectrographic analysis on silver fire-assay bead. 

3 
U.S. Geological Survey standard rocks, G-1, and 

W -1, and three samples of Hawaiian basalts, one of 

which was a basaltic glass, were each analyzed eight 

times. The results are shown in table 2. From the replicate analysis, the relative error is estimated to be 

about ± 10 percent of the amount of gold present. 

The determination of gold by this method is in good 

agreement with results determined by accepted 

methods bf activation analysis. Comparison ofresults 

for six standard rocks and several other rock samples 

is shown in table 3. 

Sampling 

The determination of gold using 1-g samples for 

analysis introduces the pro b 1 em of representative 

sampling. Wilson ( 1964) and Washington and H o 1m an 

(1966) showed that gold maynotbehomogeneouslydistributed throughout the sample. Chow and B e a m i s h 

(1967) discussed the problem of activation analysis 

versus fire assay with regard to sampling difficulties. 

The heterogeneity of gold distribution in standard rock 

W -1 is an example for which reported values vary 

considerably. It is important that the samples be ground 

to less than 100 mesh and that the portion taken for 

analysis be carefully quartered to assure a representative sample. Nevertheless, as can be seen from table 2, 

more consistent results are obtained for samples which 

are reasonably homogeneous, such as the basalts. It is 

possible to overcome some of the sampling problems 

by irradiating two large samples instead of 16 small 

ones. The only modification necessary w o u 1 d be the 

amount of fire-assay flux used to fuse the sam p 1 e. 

Appliciltion 

The method presented in this paper is being applied 

to studies of the geochemistry of gold. As it is suitable 

for rapid routine determination of low concentrations 

of gold, it extends the cap a b i 1 it y of laboratories to 

study background levels of gold in many environments. 

The method is particularly applicable to studies of 

hand-picked minerals or s amp 1 e s for which only a 

limited amount of material is available. 

REFERENCES CITED 

Baedeker, P. A., 1967, The distribution of gold and 

iridium in meteoritic and terrestrial materials: 

Lexington, Kentucky Univ. Ph. D. dissert., 104 p.; 

U.S. Atomic Energy Comm. Tech. Rept. OR0-

2670-17' 120 p. 

Baedeker, P. A., and Ehmann, W. D., 1965, The distribution of some noble metals in meteorites and 

natural materials: Geochim. et Cosmochim. Acta, 

v. 29, p. 329-342. 

4 

Beamish, F. E., 1966, The analytical chemistry ofthe 

noble metals: New York, Pergamon Press, 

p. 162-176. 

Bugbee, E. E., 1940, A textbook of fire assaying 

[3d ed. ]: New York, John Wiley & Sons, Inc., 314 p. 

Chow, A., and Beamish, F. E., 1967, An experimental 

evaluation of neutron activation, wet assay, and fire 

assay methods of determining gold in ores: Talanta, 

v. 14, p. 219-321. 

Covell, D. F., 1959, Determination ofgamma-ray 

abundance directly from total a b sorption peak: 

Anal. Chemistry, v. 31, p. 1785-1790. 

DeGrazia, A. R., and Haskin, Larry, 1964, On the gold 

content of rocks: Geochim. et Cosmochim. Acta, v. 

28, p. 559-564. 

Emery, J. F., and Leddicotte, G. W., 1961, The radiochemistry of gold: Nat I. Acad. Sci. -Nat I. Research 

Council Nuclear Sci. Ser., NAS-NS-3036, 34 p. 

Hamaguchi, Hiroshi, Kuroda, Rokuro, Tomura, Kenji, 

Watanabe, Kenju, Yasunaga, Tsutomu, 0 saw a, 

Masumi, Onuma, Naoki, Hosohara, Kyoichi, and 

Endo, Tadashi, 1961, Values for trace elements in 

G-1 and W-1 with neutron activation analysis: 

Geochim. et Cosmochim. Acta, v. 23, p. 296-299. 

Sarma, B. D., Sen, B. N., and Chowdhury, A. N., 1965, 

Platinum and gold contents of granite G-1 and diabase W-1: Econ. Geology, v. 60, p. 373-374. 

Shcherbakov, Y. G., and Perezhogin, G. A., 1964, Geochemistry of gold: Geokhimyia 6, p. 518-528. 

Simon, F. 0., and Millard, H. T., 1968,Determinatwn 

of gold in rocks by neutron activation using fire 

assay preconcentration: Anal. Chemistry, v. 40, 

p. 1150-1152. 

Vincent, E. A., and Crockett, J. H., 1960, Studies in 

the geochemistry of gold-Part 2, the gold content 

of some basic and ultrabasic rocks and stone meteorites: Geochim. et Cosmochim. Acta, v. 18, 

p. 143-148. 

Vincent, E. A., and Smales, A. A., 1956, Determination of palladium and gold in igneous rocks by 

activation analysis: Geochim. et Cosmochim. Acta, 

v. 9, p. 145-160. 

Washington, R. A., and Holman, R. H. C., 1966, A 

rapid and sensitive method for determining gold in 

rocks and other geological materials: Canada 

Geol. Survey Paper 65-7, 18p. 

Wilson, A. D., 1964, The sampling of silicate rock 

powders for chemical analysis: Analyst, v. 89, 

p. 18-30. 

GPO 864-726