An Atomic-Absorption Method 

for the Determination of 

Gold in Large Samples 

of Geologic Materials 

An Atomic-Absorption Method 

for the Determination of 

Gold in Large Samples 

of Geologic Materials 

By Gordon H. VanSickle and Hubert W. Lakin 

GEOLOGICAL SURVEY CIRCULAR 561 
United States Department of the Interior 

WALTER J. HICKEL, Secretary 

Geological Survey 

William T. Pecora, Director 

First printing 1968 

Second printing 1969 

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

Page 

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

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

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

Procedure ----------------------------------------------------------------- 2 

Preparation of standards ----------------------------------------------- 2 

Discussion of procedure -------------------------- _ ------------------------- 2 

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

Conclusions ---------------------------------------------------------------- 3 

References ------------------------------------------------------------------ 3 

TABLES 

Page 

TABLE 1. Repeatability of the method -------------------------------------- 3 

2. Comparison of gold values obtained by four 

atomic-absorption methods ------------------------------------ 3 

m 

AN ATOMIC-ABSORPTION METHOD FOR THE DETERMINATION 

OF GOLD IN LARGE SAMPLES OF GEOLOGIC MATERIALS 

By GORDON H. VANSICKLE and HUBERT W. LAKIN 

Abstract 

A laboratory method for the determination of gold 

in large ( 100-gram) samples has been developed for use 

in the study of the gold content of placer deposits and 

of trace amounts of gold in other geologic materials. 

In this method the sample is digested with bromine and 

ethyl ether, the gold is extracted into methyl isobutyl 

ketone, and the determination is made by atomicabsorption spectrophotometry. The lower limit of detection is 0.005 part per million in the sample. The 

few data obtained so far by this method agree 

favorably with those obtained by assay and by other 

atomic-absorption methods. About 25 determinations 

can be made per man-day. 

INTRODUCTION 

The Heavy Metals program of the U.S. Geological Survey requires large numbers of gold 

analyses and an amilytical capability to determine gold in very small amounts. Several rapid 

methods for gold determination that have been 

developed recently (Lakin and Nakagawa, 

1965; Huffman and others, 1967; Thompson 

and others, 1968) have proved to be very use-

;ful for reconnaissance-type sampling of most 

areas (Erickson and others, 1966; Gott and 

McCarthy, 1966) . However, these methods, 

which use samples weighing only 2-15 grams, 

are inadequate for the determination of the 

gold content of potential placer deposits because 

of the erratic distribution of particulate gold 

in these materials (Antweiler and Love, 1967; 

Clifton and others, 1967). Analysis of a larger 

sample (100 g) permits a more accurate determination of the gold content of such material. 

The 100-g sample also gives a detection limit 

for gold of 0.005 ppm (part per million), lower 

than that of the earlier methods, which have a 

detection limit of 0.02 ppm or more. 

With the recent development of atomic:-

absorption spectrophotometry the measurement 

of gold has become simple and routine. Methods 

of sample dissolution vary. In a method described by Thompson, Nakagawa, and Van1 

Sickle (1968) a mixture of hydrobromic acid 

and bromine is used to solubilize the go1d. The 

gold is then extracted into methyl isobutyl 

ketone (MIBK), and the gold concentration is 

determined by atomic-absorption spectrophotometry. 

Shima (1953) described a method of lf'aching 

gold from large samples ( 100-500 g) with a 

mixture of bromine and ethyl ether and estimating the gold colorimetrically with dithizone. 

The method which is proposed her~, and 

which is a combination and modification of 

these two methods, results in a simple, r~liable, 

sensitive, and low-cost analysis for go1d in a 

large sample. The gold is dissolved by a 1nixture 

of bromine and ethyl ether and is extracted 

into MIBK; the gold concentration is then determined by atomic absorption. 

We wish to acknowledge the help anc, advice 

of C. E. Thompson of Skyline Laboratories, 

Inc., and formerly of the U.S. Geological Survey. Arthur Kinkel, Paul Theobald, and Garland 

Gott of the U.S. Geological Survey provided the 

samples for our experimental work. 

REAGENTS AND APPARATUS 

Bromine, reagent grade. 

Ethyl ether, reagent grade. (Note: Because of 

the toxicity of bromine and the explo,~ive nature of ethyl ether these reagents should be 

used only in a well-ventilated laboratory hood. 

They should be stored in a refrigerr.tor.) 

Hydrobromic acid, concentrated, reagent grade, 

distilled. 

1.5 N HBr: Dilute 172 milliliters of concentrated HBr to 1 liter with water. 

0:1 N HBr: Dilute 25 milliliters of concentrated HBr to 2 liters with wate:--. 

Methyl isobutyl ketone (MIBK), (2 rr~thyl 4 

pentanone) , reagent grade. 

Standard gold solution, 0.1 percent: Dissolve 
exactly 1.000 g of gold in a solution of bromine in hydrobromic acid. Heat gently to 

expel excess bromine. Cool and dilute to 1 

liter with concentrated hydrobromic acid. 

Culture tubes, screwcap, 25 by 200 millimeters 

in size, with Teflon liner in screwcap. 

Evaporating dishes, porcelain, 60 millimeters 

in diameter. 

Glass jars, screwcap, 6 oz (ounces) in capacity. 

The caps must be noncorrosive. (We have had 

satisfactory results by lining each bakelite 

cap with a Teflon sheet.) 

Spatula, porcelain, 6 inches in size. 

Muffle furnace, electric. 

Atomic-absorption spectrophotometer. 

PROCEDURE 

1. Weigh 100 g of sample ground to -100 

mesh and transfer it to a porcelain dish. 

Place the dish in a muffle furnace and 

heat at 700°C for 2 hours. 

2. Cool and transfer the sample to a 6-oz glass 

jar. Add 1 ml bromine and 5 ml ethyl 

ether. Cap the jar tightly and shake 

vigorously to mix. Allow to stand for 

about 1 hour and shake intermittently. 

3. Add 20 ml MIBK to mixture. Cap and shake 

vigorously to mix. Add more MIBK if 

necessary until the sample is completely 

wet and is the consistency of a soft mud 

ball. 

4. Remove the cap and add approximately 10 

ml 0.1 N HBr to contents. Stir gently with 

a porcelain spatula. Continue to add 0.1 

N HBr in small increments and stir until 

the MIBK is separated. 

5. Pour off the MIBK into a 25- by 200-mm 

screwcap culture tube containing 25 ml 

0.1 N HBr. Cap the tube and shake it for 

30 seconds. Allow the phases to separate 

and aspirate the organic phase into 

an atomic-absorption spectrophotometer. 

Read the percent absorption and convert 

to concentration as directed for the particular instrument. The gold content, in 

parts per million, of the sample is equal 

to: 

~-tg/ml X ( ml MIBK + ml ethyl ether) 

100 

2 

PREPARATION OF STANDARDS 

Add respectively 5, 25, 50, and 100 1-tg of gold 

to four 100-ml volumetric flasks containing 25 

ml 1.5 N HBr. Add 50 ml MIBK and shake the 

flask for 1 minute. These MIBK folutions contain 0.1, 0.5, 1.0, and 2.0 1-tg per 1nl gold. 

DISCUSSION OF PROCEDURE 

In the roasting (step 1), organic carbon and 

sulfides should be completely oxicized. During 

the sample digestion (step 2), the capped glass 

jar must fill with brown vapors of bromine. 

The absence of bromine vapors indicates incomplete oxidation in step 1. An excess of bromine 

is required to ensure the solution of gold in the 

sample. 

The 5 ml of ethyl ether originally added to 

the sample (step 2) dissolves in the MIBK and 

thus, for the calculation of the result, the 

volume of ethyl ether used in step 2 must be 

added to the volume of MIBK (step 3) used in 

the extraction. 

The effectiveness of bromine ard ethyl ether 

for the solution of gold is illustrated by the 

following experiments, which sl'~W that the 

solution rate is a function of the gold particle 

size. Three pieces of gold weighing a total of 

69.4 milligrams (the largest piece was about 2 

mm long and 1 mm in diameter) were placed 

in a 6-oz jar with 100 g of fine quartz sand. 

One ml bromine and 5 ml ethyl ether were 

added. The jar was capped, shaken, and allowed to stand for 1 hour. The gold pieces 

were then recovered and weighed; they were 

found to weigh 68.3 mg-a loss of 1.1 mg. The 

above procedure was repeated, and the pieces 

weighed after overnight digestion. By this time 

32.2 mg had dissolved. Gold leaf, however, dissolves instantaneously in these reagents. 

PRECISION AND ACCURACY 

The repeatability of the method was determined on four - 100 mesh samples-three of 

phonolite breccia and one of gold- and monazitebearing sand (table 1). The repeatability as 

measured by the relative standard deviation 

may be a function of the homogeneity of the 

samples. Pulverizing the -100 mesh sample in 

a ball mill for at least 1 hour would result in 

more uniform dispersal of the gold. 

The gold content of 13 samples of oxidized 
low-grade ore and two samples of sulfide ore 

from North Carolina has been determined by 

the atomic-absorption method in combination 

with four methods of sample solution (table 2) : 

(1) Fire assay and aqu~ regia, (2) cyanide, 

(3) hydrobromic acid-bromine, and ( 4) bromine-ether as described here. The fire-assay 

method gives slightly higher gold content in 

materials containing less than 1 ppm than the 

bromine-ether procedure. Otherwise the agreement between the various methods is acceptable. 

TABLE !.-Repeatability of the method 

Gold (ppm) li'(>lative 

Description Number of Standani standanl Sample and locality determinations deviation deviation Maximum Minimum Mean (I'f't'cent) 

CC-562 Fractured phonolite 6 0.56 0.34 0.46 0.074 16 

breccia, Cripple Creek 

district, Teller County, Colo. 

567 ______ do _____________________ 8 .072 .024 .046 .016 35 

559 ______ do _____________________ 6 .018 .009 .015 .0035 24 

ABK-045 Gold and monazite placer 6 1.24 .31 .88 .34 39 

sand, Moffat County, Colo. 

TABLE 2.-Comparison of gold values obtained by fou'r atomic-absorption ntethods 

[All samples from North Carolina, and sample numbers are preceded by K-. Fire-assay analyses by 0. M. Parker, L. B. Riley, J. D. Mensik, and Claude Huffman, Jr., U.S. Geological Survey; cyanide and hydrobromic acid-bromine analyses by A. E. Hubert, U.S. Geological Sut·vey] 

Gold (ppm) 

Method of solution 

Sample Description Fire assay and aqua Cyanide Hydrobromic Bromine-ether t·egia (15-g (100-g add--b,·o:ni"le (11'0-g '!fl.mple' 

sample) 

12 Rock chips and soil _________________ _ 13 ______ do ____________________________ _ 18 ______ do ____________________________ _ 

19 Saprolite ----------------------------- 20 Hard saprolite ______________________ _ 21 ______ do ____________________________ _ 

35 Schistose (sericitic) saprolite _________ _ 36 ______ do ____________________________ _ 

57 Red soil (B zone) ___________________ _ 61 Tuff saprolite _______________________ _ 

100 Bedded tuff -------------------------- 130 Rhyolitic tuff saprolite _______________ _ 136 Cellular gossan ______________________ _ 

109 Pyritic rock -------------------------

135 Sulfide ore ---------------------------

CONCLUSIONS 

0.7 

.3 

.3 

.3 

.2 

.2 

2. 

.6 

.5 

.3 

1. 

<.05 

5.5 

4. 

7. 

The method presented here is certainly not 

a replacement for the standard fire assay for 

gold; however, the simplicity, sensitivity, and 

lower cost of the method are distinct advantages. 

We have emphasized the 100-g sample, but 

with a few modifications in the procedure and 

the equipment a much larger sample could be 

handled. The problems of grinding, roasting, 

extracting, and waste disposal by using a sample much larger than the suggested 100-g size 

increase exponentially with the sample size. 

3 

samp~e) (10-g sample) 

0.4 0.8 0.6~ 

.2 .4 .20 

.3 .7 .2r 

.1 .3 .2f 

.08 .4 .1~ 

.2 .2 .1~ 

2.4 2.8 3.1 

.7 .8 .6i 

.1 .6 .1r. 

.1 .3 .20 

1. 1. 1.2 

<.02 <.02 .0~ 

3.2 5.6 6.8 

3.8 4. 4.7 

.5 3.9 4.6 

Therefore, this method is a compromise b~tween 

the need for analysis of larger samples and the 

ability to do larger numbers of samples easily. 

A method for routine analysis of samples as 

large as 1000 g has been developed by tho. Office 

of Marine Geology and Hydrology of th~ U.S. 

Geological Survey (H. E. Clifton, writte:n commun, March 1968). 

REFERENCES 

Antweiler, J. C., and Love, J. D., 1967, Gold-bearing 

sedimentary rocks in northwest Wyoming-a preliminary report: U.S. Geol. Survey Circ. 541, 12 p. 

Clifton, H. E., Hubert, Arthur, and PhillipE, R. L., 
1967, Marine sediment sample preparation for 

analysis for low concentrations of fine detrital 

gold: U.S. Geol. Survey Circ. 545, 11 p. 

Erickson, R. L., VanSickle, G. H., McCarthy, J. H., 

Jr., Nakagawa, H. M., and Leong, K. W., 1966, 

Gold geochemical anomaly in the Cortez district, 

Nevada: U.S. Geol. Survey Circ. 534, 9 p. 

Gott, G. B., and McCarthy, J. H., Jr., 1966, Distribution 

of gold, silver, tellurium, and mercury in the Ely 

mining district, White Pine County, Nevada: U.S. 

Geol. Survey Circ. 535, 5 p. 

Huffman, Claude, Jr., Mensik, J. D., and Riley, L. B., 

1967, Determination Qf gold in geologic materials I 

4 

by solvent extraction and atomic-absorption spectrometry: U.S. Geol. Survey Circ. 5d4, 6 p. 

Lakin, H. W., and Nakagawa, H. M., 1965, A spectrophotometric method for the determiration of traces 

of gold in geologic materials, in Geological Survey 

research 1965: U.S. Geol. Surve!• Prof. Paper 

525-C, p. C168-C171. 

Shima, Makoto, 1953, Estimation of gold and silver contained in ores, study of rapid anal'Tsis for use in 

mines: Japan Analyst, v. 2, p. 96-98. 

Thompson, C. E., Nakagawa, H. M., and VanSickle, 

G. H., 1968, Rapid analysis for g<:~ld in geologic 

materials, in Geological Survey research 1968: U.S. 

Geol. Survey Prof. Paper 600-B, p. B130-B132. 

'f< U.S. GOVERNMENT PRINTING OFFICE: 1969 0-334-949