GEOLOGICAL SURVEY CIRCULAR 544 

Determination of Gold in 

Geologic Materials by 

Solvent Extraction and 

Atomic-Absorption 

Spectrometry 

Determination of Gold in 

Geologic Materials by. 

Solvent Extraction and 

Atomic-Absorption 

Spectrometry 

By Claude Huffman, Jr., J. D. Mensik, and 

L. B. Riley 

----------------------------~ GEOLOGICAL SURVEY CIRCULAR 544 

Washington 1967 
United States Department of the Interior 

STEWART L. UDALL, Secretary 

Geological Survey 

William T. Pecora, Director 

First printing 1967 

Second printing 1 968 

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

Introduction -----------------------------

Reagents--------------------------------

Apparatus-------------------------------

Cyanide atomic-absorption procedure ------

Sample preparation---------------------

Standardization ------------------------

CONTENTS 

Page 

1 

1 

1 

2 

3 

3 

4 

Fire-assay atomic-absorption procedure ---

Results ---------------------------------

Sample dissolution, cyanide procedure------

Extraction of gold with methyl isobutyl 

ketone--------------------------------

Conclusions - - - - - - - - - - - - - - - - - - - - - - --- - - --

References------------------------------

ILLUSTRATIONS 

Page 

4 

5 

5 

5 

6 

6 

Page 

Figure 1. Water bath equipped for aeration and constant temperature----------------------------- 2 

2. Constant-temperature water bath with magnetic stirrer-------------------------------- 3 

TABLES 

Page 

Table 1. Comparison of gold values obtained by atomic-absorption methods and by the 

conventional gravimetric fire-assay method ---------------------------------------- 5 

2. Comparison of gold values obtained by fire-assay atomic-absorption and 

cyanide atomic-absorption methods------------------------------------------------ ·6 

III 

Determination of Gold in Geologic Materials by 

Solvent Extracti n and Atomic-Absorption Spectrometry 

Huffman, Jr., J. D. Mens'ik, and L. B. Riley 

ABSTRACT 

The two methods presented for the deter ination of traces of 

gold in geologic materials are the cyani e atomic-absorption 

method and the fire-assay atomic-absorption ethod. In the cyanide 

method gold is 1 each e d with a sodium c y n ide solution. The 

monovalent gold is then oxidized to the trival nt state and concentrated by extracting into methyl isobutyl keto e prior to estimation 

by atomic absorption. In the fire-assay atomi -absorption method, 

the gold-silver bead obtained from fire assay· s dissolved in nitric 

and hydrochloric acids. Gold is then conce trated by extracting 

into methyl isobutyl ketone prior to determ ·nation by atomic absorption. By either method concentrations a low as 50 parts per 

billion of gold can be determined in a 15-gra sample. 

INTRODUCTION 

The U.S. Geological Survey heav metals program 

has instituted reconnaissance studie requiring rapid, 

precise, and sensitive methods for de ermining various 

elements in geologic materials. The determination of 

gold for this program has required ethods more sensitive than the conventional fire-ass y method. Lakin 

and Nakagawa (1965) described a sp ctrophotometric 

method for determining traces o gold in geologic 

materials; they found, however (oral commun., 1966), 

that thallium and iron interfere. Wals ( 1955) has shown 

that atomic-absorption spectrometry is extremely well 

suited for direct determination of low concentrations of 

certain elements in solution, but until recently no work 

seems to have been undertaken forth determination of 

gold. Olson (1965) and Simmons (19 5) used a cyanide 

solution to leach the gold from silice us carbonate ores 

prior to estimation by atomic absorpt on. Tindall ( 1965) 

described an atomic-absorption met od for the determination of gold in various types of amples dealt with 

in the copper mining industry. He u ed an aqua regia 

digestion followed by an organic ext action toconcentrate the gold into a small volume Strelow, Feast, 

Mathews, andVanZyl(1966)applied natomic-absorption method to the determination o gold in cyanide 

waste solutions. All these atomic-a sorption methods 

have been used primarily for deter:nining gold in the 

parts-per-million range. 

This paper describes two rapid andprecis,~atomicabsorption methods for gold determination currently 

used in the U.S. Geological Survey's Denver laboratory: 

the cyanide atomic-absorption method and th~ fire-assay atomic-absorption method. In developing these 

methods, sensitivity was stressed to enable g?.ochemical prospecting for gold at theparts-per-billionlevel. 

Both methods can detect concentrations as low as 50 

ppb (parts per billion) or 0.0015 ounce per ton of gold 

in a 15-gram sample. 

In the cyanide method the sample is roasted and then 

treated with a sodium cyanide solution to dis·solve the 

gold. The cyanide solution is acidified with tydrobromic acid and bromine water to oxidize the m')novalent 

gold to the trivalent state. The gold is then con7.entrated 

by extracting into a measured volume of methyl isobutyl 

ketone (MIBK). The gold concentration is determined by 

an atomic -absorption spectrophotometer. 

In the classical fire-assay method, the sil·rer ofthe 

gold-silver bead obtained by cupellation is dissolved 

and the gold residue is determined gravim<?.trically. 

The sensitivity of this method is approximately 0.02 

troy ounce per ton (600 ppb) on a 15-g (half assay ton) 

sample. In the proposed fire-assay atomic-absorption 

method the gold-silver bead from fire assay is dissolved in nitric and hydrochloric acids anc then extracted into a small volume of organic solve·1t (MIBK) 

prior to estimation by atomic absorption. This procedure thus increases the sensitivity by about a factor 

of 10 when a 15-g sample is used. The sensitivity can 

be further increased by combining beads obtained from 

two fire-assay fusions of 30 g each of the same sample. 

With this technique about 15 ppb can be determined. 

This is about the limit imposed by reagent blanks. 

REAGENTS 

Standard gold stock solution, 400 ppm (parts per million) gold. Dissolve 0.200 g of gold metal in a covered 

beaker with 30 milliliters of aqua regia on a steam 

bath, remove cover, and evaporate to dryress. Heat 

on a hotplate until fumes from HN03 are no longer 
present. Add 250 ml of ( 1:1) HCl to the beaker and 

digest on the steam bath to dissolve the gold salts. 

Transfer the solution to a 500-ml volumetric flask 

and dilute to volume with distilled water. 

Sodium cyanide solution, 0.25 percent. Dissolve 5 g 

NaCN in 500 ml distilled water, add 2 drops of a 

10-percent solution of NaOH, and dilute to 2,000 ml. 

The pH of this solution should be about 11. 

Hydrobromic acid, 48-percent reagent grade. 

Bromine water, saturated solution. 

Calcium oxide powder, reagent grade. 

Methyl isobutyl ketone (MIBK). 

r· Excess air outlet 

Santomerse S solution, wetting agen

volume;volume in vrater, obtained 

Chemical Co., St. Louis, Mo. 

APPARATUS 

Atomic-absorption spe~trophotometer

Elmer model 303 ato:-nic-absorptio

meter is used with the following in

meters: 

Wavelength: 2428 Angstroms. 

Slit: 5 (3 millimeters, 20 Angstro

Source: Gold hollow cathode. 14 m

rent. 

------

Water lev~~----- r 

Figure 1.-Water bath equipped for aeration and constant temperature. 

2 
Acetylene: 9 pounds per square inch. 

Air: 25 pounds per square inch. 

Flame adjustments: With the burner lit, adjust the 

flow meters to approximately 2.8 on the acetyHotplate and magnetic stirrer 

Figure 2.-Constant-temperature water bath with magnetic stirrer. 

3 

lene and 3.0 on the air while aspirating water. 

Minor adjustments may be necessary to obtain 

a blue flame when the MIBK is aspirated. 

Water bath equipped for aeration and constant temperature (fig. 1 ). The aeration device constructed in 

this laboratory consists of a compressor-vacuum 

pump capable of delivering air at one-third cubic 

foot per minute under a pressure of 15 po·.mds per 

square inch. This amount of air is more than sufficient for the 20 outlets shown in figure 1. The 

compressor supplies a group of air regulator valves 

(available from aquarium supply houses) which are 

connected to 3/ 16-inch-diameter Pyrex tubes by 

Tygon tubing. The valves are used to individually 

adjust the air supply to -each sample. A steady air 

flow to the tubes is maintained by means of a small 

air tank and one or more bleed-off valves. 

Constant-temperature water bath with magnetic 

stirrer (fig. 2). Teflon-coated ~-inch-diameter 

magnetic stirring bar. 

Pyrex culture tubes, 25 by 200 millimeters, with polyseal closure screw cap. 

CYANIDE ATOMIC-ABSORPTION PROCED'JRE 

SAMPLE PREPARATION 

Crush sample in a jaw crusher to less than ¥!-inch 

size, then split and grind a portion on ceramic plates 

set for less than 80 mesh. The sample is no'" sieved; 

a large portion of the sample will be less than 200 

mesh. 

1. Weigh a 2- to 15-g sample and transfer to a porcelain dish or crucible. Place the crucible in a 

cold muffle furnace and heat for 1 hour after the 

temperature reaches 600°C. Do not exceed 650°C 

because samples of high clay content may tend 

to sinter and thus trap the goldandprevent solution during the cyanide step. 

2. Cool. Add 0.25 g CaO powder and mix thcroughly; 

transfer sample to a Pyrex culture tube. Add 30 

ml of the 0.25-percent NaCN solution to the tube, 

place cap on the tube, and shake by hand to wet 

the entire sample. Place tube in an automatic 

shaker and shake for 15 minutes; let stc.nd overnight. 

3. Reshake to break up any cake formed in th~ bottom 

of the tube. Wash down sides of the tube with about 

10 ml of the NaCN reagent. Add 4 dro'Js of the 

0.25-percent Santomerse S wetting agent to the 

tube and heat for 30 minutes in the water bath at 

80° -85°C while aerating the sample (fig. 1 ). 

4. If a 2 -g sample is used, proceed to step 5. If a 

sample of more than 2 g is used, centrifuge the 

contents of the tube at 2, 000 revolutions per minute for 5 minutes, then decant the liquid phase 

into another culture tube containing a magnetic 
stirring bar. Wash the residue left in the first 

tube with 10 ml of the 0.25-percent NaCN solution 

containing 4 drops of saturated bromine water. 

Centrifuge again and combine wash solution with 

the original solution. 

5. Add 5 drops bromine water and mix. Then, working 

under a well-ventilated hood, cautious 1 y add 5 

ml of 48-percent HBr. (CAUTION: Hydrogen cyanide gas is evolved.) Heat in a water bath equipped 

for magnetic stirring (fig. 2) for 30 minutes at 

90°C, 

6. Cool. Remove the stir bar with another magnet held 

outside the tube, add 10 ml MIBK, and shake for 1 

minute. Centrifuge if step 4 has been omitted. 

7. Measure the absorption of the MIBK phase along 

with reference standards (see following section, 

"Standardization") on an atomic-absorption spectrophotometer at a wavelength of2428 Angstroms. 

Use a water -saturated so 1 uti on of MIBK as a 

reference blank. To obtain a standard calibration 

curve, plot the absorption readings against gold 

concentration expressed as parts per million in 

the organic phase. The gold content of the sample 

in parts per million is equal to: 

gold in ppm in organic phase X 10 ml 

weight of sample in grams 

If the sample has an absorbance greater than the 

lO ppm standard, a dilution of the organic layer 

can be made. The dilution method is illustrated 

in the following example: Transfer a 2-ml aliquot 

of the organic phase to a tube containing about 

30 ml of water and 2 ml of 48-percent HBr. Add 

8 ml of MIBK, shake, and measure the absorption. This dilution factor of 5 will handle a 10-g 

sample containing as much as 50 ppm Au. 

STANDARDIZATION 

Prepare reference standards containing 0.1, 0.2, 

0.4, 0.8, 1.0, 2.0, 4.0, 8.0, and 10.0 ppm Au in the 

organic phase for working curves by adding appropriate 

aliquots of a gold standard solution to a series of 

culture tubes. Add 2 ml of 48-percent HBr to each and 

adjust the volume to about 40 ml with water. Add 10.0 

ml of MIBK and shake for 1 minute to extract the gold 

into the organic phase. Measure the absorption of these 

standards on the 1X or lOX scales of the atomica b s or p t i o n spectrophotometer. P e r i o d i c a 11 y, 

standards can, and probably should, be checked by 

being subjected to the procedure starting with step 2, 

However, we have noted no difference in absorption 

readings between the standards taken through the entire 

procedure and those prepared as described. 

FIRE-ASSAY ATOMIC-ABSORPTION PROCEDURE 

The fire-assay method involves a fusion of ground 

ore with fluxes to form a slag phase and a metallic 

lead phase. The lead collects any gold, platinum metals., 

or silver and settles out at the bortmn when poured 

into a mold. After cooling., tl'le slag is broken away 

from the lead button which is then put into the furnace 

in a cupel. The lead melts and is slowly oxidized by 

the hot air. The lead oxide wets the semiporous cupel 

material, generally bone ash, and is absorbed. The 

cupel material is not wet by the molten lead nor by the 

noble metals, which remain as a bead after the gradual 

absorption of the lead oxide. 

4 

The procedure for the further treat11ent of the bead 

is as follows: 

1. Transfer the silver-gold bead obtained by fire assay 

to a 50-ml beaker. 

2. Add 5 ml of (1:1) HN03, cover, and digest on a 

steam bath for 45 minutes. Uncover and evaporate 

the solution to about 2.5 ml. 

3. Add 6 ml of concentrated HCl, cover, and let stand 

20 minutes. Place beaker on steam bath and digest 

for 30 minutes, uncover, and evaporate to hard 

dryness. 

4. Add 7 ml of concentrated HCl, cover, and digest on 

the steam bath until all salts are in solution. Uncover, add about 15 ml of water, and digest an 

additional 30 minutes. 

5. Cool. Quantitatively transfer the solution to a Pyrex 

culture tube by washing beaker with about 10 ml 

of water. Add 2 ml of 48-percerr: HBr, mix, add 

10 ml of MIBK, and shake tube for 1 minute. 

6. Determine the absorbance of the organic phase along 

with reference standards prepared in the same 

manner as described in the cyc.nide procedure. 

We routinely add 2 milligramsofsilverinthe fusion 

step. Precut lengths of lead rod, eac'1 containing this 

amount of silver, are obtainable commercially under 

the trade name Inquarts. Silver is important in this 

procedure for three reasons: (1) It has a protective 

effect and thus reduces gold losses c'uring the cupellation step (Bugbee, 1940, p. 107), (2) it allows trace 

amounts of gold to be easily transferred from the 

cupel to a suitable container for analysis, and (3) the 

addition of silver provides a silver-gold bead that is 

easily dissolved, The ratio of silver to gold should be 

greater than 3:1 to allow dissolutioncfthebead; beads 

having lower ratios of silver to gold may be difficult 

to dissolve, even in aqua regia. For s~mples that contain sufficient silver, such addition is unnecessary; 

conversely, for samples than contain considerable 

gold, the addition of more than 2 rrilligrams silver 

may be necessary to attain this 3:1 ratio. The silvergold bead is routinely weighed to allow an estimate 

or approximate check of the combined gold and silver 

content of the ore, 

For a full discussion of the fire-assay method the 

reader is referred to Bugbee ( 1940) or Beamish ( 1966 ). 
Table 1.-Comparison of gold values obtained by atomic~absorption methods and by the conventional gravimetric 

fire~assay method 

[Gravimetric fire-assay by 0, M. Parker and L, B. Riley, Atomic-absorption determinations by 

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

Gold (ppm) 

Sample Atomic absorption Sample description 

Gravimetric and locality 

fire assay Fire assay Cyanide 

96130 _______________ 4,8 4.8 

0223180 ------------ 10,3 11 

Dll9999 ------------ 13,7 16 

D100874 ------------ 15.8 18 

Dll9971 ------------ 16.5 15 

D119956 ------------ 26.4 27 

RESULTS 

The accuracy of the two proposed atomic-absorption 

methods was first tested by analyzing a suite of six 

selected samples previously an a 1 y zed by the conventional gravimetric fire-assay method. The results 

(table l) show good agreement between the two atomic 

absorption methods and the gravimetric fire-assay 

method. These samples were selected because they 

represent a wide variety of rock and ore types as 

indicated by semiquantitative spectrographic analysis. 

For example, sample Dll9956 contains 1.5 percent As, 

7 percent Fe, and 0.07 percent Zn; sample Dl19971 

contains 710 percent As, lOpercent Fe, and 0.03 percent Sb; sample Dll9999 contains> lO percent Fe, 5 

percent Cu, and 0.2 percent Mn. About 90 percent of 

sample Dl00874 is estimated to be pyrite. 

The cyanide atomic-absorption method has been 

thoroughly tested by routinely analyzing several hundred samples of varying composition. All samples that 

contained at least 0. 5 ppm Au were reanalyzed by the 

fire-assay atomic-absorption method; in addition, 

many randomly selected samples containing less than 

0.5 ppm Au were checked. Table 2 presents some of 

the data on 20 samples from Alaska and Nevada areas. 

The cyanide and fire-assay methods show concordant 

results. 

SAMPLE DISSOLUTION, CYANIDE PROCEDURE 

Simmons (1965) observed that low results were obtained on some samples if the ignition step was omitted 

prior to the cyanide treatment. We have confirmed his 

observation and routinely roast all samples. 

The cyanide process has been used commercially for 

many years. The commercial processes are based on 

5 

4.3 Sulfide ore, White Pine mine, 

Jefferson County, Mont. 

10 Channel sample, Gold Anchor 

mine, Clear Creek County, Colo. 

15 Pyrite, Billy Goat mine, Okanogan 

County, Wash. 

14 Quartz pyrite ore, Phoenix mine, 

Clear Creek County, Colo. 

16 Quartz vein, Okanogan County, 

Wash. 

25 Mineralized quartzite, Okanogan 

County, Wash. 

the following chemical reaction: 

4Au+8NaCN+2Hz0+02 -')oo 4AuCN·NaCN+4NaOH. 

The amount of gold which can be dissolved in sodium 

cyanide solution depends on the amount of oxyr-en present (Jacobson, 1949, p. 554). The aeraticndevice 

described in this paper supplies the oxygen for the 

preceding reaction and furnishes agitation to a large 

sample. It is desirable to keep the sample ir a single 

tube during the entire analysis. In the ir.itial stage of 

this investigation, 2 -g samples were aeratec, conveniently by means of magnetic stirring. However, when 

we attempted to increase our sample size to 15 g we 

found that the magnetic stirrer would not sufficiently 

agitate the larger sample. The aeration device provides 

sufficient agitation of a 15-g sample. 

EXTRACTION OF GOLD WITH METHYL ISOBUTYL 

KETONE 

Methyl isobutyl ketone (MIBK) has been used extensively in atomic-absorption work to preconcentrate 

certain elements that show insufficient sen~itivity in 

aqueous solution. Many authors favor this reagent 

because it has excellent flame characteristics and it 

is only slightly soluble in water ( 1. 9 g per 100 ml). 

We wish to emphasize that the gold is in the monovalent state after the cyanide treatment and will not be 

extracted quantitatively into the MIBK unlesE' oxidized 

to the trivalent state. In thi's method the dicyanogold 

(I) ion is oxidized to the bromaurate (III) ion by the 

addition of bromine water and hydrobromic acid. The 

reaction is as follows: 

NaAu(CN)z+2HBr+Br2~AuBr3+2HCN+NaBr. 

The bromaurate (III) ion is then extracted quan':itatively 

into MIBK prior to estimation by atomic ab"'orption. 
Table 2.-Comparison of gold values obtained by fireassay atomic-absorption and cyanide atomic-absorption 

methods 

[Analyses by Claude Huffman, Jr., J. D. 

Mensik, and L. B. Riley1 

Gold (ppm) 

Sample Sample 

Fire Cya- description 

assay nide 

D124654 __ 0,05 0.05 Altered rock. 

ABH775 __ . 09 .07 Altered diorite . 

D124913 __ .09 . 1 Altered zone . 

ABH780 __ .4 . 3 Iron -stained quartz . 

D124670 __ . 7 ,8 Altered zone . 

D124667 __ . 9 ,8 Quartz vein . 

D124665 __ 1 1 Altered zone. 

Dl24943 __ 2 2 Massive sulfide 

vein. 

ABH923 __ 2 2 Stained silicified 

quartzite. 

D124916 __ 3 3 Sulfide replacement 

lenses. 

ABH926 __ 4 4 Drusy quartz and 

iron oxide, 

D12493l__ 7 5 Altered zone and 

quartz vein, 

ABH871 __ 9 8 Gossan zone in 

quartzite, 

D124676 __ 12 13 Quartz vein and 

altered zone, 

0124675 __ 17 16 Quartz vein. 

D124920 __ 27 27 Sulfide -bearing 

quartz -calcite 

vein. 

ABH854 __ 44 40 Quartz, calcite, 

sulfides. 

D124679 __ 52 58 Altered zone. 

ABH832 __ 85 73 Iron-stained quartz. 

D124681 __ 350 330 Altered zone. 

Test solutions containing 1, 10, 50, and 100 micrograms of monovalent gold in 40 ml of cyanide solution 

were oxidized by adding 5 drops of saturated bromine 

water and 5 ml of 48-percent HBr, heated to expel 

hydrogen cyanide gas, cooled, and extracted with 10 ml 

of MIBK. All these test solutions showed by atomicabsorption spectrophotometry a recovery of greater 

than 95 percent in the organic phase. 

Recently. Strelow, Feast, Mathews, and Van Zyl 

~ 1966) c~nf1rmed our observation that monovalent gold 

m cyamde solution is not directly extractable with 

MIBK; they used KMn04 in HCl solution to oxidize the 

gold to the trivalent state. 

Atomic-absorption methods for the determination of 

gold in solution are reasonably free from interferences. No effect from diverse ions has been noted in 

6 

this work. In the fire-assay bead method, silver is 

partially extracted into the MIBK, but haS" no effect on 

the gold determination. Strelow, Feast, Mathews, and 

Van Zyl (1966) also observed no interferences; they 

stated that Tl(lll), Cr(Vl), Mo(Vl), As(V), ~IJ(V ), Sn(IV), 

Se(l V ), Te(IV), and Fe(III) are partially extracted but 

cause no interference with the determination of gold 

by atomic absorption. 

CONCLUSIONS 

The fire-assay separation combined with atomic 

absorption has two advantages over the classical fireassay method: ( 1) It eliminates the tedious and critical 

steps of transferring, parting, annealing, and then 

weighing trace amounts of gold and (2) it provides 

greater accuracy and precision in low and fractional 

parts-per-million range. 

We employ the more rapid cyanide method for scanning large numbers of samples for their gold content. 

Samples that show a gold content of greater than 1 ppm 

are then reanalyzed by the fire-assay atcmic-absorption method to confirm the results. 

REFERENCES 

Beamish, F. E., 1966, The analytical chemistry of the 

noble metals in International series of monographs 

in an a 1 y tic a 1 chemistry [1st ed. J: New York, 

Pergamon Press, v. 24. 609 p. 

Bugbee, E. E., 1940, A textbook of fire assaying [3d 

ed.]: New York, John Wiley &Sonrc,Inc.,314p. 

Jacobson, C. A., ed., 1949, Encyclopedia of chemical 

reactions: New York, Reinhold Put. Corp., v. 3, 

800 p. 

Lakin, H. W., and Nakagawa, H. M., 1965, A spectrophotometric method for the de t e r m i nat ion of 

traces of gold in geologic materials, in Geological 

Survey research 1965: U.S. Geol. Survey Prof. 

Paper 525-C, p. Cl68-C171. 

Olson, A. M., 196.5, Gold assay by atomic absorption 

spectrophotometry: Perkin-Elmer Atomic Absorption Newsletter, v. 4, no. 5, p. 278-280. 

Simmons, E. C., 1965, Gold assay by atomic absorption spectrophotometry: Perkin- Elmer Atomic 

Absorption Newsletter, v. 4, no. 2, p. 281-287. 

Strelow, E. C .. Feast, P. M., Mathews, C. J. C., and 

Van Zyl, C. R., 1966, Determination of gold in 

cyanide waste solutions by solvent extraction and 

atomic a b so r p t ion spectrophotometry: Anal. 

Chemistry, v. 38, p. 115-117. 

Tindall, F. M., 1965, Silver and gold assay by atomic 

absorption spectrophotometry: Perkin-Elmer 

Atomic Absorption Newsletter, v. 4, no. 9, p. 339-

340. 

Walsh, Ao, 1955, The application of atomic absorption 

spectra to chemical analysis: Spectrochim. Acta, 

v. 7, p. 108-117. 

RU, S. GOVERNMENT PRINTING OFFICE : 1968 0 - 285-957