Geochemistry of Gold 

in the Weathering Cycle 

By HUBERT W. LAKIN, GARY C. CURTIN, and ARTHUR E. HUBERT 

With chapters 

By HANSFORD T. SHACKLETT£ and KENNETH G. DOXTADER 

GEOLOGICAL SURVEY BULLETIN 1330 

A study of the solubilization of gold 

by the formation of chloride) bromide) 

iodide) thiosulfate) thiocyanideJ and 

cyanide gold complexes and related topics 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1974 
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary 

GEOLOGICAL SURVEY 

V. E. McKelvey, Director 

Library of Congress-catalog-card No. 74-600187 

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402- Price $1.15 (paper cover) 

Stock Number 2401-02583 
CONTENTS 

Page 

Abstract .......... -------------------------------------------------------------------------------------------------------- 1 

Introduction .............. --------------------------------------------------------------------------------------------- 2 

Scope of report---------------------------------------------------------------------------------------------··· 2 

Acknowledgments ...... ------------------------------------------------------------------------------------ 4 

Oxidation potential for gold as a function of complex stability................... 4 

Removal of gold from solution by minerals as a function of complex 

stabilitY--------------------------------------------------------------------------------------------------------------- 6 

Absorption of gold by plants as a function of complex stability.................. 14 

Gold chloride ...................... ------------------------------------····--------------------------------------------· 17 

Gold bromide and gold iodide.............................................................................. 20 

Geochemistry of bromides and iodides....................................................... 20 

Formation of gold bromide and gold iodide............................................... 23 

Gold thiosulfate..................................................................................................... 26 

Geochemistry of the thiosulfate ion........................................................... 26 

Formation of gold thiosulfate..................................................................... 28 

Gold thiocyanate.................................................................................................... 29 

Geochemistry of the thiocyanate ion.......................................................... 29 

Formation of gold thiocyanate.................................................................... 31 

Gold cyanide........................................................................................................... 32 

Geochemistry of the cyanide ion.................................................................. 32 

Formation of gold cyanide........................................................................... 36 

Tests for cyanide in some native and naturalized plants of the United 

States, by Hansford T. Shacklette.................................................................. 38 

Solubilization of gold by microorganisms and organic substances, by 

Kenneth G. Doxtader................................................................................ 47 

Introduction ...... ---------------------------------------------------------------------------------------------· 47 

Gold substrate ........ ------------------------------------------------------·-····················------------ 48 

Culture media .......... ------------------------------···--------------------------------------------------····· 48 

Culture procedures........................................................................................ 48 

Analysis of the culture liquids for soluble gold....................................... 48 

Results and discussion.................................................................................. 49 

Solubilization of gold by stock cultures of bacteria and fungi.... 49 

Solubilization of gold by microbial isolates from soil and sediments................................................................................................... 50 

Solubilization of variantly prepared colloidal gold......................... 51 

The action of peptone solutions on colloidal gold............................ 52 

Isolation of soil microorganisms resistant to high concentrations of metallic ions......................................................................... 52 

Studies of the distribution of gold in soils and vegetation............................ 53 

Preparation of samples................................................................................ 53 

Ignition and ashing....................................................................................... 54 

Solution and estimation of gold.................................................................. 54 

Precision of data............................................................................................ 54 

III 
IV CONTENTS 

Page 

Studies of the distribution of gold in soils and vegetation- Continued 

Studies of gold in soil profiles and vegetation-----------------------------------------· 57 

Mull as a sample medium in geochemical exploration for gold............. 65 

Summary of conclusions______________________________________________________________________________________ 71 

Literature cited...................................................................................................... 75 

FIGURE 1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

TABLE 1. 

2. 

3. 

4. 

5. 

6. 

ILLUSTRATIONS 

Page 

Diagram showing equilibrium distribution of unstable sulfur 

species in water ...................................................................... ------ 27 

Map showing location of sites where plants were sampled 

for cyanide testing, and the vegetation type or types present at the sites.............................................................................. 39 

Diagram showing number of particles of gold at 1 ppm in 

samples having different weights, and for a 1-g sample at 

0.1 ppm ...................... .-......................................................... ----------- 56 

Histograms of gold content of soil profiles and associated 

vegetation, showing high gold values near both the top 

and bottom of the profiles----------------------------------------------------------- 60 

Histograms of gold content of soil profiles and associated 

vegetation, showing high gold values near the top of the 

profiles.·------------------------------------------------------------------------------------------ 62 

Histograms of gold content of soil profiles and associated 

vegetation, showing high gold values near the bottom of 

the profiles .... ------------------------·---·--·········----------------------------------------- 64 

Sample locality map of the Empire district, Colorado, showing location of site EM-698 .... ---------------------------------------------------- 68 

Sample locality map showing relationship of detailed sampling around site EM-698 and reconnaissance sampling, 

Empire district, Colorado ........ ------·-------------------------------------------- 69 

Geochemical and geologic map of locality EM-698 showing 

gold content of mull ash and vein material, Empire district, 

Colorado ............................................................ ----------------------------- 70 

TABLES 

Page 

Standard electrode potentials at 25 o C.......................................... 5 

Composition of solutions of gold complexes used in tests with 

minerals ........................................................... ------------------------------ 6 

Percentage of gold removed from solutions in contact with 

various minerals and rocks_________________________________________________________ 8 

Removal, by minerals, of gold from solutions of various gold 

complexes .............................. --------------------------------------------------------- 13 

Percentage of gold complex removed from solution by a 

10-gram powder sample of carbonaceous materiaL.............. 14 

Variation of gold absorption coefficients with three gold complexes for two species of Impatiens.......................................... 16 
TABLE 7. 

8. 

9. 

10. 

11. 

12. 

13. 

14. 

15. 

16. 

17. 

18. 

19. 

20. 

21. 

22. 

23. 

24. 

25. 

26. 

27. 

28. 

29. 

30. 

31. 

CONTENTS v 

Page 

Solubilities of gold in hydrochloric acid_______________________________________ 18 

The interactions of the concentrations of Cl-1, H+1, and Mn02 

on the solubility of gold in copper leach solutions__________________ 19 

Abundance of chlorine, bromine, and iodine and calculated 

relative abundance of bromine and iodine in various earth 

materials---------------------------------------------------------------------------------------- 21 

The interactions of the concentrations of Br-1, H +1, and 

Mn02 on the solubility of gold in copper leach solutions...... 22 

The interactions of the concentrations of I-1, H+1, and Mn02 on the solubility of gold in copper leach solutions__________________ 22 

Solubility of leaf gold in 0.05 M sodium iodide solutions 

adjusted to various pH values·-------------------------------------------------- 25 

Solubility of ore gold in 0.05 M sodium iodide solutions 

adjusted to various pH values ...... --------------------------------------------- 25 

Solubility of leaf gold in 0.0124 M sodium thiosulfate solutions adjusted to various pH values-----------------------------------------· 29 

Solubility of ore gold in 0.0124 M sodium thiosulfate solution adjusted to various pH values___________________________________________ 30 

Solubility of leaf gold in 0.1 M potassium thiocyanate solutions adjusted to various pH values-----------------------------------------· 31 

Solubility of ore gold in 0.1 M potassium thiocyanate solutions adjusted to various pH values .... -------------------------------------· 32 

The interactions of the concentration of CNS-1 and H+1 on 

the solubility of gold in copper leach solutions----------------------- 33 

Some cyanoglycosides, associated enzyme source, and hydrolysis products--------------------------------------------------------------------------------- 34 

Solution of leaf gold by native cyanogenic plants macerated 

in plant-water slurries---------------------------------------------------------------- 37 

Solubility of gold in slurries of macerated peach roots, 

macerated peach roots plus emulsin, and in emulsin-water 

mixtures·---------------------------------------------------------------------------------------- 37 

Approximate cyanide concentrations in solutions, as indicated by color reactions of picric acid paper suspended 

over the solutions·------------------------------------------------------------------------ 40 

Results of tests for hydrocyanic acid (HCN) in selected 

species of plants--------------------------------------------------------------------------- 42 

Solubilization of colloidal gold by bacterial strains grown in 

glucose medium with yeast extract and with peptone__________ 49 

Gold solubilization by fungal strains grown in glucose-peptone medium·--------------------------------------------------------------------------------- 50 

Effect of shaking flasks on microbial solubilization of gold...... 51 

Gold solubilization by bacterial strains isolated from soil 

and sediment.---------------------------·---------------------------------------------------- 51 

Effect of microbial strains on variantly prepared colloidal 

gold·------------------------------------------------------------------------------------------------ 52 

Solubilization of gold by components of glucose-peptone 

medium·------------------------------------------------------------------------------------------ 52 

Precision of replicate analyses for gold in various materials 55 

Comparison between gold content of soil and that of cobbles 

and pebbles------------------------------------------------------------------------------------ 59 
VI CONTENTS 

Page 

TABLE 32. Gold content of seven size fractions of soil horizons in six 

soil profiles .. _______________________________________________________ ------· __ ----·--------- ___ .. 66 

33. Gold content of the ash of various parts of lodgepole pine 

and aspen growing near gold-bearing vein-----------------------------· 71 
GEOCHEMISTRY OF GOLD IN THE 

WEATHERING CYCLE 

By HuBERT W. LAKIN} GARY C. CuRTIN} and ARTHUR E. HuBERT 

ABSTRACT 

Gold is mobile in the supergene environment under certain weathering 

conditions. The presence of gold in plant tissues has been reported by many 

investigators. Because colloidal gold is not taken up by plants, and because 

the simple ions Au+1 and Au+3 cannot exist in appreciable quantities in aqueous solutions, gold must enter the plant as a soluble complex ion. The enrichment of gold in forest humus mull and near-surface soil also suggests that 

the mobility of gold as a complex ion is transitory. 

We have considered that the following complex ions possibly play a role in 

the migration of gold: AuC14-2, AuBr4-1, Aui2-1, Au(CN)2-1, Au(CNS) 4-1, 

and Au(S203)2-3. 

The following standard electrode potentials illustrate the marked dependence of the oxidation potential on the nature of the gold complex that is 

formed. 

Eo 

(volt) 

Au 4Cl-1= A uC14-1+3e-1 ................................. -1.00 

Au4Br-l=AuBrr1+3e-1 ................................ -0.87 

Au4CNS-1=Au ( CNS) r1+3e-1 .................... -0.66 

Au2I-1= A ui2-1+e-1 ........................................ -0.50 

Au2CN-1=Au ( CN) 2-1+e-1 ........................... -0.20 

Electrode potential for the reaction Au+2S20 3-2=Au(S203)2-3+e-1 is not 

available, but, because gold reacts with Na2S203 in aerated water, the potential of the complex is probably near -0.6 or -0.7 volt. 

Consequently, the oxidant and the pH required to oxidize gold change 

markedly. Thus, at a pH of 2 in a solution of Fe2 (S04) 3 and CuS04 containing 0.05 M (molar) CI-1, gold leaf is dissolved very slowly, if at all-

~0.004 mg/1 in 1 month; when 0.05 M Br-1 is used, 1.2 mg/1 is dissolved 

in 1 month; but with 0.05 M I-1, 72 mg/1 of gold is dissolved in 1 week. 

In contrast to the halide complexes, aqueous 0.05 M solutions of the sodium 

salts of CN-1, CNS-1,.and 820 3-2 (pH 5 to 7) dissolve 14, 0.16, and 3.4 mg/1, 

respectively, of leaf gold in 1 month, offering the possibility of solution of 

gold in soils. 

In a carbonate-free environment, the oxidation of pyrite produces an acid 

solution suitable for the oxidation of gold by chloride if Mn02 is also present. 

Although the ease of oxidation of gold increases in the order CI-1, Br-1, I-1, 

the abundance of these halides in the earth's crust decreases in the same 

order- their molecular abundance ratio in the crust is approximately 100, 

1 
2 GEOCHEMISTRY OF GOLD IN THE WEATHERING CYCLE 

0.6, 0.06. In a carbonate environment, the oxidation of pyrite may produce 

sufficient s2o3-2 to dissolve gold. 

In an oxidizing environment at pH of 5 to 8, only 82 0 3-2, CNS-1, and CN-1 

may be expected to dissolve gold. Thiosulfates are transient products of biological activity in soils but never reach substantial concentrations. Thiocyanates are even less abundant in soils. Cyanides are produced by the hydrolysis 

of cyanogenetic glycosides which are abundant. More than 1,000 species of 

plants yield H CN on hydrolysis; many arthropods produce H CN ; and some 

fungi also produce appreciable amounts of HCN. Macerated aqueous suspensions of 16 species of native plants collected in Colorado, Utah, Nevada, and 

Arizona dissolved leaf gold. 

The absorption of gold from solution by a given plant species is a function 

of the gold complex present. Gold chloride, thiocyanate, and thiosulfate are 

very weakly absorbed through the roots of Impatiens hostii; gold bromide 

and iodide are moderately absorbed (100 times more than the chloride). 

In the microbiological studies, elemental gold was added to liquid nutrient 

media which were then inoculated with pure cultures of bacteria and fungi 

either obtained from stock collections or isolated from soil. In a glucosepeptone medium containing colloidal gold, significant gold solubilization 

occurred. In other media or with powdered gold, metal solubilization was 

slight. The microbial strains showed various abilities to solubilize the noble 

metal. In sterile glucose-peptone medium gold solubilization occurred, but, in 

the same medium inoculated with various microorganisms, the solubilization 

of gold increased. 

The stability of the gold complex has a marked effect on the mobility of 

gold, as illustrated by the removal of gold from solution by various minerals. 

With radioactive gold as a tracer, 114 mineral powders were mixed for 

1 minute with solutions, at pH 6, of these six gold complexes and immediately 

centrifuged to obtain a clear supernatant liquid and a mineral centrifugate. 

The radioactivity of these separates was then measured to determine the 

amount of gold remaining in solution. Of the 114 minerals tested, the numbers 

of minerals removing 90 percent or more of the gold from solution were, 

respectively: 52 from gold chloride, 47 from gold bromide, 23 from gold iodide, 

6 from gold cyanide, 4 from gold thiocyanate, and 3 from gold thiosulfate. 

Acidic oxidation of pyritic gold deposits may result in transient mobilization of gold as AuCl4-1. Alkaline oxidation of pyritic gold deposits may result 

in transient mobilization of gold as Au(S20 3 )-3. In soils under native vegetation gold may be mobilized as Au(CN) 2-1. Whatever the circumstances, the 

gold solutions would be transitory. 

INTRODUCTION 

SCOPE OF REPORT 

Weathering is sometimes defined as the physical and chemical 

action on rocks in contact with the atmosphere. The physical 

action of frost in fracturing rocks and the abrasive action of wind 

and flowing water increase the surfaces exposed to the atmosphere 

without affecting the mineral composition of the rocks. Chemical 

weathering is defined by Loughman (1969) as follows: "Chemical 

weathering is a process by which atmospheric, hydrospheric and 

biologic agencies act upon and react with the mineral constituents 
INTRODUCTION 3 

of rocks within the zone of influence of the atmosphere, producing 

relatively more stable, new mineral species." In the process the 

sulfides, sulfosalts, and tellurides, with which gold is frequently 

associated, are oxidized, and gold commonly remains as, or is 

reduced to, the metallic state. The purpose of this study was to 

investigate some possible mechanisms of solution of metallic gold 

and its transport in solutions in the weathering or weathered 

environment. 

The presence of gold in plants and of gold dissolved in stream 

waters and in the oceans indicates that some mechanisms for 

solution of gold do exist. 

Relatively high values for the gold content of ocean water led 

Haber (1928) to believe that mining gold from the ocean might 

be a feasible way to pay the German war debts. Haber's 7-year 

study of gold in estuaries of German rivers and in the ocean was 

summarized by Jaenicke (1935). Gold has also been reported in 

stream waters (Gosling and others, 1971). 

Gold was reported in plants more than a century ago by MalteBrun (1824, p. 221), who stated: 

It is also proved by the experiments of Sage, Bertholler, Rouelle, Darcet and 

Deyeux, that there are particles of gold existing in vegetables. Bertholler has 

extracted about 2 g, 14/100 (2.14 g) or 40 grains 1/10 of gold, from 489 hectogrammes, or a quintal of ashes. Werner assures us, that at Zalatna, native 

gold has been found in half petrified wood, or rather, says he, in bituminous 

wood. 

The presence of gold in living plants and the enrichment of gold 

in the humus layer of the soil, as demonstrated by Goldschmidt 

(1937) and by Curtin, Lakin, Neuerburg, and Hubert (1968), 

suggest that gold is dissolved and transported in soil solutions and 

is taken up by plants. The summary by Jones (1970) of available 

data on the occurrence of gold in water, plants, and animals 

further substantiates the evidence that gold is mobilized in the 

weathering environment. 

The major part of the present study was an evaluation of the 

stability of some possible gold complexes that might be the means 

of transport of gold in solution in oxidizing systems. F.or many 

years the effects of transport and supergene enrichment of gold 

have been observed in the oxidized zones of ore deposits (Spurr 

and Garrey, 1908, p. 149; Ransome, 1911, p. 169). Gold chloride 

ion (AuCl4 -I) traditionally has been considered to be the principal 

gold complex in solutions in the weathering environment. Brokaw 

(1910), after making a comparative study of different possible solvents of gold, concluded that mixtures of Fe (804) a, H2S04, and 

N aCl in concentrations common in mine waters will readily dissolve 
4 GEOCHEMISTRY OF GOLD IN THE WEATHERING CYCLE 

gold to form gold chloride in the presence of manganese dioxide. 

This process has been demonstrated by other investigators, including Emmons (1917), Krauskopf (1951), and Cloke and Kelly 

(1964). 

The formation and stability of the gold chloride complex in 

nature are restricted to very special conditions in which the acidity 

and chloride concentration must be high and in which manganese 

dioxide must be present. These conditions are found only in some 

oxidizing sulfide ore deposits. The gold chloride complex is not 

formed, nor does it remain stable, in near-neutral solutions with a 

relatively low chloride content that characterizes ground water 

and water in streams and lakes. As an example, gold precipitated 

readily from a neutral solution of gold chloride when it was placed 

in contact with onion roots (Shacklette and others, 1970). The 

experiment demonstrated that gold complexes other than gold 

chloride are required for the migration of gold into plants and in 

streams and ground water. 

Results of our studies show that gold cyanide and thiocyanate 

may be the most common stable gold complexes in soils, plants, 

and natural waters. Cyanide and thiocyanate ions are products of 

hydrolysis of plant glycocides by enzymatic action (Armstrong 

and Armstrong, 1931, p. 67; Timonin, 1941, p. 404; Kingsbury, 

1964, p. 23-27). These ions have t.he ability to complex gold in 

moderately oxidizing environments (Jacobson, 1949, p. 554; Manskaya and Drozdova, 1968, p. 271). The gold cyanide and thiocyanate ions remain in solution when in contact with common rocks 

and minerals, and they are readily taken up by plants. The other 

ions studied-chloride, bromide, iodide, and thiosulfate-may form 

complexes with gold which remain stable only long enough to permit som·e restricted movement of gold under special conditions. 

ACKNOWLEDGMENTS 

We acknowledge the aid given us by several U.S. Geological 

Survey personnel. Charles W. Gale III aided us in field and laboratory studies and added to our knowledge of gold iodide systems by 

his investigation of the stability of the gold iodide complex in 

a strong oxidizing environment. Hansford T. Shacklette aided 

greatly by his identification of native plants and his joint laboratory work on uptake of gold by plants. We thank F. 0. Simon for 

the determination of gold in cement copper. 

OXIDATION POTENTIAL FOR GOLD AS A FUNCTION 

OF COMPLEX STABILITY 

Cotton and Wilkinson (1962, p. 867) introduced their discussion 

of gold compounds with the statement that the chemistry of gold 
OXIDATION POTENTIAL 5 

is essentially a chemistry of complex compounds and that no simple gold cations exist in aqueous solution. This concept is very 

important in understanding the behavior of gold. To illustrate, 

gold may be in contact with oxygenated water, such as a mountain 

stream, for a millennium without dissolving, but if 250 ppm (parts 

per million) of cyanide ion is added to this water, the gold is 

readily dissolved. The cyanide ion is not an oxidant but forms a 

complex with the gold. In its presence, the oxidation potential for 

gold (table 1) is reduced from -1.50 to -0.20 volts and the dissolved oxygen in the water becomes a sufficiently strong oxidant 

to dissolve the gold. 

The standard electrode potentials for the oxidation of gold in 

the presence of a number of anions that form stable complexes 

with gold are given in table 1. None of the ions which react with 

gold in these half cells is an oxidant of gold. The difference in 

the strength of the oxidant (indicated by the electrode potential) 

required to oxidize gold lies in the strength of the gold complex 

formed with the indicated anion. 

Gold is oxidized by Mn02 in acid chloride solutions. Krauskopf 

(1951, p. 863) emphasized that the purpose of the chloride ion 

is to form a very stable complex ion with the dissolved gold. Gold 

is not oxidized appreciably in acid (pH 2) ferric and cupric sulfate solutions containing chloride ions in the absence of Mn02. 

Gold is slowly oxidized in the same acid sulfate solutions containing bromide ions, and it is rapidly oxidized in the acid sulfate 

solution containing iodide ions. This sequence illustrates the increasing ease of oxidation of gold with increasing strength of 

the complex ion. 

TABLE 1.- Standard electrode potentials at 25°C 

[From Latimer (1952)] 

In acid solution E• (volts) 

Au= Au+3+3e-I ..................................................................................... . -1.50 

Au+4Cl-1=AuClr1+3e-1 ................................................................... . -1.00 

Au+4Br-1=AuBrr1+3e-I ................................................................. . -0.87 

A u+4CN S-1 =Au ( CNS) 4-1 +3e-1 ________ ·-----------······----·····-----····------·--· -0.66 

Au=Au+I+e-I ________________________________________________________________________________________ About -1.68 

Au+Cl-1=AuCl+e-1............................................................................. -1.17 

Au+2Br-I=AuBr!.!-1+e-1____________________________________________________________________ -0.96 

Au+2CNS-1=Au ( CNS) 2-I+e-1 __________________________ .............................. -0.69 

Au+I-I=Aui+e-1 .................... ---·------····------····-····-···-----------·-··----------··· -0.50 

In basic solution E• (volts) 

Au+2CN-1=Au ( CN) 2+e-I _______________________________________________________________ _ 1-0.20 

1 Value from Charlot (1954, p. 249). 
6 GEOCHEMISTRY OF GOLD IN THE WEATHERING CYCLE 

It is interesting to note in table 1 that the larger negative 

potentials of AuCl, AuBr2 -1, and Au (CNS) 2-1 than those for the 

corresponding trivalent complexes indicate that the univalent gold 

complexes are unstable. If the univalent gold complexes are formed, 

they would disproportionate to yield 2Au 0 +Au+3 complex. 

REMOVAL OF GOLD FROM SOLUTION BY MINERALS 

AS A FUNCTION OF COMPLEX STABILITY , 

One measure of the stability of a gold complex is given by its 

mobility in a geologic environment. In the literature on the downward migration of gold in oxidizing ore bodies, the gold· i~ presumed to be moving as gold chloride. Laboratory experiments with 

gold chloride solutions (Clarke, 1924, p. 662) have shown that 

many sulfides, carbonaceous materials, and carbonates precipitate 

gold. Pyrite is especially effective in the precipitation of gold. 

Skey (1873, p. 380) calculated that the 152-pound "Welcome Nugget" of Australia could have grown by precipitation of gold on 

the face of a 12-pound chunk of pyrite. Emmons (1917, p. 324) 

concluded that the presence of alkali minerals that neutralize the 

acid solutions formed by oxidation of pyrite would inhibit the 

movement of gold as a chloride. In favorable conditions, such as 

existed at Creede, Colo., Emmons (1917, p. 314) found gold 

enrichment at depths of 200-700 feet below the surface. 

A qualitative comparison was made of the effect of 114 minerals, rocks, and miscellaneous materials on solutions of radioactive gold chloride, bromide, iodide, cyanide, thiocyanate, and 

thiosulfate to supplement the voluminous literature on the effect 

of various minerals on chloride solutions of gold. 

Aqueous solutions were prepared with the composition shown 

in table 2. The lower gold contents of the thiocyanate and thiosulfate solutions are due to the slow rate of solution of gold in 

TABLE 2.- Composition of solutions of gold complexes used in tests with 

minerals 

Ion 

Complexing ion 

Concentration 

(M) 

Chloride._------ _______________________ . ___ 0.1 

Bromide·---------------------------------- .1 

Iodide ......... ------------------------------· .08 Cyanide ____________________________________ .002 

Do·------------------------------------ .002 Thiocyanate____________________________ .025 

Thiosulfate ______________________________ .008 

Source 

HCl 

HBr 

HI 

NaCN 

NaCN 

NaCNS 

Na2S203 

Gold in 

solutionl 

(ppm) 

18 

11 

11 

11 

12 

.15 

3 

pH of 

solution2 

6 

6 

6 

6 

8 

6 

6 

1At the beginning of the tests each solution contained 2 me (microcuries) Au198 per liter. 2pH of the halogen solutions was adjusted with sodium hydroxide, and pH of the remaining 

solutions, with dilute sulfuric acid. 
REMOVAL FROM SOLUTION BY MINERALS 7 

the sodium salts of these ions. All the solutions contained the same 

concentration of radioactive gold, and this concentration dropped 

drastically with time; therefore, the last measurements were made 

two half-lives later than the first ones. 

The minerals used in the experiment are listed in table 3 with 

the data obtained. The listed mineral was the major component 

but was rarely a pure specimen. They were ground to a fine powder ( <0.1 mm) in a ball mill but were not sized. Thus, the total 

surface of one mineral specimen may be markedly different from 

that of another. This variation is a serious factor in evaluating 

the data, and, because of it and the lack of purity of the mineral 

specimens, the data could be considered only qualitative. 

One gram of each mineral powder was placed in a test tube 

with 10 ml (milliliters) of each solution. They were shaken for 

about 30 seconds, until the powder was dispersed in the aqueous 

solution. After the contents of the test tubes had been mixed, they 

were centrifuged to separate the mineral powder as a residue in 

the bottom of the tube. The supernatant liquid was decanted into 

another test tube. The radioactivity of both the mineral centrifugate and the supernatant liquid portions were measured by a 

scintillation counter using a well-type 1.5-inch thallium-activated 

sodium iodide crystal detector. Only the measurements obtained 

with the aqueous phase were used if the mineral was naturally 

radioactive. 

The data summarized in table 4 show marked loss of gold from 

the halide solutions for many minerals, in contrast to that of the 

cyanide, thiocyanate, and thiosulfate solutions. Thus, 20-45 percent of the minerals remove 90 percent or more of the gold from 

solutions of the halide complexes, whereas only about 5 percent 

or less of the minerals remove 90 percent of the gold from solutions of the cyanide, thiocyanate, and thiosulfate complexes. 

One cannot determine from these data whether the gold complex is sorbed by the mineral or whether the gold is reduced to 

the metal. Reduction is suggested, however, by the gross relationship of removal of gold from solution to the oxidation potential 

for the complex. To illustrate, the standard electrode potentials 

for AuC14-I, AuBr4-I, and Aui~- 1 are -1.00, -0.87, and -0.50, 

and the percentages of the minerals removing 90 percent or more 

of the gold from solution are 46, 41, and 20, respectively. 

Carbonaceous materials are often associated with gold, as at 

the Getchell, Carlin, Cortez, and Gold Acres mines in Nevada, and 

gold is difficult to recover from such unoxidized ores. 

In order to observe the relative stability of various gold complexes in the presence of carbonaceous material, 10 g (grams) of 
TABLE 3.- Percent gold removed from solutions in contact with various minerals and rocks 

Percent gold removed from solution 

Sample Mineral formula1 

(where applicable) Cl-1 Br-1 J-1 

Pyrite------------------------------------------- FeS2 Pyrrhotite .. -----------·----------------------Fet-xS 

100 100 5 100 100 10 

Arsenopyrite.-----------. __________________ Fe AsS 100 100 0 Pentlandite __________________________________ ( Fe,Ni) 9S8 100 100 1 

Chalcopyrite .... ----------------------------CuFeS2 100 100 60 

Bornite .... ------------------------------------- Cu;;FeSt Chalcocite ____________________________________ Cu2S 

Galena ......... ·-------------- _________________ PbS 

Sphalerite ... ----------·---------·------------ ZnS Molybdenite·------·------------·--·--------·MoS2 

Argentite·-·----------·-·----·---------·------ Ag2S 

Orpiment .... ---------·----------------------- As2Sa Do .............. ---------------------------- As2Sa Realgar·------------·---------------·-----------AsS Cinnabar ______________________________________ HgS 

Stibnite ___________________ , _____________________ Sb2Sa Niccolite _______________________________________ NiS 

Cobaltite·--·--------------------------·--------CoAsS Alabandite ___________________________________ MnS 

Smaltite ________________________________________ ( Co,N i) As:~->· 

100 

100 

98 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

100 

Magnetite .... --------------------------------FeFe20t 40 Do __________________ ---------------·----·--· FeFe20t 98 Ilmenite ________________________________________ FeTiOa 30 

Hematite·-------···----·----------------------Fe20:< 50 Limonite .... --------···----------------·- ---·Mixed hydrous iron oxides ...... 30 

Iron oxide ................................... Mixed hydrous ir!Jn oxides ...... 70 Hydrous iron-manganese Mixed hydrous iron oxides ..... 20 oxide+quartz sand. 

Do ... ---------·-----------------·--··-----··Mixed hydrous iron oxides..... 10 Manganite·------·-------·----·-------·----·- MnO ( OH) 20 Pyrolusite .... -------------·--·------·---·----·Mn02 60 

100 100 100 100 

30 100 100 98 100 2 

100 100 100 100 100 100 100 100 100 100 

100 100 100 100 100 20 

100 100 100 100 

5 0 

98 2 30 0 

50 5 

30 5 

40 2 

10 0 

5 0 10 0 70 0 

CN-1 CN-1 (pH 6.2) (pH 8.2) CNS-1 S20 3-2 

Sulfides 

2 

0 

0 

0 100 

0 

100 

0 

0 

0 

0 

10 

30 

0 

2 

100 0 

0 

1 

0 

Oxides 

0 

0 

0 

2 

5 

0 

0 

0 

0 

0 

2 

0 

0 

0 

100 

0 

95 

0 

0 

0 

0 

10 

2 

0 

2 

95 

0 

0 

1 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

40 

0 

1 

1 100 

1 

95 

0 

0 

0 

70 

70 

40 

40 

2 

2 <1 

10 

1 

0 

0 

0 

0 

0 

10 

0 

0 

0 

0 

0 

0 

0 

0 

0 

20 

0 

0 

0 

0 

0 

30 

80 

98 

40 

2 

0 

0 

0 

100 0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

Sample No. 

CR-14-56-193 

CR-14-56-191 

CR-14-56-143 

CR-14-56-194 

CR-14-56-155 

Locality 

Rico, Colo. Falconbridge, Sudbury 

district, Ontario. Anglesia Township, Ontario. Sudbury, Ontario. Unknown. 

RT0-157 Superior, Ariz. None ____________________________ Augusta mine, Evergreen, 

Jefferson County, Colo. None ____________________________ Coeur d'Alene district, Idaho. N one ____________________________ Hyatt mine, New York. 

RT0-166 Little Cottonwood Canyon, 

CR-14-56-142 

RT0-31 

RT0-31 

RE-1 

CR-14-56-157 

Utah. 

Ansaline mine, Butte, Mont. Manhattan, Nev. Do. Getchell mine, Nevada. California. 

None .... ------------------------Idaho. CR-14-56-188 Manhattan, Nev. RT0-12 Blackbird district, Idaho. N one. ___________________________ Puebla, Mexico. 

CR-14-56-203 Cobalt, Ontario. 

CR-14-56-182 Tintic, Utah. 

None ......... -------------------J ayville, N.Y. CR-14-56-175 Kragero, Norway. CR-14-56-171 Unknown. 

CR-14-56-181 Unknown. 

E-91A 

TP-4 

TP-1 

RT0-28 

RT0-176 

Ely, Nev. Ptarmigan Peak quadrangle, 

Colorado. 

Do. Negaunee, Mich. Ironton, Minn. 

00 

!;1 

tr_j 

0 

0 

::t= 

tr_j 

~ 

-r:n. 

1-3 

~ 

~ 

0 

l'%j 

!;1 

0 

~ 

t::l 

-z 

1-3 

::t= 

tr_j 

~ tr_j 

> 1-3 

::t= 

tr_j 

~ 

-z 

!;1 

0 

~ 

0 

~ 

tr_j 
Cryptomelane .............................. "KMnsOtG 40 40 2 0 0 0 0 RT0-159 India. Hollandite .................................... MnBaMnr.Ou Hausmannite .............................. MnMnz04 85 80 0 0 0 0 0 RT0-22 Langban, Sweden. Franklinite .................................. 2 ( Zn,Mn,Fe) ( Fe,Mn) 204 90 100 1 0 0 0 0 RT0-297 Franklin Furnace, N.J. Cuprite ......................................... CuzO 100 100 100 0 0 25 1 RT0-15 Helena, Mont. 

Cassiterite ................................... SnOz 35 50 20 0 0 0 0 CR-14-56-153 Taylor Creek district, New Mexico. 

Cervantite .................................... Sbz04 20 5 0 0 0 0 0 CR-14-56-154 Mexico. 

Chromite ...................................... FeCrz04 10 15 1 0 0 0 0 CR-14-56-158 Selukew, Southern Rhodesia. Rutile ............................................ TiOz 40 30 1 1 0 0 0 CR-14-56-196 Nelson County, Va. ~ 

Brucite ......................................... Mg ( 0 H) 2 7 15 4 1 1 1 60 CR-14-56-150 Luning, Nev. t_%j 

~ Columbite .................................... :

1 (Fe,Mn) (Nb,Ta)20G 30 30 5 0 0 0 1 CR-14-56-160 Quadville, Ontario. 0 

Corundum .................................... Ab03 5 10 3 0 0 0 0 CR-14-56-161 Zoutpansberg, Transvaal. < Spinel ............................................ MgAlz04 100 99 20 0 0 0 100 CR-14-56-197 Black Lake, Quebec. > Samarskite .................................. (Y,Er,Ce,U,Ca,Fe,Pb,Th) 100 50 0 0 0 0 0 CR-14-56-200 Mitchell County, N.C. ~ ( Cb, Ta, Ti,Sn) 206 "%j Braunite ....................................... ( Mn,Si) 203 100 100 1 0 0 0 0 RT0-197 Kyushu, Japan. Psilomelane ................................. ( Mn,Si) 203 30 2 0 0 0 0 0 CR-14-56-190 No. Mountain mine, Arkansas. ~ 

0 

Silicates, except clayst ~ 

Olivine .......................................... :

1 ( Fe,Mg) 2Si04 60 40 1 0 0 0 0 15 Jackson County, N.C. rJ'). 

Helvite .......................................... ( Mn, Fe, Zn) 4Bea ( Si04) aS 10 20 0 0 0 0 0 CR-14-56-180 Iron Mountain, New Mexico. 0 

Augite ........................................... ( Ca,Na,Mg,Fe+ ,Mn,Fe+=~, 90 95 0 0 0 0 0 CR-14-56-146 Otter Lake, Quebec. ~ 

AI, Ti) 2 [ ( Si,Al) 206] c:: Hornblende ................................. ( Na,K) o-tCaz ( Mg,Fe+2, 100 100 1 1 1 0 0 24 Faraday Township, Ontario. 1-3 Fe+3,Al) s[Si&-7Ah-t ~ 

0 o~z] (OH,Fb 

Anthophyllite and tremolite ... (Mg,Fe+2) 1[Sis022] (OH, z Fb 100 100 1 0 0 0 0 25 Fullerville, N.Y. td Ca2 (Mg,Fe+2 ) s[Sis02z] t-< (0H,F)2 Biotite ........................................... K2 ( Mg,Fe+2) 6-4 ( Fe+3,AI, 80 60 10 0 - <10 0 CR-14-56-149 New Hampshire. ~ Ti) o-2[Sia-sAh-a02o] ~ 

(0H,F)4 z 

Phlogopite ................................... K2 (Mg,Fe+2) 6 [SiaAhOzo] 100 98 3 2 2 2 2 27 North Burgess, Ontario. t_%j 

(0H,F)4 ~ Garnet .......................................... Fea+2AbSL012 60 15 0 0 0 0 0 CR-14-56-145 Gore Mountain, Warren > County, N.Y. ~ Allanite ......................................... (Ca,Ce) 2 (Fe+2,Fe+3 ) 98 99 98 0 0 5 10 CR-14-56-138 Pecoema Canyon near rJ'). 

Ab0-0H[Si207] [Si04] Los Angeles, Calif. 

Beryl ............................................ BeaAh [Si60ts] 40 5 0 0 0 0 0 CR-14-56-148 Keystone, S. Dak. Microcline .................................... (K,Na) [AISiaOs] 20 30 0 0 0 1 0 CR-14-56-186 Perry Sound district, Ontario. Do .......................................... (K,Na) [AISiaOs] 2 1 0 0 0 0 0 CR-14-56-186 Do. Albite ............................................ N aAISiaOs 95 95 30 2 2 2 0 CR-14-56-137 Amelia Court House, Va. Titanite ........................................ CaTi[Si04] (O,OH,F) 80 60 0 0 0 0 0 CR-14-56-201 Westport, Ontario. Mellite ........................................... ( Ca,N a) 2 [ ( Mg,Fe+2,Al, 10 5 0 0 0 0 0 4 Unknown. 

Si)a07] 

See footnotes at end of table. ~ 
TABLE 3. -Percent gold removed from solutions in contact with various minerals and rocks- Continued 1--' 

0 

Percent gold removed from solution 

Sample Mineral formula1 Sample No. Locality C") (where applicable) CN-1 CI-1 Br-1 I-1 CN-1 CNS-1 S 0 -2 (pH6.2) (pH 8.2) 2 3 t?j 

0 

Silicates, except clays4 - Continued 0 

~ 

N epheline ..................................... N aa ( N a,K) [Al4Si40t6] 40 40 0 0 0 0 0 7 Bancroft, Ontario. t?j 

Scapolite ...................................... (Na,Ca,K)4[Ala(Al,Si)a 10 30 0 0 0 0 0 8 Templeton, Quehec. a= 

SiG024] (Cl,C0a,S04,0H) ~ 

Wollastonite ................................ Ca [SiOa] 30 10 2 1 1 0 0 32 Willsboro, N.Y. rn. Lepidolite ..................................... K2 ( Li,Al) s-6 [SiG-7 Ah-t 1 20 1 0 0 0 0 CR-14-56-179 Karibib, S.W. Africa. t-3 

02ol (0H,F)4 ~ 

Muscovite ..................................... K2Al4[Si6Ab02o] (0H,F)4 60 20 10 0 - 0 0 CR-14-56-187 ~ Hemimorphite ............................ .azn4Si20r ( OH) 2•2H20 100 100 1 1 1 1 1 CR-14-56-172 Pinal County, Ariz. 0 Do .......................................... 3Zn4Si20r (OH) 2•2H20 50 50 30 0 0 0 0 CR-14-56-173 Wentworth, Miss. l'%j Kyanite ......................................... AbSiOs 80 40 3 1 1 0 0 CR-14-56-177 Brasstown, N.C. 

Tourmaline .................................. N a ( Mg,Fe,Mn,Li,Al) aAl& 50 60 40 0 0 0 0 CR-14-56-207 San Diego, Calif. C") 

[SiGOts] (BOa)a(OH,F)t 0 

Dumortierite ................................ Ah, (BOJ) (Si04) 20a 20 10 0 0 0 0 0 CR-14-56-162 Oreana, Pershing County, ~ 

Nev. t:'j 

Topaz ............................................ Ah [Si04] ( F,OH) 2 1 0 0 0 0 0 0 12 Jefferson County, S.C. ~ 

Quartz ........................................... Si02 40 20 0 0 0 1 0 None ............................ Unknown. z Thorite .......................................... ThSi04 40 20 0 0 0 0 0 CR-14-56-205 Do. t-3 

Sulfosalts ~ 

t?j 

Tetrahedrite ................................ ( Cu,Zn,Fe,Ag) 12Sb4S1a 100 100 80 100 100 80 0 RT0-305 Coeur d'Alene district, Idaho. 

~ Sulfates t?j 

> Jarosite ....................................... .''K Fea(S04)2(0H)e} 30 50 2 1 1 1 0 None ............................ San Bernardino, Calif. north t-3 

Gypsum ........................................ CaSOt•2H20 of Barstow. ~ Anglesite ...................................... PbS04 30 30 70 0 0 2 0 CR-14-56-139 Unknown. l:_%j Barite ............................................ BaS04 1 0 0 0 0 0 0 CR-14-56-147 Danville, Ky. ~ ~ 

Elements z 

C") 

Antimony ..................................... Sb 100 100 100 0 0 75 2 CR-14-56-140 Kern County, Calif. 0 Graphite ....................................... C 100 100 100 60 50 40 5 RT0-21 Near Dillon, Mont. ~ 

Carbonates 0 

~ 

~=~~~~~----_-_·::::_·_·_·:_·_-_·:.-~:.·.-~:·.·_·_·_·_·_·_·::_·_·_·_·_·:.·.·_·g~c<g H > 2 <CO a) 2 100 100 50 100 99 2 0 CR-14-56-144 Grant County, N.Mex. t?j 

2 50 1 0 2 0 0 CR-14-56-151 Cherokee County, S. Dak. 

Hydrozincite ............................... Zns ( OH) 6 (COs) 2 90 10 2 2 2 2 0 CR-14-56-17 4 Dragoon, Ariz. 

Dolomite ....................................... Ca ( Mg,Fe,Mn) (CO a) 2 95 95 2 1 0 1 0 CR-14-56-164-165 Thornwood, N.Y. 
Phosphates 

Monazite ...................................... ( Ce,La, Y, Th) P04 <5 <5 <5 0 0 

Apatite ......................................... Cas ( P04) aF 50 40 30 0 0 

Vanadates 

Descloizite .................................... ZnPb ( V04) ( OH) 1 1 0 0 0 

Tongstates 

Scheelite ....................................... Ca WOt 100 100 100 0 0 

Fluorides 

Fluorite ........................................ CaF2 1 1 0 0 0 

Clays 

Kaolin ........................................... Al4 [Si40to] ( OH) s 30 1 2 0 0 

Glauconite .................................... ( K,N a,Ca) 1.2-2.0 

( Fe+3,Al,Fe+2,Mg) 4. o 100 65 1 0 0 [Sh-7-6Ah-o-402o] (OH)4•n 

(H20) 

Halloysite .................................... AbSbOs ( OH) 4• 2H20 20 40 1 0 0 

Carbonaceous material 

Black shale ............... -......................................................................... 100 100 30 0 0 

Carbonaceous shale ······················································· 95 95 5 0 0 

and volcanic sandstone. 

Carbonaceous channel ........................................................ 30 30 2 0 0 

filling. 

Coal. ...................................................................................................... 100 100 100 98 98 

Leonardite .......................................................................................... 95 90 85 40 50 

Do ................................................................................................. 60 70 60 50 50 Do .................................................................................................. 99 97 95 40 35 

Ore sampl~ 

AuTe ore, oxidized ........................................................................... 100 100 100 50 5 

AuTe ore, little oxidation ............................................................... 100 100 100 1 1 

See footnotes at end of table. 

0 0 

0 0 

0 0 

0 0 

0 0 

0 0 

1 0 

1 60 

10 0 

0 0 

20 0 

98 0 

90 30 

80 40 

70 2 

20 0 1 0 

CR-14-56-185 

CR-14-56-141 

CR-14-56-163 

CR-14-56-204 

CR-14-56-166 

CR-14-56-176 

CR-14-56-169 

CR-14-56-170 

Get. 1 HE-67-D8 

HE-DM 

11-V-6-2 

PPL-1 

TR-2 

GIL-3A 

AJR-916 

AJR-940 

Elk Mountain, N. Mex. Odegaard, Bramle, Norway. 

Unknown. 

Anaconda, Mont. 

W eardale, Durham, England. 

Drybranch, Ga. 

Birmingham, N.J. 

Eureka, Utah. 

Getchell mine, Nevada. 

Dewey mine, Thunder 

Mountain district, Valley 

County, Idaho. 

V -Day mine, Don ville, Ill. 

Dave Johnston coal mine, 

Glenrock, Wyo. 

Outcrop, near Sheridan, Wyo. 

Outcrop, near Gillette, Wyo. 

Cripple Creek, Colo. Do. 

~ 

tz::l 

a:: 

0 

< 

> t"l 

l'%j 

~ 

0 

a:: 

00. 

0 

t"l 

c::: 

1-3 

1-4 

0 

z 

t:d 

t-< 

a:: 

1-4 

z 

tz::l 

pj 

> t"l 

00. 

1-l 

1-l 
TABLE 3. -Percent gold removed from solutions in contact with various minerals and rocks - Continued 

Percent gold removed from solution 

Sample Mineral formula1 

(where applicable) CJ-1 Br-1 I-1 

Gossan................................................................................................. 90 40 1 Limestone........................................................................................... 40 40 2 

Rhyolite tuff ....................................................................................... 100 100 100 

Obsidian.............................................................................................. 5 5 0 

Volcaniclastic breccia ...................................................................... 100 100 5 

Phonolite............................................................................................. 80 20 1 Do................................................................................................. 0 0 0 

Gold ore in silty, ........................................................ 50 50 40 

dolomitic limestone. 

Barren, silty, ....................................................... 30 25 0 

dolomitic limestone. 

Thin laminated siltstone ........................................................ 95 100 2 and volcanic grit. 

Volcanic grit...................................................................................... 2 2 0 

Thin laminated siltstone................................................................. 70 70 0 

Pyrex stopper.................................................................................... 5 1 0 Brown glass bottle............................................................................ 80 30 0 

CN-1 CN-1 

(pH 6.2) (pH 8.2) CNS-1 S203-2 

Miscellaneous 

0 0 0 0 1 1 1 0 

0 0 0 0 

0 0 0 0 

0 0 0 0 

0 0 0 0 0 0 0 0 

0 0 0 0 

0 0 0 0 

0 0 0 0 

0 0 0 0 

0 0 0 0 

0 0 0 0 0 0 0 0 

Sample No. Locality 

E-250 Ely, Nev. CR-14-56-183 Homansville Canyon, Tintic 

district, Utah. 

HE-67-19 Pyramid Peak area, Valley 

County, Idaho. 

HE-67-22 Lookout Mountain area, Valley 

County, Idaho. 

HE-67-D2.1 Dewey mine, Thunder 

Mountain district, Valley 

County, Idaho. 

CC-387 Do. CC-388 Do. C-1 Eureka County, Carlin mine, 

Nevada. 

C-2 Do. 

HE-67-D2.2 Do. 

HE-67-D4 Do. HE-67-D7 Do. None. 

None. 

1Source of mineral formulas for sulfides, oxides, sulfates, carbonates, phosphates, vanadates, tungstates, and fluorides (except as otherwise noted) : Palache, 

Berman, and Fronde!. Vol. 1, 1944 and Vol. 2, 1951. · 2Source of mineral formulas: B. F. Leonard (written commun., 1969). · 3Source of mineral formulas: Fleischer (1971). 4Source of mineral formulas for silicates, except as otherwise noted: Deer, Howie, and Zussman (1966). 5Source of formula for jarosite: A. J. Gude III (written commun., 1969). 

...... 

1\:) 

c;1 

M 

0 

C":l 

~ 

M 

~ ~ 

00 

1-3 

~ 

~ 

0 

~ 

c;1 

0 

t"" 

tj 

z 

1-3 

~ 

M 

~ 

M 

> 1-3 

~ 

M 

~ ~ 

z 

c;1 

C":l 

~ 

C":l 

t"" 

M 
REMOVAL FROM SOLUTION BY MINERALS 13 

TABLE 4. -Removal, by minerals, of gold from solutions of various gold complexes as shown by the number of samples falling into each category of 

percentage of gold lost from solution 

"' ::I ::I "'

"' $ !] "' ~ 0 

.5 ~ $ ~ 

"' gj ~ "' r::: Mineral group .............. gj $ i "' 

= "" Ill ~ :i "" ~ 

"" ... gj $ = ..c: '"CI :s r::: '"' 1i r::: "CI Ql 0 ~ ~ 0 0 

q:: "" '"CI .8 "" E ,.Q "" '"' "' ,.Q Co) '; ~ 

~ .... 0 r::: = 0 » '"CI Co) 

"a ·~ '3 '3 Ql '"' ::I CIS '"' 0 "' ... '"' 00 &l CIS ..c: CIS :::s ~ 0 CIS i 0 Ql 

00 0 00 00 u ~ > E-4 u t!l E-4 ~ 

Total number 20 30 26 3 2 5 2 3 7 6 6 114 100 of samples. 

Percent Au Number of samples that lost gold in each percent-Au-lost category lost from solution. 

Gold chloride complex 

90+ ................................. 20 7 7 2 3 5 2 3 52 45.6 60-<90 ............................. 7 3 1 1 1 13 11.4 40-<60 ............................. 6 5 1 1 1 14 12.2 >10-<40 .......................... 3 7 2 2 1 1 16 14.0 10-0 ................................... 7 4 1 1 1 1 1 1 2 19 16.7 

Gold bromide complex 

90+ ................................. 19 7 5 2 2 5 2 3 47 41.2 60-<90 .............................. 3 2 1 1 8 7.0 40-<60 .............................. 4 6 ··-- 2 1 1 16 14.0 >10-<40 ...................... 1 9 7 1 2 21 18.4 10-0 ................................... 7 6 1 1 1 1 1 1 2 22 19.3 

Gold iodide complex 

90+ ................................. 13 2 2 2 23 20.2 60-<90 .......................... 1 1 1 2 5 4.4 40-<60 .............................. 1 1 1 3 2.6 >10-<40 ...................... 1 2 2 1 7 6.1 10-0 ............................... 5 26 23 2 4 1 1 3 2 3 5 76 66.6 

Gold cyanide complex pH 6.2 

90+ ................................. 3 6 5.3 60-<90 ............................. 1 1 .9 40-<60 .............................. 3 1 4 3.4 >10-<40 ...................... 1 1 .9 10-0 ............................... 16 30 26 3 1 4 2 1 1 1 3 3 5 6 102 89.5 

Gold cyanide complex pH 8.2 

90+ ................................. 3 6 5.3 60-<90 .............................. 0 0 40-<60 .............................. 1 3 4 3.4 >10-<40 .......................... 0 0 10-0 ............................... 17 30 26 3 1 4 2 1 1 1 3 3 6 6 104 91.2 

Gold thioeyanate complex 

90+ ................................. 2 2 4 3.4 60-<90 .......................... 2 1 2 6 5.3 40-<60 .......................... 3 4 3.4 >10-<40 .......................... 1 1 2 1.7 10-0 ............................... 13 30 25 3 5 2 1 1 1 3 2 6 6 98 86.1 

Gold thiosulfate complex 

90+ ................................. 2 3 2.6. 60-<90 .......................... 1 1 3 2.6 40-<60 .......................... 1 1 2 1.7 

>10-<40 ...................... 2 1 3 2.6 10-0 ............................... 14 30 24 1 3 2 5 2 1 1 2 5 6 6 103 90.4 

each of four different pulverized materials were separately mixed 

with 30 ml of 3 N HCl, 30 ml of concentrated HBr, and 30 ml of 

0.25 N KCN. Each of these solutions originally contained 15 J.Lg 

(micrograms) gold. Allowed to stand overnight, the series of mix-
14 GEOCHEMISTRY OF GOLD IN THE WEATHERING CYCLE 

tures was centrifuged, and gold was determined in the supernatant 

liquid. Quartz flour was also mixed with the above solutions, and 

no measurable loss of gold from solution was observed. Table 5 

shows that the AuCl4 - 1 and AuBr4 - 1 complexes are nearly completely removed (94 to 100 percent) from solution by the four 

different carbonaceous materials, whereas the Au (CN) 2- 1 complex is removed to a less extent in all but the siliceous black shale. 

Rickard's (1897, p. 978-979) experiment demonstrated the precipitation of elemental gold on a black shale from a chloride solution of gold. The removal of gold from a gold cyanide solution 

may, however, be due to adsorption of the complex on the shale 

rather than to reduction of gold. 

TABLE 5.- Percentage of gold complex removed from solution by a 10-gram 

powder sample of carbonaceous material 

Sample No ..................................................................... V-366 V-437U C-1 1769 

Gold content of sample (ppm)------------------------------- <0.04 <0.04 0.34 <0.04 

Treatment Percentage of added Au removed from solution 

15 p.g Au in 30 ml 3 N HCL................... 100 

15 p.g Au in 30 ml cone. HBr .................. 100 

15 p.g Au in 30 ml 0.25 N KCN .............. 100 

1Insufficient sample to run all three experiments. 

SAMPLE DESCRIPTIONS 

(1) 

94 

18 

100 

100 

70 

100 

(1) 

70 

V -366. Siliceous black shale from Polk Creek Shale (Ordovician) , sec. 2, T. 1 N ., R. 16 W ., Saline County, Ark. Collected by J. D. Vine. 

V-437U. Oil shale from Mahogany ledge, Green River Formation (Eocene), John Savage quarry, sec. 36, T. 7 S., R. 97 W., Garfield County, Colo. Collected by J. D. Vine. 

C-1. Carbonaceous seam from Carlin gold mine, Lynn district, Eureka County, Nev. Collected by R. L. Erickson. 

1759. Anthracite coal, Illinois. Collected by J. D. Vine. 

ABSORPTION OF GOLD BY PLANTS AS A FUNCTION 

OF COMPLEX STABILITY 

It is well established that gold is found in vegetation. The literature on gold in plants has been comprehensively reviewed by Jones 

(1970). How gold is transported from the soil into the plant has 

been the subject of much speculation. As a facet of this study of 

the geochemistry of gold, a cooperative study of the absorption of 

gold by plants was made with H. T. Shacklette. The results of this 

study and related studies are reported in this section. 

In the experiments reported by Shacklette, Lakin, Hubert, and 

Curtin (1970), colloidal gold is not absorbed by rooted or unrooted 

cuttings of two species of Impatiens. Two sols (colloidal gold 

preparations) were used: ( 1) A sol prepared by reduction of 

HAu198Cl4 with glucose in the presence of gelatin (probable maximum diameter of gold particles less than 0.05 ~-tm (micrometer)), 

and (2) a sol prepared by reduction of HAu198Cl4 with sodium 
ABSORPTION BY PLANTS 15 

oxalate (probable diameter of gold particles <0.45 p.m and >0.05 

p.m). The facts that neither of these sols produced leaf images in 

autoradiographs of the leaves of rooted cuttings of Impatiens and 

that the stems and leaves remained alive and seemed to sustain 

no cell damage discredit any assumption that colloidal gold is 

taken up by plants. 

Under certain conditions gold in the form of gold chloride at 

pH 6.2 is taken up by plants. The amount of gold absorbed from 

nonradioactive solutions of gold chloride at pH 6.2 by rooted 

Impatiens plants was found to be 1 and 7.5 ppm in the ash of 

the two plants grown in solutions containing 5.7 ppm gold. Lack 

of images on the autoradiographs of leaves taken from rooted 

Impatiens showed that nd absorption of Au198 took place from 

gold chloride solutions at pH 6.2. Perhaps gold was reduced in the 

short plant stem between the solution and the leaf. Also, gold was 

reduced quantitatively in 48 hours by onion roots, as evidenced 

by a blue suspension in the solution and coating on the roots; new 

growth onion roots were white, indicating that no gold remained 

in solution. 

Gold in gold bromide solutions is more readily taken up by 

plants. Slight reduction of gold by plant roots was observed 

with gold bromide solutions, and faint autoradiographs indicated 

absorption by the plant. Measurement of absorbed gold by atomic 

absorption gave 28 and 39 ppm gold in ash of plants grown in 

solutions containing 29 ppm gold. 

On the other hand, the gold iodide and gold thiocyanate solutions were toxic to severely toxic to the plants, and gold absorption 

was erratic. The Impatiens holstii absorbed 64 ppm gold (ash 

basis) from the gold iodide solution compared with 2.7 ppm in 

the ash of Impatiens balsamina. Less than 1 ppm gold was found 

in the ash of these plants growing in gold thiocyanate solutions. 

Gold thiosulfate behaved similarly to gold chloride in that it 

was not absorbed in the radioactive gold experiments and was 

found in the _ash of the plants grown in gold thiosulfate solutions. 

Greater amounts of gold were absorbed by the plants from gold 

cyanide solutions than from any other gold complexes tested. Of 

special interest are the results obtained by gold cyanide solutions 

prepared by the action of a slurry of the cyanogenic plant, wild 

flax (Linum lewisii Pursh), upon leaf gold. A slurry of the aboveground parts of wild flax at the flowering stage was made in a 

blender and diluted with water; leaf gold was added, and the mixture was allowed to stand for 3 days. The slurry was centrifuged, 

and the supernatant solution was found to contain 0.5 ppm gold. 

Radioactive gold cyanide was added to the supernatant liquid, and 
16 GEOCHEMISTRY OF GOLD IN THE WEATHERING CYCLE 

rooted Impatiens was grown in the liquid. Rooted Impatiens absorbed gold from this supernatant liquid. 

In a subsequent experiment mountain ash twigs and leaves were 

ground in a blender with water, gold leaf was added, and the 

slurry was allowed to stand for 1 week. After centrifuging to 

remove much of the plant material and all the undissolved leaf 

gold, the plant extract contained 0.5 ppm gold. Rooted cuttings of 

Impatiens were placed in this plant extract. The cuttings were 

severely wilted after 2 days and were dead at the end of 5 days. 

The plant tips, separated by a shield from the plant extract, were 

excised and dried. The dried plants weighed 8.5 g and contained 

3 p..g of gold (0.35 ppm gold). The solubilization of gold by a 

plant extract and the subsequent uptake of 3 p..g of gold from this 

extract demonstrates a natural process by which gold can be 

solubilized and absorbed by plants. 

The interaction of the biological system and the aqueous solutions of high ionic strength used in the experiments make interpretation of the data in a quantitative manner most difficult. Some 

relationships are shown by the data, however, in that, although 

the results of the experiments with the iodide, thiocyanate, and 

thiosulfate solutions are erratic, a correlation exists between gold 

absorbed by the plants and the stability of the chloride, bromide, 

and cyanide (table 6) complexes. The gold absorption coefficient 

for plants is defined here as the ratio of the gold absorbed by the 

plants, in parts per million, in ash of the plants to the gold content, in parts per million, of the nutrient solution. This absorption 

coefficient increases from chloride through bromide to cyanide complexes which is the order of increasing stability of the complexes. 

In summary, colloidal gold is not absorbed by the plants investigated. Gold chloride and bromide may be reduced by the action 

of the roots of plants at pH 6.2. The most strongly absorbed complex of gold is the cyanide complex, which is also the most stable 

complex studied. It is also available in auriferous areas inhabitated by cyanogenic organisms. The absorption coefficient of gold 

complex by two species of Irnpatiens (ta.ble 6) increases with the 

stability of the complex. 

TABLE 6.-Variation of gold absorpt-ion coefficients with three gold complexes 

for two species of Impatiens 

Plant 

Ratio of gold in ash of plants to gold in nutrient 

solution (ppm) 

Cl Br CN 

Impatiens holstii_ ____________________________ Q.17 

balsamina .. -------------------------------1.30 

1.0 

1.34 

5.5 

2.7 
GEOCHEMISTRY OF GOLD IN THE WEATHERING CYCLE 17 

GOLD CHLORIDE 

Gold chloride has been postulated as the probable mobile form of 

gold by many investigators. Emmons (1917, p. 323) reviewed the 

literature of the time and summarized the knowledge of the solution, 

migration, and enrichment of gold in mineral deposits as follows : 

In nature the conditions that are favorable, if not essential, for gold enrichment are (1) chloride solutions, (2) iron sulfides, (3) manganese compounds. 

Where these conditions are supplied and where no very effective precipitant 

is at hand and erosion is not too rapid, gold placers are rarely formed, and 

outcrops of gold ores are likely to be less rich than the ores that lie deeper. 

Where these conditions exist and where the lodes are fractured, gold will 

migrate downward. 

More recently Krauskopf (1951, p. 869) summarized his studies: 

(1) Solubilities of gold calculated from thermodynamic data agree reasonably 

well with experimental results. (2) In acid solutions gold may be transported 

as the ion AuClr, provide~ that reducing agents are absent. The metal is 

dissolved by an acid solutioh provided that the solution contains Cl- and that 

a fairly strong oxidizing agent is present. At high temperatures and pressures 

the hydrogen ion of the acid is a sufficiently strong oxidizing agent: at low 

temperatures a substance like Mn02, 0 2, Fe+++, or Cu++ must be present in 

addition. The requirement that an oxidizing agent be present, or at least that 

a reducing agent be absent, probably means that the gold chloride ion would 

not be transported in acid vein solutions at low temperatures, since such solutions would contain reducing agents like H2S and Fe++. On the other hand, 

solution and transport of gold in acid solution is probably the mechanism of 

supergene movement of gold. 

The solution of gold by Mn02 in 1 M HCl (Krauskopf, 1951, 

p. 862) is expressed by the equation 2Au+12H +1+3Mn0z+8Cl- 1 

==3Mn+2+2AuCl4 - 1+6H20 with E==+0.28V, dF0 ==-38,000 cal, 

and K==cal028• The positive electromotive force, the negative freeenergy change, and the large equilibrium constant for the reaction 

indicate that gold is soluble. 

Krauskopf (1951, p. 863) has given a similar equation for the 

solubility of gold in 1M HCl and atmospheric oxygen: 2Au+6H+1 

+3/2 0 2+8 Cl- 1==2AuC14 - 1+3H20. Both of these reactions are 

very sensitive to changes in the concentrations of hydrogen and 

chloride ions. 

Table 7 gives some values of the solubility of gold calculated by 

Krauskopf (1951, tables 3, 4, p. 862-863) in HCl+MnOz and in 

HCl+02 at 25°C. 

Thus, gold chloride can be formed in acid solution in the presence of Mn02, 0 2, Fe+3 or Cu+2, but it requires conditions not 

commonly found in nature. 

The calculations given by Krauskopf (table 7) show that at 

concentrations of 10-3 M H+1 and Cl- 1 equivalent to pH 3 and 

35 ppm CI-I, the solubility of gold in the presence of MnOz is 
18 GEOCHEMISTRY OF GOLD IN THE WEATHERING CYCLE 

TABLE 7.- Solubilities of gold in hyd1·ochloric acid 

[From Krauskopf ( 1951, tables 3 and 4, p. 862-863)] 

Solubility of Au in Solubility of Au in 

Concentration of Concentration of HCl + Mn02 at 2s•c HCl + 02 at 25°C 

H+1 (mole/!) C1-1 (mole/1) Au Au Au Au 

(mo1e/1) (gram/1) (mo1e/1) (gram/1) 

l0-4 1 10-4 0.02 0.1 20 

10-li ... 10-9 2Xl0-7 10-7 2Xl0-5 ..1. 

1 10-4 .1 20 10-:i .002 

l0-2 10-2 10-2 2 10-3 .2 

10-3 10-3 10-6 2x10-4 10-10 2x1o-s 

0.2 ppm. Lovering and Morris (1965, table 3, p. F16) reported 

70 ppm chloride in downward-moving acid sulfate water from the 

North Lily mine, Tin tic district, Utah. In the nearby Tin tic Standard mine, Morris and Lovering (1952, p. 693) found chalcanthite 

(CuS04·5H20) 25 feet from the ore zone that contained 2.7 ppm 

gold ( 0.08 oz;ton) . Although the gold may be in a colloidal form 

in this mineral, it is reasonable to assume that Krauskopf's conditions of solubility had been met and gold chloride had moved a 

short distance in solution. 

Cloke and Kelly (1964, p. 269) studied the solubility of gold in 

the presence of chloride solutions and arrived at conclusions similar to those of Krauskopf. 

The present practice of leaching porphyry copper dumps with 

dilute sulfuric acid solutions is carried out in sufficiently acid solutions to satisfy, in the presence of sufficient chloride ion, the 

requirements given by Krauskopf for solution of gold. Beall (1965, 

p. 81) gave data on the pH and composition of leach solutions at 

seven commercial copper leaching operations in or adjacent to 

Arizona. Using these data, we prepared solutions approximating 

the input and output solutions in copper, iron, aluminum content, 

and pH. Aluminum content was not given by Beall; however, we 

estimated it to be the same as iron. Sodium chloride and manganese dioxide, together with 0.5 g of gold shavings, were added to 

aliquots of these solutions, and the solutions were allowed to stand. 

The data were obtained from a set of confounded factorial experiments (table 8) using the design given by Youden (1951, p. 112). 

An analysis of variance can readily be made of the data in 

table 8, but this would add little to the obvious facts that the 

solubility of gold increases independently with increasing concentrations of chloride ion, hydrogen ion, or manganese dioxide. An 

increase of twentyfold in the concentration of ferric and cupric 

ions actually resulted in a decrease of solution of gold in these 

experiments owing to the formation of ferric and cupric chloride 

complexes and the resultant decrease in the effective concentration 

of chloride ion. 
GOLD CHLORIDE 19 

These experiments, reported in table 8, show that leaf gold is 

readily dissolved in sulfuric acid solutions at acidities used in 

commercial leaching of copper if sufficient chloride ion and Mn02 

are present. However, when these solutions are mixed with crushed 

ore, it appears that pockets of gold-oxidizing and gold-reducing 

regions develop, and the net result is no effective solution of gold 

because of the low stability of the gold chloride complex and its 

resultant low mobility (table 3). 

To test the effectivenes~. of the solutions used in the experiments 

reported in table 8, we added the acid solution of pH 2, containing 

3,500 ppm Cl- 1 and a large excess of Mn02, to a pulverized oxidized gold ore (3.5 oz/ton Au) from ore at Carlin, Nev. Air was 

bubbled through the mixture to provide a stirring action, and 

sulfuric acid was added, as needed, to maintain the pH at 2. After 

1 month 10 percent of the gold was in solution. At the end of 

2 months only 1 percent was in solution, and after 3 months no 

gold could be detected in solution ( < 0.08 percent of the gold in 

the ore). Thus, the gold initially dissolved was eventually lost 

from solution. The loss of gold from solution may be due to a 

reducing agent within the ore sample but is probably enhanced 

by an increase in ferric ion in solution and the consequent decrease 

in effective chloride ion, an effect which can be observed in the 

data of table 8. 

TABLE 8.- The interactions of the concent1·ations of Cl-1, H+1, and Mn02 on 

the solubility of gold in copper leach solutions 

Average porphyry ]nput sulfate solution Output (pregnant) 

sulfate solution copper leach solutions Fe+3, AJ+3, Cu+2=0.1 g/1 each Fe+3,AJ+3,Cu+2=2 g/1 each 

Add:~ Cl-1 {Normality of Cl-1 .. 0.05 0.1 0.05 0.1 NaCl ppm of Cl-t ............. 1,';'00 3,500 1,700 3,500 

pH at start .................................... :·5 I z.o 2.5 I 2.0 2.5 I 2.0 2.5 I 2.0 Approx. H+t, in ppm .................. 10 3 10 3 10 3 10 

Parts per million of gold in solution after 3 weeks of contact with gold 

No Mn02 ....................................... . 40 mg/1 Mn02 .............................. . 200 mg/1 Mn02 ............................ . 

<0.04 

3.9 

7.0 

The net effect observed in the copper industry will be loss of 

gold production as production of cement copper (product of leaching operation) increases. Gold produced by three mining companies in Arizona amounted to 0.04 to 0.35 oz;ton of copper, which 

is equivalent to 1.4 to 12 ppm Au (Au and Ag map and Cu map, 

American Cyanamid Co., 1965a, b). The gold recovered from copper in electrorefining amounted to 133,000 ounces for the three 

companies in 1965. In contrast, 13 samples of cement copper col-
20 GEOCHEMISTRY OF GOLD IN THE WEATHERING CYCLE 

lected from seven leach operations in Arizona were analyzed by 

F. 0. Simon, using activation analyses (Rowe and Simon, 1968), 

and were found to contain from 0.007 to 0.066 ppm Au. These 

data show that the increasing use of present leaching processes 

can result in the loss of many thousands of ounces of gold. 

The likelihood of gold chloride being reduced has been emphasized by many workers. Rickard (1897, p. 978) extracted the gold 

from a piece of ore (1,147 ozjton Au) from Cripple Creek, Colo., 

with a solution of ferric sulfate, sodium chloride, and "a little free 

sulfuric acid." On subsequent immersion of a piece of carbonaceous 

shale from the Rico Formation in the solution, the gold was precipitated so rapidly that "gilding of the black shale by the deposit 

of gold became visible within four hours." 

The data given in table 4 show that 45.6 percent of 114 materials removed more than 90 percent of the gold chloride from 

solution after a very short period of contact. These data also 

emphasize the relative immobility of gold chloride in a geologic 

environment. 

GOLD BROMIDE AND GOLD IODIDE 

GEOCHEMISTRY OF BROMIDES AND IODIDES 

Bromides and, especially, iodides form more stable complexes 

with gold than do chlorides. The relative scarcity of these halogens 

in the earth's crust, however, reduces the probability of their 

effectiveness in the solution of gold. The crustal abundance given 

by Vinogradov (1962, table 4, p. 649) for the three halides are 

chlorine, 170 ppm ; bromine, 2.1 ppm ; and iodine, 0.4 ppm. The 

elements do not travel together in this relative abundance as 

shown in table 9. Bromine is concentrated in residual brines 

formed from waters evaporated by solar heat, whereas iodine is 

depleted, presumably by oxidation of iodides to give volatile-free 

iodine. Both bromine and iodine are concentrated in organic debris. 

The presence of bromine in plants, marine organisms, and 

organic and oil-bearing sediments is similar to that of iodine, 

but bromine occurs in lesser amounts than iodine. Thus, in the 

iodine-rich waters of Java the highest bromine content reported 

was 68 ppm, compared with 150 ppm for iodine (Bemmelen, 1949). 

Iodine is markedly biophilic and is almost always more concentrated in the surface organic layer of ~oils than in lower horizons. 

In soils of the conterminous United States, the iodine content 

ranges from 0.015 to 40 ppm; in contrast, iodine in the soils of 

Hawaii ranges from 8.6 to 111 ppm and averages 34 ppm (Chilean 

Iodine Educational Bureau, 1956, tables 10, 12, p. 59-72, 86). The 

higher iodine content of Hawaiian soils can be attributed to iodine 
GOLD BROMIDE AND GOLD IODIDE 21 

TABLE 9.- Abundance, in parts per million, of chlorine, bromine, and iodine 

and calculated relative abu·ndance of bromine and iodine in various earth 

materials 

Relative 

Bromine/ abundance Material Chlorine Bromino! Iodine iodine Cl=100 Reference 

Br 

Earth's crust ............. 170 2.1 0.4 5.2 1.3 0.24 Vinogradov (1962). 

Ultramafic rocks ....... 85 1 .05 20 1.2 .6 Turekian and Wedepohl (1961). Basalts ......................... 60 3.6 .05 72 6.0 .8 Do. Granite (high 130 4.5 .05 90 3.6 .4 Do. calcium). Do ........ (low 200 1.3 .05 26 .65 .25 Do. calcium). 

Clays and shales ....... 160 6 1 6 3.7 .62 Vinogradov (1962). 

Shales .......................... 180 4 2.2 1.8 2.2 1.2 Turekian and Wedepohl (1961). 

Carbonate rocks ........ 150 6.2 1.2 5.2 4.1 . 8 Do • Deep sea clays .......... 21,000 70 .05 1,400 .35 . 00025 Do . Sandstones ................. 10 1 1.7 .6 10 17 Do. Soils ............................. 100 5 5 1 5 5 Vinogradov ( 1959). 

River water ................ 8.3 .006 .0018 3.3 .075 • 022 Correns ( 1956) . 

Sea water ................... 19,000 65 .05 1,300 .33 . 00025 Do . 

Hot spring in 43,790 54.8 .91 60 .13 . 0021 Miyake ( 1!)65) . 

Arizona. 

in rainwater that was contributed to the atmosphere from oceanic 

evaporation and volcanic emanations. Vinogradov (1959, p. 54) 

postulated that much of the iodine enters the plants from the air 

and calculated that 1 mg (milligram) of iodine per square meter 

per year is introduced into soils ·by plants. Iodine is also found in 

the colloid fraction of soils and in marine muds. Marine plants 

are exceptionally rich in iodine. Sediments containing the remains 

of these plants are rich in iodine, and brines squeezed .out of such 

sediments are rich in iodine. For example, Bemmelen (1949, p. 

186), in discussing the source of iodine from wells in Java (where 

iodine content averages 100-150 mgjl) stated, "the iodine-bearing 

wells of the northern belt are related to oil and the source rocks 

of oil. Iodine is derived from neogene marine sediments which 

were deposited in an open sea at some distance from the coast. 

Those of the central belt are not related to oil and issue from 

Pleistocene littoral or lagoonal deposits. These brines have a lower 

alkalinity. The source of the iodine might be seaweeds accumulated along the coast. The disintegration of these weeds after 

buria~ by sediments caused the accumulated iodine to be liberated." 

Marine muds from various Asian seas average about 25 ppm 

iodine, and the iodine content of these marine muds increases 

approximately with the eolloid content (Vinogradov, 1959). In 

oil shales the iodine content rises with increase in the oil content 

(Correns, 1956). Phosphate rocks of the world contain from 0.2 

to 280 ppm iodine, averaging about 30 ppm (Chilean Iodine Educational Bureau, 1956, table 2, p. 2). 
TABLE 10.- The interactions of the concentrations of Br-1, H+1, and Mn02 on the solubility of gold in copper leach solutions 

Average porphyry copper Input sulfate solution Output (pregnant) sulfate solution 

leach solution Fe+3, AJ+3, Cu+2=0.1 g/1 each Fe+a, Al+3, Cu+~=2 g/1 each 

Added Br- 0.005 0.01 0.05 0.1 0.005 0.01 0.05 0.1 1 {Normality of Br-2• 

as NaBr ppm of Br-1 .•.•••••••• 400 800 4000 8000 400 800 4000 8000 

pH at start ..................................... 2.5 I 2.0 2.5 I 2.0 :·5 I 2.0 :·5 I 2.0 :·5 T 2.0 

Approx. H+1 in ppm .................... 3 10 3 10 10 10 10 

Parts per million of gold in solution after 3 weeks of contact with gold 

:·51 2.0 :·51 10 

2.0 :·51 10 

2.0 

10 

4.4 

18 

16 

TABLE 11.- The interactions of the concentrations of 1-1, H+1, and Mn02 on the solubility of gold in coppe1· leach solutions 

Average porphyry copper leach solution 

Added I-1 {Normality of 1-1 .................. . as Nal ppm of 1-1 ............................. . 

pH at start ................................................. . Approx H+1 in ppm ................................. . 

No Mn02 ..................................................... . 40 mg/1 MnO~ ............................................ . 200 mg/1 Mn02 .......................................... . 

0.0001 

13 

2.5 

3 I 2.0 10 

Input sulfate solution Fe+3, Al+3 , Cu+2=0.1 g/1 each 

0.001 130 0.005 

640 

2.5 

3 1

2.0 

10 :·5 I 2.0 

10 

Output (pregnant) sulfate solution Fe+3, Al+3, Cu+2=2 g/1 each 

0.0001 

13 

2.5 I 2.0 3 10 

0.001 

130 

2.5 

3 I 2.0 

10 

0.005 

640 

:·5 I 2.0 

10 

Parts per million of gold in solution after 1 week of contact with gold 

>30 

27 

10 

~ 

~ 

~ 

tz:j 

0 

0 

~ 

tz:j 

a:: 

1-t 

00 

1-3 

~ 

to<: 

0 

":j 

~ 

0 

t"4 

t:' 

1-t 

z 

1-3 

~ 

tz:j 

$! 

tz:j 

> 1-3 

~ 

tz:j 

~ 1-t 

z 

~ 

0 

to<: 

0 

t"4 

tz:j 
GOLD BROMIDE AND GOLD IODIDE 23 

Iodine is locally concentrated in volcanic springs, reaching 

197 ppm in a mineral spring in Japan (Miyake, 1965). Thus, 

iodine is concentrated in soils, organic marine sediments, phosphatic sediments, brines from marine sediments, and volcanic 

hot springs. Locally, the iodide concentration in waters leaching 

such sources is adequate to permit the solution of gold by Fe+3 , 

Cu +2, or dissolved oxygen. 

The bromine content of soils is about 5 ppm (table 9) and the 

bromine-iodine ratio in soils ranges from 0.16 to 3 (Vinogradov, 

1959, tables 23-27, p. 46--48). Compared with the earth's crust, 

soils are enriched in bromine, but not so greatly as they are 

enriched in iodine. 

An abnormal accumulation of bromine (850 ppm) was reported 

(Yamada, 1968, p. 1137) in a Japanese humic volcanic ash soil 

containing 17.4 percent carbon. In contrast with the high iodine 

contents in Hawaiian soils, T. T. Chao (written commun., 1972) 

found that bromides released from soil fumigants were readily 

leached by water with very little retention of bromine in the soil. 

FORMATION OF GOLD BROMIDE AND GOLD IODIDE 

The same experiments were made with addition of NaBr and 

Nai (tables 10, 11) to average input and pregnant leach solutions 

that were made with NaCl (table 8). In the experiments with 

the bromide ion, the solubility of gold increases independently with 

increasing concentrations of bromide ion, hydrogen ion, or manganese dioxide, as was also observed in the chloride study; however, 

at concentrations of bromide ion of 800 ppm and greater, gold was 

dissolved in the absence of manganese dioxide. In the experiments 

with the iodide ion, the solubility of gold increases independently 

with increasing concentration of iodide ion and hydrogen ion and 

decreases with increasing manganese dioxide. The .. manganese 

dioxide oxidizes the iodide ion to iodine, which escapes from the 

system (Gale, 1971). 

The solubility of gold decreased with a twentyfold increase in 

the concentration of iron and copper in the presence of a given 

concentration of bromine or iodine. As in the chloride system, this 

effect is due to removal of halide ions by formation of iron and 

copper halide complexes. 

The increasing effectiveness of the sequence chloride-bromideiodide ions as complexing agents is apparent in these data. Thus, 

no measurable gold was dissolved with chlorides in the absence 

of manganese dioxide (table 8) ; appreciable gold was dissolved 

with bromide at 0.05 !vi and greater; more gold was dissolved with 

iodide in the absence of Mn02 • Thus, although Cu +2 and Fe+3 
24 GEOCHEMISTRY OF GOLD IN THE WEATHERING CYCLE 

are not adequate oxidants of gold in the formation of AuCI.~-1, 

they do oxidize gold to AuBr4 - 1 in 0.05 M NaBr solutions, and 

they oxidize gold in 0.0001 M N ai solutions. To dissolve approximately the same amount of gold in a given time ( 1.5 mgjl in 3 

weeks) these solutions must contain either 1,800 ppm Cl- 1 or 

400 ppm Br- 1 or as little as 13 ppm I-t, giving an equivalent 

effectiveness of 100 ppm Cl- 1=22 ppm Br- 1=0.7 ppm I- 1• 

When one considers that the relative crustal abundances of chlorine, bromine, and iodine are 100, 1.3, and 0.24, respectively 

(table 9), it is evident that considerable enrichment of bromine 

and iodine is required for these elements to be as effective in 

solubilization of gold as chloride is in the acid solutions produced 

by oxidation of pyrite in gold deposits. 

The strength of the Aui:!- 1 complex is shown by the oxidation 

of gold in the presence of N ai solutions in which the only oxidant 

is dissolved oxygen in the aqueous solution (table 12). When the 

pH of Nai solutions was adjusted with sulfuric acid or sodium 

hydroxide to give solutions ranging from pH 2.5 to 9, gold was 

dissolved throughout this pH range but was much more soluble 

in solutions of pH 2.5 than in neutral to basic solutions. Neither 

gold chloride nor gold bromide is formed under these conditions. 

The solubility of gold in four different ores in 0.05 M sodium 

iodide solutions varied with the pH of the solutions and the type 

of ore (table 13). No gold was dissolved from telluride ores, in 

contrast with 90 percent of the gold dissolved from massive pyrite 

ores, and 60-90 percent of the gold dissolved from oxidized ores. 

The decrease in dissolved gold after 14 days may be due to increased solution of iron resulting in complexing of iodide and thus 

decreasing effective iodide concentration, or the decrease in dissolved gold may be due to oxidation and subsequent loss of iodide. 

Gold is not solubilized in either chloride or bromide solutions 

under these conditions. 

Chukhrov (1947) postulated the solution of gold in sulfide deposits by free bromine and iodine. Bromides and iodides are, as 

we have seen, active complexing agents, and Fe+3 and Cu+ 2 are 

adequate oxidants of gold in the presence of sufficient bromide 

and iodide ions. 

It seems evident, however, that bromides, although forming a 

stronger complex with gold than chloride, will rarely, if ever, be 

present in sufficient concentrations to be of any importance in the 

solution of gold. 

Iodides, although much less abundant than chlorides and bromides, may, in rare instances, influence the solution of gold. One 

is tempted to speculate on the possible solution of gold in Indonesia 
GOLD BROMIDE AND GOLD IODIDE 25 

TABLE 12.- Solubility of leaf gold in 0.05 M sodium iodide solutions adjusted 

to various pH values 

Dissolved gold (ppm) 

pHt After 7 days After 14 days 

2.5 19.4 55.7 

5 1 2.2 

7 .26 .7 

9 .14 .4 

1 pH adjusted with sulfuric acid or sodium hydroxide, as required. 2 Brown color of solution indicated oxidation of iodide to iodine. 

After 21 days 

62 

21.8 

.86 

.6 

TABLE 13.- Solubility of ore gold in 0.05 M sodium iodide solutions adjusted 

to various pH values 

[ND, none detected] 

Dissolved gold (mg/1) Maximum AdjusW dissolved gold Percentage of gold 

pH After After After dissolved from ore 7 days 14 day; 21 days (ILg) 

Sample No. EM-534-9-10. Gold 25 ppm 

2.5 0.7 0.9 0.74 225 90 

5 .4 .5 .5 125 50 

7 .2 .2 .24 50 20 

9 .12 .14 .06 35 14 

SamJtle No. GBG26. Gold 162 ppm 

2.5 <0.04 <0.04 <0.04 ND 0 

5 <.04 <.04 <.04 ND 0 

7 <.04 <.04 <.04 ND 0 

9 <.04 <.04 <.04 ND 0 

Sarnple No. U75. Gold 79 ppm 

2.5 1.96 2.9 2.2 715 90 

5 1.7 2.6 2.2 650 82 

7 1.6 2.9 2.2 715 90 

9 1.2 2.9 1.8 715 90 

Sample No. D3U. Gold 130 ppm 

2.5 3.3 3.5 3.3 875 67 

5 .14 .16 .06 40 3 

7 <.04 <.04 <.04 ND 0 

9 <.04 <.04 <.04 ND 0 

SAMPLE DESCRIPTIONS 

EM-534-9-10. Fines from bedrock crevices at bottom of soil profile from Empire, Colo. Native 

gold with no observed sulfides. Bedrock is biotite gneiss containing only 0.04 ppm gold. GBG26. Volcanic breccia of phonolitt,·latite composition containing gold telluride and minor anti- mony and silver. SW'4 sec. 20, T. 15 N., R. 69 W. Cripple Creek district, Teller County, Colo. U75. Massive pyrite from dump of Atlantic shaft, Empire district, Clear Creek County, Colo. 

Collected by Maurice Chaffee. D344. Massive gray jasperoid replacing dolomite; minor pyrite and hematite. Sec. 26, T. 14 S., R. 11 W., Drum Mountains, Juab County, Utah. Collected by J. H. McCarthy, Jr. 

by iodine-bearing ground waters and its subsequent reduction in 

streams or alluvial fans. The long history of more than 1,000 

years of placer gold mining in these islands together with the 

juxtaposition of iodine-bearing hot springs and placer gold deposits in Java lends some support to the thought that the placers 
26 GEOCHEMISTRY OF GOLD IN THE WEATHERING CYCLE 

may result from precipitation of dissolved gold. The iodine content of many soils is adequate to dissolve gold if the iodine is 

available as an iodide. 

In microenvironments in soils, sufficient iodide concentrations 

may be reached to result in solution of gold. 

GOLD THIOSULFATE 

GEOCHEMISTRY OF THE THIOSULFATE ION 

Thiosulfate is a metastable ion that can be formed by the 

action of water on native sulfur in basic solutions. Thiosulfates 

and polythionates are commonly found in hot alkaline sulfur 

springs (Clarke, 1924, p. 196; White and others, 1963, table 21, 

p. F47; Day and Allen, 1925, p. 117; Blatchley, 1903, p. 76, 80, 87; 

Wilson, 1966, p. 680-681). Miyake ( 1965, p. 269), in a compilation 

of data, reported 10 springs in Japan containing thiosulfate with 

an average content of 0.11 ppm (10- 6 M) and a maximum content 

of 37 ppm (3Xl0- 4 M). Arntson, Dickson and Tunell (1960, p. 

581) suggested the possible role of thiosulfate in forming soluble 

complexes with many metallic ions in hot alkaline sulfur waters. 

Thiosulfates may be present in appreciable quantities in restricted marine environments. In an anoxic fjord on the southwestern coast of Vancouver Island, British Columbia, hydrogen 

sulfide accumulates in the deeper waters. Depletion of this hydrogen sulfide is assumed, by Cline and Richards (1969, p. 838), to 

occur in the transition zone where vertical diffusion brings oxygen 

and sulfide in contact. These authors studied the reaction of H2S 

and dissolved oxygen in sterile salt water approximating the observed concentrations of hydrogen sulfide and oxygen. After about 

50 hours, thiosulfate reached a maximum concentration of 30-35 

percent of the initial sulfide concentration, sulfite reached 10-15 

percent, and the remainder was presumed to be sulfate. 

The presence of thiosulfates in soils is not easily demonstrated 

from direct data in the literature. The ionic species of sulfur in 

soils are in a constant state of flux. Sulfates may be reduced and 

elemental sulfur oxidized. A reaction of interest here is the oxidation of S-2 to S20 3 - 2 by the enzyme oxidase (Stevenson, 1964, 

p. 278). Plant and animal residues introduce a wide variety of 

sulfur compounds, including thiosulfates. Whitehead (1964, p. 3) 

stated that thiosulfates and polythionates occur transitorily in 

soils but are subject to either oxidation or reduction and do not 

accumulate. Organisms capable of oxidizing thiosulfates are common; they occur in most soils, upon rocks, in saltwater and freshwater, and in mineral springs (Starkey, 1935, p. 201). Thus, it 

would seem likely that thiosulfates are present in soils-in amounts 

varying erratically with time. 
GOLD THIOSULFATE 27 

-0.5 

14 

pH 

FIGURE 1.- Equilibrium d:.stribution of unstable sulfur species in water. 

Total dissolved sulfur exclusive of sulfate species = 0.001 M at 25oC and 

1 atmosphere total pressure. From Granger and Warren (1969, p. 166). 

Reproduced from Economic Geology, 1969, v. 64, p. 166. 

Granger and Warren (1969, p. 166) constructed an Eh-pH diagram of the equilibrium distribution of unstable sulfur species in 

water by ignoring sulfate concentrations (fig. 1). Their interpretation shows that the thiosulfate ion is most likely to be found 

in near-neutral solutions in equilibrium with H2S and S03- 2 • 

In acid solutions the thiosulfate ion disassociates into So and 

iiS03-r, and, as the solution approaches an alkaline pH, the 

products become HS- 1 and S03- 2 • It should be kept in mind, 

however, that the stable states of sulfur in the acid oxidizing 

region are H2S, so and S04 -- 2, and in the alkaline region HS- 1 

and S04- 2 (Garrels, 1960). Valensi (1963, p. 64) presented an 

equilibrium diagram sin1ilar to that of Granger and Warren and 

discussed the reactions in more detail. 
28 GEOCHEMISTRY OF GOLD IN THE WEATHERING CYCLE 

Garrels and Naeser (1958, p. 121, fig. 19) calculated the equilibrium distribution of dissolved sulfur species for 0.1 M total 

sulfur in water at 25°C and one atmosphere total pressure and 

found that the maximum concentration of S203 - 2 would be 10-6 M 

or 0.11 ppm S203 - 2 • This amount is the average value for 10 hot 

springs in Japan reported by Miyake (1965, p. 219). The fact of 

interest in this discussion is that the thiosulfate ion is a normal 

metastable form that could be found in the aqueous environment 

of sulfur and sulfur dioxide between pH of 2 to 9 ; an environment 

that can exist in the oxidation of sulfide ores. In an alkaline situation, such as postulated by Listova, V ainshtein, and Ryabinina 

(1966), thiosulfate may result from the reaction of S03 - 2 and 

HS- 1 produced by the oxidation of pyrite in the presence of limestone and, locally, it could reach concentrations adequate to dissolve gold. 

FORMATION OF GOLD THIOSULFATE 

The gold thiosulfate complex may be of importance in the transient solubility of gold in the weathering cycle. Listova, Vainshtein, and Ryabinina (1966) reported that, during air oxidation of 

sulfides of iron, lead, and zinc, gold was more soluble in a CaCOs 

environment than in an acid medium. They suggested that calcium 

thiosulfate and polythionates, formed during the reaction of carbonates with the products of sulfide oxidation, are the solvents for 

gold. In spite of the transitory nature of the thiosulfate ion in an 

oxygenated solution, they reported that gold is retained in solution 

for a long time, and, therefore, such complexes have a large 

migration capacity. 

During a 48-hour period Stetefeldt ( 1885, p. 50) dissolved 2 mg 

of gold in a liter of 0.04 M Na2S20 3 solution containing 50 mg 

of leaf gold. Increasing the concentration of N a2S20s to 1 M made 

no appreciable change in the amount of gold dissolved. The limiting factor apparently was the concentration of dissolved oxygen 

in the water rather than the thiosulfate concentration. In our 

laboratory 3 mgjl gold (3 ppm) was dissolved in an 0.008 M 

N a2S20s solution in 1 week. 

The metastable state of the thiosulfate ion and the relative 

rarity of the ion is compensated, in part, by the stability of the 

gold thiosulfate complex. At room temperature a solution of 

NasAu (S20s) 2 does not yield the usual reaction for the thiosulfate 

ion - that is, dilute hydrochloric or sulfuric acids do not precipitate sulfur. In a like manner, gold as a thiosulfate complex is not 

reduced by stannous chloride, ferrous sulfate, or oxalic acid (Rose, 

1898, p. 31; Pascal, 1957, p. 732). The lack of loss of gold from 
GOLD THIOCYANATE 29 

solution in contact with various minerals (table 4) is another 

indication of the stability of the gold thiosulfate complex. 

The solubility of leaf gold in dilute thiosulfate solutions (table 

14) varies with the pH of the solution. At pH 5 and greater, gold 

is appreciably soluble in a solution approximately 40 times as 

concentrated (0.012 M versus 0.0003 M) as the maximum reported 

in Japanese waters (Miyake, 1965, p. 269). During the oxidation 

of pyrite in limestone adequate concentrations of thiosulfate may 

be reached to dissolve gold. 

TABLE 14.- Solubility of leaf gold in 0.0124 M sodium thiosulfate solutions 

adjust•3d to various pH values 

pH 

2.541 

4.96 

6.94 

9.15 

10.53 

pH adjusted 

withdil H2S04······· ........ . .......... do .................... . NaOH ...................... . .......... do .................... . .......... do .................... . 

Gold (ppm) in solution 

After 7 days After 14 days After 21 days 

0.28 0.46 0.8 

1.1 2.2 3.7 

1.1 2.6 3.3 

.9 1.3 2.0 

1.3 2.2 3.0 

1Sulfur was precipitated at this pll, indicating partial decomposition of the thiosulfate. 

The solubility of ore gold has also been determined in dilute 

sodium thiosulfate solutions adjusted to various pH values (table 

15). Oxidized ores (samples EM-534-9-10 and C-2) yielded 10-12 

percent of the gold content to solution in 3 weeks. Gold telluride 

(sample GBG 26) and aLiriferous pyrite (sample U-73) yielded 

1-4 percent of the total gold to solution. With these materials, the 

initial solution was followed by loss of gold from solution, suggesting an increase in acidity with time and oxidation of the thiosulfate ion. The data on the auriferous jasperoid (sample D-344), 

showing a high maximum of 63 percent dissolved at pH 2.36, 

are not understood by the authors. 

In summary, gold is dissolved by dilute sodium thiosulfate solutions at pH values greater than 5. The thiosulfate ion is present 

in environments high in .sulfur content but generally in very low 

concentrations. Many bacteria oxidize thiosulfate and are present 

in most surficial environn1ents. Gold thiosulfate is not appreciably 

absorbed by plants. 

GOI.D THIOCYANATE 

GEOCHEMISTRY OF THE THIOCYANATE ION 

Gold is readily dissolved in the presence of the thiocyanate ion, 

especially in acid solutions. It is, however, difficult to find any 

discussion of thiocyanates in geochemical or mineralogical literature except that of juliem.te, a hydrated sodium cobalt thiocyanate 
30 GEOCHEMISTRY OF GOLD IN THE WEATHERING CYCLE 

TABLE 15.- Solubility of ore gold in 0.0124 M sodium thiosulfate solution 

adjusted to various pH values 

Gold (ppm) in solution Maximum Percentage of pH adjusted gold pH with- After After After dissolved gold dissolved 

7 days 14 days 21 days (j!g) from ore 

Sample No. EM-534-9-10. Gold 25 ppm 

5.7 Not adjusted ..................... 0.06 0.08 0.1 25 10 

2.35 Diluted H:!S04 .................. . 1 .08 .12 30 12 

4.10 .......... do .............................. . 06 .06 .12 30 12 

10.28 Diluted N aOH solution ... .04 <.04 .06 15 6 

Sample No. GBG26. Gold 162 ppm 

9.14 Not adjusted ..................... 0.24 0.08 0.12 60 3.7 

2.6 Diluted H 2S04·················· .08 <.04 <.04 20 1.2 

4.9 .......... do .............................. .08 <.04 <.04 20 1.2 

10.6 Diluted N aOH solution ... .2 .3 .2 75 4.6 

Sample No. C-2. Gold 120 ppm 

8.92 Not adjusted ..................... 0.16 0.24 0.20 60 5 

2.55 Diluted H2S04·················· .5 .5 .26 125 10.4 

4.90 .......... do .............................. . 5 .16 .4 125 10.4 

10.95 Diluted N aOH solution ... .1 .56 .4 140 11.7 

Sample No. U73. Gold 92 ppm 

4.63 Not adjusted ..................... 0.12 <0.04 <0.04 30 3.2 

2.60 Diluted H2S04·················· .14 <.04 <.04 35 3.8 

7.10 Diluted N aOH solution ... .08 <.04 <.04 20 2.2 

9.80 .......... do .............................. <.04 <.04 <.04 <.04 

Sample No. D344. Gold 130 ppm 

9.30 Not adjusted ..................... 0.14 0.3 0.14 35 2.7 

2.36 Diluted H2S04 .................. 1.2 2.2 3.3 825 63 

4.90 .......... do .............................. .32 .5 .32 100 7.7 

10.60 Diluted N aOH solution ... .12 .06 .08 30 2.3 

SAMPLE DESCRIPTIONS 

EM-534-9-10. Fines from bedrock crevices at bottom of soil profile from Empire, Colo. Native 

gold with no observed sulfides. Bedrock is biotite gneiss containing only 0.04 ppm gold. 

GBG26. Volcanic breccia of phonolite-latite composition containing gold telluride and minor anti- mony and silver, SW~ sec. 20, T. 15 N., R. 69 W., Cripple Creek district, Teller County, Colo. 

Collected by A. H. Koschmann and obtained from G. B. Gott. 

C-2. Silicified carbonaceous dolomitic limestone, minor sulfides, arsenic, and mercury from Carlin 

gold mine, Lyman district, Eureka County, Nev. Collected by Ralph L. Erickson. 

U73. Massive pyrite from vein in the Tenth Legion mine, Empire district, Clear Creek County, 

Colo. Collected by Maurice Chaffee. 

D344. Massive gray jasperoid replacing dolomite; minor pyrite and hematite. Sec. 26, T. 14 S., R. 11 W., Drum Mountains, Juab County, Utah. Collected by J. H. McCarthy, Jr. 

mineral which has been found with cobaltian wad in a white schist 

at Chamibumba, near Kambove, Katanga, Belgian Congo ( Palache 

and others, 1951). One must turn to the literature on biochemistry 

to find any other discussion of thiocyanates. White, Handler, 

Smith, and Stetten (1959, p. 876) stated: "Thiocyanate is widely 

distributed in nature and occurs normally in blood, saliva, and the 

urine. Thiocyanate occurs in appreciable concentrations in many 

plants, such as members of the Brassica genus (cabbages) and 

Umbelliferae (carrot family). Of added significance is the occurrence of substances which may be transformed into thiocyanates 
GOLD THIOCYANATE 31 

by the mammalian organism. These include the isothiocyanates, 

such as mustard oil, the organic nitrites, and the widely distributed cyanogenetic glycosides." Perhaps the most convincing evidence for the existence of thiocyanate in nature is the organism 

Thiobacillus thiocyanoxidans which obtains its energy by oxidation of thiocyanate to sulfate, carbonate, and ammonia - its 

growth on organic media is not observed unless thiocyanate is 

present (Zajic, 1969, p. 59). 

FORMATION OF GOLD THIOCYANATE 

The relatively low oxidation potential for gold in the presence 

of the thiocyanate ion (table 1) suggests that dissolved oxygen 

in water would be an adequate oxidant. That this is true is illustrated in data on the solution of leaf gold in 0.1 M thiocyanate 

solutions adjusted to various pH values (table 16). Solution of 

the gold was greatest at pH 2.5 but was not particularly impressive at any pH. 

A similar experiment with four gold ores showed the same 

response to pH (table 17). Much of the gold ( 42-66 percent) 

was dissolved from ores containing free native gold, but gold in 

massive pyrite and in gold telluride was not attacked. 

TABLE 16.- Solubility of leaf gold in 0.1 M potassium thiocyanate solutions 

ad jus ted to various pH values 

Dissolved gold (mg/1) 

pH After After After 7 days 14 days 21 days 

2.5 0.3 0.7 1.0 

5 .14 .34 .4 

7 <.04 0 .2 

9 <.04 .08 .16 

Potassium thiocyanate in various concentrations was added to 

the average input and pregnant leach solutions similar to those 

used in the leaching and recovery of copper in Southwestern 

United States porphyry copper deposits (table 18). The pH was 

varied from 2.5 to 9, although the ferric iron was precipitated 

at pH of 5 through 9. The data show that the solution of gold 

in the pH 2.5 solutions is considerable and that it is a function 

of the concentration of ferric iron and thiocyanate. It is interesting, perhaps, to note that the presence of as little as 0.0001 M 

thiocyanate ion in both the input (low ferric ion concentration) 

and pregnant (high ferric ion concentration) solutions resulted 

in the solution of 2 mgjl gold during a period of 2 weeks. This 

amounts to the solution of 0.24 ounce gold per 1,000 gallons of 

leach solution to which has been added only 1 ounce of sodium 
32 GEOCHEMISTRY OF GOLD IN THE WEATHERING CYCLE 

TABLE 17. -Solubility of ore gold in 0.1 M potassium thiocyanate solutions 

adjusted to various pH values 

Adjusted 

Dissolved gold (mg/1) Maximum 

dissolved gold Percentage of gold pH After After After dissolved from ore 7 days 14 days 21 days (,_,g) 

Sample No. EM-534-9-10. Gold 25 ppm 

2.5 0.24 0.6 0.66 165 66 

5 .06 .3 .32 80 32 

7 .08 .2 .2 50 20 

9 <.04 .1 .1 25 10 

Sample No. GBG26. Gold 162 ppm 

2.5 <0.04 0.16 0.12 40 2.4 

5 <.04 .12 .12 30 1.8 

7 <.04 .10 .10 25 1.5 

9 <.04 .08 .06 20 1.2 

Sample No. U75. Gold 79 ppm 

2.5 <0.04 <0.04 <0.04 0 0 

5 <.04 <.04 <.04 0 0 

7 <.04 <.04 <.04 0 0 

9 <.04 <.04 <.04 0 0 

Sample No. D3U. Gold 130 ppm 

2.5 1.9 2.2 2.2 550 42.3 

5 .2 .6 .6 150 11.5 

7 .2 .5 .6 150 11.5 

9 .14 .5 .6 150 11.5 

SAMPLE DESCRIPTIONS 

EM-534-9-10. Fines from bedrock crevices at bottom of soil profile from Empire, Colo. Native 

gold with no observed sulfides. Bedrock is· biotite gneiss containing only 0.04 ppm· gold. GBG26. Volcanic breccia of phonolite-latite composition containing gold telluride and minor anti- mony and silver. SWI,4 sec. 20, T. 15 N., R. 69 W., Cripple Creek district, Teller County, Colo. U75. Massive pyrite from dump of Atlantic shaft, Empire district, Clear Creek County, Colo. D344. Massive gray jasperoid replacing dolomite; minor pyrite and hematite. Sec. 26, T. 14 S., R. 11 W., Drum Mountains, Juab County, Utah. Collected by J. H. McCarthy, Jr. 

thiocyanate. The marked mobility of gold thiocyanate observed 

in experiments with 114 minerals and rocks (table 4) combined 

with the solubility of gold in acid thiocyanate solutions makes 

this an attractive complex. The presence of the microorganism, 

Thiobacillus thiocyanoxidans, however, would make difficult to 

impossible the retention of even this low (0.0001 M) concentration of thiocyanate in the system. 

GOLD CYANIDE 

GEOCHEMISTRY OF THE CYANIDE ION 

Naturally occurring cyanides have their origin in biosy~tems. 

Some 70 years ago Lungwitz ( 1900, p. 501) speculated on the 

possibility of cyanide being the complex that carried gold in solution and into plants. He abandoned cyanide as a possibility 

because he could not find any evidence of its being formed in the 

decomposition of plant residues. He did maintain that vegetation 
TABLE 18.- 1'he interactions of the concentration of CNS-1 and H+1 on the solubility of gold in coppe'r leach solutions 

Average porphyry copper Input sulfate solution Output (pregnant) sulfate .solution leach solutions F~+3. -~!+a. C!.!+2-:-.:fJ.! g/! each Ft::·-\ Ai:~, Cu·::_z g/i t!ach 

Add {Normality 0.0001 0.0005 0.001 0.005 0.01 0.02 0.0001 0.0005 0.001 0.005 0.01 0.02 

CNS-1 of 

as KCNS • CNS--1. 

ppm of 5.8 29 58 290 580 1160 5.8 29 58 290 580 1160 

CNS-1. 

Parts per million of gold in solution after 2 weeks of contact with gold 

7.7 112.8 I 29 38.7 84.7 2.0 8.8 13.6 36 73 104 

.5 .4 8.1 18.7 48.4 .3 .5 .7 2.4 13 19 .......................... 1.5 1.2 4.6 ............................. .. ..................... ............................. .2 1.5 1.7 ............................ .4 .6 1.0 ............................. .. ......................... ............................ .08 .2 .5 

2.5 pH -------------------------------~1.9 

~ ~~~~~~::::::::::::::::::::::::::: ___ :~--------+-------------+ 9 pH.------------------------------

~ 

0 

t"'4 

t:1 

(j 

~ 

> z 

1-4 

t:1 

t_:l:j 

~ 

~ 
34 GEOCHEMISTRY OF GOLD IN THE WEATHERING CYCLE 

must be responsible for the solubility of gold in surface waters 

and left the identification of cyanide in vegetable residues to 

future researches. By now, hundreds of reports have been published showing that plants, fungi, and arthropods produce cyanoglycosides, which on decomposition yield hydrogen cyanide. 

Perhaps it is now appropriate to consider, as Lungwitz did, the 

possibility that the cyanide complex of gold is the effective mobile 

form of gold in soils. 

Twenty years after the discovery of hydrocyanic acid, Bohn 

(in Dillemann, 1959, p. 1050) in 1801 isolated cyanides from an 

aqueous distillate of Prunus amygdus. Amygdalin was crystallized 

in a pure state in 1830, a cyanoglycoside that is hydrolyzed by an 

enzyme to two molecules of glucose, one molecule of benzaldehyde, 

and one molecule of hydrogen cyanide (Dillemann, 1959, p. 1050; 

Armstrong and Armstrong, 1931, p. 68). The toxic character of 

cyanoglycosides was known to the early Egyptians who prepared 

a poisonous liquor by boiling peach leaves in water (Latimer and 

Hildebrand, 1951, p. 296). 

Hydrogen cyanide derived from plant tissue is a product of 

the hydrolysis of glycosides containing a nitrile group. Eleven 

naturally occurring cyanoglycosides have been identified (Conn 

and Butler, 1969, p. 47). Plants containing cyanoglycosides also 

contain enzymes that hydrolyze the glycoside. These enzymes are 

separated from the cyanoglycosides by cell walls, however, so that 

hydrolysis and formation of hydrogen cyanide occurs when the 

plant is damaged or dies. The enzymes listed in table 19 are probably mixtures of enzymes rather than a single species. Thus, in 

the hydrolysis of linamarin, a ,8-glucosidase hydrolyzes the glycoside to cyanohydrin and sugar, and this reaction is followed by 

the catalytic action of oxynitrilase to yield acetone and hydrogen 

cyanide. 

Plants producing cyanogenic glycosides are of common occurrence. About 1,000 species, representing 90 plant families and 

TABLE 19.- Some cyanoglycosides, associated enzyme source, and hydrolysis 

products 

Glycoside Enzyme Hydrolysis pro::lucts 

Amygdalin ................... Emulsin ........................ Almonds; peach Glucose, benzaldehyde, 

roots, leaves ; hydrogen cyanide. and stone fruit kernels. Dhurrin ......................... Dhurrinase ................... Sorghum and Glucose, p-hydroxybenzaldehyde millet. and hydrogen cyanide. Linamarin .................... Linamarase .................. Flax, white Glucose, acetone, hydrogen 

clover. cyanide. Phaseolunatin ............. .Amygdalase ................. Lima beans................... Do. Prunasin ............................. do ............................. Prunus sp., Glucose, benzaldehyde, many Rosaceae. hydrogen cyanide. 
GOLD CYANIDE 35 

about 250 genera, produce cyanogenic glycosides. Because of the 

toxic qualities of these plants, detailed studies have been given 

to plants of agricultural importance. Thus, sorghum (Sorghum 

vulgare) produces the cyanogenic glycoside dhurrin and the enzyme dhurrinase, which hydrolyzes the glycoside to give hydrogen 

cyanide as one of the products of hydrolysis. A large number of 

sorghum varieties are grown in the United States, and their toxicity is variable. Couch, Briese, and Martin (1939, p. 160, 161) 

examined 33 sorghum varieties grown at one or more of six 

localities in the Great Plains and found that all of them contain 

from 12 to 75 mg HCN/100 g-sufficient to be toxic to animals. 

Although the amount of hydrogen cyanide liberated by sorghums 

is a function of the variety and of the seasonal climate, perhaps 

a more important factor is the amount of the enzyme dhurrinase 

produced by the plant. Only a slow evolution of hydrogen cyanide 

was determined in many sarnples of sorghum, found deficient in 

dhurrinase (Couch and Briese, 1940, p. 419). 

The cyanoglycoside content of a plant varies with the season 

and with the plant part. In 2--year-old peach seedlings, the amygdalin content is greater in the leaves than in the branches and 

twigs and is highest in the roots. Trees sampled in the fall of the 

year had a higher concentration of amygdalin in the above-ground 

parts and a lower concentration in the roots than those sampled 

in the spring (Ward and Durkee, 1956, p. 421). Patrick (1955, 

p. 467) observed that certain microorganisms in the soil utilized 

the amygdalin from peach trE~e roots with liberation of hydrogen 

cyanide. 

Varieties of flax differ n1arkedly in their susceptibility to 

Fusarium wilt (a disease caused by Fusarium lini), and their 

susceptibility is an inverse function of their linamarin content. 

The excretion of significant quantities of hydrogen cyanide, the 

byproduct of hydrolysis of linamarin, into the soil from the root 

system of the resistant varieties of flax creates unfavorable conditions for the growth of Fusarium lini. The resistant flax plants 

generated as much as 25.3 to :37.6 mg HCN per plant when grown 

in nutrient solution (Timonin, 1941, p. 404). 

Although the fungus Fusar1:um lini is susceptible to poisoning by 

hydrogen cyanide, Marasmius oreades, a fairy-ring fungus, produces cyanogenic compounds that damage the roots of Festuca 

rubra (red fescue), Poa pratensis (Kentucky bluegrass), and 

Agrostis tenuis (colonial bent). The absorption of hydrogen cyanide in 5 ml of sodium picrate solution suspended above a 52-dayold culture of the fungi amounted to 50 to 70 ppm, or 0.25 to 
36 GEOCHEMISTRY OF GOLD IN THE WEATHERING CYCLE 

0.37 mg, HCN each 24 hours. Hydrogen cyanide was also detected 

above soil infested with the fungi (Filer, 1965, p. 574). 

Aposematic (having special means of defense) species of millipedes, such as Apheloria corrugata (Wood) and Pachydesmus 

crassiculis (Wood), excrete a defensive fluid containing hydrogen 

cyanide and benzaldehyde (Blum and Woodring, 1962, p. 512; 

Eisner and others, 1963, p. 1219). The secretion is discharged 

from paired serially arranged glands. Each gland contains two 

compartments - an undissociated cyanogenic compound is stored 

in one compartment and a catalytic compound is stored in the 

other compartment. When disturbed the animal excretes the contents of both compartments .and hydrolysis is initiated at the 

instant of discharge (Eisner and others, 1963, p. 1218). 

Hydrocyanic acid is released from crushed tissues of all stages 

in the life-cycle of species of Zygaeninal (burnets and foresters 

moths). The pupae and freshly emerged specimens of these aposematic moths release up to 0.2 percent HCN on a fresh weight 

basis and about 4 percent HCN from the crushed eggs (Jones 

and others, 1962, p. 53). 

In summary, many organisms produce compounds that yield 

hydrogen cyanide on hydrolysis, including species of plants, fungi, 

and animals. Reviews of the voluminous literature are given by 

Conn (1969), Dillemann (1959), and Tschiersch (1967). 

FORl\IATION OF GOLD CYANIDE 

Both the foregoing and the information given in the section 

on tests for cyanide in some native and naturalized plants by 

H. T. Shacklette demonstrate that hydrogen cyanide can frequently be present in soils in concentrations adequate to facilitate 

the solution of gold. Further, data given in another section indicate that the gold cyanide produced in soils is likely to be taken 

up by plants. 

We have dissolved leaf gold in suspensions of macerated flax, 

peach roots, mountain-ash, chokecherry, desert apricot, and mountain mahogany (tables 20, 21). We have also dissolved 10 percent 

of the gold in 25 g of a subsoil (EM-534-8-9, 30 ppm gold) immersed for 5 days in 250 ml of water at pH 7.5 containing 10 g 

of macerated peach roots (table 21). The addition of the enzyme 

emulsin sufficient to assure the hydrolysis of the amygdalin in 

the peach roots did not appreciably increase the solution of gold. 

Although the gold cyanide complex may be present in soils and 

may be relatively mobile, as shown both by its ready entrance 

into plant cells (table 6) and by the low percentage of rocks or 

ore minerals that significantly removed gold cyanide from solution 
GOLD CYANIDE 37 

TABLE 20.- Solution. of leaf gold by native cyanogenic plants macerated in 

plant/water slurries 

[Plants identified by H. T. Shacklette] 

Collection locality 

State County Plant species/part Gold content (ppm) of slurry 

Desert apricot ( Prtmm eriogyna) 

Nevada·---------------------·----------· 1. Leaves, twigs·-·-·---····----·-···--··--··--· 14.3 

Humboldt ______ 2. __________ do .......... -------··-----··--·-------·----- 24.2 

Colorado ________ La Plata________ 3. Root bark·-··-··---·--·-····--·---·------------ 4.0 

Coville serviceberry (Amelanchier utahensis covillei) 

Nevada __________ Humboldt ...... 1. (a) Leaves, twigs _______________________ _ (b) Root bark _____________________________ _ 

Lander........... 2. Leaves, twigs----------------··----·----·---- Utah. ______________ Duchesne ....... :3. Leaves, twigs, berries, baric. .... . 

Colorado ........ Grand. ____________ 4. Leaves, berries, bark .. ·-··---·---------

Mountain mahogany (Cercocar{ms montanus Raf.) 

17.6 

<.04 

9.9 

2.2 

13.2 

Nevada .......... Lander___________ 1. Leaves, twigs------·----------···-·------·--- 5.5 

Elko ________________ 2. Leaves, bark..·--·--·-------···-·---·---··-·- .76 

Utah _______________ Duchesne _______ 3. Twigs, bark_·--·----------·-····--··-··--···-· .36 

Wasatch________ 4. Bark.______________________________________________ .34 

Nevada __________ Lander ___________ 5. __________ do__________________________________________ 3.3 

______ do______________ 6. Root bark. ... --------------------·-----------·- 3.3 

Arizona _________ y a vapaL_______ 7. __________ do. ______ ·-----·------ __________ ---·--······ .86 

Mountain ash (Sorbm americana Marsh) 

Colorado ........ Grand _____________ 1. Leaves, twigs ... --------------·------·----·-· 3.3 

Chokecherry (Prunus virginiana) 

Utah ............... Wasatch________ 1. Cherries, leaves, bark.________________ 1.5 

Duchesne_______ 2 ........... do·--------·------·----------···-----·------ 19.8 

Nevada .......... Elko________________ 3. Root bark. ......... -----------·---·---------··· 18.3 

Blue flax (Linum lewisii) 

Nevada .......... Lander___________ 1. Seeds, tops .......... -----------------------·--· 3.3 

TABLE 21.- Solubility of gold in slurries of macerated peach roots, mace1·ated 

peach roots plus emulsin, and in emulsin-plus-water mixtwres 

[Gold values in. parts per million; pH is 5.5±0.5] 

Demineralized water 

Leaf gold-----·---·----------<0.02 Particulate 

gold in 

25 g soiL ................. <.02 

Macerated peach roots slurry 

10 g 10 g + emulsin 

45.6 31.5 

.16 .3 

Emulsin + water 

10 g 50 g 

<0.02 <0.02 

<.02 <.02 

(table 4), one must not assume a state of lasting mobility. The 

cyanide ion is itself a transient component of soils. It is hydrolyzed 

by acid or alkaline solutions. The reaction of CN- +2H20~NHs 

+HC02- 1 goes to completion at 140°C in dilute hydrochloric acid. 

At ambient soil temperatures and pH the reaction would be 
38 GEOCHEMISTRY OF GOLD IN THE WEATHERING CYCLE 

expected to proceed slowly. In the presence of a catalyst the hydrolysis of CN- 1 could be rapid. Certain parasites produce the 

enzyme rhodanase, which catalyzes the reaction of CN- 1 with go 

or S20a-2 to produce CNS- 1• Rhodanase is widely distributed in 

nature (Jones and others, 1962, p. 53). Rhodanase in the liver 

catalyzes sulfur transfer from mercaptopyruvate to cyanide to 

form thiocyanate (CNS- 1 ) and pyruvate. In the normal decomposition of plant and animal residues, it is to be expected that 

organisms will be present and capable of destroying cyanides. 

In conclusion, ample hydrogen cyanide is formed in the soil -

by hydrolysis of cyanogenic plants, animals, and fungi - to result 

in solution of gold in an oxygenated environment. The gold cyanide 

thus formed is absorbed by plants. This process provides a definite 

pathway for the migration of gold in soils. The ready decomposition of the cyanide radical by hydrolysis and by enzymic alteration 

limits the mobility of gold cyanide. 

TESTS FOR CYANIDE IN SOME NATIVE AND 

NATURALIZED PLANTS OF THE UNITED STATES 

By HANSFORD T. SHACKLETT£ 

Plants were sampled along highways and in mining districts 

on a route extending from the Front Range of the Rocky Mountains to north-central Nevada, west-central Arizona, north-central 

New Mexico, and southwestern Colorado (fig. 2) to determine 

the abundance of plant species that are cyanogenic and can, therefore, potentially influence the geochemical mobility of gold. Some 

additional species were sampled at scattered locations throughout 

the United States. A wide variety of native and naturalized plants 

were included for the purpose of testing cyanide in members of 

plant families that were known to contain many cyanogenic species 

(for example, the rose family, Rosaceae), as well as species in 

families not known to have cyanogenic tendencies. The specie3 

that were invariably cyanogenic were soon discovered; thereafter, 

the testing was restricted mostly to these species in different locations that were known or suspected to have anomalous amounts of 

gold in the substrates. The climate at many of these locations was 

so dry that only sparse vegetation was present, but testing was 

extended to these sites to determine if cyanogenic plants formed a 

significant part of this vegetation. 

Field studies were made during the latter half of August and 

the first week in September of 1969, a season when the weather 

is generally hot and dry at most of the study locations. The type 

of vegetation present at each study site was noted by using the 

method of classification and the mapped distribution of potential 
TESTS FOR CYANIDE IN PLANTS 

Reno 

lfs?ARSON 

~ITY 

,ONEVADA 

0'" ' ' ' Las '\ Vegas 

N 

1 

' 0 ' 

0 

I 

0 

1. Pine-Douglaa<-fir forest 

2. Western spruce-fir forest 

100 

I I I 

200 

1

1 I 

300 MILES 

I 

I 100 200 300 400 500 KILOMETERS 

VEGETATION TYPE 

7. Douglas-fir forest 

8. Mountain mahogany-oak scrub 

3. Alpine meadows and barren 9. Great Basin Sagebrush 4. Sagebrush steppe 10. Grama-tobosa shrub steppe 

5. Saltbush-greasewood 11. Chaparral 

6. Juniper-piiion woodland 12. Southwestern spruce-fir forest 

FIGURE 2.- Location of sites where plants were sampled for cyanide 

testing, and the vegetation type or types present at the sites. 

39 

natural vegetation as given by Kuchler (1964; fig. 2). Reference 

specimens of the species to be tested for cyanide were identified 

and later preserved in a U.S. Geological Survey herbarium in 

Denver. 

The plant samples for cyanide testing included stems, stem bark, 

leaves; roots, root bark, flowers, fruits, or the entire plant, as was 

appropriate for the species. Most samples were tested at the sites 

where they were collected by using facilities provided in a mobile 

laboratory. The samples were macerated in test tubes with a narrow stainless steel spatula or with a knife blade. A few drops of 

chloroform were added to the macerated material, then a strip 

of picric-acid paper was suspended in the test tube by means of 

a tight-fitting cork. The prepared samples were kept in a warm 
40 GEOCHEMISTRY OF GOLD IN THE WEATHERING CYCLE 

place for 24 hours before the color reactions of the test papers 

were noted. The samples were prepared and tested for cyanide 

by H. W. Lakin, G. C. Curtin, and C. W. Gale III. 

The picric-acid test paper was prepared and calibrated in the 

U.S. Geological Survey laboratories in Denver before field studies 

were undertaken. Discs of filter paper were dipped in a solution 

of 1 percent picric acid and 10 percent sodium carbonate, then 

dried, cut into strips, and the strips stored in a tightly corked 

bottle. These strips were calibrated for their color reactions when 

suspended over potassium cyanide solution of different concentrations, as shown in table 22. 

TABLE 22. -Approximate cyanide concentrations in solutions, as indicated by 

color reactions of picric-acid paper suspended over the solutions 

Cyanide concentrations (in ppm) in the solutions 

Color reactions produced 

Interpretations of color as an indicator of HCN (hydrogen 

cyanide) concentrations 

0.1 and less ................. Yellow (no change) ....... Negative ( -1). 

0.2 ................................. Yellow orange ................. HCN possibly present ( +1 ?) . 

0.5-l.O .......................... Orange ............................. Positive presence of HCN (+1). 

2.0 ................................. Dark orange .................... Strong positive ( +2). 

4.0-8.0 .......................... Red ................................... .V ery strong positive ( +3). 

16.0 and greater ........... Dark red .......................... Maximum color reaction, 

reagent consumed ( +4). 

Results of the cyanide tests are given in table 23. One or more 

parts of 150 species, distributed among 49 plant families, were 

tested. Among these species the distribution of approximate cyanide concentrations in 186 samples of one or more plant parts, as 

classified in table 22, is as follows : No HCN detected, 127 samples; 

HCN possibly present, 17 samples; positive presence of HCN 

(0.5-1 ppm), 11 samples; strong positive (2 ppm), 11 samples; 

very strong positive ( 4-8 ppm), 15 samples; and maximum color 

reaction (16 ppm and greater), 5 samples. In summary, of the 

150 species tested, 116 species (78 percent) contain no detectable 

HCN in any part sampled, 15 species (10 percent) possibly contain low concentrations of HCN in at least some parts, and 19 

species (12 percent) positively contain HCN in one or more of 

their parts. 

The cyanogenic ability of some members of the rose family 

(Rosaceae) was confirmed by our studies; furthermore, positive 

tests for cyanide were obtained of some species in this family in all 

but two vegetation types. The important cyanogenic woody plants 

in the Rocky Mountains are wild plum, chokecherry, mountain-ash, 

mountain mahogany ( CeTcocarpus rnontanus), and smooth shadbush, whereas in the Basin and Range province mountain mahogany (CercocaTpus ledifolius), desert peach, chokecherry, cliffrose, 
TESTS FOR CYANIDE IN PLANTS 41 

and Utah shadbush were of common occurrence. Judged from. the 

nature of their habitats, these trees and shrubs probably have 

extensive and deep-penetrating roots. If these roots tend to secrete 

hydrogen cyanide to the extent reported for flax roots (Timonin, 

1941, p. 407), large amounts (but probably low concentrations) of 

this compound are available to solubilize gold that may be in the 

soil near them. 

Wild flax (family Linaceae) was the only herbaceous plant 

tested that invariably contained significant concentrations of 

cyanide. The blue-flowered flaxes ( Linum usitatissimum and L. 

lewisii) produce hydrogen cyanide abundantly, whereas this compound was not detected in samples of the yellow-flowered flax 

(Linum medium var. texanum). Wild flax is widespread in the 

regions studied, but, compared with trees and shrubs, it probably 

is of only minor importance in adding cyanide to the soil because 

of its small root systems. Tests of herbaceous plants in other families indicated that either no cyanide or a very low concentration 

of cyanide is contained in these herbs. Tests of stonecrop (Sedum 

stenopetalu1n, family Crassulaceae), an herb, were inconclusive; 

usually no cyanide was found, but tests of some samples that 

included roots of the plant indicated that cyanide possibly was 

present. Profuse growths of mycorrhiza commonly surround these 

roots, and the cyanide that apparently was found may have been 

produced by the fungal mycelia. Cyanide was not found in Marchantia polymorpha (family Marchantiaceae), the only bryophyte 

that was tested. 

Species of cyanogenic plants occur in most of the vegetation 

types listed in figure 2. The extent of these types in the United 

States was mapped by Kuchler (1964). The areas not examined 

in the present study can be evaluated for the probable occurrence 

of cyanogenic species by extrapolating the following records of 

these species in the various vegetation types onto Kuchler's 

vegetation map. Some vegetation types too small in extent to be 

indicated on this map occur within the major vegetation areas, 

therefore some species in the following list are not characteristic 

species of the vegetation types under which they are listed. Some 

nondominant species reported in a particular vegetation type do 

not necessarily occur throughout the range of that type. The 

questionable cyanogenic species are listed as " ( +1 ?) ." 

1. Pine-Douglas-fir forests- Mountain mahogany ( Cercoca1·pus 

montanus), wild plum, smooth shadbush, chokecherry, wild 

flax, red-berried elder ( +1 ?) , stonecrop ( +1 ?) , columbine 

(+1 ?), and Rocky Mountain clematis (+1 ?). 
TABLE 23.- Results of tests for hydrocyanic acid (HCN) in selected species of plants 

[Results are expressed as color reactions of picric acid paper to macerated plant parts as follows: Yellow, negative (-1) test; yellow-orange, HCN possibly 

present <+1 ?) ; orange, positive presence of HCN (+1); dark orange, strong positive (+2>; red, very strong positive (+3); dark red, maximum color reaction, reagent consumed ( +4). Sp. indet., species not determined. Common names of plants omitted where unknown] 

Locality 

Family Scientific and common name Plant parts tested State County 

Aceraceae ................... Acer glabrum Torr., mountain maple ............................................... Leaves, stem bark, fruits ...................... Colorado .................. Jefferson Utah ......................... Wasatch Anacardiaceae ........... Rhus trilobata Nutt., skunkbrush ....................................................... Leaves, stems ........................................... New Mexico ........... Santa Fe Asclepiadaceae .......... Asclepias speciosa Torr., showy milkweed ................................................. do ....................................................... Colorado .................. J efferson Leaves, seed pods .................................... Utah ......................... Duchesne 

Berberidaceae ............ Berberis haematocarpa Wooton, red mahonia ................................. Leaves ........................................................ New Mexico ........... Santa Fe repens Lindl., Oregon-grape ........................................................ Leaves, stem bark ................................... Colorado .................. Jefferson Betulacea.e .................. Betula occidentalis Hook., river birch ......................................................... do ....................................................... Utah ......................... Wasatch Boraginaceae ............. Cryptantha virgata (Porter) Pays., miner's candle ..................... Entire plant .............................................. Colorado .................. Clear Creek Lithospermum ruderale Dougl., puccoon .......................................... Leaves, stems ........................................... Nevada ..................... Lander 

Mertensia ciliata (James) G. Don, tall mertensia ......................... Leaves, flowers ......................................... Colorado .................. Clear Creek Cactaceae ................... Opuntia ( ?)arborescens Engelm., cholla .......................................... Root bark, flower buds, fruits ............. New Mexico ........... Santa Fe Capparidaceae .......... PoZanisia trachysperma T. & G., clammy-weed ............................... Leaves, stems, flowers ............................ Colorado .................. Jefferson Caprifoliaceae ........... Lonicera involue1·ata (Rich.) Spreng., bush honeysuckle ............ Leaves, stems ................................................. do ........................ Clear Creek Fruits ............................................................... do ........................ Grand 

Sambucus coerulea Raf., elder ............................................................. Leaves, stems, flowers, fruits .............. Utah ......................... Wasatch Leaves, stem bark, fruits ...................... Nevada .................... Lander 

racemosa ssp. pubens (Michx.) House, red-berried elder .... Leaves, stems, fruits .............................. Colorado .................. Ciear Creek Symphoricarpos oreophilus Gray, snowberry ................................... Leaves, stems, roots ..................................... do .............................. do ........... . vaccinoides Rydb., snowberry ...................................................... Leaves, stems ................................................. do ........................ Grand 

sp. indet ....................................................................................................... do ....................................................... N evada."=·············· Eureka Celastraceae ............... Pachystima myrsinites (Pursh) Raf., mountain-lover ........................... do ....................................................... Colorado .................. Grand Chenopodiaceae ......... Atriplex confertifolia (Torr. & Frem.) S. Wats., shadscale Leaves, stems, stem bark. ..................... Utah ......................... Duchesne saltbush. 

torreyi (Wats.) Wats., saltbush .......................................................... do ....................................................... Nevada .................... Lander 

Grayia spinosa (Hook.) Moq., spiny hopsage ................................. Leaves, young stems, stem bark ............... do .............................. do ........... . Halogeton glomeratus (Bieb.) Mey., halogeton .............................. Leaves, stems ........................................... Utah ........................ Duchesne 

Sarcobatus vermiculatus (Hook.) Torr., greasewood .............................. do ............................................................. do .............................. do ........... . Compositae ................ .Achillea lanulosa Nutt., yarrow .......................................................... Leaves, stems, roots ............................... Colorado .................. Ciear Creek Arnica cordifolia Hook., heart-leaved arnica ................................... Entire plant .................................................... do .............................. do ........... . 

Arte:;/:~:g~~:e;~f!,~ f!~!~i :::=~~~-~~:.·_·_·:_·_·::_·_·_·_·_·_·:_·_·_·_·:_·_·_·_·_·_·_·:_·_·_·::.·_·:_·_·_·_·_·.·.·.·.·.t::~:: :~:::: .. ~~~~--~~~~::::::::::::::::::::::~~~~~~d~::::::::::::::::~:t~~~eCreek arbuscula Nutt., sagebrush ........................................................... Leaves, leaf buds, stem bark ...................... do ........................ Grand Chrysothamnus viscidijlorus (Hook.) Nutt., rabbitbrush ............ Leaves, stems, stem bark, flowers ............ do .............................. do ........... . Grindelia squarrosa (Pursh) DunaJ., gumweed .............................. Leaves, stems, flowers ............................ Utah ......................... Duchesne 

Results of HCN tests 

-1 

-1 

-1 

-1 

-1 

+1? 

-1 

-1 

-1 

-1 

-1 

+1? 

-1 

-1 

-1 

-1 

-1 

+1? 

-1 

-1 

-1 

-1 

-1 

-1 

-1 

-1 

-1 

-1 

-1 

-1 

-1 

-1 

-1 

-1 

~ 

l:\J 

c;l 

t_xj 

0 

0 

~ 

t_xj 

a= 

~ 

UJ 

1-3 

~ 

~ 

0 

l'%j 

c;l 

0 

t'"4 

t1 

z 

1-3 

~ 

t_xj 

~ t_xj 

> 1-3 

~ 

t_xj 

~ ~ 

z 

c;l 

0 

.....: 

0 

t'"4 

t_xj 
Senecio sp. indet., golden ragwort .. -----·------············------------------·····-··--Leaves, roots, flowers _____________________________ Colorado .................. Clear Creek -1 

Solidago multiradiata Air., goldenrod·-···--·-----···----·------------------·----····Leaves, flowers-----------·········---------------·-····----··do ..... ---·--·········--··Jefferson -1 

speciosa Nutt., showy goldenrod ... ·--------·---·-·--···----··--·---·---------··· Leaves, stems, roots.·--·--------····-----·--·--------···do ........................ Clear Creek -1 Tragopogon dubius Scop., salsifY------·-··········--------------···············-··-·····Leaves, stems, roots, flowers .... --------····---··-do ....... _________________ Grand -1 

Cornaceae ................... Cornus stolonifera Michx., red-osier dogwood ................................. Leaves, stem bark ..... ---···----·--·-··········-----Utah ......................... Wasatch -1 

Fruits ...... ----········--·-------------······-···-··-·------··------do ........ ____ .................. do............ + 1? Crassulaceae ............... Sedum stenopetalum Pursh, stonecroP----··-·--------------··----···-···-----------Leaves, stems·-··-···--------------···---···········--·- Colorado .................. Clear Creek + 1? Cruciferae .................. Arabis holboelli Hornem., rockcress ................................................... Entire plant .. -----------------------------·----········-·----··do .............................. do............ -1 Erysimum asperum (Nutt.) DC., wallflower ............................................ do .............. : ......... ----···------············-·····----··do ____________ .................. do__________ -1 

Lesquerella fendleri (Gray) Wats., bladder-pod ............................. Leaves, stems, fruits .... ---------------·····------New Mexico ........... Santa Fe +1? 

intermedia ( W ats.) Heller, bladder-pod·----------·---············-------- .......... dO-------------···········-----··----······--·----·············do .............................. do ........... . -1 

Rorippa nasturtium-aquaticum (L.) S. & T., watercress ........... Leaves, stems---·-·······-----·------········-----------Utah .... ---------·--········-Wasatch -1 

Stanleya pinnata (Pursh) Britt., prince's plume .......................... Leaves, stems, seed pods .... ---·----···········Nevada ..................... Lander -1 

Eleagnaceae ............... Eleagnus angustifolia L., Russian-olive ..... ---------··-······-·······-------·-···-Leaves, stem bark, fruits ...................... Utah ..... ---·------·----··--·Duchesne -1 

Leaves. stems-------·····-··········--·-···--·····-·····Nevada ..................... Pershing -1 

Shepherdia argentea (Pursh) Nutt., buffaloberry ......................... Leaves, stems, stem bark. ......... --------------···-do ........................ Lander -1 

canadensis (L.) Nutt., buffaloberry ........................................... Leaves, root bark-----······------···············-···Colorado .................. Clear Creek -1 

Ephedraceae .............. Ephedra torreyana Wats., joint-fir.. .. ----------------------··············--------·····Stems .... -----------·------·····------------·------·-···· .. New Mexico . __ Santa Fe -1 

viridis Coville, Mormon tea ...... ·-----····---------------·-·····-······-··········--·Young stems. old stems ......................... Utah-----·-----···-----------Duchesne -1 

Stems----··········-----·-···---------··-······-·····-------···N evada .................... Humboldt -1 

Equisetaceae .............. EQuisetum arvense L., horsetail ............................................. -----------·-Leaves, stems.----···---····-··---------------·············---do ........................ Lander -1 

Ericaceae .................... Arctostaphylos pringlei Parry, manzanita .... ----·-·······--·-··········-·------Leaves, stem bark, fruits ...................... Arizona .................... Yavapai -1 

uva-ursi (L.) Spreng., kinnikinick. ........................................... Leaves, root bark------------········----·-··-···----Colorado .................. Clear Creek -1 

Chimaphila umbellata (L.) Bart., pipsissewa ................................. Leaves, stems---·---------------··------·····-··········-·----do ........................ Grand -1 

Petrospora andromeda Nutt., pine drops ......................................... Entire plant .... ------·--------···--···-···----·····--····--·--·do .............................. do ........... . +1? 

Pyrola vir ens Schweigger, wintergreen ..................... -------------------···· Leaves, roots .... -------·········-------------······-··-· ______ do ........................ Clear Creek -1 

Vaccinium myrtillus L., myrtle blueberrY--··-····------------------------·-----·Leaves, stems. roots ..................................... do .............................. do ........... . -1 

Euphorbiaceae .......... Euphorbia dentata Michx .... ---·----·······----·-···------·········--·-·······-···-··-·------·Entire plant ... -----------······---------------·-·············-do ........................ Jefferson -1 

Fagaceae ..................... Quercus gambellii Nutt., Gambel oak·---···-------------·····----·············------Leaves, stem bark, fruits ........ --·-····------Utah .... --------···-········-Wasatch -1 

palmeri Engelm., Palmer oak--------········-·-··-----------------···········----Leaves, stems, stem bark, fruits ........ Arizona ................... .Yavapai -1 

Gentianaceae ............. Gentiana calycosa Griseb., gentjan ................. ---------··--·-·······--------·-----Leaves, stems, flowers ............................ Colorado .................. Clear Creek -1 

Gentianella amarella ( L.) Boern., amarella .................................... Entire plant.·-·····-------·--···········--·········-···· ______ do .............................. do ........... . -1 

Swertia radiata ( Kell.) Ktze., monument plant---·-···------------······---Leaves, stems, roots ..................................... do .............................. do ........... . -1 

Leaves, fruits ..... ·--·-----------·-------·······-······-Utah ......................... Wasatch Gramineae ................. Distichlis stricta (Torr.) Rydb., saltgrass.---·-·-·······-·····-------··········· Leaves, stems·---·-·----------------··················--------do ........................ Duchesne 

-1 

-1 

Poa ampla Merr ., big blu.egrass ..... ----···---·--·-····--·········-----------------······ Entire plant .... ---------···---·-···-·······-··-····-·····Colorado .................. Clear Creek -1 

Grossulariaceae ........ .Ribes americanum Mill., wild gooseberrY.---------·-······················------- Leaves, fruits .... ---·----------·---··---·····----······· Utah ......................... Wasatch -1 aureum Pursh, golden currant .......... ------------··-···----·····-········-------Leaves, stem bark, fruits·--·----····-----·····------do .............................. do............ -1 

montigenum McClatchie, alpine prickly currant .................... Leaves, stems--······---·····-----······-··-··----------Colorado .................. Clear Creek -1 Juglandaceae .............. /uglans major (Torr.) Heller, Arizona walnut ............................. Leaves, stems, stem bark, fruits ......... Arizona ................... .Yavapai -1 

Leguminosae .............. Acacia greggii Gray, catclaw acacia ......... ------····-·-··-----------·-···--········Leaves, stems, stem bark ............................ do .............................. do ........... . 

Astragalus ceramicus Sheldon, milkvetch ......................................... Leaves, stems.·--····---·········------··-···········---New Mexico ........... Santa Fe sp. indet., milkvetch ................................................................................. do .. ·-----------··-···············---------·············------·do .............................. do ........... . Gleditsia triacanthos L., honey-locust. _________________ .............................. Leaves, stems.-------·--·--·-·-····-··················--Colorado .................. Jefferson 

-1 

+I? 

-1 

-1 

~ 

tz:j 

00 

~ 

00 

~ 

0 

~ 

0 

~ 

> z 

6 

tz:j 

z 

~ 

t-t 

> z 

1-3 

00 

~ 

~ 
TABLE 23.- Results of tests for hydrocyanic acid (HCN) in selected species of plants- Continued ~ 

~ 

Locality 

Family Scientific and common name Plants parts tested State County 

Results of HCN tests ~ 

t_:rj 

Leguminosae .............. Glycorrhiza lepidota (Nutt.) Pursh, wild liquorice ....................... Leaves, stems, fruits .............................. Utah ......................... Duchesne Lupinus argenteus Pursh, common lupine ....................................... Leaves, stems. root bark, nodules ....... Colorado ................. Clear Creek Liliaceae ..................... Allium cernuum Roth, wild onion ....................................................... Leaves, roots, bulbs ...................................... do .............................. do ........... . Veratrum californicum Dur., false hellebore ................................... Leaves, stems, fruits .................................... do ........................ Grand 

-1 0 

-1 0 

-1 :I: 

-1 t_:rj 

Linaceae ..................... Linum leu•is1~i Pursh, wild flax ............................................................. Seeds ........................................................... Nevada .................... Lander +I? == Leaves, stems ................................................. do .............................. do ........... . ~ 

+3 00 .......... do ....................................................... N ew Mexico ............ Santa Fe +3 t-3 

medium var. texanum (Planch.) Fern., yellow flax .............. Entire plant .............................................. Missouri.. ................. Madison Loasaceae ................... Mentzelia stricta Stevens, small white evening-star ...................... Leaves, stems ........................................... Colorado .................. Jefferson Loranthaceae ............ Phoradendron juniperinum Engelm., mistletoe ............................... Entire plant .............................................. New Mexico ........... Santa Fe 

-I ~ -I ~ +I? 0 

Malvaceae ................... Sphaeralcea coccinea ( Pursh) Rydb., copper mallow ................... Leaves, stems ................................................. do .............................. do ........... . -I ~ 

Marchantiaceae ........ Marchantia polymorpha L., Liverwort .............................................. Entire plant ............................................. Colorado .................. Grand 

Moraceae .................... Humulus lupulus L., wild hop-vine .................................................... Leaves, stems, flowers ............................ Utah ......................... Wasatch Myricaceae ................. Comptonia peregrina ( L.) Coult., sweet-fern ................................. Leaves, stems ........................................... Wisconsin ............... Douglas 

Onagraceae ................ Epilobium angustifolium L., fireweed ............................................... Leaves, stems, flowers, fruits ............... Colorado .................. Clear Creek 

-I ~ -I 0 -1 t"'4 -I 1:' 

Orchidaceae ................ Corallorhiza maculata Raf., spotted coral-root ............................... Stems, roots, flowers .................................... do .............................. do ........... . Papaveraceae ............. Argemone po!yanthemos (Fedde) G. B. Ownbey, prickly poppy.Leaves, stems, seeds ..................................... do ........................ Jefferson -1 ~ 

-1 z Pinaceae ..................... Juniperus communis L., common juniper ........................................ Leaves, root bark, fruits ............................. do ........................ Clear Creek -I t-3 

osteosperma (Torr.) Little, one-seed juniper ......................... Stems, root bark, fruits ........................ New Mexico ........... Santa Fe Pinus contorta Dougl., lodgepole pine ............................................... Leaves, stem bark, root bark ............... Colorado .................. Clear Creek +I? :I: -1 t_:rj 

edulis Engelm., Colorado pinyon ................................................. Leaves, young cones, roots ................... New Mexico ........... Santa Fe Polemoniaceae ........... Gilia sp. indet., gilia ............................................................................... Leaves, stems ................................................. do .............................. do ........... . 

Polemonium delicatum Rydb., Jacob's ladder ............................................ do ....................................................... Colorado .................. Clear Creek Polygonaceae ............. Eriogonum jamesii Benth ..................................................................... Leaves, roots, flowers ................................... do ........................ J efferson umbellatum Torr., sulphur-flower ......................................................... do ............................................................. do .......................• Clear Creek 

-1 

-1 ~ t_:rj -I 

-I > t-3 -I :I: sp. indet ............................................................................................ Leaves, stems, flowers ............................ Utah ......................... Wasatch Leaves, stems, stem bark. ..................... Nevada .................... Lander Oxyria digyna (L.) Hill, alpine sorrel.. ............................................ Leaves, flowers ......................................... Colorado .................. Clear Creek Rumex obtusifolius L., bitter dock ..................................................... Leaves, fruits ................................................. do .............................. do ........... . Ranunculaceae .......... Actaea rubra (Ait.) Willd., baneberry ....................................................... do ..................................................... Utah ......................... Wasatch 

-I t_:rj 

-1 ~ -1 ~ 

-1 z 

-1 ~ 

Aquilegia caerulea James, Colorado blue columbine ...................... Entire plant.. ........................................... Colorado .................. Clear Creek Clematis ligusticifolia Nutt., western virgin's bower .................... Leaves, fruits ................................................. do ........................ Jefferson pseudoalpinus (Kuntze) Nelson, Rocky Mountain clematis .. Leaves, stems, roots ..................................... do ........................ Clear Creek Delphinium barbeyi Huth, larkspur ................................................... Leaves, stems, flowers ................................. do .............................. do ........... . Thalict1·um fendleri Engelm., few-flowered meadowrue ............... Leaves, stems, fruits .................................... do ........................ Grand 

+I? 0 

-1 ~ 

+I? 0 

-1 t"'4 

-1 t_:rj 

Rhamnaceae ............... Ceanothus greggii Gray ......................................................................... Leaves, stems, stem bark ...................... Arizona .................... Yavapai Rosaeeae ..................... Amelanchier pumila Nutt., smooth serviceberry ............................ Leaves ........................................................ Colorado .................. Grand -1 

+I Stem bark, fruits .......................................... do .............................. do ........... . +4 
sanguinea ( Pursh) DC., serviceberry ....................................... Leaves, stems ........................................... Wisconsin ............... Douglas +2 utahensis Koehne, Utah, serviceberry ........................................ Leaves, stems, stem bark ...................... Utah ......................... Wasatch +3 

Fruits ................................................................ do .............................. do............ +2 Leaves ........................................................ Nevada ..................... Lander +2 Stem bark ........................................................ do .............................. do............ +3 

Leaves, stems, stem bark. .......................... do ........................ Eureka +3 .......... do ............................................................. do ........................ Humboldt +3 

Cercocarpus intricatus S. Wats., mountain mahogany ................. Leaves, stems, stem bark ...................... Utah ........................ .Juab + 1 ledifolius Nutt. ex T. & G., mountain mahogany ................... Stem bark. ...................................................... do ........................ Wasatch +1 Leaves, stems ........................................... Nevada .................... Lander +2 

Stem bark ........................................................ do .............................. do ........... . Leaves .............................................................. do ........................ Elko 

Stem bark ....................................................... do .............................. do ........... . Stem bark, root bark ............................. Arizona ................... .Yavapai 

Stems, stem bark ..................................... Utah ......................... Duchesne 

Leaves, stems, stem bark ...................... Nevada ..................... Humboldt montanus Raf., mountain mahogany ......................................... Stem bark .................................................. Colorado .................. .Jefl'erson 

Chamaebatiaria millefolium (Torr.) Maxim., fern bush .............. Leaves, stem bark. .................................. Nevada .................... Nye 

Cowania mexicana Don., clifl'rose ....................................................... Leaves, stems, stem bark ...................... Utah ......................... .Juab 

Crataegus sp. indet., hawthorn ........................................................... Leaves, fruits ................................................. do ........................ Wasatch 

Holodiscus dumosus (Nutt.) Heller, ocean spray .......................... Leaves, stem bark, fruits ...................... Colorado .................. .Jefl'erson 

microphyllus Rydb.,. ocean spray ................................................. Leaves, stems, stem bark ...................... Nevada ..................... Lander PotentiUa fruticosa L., shrubby cinquefoil.. ............................................... do ...................................................... Colorado .................. Grand 

plattensis Nutt., potentilla ........................................................... Entire plant .................................................... do ........................ Clear Creek Prunus americana Marsh, wild plum ................................................. Leaves, stem bark ......................................... do ........................ .J efl'erson 

Crushed seeds ................................................. do .............................. do ........... . Fruit ................................................................. do .............................. do ........... . andersonii Gray, desert peach ...................................................... Leaves, stems, stem bark ...................... Nevada .................... Eureka .......... do ............................................................. do ........................ Humboldt 

Leaves, stems, stem bark, root bark .......... do ........................ Lander 

pennsylvanica L.f., bird cherry ................................................... Leaves, stems ........................................... Michigan ................. Cheboygan 

persica Sieb. & Zucc., peach ......................................................... Root bark .................................................. Colorado .................. .Jefl'erson 

pumila L., sand cherry .................................................................. Leaves, stems ........................................... Wisconsin ............... Douglas serotina Ehrh., black cherry .................................................................. do ............................................................. do ........................ Polk 

virginiana L., chokecherry ........................................................... Leaves, stem bark ................................... Colorado .................. .Jefl'erson 

+1 

+1 

+3 

+1 

+2 

+1 

+1 

+2 

+2 

-1 

-1 

-1 

-1 

-1 

+3 

+4 

+1? 

+3 

+3 

+2 

+2 

+4 

+1 

+1 

+4 

Fruits ............................................................... do .............................. do............ + 1 Leaves, stem bark ................................... Utah ......................... Wasatch +4 Fruits ............................................................... do .............................. do............ +3 

Leaves, stem bark ................................... Nevada .................... Humboldt +3 

Stem bark .................................................. Arizona ................... .Yavapai +3 

Purshia tridentata (Pursh) DC., bitterbrush ................................. Leaves, stems, stem bark ...................... Utah ......................... Wasatch Rosa acicularis Lind!., wild rose ......................................................... Leaves, stems, fruits .............................. Colorado .................. Clear Creek 

arkansana Porter, wild rose ......................................................... Leaves, stems, stem bark, fruits ......... Utah ......................... Wasatch 

-1 

-1 

-1 

1-:3 

t_:l:j 

00 

1-:3 

00 

":.:1 

0 

~ 

0 

to< 

> z 

1-f 

tj 

t_:l:j 

1-f 

z 

"0 

~ 

> z 

1-:3 

00 

~ 

c:n 
TABLE 23.- Results of tests for -hydrocyanic acid (HCN) in selected species of plants- Continued ~ 

~ 

Family Locality Scientific and common name Plant parts tested 

State County 

Results of 

HCN tests ~ 

t?=_j 

Rosaceae __________ __________ Rosa woodsii Lind!., wild rose ............................................................. Leaves, sterns ........................................... Colorado .................. Clear Creek 

Rubus deliciosus James, boulder raspberry ...................................... Leaves .............................................................. do ........................ Jefferson 

idaeus L., wild red raspberry ....................................................... Leaves, stems, fruits .................................... do ........................ Clear Creek 

Sibbaldia procumbens L ........................................................................ Entire plant .................................................... do .............................. do ........... . Sorbus acuparia L., European mountain-ash .. ·-······--········--··--····-···Leaves .............................................................. do ........................ Jefferson 

-1 0 

-1 0 

-1 ~ -1 t?=_j 

+2 ~ 

Stem bark, fruits .......................................... do .............................. do ........... . scopulina Greene, mountain-ash .................. --····--··--····················Leaves, stems ................................................. do ........................ Grand 

1-4 

+3 00 

+2 t-3 

Leaves, stems, root bark. ............................ do ........................ La Plata 

Spiraea caespitosa Nutt., spiraea .... ·-··-···-----·--····-··-·····································do ....................................................... Nevada .................... Lander 

latifolia (A it.) Birkh., meadowsweet ......................................... Leaves, stems ........................................... Wisconsin ............... Polk 

+3 ~ -1 ~ -1 0 tomentosa L., hardhack.·--··--··--·····--·································-········-··· .......... do ............................................................. do .............................. do ........... . -1 ~ 

Rubiaceae ................... Galium boreale L., northern bedstraw ............................................... Entire plant .............................................. Colorado .................. Grand 

Salicaceae ................... Populus angusti/olia James, narrowleaf cottonwood ..................... Leaves, stem bark ................................... Utah ......................... Wasatch 

Leaves, stems ........................................... Nevada ..................... Humboldt 

Salix bebbiana Sarg., willow ................................................................ Leaves, stem bark ................................... Colorado .................. Clear Creek 

-1 ~ -1 0 +1? t"4 -1 t1 

Scrophulariaceae ...... <J_f~!~~~~uL;::i;aM~If.~'C;;t:~:-~~~!~~~~~~~~~::::::::::::::::::::::::::::::::::~=~~:~.PJ~~~;~::::::::::::::::::::::::::::::::::::::::::::::::~:::::::::::::::::::::::::::::::~:::::::::::: -1 1-4 

-1 z 

Mimulus glabratus H.B.K., smooth mimulus ................................... Leaves, sterns ........................................... Utah ......................... Wasatch 

Pedicularis grayi Nels., Gray's lousewort ................................................... do ....................................................... Colorado .................. Clear Creek 

groenlandica Retz, elephantella ................................................... Leaves, fruits ................................................. do .............................. do ........... . 

-1 t-3 

-1 ~ -1 t?=_j 

Penstemon unilateralis Rydb., penstemon ........................................ Leaves, stems ................................................. do .............................. do ........... . virens Pennell, penstemon ............................................................ Leaves, stems, roots ..................................... do .............................. do ........... . Verbascum thapsus L., great mullein ................................................ Leaves, fruits ................................................. do ........................ J efferson 

Solanaceae ................. .Lycium pallidum Miers., wolfberry .................................................... Leaves, stems ........................................... New Mexico ........... Santa Fe rostratum DunaJ, buffalo-bur ................................................................. do ............................................................. do .............................. do ........... . 

-1 

-1 ~ t?=_j -1 

+1? > 

+1? t-3 

~ Typhaceae .................. Typha latifolia L., broad-leaved cattaiL ........................................... Leaves, stems, flowers ............................ Utah ......................... Duchesne 

Ulmaceae .................... Celtis reticulata Torr., hackberry ....................................................... Leaves, stern bark, fruits ...................... Colorado .................. Jefferson 

-1 t?=_j 

-1 ~ 1-4 

z 

~ 

0 

~ 

0 

t"4 

t?=_j 
SOLUBILIZATION OF GOLD 47 

2. Western spruce-fir forest- Mountain-ash, smooth shadbush, 

and pine drops (+I?). 

3. Alpine meadows and barren - No cyanogenic species found. 

4. Sagebrush steppe- Chokecherry, cliffrose, and desert apricot. 

5. Saltbrush-greasewood - Mountain mahogany (Cercocarpus 

montanus and C. 'intricatus), cliff rose, desert apricot, chokecherry, and narrow-leaf cottonwood (+I?). 

6. Juniper-pinyon woodland- Cliffrose, serviceberry, mountain 

mahogany ( Cercocarpus montanus), Utah shadbush, wild 

flax, desert apricot, red mahonia (+I?), cholla (+I?), milkvetch (+I?), mistletoe (+I?), one-seeded juniper ( +1 ?) , 

and bladder pod (Lesquerella fendleri) (+I?). 

7. Douglas-fir forest - Mountain-ash and red osier dogwood 

(+I?). 

8. Mountain mahogany-oak scrub- Mountain mahogany (Cercocm·pus montanus and C. ledifolius) and serviceberry. 

9. Great Basin sagebrush - Mountain mahogany ( Cercocarpus 

montanus and C. ledifolius), desert apricot, and Utah shadbush. 

IO. Grama-tobosa shrub steppe- Utah shadbush and mountain 

mahogany ( Cercocarpus ledifolius). 

II. Chaparral- Mountain mahogany (Cercocarpus ledifolius) 

and chokecherry. 

I2. Southwestern spruce-fir forest - No cyanogenic species found. 

SOLUBILIZATION OF GOLD BY MICROORGANISMS 

AND ORGANIC SUBSTANCES 

By KENNETH G. DOXTADER1 

INTRODUCTION 

The ability of microorganisms to solubilize elemental gold is 

discussed in several papers (Pares, I964a, b, c, I966; Pares and 

Cuper, I964; Pares and Giraud, I964; Pares, Hans-Moevi, and 

others, I965; Pares and Martinet, I964; Pares, Martinet, and 

others, I965). These authors presented the results of experiments 

aimed at studying the gold solubilizing activity of pure cultures 

of bacteria, either obtained from stock cultures or isolated from 

gold ore deposits. 

The aim of the present study was to test the ability of selected 

microorganisms to solubilize elemental gold. Bacterial strains of 

the same type used by Pares and associates were used in this 

study. In addition, a number of other bacterial isolates, as well 

as some fungi, were tested. 

1Associate Professor of Agronomy, Colorado State University, Fort Collins, Colo. 
48 GEOCHEMISTRY OF GOLD IN THE WEATHERING CYCLE 

GOLD SUBSTRATE 

Powdered gold itself was used as gold substrate in some experiments on the solubilization of gold by organic action, and in other 

experiments colloidal gold was used. Colloidal gold was prepared 

for selected gold substrate by dissolving a minus-100-mesh gold 

powder in boiling aqua regia. The cooled solution was neutralized 

with NH40H and treated with ascorbic acid to reduce and precipitate the gold. The suspension was centrifuged at 4,000 X G for 

30 minutes, and the gold sediment was washed with 1 N H2S04, 

re-centrifuged, washed with distilled water, centrifuged again, 

and resuspended in distilled water. In one series of experiments 

the aqua regia solution of gold was neutralized with NH40H but 

was not treated with ascorbic acid. The gold probably was incompletely reduced in this experiment. 

CULTURE MEDIA 

The basic mineral salts solution used for the media contained 

(in grams per liter of distilled water) : NH4Cl, 2.0; Na2HP04, 

6.0; KH2P04, 3.0; NaCl, 3.0; MgC12, 0.04; Na2S04, 0.161. The 

final pH of the solution was 7.0 to 7.1. Glucose was utilized as a 

carbon-energy source at a concentration of 10 gjl. Where indicated, yeast extract (Difco) was included in the medium at the 

rate of 0.5 gjl, or peptone (Difco) was included at 2.5 gjl. 

CULTURE PROCEDURES 

The general experimental approach was to incubate 150-mg 

quantities of gold with 50-ml aliquots of culture medium contained 

in 250-ml Erlenmeyer flasks. The media were sterilized by autoclaving for 20 minutes at 15 lbsjin. 2 steam pressure and a temperature of 120°C. The sterilized media were added aseptically 

to flasks containing gold sterilized by exposure to acetone which 

was then evaporated. 

The flasks of media were inoculated from stock cultures. The 

cultures were incubated at room temperature (27°-29°C). 

ANALYSIS OF THE CULTURE LIQUIDS FOR SOLUBLE GOLD 

The culture liquids were centrifuged for 30 minutes at 4,000 

X G. The supernatant fluid was evaporated to dryness, and the 

dry residue was treated with bromine water and 48-percent HBr. 

The gold in the resulting solution was extracted with methyl 

isobutyl ketone. The organic extract was analyzed on a PerkinElmer Model 303 Atomic Absorption Spectrophotometer by using 

the following instrument settings : Wavelength, 2,428 angstroms ; 

slit, 5 (3 mm, 20 A); lamp, Perkin-Elmer gold hollow cathode, 14 

milliampere current; fuel, acetylene (flow, 7.5) ; oxidizer, air 

(flow, 9.0). 
SOLUBILIZATION OF GOLD 

RESULTS AND DISCUSSION 

SOLUBILIZATION OF GOLD BY STOCK CULTURES OF BACTERIA 

AND FUNGI 

49 

Initial studies were aimed at determining the effect on gold 

solubilization of the following variables: (1) Microbial species, 

(2) culture medium, (3) incubation conditions (shaking), (4) 

incubation time, and ( 5) physical form of the gold. 

Ten bacterial strains obtained from the culture collection of 

the Colorado State University Department of Microbiology were 

inoculated into flasks of culture medium containing powdered gold. 

Three media were utilized: glucose, glucose-plus-yeast extract, 

and glucose plus peptone. 

The cultures were incubated in a stationary fashion for 8 

weeks, and at weekly intervals, soluble gold was measured. None 

of the cultures showed significant gold solubilization during the 

incubation period. At no time did the level of gold in solution 

exceed 0.5 ppm. 

Since the small surface area of the powdered gold might be a 

limiting factor in the microbial solubilization process, colloidal 

gold was prepared and incubated with the test organisms. For all 

cultures grown in the glucose medium, the level of gold in solution was extremely low ( <0.5 ppm) throughout the incubation 

period. Small but significant levels of soluble gold were detected 

in the cultures grown in glucose-plus-yeast extract medium and in 

glucose plus peptone (table 24). The amounts of gold solubilized 

were erratic. The values reported in table 24 are the highest 

soluble gold concentrations found throughout the 8-week sampling 

period. 

TABLE 24.- Solubilization of colloidal gold by bacterial strains grown in 

glucose medium with yeast extract and with peptone 

[Each culture was grown for 8 weeks ; gold was analyzed at weekly intervals ; highest detected levels ·are reported] 

Highest level of gold in solution (ppm) 

Strain 

With yeast extract With peptone 

Agrobacterium tumefaciens1 _________________________ _ 

Bacillus circulansl ___________________________________________ _ subtilis1 _____________________________________________________ _ megateriuml _____________________________________________ _ 

P1·oteus vulgaris! _____________________________________________ . 

Bacillus cereus1·-----------------------------------------------· Escherichia coli strain G1 ______________________________ _ coli strain Wl ___________________________________________ _ 

Pseudomonas fluorescens1 ______________________________ _ Sterile medium _________________________________________________ _ Sterile distilled water _____________________________________ _ 

0.51 

.83 

.22 

.71 

.53 

.59 

.49 

.99 

.83 

.62 

.00 

0.91 

.98 

1.19 

1.08 

1.21 

1.70 

.90 

1.20 

1.18 

1.13 

.00 

1Shown by Pares .and coworkers (Par~s. 1964a, b, c, 1966; Par~ and Cuper, 1964; Par~s and Giraud, 1964; Pares, Hans-Moevi, and others, 1965: Pares and Martinet, 1964; Pares, Martinet, and others, 1965) to be capable of solubilizing gold. 
50 GEOCHEMISTRY OF GOLD IN THE WEATHERING CYCLE 

On the basis of the low levels of gold detected and the sensitivity 

of the chemical assay, it is difficult to draw definite conclusions 

regarding the relative levels of metal solubilization of different 

cultures, but, apparently, some solubilization took place in both 

cultures and in sterile media. By comparison with the gold levels 

in the uninoculated media, little, if any, additional gold was solubilized by the test strains. In fact, there is a slight indication that 

some of the cultures removed gold from solution. 

The significant levels of gold in solution in the uninoculated 

culture medium suggest that organic solutions, such as peptone 

and yeast extract, may in themselves be capable of solubilizing 

gold. The glucose-peptone medium is similar in composition to 

one used by Pares and her coworkers. Although they showed that 

this medium was capable of supporting microbial gold solubilizing 

activity, they made no mention of the direct effect of the medium 

on gold. 

Three fungal strains from the stock culture collection were 

inoculated into glucose-peptone medium containing colloidal gold. 

The cultures were incubated (without shaking) for 6 weeks; gold 

analyses were performed each week. As shown in table 25, small 

but significant amounts of gold were detected in the culture broths 

and in the uninoculated medium. 

In an effort to enhance the process of gold solubilization by 

microorganisms, selected bacteria were grown in flasks of glucosepeptone medium which were placed on a shaker. In most of the 

flasks, no significant difference was noted in the levels of gold 

in solution (table 26). Cultures were grown in glucose-peptone 

medium containing colloidal gold for 8 weeks; the levels of gold 

reported are the highest found in weekly analysis of the cultures. 

SOLUBILIZATION OF GOLD BY MICROBIAL ISOLATES FROM SOIL 

AND SEDIMENTS 

Efforts were made to isolate microorganisms capable of solubilizing greater quantities of gold than those found with the stock 

culture collection strains. Standard procedures of isolation were 

TABLE 25.- Gold solubilization by fungal strains grown in glucose-peptone 

medium 

Strain 

Aspergillus fiavus ____________________________________________ 0,91 

niger·---------------------------------------------------------- .88 

Penicillium SP·-------------------------------------------------- .91 

Sterile medium·------------------------------------------------ .81 

Incubation period (weeks) 

2 

Gold in solution (ppm) 

0.82 

.81 

.91 

.93 

1.11 

.95 

.95 

.95 

6 

1.05 

1.01 

.99 

1.02 
SOLUBILIZATION OF GOLD 51 

TABLE 26.- Effect of shaking flasks on microbial solubilization of gold 

Gold in solution (ppm) 

Strain 

With shaking Without shaking 

Escherichia coli strain G ........................................ 0.90 

coli strain W ..................................................... 1.09 

Pseudomonas fiuo1·escens1 ...................................... 1.20 

Bacillus ce1·eus1 ........................................................ 1.62 

Proteus vulgaris1 ..................................................... 1.40 

Sterile medium ......................................................... 1.10 

Sterile water.·-··-·····----·-·-··---····---------------------·----·-·-··- .00 

0.99 

1.29 

1.22 

1.42 

1.44 

1.09 

.00 

1Shown by Par~ and coworkers (Pares, 1964a, b, c, 1966; Pares and Cuper, 1964; Pares and 

Giraud, 1964; Pares, Hans-Moevi, and others, 1965; Pares and Martinet, 1964; Pares, Martinet, 

and others, 1965) to be capable of solubilizing gold. 

utilized to obtain bacterial strains from gold-containing sediments 

collected near Cripple Creek, Colo., and soil collected near Empire, 

Colo. The samples of soil and sediment were suspended in sterile 

water and aliquots spread on the surface of glucose-peptone medium solidified with agar. Five major bacterial types were isolated 

from each type of sample and inoculated into glucose-peptone 

medium containing colloidal gold. As shown in table 27, the levels 

of gold solubilized were comparable to those found with the strains 

from the stock culture collection used in the previous experiments. 

TABLE 27.- Gold solubilization by bacterial strains isolated from soil and 

sedimentt 

Strain Highest level of gold 

in solution (ppm) 

C-1 ........................ 0.91 

C-2 ........................ 1.11 

C-3 ........................ 1.35 

C-4 ......................... 83 

C-5 ......................... 96 

E-1 ........................ 1.29 

Strain Highest level of gold 

in solution (ppm) 

E-2 ........................ 1.33 

E-2 ........................ 1.09 

E-3 ........................ 1.43 

E-4 ........................ 1.15 

E-5 ........................ 1.02 

Sterile medium .... 1.21 

1C strains were isolated from Cripple Creek sediment ; E strains from Empire soil. Cultures were grown in glucose-peptone medium for 8 weeks ; gold analyses were made at weekly intervals. 

SOLUBILIZATION OF VARIANTLY PREPARED COLLOIDAL GOLD 

In the preparation of colloidal gold from metal powder, on 

one occasion the ascorbic acid reducing agent was inadvertently 

omitted from the procedure. The resulting gold precipitate, which 

likely contained a slightly soluble gold compound, was extensively 

washed with 1.0 N H2S04 and water and then supplied to cultures 

in the usual way. Gold in the nutrient solution at the end of 4 days 

(table 28) may be due to the presence of soluble gold in the culture 

medium. The increase in dissolved gold in the subsequent 4-day 

period, however, must be considered as solution of gold by the 

microbial strains. 
52 GEOCHEMISTRY OF GOLD IN THE WEATHERING CYCLE 

TABLE 28.- Effect of microbial strains on variantly prepared colloidal gold 

[50-ml quantities of glucose-peptone medium or water were incubated with 0.015 g of gold] 

Strain 

Agrobacterium tumefaciens ................................ . Bacillus circulans ................................................. . subtilis ............................................................ . 1negaterium .................................................... . Proteus vulga'ris ................................................... . Bacillus cereus ...................................................... . Escherichia coli strain G ..................................... . coli strain W .................................................. . Pseudomonas fluorescens ..................................... . Sterile medium ...................................................... . Distilled water ....................................................... . 

Gold in solution (ppm) 

After 4 days 

11.0 

16.3 

17.7 

17.1 

9.5 

22.5 

21.2 

13.4 

8.3 

17.5 

.00 

After 8 days 

12.7 

21.3 

26.9 

22.5 

18.4 

25.9 

23.6 

22.4 

12.1 

24.1 

.00 

THE ACTION OF PEPTONE SOLUTIONS ON COLLOIDAL GOLD 

A study was made of the ability of the glucose-peptone medium 

to solubilize gold under sterile conditions. Colloidal gold was incubated with the complete medium and with separate organic and 

inorganic components of the medium. As shown in table 29, greatest solubilization occurred in the complete medium. These results 

would suggest that peptone acts together with certain minerals 

in the culture medium to bring about gold solubilization. 

TABLE 29.- Solubilization of gold by components of glucose-peptone medium 

[Medium components were shaken with colloidal gold (0.015 g/50 ml)] 

Gold in solution (ppm) Medium component 1 hour 5 hours 24 hours 48 hours 

Complete ....................................................... 0.46 

Mineral fraction.......................................... .28 

Peptone plus glucose fraction................... .02 

Distilled water............................................. .00 

1.13 

.49 

.05 

.00 

2.47 

.97 

.48 

.00 

ISOLATION OF SOIL MICROORGANISMS RESIST ANT TO HIGH 

CONCENTRATIONS OF METALLIC IONS 

2.52 

1.18 

.86 

.00 

A review of the literature indicates that a high proportion of 

the microorganisms in soil are potentially capable of solubilizing 

minerals, the activity not being restricted to any single microbial 

group. Microorganisms which are able to degrade minerals containing heavy metals and noble metals would appear to be adversely affected by the toxic action of the metal ions which they 

liberate. Such toxicities might effectively limit the ability of microorganisms to attack and solubilize minerals. A study was made of 

the ability of isolated microorganisms to tolerate high concentrations of metallic ions. 

One-gram quantities of Cass silt loam were inoculated into 

glucose-mineral-salts medium containing a high concentration of 
DISTRIBUTION OF GOLD IN SOILS AND VEGETATION 53 

a metallic salt. Where possible, the sulfate or chloride salt of the 

metals was used. A total of 16 metallic elements at various concentrations was studied. 

Aliquots of the culture liquids were transferred to fresh medium 

of identical composition after 5 days' incubation. 

This procedure was repeated once more prior to plating out 

some of the liquid of the final culture on glucose-salts medium containing agar and an appropriate level of metallic ion. Organisms 

that developed at the highest metal concentr~tions were isolated 

and then inoculated into liquid-culture medium in order to establish with certainty that they were capable of growth in the presence of the metals. Isolated bacteria and fungi were capable of 

growth in liquid media containing the following metallic ions at 

the indicated concentrations : 

[SS, saturated solution] 

Ion Maximum concentration at which growth 

occurred (ppm) 

Ag+l ________________________ .. 5 

A u+2 -------------------------· 25 Cd+2 ______________________ .... <50 

Co+2 __________________________ <50 

Cr+6 __________________________ 100 

Cu+2 ________________________ ;_ SS 

Hg+2 -------------------------· 5 Mn+2_________________________ SS 

Mn+4 ------------------------- .ss 

Ion Maximum concentration at which growth occurred (ppm) 

Mo+6 ________________________ .. SS 

Pb+2 __________________________ ·ss 

Pd+2__________________________ 50 Sn+2 __________________________ 500 

U+6 ............................ SS 

v +5 ···--·--····---·----·-··-··· 500 

W+6 ............................ SS 

Zn+2.......................... SS 

These results suggest some upper concentration limits for the 

growth of soil microorganisms in the glucose-mineral-salts medium containing metallic ions. It should be pointed out, however, 

that in other media or by the use of other isolation techniques 

these limits might be quite different. 

STUDIES OF THE DISTRIBUTION OF GOLD IN SOILS 

AND VEGETATION 

PREPARATION OF SAMPLES 

All soils were sieved through a 2-mm screen and the coarse 

material rejected, except in special studies of the variation of gold 

content with soil particle size. 

All rocks were ground to pass a 0.2-mm screen. 

Samples of mull were sieved through a 2-mm screen and the 

coarse material rejected. Some mull samples for special studies 

were separated by flotation in demineralized water into principally 

organic (floats) and inorganic (sinks) fractions. 

Samples of vegetation suspected of surface contamination were 

washed with demineralized water before being ashed. 

Samples of wood consisted of major limbs or trunks of trees 
54 GEOCHEMISTRY OF GOLD IN THE WEATHERING CYCLE 

that were shaved clean of bark; conversely, samples of "bark" 

contained a large percentage of wood. 

IGNITION AND ASHING 

Samples of rock, soil, and mull were ignited in porcelain dishes 

over Fisher burners to oxidize sulfides and organic matter. Occasional stirring with a glass rod facilitated the oxidation. Samples 

of vegetation were ashed in furnaces with temperature controls 

that permitted a slow increase in temperature to a maximum of 

450°C. The heating and cooling cycle was 24 hours. 

SOLUTION AND ESTIMATION OF GOLD 

The method of analysis used for gold followed that described 

by Thompson, Nakagawa, and VanSickle (1968). For rocks and 

soils the procedure was to transfer 10 g of the sample to a 25- by 

200-mm screw-capped culture tube, add 20 ml concentrated HBr 

containing 0.5 percent Br2, cap the tube, and shake for 20 minutes. 

Add 20 ml of water, 10 ml of methyl isobutyl ketone, and shake 

for 2 minutes. Centrifuge to separate the liquid phases, and transfer the ketone with an automatic pipet to a similar tube containing 

25 ml of 0.1 N hydrobromic acid and shake for about 30 seconds 

to remove excess ferric bromide from the ketone. Atomize the 

ketone in an atomic absorption spectrophotometer. Compare with 

standard solutions atomized under the same conditions. For limestones and the ash of organic materials, the procedure was to 

transfer 10 g of the sample to a 150-ml beaker, moisten with 

water, and add the HBr-Br2 solution slowly until frothing ceases. 

Then add 20 ml of the HBr-Br2 solution and boil until only 

10-15 ml remains in the beaker. Add a drop of Br2, transfer the 

contents of the beaker to a 20- by 200-mm culture tube, and proceed as above. 

PRECISION OF DATA 

The first consideration in a chemical analysis is the use of a 

sample that truly represents the material under test. If we grant 

that the collected sample is representative, we must be able to take 

an equally representative subsample for analysis. Obviously, the 

precision of the analysis of a given sample is limited by the precision of subsampling. The sampling and subsampling of oxidized 

earth materials for their gold content is complicated by the erratic 

occurrence of particulate gold within the material being sampled. 

The shape of particulate gold varies from plates, to wires~ to 

knobby spheres, but for simplicity in calculating the effect of size 

and weight of gold particles on the precision of analysis, let us 

consider cubes of gold. A single 0.1-mm cube of gold weighs 19 p.g 

-in a 10-g subsample this single particle would give 1.9 ppm 
DISTRIBUTION OF GOLD IN SOILS AND VEGETATION 55 

gold. A 0.01-mm cube of gold weighs 0.019 1-lg; in a 10-g subsample, 

21 of these particles would give 0.04 ppm gold, which is the lower 

limit of estimation reported in this paper. A micrometer (0.001 

mm) cube of gold weighs 19 picograms (10- 12 g) ; in a 10-g 

sample, 21,000 of these particles would give 0.04 ppm gold. 

Tourtelot (1968, p. F8) published a diagram (fig. 3, this report) 

that shows the grain size of gold versus the number of particles 

of gold in samples of the indicated sizes for 1-, 10-, and 100-g 

samples containing 1 ppm gold, and for a 1-g sample containing 

0.1 ppm gold. From this diagram it is evident that a 10-g sample 

containing 1 particle of gold 0.1 mm in diameter would contain 

1 ppm gold, whereas more than 1,000 particles of 0.0068 mm would 

be present at the 1 ppm range. 

Fortunately, almost all the material reported in this paper contained relatively fine grained gold. The small relative standard 

deviation shown in table 30 could not have been obtained on materials containing gold in excess of 0.01 mm in diameter. The values 

of 0.07 to 0.09 ppm for the ash of wood (sample No. CC-552) 

suggest that the gold was present as particles of less than 0.01 mm 

in diameter. 

Clifton, Hunter, Swanson, and Phillips (1969) studied the problem of sample size and meaningful gold analysis. They set as a 

goal (p. C2) "that it be 95 percent probable that the true gold 

TABLE 30.- Precision of replicate analyses for gold in various mate'rials 

Sample No. 

534-BH-4 .............. 534-3 ...................... 534-5 ...................... 

534-7 ·····--······------··· 534-8 ...................... 534-9 ...................... 

EM-772-H ............ 

EM-698-H-16 ...... 

CC-552 ................... 

CC-552 .................. 

EM-868-V -2 ........ 

Number of 

determinations 

Gold (ppm) Relative -------------standard 

Highest Lowest Mean Standard deviation 

value value value deviation (percent) 

Soil profile from Empire, Colo. 

6 0.76 0.24 0.43 0.22 51 

6 .20 .08 .12 .05 42 

5 .06 .04 .05 .01 20 

6 .20 .08 .13 .05 38 

6 36.3 13.8 20.8 8.1 39 

6 22.0 17.9 19.6 1.4 7 

Ash of humus under Douglas-fir, Empire, Colo. 

6 0.31 0.15 0.24 0.06 25 

Ash of humus under lodgepole pine, Empire, Colo. 

7 0.55 0.15 0.29 0.13 45 

Ash of wood of limber pine, Cripple Creek, Colo. 

5 0.09 0.07 0.086 0.009 10 

Ash of bark of limber pine, Cripple Creek, Colo. 

7 0.30 0.15 0.216 0.06 27.7 

Ash of bark of lodgepole pine, Empire, Colo. 

5 0.27 0.11 0.142 0.072 50 
56 GEOCHEMISTRY OF GOLD IN THE WEATHERING CYCLE 

en 

0:: 

LLI 

1-

~ 0.062 1----------+-:=---\-IHt----+--------:::1 

::::; 

..J 

i 

~ 

en 

LLI 

~ 0.031 1-----------~r---lH-+-+-----=::l 0 

z 

:::> 

0 

m 

LLJ 

0 

<( ffi 0.01& t::-:_ ..... _______________ =--_-_-_-_-_---:-_:-________ =-=lt±==t:±::±:====l 

:I: 

1-

0:: 

~ 1-

z 

LLJ 

3: 0.008 1---------------lf---\-+--+-+-----i 

0.32 

0.30 

0.20 

0.15 

0.10 

0.068 

0.050 

0.040 

0.032 

0.030 

0.020 

0.015 

0.010 

0.0068 

0.005 

0.004 

0.0032 

0.003 

0.002 L-------....I-----L----1---L-.L...-....1....-...J....._...J.......J 0.002 

w- 3 10-2 w- 1 10 ° 10 1 10 3 10 5 106 

NUMBER OF PARTICLES 

en 

0:: 

LLJ 

1-

LLJ 

~ 

::::; 

..J 

i 

~ 

o::· 

LLJ 

1-

LLJ 

~ 

<( 

0 

z 

:;;: 

0:: 

(!I 

FIGURE 3.- Number of particles of gold at 1 ppm in samples 

having different weights, and for a 1-g sample at 0.1 ppm. 

From Tourtelot (1968, p. F8). 

content of the deposit be no more than approximately 50 percent 

larger or smaller than the gold content obtained by chemical analysis of a sample." They further stated (p. C2) that "the number 
DISTRIBUTION OF GOLD IN SOILS AND VEGETATION 57 

of gold particles in the sample is the only factor controlling the 

precision of the chemical analysis if, in addition to assuming that 

the gold particles are distributed randomly, it is also assumed that 

(1) gold particle mass is uniform, (2) gold particles make up less 

than 0.1 percent of all the particles, (3) the sample contains at 

least 1,000 particles of all kinds, and ( 4) analytical errors are 

absent." They concluded (p. C2) that "a sample has the minimum 

adequate size then if it is large enough that it can be expected to 

contain 20 particles of gold." This idealized model of random distribution of gold particles of uniform mass and no analytical error 

then requires a minimum of 20 gold particles for a ±50-percent 

error. 

The difficulty encountered in soils containing coarse gold is 

illustrated by the results of the determination of gold in eighteen 

10-g subsamples of a single soil sample: 

Number of 

subsamples Gold (ppm) 

14 ________________________________ <0.04 

1________________________________ .06 

1________________________________ .12 

1________________________________ .25 

1________________________________ 1.0 

This soil, from the Copper Basin district in Arizona, contained 

wire gold that was observed by panning. One cannot calculate a 

standard deviation for these data, and it seems obvious that in 

this example a representative 10-g subsample could not be taken. 

In conclusion, the precision of the gold determinations given 

in this paper is usually represented by a relative standard deviation of less than ±50 percent. This error is a combination of subsampling and analytical error. The low relative standard deviation 

obtained in some samples suggests that gold is present as extremely fine particles in the samples, thus permitting a measure 

of the precision of the analytical method. 

STUDIES OF GOLD IN SOIL PROFILES AND VEGETATION 

Soil profiles and vegetation were sampled in 12 gold districts in 

Colorado, Utah, Nevada, and Arizona, and both the soil and vegetation were analyzed for gold content. Figures 4-6 show the gold 

content of the soil profiles including the gold humus layer and of 

vegetation at the sample sites and also give a description of the 

environment in which the soils were collected. The soil profiles 

sampled for this study are generally weakly developed because 

they are in areas where frost action or soil creep coupled with 

climate inhibit soil development. 
58 GEOCHEMISTRY OF GOLD IN THE WEATHERING CYCLE 

The soils with prefixes EM, CC, GH, B, and LP (figs. 4A-F; 

5A-C, E-1; 6E) were collected at relatively high altitudes in the 

Rocky Mountains of Colorado. These soils are characterized by a 

1- to 3-inch-thick mull (forest humus) layer described on page 65. 

The mineral soil is composed of a gray to gray-brown ash-textured 

A horizon 2-4 inches thick that is underlain by a yellow to yellowbrown colluvial layer of silt, sand, pebbles, and cobbles that does 

not change noticeably in texture and composition downward to 

bedrock. 

The soils with prefixes U, M, CA, and GA (figs. 4/, 5D, 6A-D) 

were sampled in arid localities in Utah and Nevada. These soils 

are composed of a weakly developed gray to light-brown loam A 

horizon that overlies a light-gray to yellow-gray colluvium which 

is limy and more or less cemented. 

The soils with the prefix NB (figs. 5J, 6C) were collected in an 

arid locality in central Nevada. These soils are moderately thick, 

however, because they formed from hydrothermally altered basalt 

in an area of gently rolling topography. They are characterized by 

a dark-brown sand loam A horizon 2-3 inches thick underlain by 

a red-brown clay loam B horizon ~-18 inches thick that has a 

blocky structure. The B horizon is underlain by a limy colluvial 

layer of silt, sand, pebbles, and cobbles. 

The soils sampled in the Copper basin area of west-central 

Arizona (prefix CB, fig. 4G, H) formed from quartz monzonite 

grus. This soil is composed of a yellow to gray-brown sand loam A 

horizon 3-4 inches thick that overlies a yellow sand loam B horizon 

7-16 inches thick. The grus below the A and B horizons does not 

show any noticeable soil development. 

The high concentration of gold in humus and in soil layers near 

the top of several of the soil profiles (figs. 4, 5) is due primarily 

t<? biogeochemical cycling of g~ld as described by Goldschmidt 

(1937, p. 670). 

The biogeochemical cycle is the prime factor in the concentration of gold in the upper layers of the soil profiles sampled during 

this study. Removal of soil particles by surface runoff and by wind 

erosion are secondary factors in the concentration of gold in the 

upper part of the soil profiles. Kinkel and Lesure ( 1968, p. D 17 4) 

concluded that gold in the upper layers of soil profiles collected 

in the DeKalb prospect area, Kershaw County, S.C., was concentrated in these layers by removal of soil material during surface 

runoff. 

The high concentration of gold near the bottom of the profiles 

(figs. 4, 6) may be due to the release of gold from disintegrating 

fragments of vein material or to the downward migration of gold 
DISTRIBUTION OF GOLD IN SOILS AND VEGETATION 59 

particles within the soil profiles during downslope creep of the 

soil, or both. An example of the former is soil profile EM-218-C 

(table 31) in which pebbles containing as much as 11 ppm gold 

at a depth range of 12-18 inches apparently did not contribute 

gold to the soil fraction, whereas pebbles containing 1.0 and 

3.8 ppm gold at a depth range of 79-94 inches apparently did 

TABLE 31.- Comparison of gold content between soil (2.0-mm diamete1· and 

smaller) and cobbles and pebbles (>2.0-uwt diamete1') in soil profile 

EJ.li-218-C (fig. 7 A), Empire district, Clea1· Creek County, Colo. 

Gold content (ppm) Gold content (ppm) 

Depth Depth 

(in.) Soil P<:bbles and (in.) Soil Pebbles and 

cobbles cubbies 

0 - 3 0.19 <0.04 42 55 <0.04 <0.04 

3 - 4.5 .08 <.04 55 67 <.04 <.04 

4.5- 8 .67 <.04 67 79 <.04 <.04 

8 12 .10 <.04 79 89 .18 1.0 

12 18 .12 11.0 89 94 1.5 3.8 

18 

29 

Designation 

in 

fig. 4 

29 

42 

<.04 <.04 94 -102 

<.04 .04 102 -118 

Data for soils indicated in figm·e 4 

State County District 

.46 

.32 

Soil 

profile 

A .......................... Colorado .............. Ciear Creek ............ Empire .................... EM-218C ............... .. B ................................ do .......................... do .............................. do ........................ EM-698H-14 ......... . C ................................ do ......................... do .............................. do ........................ EM-534 .................. . D ................................ do ......................... do .............................. do ........................ EM-698H-25 ......... . E ................................ do .................... Teller ........................ Cripple Creek ........ CC-552 ................... .. 

F ................................ do ......................... do .............................. do ........................ CC-565 .................... . G .......................... Arizona ............... Yavapai. .................. Copper basin .......... CB-13 ..................... .. H ................................ do .......................... do .............................. do ........................ CB-17 ..................... .. I ............................ Nevada ................ Lander ..................... Gold Acres .............. GA-1 ...................... .. 

.08 

.04 

Elevation 

(ft) 

10,000 

10,000 

10,000 

10,000 

10,400 

10,400 

5,500 

5,500 

5,600 

Designation 

in 

fig. 4 

Climate1 

Average 

annual 

precipitation2 

Vegetative cover3 Soil 

series' 

Soil parent 

material 

(in.) 

A ................... Moist subhumid .... . 20 Pine, Douglas-fir ...... Helmer .................... Granodiorite and 

biotite gneiss. 5 

B ......................... do ...................... .. 20 ...... do ................................. do........................ Do. C ......................... do ....................... . 20 ...... do ................................. do........................ Do. D ......................... do ...................... .. 20 ...... do .................................. do........................ Do. 19 Scattered pine, Underwood ............. Phonolite-Iatite 

Douglas-fir. breccia.6 E ......................... do ....................... . 

F ......................... do ....................... . 19 ...... do .................................. do........................ Do. G ................... Semiarid ................. . H ......................... do ....................... . 21 ChaparraL ................. Sierra ....................... Quartz monzonite. 7 

21 ...... do .................................. do........................ Do. I .................... Arid ........................ .. 7 Sage ............................. McCammon ............. Brecciated chert. 8 

1Thornthwaite ( 1941). 2U.S. Department of Agriculture ( 1941). 3Ktichler (1967). 4U.S. Department of Agriculture (1938). 

snraddock ( 1969) . 6Gott, McCarthy, VanSickle, and McHugh (1967). 7J ohnston and Lowell ( 1961). 8Wrucke, Armbrustmacher, and Hess in ( 1968). 
60 GEOCHEMISTRY OF GOLD IN THE WEATHERING CYCLE 

Cl) 

Ll.l 

<0.04 

10 

20 

30 

40 

50 

60 

70 

80 

90 

~ 100 

~ 

~ 110 

::£ 

t 118 Ll.l 

c 

1.0 2.0 

Ll.l 

...J 

i:i: 

0 

0: 

I Pfli.~ WOod] . I I l'i I I I I I 1 I I I I I I I I I I 

A. Profile EM-218 C 

Q.. 

...J 

0 

Cl) <0.04 1.0 2.0 

10 

20 

30 

3.0 

40 D. Profile EM-698-H 25 

50 

60 

70 

75 

<0.04 1.6 2.0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

100 

110 

1 · Pin\a Wtmd ·1 I I I I I I II ij I j I I i II I j 

B. Profile EM-698-H 14 

<0.04 1.0 2.0 

4.0 

~:!~~~~~~~I I I II 

E. Profile CC-552 

<0.04 1.0 

FIGURE 4 (above and facing page).- Histograms of gold content of soil profiles and associated vegetation showing high gold values near both the top 

and bottom of the profiles. 
DISTRIBUTION OF GOLD IN SOILS AND VEGETATION 61 

<0.04 

0 

10 

20 

30 

40 

50 

60 

70 

80 

~ 90 :I: 

u 

~ 100 

~ 

~ 110 0.. 

~ 

1.0 2.0 3.0 4.0 5.0 

<0.04 1.0 2.0 

0 

10 

20 

30 

H. Profile CB-17 

~120-L~~~~~~~~~~~~~~~~~~~A~~ 

...J 

§ f~i:rriijltj~~~el~ ~1°?~ 1 I I I I I I I I I I I I I I I I I I I I I I I I I I I I II I 1f f I I I I I I I I I j 

8: C. Profile EM-534 

...J 

0 

~ ~orr ... ~j~ ....... ,';; ...... ; ;;~ ....... ;;' ..... ~· ~~ ....... ~;' 

10 

20 

f'r;~llalclkj~c~k1 ?~~ ~~~~ 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 ; 1 1 1 1 1 1 1 1 1 1 1 1 1 I 1 1 1 1 1 1 1 1 1 1 1 

G. Profile CB-13 

1.0 2.0 3.0 4.0 7.0 8.0 

Jl"iill!!!iil 

30 --trf'T'T-fn-TTTrTT,...,..,..~I"t+rT-rTTTTTTr-TT"'rTrf+rT-+-r-rt:.f. ~i•: .;•.•: 1.·.,:;•.1••1;·•1 

GOLD CONTENT, IN PARTS PER MILLION 

FIGURE 4. - Continued. 

29ppm 

contribute gold to the soil fraction. This positive correlation of 

gold content between the rock fragments and the soil fraction 

suggests that vein material is more completely decomposed near 

the bottom of the soil profile because abrasion during creep or 
62 GJ:i~OCHEMISTRY OF GOLD IN THE WEATHERING CYCLE 

<0.04 1.0 2.0 

0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

en 100 Lo.l 

::J: 

(.) 

~ 110 

~ 

~120 

Lo.l 

c 130 Lo.l 

...I 

i:i: 

0 

a:: 

CL. 

...I A. Profile EM-314 

0 

en <0.04 1.0 2.0 

~IPii~?~~~~~ I I I I I I I I j 

E. Profile CC-564 

~~irm. W!ioEtl b I I I II I I J 

B. Profile EM-373 

D. Profile U-3 

<0.04 1.0 

10 

25 

ft· Ppnderosa pme-wood 

,,, IIIII 

<0.04 

10 

1.0 

1.0 

2.0 

20-h~+M~rnnoonnon 

GOLD CONTENT, IN PARTS PER MILLION 

<0.04 1.0 ib 

0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

100 

110 

120 

130 

140 

ISO 

~tr~ ~~ ~1' I 

C. Profile EM-852 

~~~~e, I I I I 

J. Profile NB-7 

FIGURE 5.- Histograms of gold content of soil profiles and associated vegetation showing high gold values near the top Of the profiles. 
DISTRIBUTION OF GOLD IN SOILS AND VEGETATION 63 

Designation 

in 

fig. 5 

State 

Data for Roils i11dicated in figure 5 

County Dbtrit·t Soil 

profile 

A --- --------------- Coloratlo .Clt•ar Creek ............ Empire ..................... EM-314 .................. . 

B.. - -----···---··do ... - ----·········--····do .............................. do ........................ EM-373 .................. . C ...... __ ----·- ...... do ........................ do .............................. do ........................ EM-852 .................. . D...... . ........ Utah___ ........... Juab ........................ Detroit ..................... U-3 .......................... . 

E. ----····--Colorado ............ Teller ...................... Cripp!P Creek ......... CC-564 .................... . 

F.. --------·-----··-- ...... do ................... Boulder .................... Gold Hill ................. GH-1 ....................... . G - --········-·-····--·do ......................... do .............................. do ....................... GH-12 ..................... . 11. __ -····-··--····· ...... do ................... Summit .................... Bn•ekenridge .......... ll-2 .......................... . 

I --- -·-··--·-···-··· ...... do ......................... do .............................. do ........................ B-45 ........................ . J. ---····-- N evada ............... Eureka ..................... lluckhorn ................ NB-7 ------·················· 

Elt•vntion ( ft) 

9,800 

9,800 

9,800 

6,000 

10,400 

7,600 

8,300 

11,000 

11,000 

6,800 

Designation 

in 

fig. 5 

Climate' 

Average 

annual 

precipi- Vegetative cover3 

tation2 

Soil 

series1 

Soil pan·nt 

matt•rial 

(in.) 

A ....... -----···--Moist subhumid .... . 2J Pine, Douglas-fir ..... Helmer ................... Granodioritt> and biotitl• gndss." B ........... ___ ....... do ....................... . 20 ...... do ...................... do....... Do. C ........ ·-- ........... do ....................... . 20 ...... do .... do........................ Do. D ... __ ...... Arid ......................... . 7 Sage............ . ...... McCammon ........... Lim{•stone and dolomite.6 

E.... . .. Moist subhumid ..... 19 Scattered pine, Underwood .............. Phonolite-latite Douglas-fir. breccia.7 

F .......... .............. do ....................... . 20 Pine, Douglas-fir ...... Helmer ..................... Granodiorite.s G ......................... do ...................... .. 20 ...... do .................................. do........................ Do. H ................... Humid .................... .. 22 Spruce, fir ........................ do ........................ Quartz monzonite.8 

! .......................... do ...................... .. 22 ...... do .................................. do........................ Do. J .................... Arid ......................... . 9 Sage ............................. McCammon ............. Basalt.0 

'Thornthwaite ( 1941). 2U.S. Department of Agriculture (1941). 

:;Kuchler ( 1967). 1

U .S. Department of Agriculture ( 1938). 

;;Braddock ( 1969). 

"McCarthy, Learned, Botbol, Lovering, Watterson, and Turner (1969). 

'Gott, McCarthy, VanSickle, and McHugh ( 1967). 

'Lovering and Goddard (1950). 

HWells, Stoiser, and Elliott (1969). 

chemical weathering, or both, are more active in this zone than 

in horizons nearer the surface. 

A combination of active breakdown of rock fragments near the 

bottom of the soil profile and downward migration of gold in the 

soil is suggested by the distribution of gold in soil profile EM-534 

(fig. 4C). This soil profile, in which two intervals just above the 

soil-bedrock interface contain 29 and 21 ppm gold, was collected 

on a hillside near an area where the soil has been placered to 

obtain gold that has concentrated near the soil-bedrock interface. 

Gold has been concentrated near the bottom of the soil profile, at 

least in part, by the downward migration of gold and other heavy 

constituents which results from the churning action that takes 

place within the soil during downslope creep. Accelerated breakdown of rock fragments in the lower levels of the soil is also 

taking place in this soil profile. A composite sample of the two 

gold-rich intervals was panned, and gold was visible in the panned 
64 GEOCHEMISTRY OF GOLD IN THE WEATHERING CYCLE 

~ 

-:i 

b: 

~Sage tops 

~ ~~~~ ~W. 1°1d1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 

A. Profile M-1 

i.&J 

i ~.t.~.·.;.~ ...... ·.·.•.···.·.···.· .. l I I I I l\

0 

~ 20 / J 24 ... 

ft~~~~ I II I 

,:r···.·.·.· ... 4 1.0 

20 

30 

B. Profile M-2 

<0.04 1.0 2.0 

C. Profile NB-6 ~~?~;I! D. Profile CA-2 ~~~rlu~~~~~~~ I I I I I I I 1 

E. Profile LP-78 

GOLD CONTENT, IN PARTS PER MILLION 

FIGURE 6.- Histograms of gold content of soil profiles and associated vegetation showing high gold values near the bottom of the profiles. 

Designation in fig. 6 State 

Data for soils indicated in figw·e 6 

County District Soil 

profile 

A .......................... Nevada ............... Elko .......................... Gold Circle .............. M-1 ......................... . B .......................... ...... do ......................... do .............................. do ........................ M-2 ......................... .. C ................................ do ................... Eureka ..................... Buckhorn ................ NB-6 ....................... . D-----------·----·--·------------do _________________________ do ________________________ Carlin ....................... CA-2-------·----··------·---

E .......................... Colorado ............. La Plata .................. La Plata .................. LP-78 ..................... .. 

Elevation (ft) 

6,000 

6,000 

6,800 

6,500 

10,400 

Designation in fig. 6 Climate1 

Average 

annual 

precipitation2 

Vegetative cover3 Soil 

series4 

Soil parent material 

(in.) 

A ................... Arid ........................ .. 9 Sage ............................. McCammon ............. Rhyolite.5 

B ......................... do ....................... . 9 ...... do .................................. do................... ..... Do. C ......................... do ...................... .. 9 ...... do ................................. do ........................ Basalt.6 

D ......................... do ....................... . 9 ...... do ................................. do ........................ Argillaceous limestone. 7 

25 Spruce-fir ................... Helmer .................... Red arkosic sandstone.R E ................... Moist subhumid .... . 

1Thornthwaite (1941). 2U.S. Department of Agriculture (1941). 3Kiichler (1967). 4U.S. Department of Agriculture (1938). 5Rott (1931). 

"Wells, Stoiser, and Elliott (1969). 'Hardie (1966) . 8Eckel ( 1949). 

heavy-mineral fraction. The tails were recovered and sieved. Gold 

was determined by analysis in all size fractions from colloidal size 

to greater than 0.25 mm in diameter, illustrating the release of 

gold as the rock fragments weather. The minerals muscovite (and 

possibly zinnwaldite), chamosite, and a trace of montmorillonite 
DISTRIBUTION OF GOLD IN SOILS AND VEGETATION 65 

were determined in the clay-size fraction by X-ray powder diffraction. These minerals offer further evidence that the soil at this 

depth undergoes both mechanical and chemical weathering because 

muscovite and zinnwaldite are the products of mechanical breakdown, whereas chamosite and montmorillonite are products of 

chemical weathering. 

Four of the five soil profiles showing the highest gold values 

near the bottom of the profile (fig. 6) were collected in arid 

environments where the concentration of gold in the upper part 

of the profiles by the biogeochemical cycle is restricted, owing to 

the relative sparsity of vegetation. The dominant gold concentrating processes which are effective in these environments are the 

migration of gold downward within the profiles and the dilution 

of upper soil layers by deposition of barren windblown material. 

Low gold content in the middle parts of several of the profiles 

(figs. 4A, C, E, F, H) may reflect both depletion due to the migration of gold to the bottom of the profile and dilution caused by 

the addition of barren soil material to gold-bearing material. 

The distribution of gold in sieved fractions of six soil profiles 

(table 32) shows that anomalous concentrations of gold are not 

necessarily restricted to a single size fraction. The distribution 

of gold in the size fractions of any soil profile may reflect the 

character of the associated gold deposits. As an example, wire 

gold in coarse fragments of quartz7vein material accounts for the 

relatively high gold content in the -2.01+1.04-mm and -1.04 

+.501-mm size fractions in the C horizon of profile LP-74. The 

fact that gold does occur in coarse soil material from place to 

place demonstrated the necessity either ( 1) to include in the soil 

sample all material equal to or less than 2 mm in diameter, as 

defined by the U.S. Department of Agriculture (1951, p. 207), or 

(2) to fractionate several selected soil samples in order to determine which size fraction contains the most gold when collecting 

soil samples for gold analysis. 

MULL AS A SAMPLE MEDIUM IN GEOCHEMICAL EXPLORATION 

FOR GOLD 

Mull is often an excellent sampling medium for reconnaissance 

geochemical exploration for gold and other elements. Mull is the 

partially decomposed plant debris found under trees or shrubs 

mixed with soil or rock fragments introduced by sheet wash, wind, 

and the activity of rodents and other animals. It generally occurs 

as thick, black humus-rich pads mixed with various amounts of 

mineral matter. The U.S. Department of Agriculture (1951, p. 

246) classifies such mull as the Ao horizon. 
66 GEOCHEMISTRY OF GOLD IN THE WEATHERING CYCLE 

TABLE 32.- Gold content of seven size fractions of soil h01·izons in six soil 

profiles 

[High gold value in each size fraction is shown in italic. Leaders (. ... ) in figure columns 

indicate no data available] 

Soil 

horizon Depth (in.) 

A ............................... 0-1 Bt .............................. 1-3 B2 .............................. 1-3 BC ............................. 3-12 C ................................ 12-20 

A ............................... 0-2 AB ............................ 2-2.5 B ................................ 2.5-9 B-1 ............................ 9-14 C ................................ 14-22 C-1 ............................ 22-30 

A-0 .......................... 0-1 B-1 ............................ 1-5 B-2 ............................ 5-20 c ________________________________ 20-24 

A ............................... 0-1 B ................................ 1-2.5 B-1 ........................... 2.5-7 c ................................ 7-20 

A............................... 0-1.5 B ............................... .1.5-4 B-C........................... 4-4.5 c ................................ 4.5-20 

A ................................ 0-1.5 B ................................ 1.5-5 C................................ 5-15 C-1. ........................... 15-25 

1Yellow brown. 

Gold content (ppm) of size fractions (mm) 

-2.01 -1.04 -1.00 -.503 -.249 -.119 

+1.04 +.503 +.249 +.249 +.119 +.061 -.061 

La Plata, LP-74, soil profile 

0.05 0.07 0.05 .08 .12 .16 1.3 .2 .16 1.9 .2 .3 2.86 5.06 5.28 

La Plata, LP-78, soil profile 

0.44 

5.28 

3.0 

1.80 

.34 

.76 

La Plata, LP-57, soil profile 

<0.04 0.1 0.12 

<.04 .08 .06 .1 .22 .06 .06 .10 .06 

Breckenridge, B-16, soil profile 

<0.04 0.04 <0.04 

<.04 <-04 <.04 <.04 <.04 .04 

<.04 .2 .04 

Breekenridge, B-45, soil profile 

<0.04 .12 

.1,8 

.2 

<0.04 .86 

1.0 

.5 

<0.04 

1.7 

1.0 

1.3 

Gold Hill, GH-1, soil profile 

0.18 0.11 0.15 

.14 .27 .1 

.06 .12 .09 

.04 .08 .07 

2Gray brown. 

0.07 

.16 

1.7 

1.5 

4.4 

0.14 

6.36 

1.06 

.34 

1.3 

5.16 

0.14 

2.0 

.44 

<.04 

<0.04 

<.04 

.10 

<.04 

<0.04 .5 

3.0 

4.2 

0.16 

.1 

.11 

.06 

0.18 

.42 

.2 

.9 

1.8 

0.37 

.35 

1.02 

.71 

1.34 

4.14 

0.2 

.3 

.2 

.04 

<0.04 

<.04 

<.04 

<.04 

0.3 

1.2 

.5 

2.4 

0.28 

.22 

.15 

.08 

0.14 

.2 

.14 

.1 

.7 

0.31 

.20 

.2 

.31 

.51 

1.1,8 

0.1,2 

.1 

.08 

.12 

<0.04 

<.04 

<.04 

<.04 

0.1 

.18 

.12 

.7 

0.38 

.28 

.27 

.2 

The presence of gold in humus was observed by Goldschmidt 

(1937, p. 670) in beech humus from a very old beech and oak 

forest in Germany. Goldschmidt suggested that a number of elements, including gold, which enter the plant organism through 

the roots in soil solution and which are eventually deposited on 

the ground, are immobilized and enriched in this humus layer. 

A particular advantage of the mull sample is related to the usual 

occurrence of gold as dispersed particles in the soil and to the 

tendency of particulate gold under the influence of gravity to 

migrate downward in the soil and to accumulate in crevices of the 

underlying bedrock. In analyzing soil, then, one must have very 

fine gold particles or a great amount of luck to collect a 10-g soil 

sample that contains gold. In contrast, the humus layer has acted 
DISTRIBUTION OF GOLD IN SOILS AND VEGETATION 67 

as a reductant and concentration medium which retains exceedingly fine gold at the surface that has been accumulated from a 

large volume of soil by the tree. The relative insolubility of gold 

drastically limits the portion of the total gold under the plant 

that is available to the plant roots. One would expect to find easily 

detectable gold in the humus layer only in those areas where gold 

is relatively abundant. Its detection in humus suggests the possible presence of gold ores. 

Goldschmidt (1954, p. 204) suggested that the concentration of 

gold in forest humus soil "may be used as an indication of the 

occurrence of gold in lower levels of the soil." More recently forest 

humus (mull) has been used successfully as a sampling medium 

in glacial morainal, colluvial, and alluvial terrains of the Empire 

district, Colorado (Curtin and others, 1968, 1971) , of an area 

near Stibnite, Idaho (Banister, 1970), and of the Orogrande district, Idaho (Rice, 1970). 

An. insight into the usefulness of mull samples on essentially 

residual soils is provided by reconnaissance sampling of mull and 

underlying soil at intervals of 200 feet on a ridge east of the main 

Empire district, Clear Creek County, Colo. (figs. 7, 8, 9) . 

Four sample sites (EM-696-699) on a ridge east of the Empire 

district are shown in figure 7. The gold content of soil (depth 

3-8 in.) and of the ash of mull at these four sites and the localities 

of subsequent, more detailed samples are shown in figure 8. The 

gold contents of mull ash in the subsequent mull samples and of 

the vein materials obtained by trenching are shown in figure 9. 

The ash of mull at site EM-698 contained 0.62 ppm gold, 

although no gold ( <0.04 ppm) was found in the underlying soil 

or in the sample of the dump of the adjacent prospect. This anomalous gold content of the mull ash on apparently residual soil was 

further studied by sampling and trenching, as shown on figures 

8 and 9, to determine the source of the anomaly. 

The area is underlain by lenses of biotite gneiss that have been 

intruded by Boulder Creek Granite and small dikes of pegmatite. 

The lenses of biotite gneiss strike west to northwest and dip about 

30° NE. The numerous small pegmatite dikes and associated small 

migmatite zones occur principally at or near the granite-gneiss 

contact. 

The primary vein, revealed by a small mining operation and by 

our trenching with a backhoe, occupies a small shear zone that 

follows one of the pegmatite dikes for part of its length. The vein 

strikes about N. 70° ""VV. and dips steeply to the east. This vein, 

which is composed of disseminated sulfides in a matrix of finegrained quartz, is about 3-12 inches thick. The country rock is 
68 GEOCHEMISTRY OF GOLD IN THE WEATHERING CYCLE 

39° 

47' 

30" 

47' 

00" 

39° 

46' 

30" 

105°42' 

. . . 

41'30" 

. . . . . 

. . 

.... 

. . . 

. .. . . . .. ·. . . 

.. ... .... 

0 

I 

0 

•, 

1000 

I 

. . 

500 

2000 I 

41'00" 

.. 

. . . 

3000 FEET I 

I 

105°40'30" 

.. 

, Area of • 

, figure 8 , 

•. 

1000 METERS 

FIGURE 7.- Sample locality map of the Empire district, Colorado, showing location of site EM-698 (rectangle). Heavy lines are veins. Dashed 

lines indicate placered areas. 

argillized adjacent to the vein and contains some disseminated 

sulfides. 

The granite and biotite gneiss are weathered to a sandy loam 

to depths greater than 9 feet. The pegmatite dikes and silicified 

veins are much more resistant to weathering and remain as boulders of many sizes dispersed locally in the matrix of weathered 

country rock. 

The soils here, as in the rest of the district, are inceptisols 

(U.S. Department of Agriculture, 1960, p. 136). Under the mull 

layer is a gray ash-textured layer 1-3 inches thick. Below this 
DISTRIBUTION OF GOLD IN SOILS AND VEGETATION 69 

0 

.696 

<0.04S 

0.5M 

e697 

<0.04S 

<.0.04M 

0 

0 

0 

0 0 

EXPLANATION 

e 696 Reconnaissance 

sampling site 

o Detailed sampling 

site 

<0.04 S Au content (ppm) 

in soil 

0.62 M Au content (ppm) 

in mull 

c:=::::::J Trench 

~~~ Prospect 

Shaft and dump 

Vein 

-~i_ 97 0 0 

0 u t~Blf-... 0 ~, .... ,,~0 I 

698 i~ 0 ~~~~ f..o 0 r 1

11 , ............. 0 

<0.04S '"'o U.ti2M ~ 0 0 ~ 0 

0 

e699 

<0.04S 

0.14 M 

0 

0 

0 

100 

0 

0 0 

200 

50 

0 

0 0 

0 0 

0 

300 400 FEET 

100 METERS 

00 

FIGURE 8. - Sample locality map showing relaltionship of detailed sampling 

around site EM-698 and reconnaissance sampling, Empire district, Colorado. 

surface layer is a yellow to yellow-brown gravelly sandy loam that 

varies little in texture or color from a depth of 3 inches to bedrock. 

The area is in a subalpine zone, at an elevation of 10,000 feet, 

with a vegetative cover of lodgepole pine (Pinus contorta), aspen 

(Populus t·remuloides), kinnikinick (Arctostaphylos uva-ursi), 

and subalpine grasses. It is covered with snow at least 6 months 

of the year. Rain and sleet showers are frequent throughout the 

summer. 

Mull samples were collected at 60 sites (fig. 9) under 52 lodgepole pine trees and 8 aspen trees in an area 1,500 feet long and 
70 GEOCHEMISTRY OF GOLD IN THE WEATHERING CYCLE 

CD CD 0 

0 100 200 300 FEET 

0 50 100 METERS 

EXPLANATION 

Sample site and gold 

content of mull ash 

• ?0.5ppm 

t> 0.2 to 0.49 ppm 

CD 0.04 to 0.19 ppm 

o <0.04 ppm 

85c ppm gold in cobble 

of vein material in 

overburden--bedrock 

not exposed 

63b ppm gold in bedrock 

vein material 

Exploratory trench 

Shaft and dump 

Prospect pit 

Trace of vein 

[ ~~~ J Biotite gneiss 

B Boulder Creek Granite 

N 

t 

0 

0 

0 

FIGURE 9.- Geochemical and geologic map of locality EM-698, showing gold 

content of mull ash and vein material, Empire district, Colorado. 

600 feet wide. Soil samples were collected at spade depth ( 6--8 in.) 

directly under each mull sample site. 

Gold was not detected ( <0.04 ppm) in 10 of the ashes of the 

60 mull samples (fig. 9) and was 0.5 ppm or more in 9 samples 

collected along the trace of the vein. In contrast, gold was not 

detected in 42 of the 60 soil samples and was 0.5 ppm or more 

in only 1 sample. 

Trenching with a backhoe revealed a mineralized vein in bedrock in the center trench (fig. 9) that was 2--4 inches thick and 

contained 63 ppm gold. In the western trench the gold content 

was only 3.2 ppm. Float in the eastern trench contained 85 ppm 

gold. Obviously the gold mineralization located by intensive study 

of reconnaissance site EM-698 is not extensive. The point of 

interest from this sampling is that a relatively small vein can be 

the source of a widespread anomaly in the mull samples. 
SUMMARY OF CONCLUSIONS 71 

The biological source of the gold in the mull is substantiated 

by the presence of gold in the ash of various parts of the lodgepole 

pine and aspen growing near the vein (table 33). It is especially 

significant that the ash of clean interior wood of both the lodgepole pine and the aspen contained 1 ppm or more of gold. This, in 

our opinion, removes any doubts about gold being taken up from 

soil solutions by these trees. 

TABLE 33.- Gold content of the ash of various parts of lodgepole pine and 

aspen growing near gold-bearing vein 

Sample No. Description of Gold in Sample No. Description of Gold in EM-698- ash sample ash (ppm) EM-698- ash sample ash (ppm) 

25 Pine needles ...... 0.08 15 Pine wood .......... 1.6 

15 ...... do .................. .16 25 Pine bark. ......... .39 

25 Pine twigs ......... .3 15 ...... do .................. .75 

15 ...... do .................. .82 25 Pine root wood. 3.0 

15 Pine cones ......... .22 25 Aspen leaves .... <.04 

14 ...... do .................. .40 25 Aspen wood ...... 1.0 

25 Pine wood .......... 1.96 25 Aspen bark. ...... .1 

The original sample (EM-698H) of mull containing 0.63 ppm 

gold was collected 50 feet from this minor vein. This sensitivity 

to gold-bearing rock strongly suggests that no important gold 

deposits will be missed by reconnaissance sampling with mull. 

In contrast, the underlying soil was much less sensitive to the 

local presence of gold. 

SUl\tMARY OF CONCLUSIONS 

The chemistry of gold is the chemistry of its complex compounds. The stability of complex gold anions is reflected in the 

ease with which gold metal is oxidized in the presence of the 

complexing ion. The order of increasing ease of oxidation of gold 

to form the corresponding gold complex anion studied in this 

report is Cl-I, Br-1, CNS-1, I-I, and CN-I, with 8 20 3 - 2 probably 

between CNS- 1 and I- 1• 

The removal of gold from solution by various minerals either 

by sorption or more probably by reduction of the gold complex 

to yield gold metal showed a suspectibility to removal of AuCl4 - 1 

>AuBr .. - 2 >Aui:!- 1>Au (CN) 2- 1>Au (CNS) 4 - 1 > Au (S20a) 2-::. 

A marked difference in stability of the halide complexes to the 

cyanide, thiocyanate, and thiosulfate complexes was particularly 

evident; the halide complexes were much less stable and, therefore, much less mobile in a natural environment. 

A study of the absorption of gold by plants showed a strong 

tendency for the reduction of the halides by plant roots. Colloidal 

gold was not absorbed by the plants used in these experiments, 

although colloidal gold has been suggested as the probable form 
72 GEOCHEMISTRY OF GOLD IN THE WEATHERING CYCLE 

of gold taken up by plants. The most readily absorbed complex 

was the cyanide. 

The solubility of gold in acid halide solutions also shows an 

increasing ease of solution with increasing stability of the complex. Thus, gold chloride is readily formed in strongly acid solutions in the presence of manganese dioxide; gold bromide is formed 

in acid solution in the absence of manganese dioxide but is more 

readily dissolved in the presence of manganese dioxide; however, 

with gold iodide, the solution of gold is inversely proportional to 

the amount of manganese dioxide present, being highest in the 

absence of manganese dioxide. 

A comparison of the solubility of gold in the input solutions 

used in leaching copper from porphyry copper ore to which a given 

amount of chloride, bromide, or iodide has been added shows 

increasing solubility of gold from chloride to iodide; however, for 

a given concentration of the halide the solubility of gold decreases 

with increasing iron, aluminum, and copper. Some gold dissolved 

in the input solution would be lost from solution in the output 

solution. Input chloride solutions capable of dissolving gold failed 

to maintain gold in solution in the presence of an oxidized Carlin 

gold ore. The reduction of gold chloride by many minerals, plus 

the complexing of chloride by iron and copper, hampers the recovery of gold from copper ores by present leaching processes. The 

lack of recovery of gold from copper ores by leaching processes 

will greatly decrease the production of gold in the United States 

if present leaching processes gain wide use. 

To dissolve about the same amount of gold in a given time 

(1.5 mgjl in 3 weeks) in input leach solutions, the solutions must 

also contain 1,800 ppm chloride ion, or 400 ppm bromide ion, or 

13 ppm iodide ion, giving an equivalent effectiveness of 100 Cl- 1 

to 22 Br- 1 to 0.7 I- 1• The relative crustal abundance of these elements is 100 ppm Cl to 1.3 ppm Br to 0.24 ppm I. Thus, considerable enrichment of bromine and iodine would be required for these 

elements to be as effective as chloride in solubilization of gold in 

acid solutions. Bromine and especially iodine are enriched in certain plants and the leaching of plant residues gives rise to unusual 

concentration of these halides. Iodine leached from kelp has produced waters in Java that contain 150 ppm iodine; they also contain 68 ppm bromine. Organic-rich soils of Japan contain as much 

as 850 ppm bromine. Gold chloride, gold bromide, and gold iodide, 

therefore, may be formed locally under unusual but possible circumstances. When they are formed, they are readily decomposed 

by slight changes in their surroundings; consequently, they are 

transient forms. 
SUMMARY OF CONCLUSIONS 73 

The thiosulfate ion is a metastable species that can be formed 

by the action of water on native sulfur in basic solutions, by the 

reaction of hydrogen sulfide with oxygen, and by the catalyzed 

reaction of the sulfide ion with oxygen in soils by the enzyme 

oxidase. The thiosulfate anion as a complex former with gold, 

therefore, may be considered a transition anion from the simple 

single-element halides to the complex anions largely of biological 

origin. 

Gold is soluble in thiosulfate solution over a wide range of 

hydrogen ion concentration. It is particularly of interest that gold 

is soluble as the gold thiosulfate in the normal pH range of soils 

and that once gold thiosulfate is formed it is remarkably stable. 

In the weathering of pyritic gold ores in limestones, the gold may 

be mobilized as the thiosulfate. 

The formation of gold thiocyanate takes place readily over a 

wide range of pH. Free gold is dissolved in the presence of oxygen 

and thiosulfate, but gold in massive pyrite and in gold telluride 

is not attacked. The thiocyanate ion is of biochemical origin and is 

relatively rare. In microsystems within the soil it may reach concentrations adequate for complexing gold, but it does not seem a 

particularly important facet in the solution of gold. 

Gold cyanide offers the most feasible form of soluble gold in 

soils. Gold reacts with oxygen in the presence of the cyanide ion 

to form a stable complex. The presence of the cyanide ion in soils 

is widespread and often of considerable concentration. Over 1,000 

species of plants are known to contain cyanogenetic substances, 

as do many arthopods and moths ; certain fungi release gaseous 

hydrogen cyanide in remarkably large volumes. A few varieties 

of flax excrete significant quantities of hydrogen cyanide into the 

soil from their root system. Nutrient solution in which these varieties were grown contained as much as 37 mg of hydrogen cyanide 

per plant. In order to determine the abundance of plant species 

that are cyanogenic, plants were sampled from the Front Range 

of the Rocky -Mountains in Colorado to north-central Nevada and 

less completely in some other areas. Of the 150 species tested, 116 

species (78 percent) gave no test for cyanide, 15 species (10 percent) gave questionably positive tests for cyanide, and 19 species 

( 12 percent) gave positive tests for cyanide. Leaf gold was dissolved by an aqueous suspension of macerated plant species which 

gave positive tests for cyanide. 

The fact that cyanide, the byproduct of many life processes, 

forms such a strong complex with gold would be unique if cyanide 

were, in fact, the only organic ion formed by natural processes 

that produced stable gold compounds. The work reported in the 
74 GEOCHEMISTRY OF GOLD IN THE WEATHERING CYCLE 

present paper by Kenneth G. Doxtader on solubilization of gold 

by microorganisms and organic substances demonstrates the need 

for more careful research on the solution of gold by the complexing 

action of naturally occurring organic substances. It is in this area 

that knowledge of the behavior of gold in the weathering cycle is 

notably lacking. An organic substance might be found which has 

the complexing property of cyanide but which lacks the poisonous 

character of cyanide; it would be a boon to the recovery of gold 

from low-grade deposits. 

The analytical determination of gold in geologic materials is 

made difficult because of the strong probability that a representative sample is not available to the analyst. A relative standard 

deviation of less than 50 percent cannot be obtained on 10-g subsamples if the gold particles are greater than 0.01 mm in diameter. 

Gold wires of a few millimeters in length and consisting of less 

than 1 ppm of the sample are so few that the chances of finding 

one piece of gold in a 10-g sample are remote. With few exceptions 

the materials analyzed in this work contained fine gold, and, therefore, the results are reasonably reproducible. 

Gold in some soluble form, perhaps as gold cyanide, is absorbed 

by plants but is not used as a nutrient by plants. It is therefore 

found accumulating as a reject, like barium, in the woody parts 

of the plant. The decomposition of plant debris results in the 

reduction of the gold in the plant material, and gold accumulates 

in the humus horizon of the soil. This horizon, commonly mixed 

with colluvial inorganic material, constitutes what we have called 

mull, and mull has been proved to be a very satisfactory sample 

for gold exploration. A vein only a few inches thick was located 

by mull sampling at 200-foot intervals on a ridgetop; the closest 

reconnaissance sample was more than 50 feet from the gold-bearing vein. 

The distribution of gold within soil profiles may be grouped in 

three categories: (1) high gold content in the surface humus horizon; (2) high gold content in the surface and bottom horizons; 

( 3) high gold content in the bottom horizon. 

Gold may become enriched in the surface soil because of sheet 

erosion removing lighter material. It may also become enriched by 

biogeochemical cycling of gold, a process in which gold is leached 

from the soil and bedrock, is absorbed by the vegetation, and is 

eventually concentrated in the mull and upper layers of the soil 

as the vegetation decays. This process is presumed to be the dominant one in those profiles in which the gold content is high in 

the ash of mull. 

The high density of gold particles that permits the residual 
LITERATURE CITED 75 

enrichment of the particles in sheet erosion also permits their 

accumulation at bedrock surfaces by gravity separation in the soil 

profile by much the same process as in stream sediments. When 

gravity separation is combined with biochemical or erosional surface enrichment one finds soil profiles with high gold content at 

both the surface and the rock bottom. 

When no biogeochemical or erosional enrichment of the surface 

soil is occurring, the gravity effect is dominant and the soil shows 

high gold content only near bedrock or at the interface with a 

clay pan. 

The distribution of gold with particle size is a function of the 

completeness of the weathering of the ore. Inasmuch as many gold 

ores are emplaced with silicification, the ore may be more resistant 

to weathering than are the wallrocks. In such places, fine gold may 

be enclosed in coarse particles in the soil. This fact must be considered in soil or stream sampling in gold exploration. 

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76 GEOCHEMISTRY OF GOLD IN THE WEATHERING CYCLE 

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78 GEOCHEMISTRY OF GOLD IN THE WEATHERING CYCLE 

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