GEOLOGICAL SURVEY CIRCULAR 545 

Marine Sediment Sample 

Preparation for Analysis 

For Low Concentrations 

Of Fine Detrital" Gold 

Marine Sediment Sample 

Preparation for Analysis 

For Low Concentrations 

Of Fine Detrital Gold 

By H. Edward Clifton, Arthur Hubert, 

and R. Lawrence Phillips 

GEOlOGICAl SURVEY CIRCUlAR 545 

Washington 1967 
United States Department of the Interior 

STEWART l. UDALL, Secretary 

Geological Survey 

William T. Pecora, Director 

First printing 1 967 

Second printing 1970 

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

Page 

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 

Analytical methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

Particle sparsity effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

Sample preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

Examples of calculation of gold content of samples from 

analyses of concentrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 

References............ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 

FIGURE 

ILLUSTRATIONS 

1. Map showing location of samples used in this study ......... . 

2. Diagram of suggested procedure for concentrating any 

Page 

2 

given quantity of marine sands . . . . . . . . . . . . . . . . . . . . . . . . . 9 

III 
IV CONTENTS 

TABLES 

Page 

TABLE 1. Gold content of splits of samples of unprocessed sand compared 

with actual gold content determined on concentrates of 

these san1ples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~ 

2. Gold content of size fractions of offshore, beach, marine-terrace, 

and alluvial sand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . f 

:3. Gold content of light and heavy fractions separated with 

methylene iodide (sp gr = 3.3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . f 

4. Change in gold content in gravimetric fractions with increasing 

duration of heavy liquid (methylene iodide) separation of 

six splits of the fraction smaller than 0.124 mm from sample 

M660C-56 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . f5. Gold content of the <0.124 mm heavy (sp gr>3.3) fraction of 

samples aftt>r magnetic separation 7 

6. Reproducibility of analyses of concentrates, recalculated to gold 

content of original sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 
MARINE SEDIMENT SAMPLE PREPARATION FC~'R 

ANALYSIS FOR LOW CONCENTRATIONS OF 

FINE DETRITAL GOLD 

By H. Edward Clifton, Arthur Hubert, and R. Lawrence Phillips 

Abstract 

Analyses by atomic absorption for detrital gold in 

more than 2,000 beach, offshore, marine-terrace, and 

alluvial sands from southern Oregon have shown that 

the values determined from raw or unconcentrated 

seclimen t containing small amounts of gold are neither 

reproducible nor representative of the initial sample. 

This difficulty results from a "particle sparsity effect", 

whereby the analysis for gold in a given sample depends more upon the occurrence of random flakes of 

gold in the analyzed portion than upon the actual gold 

content of the sample. 

The particle sparsity effect can largely be eliminated 

by preparing a gold concentrate prior to analysis. A 

combination of sieve, gravimetric, and magnetic 

separation produces a satisfactory concentrate that 

yields accurate and reproducible analyses. In concentrates of nearly every marine and beach sand 

studied, the gold occurs in the nonmagnetic fraction 

smaller than 0.124 mm and with a specific gravity 

greater than 3.3. The grain size of gold in stream 

sediments is somewhat more variable. 

Analysis of concentrates provides a means of greatly 

increasing the sensitivity of the analytical technique in 

relation to the initial sample. Gold rarely exceeds 1 

part per million in even the richest black sand analyzed; to establish the distribution of gold (and 

platinum) in marine sediments and its relationship to 

source and environmental factors, one commonly needs 

to know their content to the part per billion range. 

Analysis of a concentrate and recalculation to the 

value in the initial sample permits this degree of 

sensitivity. 

INTRODUCTION 

The U.S. Geological Survey, as part of its 

program to identify sources of supply of many 

minerals that are in shortage categories, initiated a geologic investigation of the origin 

and resource potential of marine black-sand 

deposits on the continental shelf of southern 

1 

Oregon (fig. 1). The gold and platinum potential of this area has long been recognized 

(Blake, 1854; Day and Richardson, 1906; Diller. 1914; Hornor, 1918; and Pardee, 1934); 

black sands are common on modern beaches 

and in 1narine terrace deposits of Pleistocene 

age (Griggs, 1945). 

At the tilne of this study, routine analyses 

of trace an10unts of platinum could not be 

made; therefore nearly all analytical effort in 

this investigation has concerned gold. Platinun1, however, generally accmnpanies gold in 

the black sand, so that auriferous sands may 

be considered also to contain some platinum. 

To date, 2,000 analyses have been made for 

gold in offshore, beach, and marine-terrace 

sands and in stream sedirnent. The results of 

these analyses and their significance will be 

reported separately; this paper deals only with 

those data that apply to sample preparation. 

Initial work indicated a disconcerting lack 

of consistency among gold values fo 1Ind by 

several analyses of a single san1ple. Sub~equent 

experi1nents showed that analyses of gold in 

splits of raw sand samples generally do not 

accurately measure the amount of golc1 in the 

original sample. Furthermore, such r.nalyses 

did not permit detection of gold in concentration smaller than 0.1 ppm, a value rarely exceeded by n1ost beach and offshore sands. The 

gold content of such sands, however small, 

must be established in order to understand 

the principles of gold distribution in tl,~se environments. 
N 

I 43" 

Port Orford 

...... . C":l • 

0 20 MILES 

-----CALIFORNIAEXPLANATION 

SAMPLE LOCATIONS 

. )( 

Offshore Marine terrace 

0 6 

Beaeh Stream 

B . 

. 

AREA OF THIS REPORT 

Figure 1.-Location from which samples used in this 

study were obtained. Each location shown represents 

one or more samples. 

Analytical Methods 

Samples were prepared for analysis either in 

a mobile sedimentology laboratory in the 

field or in the U.S. Geological Survey laboratories in Menlo Park, Calif. Nearly all the gold 

analyses were made by a wet chemical method 

(Lakin and Nakagawa, 1965) in conjunction 

with atomic absorption spectrophotometry, although a few were made spectrographically. 

Most of the atomic absorption analyses were 

conducted in the field in a mobile laboratory. 

We gratefully acknowledge the assistance of 

F. J. Swanson, David D' Armond, and R. G. 

Winkler with the preparation of the sediment 

samples and the help of Alan Chleborad and 

Francis Michaels with the atomic absorption analysis. E. M. Baldwin and Sam Boggs, 

Jr., of the University of Oregon, contributed 

stream sediment samples for the study. Off2 

shore samples were taken witr. a grab-type 

bottom sampler from the U.S. Geological Survey research vessel Polaris under the direction 

of G. A. Rusnak. 

Particle Sparsity Effect 

The major difficulty in obta.ining a gold 

analysis representative of a given sample results from the sparsity of gold particles in 

marine sediments. A single flake of gold, 

0.010-0.020 mm thick and 0.060-0.090 mm in 

diameter (a size and shape com.n10nly found in 

beach and terrace sands) in a gram of sand 

constitutes about 1 ppm (part per million). 

This is very fine gold from a 1niner's standpoint; this hypothetical flake, at about 31 million particles per Troy ounce, is one-third the 

size described as typical flour geld (10 million 

particles per ounce) by Hite ( 1933a, p. 260) 

and less than one-thirty fifth a~ large as the 

fine flour ( 885,000 particles per ounce) of Ingersoll (1932, p. 8), but even with such fine 

gold only a limited number of particles are 

required to produce the grades (mostly less 

than 1 ppm) found to date in the Oregon black 

sands. 

The analytical data can reacily be misinterpreted where samples cortain a limited number of gold particles. A 500-gram 

sample of sand containing 0.5 ppm gold in 

flakes weighing 5 X 10~ g (approximately 0.03 

mm thick and 0.1 mm in diameter) would 

contain 50 flakes. If the sample were split into 

250 2-g portions, none of which c0ntained more 

than a single flake of gold, 50 portions would 

contain one flak'e and the remaining 200 would 

contain no gold. In other words, in analyzing 

this sample, the chances are 80 percent that 

no gold will be detected; if f'. gold-bearing 

split were analyzed, its gold cortent would be 

a misleading 2.5 ppm. 

This circumstance we have termed the particle sparsity effect, wherein the detrital gold 

content of a given sample, as determined by 

analysis of a split, depends not S'J much on the 

actual gold content of the s·ample as on 

whether or not a random flake occurs in the 

analyzed portion. Obviously, nf'.tural samples 

are more complicated than the hypothetical 

sample discussed above, as they contain several 

sizes of gold flakes and any one portion could 

readily contain more than one flake. The principle, however, of the particle sparsity effect 
does apply to natural samples as noted by 

Pardee (1934, p. 34) and as may be seen 

from table 1. The gold content of splits from a 

single sample of unprocessed sand differed as 

much as 29 ppm (table 1, sample M660C-56); 

the analyses are not reproducible, nor is any 

of them representative of the initial sample. 

The particle sparsity effect is most sig:'lificant 

in samples containing relatively little ~·old. As 

the content increases, the effect will de.dine if 

particle size remains constant. It is unquestionably important in the range of cor~entrations that we have found in beach 2nd offshore sand. 

TABLE 1.--Gold content of splits of samples of unprocessed sand compo;red with actual gold content de4;ermined 

on concentrates of these samples 

[Gold reported in parts per million. Analyses by atomic absorption unless otherwise noted) 

Portion selection 

Scoop ~/ sample splitter £/ Powder ~./ 

Number of analyses Number of analyses Number of analyses 

Sample 

Actual 

gold 

content 

with the same Gold found with the same Gold found with the same Geld found 

gold content gold content gold content 

M66oc-56----- o.52 7 <0.1 6 <0.1 8 <0.1 

1 3.0 1 .25 1 3.0 

1 1.8 1 .89 1 .35 

1 29. l 4.7 1 <.1 

l 2.9 l 1.4 

55----- .31 9 <.1 9 <.1 7 <.1 

l 1.2 1 .36 1 .55 

l 1.0 

l 2.3 

2 <.1 

51----- .023 4 <,l <.1 4 <.1 

l .4 l 2.9 l .4 

l 1.8 1 1.8 

2 <,1 

67----- .07 4 <.1 6 <.l 2 <,1 

l 1.4 

1 l.l 

71----- .075 <.1 6 <.l 2 <.1 

1 1.4 

M660H7l-24--- ~/<.1 l 13. 1 3.0 

4 <.1 1 .25 

32 <.1 

T-22--------- ~~ l 7.0 

1 <,l 

2 !/<.2 

M660C-l2----- .22 9.5 

1 .3 

M660P-16----- 1.1 2 3.0 

a/ Approximately 2-g portion taken randomly from a larger split of the initial sample using a scoop with capacity of approximately 2 g of sample. 

£/ Two-g portion, weighed to 0.01 g, after being split fromlinitial sample through a riffle-type sample splitter. 

c/ Approximately 2-g portion taken randomly with a scoop from powder produced in a small ball mill from 8- tn 40-g splits 

from the initial sample. 

d/ No gold detected in concentrate. Available sample too small (28.6 g) to provide representative concentrate. 

e/ No analysis of concentrate. !/ Spectrographic analysis. 

3 
Sample Preparation 

The foregoing discussion demonstrates the 

futility of analysis for detrital gold in small 

splits of unprocessed sand. Analysis of a 

weighed concentrate which contained all the 

gold in a sample would obviate this difficulty. 

Such a concentrate can be obtained using 

sample-preparation techniques such as those 

described by Wager and Brown (1960). 

The simplest and most rapid concentrating 

mechanism is probably the gold pan. Panning, 

however, although doubtlessly an effective field 

expedient, presents some major difficulties. The 

potential for operator error is inherently high. 

The panning technique must be rigorously uniforrn from sample to sample, particularly as 

the smallest flecks of gold can easily be lost 

during panning. Ewing (1931, p. 136) compared heavy mineral concentrates taken gravimetrically from panned and unpanned samples 

and found discrepancies that he attributed to 

panning losses. Furthernwre, unless the 

weights of the original sample and the panned 

residue are carefully compared, which may be 

difficult in the field, the gold content of the 

initial sample remains unknown. It seems, 

therefore, that panning, unless done rigorously 

and expertly, is a generally impractical method 

of concentrating gold for analysis, particularly 

if flour gold is abundant. 

Son1e of the difficulties inherent in panning 

may be overcon1e by use of mechanically controlled riffles, such as a wilfiey table or a 

micropanner (Muller, 1959). Our limited experience, however, with a wilfl.ey table indicated that, although it effectively concenb·ates heavy minerals fron1 the sand, too many 

heavy grains are lost to pern1it its use for precise quantitative separation. 

A different concentrating technique is based 

on the physical characteristics of detrital gold 

in contrast to the sand in which it occurs. 

Because of its extremely high specific gravity 

(sp gr = 15-19), grains of gold are much 

smaller than hydraulically equivalent grains 

of quartz. To establish the size range of detrital gold, samples were initially dry sieved into 

three size fractions: 0.991-0.246 mm, 0.246-

0.124 mm, and smaller than 0.124 mn1. To 

further concentrate the gold, each fraction was 

separated into a heavy and a light fraction 

using the heavy liquid methylene iodide (sp 

gr = 3.3). Analysis of these fractions indicated 

4 

that in nearly every beach and offshore sand 

sample studied, gold occurs n1ostly in the 

smaller than 0.124 mm fraction (table 2). It 

should be noted, however, that in one sample 

(M660C-13) an appreciable an1ount of gold 

(6.2 ppm) is in the 0.246-0.124 mm fraction, 

whereas the finest fraction contained no gold. 

Another sample (M660P-1) C')ntained 0.02 

ppm gold in the 0.991-0.246 mm fraction; 

however, the gold content of the 2-g analyzed 

portion ( 1.5 ppm) is less than would result 

from the occurrence in this split of a single 

gold particle in this size class. Such a value 

may result either from adherenc~ of tiny gold 

flakes to coarser sand grains, from incomplete 

disaggregation, or from gold inclusions within 

quartz grains. Where significant amounts of 

coarser gold are suspected, the sieve size should 

be increased (for example, from 0.124 to 0.177 

mm) to accommodate all the gold. 

The fraction smaller than 0.124 mm of a 

few samples known to contain gold was 

further split through 0.088-, 0.062-, and 

0.038-mm sieves. Table 2 shows that the bulk 

of the gold in these samples is di~tributed 

through these size classes; the amount of gold 

passing through a 0.038-mm sieve was insignificant. 

Consideration was given, before analysis of 

gravimetric concentrates, to t~e possibility 

that the fine size and ftakinesr of the gold 

might cause it to adhere to coarser sand grains 

and to be concentrated in the lighter fraction. 

Analysis of both light and heavy fractions 

(table 3) shows that some gold does occur in 

the light fraction. Nearly all samples, however, 

with gold in the light fraction contained much 

more gold in their heavy fractic n. The occurrence of gold in the light fraction is undoubtedly due to incomplete separation; table 

4 shows how gold shifts into the heavy fraction as the time of separation is increased. 

Analysis of both light and heav;r fractions in 

several size classes indicates that, as might be 

expected, the finer gold is more difficult to 

separate gravimetrically (table 3, samples 

M660C-56, M660C__.55). Complete separation 

of the gold might be facilitated by attaching 

a vibratory motor to the heavy-liquid separatory funnel system to provide continuous slight agitation. 

Some samples, even after sie~.'e and gravimetric concentration, contain enough magnetite and ilmenite or, locally, chromite 
TABLE 2.-Gold content of size fractions of offshore, beach, marine-terrace, and alluvial sand 

[Results in parts per million. Less-than values are calculated from analysis of only the heavy fraction of the size c'la~'>] 

Sample 

Offshore-marine sand: 

P-66-13-----------

19-----------

22-----------

27-----------

53-----------

55-----------

Beach sand: 

M66oP-l-----------

3-----------

4-----------

5-----------

6-----------

7-----------

8-----------

9-----------

M660C-13----------

14----------

15----------

51----------

52----------

66Ahp35A----------

M660P-77----------

Marine-terrace sand: 

M66oC-12a---------

12b---------

21----------

23----------

55----------

56----------

Alluvial sand: 

M660EB-126--------

134--------

139--------

~/ <0.124 mm. 

Gold content of indicated size fractions (mm) 

0.991-0.246 o.246-0.l24 o.l24-o.o88 o.o88-o.o62 o.o62-o.o38 '~.o38 

<0,02 

<,02 

<,02 

<,02 

c,o6 

c,08 

.02 

<.0009 

<,008 

<,003 

<.03 

<.008 

<.008 

<,04 

<.02 

<.07 

<,02 

<.07 

<,04 

<.13 

<.02 

<,06 

<.03 

<.03 

<.03 

<.05 

.3 

<.008 

<0,004 

<,005 

<,007 

<,005 

<,006 

c,004 

<,06 

<,0003 

<.05 

<..02 

c.08 

<..09 

<.03 

<.07 

6.2 

<,1 

<,08 

<,06 

<.05 

<.05 

<.09 

<.08 

<,07 

c,07 

<.03 

.2 

.03 

8.1 

.02 

~/0.03 

~/ .03 

~/ .01 

~/ .02 

~/ <,02 

£./<,02 

~/22 

~/5.0 

~/17.5 

~/4.0 

~/ll.O 

~/20 

~/1.8 

~./22 

~/ <.1 

~/2.4 

~/1.2 

~/ .5 

!1 .6 

~/ .1 

4.7 

!/7.1 

!/1.3 

!/6.5 

!1 .4 

.8 

7.2 

,!/<.01 

,!/<,07 

!I .1 

£./ 0.124-. o62 mm. 

5 

18 

<,0003 

150 

~/0.03 

~/ .08 

8.6 

6.9 

140 

~/cO.o62 mm. 

<0,1 

.3 
TABLE 3.-Gold content of light and heavy fractions 

separated with methylene iodide (sp gr=3.3) 

[Results in parts per million] 

Sample 

M66oc-46a-----

b-----

d-----

M660C-47a-----

b-----

d--- --

M66oc -12a -----

b--- --

d--- --

M66oc -56------

M66oc -56------

(recheck) 

M660C-55------

!_I Duplicate. 

Size fraction 

{mm) 

<0,124 

!.1<.124 

<.124 

!.1 <.124 

<.124 

!.1<.124 

<.124 

!_1<.124 

<,124 

<.124 

<,124 

<,124 

<.124 

<,124 

<.124 

<,124 

<.124 

0.124-.088 

.o88-.o62 

<.o62 

.124-.088 

.088-.062 

.o62-.038 

<.038 

<.124 

.124-.088 

.o88-.o62 

<.o62 

Light Heavy 

fraction fraction 

{sp gr =<3.3) {sp gr = .. 3.3) 

1.0 11 

<,1 15 

<.1 8.3 

<,1 15 

<,1 16 

<.1 7.2 

4.5 8.9 

<,1 5.8 

3.3 230 

<.1 230 

3. 3 240 

<,1 240 

<.1 1.3 

<,1 7.5 

3.3 24 

<,1 6.5 

<,1 15 

<.1 8.0 

<.1 220 

.9 930 

<.1 3.4 

1. 5 230 

93 680 

. 35 <.1 

.27 5.2 

<.1 <.1 

<.1 c.l 

6o 26 

(Griggs, 1945) to dilute the gold below the 

detection level. For such samples, further concentration can be made magnetically. A hand 

magnet readily removes the magnetite, and a 

Frantz magnetic separator (Gaudin and Spedden, 1943) removes ilmenite and chromite 

when set at 0.6 amps, with a forward slope of 

25 ° and a side slope of 15 °. Some of the gold 

in the southern Oregon black sands reportedly 

is coated with iron oxide (Pardee, 1934, p. 

38). This coating, however, seems not to increase significantly the magnetic susceptibility 

of the gold, which occurs almost entirely in the 

less magnetic fraction (table 5). The occurrence of gold in the magnetic fractions of 

sample M660C-56a is thought to result from 

incomplete magnetic separation, a conclusion 

supported by the results of analysis of M660C6 

56b, a carefully processed split of the same 

sample. 

A few concentrates-even aft~r sieve, gravimetric, and magnetic separation-are too 

large for a single analysis. Such concentrates 

can be split into analyzable portions with a 

riffle-type sample splitter. Analy-sis of all the 

portions establishes the total gold content; 

analysis of one provides a value presumably 

representative of the total concentrate. The 

particle sparsity effect, howev~r, may still 

apply if the sample contains v~~ry little gold. 

Splits of such concentrates show less consistency in their values than do concentrates of 

large splits of a single sample (table 6). 

Therefore, analysis of the tot~.l concentrate 

provides a better gold determination than does 

analysis of a presumably repre.,.~ntative split 

from the concentrate. It is most important that 

the analyst utilize the entire p'lrtion of concentrate submitted for the analysis, lest the 

particle sparsity effect reappear even after the 

most careful concentration of th~ gold. 

A concentrating technique for marine sands 

is suggested in figure 2. This m~thod requires 

modification depending on the size of the gold 

and the sorting of the sedimert. Beach, offshore, and marine-terrace sand"' of southern 

Oregon contain very little gold coarser than 

0.124 mm. These sands lie tens of miles from 

the inland original source of the gold; therefore, their detrital gold may be expected to be 

very fine. In beach sand in Alr~ka, however, 

gold less than 2 miles from the source (D. M. 

Hopkins, oral commun., 1966) is of similar 

grain size. Additional data may show that 

the grain size of gold in beach and shelf sands 

is everywhere smaller than 0.124 mm. 

Gold in stream sediments shows less consistency in size (table 2). The concentration 

TABLE 4.-Change in gold content in gravimetric fractions with increasing duration of heavy liquid 

(methylene iodide) separation of six splits of the 

fraction smaller than 0.124 mm frorn sample M660C56 

[Results in parts per million. Sample stirred after 

15 and 30 minutes] 

Change in gold cortent with indicated 

length of separation time (minutes) 

15 30 4; 60 90 120 

Light fraction (sp gr <3.3)-- 2.9 1.7 <0,1 <O.l <0,1 <0.1 

Heavy fraction (sp gr >3.3) --60 35 26 36 25 34 
technique, obviously, must take the size factor 

into account. In stream sediments, for example, the material greater than 2 mm may be 

eliminated by sieving, clay-size particles may 

be removed by washing and decanting, and the 

light fraction may be separated gravimetrically, thus concentrating the gold in the 

heavy intermediate grain-size fraction. In the 

Snake River, Idaho, the finest detrital gold 

flake observed by Hite (1933b, p. 698) was 

approximately 0.01 mm across and 0.002 mm 

thick. Such a particle would weigh about 

0.0033 millionths of a gram, and 300 of these 

particles in a gram of sample (or 600 in 2 g) 

would constitute 1 ppm, enough to eliminate 

the particle sparsity effect and the n~ed for 

concentration. Our studies, however, have not 

yet established the presence of any appreciable 

amount of such extremely fine gold. 

Similarly, the sorting of a sediment may determine the concentration technique. The wellsorted beach sands of the Oregon coasts contain very little light material in the smaller 

than 0.124 mm fraction. Because the gold is 

nearly always restricted to this size fraction, 

TABLE 5.-Gold content of the< 0.124 mm heavy (sp gr>3.9) fraction of samples after magnetic separation 

[Results in parts per million] 

Gold content Gold content of <0,124-mm heavy fraction (sepaution 

(.hand-magnet with Frantz isodynamic magnetic separator after handSample separation) magnet separation--setting in amperes indicated) 

Magnetic Nonmagnetic 

portion portion <0.4 0.4-0.6 0.6-1. 2 1. 2-1.5 >1.5 

M66oc-46a------ <0.1 11 

<,1 15 

b------ <,1 8.2 

<,1 15 

c------ <,1 16 

<,1 7.2 

d------ <.1 8.9 

<.1 5. 7 

47a------ <.1 230 

b------ <.1 230 

51------- <,1 3.5 <.l ~/1.3 

52------- <,1 3.5 <.1 ~/1.5 

55------- <,1 1.6 <.1 ~/120 

56a------ 8.5 34 5.5 ~/540 

b------ <.l <,1 <.1 3.5 470 <0,1 

71------- <.1 1.7 .83 <.1 <.l 42 

67a------ <.1 6.2 <.1 <.l <,l 5.8 <.1 

b------ <,l <.1 <,1 <.1 11 

c------ <.1 .3 <,1 <.l 

d------ <.1 .4 <.l <.1 

~I >0.4 amp. 

7 
TABLE 6.-Reproducibility of analyses of concentrates, 

recalculated to ,qold content of original sample 

S11mple 

M66oct46a------

b------

d------

47a------

b------

d------

b------

S6a------

b------

d------

f------

Gold 

(ppm) 

!1 .21 

'!I .26 

'!I .18 

.CJ6 

1,1 

1.2 

1.2 

,ll 

.22 

~_/0.15-.16 

!./ .ll 

!1 .49 

.27 

. 35 

.25 

g------ .25 

h---·-- ,18 

i-----· .20 

j------ .27 

57-------

Splits of 

concentrate 

Gold 

(ppm) 

0.18 

.27 

.19 

. 35 

. 35 

.16 

.23 

.15 

.20 

.21 

.05 

<.03 

. 46 

<.03 

.17 

.05 

.66 

.32 

.49 

.39 

. 35 

.14 

<.005 

c.Ol 

.24 

<,01 

<,01 

<.005 

e,l B11sed on calculllti011 frOl!l the values of the splits nf rancentretes. Where 

lei3s-than values exist for splits nf concentrates, the averaged value given 

indicates maximum 11nd 1l!inimum gold content. 

sieving commonly is the only concentration 

required. 

Sieve, gravimetric, and magnetic separations constitute an effective concentration 

method for gold. Reproducibility is good, as 

indicated by analysis of concentrates of several splits of the same original sample (table 

8 

6). The gold content of the initial sample as 

calculated from the analysis will be accurate, 

providing the concentrate contains all the gold 

in the sample. Examples of this calculation 

are shown under "Examples of calculation of 

gold content of samples from analyses of concentrates." 

The concentration method de~cribed herein 

has several disadvantages. Very large san1ples 

are difficult to handle with standard laboratory 

facilities. The method is rather slow, although with proper facilities t"vo technicians 

can readily process 30 1,000-g samples a day. 

Currently, we use a stack of duplicate sieves 

separated by insert pans in orier to process 

several samples simultaneously. Heavy liquid 

separation is done in a battery of large funnels. Magnetic separation can doubtlessly be 

improved by using an electrom~.gnetic separator (Holmes, 1930, p. 89), as separation by 

hand magnet is particularly tiJne consuming . 

The method has provided a byproduct of useful information about the size of gold particles 

and the size distribution and heavy-mineral 

content of auriferous sand. 

Examples of Calculation of Gold 

Content of Samples from ..Analyses of 

Con cent rates 

Exarnple 1.-M660C-41, a sample of 368 g, 

yields a concentrate of 0.44 g, that, as analyzed, contains 6.6 ppm gold. What is the gold 

content of this sample, assuming that the concentrate contains all the gold in the sample? 

Gold content of the sample= weight 

of the portion analyzed times its 

gold content (ppm) divided by the 

weight of the initial sample, that 

is, (0.44 X 6.6) /368=0.008 ppm. 

Exarnple 2.-M660C-39, a sample of 155 g, 

yields a concentrate of 4.13 g. A 2.02-g split 

of the concentrate, as analyzed, contains 0.7 

ppm gold. What is the gold content of this 

sample, assuming that the concentrate contains all the gold in the samph~ and that the 

gold content of the split is representative of 

that of the concentrate? 

Gold content of the sample=weight 

of the whole concentrate times the 

gold content of the analyzed portion divided by the weight of the 

initial sample, that is, ( 4 .. 13 X 0.7) I 

155=0.019 ppm. 
Example 3.-M660P-77, a sample of 2,530 

g, yields heavy (sp gr >3.3) fractions that 

were analyzed for gold with the following 

results: 

e. 1.27 g, 4. 7 ppm 

f. 1.95 g, <0.1 ppm 

0.088-0.062 mm size class, 1.95 g, 18 ppm 

0.062-0.038 mm size class, 0.44 g, 8.6 ppm 

0.124-0.088 mm size class, 128 g from <0.038 mm size class, 0.50 g, <0.4 ppm 

which six portions were split for 

analysis: 

What is the gold content of this sr.mple? 

Calculation of the contribution of g')ld from 

a. 2.09 g, <0.1 ppm the 0.124-0.088 mm heavy fraction: 

b. 1.22 g, <0.2 ppm Maximum gold content of the portions analyzed= sum of the products of the W3ights of the portions 

c. 1.13 g, <0.2 ppm 

d. 1.23 g, <0.2 ppm 

INITIAL SAMPLE 

A. Weigh 

t 

B. Pass through 0.124 mm sieve 1 

C. Weigh fraction smaller than 0.124 mm 2 

I I 

If smaller than maximum If larger than maximum analyzable portion 

analyzable port1on 

ANALYST 3 

Spht into portions for separate 

individual analysis 

D. Separate with methylene iodide (specific gravity= 3. 3) 

E. Weigh heavy fraction 

I I 

If smaller than maximum If larger than maximum 

analyzable po~:,~ onto Po•tions fo .. ~~~~~:~•A ondividual analysis - 1 

F. Separate magnetically 

G. Weigh the less magnetic fraction 

I .1 

If smaller than max1mum If larger than max1mum 

analyzable portion analyrble portion 

Split into portions for separate 

individual analysis 

1 The sieve size used in this separation should be large enough 

to allow passage of all the gold particles in the sample. 

2 If sieving yields only a small fraction, resieving through a 

slightly coarser sieve may be advisable before submittal for 

analysis or before further processing. 

3 It must be recognized that. where different concentrating 

techniques are used, the analytical results may differ 

accordingly. 

Figure 2.-Suggested procedure for concentrating any given quantity of marine (beach, offsho,.e, 

and marine-terrace) sands. Quantity of sample used for analysis may be modified to confo~ 

with analytical technique. 

9 
analyzed times their maximum gold 

content divided by the sum of the 

weights of the portions analyzed, 

that is, [ (2.09 0.1) + ( 1.22 0.2) + 

( 1.13 0.2) + ( 1.23 Y 0.2) + ( 1.27 X 

4.7) + (1.95 0.1)]1[2.09 + 1.22+ 

1.13 + 1.23 + 1.27 + 1.9'5] = 7.091 

8.89 = 0.80 ppm. 

Minimum gold content of the portions 

analyzed=sum of the products of 

the weights of the portions 

analyzed times their minimum gold 

content divided by the sum of the 

weights of the portions analyzed, 

that is, ( 1.27 X 4.7) I (2.09 + 1.22 + 

1.13 + 1.23 + 1.27 + 1.95) = 5.971 

8.89=0.68 ppm. 

Assuming the gold content of the portions 

analyzed to be representative of the whole 

0.124-0.088 mm heavy fraction, this fraction 

contains between 0.80 and 0.68 ppm gold. The 

contribution of this fraction to the gold in the 

total sanlple may be calculated as follows: 

Contribution of gold from a fraction 

to the total gold content of the sample= weight of the fraction times 

its gold content divided by the 

weight of the total sample, that is, 

( 128 X 0.80) 12,5,30 = 0.041 ppm 

(maximum), and (128 0.68) I 

2,530 = 0.34 ppm (minimun1) 

Similarly the contributions of . the 

other fractions are as follows: 

0.088-0.062 mm heavy fraction= 

( 1. 95 X 18) /2,530 = 0.014 ppm 

0.062-0.038 mm heavy fraction= 

(0.44 8.6)/2,530=0.001 ppm 

< 0.038 mm heavy fraction maximum contribution is insignificant 

(0.0001 ppm) 

The gold content of the initial sample is the 

sum of the contributions from the different 

fractions: 

Maximum= 0.04 + 0.014 + 0.001 ppm= 

0.055 ppm 

Minimum=0.034+0.014+0.001 ppm= 

0.049 ppm 

The gold content of this sample is between 

0.055 and 0.049 ppm. 

Conclusions 

Analyses of splits of raw samples containing low concentrations of detrital gold are not 

10 

reproducible, nor in general is any single 

analysis representative of the gold content of 

the sample. Reliable analysis, however, may be 

made on concentrates. The method described, 

which combines sieve, gravime~.ric, and magnetic separations, has generally been adequate for beach and offshore sands. With modification, it may prove effective for analysis of 

stream sediments. 

At the time the concentratiiJg method was 

being developed, the size of the analyzed portion was restricted to 2 g. Presently the analytical technique is being modified to accommodate 10-g samples (H. W. Lakin, oral commun., 

1967). This increase greatly facilitates the 

concentrating procedure; for example, it will 

be possible to quickly concentrate a sample to 

50 g which can be split into five separately 

analyzable portions. 

The described concentrating method works 

well in the field within a m9bile sedimentology laboratory and requires no highly spe ... 

cialized equipment. Other methods will almost 

certainly be required to conce:r1trate detrital 

gold from sedimentary rocks. Techniques such 

as flotation or electrostatic separation may 

prove to be highly practical. Industrial-scale 

facilities may provide the quickest and most 

convenient means for concentration, particularly for large samples. 

Our data are taken only frmn analyses for 

detrital gold. The principle of the particle 

sparsity effect, however, is certain to apply to 

other minerals of high specific gravity, such 

as platinum, which will likewiEe require concentration of the initial sample. 

This paper deals only with the problem of 

obtaining an analysis that is representative of 

the gold content of a given sample. The problem of taking meaningful samples, of course, 

remains with the geologist. 

References 

Blake, W. P., 1854, On gold and rlatinum of Cape 

Blanco [California]: Am. Jour. fd., 2d ser., v. 18, 

p. 156. 

Day, D. T., and Richardson, R. P., 1906, [Useful 

minerals in the] Black sands of the Pacific slope: 

U.S. Geol. Survey Mineral Resources, 1905, p. 

1175-1246. 

Diller, J. S., 1914, Mineral resourcef of southwestern 

Oregon: U.S. Geol. Survey Bull. 546, 14 7 p. 

Ewing, C. J. C., 1931, A comparison of the methods of 

heavy mineral separation: Geol. Mag. [Great 

Britain], v. 68, p. 136-140. 

Gaudin, A. M., and Spedden, H. R, 1943, Magnetic 
separation of sulphide minerals: Am. Inst. Mining 

Metall. Engineers Tech. Pub. 1549, 13 p. 

Griggs, A. B., 1945, Chromite-bearing sands of the 

southern part of the coast of Oregon: U.S. Geol. 

Survey Bull. 945-E, p. 113-150. 

Hite, T. H., 1933a, Fine gold and platinum of Snake 

River, Idaho: Eeon. Geology, v. 28, no. 3, p. 256-

265. 

1933b, Special features of fine gold from 

Snake River, Idaho: Econ. Geology, v. 28, no. 7, p. 

686-691. 

Holmes, Arthur, 1930, Petrographic methods and calculations; with some examples of results achieved, 

rev. ed.: New York, D. Van Nostrand Co., Inc., 516 

p. 

Hornor, R. R., 1918, Notes on the black sand deposits 

of southern Oregon and northern California: U.S. 

Bur. Mines Tech. Paper 196, 42 p. 

Ingersoll, G. E., 1932, Small scale methods of placer 

mining and placer mining districts of Washington 

and Oregon: Washington State Coll. Monthly Bull. 

(Eng. Expt. Sta., Eng. Bull. 43, 1933), v. 15, no. 

11 

6, 59 p. 

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

traces of gold in geologic materials, in Geological 

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

Paper 525-C, p. C168-C171. 

Miesch, A. T ., 1967, Chart showing number of rarticles 

of metal expected per 5-pound sample for various 

levels of grade and average particle size (regardless of the spatial distribution of particler): U.S. 

Geol. Survey open-file report. 

Muller, L. D., 1959, The micropanner-an appar~.tus for 

the gravity concentration of small quantities of 

materials: Inst. Mining and Metallurgy [London] 

Trans., v. 68, pt. 1, p. 1-7. 

Pardee, J. T ., 1934, Beach placers of the Oregon coast: 

U.S. Geol. Survey Circ. 8, 41 p. 

Wager, L. R., and Brown, G. M., 1960, Collection and 

preparation of material for analysis, Chap. 2, in 

Smales, A. A., and Wager, L. R., eds., Methods in 

geochemistry: New York, Interscience Publishers, 

Inc., p. 4-32. 

u U. S. GOVERNMENT PRINTING OFFICE: 1970 0- 384-382