Distribution of Placer 

Gold in the Sixes River, 

Southwestern OregonA Preliminary Report

By SAM BOGGS, JR., and EWART M. BALDWIN

CONTRIBUTIONS TO ECONOMIC GEOLOGY

GEOLOGICAL SURVEY BULLETIN 1312-1

Prepared in cooperation with the 

University of Oregon

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1970
UNITED STATES DEPARTMENT OF THE INTERIOR 

WALTER J. HICKEL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

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

Washington. D.G. 20402 - Price 25 cents (paper cover)
CONTENTS

Page

Abstract.._ ___.__.__._____._.________.____.__._.__.____.__________ II

Introduction-_____________________________________________________ 1

Methods.---_----_----------------------._-------..----_---------- 2

Evaluation of analytical precision..__________________________________ 3

Gold distribution._.___ ________-__-_-__-_-_ -.-__-____-_____-___.-_ 11

Gold concentration in relation to gold size-_____---_-_-___-___-___ 20

Source of gold.________-_--_-______-__,------_-___-____--__-__ 22

Conclusions.______________________________________________________ 26

Selected references.____-____-__-_----_--_-_,--_-_--_-__-____-__-___ 27

ILLUSTRATIONS

Page 

FIGURE 1. Plot of </> gold size against cumulative percentage for a

typical sample.____________________________________ 14

2-6. Sketch maps:

2. Average concentration of gold at each sample

locality _____________________________________ 12

3. Average size of gold at each sample locality _______ 14

4. Topographic map of pilot-study area at the mouth

of the South Fork showing location of sample sites _ 16

5. Distribution of gold at pilot-study area at the mouth

of the South Fork______._.__.________________ 17

6. Mean size of gold at pilot-study area at the mouth

of the.South Fork_-____----__-________________ 19

7. Histogram showing frequency of occurrence of the modal

gold-size fraction..______________ ___________________ 21

8. Plot of gold concentration, in parts per billion, in the 

total sample in relation to $ mean gold size for 126 

samples...__-___-_-____.______---___________.______ 23

9. Generalized rock-type map of the Sixes River drainage

basin _________--___-____-___-___.--_________------ 25

TABLES

Page 

TABLE 1. Size and grade of gold in surface alluvium of the Sixes

River _____________________________________________ 16

2. Frequency of occurrence of modal gold-size fraction shown

as a function of total gold concentration.-__-___--_-__ 22

m

CONTRIBUTIONS TO ECONOMIC GEOLOGY

DISTRIBUTION OF PLACER GOLD IN 

THE SIXES RIVER, SOUTHWESTERN 

OREGON A PRELIMINARY REPORT

By SAM BOGGS, JR./ and EWART M. BALDWIN x

ABSTRACT

A study of the Sixes River, a known source of placer gold, was undertaken in 

order to gain a better understanding of the factors controlling transportation and 

accumulation of gold in fluvial environments. As part of this study, 158 samples 

were analyzed for gold content. Data from these analyses show that gold in 

particles larger than about 0.15 millimeter is concentrated mainly in the South 

Fork of the Sixes River with lesser concentrations in the Middle Fork, Hays 

Creek, and the main stream between Hays Creek and the South Fork. Minor 

amounts of finer gold occur in all tributaries. The origin of most of the coarse 

gold particles appears to be related to the formation of quartz veins and the emplacement of small diorite dikes that locally intrude argillites of the Galice 

Formation. Some finer gold is furnished by conglomerates and sandstones of 

the Otter Point and Rocky Point Formations of Koch (1966) and the Umpqua 

Formation.

The tenor of the placer gold is highest in those samples which contain the 

coarsest gold, and the analyses indicate that increasing tenor generally results 

mainly from an increase in size of gold particles rather than an increase in 

number of smaller gold particles. Both the size and content of gold decrease in 

the lower part of the Sixes River below the junction with the South Fork. The 

average concentration of gold in the upper part of the alluvial fill in the lower 

8-10 miles of the river is less than 1 part per billion, and the average size of the 

gold is less than about 0.15 millimeter. Coarser gold appears to lag behind in the 

upper reaches of the stream and apparently is not transported into the intertidal 

zone.

INTRODUCTION

A study of placer gold distribution in the Sixes River in southwestern Oregon was begun in 196Y as part of the Heavy Metals program initiated by the U.S. Geological Survey to evaluate and improve 

domestic sources of metals and minerals in critically short supply.

1 University of Oregon, Eugene, Oreg.

II
12 CONTRIBUTIONS TO ECONOMIC GEOLOGY

Placer gold has been mined from terrace gravels and recent alluvial 

fill in the Sixes River since the late 1800's. The present investigation 

was begun in a known gold province in order to develop a better understanding of the factors controlling transportation and accumulation 

of gold and heavy minerals and to evaluate the relative importance of 

the Sixes River in supplying gold to offshore areas. This report describes the distribution of gold in the upper part of the alluvial fill in 

the Sixes River and its major tributaries, and the probable source 

rocks from which the gold was derived.

Gold analyses were made by Kam W. Leong and Arthur Hubert; 

samples were preconcentrated under the direction of H. E. Clifton 

prior to analysis (see Clifton and others, 1967). Richard Robertson, 

LeRoy Maynard, Saleem Farooqui, and Michael Brownfield, graduate 

students at the University of Oregon, assisted with field mapping 

and sampling. Funds for the research were provided by U.S. Geological Survey Contract 14-08-001-11058 with the University of Oregon.

METHODS

The location of sample sites on the Sixes River and its major tributaries is shown in figure 2. The number of samples collected at each 

sample locality, excluding locality 1 where about 40 samples were 

taken, ranged from one to seven depending upon local conditions. 

Samples, where possible, were collected at depths of 14-18 inches below 

the surface to insure some degree of uniformity and to facilitate comparison of results. During the summer months, at the time samples 

were collected, only fine silt and clay are transported by the Sixes. 

Water velocities become high enough to move coarser sediment and 

gold only during high-water conditions in winter and spring; thus 

the gravelly materials sampled represent flood deposits formed during 

the preceding season or seasons.

Clifton, Hunter, Swanson, and Phillips (1969) point out that a 

sample taken from a deposit assumed to contain randomly distributed 

particles of gold must be of a minimum specified size for meaningful 

analysis; that is, the sample must be large enough to insure that the 

analysis will have a specified degree of precision. The degree of precision arbitrarily chosen required a 95-percent probability that the 

true gold content be not much more than 50 percent larger or smaller 

than the gold content obtained by chemical analysis; this degree of 

precision is met if the sample is large enough to contain 20 particles of 

gold. If the approximate size and tenor of gold in a given deposit is 

known from reconnaissance sampling, the approximate minimum 

size of sample required for analysis can be estimated by reference to 

figure 3 of Clifton, Hunter, Swanson, and Phillips (1969). A prelim-
DISTRIBUTION OF PLACER GOLD IN THE SIXES RIVER I 3

inary analysis of samples from the Sixes River indicated that samples 

ranging in size from about 5 to 15 kilograms would be required. 

Because 10 kg is about the maximum practical size that can be transported in the field, 5-10 kg of material was collected from most 

deposits.

Prior to gold analysis, the samples (all mainly sandy gravel) were 

dried, weighed, and wet-sieved through a 1-millimeter sieve. Material 

larger than 1 mm was concentrated by panning and visually examined 

for gold. Material smaller than 1 mm was analyzed by wet chemical 

and atomic absorption spectrophotometry methods in U.S. Geological 

Survey laboratories in the field and in Menlo Park, Calif., by methods 

described by Thompson, Nakagawa, and VanSickle (1968) or by 

VanSickle and Lakin (1968). As part of the analytical process, 103 

samples were sieved into five size fractions (1.0-0.5 mm, 0.5-0.25 

mm, 0.25-0.125 mm, 0.125-0.0625 mm, <0.0625 mm) and separate 

analyses were made of each size fraction. The remaining samples were 

analyzed in three size fractions (>0.5 mm, 0.5 mm-0.177 mm, and 

<0.177 mm).

The size distribution of gold in each sample was determined from 

the data supplied by the gold analyses. Mean size of the particles, 

in 0 units, was computed from cumulative curves of the gold size 

distribution using the following relationship (Folk, 1961) :

Mz= (016+050+084) /3

where Mz is mean size, in 0 units, and 016, 050, and 084 are the 16th, 

50th, and 84th percentiles, respectively. Figure 1 shows a sample 

curve and the method of computing mean size from this curve. Size, 

in 0 units, was converted to millimeters by use of appropriate tables. 

Mean gold size may also be computed from the formula:

where M0 is mean size, in 0 units, d is the midpoint of each size group, 

and w is the weight of gold, in micrograms, in each size group.

EVALUATION OF ANALYTICAL PRECISION

Table 1 shows the tenor and mean size of gold particles in surface 

alluvium of the Sixes Eiver and its tributaries based on 158 samples 

analyzed to date. The degree to which the analyzed values represent 

the true gold content of the deposits sampled is a function of gold size, 

tenor, the degree to which the distribution of gold particles is random, 

the size of the samples analyzed, and the extent of analytical errors 

(Clifton and others, 1969). Assuming random distribution of the gold 

particles through the deposit and the absence of analytical errors,
14 CONTRIBUTIONS TO ECONOMIC GEOLOGY

100

ol

-1.0

</> GOLD SIZE

FIGURE 1. Plot of # gold size against cumulative percentage for a typical 

sample showing method of computing mean gold size from analyses of sieved 

samples (see text for interpretation of equation).
DISTRIBUTION OF PLACER GOLD IN THE SIXES RIVER 15

the precision of the analysis is a function of the number of gold particles present in the sample. The number of gold particles can be estimated if the particle sizes and concentrations are known; from these 

data the approximate relative error of the analyses can be calculated. 

For example, a sample containing 20 particles of gold will have a 

relative error of about minus 35 to plus 50 percent at the 95-percent 

level of confidence.

Calculation of the relative error of an analysis is based on the 

assumption that the gold particles are of uniform size. Variations in 

gold size add an additional source of error to that resulting from 

variations in the number of gold particles per sample. This error can 

be compensated for by assuming that the gold particles have some 

uniform size larger than the average size per gold particle in the 

sample. For this purpose, Clifton, Hunter, Swanson, and Phillips 

(1969) suggest the use of the following equation taken from the work 

of Pierre Gy and A. Prigogine:

where de is the effective gold size, Mj is the mass of gold in size grade 

y, dj is the midpoint diameter of size grade j, and M is the total mass 

of gold in all size grades; the proper value for djs is one-half the sum 

of the cubes of the sieve openings bounding size grade j. The equation 

places unequal weight on material in the coarser size grades, and thus 

effective gold size may be considerably larger than mean gold size. 

This provides a safety factor for determining adequate sample size 

when dealing with deposits containing gold of variable size.

In table 1 both the effective size and mean size of gold are given for 

those samples that were separated by sieving into five size fractions 

before analysis. The relative error and the confidence ranges of the 

concentrations, in parts per billion, at the 95-percent confidence level 

were calculated using both the effective gold size and mean gold size. 

The remaining samples were analyzed in only three size fractions 

(>0.5 mm, 0.5-0.177 mm, and <0.177 mm); because this distribution 

is largely open ended, effective gold size was not calculated for these 

samples, and the relative error and the 95-percent confidence ranges 

of the concentration are shown as a function of the mean gold size 

only. Use of mean gold size in calculating precision of the analyses 

gives a relative error which, for most samples, is probably too small. 

These values are useful, however, for comparison with those obtained 

using effective gold size in the computations, and they provide an 

approximation of the true precision of the analyses for those samples 

for which effective size could not be accurately computed.

373-710 O 70 2
TABLE 1. Size and grade of gold in surface alluvium of 

I Gold in total sample: BD, below level of detection of chemical analysis. Effective size of gold: NC, effective s

fractions only]

Sample No.

Locality 1: 

SR-1-. .................

2...................

3..--....-..-.......

4. ..................

6-. -- -.. 

7. ................. .

S. ..................

9.. .................

10..-:... .........

11. .................

12..................

14..................

15...... ...... .......

16..................

17..................

18..................

19--..-.-.....-.....

22..................

23.....-...... ..,

1A.. ...... ........ ..

2A-... .............

3A.. ................

4A __ .............

5A._ ................

6A..................

7A......... .........

10A ... ...........

11A....... ..........

12A.... .............

13A.. ..---... .......

14A.. ...... .........

ISA.................

19A.. ...............

20A-. .............

Gold in Weight of 

total total 

sample sample 

(milligrams) (grams)

............ BD

............ 55.0

-.-.-......- 19.0

............ 6,307.1

-....--..-.. 26.0

............ BD

.-....---..- 181.0

-... .. 3.0

............ BD

............ BD

............ 131.5

 _- -.- 50.0

............ 20.0

............ 1,040.8

............ 9,840.0

............ 6610

............ 39.9

............ 1,396.3

............ 22.8

............ 17.0

............ 92.0

............ 1,959.8

............ 16.8

............ BD

............ 1,390.2

............ 792.0

............ 41.4

............ 21,554.0

............ 8.9

............ 2,118.4

............ 4,258.0

............ 146.5

............ 30.0

............ 3.165.4

5,501 ..

5,027 

7,823 

4,536 

7,213 

7,333 ..

6,715 

6,216 

2,872 ..

7,888 ..

7,041 

5,207 

6,383 

5,099 

4,923 

6,530 

6,336 

7,236 

6,301 

6,882 

5,498 

4,636 

6,165 

6,628 ..

7,313 

6,150 

2,916 

7,432 

6,705 

7,369 

4,699 

6,676 

6,652 

6.423

Percentage 

Gold in Effective relative error 

total size of at 95-percent 

sample gold confidence 

(parts per (millimeters) level based on 

billion) effective size 

(minus to plus) 

10.8 

2.4 

1, 390. 5 

3.6

26.9 

0.48

18.7 

9.6 

3.1 

202.2 

1, 998. 7 

101.7 

6.0 

193.0 

3.6 

2.5 

16.7 

419.7 

2.7

193.1 

128.8 

14.2 

2, 900. 2 

1.3 

287.5 

906.2 

21.9 

4.5 

492.9

0.25 

.22

.77 

NC ...

.36 

.41

NC ...

NC ...

.22

.74 

NC ...

.32 

.26 

.40

NC ...

.10 

.68 

.76 

.21

.82 

.41 

.21 

.82 

.13 

.68 

NC ...

NC ...

NC ...

.73

62-165 

78-390 

45-86

70-225 

>80->400

77-385 

75-300

39-65 

75-300 

36-58

40-68 

>80->400 

60-150 

78-390

70-225 

53-110 

65-180 

29-42 

67-200 

53-110

52-105
DISTRIBUTION OF PLACER GOLD IN THE SIXES RIVER I 7

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TABLE 1. Size and grade of gold in surface alluvium of the Sixes 

Sample No.

Locality 10: 

8RS-1A..... ----..-. ------

2A-...-- ............

3A..... ..............

4A._ ................

Locality 11: 

SRER-1A.. --------------

2A. .............. .

3A_. ............ ..

4A.. ----------...-

5A.. ..............

Locality 12: 

SRD-1A..-. --.- .......

2A....... ...........

3A.. ................

4A-. ...... ----------

5A. -..-.-...-----..-

Locality 13: 

SRE-1A.. ._-_--...-..._. .

2A.. ........ ....... .

3A..................

4A..... ..........

5A. ........ .........

Locality 14: 

SRX-IA...... ............

2A

3A......... .........

Locality 15: 

SH-26A.-.. ...........

27A-... ...............

28A. ..................

29A. ..................

30A...................

Locality 16: 

SB-31A-.... .............

32A....... ............

33A.. ...... ...........

34A--... ..............

35A.. .................

Percentage 

Gold in Weight of Gold in Effective relative error 9

total total total size of at 95-percent c

sample sample sample gold confidence lev

(milligrams) (grams) (parts per (millimeters) level based on ef

billion) effective size 

(minus to plus) 

........-- 121.1

........... 42.9

.......... 1,185.9

.......... 60.4

.......... BD

.......... BD

.......... BD

.......... 2.5

.......... 1.5

.......... 1.0

.......... 3.6

.......... 2.8

.......... 0.2

.......... 1.5

.......... 2.7

.......... 2.0

.......... BD

.......... BD

.......... 2.5

.......... 0.4

.......... 9.1

........... 0.8

.......... BD

........... 702.0

........... 64.8

........... 11.8

.......... .3

........... BD

........... 1.9

..... ..... 2.2

........... 1.0

........... 3.0

.......... BD

9,208 

9,534 

8,618 

9,963

9,507 ...

9,883 ...

8,499 ...

9,418 

10,401

10,010 

9,514 

10,601 

10,659 

8,862

11, 352 

10,180 

10,590 ...

7,412 ...

9,640

9,289 

9,147 

9,031 

9,549 ...

9,272 

9,850 

9,637 

9,153 

9,453 ...

8,693 

9,317 

11, 789 

10,441 

8.930 ...

13.4 

4.5 

137.8 

6.1

.3

.1

.1 

.4 

.3

.02 

.2

.2 

.2

.3

.04 

1.0 

.1

75.7 

6.6 

1.2 

.03

.2 

.2 

.1 

.3

.22 

.39 

.76 

.21

NC ..

NC ...

.05 

.14 

.18 

.05 

.10

NC ..

NC ...

NC ..

.09 

.19 

.05

.82 

.39 

.21 

.10

NC ..

NC ..

NC ..

NC ..

47-92 

>80->400 

75-300 

55-125 

65-180 

>80->400 

>80->400 

>80->400 

>80->400 

>80-7400 

78-390 

>80->400 

78-390 

>80->400 

>80->400 

>80->400 
Locality 17:

SR-36A- ------------------------ BD 9,538

37A- - - - - 1.4 10,848 

38A-. ------------- BD 10,318

39A--- - -- - 7.5 8,945 

40A----------------------------- BD 9,883

locality 18:

SR-41A- -- 4.7 10,624

42A- -- - - - - 13.0 10,097 

43A-.----------------------- 2.1 10,771

44A...--------------- -- 5.0 10,352

SB-45A-. - - - -- --- BD 10,519 

46A-- ---- .8 10,658

47A-.------------------------ 3.4 11,421

Locality 19:

SK-48A-.--------------------- 2.3 10,732

49A-.---------------------- .8 10,020

50A-.------------------------ 7.4 11,809

51A-.------------------- 1.6 11,979

52A-.---- --------- .8 9,410

Locality 20:

SR-53A-.----------------------- 47.4 9,666

54A-.-------------------------- 4.3 10,020

55A. . - 31.5 9,527 

56A-......... --_._-.-_-- --- 7.5 9,901

57A-.-.-.....---.-...-_-...--. 10.7 13,076

Locality 21:

68-1----......- . .-__ .___- 4.2 7,767

Locality 22:

68-2.-...-.-.-.---...---...--....-..- 4.6 7,828

Locality 23:

68-3................................. 2.8 7,312

Locality 24:

68-4,---.---......-.-..--.- ...... 5.8 7,601

Locality 25A:

68-5............................... 45.4 7,612

Locality 25B:

68-6............................... 1.7 6,247

Locality 26:

68-7................................ 7,852.4 7,661

Locality 27 A:

68-8-...-.-..-.........--..-.... 22.8 6,994

Locality 27B:

68-9..........-...-- ............ 399.0 7,751

Locality 28A:

68-10 .--...- .--..-..-.....-.. 14.6 7,464

Locality 28B:

68-11 .. 2.0 8,016

Locality 29:

68-12 .. ... ............. 26.0 9,228

13 5.0 9,286

.4

1.3

.2

.5

.1 

.3

.2 

.1 

.6 

.1 

.1

4.9 

.4

3.3 

.8 

.8

.5 

.6 

.4 

.8

5.96 

.3 

1,024.9

3.3 

51.5

2.0 

.3

2.8 

.5

NC

"NC"

. 10 

.38 

.14 

.39

65-180 

>80->400 

>80->400 

>80->400

NC 

.10

.09 

.16 

.09 

.07 

.09

.53 

.22 

.20 

.10 

.12

.66 

.64 

.10 

.29 

.45 

.45 

.82 

.47 

.74 

.55 

.82

.45 

.30

62-165

67-200 

>80->400

50-100

78-390 

>80->400

>80->400

>80->400

70-225

50-100

60-150

>80->400 

>80->400

70-225 

>80->400 

>80->400 

>80->400

40-75 

>80->400 

>80->400 

>80->400 

>80->400

>80->400 

>80->400
Sample No.

Locality 30: 

68-14- -- 

15----.--~.-~-- -~~~

Locality 31: 

68-16------. ----------

17--.. ---------------------

Locality 32: 

68-18---.. -------------------

19

Locality 33: 

68-25 

26------- - -------

Locality 34: 

68-27.. -----------------------

oo

Locality 35: 

68-29.. -----------------------

30.. -----------------------

TABLE 1. Size

Gold in 

total 

sample 

(milligrams)

------- 2.0

------- 16.0

--....- 1,249.6

-.-...- 193.2

------- 1.0

------- BD

.-...-. 389.0

.--.-.. 707.0

-...-.; 2.0

------- BD

....... 5.5

------ BD

and grade

Weight of 

total 

sample 

(grams)

8,155 

9,493

8,475 

8,830

8,281 

7,659

9,800 

9,233

7,553 

7,518

9,881 

9.473

of gold in surface allu

Gold in 

total 

sample 

(parts per 

billion)

.3

1.7

147.4 

21.9

.01

39.7 

76.6

.3

.6

Effective 

size of 

gold 

(millimeters)

.35 

.09

.57 

.38

.82

.60 

.57

.09

.18

vium of the S

Percentage 

relative error 

at 95-percent 

confidence 

level based on 

effective size 

(minus to plus)

>80->400 

40-70

60-150 

67-200

>80->400

>80->400 

70-225

70-225

>80->400
DISTRIBUTION OF PLACER GOLD IN THE SIXES RIVER 111

If the allowable relative error for a meaningful analysis is taken 

to be ±50 percent at the 95-percent level of confidence that is, the 

true gold content of the sample is no more than about 50 percent larger 

or smaller than the gold content obtained in the analysis then table 

1 indicates that analysis of many of the samples collected to date would 

not be meaningful. In fact, some of the deposits of very low concentrations would require sampling of more than 50 kg of material in 

order to insure the specified degree of precision. Collecting and transporting samples of this size is extremely difficult and would necessitate 

some type of preconcentration in the field.

The extent of analytical error is uncertain. Undoubtedly some 

random analytical error exists, and its effect is to decrease the precision 

of the analyses more than predicted from the calculated effective number of gold particles per sample. Moreover, the method of analysis 

used here may result in sysematic error or bias because gold included 

within grains not dissolved during the chemical treatment would not 

be revealed by the analysis. This type of systematic error, which leads 

to analyzed gold concentrations lower than the true gold concentration 

of the sample, may not be disadvantageous in an investigation such as 

this, in which determining the distribution of particulate gold is the 

objective. Other causes of systematic error are possible but cannot be 

evaluated at the present time.

GOLD DISTRIBUTION

Figure 2 gives the mean or average concentration of gold at each 

of the localities sampled. The value shown at each locality was obtained 

by averaging the gold concentrations of all the samples taken at that 

locality. Table 1 shows that the concentration of gold in individual 

samples varies markedly at many localities and that many of these 

variations are too large to be accounted for on the basis of relative 

error in the analyses alone. The variations indicate that gold is probably not distributed evenly or randomly within the surface alluvium 

at many localities, and therefore the assumption of random spacing of 

gold particles made in calculating relative error of the analyses can 

only be taken to hold true for a limited area immediately surrounding 

the sample site. Despite these local variations in gold content, figure 2 

indicates that average gold concentration varies by several orders of 

magnitude between different localities in the Sixes drainage system.

The richest concentrations of gold occur in parts of the South Fork 

drainage, which appears to encompass a major gold source. Note the 

particularly high values in the area near Rusty Creek and also at 

the mouth of the South Fork. Other concentrations of interest occur 

in the Middle Fork, Hays Creek, and the main channel of the Sixes
112 CONTRIBUTIONS TO ECONOMIC GEOLOGY

-coi
DISTRIBUTION OF PLACER GOLD IN THE SIXES RIVER I 13

between Hays Creek and Elephant Rock Creek. All other tributaries 

sampled have minor amounts of gold, but most samples contain less 

than 1 ppb (part per billion). The tributaries downstream from South 

Fork furnish little gold to the main channel, but they do contribute sizable quantities of gravel and sand. This appears to dilute the 

gold content of the Sixes below the South Fork, and the average gold 

content of samples taken from the main channel between the South 

Fork and the intertidal zone, extending about 3 miles upstream, is less 

than 1 ppb. To date, samples taken fom the intertidal zone have an 

average gold content of about 2 ppb.

Figure 3 shows the distribution of gold by particle size in the Sixes 

River drainage basin. The values given in millimeters are the averages 

of the mean size of gold in all samples taken from the same sample 

locality. Gold particles exceeding 1 mm were not detected in the 

samples analyzed; however, some gold larger than 1 mm was panned 

from localities on the South Fork by scraping material directly from 

the bedrock. Most gold in the samples collected is less than 0.5 mm.

No specific pattern of size distribution is apparent in the various 

tributaries, but, in general, samples having the highest gold content 

also contain the largest gold particles. Thus the coarsest gold particles 

were taken from the South Fork, the Middle Fork, Hays Creek, and 

the main channel between Hays Creek and the South Fork. The average 

mean size of gold in this part of the drainage basin ranges from 0.09 

to 0.84 mm; however, table 1 shows that many of the individual 

samples contain gold particles having a larger mean size than the 

average means given in figure 3.

The mean size of gold particles in samples collected from the main 

channel of the Sixes below Elephant Rock Creek is smaller than that 

from the upper Sixes region, and mean gold size diminishes rather 

abruptly below the junction with the South Fork. This may be due 

mainly to the decreased gradient of the Sixes below this point and to 

the fact that very little coarse gold is supplied to the downstream part 

of the main channel by tributaries. Because of the dilution effect mentioned above, there is also less likelihood of finding coarse gold particles in a particular sample at a given locality in the lower part of 

the stream should such coarse gold be present. Therefore, gold coarser 

than these data indicate may be present in the surface alluvium of the 

downstream part of the main channel. However, in 27 samples taken 

from the lower part of the channel, only four samples contained gold 

particles having a mean size in excess of 0.15 mm. Although further 

data are needed for confirmation, these data suggest that most gold 

particles in the upper few feet of surface alluvium in the intertidal 

zone, and thus the particles most likely to be transported ultimately

373-710 O 70 3
114 CONTRIBUTIONS TO ECONOMIC GEOLOGY
DISTRIBUTION OF PLACER GOLD IN THE SEXES RIVER I 15

into the ocean, are probably less than about 0.15 mm in size. However, 

gold coarser than this may be present in the deeper channel fill.

The surface detritus on the bars and in the stream in the Sixes Kiver 

is mainly granule and gravel size (2 mm and larger). This material 

does not move during the extremely low water conditions existing 

during the summer months, when collections were made, but is transported only during flood stage of the river in winter and spring. In 

order to learn more about the factors favoring concentration of gold 

in such gravelly "flood" deposits, a detailed study was made of a small 

area at the mouth of the South Fork of the Sixes. This area was 

selected for study because preliminary panning indicated that both 

the streambed and the gravel bars contained gold. The area was 

mapped by plane-table methods (fig. 4) and a grid sampling pattern 

laid out to facilitate collection of random samples. Additional samples 

were taken between grid points where additional control appeared 

to be necessary. The size distribution of the surface detritus was 

mapped onto this base (fig. 5) to aid in determining whether a relationship exists between gold size, tenor, and the size of the detritus with 

which gold occurs.

Figure 5 shows the distribution of gold in the pilot-study area 

based on 39 samples. The greatest concentrations of gold occur in 

three places: (1) in the exposed bar adjacent to bedrock along the 

east side of the Sixes above the junction with the South Fork, (2) in 

the middle and upper parts of the large bar south of the junction at 

the forward (stream) edge of the bar and in the stream channel at 

the upstream end of this bar in the South Fork, and (3) near the 

back of the upstream part of the bar on the north side of the Sixes 

below the junction with the South Fork. The highest gold concentrations occur with surface materials that are either mainly cobbles 

and boulders or mixed pebbles, cobbles, and boulders. Below the immediate surface layer of pebbles, these deposits also contain considerable interstitial sand (15 45 percent).

The greatest concentration of gold in the large bar south of the 

junction is in the upstream portion of the bar. Where the bar broadens 

out below the junction, gold is concentrated near the stream edge of 

the bar rather than at the back edge. This bar has a relief of about 

6-7 feet above stream level at this point, and the gold apparently 

concentrates near the stream level rather than in the higher part of the 

bar. In the Sixes River above the junction, gold is also concentrated in 

the coarser material. The exposed bar is 2-3 feet above stream level, 

and richest concentrations of gold are toward the back side of the bar. 

only a few feet of detritus is present here above bedrock, and irregularities in the underlying bedrock probably have helped concentrate
116 CONTRIBUTIONS TO ECONOMIC GEOLOGY

z 

0

< r

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X

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O-
DISTRIBUTION OF PLACER GOLD IN THE SIXES RIVER I 17
I 18 CONTRIBUTIONS TO ECONOMIC GEOLOGY

the gold. In the bar downstream from the junction on the north side 

of the stream, best concentrations were found in samples collected from 

sites at which the surface material was either sand or mixed pebbles, 

cobbles, and boulders. The sand at these sites is only a thin veneer, 

2-4 inches thick, and below this the deposits become quite gravelly. 

Relief on this bar is about 3 feet above stream level, and gold is concentrated in the upstream part of the bar toward the back side. Bedrock here also is only a few feet below the surface.

Figure 6, which is plotted on the same base as figure 5, shows the 

distribution of gold by size in the pilot-study area. A comparison of 

figure 5 and figure 6 indicates a correlation between the tenor of gold 

in the samples and the mean size of the gold; samples having the 

highest concentrations of gold also contain the coarsest gold particles. 

Thus the coarsest gold tends to occur with the coarsest detritus, in the 

upstream parts of bars, toward the back side of the low-relief bars and 

the front (stream) edge of the high-relief bars, and in areas where 

bedrock is fairly close to the surface. The concentration and size of gold 

in the active stream bed (now covered with water) is generally similar 

to that in the near-stream part of the adj acent bars.

Except for the pilot-study area, one to seven samples were collected 

from all sample localities. Sample sites were chosen from different 

parts of the stream and bars in order to provide additional information 

regarding the local distribution of gold in stream channels and bars. 

At most sample localities the highest concentrations of gold occur 

in the coarsest material available at the sample site; however, a few 

exceptions were found. Also higher concentrations generally came 

from the upstream parts of the bars rather than the downstream parts, 

although again some exceptions were found. Many of the samples 

collected from the coarse detritus in the active stream channel also 

had relatively high concentrations. In addition the position above 

bedrock at which samples were collected is important. Samples collected from immediately above bedrock are almost always richer in 

gold than other samples collected in the same area but at a greater 

height above bedrock; thus the back sides of bars, where bedrock is 

generally closest to the surface, generally show relatively higher concentrations than other parts of the bar or stream.

Although some deposits consisting mainly of sand occur in the 

Sixes, most of the sand is deposited in the interstitial spaces in the 

gravel deposits. The amount of sand in such deposits ranges from 15 

to 45 percent in most samples; in a few samples, it makes up more 

than 50 percent. Highest gold concentrations are generally in samples 

that have relatively low sand content, and the concentrations of gold 

in the few samples that contain more than 50 percent sand is rather low.
DISTRIBUTION OF PLACER GOLD IN THE SIXES RIVER I 19
120 CONTRIBUTIONS TO ECONOMIC GEOLOGY

GOLD CONCENTRATION IN RELATION TO GOLD SIZE

The tenor of a gold deposit may increase as a result of increasing 

numbers of fine gold particles of a given size or sizes, increasing size 

of gold particles without significant increase in numbers of particles, 

or any combination of these. Comparison of figures 2 and 3 and of 

figures 5 and 6 seems to show that the increased tenor of most of the 

deposits of high concentration in the Sixes River is associated with 

increased grain size. In order to determine whether this may be purely 

fortuitous, the data from 93 samples, each of which was sieved into five 

size fractions before analysis, were examined to determine whether 

gold tends to occur preferentially in any particular size range. The data 

indicate that 76 percent of the samples contains gold in two or more size 

fractions, and 11 percent of the samples contains gold in all five size 

fractions. The data show further that the size fraction in each sample 

in which the amount of gold is greatest (modal size fraction) varies 

from sample to sample. Figure 7, in which the frequency of occurrence is plotted as a function of the modal gold-size fractions, shows 

that highest amounts of gold occur in size fractions ranging from 

the <0.0625-mm fraction to the 1.0-0.5 mm fraction; however, the 

"modal" size of the gold in two-thirds of the samples does lie between 

0.0625 and 0.5 mm.

Because these data show that gold in the Sixes River occurs in a 

range of sizes, a further check was made to see if a relationship exists 

between the modal gold-size fraction and the total concentration of 

gold in the sample. For 89 of the 93 samples used (four samples do not 

have a single modal size fraction), table 2 shows the frequency of 

occurrence of the modal gold-size fraction in relation to the concentration of gold, in parts per billion, in the total sample. Italic figures 

represent the maximum number of samples in which a particular 

modal size fraction occurs in each concentration group. For example, 

in 21 samples with total concentrations of gold ranging from 1 to 10 

ppb, the modal gold-size fraction of 11 of the samples is the 0.25- 

0.125 mm size fraction. Note that the italic values form a distinct 

linear progression from lowest to highest concentrations; that is, in 

deposits of low gold concentration the "modal" size occurs preferentially in the fine-size fractions, and in deposits of higher concentrations it occurs preferentially in the coarse-size fractions.

It is expected on the basis of the relationship of particle mass and 

size (see fig. 3 of Clifton and others, 1969) that most of the gold in 

samples of very low tenor would be of very fine size. The few seeming 

exceptions to this shown in table 2 are probably the result of fine 

grains of gold included on the surface of larger silicate grains. In 

samples of higher concentrations, the gold might be either coarse
121

20

10

0

1

66 percent

0 0.50 0.25 0.125 0.0625

MODAL GOLD-SIZE FRACTION, IN MILLIMETERS

FIGURE 7. Histogram showing frequency of occurrence of the modal gold-size fraction. (Four of the 

93 samples do not have a single modal size fraction 

and therefore are not included.)

or fine. The relationship shown in table 2, however, seems to indicate 

that the increase in tenor of the samples studied is due mainly to 

increasing gold-particle size rather than increasing numbers of fine 

gold particles.

As a last check on the relationship of gold size in relation to gold 

concentration, mean gold size was plotted against total gold concentration, in parts per billion, as shown in figure 8. In order to give some
I 22 CONTRIBUTIONS TO ECONOMIC GEOLOGY

TABLE 2. Frequency of occurrence of modal gold-size fraction shown as a function

of total gold concentration

[Italic figures are maximum values]

Number of samples having indicated gold concentration, in parts per billion, and having Number PercentSize fraction (millimeters) modal gold size in given fraction of samples age of 

 total 

<0.1 0.1-1 1-10 10-100 100-1000 >1000

1.0-0.5.-. -----------

0.6-.25.. ............. .

0.2&-.125. --------

0.125-.0625.. ------------

<.0625... ..---....--....

Total..-....--.....

........ 1

........ 0

......... 0

.--...- -. 3

......... 4

......... 8

3

4

4

IB

9

35

0

7

11

3

0

21

4

5

3

0

0

12

6

2

2

0

0

10

3

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0

0

0

3

17

18

20

21

13

89

19.0

20.3

22.5

23.6

14.6

100.0

idea of the relative precision of the analyses, the data points are identified by symbols that indicate the range in relative error of the analyses. 

In this plot all samples that contained gold were used. The values show 

an appreciable amount of scatter, and the precision of some of the 

analyses is low; nevertheless, a clear trend is evident, and an approximate best-fit line has been visually adjusted to the data. The slope of 

this line indicates a general increase in mean gold size with increasing 

total gold concentration in the samples. Increasing </> values indicate 

decreasing grain size: </>= Iog2 grain size, in millimeters.

SOURCE OF GOLD

Although parts of the South Fork and the lower Sixes have been 

worked extensively for placer gold, efforts to locate the lode source 

of the gold have been largely unsuccessful. Diller (1903) reports some 

lode mining on Rusty Butte (Harrison Property) both in igneous 

rock and in quartz veins intruded into "slate." Most of the gold was 

found in small pockets and veins, and Diller concludes, "It is pockets 

and seams of this character chiefly that have supplied the placer gold 

of the streams and beach gravels. Their small size, irregularity, and 

lack of persistence are not encouraging features." Butler and Mitchell 

(1916) report a claim (P. L. Wallis Quartz Claim) somewhere between 

the mouth of the South Fork and Kusty Creek but state, "the prospect 

was not sampled, and no information concerning the grade of the 

vein matter was procurable." Brooks and Ramp (1968) report two 

other lodes in the Rusty Butte area of the South Fork that are listed 

in Oregon State Mine File Reports, published in 1938 and 1950. However, they state that production from lode mines in this area has been 

negligible. A map included with the Brooks and Ramp report shows 

the location of some of the placers worked on the Sixth River. Almost 

all the placer workings are either on the South Fork or on the main 

stream below the junction with the South Fork, and according to
DISTRIBUTION OF PLACER GOLD IN THE SIXES RIVER I 23

Brooks and Ramp, many of the placers were in terrace gravels from 

50 to 130 feet above the present stream level.

Four types of potential source rocks for placer gold crop out within 

the Sixes Kiver drainage basin (see fig. 9).

1. Intrusive igneous rocks, mainly diorite.

2. Sandstones, argillites, and greenstone of the Galice Formation, 

Jurassic, locally intruded by diorite dikes.

3. The Humbug Mountain Conglomerate of Koch (1966), Cretaceous.

4. Sandstones and conglomerates of the Otter Point Formation of 

Koch (1966), Jurassic; Eocky Point Formation of Koch (1966), 

Cretaceous; and Umpqua Formation, Tertiary. 

In order to determine the relative importance of each of these 

potential source rocks, samples were collected from various tributaries 

which drain exclusively or almost exclusively each of these rock types. 

Analyses of these samples show that tributaries whose sources are in 

sandstone units have very low concentrations of fine gold ranging 

from values below the level of detection of chemical analysis up to a 

maximum of about 1 ppb. Tributaries that drain conglomeratic units 

exclusively, or in which extensive conglomeratic deposits crop out, have 

somewhat higher concentrations ranging from about 1 to 3 ppb. The 

major intrusive body in the Sixes River drainage basin is a small 

diorite stock which intrudes the Galice Formation and crops out in 

Josh Creek and Benson Creek in the headwaters of the South Fork. 

Samples collected from Josh Creek and Benson Creek contain very low

1 1 I 1 1 1 1 1

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 A 

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nalysis (percent) _

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-

-

A

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§2

-1

10' 10° 10 1 10 2 103 

GOLD CONCENTRATION, IN PARTS PER BILLION

10"

FIGURE 8. Plot of gold concentration, in parts per billion, in the total sample in 

relation to <p mean gold size for 126 samples.
124 CONTRIBUTIONS TO ECONOMIC GEOLOGY
g;

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if

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a g g s S- ^ ^ S 

JURASSIC, CRETACEOUS

CRETACEOUS,

AND 

TERTIARY

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QUATERNARY

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JURASSIC

QTLOO XI 3HI, S3XI8 H3AIH 
126 CONTRIBUTIONS TO ECONOMIC GEOLOGY

gold concentrations (less than 1 ppb), however, as do samples from 

the upper part of Benson Creek, which drains mostly nonintruded 

Galice argillite. Samples taken from the lower part of the South Fork, 

particularly from around Rusty Creek and below, contain the richest 

concentrations and the coarsest gold found within the Sixes drainage 

basin. In the area around Rusty Creek, Galice argillite is locally 

intruded by numerous small dikes, probably emplaced at about the 

same time as the Benson Creek stock. The origin of most of the placer 

gold in the South Fork thus appears to be related to these small intrusive bodies rather than to any large lode source.

CONCLUSIONS

The richest concentrations of placer gold in surface alluvial fill in 

the drainage basin of the Sixes River are in the South Fork, the 

Middle Fork, Hays Creek, and the main stream between Hays Creek 

and the South Fork. Bedrock in the drainage basin includes intrusive 

igneous (diorite) bodies, argillite and sandstone of the Galice Formation, and conglomerate and sandstone of the Otter Point and 

Rocky Point Formations of Koch (1966) and the Umpqua Formation. In order of decreasing relative importance, source rocks for the 

placer gold are (1) fractured and intruded Galice argillite, (2) conglomerates, and (3) sandstones. Although no lodes of importance have 

been discovered, the principal source of primary coarse gold in the 

drainage basin of the Sixes River appears to be the Galice Formation 

in the headwaters of the South Fork, where this formation is locally 

fractured and permeated with small diorite dikes and associated intrusives. Conglomerates and sandstones are a much less important 

source of fine second-cycle gold.

Tenor of surface alluvium in the Sixes River varies markedly 

from locality to locality. It is expected on the basis of the relationship 

of particle mass to particle size (Clifton and others, 1969) that 

deposits of very low tenor will contain only very fine gold, whereas 

deposits of higher tenor may contain either large numbers of very 

fine gold particles or a few much larger gold particles. Data from the 

analysis of 158 samples indicate that increase in tenor of deposits 

in the Sixes River is related more closely to increase in size of gold 

particles than to increase in number of small gold particles.

Highest concentrations of gold and the coarsest gold in the upper 

part of the alluvial fill are generally associated with the coarsest 

detritus at a given locality and occur preferentially in the upstream 

parts of the gravel bars toward the back side of low-relief bars, where 

alluvial fill above bedrock is shallowist, and toward the front (stream) 

edge of high-relief bars. Both size and tenor of gold in the Sixes
DISTRIBUTION OF PLACER GOLD IN THE SIXES RIVER 127

River decrease in the lower parts of the stream below the junction with 

the South Fork. The average concentration of gold in the surface 

alluvium of the lower 8 to 10 miles of the Sixes River is less than 1 

ppb, and the mean size of most gold particles is less than 0.15 mm.

Thus the mean size of most gold particles furnished to the intertidal zone, and eventually to the ocean, is "probably less than about 

0.15 mm.

SELECTED REFERENCES

Baldwin, E. M., 1965, Geology of the south end of the Oregon Coast Range

Tertiary basin: Northwest Sci., v. 39, no. 3, p. 93-103. 

Brooks, H. C., and Ramp, L., 1968, Gold and silver in Oregon: Oregon Dept.

Geology and Mineral Industries Bull. 61,337 p. 

Butler, G. M., and Mitchell, G. J., 1916, Preliminary survey of the geology and

mineral resources of Curry County, Oregon : Oregon Bur. Mines and Geology,

Mineral Resources Oregon, v. 2, no. 2,134 p. 

Clifton, H. E., Hubert, Arthur, and Phillips, R. L., 1967, Marine sediment sample

preparation for analysis for low concentrations of fine detrital gold: U.S.

Geol. Survey Cir. 545,11 p. 

Clifton, H. E., 1968, Gold distribution in surface sediments on the continental

shelf of southern Oregon, a preliminary report: U.S. Geol. Survey Cir.

587, 6 p. 

Clifton, H. E., Hunter, R. E., Swanson, F. J., and Phillips, R. L., 1969, Sample

size and meaningful gold analysis: U.S. Geol. Survey Professional Paper

625-0,17 p. 

Diller, J. S., 1903, Description of the Port Orford quadrangle, Oregon: U.S.

Geol. Survey Atlas, folio 89.

Folk, R. L., 1961, Petrology of sedimentary rocks: Austin, Tex., Hemphill's, 170 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. 

Koch, J. C., 1966, Late Mesozoic stratigraphy and tectonic history, Port OrfordGold Beach area, southwestern Oregon coast: Am. Assoc. Petroleum Geologists Bull., v. 50, no. 1, p. 25-71. 

Thompson, C. E., Nakagawa, H. H., and VanSickle, G. H., 1968, Rapid analysis

for gold in geologic materials, in Geological Survey research 1968: U.S. Geol.

Survey Prof. Paper 600-B, B13Q-03132. 

VanSickle, G. H., and Lakin, H. W., 1968, An atomic-absorption method for

determination of gold in large samples of geologic materials: U.S. Geol.

Survey Circ. 561,4 p. 

Wells, F. G., 1955, Preliminary geologic map of southwestern Oregon west of

meridian 122° West and south of parallel 43° North: U.S. Geol. Survey

Mineral Inv. Field Studies MF-38, scale 1: 250,000.

U.S. GOVERNMENT PRINTING OFFICE: 1970 O 373-710