GEOLOGICAL SURVEY CIRCULAR 566 

Tertiary Gold-Bearing 

Channel Gravel in Northern 

Nevada County, California 

Tertiary Gold-Bearing 

Channel Gravel in· Northern 

Nevada County, California 

By Donald W. Peterson, Warren E. Yeend 

Howard W .. Oliver, and Robert E. Mattick 

G E 0 l 0 G I C A l 5 U R V E Y C I R C U l A R 566 

Washington 1968 
United States Department of the Interior 

WALTER J. HICKEL, Secretary 

Geological Survey 

William T. Pecora, Director 

first printing 1968 

Second printing 1968 

Third printing 1970 

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

Geology of the channel gravel, by 

Donald W. Peterson and Warren E. 

Y eend - - - - - - - - - - - - - -

Introduction - -. - - - -

Previous work - - - -

Acknowledgments - -

General geology- - - -

Bedrock - - - - - - - - - - - -

Auriferous gravel- - -

Volcanic rocks - - -

Colluvium- - - - -

Tertiary channel near North Columbia - -

Configuration of the channel - - - - - -

Grade and distribution of gold - - - - -

Geophysical investigations of the channel 

gravel, by Howard W. Oliver and 

Robert E. Mattick - -

Introduction - - - - -

AcknowledgmentsCONTENTS 

Page 

1 

1 

1 

2 

3 

3 

3 

3 

8 

8 

8 

9 

9 

11 

11 

12 

Geophysical investigations of the channel 

gravel--Continued 

Instrumentation and field procedures- - -

Determination of bedrock configuration - - - - - - - -

Seismic method -

Gravity method - -

Magnetic method - - - - -

Resistivity method 

Electromagnetic method 

Induced-polarization method -

Detection of Tertiary channels under 

volcanic cover - - - - - - - - - -

Recognition and delineation of the 

blue gravel - - - - - - - - - - - - -

Location of concentrations of magnetic 

·-

minerals - - - - - - - - - - ·· -

Conclusions - - -

References cited- - -

ILLUSTRATIONS 

Page 

12 

12 

12 

13 

13 

13 

13 

16 

17 

17 

19 

19 

22 

Page 

Figure L. Index map showing location of area studied - - - - - - - - - - - - - - - - - 2 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

10. 

1L. 

12. 

13. 

Geologic map of part of area between the South and Middle Yuba Rivers - - - - - - 4 

Diagram showing inferred path followed by the Tertiary Yuba River - - - 6 

Diagrammatic section across an idealized Tertiary channel - - - - - - - 7 

Detailed geologic map showing location of hydraulic pits and drill holes in the 

Tertiary gravel in the vicinity of North Columbia and Badger Hill- - - - - - - 9 

Subsurface contour map of bottom of channel near North Columbia as indicated by 

drill holes - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 10 

Cross section of channel near North Columbia as indicated by one line of 

drill holes - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Contour map of the foot-assay product as indicated by drilling information 

Seismic, resistivity, gravity, magnetic, and electromagnetic data and the 

interpreted geologic section along A-A' (figs. 2 and 5) and gold values - - -

Resistivity soundings and interpreted layers at R1 and R1 1 near profile A-A' 

(fig. 9) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Geologic section, resistivity data, and seismic traveltimes along B-li' (fig. 2) -

Electromagnetic survey of Badger Hill diggings - - - - - - - - - - - - - - - - - - - - -

Magnetic, electromagnetic, induced-polarization, and geologic data along ..C.-.k' 

(figs. 2, 5, and 12), Badger Hill diggings - - - - - - - - - - - - - - - - - - - - - - -

Ill 

10 

11 

15 

16 

18 

20 

21 

Tertiary Gold-Bearing Channel Gravel 

in Northern Nevada County 

California 

By Donald W. Peterson, Warren E: Yeend, Howard W. Oliver 

and Robert E. Mattick 

Abstract 

The remains of a huge Tertiary gravel-filled channel lie in the 

area between the South and i\tiddle Yuba Rivers m northern Nevada 

County, Calif. The deposits in this channel were the site of some of 

the most producnve hydraulic gold mines m Calif(!rnia between the 

1850's and 1884. 

The gravel occupies a major ·channel and parts of several 

tributaries that in Tertiary tlme cut into a surface of Paleozoic 

and Mesozoic 1gneous and metamorphic rocks. The gravel is partly 

covered by the remains of an extensive sheet of volcanic rocks, but 

it crops out along the broad crest of the r 1dge between the canyons 

of the South and Middle Yuba Rivers. The lower parts of the gravel 

deposits generally carry the h1ghest values of placer gold. Traditionally. the richest deposits of all are found in the so-called blue 

gravel, which, when present, lies just above the bedrock and consists of a very coarse, poorly sorted mixture of cobbles, pebbles, 

sand, and clay. It is unoxidized, and, at least locally. contains 

appreciable quantities of secondary sulfide minerals, chiefly 

pynte. 

Information in drill logs from private sources indicates that a 

2 -mile stretch of the channel near North Columbia contains over 

half a ~illion ounces of gold dispersed through about 22 million 

cubic yards of gravel at a grade averaging about 81 cents per 

cubic yard. The deposit is buried. at depths ranging from 100 to 

400 feet. 

Several geophysical methods have been tested for their feasibility 

in determming the configuration of the buried bedrock surface, in 

delineating channel gravel buried under volcanic rocks, and in 

identifying concentrations of heavy minerals within the gravel. 

Although the data have not yet been completely processed, preliminary conclusions indicate that some methods may be quite useful. A combination of seismic-refraction and gravity methods was 

used to determine the depth and configuration of the bottom of the 

channel to an accuracy within 10 percent as checked by the drill 

holes. Seismic-refraction methods have identified depressions 

which are in the bedrock surface, below volcanic rocks, and which 

may be occupied by gravels. Seismic methods, however, cannot 

actually recognize the presence of low-velocity gravels beneath 

the higher v e 1 o city volcanic rocks. Electromagnetic methods, 

supplemented in part by induced-polarization methods, show 

promise of being able to recognize and trace blue gravel buried 

less than 200 feet deep. A broad vague magnetic anomaly across 

the channel suggests that more prec1se magnetic studies might 

delineate concentrations of magnetic material. The us:-fulness of 

resistivity methods appears from this study to be quite restricted 

because of irregular topography and the variable conductivity of 

layers within the gravel. 

GEOLOGY OF THE CHANNEL GRAVEL 

By Donald W. Peterson and Warren E. Yeend 

INTRODUCTION 

Some of the largest and most productive mines in 

California operated in the Tertiary gold-bearing 

gravels between the South and Middle Yuba Piver s, 

Nevada County, Calif. These mines were in coeration 

from the late 1850's until 1884, when a court decision 

halted the dumping of debris into streams tributary to 

the Sacramento and San Joaquin Rivers. Thereafter, 

small hydraulic mines operated sporadically, and the 

high-grade deposits just above bedrock were mined 

locally by underground methods, but product:on from 

the gravels never again reached the pre-18134 level. 

The region is being studied to learn the extent of 

possible gold deposits remaining in plac~ and to 

develop methods for establishing the location and size 

of gravel deposits that may lie undiscovered beneath 

younger rocks. This report is preliminary an·i briefly 

summarizes the results of current geological and geophysical studies of the gravels in the area shown in 

figure 1. The work was done as part of tl·~ Heavy 

Metals program of the U.S. Geological Survey. 

One particularly attractive area, long recognized 

as representing a large, only partly exploited block of 

ground, lies between North Columbia and Bacqer Hill 

(Lindgren, 1911, p. 139; MacBoyle, 1919, p .. 48, 50, 

100; Haley, 1923, p. 118; Averill, 1946, p. 266-267). 

Drilling records in the files of the property owners 
Figure 1.--Index map of part of northern California showing location of report area (shaded). 

indicate that a 2-mile stretch of channel within this 

block· contains a placer deposit of over half a million 

ounces of gold dispersed through about 22 million cubic 

yards of gravel at a grade that averages about 81 cents 

per cubic yard. This deposit is bur i e d at depths of 

from 100 to 400 feet by gravels containing gold values 

of just a few cents a yard.!/ 

Previous Work 

The classic geologic studies of the Tertiary gravels 

of the Sierra Nevada are those by Whitney (1880) and 

Lindgren (1911); most of the subsequent published 

:!./ \11 values cited in this report are based on a price of $35 

per ounce for gold. 

2 

reports draw heavily on Lindgren for much of their 

basic in formation. The area covered b? this report is 

inc 1 u de d in maps by Lindgren and Turner (1895), 

Lindgren (1900), and Burnett and Jennin-;s (1962). Summaries of local information on the gravels and the 

mines in Nevada County are given by Mac Boyle ( 1919) 

and Logan (1941, p. 431-436). General information on 

the geology of the Tertiary gravels and their exploitation is given by Haley (1923), Averill (1946), and 

Jenkins ( 1946). Jarman ( 1927) reports on the feasibility 

of resuming hydraulic mining and citef' figures on reserves. An excellent brief summary of the Tertiary 

channels is given by Clark ( 1965). A sum m a r y of the 

geology of the Sierra Nevada by Bat~man and 

Wahrhaftig (1966) provides a compreh.>nsive discussion of the geologic framework of the Tertiary gravel.· 
Acknowledgments 

The interest and cooperation of property owners 

throughout the area are gratefully acknowledged. The 

San Juan Gold Co., D. R. Schiffner, Harold Helland, 

M. J. Meredith, the Coughlan brothers, Herbert 

Jeffries, C. R. Echlin, and E. J. Colley have granted 

access to their lands and operations and have offered 

valuable suggestions and information. Special thanks 

are extended to the San Juan Gold Co. for permission 

to exam in e and use information in their files. The 

California State Division of Beaches and Parks, the 

California State ivision of Forestry, and the U.S. 

Forest Service ha' aided this study by their interest 

and cooperation. 

GENERAL GEOLOGY 

The geologic map (fig. 2) shows the distribution of 

the gravel and adjacent rocks in a strip from 2 to 6 

miles wide extending from French Corral on the west 

to Bowman Lake on the east, a distance of about 30 

miles. The channels were cut into a bedrock surface 

of metamorphic and igneous rocks ·of Paleozoic and 

Mesozoic age. The winding main channel descends 

from east to west and ranges from 1,000 to 7,000 feet 

in width and from a few tens to nearly 500 feet in 

depth. The tributary channels are smaller. Few exposed stretches of gravel remain entirely undisturbed 

by mining, but some of the partly mined deposits still 

contain large amounts of unexploited material. 

·After deposition of the gravel, the area was extensively blanketed by volcanic breccia. Although there 

has been ~orne erosion, this breccia still cqvers the 

gravel over much of the area, and in the eastern part 

of the area, glacial depc:>sits partly conceal the older 

rocks. 

Bedrock 

Steeply dipping metasedimentary and metavolcanic 

roGks constitute the bulk of the exposed bedrock. In 

the western part of the area, these rocks are part of 

the late Paleozoic Calaveras Formation (Lindgren, 

1900; Ferguson and Gannett, 1932). In the eastern part 

they consist of the Silurian(?) Shoo Fly Formation 

(Clark and others, 1962, p. B16, B17). Siliceous slate, 

quartzite, phyllite, and greenstone crop out in belts 

striking north-northwest and dipping northeast. Serpentine, gabbro, diorite, granodiorite, and quartz veins 

locally intrude the metamorphic rocks. Large bodies 

of Mesozoic granitic rocks intrude the metamorphic 

rocks at the east and west ends of the mapped area. 

Mesozoic metavolcanic rocks possessing a structural 

attitude similar to that of the older Calaveras Formation crop out between French Corral and Birchville. 

The steep dips, multiple intrusions, and metamorphism indicate a complex structural history for the 

pre-Tertiary rocks. At the east end of San Juan Ridge 

the north-striking Melones fault zone cuts the metamorphic rocks and brings serpentine into contact with 

them. The volcanic rocks capping the ridge, however. 

are not offset by the fault (fig. 2). 

3 

Auriferous Gravel 

Gravel deposited by a major river system crops out 

on the divide between the Middle and South Yuba Rivers 

and is p e r'c he d 500 to 1,000 feet above the present 

major streams. Although large sections of the old river 

channel have been removed by erosion, the path of the 

ancient river can be reconstructed with reasonable 

certainty. Its path as determined in the present study 

(fig. 3) is in general similar to the path portrayed by 

Lindgren (1900, 1911) and Lindg1·enandTurner (1895). 

The geologic map (fig. 2) shows the boundaries of the 

preserved gravel and the outline of the hydraulic pits. 

P a 1 eo current indicators such as crossbeds and 

imbricated pebbles show that drainage was from east 

to west. 

The major channel of the ancestral Yuba River can 

be traced, with minor interruptions, from the town of 

French Corral in the western part of the map area to 

Orleans Flat in the northeastern part of the area 

(fig. 2). Between North San Juan and the Badger Hill 

diggings, a distance of approximately 4 miles, the old 

river channel has been completely eroded by the Middle 

Yuba River. Large hydraulic pits oc~;upy the channel 

from the Badger Hill diggings to North Columbia. The 

greatest volume of gravel lies near North Columbia, 

and this deposit is probably the largest in the Sierra 

Nevada (Lindgren, 1911, p. 139).In this area the gravel 

deposit reaches a width of 7,000 feet and a thickness 

of nearly 500 feet. Lindgren (1911, p. 139) estim.ated 

that 25 million cubic yards of gravel were removed 

and that 165 million cubic yards remained in the North 

Columbia area. Despite the extensive hydraulic mining, 

bedrock was not reached. A major tributary entering 

from the south and a minor tributary flowing from the 

north presumably helped produce the large gravel 

deposit. Eastward, the main channel is buried beneath 

volcanic breccia and colluvi urn for part of the distance 

between the diggings at North Columbia and those at 

Malakoff. A part of the channel is also buried beneath 

colluvium at the Malakoff diggings, wh~re hydraulicking has resulted in a colorful, awe-inspiring hole 

in the ground that is now a State park. The gravel here, 

which reaches the extreme thickness of 500 feet, is 

probably the result of the confluence of several rivers. 

Because of the extensive colluvial cover in the Malakoff 

area, it has notbeenpossibletomapthe major channel 

and its tributaries in detail. Information from underground drift mines, such as the Derbec and Union Blue, 

indicates that the major channel extends northeastward 

from the Malakoff diggings, underneath colluvium and 

volcanic breccia, and finally emerges at Woolsey Flat 

(Lindgren, 1911, p. 140-141). Approximately 3 miles 

of the channel is concealed beneath San Juan Ridge. 

A tributary of undetermined size, draining from the 

north and covered by v o 1 can i crocks, and a minor 
Figure 2.--Geologic map of part of area between the South and Middle Yuba Rivers from French Corral to Bowman 

Lake, 

4 
2 

EXPLANATION 

Colluvium 

r+:+--:+1 

L~ Andesite breccia 

Volcanic-rich sedimentary rocks 

Gravel 

Hachured lines indicate boundaries 

of hydraulic pits 

0 2 

) 

Figure 2.--Continued. 

5 

>-

0: 

<( 

z 

0: 

w 

1-

<( 

:> 

0 

>-

0: 

<( 

i= 

0: 

w 

1-

4 

) 

u 

0 

N 

0 

Ul 

Igneous rocks w 

~ 

) 

u 

M 0 

N 

0 

w 

Metamorphic rocks ..J <( 

c.. 

----

Contact 

Dashed where approximately located 

~ 

Fault 

Dashed where approximately located. 

U, upthrown side; D, downthrown side 

85 ~ -+ 

Strike and dip of cleavage in 

metamorphic rock 

Cleavage appears to be parallel to bedding 

A---A' 

Location of geophysical profile 

® 

Center of buried depression indicated 

by seismic study 

6 MILES 
121'15' 

T. 

17 

N. 

39'15' 

_ _, 

R. 7 E. 10' 

French 

Corral 

r 

___ It // 

I,.---/ 

R. 8 E. 5' 

0 

121'00' R. 9 E. 

EXPLANATION 

Volcanic rocks and colluvium 

Tertiary gravel 

Channel boundary 

Dcuhed where in/erred; 

dotted where concealed 

5 MILES 

B B ' 

Location of geophysical profile 

Buried depression - as indicated 

by seismic study 

Alm!g profile B- B' 

M 

Location of magnetic anomaly 

Figure ),--Reconstruction of the path followed by the Tertiary Yuba River and its tributaries, Inferred boundaries show areas where the gravel has been removed by erosion. Concealed boundaries indicate probable position 

of gravel where it is buried by younger rocks, 

tributary coming from the southeast probably joined 

the major channel at Malakoff. Another large channel 

appears beneath the volcanic cover on a small spur 

above the South Yuba River at Relief and must join the 

rna jor drainage somewhere beneath the volcanic cover 

of San Juan Ridge. Small remnants of the major channel are preserved on bedrock spurs above the Middle 

Yuba River at Orleans Flat and Moores Flat. The channel probably continues northeastward from Orleans 

Flat, but it has been eroded by the Middle Yuba River. 

We have not yet studied the extension of the channel on 

the north side of the Middle Yuba River. 

Most of the exposed parts of the channel have been 

modified by mining activities, so that its original configuration cannot be seen. At the few localities where 

gravel is undisturbed, the channel is expressed as 

a low topographic ridge-a topographic reversal of 

what originally must have been a linear depression. 

The ridge can be explained by the high resistance of 

the gravel to erosion relative to the surrounding bedrock. The gravel has high porosity and permeability, 

so that rqinwater tends to percolate into the gravel 

instead of carrying it away in surface runoff. Figure 4 

is a diagrammatic cross section of the channel and its 

gravel fill. 

The gravel-bedrock contact is well exposed along the 

chan n e 1 from French Corral to North San Juan and 

6 

again at the Badger Hill diggings. No bedrock is exposed along the channel floor between Badger Hill and 

Woolsey Flat, a distance of about ll miles. The bedrock floor of the Malakoff pit was exposed during the 

days of hydraulic mining (MacGinitie, 1941, fig. 1), but 

the pit has subsequently been partly filled with fine 

sand, silt, and clay from successive mudflows from 

the steep surrounding cliffs. Where exposed, the bedrock surface beneath the channel shows characteristic 

river scour, polish, fluting, and grooving. Some sections of the channel possess a narrow, steep-walled 

gut within the major broad channel (fig. 4). Generally, 

this gut is 30-50 feet across and 20-40 feet deep. It 

is thought to have contained many of the high-grade 

deposits. Quartz veins in the bedrock stand as low 

ridges in a few places along the exhumed channel floor. 

Such obstructions along the river bed would act as 

giant riffles and were probably important concentrators of coarse gold. 

The gradient of the channel has been determined 

where the floor is sufficiently exposed to establish its 

elevation. Between French Corral and North San Juan 

the gradient is 67 feet per mile. The reconstructed 

gradient along the eroded stretch between North San 

Juan and the Badger Hill diggings is 61 feet per mile. 

Lindgren (1911, pl. 10) gives gradients of 65 and 80 

feet per mile, respectively, for these two segments of 

the channel. Drill holes in the hydraulic pit near North 
Hydraulic pit Upper 

gravel 

Bench gravel 

lower 

gravel 

Figure 4.--Diagrammatic section across an idealized Tertiary channel. The deeper part of the channel is occupied 

by lower gravel, which is coarse and poorly sorted. The lower gravel can be divided into two parts: (1) blue 

gravel, which is below the water table, has a characteristic bluish color, and carries secondary sulfides, 

(2) red gravel, which has been oxidized, is stained red or brown by iron oxides, and generally lacks sulfides. 

It is not known if blue gravel everywhere occupies the base of the channel. The upper gravel, bench gravel, or 

white gravel is considerably less coarse and better sorted and contains many beds of clay- and sand-sized material. Not all channels have a gut but 1 where present, the gut may contain rich gold deposits. 

Columbia indicate a gradient of 66 feet per mile along 

2 miles of the channel. The gradient of the Tertiary 

channel is probably a function of both the original river 

gradient and the subsequent tilting of the Sierran block. 

Apparent gradients could be significantly affected by 

any vertical displacements along faults. In the area 

covered by figure 2, however, only one fault offsetting 

the channel has been r e cog n i z e d. This northwesttrending fault cuts the tributary channel south of North 

Columbia near Spring Creek. The northeast block is 

displaced upward, and if the channel gradient is similar 

to that determined for the North Columbia area, the 

throw of the fault may be as much as 150 feet. 

Although the channel gravel was mapped as a single 

unit, it is far from uniform in texture and composition. 

The lower 100 to 150 feet of gravel is coarser than 

the upper part and is compositionally less mature. This 

lower gravel is exposed only at Badger Hill, Birchville, French Corral, and nor the a s t of Edwards 

Crossing on the Spring Creek tributary. Size analyses 

of the lower gravel give the following averages, based 

on the Wentworth size classes: cobble and· boulder 

( > 64 mm), 13 percent; pebble (64 to 4 mm), 56 percent; granule and sand (4 to 1/16 mm), 28 percent; and 

silt and clay ( < lj 16 mm), 3 percent. Pebble counts 

of the 16- to 32-mm-size fraction give: bluish-black 

siliceous slate, 52 percent; undetermined weathered 

rocks, 31 percent; milky quartz, 12 percent; and other 

constituents, 5 percent. 

Two separate units can be recognized within the 

lower gravel-blue gravel and red gravel. The gravel 

within 50 feet of bedrock is commonly water saturated. 

This gravel is unoxidized and has a bluish-gray color 

that is especially conspicuous when wet; it has been 

called the blue gravel, blue lead, or blue clay. The 

color is due to the high content of bluish-gray slate 

derived from the Paleozoic metamorphic rocks. The 

blue gravel contrasts markedly in color with overlying 

7 

oxidized gravel, .sometimes called red gravel, but the 

two units are similar in sorting characte-:-istics and 

lithology of their components. As Lindg:ren (1911, 

p. 76) points out, the blue gravel seems to be a zone 

lying below the water table and is associated with reducing conditions, rather than being a litt·ologic unit 

distinct in composition from the overlyir?-" oxidized 

gravel. Secondary sulfides, chiefly pyrite, coat the 

pebbles and are disseminated through the rratrix of the 

blue gravel in several localities. Gravel-concentrating 

plants operating near French Corral and Birchville 

are producing gold from undisturbed blue gravel along 

the main channel. 

The light-colored, compositionally more mature 

upper gravel overlies the lower gravel. This white 

gravel or bench gravel is well exposed in most of the 

hydraulicked cliff faces and constitutes the bulk ofthe 

deposit in most areas. The term "bench gravel" comes 

from its frequent occurrence on bedrock benches bordering the deeper channel trough. Size analyses of the 

upper gravel at numerous localities giveth~ following 

averages: cobble and boulder, 1 percent; pebble, 41 

percent; granule and sand, 53 percent; and silt and 

clay, 5 percent. Pebble counts of the 16- to 32-mmsize fraction are: white quartz, 52 percen':; siliceous 

slate, 40 percent; quartzite, 7 percent; and other constituents, 1 percent. Clay beds are common in the upper 

part of the gravel. The chief differences }·~tween the 

upper and lower gravels are thatthe upper ~ravel contains: ( 1) fewer clastic fragments of cobble and boulder 

size, (2) sand and clay beds, and (3) abundant milkywhite quartz pebbles. Large fragments of silicified 

wood are common in the upper gravel, wh~reas wood 

fragments in the lower, water -saturated gravel are 

carbonized. 

Leaf imprints (as yet unidentified) have been collected from clay beds in the pit near North Columbia. 

MacGinitie (1941) studied leaf flora at Chtlk Bluffs, 
about 11 miles south of North Columbia, at a stratigraphic position similar to that of the channel, and 

he dated the flora as early middle Eocene. 

When one studies or visits this area, it does not take 

him long to gain an appreciation for the magnitude of 

the gravel deposits and a fee 1 in g for the enormous 

physical size of the river system responsible for their 

development. Maximum relief on the early Tertiary 

surface was at least 1,000 feet, but local relief was 

generally 200 feet or less. A much different stream 

regimen, with a correspondingly larger drainage basin, 

must be postulated for the Tertiary rivers than currently exists for the present Yuba River. This leads 

to speculation that the headwaters of the ancient river 

system may have been far east of the present crest of 

the Sierra Nevada, possibly as far east ~s Nevada. 

Volcanic Rocks 

Fragmental volcanic rocks overlie and are locally 

gradational with the auriferous gravels. Two separate 

volcanic units have been mapped (fig. 2). The lower 

unit is composed of volcanic-rich sedimentary rocks, 

including claystone, siltstone, sandstone, and conglomerate; it crops out in two locations. In the vicinity 

of Columbia Hill it is overlHin by volcanic breccia and 

is about 120 feet thick along the Graniteville Road 

below Columbia Hill Lookout. In the southwest corner 

of the map area, weakly cemented volcanic gravel as 

much as 400 feet thick forms a cap on Montezuma Hill 

and Bunker Hill. Clasts are composed predominantly 

of andesite with minor amounts of nonvolcanic rocks 

similar to those found in the underlying gravel. Clasts 

are rounded, the matrix is moderately well sorted, and 

cut-and-fill structure and crossbedding are common, 

all of which suggest an alluvial origin for this material. 

The upper volcanic unit is an andesite breccia that 

caps much of San Juan Ridge (fig. 2 ). The breccia is 

poorly sorted and fragments are angular; some blocks 

are as much as 6 feet in diameter, but more commonly, 

the fragments are 6 inches to a foot in diameter. The 

moderately well indurated light-gray matrix is composed of fine sand-size andesitic debris. The unit 

resembles the late Tertiary Mehrten Form at ion 

described from other areas in the Sierra Nevada (Piper 

and others, 1939; Curtis, 1954), but correlation has 

not yet been established. This unit is at least 500 feet 

thick over much of San Juan Ridge and probably is 

locally thicker over the Tertiary river channels. Zones 

containing rounded, apparently waterworn cobbles and 

boulders are intercalated in the breccia at several 

localities. These zones are good aquifers and springs 

are common, particularly along the Relief Hill Road 

above the old town of Relief. The surface of the volcanic breccia is generally flat and slopes gently to the 

west. 

The age of similar andesitic material in other parts 

of the Sierra Nevada has been determined to be late 

Tertiary (Piper and others, 1939; Dalrymple, 1964). 

The volcanic rocks were originally much more wide8 

spread and probably constituted a rearly continuous 

sheet over this part of the range. Pa:t1:s of the Tertiary 

river system have been exposed only after subsequent 

erosion of the volcanic rocks. 

Colluvium 

A mantle of landslide debris, here termed "Colluvium," is composed largely of fragments of andesite 

breccia and borders some of the steep slopes capped 

by breccia. The colluvium has an irregular, hummocky, 

and sometimes blocky surface, indicating little modification since its de v e 1 o p m en t. The colluvium is 

thickest near the steep slopes of the undisturbed volcanic breccia where it commonly grades into talus. 

Generally, the colluvial mantle is se''eral tens of feet 

thick and may extend as much as a mile from the volcanic cliffs. 

Curiously, the colluvium is alm'Jst restricted to 

those places, such as Relief, Moores Flat, Woolsey 

Flat, M a 1 a k off, Lake City, and Bloody Run Creek, 

where the early Tertiary river channels extend beneath the thick volcanic cover. This in t e r e sting 

relationship can perhaps be explained by the fact that 

the upper part of the gravel is rich in clay. Slope failure could be expected under conditions of a thick, 

relatively porous, water-saturated v o 1 can i c layer 

overlying weak clays; therefore, the presence of colluvium might be a useful indicator to otherwise 

unsuspected gravel throughout the Sierra Nevada. 

Colluvium undoubtedly a c cum u 1 ate d as soon as 

erosion began to remove the volcani~ breccia, and, as 

the breccia scarp retreated, a mantle of colluvium 

continued to form at its base. The rate of erosion and 

colluvium accumulation probably reached a maximum 

during the wet periods of the Pleisto-::ene, and most of 

the present colluvium probably formed at that time. 

TERTIARY CHANNEL NEAR NORTr,• COLUMBIA 

Gravel in the Tertiary channel from North Columbia 

to Badger Hill has been partly removed by hydraulic 

mining, but a large,amount of gravel remains in place. 

No records indicate that any appreciable underground 

mining has exploited the higher grade gravel in the 

lower part of the channel in this vicinity; consequently, 

the area is an attractive one for investigation. Figure 

5 is a geologic map showing the extent of both undisturbed and partly removed gravel in the area from 

North Columbia to Badger Hill. 

An extensive drilling program was conducted near 

North Columbia during the late 1930's-, and the property 

owners have kindly made the drill records available. 

These records indicate that 59 holes totaling more 

than 20,000 feet were drilled; the location ofthe holes 

is shown in figure 5. Two categories of information 

from the drill holes are of particular interest: (1) the 

shape and configuration of the botto'll of the channel, 

and (2) the distribution and amount of gold in the gravel. 
CONTOUR INTERVAL 25 AND 40 FEET 

DATUM IS MEAN SEA LEVE L 

EXPLANATION 

Unmined gravel 

Hydraulic 

pit 0 

Drill hole 

M 

a a' A _ _:_:_ __ -=-- A' 

Location of magnetic Geophysical profile 

anomaly a-a: is line of cross section shown on figure 7 

Figure 5,--Detailed geologic map showing location of hydraulic pits and drill holes in the Tertiary gravel in 

the vicinity of North Columbia and Badger Hill, Unmined gravel remains in place in most of the hydraulic pits, 

and only in the lower pit of the Badger Hill diggings has bedrock been exposed by hydraulicking. 

Configuration of the Channel 

The drill holes range in depth from 100 to 484 feet. 

They outline a channel bottom broadly concave in cross 

section that descends westward with a gradient of about 

66 feet per mile. A contour map showing the topography 

of the bottom of the channel is shown in figur e 6. A 

typic~l vertic_al cross section across the channel along 

one lme of dnll holes is shown in figure 7. This section 

is part of one of the profiles that was examined in 

detail by geophysical methods. 

9 

Grade and Distribution of Gold 

The drilling records indicate that the highest gold 

concentration is near the bottom of the deepest part 

of the channel. Values in the upper part of the gravel 

deposit vary from a fraction of a cent to a few cents 

per cubic yard, but in the lower part of the deposit the 

grade increases to several tens of cents per cubic yard 

and locally attains sever a I dollars per yard, The 

richest grade indicated in the records is an interval 

of 42 feet in one hole, which assayed $9.21 per cubic 

yard. 
EXPLANATION 

0 

Drill hole 

Contour 

Dad.ed where i nterpretati on il 

uncertai n . Contour i nter val 

to f eet 

SECTION 6 

SECT ION 7 

. 800 0 800 1600 FEET 

~~~-L------L-- ~ 

SECTION 5 T. 17N ., R. 9E. SECT I ON 4 

SECTION 8 SECTION 9 

Figure 6.--Contour map of bedrock surface at base of gravel in North Columbia hydraulic pit. Contours based on 

drill-hole data. 

EXPLANATION 

100 0 100 200 FEET 

Sti'JipU.d WMn GUGW aboot 

10 c..U ,.,- e..wt .. ni 

Figure 7.--Cross section of channel near North Columbia as indicated by one line of drill holes. Line of section 

~-~· is shown in figure 5. 

Calculations made from the drill records indicate 570,000 ounces of gold, or about $19.9 million in gold. 

the total amount of gravel and the value of its gold The average grade of the total block is 13.7 cents per 

content. The ground was divided into rectangles, each cubic yard, but this includes much material that assays 

with a drill hole at the center. The volume of gravel at only 1 or 2 cents per cubic yard or less. Therefore 

underlying each rectangle was determined, and from further calculations have been made, arbitrarily asthe assay values given for specific intervals in each signing a cutoff grade of 10 cents per cubic yard. About 

hole , a figure was obtained for the value of the gold 21.8 million cubic yards of gravel, containing values 

under each rectangle. richer than this cutoff grade, has a total gold content 

The summation of these numbers indicates that the 

entire block of drilled ground consists of a total of 

145.2 million cubic yards of gravel containing about 

of about 506,000 ounces, or about $17.7 million in 

gold, at an average grade of 81.2 cents per cubic yar d. 

This higher grade gravel is buried at depths ranging 

from 100 to 400 feet, and a representative cross section 

10 
-- / / / _ _.....-0 

/ 

// ---

0 

SECTION 6 

SECTION 7 

0 

SECTION 5 T 17 N, R 9 E. 

SECTION 8 

0 

0 

EXPLANATION 

0 

Drill hole 

800 1600 FEET 

SECTION 4 

SECTION 9 

d b d Uli information. Contour figures are in Figure a.--Contour map ef the foot-assay product as indicate Y r ng thousands of foot-cents. 

of its distribution is shown in figure 7. Figure 5 shows 

that over half the length of the channel in the North 

Columbia-Badger Hill area is not included in the block 

of drilled ground. It is reasonable to speculate that the 

potential total volume of the deposit and the gold content are considerably greater than the above figures. 

The foot-assay product.Y has been determined for 

each drill hole that contains gold values exceeding the 

cutoff, and a map giving contours offoot-assayproducts 

is shown as figure 8. A comparison of figures 6 and 8 

shows that each peak locality of the foot-assay product 

lies in the trough of the channel and that the maximum 

foot-assay product of any cross section generally 

coincides with the lowest part of the trough. The three 

major peaks of the foot-assay product along the central 

and e a s t e r n stretches of the channel lie at or just 

downstream from a bend in the channel. Considerable 

variation occurs lengthwise along the channel, and 

values drop abruptly toward the sides. These observations are based on drill records that have not been 

confirmed by additional tests. Inasmuch as reliable 

sampling of placer deposits is notoriously difficult, 

these records must be interpreted with care. 

In view of the waste-disposal problems, water costs, 

and land values, it is unlikely that hydraulic methods 

can again be employed on a large scale to recover gold 

in this area. Furthermore, the grade is too low to permit conventional underground mining methods. The 

challenge of a large target, however, may stimulate 

the imagination and ingenuity of gold seekers to devise 

a low -cost mining method that will permit recovery 

of the gold in this and other deposits in the Tertiary 

channels of the Sierra Nevada. 

~/The foot-assay product is the number obtained by multiplying 

the length in feet 'or each segment of the drill hole by the assay 

value of that segment in cents per cubic yard. The total foot-assay 

product for each hole is obtained by adding together the products 

of the segments. The foot-assay product is an expression of the 

total quantity of mineral in the block but does not necessarlly express the grade. Ten feet of 10-cent gravel has the same footassay product as l foot of $1.00 gravel. 

GEOPHYSICAL INVESTIGATION! OF 

THE CHANNEL GRAVEL 

By Howard W. Oliver and Robert E. Mattick 

INTRODUCTION 

A series of tests have been conducted in parts of 

the Tertiary channels in northern Nevada County, 

Calif. (fig. 2), to determine the feasibility of using 

geophysical methods to ( 1) determine the configuration 

of the bedrock surface beneath the exposed gravels, 

(2) find and trace Tertiary channels where they are 

covered by volcanic breccia, (3) 1 ocate structures, 

such as transverse ridges, faults, and litholor;ic boundaries in the bedrock, that might have served as natural 

traps for gold concentrates, and (4) determine the 

distribution within the gravels of other relati,•ely heavy 

minerals such as magnetite and pyrite that are known 

to be associated with concentrations of gold in this 

area (Lindgren, 1911, p. 73-74). Initial tests of seismic, gravity, ground magnetic, resistivit~', ground 

electromagnetic (EM), and induced-polariz:11tion (IP) 

methods were made during July to mid-Augu~t 1967. In 

addition, tests of three airborne methods-magnetic, 

EM, and radiation-were carried out in Octcher 1967. 

The compilation of all these data is incomplete but 

several important preliminary results are reported 

herein. 

The geophysical methods employed were selected 

in an attempt to obtain as varied and detailed informa.:.. 

tion as possible on the gravel deposit E' .. Seismic 

refraction supplemented by gravity and resistivity 

techniques was used to find out if bedrock configuration could be determined and if gravel depo~its could 

be traced under a cover of volcanic rocks; resistivity 

methods, EM and IP, based on a suspected vrriation in 

the water and (or) clay content and hence on the electrical properties of the blue, red, and bench gravels, 

were used to see whether structures in tb~ gravels 

and the distribution of the gold-rich blue gravel could 

11 
be revealed; and IP and magnetic methods were tested 

in order to determine whether they could detect disseminated sulfides or concentrations of magnetite, 

respectively, that might be associated with placer gold 

deposits. 

Acknowledgments 

The g eo physic a 1 tests were under the overall 

direction of H. W. Oliver. Investigators responsible 

for the fieldwork and the interpretation of data for the 

respective methods are as follow: R. E. Mattick, seismic; D. B. Jackson, resistivity; Peter Popenoe, EM; 

D. B. Hoover, IP; S. L. Robbins, gravity and ground 

magnetic; F. C. Frischknecht, airborne; and E. D. 

Seals, surveying. Field parties inc 1 u de d Cletus 

Wotorson from Liberia, J. H. Seo from South Korea, 

and C. E. Shipp, J. R. Matis, D. R. Shoenthaler, R. N. 

Babcock, J. T. Page, Jr., C. L. Tippens, Arthur 

Conradi, Jr., and A. W. Savage, Ill. Herbert Jeffries 

kindly provided magazine storage at North Columbia 

for explosives. We are particularly grateful to Clyde 

Newlin and Eric Leffingwell for expediting approval by 

the California Division of Beaches and Parks to carry 

out tests in the Malakoff Diggins State Historic Park. 

INSTRUMENTATION AND FIELD PROCEDURES 

Tl1c ._. e ism i c tests were made with a 7000-B recording unit built by Houston Technical Laboratories. 

This unit recorded the output of 12 vertical seismometers on photographic paper. The seismometer 

spacing ranged from about 100 feet (fig. 9) to 650 feet 

(fig. 11), and energy was propagated from dynamite 

charges of 50 to 200 pounds buried at depths of 5 to 15 

feet. Ground elevations of the seismometer stations and 

shot points were surveyed and staked to the nearest 

0.1 of a foot and used in conjunction with the gravity 

survey. 

Gravity, magnetic, and EM data were obtained at 

every seismometer stake as well as along extensions 

of the seismic lines. LaCoste and Romberg meter G62 

was used for the gravity survey, and three-frequency 

slingram equipment built by the U.S. Geological Survey 

was employed for the EM measurements. The vertical 

magnetic measurements shown in figure 9 were obtained using two Sharp flux-gate magnetometers in 

tandem with a Barringer precession magnetometer 

which provided the total-intensity data. The Sharp 

magnetometers were placed on leveled planetables and 

the Barringer sensor set on a wooden staff to insure 

the greatest possible accuracies of the respective 

instruments. Aeromagnetic surveys were made with 

the U.S. G eo 1 o g i c a 1 Survey's flux-gate equipment 

mounted in the tail of a Convair CV -240. 

Standard direct-current equipment built by the u.s. 

Geological Survey was used for the resistivity measurements. Commercial instrumentation manufactured 

by Burr-Brown Corporation was used for the IP survey. 

12 

DETERMINATION OF BEDROCK CONFIGURATION 

Seismic Method 

The drill-hole data furnished by the property owners 

provided an excellent opportunity to test methods of 

finding depths to bedrock. Two seismic refraction 

profiles were shot in the North Columbia diggingsone along A-~' in the upper pit and the other along 

the line of drill holes located about 0.4 mile east of 

A-A' in the lower pit (fig. 5). The seismic results, as 

well as resistivity, gravity, magnetic, EM, and drillhole data obtained along profile A-A' are shown in 

figure 9. 

The total length of the seismic profile within A-A' 

is about 4,200 feet, and the average geophone spacing, 

although varia b 1 e, was about 100 fe~t. Weathering 

spreads have been shot in the vicinity of each of the 

five shot points and elevation corrections made to the 

unreduced traveltimes shown in figure 9 for a nearsurface velocity of about 1,000 fps (feet per second). 

Except at the northern end of the pro~ile where bedrock arrivals were recorded in only one direction, 

depths to bedrock were computed at each geophone 

by the time-depth method (Hawkins, 1961). Although 

the drill-hole data were available prior to the seismic 

tests, no referral to specific drill-hole depths was 

made until completion of the computed seismic cross 

section. Disregarding the thin weathering layer (about 

20-25 feet thick), the average veolocity of the layer 

of sand, clays, and gravel which overlies the bedrock 

surface is about 6,150 fps. In contraf't, the average 

velocity of the bedrock, as computed by the method of 

Willmore and Bancroft (1960), is about 18,500 fps. 

This simple two-layer case, together with the large 

velocity contrast between the bedrock and overlying 

material, suggests that seismic-refraction methods 

should yield accurate bedrock depths. In this case, a 

comparison of the depths to bedrock computed from the 

seismic data and the depths found in drilling shows that 

the average error in the seismic denth data is 4.6 

percent. This agreement along one rrofile is very 

close but should be considered as posf'ibly fortuitous 

until all the test data have been analyzed. 

Most of the scattering in bedrock arrivals about 

straight-line segments shown on the time-distance 

curves (fig. 9) can be explained by ground surface 

relief or better interpreted as result;ng from nearsurface lateral changes rather than from relief on the 

bedrock surface. One possible except:':>n is the 1 ate 

arrivals marked "an" and "a 5 " near the south end of 

the profile. These arrivals were recorded in an area 

where the ground surface is nearly flat and where a 

detailed weather in g spread indicated no velocity 

anomalies within the upper 30 feet of gravels; moreover, the scatter of the overburden arrivals marked 
"ct 0 " (fig. 9) is inconsistent with a near-surface explanation for the late bedrock arrivals. On the other 

hand, if the late arrivals ct nand ct 5 are caused by a 

local depression in the bedrock, as shown by the dashed 

curve in the bedrock surface, the delayed ct n arrival 

recorded from the north shot point should theoretically 

be displaced a distance of 180 tan ( 19° -12°) or about 

20 feet south of the bedrock depression; similarly, the 

delayed ct 5 arrival recorded from the south shot point 

should be displaced 180 tan (19°+12°) or about 110 feet 

to the north of the depression and 130 feet or about 1 

geophone spacing north of "n. As the maximum delay 

times an and ct 0 are not displaced relative to each other 

within the limits of these data, the interpreted bedrock 

depression "«" should be considered as questionable 

pending more detailed data. An alternate interpretation 

of the seismic anomaly is a low-velocity (<6,150 fps) 

lens 1 ocate d within the lower part of the Tertiary 

gravels at "a". 

Other more subtle examples of late arrivals more 

likely caused by local relief in the bedrock surface are 

marked "~." Although not considered in the firstorder an a 1 y sis, they occur at the correct surface 

location for critical rays generated at the south shot 

point and emerging at an angle of 19° from the center 

of the channel as shown by the drill-hole data. 

Gravity Method 

Regional gravity data were obtained in the area of 

figure 2, and six detailed profiles were made across 

the Tertiary gravels at North Columbia, Malakoff, and 

San Juan Ridge. Simple Bouguer anomalies along one 

of the profiles at North Columbia (A-A', fig. 5) are 

shown in figure 9, one curve for an assumed bedrock 

density of 2.67 gjcm3 (grams per cubic centimeter) 

and the other for a gravel density of 2.1 gjcm3. The 

2.1 gjcm3 reduction density is based on actual measurements of the blue gravel at Birchville (fig. 2) which 

average 3,500 pounds per cubic yard (Herbert Jeffries, 

oral commun., 1967). However, bucket-size samples 

of the ben~h gravel at North Columbia yield densities 

in the range 1.4-1.8 gjcm3 for dry samples and 1.8-

2.2 gjcm3 for wet samples. The porosity of the bench 

gravels is evidently about 40 percent. 

The gravity low of about 1 milligal, which is presently uncorrected for terrain, corresponds to the thick 

part of the channel fill and qualitatively demonstrates 

the potential usefulness of the method. Quantitative 

refinements and extensions to the north and south of 

the bedrock surface as interpreted from the seismic 

data along A-A' should be possible after terrain corrections are completed. 

Magnetic Method 

Inasmuch as slates, phyllites, and quartzites of the 

Calaveras Formation are generally nonmagnetic, as 

shown by 17 east-west high-level aeromagn~tic profiles with a north-south spacing of about 5 miles across 

the centra 1 Sierra Nevada (Isidore Zietz, written 

commun., 1967), the rr.~gnetic method was not expected 

to be of much help in determining bedrock configuration. However, a north-south ground magnet~(: profile 

crossing the North Co 1 u m b i a pit along A-A' and 

extending half a mile into bedrock on both ends shows 

a broad magnetic low of several hundred ganmas associated with the Tertiary channel. The central part 

of the anomaly is illustrated in figure 9. Th~ form of 

the anomaly, with the magnetic minimum die,placed to 

the south of the deepest part of the chan n e 1, is 

qualitatively consistent with a homogeneous basement 

magnetized in a direction parallel to the earth's magnetic field. However, a rather high basement susceptibility of about 10-3 cgs (centimeter -grarr -seconds 

system) would be required to explain the anomaly if 

caused entirely by the basement configuratio:'l. Results 

from the detailed airborne survey, when available, 

should help resolve this apparent problem: 

Resistivity Method 

Tests of the resistivity method for de<:ermining 

depths to bedrock were somewhat disappointing in the 

North Columbia area. However, at the Malakoff diggings where bedrock is shallower and the terrain more 

favorable, it does seem likely that the depth to bedrock 

was obtained at three places across the pit. At North 

Columbia five soundings were tried and a 1,300- by 

700-foot area of the lower pit was mapped., The data 

from and the interpretation of two of the soundings 

are shown in figure 10 and the interpreted resistivity 

layers within the Tertiary gravels are plotted in the 

geologic section A-A' (fig. 9). 

With the exception of the sounding at R11 illustrated 

in figure 10, the current electrodes could not be expanded far enough to pick up the effect of basement 

rocks because of the hydraulicked cliffs and deep 

gullies within the pits. Moreover, soundings expanded 

across the Tertiary channel, such as in figure 10, may 

have dip effects from the rising basement rocks to the 

north and south. These problems preclude the use ot 

resistivity for determining bedrock configurations in 

this area, although the method may be useful f:>r tracing 

layers within the gravels. Possibly the fom· Tertiary 

layers with sharply decreasing resistivities of 6, 100, 

3,500, 1,000, and 25 ohm-m (ohm-meters) (figs. 9 and 

10) correspond in some way to the bench, red, and 

blue gravels (fig. 4). 

Electromagnetic Method 

EM has some advantages over resistivity at North 

Columbia because it is not affected by the highly resistive washed gravels (fig. 10). However, the method 

is able to "see" only as deep as about o'1e-half to 

three-quarters of the loop spacing, which was 300feet 

13 
1-

Iii 

Iii 

IL 

~ 

i 

0 

~ > Iii 

..J 

Iii 

:; 

3000A 

2800 

2600 

2400 

(/) 

a 

z 

0 

0 

Iii 

(/) 

:J 

..J 

i 

Tertiary 

?~ravels 

300 

250 

200 

150 

100 

50 

0 

North 

shot point 

t; 8,000 ::I 

Q~ 

~as 6,ooo (..) 

a~ 4,000 

~~ ...:.. z: 2,000 o-

~ 0 

GOLD VALUES 8,000 

6,000 

4,000 

2,000 

~~~--~~~----~~0 

GEOLOGIC SECTION AND RESISTIVITY DATA DH-~ DH-4 DH-• Positions of resistivity soundln(ls 

DH- 1 DH-3 DH-!S DH-7 } DH-8 DH-11 Ground surface 

Velocity=6150 fps 

Density= LS-2.2 s/cm3 '" 6700 R, 1 

0 Resistivity, In ohm-meters C , " ------------. 

Velocity:= 18,500 fps 

Density=2.7 s/cm 3 Center of channel 

Shot 

point 

1000 

25 

""-Gravel-bedrock contact as shown 

by drill-hole data 

SEISMIC 

Travel times 

__ ,. 

\.II "" 

Overburden arrivals 

Shot Shot 

point point 

300 

250 

200 

150 

100 

50 

t 

South 

shot point 

A' 3000 

2800 

2600 

2400 
-

c.n 

Ill 

Ill 

.J 

~ 

CJ 

3-

1.5 

1.0 

i 0.5 

0 

52,550 

52,500 

~ 52,400 

:1: 

~ 

CJ 52,300 

52,200 

52,100 

1-

z 

w 

z 

OIILZ 

;~:W 

oo 

00:: lal 

wiL 

lllz 

~-IL 

~ 

North 

110 

100 

90 

GRAVITYRegional gradient for p ==2.1 g/crn3 ---------------lr-----1---------------

ii i r e 

Simple Bouguer anomaly 

for p =2.67 s/cm3' 

"" Simple Bousuer anomaly 

for p =2.1s/cm 3 

MAGNETISM 

Vertical component~ 

EM 

Horizontal coplanar coils 

500 0 500 1000 FEET 

1.5 

1.0 

0.5 

0 

52,550 

52,500 

52,400 

52,300 

52,200 

52,100 

South 

-10 

..... 

z 

w 

0 

+10 

z 

0 

IL 

;~:1- oZ 

ow 0 wa: 

Ulw 

~IL 

Iz 

ILIi. 

0 

..:. 

::J 

0 

figure 9.--Seismic• resistivity. gravity. magnetic, and electromagnetic data and the interpreted geologic section along~--~· (figs. 2 

and 5). The average error in the computed depths to bedrock at the nine drill-hole locations is 4.6 percent with a maximum error of 8.6 

percent or 28 feet at the southernmost drill hole. The amount of gold in each drill hole is shown as a graph of the foot-assay product. 

This graph is a profile through ~~· of figure 8. 
100,000 __..._ 

50,000 

5 feet V' ...,.,... ~ 

~ \ 

1\ Codected resistivity 

curve at R1 _, 

__) 

Ill 

0:: 

w 

1-

w 

~ 

:t 10,000 

:r 

0 

~ _] 

r--.~ 

~ 

~ ~ 

"~ \ \ V Correcte:d resistivity 

I curvn at R11 I 

\ t ~ 

~ 5000 ..... 

> i= 

Ill 

iii 

w 

0:: 

..... 

z 

w 

0:: 

c( 

a.. 

a.. 

c( 

1000 

" 1\ 

~ 

' ~ ~ 2000 feet 

'" L 500 

1\ """'41 

200 1 5 10 50 100 500 1000 3000 

HALF-DISTANCE BETWEEN CURRENT ELECTRODES, IN FEET 

Depth, in feet 10 75 230 320 430 

Resistivity, in I J ohm-meters 80,000 6100 3500 toool 2sl >2000 

I Washed gravels Tertiary gravels I Bedrock 

INTERPRETED RESISTIVITY LAYERS 

Figure 10.--Resistivity soundings at R1 and R11 (geologic section, fig. 9) made with Schluw ~rger arrays 

(Keller and Frischknecht, 1966, p. 95) expanded parallel to profile~~·· The six-layer interpretation 

using the auxiliary point method (Zohdy, 1965) is constrained to fit the depth to bedrock determined by drilling. 

in most of our work. The EM data illustrated in figure 

9 were obtained using a 200-foot loop spacing. They 

indicate that there are no electrical conductors within 

about 150 feet of the surface over the deep part of the 

channel, a conclusion which is consistent with the resistivity sounding (fig. 10). EM anomalies do appear 

as the bedrock surface begins to rise toward both the 

north and south ends of the profile, which indicates 

that some conductor has come within about 150 feet 

of the surface (zero for the out-of-phase component 

is off by about 5 percent). Previous study of electrical 

effects over buried channels in the search for uranium 

(Black and others, 1962) indicates that some of these 

anomalies probably represent elongate clay lenses in 

the stream gravel. Perhaps the 25-ohm-m layer (blue 

gravel?) discovered by the resistivity sounding does 

not pinch out toward the side of the channel but continues to overlie the bedrock at least intermittently 

where it comes to within 150 feet of the surface. 

Induced-Polarization MethN:I 

Tests of the induced-polarization (lP) method were 

made in the diggings at French Corral, Badger Hill, 

North Columbia, and Malakoff (fig. 7. ). Rough terrain 

severely restricted the depth of exploration in most of 

the diggings by limiting the electrode separation. Thus 

IP profiles were essentially limited to 1 o o king for 

clays, disseminated sulfides, and other anomalous 

materials within about 200 feet of the ground surface. 

Indications of bedrock configurations were obtained at 

both the Malakoff and Badger Hill diggings. The evidence at Badger Hill is illustrated in figure 13, which 

shows contours of apoarent direct-current resistivities bending upward to the west on the west flank of 

the hydraulicked channel. Although t~e exact bedrock 

configuration is unknown in this area, this pattern is 

similar to the general form of the bedrock surface as 

depicted in the geologic section (fig. 13). 

16 
DETECTION OF TERTIARY CHANNELS UNDER 

VOLCANIC COVER 

A seismic refraction profile was run a 1 on g the 

Graniteville Road between Willow Spring and the 

Moores Flat road junction (profile 12-:61

, fig. 2) to 

determine whether a gravel channel could be detected 

beneath a thick cover of volcanic rocks. Profile E.-Jl 1 

is about 10,800 feet long and was chosen so as to cross 

the concealed position of the main Tertiary channel as 

projected between the exposed gravels at the Malakoff 

diggings and Woolsey Flat (figs. 2 and 3). As projections from outcrop and from workings of the Derbec 

mine (Lindgren, 1911, pl.20)suggestthatthethickness 

of volcanic cover in this area is over 500 feet, it was 

thought that this locality would provide a firm test of 

geophysical methods. No drill data are available in 

this area. 

Three shot points were employed and a geophone 

spacing of about 650 feet was used for profile 12-!2' 

(fig. 11). The time-distance plot shows that the average 

velocity through the overlying volcanic material is 

about 8,350 fps and the velocity through the bedrock is 

about 18,700 fps. If it is assumed that only a thin layer 

of lower velocity material (sand, clays, and gravel 

with a velocity of about 6,500fps) lies between the volcanic rock and the bedrock surface, then, although no 

indication of this layer would appear on the timedistance plot, the error in computing the depth to bedrock would be small. A hypothetical 1 a y e r of t hi s 

nature 200 feet thick would induce an error of about 

5 peq:ent. Furthermore, if we assume that the thickness of a layer of this type remains constant, then the 

relative error in calculated depths between geophones 

would be negligible. · 

Two channels appear on the profile. The smaller 

channel, located about 3,000 feet west of the east shot 

point, is based on one arrival at one geophone and 

hence is questionable. The larger channel, centered 

about 2,000 feet east of the west shot point, is based 

on arrivals at about six consecutive geophones, which 

suggests that the channel exists and that near-surface 

changes in velocity have not caused a faulty interpretation. Although a lens of sand and clay at depth with 

a velocity of about 6,500 fps would appear as a channel 

on the computed cross section, it would require a lens 

about 700 feet thick to give rise to an apparent channel 

of this magnitude. 

The position of the center of the larger channel ~s 

interpreted from the seismic data along the Granite·:.. 

ville road is about 900 feet west of Upper De r be c 

Spring and is shown in figures 2 and 3. The dotted horizontal line at 3,600 feet in figure 11 defines the lateral 

borders of the channel as obtained by seismic means. 

Its position agrees remarkably well with the position 

inferred by geological projection shown in figure 3. 

This agreement indicates that the seismic method may 

be very useful in locating channels buried beneath volcanic rocks. The thickness of the gravel at this locality, according to the interpretation, is about 200 feet. 

The resistivity sounding on the east flank of the 

Tertiary channel (fig. 9) was obtained prior to the 

seismic tests. A sounding directly over the center of 

the seismic channel would be useful for C':lmparison 

although there is some question whether th~ Tertiary 

gravels could be distinguished from the 50 ohm-m 

layer, which presumably represents the hwer section of the Tertiary volcanic breccia. 

Gravity data in the area of San Juan Rid:-re are now 

being compiled, and no results are available at this 

time. 

RECOGNITION AND DELINEATION OF THE BLUE 

GRAVEL 

Because the richest deposits of p 1 ace r gold are 

formed in the lower part of the channel, and because 

at least some of the lower part of the channel consists of blue gravel, tests were made to determine if 

any of the geophysical methods could recognize and 

trace these wet, clay-rich, sulfide-bearir'! gravels. 

Blue gravels are currently exposed by miring operations at French Corral and Birchville, and a small 

outcrop appears at the base of the cliff face that separates the lower and upper pits at Badger Hill (figs. 5 

and 12). 

Tests of resistivity, EM, IP, andmagnet;csdirectly 

on the blue gravels at French Corral show that these 

gravels have a relatively low resistivity of about 

20-30 ohm-m, produce a small metal-factor IP anomaly of 265 (ohm-m)-1, and cause a magnetic anomaly 

of about 70 gammas. The grave 1 s are sufficiently 

conductive to produce an EM anomaly of 45 percent 

in the out-of-phase component as determined by a 

vertical coplanar loop orientation, a spacing of 300 

feet, and a frequ·ency of 3,600 cycles per second. 

Pyrite is clearly visible throughout the bJue gravels 

in this area, although quantitative data are presently 

unavailable. 

An EM test was also made at Birchville where the 

sulfide content of the blue gravels averages about 10 

pounds of sulfides per cubic yard of graY~l or about 

0.3 percent by weight (Herbert Jeffries, orrlcommun., 

1967). A smaller EM anomaly of 33 percent was observed using the same orientation and frequency as 

at French Corral. The interpreted resisti,·ity is about 

50 ohm-m, nearly double that at French Corral. 

In view of these results, tests were des;rable in an 

area where the blue gravel is buried by varying 

amounts of red and bench gravels up to the approximate 200-foot-depth limit of our EM and IP instrumentation. Such an area is the upper pit of the Badger 

Hill diggings (fig. 12), in which thicknesse:;- of the remaining gravels vary from about 100 feet in the 

northern part to possibly 200 feet at the scuthern end. 

An EM survey was made of the upper rit, the contours of the out-of-phase component being shown in 

figure 12. IP and ground-magnetic data we'¥'e also obtained along several of the EM traverse~.. In figure 

13, the three sets of data are shown alo"lg traverse 

17 
West 

lUI 

w 

... 

5000 

~ 4500 

z 

0 

~ 4000 

(;j 

.J 

w 

3500 

(/) 

0 

z 

0 

"""" u 00 w 

(/) 

:J 

800 

600 

.J 400 

i 

z 

tJ 

~ 

i= 

200 

0 

B 

West 

shot point 

Center of 

channel 

Bedrock surface as 

computed from 

seismic data 

li 

~ 

§ 

Resistivity 

sounding 

=~290 

50 

CXl 

Velocity= 18,700 fps 

Ground surface 

Velocity:8350 fps 

Tertiary volcanic breccia 

Channel(?) _ 

Pre-Tertiary bedrock 

0 500 1000 FEET 

~ !).~ 

- cs>~cs>o 

~., 

I I II I I 

Center 

shot point 

EXPLANATION 

Bedrock arrivals 

C>-----0---<> 

Overburden arrivals 

t 

~ 

8 

01 

East 

B' 

5000 

4500 

1-4000 

800 

600 

400 

0 East 

shot point 

Figure 11.--Geologic section, resistivity data, and seismic traveltimes along!--!' (fig. 2). The buried Tertiary channel postulated from the seismic 

data apparently crosses this section at an angle of about 45° (see fig. 3). The depth to bedrock computed from the resistivity sounding is 1,120 

feet, about 12 percent larger than the 1,000 feet interpreted at that point from the seismic data. 
C.-.C.', located several hundred feet south of the cliff 

face separating the lower and upper pits (fig. 12 ). The 

depth to bedrock along the central part of Q-..Q' is 

about 110±15 feet based on a projection of the elevation of exposed bedrock in the lower pit. 

A significant EM anomaly of about +~4 p e r c en t 

occurs at the north end of the upper pit and decreases 

widens, and meanders toward the southern end of th~ pit (fig. 12). The anomaly amounts to 17 percent along 

Q.-Q.' and correlates with a moderate IP anomaly of 

18~20 percent in the frequency effect. As 1 i gh t magneue anomaly of possibly 20 gammas also occurs at 

about the same location, although other magnetic 

variations along the profile tend to obscure the significance of this apparent correlation. 

The correlation between the EM and IP anomalies 

is significant and the EM anomaly in figure 12 may 

define the trace of the blue gravel. This is potentially 

an important finding because EM is easily the most 

portable, rapid, and economical ground-geophysical 

technique that we have tried. Unfortunately, additional 

IP data obtained in the diggings show that limonitecemented gravels also produce f r e que n c y effect 

anomalies of the same order of magnitude. 

Seismic tests or drilling would be required to fur-

~her c~eck on the feasibility of using EM, possibly 

m conJunction with IP, to trace the blue gravels at 

depths less than about 200 feet. For depths of over 

300 feet, the data at North Columbia (fig. 9) suggest 

that the geophysical methods used cannot trace the 

blue gravel. However, the blue gravel would most 

likely be in the deepest parts of the channel, which 

apparently can be located by seismic and gravity 

methods. 

LOCATION OF CONCENTRATIONS OF MAGNETIC 

MINERALS 

The application of magnetic methods for locating 

concentrations of black sand, with which gold is associated in placer deposits, has been used successfully in California, Arizona, Colorado, and Alaska 

over the last 40 years (Jakosky, 1950, p. 228-230). 

Jakosky points out that, in some cases, as in Trinity 

County, Calif., it was possible to outline an old stream 

bed completely with a ground magnetometer. Joe sting 

(1941, figs. 1-3, 5) documents a rather striking relationship between magnetic anomalies and gold values 

across four different placer channels in Alaska. He 

reports, however, that the magnetic method was 

valuable in preliminary prospecting for only about 

half of the gold placers in the interior of A 1 ask a. 

According to Joesting, some of the Alaskan placers 

contain little or no magnetite, and other chan n e 1 s 

were incised into a magnetic basement which tends 

to mask the limited effect, generally less than 100 

gammas, of the black sand. 

Initial ground magnetic tests are inconclusive. Profiles with 100-foot spacing were obtained across the 

Tertiary gravels at French Corral, Badger Hill, North 

Columbia, Malakoff, and the concealed gravels under 

19 

the volcanic rocks of the San Juan Ridge (figs. 2 and 3). 

In general, the magnetic profiles are simikr to those 

published for other gold-bearing placer deposits by 

Joesting (1941) and Jakosky (1950). Small magnetic 

anomalies occur near the center of the exp')sed Tertiary gravels at all the above locations and are shown 

by the symbol M in figure 3. The anomalies range from 

20 gammas at Badger Hill (fig. 13) to about 70 gammas 

at French Corral. At the North Columbia diggings, 

where the center of the channel is kiJown accurately 

from drill holes, the anomaly is about 40 gammas 

along A-_&_' in the upper pit and is marked "M" on the 

total intensity profile (fig. 9). This small magnetic 

high is displaced several hundred feet ncrth of the 

center of the channel, which coincides with the maximum gold values along profile A-A . 

The most significant magnetic anomaly found over 

the gravels is 0.4 mile east of section A -A' in the 

lower pit of the North Columbia diggings (~arl:ed "M" 

in fig. 5). Here the depth to bedrock in the central part 

of the channel is 250-300 feet about 100 feet 1 e s s 

than along .A-A' through the upper pit. Tt:~ anomaly 

is about 60 gammas and is located directly over the 

center of the channel. 

The bedrock contour map (fig. 6) and th~ contours 

of gold values in the North Columbia diggings (fig. 8) 

provide an unusual opportunity to test the correlation 

of magnetic anomalies with both of these parameters, 

but an adequate test should measure magn~tic variations at or within SO feet of the ground su-:-face to an 

accuracy of ±2 gammas. Such a test might be conducted by helicopter. 

Two aeromagnetic profiles at 400 feet at'OVe ground 

level have been made across the Tertiary gravels at 

North Columbia with fixed-wing aircraft. The data are 

now in compilation. 

CONCLUSIONS 

Although many of our data arestillbeingprocessed, 

particularly from the gravity and airborne studies, the 

following positive conclusions can now be made: 

1. By using shallow seismic refraction in conjunction 

with gravity methods, the configuration of the bedrock beneath the exposed Tertiary gravels has 

been determined to an accuracy of wit'lin 10 percent. 

2. Bedrock depressions that are under several hundred 

feet of volcanics, and that may be occupied by 

channel gravels, can be recognized by seismic 

refraction. Application of the method may be made 

difficult by high surface relief, and th~ problems 

of laying geophone cables along shot lines through 

chaparral and timber can be severe. 

3. The slingram EM method, possibly ue-ed in conjunction with IP, offers some promise of being 

able to trace the blue gravel where it is not more 

than 200 feet deep. Other EM and IP methods and 
EXPLANATION 

I I 

EM traverse lines 

--+2--

0ut-of-phase component of 

EM response, in percent 

~\ 

500 0 500 

LOWER PIT 

(hydraulicked to bedrock) 

1000 FEET 

Figure 12.--Electromagnetic survey of Badger Hill diggings (fig. 5). Contour values refer to t~4 outof-phase component obtained by using a vertical coplanar loop orientation, a 300-foot loop s~acing• 

and a frequency of 3•600 cycles per second. The contours are controlled by stations spaced at 100-foot 

intervals along the seven transverse and two longitudinal traverses indicated. 

20 
West 

c 

(/) 

<( 

100 

~ 50 <( 

(!) 

·0 

12W llW lOW 9W BW 7W 6W 5W 4W 3W 2W lW 0 

I 

0 1- 18 !Lz 

~LLI 

Ou 

Ua::: 

wlf 

(/) 14 <Cz 

IlL -

U:.~ 

C?w j ZlO 

0 

a 

----~.h"e oomponent 

\.__.____ ....:-

102 ~ 

ziOz 

!Lw 

~u 

106 8 ffi LLI!L 

rJlz <CI 

110 II;-

~ 

N =2 ________ 13 12 12 

N=3---12-13 11~7 14 9 

10 ---Frequency-effect anomalies, 

---- in percent 

11 18 18 10 

N=4 11 17 16 

1Y.~"s 1.1\ 

N=5------- 14 

IP 

Frequencies-0.05 and 5 

cycles per second 

1;00 ~I 200 N ~ 

~ r<?"J ' / " / 

N=2--------31~ ' / 25 '-, // 

63 ,47 "35 34 31-32-30 \/ 

N=3---62,........_50 "'-39 37 39 .40 Plotting point 

64 ~~ ---------

N= 4 ------· " 70~-~50 ~60 Apparent d-e resistivities, 

-s3-82_,g in oh;;eet 

N=S--------110 

200 0 200 400 600 800 FEET 

NO VERTICAL EXAGGERATION 

East 

C' 

2600 

2500 

2400 

Figure 13.--Magnetic, EM, I~, and geologic data along~£' (figs. 2, 5, and 12), Badger Hill diggings. The EM data were obtained using a vertical coplanar loop orientation, a 300-foot loor spacing, and a frequency of 3,600 cycles per second. The location of the peak of the EM anomaly 

(markeda) is also shown on the magnetic profile and the geologic section. 

21 
equipment might be used for deeper exploration; 

they would be slower and more costly to use. 

4. Although the present magnetic data are inconclusive, there is some evidence that magnetic concentrates occur in the central parts of at least 

some of the Tertiary channels, and the magnetic 

method may provide an economical me an s of 

locating the richer deposits. 

The resistivity method is not as useful as in many 

other areas because it is limited by the moderate to 

locally rugged topography in the hydraulic pits and in 

dissected volcanic terrain. We did not observe any 

large IP effects; moreover, the contri~ution of the 

clays and limonite-cemented materia 1 within the 

gravels may tend to mask the anomalies that might 

be caused by the disseminated sulfides. 

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Geology of northern California: California Div. 

Mines and Geology Bull. 190, p. 107-172. 

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22 

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-:, U. S. GOVERNMENT PRINTrNG OFFICI~ : 1970 0- 408-913