Ul S1LE 

I

Geologic Setting of

Gold Occurrences in the

Big Canyon Area,

El Dorado County, California

By ROBERT L. EARHART

U.S. GEOLOGICAL SURVEY BULLETIN 1576
DEPARTMENT OF THE INTERIOR 

DONALD PAUL MODEL, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1988

For sale by the Branch of Distribution 

Books and Open-File Reports Section 

U.S. Geological Survey 

Federal Center 

Box 25425 

Denver, CO 80225

Library of Congress Cataloging in Publication Data

Earhart, Robert L.

Geologic setting of gold occurrences in the Big Canyon area, El 

Dorado County, California.

(U.S. Geological Survey bulletin ; 1576) 

Bibliography: p. 

Supt. of Docs. No.: I 19.3:1576

1. Gold areas California El Dorado County. I. Title. II. Series. 

QE75.B9 no. 1576 557.3 s 87-600222

[553.4'1'0979441] [QE390.2.G65]
CONTENTS

Abstract 1 

Introduction 1

Previous work 1

Present investigations 3 

Regional setting 3 

Geology of the Big Canyon area 5 

Carbonate alteration 9 

Gold occurrences 10

Big Canyon mine and related occurrences 10

Vandalia mine 12 

Conclusions 12 

References cited 13

FIGURES

1. Index map of the central Sierra Foothills, Calif. 2

2. Geologic map of the Big Canyon area 4 

3-8. Photographs showing:

3. Mt. Aigare, a detached block of layered chert

4. Brecciated chert overlying serpentinite 7

5. Banded iron-formation 8

6. Debris flow deposits 9

7. Pillow structures in basalt 10

8. Lapilli tuff 11

TABLE

1. Major-element composition of selected rock samples 11

Contents III

Geologic Setting of

Gold Occurrences in the

Big Canyon Area,

El Dorado County, California

By Robert L. Earhart

Abstract

The Big Canyon area is on the eastern margin of the 

melange belt, which is one of four tectono-stratigraphic belts 

that trend northerly along the western flank of the Sierra 

Nevada. The melange belt is separated from the Mother Lode 

belt to the east by an unnamed fault zone in the eastern part 

of the study area. The Mother Lode belt contains volcanic, 

volcaniclastic, and sedimentary rocks of island-arc affinity. The 

Bear Mountains ophiolite to the west and northwest of the study 

area contains ultramafic and mafic rocks derived from oceanic 

crust and upper mantle. The Bear Mountains ophiolite is one 

of a series of tectonically disrupted ophiolite sequences on the 

western flank of the Sierra Nevada Mountains.

Gold deposits in the Big Canyon area occur in a melange 

that consists of lithic fragments derived from oceanic crust and 

island-arc terranes. Serpentinite and basalt that compose the 

matrix of the melange are vestiges of dismembered ophiolite. 

The melange represents deposition in a trench that formed as 

a result of the subduction of oceanic crust beneath island-arc 

terrane.

The area contains two distinct types of gold deposits. The 

older gold deposits are hosted by detached blocks of pyritic 

chert and associated banded iron-formation. These deposits, 

formed by sea-floor exhalative processes in an island-arc 

environment, were moved by gravity to their present chaotic 

setting during formation of the melange. Younger gold occurrences, including the Big Canyon deposit, are associated with 

hydrothermally altered fault zones that are younger than the 

formation of the melange. They are akin to the fault-controlled 

auriferous quartz veins in the Mother Lode belt east of the Big 

Canyon area.

Undiscovered deposits similar to those in the Big Canyon 

area may occur in parts of the 235-mile-long melange belt in 

the western Sierra Foothills where the geologic environment 

is similar. Chert and iron formation in island-arc terranes east 

of the study area, which were the inferred source of the goldbearing chert blocks, may also be favorable targets for gold 

exploration.

INTRODUCTION

The Big Canyon area is about 10 mi southeast of 

Placerville and 4 mi south of Shingle Springs in El Dorado 

County, California (fig. 1). The area is accessible from 

Shingle Springs, which is on U.S. Highway 50, via

Frenchtown Road, a paved, secondary county road. The 

area contains two inactive gold mines, the Big Canyon 

and the Vandalia, as well as numerous gold prospects.

The Big Canyon (Oro Fino) and Vandalia gold 

deposits were discovered in the mid-1880's. The Big Canyon mine, one of the largest in El Dorado County, produced $720,000 in gold from 1893 to 1901 (Clark and 

Carlson, 1956). In 1934, the mine was acquired by Mountain Copper Co., and from 1934 to 1940 it produced gold 

valued at $2,368,000. The mine has been idle since 1940. 

The Vandalia mine produced minor amounts of ore in 

1888, about 1900, and from 1926 to 1928. The mineral 

rights on both properties were acquired in 1983 by Gold 

Fields Mining Corp., which reevaluated the gold potential. The work by Gold Fields indicated the presence of 

additional gold ore of a grade comparable to that mined 

in the past, between 3 and 7 ppm; however, the tonnage 

of reserves was not established (R. Ridler, oral commun., 

1985).

Gold Fields Mining Corp. provided access to the 

Vandalia mine and geologic data for the Vandalia and 

Big Canyon mines to supplement ongoing studies by the 

U.S. Geological Survey. Mr. Ernest 'Doc' Schieber 

graciously allowed access to his ranch, on which are 

situated the Big Canyon Mine and about 70 percent of 

the map area. The west-central part of the area is on the 

Doug Albright Ranch and the southern part is on the 

White Ranch; the landowners are gratefully acknowledged for their help and cooperation. Messrs. Tom 

Anderson and Ralph Loyd of the California Division of 

Mines and Geology were particularly helpful during the 

present studies. I thank them for sharing with me their 

insight on the geology of the region.

Previous Work

The Sierra Foothills, a major gold-producing region 

in the past, has been studied by geologists since the late 

1800's; however, until recent exploration activities were 

undertaken by Gold Fields Mining Corp., the rocks in 

the Big Canyon area of the Foothills region had been 

neither mapped nor studied in detail. Early workers 

(Becker, 1885; Turner, 1893;Taliaferro, 1942) considered

Introduction
38°30' 

Figure 1. Index map of the central Sierra Foothills, Calif., showing general distribution of tectonostratigraphic belts.

2 Geologic Setting of Gold Occurrences in the Big Canyon Area, El Dorado County, California
the rocks in the Foothills region to be conformable 

sedimentary-volcanic sequences, which they divided into 

three principal stratigraphic units: the Calaveras Formation, the Mariposa Formation, and the Amador Group 

(now called the Logtown Ridge Formation). Until 1964, 

most workers in the Foothills correlated the rocks in their 

study areas with these stratigraphic units or with subdivisions of these units, based on similarities of lithology and 

metamorphic grade. The results were often ambiguous 

owing to similar lithologies in various units. Clark (1964) 

restricted the use of the stratigraphic names to the type 

localities of the formations and to areas where stratigraphic continuity could be demonstrated.

With the broad acceptance of plate tectonic theory, 

most modern workers have considered the Foothills 

region to consist of three or more tectono-stratigraphic 

belts that were juxtaposed as a result of plate tectonic 

processes along the western continental margin in the 

Triassic and Jurassic Periods. Duffield and Sharp (1975), 

during studies of the Foothills region south of the 

Cosumnes River, recognized an eastern belt, a Mother 

Lode belt, a melange belt, and a western belt (fig. 1). The 

Big Canyon area discussed in this report is in the northern extension of a part of their melange belt. Behrman 

(1978), who studied the rocks adjacent to the study area, 

divided the melange belt of Duffield and Sharp into a 

central belt and a Bear Mountains ophiolite belt. The Bear 

Mountains ophiolite is one of a series of ophiolite assemblages that trends northerly within or adjacent to the 

melange belt along the western flank of the Sierra Nevada 

for about 235 mi. Saleeby (1982) demonstrated that the 

ophiolites in this belt are tectonically disrupted sequences 

which have contrasting metamorphic and deformational 

histories and which range in age from about 300 m.y. to 

about 150 m.y. For the most part, the rocks in the study 

area appear to belong to the 300-Ma ophiolite terrane 

of Saleeby.

Present Investigations

Present investigations by the U.S. Geological 

Survey were undertaken in order to document the geologic setting, petrography, and geochemistry of gold occurrences in the melange belt. This report describes the 

geologic setting; petrographic and geochemical studies are 

still in progress. Past studies of gold occurrences in the 

Foothills region focused on the Mother Lode belt to the 

east of the study area (Lindgren and Turner, 1894; Knopf, 

1929). The settings of less well documented gold occurrences in the melange belt differ from the auriferous 

quartz veins and adjacent gray ore zones typical of the 

Mother Lode belt. The Big Canyon area was mapped 

during the present studies in 1985 and 1986, and outcrop samples were collected for analysis in order to

characterize the rocks in the area. Maps by Mr. Mark 

W. Osterberg of Gold Fields Mining Corp., which were 

made available, proved to be invaluable. However, the 

interpretation of geological data utilized in this study may 

not reflect the conceptual geologic model favored by Gold 

Field Mining Corp. geologists familiar with the area.

REGIONAL SETTING

The Big Canyon area is within the Sierra Foothills, 

which consist of a series of north-trending tectonostratigraphic belts of metamorphosed sedimentary, 

volcanic, and intrusive rocks that range in age from late 

Paleozoic to Mesozoic (fig. 2). Locally, the Mesozoic 

rocks are capped by erosional remnants of a onceextensive conglomerate and tuff of Tertiary age. The 

structural belts, which extend for about 235 mi along the 

western side of the Sierra Nevada, are flanked to the east 

by the Sierra Nevada batholith and to the west by 

sedimentary rocks of the Great Valley sequence of 

Jurassic and Cretaceous ages. The structural belts are internally bounded by the Melones and Bear Mountains 

faults (Clark, 1964; Duffield and Sharp, 1975). These 

steeply dipping major faults, along with an unnamed normal fault that parallels Big Canyon Creek in the study 

area, separate diverse geologic terranes of oceanic-crust 

and magmatic-arc affinities. All the belts are characterized by extensive faulting, shearing, and folding, and 

metamorphic grades range from greenschist to amphibolite facies (Clark, 1964).

The following lithologic descriptions from the eastern, Mother Lode, melange, and western belts are mostly summarized from Duffield and Sharp (1975). The 

eastern belt is dominantly phyllite, phyllonite, and chert 

of Paleozoic age. The phyllite and phyllonite are dark 

gray to silver gray and appear similar in outcrop. The 

chert is mostly thin bedded with phyllite partings and, 

for the most part, is indistinguishable from chert in the 

melange belt. Other rocks in the eastern belt include an 

Upper(?) Jurassic granodiorite pluton near the Cosumnes 

River and small bodies of Upper(?) Jurassic serpentinite, 

gabbro, diorite, and limestone. The Paleozoic metasedimentary rocks of the eastern belt have been assigned to 

the Calaveras Formation by most investigators.

The Melones fault zone separates the eastern belt 

from the Mother Lode belt, which consists of the Upper 

Jurassic Logtown Ridge and Upper Jurassic Mariposa 

Formations. The Logtown Ridge, which at the type locality on the Cosumnes River is about 6,500 ft thick, consists of volcanic and volcanic-sedimentary rocks of 

island-arc affinity. These rocks are mostly basaltic and 

include flows, breccias, and a variety of layered pyroclastics, including lapilli tuff. The overlying Mariposa Formation contains a distal-turbidite, hemipelagic sequence

Regional Setting
120°55'

38°37'

3B°36' -

Unmapped

Unmapped

451-55^75

X ' "\

Vandalia n

-j' \, i j \ A I: \707°\X "\ 75 ^ 7>1,

4 Geologic Setting of Gold Occurrences in the Big Canyon Area, El Dorado County, California
ch 

df1

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sp

EXPLANATION

Chert, banded iron-formation, and felsic tuff 

Debris flow of mostly basaltic rocks 

Debris flow of mostly cherry rocks

Flows, hyaloclastites, pyroclastics, and dikes of basaltic 

composition

Serpentinite Includes harzburgite and dunite

Metasedimentary rocks of pelagic affinity Includes slate 

and laminated argillite

- -^ -L ~ Inferred contact of olistolith Hachures in detached 

block

Strike and dip 

Beds

Foliation 

Shears

Anticline

Syncline

 Inferred fault Queried where doubtful

A A A

A A A A Tectonic breccia 

/Q --*-- - Quartz vein or stockwork

Carbonate-altered rock

Adit

Shaft or incline

Mine

Prospect

Rock sample site

Figure 2 (above and facing page). Geologic map of the Big 

Canyon area.

of black slate, graywacke, conglomerate, fine-grained 

tuffaceous rocks, and subvolcanic intrusive rocks. The 

thickness of the Mariposa is difficult to determine because 

of structural complexities, but it is probably about 2,600 

ft thick at the Cosumnes River.

The Mother Lode belt is separated from the 

melange belt by an unnamed fault along the eastern 

margin of the study area (fig. 2). In the vicinity of the 

study area, the melange belt consists of detached blocks 

that were derived from oceanic-crust and island-arc terranes. The detached blocks are as much as 2 mi long, in 

a matrix of mostly serpentinite. The melange, at least in 

the study area, represents fill in a subduction trench that 

formed adjacent to an island-arc. In adjacent areas, the 

melange belt contains epiclastic rocks not present in the 

study area. The melange belt broadly corresponds to the 

Sierran ophiolite belt of Saleeby (1982), who subdivided 

the belt into two groups based on structural setting. As 

deduced from Saleeby's descriptions, the study area is 

within his oldest group which yields zircon ages in the

300-200-Ma range. This group crops out locally along 

the length of the western margin of the Sierra Nevada 

and constitutes the oldest basement element west of the 

mountains. Past workers have included the rocks of the 

melange belt in the Calaveras Formation; most modern 

workers, however, do not assign this chaotic assemblage 

to any formation.

The melange belt is separated from the western belt 

by the Bear Mountains fault. All the rocks in the western 

belt were assigned to the Upper Jurassic Copper Hill 

Volcanics by Clark (1964). The Copper Hill Volcanics 

consist of mafic to felsic flows and pyroclastic rocks that 

are metamorphosed to greenschist and amphibolite facies. 

Locally, this formation contains some slaty argillaceous 

rocks. In general, the Copper Hill Volcanics contain a 

greater amount of felsic volcanic rocks than do the otherwise similar strata in the Mother Lode belt. The Copper 

Hill Volcanics are highly deformed and intensely sheared 

and, according to Duffield and Sharp (1975), may contain tectonic intermixing similar to that found in the 

melange belt to the east.

GEOLOGY OF THE BIG CANYON AREA

The Big Canyon area contains a melange of detached blocks of debris flow deposits, of banded chert 

and banded iron-formation (BIF), and of flows, pyroclastics, and hyaloclastites, mostly of basaltic composition. The largest identifiable detached block is the banded 

chert block that composes Mt. Aigare in the west-central 

part of the map area (fig. 2). This block is about 1 mi 

long and as much as 0.4 mi wide; drill hole data indicate 

that the block is at least 500 ft thick. The detached blocks 

of banded chert and associated siliceous rock types form 

prominent peaks and knolls in the area (fig. 3).

The detached blocks are in a matrix that is predominantly peridotite, partly to completely altered to serpentinite, and that also includes basaltic flows and related 

pyroclastic rocks. Rock types in the matrix are strongly 

suggestive of an oceanic-crust and upper-mantle protolith 

and are thought to represent a dismembered and incomplete ophiolite sequence.

The structure within the melange is highly complex. 

Where exposed, the margins of the detached blocks are 

seen to be fractured and brecciated. For example, the 

lower contact between a chert block and serpentinite in 

the central part of the area is highly fractured and brecciated (fig. 4). Chert breccia fragments are embedded in 

plastically deformed serpentinite for 2-3 ft below the 

contact. Blocks of cherty metasedimentary rocks are intensely deformed and contain numerous asymmetric, 

isoclinal to subisoclinal folds with associated brittle fracturing; the folds were apparently formed during the detachment and emplacement of the blocks in the melange.

Geology of the Big Canyon Area
Figure 3. Mt. Aigare, center background, is formed from a mile-long detached block of layered chert. Low-lying area in foreground 

is mostly underlain by serpentinite. View to the west.

After formation of the melange, the rocks were sheared 

and faulted. To the east, the melange is terminated by 

a major high-angle fault that parallels Big Canyon Creek 

(fig. 2). Rocks east of the fault in the study area are mostly slate and argillite of the dominantly volcanic Logtown 

Ridge Formation. The fault forms the eastern margin of 

the melange belt and is host to gold deposits at the Big 

Canyon mine in the northeastern part of the map area 

and at prospects in the southern part (fig. 2). Prominent 

shear zones, some of which are altered and mineralized, 

trend north to northwest in the melange.

The largest and most conspicuous of the detached 

blocks consist of banded or layered chert and related 

rocks. This unit is mostly thinly layered to parallellaminated, light-gray, recrystallized chert and some pink 

ferruginous chert. The banded appearance of the chert 

is due primarily to phyllite partings along bedding planes. 

In the absence of phyllite, the chert appears massive. 

Locally, the chert contains intercalations of BIF that are 

as much as 35 ft thick, and that contain as much as an 

estimated 35 percent magnetite (fig. 5). The magnetitebearing BIF grades laterally, probably over a few tens 

of feet, to hematitic ferruginous chert, which in turn 

grades to nonferruginous chert. In places, the BIF and 

the ferruginous chert contain as much as an estimated 

15 percent layered pyrite, some of which has associated 

native gold. BIF and pyritic chert are host rocks to gold 

deposits at the Vandalia mine in the northern part of the 

area and also at numerous localities elsewhere in the chert 

blocks (fig. 2). The BIF is not a mappable unit at the scale 

of figure 2 and is therefore not differentiated from the 

layered chert unit on the map. It is best developed in the 

vicinity of a tightly folded syncline northeast of Mt. 

Aigare and in the vicinity of the Vandalia mine. It also 

occurs at numerous other chert localities, some of which 

have been prospected for gold.

In addition to BIF, the chert in the northern part 

of the map area contains minor interlayers of felsic rock 

that are interpreted to represent reworked rhyolitic tuff. 

The felsic rock is weakly sericitic and locally limonitic 

and has a fine-grained fragmental texture. Adjacent 

to the study area, chert blocks contain a tuffaceous

Geologic Setting of Cold Occurrences in the Big Canyon Area, El Dorado County, California
Figure 4. Brecciated chert along lower margin of a detached block overlies serpentinite in the central part of map area.

component that is as much as 40 percent hornblende and 

epidote and also contain some pelagic metasedimentary 

rocks (Behrman, 1978).

Within the study area, the chert has abundant veins 

and stockworks of quartz. Older quartz veins are blue 

gray, weakly pyritic, and not more than about 2-3 ft 

thick. Younger quartz is milky white and forms much 

thicker veins and stockworks. Unlike the quartz veins in 

the Mother Lode belt to the east, those in the study area 

are not known to be gold bearing. Although complex 

folding makes stratigraphic thickness difficult to determine, as much as 2,000 ft of chert may be present in the 

Mt. Aigare block.

The chert, BIF, and felsic tuff are considered to be 

more characteristic of chemical sediments and pyroclastics deposited in an island-arc environment than in 

an ophiolite sequence of a mid-oceanic ridge or a spreading back-arc basin such as represented by the matrix of 

the melange. Similar chert appears to be part of an islandarc sequence in the foothills to the east of the melange 

belt; therefore, an island-arc terrane to the east was the

likely source of the chert blocks that now occur in the 

melange belt. Behrman's map shows no chert in the 

western part of the melange belt, and he observed that 

the detached blocks of chert become increasingly abundant toward the east a distribution that further suggests 

a source to the east (Behrman, 1978).

Detached blocks of coarse debris flow deposits in 

the study area strongly suggest that the melange formed 

by gravity depositional processes in a submarine trench. 

Two types of debris flow deposits are present. One type, 

shown as df 1 on figure 2, consists of randomly oriented 

fragments of basaltic rocks and minor peridotite (fig. 6A). 

The fragments in this type are of greatly varied size but 

are rarely more than about 15 in. in diameter. Chert fragments are completely lacking in this type. The clasts are 

representative of the partial ophiolite assemblage that 

composes the matrix of the melange. This debris flow 

deposit probably is derived from oceanic crust on the 

western side of the subduction trench. A large detached 

block of this deposit is in the southern part of the map 

area (fig. 2). The second type of debris flow deposit,

Geology of the Big Canyon Area
Figure 5. Banded iron-formation on ridge north of Mt. Aigare.

shown as df2 on figure 2, consists of about 75 percent 

chert fragments (fig. 6B); other fragments include basalt 

and exotic metamorphic rocks. Fragments in this type of 

debris flow deposit are rarely larger than 20 in. in diameter. This type is inferred to represent deposition from 

source rocks to the east of the trench. A small detached 

block of df2 crops out to the north of the df 1 unit in the 

east-central part of the map area (fig. 2).

Blocks of basaltic volcanic rocks of olivine tholeiite, 

tholeiite, and calc-alkalic compositions are also present 

in the melange. The basaltic rocks are shown as a single 

unit on figure 2 because it is difficult to differentiate 

basaltic rocks that are a part of the dismembered ophiolite 

from those that represent deposition in the subduction 

trench. In the western part of the area, the basaltic rocks 

are dominantly pillow lavas (fig. 7). The pillows are commonly rimmed with hyaloclastite, but pyroclastic rocks 

are rare in this part of the area. Basaltic volcanics in the 

eastern part of the area include basalt and olivine basalt 

flows, mafic tuff including lapilli tuff (fig. 8), mafic volcanic breccia, and hyaloclastic material. Pillow structures

are rarely discernible in the eastern part of the area, probably due to widespread shearing and faulting. Locally, 

sheared basaltic rocks in the eastern part are metamorphosed to actinolitic amphibolite, and where the basalts 

are highly sheared, they are converted to actinolite schist. 

Amphibolite dikes, which were probably feeder dikes, 

form resistant outcrops, and locally, chilled dike margins 

are preserved.

Other rock types not recognized in outcrops were 

observed by J.T. Nash (written commun., 1986) in drill 

cores from the eastern part of the area. Analytical results 

of the drill core samples confirm the presence of small 

amounts of dacite or rhyodacite, andesite, and vitric tuff, 

all of which are here inferred to be derived from islandarc terranes to the east. Lapilli tuff that may be similar 

to that in this area has been described by Duffield and 

Sharp (1975) from the Logtown Ridge Formation to the 

southeast of the study area.

The major-element composition of a variety of 

basaltic rocks in the area is given in table 1. These data 

show that the low-silica basalts also contain low amounts

8 Geologic Setting of Gold Occurrences in the Big Canyon Area, El Dorado County, California
 JFigure 6. Debris flow deposits. /A, Deposit with predominantly basaltic rock fragments dfl unit on figure 2. 6, Deposit with 

predominantly chert rock fragments df2 unit on figure 2.

of K2O and TiO2 and are chemically similar to olivine 

tholeiites from most deep sea floor-oceanic ridge environments (Charmichael and others, 1974).

Rocks of ultramafic composition are dominant in 

the matrix of the melange, and they include serpentinite, 

talcose serpentinite, partially serpentinized olivineorthopyroxene rock (harzburgite?), and dunite. The 

serpentinite is massive and commonly sheared, and it 

completely lacks relict textures. Talcose serpentinite occurs in intensely sheared zones. Locally the serpentinite 

contains chrysotile in multiple veinlets as thick as onequarter inch. Partly serpentinized harzburgite(?) contains 

relict cumulate texture and thick discontinuous layers that 

reflect the differential concentration of olivine. Dunite 

occurs as small, discontinuous to lenticular bodies in harzburgite(?) and is conspicuous by the rusty appearance of 

the outcrop surface. The compositions of ultramafic 

rocks in the area are given in table 1 (samples 17, 18, and 

19).

Argillaceous and siliceous pelagic metasedimentary 

rocks occur within and east of the fault zone that parallels

Big Canyon Creek and forms the eastern boundary of 

the melange belt (fig. 2). They are tentatively assigned 

to the Upper Jurassic Logtown Ridge Formation and may 

be a basinal facies of the Goat Hill Member of Duffield 

and Sharp (1975). The Goat Hill Member, where described about 22 mi to the south near the Mokelumne 

River, consists in large part of well-bedded marine 

pyroclastic rocks. Near the Big Canyon Creek fault zone 

in the study area, the member consists of parallellaminated, gray to greenish-gray argillite with 2-3-mmthick beds and of dark-gray slate that locally occurs as 

pencil slate.

CARBONATE ALTERATION

Carbonate rock in the alteration zone by the Glory 

Hole at the Big Canyon mine is similar to the ophicarbonate described by Behrman (1978) from the melange 

belt adjacent to the study area. Ophicarbonate is a 

carbonate-rich rock that has been observed in some

Carbonate Alteration
Figure 7. Pillow structures in basalt in western part of area. 

Hyalaclastic material occurs along pillow margins.

ophiolite sequences and is assumed to be cogenetic with 

other rocks of the ophiolite suite. The carbonate rock in 

the study area, however, appears to be younger than the 

ophiolite. Behrman's mapping shows that carbonate rock 

occurs exclusively along or near the eastern boundary of 

the melange belt, and that it is everywhere fault bounded. 

The carbonate rock at the Big Canyon mine and at other 

localities in the eastern part of the area is similar in composition and in geologic setting to that described by 

Behrman. In the study area, carbonate-cemented breccia with basaltic clasts occurs within the fault system that 

parallels Big Canyon Creek and forms the eastern limit 

of the melange belt. Calcite also partially replaces sheared 

basalt in the fault zones. The occurrence of the carbonate 

rock in cross-cutting fault zones suggests that it is younger 

than the development of the melange which, in turn, is 

younger than the formation of the ophiolite. In other 

areas of similar geologic setting, carbonate is commonly 

derived from the release of calcium from epidote or other 

calcium-bearing minerals during serpentinization.

The fault-associated carbonate zones in the study 

area contain pyrite and free gold as do the fault-associated

quartz veins in the Mother Lode belt to the east. According to Landefeld (1985), the quartz veins and associated 

gold deposits in the Melones fault zone of the Mother 

Lode belt formed during late-stage brittle faulting in Late 

Triassic time. A similar origin may be inferred for the 

gold-bearing carbonate zones of the study area. The fault 

zone along Big Canyon Creek, however, contains strata 

of inferred Jurassic age and therefore, the carbonate 

alteration zones were formed after the emplacement of 

these strata.

GOLD OCCURRENCES

Gold occurrences in the study area are associated 

with pyrite and arsenopyrite in highly altered fault zones 

and with pyrite in banded iron-formation and chert. Eight 

samples collected from outcrops and surface mine workings in the fault zones during mapping contained as much 

as 3 ppm gold. Core samples from the fault zone, which 

were collected and analyzed by Gold Fields Mining Corp., 

reportedly contain as much as 8.5 ppm gold. Similarly, 

19 outcrop and mine samples of BIF and pyritic chert 

contained as much as 0.7 ppm gold, whereas core samples 

of these rocks in the vicinity of the Vandalia mine contained as much as 4.2 ppm. Gold-bearing outcrop samples 

from both types of occurrences contained 1 ppm or less 

silver.

Big Canyon Mine and Related Occurrences

At the Big Canyon mine, which is the largest producer in the area and one of the largest in the melange 

belt, gold occurs in a carbonate alteration zone at the intersection of three branching faults which are parts of 

a major fault zone that parallels Big Canyon Creek (fig. 

2). The steeply dipping ore zone is exposed for about 150 

ft in a northerly direction along strike on the western side 

of the Glory Hole. The width is indeterminable owing 

to scarcity of exposures. The ore zone is on a strike projection of laminated argillite that is part of a sequence 

of metasedimentary rocks that crop out on the southeastern side of the Glory Hole. The ore evidently occurs along 

a fault that separates metasedimentary rocks to the east 

from basalt and basaltic tuff to the west. The host rock 

is a breccia that contains silcified and pyritized fragments 

primarily from the basaltic wall rocks. The breccia matrix 

consists of calcite, ankerite(?), quartz, chlorite, and 

tremolite. Sulfide minerals occur as disseminations and 

fracture-fillings in both the matrix and the fragments of 

the breccia, but are most abundant in the matrix. Surface exposures contain as much as 5 percent sulfides, and 

intervals of 2-3 ft in the drill core contain as much as 

15 percent. Gold values appear to vary directly with

10 Geologic Setting of Gold Occurrences in the Big Canyon Area, El Dorado County, California
Figure 8. Lapilli tuff in northeastern part of map area.

Table 1. Major-element composition of selected rock samples from the Big Canyon area, El Dorado County, California 

[Total iron reported as FeO; analyzed by X-ray fluorescence; A.E. Hubert, analyst]

Sample 

No.

i

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

Sample 

lithology

Oli ine basalt ........

Oli ine basalt ........

Oli ine basalt ........

Oli ine basalt ........

Oli ine basalt ........

Oli ine basalt ........

Oli ine basalt ........

Basalt ...............

Basalt ...............

Andesitic basalt ......

Basaltic tuff .........

Lapilli tuff ..........

Hyaloclastite .........

Basaltic breccia ......

Amphibolite .........

Amphibolite .........

Serpcntinized ........

harzburgite(?) ......

Serpentinized ........

harzburgite(?) ......

Serpentinite ..........

Si02

45.63

47.05

44.29

49.51

47.28

45.72

48.82

43.55

40.43

52.17

47.81

46.59

48.31

50.16

56.97

56.88

43.55

44.02

42.59

AI 2 03

18.01

14.18

13.52

14.63

14.72

17.96

13.58

15.51

14.70

13.24

20.10

18.22

28.18

16.66

15.25

15.52

0.10

0.10

0.47

MgO

6.52

11.65

7.17

7.72

9.31

6.15

8.92

8.36

7.98

8.13

5.53

7.06

2.07

4.69

6.56

4.78

38.10

34.94

32.61

Na2O

1.86

2.06

2.67

2.94

2.17

2.34

2.23

2.39

1.86

3.56

3.15

0.93

3.09

1.67

5.86

2.67

0.10

0.10

0.10

CaO

12.40

9.55

8.42

8.57

9.64

12.08

8.80

14.43

18.66

7.83

10.60

10.47

11.51

9.94

4.60

5.26

0.02

0.54

1.09

K2O

0.59

0.23

0.31

0.53

0.1!

0.27

0.15

0.14

0.08

0.18

0.13

0.24

0.53

0.24

0.22

0.30

0.10

0.02

0.02

FeO

14.56

12.33

14.10

11.48

14.42

11.64

15.26

12.14

10.99

15.13

10.52

16.42

7.51

14.02

9.13

11.45

8.96

10.81

9.04

TiO2

2.14

1.14

2.34

.29

.70

.60

.74

.77

.65

2.28

1.10

2.12

0.68

2.87

0.53

0.42

0.01

0.08

0.04

Cold Occurrences 11
sulfide content. Pyrite is by far the most abundant sulfide 

mineral, and trace amounts of arsenopyrite may be in the 

ore. Spectrographic analysis by B.M. Adrian and R.W. 

Baker that show as much as 500 ppm arsenic probably 

reflect small amounts of arsenopyrite. A trace of 

chalcopyrite was noted in one sample; however, the ore 

contains very low concentrations of copper and other base 

metals.

Pyritic carbonate alteration zones that are smaller 

than but similar to the zone at the Big Canyon mine occur at several faulted or sheared localities within the Big 

Canyon Creek fault zone (fig. 2). The mineralized locality 

in the southern part of the map area, locally known as 

the Live Oak mine, was developed by prospects. Core 

samples obtained by Gold Fields Mining Corp. from this 

locality reportedly contained gold values comparable to 

those at the Big Canyon mine (R. Ridler, oral commun., 

1985). Outcrop samples collected from other carbonate 

alteration zones that were found during the course of the 

present studies contained less than 0.05 ppm gold.

Vandalia Mine

Pyritic banded iron-formation, pyritic ferruginous 

chert, and pyritic chert were mined for gold at the Vandalia mine in the northern part of the area shown in figure 

2, and several localities of pyritic rocks in the detached 

chert blocks have been prospected for gold. The BIF occurs as intercalations in the chert and consists of alternating bands of magnetite and chert in units that are as 

thick as about 35 ft, south of the mine area. Apparently, 

the magnetic BIF grades laterally and vertically to 

hematitic ferruginous chert and then to iron-poor chert 

over a few tens of feet. In places, the iron bands, in addition to magnetite or hematite, contain fine- to mediumgrained euhedral pyrite that composes as much as 10 percent of the rock. Pyrite also occurs in chert beds that are 

devoid of magnetite or hematite. Pyritic chert-BIF intervals are commonly about 6 ft thick and may persist along 

strike for several hundred feet; however, gold distribution in the pyritic beds, as judged from the assay results 

of outcrop samples, is apparently localized. In most 

places the BIF contains even, parallel laminations of 

magnetite and chert (fig. 5). At some localities where the 

BIF is gold-bearing, however, the laminae are deformed, 

and the BIF contains boudinage structures, small extension fractures, and detached folded laminae. Additional 

studies are needed to determine the relationship, if any, 

between the distribution of gold and the deformation 

features in the BIF. Gold-bearing chert and BIF at the 

Vandalia mine are underlain by a zone of extensive quartz 

veining. In places the veins are pyritic; however, none of 

the vein samples collected during the present studies contained a detectable amount (0.05 ppm) of gold. The

spatial association of the veins with the ore zone suggests 

that they may represent a feeder zone, but direct evidence 

that would genetically link the veins to the ore zone is 

lacking.

The BIF at the Vandalia mine contains as much as 

5,000 ppm manganese and as much as 100 ppm lanthanum, but it contains very low amounts of base metals. 

Gold-bearing core samples that are currently under study 

by J.T. Nash (written commun., 1986) contain as much 

as 2,700 ppm arsenic. Outcrop samples contained only 

normal amounts of arsenic, and no arsenopyrite was 

noted.

The chert and associated BIF are characteristic of 

chemical sediments formed by sedimentary exhalative 

processes in an island-arc setting. The association of gold 

with stratiform pyrite suggests that it was deposited during sulfurous emanations on the seafloor.

CONCLUSIONS

Gold occurrences in the Big Canyon area are in a 

subduction melange complex that trends northerly along 

the western flank of the Sierra Nevada. The area lies on 

the northerly projection of the melange belt as defined 

by Duffield and Sharp (1975) from outcrops south of the 

Cosumnes River, and it is within the Sierran ophiolite belt 

of Saleeby (1982). In the study area, the melange consists of small to large detached blocks as much as one 

mile in length of volcanic, volcaniclastic, and siliceous 

sedimentary rocks. The matrix of the melange is serpentinite and basaltic rocks that are vestiges of a 

dismembered ophiolite. Dismemberment of the ophiolite 

is inferred to have occurred during the formation of a 

submarine trench associated with the subduction of 

oceanic crust along the continental margin in early to 

middle Mesozoic time. During and shortly after trench 

formation, coarse debris flow deposits and large blocks 

of cherty and locally iron-rich sedimentary rocks with 

some interlayered felsic tuff filled the trench from islandarc terrane to the east, while debris flow deposits and 

slabs of basaltic rocks, all derived from oceanic crust, 

filled the trench from the west.

Two types of gold deposits occur in the Big Canyon 

area. The older deposits are represented by the Vandalia 

deposit which occurs with stratiform pyrite in chert and 

in banded iron-formation. The host rocks are a part of 

a thick sequence of mostly layered chert that occurs in 

the study area as detached blocks. The chert, BIF, and 

associated iron sulfide-gold deposits are inferred to represent sea-floor exhalites deposited east of the study area 

in an island-arc terrane of late Paleozoic to early 

Mesozoic age. Chert sequences indistinguishable from 

those in the study area have been described from the 

Paleozoic eastern belt (Duffield and Sharp, 1975). This

12 Geologic Setting of Gold Occurrences in the Big Canyon Area, El Dorado County, California
suggests that cherts within and east of the Mother Lode 

belt may also be favorable targets for gold exploration. 

The second type and younger of the gold deposits 

in the study area is represented by the Big Canyon deposit 

which, like occurrences in the Mother Lode belt to the 

east, is fault controlled (Landefeld, 1985). In the Big Canyon area, occurrences of this type are characterized by 

the effects of pyritization, silicification, and strong carbonate alteration in brecciated fault zones. The principal 

occurrences are associated with a major fault that trends 

parallel to Big Canyon Creek and forms the eastern 

margin of the melange belt. A zone of carbonate alteration in a relatively minor fault that branches from the 

Big Canyon Creek fault zone (fig. 2) may also be a 

favorable target for gold exploration, although outcrop 

samples from this alteration zone did not contain detectable amounts of gold. Structural relationships show that 

the gold-bearing breccia zones are younger than the formation of the melange and that, like the gold-quartz veins 

in the Mother Lode belt to the east, the breccia of the 

study area probably formed prior to the emplacement of 

the Cretaceous Sierra Nevada batholith.

REFERENCES CITED

Decker, G.F., 1885, Notes on the stratigraphy of California: 

U.S. Geological Survey Bulletin 19, 28 p.

Behrman, P.G., 1978, Paleogeography and structural evolution of a Middle Mesozoic volcanic arc-continental margin; 

Sierra Nevada Foothills, California: Berkeley, Calif., 

University of California, Ph. D. thesis, 268 p.

Charmichael, I.S.E., Turner, F.J., and Verhoogen, John, 1974,

Igneous Petrology: McGraw-Hill, 737 p. 

Clark, L.D., 1964, Stratigraphy and structure of part of the

western Sierra Nevada Metamorphic Belt, California: U.S.

Geological Survey Professional Paper 410, 70 p. 

Clark, W.B., and Carlson, D.W., 1956, Mines and mineral

resources of El Dorado County, California: California

Division of Mines, California Journal of Mines and

Geology, v. 52, p. 369-591. 

Duffield, W.A., and Sharp, R.V., 1975, Geology of the Sierra

Foothills melange and adjacent areas, Amador County,

California: U.S. Geological Survey Professional Paper 827,

30 p.

Jennings, C.W., 1977, Geologic Map of California: California Division of Mines and Geology Geologic Data Map 2,

scale 1:750,000. 

Knopf, Adolph, 1929, The Mother Lode system of California:

U.S. Geological Survey Professional Paper 157, 88 p. 

Landefeld, L.A., 1985, Tectonostratigraphic setting of the

Mother Lode gold belt, south of Placerville, California:

Geological Society of America Abstracts with Programs,

98th Annual Meeting, p. 638-639. 

Lindgren, Waldemar, and Turner, H.W., 1894, Placerville,

California: U.S. Geological Survey Geologic Atlas, Folio

3, 3 p. 

Saleeby, J.B., 1982, Polygenic ophiolite belt of the California

Sierra Nevada geochronological and tectonostratigraphic

development: Journal of Geophysical Research, v. 87, no.

B, p. 1803-1842. 

Taliaferro, N.L., 1942, Geologic history and correlation of the

Jurassic of southwestern Oregon and California: Geological

Society of America Bulletin, v. 53, no. 1, p. 71-112. 

Turner, H.W., 1893, Some recent contributions to the geology

of California: American Geologist, v. 11, p. 307-324.

U.S. GOVERNMENT PRINTING OFFICE: 1988 573-0^7/66087 REGION 8 References Cited 13