The relationships of metallogenic zones and local geological features to lode gold orebodies, central Sierra Nevada foothills, California
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Authors Sullivan, Jeffery Alan
Publisher The University of Arizona.
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Link to Item http://hdl.handle.net/10150/557587 THE RELATIONSHIPS OF METALLOGENIC ZONES AND LOCAL
GEOLOGICAL FEATURES TO LODE GOLD OREBODIES,
CENTRAL. SIERRA NEVADA FOOTHILLS, CALIFORNIA
by
Jeffery Alan Sullivan
A Thesis Submitted to the Faculty of the
DEPARTMENT OF MINING AND GEOLOGICAL ENGINEERING
In Partial Fulfillment of the 'Requirements For the Degree of
MASTER OF SCIENCE WITH A MAJOR IN GEOLOGICAL ENGINEERING
In the Graduate College
THE UNIVERSITY OF ARIZONA
19 8 0 STATEMENT BY AUTHOR
This thesis has been submitted in partial ful fillment of requirements for an advanced degree at the University of Arizona and is deposited in The University Library to be made available to borrowers under rules of the Library.
Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for per mission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his judgment the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author.
SIGNED
APPROVAL BY THESIS DIRECTOR
This thesis has been approved on the date shown below:
r . ( u z z z : ? ~ / s . / W. C. PETERS Hate Professor of Mining and Geological Engineering TABLE OF CONTENTS
Page
LIST OF ILLUSTRATIONS...... v
LIST OF TABLES ...... vi
ABSTRACT ...... vii
1. INTRODUCTION ...... 1
Geographical and GeologicalSetting ..... 1 H i s t o r y ...... 5 Previous Work . 8
2. REGIONAL GEOLOGIC SETTING ...... 10
Nature and.Distribution of the. Principal Rock T y p e s ...... 10 Metamorphic Rocks ...... 11 Western Belt ...... 12 > Eastern Belt ...... 16 Intrusive Rocks ...... , . . . 17 Serpentinite and Associated Rocks . . 17 Granitic Rocks ...... 18 Structural Geology ...... 18 Summary of Geologic H i s t o r y . 21
3. ROCK UNITS ...... 23
Western B e l p ...... 23 Unit A ...... 23 Unit C ...... 24 Unit B ...... 27 Smartville Block ...... 28 Rocks of the Melones and Bear Mountains Fault Zones ...... 29 Eastern B e l t . 29 Intrusive Rocks ...... 32 Ultramafic Rocks ...... 32 Granodiorite ...... 34
iii iv
TABLE. OF CONTENTS--Continued
Page
4. GOLD 0REBODIES ...... 35
Introduction ...... 35 Quartz Veins ...... 37 General F e a t u r e s ...... 37 Vein Filling ...... - . . . 40 Gouge . '...... ■ ...... 42 Ore Shoots ...... 43 Mineralized Greenstone ...... 44 Mineralized Schists ...... 45 Alteration ...... 46 Origin of the Gold Orebodies ...... 47
5. METALLOGENIC STUDY ...... 49 -
The West Belt ...... • . . 49 Southern Area ...... :. . . 51 Northern Area ...... 52 Characterization of West Belt Deposits . 54 The East Belt ...... 56 Southern Area ...... 57 Central Area ...... 58 . Northern Area ...... 60 Characterization of East Belt Deposits . 60 The Mother Lode B e l t ...... 62 Northern A r e a ...... 63 Jacks on-Plymouth ...... 66 San Andreas ...... ■...... 67 Angels Camp . . . . . -...... 68 Characterization of the Mother Lode . Deposits ...... 69 Comparison of Belts ...... 74 Comparison Between the Mother Lode and Other Mesozoic Lode Gold Belts . . . . . 81 Nature.and Distribution of Mesozoic Lode Gold Deposits ...... 81 Canadian Deposits ...... 83 Deposits of Eastern Asia ...... 84 Summary of Relationships ...... 84
REFERENCES ...... 86 LIST OF ILLUSTRATIONS
Figure Page
1. Index Map Showing Location of the , . Thesis Area ...... Y...... 3
■ 2. Map of California Showing Gold Bearing Areas and the Thesis Area . 4
3. Geologic Map of the Central Sierra Foothills Showing the Location Of the Three .Gold Belts ...... 6
4., Simplified Geologic Map of the Central Sierra Nevada Foothills ...... 13
5. Stratigraphic Nomenclature Used in Amador County ...... 14
6. Geologic Map Sacramento A.M.S. Sheet, California (in pocket) ......
7. Attitude of the Quartz Veins and Mine Locations Sacramento A.M.S. Sheet, California (in pocket) • ......
8. Vein Systems and Approximate District Locations ...... 50
9. Location and Trend of Veins in the Ophir District ...... 77
10. Location and Trend of Veins in the Central A r e a ...... 78
11. Distribution of Mesozoic Gold Deposits . .• . . 82
v LIST OF TABLES
Table Page
1. Structural and Lithologic Controls in the Mother Lode R e g i o n ...... 80
vi ABSTRACT
The gold quartz veins of the Mother Lode region, central Sierra Nevada foothills, California, are distrib uted among three north-trending belts, the West belt, the
East belt and the Central, or Mother Lode, belt. The structural.and lithologic controls of the gold deposits along each belt are examined and analyzed. Two of the belts, the West belt and the Mother Lode belt, have strong, spatial associations with regional structures, while a third belt, the East belt, is associated only with local structures. Because of the lack of a regional structure, the use of the term East belt should be discontinued. In the West and Mother Lode belts, there is little relation ship between productivity and specific locations along a belt, therefore location cannot be used to predict productivity. The productivity of the gold deposits in a particular area is dependent on local structure and lithologic features rather than on regional features. CHAPTER 1
INTRODUCTION
This thesis concerns a major portion of one of the more important lode gold districts in the history and development of the western United States, the Mother Lode.
The district has been unproductive for several years but takes on a renewed importance because of the present high price of gold. Primarily the structural and metallogenic features of the northern Mother Lode region are studied.
These features are characterized and compared to features both within and outside of the Mother Lode region.
Information has been obtained from a variety of sources including publications of the California Division of Mines and the United States Geological Survey. Field work was limited to a two-week geological reconnaissance of the region.
The ideas presented are meant to help exploration geologists identify targets for detailed field study in this and other similar lode gold districts.
Geographical and Geological Setting
The area studied covers approximately 7,500 square
miles in central California. It is bounded to the west and east by 122°00' and 120°00' west longitude and to the
south and north by 38°00' and 39°00' north latitude
(figure 1). The fraction of the area actually involved in gold mineralization is small. Gold quartz veins are con
fined to a 20-mile-wide belt of north- to northwest- trending metamorphic rocks' and of small plutons, having varying composition, that intrude these rocks. This mineralized belt occurs west of the Sierra Nevada batholith
in a region commonly referred to as the Sierran foothills.
The topography of the. foothills is rolling, with an average elevation of 1,500 feet. Vegetation normally in cludes grass and small types of brush along with various
types of pine and fir trees. West of the foothills the metamorphic rocks are covered by younger sediments o f Xthe
Great Valley sequence.
. ' / - . The thesis area includes only a portion of one of
the gold-bearing regions of California. The two most
extensive gold areas in the state are found in Mesozoic
rocks within the Klamath Mountains and Sierra Nevada geo- morphic provinces (figure 2). The deposits, both placer
and vein type, in the Sierra Nevada province occur in a
zone that stretches from Butte.County in the north to
Mariposa County in the south. The southern and central
sections of this mineralized zone contain a series of
structurally related quartz veins which are termed the 3
X.trjy
t ^ O o i- ^ e to'-.Ti AREA
L t'J D O R A D O
' I . y S A C R A M E N T O ' ^ T" M^Hymouth— "q 9 '---- - ^ S A C R A M E N T O i Vv 3 hr‘^ r L / j '''^-<>> JACKSON ^ k v % . i rX '. , -VvVi^fl e C &mp ! (//-- ^STOCKTOW j ^ ^ ’^-vCONTRA COSTA ) SAN^"JOAOUI'
\) X'—_ I .-, PAN FRANCISCO ,— ^
A M E D A i /_ v I I i T > ^ - V L MARIPOSA
—A
Figure 1. Index Map Showing Location of the Thesis Area. After Knopf (1929). 4
MAP OF CALIFORNIA SHOWING GOLD-BEARING AREAS AND GEOMORPHIC PROVINCES
SCALE 40____80 120 Miles
THESIS AREA
EXPLANATION I KLAMATH MOUNTAINS H CASCADE RANGE MODOC PLATEAU COAST RANGES GREAT VALLEY SIERRA NEVADA BASIN RANGES MOJAVE DESERT TRANSVERSE RANGES PENINSULAR RANGES COLORADO DESERT GOLD-BEARING AREA
Figure 2. Map of California Showing Gold Bearing Areas and the Thesis Area. After Clark (1969). Mother Lode vein system. This thesis examines the northern part of the Mother Lode system where three north- to northwest-trending belts of gold-bearing quartz veins have been defined. The central belt is by far the most productive and is known as the Mother Lode belt. The belts flanking the Mother Lode belt are known as the West and East belts (figure 3).
: ' History
Gold mining in the Mother Lode region has a long and colorful history. Some of the color and vitality of the region during its boom period is recorded in. stories
Such as "Roughing It" and "The Jumping Frog of Calaveras
County" by Mark Twain and in several short stories by Bret
Harte. Placer gold was discovered in the Mother Lode region at Sutter s Mill on the American River near Coloma by James Marshall. The exact date of the discovery is unknown, but it is officially designated as January 24,
1848. This discovery set off the famous California gold rush, and thousands of gold seekers poured into California.
Quartz vein mining began in 1849 in Mariposa County.in the southern Mother Lode, with mining in the northern Mother
.Lode beginning shortly thereafter. By 1852 the great placer deposits of the region were producing vast quan tities, of gold as the yearly output of the region reached its maximum value of $80 million. . At that time towns (Manoosa fo'mation'
Greensrone ampnioohte _v-v XA cnionte scnis:
-T^ Slate mica scnist. guamite some nomteis and limestone (Ca’averas Formation in pan)
Goid Belt or vein system.....
Tertiary rocks are omitted
Northern Mother Lode Southern Mother Lode
s.m ^ Fr«ncisco»l x
; v $
Kilom etres
M ile s
Figure 3. Geologic Map of the Central Sierra Foothills Showing the Location of the Three Gold Belts After Boyle (1979). along the Mother Lode such as Jackson and PlacerviHe were among the most populous in the state. By 1864 the rich placer deposits were essentially exhausted, and a larger percentage of the production after that date came from lode deposits. Gold production maintained a steady rate of • 'V . ' ' ' approximately $5 million per year from 1864 until 1933, when the price of gold increased from $20.67 to $35.00 per troy ounce. This increase in the price of gold caused a doubling of the dollar value of the output which lasted until 1942, when the mines were closed down by order of the War Production Board. When the order was rescinded in
1945 a few mines reopened, but in reality the closing of the mines in 1942 signaled the end of mining in this region. The few mines which did reopen had difficulty making a profit because of higher postwar costs; the last operating mine, the Central Eureka, was shut down in 1953.
Total production from the northern Mother Lode region was approximately $460 million, or 21 million troy ounces of gold.
Little visible evidence of the gold quartz mines remains, aside from the head frames at some of the larger mines such as the Kennedy, near Jackson, Amador County.
The surface plants and stamp mills were dismantled long ago and the dumps now have a cover of vegetation. However, the rich history of the region has been preserved to a certain ■ 8 extent. Old gold mining towns such as Columbia and Coloma are now California state parks, and the buildings in other towns along the Mother Lode have been preserved and are now something of a tourist attraction. Annual festivals such as the frog jumping contest at Angels Camp, Calaveras
County, attempt to recapture the flavor of the gold rush era.
Because of the current price of gold a few mines
in the Mother Lode region have reopened, and exploration activity has increased. As of 1979 the operating mines in the region were the Blackstone and Blazing Star mines of
Calaveras County . (Rapp, 1979). The Blazing Star mill has a capacity of 163 tons per day, but no production data are available (McWilliams et al., 1979).
■ Previous Work
The first geologic study of the Mother Lode belt was made by Fairbanks in 1890. An area four miles wide was mapped, and mines were examined. Because of time restrictions, the study was incomplete, and the geologic
terminology used by Fairbanks has been superseded. In
1900, Ransome completed a more comprehensive and detailed
series of geologic maps, which were published .in the
Mother Lode folio. In 1929, Knopf completed the most
comprehensive study of the economic geology of the Mother
Lode region. Knopf's paper, written at a time when the ’ 9 workings of almost all the mines were accessible, contains a significant number of the available published detailed, mine maps and sections. His work cannot be improved upon, since almost all of the mines in the region are now inac cessible owing to caved ground. Papers subsequent to
Knopf's have been summaries of the productivity, economic geology, and mining and milling techniques at individual deposits. These works include a report on the mining technology by Logan (1935) and reports on the character istics of lode gold deposits in the counties along the belt by Carlson and Clark (1954) (Amador County), Clark and
Carlson (1956) (El Dorado County), and Clark and Lydon
(1962) (Calaveras County). The characteristics of indi vidual districts in the region are summarized by Clark
(1969). CHAPTER 2
REGIONAL GEOLOGIC SETTING
Nature and Distribution of the Principal Rock Types
The area within and surrounding the Mother Lode belt is underlain by crystalline rocks of the Mesozoic and
Paleozoic ages. These rocks are commonly called the
"Bedrock Series" to distinguish them from a group of
Tertiary volcanics and sandstones, the "Superjacent
Series," which occurs to the east and west of the Mother
Lode and is not associated with lode gold orebodies. The
"Bedrock Series" consists of alternating bands of phyllite, amphibolite schist, black slate, and greenstone which have been complexly deformed and.faulted. These belts range from narrow strips to zones over a mile wide, and many extend over the entire length of the Mother Lode. The
"Bedrock Series" has been intruded by several types of plutonic rocks, chiefly granite, which are exposed in large areas east of the Mother Lode. Smaller amounts of basic and ultrabasic rocks, largely altered to serpentinite, are found in narrow northwest-trending lenses west of the
Mother Lode.
10 t . 11
Metamorphic Rocks
The metamorphic rocks of the Mother Lode, region form a series of elongate northwest-trending belts sepa rated by steeply dipping fault zones. The beds within the belts commonly strike northwest to north-northwest and dip steeply to the east. Presently these rocks are interpreted as remnants of ancient volcanic arcs, subduction zone complexes, and sequences of oceanic crust and upper mantle
(Schweickert and Cowan, 1975). Formerly, all of the meta morphic rocks of the Mother Lode region were assigned to one of three formally named stratigraphic units: the
Calaveras formation, the Amador group, or the Mariposa formation. The Calaveras formation originally included all metasedimentary and metavolcanic rocks of Paleozoic age and was identified by Turner (1893). The Jurassic Amador group was identified and separated from the Calaveras and
Mariposa formations by Taliaferro (1942). In addition,
Taliaferro separated the Amador, group into two formations: the Cosumnes and the Logtown Ridge. The Upper Jurassic
Mariposa formation was identified by Becker (1885).
The Upper Jurassic dark slates enclosing the Mother
Lode quartz veins in the northern Mother Lode region are referred to as slates of the Mariposa formation. The greenstones and amphibolite schists immediately west of the Mariposa formation are considered part of the Logtown 12
Ridge formation. Paleozoic rocks east of the Melones fault
zone are generally assigned to the Calaveras formation;
however', the names Amador group and Cosumnes formation have
been abandoned (Sharp and Duffield, 1973). The metamor-
phic rocks are divided into tectonically significant
Western and Eastern belts, based on their interpretation
as island arc and subduction zone sequences.
Western Belt. The Western Belt has been divided
into three tectonically significant units by Schweickert
and Cowan (1975). These units have been informally named
Units A, B, and C. South of the American River Unit B
separates Unit A from Unit C (figure 4). To the north Unit
B is not present and Units A and C are separated by the
Smartville ophiolite sequence. Previously, Unit A has been
termed the Western belt (Duffield and Sharp, 1975), the
Cosumnes and Mariposa formations (Taliaferro, 1942; Clark
and Carlson, 1956), and the Mariposa and Calaveras for mations (Knopf, 1929; Turner, 1893) (figure 5). Unit A is
bounded by Tertiary "Superjacent Series" rocks to the west
and the Bear Mountains fault zone to the east. It consists
primarily of Jurassic fine-grain pyroclastic greenstones
and Upper Jurassic dark colored slates which trend north
to northwest along the entire length of the northern Mother
Lode region. Currently, Unit A is interpreted as an island 13
12V >39
JC
^- Veones Fault \ Zone Bear Mcunta ns — Fault Zone
120 EXPLANATION
Superjacent Series Calaveras Formation
1 J* | Unit A INTRUSIVE ROCKS c | Granitic rocks | jc | Unit C f ut, I Serpentinite js | smartville Block
| Pup 1 Unit B o to i°
Figure 4. Simplified Geologic Map of the Central Sierra Nevada Foothills. After Schweickert and Cowan (1975). 14
Mariposa formation Mariposa formation Mariposa formation Mariposa formation
Island Arc Logtown Ridge* Sequence Logtown ► Mother Lode "C" Calaveras formation Logtcvm Rioge Logtown Ridge . Belt Ridge formation formation ^ formation
Previous Nomenclature for Island Arc Sequence C.
Cosumnes formation Copper Hill Voicancs Copper Hill Votcanics
Island W est Salt Spring ^ Arc Calaveras formation Mariposa formation Salt Spring Slate "B elt Slate Sequence "A"
Gopher Ridge Voicancs Cosumnes formation Not Mapped
Previous Nomenclature for Island Arc Sequence A
Figure 5. Stratigraphic Nomenclature Used in Amador County. 15 arc sequence. Some West belt mines are found within this
unit.
Unit C is bounded by the former Cosumnes formation
/ contact to the west and the Melones fault zone to the east.
This unit is also interpreted as an island arc sequence and
consists of. greenstone, slate, sandstone and amphibolite
schists. Rocks of this unit were previously mapped as the
Logtown Ridge-Mother Lode belt (Duffield and Sharp,. 1975) ,
the Amador group and Mariposa formation (Taliaferro, 1942;
Clark and Carlson, 1956) and Calaveras and Mariposa for mations (Knopf, 1929; Turner, 1893) (figure 5). The rocks
of this unit are currently divided into the Logtown.Ridge
and Mariposa formations. The bedding of these formations
is very persistent, with north to northwest strike and
steep eastern dip. The mines of the Mother Lode belt are
found within this belt, with the very productive quartz
veins of the northern part of the belt enclosed by slate
of the Mariposa formation.
To the south of the American River, Units A and C
are separated by a tectonic melange constituting Unit B
(Schweickert and Cowan, 1975; Duf field and Sharp, 1975).
Previously the unit was not recognized as a melange and •
was mapped as the Calaveras and Consumnes formations
(Taliaferro, 194.2) or the Calaveras formation (Knopf,
1929; Turner, 1893). The melange, a narrow strip.less 16
than 1% miles wide, is .composed of phyllite, slate, gray-
wacke, quartzose sandstone, chart, and conglomerate, all
of which have been pervasively sheared. Very few mines are .
located in Unit B .
North of the American River, the■Smartvilie block, named by Cady (1975), separates island arc sequences A and
C. The block averages 7 miles across and is bounded by
faults to the east and west. The block consists of meta morphosed pillow basalts, basaltic dikes, and gabbro. This block has been recognized as an ophiolite and has been -
assumed to be a fragment of lithosphere which formed in a
Jurassic marginal basin (Schweickert and Cowan, 1975).
According to Xenophontes and Bond (1978), the ophiolite
did not form in a marginal basin but is a very young inter
arc basin. • '
-.: Eastern Belt. The Eastern belt contains all ineta- morphic rocks east of the MelOnes fault, which is inter
preted as a complex Paleozoic continental margin
(Schweickert and Cowan, .1975). Davis et al. (1978), however, believe.that the continental margin lies' to the
east and the Paleozoic rocks.of this belt form, a melange.
The. name "Calaveras"., formation is applied to these rocks in
keeping with the historical method of naming.Paleozoic
rocks in the central foothills (Turner, 1893). This belt
occurs over a wide region east of the Melones fault zone 17 and is very persistent from north to south. The belt consists of a monotonous succession of beds of black clay slate,’ metamorphosed sandstones, and quartzites with scattered lenses of crystalline limestone (Turner and
Lindgren, 1894). Beds within the belt strike north to northwest and dip steeply to the east except in the vicinity of Murphys, Calaveras County, where they strike westward and dip steeply to the north. All of the East, belt mines are found within.this belt.
Intrusive Rocks
Plutons of both relatively deep-seated and hypabyssal origin intrude the metamorphic rocks of the
Mother Lode region. Composition ranges from ultramafic to granitic. The majority of the plutons west of the Melones fault range in composition from ultramafic rocks to diorite, while those east of the fault range from quartz
j ■ - - -. - - diorite to granite.
Serpentinite and Associated Rocks. . Serpentinite is commonly found as thin north- to northwest-trending lensoid bodies within the melange belt, in parts of the Melones fault zone and in the Smartville block. Serpentinite is found near most West belt mines and in the mines of the southern Mother Lode belt near Angels Camp, Calaveras
County. The serpentinite is derived from ultrabasic 18
intrusions of pyroxenite, peridotite, and dunite.
Occasionally unaltered ultrabasic rocks are found within
the serpentinite along with considerable amounts of gabbro
(Cater, 1951).
Granitic Rocks. Numerous bodies of granitic rocks,
which are associated with the emplacement of the Sierra
Nevada batholith, occupy an extensive region east Of the
Melones fault zone. Hornblende granodiorite. is by far
the most abundant; however, granite and quartz diorite are
also found. Small bodies of fine grain diorite are found
in various locations, especially between Railroad Flat and
Mountain Ranch, Calaveras County. Gold-bearing quartz
veins within fissures in the granodiorite are most com-• monly found in northern Calaveras and southern Amador
Counties, but are rarely found elsewhere.
Structural Geology
The structural geology of the Mother Lode region is
extremely complex owing to the folding and faulting of the
metasediments and metavolcanics. Early studies of the
region, such as those by Turner and Ransome (1898),
designated the region as a series of steeply dipping north-
to northwest-striking beds interrupted only by granitic
intrusions'. Taliaferro (1942) was able to determine the
structure along the Cosumnes River by separating the Amador 19 group from the Calaveras and Mariposa formations. Talia ferro found that the region is a series of steeply inclined
isoclinal folds which have been faulted, causing repetition of the beds. Clark (1964) has interpreted the structure of a region of central Amador County as a homoclinal sequence with tops facing eastward. Units within the homoclinal sequence are intensely folded and faulted.
Duffield and Sharp (1975) believe Clark's interpretation of the structural geology is valid only locally, but they agree that any interpretation must involve abundant
faulting and shearing both within and between lithologic units.
Faults of the foothill fault system strike north- northwest and dip steeply, approximately 80° to the east, as indicated by "attitudes of schistosity within the fault zone and by straight traces of the fault zones across■ rugged topography" (Clark, 1960). In Calaveras and Amador counties two north-northwest trending faults are the prin cipal components of the foothill fault system. The eastern
fault, which is associated with the deposits of the Mother
Lode belt, is named the Melones fault zone.'- The western
fault; which is associated with deposits of the West belt,
is named the Bear Mountains fault zone. In El Dorado
County the Melones fault continues slightly sinusoidally along its north-northwest trend. Three miles south of 20
Georgetown, however, the fault bifurcates into northwest- and northeast-trending branches. The Bear Mountains fault is not well defined in El Dorado County; rather, a series of small, unnamed north- to northwest-trending fault zones are found. Eight miles west of Placerville these faults abruptly change to a westerly trend and continue into
Placet County (figure 6).
Faulting in the Mother Lode region is younger than the final emplacement of the plutons, as demonstrated near the Melones fault zone in northern Amador County. At
Plymouth the western border of a pluton is parallel to the trend of the Melones fault zone,, indicating that the pluton has been truncated by the fault (figure 7). In addition, the granitic rock near the fault zone has been altered to a lineated schist containing saussurite, quartz, and horn blende (Clark, 1969).
There is no direct conclusive evidence to indicate either the nature or the amount of offset along the foot hills faults because of the poorly understood small-scale stratigraphic sequence and the near parallelism of the faults and the bedding. Several different interpretations of the faulting appear in the literature. Knopf (1929) found that the major faults in the region were dip slip reverse faults. Taliaferro (1942) and Chandra (1953) have found evidence of thrust faulting, while strike slip ■ 21 faulting is advocated by Clark (1960), Cebull (1972), through a structural analysis of the Melones fault near
Downieville, found evidence of strike slip preceded by dip slip or oblique slip movement. Most recently Davis and others (1978) interpret the Melones fault as a right lateral strike slip fault and the Bear Mountains fault as a left lateral strike slip fault.
. Although the nature of the faulting is unknown,.the offset must be quite large because points formerly on oppo site sides of the fault have not been located. Clark
(1960) assumes a displacement much greater than a few thousand feet along the Melones fault because if the dis placement were a few thousand feet, "at least its order of magnitude would be known." The horizontal displacement of the Melones fault may be greater than 100 miles if indeed it is a strike slip fault.
Summary of Geologic History
During late Paleozoic time the rocks of the
Calaveras formation were part of an island arc. Near the close of the Paleozoic, the island arc collided with the
North American plate, triggering the Sonoma orogeny, which formed an ancestral Sierra Nevada. During late Triassic time another island arc formed as subduction continued along the western edge of the North American plate. During middle to late Jurassic time the Mesozoic island arc was 22 fragmented into two arcs separated by an interarc basin.
The Smartville block is interpreted as the floor of the basin. During Upper. Jurassic time vulcanism ceased and detrital sediments were deposited in the calm offshore environment. Near the end of Jurassic time the island arcs collided with the North American plate. This col lision marks the Nevadan progeny, during which a second ancestral Sierra Nevada was established. During Cretaceous time large-scale plutonic activity began. First to form were small ultrabasic plutons, which were later altered to serpentinite; batholithic grandiorites then intruded the metasediments and metavolcanics. The intrusion of the
Sierra Nevada batholith during the Nevadan orogeny de stroyed most of the Paleozoic rocks in the region. During this period of plutonic activity many types of deposits formed, including chromite, copper, and tungsten bodies.
However, the most economically important deposits formed during this time were the auriferous quartz veins of the
Mother Lode system. CHAPTER 3
ROCK UNITS
Western Belt
Unit A
Unit A is the least economically important unit in the Mother Lode region and is discussed only briefly.
The lowest formation of Unit A, the Gopher Ridge Volcanics, consists chiefly of pyroclastic rocks of basaltic and ande- sitic compositions; however, lavas, pillow basalts and rhyolitic pyroclastic rocks are abundant locally. The .
Gopher Ridge volcanics are overlain by, and intertongue with, the Salt Spring slate. The Salt Spring slate con tains graywacke, conglomerate, and limestone in addition to dark, carbonaceous slate. Fossil evidence from the lime stone lenses indicates an Upper Jurassic age for this for mation. To the east the Salt Spring slate intertongues . with the Copper Hill Volcanics, which contains andesitic and basaltic fine-grained pyroclastic rocks very similar to. the Gopher Ridge volcanics.
23 24
Unit C
In contrast to Unit A, the rocks of Unit C are very important economically, since the very productive
Mother Lode belt quartz veins of Amador and El Dorado counties are enclosed by rocks of this unit. Unit C con sists of the metavolcanics of the Logtown Ridge formation and the metasediments of the Mariposa formation.
The Logtown Ridge formation is a late Jurassic sequence of mafic pyroclastic rocks and inter layered flows and sills. The unit has been divided into four members by
Duffield and Sharp (1975). The basal Rabbit Flat member consists of coarsely porphyritic basalt breccia and massive flows. The base of this member, is in fault contact with
Unit B to the south of the American River and Smartville block to the north.
The Rabbit Flat member is conformably overlain by the Goat Hill member. This member consists of three principal rock types: "thin-to-thick-bedded fine-to- medium-grained tuff, coarse pumice lapilli tuff in commonly thick locally graded beds that are interlayered with the finer tuff, and thick-bedded fine to coarse volcanic breccia that grades upward into medium- and fine-grained tuff" (Duffield and Sharp, 1975, p. 10). Much of the vol canic breccia was deposited by subaqueous mudflows, as evidenced by the very poor sorting and lack of internal 25 bedding (Clark, 1964). In addition to the tuffs and breccias, minor amounts of massive and pillow-structure
lavas occur in northern Amador and El Dorado counties in sheet-like flows and sills. The age of this member, determined from fossils found along the Cosumnes River, is
Callovian.
The Pokerville member overlies the Goat Hill mem ber slightly unconformably north of Drytwon but conformably to the south. This member is dominantly a breccia, but coarsely porphyritic augite basalt which occurs in massive
flows, pillow lavas and bedded pyroclastic deposits is also present. Chemical analyses of this member indicate that it is very similar to the Rabbit Flat member (Duffield and Sharp, 1975). The thickness of this member is extremely.variable. In northern Amador and El Dorado counties this member constitutes 75 percent of the for mation; however, to the south the thickness diminishes rapidly so that in Calaveras and southern Amador counties
it constitutes only 10 percent of the formation.
In Calaveras and southern Amador counties, the New
Chicago member overlies the Pokerville member. It occurs only as thin discontinuous bodies. .. Again, this member is dominantly a volcanic breccia.
The Mariposa formation overlies the Logtown Ridge
formation. It is a monotonous sequence of primarily dark 26 clay slates which contain minor amounts of sheared sand stone and conglomerate. Volcanics, primarily andesite flows and pyroclastics, are common in certain locations, providing evidence for local submarine explosions. The volcanics are termed the Brower Greek member (Clark, 1964).
The character of the slate varies little from north to south, although a slight color change is noted in
Amador County, where the slates take on a bluish tint.
The slates are fine-grained and cleavable, occurring in uniform, nearly vertical beds. Interbedded with the ■ slates are harder and coarser thin beds of graywacke
(Knopf, 1929). These beds were formerly called, grauwacke by Fairbanks (1890) and sandstone by Ransome (1900) . These beds are easily distinguishable from' the slates because of the higher quartz content, and they can be used to deter mine small-scale structure. The graywacke interbeds occur rarely in El Dorado County but become more abundant towards the south.
The conglomerate occurs in small quantities in El
Dorado and Calaveras counties, but in Amador county, one- half mile west of Plymouth, it reaches a thickness of 200 feet but thins rapidly to the north and south. . The well- rounded to subangular pebbles in the conglomerate represent a wide variety of rock types including chert, quartz 27 schist, and several types of igneous rocks (Taliaferro,
1943).
The black color of the slates is produced by a carbonaceous pigment that is capable of generating methane.
The Plymouth mine in Amador County contained an average of
.21 percent methane according to air samples taken by the
United States Bureau of Mines. Methane can reach explosive levels. In one stope within the Plymouth mine a small explosion injured one worker (Knopf, 1929).
Fossils found within the slates at several locations indicate aviate Oxfordian to early Kimmeridgian age for the Mariposa formation (Imlay, 1959).
Unit B
Unit B is a tectonic melange which underlies the
Logtown Ridge formation south of the. American River. The melange occurs as a north-northwest-trending narrow strip averaging 1% miles wide. A wide variety of pervasively sheared rock types including phyllite, slate, quartz sand stone, chert, greywacke, limestone, and conglomerate are found in the m e l a n ge.The melange has not been divided into any single lithology map units; rather, division has been made on the basis of rock assemblages (Duffield and
Sharp, 1975). The most common assemblage is quartzose phyllite with thin chert interbeds. This assemblage may 28 also contain thick chert, quartz sandstone, or limestone beds. All of the fossils used in the dating of this unit have come from the limestone lenses. These fossils indi cate a Paleozoic , most likely Permian, age and have been used to date unfossiliferous rocks, Schweickert and Cowan
(1975), however, believe that the "tectonic contacts of
Unit B and of the limestone blocks themselves preclude such a correlation " Therefore Unit B may contain both Paleo- zoic and Jurassic age rocks.
Smartville Block
The Smartville block is a fault-bounded wedge found in northern Amador and El Dorado counties. It con- sists primarily of pillow basalts, although swarms of basaltic dikes are common. Mapping by Schweickert and
Cowan (1975) shows that the block is an antiform with gently dipping limbs, They postulate that the core is composed of metagabbro and that the block should therefore be termed an ophiolite. The block contains many mines of the northern West belt, but the great majority of these mines are associated with small granodiorite and diorite plutons which intruded after the block had been deformed and metamorphosed. 29
Rocks of the Melones and Bear- Mountains Fault Zones
The rocks of the Melones and Bear Mountains fault
zones occur in narrow, 800- to 3,200-foot bands. Despite
the small areal extent of these rocks, they are very
important economically, as all of the southern West and
Mother Lode belt mines occur within the fault zones. These rocks are easily separated from the adjacent rocks because
they are sheared and recrystallized to a much greater
extent. Within these zones greenstones have been altered
to a mica schist containing abundant epidote, chlorite, hornblende and quartz. Bodies of strongly mylonitized
rocks containing large hornblende augen and light-colored
foliated alaskite are abundant locally (Duffield and Sharp,
1975).
Eastern Belt
The eastern belt contains Paleozoic age rock.
Following the historical method of mapping Paleozoic rocks
in the central Sierran foothills-, this belt is referred to
as the Calaveras formation. The Paleozoic rocks of the
central Sierra Nevada were first mapped in detail by
Diller (1908), 110 miles north of Plymouth in the Taylors
ville, Plumas County, region. Diller described formations
ranging from Silurian to Carboniferous and assigned all of
the Carboniferous units to the Calaveras formation. To the 30 south of Taylorsville, all Paleozoic rocks were assigned to the Calaveras formation, sometimes with very little evidence. The exact age of the Calaveras formation is impossible to determine since fossil recovery from lime stone lenses is poor because of recrystallization, and recovered fossils indicate little more than a Paleozoic age. These rocks are now considered to be of Permian age
(Clark, 1964). However, geologists have previously con sidered the rocks to be of Carboniferous (Heyl, 1948),
Triassic through Carboniferous (Knopf, 1929), or Permian and Carboniferous age (Taliaferro, 1943).
The Calaveras formation is currently divided into four unnamed members: a clastic member, a volcanic member, an argillaceous member, and a chert member (Clark, 1964).
The individual members will not be considered here. The most common Calaveras rock type, originally a dark shale, is presently distributed in a wide variety of forms. In its least altered form it appears in an advanced slate stage, but generally it occurs as a phyllite. Interbedded with the phyllites are white, blue-grey, or black recrys- tallized limestone and sheared sandstones: Both the al tered sandstones and the limestones may occur in thick bands with few phyllite intercalations. With increasing grade of metamorphism the sandstones and limestones become
fine- to coarse-grained mica schist and marbles. The 31
sandstones appear to be feldspathic, and they may pass into
gneisses.' Graphitic schists result from the metamorphism
of highly organic shales or shaly limestone. Conglomerates
occur in many locations with crushed or elongate pebbles.
With extreme metamorphism, the conglomerates may pass into
augen or flaser gneiss. Metacherts are commonly present
and range from rhythmically bedded and slightly crystal
lized red radiolarian cherts to completely recrystallized
white to grey, quartzites.
Interbedded with the sediments are volcanic rocks
in various stages of metamorphism. Some flows are unal
tered so that top and bottom can be distinguished by the
characteristics of the vesicles (Taliaferro, 1943), while
others have been completely converted to schist. Rhyolitic
flows and tuffs sometimes are present, but the majority
appear to be andesitic and basaltic. Pillow basalts are
not uncommon, especially in association with the little-
altered red radiolarian cherts, but metamorphism and
crushing have generally obliterated the pillow structure.
The great majority of the metavolcanic rocks are schistose,
occurring most commonly as chlorite, chlorite-amphibolite,
or amphibolite schists. In some cases the rocks were
clearly augite andesites, but in other cases the exact
original composition is unknown because of the changes
caused by the metamorphism. The majority of the green 32 schists appear to have.been intermediate and basic volcanics.
Intrusive Rocks
Intrusive rocks exposed in the Mother Lode region range from ultramafic to granitic composition. The ultra- mafic rocks are largely altered to serpentinite and occur as thin north-northwest-trending belts west of the Melones fault zone. The less mafic plutonic rocks form the Sierra
Nevada batholith east of the Mother Lode region and small isolated plutons elsewhere. Original mineralogy has been altered in many plutons, but "despite local overprinting of metamorphic texture or cataclastic structural fabric the initial textures are commonly intact" (Duffield and
Sharp, 1975, p. 17).
Ultramafic Rocks '
The metamorphosed ultramafic rocks of the region include serpentinite, talc-antigorite-ankerite schist, and less commonly unmetamorphosed peridotite and dunite.
Serpentinite is by far the most abundant.
The areas underlain by serpentinite are easily identified because they support a thick growth of green brush which sets them off from the rest of the region.
Two varieties of serpentinite have been identified. The most abundant type is a dark green rock that contains 33 numerous plates of bastite which give the rock an almost metallic luster. The bastite is scattered throughout a dark aphanitic matrix, giving this variety a porphyritic
texture. The second variety is a fine-grained homogeneous
light- to medium-green-colored rock. It is massive, with
few slickensides, and is common in the Angels Camp and
Carson Hill areas of southern Calaveras County.
The serpentinite bodies are elongate parallel to the strike of the bedding or fault zones. Serpentinite is especially common in extensively faulted areas and has been used as an indicator of faults. It is suggested that serpentinite and the fault zones are somehow related
(Clark, 1964).
In many locations throughout the Mother Lode region, but primarily in the Angels Camp and Carson Hill
areas, the serpentinite has been altered to a mariposite-
ankerite rock containing many quartz veinlets and
stringers. These altered masses are economically impor
tant because the entire mass is often auriferous.
The ultramafic rocks are assumed to be of late
Jurassic age because the faults which controlled: their
emplacement cut all the metamorphic rocks in the region.
Additionally, granitic intrusions cut serpentinite bodies
in, several locations,indicating that the serpentinite is
younger than the granitic intrusions. Granodiorite
Granodiorite occurs throughout the area east of the Mother Lode within the Sierra Nevada batholith and its extensions. The large intrusive masses, however, are economically unimportant, as no gold-bearing quartz veins are found within the batholithic granodiorites. Many East belt mines are found near the contact or in the center of small irregularly shaped plutons west of the batholith.
The granodiorite is uniform in composition and texture. It contains approximately 20 percent mafic minerals, primarily hornblende and biotite. Andesine
(Ab^gAn^y) is the primary light-colored mineral (Knopf,
1292), although quartz is abundant. Accessory minerals include titanite, magnetite apatite, and zircon. CHAPTER 4
GOLD 0REBODIES I Introduction
Gold orebodies are found in three nearly parallel north- to northwest-trending belts: the East, the West, and the Central belt. The Central belt, which is easily the most productive, is termed the Mother Lode, although it is not a continuous vein as the name implies but a belt of parallel gold-bearing quartz-filled fissures. The Mother
Lode is associated with a north-trending zone of fissures caused by reverse faulting. The belt commonly has the same trend as that of the wall rock bedding, although the dip of the fault zone and bedding are commonly in different directions. The vein system is very persistent, and it appears to be independent of rock type since the quartz veins are found in many different rock types including slate, schists, greenstones, and, in the southern Mother
Lode region, serpentinite.
The veins of the Mother Lode belt occurring within the slates of the Mariposa formation are of a relatively uniform nature, especially in Amador County between
Jackson and Plymouth. However, there is a great amount of
35 36
variation in the size and grade of the deposits along the
entire length of the Mother Lode due to the occurrence of mineralized country rock. Both mineralized schists and
greenstones are found, with the schists being more
diverse. Auriferous schist is found as stockworks con
sisting of gold-bearing quartz veinlets within the schists
and replacement orebodies in which the silica in the
schists was replaced by carbon dioxide to form ankerite.
Gold was introduced along with the carbon dioxide. Auri
ferous country rock is most common in southern Calaveras
County, where it constitutes a major number of the ore- bodies in the Angels Camp and Carson Hill regions; however,
to the north it becomes less common. It is found at depth beneath discrete quartz veins in the mines of central
Amador County, but it is rarely found within the Mother
Lode belt in El Dorado county.
The East belt is located 6 to 18 miles east of
the Mother Lode and comprises a discontinuous belt of mines. Unlike the Mother Lode, there is no structural
feature which can be traced along its entire length. The
East belt consists of a series of discrete districts
separated by large.distances. The districts do not occur
in a linear belt, but rather in two or three parallel
belts often separated by distances greater than one mile.
The veins of the East belt are found in the same country 37
rock as the southern Mother Lode veins, but they are also
found in granodiorite and at granodiorite contacts. In general, the veins of the East belt are narrower and less persistent than the Mother Lode veins. Also, the East belt veins contain more sulfides and are higher in grade.
The West belt is even more discontinuous than the
East belt. Few gold-bearing veins are found along this belt. The majority of the production from West belt mines occurred in southern Calaveras County. The remainder of
the production occurred in northern El Dorado County.
Almost no production came from Amador and northern
Calaveras counties along the West belt. The largest deposits in this belt are large low-grade bodies of mineralized country rock which are associated with the
Bear Mountains fault. These deposits are found in Unit A
near the contact of the Salt Spring slate and the Copper
Hill Volcanics in southern Calaveras County. In the
northern area gold-bearing quartz veins associated with
granodiorite are found.
Quartz Veins
General Features
In the Mother Lode belt, quartz veins found within
the metamorphic rocks are generally tabular masses which
strike north to northwest and dip eastward. The veins are 38 normally 10 feet wide and can extend thousands of feet along both strike and dip. They carry approximately 1/3 oz gold per ton. In local occurrences, such as in mine drifts, the veins appear to have the same attitude as the bedding of the enclosing rocks. However, on a broader scale it is seen that the veins actually cut the bedding in both strike and dip (Knopf, 1929). The bedding of the country rock appears to conform to the veins because the wall rock has been bent and flattened by quartz veins during post-mineralization tectonism so that a small zone of wall rock immediately surrounding the vein conforms in both strike and dip. The actual divergence of the veins from the attitude of the wall rock is 15 to 20 degrees in dip and 5 to 10 degrees in strike. The divergence is greater in central Amador county where the Mariposa for mation dips westward (Storms, 1900).
Because the veins have a different attitude than that of the enclosing rocks, veins which occur in slate at an outcrop may be found in greenstone at depth. As the vein passes from one wall rock type to another, it is bent or refracted much the same way as light bends as it passes from one medium to another. By observing the refraction of the quartz veins, Knopf (1929) was able to assign indices of refraction to several types of rock. If slate is assigned an index of refraction of 1.0 , greenstone is 39 found to have an index ranging from 1.4 to 1.6. There fore, when a vein passes from slate into greenstone the vein will be bent toward the normal; that is, the dip will increase. The amount of refraction.can be calculated by applying Snell's law:
sin 8 % n = ------. sin 82
The quartz veins of the Mother Lode and West belts are found in fissures associated with the Melones and Bear
Mountains faults. Both reverse and dip-slip displacement have occurred along these fissures, but the magnitude of the displacement is much less than the assumed displacement along the Melones or Bear Mountains faults. The largest dip-slip motion along a fissure, measured at the Fremont mine, was 375 feet, while the largest strike slip dis placement, measured at the Central Eureka mine (figure 7), was 120 feet (Knopf, 1929). The orebodies within these fissures must postdate significant movement along the major fault or the fissure; otherwise the veins would have been broken by the movement into a series of horses or boudins. Displacement before or during emplacement of the orebodies, however,' buckled and kinked the slates near the veins. Greenstones near the veins have been shattered and traversed by quartz veinlets. 40
Quartz veins in granodiorite are common in the East
belt and are somewhat different from those in the Mother
Lode belt. These veins are found in fissures in the grano
diorite. These fissures can be confused with joints caused
by the release of residual stress, especially since the
joints can also contian quartz (Logan, 1923). The veins
can be very persistent considering their narrowness. The
Sawyer vein, which averages under two feet in thickness, is
traceable for more than a mile. Unlike the veins of the
Mother Lode, the veins in granodiorite have no uniform
orientation, and the dip is no more uniform.
The fissures are often planes of movement, as shown
by the slickensides and gouge seams (Storms, 1900) . The
fissures contain crushed granitic material which has been
altered so that the feldspars have been kaolinized and the
entire mass softened. However, the solid wall rock is not
generally affected by alteration because of the small size
of the quartz veins.
Vein Filling
The great majority of the veins found in the Mother
Lode are composed of coarse, milk white quartz. The veins
are lenticular bodies which end abruptly. Beyond the edge
of the lenses the. orebodies fray out into stringer lodes.
The. stringers are often found in both the hangingwall and
footwall of the vein. The quartz filling is normally 41
banded, particularly in Amador County, because of inter
calations of black slate or dark schist. : These bands range
in thickness from a few inches to paper thin (Knopf, 1929).
The bands, or ribbons, are parallel to the attitude of the
vein and were probably caused by successive openings of
the vein. The bands are so common in certain parts of the
Mother Lode that they have been given a special name:
ribbon quartz.
Wall rock inclusions, generally of slate and green
stone, are common. The inclusions are angular and contain
anomalous amounts of pyrite and arsenopyrite. Also, they
are surrounded by a fringe of ankerite. Inclusions may be
so abundant that the vein becomes angular rubble cemented
by quartz (Knopf, 1929).
The dominant vein mineral is quartz, followed by
ankerite • and alb.ite. , Sulfides comprise one to two per
cent, with pyrite predominating. Arsenopyrite is next in ■
abundance, followed by sphalerite, galena, chalcopyrite,
tetrahedite, gold, and petzite. . No size analysis of the
gold has been performed, but gold commonly occurs in easily
visible particles and small masses even though the majority
is microscopic. Gold may be found alone embedded in quartz
or it may be intergrown with pyrite, arsenopyrite and,
rarely, tellur ides (Knopf, 1929). 42
In the granodiorite veins of the East belt, quartz again is the most abundant, but it is of a bluish color and of greasy appearance (Logan, 1923). Sulfides are more abundant than in the Mother Lode veins, with concen trations up to 20 percent. Again pyrite dominates, with pyrrhotite, which is not found in Mother Lode veins, second in abundance, followed by galena, arsenopyrite, and Chalcopyrite.
Gouge
Quartz veins of the Mother Lode belt are usually accompanied by large amounts of black gouge, especially in
Amador County. The gouge is composed of crushed slate and sometimes contains fragments of weathered quartz. The gouge can be found on either side of the vein or on both sides; it can also pass through the vein (Knopf, 1929).
The gouge pinches and swells from a minimum thickness of a few inches up to a maximum of eight feet, with an average thickness of two to three feet. When gouge occurs.on both sides of the vein the quartz is completely crushed.
Slates found adjacent to the gouge are buckled, faulted, and polished (Knopf, 1929). The gouge causes swelling ground which proved to be a problem for miners in this region. Drifts had to be retimbered every few weeks or they were.lost. Because of the unstable ground caused by the gouge, most mines are inaccessible today. Ore Shoots
Gold is not uniformly distributed within veins; rather, the majority of the gold is concentrated in ore shoots, which constitute a small segment of the total vein length. Typically a vein with a 2,000-foot, strike length may contain a 200-foot-long ore shoot. As a rule the ore shoots are thicker than the unproductive parts of the vein, indicating that the ore shoots formed primarily in open zones caused by movement along curved or irregular fault surfaces. The open space was an area of low chemical potential, which made precipitation of gold more likely
(Boyle, 1979) . The grade within the ore shoot is not consistent with depth, so the shoot may become barren at depth. Some of these barren zones have 300- to 400-foot vertical lengths, and in some of the deeper mines two or three of these zones have been encountered. These barren zones ended the production in many smaller mines. The depth of occurrence of the barren zones is unrelated to the depth from outcrop or the depth from any arbitrary datum. Therefore, the depth at which a barren zone may be encountered cannot be predicted.
The quartz in the shoots can be massive, or it may occur in ribbons. In some mines ribbon quartz appears to be an indication of high-quality ore, but in most mines ribbons extend beyond the boundaries.of the ore shoots. 44
Occasionally, wide zones of barren ribbon quartz are found, as at the Central Eureka mine, Therefore, although ribbon quartz appears to indicate a higher possibility of an ore shoot occurring, no general rules can be made concerning the type of quartz found in a shoot and its grade.
Mineralized Greenstone
Mineralized greenstone, or grey ore (Prichard,
1.904) , is greenstone which has a grey color caused by ankerite alteration. The main constituents of the altered rock are fine-grained ankerite, sericite, albite, and quartz, along with three to four percent pyrite and arsenopyrite. The grey ore is crossed by many quartz veinlets which contain albite and anker ite. The structure of the grey ore is inherited from the parent greenstone so that the texture of the mineralized rock may be massive to schistose.
Grey ore occurs in shoots adjacent to quartz veins in the hanging wall or the footwall of the vein, but never in contact with the vein (Knopf, 1929). The boundaries of the mineralized greenstone orebody are difficult to determine by sight, so the boundaries are usually assay boundaries. The grey tone of these bodies is due to the amount of ankerite present. The center of the body is darkest in color and there is a gradual lightening toward 45
the edges. The size of a grey ore body is not related to
the size of the associated quartz vein.
The grey ore was formed by a replacement process in
which silica in the greenstone was replaced by carbon
dioxide. Along with carbon dioxide, sulphur and arsenic
were added to form pyrite and arsenopyrite in the grey ore.
Mineralized Schists
Mineralized schists are similar to grey ore in that
the schists have also been altered by ankerite, but the
mineralized schists have a more diverse character than the
grey ores because of the diverse nature of the parent
schists. The degree of alteration found in the schists can
be extremely variable. Schists which contain little or
no carbon can be altered as strongly as the greenstones,
while graphitic schists are weakly affected. The mine
ralized schists are low in grade, approximately one-tenth
oz of gold per ton, with high amounts of sulfides, three
to six percent. Pyrite is found in disseminated cubes
which are up to one-half inch long on each edge. Stringers
occur, but they are rarely thicker than a few inches.
Quartz and ankerite veinlets contain coarse white quartz
with minor ankerite, while those of less than one-half
inch are entirely ankerite (Knopf, 1929). Accessory
minerals include pyrite, sericite, quartz, albite, and
■rutile. 46
Alteration 1
By far the most common type of alteration occurring
in the Mother Lode region is ankeritization of the wall rocks. The amount and degree of alteration depend, on the wall rock, but all types are affected, including those containing high amounts of carbon: black carbonaceous
slate and graphitic schists. The slates of the Mariposa
formation are the least altered. The alteration appears as small lenticular grains of carbonate which cause small depressions or pits on the weathered cleavage surfaces of
the slates (Knopf, 1929). Slates have been altered in this manner up to 10 feet away from the vein. On the other hand, the greenstones and green schists have been
intensely altered. Near the veins the greenstones and
schists are solid ankerite rocks. The original texture of
these rocks is usually well preserved despite the intense
alteration (Knopf, 1929). Sericite is present in some of
the altered rocks, producing ankerite-sericite rocks.
Serpentinite rarely occurs near the veins of the northern
Mother Lode, but when it does, as in the Carson Hill mines
of southern Calaveras County, it is intensely altered to a
green-colored ankerite-mariposite (green, chromiferous
sericite) rock. Oxidation of the altered serpentinite
produces a limonitic gossan. . 47
The alteration was caused by quartz-deficient solutions which•removed silica from the wall rocks and added carbon dioxide, sulphur, potassium, and arsenic.
The amount of carbon dioxide added to the wall rock was dependent on its iron, magnesium, and calcium content. The amount of quartz found in the veins is far less than the amount removed from the wall rock.
Origin of the Gold Orebodies
The Mother Lode veins are found in fissures which dip less steeply than the wall rock. The fissures were opened intermittently, as shown by the ribbon structure of the veins (Ransome, 1900). Movement along the fissures caused the formation of cavities. Subsidence and subse quent jointing occurred in an attempt to fill the voids left by the faulting. The joints provided a location for the formation of stringers. Classical theory maintains that the ore deposits were formed by hot ascending fluids which carried gold, zinc, lead, potassium, sulphur, arsenic, and carbon dioxide in solution. The carbon dioxide replaced large amounts of silica in the wall rock.
Part of the silica precipitated in the vein channel while the rest was carried in solution. The carbon dioxide caused the formation of ankerite in the wall rock, while potassium caused the formation of sericite. The fluids which carried the gold in solution have been, described as 48 probably meteoric waters which obtained the carbon dioxide, sulphur, arsenic, and gold along with heat from an exha lation from an unknown deep-seated consolidating magma
(Knopf, 1929).
Current ideas, however, discount the possibility of an igneous source because the ore bodies are fairly uniform in strike, grade, and thickness along the entire length of the Mother Lode system. According to Worthington
(1978, p. 1), the ore bodies were formed by metamorphic waters which "remobilized the gold from pre-existing deposits or even from weakly auriferous peridotites and deposited it in favorable structural loci along, the Mother
Lode fault system." CHAPTER 5
METALLOGENIC STUDY
This chapter identifies and examines the charac
teristics of each area along the East, West, and Mother
Lode belts. Utilizing the analysis of each area, I will
identify and analyze the changes occurring within each belt. Of particular interest are the grade and thickness
of the orebody, the amount and type of accessory minerals,
and structural information. After each belt is analyzed,
the belts will be compared with each other to determine what relationships exist between the belts. Finally, the
Mother Lode region as a whole will be compared to other
Mesozoic gold quartz vein deposits.
The West Belt
The orebodies of the West belt can easily be
separated into two areas, northern and southern, on the
basis of location and ore deposit geology. The northern
area is more complex and contains the Shingle Springs,
Deer Valley, and Rattlesnake Bar districts of El Dorado
County along with the Ophir and Penryn districts of Placer
County (figure 8). The two areas are separated by a
distance of approximately 40 miles. The region between the
49 . 50
GEORGETOWN
RATTLESNAKE \ BAR
DEER valley
EL DORADO
SHINGLE Northern SPRINGS A rea
JACKSON - PLYMOUTH
.A NGELS CAM P
38
EXPLANATION
Vein System hooscn Gold District •Plymoutn T O U T . s c a l e t— - — >— — ------1------1______I 0 >0 20
Figure 8. Vein Systems and Approximate District Locations. After Clark (1969). ' . ' ■ . 5 1 two areas contains a few very small deposits, but for practical purposes this region is considered barren.
Southern Area
The deposits of the southern area in the West belt are found approximately 10 miles west of Angels Camp within the northwest-trending Hodson fault zone. The Hodson district, which.is associated with the fault zone, extends beyond the southern boundary of the study area, so the district is much larger than it appears in figure 8. The
Hodson fault zone, an extension of the Bear Mountains fault zone, is near the contact between the Salt Springs ■ ' ■ . ■ ' „ ') slate and a belt of the Logtown Ridge formation, so slate. is found to the west of the area and massive greenstones and amphibolite schists lie to the east. The central portion of the area has been intruded by several serpen- tinized bodies which have been altered to mariposite- ankerite rock. .. The occurrence of serpentinite is so per vasive that all deposits within the district are associ ated with serpentinite or mariposite-ankerite rock.
Both vein and mineralized wall rock type orebodies are found within the area, but the mineralized wall rock deposits are more common. The mineralized wall rock ore bodies can be quite large, as demonstrated by the Royal
Mine orebody, which is 4,000 feet long, 500 feet wide, and mined to a depth of 3,000 feet (Clark and Lyndon, 1962). 52
Although not as large, the average thickness being 40 . feet, most of the deposits are similar in other ways to that at the Royal Mine. They are large, low-grade (1/7-
1/5 ounce per ton) deposits. Sulfides constitute one to two percent of the ore, with pytite by far the most abundant. No rare accessory minerals are found. When gold occurs in vein type deposits, most of the gold is found in stringers rather than in the vein itself. The veins and . adjacent mineralized wall rock strike northwest and dip steeply to the northeast.
Northern Area
The deposits of the Northern area occur near and within granodiorite and diorite intrusions found in north western El Dorado and southeastern Placer counties. The deposits are found in a four- to six-mile-wide belt which begins in the south at the town of Shingle Springs and
trends northward -for. seven miles, forming the Shingle
Springs district. To the north the belt abruptly changes direction so that it trends northwest through a diorite
intrusion, the Deer Valley district,and a section of the
Smartville block, the Rattlesnake Bar district. The belt continues its, northwest trend and enters a granodiorite
intrusion where it appears to be offset to the north to
form the Ophir district. The Penryn district is found within the same granodiorite body as a north-trending belt south of Ophir.
The deposits of the Shingle Springs district are found in green schist, greenstone, and slate of the Log- town Ridge formation, which has been intruded by serpen- tinized bodies of various sizes. The serpentinite has subsequently been altered to mariposite-ankerlte rock. The deposits are associated with the Bear Mountains fault zone and are primarily large, low-grade (1/7 to 1/5 ounce per ton) bodies of mineralized wall rock containing numerous quartz veinlets and stringers. The deposits are normally
15 feet.thick but can.range up to 60 feet, as at the Big
Canyon Mine (Clark and Carlson, 1956). Gold is most commonly found in association with disseminated pyrite in both the wall rock and the vein. Other sulfides found in the district include galena, pyrrhotite, arsenopyrite and chalcopyrite. The veins strike north, but there is a variance between north-northwest and north-northeast. Dip is moderate (40o-60°) and towards the east (figure 7).
The Deer Valley and Rattlesnake Bar districts are found within the Deer Valley extension of the Bear Moun tains fault, zone. The deposits are shallow, narrow, moderate grade (1/4 ounce per ton), vein type deposits which have produced little gold. However, these districts are unique in that the veins are en echelon to the trend of the belt. The Deer Valley fault zone trends northwest, while the veins strike northeast and dip moderately , (40°-
60°) southwest. Sulfides, including pyrite, galena, and chalcopyrite, constitute 10 percent of the ore, with pyrite by far the most abundant..
The Ophir and Penryn districts are found within a granodiorite intrusion and surrounding amphibolites and consist of a large number of narrow, moderate- to high- grade (1/4 to 1 ounce per ton) veins. The veins of the
Ophir district strike west-northwest to west-southwest and dip to the south. These veins are more persistent at depth than other veins found in granodiorite and have been mined to depths of 1,000 febt. Although most of the deposits are small, this district has had relatively high production because of the number of veins and their grade.
Sulfides constitute 10 percent of the ore, with pyrite and galena most abundant. The deposits also contain rare accessory minerals including stibnite and tellur ides. ■ The
Penryn district is almost identical except that the veins strike north and have a lower grade (1/4 to 1/2 ounce per ton) .
Characterization of West Belt Deposits
The deposits of the West belt occur in essentially four litholigic environments, all of which are structurally related by the Bear Mountains fault zone. The deposits of 55
the Hodson and Shingle Springs subareas are in regions
where serpentinite intrudes the fault zone and the fault
zone splays out into numerous smaller faults. Both of
these factors appear to have been conducive to the depo
sition of gold orebodies. The splaying of the fault
caused the formation of large fractured zones and therefore
provided porous rocks. Precipitation of gold is considered
likely in porous rock since the rock provides large sur
face areas for chemical reaction with migrating solutions.
Also, the serpentinite country rock in these localities
is chemically very favorable for precipitation of gold.
When mineralizing solutions containing H2O, CC^, S, and As
react with serpentinite, "extensive chemical changes take
place resulting in the binding of Fe, Ga, Mg, and Mn in
the rocks by CC^, S, and As to form carbonates,
pyrite, and arsenopyrite" (Boyle, 1979, pp. 409-410). As
this reaction occurs in the rock, the sulfide-arsenide
gold complexes are broken down and gold is precipitated in
pyrite, arsenopyrite, or as native gold.
Between the Hodson and Shingle Springs districts
the West Belt is virtually barren even though the Bear
Mountains fault and an associated belt of serpentinite are
found along the entire length of the barren zone. The
barren zone may be due to the discrete nature of the
fault in this region. Because the fault is discrete, the 55 associated serpentinite is not fractured as extensively as the serpentinite in the Hodson and Shingle Springs dis tricts. Thus, the mineralizing solutions were unable to affect large volumes of country rock in this region.
The deposits of the Deer Valley and Rattlesnake Bar districts are associated with two types of litholigic environments, diorite and greenstone, and both districts are associated with an extension of the Bear Mountains fault. The number and productivity of the deposits in these districts is independent of lithology, as both - districts contain small veins en echelon to the major structural trend. The veins are in tension fractures within the greenstone and diorite and are similar to those in the Beresovsk gold belt of the USSR (Boyle, 1979). In these deposits, as in the Deer Valley and Rattlesnake Bar districts, structural features apparently were much more important than lithology for the localization of orebodies.
The deposits of the Ophir and Penryn districts occur within a fractured granodiorite pluton and in sur rounding amphibolites. The fractured granodior ite is a favorable site for gold deposition because it contains large amounts of pore, space, as has been discussed.
However, the deposits in the amphibolite are not solely dependent on structural factors such as fracturing, because the amphibolite is highly pyritiferous. Pyritiferous 56 formations have shown a marked ability to precipitate gold within the pyrite or as the native element (Boyle, 1979).
The serpentinite lighologic environment exerts a much different effect on the mineralizing solutions than do the rocks of the other lithologic environments, as evi denced by the large difference in the deposits found in the serpentinite environment. The gold deposits found in altered serpentinite commonly occur in 20- to 35-foot-wide zones of mineralized rock containing numerous quartz veinlets. These deposits are of low grade (1/5-1/7 ounce per ton) and are low in sulfides (1-2%). The West belt deposits which are not associated with serpentinite occur in discrete quartz veins and are much narrower, rarely exceeding 10 feet, higher in grade (1/2 ounce per ton), and higher in sulfide content (up to 10%).
The East Belt
The deposits of the East belt can be divided into three areas which conform to the gross features and the distribution of the vein Systems within the belt. The
Southern area contains the deposits of southern and central
Calaveras County and northern Tuolumne County. The Central area includes the deposits of Northern Calaveras and Amador counties and the Northern area includes the deposits of
El Dorado County. The deposits within these areas are variable, so it is possible to subdivide the areas into 57 smaller districts. The need for subdivision becomes obvious when the number of vein systems in each area is observed. According to Clark (1969), the Central area contains seven north-trending vein systems which occupy a 20-mile-wide band. The Southern area contains two vein systems, and the Northern area contains one (figure 8).
Southern Area
The vein systems within the Southern area terminate and change direction often so that districts can be iden tified along different segments of the vein systems. The southernmost vein system begins in Tuolumne County near
American Camp Station with the north-northwest-trending
American Camp district. The vein system abruptly termi nates and veers to the west five miles east of Murphys, forming the Murphys district. The northern vein system trends southeast, with some variation, from Jesus Maria in the north through Mountain Ranch to a point six miles southeast of Mountain Ranch. The Sheep Ranch and Mountain
Ranch districts are found along this vein system.
The districts found along the southern vein system are underlain by a phase of the Calaveras formation which contains a large number of recrystallized limestone and dolomite lenses in addition to the graphitic and green schists, slate, and quartzite normally associated with the
Calaveras formation. The geology of the western portion of 58 this vein system is very unusual because the beds of the
Calaveras formation and then enclosed quartz veins strike west rather than north-northwest. The major structures in this area also have a westerly trend, as evidenced by a shear zone. The deposits along this system are small and shallow, but they are numerous, so the production from this system is fairly large. There are numerous auriferous ■ quartz veins, averaging eight feet in thickness, which contain white, rose, and occasionally black-colored quartz along with varying amounts of pyrite, galena, chalcopyrite stibnite, uranite, cinnabar and tellur ides.
Fine-grained diorite dikes are common and are often associated with the veins. '
The northern vein system has far fewer deposits than the southern system, and the trend of the system is not well defined because the deposits are scattered.
However, the production of the northern system is larger because the mines are much larger. The most productive mine in the entire East belt, the Sheep Ranch mine, is found in this vein system. The Northern vein system is underlain by slate, impure quartzite, and graphitic schists of the Calaveras formation, with gabbroic stocks common.
Diorite, in the form of dikes, is often associated with the quartz veins. The veins range from one to five feet in thickness, and contain between 1/3 and 1 ounce gold per ton. 58
Quartz, the major constituent of the veins, ranges from white to smoky grey in color. Sulfides are abundant, 10 percent at the Washington mine, with pyrite, galena,
chalcopyrite, and arsenopyrite common. Cobalt minerals
are found at the Mar John mine.
Central Area
The Central area geology is much more complicated
than the Southern area because it contains seven vein
systems (figure 8). Each vein system is separated by more
than a mile., so neighboring veins may occur in totally
different geologic environments. The veins can be divided
into two.general classes: veins found in granodiorite or
at granodiorite contacts and veins found within the
Calaveras formation.
The deposits occurring in granodiorite are most
numerous in the vicinity of the town of West Point, where
three major vein systems are associated with a west
elongated body of granodiorite which is 5 miles wide and
15 miles long. The quartz veins are found in fault-related
fissures in the granodiorite,. The veins have a general
northerly trend, but; the variance of the trend is large.
The veins are high grade, averaging 1/2 to 1 ounce per
ton, with higher grades at fault-vein intersections. The
highest.reported grade, 2.7 ounces per ton, occurred at 59 such an intersection in the Black Prince mine. Other indi cators of high-grade ore include the presence of anoma lously high amounts of galena and the presence of diorite dikes. Sulfides are abundant and generally auriferous, with 30 percent of the gold found in sulfides. Both the strike and dip of the veins are extremely variable.
: The deposits which are not associated with grano- diorite are found in the vicinity of the towns of Railroad
Flat and Pine Grove. These areas are underlain by graphitic schist, slate, and quartzite of the Calaveras
formation. Grsnodiorite bodies are found near all of these towns. The deposits occur within shear and small fault zones caused by the intrusions or near the intrusive con tact. These deposits are similar to those found in grano- diorite. The attitude of the veins is erratic. The strike varies from westerly to northerly, with the dip normally greater than 60° (figure 7). The veins are narrow, averaging between one and three feet in width, and are of high grade near the surface, but the grade decreases
rapidly with depth. These factors have prevented any one
single deposit from becoming highly productive, but the
large number of mines within each of the vein systems makes
the overall district productive. The veins contain white
to dark grey quartz and varying amounts of sulfides which
include pyrite, galena, chalcopyrite, arsenopyrite, and ■ , . ' 60
stibnite. Dark and buff-colored, fine-grained diorite dikes are associated with the veins.
Northern Area '
The deposits of the Northern area occur in both granodiorite and slate, phyllite and mica schist of the
Calaveras formation. The Calaveras veins are also asso-
'i elated with the granodiorite since the veins occur in shear or fault zones caused by the intrusion or near grano diorite contacts. This district does not contain many mines, and the veins that do occur are usually narrow, one to three feet, and shallow. However, one large de posit with a five- to eight-foot-wide vein, the Hazel
Creek mine, has produced enough gold to make this area as productive as the Central area. Sulfides are abundant and
include pyrite, galena, chalcopyrite, arsenopyrite, and
stibnite.
Characterization of East Belt Deposits
The East belt does not form a linear belt from north to south; instead, the deposits are scattered over
a wide band 5 to 20 miles east of the Mother Lode belt.
Because the deposits are so widely spaced, they are found
in different geologic environments. However, the deposits
of the East belt are remarkably similar, indicating that
the diverse geologic environment does not greatly affect 61 the individual deposits. The grade, averaging 3/4 ounce per ton, the vein thickness, one to three feet, and the nature and amount of accessory minerals are consistent from north to south.
The areas of the East belt show an interesting pattern. Those areas with large numbers of deposits do not have any large mines. Examples include the Murphys and
West Point districts. These districts contain up to 10 mines per square mile, yet no mine in either district had an output greater than $500,000 (approximately 24,000 troy ounces). On the other hand, localities with few deposits, such as the Sheep Ranch district and the Northern area have one large mine; therefore, the production of these localities equals or exceeds that of the high-density areas.
The Sheep Ranch district is the most productive along the entire East belt, yet only 15 deposits are found over its
15-mile linear extent. The largest mine in this district, the Sheep Ranch mine, has had an output of $7 million
(approximately 3.30,000 troy ounces).
The deposits of the East belt are more or less independent of the country rock, indicating a similar structural control throughout. The lithological control on these deposits is minimal since the country rock, with the exception of the graphitic schists of the Calaveras for mation, are not favorable for the precipitation of gold. 62
The deposits are localized by fracturing of the country
rock, which provides large surface areas for precipi
tation. In the southern area this fracturing was caused by
a west- to northwest-trending shear zone. The deposits of
this area are generally aligned with the trend of this structure. In the northern and central areas the country rock was extensively fractured by the emplacement of the
-smal1 granodiorite plutons. The fracturing Caused by
these plutons does not have any strong orientation,
although a weak northerly trend is present. These areas have been interpreted by Clark (1969) as forming discrete north-south veins (figure 8). Strong structural evidence does not exist, however, for proposing north-south-
trending belts or vein systems through the northern and
central areas because of the somewhat random distribution
of vein orientations (figure 7).
The East belt is actually two separate "belts,"
since the deposits of the southern area are not struc
turally related to the northern and central areas. This •
fact makes the East belt different from the West and
Mother Lode belts in which all the deposits are struc
turally related by a major:fault.
The Mother Lode Belt
The Mother Lode belt can be divided into four
areas: the Northern area, which includes the deposits of 63
El Dorado and northernmost Amador counties, Jackson-
Plymouth, which includes the deposits of central Amador
County, San Andreas, which includes deposits of northern
and central Calaveras County, and Angels Camp, which
includes the deposits of southern Calaveras County. These areas form a continuously mineralized belt from Georgetown
in the north to Angels Camp in the south.
Northern Area
The deposits of the Northern area are found one mile west of the Melones fault zone in eastern-dipping
fissures enclosed by a belt of Mariposa slate which bifurcates three miles south of Georgetown, forming two
parallel belts of mineralized Mariposa slate north of
Georgetown. The deposits within the eastern slate belt .
and the surrounding Calaveras formation form the George
town district. This district marks the northernmost
extent of the Mother Lode. The orebodies occur as 12-
to 15-foot-wide zones of mineralized green schist and slate
with a minimum width of 50 feet. Numerous quartz veins -
and veinlets are associated with the deposits. A number
of faults and basic dikes are found in the areas. Where
faults intersect the veins, high-grade ore shoots can be
expected. The strike of the veins varies from northeast
to northwest. The dip is greater than 60° to the east. 64
Directly west of the Georgetown district is the
Greenwood district. It is very similar to the Georgetown
district because the deposits are also found within the
Mariposa and Calaveras formations. Serpentinite, however,
is present in many mines in this district. In fact, the
largest mine in the district occurs within a fault contact . between Mariposa slate and serpentine. The mineralized
zones average 15 feet in width and contain 1/5 to 1/2
ounce per ton gold. Sulfides constitute approximately
two percent of the deposit, with pyrite predominating..
Minor galena and chalcopyrite are found with rare arseno-
pyrite.
South of Georgetown in central El Dorado County
the Placerville district is found. Northwest-striking
quartz veins occur in a belt of Mariposa slate which is
flanked on the west by greenstone- and green schist of the
Logtown Ridge formation along with granodiorite and on the
east by mica schist and slate of the Calaveras formation.
The veins are less than 20 feet thick and contain numerous
quartz stringers. The deposits are low to moderate in
grade, with anomalously high sulfides. In some deposits,
shoots containing up to 10 percent copper are found. The most important sulfides are pyrite, galena, chalcopyrite,
and aresonpyrite. Galena is an indicator of high-grade
ore. 65
South of Placerville the Mother Lode bends to the
southwest to form the El Dorado district. This district
is very similar to the Placerville district. The veins, are somewhat narrower, ranging from 5 to 10 feet, but generally there are no large changes. This district is
important because it shows that the underlying structure of the Mother Lode region has a greater influence over the attitude of the veins than the enclosing country rock. In this district, the strike of the Mariposa slate which encloses the veins changes from north to northeast, but the . veins continue to strike northwest to north, reflecting the underlying structure of the region.
Near the El Dorado-Amador county boundary the belt of Mariposa slate which contains the Mother Lode returns
to a northerly trend. Deposits in this area constitute
the Nashville district. This district contains mines which
are larger and more productive than those found to the
north. Gold occurs in several long north-striking massive
quartz veins which average 15 feet in thickness and range
up to 25 feet. The deposits of this district are also more
persistent at depth and have been mined to depths of 2,000
feet. Sulfides are present in moderate amounts near the
surface,. but the "amount increases with depth to approxi mately 4 percent. Pyrite is most abundant, followed by
chalcopyrite and sphalerite. Gold is found in low to 66 moderate grades (1/4 to 1/2 ounce per ton) with higher
grades found at intersections of veins and contacts or
faults, abrupt changes in the attitude of the vein, and
points where the vein swells abruptly. Fault gouge is
abundant.
Jackson-Plymouth
The Jackson-Plymouth district is located just south
of the Nashville district on the same vein system.■ This
district is the most productive in the Mother Lode region
and the second most productive in the state of California.
Production totaled approximately 180 million dollars from
1855 until production ceased in 1953 (Clark, 1969). Also,
this was one of the more important districts in the United
States from 1890 until 1942.
The deposits in. the district occur within a 20-
mile-long mineralized quartz vein system which stretches
from two miles north of Plymouth to the Amador-Calaveras
county boundary. The vein system is located one mile west
of the Melones fault zone and is enclosed by a north- to
northwest-trending belt of Mariposa slate which is occas-
sionally interbedded with a coarse conglomerate. The belt
of slate is flanked on the west by massive greenstone of
the Logtown Ridge formation and on the east by schist, •
slate, and metachert of the Calaveras formation. Slate
outcrops along the entire length of the district except 67 three miles north of Jackson, where it is covered by
Tertiary andesites.
The deposits of this district are very uniform.
Gold occurs in sheared quartz veins which are enclosed by slate at the surface. However, at depth the vein may be enclosed by greenstone because the veins dip less steeply than the strata. The veins are found in fissures which are the result of reverse faulting. Because of the faulting, thick zones of black fault gouge are associated with the veins. The veins are composed of massive or ribbon quartz and average 10 to 50 feet thick, with a maximum of 200 feet. Pinch and. swell structures are common.
Associated with the veins are numerous stringers which contain a higher amount of sulfides than the veins.
Sulfides are a minor constituent of the veins, constituting one to two percent. Pyrite, galena, and arsenopyrite are the most common. Grade is low to moderate (1/7 to 1/3 ounce per ton), with galena and in some mines ribbon quartz indicating higher grade ore.
San Andreas
The San Andreas district marks the departure of the Mother Lode belt from the Mariposa slate. The quartz- bearing fissures in this district and those to the south are found within the Melones fault zone rather than one mile to the west of the fault zone, as is the case in the 68 northern districts. In this district the veins are enclosed by amphi.bolite schist of the Me lone s fault zone and occa sionally greenstone of the Logtown Ridge formation. Tine deposits in this district consist of northwest-trending quartz veins and stringers occurring primarily in schist.
This district is anomalous in that no large bodies of mineralized schist or greenstone are associated with the veins as is common in other areas.
The output of this district is much less than the neighboring Jackson-Plymouth district. The veins are slightly lower in grade and they are definitely less persistent. These factors indicate that either the lithology or structural setting of this area is much less conducive to vein type gold mineralization than the structural setting and lithology of. areas to the north.
The veins of this district contain the same accessory minerals as other districts in the Mother Lode, and in addition tellurium and uranium minerals are found.
Angels Camp
The Angels Camp district includes the deposits in and around the town of Angels Camp plus the mines on
Carson Hill. The Angels. Camp area is underlain by a
series of northwest-striking beds of amphibolite schist, phyllite, greenstone, and metagabbro. Much of the green
stone was formerly volcanic breccia. The orebodies, which 69 consist of massive quartz veins', zones of parallel quartz stringers, and mineralized greenstone and schist, are found in three parallel northwest-striking vein systems. Each of the vein systems is related to a fault zone. The gold in this district is not contained within the veins, so the average grade is low (1/7 to 1/5 ounce per ton). Large areas are mineralized, however, so that the deposits in this district are large producers. A typical orebody is
80 feet wide, 500 feet long and extends 2,000 feet in depth. This district contains some of the more productive mines in the Mother Lode, and overall it is second only to the Jackson-Plymouth district in production.
Carson Hill is found three miles southeast of
Angels Camp on the same fault system. The deposits on
Carson Hill are similar to those near Angels Camp, in size and output; however, instead of mineralized greenstone and schist orebodies, the deposits on Carson Hill are asso ciated with serpentinite. The serpentinite has been hydrothermally altered to mariposite-ankerite rock which contains numerous thin quartz seams and veinlets.
Characterization of the Mother Lode Deposits
Three general types of orebodies occur within the
Mother Lode vein system. The northern and central Mother - ‘ . i Lode orebodies are generally quartz veins enclosed by 70
slates of the Mariposa formation. The orebodies near,San
Andreas are quartz veins in greenstone and amphibolite
schist and the orebodies of the southern Mother Lode, which
are large bodies of altered greenstone, schist, and
serpentinite. All three types of orebodies are associ ated with the Melones fault zone.
The most productive orebodies are the quartz veins enclosed by Mariposa slate; however, the slates are not equally mineralized. The deposits of the central
Mother Lode are much more productive than the deposits to
the north. One difference between the veins of the
Jackson-Plymouth district and the veins to the north is
the large amount of ribbon quartz found in the Jackson-
Plymouth deposits but not in the north. . Another difference
is the amount of fault gouge associated with the Jackson-
Plymouth deposits. Fault gouge is not commonly found in
the northern district, while in the Jackson-Plymouth dis
trict the fault gouge may be as thick as the vein. The
ribbon structure and gouge indicate that the veins were
opened and exposed to gold-bearing solutions several times.
The difference between the deposits of the Jacksod-
Plymouth district and the Northern area is not great, and
the change between the areas is gradual. South of George
town the veins become thicker.and more persistent, the
amount of sulfides decreased, and pinch and swell 71 structures along with gouge become more abundant as the deposits become more like those in the Jackson-Plymouth district. These factors indicate that one or all of the following changes occurs gradually to the north: the mineralizing solutions were less potent, activity along the fissures decreased, some lithological factor, such as carbon content of the slates, varies.
The change between the Jackson-Plymouth and San
Andreas districts is not at all like the gradual change which occurs between the Northern and Jackson-Plymouth areas. The productivity and geological environment of the ore deposits change abruptly upon entering the San Andreas district. The decrease in productivity is due to a decrease in the number and grade of ore shoots, which may be due to changes in lithology or structural control, since both occur.upon entering the San Andreas district.
The lithologic change, from carbonaceous slates to greenstone, between the two areas has a great effect on the productivity of the deposits. Carbonaceous material such as is found in .the slates of the Mariposa formation
"has a marked reduction and precipitation effect on gold solutions" (Boyle, 1979,' p. 410), whereas greenstone, especially if it is not highly fractured, does not have a tendency to precipitate gold. Mine operators have long
recognized that the carbonaceous slate of the Mariposa 72 formation is very important for the formation of ore shoots, and published evidence substantiates their position. A correlation between the rich and poor zones of a vein dr shoot and the type of enclosing rock exists. In several mines, including the Eureka, the Kennedy, and the Argonaut, a vein passing from slate to greenstone abruptly decreases its gold content by half (Knopf, 1929).
The change in lithology is not the only factor causing the low production of the San Andreas area. The structural difference between the two areas may be just as important. Unlike the fissures of the Jackson-Plymouth and Northern areas which are found one mile west of the . -
Melones fault zone, the deposits of the San Andreas area are found within the fault zone.: If any post-ore movement occurred along the Melones fault, the veins would be broken into a series of horses or boudins which would be difficult to mine. Also, the largest deposits, in general, do not occur in large fault zones; rather, they occur in surrounding fissures. This pattern is observed in several other gold vein districts including Yellowknife, Northwest
Territories (Boyle, 1961), Timmins, Ontario (Jones, 1948),
Kalgoorlie, Western Australia (Tfavis, et al., 1971), and
Kolar, India (Narayanaswami et al. .. 1960) Considering both the structural and the lithological setting of the San
Andreas area, neither of which is conducive to the 73
formation of lode gold orebodies, it is clear why the out put of the San Andreas area is so low.
To the south of the San Andreas area anotherabrupt change in the productivity of the orebodies occurs. The country rock of the northern Angels Camp area is very similar to the country rock of the San Andreas area, yet the production of the Angels Camp area is at least 100
times greater. The reason for the difference in pro duction again involved both structural and lithological
factors. Within the Angels Camp area the Melones fault
splays into a series of smaller faults, some of which are
shown in figure 6. These faults have intensely fractured
the greenstones and schists of the area, forming an
extremely porous rock. In addition, the country rock of
the Angels Camp area contains a large amount of volcanic breccia which adds to the porosity. Porous rock is an
extremely favorable site for gold deposition, and many
rich ore shoots in other districts, including Cripple
Creek, Colorado, occur in this type of environment. The
intense fracturing in this area allowed the mineralizing
fluids to affect, a large volume of rock and form the large mineralized country rock orebodies of this area. In the
southern half of this area, near Carson Hill, serpentinite
occurs and provides an additional positive lithological
control, as in the southern West belt at Hodson. 74
.The veins of the Mother Lode belt follow the strike
of the Melones fault closely but with some small deviation over the entire Mother Lode belt except in northern El
Dorado County, where the Melones fault forks into north east- and northwest-trending branches. At this location some deviation from the trend of the fault occurs (figure
7). The dip of the veins found in Calaveras County is less than the dip of the veins found in El Dorado and
Amador counties. The lessening of the dip corresponds to
the movement of the fissure veins from one mile west of
the Melones fault zone to within the Melones fault zone
south of the Mokelumne River (figure 7).
Comparison of the Gold Belts
The three gold belts of the Mother Lode region are dissimilar in production, continuity of mineralization
along the belt, and structural relationships. The deposits
of the Mother Lode belt are structurally related to the
Melones fault, and mineralization is fairly continuous
except for the weak mineralization of the San Andreas area,
along the entire north-south length of the belt. The West
belt has the same type Of structural relationship as the
Mother Lode belt in that the deposits are associated with
the Bear Mountains fault or its extensions. In Amador
County, however,,the fault zone is barren, which destroys
the continuity of mineralization along the West belt. On 75 the other hand, the East belt is continuously mineralized, south of El Dorado County at least, but no regional structural, feature relates all of the deposits of the East belt; rather, smaller localized structures control the
East belt deposits.
Since the East belt deposits are not related by any regional structural feature as are the West and Mother
Lode belts, labeling these deposits a belt is inappropriate.
These deposits should be termed the deposits east of the
Melones fault rather than the East belt deposits, since the word "belt" implies a regional structural relationship which does not exist.
Even on a small scale the structures controlling the deposits east of the Melones fault are different from those controlling the Mother Lode or West belts. The con trast in structure can be shown by examining two districts consisting of similar lithologies that are located in different belts. The Ophir district of the West belt and the Central area of the "East belt" have similar litho logies in that the quartz vein deposits are found in the fractures of small granodiorite plutons.• There are> how ever ,. some significant differences between the two local ities because of the relationship between the Ophir district of the West belt and the Bear Mountains fault.
The veins of the Ophir district have a definite east-northwest trend, and they are confined to a narrow belt (figure 9). In comparison, the veins of the Central area do not have any definite trend although a slight northern trend is suggested (figure 10). The northern trend has a wide variance; one can also find northwestern, northeastern, or even western,trends in some locations within the Central area. The deposits of the Central area are widely spaced, so no single continuous vein system can be traced, as can be done in the Ophir district. Finally, the veins of the Ophir district are more persistent and can be followed to much greater depths than the veins of the
Central area. The differences between these portions ;of the belts can be attributed to the lack of a large con trolling structure east of the Melones fault, and they provide further evidence for discontinuing the usage of the term "East belt."
Because.the deposits east of the Melones fault zone are not related to a continuous north-south-trending feature, they cannot be compared with the deposits of the
West or Mother Lode belts. Although both the West and
Mother Lode belts are related to continuous structural features and thus qualify as belts, the individual deposits found along these two belts are, for the most part, dis similar. In the southern areas, the deposits are related to a splaying of the major fault zones,with serpentinite 77
AU 8U RN rr-GOLD V HILL
NEWCASTLE
80
EXPLANATION
. . . GronitiC rocks PENRYN .I'/
V 'I' ‘ Amphibohte
>X N Gold-quortz v<
SCALE
Figure 9. Location and Trend of Veins in the Ophir District. After Clark (1969). iue 0 Lcto ad rn o Vis n the in Central Veins of and Trend Location 10.Figure I MILES M 2 I 0 oe od mine gold Lode A N O I T A N A L P X E Soe schisf, quorfzite Slore, ; Gronodionte i
Attitude of theofquartz Attitude veins C Uto n metocnerf one SCALE BLUE LNO: 1 ’ GLENCOE: ■ .' .' ■ ra Atr lr (99, alo and Clark (1969), Carlson Clark After Area. A PE3RC SAN 15) Cak n Ldn (1962). Lydon(1954), and Clark - STATIC - . CL ICE NO.3 RINCGE iCEL: r LH/ 1 ALRHA/X blackstone PIONEER « e * */%*■* X ■ ■ « * v>> ■ LONE WILLOW WILLOW LONE ■ v>> * « ■■
YELLO l ASTE N STA 3NE
OUBS MONMOUT COLUMBUS^ AMADOR- A S T SARISAuOl GLENCOE ALEXANDER -i * * « \ ' z * t STA t» * » » * 1 »z ' \ « « ■ * « * O-i_ UN VISTA3UENA . V 7 ^ , i OOOHOUSE . SWALLOW e o N A I S S U R MILL / r e o n e HM,N>‘ 0 O b w K V v ^ A * " i ‘ CHAMP,ONX> A X HAZE. CELL . E Z A T H / I £R E N »IC w UC y k *1 *. ■ - M CONTINENTAL 11 C r a t s
lawson * / ■ ' ■ * m« * ■ / ■ ' ■ / *m« * * * U * .WILSEYVILLE ■ v CARLTON . GOE s V , , , , « ^ , * V . GROVE' s ^ . . . 7cFf V/V. . V / V f F c R7 ! s V k c a A « * X r * « m * rn X # /X * « « ■ AX s I ' V . ^ s J T =»OOT ' ;V AGL ^r" ^ L1 G EA ' - ‘ f^OR SES N ASCERSO S A ‘ a ■ / . ; e h t ■-txX- -'A>T,?IA.' -.'.AU> - . X x t >■ - T ■ l a R-0 FLAT LR0-10 MOrAW< 1 C V ^ 0 3 O T A « e. AQI ‘ e e ‘ MARQUIS e e%. ' _^Vf4 MME.RVILLE M UM n h a . - A L
y r n e h ^\ ■ ■ X^,\ . . « . * . . . Z T L £ < n e d l o o Q Q2>/'G0" l a e m
l u r
78 ■ 79 present in both areas. Large low-grade bodies of miner alized country rock are found in both areas.■ To the north, the deposits of the West and Mother Lode belts are very dissimilar, mainly because the deposits of the Mother Lode belt are found in fissures- while the deposits of the West belt still occur within the fault zone. The production of the deposits of the northern Mother Lode is much greater than the production of the deposits in the northern West belt. Thus, in Amador and El Dorado counties there is no similarity between the Mother Lode and West belts aside from their structural relationship. In Calaveras County a much stronger similarity exists between the belts.
No strong relationship exists between the location along a belt and the productivity of the deposits in that location, so location along a belt cannot be used to predict productivity. Therefore, the localization of gold within a particular belt must be strongly dependent on local, rather than regional, features. To aid in the
further study of local features or in exploration within the Mother Lode region, a list of the more important
structural and lithological controls is provided in table
1. It can be seen that the more productive deposits occur
in areas containing many of the favorable controls. < Table 1. Structural and Lithologic Controls in the Mother Lode Region
Belt Area Remarks
Relation to Pre- West All The deposits of the West and Mother Lode are associated mineral Faults with Bear Mountains and Melones faults respectively. Mother Lode All
Relation to Minor West All Deposits related to minor faults and fractures which are Fractures, Faults, associated with the Bear Mountains fault. Shear Zones West Deer Veins occur in tension fractures in diorite which are Valley en echelon to the major trend.
Mother Lode Southern Orebodies related to fractures associated with the Melones fault.
Mother Lode Jackson- Veins found in the fissures a mile west of Melones fault Plymouth zone. Ore shoots related to pinch and swell structures.
East Southern Orebodies related to an east- to northeast-trending shear zone *
East Northern, Veins found in fractures related to plutonic emplace Central ment, high-grade shoots occur at fracture or fault- fracture intersections.
Relation to West Southern, Favorable: Serpentinite Favorable and Shingle Unfavorable: Greenstone Unfavorable Springs Rocks West Ophir Favorable: Pyritiferous-amphibolite Mother Lode Southern Favorable: Fractured breccia greenstone Unfavorable: Unfractured greenstone Mother Lode Jackson- Favorable: Carbonaceous slate Plymouth Unfavorable: Greenstone, conglomerate
Relation East Southern Diorite dikes associated with the orebodies. Ore shoots to Dikes Central occasionally within or in contact with the dikes. . Comparison Between the Mother Lode. Belt and Other Mesozoic Lode Gold Belts
Nature and Distribution of Mesozoic Lode Gold Deposits
Mesozoic gold deposits are confined to the western cordillera of North and South America and Eastern
Asia (figure 11). Lode gold deposits of this age are associated with the various Mesozoic orogenies. In North
America these deposits are related to the Nevadan orogeny and are found from Alaska to Central America. The most productive of these deposits, besides the Mother Lode, are found in British Columbia, Canada. Smaller deposits are found in western South America and eastern and southeastern
Asia. Europe and Africa were largely unaffected by the
Mesozoic orogenies.
Mesozoic lode, gold deposits are found in a variety of host rocks including slate, schist, carbonate rocks, and various types of volcanic and plutonic rocks or their meta morphosed equivalents'. Nearly all of these deposits are associated with plutonic activity of some kind. Quartz is by far the dominant gangue mineral, followed in abundance by ankerite, pyrite, arsenopyrite, chalcopyrite, galena, and sphalerite (Boyle, 1979). Gold occurs as the native
81 J Exposures of Mesozoicand older rocks aflected Areas of primary gold deposits t— Areas of pi imary gold occurrences by Mesozoic orogenies (Nevadian. Palisade, and gold mines (some contain commercial placers) Cimmerian, Laramide, Columbian, Yen Shaman, etc.) (includes some rocks ullected by Cenozoic orogenies)
Figure 11. Distribution of Mesozoic Gold Deposits. After Boyle (1979). 83 element or in association with pyrite or arsenophrite. The
Au/Ag ratio is typically between five and nine.
Canadian Deposits
The Mesozoic gold quartz vein deposits of western
Canada are similar in size and structural control, although the host risk is variable. The structural controls of a typical cordilleran Canadian area, the
Barkerville area, British Columbia, are summarized below.
The Barkerville area contains the Cariboo Gold
Quartz, Island Mountain mines, and numerous other deposits.
The veins are enclosed by metamorphosed sediments, pri marily schist, argillite and quartzite, of late Precam- brian age. Small Mesozoic plutons ranging in composition from gabbro to quartz monzonite intrude the sediments. The sediments have been complexly folded into numerous minor folds which are superimposed on a major anticlinal struc ture. Numerous minor reverse, strike slip and normal faults occur throughout the area. Fracturing is extensive, with one set of fractures parallel and the other set per pendicular to the bedding. The mineralized quartz veins are essentially parallel to the bedding, and ore shoots are assumed to be controlled by the intersection of fractures and minor faults. Some lithological control is provided by a brittle argillite-quartzite which is pre ferentially fractured (Lang,. 1948) . The deposits of this 84 area average .4 ounce gold per ton and have an Au/Ag ratio of eight.
Deposits of Eastern Asia
The deposits of eastern Asia are in general less productive than those in western North America. Deposits in the USSR such as in the Stanovoy Range (Kogen, 1969) or the Alden Shield (Perelygina, 1973) are two examples.
Large Mesozoic plutons of granodioritic to granitic com position intrude Precambrian rocks of variable composition including crystalline schists and dolomites. The emplace ment of the plutons revitalized ancient faults and caused new faults and. fractures to form in the Precambrian host rocks. Gold, veins occur in steeply dipping fracture zones and have a definite alignment. The veins contain 90 to
99 percent milky white quartz plus varying amounts of ankerite, sericite, and sulfides. Galena is an indicator of high grade ore. . . "
Summary of Relationships
On a broad scale the Mesozoic lode gold districts .
Of Western Canada and Eastern Asia appear very similar to
those of the Mother Lode. The presence of a Mesozoic
pluton is almost universal. Faulting and fracturing
resulting from the emplacement of the pluton formed avenues REFERENCES
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86 87
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Xenophontes, C ., and Bond, G. C ., 1978, "Petrology, Sedimentation, and Paleogeography of the Smartville Terrane (Jurassic)--Bearing on the Genesis of the Smartvilie Ophiolite," Ln Mesozoic Paleogeography of the Western United States, Howell, D . G ., and McDougall, K. A., editors, pp. 291-302. 5144 4 122 00 3 0 00 39 00 3 9 00
Arr.e'-'can Camp Station
3 8 0 0 t2 2 "0 0 121 0 0 120 00
FIG. 7 ATTITUDE OF THE QUARTZ VEINS AND MINE LOCATIONS SACRAMENTO A.M.S. SHEET, CALIFORNIA
Scale 1:250.000
15
EXPLANATION
Strike and dip of the quartz veins MINES MENTIONED IN TEXT X dip 0 to 40 1 A r g o n a u t 7 F r e m o n t 2 Big C anyon 8 Hazel Creek dip 40° to 65 3 Black Prince 9 K e n n e d y X' dip over 65 4 Blackstone 1 0 R o y a l v e r t i c a l 5 Blazing Star 11 Sheep Ranch / J. A. Sullivan. M.S. Thesis. Geological Engineering.I980 6 Central Eureka 1.VG0 ' *9 00 39 0C
V
SUPERJACENT SIERRA BAT HOLITH AND SERIES TERTIARY VOLCANICS
m EXPLANATION
T v J v o l c a n i c s
J a U n i t A
jm Mariposa formation
Slj Jlf Logtown Ridge formation
Js Smartville Block
P J b U n i t B
Pc Calaveras formation
intrusive rocks
gr granitic rocks
diorite. gabb r o
s e rp e n t inite
c o n t a c t
122 (X) f a u l t FIG.6 GEOLOGIC MAP SACRAMENTO A.M.S SHEET, CALIFORNIA
oo 10 20 Statute Miles
JP 15 20 30 Kilometers & d
AFTER JENKINS(1965) SCHWEICKERT AND COWAN(1975).
DUFFIELD AND SHARP(1975)
J A Sullivan M S. Thesis. Geological Engineering. 1980 a 0 n\