Origin of gold in placer deposits of the Sierra Nevada Foothills, California
Christensen, Odin D., Hardrock Mineral Exploration Inc., Mancos, CO
Henry, Christopher D., Nevada Bureau of Mines and Geology, Reno, NV
Wood, Jim, Sierra Geological Services, Colfax, CA
ABSTRACT
The Sierra Nevada Foothills (SNF) region of northern California is one of the world’s
premier gold provinces. Gold is present both in bedrock lode deposits and in major
Tertiary, primarily Eocene, and Quaternary alluvial placer accumulations. Gold nuggets
in the auriferous gravels of northern California traditionally have been interpreted to be
masses of native gold released by physical weathering from underlying lode deposits
and incorporated in the stream bed load. This interpretation has numerous problems.
(1) Large gold nuggets are extremely uncommon in primary hydrothermal gold deposits,
even within the orogenic gold-quartz veins of the SNF. (2) Historic lode gold production
from the SNF has been about 35 million ounces, while Tertiary placer production
exceeded 65 million ounces with yet another 50 million ounces of placer resource
stranded by extraction restrictions. (3) A large part of the placer gold accumulation is
situated upstream and uphill of the lode deposits. (4) Tertiary channel gold averages
920 fineness, whereas gold from primary veins is 600-900 fineness.
We suggest an alternative hypothesis that much of the placer gold in the SNF Tertiary
gravels was sourced from the east, either from the great gold districts of Nevada or from
districts long ago removed by erosion.
Gold in most primary hydrothermal deposits is widely disseminated in host rock at ppmlevel concentrations within pyrite or other sulfide minerals. As demonstrated in modern
tropical environments, the transformation of gold from trace concentrations in sulfide to
grains or masses of native metal occurs primarily by supergene processes during
intense chemical weathering in the soil profile. Gold is readily dissolved, mobilized and
reprecipitated by chloride, bisulfate or organic-acid solutions, often with microbial
mediation. These supergene processes significantly increase the fineness of the
supergene gold grains relative to that of gold in the primary source.
The Paleogene climate of western North America was considerably warmer and wetter
than present, much like modern tropical environments. Tertiary river channels were
eroded into a surface of intense weathering, as evidenced by remnant laterite soil
profiles throughout northern California. The lower Ione Formation, the distal marine
equivalent of the proximal alluvial auriferous gravels, is comprised largely of kaolinite
and quartz: the products of intense chemical weathering. Plant and animal fossils in
Tertiary sedimentary rocks record a distinctly more tropical climate.
Multiple lines of evidence indicate that the Cretaceous to middle Cenozoic Sierra
Nevada formed the steeper western flank of a gradually sloping high-elevation plateau the Nevadaplano - that extended to central Nevada through the Oligocene time, and
that the gold-bearing river channels extended from the Great Valley shoreline to head in
what is now eastern Nevada.
We propose that gold nuggets, formed from by pedogenic processes during intense
weathering of disseminated Eocene and older gold deposits in Nevada during a warmer
and more humid Tertiary climate, were transported westward in great river systems
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Christensen, Henry, Wood 2015
draining the Nevadaplano, and were deposited in the Tertiary channels as stream
gradients decreased at the foot of the range. The SNF auriferous gravels combine
these distal nuggets with gold nuggets derived locally from intense weathering of the
bedrock veins.
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INTRODUCTION
Although the presence of gold in the area that became California had long been known
to Native Americans and early Spanish settlers, the discovery and promotion of placer
gold by James Marshall at Sutter’s Mill on the South Fork of the American River near
Coloma, California, in 1848 sparked the largest voluntary migration in American history.
The westward rush of Americans ensured that California became part of the United
States, and the wealth generated by the California gold rush established California as
an economic powerhouse for the next century (Walker, 2001).
The spirit of the event is wonderfully summarized by Waldemar Lindgren in his 1911
U.S. Geological Survey Professional Paper on the auriferous gravels of California.
“The peace of the wilderness was interrupted in 1849. An army of gold seekers invaded
the mountains; at first they attacked the auriferous gravels of the present streams, but
gradually the metal was traced to the old Tertiary river beds on the summits of the
ridges and to the quartz veins, the primary source of all the gold in the Sierra Nevada.
The Tertiary stream beds - the "channels," as they are called - proved rich but difficult to
mine. New methods were devised; by hydraulic mining the gravel banks were washed
down by the aid of powerful streams of water, and by drift mining the bottoms of the old
stream beds were followed by tunnels underneath the heavy volcanic covering. Millions
of dollars were annually recovered from these Tertiary channels…”
Exploitation of the extensive and rich placer deposits quickly progressed from hand
panning to a variety of sluice-boxes to massive hydraulic washing of Tertiary river
channels to drift mining paleochannels buried beneath younger Tertiary volcanic rocks.
Hydraulic washing required massive amounts of water; the surface hydrology of the
entire Sierra Nevada Foothills was quickly reconfigured by flumes, pipelines, ditches
and dams.
The massive washing of stream beds and bank deposits choked the rivers with
sediment, destroyed aquatic life, and soon threatened agriculture and communities
downriver in the Great Valley of California. Parts of the valley were aggraded by as
much as 2 meters, and a layer of sediment some 6 meters thick had accumulated in
San Francisco Bay. The process of hydraulic mining, because of its introduction of
sediment into the rivers of California, was banned in 1884 by court action - the Sawyer
Decision - putting an end to the era of hydraulic mining (Peterson et al., 1968; Yeend,
1974).
Large-scale dredging of rivers in confined ponds began in the late 1890’s. During the
1930’s, there were as many as 12 dredges working the Yuba River valley. In total,
some 1 billion cubic yards of gravel were disturbed. Elsewhere, conventional mining of
Tertiary river gravels has continued intermittently, but at a much reduced level.
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The geology of the Sierra Nevada Foothills placer gold deposits was intensively studied
during the first decades of the 20th century. The best documentation was that of
Lindgren (1911). Other important references are those of Yeend (1974) and Peterson
et al. (1968). Classic historical references to the lode gold deposits are those of Knopf
(1929), Ferguson and Gannett (1932), and Johnston (1940). Boyle (1979) provides a
good summary of both lode and placer occurrences, notably within the context of gold
deposits worldwide. More current important reviews of the gold province are those of
Landefeld (1990), Bohlke (1999a, b), and Beirlein et al. (2008).
To identify the ultimate source of the placer gold, and the geological processes by which
the gold grains and nuggets were formed, eroded, transported and deposited requires a
solid understanding of the landscape and climate of western North America during the
Tertiary time. In turn, these unique fluvial sediments can enhance our understanding of
the topography, tectonics, and climate of western North America in the early Tertiary.
GEOLOGY AND GOLD DEPOSITS OF THE SIERRA NEVADA FOOTHILLS
Lode gold deposits
The Sierra Nevada Foothills gold province is host to some 13,000 mines and prospects
that are distributed for some 300 kilometers throughout the northern and central Sierra
Nevada Foothills metamorphic belt in California (Beirlein et al., 2008; Clark, 1970;
Bohlke, 1999b) (Fig. 1). About 80-90% of the gold production, however, was
concentrated within three districts: (1) the 195 km-long by 1.5 km-wide Mother Lode
district, (2) the Alleghany district, and (3) the Grass Valley district (Goldfarb et al., 2008)
(Fig. 2).
The Sierra Nevada Foothills metamorphic belt is part of the North American Cordillera
that extends from Alaska to California and comprises a diverse collage of Paleozoic and
Mesozoic turbidite terranes, island arc complexes, and backarc basins accreted to the
North American continent. The terranes are bounded by a series of transcrustal faults
and fault zones, traceable for hundreds of kilometers (Goldfarb et al., 2008; Snow and
Scherer, 2006).
Extending along the entire length of the Sierra Nevada Foothills metamorphic belt, the
Melones and Bear Mountain fault zones are mid-Jurassic structures that subsequently
defined zones of failure between accreted terranes The Melones fault zone exhibits
first-order, province-scale, control over the location of much of the lode gold
mineralization (Beirlein et al., 2008). In the northern Sierra Nevada Foothills
metamorphic belt, the Melones fault zone comprises a 3.5 km-wide zone of serpentinite
and mélange. To the south, the fault narrows to a 1 km-wide shear of mylonite and
phyllonite (Fig. 2).
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Figure 1. Generalized geological map of California showing the distribution of principal terranes and basement rocks.
The Sierra Nevada Foothills gold province is hosted within the Sierra Nevada metamorphic belt (after Wallace, 1990).
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Christensen, Henry, Wood 2015
Figure 2. Generalized geology of the Sierra Nevada Foothills metamorphic belt showing locations of significant lode
gold deposits and major districts. Terrane boundaries are from Snow and Scherer (2006).
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Christensen, Henry, Wood 2015
Magmatism in the Sierra Nevada Foothills metamorphic belt was concentrated in two
magmatic episodes that correspond to changes in the rate and angle of subduction
between the North American and Farallon Plates. These episodes mainly pre-date and
post-date the age of gold mineralization. The first magmatic episode was in Late
Jurassic time with emplacement of isolated plutons across all tectonic terranes between
165 and 140 Ma (Goldfarb et al., 2008). The second magmatic episode, in Cretaceous
time, resulted in the emplacement of the massive Sierra Nevada Batholith, located to
the east of the Sierra Nevada Foothills gold districts, at about 120-80 Ma (Goldfarb et
al., 2008).
The major gold deposits of the Sierra Nevada Foothills are classic examples of the style
of gold deposits classified as Orogenic Gold Deposits (Goldfarb et al., 2005; Goldfarb et
al., 2008; Groves et al., 2003). Orogenic gold deposits formed as an integral part of the
evolution of subduction-related accretionary or collisional terranes in which the hostrock sequences were formed in arcs, back arcs, or accretionary prisms. Country rocks
are regionally metamorphosed into belts with extensive greenschist to lower-amphibolite
metamorphic-facies rocks. Mineralization formed synkinematically and invariably has a
strong structural control involving faults, shear zones, or zones of competency contrast.
Veins and vein zones have vertical dimensions of 1-3 kilometers with only subtle vertical
metal zonation; in contrast, strong lateral zonation is typical within wall-rock alteration.
Although lode gold deposits occur along the length of the Sierra Nevada Foothills
metamorphic belt, the northern and central portions of the belt were most productive.
Towards its northern end, the Sierra Nevada Foothills gold province is about 12 km
wide, and includes several important gold districts. By contrast, most of the significant
districts in the 200-km long Mother Lode belt to the south occur within a <4 km-wide
corridor centered on the Melones Fault Zone (Fig. 2).
Gold deposits occur within all rock types, including serpentinite, granitoids, mafic to
felsic metamorphic rocks, and turbidite metasedimentary rocks. Host rocks are variably
faulted and sheared, and range from greenschist to amphibolite metamorphic facies.
Dating of many of the foothills gold deposits has narrowed the age range of vein
formation to about 125±10 Ma (Marsh et al., 2008). An age of 152±1.2 Ma in the Grass
Valley district provides evidence of an older gold event in this one part of the gold
province (Goldfarb et al, 2008).
The gold ore bodies are of two principal types: gold-quartz veins and gold-mineralized
country rock (Knopf, 1929). The second type is more diverse as it includes auriferous
greenstone (“gray ore”) and auriferous schist.
As might be expected, the quartz veins exhibit considerable variability in dimension and
character. The veins are typically steeply-dipping tabular masses accompanied by both
hanging-wall and footwall zones of quartz stringers; quartz is the dominant mineral;
ankerite and albite occur very subordinately. Sulfide minerals vary between 1 and 2 per
cent: pyrite predominates, followed in abundance by arsenopyrite and minor sphalerite,
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Christensen, Henry, Wood 2015
galena, chalcopyrite and tetrahedrite. Alteration minerals in selvages surrounding the
quartz veins include ankerite and sericite followed in abundance by albite, quartz, pyrite
and arsenopyrite.
Within the quartz veins, gold is highly localized within ore shoots, which typically
constitute a mere fraction of the vein volume, and, even within these ore shoots, the
distribution of gold is highly irregular. The gold is mainly present as native gold of very
small size. Grade control within most operating mines was, of necessity, by assay.
Few mines had gold of sufficient size that high-grading by miners was possible or a
problem (Knopf, 1929).
“Gray ore” was the name given to ankerite-altered greenstone that carried sufficient
gold to be mined economically. The gray ore is comprised largely of fine-grained
ankerite with more or less sericite, albite and quartz, and 3 to 4 per cent pyrite and
arsenopyrite. The gray ore occurs in shoots that lie parallel to quartz fissure veins, with
gradational margins to unaltered rock. Gold grade distributions are highly irregular; gold
occurs as very fine grains of native metal.
Lode gold deposits throughout the Sierra Nevada Foothills gold province were mined to
depths of more than 3 kilometers, with little change in the character of the deposits and
no obvious decrease in resource potential at the deeper mining levels.
Of course there were some significant masses of gold discovered during mining. The
largest mass of gold ever found in California came from Carson Hill. Found in 1854, it
weighed 2340 troy ounces (72 kg). Several other nuggets weighing 6 to 7 troy pounds
(2.2-2.6 kg) were found at the same locality. Notably, these nuggets were taken from a
quartz vein, but within the zone of weathering immediately beneath surface outcrops
(Knopf, 1929). A number of other significant gold masses and rich pockets were
discovered during mining, particularly in the Alleghany District. While these were
exciting discoveries, and certainly sparked enthusiasm during the early days of the Gold
Rush, these large gold masses were exceptions to the observation that by far the
greatest amount of gold in the Sierra Nevada Foothills lode gold deposits occurs as very
small gold particles, often not visible to the unaided eye.
Placer gold deposits
The character of the Tertiary placer gold accumulations and the Tertiary river channels
are described at length by Lindgren (1911), Haley (1923), Peterson et al. (1968), Clark
(1970), and Yeend (1974). The channel-filling alluvial deposits are here called the
Tertiary “gravels” following the historical use of this term when referring to the deposits.
It should be emphasized, however, that the bulk of the alluvial sediment, particularly
from upper stratigraphic levels, is not strictly gravel but is predominantly sand, silt and
clay.
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The sedimentary regime of the SNF Tertiary auriferous gravels consists of two
components, which provide complementary evidence of the character of the fluvial
system and its contained sediment load:
• The alluvial gravels localized within paleochannels of Tertiary river systems,
representing a more proximal fluvial depositional environment; and
• the Ione Formation, fluvial-deltaic and marginal marine sediments representing a
more distal depositional environment where the Tertiary rivers discharged their
remaining sediment load into shallow marine waters.
The gold-bearing Tertiary gravels occur in paleochannels of once-great river systems
that coursed down the western slope of the Sierra Nevada range and in deltas where
these rivers met the sea that occupied the area that is now the Great Valley of
California. The Tertiary channels are cut into the Paleozoic and Mesozoic rocks of the
Sierra Nevada Foothills metamorphic belt, including the gold-quartz veins. The Tertiary
gravels were in turn covered to a great extent by younger Tertiary volcanic and
sedimentary rocks.
The Tertiary gravels accumulated over some considerable period of time during which
time the form of the channels evolved, and the character, texture, and mineral
composition of the alluvial sediments changed. The nature of the deposits and form of
the paleochannels, as discussed by Lindgren (1911), is summarized in Figure
Figure 3. Schematic illustration of typical relationships in the Tertiary channels: a) Lower or Channel Gravels; b)
Upper or Bench Gravels; c) rhyolite tuffs and inter-rhyolite channels; d) andesite tuffs and intervolcanic channels
(after Lindgren, 1911).
In broad terms, the Tertiary gravels can be divided into Upper and Lower units.
Although the units are lithologically and texturally distinct, the contact between the units
is normally transitional and no distinct contact can be picked in the field (Lindgren, 1911;
Yeend, 1974).
Lower Gravels
The deepest trough-shaped channels in the paleovalleys are filled to a depth of 15-60
meters by coarse gravel, which is normally well cemented: the so-called Lower or
Channel Gravels. The Lower Gravels typically rest on a polished surface incised into
metamorphic bedrock. Clasts range in size from pebbles to boulders as large as 2-3
meters in diameter, all well-rounded with polished surfaces. Clast lithologies are largely
those of the regional metamorphic basement: slate, phyllite, greenstone and
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Christensen, Henry, Wood 2015
granodiorite. Quartz cobbles constitute part of the clasts but are not dominant. The
abundance of quartz pebbles and cobbles increases up-section. There is also a notable
increase in the angularity of cobbles and boulders up-section.
The fine matrix of the gravel is mainly sand-size. Claystone or siltstone beds are
absent. While clay-size sedimentary material is sparse, the gravels contain significant
amounts of clay minerals in the sand fraction, in the form of transported bedload
kaolinite clasts, soil debris and altered saprock pebbles. The clay is dominantly
kaolinite (Wood and Glasmann, 2013a, b).
The early miners divided the Lower Gravels into subunits based on coloration: deeper
“blue gravel” and overlying “red gravel.” The coloration of the rock is a consequence
both of the oxidation state of iron and differences in clast composition. Pyrite is
common deeper in the Lower Gravels, restricted to zones beneath the water table.
Authigenic pyrite formed by the reducing action of organic matter on sulfate carried in
groundwater. In places, pyrite coats the surface of pebbles; assays of this pyrite
frequently show it to contain low concentrations of gold. The immediately overlying
gravel has a deep red color due to the later oxidation of pyrite. This, combined with the
differing clast compositions – dark blue-gray lithic clasts at depth grading to more white
quartz in the gravel above – contributes to the noted color change.
Upper Gravels
Covering the Lower Gravels, and attaining thicknesses to 100 meters, are the Upper or
Bench Gravels. Deposits of the Upper Gravels occur both as channel fill incised into
the Lower Gravel deposits and as sheets of alluvial sediment on a wide, gently sloping
strath terrace. The terrace deposits reach widths of 3 km in places and occur on one or
both sides of the underlying trough-shaped channel.
Attributes that distinguish the Upper Gravels from the Lower Gravels change
gradationally. The Upper Gravels contain abundant silt and clay in the form of
mudstone lenses. Lithic clasts, including slate, phyllite, hornfels, quartzite, greenstone
and granodiorite, remain common but decrease in abundance up-section. The majority
of the clasts represent a mixture of lithologies present within bedrock of the western
Sierra Nevada slope, but some clasts have indicated provenance in Nevada
(Slemmons, 1953; Bateman and Wahrhaftig, 1966; Yeend, 1974). The abundance of
quartz, both as white quartz and quartzite, increases up-section, matched by a
decrease in the abundance of lithic clasts and iron-oxide-stained soil debris.
The Upper Gravels are also distinguished by clasts of rhyolite, andesite and tuffaceous
sediments at the top of the section. While the size of the clastic material within the
Upper Gravels generally decreases downstream from east to west, the Upper Gravels
locally contain enormous boulders up to 3 meters in diameter.
The clay mineralogy changes from dominantly kaolinite in the Lower Gravels to
dominantly smectite in the Upper Gravels. In the Upper unit, the kaolinite component
decreases until a ratio of smectite to kaolinite of about 70:30 is reached. This ratio is
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Christensen, Henry, Wood 2015
similar to the ratio in paleosols preserved along the paleochannel margins and locally
below the terrace gravels (Wood and Glasmann, 2013a, b).
The differences in the compositions and textures of the sediments between the Lower
Gravels and the Upper Gravels record a dramatic change in sediment source, channel
dynamics, and environmental conditions. The coarse boulder and cobble
conglomerates with little fine sediment represent only a small fraction of the total
sediment load that would have been transported within the Tertiary river systems. The
rivers were actively eroding the land, cutting their channels, and carrying much of the
sediment away. Large resistant rounded cobbles and boulders – and placer gold –
remained as channel deposits, while finer sediment was moved down the channels to
be deposited in lower energy depositional environments downstream. By contrast, the
Upper Gravels were deposited by streams choked with sediment, meandering across
broad floodplains and depositing much of their sediment load. This change could reflect
a change in source area, climate and weathering, depth of erosion in the source area,
tectonism, volcanism, or likely some combination of these influences. The transition
from angular to well-rounded white quartz cobbles and boulders is indicative of
gradually greater distances of transport. The decreasing abundance of lithic clasts of
local derivation suggests that erosion of local terranes was minimal and that sediment
was sourced further to the east.
The clay mineral composition of the auriferous gravels is distinctive. Quartz and
kaolinite are the primary residual minerals in lateritic soil of warm-humid climates.
Similarly, the kaolinite paleosols of this same composition that underlie Tertiary gravels
throughout this region record the influence of warm-humid climatic conditions that
prevailed during, or preceding, the period of erosion and deposition of the Tertiary
gravels.
The Upper Gravels are overlain by rhyolite ignimbrite, tuffaceous paleosol horizons, and
volcaniclastic fluvial sand generally referred to as the Delleker (northern Sierra) or
Valley Springs (central Sierra) Formations (Bateman and Wahrhaftig, 1966; Wagner et
al., 1981; Saucedo and Wagner, 1992). This unit reaches a thickness of 75 meters
near Chalk Bluff (Fig. 11).
The Delleker Formation is unconformably overlain by andesite volcanic breccia, debris
flows, and interbedded fluvial volcaniclastic sediments of the Miocene Mehrten
Formation.
It was well known to the early miners that gold is not at all uniformly distributed within
the gravels, and considerable effort was made to understand its distribution (Lindgren,
1911; Yeend, 1974). Conventional exploration wisdom was that the highest gold
concentrations were located on and immediately above the bedrock surface (Fig. 4).
Placer gold concentrations were found to wind from one side of the channel to the other,
and were, as well, irregular and discontinuous along the lengths of the channels. The
precise location of gold concentration in the ancient river systems depended upon the
abundance of gold in the sediment load and the highly localized hydraulic and
sedimentary regime present at the moment of deposition.
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Christensen, Henry, Wood 2015
Figure 4. Gold values in three drill holes testing the North Columbia hydraulic mine, Nevada County, California.
Average gold concentrations in the Upper Gravel and Lower Gravel in each drill hole are indicated in ppm gold. (after
Yeend, 1974).
Although nuggets of gold receive much attention, the largest quantity of gold is of finer
grain size. Most of the grains are flattened due to pounding of the particles by cobbles.
At San Juan Ridge, Yeend (1974) documented that most of the gold grains in the lower
gravels are 1-2 mm in diameter and 0.1-0.2 mm thick. The largest flake was 3 mm and
the smallest 0.3 mm. Approximately 80% of the total gold weight was contained in
grains of a diameter greater than 1 mm diameter. During more recent mining in the
same locality, about 95% of the gold particles were less than 3.35 mm (6 mesh), with
occasional particles up to 6 mm diameter (Pease and Watters, 1996).
Lindgren (1911) presented convincing documentation that the fineness of gold in the
placer deposits – the purity of the gold expressed in parts per thousand - is always
greater than that of the bedrock gold-quartz lode deposits in the nearby districts. The
fineness of gold also varies considerably between different mineral districts. Figure 5
shows the range of fineness for lode and placer gold within six counties along the Sierra
Nevada Foothills gold province. Gold fineness in the lode deposits ranges from about
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Christensen, Henry, Wood 2015
600 to 900. Overall, the fineness of alluvial gold in the Tertiary channels averages
about 920 (Lindgren, 1911). The distinct differences in gold grain chemistry between
placer deposits and lode deposits in the same district indicate that the placer gold grains
were not simply eroded from the bedrock lode source and deposited in the immediately
overlying alluvial deposits.
Figure 5. Ranges in gold fineness for lode and placer gold deposits for six counties within the Sierra Nevada Foothills
gold province. The fineness of the gold in placer concentrations is always greater than the fineness of gold in the
nearby lode deposits. (Data from Lindgren, 1911)
Although Lindgren (1911) discounted the significance of supergene redistribution of gold
within the placer gold deposits, he noted the well-documented occurrence of secondary
gold crystals growing on particles of magnetite and ilmenite and documented that
authigenic pyrite and marcasite in the Tertiary gravels are in places auriferous.
Ione Formation
The Ione Formation constitutes the western distal facies of the Tertiary fluvial
depositional regime of the Sierra Nevada Foothills province. The Ione Formation is an
Early to Middle Eocene sequence of clastic sedimentary rocks exposed discontinuously
for 320 km along the western foothills of the Sierra Nevada (Fig. 9) (Creely and Force,
2007; Wood et al., 1995). The unit is characterized by kaolinite-rich sandstone and
mudstone with interbedded lignite. These clay-rich sediments were deposited primarily
in deltaic to lagoonal conditions at the western margin of a landscape of relatively low
relief, with local coal swamps accumulating in quiet water.
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The Ione Formation has been shown to consist of two end members separated by an
erosional unconformity with as much as 40 meters of relief (Pask and Turner, 1952;
Creely and Force, 2007). Sediments of the lower unit are compositionally more mature
than the upper member (Pask and Turner, 1952). Allen (1929) correlated the upper 30
m feldspathic Ione sands with the Upper Gravels mapped by Lindgren (1911) and the
lower 150-180 m of kaolinitic sands with the Lower Gravels.
The mineralogical composition of the lower Ione Formation is predominantly kaolinite
and quartz. This distinctive clay mineral assemblage represents the climax mineral
assemblage of intense chemical weathering, interpreted to have developed in response
to a unique set of Tertiary conditions that included long-term continental stability
associated with intense tropical weathering, followed by an erosional period initiated by
tectonic activity and changing landscape drainage (Wood et al., 1995)
The clay mineral assemblage of upper Ione deposits contains more feldspar, biotite,
and a clay mineral assemblage with significant amounts of smectite. The change in
mineralogy within the Ione Formation is similar to that observed between the Lower
Gravels and the Upper Gravels.
The Ione Formation itself was deposited upon a deeply weathered surface developed
on crystalline basement rock with local relief of as much as 300 meters. The surface is
characterized by a paleosol, 30 meters or more thick, with clearly preserved laterite and
saprolite. Transported laterite, some containing as much as 44% iron, is interbedded in
the basal Ione beds on the flanks of buried highlands (Bateman and Wahrhaftig, 1966).
Both the thickness of this paleosol and the mineral suite present are evidence of the
intensity and duration of chemical weathering prior to deposition of Ione sediments.
Wood (1994) and Wood et al. (1995) documented the pedogenic origin of the kaolinite
in the Ione Formation. The mineral assemblage of the Ione Formation, and the thick
laterite soil on which it rests, indicate a tropical climate with perhaps alterations of wet
and dry seasons for the period of weathering that led to the formation of its sediments
(Allen, 1929). During the early Tertiary, thick kaolinite-quartz-rich soil mantled a
landscape of generally low relief with intense warm-humid weathering. In response to
tectonism or changing erosional patterns, this extensive blanket of soil provided the
source of kaolinite, carried down the Tertiary river channels to be deposited as the lower
Ione Formation. The change in clay mineralogy within the Ione section suggests that
the climate became less tropical or that an accelerated rate of erosion exposed and
transported less-weathered material from the base of the source area weathering
profile.
Age of the gold-bearing Tertiary gravels
The depositional age range of the Tertiary gravels, particularly the maximum age, has
proven difficult to establish with certainty but is essential to evaluating potential sources
of alluvial gold. The Lower or Channel Gravels contain neither fossil nor volcanic
material that can be dated. The auriferous gravels, in general, are commonly correlated
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with the Ione Formation, which is in turn correlated with the marine, middle Eocene
Domengine Formation of the California Coast Ranges (Creely and Force, 2007;
Cherven, in press). Cherven (in press) places the Domengine astride the Early –
Middle Eocene boundary and indicates an age of ~50-47 Ma. Sullivan (in Creely and
Force, 2007) interprets “that the Ione is somewhat younger than the Domengine.” Data
presented here indicate that much of the “auriferous gravel” section is significantly
younger than Domengine Formation.
The stratigraphic section at Chalk Bluff north of I-80 in the Sierra Nevada Foothills is
representative of the region’s overall stratigraphy (Fig. 6). Lindgren divided this
sequence and many similar ones into, from youngest to oldest, a) andesite tuffs
(subsequently Mehrten Formation), b) rhyolite tuffs (Delleker or Valley Springs
Formation), c) Upper or Bench Gravels, and d) Lower or Channel Gravels (Fig. 3).
Placer gold occurs throughout the section but is primarily concentrated in the lower
gravels, with progressively decreasing concentrations in overlying units (Fig. 4). The
lower auriferous gravels lack fossils or isotopically dateable material, but the other units
have been dated either regionally or specifically at Chalk Bluff.
Intermediate to mafic lavas of the Mehrten Formation erupted from vents in what is now
the high Sierra Nevada and flowed variable distances down the paleochannels. Debris
flows commonly extended farther down these channels. The oldest dated flows or
debris deposits of the Mehrten Formation are 16 to 18 Ma, and they range to as young
as ~3 Ma (Wagner and Saucedo, 1990; Saucedo and Wagner, 1992; Cousens et al.,
2008).
Early KAr dating has, unfortunately, somewhat confused the age of the rhyolite tuffs, but
recent 40Ar/39Ar dating demonstrates that the tuffs are, with one exception, no older than
31.5 to as young as 25.4 Ma near I-80, and from ~31.6 Ma to as young as 22.9 Ma
farther south near Valley Springs Peak (Henry and John, 2013 and unpublished data;
Figure 6). The early problematic KAr dates, which were of bulk samples of biotite,
sanidine, or plagioclase, suffered from two problems. Sanidine dates were commonly
too young because of incomplete extraction of radiogenic Ar (McDowell, 1983;
Hausback et al., 1990). An example is the lower tuff at Valley Springs Peak, for which
Dalrymple (1964) reported a sanidine date of 23.4 Ma and for which we find 28.95±0.01
Ma and correlate it with the tuff of Campbell Creek (Henry et al., 2012). Other samples
are probably too old because of contamination by xenocrysts from Mesozoic granites.
This interpretation probably applies to the 38.7±1.0 Ma biotite KAr age reported for tuff
from a drillhole at North Columbia (Yeend, 1974). Unfortunately, it is not currently
possible to resample and redate the North Columbia tuff.
Sierra Nevada Foothills placer gold
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Christensen, Henry, Wood 2015
Figure 6. Stratigraphic summary of the Tertiary gravels of the Sierra Nevada Foothills and associated formations with
known and best interpreted ages.
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Christensen, Henry, Wood 2015
The only tuff conclusively older than 31.5 Ma is a reworked dacite tuff at La Porte (Fig.
8), where a KAr plagioclase date is ~33.2 Ma (no reported ±; Evernden and James,
1964). Detrital zircons in sedimentary deposits below the La Porte tuff are as young as
33±2.6 Ma (Cassel et al., 2012; AJS). The La Porte dacite tuff is unlike the rhyolite tuffs
that make up all other tuffaceous deposits (Garside et al., 2005; Cassel et al., 2012;
Henry and John, 2013), may have been derived from intermediate volcanoes of the
western Cascades far to the north, and has not been found anywhere else in the Sierra
Nevada.
The sedimentary sequence that underlies the rhyolite tuffs can only be constrained to
be older than ~31.5 Ma based on isotopic dating, except at La Porte, where it is ~33Ma
and older. The upper gravels do contain floral and faunal data, which, unfortunately,
have further confused the estimated age of the auriferous gravels. The best known work
is that of MacGinitie (1941), who interpreted an Eocene age (~Capay: 52-49 Ma) for
fossil leaves at Chalk Bluff based on correlation with a Capay-age marine unit near
Oroville that MacGinitie thought equivalent to the Ione Formation. As pointed out by
Schorn (2012), this correlation is incorrect. The Capay-age “Dry Creek” formation
underlies Ione Formation near Oroville (Creely, 1965). MacGinitie’s incorrect correlation,
however, has proliferated so that many subsequent publications assign the Chalk Bluff
flora to late Early Eocene (~56-49 Ma; Wing and Greenwood, 1993; Fricke and Wing,
2004; Hren et al., 2010). As further pointed out by Schorn (2012), Wing and Greenwood
(1993) estimated a MAT (mean annual temperature) for the Chalk Bluff flora that is
~14.5°C too cool for their latitude, coastal position, and inferred age. A reasonable
conclusion is that the Chalk Bluff flora grew at a younger and cooler time.
General approximation of the age of clay-rich sediments is possible by correlation with
the global climatic record. The clay mineral content of paleosols and sediment derived
from weathering of soils is an integrated record of pedogenic processes at the time of
formation. The Early Tertiary paleosols and sedimentary rocks in the Sierra Nevada
Foothills record a change in the dominant clay mineral assemblage from kaolinite in
lower units to smectite in upper units. This shift reflects a decrease in the intensity of
the hydrolytic activity of weathering affecting regional soils, probably in the Mid- to LateEocene. Age estimation using soil mineralogy is imprecise; once formed, kaolinite-rich
laterite soil remains largely unchanged until it is removed by erosion, which time interval
cannot be determined.
The Chalk Bluff flora occur in mudstone beds enveloped by sandstone in the Upper
Gravel section. These sandstone beds possess a matrix with a clay mineral
assemblage that is a transition from the dominantly kaolinite assemblage typical of the
Lower Gravel section to the dominantly smectite assemblage typical of the Upper
Gravel. The smectite gravels are probably equivalent to the upper member of the Ione
Formation (Fig. 6) (Allen, 1929; Pask and Turner, 1952). The smectite clay mineralogy
indicates that these deposits were weathered during younger, cooler times than during
the Early Eocene Climatic Optimum (~52-50 Ma) inferred from stable isotope and other
data (Fig. 7) (Greenwood and Wing, 1995; Zachos et al., 2001, 2008; Hyland and
Sheldon, 2013; Mudlesee et al., 2014). This clay mineralogy may also have been
Sierra Nevada Foothills placer gold
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Christensen, Henry, Wood 2015
derived from tuffaceous deposits, which typically alter to smectite, although the
presence of a tuffaceous component in these deposits has not been confirmed. The
maximum age of the upper gravels could have been sometime in the Late Eocene (~4134 Ma).
Figure 7. Chart of Cenozoic climate evolution based on the proxy of oxygen isotopic composition (δ18O) obtained
from benthic foraminiferal shells found in marine sediment cores (from Mudelsee et al., 2014)
The age of the oldest, kaolinite-rich part of the gravels is still less constrained by direct
dating. The best that can be done is to attempt to correlate sedimentary deposits and
their mineralogy with the Eocene climatic record. We presume that the lower gravels
are equivalent at least in part to the lower member of the Ione Formation as interpreted
by Allen (1929) (Fig. 6). Climate underwent a long-term and mostly gradual cooling of
~12°C from the Eocene Climatic Optimum at ~52-50 Ma to the beginning of the
Oligocene (~34Ma; Zachos et al., 2001, 2008; Hyland and Sheldon, 2013; Mudlesee et
al., 2014) (Fig. 7). The Eocene-Oligocene boundary coincides with an abrupt cooling of
another 4° to 6° C. The gradual Eocene-Oligocene cooling was interrupted by an
abrupt warming of ~4°C during the Mid-Eocene Climatic Optimum.
Development of kaolinite-rich soils by extreme weathering probably would have been
most intense during the 52-50 Ma Eocene Climatic Optimum. However, when the
intensity of weathering transitioned from kaolinite-dominant to smectite-dominant is
unclear. Neither is it known whether the transition was relatively abrupt or gradual over
millions of years. The gradual climate record, along with the ~41 Ma Mid-Eocene
Climatic Optimum, suggest a weathering continuum, possibly even with an episode of
more intense weathering during the Mid-Eocene. Unfortunately, the length of time
required to develop a climax kaolinite soil mineralogy is not known.
Sierra Nevada Foothills placer gold
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Christensen, Henry, Wood 2015
A similar Eocene transition from kaolinite-dominant weathering is recognized in the San
Diego region of southernmost California and northernmost Baja California (Abbott et al.,
1976; Peterson and Abbott, 1979; Link and Abbott, 1991). In that region, a thick
kaolinite-rich paleosol is developed on an erosional surface that truncates all preMiddle Eocene units. Middle and Upper Eocene units dominanted by smectite clay
overlie the paleosol, indicative of a climate beginning to cool and become dryer. This
would place the kaolinitic paleosol as older than ~49 Ma and suggest it developed
during the Early Eocene Climatic Optimum and the Paleocene – Eocene warming trend
that preceded it.
If the southern California data are indicative of the age of the auriferous gravels, the
Lower gravels of the Sierra Nevada Foothills could be mostly 49 Ma or older. The
gravels could range to slightly younger, because they consist of transported kaolinite
eroded from kaolinitic paleosols. How much younger is unknown.
Figure 6 gives our best assessment of the possible age(s) of the Lower and Upper
gravels. Clearly, the timing is imperfectly known. Implications of the ages for the source
of placer gold are discussed below.
The Tertiary Climate of Western North America
The Tertiary climate at the time the placer gold is proposed to have formed and the
Tertiary auriferous gravels were deposited can be ascertained by the record of plant
and animal fossils contained within. The classic documentation of the flora of the
Tertiary gravels is that of MacGinitie (1941). A concise summary of the Tertiary climate
record for the Sierra Nevada is that of Millar (1996), upon which the following summary
is based. During the early Tertiary, warm-humid, subtropical to tropical conditions
prevailed in the area now the Sierra Nevada. Among the trees, ginkgo (Ginkgo biloba),
avocado (Persea), cinnamon (Cinnamonum), fig (Ficus), and tree fern (Zamia) were
common. At the end of the Eocene - about 34 Ma - global climates changed rapidly
from consistently warm to cool-seasonal temperate conditions. In response, vegetation
also shifted to cool-dry-adapted conifers and hardwoods. This new flora contained
many of the taxa now native to the Sierra, plus relicts from the subtropical forests of the
earlier Tertiary. By the late Tertiary, in response to continued drying, winter cooling,
and increasing summer drought, replacement of early Tertiary flora by modern taxa and
associations had occurred.
Mammalian fossil remains recovered from the Tertiary gravels of California include
species of rhinoceros and pachyderms, painting a similar paleoclimate picture.
Lindgren (1911) believed the consistent evidence from all the fossil localities showed
that the climate of the region at the time the auriferous gravels were deposited was like
that of the southern temperate zone of the Atlantic coast region today, with warmer
temperatures and assuredly characterized by heavy rainfall.
Remnant Cretaceous-Tertiary lateritic paleosols preserved throughout the Sierra
Nevada Foothills (Evans, 1981; Wood and Glasmann, 2013a, b; Wood et al., 1995)
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Christensen, Henry, Wood 2015
provide qualitative confirmation of intense chemical weathering within a warm-wet
climate regime. The paleoclimatic history interpreted for the Sierra Nevada is consistent
with interpreted worldwide climate change (Fig. 7)
Gold endowment of the Sierra Nevada Foothills gold province
The Sierra Nevada Foothills gold province (Figs. 2 and 9), including both lode and
placer gold deposits, is one of the world’s great gold provinces. Ascertaining exactly
how much gold has been produced, however, is difficult. During the early years, gold
was currency, the west was wild, and no one was keeping records. Important production
estimates and database summaries of past production are those of Boyle (1979),
Bohlke (1999b), Beirlein et al. (2008), Long et al. (1998), and Koschman and
Bergendahl (1968) (Table 1).
Table 1. Estimates of California and Sierra Nevada Foothills historic gold production, in millions of ounces.
Total California
Gold Production
Total Production
SNF
Placer Production
SNF
55
Lode Production
SNF
106
106
100
65.7
66.8
32.9
39
37.6
43
35
Clark, 1998
Long, 1998
32
Bohlke, 1999
Beirlein et al., 2008
Boyle, 1979
106
115
Reference
86
128
Koschmann and Bergendahl, 1968
Evans 1981
Perhaps the most comprehensive source, Koschman and Bergendahl (1968),
documented gold production in all of California from 1848 to 1965 of 106 million ounces
(3297 t), of which 68 million ounces (2115 t) were from placer occurrences and 37.9
million ounces (1178 t) from lode production. Of this total California state production, the
Sierra Nevada Foothills produced an estimated 65.7 million ounces (2043 t) of placer
gold and 32.8 million ounces (1020 t) of lode gold.
The estimates in Table 1 differ considerably. We conclude that total lode gold
production from the Sierra Nevada Foothills gold province was at least 35 million
ounces (1088 t). Placer gold production from the same area likely exceeded 65 million
ounces (2022 t).
As in most orogenic gold provinces, the majority of primary lode production came from a
small number of deposits, with the 12 largest deposits accounting for almost 60% of the
gold (Beirlein et al., 2008). The two largest lode gold deposits were the Empire and
Idaho-Maryland deposits near Grass Valley (Fig. 2) with a combined production of 8.9
million ounces (275 t). Of 814 lode gold deposits in the database of Northover (2006),
only 8 had gold production greater than 1 million ounces (31 t) (Fig. 9).
Sierra Nevada Foothills placer gold
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Christensen, Henry, Wood 2015
Haley (1923) reviewed the gold resource in Tertiary gravels stranded by the Sawyer
decision, which ended large-scale hydraulic mining in 1884. His estimate considered
both the total gold resource, and a smaller economic gold resource, reduced by that
volume of material not then economic due to topography, elevation or lack of water. He
estimated an “economic resource” of 30 million ounces (930 t) gold within a total
remaining resource of 50 million ounces (1550 t) gold. Nearly as much placer gold
remains in place today as was produced during more than a century of production!
We estimate that the total amount of gold contained within the Sierra Nevada Foothills
gold placer deposits, combining past production and remaining resource, was greater
than 115 million ounces (3,577 t).
However imprecise the historical gold production figures might be, several important
observations can be made. First, the quantity of gold in either the lode or the placer
deposits constitute a giant gold province, defined as a province containing more than
250 tonnes (~8 million ounces) of gold (Laznicka, 1999; Groves et al., 2003). Second,
the quantity of gold contained within the placer deposits was more than three times as
great as that within known lode deposits. This disparity in abundance makes it difficult
to imagine the lode gold deposits to have been the source of the extensive overlying
placer gold deposits.
ORIGIN OF GOLD IN THE PLACER DEPOSITS – AN ALTERNATIVE
INTERPRETATION
The Sierra Nevada Foothills region of northern California is one of the world’s premier
gold provinces. Gold is present both in bedrock lode deposits and in major Tertiary and
Quaternary alluvial placer accumulations. Native gold in the auriferous gravels of
northern California traditionally has been interpreted to be native gold released by
physical weathering from underlying lode deposits and incorporated in the stream bed
load. This interpretation has numerous problems. (1) Large gold grains and nuggets
are extremely uncommon in primary hydrothermal gold deposits, even within the
orogenic quartz-gold veins of the Sierra Nevada Foothills. (2) Historic lode gold
production from the province has been about 35 million ounces, while Tertiary placer
production exceeded 65 million ounces with yet at least another 50 million ounces of
placer resource stranded by extraction restrictions estimated to remain. (3) A large part
of the Tertiary alluvial placer gold accumulation is situated upstream and uphill of most
of the lode deposits. (4) Tertiary channel gold averages 920 fineness, whereas gold
from primary veins varies from 600-900 fineness.
We suggest as an alternative hypothesis that much of the placer gold in the Sierra
Nevada Foothills Tertiary auriferous gravels was sourced from the great gold districts of
Nevada. The native gold grains and nuggets formed through supergene weathering of
primary hydrothermal gold deposits in a warm-humid environment on a tectonically
stable high plateau. Subsequent accelerated physical weathering due to tectonism,
climate change, or river incision, stripped the tropical soils with their contained gold from
a large source area and transported it westward in large river systems. The dense gold
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Christensen, Henry, Wood 2015
particles were deposited within the high-energy stream channel deposits at the base of
the steep pre-Sierra slope, while the finer grained and less-dense, kaolinite-rich silicate
soil minerals were carried westward to be deposited as the kaolinite-rich sedimentary
units of the Ione Formation.
Genesis of placer gold from chemical weathering primary gold deposits
Few primary hydrothermal gold deposits anywhere in the world contain abundant
masses of native gold that can be released by physical weathering to form the gold
grains and nuggets found in alluvial placer deposits. In those few deposits in which
masses of native gold are found, the coarse fraction constitutes but a minor part of the
total gold resource. It appears highly unlikely that the orogenic quartz-gold veins of the
Sierra Nevada Foothills gold province were the source for all or even most of the gold in
the overlying placer gold deposits. So where did the gold come from? An examination
of where gold nuggets are known to form in the modern environment can usefully inform
this discussion.
Generally, placer gold deposits are considered to be of two types: eluvial and alluvial.
Residual or eluvial gold placers are concentrations of gold that remain when other rock
or soil material has been removed by chemical and physical weathering, leaving a nearsurface residuum with abundant native gold. Gold grains from eluvial placers can in
turn be eroded, physically transported, and deposited by water to form alluvial placers.
Once formed, concentrations of placer gold may be repeatedly eroded, transported and
redeposited (Henley and Adams, 1979). In North America, the well-known great placer
gold fields of California, the Klondike, and Alaska are all alluvial placer gold deposits. It
is important to appreciate, however, that the gold grains and nuggets in these alluvial
deposits may have been formed, transported, and deposited in more than one cycle of
erosion and deposition before coming to rest in their current locations. The ultimate
source areas may have been distant from the locations where the alluvial
concentrations now rest.
Elsewhere in the world, great quantities of gold are recovered from eluvial placer
concentrations. There are thousands of residual placer gold deposits within the
Amazon Basin, within the zone of tropical forested Africa, in Australia, and in Southeast
Asia. These eluvial gold placer concentrations are typically restricted to the nearsurface weathering profile and are often mined by artisanal miners using a variety of
methods from simple panning to larger-scale hydraulic washing. The gold in these
deposits occurs as grains to nuggets, which are recovered by gravity separation. In
some occurrences, artisanal miners recover the gold underground, digging shafts down
through the thick tropical soil profile, then tunneling along the soil-bedrock interface to
recover the gold concentrated at the base of the soil profile.
The fact that some chemical elements can be chemically moved and concentrated by
pedogenic processes within tropical soils is well known. Laterite concentrations are
important ores for aluminum (bauxite), iron (direct-shipping hematite ores), and nickel
(nickel laterite). The importance of intense chemical weathering in the formation of
Sierra Nevada Foothills placer gold
22
Christensen, Henry, Wood 2015
many gold deposits has only been widely appreciated within the past decades.
Although most modern eluvial gold deposits are found in tropical locations, Taylor et al.
(1992) document the formation of cool climate laterite and bauxite weathering under
conditions of high precipitation in New Zealand.
Boyle (1979) presents a comprehensive review of supergene gold processes and
deposits, current as of that date. Butt (1998) reviewed supergene gold deposits with an
emphasis on Australia. Good reviews of specific deposits and districts are those of
Clough and Craw, 1989; Bowell, 1992; Santosh and Omana, 1991; Hughes et al., 1999;
Mann, 1984; DeOliveira and Campos, 1991; Freyssinet, 1993, 1994; Hocquard et al.,
1993; Lawrance and Griffin, 1994; and Webster and Mann, 1984.
Soils can be described as geomembranes – open biogeochemical systems in which
there is a dynamic redistribution of elements in response to changing physico-chemical
conditions (Colin et al., 1992). The typical product of intense chemical weathering is a
laterite soil (oxisol), marked by the progressive development of variations of saprolite,
mottled zone, and laterite or ferricrete soil horizons (Fig. 8). Every chemical element is
mobile to some extent; differences in element mobilities result in removal, residual
concentration or secondary concentration of elements. Geochemical processes in the
pedogenic environment are functions of primary lithology, geomorphology, biology,
climate, hydrology and time.
Figure 8 summarizes common terminology for a deeply weathered regolith profile and
summarizes typical mineralogy and major-element chemistry. Through chemical
weathering processes, the chemistry and mineralogy of the soil profile is dramatically
changed through time. In general, the oxides of iron, aluminum and titanium are least
mobile and are concentrated in the residual soil. Alkali and alkali earth elements (Na, K,
Ca, Mg) are highly soluble and readily moved away in groundwater. Notable for this
discussion is the importance of the alumina-rich clay mineral kaolinite as a stable climax
mineral in the laterite soil profile.
Although the concentration of silica decreases with weathering, some quartz remains as
a residual mineral. Silica removed from the soil in groundwater may precipitate as
crystalline quartz veins and fracture-fill in underlying bedrock, or may move laterally in
groundwater to form silcrete beds or quartz overgrowths on quartz grains.
Sierra Nevada Foothills placer gold
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Christensen, Henry, Wood 2015
Figure 8.Regolith terminology for deeply weathered laterite profiles with mineralogy and major-element chemistry;
schematic section of a laterite profile with supergene gold concentrations developed over a deeply weathered primary
vein deposit. The specific dimensions and mineralogy of any tropical soil profile are uniquely related to the nature of
the bedrock and local topography, hydrology, and climate. (after DeOliveira and Campos, 1991; Hocquard et al.,
1993; Smith et al., 1997; and Valeton, 1994)
Gold is generally considered chemically inert under most weathering conditions, but
may be chemically quite soluble and mobile where certain complexing ligands are
present in soil and groundwater. Field evidence suggests that oxidation of primary goldbearing sulfide minerals releases gold in an extremely fine-grained soluble state to the
weathering environment. Significant chemical complexes for solubilizing gold are
thiosulfate generated by pyrite oxidation, organic acids formed by the interaction of gold
with organic matter, and halide complexes in saline soils (Butt, 1988). In tropical forests,
organic acid complexes are an important ligand (Freise, 1931; Boyle, 1979), while in
more arid terranes, such as present-day Australia, chloride complexes are dominant.
Once in solution, gold moves with groundwater. In continually wet environments, gold
moves downward until it is precipitated by reduction, often by ferrous iron, at the
weathering front. In tropical climates marked by wet and dry seasons, gold in
groundwater may move also upward by capillary action during dry seasons to
precipitate within the ferruginous laterite cover. Thus, two distinct modes of supergene
gold concentration are distinguished: concentrations of native gold intergrown with ironSierra Nevada Foothills placer gold
24
Christensen, Henry, Wood 2015
oxides in near-surface laterite, and saprolite gold concentrations formed at the
weathering front (Fig. 8).
Because silver is more soluble than gold, the two metals are parted during weathering
and secondary concentration, removing silver in solution and leaving a gold-enriched
residue behind (Jaireth, 1994; Mann, 1984).
As the weathering front progresses downward, supergene gold is repeatedly dissolved
in the zone of oxidation, moved downward in groundwater, and precipitated at the redox
boundary, only for the cycle to be repeated as the weathering front recedes. The
morphology of gold grains observed in tropical soils around the world is quite consistent
with this model. In the upper oxidizing horizon, gold occurs as corroded and pitted
remnant grains, evidence of dissolution processes occurring in the unsaturated zone
high in the weathering profile. Lower in the profile, in the zone of accretion, gold occurs
as subhedral to euhedral crystals and crystal aggregates described as delicate
euhedral, filimental or dendritic, evidence of grain growth in the supergene environment.
(Santosh and Omana, 1991; Freyssinet, 1993, Wilson, 1984) The fineness of gold is
almost always greater in these secondary crystals (Jaireth, 1994; Boyle, 1979, Mann,
1984).
It is common knowledge among placer miners that gold “grows” in placers, and modern
studies confirm this (Freise, 1931; Boyle, 1979; Clough and Craw, 1989; Hughes et al.,
1999). Alluvial gold placers, worked and thoroughly exhausted, may after a period of
years once again be panned and yield a profitable amount of newly accumulated gold
(Friese, 1931). Other evidence for chemical migration within alluvial placer deposits
include crystalline gold overgrowths on abraded alluvial gold nuggets; the presence of
gold in diagenetic pyrite that has replaced roots and branches in the alluvial placer
deposits; occurrences of fine wire-gold within alluvial cobbles; fine crystalline gold
crystals in moss mats; and gold impregnations of detrital wood and coaly material.
Hughes et al. (1999) document widespread and abundant diagenetic pyrite in the
sediments of paleoplacers, together with native copper and at least some perfectly
crystalline diagenetic arsenopyrite. Schuster and Southam (2014) and Reith et al.
(2010) have documented the importance of bacteria and biofilms in mediating the
dissolution and precipitation of gold in weathering and sedimentary environments. In
laboratory simulations, Lengke and Southam (2007) document the growth of millimeterscale gold crusts on sedimentary grains during experiments as short as 150 days.
It is the conventional interpretation of many placer miners that gold grains and nuggets
with delicate crystal forms have not traveled far from a bedrock vein source. Abundant
evidence, however, suggests it more likely that often these delicate gold crystals were
formed by supergene processes within a soil profile, or have recrystallized or grown insitu within the alluvial environment in which they are found.
As gold nuggets are being formed today by pedogenic processes under conditions of
intense chemical weathering, we propose that most of the gold nuggets within the
Tertiary placer deposits of the Sierra Nevada Foothills gold province were formed by
Sierra Nevada Foothills placer gold
25
Christensen, Henry, Wood 2015
weathering of primary gold deposits, supergene enrichment, and reconstitution of gold
by pedogenic processes, likely with microbial mediation. Crystalline gold grains and
nuggets were concentrated as eluvial placer deposits within the tropical soil mantle. In
response to changing climate or topography, the gold nuggets were eroded from the
source area, transported by large west-flowing river systems, to be deposited in the
great Tertiary alluvial deposits of California. Some portion of the placer gold may have
been formed by chemical weathering of the gold-rich terrane of the Sierra Nevada
Foothills metamorphic belt to be locally redistributed by the Tertiary river systems
(Evans, 1981).
Consider the sequential sediment yield from physical erosion of a plateau surface
mantled by a thick auriferous laterite soil profile as in Figure 8. The early sediment load
would contain abundant particulate gold with fine-grained kaolinite and quartz and few
or no resistant lithic clasts. As erosion cuts deeper, the abundance of kaolinite would
decrease, while that of smectite would increase. The abundance of gold, great during
early erosion, would decline abruptly. As erosion reaches the unweathered bedrock
surface, lithic clasts from the source area will enter the sediment stream. This is the
general sequence of sediment composition noted up-section in the Sierra Nevada
Tertiary gravels.
Tertiary river channels, the Sierra Nevada, and Nevadaplano
Early miners and geologists quickly appreciated the importance of following the Tertiary
paleochannels, both at the surface and in the subsurface beneath younger cover. The
channels were prospected and mapped (Lindgren, 1911). Lindgren recognized and
documented that many of the major auriferous Tertiary channels are located uphill, and
upstream of the major lode gold deposits from which they were assumed to have
derived (Figs. 9 and 10). This evidence strongly suggests a yet more eastern source for
the great alluvial accumulations with their contained gold.
As Lindgren (1911) and other early geologists sought the source of the placer gold, they
were constrained by their understanding of the tectonic and topographic history of the
Sierra Nevada. It was then accepted that the mountain range was generally
symmetrical throughout Cretaceous time, with a crest located near to the current
summit line. At some time at the end of the Cretaceous, the structure changed from a
symmetrical uplift to a west-dipping monoclinal fault block, yet the location of the crest
remaining largely unchanged. Constrained by this interpretation, the potential source
area for all of the gold contained within the Tertiary gravels was restricted to the narrow
slope between the crest of the Sierra Nevada and the location of the placer deposits.
Sierra Nevada Foothills placer gold
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Christensen, Henry, Wood 2015
Figure 9. . Map showing the location of significant lode and placer gold deposits in the Sierra Nevada Foothills gold
province. Note that many of the significant placer gold deposits are upstream and uphill from the major lode gold
deposits. Outcrop occurrences of Ione Formation from Creely and Force (2007). Locations and gold production data
are from Clark (1970) and Northover (2006).
Sierra Nevada Foothills placer gold
27
Christensen, Henry, Wood 2015
Figure 10. Map of part of the Tertiary Yuba River drainage, showing Tertiary channels and locations of lode gold
mines. The topography of the area is higher to the east, with rivers draining to the west. The named major placer
mine locations are uphill and upstream of the major lode gold mines at Grass Valley and Nevada City (after Yeend,
1974).
More recently, the area that is now the Great Basin is widely interpreted to have been
an erosional highland in the middle Cenozoic, following crustal thickening to as much as
70 km during Mesozoic–early Cenozoic shortening, batholith emplacement, and shallow
slab subduction (e.g., Wolfe et al., 1997; Dilek and Moores, 1999; Ducea, 2001;
Humphreys et al., 2003; DeCelles, 2004; Dickinson, 2006; Henry, 2008; Best et al.,
2009; Cassel et al., 2009a, b, 2011, 2012, 2014; Colgan and Henry, 2009; Ernst, 2010;
Henry and Faulds, 2010; Long, 2012). This highland, commonly referred to as the
Nevadaplano based on an analogy to the Altiplano of the Andes (DeCelles, 2004),
probably had maximum elevations of 3-4 km in east-central Nevada (Fig. 11). The
Nevadaplano was maintained at high elevation until major extension and elevation loss
began in the middle Miocene (Horton and Chamberlain, 2006; Colgan and Henry, 2009;
Henry et al., 2011, 2012). Major paleoriver systems drained this highland both west to
the Pacific Ocean, connecting to the Tertiary channels of this paper, and east to the
Uinta Basin (Fig. 10; Henry, 2008; Cassel et al., 2009a, b, 2014; Henry and Faulds,
2010; Henry et al., 2012).
Sierra Nevada Foothills placer gold
28
Christensen, Henry, Wood 2015
Figure 11. The Sierra Nevada and western Great Basin showing known paleovalleys and an approximate
paleodivide, the location of the Sierra Nevada Foothills gold deposits and paleoplacers in the Sierra Nevada, and
major pre-Miocene gold deposits with their ages in Nevada that could have contributed to placer gold in the Tertiary
channels. Until ~16-17 Ma, the Great Basin was a topographic high (Nevadaplano) that had major rivers draining
westward across what is now Sierra Nevada to the Pacific Ocean, which was in the Great Valley in the Eocene and
Oligocene. Paleovalleys in the Sierra Nevada (the ancestral Yuba, American, Mokelumne, Calaveras, and Tuolumne
Rivers; Lindgren, 1911; Jenkins, 1932; Clark, 1970) were mapped in great detail because they contained substantial
gold but probably include both main channels, tributaries, and channels within main channels. Paleovalleys east of
the Sierra Nevada (Henry and John, 2013) are much less completely known because of incomplete mapping and
cover beneath basins. CR = Carson Range, SV = Spring Valley, S = Sierra District.
Despite this consensus, the absolute elevation and structural-topographic evolution of
the Nevadaplano, particularly of what is now the Sierra Nevada, remain highly
controversial (Wakabayashi and Sawyer, 2001; Mulch et al., 2006, 2008; Cassel et al.,
2009a, b, 2014; Molnar, 2010; Wakabayashi, 2013). For most of the past century, the
high topography of the northern Sierra Nevada was thought to be the result of uplift
during the late Cenozoic, that the Sierra Nevada was a great uplifted and westwardtilted fault block like fault-block ranges of the Basin and Range (LeConte, 1886;
Christensen, 1966; Huber, 1981; Unruh, 1991; Wakabayashi and Sawyer, 2001; Jones
Sierra Nevada Foothills placer gold
29
Christensen, Henry, Wood 2015
et al., 2004; Wakabayashi, 2013). Recently, based on crustal thickness, stable isotope
and organic molecule paleothermometry and altimetry, and detrital zircon
geochronology, Wernicke et al.,(1996); Horton et al. (2004), Mulch et al. (2006, 2008),
Cassel et al. (2009a, b), Cecil et al. (2010), and Hren et al. (2010) interpreted that the
Sierra Nevada initially uplifted during Late Cretaceous arc magmatism and, during the
Eocene-Oligocene, was approximately the same elevation (ca. 2.5-3 km at the latitude
of Lake Tahoe) as it is today. After cessation of Cretaceous magmatism at about 80
Ma, the present Sierra Nevada underwent significant erosion – as much as 5-8 km over about 30 million years creating a pre-Eocene erosional unconformity surface. This
surface underwent significant chemical weathering due to the wet, warm Paleogene
climate, leaving the surface, at least locally, covered by a thick lateritic soil profile
(Evans, 1981; Wood et al., 1995). The auriferous gravels were deposited on the preEocene erosional unconformity.
The Tertiary channels and their contained sedimentary deposits and gold are essential
parts of the evidence for the “Nevadaplano” and bear on the evolution of elevation. For
years, geologists have recognized that some clasts in the channels had to have come
from farther east, in what is now the Great Basin of Nevada (Slemmons, 1953; Bateman
and Wahrhaftig, 1966; Yeend, 1974). Our work has shown that these channels
extended far to the east to an interpreted paleodivide extending across east-central
Nevada (Faulds et al., 2005; Garside et al., 2005; Henry, 2008; Cassel et al., 2009a, b,
2012, 2014; Henry and Faulds, 2010; Henry et al., 2012; Fig. 11). Most notably,
voluminous, mostly Oligocene (~34-19 Ma), rhyolite ash-flow tuffs erupted from calderas
in central Nevada and flowed great distances down the paleochannels (Henry, 2008;
Cassel et al., 2009a, b, 2012; Henry et al., 2012). The ash-flow tuffs and reworked
tuffaceous deposits constitute part of the Tertiary gravel section. Several tuffs probably
reached the former shoreline in what is now the Great Valley. The distribution of these
tuffs demonstrates that the Sierra Nevada was a lower, western slope to the
Nevadaplano to the east. Probably all of Lindgren’s (1911) named Tertiary rivers had
headwaters in central Nevada, although they certainly had local tributaries restricted to
the Sierra Nevada. Tuff distribution is also consistent with, but does not prove, the
interpretation from stable isotope data of a 3-4 km high Nevadaplano (Cassel et al.,
2014), because channelized tuffs can flow great distances even down low topographic
gradients (Henry et al., 2012).
Possible sources of gold to the Tertiary Channels
Drainage of the Tertiary channels of the Sierra Nevada from much of the Great Basin
allows for many additional possible sources of gold to the channels (Fig. 11). Indeed,
the Great Basin contains one of the greatest gold endowments of the Americas (Sillitoe,
2008). Numerous major Mesozoic to late Cenozoic precious-metal deposits lie in the
basins that drained through the Sierra Nevada Tertiary channels. This discussion
mostly considers Eocene and older deposits, because younger deposits obviously could
not contribute to the main auriferous gravels.
Sierra Nevada Foothills placer gold
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Christensen, Henry, Wood 2015
Garside et al. (2005) discussed numerous possible sources of Au in the easternmost
Sierra Nevada and western Nevada. These include porphyry Cu mineralization at Lights
Creek, Genesee, and Meadow Lake (Fig. 11). Hypabyssal quartz monzonite at Lights
Creek is 178.1±3.9 Ma (Dilles and Stephens, 2011). Detrital zircons from sediments
underlying the La Porte tuff are dominantly 160-180 Ma, which indicates that the Lights
Creek intrusion or a similar age intrusion was exposed and shedding material into the
Tertiary channels (Cassel et al., 2012). Numerous small “probably gold-bearing” quartztourmaline veins are scattered around western Nevada, but none had significant
production of any metal. Garside et al. (2005) also pointed out that placer Au is found in
paleovalleys in western Nevada below 27 Ma ash-flow tuffs in the Carson Range and
near Yerington. The upper parts of the Jurassic porphyry copper system at Yerington
could have been a source of gold to these placer deposits.
Probably Late Cretaceous, orogenic precious-metal veins are widespread in western to
northwestern Nevada, particularly in the Humboldt Range (Fig. 11). Many are silverrich, with Ag:Au ratios around 100:1 (Vikre, 1981, 2014; Crosby, 2012; Davis and
Muntean, 2014a, b; Muntean and Davis, 2014), and mine production has been
dominantly Ag. Nevertheless, Au production has been significant, recent exploration
indicates major gold prospects, and the veins were major sources of Au to nearby
Tertiary placer gold deposits (Johnson, 1973; Peters et al., 1996; Cheong et al., 2000).
The largest producer, the Couer Rochester mine in the southern Humboldt Range, has
produced at least 130,000,000 ounces Ag and 1,500,000 ounces Au through 2012 and
has total reserves and resources of 186.1 million ounces (5788 t) silver and 1.24 million
ounces (38 t) gold (Coeur Mining, 2015; Davis and Muntean, 2014). The nearby goldrich Spring Valley deposit has ~4 million ounces (~124 t) Au in the measured and
indicated categories with no reported Ag (Midway Gold Corp., 2015). A Tertiary placer
deposit at Spring Valley produced about 500,000 ounces Au, and one in the Sierra
district to the northeast produced about 200,000 ounces (Fig. 11) (Johnson, 1973). The
southern Humboldt Range underwent extensive early Tertiary erosion (Vikre, 1981).
Orogenic veins in the Humboldt Range are at least Late Cretaceous, ~70 Ma old, but
have been difficult to date more precisely. Based on U-Pb zircon dating, host rocks are
mostly Triassic (~249 Ma) volcanic rocks, and 92 and 94 Ma intrusions lie within 10 km
of Rochester and Spring Valley in the southern Humboldt Range (Vetz, 2011; Crosby,
2012; Vikre, 2014). 40Ar/39Ar dating of hydrothermal minerals generally give disturbed
spectra that suggest Late Cretaceous alteration (Cheong et al. 2000; Vikre, 2014).
Disturbed 40Ar/39Ar and KAr dates of vein muscovite from Rochester scatter around
103-86 Ma, and all other KAr and 40Ar/39Ar dates of muscovite and alteration(?) K
feldspar from the southern Humboldt Range range from 70 to 80 Ma (Vikre, 2014). Four
of five samples of sericite associated with Au-quartz veins north of the Sonoma Range
gave dates between about 90 and 107 Ma, and the fifth showed a maximum age of ~80
Ma with considerable Ar loss (Cheong et al., 2000).
Carlin-type gold deposits (CTDs) are the best known and largest producers of Au in
Nevada with an identified gold endowment greater than 211 million ounces (6560 t) in
Sierra Nevada Foothills placer gold
31
Christensen, Henry, Wood 2015
deposits concentrated within four linear belts in north-central Nevada (Fig. 11) (Sillitoe,
2008; Muntean et al., 2011; Cline et al., 2005). Despite the immense size of these
deposits, their age, structural-stratigraphic setting, physical and mineralogical
characteristics, and possibly location make their contribution to the “auriferous gravels”
highly unlikely.
All reasonably well-dated Carlin-type deposits formed in the Eocene, between about 43
and 35 Ma (Fig. 11) (Cline et al., 2005; Ressel and Henry, 2006). Mineralization
generally becomes younger southward, following the younging of Eocene magmatism,
which was at least the heat source and possibly the source of fluids and metals to the
deposits (Ressel and Henry, 2006; Muntean et al., 2011). If the deep channel gravels of
the Sierra Nevada are older Eocene, ≥45 Ma, then CTDs could not have contributed to
them. CTDs also would have been available to weathering only after the Eocene
Climactic Optimum, when the climate was becoming cooler and dryer.
Carlin-type deposits are primarily hosted in largely carbonate, shelf- and slope-facies,
Paleozoic sedimentary rocks in the lower plate of the Roberts Mountain allochthon,
structurally below mostly siliciclastic, deeper-water, upper plate Paleozoic rocks that
had been thrust over them. Lower-plate carbonate rocks are more favorable physical
and chemical host rocks; upper-plate rocks often served as less-permeable caps.
Upper-plate gold deposits are usually much smaller and of lower grade than lower-plate
deposits. Although mineralization formed at relatively shallow depths, most exposure in
the Eocene soon after gold deposit formation may have been restricted to upper plate
rocks, so only a small part of the total mineralization was available for weathering,
enrichment, or transport.
Gold in CTDs occurs as submicron substitution in arsenian pyrite, not as discrete Au
grains (Cline et al., 2005). More recent oxidation of some deposits has generated
extremely fine-grained free Au, but erosion and reworking of this Au into modern placer
deposits has been minor. The “invisible Au” that is characteristic of CTDs meant that
they were not discovered by early prospectors, in contrast to the discovery of most
major gold deposits in Nevada and elsewhere. CTDs were only recognized as a distinct
deposit type and large-scale mining undertaken in the 1960s.
CTDs are also the most distal potential sources to the “auriferous gravels” (Fig. 11). The
Carlin trend and parts of the Battle Mountain-Eureka trend lie astride or even east of the
inferred Tertiary drainage paleodivide. The precise location of the paleodivide and its
evolution through time are uncertain, but many CTDs probably were in east-draining
regions during Eocene and Oligocene time.
The paleodrainage network off the Nevadaplano probably lasted until the onset of major
extension in the middle Miocene, ~17-16 Ma (Dilles and Gans, 1997; Wolfe et al., 1997;
Horton and Chamberlain, 2004; Colgan and Henry, 2009). Deposits younger than that
could not have contributed gold to Sierra Nevada gravels regardless of the influence of
climate on Au mobilization. With the notable exception of the 26 Ma Round Mountain
deposit (16 million ounces; 500 t Au), large deposits of Oligocene age are sparse in
Sierra Nevada Foothills placer gold
32
Christensen, Henry, Wood 2015
Nevada (Henry et al., 1997). Some gold at Round Mountain is relatively coarse and
includes high-grade veins that make exceptional sources for placer deposits.
Quaternary placers were prominent producers during early mining in the district.
Precious-metal deposits of the Ancestral Cascade magmatic arc in what is now the
Walker Lane belt of western Nevada range in age from ~22 to 4 Ma (John, 2001; Vikre
and Henry, 2011). Only the oldest of these, e.g., the 20-22 Ma Goldfield (Ashley and
Silberman, 1976; Vikre and Henry, 2011), 19-20 Ma Tonopah (Bonham and Garside,
1979), and 18-19 Ma Paradise Peak (John et al., 1989), could have contributed to upper
parts of the Tertiary gravels.
We should further consider deposits that were likely present in the Cretaceous or Early
Tertiary but that have been completely removed by erosion. It is an underlying premise
of this paper that to understand the origin of the gold in the Tertiary placer deposits of
the Sierra Nevada Foothills, we must imagine landscapes and climates different than
those of today. At the end of the Cretaceous, the Sierra Nevada stood as the high
volcanic arc of the North American Cordillera. We expect that the Cretaceous North
American Cordillera presented geology and topography similar to that of the modern
South American Andean Cordillera. By analogy with the younger Andean Cordillera,
there likely could have existed within the North American volcanic arc, giant gold
districts, which have been reduced and removed by tens of millions of years of
weathering and kilometers of erosion.
The Cajamarca-Huaraz mineral district in Peru has a gold endowment of 86 million
ounces (2675 t) in high-sulfidation epithermal and porphyry copper-gold deposits, and
the El Indio-Maricunga mineral district in Chile has an endowment of 81 million ounces
(2520 t) gold in high-sulfidation epithermal deposits. These two districts are among the
largest gold systems in the American Cordillera (Sillitoe, 2008). Both districts are about
400 km long, stretching along the crest of the Andean magmatic arc. High-sulfidation
gold deposits are hosted by voluminous long-lived volcanic systems of andesite-daciterhyolite composition. It is reasonable to consider that comparable districts once existed
in the similar, but older, arc setting of the North American Cordillera. Weathering and
erosion of even 3 kilometers of the uppermost volcanic arc would have completely
reduced such deposits. Weathering of these deposits during warmer wetter Paleogene
climate, under lush vegetative cover, may have released finely disseminated gold from
auriferous sulfide minerals to be reconstituted as native gold particles and nuggets,
accumulating as residual placer deposits. Near-surface supergene redistribution and
concentration of gold is significant in some Andean gold deposits.
Chemical Fingerprinting of Gold to Evaluate Sources
The ability to chemically or isotopically fingerprint gold grains both in primary lode
deposits and placer gold concentrations could greatly help evaluate sources, and a few
attempts have been made in other districts. Knight et al. (1999) and Chapman et al.
(2010a, b) used combined microchemical analysis and inclusion mineralogy of gold
grains to connect placer accumulations in separate drainages with bedrock source
areas elsewhere within the Klondike District, Yukon. Placer gold grains within individual
Sierra Nevada Foothills placer gold
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Christensen, Henry, Wood 2015
drainages have distinct ranges of composition and inclusion mineralogy, which can be
correlated to lode gold centers within the district (about 2500 square kilometers area).
The technique required analysis of thousands of individual gold grains by
microanalytical techniques. Placer gold is typically an alloy of Au, Ag, Cu, Hg, and
sometimes platinum group elements, and grains commonly exhibit significant chemical
zonation from core to rim. In the Klondike studies, the chemistry of placer gold grains
within any location exhibited significant variability, and it was only through analysis of
large number of grains that compositional ranges could be defined.
The relative Re and Os concentrations in gold have been shown to be distinctive for
deposits of different ages and geological settings and to reflect the degree of tectonic
recycling of the gold (Frimmel et al., 2005, 2015).
For chemical fingerprinting to reliably identify the source(s) of gold grains in Sierra
Nevada gravels, where potential source areas may be hundreds of kilometers distant,
several requirements need to be met.
1) Different sources need to have distinct chemical or isotopic characteristics. There
must be distinct chemical differences between, for example, Mesozoic lode gold
deposits of the Sierra Nevada, epithermal deposits that may have existed on the North
American Cordillera volcanic arc, and Carlin-type gold deposits of central Nevada. An
immediate corollary is that a database of sufficiently representative analyses is required
to determine the homogeneity and distinctiveness of gold alloy chemistry in these
different deposit types. This data currently does not exist.
2) All or at least part of the distinct chemical signature of gold from the original lode
deposits would need to be retained through the repeated supergene dissolutionprecipitation process called upon here. It is well documented in references already cited
that the fineness – the alloy chemistry - of gold is dramatically changed by supergene
processes, both with and without microbial mediation. In the Klondike studies, Knight et
al. (1999) concluded that such supergene modification was minor and restricted to thin
rims in which Ag, Hg, and Cu were depleted. In other locations, chemical change is
undeniable and this approach will not be reliable. Since heavy isotopes are not
fractionated by low-temperature chemical processes, the lead isotopic composition
should be preserved even if nuggets were chemically altered. Lead isotopes have been
used extensively to identify crustal provinces and could correlate with the gold deposit
areas.
3. Analytical methods with sufficient spatial resolution and analytical sensitivity are
required, since placer gold grains exhibit significant chemical variability and frequently
contain inclusion minerals. Knight et al. (1999) found that X-ray microprobe analysis of
polished samples of lode and placer gold from the Klondike District, Yukon was practical
only for Au, Ag, Hg, and Cu. Bulk analysis of individual grains is not likely to provide the
chemical detail required for a reliable fingerprint.
Sierra Nevada Foothills placer gold
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Christensen, Henry, Wood 2015
We conclude that chemical or isotopic fingerprinting of placer gold might be possible but
will require considerable effort just to test its potential. Until that technology is
developed, the history of these nuggets is best interpreted from the geological context in
which they are found, appreciating that the history thus interpreted may be only the
latest chapter in a much longer geological history.
CONCLUSIONS
The discovery of gold in California in 1848 quickly led prospectors from modern river
gravels to Tertiary river gravels to the bedrock orogenic gold-quartz vein deposits.
Following a similar path, it has been the traditional interpretation that gold nuggets in
modern alluvial gravels were derived from the auriferous Tertiary gravels, and all of the
gold was ultimately sourced from the bedrock quartz-gold veins of the Sierra Nevada
Foothills metamorphic belt.
This interpretation is difficult to accept for a number of reasons. 1) The amount of gold
in the gravels greatly exceeds that known in the lode deposits by a factor of at least
three. It would have required kilometers of erosion to expose that amount gold, which
fortuitously would have remained atop the bedrock source. 2) Nuggets or grains of
native gold are uncommon in most primary gold deposits, including the orogenic gold
deposits of the Sierra Nevada Foothills. 3) Much of the gold in the Tertiary gravels
occurs upstream, uphill, and laterally displaced from the major potential local bedrock
lode gold sources. 4) The fineness of the placer gold is significantly greater than that of
the underlying lode deposits – strong evidence for chemical reconstitution in the
supergene environment.
Modern eluvial placer gold accumulations are found in environments with intense,
commonly tropical, chemical weathering and thick laterite soil development. These
deposits typically overlie bedrock containing geochemical enrichment in gold, but not
necessarily over significant economic gold deposits. Over long periods of geologic time,
gold has been concentrated by removal of other elements. As the weathering front
advanced progressively downward with time, gold was repeatedly dissolved, mobilized
locally within groundwater, and reprecipitated as the native metal. With time, widely
dispersed gold was aggregated into grains and nuggets, often with delicate crystal
forms and with increasingly higher fineness. Conditions favoring this process are a
warm-wet climate, with lush vegetation, and tectonic stability.
The Late Cretaceous to mid-Tertiary climate of western North America was
demonstrably warmer and wetter than present. The evidence is clear in the floral and
faunal fossil record preserved in the upper Tertiary gravels. The interpreted climatic
conditions mirror those interpreted for worldwide climatic conditions. Remnant laterite
paleosols and the abundance of detrital kaolinite in the Ione Formation sediments
indicate conditions of intense chemical weathering during deposition of the Lower
Gravels, with moderating intensity of weathering during deposition of the Upper Gravels.
Most of the gold within the Tertiary channels is concentrated at the bedrock contact and
within the immediately overlying few meters of gravel. This suggests that as incision of
Sierra Nevada Foothills placer gold
35
Christensen, Henry, Wood 2015
the Tertiary channels began, there already existed in some source area, a large quantity
of gold nuggets and grains, readily available for erosion, transportation and deposition
within the Lower Gravels. An expansive plateau mantled by meters of laterite soil with
eluvial placer gold is the landscape we imagine to have been the source.
Clay mineralogy tells a consistent story. The mineral assemblage kaolinite-quartz is the
climax assemblage in a typical laterite profile. Early erosion of an auriferous laterite
plateau carried a distinctive sedimentary assemblage containing abundant kaolinite,
quartz, and native gold. The sediment load was carried within major west-flowing river
systems heading in central Nevada. At the break-in-slope at the base of the western
sierra, dense placer gold was concentrated among locally-derived boulders in the base
of the channels, while the finer quartz and kaolinite sediment was carried along to be
deposited in the littoral Ione Formations as the Tertiary rivers entered marine waters to
the west.
It is now appreciated that the Sierra Nevada Foothills were the western slope of the
high-elevation Nevadaplano that extended across the area that is now Nevada, with a
drainage divide in central Nevada. The Tertiary channels, which contain the auriferous
gravels in the Sierra Nevada Foothills, are known to have extended well into what is
now the Great Basin. Some rock clasts identified in the Tertiary placers were
demonstrably sourced in Nevada. In addition, the very well-rounded and polished
durable cobbles and boulders in the Lower Gravels demonstrate long distance
transport.
The age of the gravels is, unfortunately, poorly constrained. The Upper Gravels are
demonstrably older than about 31.5 Ma. The Lower Gravels are estimated to be
younger that about 50 Ma. This is too great an interval to be very useful, and certainly a
question worthy of further investigation.
Although it is clear that the bulk of the gold contained within the Tertiary gravels was not
sourced from the underlying quartz-gold veins and likely was sourced to the east of the
Sierra Nevada, no specific source has been identified. The amount of gold contained
within the placer deposits is greater than all but the largest of gold districts worldwide. It
is possible that the source of the gold may have been the combination of numerous
eluvial gold concentrations overlying a number of separate gold deposits across the
broad area of the Nevadaplano.
Carlin-type gold deposits are the most significant bedrock gold deposit type in Nevada
at the present time and current level of erosion. Most of these deposits formed at about
39 Ma. Although the age of the California Tertiary gravels is not well constrained, it is
likely that the maximum gold concentrations in the Lower Gravels are older than this,
and these CTDs are not likely bedrock sources.
A speculative possibility is that the gold was sourced from giant volcanic rock-hosted
high-sulfidation gold districts over the North American magmatic arc, similar to gold
districts presently exposed in the Andean Cordillera. Over millions of years, and
kilometers of weathering of the Sierran arc, gold grains and nuggets may have formed
by pedogenic processes to form eluvial or alluvial placers. Changes in topography,
Sierra Nevada Foothills placer gold
36
Christensen, Henry, Wood 2015
tectonics, or climate may have accelerated physical erosion, stripping the laterite
surface and flushing gold-rich gravel down the Tertiary river channels, to be deposited
at the break-in-slope where we find the deposits today. The occurrence of the placer
gold accumulations near to, and in some places overlying the lode quartz-gold veins, is
likely fortuitous, not genetic.
The model proposed here brings together observations of sedimentology, volcanology,
geochronology, paleoclimatology, clay mineralogy, ore deposits and tectonics in a broad
framework explanation with few details. The Tertiary auriferous sediments of the Sierra
Nevada Foothills have received surprisingly little careful study beyond the quest for
gold. Numerous burning questions of geochronology, clast provenance, clay
mineralogy, geomorphology and paleontology beg for further research. We emphasize
the critical importance of understanding past climates and former landscapes to
understanding the alluvial placer gold deposits of the Sierra Nevada Foothills, and other
placer gold fields worldwide.
ACKNOWLEDGEMENTS
Early versions of this story were presented by Odin Christensen to the Geological
Society of Nevada, the Four Corners Geological Society, and the Volcanological Society
of Sacramento. The questions, comments, and observations of numerous individuals
have contributed to our understanding. The authors acknowledge the contributions of
Reed Glasmann for elucidating the clay mineral distribution within the Tertiary gravels
and Ione Formation, of Bob Pease for sharing his experience with the auriferous gravel
deposits at North Columbia, and of Brian Hausback and Tim McCrink for observations
about the Sierra Nevada channels and their deposits. A review of the manuscript by
Roger Steininger improved the quality of the final paper.
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