9.4: Gold of the Sierra Nevada

How Gold Forms

A mineral deposit is a place in Earth’s crust where geologic processes have concentrated one or more minerals at greater abundance than in the average crust. An ore deposit is a mineral deposit that can be produced to make a profit. Thus, all ore deposits are mineral deposits, but not all mineral deposits are ore deposits.

Ores and ore minerals vary greatly in quality. Ideal ores contain 100% ore minerals. Such ores do not exist, but some come close. Ideal ore minerals contain 100% of the commodity of interest. Native copper, for example, is an ideal copper ore mineral. Ideal ore minerals are, however, uncommon. The most commonly mined ores are not ideal. Instead they are rich in ore minerals that can be processed relatively inexpensively to isolate desired components.

Gold is a unique ore in that it comes in two forms: as native gold (like the gold nugget in Figure 9.4.1), and as the mineral calaverite (AuTe₂) (Figure 9.4.2). The name "calaverite" originates from its discovery in Calaveras County, California. The foremost is a large natural nugget from a placer deposit in California. The discovery of gold nuggets in rivers and streams of California's Sierra Nevada foothills triggered the famous "Gold Rush" of 1848 and 1849. The gold is ultimately derived from quartz-gold hydrothermal veins in the bedrock of the Sierra Nevada Mountains of eastern California--but more on that in a moment.

Figure \(\PageIndex{1}\): Large gold nugget (placer gold) from California by James St. John is licensed under CC BY 2.0. Access a detailed description.

Figure \(\PageIndex{2}\): Gold-colored mineral = calaverite (AuTe2); Purplish areas = fluorite (CaF2) by James St. John is licensed under CC BY 2.0. Access a detailed description.

Gold, commonly mined as nuggets, flakes, wires, or scales, is often found in quartz-rich hydrothermal veins, typically alongside silver. While large, visible gold specimens, like those shown in Figure \(\PageIndex{3}\), are rare, most gold occurs in very fine grains, often microscopic. These precious metals are usually embedded within the quartz, forming part of intricate mineral veins.Within these veins, concentrated pockets of gold, known as ore shoots, can form large, vertical bodies rich in gold and silver.

Figure \(\PageIndex{3}\): Placer gold flakes by James St. John is licensed under CC BY 2.0. Access a detailed description.

In the western Sierra Nevada foothills, gold is predominantly found in these quartz veins, rather than being alloyed with other elements to form gold-bearing minerals. These veins can range from 30 cm to 30 meters (~12 inches to 98 feet) in thickness and often include other minerals like calcite and pyrite, also known as "fool's gold."

Lode Deposits

Sometimes geological processes concentrate ore minerals in vein deposits consisting of veins that are centimeters to meters thick. If ore is distributed in many small veins, geologists call a lode deposit. Vein deposits account for most of the world’s gold and silver mines, and also some copper and lead-zinc mines. Figure 9.4.4 shows a gold bearing quartz veins from the Mother Lode, California. In still other kinds of igneous deposits, ore minerals become concentrated in layers.

Figure \(\PageIndex{4}\): Gold bearing quartz from the Mother Lode, CA by United States Geological Survey is licensed under public domain. Access a detailed description.

Lode deposits represent one of the primary forms of gold mineralization in the Sierra Nevada. These deposits typically occur within quartz veins, faults, and shear zones, reflecting the legacy of tectonic forces that have shaped the region. As hot fluids rich in dissolved minerals migrate through fractures in the earth's crust, they encounter favorable conditions for gold deposition. Upon cooling, these fluids precipitate gold along with other minerals, forming veins of economic significance.

Figure \(\PageIndex{5}\): Tectonic Setting of Most Common Gold Deposits by Nguimatsia et al. is licensed under CC BY 4.0 DEED. Access a detailed description.

As Figure \(\PageIndex{5}\) shows, granitic intrusions buoy up through the crust carrying hot, mineral saturated fluids. The granitoids in Figure \(\PageIndex{2.1A}\) would be a portion of these hot fluids. The black dots and squares that are located in the continental arc would be the lode deposits that are injected into place along the Sierra Nevada.

The formation of lode deposits in the Sierra Nevada is intricately linked to the region's geological history. During periods of mountain building and tectonic activity, such as the Sierra Nevada's uplift during the Cenozoic era, fractures and faults provided conduits for mineral-rich fluids to migrate upwards from deep within the earth. Over time, the deposition of gold within these structures resulted in the formation of economically viable ore bodies.

Lode deposits in California are primarily found in the western foothills of the Sierra Nevada, often associated with regions of active or past tectonic activity. These deposits occur in a variety of geological settings, including fault zones, shear zones, and igneous intrusions. The gold mineralization in lode deposits is thought to be primarily of hydrothermal origin, with gold-bearing fluids migrating through fractures and faults in the surrounding rocks. Over time, the deposition of gold within these structures forms economically viable ore bodies, which have been extensively mined throughout California's history.

Lode Deposits and California’s Gold Rush

The Mother Lode, an uninterrupted expanse of gold-bearing quartz veins and gold-enriched rock, extends for approximately 200 kilometers along the Melones fault zone. Journeying along California's historic Highway 49, which traverses Gold Rush towns and mines, also means traveling along the boundary, or suture, between accreted oceanic terranes. Numerous mines were developed to trace the quartz veins within this 1.5-kilometer-wide belt. Given the inclined nature of most veins, the lode deposits have the potential to extend to depths reaching thousands of meters beneath the Earth's surface.

While the initial discoveries of gold in California were in the form of nuggets within stream gravels, early miners quickly realized that the origin of the stream sediments lay in gold-bearing quartz veins. These veins were found to be predominantly situated in the metamorphic rocks of the western foothills. Subsequently, geologists established a connection between lode deposits and submarine volcanic rocks originating from accreted terranes, which had been intruded by more recent granitic rocks.

Geologists theorize that the gold in the Sierra Nevada region was initially deposited within rocks from accreted oceanic terranes. The migration of fluids enriched with gold likely occurred in the vicinity of submarine volcanic vents, leading to the creation of mineralized zones akin to those observed in contemporary submarine spreading centers, known as "black smokers." Following the accretion of these rocks to North America, the intrusion of the Sierra Nevada Batholith generated heat that mobilized and concentrated the gold in the veins observed today. As previously mentioned, Figure \(\PageIndex{4}\) exhibits gold bearing quartz from the Mother Lode of California.

Box: Hydrothermal Vents - "Black Smokers"

Hydrothermal vents, known colloquially as black smokers, are captivating geological phenomena discovered in the depths of the ocean. Formed by the dynamic interplay between seawater and the intense heat of subterranean magma, these vents release jets of superheated water laden with minerals and gases. The moniker "black smokers" originates from the appearance of these vents, which emit dark plumes resembling smoke as they disperse into the surrounding oceanic depths (Figure \(\PageIndex{6}\)).

While predominantly situated along mid-ocean ridges, where tectonic plates diverge, hydrothermal vents are also found in unexpected locales, such as subduction boundaries. Here, the process of plate subduction creates conditions conducive to the formation of these remarkable features. These vents, whether nestled in the seafloor trenches or dotted along volcanic arcs, serve as vital ecosystems, harboring a plethora of life uniquely adapted to thrive in the extreme conditions they offer.

These ecosystems are home to an astonishing array of life, from resilient extremophiles like thermophiles to complex organisms such as giant tube worms and deep-sea crabs. The high temperatures and chemical composition of the vent fluids provide the necessary energy and nutrients to sustain this diverse community, which forms intricate food webs. The study of hydrothermal vents not only deepens our understanding of geological processes shaping our planet but also broadens our perspective on the potential for life in harsh environments, both here on Earth and beyond.

Figure \(\PageIndex{6}\): Venting black smoker emits jets of particle-laden fluids by NOAA is licensed under public domain. Access a detailed description.

These gold-bearing veins traverse Mesozoic metamorphic rocks and some plutonic rocks, indicating that the gold mineralization occurred subsequent to the formation of these rocks. Through the dating of rocks predating and succeeding the gold-bearing veins, geologists estimate that the emplacement of the gold veins took place approximately between 140 and 160 million years ago.

Placer Deposits

In addition to hard-rock deposits, gold is also found in placer deposits, or simply, placers: accumulations in river, stream, or other kinds of sediments.

Placer deposits develop when heavy minerals, weathered from igneous, sedimentary, or metamorphic rocks, are transported by streams. Eventually, the water slows enough for those dense minerals to be deposited, usually along point boars, in braided streams, or in alluvial fans. Placer deposits form when one or more minerals concentrate in this way to become an ore deposit. The word placer is Spanish for alluvial sand. Figure 9.4.7 shows the general origin of placer gold deposits originating from "Mother Lode" deposits as they erode into streambeds and are carried down slope.

Figure \(\PageIndex{7}\): The origin of placer gold deposits. Gold in these veins is liberated by weathering and erosion into streams. Gold is dense and naturally settles out of water when it slows down. This work by Dexter Perkins is licensed under CC BY-SA. Access a detailed description.

Placer gold set off the historically important California Gold Rush of 1849. The original sources of minerals in placers are often difficult to determine but, in California, the source has been identified. The California gold weathered from extensive vein deposits, called the mother lode, in the Sierra Nevada Mountains. The lode is in a zone east of Sacramento and San Francisco that is 1 to 6 km (0.62 to ~3.7 miles) wide and extends almost 200 km (~124 miles) north-south. Mother Lode gold is in quartz veins up to 20 meters (~66 ft) thick and thousands of meters long.

The California gold rush resulted in California being admitted to the United States in 1850 and later provided a name for a San Francisco football team (the 49ers). Although once one of the most productive gold-producing districts in the United States, the Mother Lode is presently mostly a tourist destination and the home of wineries.

Placer minerals must be both dense and durable to be deposited and remain in place without decomposing. Native metals such as copper or gold, sulfide minerals such as pyrite or pyrrhotite, and oxide minerals such as magnetite or ilmenite are all dense and likely to be found in placers. Metal oxides, especially magnetite (iron oxide), are common and especially dense and durable, and often dominate such deposits. And gold is dense and extremely resistant to any kind of weathering and so can accumulate in stream and river sediments. Gold in placer deposits is found as nuggets that range from microscopic size to basketball size.

Box: Timeline of the 1849 Gold Rush

The discovery of gold at Sutter's Mill in January 1848 triggered the California Gold Rush, leading to a massive influx of prospectors and settlers seeking fortune in the Sierra Nevada foothills. By the end of 1848, news of the discovery had spread across the United States and internationally, sparking "gold fever" and prompting thousands to embark on the perilous journey to California. In 1849, known as the peak of the Gold Rush, approximately 80,000 "49ers" arrived, transforming San Francisco from a small town into a booming city.

Throughout 1849, mining camps sprang up along the Sierra Nevada's rivers and streams, where prospectors used pans and simple tools to extract gold from placer deposits. The rapid population growth and intense mining activity led to significant environmental changes, with rivers being diverted, forests cleared, and hillsides stripped bare. By the end of the year, California had become a bustling hub of economic activity, leading to its admission as the 31st state in 1850. The Gold Rush era left a lasting legacy on California's development and the cultural and physical landscape of the Sierra Nevada.

Figure \(\PageIndex{8}\): 1852 miners in California work their "long tom" to separate gold from gravel by the National Park Service is in the public domain. Access a detailed description.

Gold Recovery

One of the simplest and enduring methods for separating gold from sediments was through panning, a technique that remains popular among hobbyists today. California miners employed either a flat-bottomed pan crafted from iron, tin, or a wooden bowl. The process involved filling the pan with sediment and water, extracting larger gravel, and then submerging the pan into a stream or pool at a slight tilt while swirling the sediment. Lighter materials were carried away, leaving behind the heavier grains or "pay dirt," which often included heavy minerals like magnetite and pyrite, along with the desired gold flakes or nuggets. The miner would be optimistic upon seeing a glint in the pan, but there were occasions when the sediment did not yield the desired results.

Panning, although a simple and cost-effective method of placer mining, is limited in its ability to handle large volumes of sediment. In response, miners developed more efficient methods for processing substantial amounts of material. One such innovation was the rocker or cradle, employed as early as 1848. The rocker, resembling a long box mounted on rockers akin to a baby's cradle, operated on a slope with one end positioned downhill and the other uphill. At the top end of the cradle, there was a hopper equipped with a sieve at its base.

To facilitate the process, water was continuously poured onto the hopper, generating a current. Gold, being heavier, tended to settle in the rocker box behind cleats, effectively trapping it. The cradle's design allowed three or more individuals to collaborate in washing a substantial volume of sediment. Many of California's most lucrative and easily accessible placer deposits were worked using a combination of panning and the cradle.

In the latter part of 1849, miners initiated the extensive movement of sediment by diverting rivers to access the gold concealed within the stream beds. Figure \(\PageIndex{9}\) displays a map published by the USGS in 1968 and shows a shaded focused area along the Yuba River of placer deposits in Nevada County, CA.

Figure \(\PageIndex{9}\): Index map of part of Northern California showing location of 1968 Placer Deposits in Nevada County, CA by the USGS is in the public domain. Access a detailed description.

Environmental Impacts of Lode Extraction

The extraction of lode gold deposits in the Sierra Nevada has historically had significant environmental impacts, many of which continue to affect the region today. Lode mining involves the excavation of hard rock to access veins of gold embedded within the bedrock. This process often requires extensive tunneling and the removal of large volumes of rock, leading to substantial landscape alteration and habitat disruption.

One of the primary environmental concerns associated with lode mining is the generation of mine tailings, which are the waste materials left after the gold has been extracted from the ore. These tailings, often containing heavy metals and other toxic substances, are typically stored in tailings ponds or heaps. Over time, these storage sites can leak or fail, releasing harmful contaminants into nearby soil and water bodies, referred to by geologists as acid mine drainage. The leaching of heavy metals such as mercury, arsenic, and lead from mine tailings into streams and rivers poses a significant threat to aquatic ecosystems and human health.

While not directly related to lode extraction, the Spring Creek acid mine drainage event has been an ongoing issue since the mid-20th century, with the most significant impacts occurring in the 1960s and 1970s and is an example of potenital environmental impacts from mining (Figure \(\PageIndex{10}\)). The problem originated from the Iron Mountain Mine, where extensive mining activities exposed sulfide-rich ore bodies (e.g. copper, zinc, and pyrite), leading to severe acid mine drainage. The peak of the environmental impact occurred during heavy rainfall events, which exacerbated the flow of acidic water into Spring Creek and subsequently the Sacramento River. The situation was particularly dire in the late 1960s, prompting environmental cleanup efforts that have continued to this day.

Figure \(\PageIndex{10}\): Acid mine drainage into Spring Creek, in Shasta County by NOAA is in the public domain. Access a detailed description.

Water usage in lode mining is another critical issue. The processing of ore to extract gold requires large quantities of water, which can deplete local water resources and affect downstream water availability. Additionally, the diversion of rivers and streams to facilitate mining operations can alter natural water courses, impacting aquatic habitats and the organisms that depend on them. The historical practice of diverting rivers, as mentioned in the preceding text, often led to increased sedimentation and erosion, further degrading water quality and riparian environments.

Acid mine drainage (AMD) is a particularly severe environmental impact associated with lode mining. When sulfide minerals in the excavated rock are exposed to air and water, they can oxidize, producing sulfuric acid. This acid can then leach heavy metals from the surrounding rock, creating highly acidic runoff that can contaminate nearby water bodies. AMD can persist for decades or even centuries after mining operations have ceased, making it a long-term environmental challenge.

The physical alteration of landscapes due to lode mining, including the creation of open pits, waste rock piles, and mine shafts, can lead to habitat destruction and fragmentation. These changes can have profound effects on local wildlife, disrupting migration patterns, breeding grounds, and food sources. In the Sierra Nevada, where biodiversity is rich and includes numerous endemic species, the loss of habitat due to mining activities has had lasting ecological consequences.

In response to these environmental challenges, regulations and remediation efforts have been implemented to mitigate the impacts of lode mining. The U.S. Geological Survey (USGS) and other agencies have conducted studies to assess the extent of contamination and to develop strategies for restoring affected areas. Remediation efforts often involve the stabilization of tailings, treatment of contaminated water, and rehabilitation of disturbed landscapes. However, the legacy of historical mining activities continues to pose environmental risks, highlighting the need for ongoing monitoring and management.

Overall, while lode mining has played a crucial role in the economic development of the Sierra Nevada region, it has also left a complex legacy of environmental degradation. Addressing these impacts requires a comprehensive understanding of the historical and current practices of mining, as well as a commitment to sustainable and responsible resource management.

Environmental Impacts of Placer Extraction

In the early 1850s, a significant portion of the easily accessible placer gold had been extracted from the riverbeds in the Western Foothills. The entire beds of most stream channels had been extensively worked, with some areas being overturned more than once. The rivers underwent substantial modifications, including diversions and damming, to access the placer gold. Even in the present day, remnants of this intensive mining activity are evident throughout the foothills, where piles of sediment persist, resembling the excavations of numerous large-scale prospectors.

Following the depletion of easily accessible placer deposits, miners shifted their focus to older placer formations created by streams flowing westward from a previous Sierra Nevada range. Cornish miners were among the first to explore these older, buried gravel deposits, which geologists referred to as Tertiary gravels. Due to the uplift of the Sierra Nevada, these Tertiary gravels are now situated above contemporary river channels. Consequently, miners needed a method to wash these gravels effectively to access the gold concentrated near the bottom layers of sediment.

In certain regions, miners took to tunneling beneath the Table Mountain volcanic flows to reach the ancient gravels beneath. The Omega Mine near Jamestown stands as an example of successful tunneling efforts in this pursuit.

In 1853, resourceful miners conceived the idea of affixing a nozzle to a hose and using forceful water jets to wash sediment down from the hillsides, giving rise to hydraulic mining. This method, which drastically altered the landscape from the Sierra Nevada foothills to San Francisco Bay, represented a significant departure from the slower processes of earlier mining practices. Instead of being limited to washing 1 cubic yard of sediment in a day, or 4 cubic yards with a sluice, a miner employing hydraulic systems alongside a sluice could wash 50 to 100 cubic yards per day. Movement of such materials is exhibited in Figure \(\PageIndex{11}\).

However, the widespread use of hydraulic mining had unintended consequences. The massive influx of sediment into the streams of the Sacramento Valley and the San Francisco-San Joaquin Delta began to impact navigation and irrigation. In response to these issues, the Sawyer Decision of 1884 prohibited the dumping of mining-related sediment into the Sacramento and San Joaquin Rivers and their tributaries. Even today, the effects of sedimentation from hydraulic mining in the late 1800s are still evident in the eastern part of the San Francisco Bay, more than a century after the mining activities took place.

Figure \(\PageIndex{11}\): Hydraulic Mining, California, 1870s by United States Geological Survey is licensed under public domain. Access a detailed description.

Gold Production Since the Mid 1800s

California boasts notable instances of substantial gold finds, such as the 195-pound mass uncovered at Carson Hill in 1854 and a 54-pound pure gold nugget discovered at Magalia in the same year, albeit the latter was melted down shortly after its revelation. However, such large masses are exceptional. Most of the gold extracted in the Sierra Nevada region has originated from deposits containing between 1/4 and 1/2 ounce of gold per ton of ore.

In geological terms, ore refers to any accumulation of a specific element, including metals like gold, silver, or mercury, that has been adequately concentrated to be economically viable for mining. It is evident from this definition that economic factors, accessibility, and technological advancements play crucial roles in determining what qualifies as ore. Materials discarded as waste rock during one period may become profitable to mine in later years, influenced by fluctuations in commodity prices or the introduction of new mining methods and technologies.

California's gold production has experienced fluctuations driven by changes in economic conditions, accessibility, and technological advancements. Ventures and investments in precious metals, like gold, are inherently financially risky due to the intricacies of market conditions and the geological uncertainties associated with ore deposits.

In the 1980s, a surge in the price of gold, coupled with the adoption of a novel extraction method, triggered a renewed interest in mining operations at previously abandoned sites in the Sierra Nevada. Today, mining companies are employing a chemical process known as heap leaching to extract gold from deposits that were previously considered unprofitable. Heap leaching, which utilizes cyanide to dissolve gold from quartz veins, has significantly enhanced gold recovery, surpassing other methods for lode deposit mining by a factor of 100. This method is presently in use at various mines in California and Nevada. As of 2023, there were 15 active and/or newly permitted gold mines in California. Figure \(\PageIndex{12}\) shows the locations of active and close gold mines from 1995 to the start of 2023.

Figure \(\PageIndex{12}\): CA Gold Mines by Type and Status 1995-2023 by Cole Heap (2023) is licensed under CC BY-NC 4.0. Constructed in a GIS using data from data.ca.gov. Access a detailed description.

References

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