Geologic map depicting the Smartville Complex and other accreted terranes in California

The Smartville Block, also called the Smartville Ophiolite, Smartville Complex, or Smartville Intrusive Complex, is a geologic

terrane In geology, a terrane (; in full, a tectonostratigraphic terrane) is a crust fragment formed on a tectonic plate (or broken off from it) and accreted or " sutured" to crust lying on another plate. The crustal block or fragment preserves its d ...

formed in the ocean from a volcanic

island arc Island arcs are long archipelago, chains of active volcanoes with intense earthquake, seismic activity found along convergent boundary, convergent plate tectonics, tectonic plate boundaries. Most island arcs originate on oceanic crust and have re ...

that was accreted onto the North American Plate during the late

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...

(~160–150 million years ago). The collision created sufficient crustal heating to drive mineral-laden water up through numerous fissures along the contact zone. When these cooled, among the precipitating minerals was

gold Gold is a chemical element; it has chemical symbol Au (from Latin ) and atomic number 79. In its pure form, it is a brightness, bright, slightly orange-yellow, dense, soft, malleable, and ductile metal. Chemically, gold is a transition metal ...

. Associated with the Western Metamorphic Belt of the Sierra Nevada foothills it extends from the central

Sierra Nevada The Sierra Nevada ( ) is a mountain range in the Western United States, between the Central Valley of California and the Great Basin. The vast majority of the range lies in the state of California, although the Carson Range spur lies primari ...

mountain range, due west, under a section of the Central Valley and

California Coast Ranges The Coast Ranges of California span from Del Norte County, California, Del Norte or Humboldt County, California, south to Santa Barbara County, California, Santa Barbara County. The other three coastal California mountain ranges are the Trans ...

, in northern California. The ophiolitic sequence found in this terrane is one of several major ophiolites found in California.

Ophiolites An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle (Earth), upper mantle that has been uplifted and exposed, and often emplaced onto continental crustal rocks. The Greek word ὄφις, ''ophis'' (''snake'') is ...

are crustal and upper-mantle rocks from the ocean floor that have been moved on land. Ophiolites have been studied extensively regarding the movement of crustal rocks by

plate tectonics Plate tectonics (, ) is the scientific theory that the Earth's lithosphere comprises a number of large tectonic plates, which have been slowly moving since 3–4 billion years ago. The model builds on the concept of , an idea developed durin ...

Gold

Gold was discovered in California in the mid 1800s. The Smartville Block is named for the small Gold Rush town of Smartville in Yuba County, California. Gold found in the Smartville Block and within the metamorphic belts of the Sierra Nevada foothills were created through hydrothermal processes. Mineral rich groundwater is heated geothermally and injected towards the surface through fractures in the rock created from faulting. Gold and other minerals are precipitated from this fluid and have accumulated in the Sierra Nevadan metamorphic belt region in features called

veins Veins () are blood vessels in the circulatory system of humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are those of the pulmonary and fetal c ...

. The Smartville Block's eastern margin is the Rescue Lineament-Bear Mountains fault zone, and generally defines the range of gold-bearing veins of the Mother Lode region, important in the history of the

California Gold Rush The California gold rush (1848–1855) began on January 24, 1848, when gold was found by James W. Marshall at Sutter's Mill in Coloma, California. The news of gold brought approximately 300,000 people to California from the rest of the U ...

Geology

The Smartville Ophiolite is divided into sections based on

lithology The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lit ...

that describe the origin of the rocks it is made up of. In order of increasing depth: the top most unit is called the upper volcanic unit, then the lower volcanic unit, followed by the dike complex, and the plutonic suite at the bottom. The upper volcanic unit consists mainly of

pillow lava Pillow lavas are lavas that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava underwater, or ''subaqueous extrusion''. Pillow lavas in volcanic rock are characterized by thick sequences of discontinu ...

and massive lavas with

turbidites A turbidite is the geologic Deposition (geology), deposit of a turbidity current, which is a type of amalgamation of fluidal and sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean. Sequencing ...

also being common. The turbidites of the upper volcanic unit are made of a

siliceous rock Siliceous rocks are sedimentary rocks that have silica (SiO2) as the principal constituent. The most common siliceous rock is chert; other types include diatomite. They commonly form from silica-secreting organisms such as radiolarians, diatoms, o ...

type and have

sandstones Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains, cemented together by another mineral. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed o ...

and conglomerate tops. Next is the lower volcanic unit, which is mostly pillow lava,

breccia Breccia ( , ; ) is a rock composed of large angular broken fragments of minerals or Rock (geology), rocks cementation (geology), cemented together by a fine-grained matrix (geology), matrix. The word has its origins in the Italian language ...

, and intrusive rocks. Metasediments of mostly

mafic A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...

and intermediate

tuffaceous Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock cont ...

composition are cut by dikes (intrusive rock). Below the lower volcanic unit is the dike complex. The dike complex has

dacite Dacite () is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyolite. ...

and

rhyolite Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture (geology), texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained matri ...

dikes that cut into the rocks of the lower volcanic unit. At the base is the plutonic suite that are made up of

gabbro Gabbro ( ) is a phaneritic (coarse-grained and magnesium- and iron-rich), mafic intrusive igneous rock formed from the slow cooling magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is ch ...

diorite Diorite ( ) is an intrusive rock, intrusive igneous rock formed by the slow cooling underground of magma (molten rock) that has a moderate content of silica and a relatively low content of alkali metals. It is Intermediate composition, inter ...

, and other plutonic rocks that are intruded by dikes as well as granitic plutons from the

Nevadan Orogeny The Nevadan orogeny occurred along the western margin of North America during the Late Jurassic to Early Cretaceous approximately 155 Ma to 145 Ma. Throughout the duration of this orogeny there were at least two different kinds of orogenic process ...

References

Additional Reading

* ''Assembling California'',

John McPhee John Angus McPhee (born March 8, 1931) is an American author. He is considered one of the pioneers of creative nonfiction. He is a four-time finalist for the Pulitzer Prize in the category General Nonfiction, and he won that award on the fourt ...

, 1993. Farrar, Straus and Giroux, New York. * Day, Howard W., E. M. Moores, and A. C. Tuminas. "Structure and tectonics of the northern Sierra Nevada." ''Geological Society of America Bulletin'' 96.4 (1985): 436–50. * Dilek, Yildirim. “Tectonic Significance of Post-Accretion Rifting of a Mesozoic Island-Arc Terrane in the Northern Sierra Nevada, California.” ''The Journal of Geology'', vol. 97, no. 4, 1989, pp. 503–518. * Dilek, Yildirim. "Structure and tectonics of an Early Mesozoic oceanic basement in the northern Sierra Nevada metamorphic belt, California: Evidence for transform faulting and ensimatic arc evolution." ''Tectonics'' 8.5 (1989): 999–1014. * Menzies, Martin, Douglas Blanchard, and Costas Xenophontos. "Genesis of the Smartville arc-ophiolite, Sierra Nevada foothills, California." ''American Journal of Science'' 280 (1980): 329–44. * Saleeby, Jason B. "Polygenetic ophiolite belt of the California Sierra Nevada: Geochronological and tectonostratigraphic development." ''Journal of Geophysical Research: Solid Earth'' 87.B3 (1982): 1803–24. * Taylor, Ryan D. (2015). ''Orogenic gold formation and tectonic evolution of the Grass Valley gold district and temporal correlations of gold deposits in California''. Diss. Colorado School of Mines. Arthur Lakes Library. * Xenophontos, Costas, and Gerard C. Bond. "Petrology, sedimentation and paleogeography of the Smartville terrane (Jurassic)-bearing on the genesis of the Smartville ophiolite." (1978): 291–302. {{California Faults Terranes Geology of California California Gold Rush Sierra Nevada (United States) Geography of Amador County, California Geography of Calaveras County, California Geography of El Dorado County, California Geography of Mariposa County, California Geography of Tuolumne County, California Geography of Yuba County, California Seismic faults of California

Gold

Geology

See also

References

Additional Reading