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CONTENTS

Abstract Introduction Background Geologic terranes-descriptions, origins, and mineral deposits Cratonic platform terrane Shallow marine terranes (cratonic)

Batholithic terranes Island-arc terranes Oceanic terranes Composite terranes Cover terranes Tectonic history of California California mineral potential California desert region

Peninsular Ranges and Salton Trough Transverse Ranges Sierra Nevada Great Valley Coast Ranges Klamath Mountains Modoc Plateau and Southern Cascade Range References cited Appendix-geologic time chart

FIGURES

1. Map of generalized geologic terranes of California
2. Map of physiographic provinces, places noted in text, and major active faults of California

Cover-View east toward folded outcrop of limestone of the Lost Burro Formation of Middle and Late Devonian age in the southern Inyo Mountains, California. Ore bodies within this formation at the Cerro Gordo mine, 8 mi southeast of the locality, contained silver, gold, lead, zinc, and minor amounts of copper. Saline Valley, at lower right, contains numerous known saline deposits. Photograph by Michael F. Diggles. Shallow marine carbonate terrane Shallow marine noncarbonate terranes

Introduction Geologic history of the region History of mineralization History of mineral resource production Metals Special metals (rare-earth elements, uranium) Nonmetallic commodities

BACKGROUND

mineral deposit types; they divided the State into eight types of geologic terranes, each of which contains a characteristic set of deposit types.

The term "geologic terrane," as used here, refers to a discrete portion of the Earth's crust that was created in a specific plate-tectonic setting; in some cases, these terranes formed far from the place where the terrane is now exposed. Each type of geologic terrane is composed of a particular assemblage of rock types; the assemblage found in a given terrane reflects the geologic processes characteristic of the plate-tectonic environment or setting in which it formed. Similarly, the formation of mineral deposits in these rock assemblages results from the action of specific geologic processes associated with particular plate-tectonic settings. Thus, each type of geologic terrane has a unique set of rock types and associated mineral deposit types that reflect the geologic processes characteristic of a particular plate tectonic setting. Specific geologic terrane types and their associated suites of mineral deposits are described below.

In this report, deposits of the metallic commodities are given the most thorough consideration. Occurrences of, and areas permissive for, oil and gas deposits and geothermal energy resources are mentioned briefly if they occur in the province under discussion. Consideration of industrial materials (sand and gravel, aggregate, building stone, and specialized deposits like zeolites, gypsum, and perlite) and other nonmetallic deposits (evaporite or saline deposits, for example) is generally restricted to discussions of resources in the California desert region.

Estimates of mineral resource potential noted in this report are based largely on the compilations of Albers (1981) and Albers and Fraticelli (1984); numerous other published sources have also been used, especially for resource data on the California desert region.

GEOLOGIC TERRANES-DESCRIPTIONS, ORIGINS, AND MINERAL DEPOSITS

The major characteristics, origins, and typical mineral deposit types of each of the geologic terrane types in California (fig. 1) are outlined in this section. A more complete account of the mineral resources and potential of each major physiographic region in the State (fig. 2) is presented in the last part of this report.

Cratonic Platform Terrane

Cratonic platform terrane makes up a large part of the eastern California desert region and is therefore considered in some detail. A cratonic platform is part of the stable, tectonically inactive core of a continent, usually consisting of a complex of deformed metamorphic and igneous rocks overlain by flat-lying, stratified sedimentary rocks. In California, the cratonic platform terrane consists of a basement of Precambrian igneous and metamorphic rocks overlain by a thin sequence of upper Precambrian and Paleozoic sedimentary rocks, all intruded by granitoid plutons, mostly of Mesozoic age (Albers, 1981). The complex history of the Precambrian basement rocks is poorly understood at present, due to lack of intense study, scarcity of outcrop, and deformation during and since late Precambrian time. The igneous and sedimentary protoliths of these basement rocks, however, probably formed in plate tectonic environments analogous to those of the present. The most significant mineral deposits associated with the cratonic platform terrane include disseminated-gold deposits, contact metasomatic (replacement) deposits of iron, replacement and vein deposits of silver, lead, or zinc, and the single largest rare-earth-element deposit in the United States (Albers, 1981; Warhol, 1980).

Shallow Marine Terranes (Cratonic)

Shallow marine terranes consist of rocks that were deposited as a growing wedge of sediments along the continental shelf at the margin of a craton. In California, the thin layer of upper Precambrian and Paleozoic sedimentary rock that overlies the cratonic platform terrane of the Mojave and Sonoran Deserts thickens to the north and west, where it is part of a band of shallow marine sediments that extends from northern Canada to northern Mexico (Albers, 1981). This band can be divided into two parts: an eastern carbonate terrane, discussed first, that grades westward into a noncarbonate terrane dominated by clastic rocks.

The shallow marine carbonate terrane consists primarily of thick carbonate units (limestone and dolomite) with very subordinate quartzite and shale. These rocks crop out primarily in the Basin and Range province and the northern Mojave Desert. The most significant mineral occurrences within this terrane are replacement and vein lead-silverzinc deposits and contact metasomatic and vein tungsten deposits, all found at or near contacts with Mesozoic intrusive rocks (Albers, 1981).

The western part of the late Precambrian and Paleozoic band of shallow marine deposits, in contrast to the eastern part, consists mostly of quartzite and fine-grained clastic sedimentary rocks. This group of rocks, which crops out north, south, and west of the carbonate terrane, is transitional to deep marine sedimentary rocks found in western Nevada and in isolated exposures in the Sierra Nevada. These rocks host contact metasomatic and vein tungsten deposits, replacement and vein deposits of silver, lead, and zinc, and some gold-quartz vein deposits. The area southwest of the San Andreas fault between Point Arena and the Transverse Ranges, commonly called the Salinian block, is underlain by metamorphosed shallow marine clastic rocks intruded by Mesozoic granitoid plutons. Metallic mineral deposits are extremely rare in the Salinian block.

Batholithic Terranes

Batholithic terranes are composed primarily of granitoid plutons that rose from zones of melting in and above long-lived subduction zones. Granitoid rocks commonly intrude other terranes, especially those adjacent to the batholiths themselves, but only those areas where the plutons occupy nearly all the exposed surface are referred to as batholithic terranes in this report. There are two such terranes in California, both of Mesozoic age: the Sierra Nevada and the Peninsular Ranges batholiths. The granitoid rocks of batholithic terranes are usually devoid of mineral resources except where they are in contact with other rocks, especially those composed of metasedimentary sequences that contain carbonate rocks. Commodities found within the batholithic terranes include tungsten, molybdenum, and tin in contact metasomatic and vein deposits, iron in contact metasomatic deposits, and gold in quartz veins.

Island-Arc Terranes

Island-arc terranes are typically dominated by volcanic and volcaniclastic rocks of basaltic to andesitic composition, locally intruded by gabbroic to dioritic plutons. These terranes commonly show evidence of compressive deformation, apparently related to the accretion of the terrane onto the continent and to later joining of other terranes. Island-arc terranes in California occur in the Klamath Mountains, the northwestern Sierra Nevada, and the Peninsular Ranges. In the northern areas they are present within larger zones that also include accreted oceanic and composite terranes, all of Paleozoic to early Mesozoic age. Island-arc terranes typically contain lead-silver-zinc-copper-iron massive sulfide deposits, gold-quartz veins, vein and disseminated deposits of copper, contact metasomatic and vein iron and tungsten deposits, and chert-associated and fissure deposits of manganese.

Oceanic Terranes

Modem oceanic crust is typically composed of a simple sequence of igneous rocks, crystallized at midocean ridges, overlain by fine-grained sediments deposited as the ocean floor moves away from the ridge. Ancient oceanic terranes now preserved within continents, however, are usually highly deformed, reflecting the extremely disruptive nature of the processes by which oceanic crust becomes joined to continents. Some oceanic terranes consist

Cover Terranes

plutons in the Salinian block of the southern Coast Ranges. Additional oceanic, island-arc, and composite terranes of the western Sierra Nevada and Klamath Mountains were also added to the continent during this period, as one or more island arcs collided with the continental margin. Rocks of the oceanic terranes of the Coast Ranges, consisting mostly of melange (but including less-disrupted sequences of sediments), formed just landward of the trench during late Mesozoic time, and the Great Valley sequence of sediments filled a forearc basin that separated the magmatic arc from the trench.

During the later part of the Cenozoic era, subduction in the southern half of present-day California began to give way to transform motion along faults of the San Andreas system, and the rocks southwest of the fault were transported hundreds of miles northwestward. North of Cape Mendocino, where the San Andreas fault veers westward into the Pacific Ocean, subduction continued, and the accompanying magmatism resulted in the late Cenozoic and Quaternary volcanism that characterizes the Modoc Plateau and the Cascade Range. Crustal extension (stretching) in eastern California and Nevada during this period led to the formation of the elongate mountains and valleys that characterize the Basin and Range province.

The coastal region from Morro Bay to Mexico is covered by Cenozoic sedimentary rocks, and the Great Valley is filled to great depths with late Mesozoic and Cenozoic sediments shed from the Sierra Nevada and the Coast Ranges. Older terranes beneath the Cascade Range and Modoc Plateau areas are completely hidden beneath late Cenozoic to Holocene volcanic rocks. All of these areas are classified as cover terrane.

CALIFORNIA MINERAL POTENTIAL

In this section, the mineral resources of each of the major physiographic regions in California (fig. 2) are considered separately. The province names and boundaries follow or are modified from those of Fenneman and Johnson (1946), Hinds (1952), and Norris and Webb (1976). Most of the physiographic provinces are composed of more than one geologic terrane, so they often have more than one set of associated mineral deposit types. The California desert region is discussed here in greater detail than the other regions in the State.

A simple classification system for mineral resource potential is used in this report. Areas are considered to be permissive for the occurrence of mineral deposits if they contain rock assemblages commonly associated with one or more types of mineral deposits. In general, areas are classified as permissive for the occurrence of mineral deposits if they were classified as geologically favorable by Albers and Fraticelli (1984). (The word "favorable," in reference to mineral potential, is now usually applied to relatively small areas that have other characteristics, in addition to rock assemblage, that are similar to those of areas known to contain mineral deposits.) In some cases, the areas classified as permissive in this report contain smaller areas that are considered favorable for the occurrence of mineral deposits, but the identification of such areas is beyond the scope of this report.

California Desert Region

The California desert region includes all or parts of three different physiographic provinces: the Basin and Range province in the north, the Mojave Desert province in the middle, and the Sonoran Desert in the southeast. The region is bounded on the northwest by the Sierra Nevada and Tehachapi Mountains, on the southwest by the eastern Transverse Ranges and the Peninsular Ranges, and on the east by the Nevada State line and the Colorado River. The Basin and Range province is separated from the Mojave Desert by the Garlock fault and a projected eastward extension of the fault; the boundary between the provinces is gradational in the east, but quite sharp along the Garlock fault itself. The boundary between the Mojave and Sonoran Deserts is arbitrary but corresponds roughly with a change in average elevation; the mean elevation of the Mojave Desert is about 2,500 feet, while elevations in the Sonoran Desert are generally much lower. The Mojave and Sonoran Deserts are characterized by low mountain ranges rising out of an alluvial cover of varying thickness; these low ranges have no particular orientation in most areas, but in some places they have a general northwesterly trend. In contrast, the Basin and Range province is characterized by elongate, approximately north-south-trending mountain ranges and basins. In California, elevations in the Basin and Range province range from below sea level in Death Valley to over 14,000 feet in the White Mountains. Drainage is internal throughout the California desert region except in the eastern part of the Sonoran Desert province, which is drained into the Gulf of California via the Colorado River.

The desert region in California is geologically different from the rest of the State in several aspects, which are reflected in the variety and types of mineral deposits of the region. First, the crystalline rocks that form the basement of the region are the oldest rocks in the State. One type of important mineral deposit (rare-earth-element-rich carbonatite) is restricted to a single known occurrence in these rocks, and the potential for additional rare-earth-element deposits of this type in the State is restricted to the California desert region. Second, during parts of the late Cenozoic era an unusual combination of climatic and geologic conditions led to the formation of extensive boron-rich evaporite

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deposits. Borate deposits in the United States are confined to the California desert and the adjacent region of Nevada, and all present production is from California. Finally, the region has an unusually complex geologic history. Many rock types are present, and the entire area has been subjected to long episodes of intrusive and extrusive magmatism, accompanied by periods of both compressional and extensional deformation. Most areas are cut by faults related to one of the deformational episodes, and many of the rocks that are not themselves magmatic in origin show evidence of alteration due to magmatic heat and fluids. Most types of metallic mineral deposits, worldwide, are related (directly or indirectly) to heat and fluids associated with the rise and emplacement or eruption of magma; faults commonly serve both as channels for mineralizing fluids and as sites of mineral deposition. In summary, the California desert region has the necessary ingredients for the formation of many different types of mineral deposits: a great variety of rock types and compositions, long episodes of magmatism, and an abundance of faults in a variety of orientations.

The beginnings of the geologic history of the California desert region are enigmatic. The oldest rocks in the region are schists and gneisses that formed during a metamorphic event about 1. 7 billion years ago, but the ages of the igneous and sedimentary rocks from which they formed are unknown. At least two intrusive episodes apparently occurred during the approximately 1-billion-year interval between the earliest metamorphism and the beginning of shallow marine deposition in late Precambrian time, but little else is known about the history of the region during this time interval. From an economic standpoint, the most significant of these intrusive pulses was the emplacement of alkaline plutonic rocks, including carbonatite, about 1.4 billion years ago. Some workers (for example, Stewart, 1972, and Dickinson, 1981) hypothesize that the continent extended much farther west during this period and that the western part was rifted away during the late Precambrian, thus setting the stage for shallow marine sedimentation. The remaining Precambrian crystalline rocks form the basement of the California desert's cratonic terranes.

Beginning in the late Precambrian and continuing throughout the Paleozoic, shallow marine sediments were deposited on the margin of the craton. These rocks are thin on the cratonic platform terrane (south of the Garlock fault), but they thicken considerably in the Basin and Range province to the north. Oceanic sedimentary rocks and island-arc(?) volcanic rocks of about the same age are present in the northwestern Mojave Desert, but their origin is uncertain; for the purposes of this report they are grouped as composite terrane.

Beginning in the late Triassic, the magmatic arc that formed the Sierra Nevada batholith also extended through the present-day desert region. Much of the Mesozoic era was characterized by repeated episodes of deformation, evidence of which is preserved in the geologic record as east-directed thrust faults in the northeastern part of the region and east-verging folds in the southwest Mesozoic plutonic rocks make up more than half the exposed rocks in the Mojave and Sonoran Deserts (Dibblee, 1980) and are less common (but nevertheless abundant) in the Basin and Range province. Mesozoic volcanic rocks are also present but not widespread, probably because most were removed by erosion during the Cenozoic era. Magmatic activity in the California desert region continued into the Cenozoic, but most of the exposed magmatic rocks of Cenozoic age are volcanic rather than plutonic. In most cases the upper levels of these relatively young magmatic systems have not yet been removed by erosion, and their deep intrusive roots remain unexposed.

In late Cenozoic time, the entire region was transformed into an area of internal drainage by uplift along its west margin. Local uplifts and depressions associated with crustal extension occurred at many places in the region, creating numerous closed drainage basins. These basins became sites of deposition for lacustrine (lake) sediments, volcanic and volcaniclastic rocks, and (or) evaporite deposits similar to those now forming in dry lakes or playas at many places in the California desert region. As a result of the lack of external drainage, thick Quaternary sedimentary sequences cover much of the region, especially the western part of the Mojave Desert, and relatively thin sequences of sediment locally cover bedrock adjacent to the mountains.

The history of mineralization of the California desert region parallels its geologic history. The length and complexity of the geologic history has provided numerous opportunities within the past 1.7 billion years for the concentration and deposition of mineral resources. Two of the most notable deposit types--the rare-earth-element deposit at Mountain Pass and numerous borate deposits in the region-are, respectively, among the oldest and youngest kinds of deposits found in the region.

The most significant Precambrian deposit-forming event was the intrusion of alkaline plutons about 1.4 billion years ago. Among these plutons was at least one carbonatite intrusion, now exposed at Mountain Pass, near the Nevada border. Other rare-earth-element-bearing carbonatite bodies may exist in the California desert region, but none are presently known.

Little mineralization of late Precambrian or Paleozoic age is known. The thick sequences of limestone and dolomite that were deposited in the shallow marine carbonate terrane of the northern Mojave Desert and Basin and Range province, however, later served as host rocks to numerous deposit types associated with the intrusion of magmas. Reaction between the carbonate rocks and younger hydrothermal fluids was an essential part of the deposit-forming process.

Most of the metallic mineral deposits in the California desert region were formed in association with Mesozoic and Cenozoic magmatism and deformation. Emplacement of magma into the upper crust provided both the heat and the fluids that are critical for the formation of many types of mineral deposits, and faults provided channels for the movement of the fluids. The details of mineral deposit genesis are beyond the scope of this report, but a short description of the general features of magmatic mineral deposit formation may be useful.

During the rise and emplacement of magma bodies into overlying rocks, magmatic fluids are released into the surrounding rocks, and meteoric water already in the rocks becomes heated. Under appropriate conditions, reactions between the host rocks and these fluids result in mineral deposits. In some cases these deposits occur adjacent to magmatic intrusions; the tungsten and the iron contact metasomatic (skarn) deposits of the California desert region, for example, occur along the contacts between granitoid rocks and carbonate rocks. Often, however, mineral deposits occur at some distance from intrusions; many kinds of vein deposits and replacement deposits form where hydrothermal fluids, traveling along faults or fractures, react with appropriate host rocks or simply precipitate metallic minerals as they cool. The lead-silver-zinc replacement deposits and many kinds of vein deposits (including gold, silver, tungsten, lead, zinc, and copper) are examples found in the California desert region.

More than 60 percent of the California desert surface is covered by alluvial deposits that prevent direct examination and exploration of bedrock. Assuming that the rock types and geologic structures seen in exposures of bedrock continue under this alluvial cover, it is very probable that undiscovered mineral resources similar to those already discovered in exposed bedrock are present beneath the alluvium. Exploration of bedrock beneath sedimentary cover is difficult at present, and production of resources from depth is more expensive than surface mining. Because subsurface data is very scarce, no assessment of the mineral resource potential of bedrock beneath alluvium is included in this report. Future developments in exploration and mining technology, however, are likely to make the discovery and production of buried resources in the California desert region possible and economically feasible.

The earliest development of mineral resources in the California desert region was probably by Native American populations who gathered such materials as clay and turquoise (Davis and Anderson, 1980). Gold and silver mines were in operation by the end of the 18th century, and production has continued intermittently to the present. Most production has been from bedrock, but gold has also been produced from placer deposits in at least three mining districts in the California desert region (Clark, 1980). Accurate production figures for gold and silver are not available, but total production of gold from the California desert region certainly exceeds 3 million ounces, and annual production from the desert region has been rapidly rising during the past 5 years. Gold production from the desert was about 5,000 ounces in 1982, while anticipated production totals for 1987 and 1988 are 255,000 ounces and 305,000 ounces, respectively (Anderson, 1987). Total gold production for the State was about 108 million ounces as of 1987, with about a third of that taken from gold-quartz vein deposits in the northern Sierra Nevada Since 1982, the rate of gold production in California has been more than doubling every year (Lucas, 1988). The largest contact metasomatic iron deposits in the State are located in the California desert region, where well over 100 million tons of ore have been produced (Scott and Wilson, 1980). Other metallic mineral commodities that have been produced in smaller amounts include antimony, bismuth, cadmium, lead, lithium, manganese, magnesium, mercury, tin, tungsten, uranium, vanadium, and zinc (Davis and Anderson, 1980).

Production of nonmetallic mineral commodities also has a long history in the California desert region. Development of borate resources in the region began with production from playas in the 1870's. Most borate production shifted to richer bedded deposits about 1890, except at Searles Lake, where production of borates and other salts from brine continues. Bedded borates (kernite, ulexite, probertite, or colemanite) are mined at Boron and were mined in Death Valley until 1986. In recent years, the California desert region has been the source of all U.S. production and one-third to one-half of the worldwide production of borates (Lyday, 1988; Carillo and others, 1987). The largest producing rare-earth deposit in the western world is at Mountain Pass (Warhol, 1980); it was discovered in 1949, and development started soon thereafter. During the 20th century, population growth in the California desert region and adjacent areas, especially the Los Angeles and San Diego metropolitan areas, has increased the demand for significant production of industrial materials such as sand, gravel, aggregate, perlite, clay, limestone, and gypsum. At present, the value of nonmetallic mineral commodities produced from the California desert region (including borates, rare-earth elements, and industrial minerals) exceeds that of the metallic commodities from the region (Carillo and others, 1987).

More than half of the exposed bedrock in the California desert region is considered geologically permissive for the existence of undiscovered mineral deposits (Albers and Fraticelli, 1984). These permissive areas, widely scattered throughout the desert, are separated by alluvial cover and by bedrock that is not considered geologically permissive for mineral resources and that lacks known deposits. Alluvium may contain placer deposits of gold or other commodities. About a third of the area underlain by permissive bedrock has numerous known deposits. In order to facilitate description of the mineral resources and potential of the five geologic terranes that underlie this large region, each terrane is discussed separately.

The cratonic platform terrane, which underlies most of the Sonoran Desert and the eastern third of the Mojave Desert, has numerous areas that are geologically permissive for mineral deposits of gold, iron, copper, tungsten, manganese, lead, zinc, or silver. About a third of this area contains known deposits of one or more of these metals. For more than 30 years, most mining activity in the cratonic platform terrane was focused on contact metasomatic iron deposits, but major production of iron ore has ceased. The largest producer of iron ore was the Eagle Mountain iron mine, located in the Eagle Mountains in the southern part of the cratonic platform terrane. It opened in 1948 and produced more than 100 million tons of ore before closing in 1984 (Scott and Wilson, 1980; Burnett, 1985). At present, disseminated-gold deposits are the most important metallic mineral deposits in the region; most present exploration and mining activity in the area is directed toward finding and developing these deposits. Large disseminated-gold deposits include the Mesquite and Picacho mines in the Chocolate Mountains, the Morning Star mine in the Ivanpah Mountains, and the Colosseum mine in the Clark Mountains. These four mines are expected to have combined production of at least 300,000 ounces of gold per year (Anderson, 1987; Lucas, 1988; Carillo and others, 1987). In these deposits, the gold occurs as extremely fine grained particles, usually invisible to the naked eye. The characteristics and settings of disseminated-gold deposits are apparently quite varied. Tosdal and Smith (1987) describe the the Mesquite deposit as a large gneiss-hosted epithermal vein deposit in brecciated or fractured host rock. They associate deposits in the Cargo Muchacho Mountains with aluminous, kyanite-bearing gneisses and schists. Some of the Cargo Muchacho deposits may be metamorphosed gold-skarn deposits (Orris and others, 1987), closely associated with the metasomatic iron (iron skarn) deposits of the region. Bouley (1986) describes the occurrence of disseminated gold in breccia on low-angle detachment faults that cut crystalline and volcanic rocks and cites the Picacho mine as an example. In the Hart mining district, located in the Castle Mountains, disseminated-gold deposits are hosted by volcanic rocks.

Shallow marine noncarbonate terrane underlies the central part of the Mojave Desert, west of the cratonic platform terrane. Much of the exposed bedrock in this area is considered permissive for iron, gold, lead, zinc, or copper, and a few areas are considered permissive for silver, tungsten, or mercury. About a quarter of the area considered permissive for one or more of these commodities contains known mineral deposits. As in the cratonic platform terrane, most known metallic mineral resources occur in contact metasomatic iron deposits and in replacement or vein deposits of the other commodities.

The western Mojave Desert is underlain by terrane of debatable origin. In this report it will be considered to be composite (oceanic and island-arc) terrane, following Albers and Fraticelli (1984). Most of the region is not considered permissive for metallic mineral deposits, but there are significant exceptions to this generalization. Several areas within the region are considered permissive for gold, tungsten, lead, or zinc, and some of these permissive areas contain known deposits of gold. The mineralized area in the vicinity of the Rand Mountains is considered permissive for deposits of tungsten, gold, silver, manganese, and antimony and contains known deposits that include the vein-type tungsten deposit at Atolia and the gold-quartz vein deposits of the Randsburg mining district.

Shallow marine noncarbonate terrane underlies the part of the California Basin and Range province north and west of the carbonate terrane. Bedrock there is considered permissive for gold, tungsten, lead, silver, zinc, or copper. About a third of this area contains known deposits of one or more of these commodities. As in the adjacent shallow marine carbonate terrane, most of the deposits in this area are contact metasomatic tungsten deposits and lead-silverzinc replacement and vein deposits.

Precambrian rocks at Mountain Pass contain the largest producing rare-earth-element deposit in the western world. Economic deposits of rare-earth elements are commonly found in carbonatite, a rare kind of igneous rock containing carbonate minerals. According to Singer (1986), these deposits typically occur in cratonic terranes and are spatially related to fault lineaments and alkaline volcanism. In the deposit at Mountain Pass, the rare-earth elements occur in the mineral bastnaesite (a fluorocarbonate) that makes up about 10 percent of an approximately 1.4-billion-year-old carbonatite body that intrudes still older gneiss (DeWitt and others, 1987; Woyski, 1980). The rare-earth elements in the Mountain Pass deposit are cerium, lanthanum, neodymium, praesodymium, samarium, gadolinium, and europium, in order of decreasing abundance; cerium and lanthanum make up about 85 percent of the rare-earth elements in the deposit (Warhol, 1980). Although other rare-earth-element deposits are not known in the California desert region, Precambrian crystalline rocks are considered geologically permissive for additional deposits.

Cenozoic intermediate to silicic volcanic rocks throughout the region are considered geologically permissive for uranium deposits, but known deposits are sparse. Sediments derived principally from these rocks are also considered geologically permissive (for example, uraniumbearing volcanogenic sediments are present near the Coso Range).

Among the most important nonmetallic commodities found in the California desert region are borate and associated evaporite minerals and saline brines, collectively known as saline deposits. The origins of these deposits are discussed above. The lower parts of most of the closed basins in the California desert region are considered geologically permissive for saline deposits, and most of them contain known deposits. Examples include Death Valley, Saline Valley, Searles Lake, Cadiz Lake, Danby Lake, and Koehn Lake. Borate minerals do not occur in all of these areas; common salt (sodium chloride) is recovered from saline brine by evaporation at several locations.

Deposits of industrial minerals occur throughout the California desert region. Sand and gravel deposits are abundant in the alluvium that covers much of the region (Leighton, 1980). Notably pure and uniform deposits of limestone and dolomite are present in many of the late Precambrian and Paleozoic carbonate sequences that occur in places in the cratonic platform terrane of the Mojave and Sonoran Deserts and form the shallow marine carbonate terrane of the Basin and Range province (Gray and Bowen, 1980). Development of these deposits has been hindered by the high cost of transporting them to markets along the Pacific coast, but as sources closer to the coastal markets are depleted and desert population centers grow, these resources may become more important. Pumice, perlite, and cinder deposits are mined in several places in the California desert region, and Cenozoic volcanic rocks are geologically permissive for additional deposits of these commodities. Eleven areas in the western Mojave Desert and two areas in the Basin and Range Province contain known zeolite resources (Stinson, 1984).

There are four Known Geothermal Resource Areas (KGRA's) in the California desert region. The largest of these is the Coso field, adjacent to the Coso Range, which is being developed for electric power generation (Collie, 1978; Anderson, 1987). Eight other areas, scattered throughout the California desert region, have known nearsurface thermal waters (Majmundar, 1983).

Peninsular Ranges and Salton Trough

The Peninsular Ranges and Salton Trough areas of California are located in the southern part of the State, primarily southwest of the San Andreas fault The geologic history of the Peninsular Ranges should be considered along with that of Mexico and Central America, as the area has been moved northwestward at least 200 miles since motion along the San Andreas fault began in the Miocene; some evidence, based on paleomagnetic measurements, suggests that the Peninsular Ranges have moved much farther than 200 miles. Most of the region is underlain by a batholithic terrane similar in age and composition to the Sierra Nevada batholith. Granitoid plutons of this terrane intrude an island-arc terrane along the west edge of the range and a shallow marine noncarbonate terrane in the northern part of the area. Tertiary volcanic rocks cover scattered parts of these areas. Details of the geologic history of the Peninsular Ranges are beyond the scope of this report, but the same kinds of geologic events (if not the same sequence) that created the terranes of the Sierra Nevada northeast of the San Andreas fault must also have led to the formation of the geologic terranes of the Peninsular Ranges.

The Salton Trough is a pull-apart basin created by strike-slip (horizontal) motion on the San Andreas fault. The basin is filled to considerable depths with sediment, but mineral resource potential is restricted to saline (evaporite) deposits at or near the surface. The Salton Trough also has significant known and potential geothermal energy resources.

The most notable mineral deposits in the Peninsular Ranges are lithium minerals and gemstones in pegmatites. Pegmatites are found only in areas underlain by plutonic rock; batholithic terrane in the region is considered geologically permissive for undiscovered lithium and gemstone deposits.

Large metallic mineral deposits have not been discovered in the Peninsular Ranges or Salton· Trough provinces, but two areas in the Peninsular Ranges do have numerous small deposits of gold, tungsten, copper, and other metals. Part of the northern Peninsular Ranges, near Lake Elsinor, has deposits of gold, tungsten, copper, antimony, manganese, lead, or zinc. The area northeast of Cayamaca Peak (about 40 miles from San Diego) has deposits of copper, nickel, silver, gold, and manganese. Both areas are considered geologically permissive for the discovery of additional deposits.

Most of the western Peninsular Ranges area is considered geologically permissive for copper, gold, manganese, iron, or nickel, but there are few known deposits in this area. Tertiary volcanic rocks, which occur in scattered localities throughout the eastern Peninsular Ranges, are considered geologically permissive for deposits of uranium, but known deposits are sparse or absent. Areas within several basins in the eastern Peninsular Ranges and parts of the Salton Trough are considered geologically permissive for saline (evaporite) deposits. Some of these areas, including a large area southeast of the Salton Sea, contain known deposits of evaporite minerals or saline brines. The large gypsum deposits near El Centro are an example. The Salton Trough also contains seven KGRAs that together have substantial identified electric-energy potential (Nathenson and Muffler, 1975; Collie, 1978; Majmundar, 1983). Five geothermal power plants are currently operating in these areas. The rest of the Salton Trough area is considered geologically permissive for additional geothermal energy resources.

Transverse Ranges

The Transverse Ranges trend approximately east-west, nearly perpendicular to the general north-northwesterly grain of most geologic and physiographic features in California. The ranges extend from Point Arguello on the Pacific Ocean to the San Bernardino Mountains in the east, where the province grades into the Mojave and Sonoran Desert provinces. The western part of the Transverse Ranges province is underlain by Cenozoic marine and nonmarine sedimentary rocks. These rocks are not known to contain metallic mineral deposits, but they do contain many reservoirs of oil and gas from which considerable production has taken place. The offshore extensions of the producing formations are also known to contain oil and gas, and development of these deposits is underway.

Noncarbonate shallow marine terrane underlies the eastern Transverse Ranges. About half of this area is considered geologically permissive for gold, copper, lead, zinc, manganese, iron, or titanium, and about half of the geologically permissive area contains known deposits of one or more of these metals. Deposit types include chert-associated manganese, gold-quartz veins, and anorthosite titanium.

Sierra Nevada

The Sierra Nevada extends 400 miles from the Modoc Plateau in the north to the Mojave Desert in the south; it varies in width from 40 to 100 miles. The range is highest and most rugged along much of its east edge, and overall elevation gradually decreases to the west. The range can be divided into western and eastern belts that have geologically different characteristics. The western belt, along the west edge of the north half of the range, consists of Paleozoic to Mesozoic oceanic, island-arc, and composite terranes that are intruded by Mesozoic quartz dioritic to granodioritic plutons. These three terranes form bands that parallel the trend of the range; generally the oceanic band is closest to the core of the range, and the island-arc band lies along the west edge. The eastern belt of the Sierra Nevada is batholithic terrane, composed almost entirely of Mesozoic granodioritic to granitic plutons that enclose remnants of older Mesozoic deep marine volcanic and clastic rocks in its western part and Paleozoic shallow marine quartzose and carbonate rocks in the eastern part. The different geologic characters of the eastern and western belts are reflected in the different mineral deposit types found in these two regions.

The western belt is historically important as the center of the California gold-rush activity in the middle 19th century. The Mother Lode is in the western belt, and about a third of California's total gold production was taken from gold-quartz veins in this region; the belt contains nearly all the large lode-gold deposits in California (Albers, 1981). Virtually the entire western belt is considered geologically permissive for mineral deposits containing gold, chromium, nickel, copper, zinc, manganese, or mercury, and about a

Great Valley

inces. In this report they are included in the discussion of

Coast Ranges

that are not considered pennissive for mineral deposits. Some of the Cenozoic volcanic rocks in the central part of the northern province, between Clear Lake and the San Francisco Bay, are considered pennissive for epithennal hot-spring gold and mercury deposits. The McLaughlin gold mine southeast of Clear Lake is an example of a large mine of this type. Significant geothennal energy resources are present in the northern Coast Ranges. The Geysers KGRA, south of Clear Lake, contains the largest geothermal electric-power-generation installation in the world (Majmundar, 1983), and the region surrounding the KGRA is considered geologically permissive for additional geothennal resources (Collie, 1978).

The shallow marine noncarbonate terrane of the southem Coast Ranges (also known as the Salinian block) is composed principally of metamorphosed clastic sedimentary rocks intruded by Mesozoic plutons. Known deposits are extremely rare in this terrane.

Cenozoic sedimentary sequences cover the southern end of the Coast Ranges province and parts of the Salinian block. Some areas within this cover terrane contain known deposits of oil and gas.

Klamath Mountains

The Klamath Mountains province of northern California is a rugged, mountainous region that has a complex geologic history extending from early Paleozoic time to the present. It is composed of oceanic and island-arc terranes of Paleozoic and early Mesozoic age, stacked in a series of generally east-dipping thrust sheets, that are intruded by granitoid plutons of Ordovician to Early Cretaceous age. Most of the rocks in the Klamath Mountains are marine island-arc-related volcanic and sedimentary rocks, but ultramafic and related oceanic igneous rocks also occur throughout the region, especially at the major thrust faults that mark boundaries between the terranes (Irwin, 1981). Two unusually large ultramafic masses are geologically pennissive for chromite deposits: the Josephine Peridotite, which straddles the California-Oregon border northeast of Crescent City, and the Trinity ultramafic sheet southwest of Mount Shasta.

The distribution of known mineral deposits and geologically pennissive terrane in the Klamath Mountains province is complex, reflecting the complexities of the geology. About three-fourths of the region is considered geologically pennissive for gold, chromium, silver, manganese, copper, lead, mercury, iron, tungsten, nickel, zinc, molybdenum, or magnesium, and about a quarter of the area contains known deposits of one or more of these commodities. Bodies of ultramafic rock, many of them quite small, occur throughout the region. All of these bodies are considered geologically pennissive for podifonn chromite deposits, but most do not contain known deposits. Nickeliferous laterites overlie ultramafic rocks in some places, especially in the western part of the Klamath Mountains province. Gold-quartz vein deposits are also widespread throughout the Klamath Mountains, apparently associated with bodies of granitoid rock that intrude the region. The largest of these is the French Gulch-Deadwood vein system, northwest of Redding, which has yielded about 800,000 oz of gold. Massive sulfide deposits (copper and zinc with associated lead, silver, iron, manganese, and tungsten) occur in Devonian, Pennian, and Triassic island-arc terrane (West and East Shasta districts) in the southeastem part of the Klamath region; smaller massive sulfide deposits also are present in metavolcanic rocks farther west. In the western part of the province, small lode-gold deposits are associated with a late Paleozoic or Triassic island-arc terrane, and chert-associated manganese deposits occur in oceanic terrane.

Modoc Plateau and Southern Cascade Range

These two provinces that make up northeastern California are underlain by relatively young volcanic rocks and volcanogenic sediments. The Cascade Range is a chain of high, young volcanos, distributed along an older, late Cenozoic volcanogenic range; it extends from Canada through Washington and Oregon into northern California. The late Cenozoic to Holocene volcanic rocks of the Modoc Plateau fonn an upland of subdued topography. Geologically, the Modoc and Cascade areas differ in style of volcanism; the Cascades are dominated by the products of explosive volcanism, whereas the rocks of the Modoc Plateau are more commonly lava flows. Both of these areas are Cenozoic cover terranes that conceal northern extensions of the Sierra Nevada province and the eastern Klamath Mountains province beneath considerable depths of volcanic rock and volcanogenic sediment.

A few relatively small areas in the Modoc Plateau contain known deposits of gold, mercury, copper, or silver and are considered geologically pennissive for the discovery of additional deposits. These deposits are related to rhyolitic intrusions that are scattered throughout the region. Most of the region, however, is devoid of known deposits and is not considered to be permissive for the discovery of mineral resources. Several KGRA's are present in the region, however, and most of the Modoc Plateau is considered geologically pennissive for additional geothennal resources (Collie, 1978; Majmundar, 1983).

REFERENCES CITED

p.

Terms and boundary ages used by the U.S. Geological Survey in this report

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