Geology 101

Geology 101
THE GEOLOGY OF NEVADA ORE DEPOSITS
By BERNARD YORK 1944

1. THE ELEMENTS
The elements that are combined to form the earth's "crust" are not uniformly distributed and are rarely present singly. They occur in chemical combinations that are stable under the conditions existing at the time of their union. Their rearrangement into new compounds may be brought about by a change in their environment with both chemical and physical processes influencing their arrangement. Combinations which are stable under the new conditions have remained unchanged. Those unstable are affected and changed to a stable composition either by the addition, removal, or interchange of one or more elements. These natural occurring compounds, when of definite chemical composition, are called minerals.

The earth's crust is that solid portion to a depth of approximately ten miles and made accessible to man either through boring and mining operations or exposed by faulting, folding, or erosion. It is composed principally of minerals and a few which predominate are normally called the rock-forming minerals.

The crust consists mainly of igneous rocks, or their derivatives, the bulk of which are composed of a relatively few mineral species, mostly silicates and oxides.

The predominate minerals are quartz, feldspar, pyroxene, amphibole, mica, olivine, garnet, magnetite, and ilmenite. Eight elements, oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium, make up approximately 98 percent of the rocks. Only three of these, iron, aluminum, and magnesium, are metals of economic importance.

The other metals are present in amounts of less than 0.1 percent. Many of the most useful metals occur in quantities of less than 0.01 percent. The amount of useful metals present in a rock does not represent that which is available to industry.

Only a small fraction of these metals occur in sufficient concentration to be recoverable by man. It is indeed fortunate that natural processes have accumulated a portion of the useful metals and localized them into concentrations containing many times the general rock average.

In general, igneous rocks contain more of the useful metals than do sedimentary rocks. This fact, and the conditions under which some of the local concentrations occur, has led to the belief that igneous rocks are the original source of the metals. However, sedimentation under proper conditions may bring about local concentration of metals.

3-THE ROCKS
Rocks are composed of an aggregate of minerals grouped together to form a relatively solid mass. Rocks belong to three main groups:
igneous, formed through the solidification of molten material;
sedimentary, rocks derived from erosion and deposition;
and metamorphic, rocks derived by modification of either igneous or sedimentary rocks.

All rocks were originally of igneous origin. At present igneous rocks compose approximately 95 percent of the crust. Sedimentary rocks, though widespread, form only a relatively thin veneer on the earth's surface, and metamorphic rocks are present in even less quantity.

Classification of rocks into groups has been based upon mode of formation, texture, and the mineralogic composition rather than on the chemical composition. The combined average chemical composition of all sedimentary rocks agrees closely with that of the igneous rocks from which they were derived.

Metamorphic rocks have been formed through prolonged heat and pressure which have changed the original rock, either sedimentary or igneous, to such an extent that it retains but few of its original physical characteristics. The rocks of each main group have been classified according to composition and texture and given a distinct name.

It is of interest to note that in the early days of the State many types of rock were used as building stones, and the earliest mining bulletin published by the University of Nevada \\?as a "Preliminary Report on the Building Stones of Nevada," by John A. Reid in 1904.

The State capitol building and the State prison buildings at Carson City are excellent examples of early sandstone buildings erected in the State. Bower's Mansion in Washoe Valley is a beautiful example of granitic rock masonry work.

The Museum of the Mackay School of Mines contains a piece of tuff inscribed "1847" that came from an old fireplace near Wamonie in Nye County, its origin being a mystery. Some of the early buildings of Eureka, and, at a later date of Tonopah, are constructed of tuff, and a quarry in tuff alongside the highway east of Sparks has furnished the rock for recent residences and service stations in that city and Reno.

3b- INTRUSIVE IGNEOUS ROCKS

Only the most common types of the intrusive igneous rocks will be defined.

Alaskite is a granular rock of white or light-gray color. It is composed almost entirely of quartz and orthoclase feldspar. It closely resembles granite, but lacks mica and hornblende. Alaskite is common at Silver Peak, and is found at Goldfield, near Reno, and in many other Nevada localities.

Granite is a granular rock of pink or gray color, composed of orthoclase, quartz, and either muscovite, biotite, or hornblende.
Granite porphyry is a rock of the composition of granite with abundant phenocrysts of quartz arid orthoclase imbedded in a granular groundmass.

Monzonite is a granular rock resembling both granite and diorite in appearance and composition. It contains orthoclase and plagioclase in approximately equal proportions. If quartz is present, the rock is called a quartz monzonite. It is usually of a gray color with a few prominent pink crystals. The monzonites are common in Nevada and are particularly abundant at Ruth and Kimberly.
Monzonite "porphyry" is a rock of the composition of monzonite with abundant phenocrysts of orthoclase and plagioclase.

Granodiorite is a granular igneous rock with the minerals contained in a quartz diorite but also containing considerable orthoclase. It has less orthoclase than quartz monzonite. Granodiorite is a common rock throughout Nevada as at Austin, Steamboat Springs, and in the Sierra Nevada in western Nevada.

Diorite is a gray crystalline igneous rock, composed essentially of plagioclase, feldspar, and hornblende. Biotite or augite may also be present. When much quartz is present the rock is termed a quartz diorite.
Diorite "porphyry" is a rock of the composition of diorite with abundant and prominent crystals of plagioclase, hornblende, and biotite in a granular ground mass. Diorite makes up the upper portion of Mount Davidson at Virginia City.

Gabbro is a granular rock usually of dark color. It is composed of plagioclase and pyroxene, diopside or augite. The ferromagnesian mineral commonly has a laminated parting. Olivine, biotite, or hornblende may be present in small amounts. Gabbros are found in the Pine Forest, Jackson, and other ranges of the northern and northwestern part of the State.

Peridotite is a granular rock usually of dark color. It is generally composed essentially of olivine, with some form of pyroxene, and with or without hornblende or biotite. Plagioclase feldspar may be present in small amounts. This rock occurs as dikes in eastern Clark County.

3d- Sedimentary Rocks
Transporting agents in the wearing away of a land mass, and explosive volcanism scatters unconsolidated material over large areas. This redistribution of the material at the earth's surface has been continually in progress since the beginning of geologic time. Material that was once deposited on the bottom of the sea and then elevated into a land mass is again subjected to erosive action.

The bulk of the sedimentary material has been deposited in basins occupied by bodies of relatively quiet water, therefore the discussion here will be limited to a few brief statements of the processes of deposition in seas and lakes. Distinct terms are used to designate the sediments deposited in each type of water:
"lacustrine" for fresh water sediments, and "marine" for sediments deposited in the sea.

The disintegration and decomposition of rocks sets free material that is transported by surface waters, either in solution or in suspension. Material in suspension settles out as the stream loses carrying capacity through loss of velocity. There is a sudden loss of velocity as the stream enters a body of water and the coarse material settles near shore. The extremely fine material remains in suspension longer and therefore is transported far from the point of entry. In this way, there is a segregation of the material according to size, and to a lesser extent a segregation according to chemical composition. Quartz crystals, and grains, are highly resistant to disintegration or solution, therefore much of the quartz is transported as pebbles or grains and deposited in layers or beds near shore. Minerals of the feldspar and ferromagnesian groups are readily decomposed by processes of weathering and break down into both soluble and insoluble products.

The dissolved material remains in solution until finally precipitated by chemical reactions or removed either by evaporation or by organisms. The insoluble products are transported as extremely fine grains, some of which are of colloidal dimensions, and are carried in suspension far from shore. Soluble material removed from solution by plant and animal life of the sea or lake is incorporated into their supporting structures. Upon the death of the plants and animals the skeletons and body structures settle to the bottom. Layers or beds of diatomite and occasionally of limestone have accumulated in this way.

Varying conditions in a large body of water as a change in direction or velocity of the currents, change in depth of water, or a change in the volume of material being delivered, cause material of different size or composition to be deposited over the same area at different times. This forms distinct layers or beds. There may be great diversity in the character and composition of separate beds, but the material within any individual bed is relatively uniform. The sediments as we see them today, after they have been elevated, tilted, and partially eroded, are a series of layers of reconsolidated material. Some layers are only a few inches in thickness while others are several hundred feet. Some sedimentary rocks are thin bedded, others are massive. Shales, limestones, and sandstones are the most common reoccurring sedimentary beds.

The material as laid down was loose, as coarse material intermixed with moderately fine grains as a layer of gravel, as fine grained material as a layer of sand, as clayey material as a mud or ooze, or as plant and animal remains in a porous mass. Prolonged pressure from the weight of great thickness of overlying material has in time consolidated the loose fragments into rock.

Deposition took place on a nearly horizontal surface so the normal attitude of the beds was approximately horizontal. Faulting and folding have warped, tilted, and elevated large blocks to such an extent that, in many places, the beds are now inclined at steep angles. In some cases folding has been so extreme that the beds have been overturned, placing the older above the younger, and beds that were once buried to a depth of tens of thousands of feet have been so elevated and tilted that they are now exposed at the surface.

This discussion on the origin and formation of sedimentary rock is extremely brief and generalized. The reader should consult the published literature on the subject for detail. Books on sedimentation and sedimentary rocks are available at any good library.
Here again, as in the case of the igneous rocks, only the common types are defined. These terms are loosely used and in many cases indicate structure rather than chemical or mineralogical composition.

Chert is a term for a compact, microcrystalline silica, usually of light color. It occurs in rounded, nodular, concretionary masses, replacing limestones as in the Pogonip limestone at Cortez, Eureka, and Ely.

Conglomerate is formed of rounded pebbles cemented together. It differs from ordinary sandstone only in size of particles.

Limestone is a rock composed almost entirely of calcium carbonate. Magnesium sometimes replaces part of the calcium. It can be distinguished from dolomite by its effervescence when treated with dilute hydrochloric acid. Limestones are formed both through the accumulation of fragmental remains of marine animals and from chemical precipitation. Calcium carbonate readily undergoes crystallization, even at low temperature and pressure, so that many limestones show a crystalline structure with little evidence of their origin remaining. When crystalline, limestone is known as marble. Limestones are present in the sediments in many parts of the State, and ores occur in these rocks at Eureka, Goodsprings, Pioche, Cortez, Yerington, and elsewhere.

Dolomite is a magnesian limestone. It can be distinguished from limestone by its increased hardness and specific gravity or by chemical test for the presence of magnesia and by its reaction with acid. Limestone effervesces readily when treated with dilute hydrochloric acid, dolomite does not. Dolomite is common in the sediments in eastern and southern Nevada. Large quantities of dolomite are present at Gabbs, in the Paradise Range, associated with magnesite and brucite. Large deposits of very pure dolomite occur at Sloan in Clark County. Lead ores at Eureka occur in the Eldorado dolomite.

Sandstone is a rock composed of an aggregate of small to medium sized quartz grains bound together by material of a calcareous. ferruginous, siliceous, or argillaceous nature. Sandstone can be distinguished from quartzite by the weakness of the binding material.

Quartzite is a sandstone thoroughly cemented by silica. It can I)e distinguished from the crystalline igneous rocks by the predominance of rounded grains, most of which are of quartz. It differs from sandstone in the strength of the cementing material, as quartzite breaks through the grains rather than around them as is the case with sandstone. The most widespread quartzite is the Prospect Mountain quartzite which lies at the base of the Paleozoic sedimentary rocks in many regions, particularly at Eureka and Pioche. The Eureka quartzite is present in many mining districts. The gold ores of Delamar occur in quartzite, while those at Cortez are in places terminated against the Eureka `quartzite.

Shale is a more or less laminated rock formed by the induration (hardening) of muds, silts, or clays. It can be distinguished from the other rocks by its laminated structure, comparative softness, and lack of visible grains. Shale is the wall rock of the gold ore at the Getchell mine in Humboldt County and also the copper ores at the Rio Tinto mine in Elko County. Shales are also found at Ruth, Eureka, and Pioche.

Slate is a metamorphosed shale which splits readily along cleavage planes. A slate of roofing quality occurs both in Humboldt County and in Clark County.

Tufa is usually a calcium-carbonate rock deposited from solution in spring or lake water. It occurs at Pyramid Lake and along the shoreline of old Lake Lahontan.

Tuff is a fine-grained fragmental rock resulting from explosive volcanic activity. When the volcanic material is deposited on a water surface, it forms a sedimentary bed.

Much of the lacustrine sediments of Nevada contain a large proportion of volcanic dust or ash.

3e- METAMORPHIC ROCKS
By metamorphism, the geologist means any change in the constitution of any rock, usually sediments, induced through physical and chemical agencies. Sedimentary rocks may become deeply buried and subjected to intense and prolonged heat and pressure causing induration of the fragmental material, accompanied by crystallization. The resulting metamorphic rock may have little resemblance to the original rock.

Cleavage that was once parallel to the beds may no longer exist or may now be inclined to the original bedding. Massive igneous rocks have been converted into schistose aggregates having the appearance of well-stratified sediments. These changes are rarely brought about by physical agencies alone, as chemical action has also been active, causing some rearrangement of the elements into new combinations, but the chemical composition of the rock usually remains essentially unchanged.

The commonest evidence of the metamorphic effect of prolonged pressure is manifested by flattened and elongated grains or crystals. Elongation takes place in a direction normal to the direction of greatest pressure. This elongation without fracturing has been accomplished by molecular flow.

Another common form of metamorphism is indicated in the change of siliceous sandstone to quartzite. This is brought about by the deposition of silica in the pore space or the partial solution and intergrowth of the original sand grains.

Localized metamorphism of the invaded rocks accompanies the intrusion of a molten igneous mass or magma. Heat, together with the chemical action of vapors and solutions, is responsible for the changes. New minerals are formed through the introduction and interchange of elements, resulting in a rock differing from the original material in both texture and chemical composition.
Since these changes are most pronounced along the contact between the two rock bodies. the phenomena has been descriptively termed contact metamorphism.

All gradations of metamorphism occur from minor changes to those so extreme that none of the characteristics of the original rock remain. Few of the older rocks have escaped metamorphic action, and many geologists prefer to classify quartzite and slate as sedimentary rock, even though metamorphism has been responsible for their physical characteristics.

The mode of' origin of the material from which the schists and gneisses were formed is not so readily ascertained, so they are normally classified as distinctly metamorphic rocks.

Those aggregates, of varying mineral composition, formed by the processes of contact metamorphism have not been given specific rock names because the variation in texture and mineral composition makes their classification impracticable. Several general terms are used that signify contact-metamorphic material, but they are used primarily for the sake of convenience.

Herewith are defined under the heading of metamorphic rocks. only those considered to be strictly of metamorphic origin:

Gneisses are rocks of banded structure, and of variable chemical composition. Gneisses have been formed through the metamorphism of sedimentary or igneous rocks. Their composition corresponds closely to that of the rock from which they were derived. Accessory minerals, formed during metamorphism, are present in abundance.

Hornfels is a compact, fine-grained, silicated rock having an appearance similar to that of flint, formed principally through the, contact metamorphism and metasomatism of clay-rich rocks, and is composed chiefly of biotite, andalusite, staurolite, garnet, and feldspar. It is highly resistant to erosion and forms prominent outcrops. Hornfels is abundant in many areas, and it is the wall rock of the tungsten ores at Mill City.

Marble is crystalline limestone or dolomite altered in texture but not changed chemically. It is granular in texture and often of pleasing color. Marble is present in many places, and was once mined at Carrara in Nye County.

Schists are a group of rocks characterized by a pronounced foliated structure, due to the parallel arrangement of the various constituents. They are crystalline in contrast to slate, a rock which they resemble structurally. The gold ores of Manhattan are mainly in schist.

Slate (see Sedimentary Rocks).

Tactite is a rock of more or less complex mineralogy formed of lime silicates through the contact metamorphism of limestone, dolomite, or limy shales, into which foreign matter has been introduced by hot solutions from the intruding magma. It is present in large quantities in most contact-metamorphic regions such as Nightingale, Adelaide, Yerington, and Contact.

4- Forms and Origin of Mineral Deposits.
Mineral deposits occur in diverse forms. Some of them are thin and tabular, some pipe-like, others massive and irregular. They usually occur inclined at various angles from the horizontal and seldom maintain the same strike or dip for more than a few hundred feet. Few deposits are uniformly mineralized throughout their length and depth; the richer or economically valuable portions are called ore shoots. The terms commonly used to designate the forms of mineral deposits are briefly defined as follows :

A vein is a single tabular deposit of minerals occupying a fissure, one or both walls of which generally are well defined. Veins following the bedding planes in sedimentary rock are called bed veins. A number of parallel veins are called a vein system. Vein deposits with sharply defined walls are well exemplified in the Austin and Belmont districts.

The term lode is used by miners as nearly synonymous with the term vein, and is little used in late geological literature.

Contact-metamorphic deposits are deposits occurring at or near the contact of intrusive rocks with sedimentary rocks and carrying minerals characteristic of contact metamorphism. Some of these deposits are roughly tabular, others are quite irregular. The copper mines of the Yerington district are mainly of this type, as are also most of the tungsten deposits of the State.

Replacement deposits are masses of mineral formed by the alteration and replacement of rocks, particularly limestones and dolomites. They are often extremely irregular in form and in many places grade into country rock. The lead ores of the Eureka district and the zinc ores of the Mt. Hope mine in Eureka County are examples of this type of deposit, as is also the lead-zinc ore of the Combined Metals bed in the Pioche district.

Disseminated deposits contain ore minerals disseminated throughout a localized body of rock. The so-called "porphyry copper" deposits are of this form as a t Ruth, Nevada.

A stockwork deposit is a complex system of small fissure veins, not of tabular or sheet form, so interpenetrated that the whole must be mined together. The deposit of gold ore at the Dexter mine at Tuscarora has been termed a stockwork.

A placer deposit consists of heavy resistant minerals formed as a result of mechanical concentration of detrital material through the action of surface waters. The gold placers of Manhattan and Round Mountain in Kye County are striking examples.

A bedded deposit is a sedimentary deposit mined for its mineral. The best illustration in Nevada is to be seen in the mining of gypsum beds in the Arden district of Clark County.

The mode of origin of these forms of mineral deposits varies from those formed by pure mechanical processes, to those formed from minerals in solution, to those of magmatic affiliation.

4a-1- Placer Deposits
Placer deposits are formed by the accumulation and concentration of heavy and resistant minerals in stream channels or along beaches. The products are concentrated by the churning and jigging action of water in motion. The jigging action causes the heavier particles to settle through and accumulate at or near the bottom of the detritus. This accounts for the occurrence of economical quantities of valuable minerals on bed rock in some stream gravels. Where the force of the currents was insufficient to cause jigging action or movement in the whole mass of detritus, concentration of any consequence was accomplished near the top of the detritus, leaving a concentration of minerals in layers high above bedrock; a type common in the channels of intermittent streams.

Some of the least abundant minerals have been concentrated into placer deposits of great commercial value. The valuable minerals can be readily and inexpensively recovered from such deposits through the use of concentrating processes which are very similar to those that have accomplished the concentration in the stream channels. The free state of the mineral grains, their purity and high specific gravity,'and the high unit value of the minerals may make possible their economic recovery from low grade deposits.

Gold is the most important metal occurring in placer deposits and has been recovered in quantity from both ancient and present stream channels and from ocean beaches. The source of placer gold is not always apparent on account of the distance that the gold has been transported. The gold was derived through the disintegration of gold-bearing veins, lodes, or shear zones. These deposits were not necessarily rich and, in many cases, they have been entirely removed by erosion.

Few of the gold placers in Nevada occur in the well-defined channels of permanent streams. Most of them are found in the alluvium on hillsides or in the channels of intermittent streams. Disintegration of the gold-bearing deposits has set much of the gold free, but there has been insufficient water to transport and sort the material except during "cloudbursts," torrential rains of short duration. The water from these heavy rains runs off the steep mountain slopes in such volume, and with such velocity that it has extremely high transporting power and tends to scatter rather than to sort and concentrate the material in the stream bed. The particles of gold and detrital material in the placer deposits formed on hillsides, and in intermittent stream channels, have not been subjected to the prolonged pounding and grinding action of a permanent stream, consequently they are sharp and angular. Several Nevada streams have cut their channels down through ancient and buried stream channels. Where this has taken place, the well-worn material from the ancient channel has been carried into the present stream and deposited along with the angular fragments from recent erosion.

The subject of gold placers is covered in the LTniv. of Nevada Bull., 1'01. 30, Yo. 4, Placer Mining in Sevatla. by IYilliam 0. Vanderburg, 1936.

Deposits of minerals are formed in the same way and under much the same conditions as the gold placers. The source of the mineral is also much the same. In every case, it has been derived through the disintegration of primary deposits. As examples, platinum occurs in minute quantities in the sands of the Colorado River near the Boulder Dam, and interesting placer deposits of cassiterite (the oxide of tin) occur at Rabbit Hole in Pershing County, at Tuscarora in Elko County, and near the Izenhood Ranch in Lander County, but none have been proven to be of commercial importance. Minerals of high specific gravity such as scheelite, cinnabar, and several of the lead minerals are sometimes found in detrital material. While they have not been found in commercial quantity in Nevada, they can be readily concentrated and recognized in the miner's pan, and the source of the minerals has often been located in Nevada by panning the alluvium up the stream bed or hillside.
Residual concentrations formed on hillsides through the disintegration of rock, without the sorting effect of running water are commonly called "eluvial" deposits to distinguish them from stream placers. In a strict terminology, many of Nevada's productive gold placers have been eluvial deposits. The original placer worked at Round Mountain by dry washers was of this type.

4b- Deposits Formed in Bodies of Surface Water
During decomposition of the rocks, part of the material goes into solution as soluble salts and is transported by water to the sea or into lakes. Much of this material remains in solution as is evident from the high mineral content of the ocean and also of many lakes, such as Pyramid and Walker Lakes. Deposition of the dissolved salts is brought about
(a) by precipitation as a relatively insoluble compound through chemical reaction between dissolved substances or between such substances and carbon dioxide or oxygen absorbed by the water,
(b) by precipitation as a result of evaporation of the water, and
(c) indirectly by the accumulation of the boney structure of plant and animal remains.
Deposits of limestone, magnesite, diatomite, colemanite, and manganese oxides in Nevada are of these types.

Limestones are sedimentary rocks, composed principally of calcium carbonate. They may contain minor amounts of magnesium and iron, and other impurities such as clayey or sandy material.
Limestone has been deposited either by the accumulation of the shells of animals and the supporting structures of plants, or by precipitation from solution by chemical reaction. Many fossiliferous limestones have been so thoroughly crystallized by metamorphism that their type of deposition remains in doubt as the fossils have been practically obliterated.
Limestone, to be of commercial grade, must be comparatively free of clayey, sandy, or cherty material. The deposit at Sloan, Nevada, has been noted for its purity and has been mined steadily for many years.

Dolomite is composed of both calcium and magnesium carbonate, and it also occurs as thick sedimentary beds. Pure dolomite contains 54.35 percent calcium carbonate and 45.65 percent magnesium carbonate.
Dolomitic limestones contain varying proportions of the two compounds., Most dolomites, and dolomitic limestones, have been formed by the replacement of part of the calcium carbonate of limestone by magnesium carbonate. It is of common occurrence in the eastern and southern part of the State.
Dolomite is used as a substitute for magnesite and brucite in the manufacture of refractory brick. It is burned as a source of quicklime and may ultimately be used as a source of magnesium metal in Nevada. A deposit of excellent purity has been mined at Sloan, Nevada.

Magnesite sometimes occurs as stratified deposits, but these deposits, on account of impurities contained, are seldom commercially valuable. Such a deposit occurs southeast of Overton, Nevada, on the edge of Lake Mead.
The commercial deposits of magnesite at Gabbs, Nevada, mined both for refractory uses and for reduction to metal magnesium, are closely associated with dolomite and are generally considered to be an alteration product of dolomite. It occurs as a massive rock similar in appearance to the surrounding dolomite, while the magnesite of California occurring in serpentine is commonly white in color, with a vitreous luster and a distinctive conchoida1 fracture. The relatively rare mineral, brucite (magnesium hydrate) is found in large tonnage adjacent to the magnesite at Gabbs.

Diatomite (diatomaceous earth) is a chalky appearing, often pulverulent, sedimentary deposit, composed almost wholly of diatoms. The diatoms, a class of small aquatic plants, construct protective coatings or tests of opaline silica extracted from solution. These tests accumulate as stratified deposits in fresh-water lakes as well as in the sea. Diatomite is of widespread occurrence in Nevada and has been mined in many places at frequent intervals, mainly for insulation use.

Colemanite, a calcium~ borate, occurs quite extensively in Clark County as a bedded deposit associated with beds of limestone and clay. It was actively mined until the discovery of thicker beds of borate minerals near Mojave, California. There are "Bedded Deposits of Manganese Oxides near Las Vegas, Nevada," according to the Univ. of Nevada Bull., Vol. 25, No. 6, of that title. The origin is given as precipitation from solution and the occurrence as "lenticular beds interstratified with tuffaceous material." The tonnage available resulted in the construction of a large metallurgical plant for its treatment by the Defense Plant Corporation in 1943.

The evaporation of bodies of surface waters form deposits of easily soluble mineral salts that accumulate as saline residues. The easily soluble salts, leached from the weathered rocks, are transported to basins occupied by lakes or seas where evaporation leaves an accumulation of salts. Saline deposits may also form on playas of intermittent lakes, marshes, and on slopes below mineral springs.

Precipitation takes place in a saturated solution, therefore complete evaporation is not necessary for deposition. Fine silt and sand often settle out of suspension and accumulate with the salts, resulting in impure deposits. A dry climate accelerates evaporation.

A closed basin prevents dilution and dissipation, thereby confining the deposition to a limited area. Such conditions existed throughout the Great Basin area of western United States, including Nevada.
A great variety of mineral salts are present in the saline deposits although many of them are found only in minor amounts. The more abundant ones in Nevada are common salt, gypsum, anhydrite, sodium carbonate, sodium sulphate, and the borates, while the scarce, but yet important ones, in other localities are bromine salts, potassium salts, calcium chloride, and sodium nitrate.

Gypsum and anhydrite, as saline residues, occur abundantly in Nevada in sedimentary beds. Gypsum contains water of crystallization as one of its constituents, and when calcined it combines with water to make an excellent cementing material. It finds extensive use in industry as wall plaster and plaster board.
Also, large tonnages of raw gypsum are used in the manufacture of Portland cement.
In the past forty years large deposits of very pure gypsum have been worked in the vicinity of Yerington and Mound House in Lyon County, near Gerlach, but in Pershing County, and along the Union Pacific Railroad in Clark County.

Anhydrite, the nonhydrous calcium sulphate, finds its only use as a low-cost fertilizer. Anhydrite, through absorption of water, slowly alters to gypsum. Therefore, the top portion of many deposits in Nevada is gypsum, grading with depth into anhydrite.
Both of these minerals are also common as gangue minerals in some ore deposits. Some minor occurrences are the products of reactions of acid solutions on lime-bearing minerals.

Sodium chloride (common salt) occurs in many of the playas or marshes in Nevada. In the days of the early silver mills at Virginia City, Candalaria, Tybo, and other camps, much salt was produced at the closest marshes such as the Eagle Salt, Dixie, Diamond, Columbus, Rhodes, Teels, Butterfield, and Sand Springs marshes. The latter has produced salt in recent years, as has also the rock salt deposit southeast of Overton in Clark County.
Sodium carbonate and sodium sulphate are present in the muds of playas in many Nevada valleys, and in some localities they are combined in various proportions with calcium carbonate, and calcium and magnesium sulphates. The presence of these salts is evident from the white coating on the ground. Few of these numerous deposits are of commercial value.

Deposits of sodium carbonate are relatively rare in Nevada. In the days of the early mills, deposits were worked at Soda Lakes in Churcllill County and at Double Springs in Mineral County. Soda from chemical plants now supplies the market.

Sodium sulphate is used mainly in the glass and paper manufacturing industry, and attempts have been made to utilize Nevada deposits. Deposits have been worked at Sodaville, in Mineral County, and at Wabuska, in Lyon County, and many other deposits are known.

The borates occur in notable quantities in the playas of intermittent lakes as natural water soluble borax and as ulexite ("cotton ball") mixed with other salts, silt, and sand. All of the early borax was recovered from this type of deposit, and in Nevada the marshes of Churchill, Mineral, and Esmeralda Counties were important sources until the bedded deposits of colemanite were discovered in Death Valley. In recent years a small quantity of borax was produced from a marsh in Fish Lake Valley.

At frequent intervals over the years extravagant claims are made as to the fabulous content of gold and mercury in the muds of the playas and dry lakes and its easy recovery. The fallacy of these claims is set forth in the Univ. of Nevada Bull., Vol. 35, So. 4, Geol. and Min. Ser. KO. 35, "An Investigation as to the Presence of Commercial Quantities of Mercury and Gold in the Dry Lakes of Nevada," by Jay A. Carpenter, 1941.

4d- Deposits Formed by Circulating Meteoric water
Atmospheric water percolates down along available channel ways in the rocks of the earth's crust and is then termed meteoric water. Readily soluble minerals are taken into solution and transported until deposited from solution by evaporation or precipitation.

That circulating water has dissolved and removed large volumes of material from below the earth's surface is apparent from the large caverns that now exist in some of the calcareous rocks. Some of this water penetrates to considerable depth where a rise in temperature increases its dissolving power, and much of this water then returns to the surface as springs, bearing a heavy load of soluble salts. Part of its load is deposited along the channel ways by precipitation, part around the spring vent by evaporation, as spring deposits, and the remainder is carried away in surface streams to accumulate in a lake or sea.

Calcium, sodium, potassium, and magnesium salts are easily soluble, and have been taken into solution in large amounts and either removed entirely or deposited where conditions were favorable.

Silica and many of the metallic minerals are but slightly soluble in meteoric water, but some important deposits of copper, zinc, and vanadium have been formed as a result of concentration by circulating water.

Indeed, it would be a unique mineral deposit that did not show the effects of circulating water in its near-surface portion, but no economically important mineral deposits, formed strictly by circulating meteoric waters, so far have been found in Nevada.