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By R. P. Fischer

The Colorado Plateau region has been the principal domestic source of uranium, vanadium, and radium. The value of these commodities produced from the region through 1964 slightly exceeds $2 billion. Most of the deposits occur in streamlaid lenses of sandstone in the Chinle and Morrison Formations. The ore minerals below the zone of oxidation are low-valent oxides and silicates of uranium and vanadium; all of these, except the quite stable vanadium silicates, oxidize readily to a variety of high-valent uranium and vanadium minerals. The ore minerals partly replace fossil wood but mainly impregnate sandstone, typically forming tabular ore bodies that are nearly concordant to the bedding of the host rock. The deposits range in size from those containing only a few tons of ore to those containing more than 1 million tons. Stratigraphic, lithologic, and sedimentary structural factors obviously influenced the localization of these deposits, but igneous activity, hydrothermal mineralization, and tectonic structures show no similarly consistent influence on localization. The deposits are epigenetic, but the source of the ore metals, the nature of the ore-bearing solutions, and the time of ore emplacement are uncertain. A few uranium deposits in the region are dominantly discordant to bedding and occur in collapsed breccia pipes or along linear fractures. Their origins are also uncertain.

Jan 1, 1968

By Paul E. Melancon, Vincent C. Kelley, Dale F. Kittel

Uranium of the Grants region occurs predominantly in continental sandstones of the upper part of the Jurassic Morrison Formation, but significant lesser deposits are found in limestone of the Jurassic Todilto Formation and in black shale of the Cretaceous Dakota Formation. The deposits are disseminations that form runs ranging from a few hundred tons to several million tons. The ore consists mainly of uraninite, uraniferous carbonaceous material, coffinite, and such secondary oxidized minerals at tyuyamunite, carnotite, and uranophane. In sandstone, the ore runs were locallized by mudstone, interstitial carbonaceous material, and primary sand channel trends. In limestone, ore deposition was related to folds and fracture zones. The great bulk of production has come from the Ambrosia Lake mines in the Grants district and the Jackpile and Paguate mines in the Laguna district. The ores, which generally have been 0.20 to 0.30 per cent U3O8, are treated at mills operated by Kerr-McGee Corporation, Homestake-Sapin Partners, and The Anaconda Company, all in the Grants district. In 14·years after discovery in 1950, the deposits had yielded concentrates valued in excess of $800,000,000. Since deposition of the Jurassic host rocks, a geologic history involving tilting, faulting, erosion, changing ground-water environments, and oxidation has considerably influenced the existence and character of the deposits. It is evident that their ongm is not simple. However, a degree of agreement appears probable for a pre-Dakota age for the original ore runs, and, at Ambrosia Lake, two principal stages of ore formation, separated by Laramide faulting, are recognized. All earlier ores have been modified by Quaternary oxidation, solution, and enrichment during which time much secondary tyuyamunite, metatyuyamunite, carnotite were formed.

Jan 1, 1968

By Hiram B. Woon

Uranium ore deposits in the Lisbon Valley area are in an arcuate belt, 15 miles long by one-half-mile wide, on the southwest flank of the Lisbon Valley anticline. They range in size from 500 to 1,500,000 tons and by mid-1965 had yielded 6,147,000 tons containing 48,530,000 pounds of U3O8. Ore bodies average 6 feet thick, are tabular, amoeba-shaped masses, concordant to the bedding, and are in the thickest, lowest sandstone unit of the Moss Back Member of the Chinle Formation of Triassic age. The host rock is predominantly a fluviatile, calcareous, fine-grained to conglomeratic sandstone. Uraninite, the principal ore mineral, fills the pores in the sandstone and partly replaces sand grains and fossil wood fragments. During Late Permian and Early Triassic time pre-Chinle Formations were arched to form the ancestral Permian anticline. Moss Back streams truncated Moenkopi and Cutler beds which capped the ancestral anticline and then covered it with fluviatile elastics, which were subsequently covered by thousands of feet of Triassic, Jurassic, and Cretaceous sediments. Uranium, probably derived from the Chinle Formation by diagenetic processes, was transported in connate ground waters, was moved by compaction or hydrostatic forces, and was deposited under reducing conditions. Uranium was emplaced around the crest of the ancestral anticline, prior to the Laramide orogeny. During the orogeny, the Tertiary-Lisbon valley anticline was super induced on the Permian anticline and, penecontemporaneous with uplift, was faulted parallel to its longitudinal axis. The Big Indian ore belt on the southwest flank footwall block was elevated to approximately its present position, and the northeast flank hanging wall block was elevated only slightly. An extension of the Big Indian ore belt around the northeast flank of the ancestral Permian anticline may exist in the Moss Back sandstone in the downthrown block northeast of the Lisbon Valley fault.

Jan 1, 1968

By Roger C. Malan

The Monument Valley and White Canyon districts are in northeastern Arizona and southeastern Utah. Exploration and mining for uranium has been conducted in these districts since the late 1940's. In July 1965, ore reserves plus ore production at 174 properties were approximately 3.3 million tons of ore containing about 19 million pounds U3O8, of which about 10 per cent remained in reserves. The two largest mines, Monument No. 2 and Happy Jack, together account for about 45 per cent of the sum of production and reserves. Approximately half the deposits in these districts contain less than 1000 tons of ore. Nearly 5000 feet of Permian, Triassic, and Jurassic sedimentary rocks, mainly continental in origin, are exposed in these districts. All the important uranium deposits are in the Shinarump, the basal member of the Chinle Formation of Triassic age. The Shinarump lies on a widespread unconformity. It is composed of fluvial sediments, generally less than 100 feet thick, that were deposited by streams flowing from a source to the south. Most of the deposits are in an arcuate belt, convex to the west. This belt, 3 to 12 miles wide, extends from Monument Valley northward nearly 130 miles. It is along the western flank of the ancient Monument Valley-Monticello upland, an area that was slightly uplifted at the beginning and again at the end of deposition of the Shinarump. Erosion and subsequent reworking of the Shinarump sediments in the vicinity of this upland are postulated to have made possible the transportation, in solution, of the uranium contained in these sediments. The soluble uranium probably was carried by migrating ground water into sites favorable for precipitation in Shinarump beds bordering the upland. Thus, the original very small amounts of dispersed uranium in the early Shinarump sediments were accreted in the favorable belt where the accumulations of carbonaceous plant remains and their decay products provided a persistent reducing environment.

Jan 1, 1968

By E. Motica

Ores containing uranium and vanadium minerals have been mined from the Salt Wash Member of the Morrison Formation from many localities in the Colorado Plateau region since about 1900. The most productive deposits are in a relatively small area in Southwestern Colorado referred to as the Uravan mineral belt. The mineral belt is a narrow elongate area in which uranium-vanadium deposits in the Salt Wash have a closer spacing, larger size, and higher grade than those in adjoining areas. Pre- 1948 ore production from the mineral belt was 655,000 tons averaging 1.91 per cent V2O5 and 0.28 per cent U3O8. Production during the 1948 to 1964 period was 7,900,000 tons of ore averaging 1.46 per cent V2O5 and 0.27 per cent U3O8. The ore deposits occur principally in the uppermost sandstone unit of the Salt Wash. This unit consists of sandstone lenses formed by a system of aggrading braided streams flowing in an easterly direction generally normal to the mineral belt. These lenses, referred to as channels, are as much as one mile in width and average about 50 feet in thickness and can be traced for several miles along their courses. Areas favorable for one deposits are recognized principally by the following criteria: ( 1) a host sandstone thickness of over 30 feet, (2) the presence of carbonaceous material in the host sandstone, and ( 3) gray mudstones and clays associated with the ore-bearing sandstone. The ore minerals are believed to have been precipitated from laterally migrating solutions; there is no apparent genetic relationship between ore deposits and tectonic structural features. Exploratory and development drilling from the surface is conducted in three phases: first, wide-spaced drilling to find and outline favorable channels; second, moderate-spaced drilling in favorable ground to locate ore deposits; and third, close-spaced drilling to block-out ore. Long-hole drilling from underground stations is used to find and delineate extensions of ore bodies being mined. Due to intense production since 1948 from shallow deposits, best potential for new deposits exists in areas where the ore-bearing unit is 600 to 800 feet deep.

Jan 1, 1968

By W. G. Fischer, S. R. Felde

Trona ore at the Westvaco, Wyo., mine of FMC Corp. has been obtained by room-and-pillar methods since the mine began operation. The flat- lying, 10-ft thick trona (Na2CO3.NaHCO3.2H2O) is covered by 1500 ft of weak shale and sandstone, and extraction of pillars on the retreat quite often throws large amounts of weight onto development workings. Weak shales in the roof and floor fail rapidly, and the mine ribs slough to such an extent that mining often becomes very difficult. When this occurs, efficiency goes down, costs go up, and the risk to men and machinery increases greatly. Difficult mining conditions in the early '60s forced a re-evaluation of all presently known mining techniques in bedded deposits. Many room-and-pillar mining plans were compared with the system in use at that time, but none of them offered a better solution to the roof control problem. Longwall techniques were also seriously considered, but machinery was not available that would cut the hard trona seam and still maintain the productivity obtained from room-and-pillar sections. However, since roof control in the longwall methods appeared to be more satisfactory than that found in room-and-pillar techniques it was believed that a new technique using the longwall principle and based on rock mechanics should be tried rather than previously known pillaring methods.

Jan 4, 1966

By Robert Glass Cleland

[NEW YORK OFFICE James Douglas President Chairman of Board Walter Douglas Vice-president President Louis S. Cates President Cleveland H. Dodge Vice-President Chairman of Board]

Jan 1, 1952

By Davis M. Lapham

Ore bodies at Cornwall, Pennsylvania, have been mined since 1742 principally for iron from magnetite, but also for copper (in chalcopyrite), silver (in chalcopyrite), gold (in chalcopyrite), cobalt (in pyrite), sulfur, and crushed limestone aggregate. Metallization occurs as a limestone replacement above the essentially conformable, south dipping portion of a discordant saucershaped diabase sheet. Pre-Triassic and Triassic faulting have emplaced host Cambrian limestone and two Ordovician(?) hornfels units, now for the most part resting above diabase. Thermal metamorphism of carbonate-bearing country rocks largely to calc-silicates, is ubiquitous. Metasomatism, particularly of potassium, is unique to ore zones. Associated magnetite- actinolite mineralization replaces, in a preferential sequence, all earlier metamorphic and metasomatic minerals and rarely diabase chilled margin. Mineralogical (diopside, actinolite, mica, sulfides, oxides) and chemical (Fe, Cu, S, Co) zoning of the eastern ore body is focused down dip, near its western edge, along a major vertical fault through diabase. This zoning is spatially related to magnetite veins in diabase, to an ore body within diabase, and to a shear zone that probably extends through the sheet.

Jan 1, 1968

By Alvin Kaufrnan

This chapter will deal with the policy issues relating to mineral land disposal. The first portion of the chapter will discuss in general terms the legal systems utilized in disposing of mineral rights and the major problems encountered; the second section will comprise an analysis of the policy alternatives with emphasis on the pricing aspects. In this section, we are not attempting to provide a legal text, but simply to outline very briefly and sketchily the details of the systems. More detailed information can be obtained by contacting the appropriate governmental agency for specific information. Mining laws throughout the world are based on two major legal groupings: English Common Law and the Civil Law. These basic systems are often modified by Islamic Law and by negotiated -mixtures of these. Under English Common Law, which is embraced by the United States, Canada, and many of the members of the British Commonwealth of Nations, individuals can hold property and can extract minerals from it. The owner can dispose of minerals as he sees fit, either through contract, deed, or whatever other system is appropriate. CIVIL LAW Under Civil Law (also known as Regalian Law), mineral ownership is normally placed in the state, regardless of who owns the surface. This legal system applies in most European countries and their former colonies. It is derived from Roman law. The mineral deposits, thus, are worked under concessions. Most countries use a negotiated concession system and sometimes a combination of negotiation and competitive leasing. The negotiated system has advantages as well as disadvantages for both government and industry. In a system where the government administratively determines who is to get what, the concessions are normally large. Frequently they range up to several millions of acres and involve a considerable investment on the part of the concessionaire. There is no duplication of effort on the part of producers to obtain information, since a single concessionaire controls a given area. Consequently, less money is spent in toto because several prospective operators are not required to seek the same information. The system also has the advantage to the industry of permitting the concessionaire to select and retain the best producting area. Thus, major developmental expenditures are made on the production unit only, with a resulting reduction in the risk factor. The large initial size of the concession also minimizes the possibility of expending a large bonus payment for a barren property. The major governmental advantage of a concession is its flexibility in accomplishing other national objectives. For example, a lease could be negotiated at less than fair market value, but requiring substantial de-

Jan 1, 1976

By Wallace F. Lovejoy

The conservation of mineral resources as a public policy question has received a great deal of attention in the area of petroleum and practically no attention in other mineral areas, except, of course, soil and water. Some insights into mineral conservation problems and how these problems can be approached in the social decision-making process are significant. THE MEANINGS OF CONSERVATION Conservation means many things to many people .2, 11 Too often it is a label attached to a proposed program or policy to add respectability and to gain public acceptance. It is a term applied to specific resources in some contexts and to resources generally in other contexts. The oft-quoted comment of President William Howard Taft that conservation is something everybody is for, no matter what it means, succinctly illustrates the confusion over both the meaning of the concept and the policies required to implement a conservation program. This confusion goes quite deep and is more than a matter of semantics or emphasis. For example, the efficient exploitation of a mineral deposit which might be applauded as good conservation in terms of that mineral, might well result in destruction of or harm to the environment which might be viewed as bad conservation. Economics provides some tools to help solve this sort of problem, but there may be fundamental differences in value systems among groups in this society, so that ultimately society must collectively make a subjective political choice as to what is good or bad conservation policy.a An Economic Definition of Conservation The economic concepts of conservation have more relevance and are more easily applied to minerals than to many other natural resources. This does not mean that economics goes all the way in understanding mineral conservation, but at least it provides a framework for analysis which later can be altered to include noneconomic variables. Perhaps the greatest confusion in discussing mineral conservation arises in attempting to relate purely physical concepts with economic concepts. An economist is interested in maximizing or conserving values; he is not interested in maximum physical recovery or utilization. He wants to know the costs and benefits of using, or not using, resources. The economic theory of mineral-resource conservation can be labeled variously as the theory of exhaustion, the theory of resource exploitation, the theory of resource utilization, or simply as a special case of the broad theory of capital.4,28 Consideration of these labels illuminates the concerns of the econo-

Jan 1, 1976

By Henry N. Doyle

Basic procedures in the mining industry have changed only slightly over the centuries since metals and fossil fuels became an essential part of man's economic and cultural life. Mining techniques of the 20th century bear a close resemblance to those of the ancients: exploration, ore breakage and removal, hoisting, and processing. The great advances, however, have been in equipment and machinery of greater size and speed which allow greater efficiency in each of these operations, with a corresponding reduction in manpower requirements. Yesteryear production was measured in pounds per man-day; today a giant and complex, but highly efficient, continuous mining machine can remove as much as eight tons of material a minute from the coal face. If our energy and metal demands of the future are to be met, and at this time it can be assumed that coal will continue to supply a significant part of the energy demand, other dramatic and spectacular technologic advances must be accomplished. Along with mechanization, however, public policy will require the industry to place greater emphasis on health and safety protection, as has been well demonstrated by recent actions of Congress. Together, fossil fuels and metals formed the basis for the industrial revolution-coal to develop power and metals to transform power into useful energy. Strangely enough, however, the mining industry the world over has lagged behind the continuous social changes brought on by the industrial revolution, such as preventive health and industrial hygiene services, and other programs directed to the well-being of the worker. In some countries this resistance, to change may have led in part to the nationalization of the coal industry. Whether this disinclination to social change in the industry of the United States reflects managerial attitudes, inept labor leadership, government neglect, or economic relationship would be the subject of an interesting study. Doubtless the mores of miners and their resistance to change, as well as their geographic isolation from other industrial groups, may have had a strong influence on these factors. In every major coal mining country the accident rate in the coal industry is substantially higher than other industrial groups, and the cost of pneumoconiosis compensation in some countries is greater than the combined costs for all other occupational diseases from all other industries. The high social cost of occupational accidents and diseases may have been a factor in the closing of the industry in at least two countries. Despite diminishing manpower needs, the industry is faced with critical manpower shortages. Miners once reared their sons to be miners; now it appears that the opposite is true. This factor alone, however, does not explain an acute labor shortage in the industry. Traditionally, the industry drew much of

Jan 1, 1976

By E. N. Harshman

Uranium mining and milling are rather new industries in Wyoming and South Dakota, but, in the past 15 years, ore valued at more than $400,000,000 has been mined and milled in the region. The major uranium deposits are in structural and/ or erosional basins or on the flanks of uplifts. The principal host rocks are medium- to coarse-grained sandstones that were deposited in continental or brackish-water environments and were derived in large part from the granitic cores of the uplifts and the sedimentary rocks that flank the cores. The ages of the host rocks range from Cretaceous to Miocene, but more than 95 per cent of the known reserves in the region are in rocks of early Eocene age. The bulk of the ore in the region is unoxidized and lies below the water table; some oxidized ore has been mined in the Powder River Basin and Black Hills districts. The unoxidized ore comprises pyrite, uraninite and/ or coffinite, marcasite, hematite, jordisite(?), and selenium. Minerals in the oxidized ore include hydrous uranium carbonates, phosphates, and sulfates and hydrous uranium and/ or vanadium silicates. The deposits are in the form of ore rolls that contain from a few hundred to a few hundred thousand tons of ore, ranging in grade from 0.1 per cent to 1.0 per cent U3O8. The rolls are at the margins of large tabular bodies of sandstone that has been altered by the orebearing solutions. The deposits are believed to have resulted from the interaction of an alkaline; oxidizing, ore-bearing ground water and a reducing environment. Exploration guides include the spatial relations of ore to altered sandstone, favorable lithology in the host rock, the content of certain elements and/ or compounds in ground water, and the relation of deposits to major paleodrainages.

Jan 1, 1968

By Olin M. Hart

Uranium ores occur in the Lower Cretaceous Inyan Kara group of heterogeneously stratified fluvial and fluvial-marine sandstones in the Black Hills of western South Dakota and northeastern Wyoming. There are three principal mining districts, each in the ancestral and present drainage area of a major river partially responsible for stripping the region to its present topographic form. Primary minerals in the deposits are coffinite and uraninite with minor amounts of paramontroseite and haggite. The ore minerals coat sand grains and fill interstices of complexly cross-stratified sandstone along solution fronts similar to "roll" type deposits of the other districts. Minerals of oxidized deposits are typically carnotite and tyuyamunite with different proportions of secondary vanadium accessory minerals. Ground water was the transporting medium and deposition of primary uranium and vanadium minerals occurred in reducing environments produced and controlled by physio-chemical characteristics of the sedimentary rocks.

Jan 1, 1968

By Vernon A. Mrak

The Powder River Basin of northeast Wyoming was the first area in the state to receive attention during the early days of uranium exploration. Although the uranium occurrences are many and widespread, most of the known ore deposits are quite small and difficult to mine. As a result of this, detailed regional exploration in the Powder River Basin has been neglected. The uranium ore deposits occur in that part of the Wasatch Formation that is of earliest Eocene age, and they are scattered over an area of about 400 square miles. All uranium ore deposits are associated with red beds or altered sandstones within the Wasatch Formation. Uranium ore was first shipped from the region in 1953. Total production through June of 1965 is 506,257 tons of uranium ore containing 1,922,527 pounds of U3O8. The average grade of ore is about 0.19 per cent U3O8. The major producing area of the Powder River Basin is the Monument Hill district where about 91 per cent of the basin's total uranium ore has been mined. The source of uranium is presumed to have been the Oligocene, Miocene, and perhaps Pliocene tuffaceous rocks that once overlay the area. Joints, fractures, and permeable beds or zones were the passageways for the migration of uraniferous waters from the overlying beds to the Eocene host rocks.

Jan 1, 1968

By E. N. Harshman

The Wind River Formation of Eocene age is the host rock for large high-grade uranium deposits in the Shirley Basin. The major deposits are in a northwest-trending belt of sandstones that were deposited in stream channels and that were lithologically favorable for uranium accumulation. Paleotopography influenced the character and position of the belt. The major deposits within the belt are at, or near, the margins of large tongues of altered sandstone formed in the most transmissive parts of thick sandstone beds. At least two tongues of altered sandstone are present in the belt, one of which is 5 miles long, 3 miles wide, and 70 feet thick; the other tongue is somewhat smaller. Ore bodies consist of a few hundred tons to several hundred thousand tons of material containing from 0.10 to about 2.00 per cent U3O8. The ore mineral is uraninite associated with pyrite, marcasite, hematite, and calcite. Deposition is believed to have taken place when neutral to slightly alkaline, oxidizing, uranium-bearing ground water, on entering the basin, encountered a reducing environment. The reductant may have been H2S of biogenic origin. Geology played an important role in discovery, exploration, and development of ore in the Shirley Basin.

Jan 1, 1968

By William Paxton Hewitt

Mineral deposits of western Utah and eastern and central Nevada have produced in excess of $8,500,000,000 since 1871. Through 1965, Bingham Canyon had produced over $4,600,000,000 and seven other camps over $100,000,000 each, principally from base metals. In addition, they yield iron and uranium ores; contain the world's largest beryllium reserves; and have produced the United States' most important gold discovery in 25 years. The area is famous for its silver production and its bonanza-type precious metal deposits, but such production has become unimportant. The area's greatest producers of metallic wealth are base-metal deposits in limestones and porphyry-type copper ores. In contrast to silver's decline, gold production has been rising-the result of by-product operations at Bingham Canyon. The area also contains gold deposits in which the gold is so finely disseminated it is not recoverable by panning. The area's gold ores frequently are associated with realgar, cinnabar, and carbon. The ore deposits generally are attributed to Tertiary intrusives,* but, at Mountain City, they may be related to Paleozoic volcanism. Mountain City has yielded rich chalcocite from quartz-pyrite-chalcopyrite bodies in siliceous sediments. Regional stratigraphy and structure are difficult to interpret, 69 per cent of the area being obscured by alluvium or volcanics. In general, central Nevada consists of chert, shale, silty sandstones, lavas, and pyroclastics; eastern Nevada and western Utah of many alternations of limestone, shale, and sandstone. Central Nevada has been structurally active since late Devonian. Although granitic stocks may have intruded eastern Nevada and western Utah prior to the Cretaceous, these areas remained quite stable until affected by the Laramide Revolution. Precambrian exposures, occupying 3 per cent of the area, have not been important hosts. However, the question is raised as to whether Precambrian carbonates, properly situated with respect to invading mineralizers, might yet prove to be important. This paper describes 16 precious metal camps, 6 limestone replacement districts, 2 silver-base metal vein deposits, and 9 miscellaneous types and discusses briefly the 11 * Some ores, as at Ely, are associated with intrusives of. Cretaceous age. major camps that are described in detail in this Volume.

Jan 1, 1968

By R. D. Rubright, Owen J. Hart

In the Bingham district over a span of more than 90 years, 43,947,104 tons of "non-porphyry" copper, lead, zinc, gold, and silver ore have been mined from a folded and faulted alternating series of Pennsylvanian and Permian limestones and sandstones and from some Tertiary intrusives. This ore came from six major mine areas located east, south, and west of the Bingham stock, site of Kennecott's porphyry copper deposit. The sediments were affected by varying degrees and types of metamorphism within 2000 feet or more of the Bingham stock. The intrusives consisting of the mineralized Bingham and the unmineralized Last Chance stocks and numerous dikes and sills of granite and monzonite were intruded into the asymmetrical northwest-trending and plunging Bingham syncline. Extrusives, although present, contain no ore. The bulk of the non-porphyry ore in the district was found above the Midas thrust. The primary ores are found in three major types of deposits: concordant bedding fissures, crosscutting fissures, and replacement deposits. These deposits consist of both copper and lead-zinc ore bodies that contain various amounts of gold and silver. These are in places roughly zoned both laterally and vertically and have been mined for up to 10,000 feet outward from the center of the Bingham stock and up to 5000 feet vertically below the surface. Secondary ore was of minor importance and was found in and under the oxidized zone at shallow depths. Genetically the Bingham stock, the ores, and related phenomena have had a common source.

Jan 1, 1968

By Paul F. Kerr, Donald M. Hausen

Fine colloidal gold near Carlin, Nevada is disseminated in leached carbonate strata of the Roberts Mountains Formation in the Lynn "window" of the Roberts Mountains thrust fault. The ore body is generally stratiform and is more or less conformable to altered beds near the top of the formation, underlying Devonian limestones. Two sequences of mineralization are recognized: ( 1) an earlier base metal-barite assemblage related to early Cretaceous intrusives (121 ± 5 m.y.), and (2) a later low temperature Au-As-Hg-Sb assemblage of near surface emplacement. The earlier sequence consisting of sparse galena and sphalerite in barite with anomalous amounts of zinc, lead, nickel and copper, associated with dikes of dacitic composition, is of little economic importance. The later sequence of gold, realgar, cinnabar, and stibnite associated with extensive silicification and argillic alteration of limestone beds, has resulted in important deposits of gold. Argillic alteration of hydrothermally leached carbonate strata has provided the environment in which the most prominent gold deposition took place. Carbonate minerals in the limestone host rock have been replaced by microcrystalline quartz and chalcedony to form stratiform silica masses and recrystallized lenses of euhedral quartz. Zones of porous silicification are light gray to white, elliptical in shape, and more or less follow bedding. Silicification is bordered by argillic alteration and pyritization. Deposition of gold usually lies in a zonal pattern that encircles chimney areas of silicification.

Jan 1, 1968

By D. R. Cook, W. M. Shepard, H. T. Morris

The East Tintic district in central Utah has produced ores of gold, silver, copper, lead, and zinc valued at more than $120,000,000. All of this ore has been produced from blind ore bodies in Paleozoic sedimentary rocks that are concealed beneath hydrothermally altered volcanic rocks and cut by intrusive bodies of Eocene age. The Paleozoic rocks, which range in age from Early Cambrian to Mississippian form the core and limbs of a large, northtrending asymmetric anticline that is cut by low-angle thrust faults, high-angle transcurrent faults, mineralized fissures and faults, and normal faults that are both older and younger than the mineralized fissures. The ore bodies generally may be grouped into two classes: ( 1) massive replacement bodies that are rich in silver, lead, zinc and manganese and (2) fissure veins that are valuable primarily for their content of gold, copper, and silver. The replacement ore bodies are localized chiefly in the Middle Limestone Member of the Ophir Formation of Middle Cambrian age where it has been thrust-faulted against older or younger rocks and cut by northeast-trending mineralized fissure zones. The fissure veins primarily are productive only in the Lower Cambrian Tintic Quartzite. Extensive lowgrade deposits of lead-zinc-silver ore recently have been discovered in Devonian carbonate rocks in the Lower plate of the East Tintic thrust fault but as yet are unevaluated. The principal guides to ore include: ( 1) intersections of northeast-trending fissures and crossbreaking faults that brecciate the carbonate rocks; (2) zones of late-stage pyritic, calcitic, and sericitic alteration in the lavas; ( 3) primary geochemical anomalies in the altered rocks; and ( 4) mineral zoning patterns of the known ore bodies. The present development of the Burgin mine would seem to assure the continued productivity of the district for at least a decade.

Jan 1, 1968

By T. B. Nolan, R. N. Hunt

In terms of present metal prices, analysis of extant records of the Eureka district indicate past production of the magnitude of $200,000,000 in recovered silver, lead, and gold. Production to date has been thoroughly oxidized ore from massive replacements, fissures, manta, and pipe-like bodies-in large part from the massive Eldorado (middle Cambrian) and Hamburg (middle to late Cambrian) dolomite horizons. In the productive area of about 6 square miles carbonate rocks comprise 80 per cent of the section, which includes early Cambrian to late Ordovician horizons. Four-fifths or more of the district's production came from a block of Eldorado dolomite in Ruby Hill that was moved in over younger horizons on strands of a regional thrust fault. The lower 500 to 600 feet of the block was much fractured and brecciated and became host to the ores. The Ruby Hill fault, a late normal fault, limits the Ruby Hill block of dolomite on the north for its entire length of nearly a mile and terminates the downward extent and exploitation of its ores. It introduced an hiatus of some 40 years in the district's development. Drilling of the past 20-odd years, followed by underground work, has now exposed the down-faulted extension of the Ruby Hill block of Eldorado dolomite and its contained ores, dropped 1800 feet in a dip shift to depths beneath the surface of 2000 to 3000 feet. The ores at depth are the sulfide counterpart of those mined in early years on the footwall side of the fault. The ores of the district may be attributed to the same source as the intrusive quartz diorite outcropping in a limited way south of Ruby Hill but found at depth elsewhere in drill holes. The long north-south axis of the district, its anticlinal structure, its major faults, and the distribution of the intrusives and, in their relation to the surface, the ores themselves, all suggest that erosion has not cut deep enough to expose the extent of intrusives and the ores consequent upon them.

Jan 1, 1968