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By Ogden Tweto

The classes of ore deposits in the mountain province of Colorado that have been the most productive in the past and that offer the greatest promise for the future are: (1) disseminated or stockwork molybdenum deposits associated with Tertiary stocks; (2) combination precious- and base-metal veins in Tertiary volcanic rocks; and (3) base-metal replacement deposits and veins in Paleozoic sedimentary rocks. Veins in Precambrian rocks, including precious metal-bearing sulfide veins, gold-silver telluride veins, and tungsten veins, have been mined extensively in the past, but their aggregate output has been far subordinate to that of the groups named above, and the outlook for major output from them in the future is poor. Principal ore deposits of south-central Wyoming are sedimentary iron ores of Precambrian age and uranium deposits in Tertiary sedimentary rocks. Precambrian deposits of several kinds are scattered through the mountains of both states. The most promising of these, based on present indications, are vanadiferous titaniferous magnetite in southern Wyoming and southwestern Colorado and thorium-niobium rare-earth deposits associated with alkalic intrusive complexes in Colorado. Most of the mining districts of Colorado are in the Colorado mineral belt, a generally narrow but somewhat irregular belt that extends northeastward across the north-trending mountain ranges and the north-northwest trending geologic grain of the state. The belt is characterized by intrusive rocks of Laramide and younger age and by ore deposits, and it occupies environments ranging from Precambrian crystalline rocks through Paleozoic and Mesozoic sedimentary rocks to Tertiary volcanic rocks. Two ages of intrusion and ore deposition recently have been recognized in the mineral belt. One is Laramide (late Cretaceous and early Tertiary), and the other is Oligocene. As interpreted herein, the major replacement deposits and most of the sulfide vein deposits in Precambrian and sedimentary rocks are Laramide in age, and the molybdenum deposits, the tungsten and the gold telluride veins in Precambrian rocks, and the precious- and base-metal veins in volcanic rocks are Oligocene in age. In general, the Laramide deposits are mesothermal in aspect, and the Oligocene deposits are principally epithermal. The existence of two mineralization stages, in considerable part characterized by different suites of metals, has many applications in exploration.

Jan 1, 1968

By R. K. Davey

Before the introduction of computerized procedures, mineralization modeling (compilation of level maps), ultimate pit studies, ore reserve compilations, and long-range mine planning all were performed manually. In fact, much of the basis for today's computer programs was derived from manual procedures and actual experience of mining engineers and geologists.

Jan 1, 1979

When exploration shows a mineral deposit to be of importance, the subsequent work of defining the limits or boundaries of the ore body and preparing it for the extraction of the mineral is termed development (Young, 1946). In underground mining, development also connotes the work that is involved in opening up a deposit preparatory to stoping or productive operations. In this section, mine development is defined to include all studies and operations that will enable a sound evaluation of an explored ore body from the technical and financial standpoints. Though the ultimate objective of all mine development activities is the same, the scope and detail of these activities vary depending on the type of ore body (e.g., flat bodies, lenses and sheets, massive deposits, narrow veins, and bedded deposits), and the method of working the deposit (e.g., under- ground or surface methods or a combination of both of the methods). Therefore, the various contributions in section 3 address such topics as long-range planning, feasibility studies, and valuation for many types of ore bodies and different mining methods. This introductory review is divided into three major parts. In the first part, long-range planning aspects are briefly discussed with .an emphasis on current trends that affect mine planning. In the second part, an overview is provided of the contents in the mine development and valuation section. In the last part, remarks are made on the status, scope, and future of quantitative and computer methods in mine planning and development.

Jan 1, 1979

By Y. C. Kim

One aspect of long-range mine planning in an open-pit mine is first to define the ultimate pit limit, i.e., the projected size and shape of the pit at the end of its life. This chapter is concerned with various techniques that are currently in use today in determining the ultimate pit limits using the computer. Prior to the early 1960's, open-pit limits analyses were entirely performed manually. This manual technique has been well explained by Soderberg and Rausch in Surface Mining (1968). Soderberg and Rausch also explain clearly the need for establishing the ultimate pit limits in long-range mine planning, i.e., location of surface facilities, dumps, etc. Consequently, the need for ultimate pit limit design will not be reiterated here. Nor will the definitions of the most commonly accepted terminologies such as break-even stripping ratio, economic cutoff grade, ultimate and working slope angles, preproduction stripping, etc. be repeated here. Briefly, the objective of the ultimate pit limit design (which is also frequently referred to as pit design in short) is to determine the projected final pit limits of an ore body and its associated projected grade and tonnage, which will maximize some prespecified economic criteria while satisfying practical operational requirements.

Jan 1, 1979

By R. K. Davey

Subsequent to the determination of an ultimate pit and ore reserves for metallic and nonmetallic deposits, it is necessary to develop the most profitable long-range mining sequence for the deposit. Long-range planning is a dynamic process because product prices and costs change, additional assay and metallurgical data become available, mining technology and operating prices and requirements change. The three case studies included in this subsection present different techniques for dealing with the objectives of long-range planning. To be able to forecast production, profitability, and capital requirements, it is essential to have a long-range mining plan.

Jan 1, 1979

By S. C. Suboleski

Underground long-range planning and mine design is an iterative process beginning with a specified production level, quality criteria, and geology of the deposit. Given these factors, face equipment is selected, the number of units needed and production per unit are calculated, and the mine infrastructure needs are developed. The scheduling of units then follows, with the design of the various support systems coming next. The support systems needed to maintain production can be grouped as follows: (1) outby haulage of mined material, (2) supply and manpower haulage, (3) ventilation and associated environmental considerations, (4) drainage, fresh water, and related considerations, (5) rock mechanics, and (6) power, supply. Thus, long-range planning may be viewed as a three-tiered structure, with the data on required production, product quality, and geology at the upper level; the development of face production capacity and the scheduling of units sufficient to meet the needs of the first two at the second level; and the development of support operations for the production units at the third level. Changes at any level may affect not only the items on that tier but may also cause a change to the items on the tier above and below. For example, if the rock mechanics data indicate that desired opening sizes will not be stable, then this situation will affect not only the ventilation design but also the production rate and possibly the selection of equipment itself. These, in turn, may affect the scheduling of units and the number of units, ultimately influencing the entire third tier of systems.

Jan 1, 1979

By Donald W. Gentry

Mine valuation is a multifaceted, interdisciplinary process by which the economic viability of an old, existing, or new mining property or venture is determined. The problems associated with arriving at a representative and accurate value for a mining property are complex and numerous, and the procedures utilized in determining these values vary from simple, back-of-envelope calculations to sophisticated mathematical manipulations requiring computer modeling and analysis. Regardless of the complexity or the degree of sophistication associated with any particular project, the evaluation process must ultimately provide management with two basic types of information. First, the evaluation process should indicate whether or not the mining project is economically viable after careful consideration of all engineering, environmental, geologic, and other variables pertaining to the project. Second, the evaluation process should provide management in- formation on the relative attractiveness of the project when compared to other alternatives. In some cases, it may be extremely difficult to supply this information for a project directly. Nonetheless, the evaluation process should and must present all information necessary in a format which can be incorporated into the decision-making process as expeditiously as possible.

Jan 1, 1979

By Joel Moskowitz, George J. Tamaro

"A new office complex is being constructed within 25m of the Hudson River in New Jersey. The project, about 140m by 100m, will include 4 basements under the entire footprint. The perimeter was to be secured by a 0.8m thick diaphragm wall, laterally supported by high capacity tieback anchors to rock. Columns were to be supported on footings or piers to rock at a nominal bearing pressure of 3.8MPa at depths up to 17m. Although some variation in rock quality was observed in borings, a substantial vertical seam of weathered and/or decomposed rock, 1 to 2 meters wide by as much as 8 meters deep, was encountered during construction necessitating field redesign. In some locations, the seam dipped below rock that is more competent. Footings were redesigned for support on drilled caissons, or soft rock was excavated and replaced with concrete necessitating a reduction in bearing pressure of the footings by one half. The tower covering the south half of the site was topped out in 2002 and the garage levels in the north half of the site were completed in 2003.PROJECT DESCRIPTIONIn September 1999, a major investment company began the due diligence process to evaluate a property on the shore of the Hudson River in Jersey City, New Jersey for a new multi-use complex. The site offers a spectacular view of lower Manhattan directly across the river from the site of the former World Trade Center. The site, covering two city blocks, was previously occupied by manufacturing and office facilities of Colgate-Palmolive.The building footprint is approximately 140m by 100m, the southern half of which is occupied by a 260m tall office tower, shown in Fig. 1. As of this writing, design of the structure for the northern half of the site has not been finalized, but it is expected to comprise an 11-story office building. A glass-covered atrium will occupy the street that formerly separated the two blocks. As part of the due diligence process and preliminary design evaluation, alternatives were considered to accommodate various numbers of parking places, both above and below existing grade. A major consideration for below grade parking is the proximity of the Hudson River some 25m east of the east building line."

Jan 1, 2004

By Brian W. Hester

Ore was first discovered at the Kidd Creek mine site during the first few days of December 1963 when a diamond drill hole there cut sulfide mineralization. This event marked the end of the search started 11 years earlier with the conception of a geological idea. Economic forces provided the stimulus needed for the application of this idea but other-events, and legal considerations, also played arole in delaying the ultimate discovery. The location of Kidd Creek is shown in Fig. 1. Management of the then Texas Gulf Sulphur Co. had become concerned in 1951 at the company's declining reserves of native sulfur in Texas. This problem was to be alleviated by new discoveries and development of methods to recover sulfur from sour gas. At the time, however, the solution appeared to lie in the recovery of sulfur from pyritic ores. Canada was chosen as the best country in which to find these ores. Walter Holyk was hired in 1951 to conduct the search. By 1952, he was working in the area near Sherbrooke, Quebec, known as the "Eastern Townships" where mineralization consisting of base metal sulfides in volcanic rocks has long been known. His observations here led him to the then revolutionary conclusion that such mineralization could have formed as part of the volcanic process and not, as was generally supposed at the time, by replacement. On learning of the discovery of similar mineralization near Bathurst, NB, he moved there in the spring of 1953. He was able to utilize his new concept and found the Half-Mile Lake sulfide deposit.

Jan 1, 1991

By J. J. McDougall

The Windy Craggy copper-cobalt-bearing massive sulfide lies in the Tatshenshini map area of northwestern British Columbia, where British Columbia joins Alaska and the Yukon. The area is covered with numerous glaciers and ice fields that mask geologic detail and hamper ground access. In 1957, Ventures Ltd., the predecessor of Falconbridge Ltd., undertook a reconnaissance of the area using a float-equipped Supercub. At that time, large lakes and glaciers were unnamed and little was known of the geology. The prospecting team was led by J.J. McDougall with Stan Bridcut as pilot-prospector. Prospecting was directed at the St. Elias Mountains between the rich Kennecott copper deposits to the northwest and the Maid of Erin silver-copper mine on the Haines Road, BC. Reconnaissance flying in 1957 and early 1958 eliminated vast segments of the Kennecott-Haines Road terrain as being too inaccessible for mining. In 1958, the Supercub landed at an unnamed lake and the terminal moraine of an unnamed 12.9-km (8-mile) long glacier was prospected. The lake was called Tats Lake and the glacier named Tats glacier (short for Tatshenshini). Float of sulfide was found in the moraine. Follow-up investigation along Tats glacier led to the discovery of several small sulfide deposits and these were staked. Prospecting active glaciers in this area can be complicated because of drainage capture by eroding valley glaciers. Glacier capture resulted in the float from one mineralized area being deposited at different times in three widely separated moraines. Float of the Windy Craggy deposit, therefore, occurs along the course of older glaciers and at the base of the currently active ones. The problem of sourcing caused much additional Supercub reconnaissance. It is interesting to note that about 40 geographic names were given various features in this unmapped portion of the St. Elias Range, and these names were ultimately officially adopted.

Jan 1, 1991

By Ogden Tweto

The Leadville district, on the west flank of the Mosquito Range in central Colorado, has produced silver, zinc, lead, gold, and minor metals valued at $512,000,000. The ore deposits are in a sequence of dolomites and quartzites, Cambrian through Mississippian in age and about 500 feet thick, which is extensively intruded by porphyry sills, dikes, and plugs of Tertiary age. The sedimentary rocks and sills dip about 15°E and are broken by many faults, which are predominantly of nearnorth trend and downthrown to the west. The ore deposits are principally blanket or manto replacement deposits, but in the eastern part of the district many veins occur also. The replacement deposits are largely confined to three dolomite units in the stratigraphic sequence. Of these, the uppermost or Leadville Dolomite is the most productive. Most ore bodies are on the underside of porphyry sills and, particularly, beneath sills that occupy unconformities. Many replacement bodies have vein roots or branch from veins. The veins occupy faults and are productive mainly in the section of quartzites and dolomites and included sills. The primary ores consist principally of pyrite, marmatitic sphalerite, and galena but locally contain chalcopyrite, silver minerals, bismuth minerals, and gold. Principal gangue minerals are manganosiderite and jasperoid. The ore deposits have a crude zonal pattern centered around an intrusive center at Breece Hill, which evidently influenced ore deposition thermally. The ore deposits were oxidized to depths of several hundred feet in Miocene time. Bonanza ores mined in the early days were characterized by cerussite, cerargyrite, and embolite. Later, zinc carbonate ores were identified and mined on a large scale. The Leadville district is a complexly faulted and intruded area at the intersection of major fault systems trending N20°W and N15°E in the Mosquito Range. The intensely fractured area evidently localized igneous intrusion and subsequent mineralization. Ore solutions rose on many of the faults in the area, presumably from the same source as the porphyry magmas. Movement on these faults continued into the Pleistocene, displacing ore bodies, oxidation zones, Pliocene sediments, and glacial deposits.

Jan 1, 1968

By Wilbur S. Burbank, Robert G. Leudke

The impressive western San Juan Mountains of Colorado were carved by Pleistocene and Recent erosion from a thick blanket of Tertiary volcanic rocks that rests upon a basement of metamorphic, sedimentary, and igneous rocks ranging in age from Precambrian to Tertiary. These rocks record a long, complex, and fairly complete sequence of geologic events that have affected the region through much of geologic time. The geologic history of the region includes several periods of deformation, erosion, and igneous activity; closely associated with two of these periods were two periods of mineralization. The older of these was related to Late Cretaceous-early Tertiary (Laramide) intrusive activity, and its deposits occur in the older sedimentary units. The younger, of later Tertiary age and related to the widespread middle Tertiary extrusive and intrusive activity, is the chief concern of this paper. Tertiary volcanism built a great plateau surrounding the San Juan volcanic depression, a complex of subsided crustal blocks in the vent areas. The Silverton and Lake City cauldrons, nested within the larger depression, were domed by resurgent magma that created keystone grabens along the distended crests of the domed floors and many postcauldron radial and concentric fractures, dikes, and intrusive plutons within and marginal to the cauldron sites. The postvolcanic evolution of large quantities of gases, chiefly carbon dioxide and water, propylitically altered the rocks in and around the volcanic depression prior to the younger period of ore deposition. The ore deposits, localized mainly in the radial and concentric fractures about the cauldrons and in the associated graben faults, include fissure veins, chimneys, replacements, and disseminations in volcanic and underlying basement rocks. Fissure veins, constituting the major economic source, yielded precious- and basemetal ores. Most veins are compound with sphalerite, galena, and chalcopyrite; some veins contain silver-bearing arsenical sulfosalts and free gold. These minerals occur in predominantly quartz gangue. However, gangues composed of several manganese silicates and carbonates and relatively· barren of metals are abundant locally and probably originated by alteration of rocks at roots of faults and fissures during evolution of carbonatic solutions. The bulk of the ores are hypogene and probably had their source in deep parent magmas of the shallow eruptive rocks. Total production since the early 1870's has exceeded half a billion dollars in recovered metals, with a ratio of precious- to base-metal values of about 3 to 2. Since 1930, improvements in metallurgical treatment and mining techniques have increased the base-metal yield.

Jan 1, 1968

By J. D. Miller, C. L. Lin

In general, x-ray computed tomographic (CT) techniques are able to provide 3D images of the internal structure of opaque materials in a nondestructive manner. The unique cone beam geometry allows acquisition of all 2D projections with only one rotation of the sample thus providing for fast data acquisition and better x-ray utilization, as a complete 2-D detector array receives the cone-shaped flux of rays. Thus, an isotropic 3D volume can be reconstructed without the mechanical translation and the stacking of sequential slices as is the case for more conventional CT scanners. In this regard, a state-of-the-art, custom designed xray microtomography facility to provide very detailed 3D spatial analysis of multiphase particles has been installed and is in operation at the University of Utah. The reconstructed 3D tomographic volume allows for an adjustable spatial resolution from 5 to 20 microns for sample dimensions from 10 to 40 mm in diameter, respectively. Utilization of this custom designed cone-beam x-ray microtomography system is discussed. Applications described include: coal washability analysis for the design and operation of coal preparation plants, liberation analysis for evaluation of grinding practice and separation efficiency, mineral exposure analysis for prediction of the ultimate recovery from heap leaching operations, and analysis of the pore structure network of packed particle beds for simulation of flow through such porous structures as encountered in filtration and heap leaching.

Jan 1, 2003

By N Madani

Linear interpolation of geo-related variables is important to different aspects of mining engineering such as long and short-term mine planning. The ore deposit description attained from spatial modeling of blast hole datasets has been widely taken into account, particularly, for short-term mine design and grade control, so as to define the ultimate destination of a mined block. It is approved in practice that making the wrong decision leads to losing a huge amount of money for choosing the destination of a mined block. In the case of complex ore deposits, the issue of estimation of variables in the region is questionable and somehow tedious. Due to this fact, modeling those complications in the co-spatial behavior of the grades in multi-element deposits prompts one to employ enhanced geostatistical techniques. Traditional approaches of this purpose such as polygon, inverse distance weighted and even (co)-kriging approaches, are incapable of reproducing those complexities such as non-linearity, heteroscedasticity and geological constraint that frequently exist among the variables of interest in those deposits. Another difficulty in employing those linear interpolations in grade estimation is smoothing effect, in which the estimated grades are over and under estimated. CITATION: Madani, N, 2018. 3D estimation of variables with complexity in cross-correlation structures, in Proceedings Complex Orebodies Conference 2018, pp 49–52 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Nov 21, 2018

By D. Kruse, M. Sanaei

"A three dimensional nonlinear finite element model was constructed to simulate and analyze the behavior of large diameter segmentally lined shafts when subjected to seismic waves. To reduce computation time, a less complex shell model was also constructed. Transient excitation was applied to the supported nodes of both models, at the base of the shaft, with a time-function representing the 1994 Northridge earthquake. The models are developed at the end of an original phased construction-stage analysis of the shaft. Displacements, forces and moments are monitored and compared during the analyses steps. During the initial 15 seconds of the earthquake, the top and bottom of the shaft show similar displacements. The maximum horizontal displacement within the shaft is approximately 3.5 inches in the solid model and the shell model results conform to this by a factor of 50%. Keywords: Segmentally Lined Shaft, Soil Structure Interaction, Seismic Analysis, Finite Element Modeling, Ground Modification INTRODUCTION TO SEGMENTALLY LINED SHAFTS Large diameter segmentally lined shafts (SLS) are underground systems, constructed sequentially using precast concrete segments to form a ring which provides both temporary earth support during excavation and permanent structural support. The segments are connected via alignment pins, bolts, packer material and other structural components that ensure the continuity of the SLS. In this analysis, the construction process uses a top-down sequence wherein each soil layer is excavated, damp-proofing is installed and concrete segments are assembled to form each ring. This process is repeated until design depths are achieved completing the shaft. It is known that cylindrical structures sustain lateral (earth) pressure mainly through the mechanism of hoop-stress. As Timoshenko’s ring theory goes, a cylinder shell structure is prone to buckling under external pressure [1]. In typical designs of small diameter shafts and tunnel linings with small diameter to wall thickness ratios, nonlinear buckling analysis (i.e., nonlinear geometric analysis) is not considered, since the buckling capacities for those structures are much higher than typical pressures encountered at their design depths. However, in shafts with large diameter to wall thickness ratios, buckling could become a critical issue. In any given mode, as buckling occurs in a ring, some sections of the structure move inward while alternating sections protrude. Although the presence of the surrounding soil could mitigate the buckling by resisting the movement (Soil structure interaction) seismic loads can cause ground movements and lateral pressures, exceeding the capacity of the structure. It is therefore essential to design the load bearing capacities of the shaft to minimize the possibility of any catastrophic failure due to earthquakes. This study explores the structural response of the SLS when subjected to seismic loads."

Jan 1, 2016

By G Guy, J Summerville

Over the past five years in coalmine slope evaluation the application of 3D modelling has increased significantly as software has become more user friendly but also areas requiring evaluation have become increasingly complex. Often simplistic 2D Limit Equilibrium modelling and analysis has been poorly used to solve slope stability issues by back analysis and subsequent forward predictive analysis. A plane strain condition with slope movement perpendicular to the slope strike is assumed where in reality the problem is more complex. This often leads to a misinterpretation of the failure mechanism because the failure mechanism being modelled in 2D does not generally represent the actual failure mechanism observed in the pit, with 2D modelling often oversimplifying a slope. This paper presents a case study where the initial slope assessment and remedial design evaluation was undertaken as a 2D Limit Equilibrium model which led to an overly conservative and costly remedial solution being implemented. Subsequent modelling of the slope using both 3D Limit Equilibrium and Finite Element methods were able to provide a better understanding of the structural complexities associated and correlate well with slope monitoring data to create a more realistic outcome.

Nov 29, 2022

By Z Bagheri, E Moosavi

Identifying the exact borders of geological domains and assessing the uncertainty of these borders are crucial steps in resource estimation. This potential field method was designed to build 3D geological models from data available in geology and mineral exploration, namely the geological map and a digital terrain model, structural data, borehole data, and the geologist’s interpretations. The resource evaluation of a copper ore deposit is often performed in three steps: (i) delimitation of the boundaries of the units corresponding to the various geological formations or ore types; (ii) estimating densities; and (iii) estimation of grades within each unit. In simple cases (e.g., a series of sub-horizontal layers), the geometric model can be built using 2D geostatistical techniques (kriging or co-kriging of the elevations or thicknesses of the various horizons) which also quantify the uncertainty of the model. A 3D model of Cu grade is generated using ordinary kriging; however, the indicator kriging method is run to design a 3D model of rock types by incorporating the drilling results. Block models of geophysical and geological characteristics are included in a similar 3D mesh to investigate their relationships in copper mineralisation.

Sep 1, 2021

By Jean François Stumper

This paper presents the TMT “3D TopScan” profilemeter, which is a Radar-based profilemeter that measures the total burden surface within a few seconds and on a continuous basis. This equipment is based on an electronically steered solid-state radar scanner, such that a very large amount of surface data is collected within short time intervals. These new electronically steered antenna systems are the key to meet the demands for both accuracy and speed. The TMT probe is installed at a fixed location at the top cone, such that it does not interfere with the charging, enabling a continuous measurement and a compact design.Blast furnace charging optimization can be achieved based on its measurement data. The burden surface is continuously measured every few seconds, enabling the blast furnace operator to detect any details including asymmetries or irregularities in the surface pattern. A variety of process analysis tools are available. Due to the continuous and fast measurement, the burden descent speed can be supervised without process interruption on an ongoing basis. More accurate process models (layer models) than ever are obtained which fully consider the unsymmetries.This technology, developed by TMT Tapping Measuring Technology, is successfully in operation at Salzgitter Flachstahl GmbH at their blast furnace B. The probe and the data evaluation tools combine today’s technological capabilities in measurement technology with the practical requirements of blast furnace operation.

Aug 16, 2017

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 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