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By J. M. Ríos

"Without conscious effort and understanding, a number of factors unique to mineral exploration often hinder building sustainability in host communities. These include the episodic and transitory presence of an exploration team in the community and the inherent uncertainty of outcomes— most projects fail from an exploration perspective. Experience in rural communities across Mesoamerica, however, has shown that it is possible to go beyond pragmatic philanthropy and infrastructure improvements to promote positive, sustainable outcomes in communities. Examples are provided of lessons learned and proven practices that improve quality of life and support a strong social license to operate.RÉSUMÉ Sans une compréhension et un effort réfléchis, de nombreux facteurs uniques à l’exploration minérale nuisent souvent à l’établissement d’un développement durable dans les communautés d’accueil. Ces facteurs comprennent la présence épisodique et transitoire d’une équipe d’exploration dans la communauté et l’incertitude inhérente des résultats – la plupart des projets échouent après l’étape de l’exploration. L’expérience dans les communautés rurales à travers la Méso-Amérique a toutefois démontré qu’il est possible de dépasser la philanthropie pragmatique et les améliorations des infrastructures pour promouvoir des résultats positifs et durables dans les communautés. Des exemples qui améliorent la qualité de vie et soutiennent un solide permis social d’exploitation sont présentés; ils sont tirés de leçons apprises et de pratiques éprouvées."

Jan 1, 2015

By Solar D. Andrés

Several large open pit mines are facing the challenge to design and plan an underground transition to sustain the long term future of their operations. The desired underground method is Block Cave since its natural high productivity and low operating cost that could compete against the open pit cost to mine the final open pit pushbacks. It has been found at several operations where this transition is taking place that the pit limit at which transition is performed is rather deep. This aims to think that perhaps several transition projects have started late in the life of an operation producing an over stress in the transition project and reducing the flexibility in the design of the overall infrastructure and a substantial lost in the complete project NPV. Traditionally the approach to plan open pit underground transition consists of sequentially deciding the final pit using a Lerchs and Grossman type of optimization to find the overall maximum pit shell and then running block cave simulations to find the best footprint for the underground option. The research summarized in this paper is related to the development of an integer linear programming model that aims to compute simultaneously the maximum NPV of the combined option final limit open pit and block cave underground footprint. The model has been tested using two large block models representing two of the current operations in the process of transiting from open pit to underground operations. Finally, the integrated open pit underground transition model is going to be compared to the traditional sequential approach in order to define which method achieves the best NPV. The paper illustrates both case studies in depth.

Oct 1, 2009

By V. A. Temeng, M. Owusu-Tweneboah, K. Awuah-Offei

The mining industry has long recognized the value of dispatch systems in open pit mines as they reduce load and haul costs. Over the years, researchers have proposed many dispatch systems with various limitations and advantages. The simplest dispatch algorithms are the so called 1-truck-for-N-shovels dispatch strategy. These algorithms are limited by the fact that their objective functions do not consider all the objectives of a mine and cannot be applied to all possible truck-shovel configurations. They are also myopic in nature. However, they are simple and computationally efficient and do not require occasional updates of the upper stage problem as required in multi-stage dispatch algorithms. In this work, an agent-based truck dispatch algorithm that conceptualizes trucks as intelligent agents that make autonomous dispatching decisions to maximize their utility is proposed. The advantages of this algorithm includes utility functions that encapsulate all of management’s objectives and agent’s with broad situational awareness. They are also more suitable for autonomous trucks. We evaluate the new algorithm against a simple 1-truck-for-N-shovels dispatch strategies using discrete event simulation. The simulation results show that the new utility function has significant advantages over 1-truck-for-N-shovels inspired utility functions. Future work will incorporate adaptive behavior into the model via reinforcement learning algorithm. INTRODUCTION In an attempt to optimize truck shovel operations in open pit mines in order to reduce the cost of haulage operation, which according to (Alarie and Gamache, 2002) is estimated to be about 50-60% of the total cost of mining, several effective and efficient strategies have been developed. Optimizing truck haulage systems in an open pit mine is one of the greatest challenges of mining engineering and has been the subject of many research projects. It is important that haulage systems are designed to be efficient, in order to minimise haulage cost, improve profitability and increase the total mine value. Although optimizing the number of trucks required in the operation of open pit mines can help reduce the cost, an effective and efficient truck dispatching system add value by ensuring that the available trucks are used efficiently. With truck dispatching systems, the trucks are normally dispatched between the crusher and dumping station (e.g. waste dumps and stockpiles), and the shovels in order to maximize the total output, obtain grades targets, reduce fuel consumptions and other objectives. Truck dispatching, which can have one or more objectives, is typically subjected to certain operational constraints such as truck and shovel capacities, empty and loaded trucks speed, among others. The problems associated with truck dispatching was first mentioned by Dantzig and Ramser (1959) after they made an attempt to determine the optimum routes of a fleet of delivery trucks between bulk terminals and a service stations. The problem occurs in several industrial systems besides mining (Alarie and Gamache, 2002; Gendreau et al., 2006; Rego and Roucariol, 1995). The dispatching problem, also referred in literature as the vehicle routing problem, has seen a number of modifications and extensions with Bertsimas and Ryzin (1991) presenting a dynamic and stochastic vehicle routing problem, and Ralphs et al. (2003) also adding to the original concept, the concept of vehicle capacity routing.

Jan 1, 2019

By P Holtham, K C. Runge, E Tabosa

Traditionally, comminution and flotation have been surveyed, analysed and optimised separately. This often results in less than optimal outcomes because of the complex interactions between the two processes. Flotation recovery is a strong function of the valuable mineral particle size distribution with lower recovery of ultra-fines due to poor flotation kinetics, optimum recovery for the intermediate sized particles and lower recovery for the coarser particles due to poorer liberation. Thus, to optimise flotation recovery and grade it is usually best to grind to a fine size. This requirement needs to be balanced by the cost of grinding finer and the revenue that results from an increase in grinding throughput, which often results in a coarsening of the flotation feed.This paper demonstrates a technique which can be used to predict the effect of a change in feed grind size on flotation performance. It can be used to determine the optimum grind size for a particular operation which optimises total circuit profitability. Analysis is relatively easy to perform, as a minimum, requiring only size by assay data of the feed and product streams of the process. Case studies will be presented that demonstrate the usefulness of this technique in feed grind applications. Potential use of the technique for regrind evaluation is discussed. The limitations associated with this technique in certain circuit scenarios will also be outlined.CITATION:Runge, K C, Tabosa, E and Holtham, P, 2014. Integrated optimisation of grinding and flotation circuits, in Proceedings 12th AusIMM Mill Operators’ Conference 2014 , pp 77–84 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Sep 1, 2014

By Joel Lööw, Jan Johansson, Lena Abrahamsson

Industry 4.0 offers new possibilities to combine increased productivity with stimulating workplaces in a good work environment. Used correctly, digitalization can create attractive jobs in safe control room environments, which provide space for the employee’s full expertise and creativity. This is true also for the mining industry. But, to succeed, it is important to analyze the development from a worker’s perspective. What will happen to their work? What skills will be needed in the mine of tomorrow? We must also consider the risks, such as privacy issues, increased stress, and work-life boundaries. These questions must be understood if we are to create workplaces that can attract a young and diverse workforce to tomorrow’s mining industry. In this article, we try to illustrate what the new technology can mean for the individual miners. We formulate the notion of Mining 4.0 (Industry 4.0 in the mining industry), where we try to create an image of how the future might look from a miner’s perspective and how mining companies may navigate their way to a future that works for all miners. To illustrate the range of possible outcomes, we formulate two scenarios: one utopian and one dystopic. At the end of our article, we bring forward six recommendations that can be considered a beginning of a road map for the human side of Mining 4.0.

By K. C. Corin, Z. G. Song, J. G. Wiese

In the present investigation flotation was used as a diagnostic to understand the effects of changes to the media during the grinding stage. The observations from flotation for different media are discussed in terms of particle size distribution, water recovery, solid recovery and valuable mineral recovery. The chemical parameters; pH, redox potential (Eh) and dissolved oxygen (DO) levels were measured. A Base Metal Sulphide (BMS) ore was ground under media of different shapes and compositions to determine the effect of different rod/ball material on the flotation performance of the ore. For a given grind (D80 = 75 µm), media shape had no effect on flotation performance of a BMS ore. Different grinding media had significant effect on slurry chemical properties and flotation performance. Mild steel rods (RMMS) produced the highest solids recovery, nickel and pyrrhotite (Po) recovery, with the highest ethylenediaminetetraacetic acid (EDTA) extractable iron and the lowest DO level. Chrome steel balls and stainless steel rods produced the lowest metal recovery, EDTA extractable iron and the highest DO level. A complementary study was conducted to investigate the effect of using different grinding media compositions, viz, ceramic, forged steel, 15% chrome steel, stainless steel and a mixture of stainless steel and 15% chrome steel (50:50) balls were investigated using a Platinum Group Mineral (PGM), UG2 ore. Grinding products of five different media produced the same solids and water recovery, but the grades and recoveries of Platinum Group Elements (PGE) and chromite were different. Grinding products of ceramic media showed the best flotation performance, with the highest PGE recovery and the lowest Cr2O3 recovery. While grinding products of stainless steel balls (SS) produced the lowest PGE recovery and highest Cr2O3 grade and recovery in the concentrates.

Jan 1, 2016

By John A. Cordes, Thomas F. Torries

INTRODUCTION Nonfuel mineral markets have always tended to be volatile due to the cyclical nature of demand and the difficulty of fine-tuning capacity and output levels. Experiences over the past two decades, however, have been especially perverse. This short volume attempts to provide insights into this era and some guidance for the future. The papers are linked together by their focus on issues of productive capacity. At the same time, they provide a useful description, explanation and evaluation of the industry's evolution and its implications for the near-term. The regnant pessimism of the 1980s was very much alive during the early months of 1987 when the theme of this symposium was formalized. Indices of real metal prices had only inched-up from all-time lows in 1986. The hemorrhage of corporate red-ink was still a powerful memory. Frustration with the apparent inability of closures and reduced capacity utilization to re-establish market balance was strong. To the more pessimistic, the industry appeared destined to a quarter century of persistent excess capacity and nonremunerative prices. The conditions to support this pessimism were readily apparent. Economic literature tells us that surplus capacity is a complex, usually unexpected and often long-lived phenomenon caused by the interaction of at least three factors. The first is a rising capital-intensity of production. The second is the presence of aggressive new entrants whose capacity additions are typically (1) trade- dedicated because local market absorption is low; (2) cost-competitive for natural or policy-support reasons; (3) heavily burdened by debt-repayment obligations; and (4) viewed as important sources of employment, regional development and foreign exchange. The third, and often decisive, factor is an unanticipated decline or stagnation in demand growth for the sector's output. The first two factors combine to increase competitive pressures and to raise the actual or perceived cost of idle capacity. Market rigidity reduces responsiveness to price signals and distorts the burden of adjustment. Demand atrophy exacerbates these pressures especially on unprotected producers in the higher quadrants of the global supply function. In the mid-1980s, evidence of the existence of all these factors were easy to cite. Culprits were often identified. Claims for protection, global sharing of adjustment burdens, and imposition of disincentives to new capacity regardless of inherent economic status were co-dominant themes with the buzz-words of "competitiveness" and "level playing fields." This, notwithstanding the suggestions of the same economic literature that excess capacity persists longest when interventionist remedies permit it to continue. Today, only eighteen months later, the mood is decidedly more buoyant and most analysts are cautiously optimistic about the next few years. Many informed analysts have concluded that the excess capacity of the recent past has disappeared. For the present, the marketplace seems to have restored a balance between supply and demand. That the adjustment took five or six years longer than anticipated attests to the dramatic structural changes which have characterized the industry. The implications of these changes for the future are unclear. Were the macroeconomic shocks, coupled with industry transitions, so extraordinary in magnitude and timing that experiences were unique? If so, the next years may hold more stable and cautious times for the industry. Alternatively, is the inherently volatile metals industry facing a future of cycles equally perverse in terms of amplitude and duration due to economic uncertainities and inflexible industry response capabilities?

Jan 1, 1989

By Craig C. Hustwit

A pilot-scale treatment study was conducted on a copper-zinc bearing metal mine drainage using a pipeline treatment system called the In-Line System (ILS). The objective of this U.S. Bureau of Mines study was to determine whether the ILS could be extended from the treatment of coal mine drainages to the treatment of metal mine drainages. Hydrated lime (Ca(OH)2), quick lime (CaO), and caustic soda (NaOH) were used to neutralize the drainage. A novel reagent slurry of Ca(OH)2 and type C fly ash was also evaluated. Except for Ca(OH)2-fly ash slurry, treatment performance was evaluated for all reagents at pH values of 7,8,9, and 10. The Ca(OH)2-fly ash slurry was evaluated at a pH of 10. All reagents tested neutralized the metal mine drainage and substantially reduced the dissolved metal concentrations. It was concluded that the ILS could be useful in the treatment of metal mine drainages.

Jan 1, 2010

By T. Plikas, L. H. Gunnewiek, L. Oshinowo

"Electric furnace smelting is one of the principal unit operations for ferroalloy production, and increased process intensity, improved availability, minimal maintenance and a longer campaign life are common objectives for electric furnace operation in order to obtain favourable economics of production. Furnace designers have continued to develop innovative solutions, e.g., furnace cooling systems, that have allowed these objectives to be realised and CFD modelling plays a key role in the design process. Enhanced understanding of the energy transfer process in the furnace and the ability to develop, evaluate and optimize components of the crucible design to match process requirements are the key motivators for modelling.This paper provides an overview of recent applications of CFD to the design of electric furnaces. The aim is to illustrate what can be done, and identify areas where further work is needed in this field. Examples include furnace bath simulation, crucible cooling system design, taphole design, baking of Soderberg electrodes, off-gas systems, and fume control and building ventilation. It is argued that the trend is for numerical models to become more encompassing while at the same time more accessible, and therefore further improvements in furnace design will be realised with continued and well validated modelling efforts.IntroductionOne of the main processing steps in ferroalloy production is the smelting of ore in electric furnaces. The three-electrode, AC submerged arc furnace is most prevalent method for smelting in the industry, with shielded-arc and DC plasma arc furnaces achieving limited application (1,2). Commercial furnaces range in size from a few megawatts to more than eighty megawatts of supplied power, and operate with the objective of maximising the return on capital invested. Maximising productivity and energy efficiency are two pathways to achieving this objective, through process intensification, increased availability, minimal maintenance, and increased campaign life for the furnace and associated equipment. Concurrently, the need to reduce the environmental impact of metal production by controlling the emission of particulate emissions, greenhouse gases, and other harmful by-products must also be met."

Jan 1, 2004

By M. Dworzanowski

The difference in the magnetic properties of minerals is the basis for magnetic separation. All minerals can be generally classified as ferromagnetic (strongly magnetic), paramagnetic (weakly magnetic) or diamagnetic (non-magnetic). Magnetic separation can be conducted dry or wet. The majority of the applications of wet magnetic separation in the mining industry are based on the wet drum magnetic separator. The wet drum magnetic separator has been in use for over 50 years and its design is based on a rotating drum installed inside a tank. Inside the drum are stationary, permanent magnets arranged in an arc to provide the magnetic field. These magnets can be of the ceramic ferrite type providing a low intensity magnetic field or of the rare earth type providing a high intensity magnetic field.

Jan 1, 2010

By Keith A. Heasley

The objective of this paper is to back-analyze the August 6th ,2007 collapse at the Crandall Canyon Mine in order to better understand the geometric and geo- mechanical factors which contributed to that collapse. In this work, the LaModel boundary-element program along with the best available information is used for the back- analysis. To initiate the analysis, a six step base model of the mining in he Main West area was developed. This base model included a step for each of the critical stages in the mining of this area. Next, calibrated values for the critical input parameters: rock mass stiffness, gob stiffness and coal strength, were developed and optimized based on the best available information. Finally, the model was used to simulate the final mining scenario. As a result of this back analysis , a number of conclusions can be made concerning the mine design and the August 6th collapse: 1) Overall, the area was primed for a massive pillar collapse because of the large area of equal size pillars, 2) The abutment stress from the active retreat sections and the adjacent longwall panels were significant contributors to the collapse, 3) From the modeling alone, it is not clear exactly what triggered the August collapse, although the active mining is a reasonable assumption.

Jan 1, 2012

By Jerry Wang, Greg Bold, Mahmood Khwaja, Rebecca Caldon, William Simcoe

Phase III and Phase IV of the Beaver Creek Clean River Project, Albany, NY, includes microtunnel construction of 2,320 feet of 48-inch/36-inch internal diameter (ID) conveyance pipe and a 525 foot long 9.5-ft horseshoe shape roadheader excavated tunnel. Notable challenges were encountered during the construction including mixed face tunnel conditions, undocumented existing fill ground conditions that resulted in settlement during shaft construction, and unstable tunnel face conditions. This paper discusses several of the key challenges encountered during construction and selected solutions to construction sequencing and methodology to successfully complete Phase III and Phase IV of the Beaver Creek Clean River Project.

Dec 1, 2022

By A. J. Dudzik

In 2006 Arch Materials initiated the development of an underground limestone mining complex. The decision to proceed with the project was the end result of several years of market research and geological reconnaissance. The scope of the project included the construction of two declines to the mining horizons located approximately 900 feet below the surface, the means and methods of this phase will be the primary focus of this discussion. The design, fabrication and erection of a state of the art crushing and screening processing plant which when completed will have the primary crushing located on the mine level allowing uniformly sized material to be brought to the surface by means of a highly efficient conveyor system and a surface portion of the plant where material would be additionally processed, screened and stockpiled into specific product types. Also included in the scope of the project is the construction of offices, warehouse and shop facilities necessary to support the operations, commercial sales and distribution of the finished products.

Jan 1, 2010

By William P. Diamond

This report presents results of a cooperative research project between the U.S. Bureau of Mines and BethEnergy Mines, Inc. Four coreholes (two before mining and two after mining) were drilled at a longwall mine operating in the Lower Kittanning Coalbed to obtain coal and rock samples from over-lying strata to determine their gas content at various times in the mining cycle. The results of those tests indicate that 91 pct of the gas removed from the overlying strata came from coalbeds. Material balance calculations were made to compare the volume of gas produced both from gob gas vent holes drilled on the panel and gas removed by the mine's ventilation system with the volume of gas removed from strata directly overlying the panel to a height of 275 ft. This analysis indicates that only 40 pct of the total gas produced from the panel actually came from the strata directly overlying the panel. The remaining volume of gas production probably migrated to the longwall gob from overlying, and perhaps underlying, strata immediately adjacent to the panel and perhaps from greater distances downdip because of the establishment of a long-term pressure gradient in the study area.

Jan 1, 2010

By F. J. Cservenyak

The Bureau of Mines Solid Waste Program is directed toward three main areas of research: (1) Extraction of mineral, metal, and energy values from urban refuse; (2) recovery of mineral and metal values from wastes generated in the industrial extraction, processing, and utilization of raw materials and mineral-base products; and (3) upgrading and utilization of automotive scrap. The Bureau also has a contract and grant program which supplements the in-house research and aids in the training of personnel in the proper management of mineral- and metal-based solid wastes.

Jan 1, 1970

By S. Cotten

The concept for a Mine Escape Vehicle (MEV) is motivated by the desire to enable personnel to evacuate a mine rapidly as the first and most desirable response to any underground emergency. Under a NIOSH contract, Raytheon UTD developed a conceptual design for a vehicle that coal miners could use to facilitate their rapid exit from an underground coal mine in the case of emergency, including fire or explosion. The vehicle requirements include the capacity to provide life support functions for a number of miners, operate in an oxygen deficient, low or no visibility atmosphere, and travel at speeds faster than miners can walk out of the mine. To develop the MEV concept, three primary tasks were identified and executed. The first task focused on developing a realistic concept of operations (CONOPS) and associated high level vehicle requirements. The second task identified and evaluated currently available equipment and technologies that could be applied to address the CONOPS. The final task developed a design concept that integrated the identified components for the five (5) MEV sub-systems to provide the foundation for future detailed engineering design and prototype construction. The resulting MEV design concept is centered upon retrofitting existing mine personnel carrier chasses with available, mine-worthy equipment capable of addressing the MEV requirements. This approach offers a practical and economic avenue to prototype development and delivery with minimum lead-time and risk. Furthermore, by integrating familiar and available equipment into the design and CONOPS, the resulting Mine Escape Vehicles should also be more readily accepted by the mining industry.

Jan 1, 2010

By J. M. Mutmansky, C. Lee

"IntroductionRecognition of Coal Workers' Pneumoconiosis (CWP) as a major occupational disease has affected various aspects of the coal mining industry in the United States as well as in almost all major coal producing countries, Regulations related to the control of coal mine dust forced mine operators to modify their mining and ventilation practices. As the coal production record of the last 15 years shows, this had both direct and indirect impacts on the productivity and production costs.To identify the causes of CWP, many research efforts have been made throughout the world. Most of these studies were epidemiological in nature, attempting to pinpoint exposure and seam variables that affected the incidence of the disease. Other studies have been oriented toward the elemental and mineralogical composition of the coal dust in the mine atmosphere and in the miners' lungs. These studies indicate that there may exist some relationships between the characteristics of the coal mine dust and the prevalence of the disease. Some of the variables that are thought to affect the occurrence of CWP are the mass of the dust in the working place, the rank of the coal, the existence of certain trace elements, the free silica content of the coal, and the existence of other minerals in the respirable dust. A more complete description of these previous research studies has appeared elsewhere (Mutmansky and Lee, 1984).The behavior of particles in the human lung is known to be governed by their aerodynamic diameter, which is defined as the diameter of a hypothetical spherical particle with unit density that exhibits the same settling velocity as the given particle. Because of this fact, the aerodynamic size distribution of coal mine dust is one of the parameters to be determined along with the other physical, elemental, and mineralogical characteristics. The objective of this paper is to establish a strategy for providing coal mine respirable dust samples for characterization purposes using instruments that classify according to the aerodynamic diameter. In addition, several findings concerning characterization methods will be presented"

Mar 1, 1989

By A. Ghalambor

The objective of this investigation was to determine the sensitivity of three selected input parameters in the operational control of continuous bottom-fed thickeners used for the treatment of coal refuse slurries. The three variable parameters were "Feed Flow Rate," "Feed pH," and "Feed Solids Concentration." These three input parameters were studied under three polymeric conditions which were cationic polymer, anionic polymer, and a combination of cationic-anionic polymer. For each polymeric condition, two mathematical models were developed: one for overflow response, the other for underflow response. The combination polymers produced the best desired quality of overflow and underflow. For this polymeric condition, changes in the feed solids concentration was the most sensitive factor in the maintenance of the overflow quality, and the feed pH was the most sensitive factor for the maintenance of the underflow quality. The mathematical models generated a set of plots which would allow an operator to control and/or adjust any of the three variables to achieve optimum conditions.

Jan 1, 1989

By N. A. Warner

Virtually zero gas emission smelting is promoted as a means for securing environmental advantages and the general public’s acceptance. Concerns about climate change will increasingly make it more difficult for new primary metals projects to come to fruition unless greenhouse gas emissions are decreased. Assuming a sustainable market for sulphuric acid is not locally available, the next best option is high temperature reaction of strong SO2 with limestone or dolomite. This is followed by CO2 liquefaction and disposal either underground or in the deep ocean. The Voisey’s Bay nickel project is cited as an example where appropriate location of a new smelter close to large dolomite or limestone deposits would lead to a virtually zero gas emission operation. Most of the electrical energy required for producing technically pure oxygen and for liquefying CO2 and transporting it to disposal can be derived from smelting a bulk sulphide concentrate and then reacting SO2 at high temperature. Provided the smelter gases are properly cleaned before sulphur fixation, the sulphated stone should be uncontaminated and environmentally benign and possibly of use in the building and construction industry.

Jan 1, 1999

By Brian G. White

The Coeur d'Alene Mining District of northern Idaho is remarkable for its number of large ore bodies and for the amount of its historical production of lead, zinc and silver (Fig. 1). Since veins were first discovered in 1884, more than 127 Mt (140 million st) of ore have been produced from narrow, high-grade veins. Mining continues at the Lucky Friday, Galena and Sunshine mines (Fig. 2). All have been in production for decades. At these mines, more efficient mining and exploration techniques have reduced costs while adding significant new reserves. Even the old Bunker Hill Mine, where ore was first found in 1875, continues operation on a limited basis, partly involving production of world-class pyromorphite specimens. Until recently, the Coeur Mine was also a profitable silver producer. Other large producers have included the Star-Morning, Hecla, Page, Standard-Mammoth, Hercules and Gold Hunter mines (Fig. 2). The first deposits were discovered by prospectors who spilled over from the Murray gold mining district, 24 km (15 miles) to the north. The Bunker Hill Mine was founded the year after the first veins were discovered. Bunker Hill has now operated for more than 100 years. Production includes 2.3 Mt (2.6 million st) of lead, 1.1 Mt (1.2 million st) of zinc and 4.1 kt (132 million oz) of silver. Total district production through 1996 (Springer, 1997) amounted to 7.3 Mt (8.1 million st) of lead, 3 Mt (3.3 million st) of zinc, and 34.5 kt (1.109 billion oz) of silver. This record establishes the district as the largest recorded producer of silver in the world, while also yielding major quantities of lead and zinc. Only Potosi, Bolivia, may have produced more silver, although records are unclear. Despite the district's longevity, historic production and current activity, the published literature on the district lags behind with regard to recent concepts of district geology. The most recent comprehensive US Geological Sur- vey (USGS) bulletins (Hobbs et al., 1965; Fryklund, 1964) are dated. Much research in the last half century has been short-lived or geographically restricted because the district is large and its geology is complex - and even this research tends to remain unpublished. Thus, it has been difficult for in- [ ]

Jan 1, 1998