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By Joseph R. Wojcik

Introduction Placer deposits account for one-half to two-thirds of the total gold produced worldwide. As many as 311 t (10 million oz) have been produced from some major districts. Several others are credited with 155 t (5 million oz). All the major districts and many lesser ones have similar characteristics. Consideration of these characteristics offers insight into the origin of gold in the deposit. Knowing the gold deposit's geologic history leads to useful conclusions in exploring for placer deposits and for deposits in consolidated areas. Placer Deposit Characteristics Table 1 lists 17 placer gold districts around the world and their characteristics. These include type and age of, mineralized source rocks, associated intrusive rocks, and age of intermediate sedimentary cycles. Although quartzite or sandstone occurs in the bedrock sequence in four of the 17 districts, slates, phyllites, or schists are present in all of the districts. Additionally, in all but three, the slates are pre-Mesozoic. And, in six of the remaining 14, the slates are Precambrian. Rocks intrusive into the slates are all of intermediate to acidic composition and are emplaced as stocks, dikes, or sills, rather than being batholithic in proportions. Intermediate interceptors have not been recognized in only three of the 17 districts. Data from China and Mongolia are inconclusive as to this characteristic. Black Shale Gold Higher than average gold values are found in modern sedimentary rocks in coarse granulites, conglomerates, sandstones, and gray-wackes. Not so predictably, higher than average gold values also occur in black shales and carbonaceous argillities. Li and Shokena (1974) found that, in the Proterozoic sedimentary and equivalent metamorphic rocks of the Enisei Ridge, carbonaceous and graphitic phyllites and schists were particularly enriched in gold. Korotayeva and Polikarpochkin (1969) reported gold content in organic rocks to be enriched up to 15 times more than the organic free shales and siltstones of the Nerchinsk Zavod region of eastern Transbaikal. Black shales can be gold interceptors as are rocks of the granulite facies. Gold occurs in shales in particulate, free form, in pyrite, adsorbed on clays, and precipitated on carbon. In copper-bearing shales of Poland's Fore-Sudetic monocline, Kucha (1973) reported gold present in organic compounds as well as in silver bearing minerals. Anoshin (1969) found a strong correlation between gold and the pelitic fraction of sediments in certain rocks of the Atlantic Ocean basin. Heavy metal absorption on clays results in agglomeration to the extent that the particles settle with the heavy mineral fraction in enriched strata. Piper and Graef (1974) think that gold in sediments on the flank of the East Pacific rise is of lithogenic origin. Crockett (1973), however, proposed a volcanogenic exhalative origin. A strong correlation between gold and volcanic activity exists in rocks of the Canadian Shield where tuffaceous sediments and porphyries are more frequently enriched in gold than rocks with no volcanic components. Volcanic rocks of intermediate to acidic composition are considered here to be the primary source of gold in black shales. Gold is concentrated in shales as detrital free grains with diameters less than 50 µm (270 mesh), as precipitates, and in pyrite or other sulfide minerals. This is the first step in the history of gold placer deposits. Grain Size Modification Gold, as free grains with diameters less than 50 µm (270 mesh), is too light to be concentrated hydraulically into placer deposits. Some process has increased the average size of the gold grains. Mobilization during regional metamorphism converts most of the gold into particulate grains or entrains it in microfractures in the sulfide minerals. With moderate folding, small, white, vitreous quartz veins develop. As deformation continues, larger, more conspicuous quartz veins are formed. The larger veins are folded and faulted and contain native gold as minute specks and flakes not visi-

Jan 11, 1984

By Robert A. Jankowski, Charles A. Babbitt, Natesa I. Jayaraman

Today, approximately 31% of underground coal mining operations have designated occupations or areas with more stringent (below 2.0 mg/m3) respirable dust standards in place. Many U.S. coal mines have been able to maintain compliance with very low dust levels, and the Bureau of Mines, in an effort to identify those factors contributing to low dust levels, completed a survey of twelve continuous miner sections which were able to maintain their dust levels at or below 0.5 mg/m3. The quantity and quality of face ventilation appeared to be the most important factor in controlling dust exposure. The results directed Bureau research towards identifying improved face ventilation techniques for reducing quartz dust levels on continuous mining operations. This research has involved the evaluation of a half-curtain face ventilation technique which reduces the cross-sectional area of the entry, thus increasing the air velocity over the operator. The Bureau has also conducted extensive laboratory studies to compare tubing versus brattice, including an evaluation of both systems in a crosscut turn. In an effort to improve ventilation and reduce dust exposures of roof bolter operators working downwind of the continuous mining machine, the Bureau has evaluated two novel face ventilation systems. The first system consists of a fan discharge tubing technique which routes dusty air away from the return entry being bolted. In a technique similar to the fan/tubing concept, exhaust air from the upwind continuous miner face can be scrubbed using a dust collector to capture a portion of the dusty air, clean it, and supply it to the bolter operators as a split of clean intake air. A better understanding of face ventilation techniques should lead to solutions to the problem of more stringent dust standards due to quartz.

Jan 1, 1989

By David Sainsbury, Bevan Thompson, Yuejun Cai

The practice of placing loose rockfill as passive support in open stoping voids, of narrow vein mineral deposits, lends itself to the sterilisation of substantial mining reserves in the requirement for regional crown/sill pillars. Significant benefits are to be gained by the Australian mining industry via the development of improved guidelines for the extraction of such sterilised mine pillars. The application of selectively injecting specifically developed ettringite (calcium sulphoaluminate hydrate) type cement grout into loose rockfill, enables an artificial pillar to stabilise the surrounding rockmass and prevent the caving of loose rockfill from the previously backfilled void during extraction of remnant crowdsill pillars. This paper presents the results of laboratory testing used to reduce the technical risks associated with the design of cemented rockfill pillars. A laboratory testing program was devised to investigate the critical parameters influencing the performance of a stabilised rockfill matrix, along with the critical parameters influencing the interpretation of laboratory test samples. The factors investigated include; sample size and shape, particle size distribution, curing time, water quality, cement : water ratio and the confinement effect.

Jan 1, 2001

By Subash Chander, B. Arnold

Many developments have occurred in coal flotation with regard to increasing recovery from the flotation circuit and its impact on overall coal preparation plant recovery. This includes innovative design of flotation circuits, equipment modification and development, methods of enhancing the use of reagents, effect of slimes, and recognizing the effect of water chemistry. This paper will review recent advances and present some guidelines to improve process efficiency.

Jan 1, 2005

By C. Bemelmans

Lead alloys function effectively as oxygen evolving anodes in copper SX-EW operations and are highly resistant to corrosion. The presence of organic from solvent extraction in the electrolyte seemed to enhance solution line attack. Laboratory tests showed that lead is soluble in D2EHPA, Acorga and LIX 984 to a limited, yet measurable degree. Lead alloys and various lead phases formed on the anode were tested to evaluate their effect on the corrosion process in organic media. Possible causes for anode corrosion and mechanisms responsible for their initiation will be discussed.

Jan 1, 1996

By J. T. Crawford

A comprehensive strategic-planning model was developed for the phosphate strip-mining and chemical operations at White Springs, FL. The computerized model covers orebody modeling, production simulation and financial analysis. The model has several innovative features, including the use of NPV techniques in orebody block economics, the adaptation of the floating-cone method to a floating template for generating dragline mining units automatically, the use of interactive graphics for designing mining units, and the use of a dragline move economics algorithm for the automatic scheduling of mining-unit assignments and moves. The simulation of chemical-plant production uses linear programming and concentrate inventory levels to control mine-operating schedules.

Jan 1, 1999

By G. T. Adel, W. Z. Choi

A model describing size reduction and mineral liberation during grinding has been extended to include multiple classes of composite particles. The majority of the model parameters have been directly determined using computerized image processing equipment to analyze the products from batch grinding tests. A coarse-grained sphalerite ore from Tennessee, representing a nearly ideal binary mixture of sphulerite and dolomite, has been used to experimentally validate the model for the case of dry batch grinding. In all cases, excellent agreement has been observed between the model predictions and the experimental results.

Jan 1, 1988

By Jan E. Stache

Commercial software programs are being utilized to create and maintain a mine subsidence database in conjunction with the U.S. Bureau of mines research on mine subsidence in Illinois. A three-year survey net of monitored structures in an Illinois subdivision was entered as textual information (attributes), on an existing AutoCAD drawing. These attributes can be optionally displayed on the corresponding drawing or extracted as a text file for import to a database program. The database program, dBASEIV, allows combinations of multiple database files for extensive querying capabilities. Subsurface database files can be merged with surface database files to enhance the effectiveness of surface response analyses in mine subsidence research. The geographic information system (GIS), pcARC/INFO, was chosen to integrate existing AutoCAD drawings and dBASEIV database files into a relational database with a graphical representation. The relational database allows users to update either the graphical representation or the database, with the changes being reflected automatically in its alternate format. The increased capacity, through use of a GIS, to query combinations of database files and have the results reflected graphically has significant potential for expediting mine subsidence research.

Jan 1, 1986

By D. G. Krige

In the world mining scene, precious metals (particularly gold) have recently monopolized the attention in exploration as well as new production activities. As the major world producer of gold, South Africa plays and will continue to play a very significant role in this commodity. This role will, of course, have its consequential effects on the world's gold supply and, hence, also on the international price of gold. It is essential, therefore, and in the interests of all gold producers that the likely future pattern of gold production in South Africa be estimated such that some measure of production uncertainties can be predicted. For this purpose, some elementary geostatistical models will be used, and the main factors affecting the situation will be analyzed in broad outline only under the following headings: South Africa's basic gold ore potential in the three categories of: (1) ore remaining in existing mines; (2) extensions to existing mines via unexplored areas and/or reef horizons of lower grade; and (3) new mines, potentially deep or ultradeep. Cutoff grades - operating, marginal, and flotation, for the foregoing three ore categories, respectively. The availability of production, financial, and other resources required. Cutoff grades will, of course, depend on cost levels (operating and capital), future inflation and cost escalation trends in South Africa (SA), and/or gold price trends (in US$ and via the exchange rate in SA Rand terms). The differentials in inflation and escalation trends with those in overseas countries, mainly the USA, will, in general, set the scene for future exchange rate levels of the Rand. To predict the effect of various cutoff grades on future gold production from South Africa, it is essential to use geostatistical models. The applicability of such models is tested against past records of price, cost, and grade trends and then applied to the estimated ore potentials. These models were referred to briefly in a paper at a recent seminar on gold (Krige, 1986) but are discussed in detail herein. This lecture was intended originally to cover only technical aspects, but, in view of South Africa's position internationally and the possible effect of sanctions, internal unrest, and strikes on gold production, a brief reference to the third factor above (availability of production, financial, and other resources) seems appropriate.

Jan 1, 1988

By Jean-Luc Cornet

Advances in conveyor technology over the past decade have enabled the design and erection of very long overland belt conveyors. Such conveyors usually require head and tail drives to lower the belt tension and belt strength required. This paper provides an overview of the problems encountered in controlling the head and tail drives on such conveyors and some of the solutions developed by CDI for several such projects.

Jan 1, 2002

By John H. Raymer

The accuracy of inflow estimates depends largely on how well permeability is characterized. Permeability is best evaluated as a statistical distribution. For the Chattahoochee Tunnel, this distribution was obtained with a large number of packer tests performed in 50 boreholes along the alignment. The distribution of packer-test data may not accurately reflect permeability, however, due to the limits of the test method and the luck of the field investigation. These discrepancies may be overcome by using log-normal plots of the data to develop a permeability model that will be use din lieu of the data to calculate inflow.

Jan 1, 2001

By Ronald L. Wiegel

The United States produces approximately 40 mil- lion tons of phosphate rock per year. Most of this rock is converted to diammonium phosphate and used as a fertilizer component for the domestic and international agricultural market. Much of this phosphate rock production is concentrated in the southeastern United States, with about half coming from washing and screening operations and half from flotation separation from sand and clay. This paper summarizes current operating practice in the industry and touches on some of the emerging technologies required to solve the problems associated with a diminishing high- grade reserve and the need to implement dolomite/francolite separations on a commercial scale.

Jan 1, 1999

By Joseph R. Hatch

Previous studies of the Thompson and Temperly ore bodies show alteration aureoles in the Middle Ordovician host-rock limestones, dolomites and shales extending laterally as much as 60m from the ore. Thickness, texture, carbonate and clay mineralogy, major-, minor-, and trace-element contents; and carbonate carbon and carbonate oxygen Isotope compositions of the host rocks all have been significantly altered by hydrothermal processes. Organic geochemical analyses of samples from the basal 8-cm-thick carbonaceous shale bed and immediately overlying limestone bed of the Quimbys Mill Member of the Platteville Formation, collected from within the ore, the alteration aureole, and the unaltered country rock show: 1) solution of carbonates and residual concentration of organic matter, 2) decreases in the volatile (Si) and pyrolytic (S2) hydrocarbon yields In samples of the basal shale from within the ore, 3) decreases in the relative amounts of the lower molecular weight (<nC25) saturated hydrocarbons in samples from the ore, and 4) no changes in either the organic carbon isotope composition or the Rock-Eval or biomarker thermal maturation Indicators. In light of the 125-170°C depositional temperatures documented for the early marmatitic sphalerltes in these deposits by fluid Inclusion studies, the lack of thermal alteration of the organic matter suggests a relatively short mineralization episode, a conclusion supported by the relatively simple sphalerite band stratigraphy. The decreases in the volatile and pyrolytic hydrocarbon yields for shale samples from the ore supports a model for mineral deposition which obtains sulfide through reaction of brine sulfate and indigenous organic matter.

Jan 1, 2013

By A. K. Bakshi

In cold regions, rinsing of spent cyanide from leach heaps is stopped during winter and is resumed again after the snow melts. The effect of this idle time on the overall removal process was studied in this experiment. Samples were collected for rinsing tests from one heap at a gold mine near Fairbanks, Alaska. Continuous and interrupted rinsing of this sample were carried out in test columns. The rinse solution was analyzed at regular intervals to determine the rate of cyanide removal from the solids. It was found that total cyanide concentration per rinse cycle for interrupted rinsing test was higher than for the corresponding continuous rinsing test. As a result, interrupted rinsing required less rinsing cycles than continuous rinsing.

Jan 1, 1995

By Rolf Hindstroom

Outokumpu Mintec started development of the pressure filter in 1991, and the first unit was delivered to Tara Mines Ltd. Ireland in 1992. The filter replaced a conventional disc filter and a thermal dryer. The replacement of the thermal dryer has improved both environmental and working conditions. The filter is currently dewatering 80,000 tons per year of lead concentrate, producing a cake with less than 6 wt % residual moisture.

Jan 1, 1995

By E. R. Reeves

The mining industry has recognized the importance of engineering controls as a primary means of preventing hearing loss. The noise control treatments most commonly observed on drills and bolters during this study were windshields, sound absorbing material, and hydraulic motor covers. These controls were evaluated on machines at underground metal mines to determine the amount of noise reduction achieved by each control. The results indicate that absorbing material has very little effect on the noise level. The noise reduction attributable to the motor covers was dependent on the material used to create the cover. Properly installed windshields were the most consistent control.

Jan 1, 2004

By H. S. Chang

A three-dimension two-phase (air and dust) computer model was developed to model air velocity and respirable dust distributions in the longwall faces. This paper describes the derivation of the model including the model features, mathematical equations and boundary conditions. Example simulations were also performed for two longwall faces where measured respirable dust and air velocity distributions were available. The results indicated the model can simulate the mine conditions fairly accurately.

Jan 1, 1996

By A. J. Sinclair

Los métodos de desmuestre para estudios de viabilidad dependen de las características geológicas del yacimiento que ha de ser evaluado. Se requiere un estudio geoestadístico preliminar, teniendo en cuenta los aspectos de autocorrelación de las variables sometidas a estudio, con objeto de que el plan de diseño de muestras pueda ser optimizado para producir las estimaciones de ley y tonelaje dentro de los límites de errores establecidos. En general, debe de tenderse a una distribución regular de muestras para ello, y longitudes regulares de las mis¬mas (a partir de los testigos o cortes, por ejemplo) son preferidas con objeto de facilitar los análisis estadísticos. Los planes de desmuestre son modificados y mejorados (bien técnica o económicamente) a medida que avanza el programa y es obtenida información adicional. Las muestras a granel para pruebas metalúrgicas pueden ser evaluadas en término de va¬riaciones señaladas por todas estas muestras en un depósito y la variabilidad de la ley dentro de las mismas. Una atención adecuada sobre estas fuentes de variabilidad asegurará una evaluación adecuada de los resultados de las pruebas piloto de las muestras a granel. Por otro la¬do, un estudio mineralógico general del yacimiento es un requerimiento importante, a veces ignorado, en una evaluación más general pero semicuantitativa de la representatividad de una muestra a granel; y, además, proporciona una información muy útil para optimizar el diseño de la planta.

Jan 1, 1982

By Laurel E. Burns

Alaska is in competition with the rest of the world for exploration dollars. Providing a "seed package" of airborne geophysical data for the state&apos;s numerous mining districts encourages companies to begin more detailed exploration in more areas than they could have afforded on their own. When data are already available, the initial risk decreases for people involved in exploration. Airborne magnetic and electromagnetic data help ge¬ologists and geophysicists determine what rocks and structures are present under overburden and vegetation. These data form a fundamental package that exploration companies want before intensively exploring most regions. Individual mining companies have conducted their own geophysical surveys during the past several decades. However, these data are not available to other companies. The companies able to afford geophysical surveys have less money for ground exploration after purchasing expensive, private geophysical data. Therefore, they are not able to explore as many places as they might. Additionally, since data are not shared with other exploration companies, the proprietary surveys do not foster intercompany competition to move projects forward. Individual prospectors and small companies can not afford their own geophysical surveys. "It is more often than not the grassroots exploration efforts of the smaller operator that leads to significant discoveries," said Klaus M. Zeitler, senior vice president for development and exploration at Teck Exploration. Acquiring and releasing the geophysical data to the public produces several benefits. The increased explo- [ ]

Jan 1, 1998

By R. L. Wiegel, G. R. Oswald

The processors of Florida phosphate rock face constraints similar to those of other mineral operations--leaner ore, more complex treatment schemes and a product which sells for a penny a pound. IMC Fertilizer is a dominant producer in this market with its current operation of five processing plants. An emphasis by their management in maintaining and improving plant performance has continued unabated for some 15 years. This has resulted in a commitment to the importance of routine automatic sampling of product and in process streams, the generation of reliable assay and size distribution data, the accurate measurement of process flows, and the computerized "rendering" of this information into weekly and monthly material balances which provide consistent measures of process performance. In addition, there has been a parallel program to monitor and improve the performance of flotation reagents and to evaluate process modifications or changes in equipment. This paper will describe the evaluation techniques that have evolved over this period and indicate how the historical data base is used to identify plant problems early and help in their solution.

Jan 1, 1991