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By Dan Kump

This paper provides an overview of Barrick Gold's Bullfrog underground mine backfill program. The Bullfrog underground mine was a gold mine that primarily used the underhand cut-and-fill mining method. The first portal was established June 18, 1991, and mining continued until Dec. 29, 1998. This paper begins with a discussion of the events that took place to reach about 1.8 kt/d (2,000 stpd) of backfill. It ends with a commentary of what was learned. The Bullfrog experience suggests that there are two essential factors to a successful backfill program: a well-graded aggregate supply and proper placing procedures.

Jan 1, 2002

By H. Kandemir, G. Neeling, P. A. Spencer

Gold extractions from three refractory sulphide concentrates have been increased by bacterial oxidation with T. ferrooxidans prior to cyanide leaching. Arsenopyrite, pyrite and stibnite were the major sulphide components associated with gold in the concentrates examined. Kinetic oxidation tests were conducted for each concentrate by monitoring iron and arsenic levels in leach slurry. A correlation has been established between the level of bacterial oxidation and gold extraction from cyanide leaching of oxidised residues. High levels of bacterial oxidation have been achieved in approximately 8 days at pulp densities of 20-25/ w/v solids, resulting in 70-90% gold extraction.

Jan 1, 1987

A study was undertaken to determine the effects of five industrial frothers on the relationship between entrainment recovery and water recovery. The experiments were conducted using a Batequip rectangular 60L steel flotation cell that was run in parallel with the first cell in a copper/silver prefloat rougher circuit. The concentration of the frothers ranged from 2.5 to 8.4 g/t. The experiments were repeated at three different froth depths. A power function was found to fit this relationship with no discontinuity in the relationship caused by the changes in frother type and concentration and froth depth.

Jan 1, 2002

By D. R. Schmitt, B. L. Jensen, T. J. Ahrens, C. L. Smither

The in situ elastic moduli of rock are measured holographically by a borehole instrument in marble and oil shale. A point force induced displacement field is recorded with double expo- sure holography. Data consists of a fringe pat- tern on the viewed hologram. The elastic moduli are determined by modelling of the fringe pattern. Only Young's inodulus is fairly constrained by the present data reduction. For an applied force of 520 [ ] 50 Newtons, a marble was found to have a Young's modulus of 56 [ ] 8 GPa, near an ultrasonically determined value of 48 [ ] 6 GPa. In an oil shale, an applied force of [ ]

Jan 1, 1986

By Gao Zhizhong, Zheng Shibo, Gu Zhengxing

Extensive research on the beneficiation of Florida dolomitic phosphate pebble has been carried out by Lianyungang Design and Research Institute of China (CLDRI) in recent years. Test results indicate that the fine-particle flotation process could be used to effectively recover the phosphate values from Florida high-dolomite phosphate resources. With the CLDRI fine-particle flotation technology, a phosphate concentrate with 31% P2O5 and less than 1% MgO could be obtained from a flotation feed containing 25.8% P2O2 and 2.2% MgO at an overall P2O5 recovery of up to 85%.

Jan 1, 1999

The American Mining Congress International Coal Show, this year’s largest gathering of coal industry leaders, convened May 5-8 in Chicago. Against a 73 000-m2 backdrop of the coal industry’s most productive equipment,* some 23 000 people heard government and industry executives give guardedly optimistic reports of coal’s near future gains. Although a rocky road lies ahead, operators and suppliers still continue their search to improve efficiency. Among the ideas discussed at the show were better underground communications systems, continuous on-line coal analysis, and recovery of hard-to-float coal.

Jan 7, 1980

By Frank F. Aplan

Introduction Heavy media separation (HMS), also called dense media or float¬sink separation, is one of the newer forms of gravity concentration. Though the concept can be traced to the last century, the process has enjoyed its major growth since 1940. Heavy liquid separation is a mutation. The heavy media process is used extensively to clean coal and for the concentration of a wide variety of ores such as those of iron, lead-zinc, chrome, manganese, tin, tungsten, fluorspar, magnesite, sylvite, garnet, diamonds, gravel, etc. It may be used where ever a significant density difference occurs between two minerals, and commercial separations are typically made in the range of 1.3 to 3.8 sp gr. The particle size treated ranges downward from 6-8 in. top size. Particles greater than about 1/16-in. (10 mesh) may be treated in a "static" bath, though for reasons of separation efficiency, + 1/2 -in- feed is usually preferred. For particles less than this size, separation in a heavy media cyclone is generally used. The flowsheet of a typical heavy media process, in this case one using a ferrous medium, is shown in Fig. I. In essence, the process consists of: (1) preparation of the feed usually by wet screening to remove undesired fines, (2) heavy medium separation, and (3) removal and recovery of the medium from the separated products. Many muta¬tions of the basic scheme are possible and numerous options are possi¬ble. HMS offers the following potential advantages:12 1) Ability to make sharp separations. 2) Ability to change the specific gravity of separation quickly to meet changing conditions. 3) Ability to remove products continuously. 4) Ability to treat a broad size range of products. 5) Ease of start-up and shutdown without loss of separating efficiency. 6) Relatively low medium cost and low media losses. 7) Low operating and maintenance costs. 8) High capacity with the use of relatively little floor space. 9) Relatively low capital investment per ton of capacity. The process may be used to produce a finished concentrate, two finished concentrates, or a concentrate and a middling of differing quality, or a preconcentrate by rejection of unwanted gangue. It is an ideal method for the reprocessing of coarse waste dumps. The greatest use for the process lies in coal cleaning and in the preconcentration of ores. The relatively inexpensive heavy media process may be used advantageously to reject large quantities of coarsely crushed gangue. When used in this way, the process will allow: (1) the use of lower cost but less selective mining methods with the "overbreak" material being removed at the front end of the concentrator or preparation plant; (2) a substantial reduction in the quantity of ore that must be finely ground for subsequent mineral liberation and separa¬tion. Since comminution is often the single most expensive step in beneficiation, it is desirable to eliminate as many essentially barren pieces of rock as possible before the grinding step, (3) a decrease in overall plant capital cost per ton of concentrate since the size of the plant from the dense medium step onward will be smaller. Several general references are available,12-18 though much of the technical data on the process is widely scattered in the general litera¬ture. Heavy Liquid Separation Organic Liquids Given sufficient settling time, it is possible to make a perfect separa¬tion between two particles of differing density by placing them in a liquid whose density is intermediate between the two. This means of achieving a perfect separation has proven to be elusive because of problems in feed preparation, particle settling rates, operational considerations, and economic constraints. There are a wide variety of heavy liquids that could be used, most of them halogenated hydrocarbons, and a few typical examples are given in Table 4. These liquids are most commonly used in ore dressing for the laboratory fractionation of ore particles on the basis of specific gravity. Laboratory Separations. Using liquids typified by those given in Table 4, separations are made to develop either the standard washability curves used to estimate the response of a given sample to gravity concentration or to prepare a partition curve to evaluate the effective¬ness of a given gravity separation process or piece of equipment. A typical washability curve is given in Fig. 2.19 Such curves are generated for raw coal, e.g., by treating either the whole or various size fractions of the sample in a series of heavy liquids and analyzing the various specific gravity fractions so produced. The procedure is relatively simple for coal samples because of the ready availability of a wide variety of relatively low cost heavy liquids in the density range 1.2¬-2.0. For ores the problem is much more complicated, because only a few high density liquids, all of rather high cost, are available. Parti¬tion curves are generated in the same manner by treating the separated products in the same liquids. Greater details on the procedures to be used in heavy liquid separa¬tions are to be found in the literature (for coal, Refs. 13, 14 and 19 and for ore, Refs. 20 and 21). For testing coal, calcium and zinc chloride solutions have been used extensively in the past, though today halogenated hydrocarbons (available under the trade name Certigrav) are the preferred media. The liquids shown in Table 4 may

Jan 1, 1985

By E. A. Martell, K. S. Sweder

INTRODUCTION Lung cancer in uranium miners has been attributed to alpha irradiation of basal cells of the bronchial epithelium by radon daughters, primarily by 7.7 MeV alphas from polonium-214 (Altshuler et al., 1964). It also has been was observed that for a given cumulative radon progeny exposure, uranium miners who smoke cigarettes have an incidence of lung cancer about 10 times higher than nonsmoking miners (Lundin et al., 1969). It has been pointed out that the large excess of lung cancer deaths among smoking uranium miners is a multiplicative effect (Doll, 1971), which suggested possible synergistic interactions between airborne radon progeny and cigarette smoking. Experimental studies of the complex pattern of interactions between radon progeny, cigarette smoke particles, and the cigarette smoking process are in progress in our laboratory. Preliminary results, reported elsewhere (Martell, 1981), implicate alpha radiation from indoor radon progeny in the etiology of lung cancer in all cigarette smokers. Cigarette smoking produces high concentrations of smoke particles of low mobility and respirable size--particles between 0.5 and 4.0 µm in aerodynamic diameter (see below). The attached fraction of indoor radon progeny is highly dependent on the air concentration of small particles from cigarette smoking and from other combustion sources (Martell, 1981). The size distribution and other properties of radon progeny associated with cigarette smoke particles enhances their effectiveness in the induction of bronchial cancer in man. In this paper we discuss the properties of radon progeny associated with cigarette smoke, the fractionation of radon progeny and modification of their aerosol properties in burning cigarettes, the role of 218Po in these processes, the production of insoluble 214Pb and 212 Pb enriched particles in burning cigarettes, and the consequent differences in the patterns of polonium isotope alpha irradiation in the bronchial epithelium of smokers. EXPERIMENTAL PROCEDURES Experimental methods used in these studies involve the use of small experimental chambers of known radon and radon progeny concentrations in combination with aerosol collection and sizing techniques and sensitive radioactivity detection methods. The use of low-level [ß-] counting for radon progeny determination, providing a measure of 214 Pb plus 214Bi activity, makes it possible to carry out chamber experiments with small radon emanation sources and relatively low air concentrations of radon and radon progeny concentrations in the range from 100 to 1,000 pCi per liter. Thus, for example, in a typical experiment we use a 10 nanocurie 226Ra solution standard in a 10 liter chamber, providing an equilibrium concentration of 1,000 pCi of radon per liter. In small sealed chambers, radon progeny plate out rapidly on the chamber walls, with steady-state concentrations of airborne progeny less than 2 percent of equilibrium levels. This is experimentally convenient because, upon introduction of high concentrations of cigarette smoke particles or small particles from other sources, there is a systematic ingrowth of attached radon progeny, providing a tagged aerosol source of known age and radon progeny composition. In some chamber experiments a 226Ra solution standard of small volume, acidified to O.1N HNO3, was used as the radon emanation source. When used with a bubbler the holdup of radon in an 8 ml volume of 226Ra solution standard at 0.1N HN03 was only 2% of the total radon in the chamber at equilibrium. For experiments with 212Pb-tagged aerosols, we used a dry Ba(228Th) stearate emanation source prepared by the method of Hursh and Lovaas (1967). 226Ra and 222Rn determinations were made by radon gas counting. The 222Rn in a sealed air or water sample is transferred, using helium gas as a carrier, successively through a dry ice cooled trap at -80°C to remove water, through ascarite to remove C02, and through a small activated charcoal trap at -80°C to collect the 222Rn. Subsequently, by heating the charcoal to 400°C, the 222Rn is transferred next to an LN2-cooled capillary trap, and finally into an alphascintillation counting cell of the type described by Lucas (1957). As already stated, radon progeny activities were determined by low-level [ß-] counting, which provides a measure of 214Pb plus 214Bi. The radon progeny samples, collected on efficient Delbag polystyrene micro-fiber filters or on impactor foils, are placed in close, sandwich geometry between two thin-walled flow counters inside shielding anticoincidence counters and a 15 cm thickness of steel shielding. This configuration provides nearly 4II geometry and a low background of only 0.25 to 0.30 cpm. Aluminum absorber was added to provide a combined thickness of absorber and counter wall exceeding 7.0 mg/cm2 to eliminate the variable contribution of 7.7 MeV alphas from 214Po. 212 Pb determinations also were carried out by low-level [ß-] counting, in this case using a combined absorber and counter wall thickness of 9.0 mg/cm2 to eliminate contribution of 8.8 MeV alphas of 21 Po. In each experiment the [ß- ]activity data were corrected for decay to an appropriate common reference time for assessment of activity distributions.

Jan 1, 1981

By Seppo] [Kreula

Outokumpu Oy has acquired considerable experience with on-stream X-ray fluorescence analyzers and with computer control of flotation processes over several years. It became evident at the outset of this work that a reliable and accurate on-stream analyzer was more than a collection of different pieces of hardware linked together. Extensive work was done to develop an integral analyzer system, beginning with process samples through the spectrometer to report typewriters, recorders and the control computer. To date, over ten Courier-300 analyzer systems based on this development have been installed world-wide, several of which provide the key information for successful computer control systems. Computer control of flotation processes has been under development at the Pyhäsalmi concentrator since 1968. Two different control strategies have been implemented. The copper flotation is controlled by means of a mathematical model and economic criteria, whereas the "evolutionary operation principle" is applied to the zinc flotation. The computer control system installed at' the Vuonos-mine has been developed according to the same guidelines. In order to consolidate this wide experience, a flotation computer control system, Proscon-100, was developed by Outokumpu Oy, and the first system was installed at Kotalahti in 1972. In addition to the technological experience, enough information has also been gathered to illustrate the economical, benefits accruing through the use of these advanced tools of automation.

Jan 1, 1973

By John O. Marsden

"This article is an attempt to capture a presentation given at the 2016 SME Mineral & Metallurgical Processing Division (MPD) Luncheon held on Feb. 24, 2016, in Phoenix, AZ. It was presented as a tongue-in-cheek, lighthearted view of several of the issues facing our industry, while identifying some of the challenges and opportunities available to new engineers entering the industry today.Disclaimer• What you are about to read includes personal preferences, biases and occasionally arbitrary opinions.• Some readers may have dissimilar opinions on the contents and/or the rank order of the lists.There are eight top ten lists offered in this article: (1) Industry realities (2) Things a metallurgical or mineral process engineering graduate should do (3) Books every metallurgist should have on their shelf (4) Things I wish I had known (5) Legends of metallurgy (posthumous, with highlights) (6) Things we’ve forgotten (7) Mineral processing innovations of the past 20 years and, finally, (8) The future? For some items, additional context or clarification is provided after the item, while for other items no further explanation is warranted and none is offered. Top 10 industry realities1. Metal price cycles.2. Bigger is better (except when it isn’t).3. Ore grades declining across the board of metals/commodities.4. Ores becoming more complex.5. Three-fold increase in capital costs between 2004 and 2015.6. Three-fold increase in operating costs between 2004 and 2015.7. Less capital availability/challenging financial markets.8. Environmental constraints increasing.9. Socio-political constraints increasing.10. The mine manager is always impossible to work with.As Table 1 shows, there was more than a three-fold increase in gold operating costs between 2004 and 2014. The increases for copper, iron ore and other commodities were similar."

Aug 1, 2016

By Howard E. Banta

From Alaska to Florida and California to Maine, the National Forest System (NFS) is host to unprecedented levels of prospecting and development. The NFS is large (191 million acres), geologically attractive from the mineral standpoint and relatively unexplored. Significant recent finds and developments involve silver, gold, platinum group metals, molybdenum, cobalt and phosphates. Exploration and development are closely regulated, but most of the NFS is open. The Forest Service endeavors to be a good host to the mineral industry, but increasingly is asked or forced to delay or disapprove mineral projects to protect the environment.

Jan 1, 1984

By F. Cappel

Except in a few special cases, it is necessary for solid fuel direct reduction in rotary kilns that iron-bearing materials be charged with a minimum grainsize of approx. 5 mm. For fines not usable in such form, the combination of sintering - as agglomeration step - and direct reduction were investigated. During reduction of sinter in rotary kilns prevail dependencies on sinter quality and varying reaction parameters, owing to the use of different coals, which led to a definite statement as to what quality sinter must have and which coals can be used.

Jan 1, 1983

By S. J. Schatzel, K. M. Ajayi, E. N. Rubinstein, Z. Khademian

This study simulates the impact of a shale gas well casing breach near a longwall mine. Field studies are conducted to measure mining-induced permeability changes over the abutment pillar of a longwall mine, and a geomechanical model is developed in 3DEC, a three-dimensional numerical modeling code, to predict the aperture of fractures in the overburden at the study site. The predicted aperture values are used to determine mining-induced permeabilities, and the results are compared with the field measurements. These aperture values are provided as inputs into fracture flow code (FFC), which generates a stochastic discrete fracture network (DFN) model for the study site and predicts the potential shale gas flow to the mine. Results from 100 DFN realizations are statistically analyzed using the bootstrapping method to compensate for notable variation in fracture geometry. The results show a significant difference between the gas inflow for nearby panels due to increase in the induced permeability during mining of the second panel. The average gas flow to the mine was calculated as 4.72 × 10 m3/s (49 cfm) for a hypothetical breach at the Sewickley horizon during the first panel mining, 8.97 × 10 m3/s (19 cfm) for a hypothetical breach at the Uniontown horizon during the first panel mining, 2.16 × 10 m3/s (458 cfm) for a hypothetical breach at the Sewickley horizon during the second panel mining, and 8.07 × 10 m3/s (171 cfm) for a hypothetical breach at the Uniontown horizon during the second panel mining. Depending on the mine ventilation system, this could result in methane concentrations exceeding regulatory limits. Hence, these findings provide insights into the potential risk of an unconventional gas well casing breach near a longwall mine under shallow cover.

Feb 1, 2024

By J. D. Allen

The Concord Mine Coal Preparation Plant of United States Steel Corporation, located near Bessemer, Alabama, began operations in June, 1950. The basic cleaning circuit consisted of a Chance Cone for cleaning the 2-inch x 3/8-inch fraction and concentrating tables for cleaning the minus 3/8-inch coal fraction. Dewatering of the final product was accomplished with 35-mesh vibrating screens. However, because Concord coal is very friable (Figure 1), 85 per cent passing three mesh and 25 per cent passing 28 mesh, there was a considerable amount of fines passing through the dewatering screens. To recover these fines, the dewatering screen effluent was sent to two 70-foot diameter hydro-separators with the underflow returning to the dewatering screens and the overflow going to a 175-foot diameter thickener for water clarification, (Figure 2). As might be expected, there was a considerable amount of recirculation of the minus 35-mesh coal in this system. This degraded the coal and led to higher moistures. It also resulted in a fines bleed-off of plant water and thickener underflow to a fine-coal settling pond. In 1961, vacuum filters were installed to supplement the screens and to recover the fine coal lost in the bleed-off. The above system resulted in a product moisture of about 20 per cent. Further reduction to approximately 11 per cent was accomplished by drainage from railroad cars during transit to the coke plant. Control of the moisture at unloading was poor because of variations in operating levels and railroad car availability, but since the Concord coal constituted a smaller percentage of the coal previously used at the coke plant, the high moisture and fluctuations did not seriously hamper coking operations.

Jan 1, 1966

By John A. Wolfe

The concepts of sea floor spreading and plate tectonics provide more logical explanations of some geologic processes than were possible a few years ago. I plan to premise my discussion of the regional features of Philippine porphyry copper deposits on this tectonic basis, recognizing that parts of the picture can be explained in other ways, but in the belief that this is the best explanation of the whole. The sequence of events which results in the formation of Porphyry deposits (Figure 1) can be summarized as the result of collision of crustal plates, subduction of the oceanic plate, emplacement of a serpentine ultrabasic diapir, formation of a dioritic magma at depths of 300 to 500 km, rise of the magma to a sub crustal or intra-crustal chamber, eruption of andesitic volcanoes, emplacement of diorite or differentiation of the magma into more acidic products, multiple intrusion of hypabyssal rocks and hydro-

Jan 1, 1973

By Frank C. Appleyard

As the environmental movement gained force during the past few years, it was inevitable that attention would be focused on surface mining, and we have seen this method of extracting minerals from the ground come under heavy attack by conservation groups, the press and television, legislators at all levels of government,, and the public at large. It is difficult, and in some situations, perhaps impossible, to measure either the need for, or the results of, land reclamation in absolute terms. Therefore, much of the criticism of surface mining has been based upon subjective analysis rather than hard data, with the result that discussion of the subject often includes a fair amount of emotional response. To be fair, this reaction is not limited to just our critics; even we hard headed engineers sometimes respond in the same way, an observation with which you may agree when I finish this talk. This leads to the thought that whether you are for it, or against it, land reclamation in many respects is a state of mind. That is, our reaction to it can vary considerably, depending on how far we have thought it through. And think it through we must, because the pressures being brought to bear need to be met with positive actions if the problem is to be brought under control, and in the next few minutes I would like to discuss from a management point of view some ideas which might aid in this thinking process. It would be a mistake, I believe, to think that the current energy crisis will suddenly bring the environmental movement to a stop, and therefore, we can relax and go back to running our busi¬ness the way we used to. This is not realistic; on the contrary the energy crisis is focusing attention on the limitations of our natural resources as nothing else could, and with respect to land

Jan 1, 1974

By W Frint

An area in Southwestern Wyoming is rapidly becoming the soda ash capitol of the world. Starting only 20 years ago from a unique deposit of trona, the out-put has rapidly increased until now about half of the soda ash produced in the United States comes from this area. Chemically, soda ash is sodium carbonate, a large tonnage, widely used alkali. The largest single industrial use is in glass manufacture, where 25 tons of soda ash are used in producing 100 tons of glass. Other big users are the detergent and chemical industries. The trona deposits in the Green River basin of Wyoming, were formed about 75 million years ago as part of a geologic period when an ancient lake alternately went through periods of being a large fresh water or smaller brine lake. Today these deposits lie from 800 to 3000 feet below the surface in a semi-arid, almost desert region just west of the Rocky Mountain Continental Divide at an elevation of 6200 feet. In this area there is a prevailing westerly breeze and almost every afternoon the velocity reaches very windy conditions. Rainfall is minimal, being only 6-8 inches of moisture per year with an evaporation rate exceeding 24-inches. Most of the moisture comes in winter snows or summer thunderstorms. Temperatures are low with below zero weather dur¬ing most of the winter and early morning frost is not uncommon during the summer months. There are many trona beds varying in thickness from a few inches up to 15 feet. The deposits now being mined vary from 8 to 10 feet in thickness. The ore is mined by driving tunnels either via conventional explosives or with continuous boring machines. It is transported to the shafts via conveyor belts

Jan 1, 1972

By Michel Dagbert

With the growing demand for energy, explora¬tion for uranium deposits has dramatically increased in the past few years. Since the early ages of uranium exploration, natural gamma ray logs has proved fast and convenient in locating and evaluating mineralized bodies. However, natural gamma ray logging is an indirect measure of the uranium content and in some circonstances, the radiometric signal can be 'altered in a very significant way. To check the presence of any distortion, statistical comparision of radiometric counts and chemical assay values for the same samples is generally performed. Of course in such studies, one inevitably mixes samples where the radiometric signal is really proportional to the uranium content and samples where this signal is influenced by many other sources or distorted before being recorded. Nevertheless, the average tendency leads to correction factors which are then applied to every radiometric reading in the deposit or portion of the deposit where the statistical comparison is made. To account for the "average" character of these correction factors, a parameter of uncertainty like a standard deviation of possible errors is associated to each corrected value. This parameter can be latter used in an ore reserve estimation when the weights assigned to samples should depend on the precision of their grade. The purpose of this paper is to present a new regression technique which can be used without any restrictive hypothesis like a normal or lognormal distribution. Different standard ways which can be used to statistically compare radiometric and chemical assay values for the same sample and to derive a correction factor for the latter will first be reviewed, after a general discussion of disequilibrium problems. The discussion and the examples will be restricted to the case of natural gamma ray logging in drill holes but the methods presented in this paper can also be applied to data from the radiometric probing of a working face or a truck load in an operating mine.

Jan 1, 1979

By Ronald W. Marrs

Kimberlite contains a unique assemblage of minerals and exhibits a characteristic pattern of spectral reflectance in the visible and near-infrared wavelength region. This spectral pattern is readily discerned in well-exposed kimberlite outcrops, but becomes increasingly difficult to identify when outcrops are partially covered by vegetation or soils derived from non-kiaberlitic materials. Detection of poorly exposed kimberlite diatremes is possible using multispectral imaging techniques in which spectral bands are chosen to optimize contrast between he kimberlitic soils, non-kimberlitic soils and vegetation. Careful processing of the multi-spectral data is also essential in order to separate the spectral signature of kimberlite from the signatures of other components of the scene.

Jan 1, 1984

By D. N. Hillhouse

The economics of exploration target selection can be measured ultimately only against a company's success at developing mines. In the following brief history of the mine acquisitions of one company, Placer Development Limited, I try to identify the principal targeting parameters and the principal skills that came into play for each property and examine how these have changed with time. The process of target selection in exploration is directed: - first by corporate investment criteria which define objectives and acceptable levels of risk for pursuit of those objectives; - second by experience and skills which permit us to minimize risk; and - third by anticipation of future events relating to ore discoveries and changes in external risk factors.

Jan 1, 1984