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By Heinz W. Duddeck, Hans-Konstantin Nipp

The design of storage space or deep mining in rock salt should take into account not only time dependency of the rock salt behavior but also temperature effects. For deep mining of potassium salt the natural increase of temperature with depth may no longer be negligible. For the storage of cryogenic liquid gas in salt caverns more knowledge is nezded about the propagation of deep temperatures and their effects on the stress-strain-creep behavior of the surrounding salt. High temperature problems arise for nuclear waste storages in caverns or drill holes emitting radiation. A numerical model is proposed for the analysis of displacements and stresses including their time and temperature induced change. For the material behavior of salt constitutive equations are chosen composed of a combination of Hooke-Kelvin-Bingham-Newton-model based on experimental results (excluding at present deep cooling), Langer 1980. The parameters involved are depending on the stress level and the temperature. The finite-element-method is applied to rotationally symmetrical three-dimensional problems. The non-linear calculation method is based on an implicit method with respect to time. A modified Newton-Raphson-method and the initial strain approach are employed. Along with some experiences on suitable numerical procedures, examples are shown for the characteristical effects of time-temperature dependency. Having achieved convergency and reliability a method is now available for practical design and parametric sensibility studies.

Jan 1, 1982

Mining companies around the globe have long struggled with the need to ensure that earth moving equipment is designed to be operated and maintained under all site conditions without causing harm to people. Incident and accident data from a range of sources clearly identify mining equipment related issues as major causal factors. One Australian source, the NSW Department of Primary Industries? International Mining Fatality Database Project Report (2008) noted that ?Approximately 77% of mining accidents resulting in a fatality have? involved equipment?. The largest number of fatalities were associated with trucks; the next with LHDs. Many of the technical advances in equipment design over the past few decades have not been matched by advances in the human factors design of mining equipment, resulting in a ?design vacuum? that must be filled prior to commissioning new equipment. This ?design vacuum? is generally filled by local dealers. They retrofit equipment according to customer requirements to mitigate risks that have not been designed out by the Original Equipment Manufacturer (OEM). Mining companies have been frustrated by lost time and additional costs associated with this need to retrofit ex-factory designs. Some companies have attempted to address the problem by negotiating company or site-wide ?solutions? to equipment designs, but this has not simplified or clarified the issue from the point of view of the equipment manufacturers. A number of mining companies operating in Australia first discussed the concept of a joint customer approach to improve the Human Factors design of earth moving equipment at the factory level in 2004. They agreed that a new approach to engaging with OEMs was needed; one that would define the ?Problem?, in other words, the basis of the risks, rather than dictate the ?Solution?. The Earth Moving Equipment Safety Round Table (EMESRT) was formally established in 2006 by six global mining companies to establish a process of engagement between Original Equipment Manufacturers (OEMs) and mining customers.

May 1, 2009

Kalgoorlie Consolidated Gold Mines Pty Ltd (KCGM) is a management company, formed in 1989, to manage the assets and operate the Kalgoorlie joint venture for the owner companies Gold Mines of Kalgoorlie Limited (GMK) and Homestake Gold of Australia Limited (HGAL). Mining is still in progress on the 'Golden Mile' and surrounding areas more than 100 years after Paddy Hannan's discovery adjacent to the active Mt Charlotte mine. From 1893 until 30 June 1995 the Kalgoorlie Goldfields has produced approximately 45 million ounces (-1400 tonnes) of gold from mining and treating about 179 million tonnes of ore. This production equates to about 20 per cent of the total Australian gold production and about one per cent of the world's

Jan 1, 1996

By S. Kashani-Nejad

The kinetics of the thermal decomposition of samples of Mg2(OH)3Cl.2H2O that were obtained by a novel leaching technique applied to a commercial magnesium electrolysis feed were investigated under ambient atmosphere. Thermogravimetric (TG) Analysis showed that decomposition of Mg2(OH)3Cl.2H2O exhibited a single intermediate stage wherein crystalline water was lost at roughly 108 °C. Conversion of the resulting Mg2(OH)3Cl to MgO, HCl and 1120 occurred at temperatures above 398 °C. Thermal decomposition of anhydrous Mg2(OH)3Cl was seen to be consist of two stages. Both stages exhibit first order kinetics with respect to the weight percent of Mg2(OH)3Cl remaining in the sample. However, the rates of decomposition in the two stages were significantly different. Detailed analysis of the kinetic data suggested that the rate of decomposition of Mg2(OH)3C1 was controlled by the rate of removal of the gases released. The change in decomposition rate during the course of the experiments was explained by HCI being released in stage 1 and followed by release of H2O in stage 2.

Jan 1, 2004

By Patrick R. Taylor

A novel method for Chemical Vapor Infiltration (CVI) using plasma technology is proposed, Plasma Enhanced CVI (PE-CVI). Due to the low rates at which CVI is currently performed, the use of plasma as a source of heat may be considered to be a faster and less expensive production method. Precursors fed in the hot region of the plasma gas are completely vaporized and forced by the same flow through a porous preform where it condenses as a new material. In this research, silica and methane where used as precursors to form SiC/on a / graphite felt (preform). To enhance the infiltration process, vacuum was used to increase the flow through the preform. Characterization of infiltrated graphite was perfontied using Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD). A uniform distribution of Sic was observed along the felt with only twenty minutes of infiltration process, yielding a 178% increment in weight.

Jan 1, 1997

By C. W. Spencer, J. T. Smith

GRAIN-BOUNDARY surfaces are penetrated by a liquid phase when the liquid-solid interfacial tension is less than one-half the tension of the grain-boundary surfaces. Grain growth may occur in the presence of this liquid phase. burke1 reports that grain growth of this nature has been observed in two-phase ceramic systems. He suggests that the presence of a grain-boundary liquid may accelerate or decelerate the

Jan 1, 1963

By Leon H. Nelson

Several factors which affect the segregation and solidification of killed hot-topped steel ingots are: (1) pouring temperature, (2) volume in the hot top, (3) taper in the ingot, (4) pouring rate by varying the nozzle size, (5) segregation of several elements, (6) moving ingots before completely solidified. In most of this discussion, segregation will be measured in terms of carbon variation only. All the data given are results on S.A.E. 1040 analysis of fully killed steel direct-poured in hot-topped molds giving 20-in, square ingots. Thus the variables of analysis and ingot size and contour are eliminated. Segregation results are all on the rolled-bloom size, not on the ingot itself. They are results from plant-control work and not results of special research on ingot segregation. Pouring Temperature The temperature (or, more correctly, the amount of superheat above the liquidus-solidus) of the steel entering the ingot is, of course, one of the most important factors influencing segregation and solidification. Table 1 gives the average segregation of a number of heats of S.A.E. 1040 steel with normal pouring temperatures of 2775" to 2825" F. The table also gives some data on segregation in hot heats (pouring temperature 2840" to 2870" F.) and in cold heats with some skull (poured at 2725" to 2760" F.). The results in Table 1 are on steel ingots with 20-in. square cross section and 60 in. long to the hot top. The ingot weighs 7100 lb. and the hot top contains 14.3 per cent of the total ingot volume and 11.2 per cent of the total ingot weight. The results are from drillings after rolling the ingots to bloom size. Segregation results obtained on the bloom are probably not as great as they would be in the ingot itself, since heating the ingots for rolling can perhaps cause some migration of carbon from regions of high and low carbon to those of average carbon content.

Jan 1, 1937

By Shail J. Butani

In 1986 the Bureau of Mines conducted a probability sample survey, Mining Industry Population Survey, to measure such employee characteristics as occupation; principal equipment operated; work location at the mine; present job, present company, and total mining experience; job-related training during the last 2 years; age; sex; race; and education. The population estimates are necessary to properly analyze the Mine Safety and Health Administration (MSHA) injury (includes illness and fatality data) statistics; that is, to compare and contrast injury rates for various subpopulations in order to identify those groups that are exhibiting higher than average injury rates. This report uses the survey's results to characterize the U.S. stone mining workforce from March through September 1986. Similar reports have been published for the metallic, sand and gravel, and nonmetallic mining industries, as well as for the entire metal and nonmetal mining (includes metallic, stone, sand and gravel, and nonmetallic industries) sector and the coal mining sector.

Jan 1, 1988

By E. J. Roberts

F. C. Bond (Allis-Chalmers Mfg. Corp., Milwaukee) —This paper considers comminution as a first order process, with the reduction rate depending directly upon the amount of oversize material present. The data show that other factors should be taken into account, and it is possible that in time these may be evaluated as simultaneous or consecutive reactions: Development of the theory of comminution has been retarded for many years by the assumption that surface area measurements constitute the sine qua non of the work done in crushing and grinding, and it is encouraging to note the belated growth of other ideas. In the Abstract the term "net power" should be changed to "net energy." Throughout the paper the term "hp per ton" should be changed to "hp hrs per ton", or "hp hr t." The term "Probability Theory" in the title does not seem appropriate, since it is not clear how the probability theory is used in developing the ideas in the paper. There seems to be a contradiction between the large calculated advantages of closed circuit operation and the statement following that the closed circuit test results showed no significant change in grinding behavior, when compared with the batch grind curves. Tables I and II show that between 75 pct and 50 pct solids the energy input required decreases with increasing moisture content and may indicate the advisability of grinding at higher dilutions in certain cases. The calculation of the hp-hr per ton factor indicates an input in the laboratory mill of only 7.32 gross hp per ton of balls; this casts some doubt upon the accuracy of the factor used, since the power input in commercial mills at 80 pct critical speed is customarily much higher. The tests show that within fairly wide limits the amount of ore in the laboratory mill may be varied and a product of constant fineness obtained, provided that the grinding time is varied in the same proportion. This has often been assumed, and confirmation by actual testing is of value. The Cavg corrections for differences between the plant and laboratory size distributions do not seem very satisfactory, since in many cases the plant/laboratory ratio is farther from unity after correction than before. The following equation has been derived from the data in Table VI: Relative Energy (log new ball diam in in. + 0.410) Input = --------------—--------------- from which the relative energy inputs for balls of different sizes can be calculated and compared. The relative energy input is unity for balls of 2.715 in. diam. The equation indicates that the work accomplished by a ton of grinding balls per unit of energy input is roughly proportional to the square root of the total ball surface area; provided, of course, that the balls are sufficiently large to break the material. The data in support of this statement are admittedly meager, but are fairly consistent when plotted. The relative grindability values listed in Table VI for 200 mesh multiplied by 4/5 apparently correspond approximately to the A-C grindability at 200 mesh.' It would seem that for open circuit tests comparable accuracy could be obtained much more simply by the old method' of plotting the test grind, extending the mesh grinds to the left of zero time if necessary, and determining from the plot the equivalent time required to grind from the plant feed size to the plant product size, using the average of several mesh sizes. The en- ergy input value of one time interval could be determined by tests on materials of known grinding resistance, and this multiplied by the interval required should give the desired energy input value. The relative grindabilities would be the relative time intervals required for a specified feed and product size. When the plotted mesh size lines of a homogeneous material are extended to the left beyond zero time they meet at one point at zero pct passing. The horizontal distance of this point from zero time indicates the equivalent energy input required to prepare the mill feed. The author's results show that the closed circuit grinding tests give about the same K values as open circuit tests, from which he concludes that open circuit tests are satisfactory in many cases. The value of the closed circuit test is its ability accurately to predict energy requirements in closed circuit grinding for both homogeneous and heterogeneous materials. If the material is homogeneous, the open circuit test gives satisfactory results; but if the material contains appreciable fractions of hard and soft grinding ore, the open circuit tests will not be accurate because of the accumulation of hard grinding material in the circulating load. Since in most cases it is not possible to determine a priori whether the material contains hard and soft fractions, the closed circuit tests are preferable and more reliable. B. S. Crocker (Lake Shore Mines, Ontario)—Dr. Roberts probability theory of grinding is very similar to our log pct reduced vs. log tonnage method of plotting and evaluating grinding tests at Lake Shore. However, although we both seem to start at the same point we finish with different end results. Shortly after publishing our grinding paper (referred to by Dr. Roberts) in 1939, we did pursue the subject of the "constant pct reduction in the pct +28 micron material for each constant interval of time. We ran innumerable tonnage tests on the plant ball mills, rod mills, tube mills with 11/4 and 3/4 balls, and lastly pebble mills, with tonnage variations from 180 tons per day to 950 tons per day. We found that when we plotted the log of the tonnage against the log of the pct reduced of any reliable mesh, we had a straight line up until 90 pct of the mesh is reduced. We have also tested this in our 12-in. laboratory mill with the same results. We have used this method of evaluating grinds for the past 8 years and developed the recent four stage pebble plant on this basis. By pct reduced we mean the percentage of any given mesh that is reduced in one pass through a mill at a given tonnage (or time). For example, if the feed to a rod mill is 90 pct +35 mesh and the discharge at 500 tons per day is 54 pct +35, the pct reduced is 90 — 54/90 = 40 pct. If the feed had been 80 pct +35 the discharge would have been 48 pct +35 or pct re- duced 80-48/80 = 40 pct as long as the tonnage re- mained constant at 500 tons per day. Thus we can easily correct for normal variation of mill feeds. This log — log relationship derived from the tonnage tests of all our operating mills has proved of tremendous help in checking laboratory work and in designing alternate layouts or new plants. The difference between the log — log and the semi-log plot is only shown up when the extremes in tonnages are plotted. When the relationship between the pct reduced and the tonnage was first investigated, we used semilog

Jan 1, 1952

By Sandra Villacorta

"El presente trabajo describe los aspectos geoambientales de la cuenca del río Huaura, ubicada al norte de la región de Lima con el fin de establecer una línea de base geoambiental. Dicha cuenca destaca por la actividad agroindustrial, la minería representada por minas como Raura y Ucchucchaucua, la presencia de áreas geoturísticas como los baños termo-medicinales de Churín y los centros poblados como Huacho, Huaura y Oyón. La caracterización geodinámica y el análisis de susceptibilidad a movimientos en masa e inundaciones, han permitido establecer que las zonas más susceptibles a estos procesos se encuentran en los distritos de: Sayán, Naván, Andajes, Pachangara, Oyón, Maray, Santa Leonor, Caujul, Huaura, Huacho, Leoncio Prado e Ihuarí. En este sentido, el estudio geambiental señala que la zona más impactada está ubicada en Caujul (alrededores de la mina abandonada Santa Rita) y que se debe monitorear los siguientes sectores: Uchucchacua, Churín e Iscaycruz en Oyón; y Vilcahuara, Sayán, Huamboy Grande, Chiuchín y Huacho, en Huaura; donde en algunos casos se ha superado los límites permisibles para aguas y sedimentos de normativas internacionales, e igualmente se ha registrado disposición de residuos sólidos a la intemperie, salitrización de los suelos por uso de insecticidas en actividades agrícolas, vertimiento de gases tóxicos de las actividades agroindustriales y disposición inadecuado de desmontes de construcción o de mina."

Sep 14, 2009

By R. V. Ramani, C. B. Manula, A. Owili-Eger

That a close analogy exists between the flow of air in mines and the flow of current in electrical networks- has been known for a long time. The laws that have been used to solve series and parallel electrical circuits have been used to solve for the equivalent resistance in mine ventilation problems. Kirchhoff's current and voltage laws have also been adopted in solving mine ventilation network problems. It will be best to define a few terms here regarding networks, and with reference to Figure 1, these definitions can be elucidated. Figure 1 can be represented by the network in Figure 2 without losing any essential details. 1. A junction is a point where two or more airways meet. 2. The segment of a roadway connecting any two junctions is known as a branch. 3. A mesh is a closed path through the network. In the above network, B, C, D and F are the junctions, CF and CD are two of the branches and CFDC is a mesh. Kirchhoff's current law states that the sum of currents entering a junction. Applying this law to the junctions in the ventilation network, one obtains for junctions C and F, the following relationships: JUNCTION C: Q1 = Q4 + Q3 (1) JUNCTION F: Q2 + Q3 = Q5 (2) Similar relationships can be drawn for other junctions. Kirchhoff's voltage law states that in any complete electric circuit, the algebraic sum of the emf's must equal the algebraic sum of the voltage drops. In mine ventilation, the square law (Atkinson's

Jan 1, 1975

By Ihsan Berk Tulu, Ted Klemetti, Deniz Tuncay

"The Analysis of Retreat Mining Pillar Stability (ARMPS) program was developed by the National Institute for Occupational Safety and Health (NIOSH) to help the U.S. coal mining industry to design safe retreat room and pillar panels. ARMPS calculates the magnitude of the in-situ and mining-induced loads by using geometrical computations and empirical rules. In particular, the program uses the “abutment angle” concept in calculating the magnitude of the abutment load on pillars adjacent to a gob.In this paper, stress measurements from U.S. and Australian mines with different overburden geologies with varying hard rock percentages were back analyzed. The results of the analyses indicated that for depths less than 650 ft, the ARMPS empirical derivation of a 21° abutment angle was supported by the case histories; however, at depths greater than 650 ft, the abutment angle was found to be significantly less than 21°. In this paper, a new equation employing the panel width to overburden depth ratio is constructed for the calculation of accurate abutment angles for deeper mining cases.The new abutment angle equation was tested using both ARMPS2010 and LaModel for the entire case history database of ARMPS2010. The new abutment angle equation to estimate the magnitude of the mining-induced loads used together with the LaModel program was found to give good classification accuracies compared to ARMPS2010 for deep cover cases.INTRODUCTIONIn the early 1990s, the Analysis of Longwall Pillar Stability (ALPS) was introduced by Mark (1990) as a chain pillar design software and was generally accepted and used by the U.S. coal mining industry. Following the success of ALPS, the National Institute for Occupational Safety and Health (NIOSH) developed the Analysis of Retreat Mining Pillar Stability (ARMPS) program for designing retreat mining pillars using a similar approach as ALPS (Mark and Chase, 1997). The Australian mining industry also recognized the success of ALPS, and Colwell, Frith, and Mark (1999) calibrated the program to Australian conditions. The ALPS and ARMPS programs draw their strengths from the large databases that are used to calibrate them (Mark, 2009). However, following the Crandall Canyon Mine collapse in 2007, NIOSH had to reconsider the pillar design criteria used in deep-cover retreat mining (Mark, 2010). The ARMPS overburden load prediction algorithm was improved to more accurately predict the loading of narrow panels with high overburden depths by implementing the pressure arch concept, and this new version is called ARMPS2010."

Jan 1, 2019

By W. Clark Ashby

Productivity is a fascinating concept. Like motherhood or apple pie, we know it as a good thing. Just what it means is not so clear. To a coal miner we assume it means tons of coal hauled from the mine. To the company president it means profit after capital, labor, and other costs have also been reckoned. To the government economist it means output in a balance of payments. What does it mean to the biologist, and can reclamation really be productive? In this paper we discuss productivity from a biological viewpoint, give some yield data for various types of reclamation plantings, and discuss how our demand for or acceptance of differing kinds of productivity is tied up in bigger questions of land use, environmental stability, and resource allocation.

Jan 1, 1981

By P. E. J. Pitfield, S. D. G. Campbell

The regional shear zone runs along or near the southern margin of the Chinhoyi greenstone belt. It hosts four of the seven mines in the southern part of the belt with recorded production above 100 kg Au, including the top two current producers. Displacement occurred in the late Archaean, producing a shear zone with a trend ranging from northeast at Golden Kopje to east at Muriel. The orebodies at the two mines formed synchronously with the shearing and show strong but contrasting structural controls on mineralisation. The Golden Kopje orebodies plunge to the southwest, parallel to the intersection of curved foliation with discrete shear planes and sub-perpendicular to local stretching lineations. The Muriel orebodies plunge steeply west but parallel to local stretching lineations. The greatest fluid flow during mineralisation of the shear zone was along either the intersection of fabrics (Golden Kopje) or the extension direction (Muriel). There is no unique relation between the orebodies and the extension or shear direction. Where earlier mining has not established the directions of the oreshoots it is essential to understand the possible structurally controlled geometries of orebodies before planning a drilling programme to evaluate their potential

Apr 1, 1996

By Paul J. Remington

The field data gathered as part of the measurements described in Volumes I and II were re-analyzed in the low frequency regime (.1 Hz to 1 Hz) where motion sickness normally occurs. These data were compared co the international standard ?ISO 2631 Addendum 2: Evaluation of exposure to whole body z-axis vertical vibration in the frequency range 0.1 to 1.0 Hz." For both underground mining machines and surface mining machines the results showed that one would not expect motion sickness to .be a severe problem. New field data were gathered on the vibration reduction performance of various seat designs found on surface coal mining machines. The vibration during normal operation was measured on the seat using the standard seat pad accelerometer package described in the previous two volumes and simultaneously on the floor of the machine at the base of the seat. Preliminary indications are that the so-called high performance seats used for operator vibration reduction do not offer significant improvement over conventional seats.

Jan 1, 1984

By W. J. Guay

Carbonaceous gold ores containing gold dispersed in sulfides are refractory to cyanidation because: ( 1) the carbonaceous materials adsorb gold during cyanidation; (2) a portion of the gold is so finely dispersed in the sulfide mineral lattice as to preclude contact with cyanide solution; and (3) a portion of the gold is chemically combined with carbonaceous material. Techniques which have been used to treat these ores are: (1) roasting or chemical oxidation to decompose sulfides and carbonaceous materials; (2) flotation to reject carbon; (3) the use of kerosene or other reagents to passivate the adsorptive carbon compounds; (4) carbon or resin in pulp; (5) flotation of a bulk concentrate which is treated by one of the above methods; and (6) the use of chelating agents which have a stronger affinity for gold than the activated carbon in the ore.

Jan 1, 1981

By J. Hanna, H. R. Gonzalez

Four Ca-bentonitic clays from Alabama were evaluated for use in diammonium phosphate (DAP) or urea ammonium nitrate (UAN) fertilizer suspensions. The viscosity and gel strength of suspensions prepared with different proportions of a Western bentonite clay and sodium-exchanged Alabama clays were measured as a function of clay content and dispersant concentration. The results obtained suggested that three of the Alabama Ca-bentonites would replace about 40% of the more expensive Western bentonite in UAN and DAP fertilizer suspensions. The performance of the Nu-modified clays, prepared according to the Tennessee Valley Authority soda ash method, was as good or even better than those observed for the Mg-, NaIMg- or K-modified clays. The TVA clay modification procedure appears to be reliable and cost effective.

Jan 1, 1991

By N. S. Keith

(Read at the Wilkes-Barre Meeting, May, 1877.) THIS question is suggested by a statement made by Dr. Siemens, widely printed in the journals of the day, that a continuous rod of copper, thirty miles in length and three inches in diameter, is capable of conveying that distance, electrically, energy equal to 1000 horse-power. It is not attempted to advance the statement that the source of power shall be zinc, nor even coal, but waterfalls, which from their situation are not practically available for manufacturing establishments in their immediate vicinity. In order to fully consider this subject., we must understand the doctrine, I may say the science, of the correlation of the forces, so called. We must understand that all matter is endowed with an amount of force, and that each atom and molecule, simple and compound, has its specific portion of the whole. This force, at rest, is called latent heat, intrinsic energy, or potential. In motion it is called heat, light, electricity, chemical affinity, attraction, magnetism, power, etc., according to its sensible manifestations. These are the effects of the one force active in different substances, or in different assemblages of matter. Force put in motion comes to rest by reason of the resistance to motion which it encounters; in overcoming

Jan 1, 1878

By Leonard Sugin

As we approach the end-of the 20th century we are made more aware of the tremendous role that energy plays both directly and indirectly in our varied way of life. The industralized nations of the world have already established industries and economies which are greatly dependent upon the availability of energy. The underdeveloped and developing nations of the world are slowly and surely moving to the same utlimate requirements. The world wide need for distribution of energy sources is continuing to grow. At the present time there is a great imbalance between the sources of energy and the consuming economies In order to bring about greater equality between supply and demand major efforts and large capital expenditures are constantly being made. Industralized areas are seeking their own independent sources of energy whereas those areas of the world which contain great reserves of energy are limiting its export as well as developing its own industralized capacity. Nevertheless, energy bearing materials are still transported great distances in order to satiate the appetites of power hungry economies.

Jan 1, 1975

By A B. Christie

Known epithermal deposits in the Coromandel region belong to the low-sulphidation type, represented by three subtypes: (1) andesite-hosted epithermal Au-Ag, (2) rhyolite-hosted epithermal Au-Ag, and (3) andesite-hosted polymetallic veins. Paleo-depths for a range ofdeposits are estimated using various features including: paleo-surface indicators (e.g. sinters), hydrostatic pressures estimated from homogenisation temperature measurements on fluid inclusions, and alteration mineralogy. Paleo-depth estimates to the base of the ore zone for the rhyolite-hosted Coromandel deposits are in the range 200 to 500 m. Andesite-hosted Au-Ag deposits have more variable paleo-depths, and are deeper (up to 950 m) in those deposits with base metals at depth. The majority of Coromandel Au-Ag deposits have limited depth extents (<200 m) of economic grade mineralisation, although Waihi and Karangahake are notable exceptions with depth extents of 575 m and 700 m respectively. Limited depth extents of economic ore are a feature of epithermal systems worldwide and are commonly attributed to gold deposition from boiling fluids. Under hydrostatic conditions in epithermal/geothermal systems, boiling in an up-flowing hydrothermal fluid is controlled by the boiling point for depth (BPD) curve. From fluid inclusion studies of many low-sulphidation epithermal systems, deposition of electrum in economic concentrations appears to be confined to a temperature window of 260 to 180¦C. This means that for dilute, gas-poor fluids that follow the BPD curve, gold deposition is limited to a vertical depth zone of about 400 m, with its upper boundary at about 100 m below the ground surface. The greater depth extent at Waihi and Karangahake, can be explained by mixing of/a deep, hot (300¦C) fluid with cooler (c. 200¦C) heated ground waters. This has the effect of lowering the 260¦C isotherm by up to 300 m, thereby increasing the depth range of economic grade gold deposition within the 260 to 180¦Cwindow of electrum deposition.

Jan 1, 2000