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By Miller F. S

The famous Golden Mile of Kalgoorlie in Western Australia was, between 1888 and 19ft, one of the richest and most important gold camps in the world. Annual production, which reached a peak of nearly 1,300,000 oz. in 1903, however dropped to below 400,000 oz. after 1920.Depression seized the camp, and many mines closed down. The output from this old field is now steadily mounting under the stimulus. of a higher price for gold. In 1935 300,000 oz. of precious metal were recovered and the future is bright. Total production from the field is 21,400,000 oz., whereas something like £23,600,000 have been paid in dividends.The Golden Mile has been studied by many geologists. Their somewhat diverse findings were published for the most part in the Australian technical press which, unfortunately, is not sufficiently represented in foreign libraries.The following article is based on an eighteen months' study in the district for Gold Mines of Kalgoorlie Ltd., preceded by four years' experience with almost identical rock types in the, Porcupine district of Ontario. The excellent record of observed fact left by previous invesigators, augmented by talks with observant mining men...

Jan 1, 1937

Drilling and blasting are an essential part of most mining operations, and are commonly major operating cost centres. The effectiveness of blasting can have a significant influence on all subsequent activities, and hence overall operating costs. Drilling and blasting systems must be compatible with the overall mining method, to minimise total costs and ensure operational efficiency. Selection of suitable equipment and techniques requires an understanding of the total operation, plus current skills in specialised technologies. This paper examines the contribution of drilling and blasting to total costs, including the balance between capital and operating costs. The use of computer modelling is discussed, to demonstrate how blast performance and costs can be predicted and alternatives analysed. Case studies illustrate how cost savings can be made in operating mines, and alternative systems evaluated to optimise new projects.

Jan 1, 1990

By C Maloloy-on, S Morgan, M Deiparine, H Mackie

There is a strong focus in the mining industry to limit its impact on water quality and usage. This is even more evident in developing countries, where mining companies operate within local communities and have to be diligent not to affect them negatively. In mineral processing operations, the vast majority of water reclamation occurs at the tailings thickener area and tailings storage dam. It is often more effective to maximise water recovery at the tailings thickener before material transport to the tailings dam. Tailing dams are relatively inefficient, and experience a high degree of water loss due to evaporation, seepage and water retention. In addition, tailings dams present the biggest environmental liability for mineral resource projects, so it is beneficial to limit their size and loading.In the past, it was common for tailings thickener areas to have a significant footprint with several large diameter conventional thickeners, producing underflows of between 45-55% solids (w/w). At times, these multiple thickeners were difficult to operate, leading to inefficiencies in water recovery and quality. The development of High-Rate Thickeners (HRT) has resulted in improved settling properties and HRTs usually operate at significantly higher solids loadings (= 1.2 t/m2.h) when compared to conventional thickeners (= 0.1 t/m2.h). Subsequently, thickener diameters have reduced in size and HRT technology has become the preferred option for thickener circuit design in both new and existing operations. HRTs also produce higher water recoveries with increased underflow densities = 55% solids (w/w).The Carmen Copper Corporation (“Carmen Copper”) circuit upgrade is a recent example of these HRT benefits, where one, 39 m diameter, Outotec HRT treats the solids flow rate of five 76 m diameter conventional thickeners. This has allowed operations to increase dry plant tonnage by 50%, maintain key performance targets and provide a framework for further plant expansion. This paper details the sustainable approach to the Carmen Copper tailings thickener upgrade, where metallurgical performance, cost-effectiveness and environmental impact were key considerations.CITATION:Morgan, S, Mackie, H, Maloloy-on, C and Deiparine, M, 2015. Carmen Copper – a sustainable approach to tailings thickener upgrades, in Proceedings MetPlant 2015, pp 416–430 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Sep 7, 2015

The epithermal Au-Ag deposits of the Hauraki Goldfield have formed in the convergent plate margin setting of the Coromandel Volcanic Zone (CVZ). The deposits form as siliceous lodes in the upper parts of geothermal systems associated with andesitic and rhyolitic volcanism. Two types of low sulphidation epithermal mineralisation are recognised in these volcanic terranes. Vein deposits within the rhyolitic terrane in the east and south of the CVZ (Whitianga Province) are typified by Chalcedonic Silica-Adularia-Illite type mineralisation. The veins are hosted by both rhyolite-dacite volcanic rocks and the underlying andesitic basement. The mineralisation formed by episodic violent decompressional boiling of deeply circulating meteoric water, with some magmatic fluid contribution; this boiling was associated with hydraulic fracturing and hydrothermal eruptions in geothermal systems where the piezometric surface of the deep geothermal reservoir lay near-surface. Such episodic boiling provides chalcedonic veining, banding and brecciation textures, adularia stability, bladed calcite, and extreme loss of H2S gas from the fluid to cause major gold deposition.   Crystalline Quartz-Illite-(Adularia-Kaolinite) type mineralisation is widespread in the andesitic terrane forming the northern and western portions of the CVZ (Kuaotunu Province). There is a spatial association with weak mineralisation of the high sulphidation (Vuggy Silica-Alunite-Kaolinite-Illite) epithermal type and porphyry eu-Au type. Crystalline Quartz mineralisation is probably formed by the mixing of shallow acidic groundwaters with the ascending deep geothermal fluid in geothermal systems beneath andesitic stratovolcanoes. The piezometric surface to the deep geothermal reservoir in these systems is likely to lie at considerable depth, with acidic gases rising from the geothermal reservoir condensing within the overlying vadose zone. The cooling effect, oxidation and pH decrease accompanying mixing apparently overcomes the dilution effect, and results in quartz deposition, illite (and locally kaolinite) alteration, and precipitation of gold. A high level of base-metal sulphide precipitation also enhances gold deposition. Violent boiling of the mixing fluids may occur at local structural dilation sites, producing high grade "bonanza" gold mineralisation.

Jan 1, 1991

The problem of ventilating an advancing drive which finally reached 5,550 ft from the fresh air base in rock with virgin temperature of 83°F was solved economically by the use of two 15 h.p. electric fans forcing ventilating air through 281/2 in. dia. fiexible polythene tubing of wall thickness 0·010 in. The saving in cost over conventional metal duct was estimated to be £3· 24 per ft of drive.INTRODUCTIONAs described by Dutton (1965) and Newton (1965) the existing 1,480' ft level drive at the South Mine, Broken Hill, was extended in two stages. The first extension was 3,790 ft, the face then being 4,790 ft from the fresh air inlet. Later, after a vertical ventilation connection was made 1,000 ft back from the face, the drive was extended a further 4,550 ft. The rate of face advance in both stages was about 15 ft per day (3 shifts of 5 ft), 5 days per week. Drive size was 8 ft by 9 ft 6 in. high.The Mines Inspection Act of New South Wales requires 6 hr shifts (instead of the normal 7 in Broken Hill) to be worked if the wet bulb temperature exceeds 80°F. Virgin rock temperature on the 1,480 ft level was 83°F. Inlet air temperature varied with seasons from 58 to 72,oF wet bulb.To adequately ventilate the face, a flow of air of about 11,000 c.f.m. was thought to be necessary.Re-entry time was about 75 min, being governed by the 7 hr length of shift (3 shifts per day) and travelling time.Blasting was permitted only at the end of the shift, in accordance with local industrial restrictions.

Jan 1, 1965

A description is given of preliminary work done to identify the problems inherent in electrochemical oxidation of metal sulphides, and particularly in the use of powder compacts as anodes in electrolysis. Lead flotation concentrate pressed into a compact can serve as an anode for direct electrowinning of lead according to the reaction PbS -7 Pb2+ +So +2e. No pYrometallurgical step is required, and sulphur is recovered in elemental form.In the experimental work, electrical conductivity of the compacts was achieved by addition of 5·5 per cent of graphite. After compaction at 10-20 tons/sq. in. the anodes were strong and remained intact throughout the electro-oxidation period. Perchlorate electrolyte was used, but fluosilicate electrolyte is also applicable. A sheet of pure lead formed the starting cathode.As anode potential increased above 1·0 V (v. SHE) PbS04 was formed in increasing amount. At 0·90 V, Pb2+ extraction efficiency was 85 per cent. Anode potential was kept low by electrolysing at 60°C and 15 mA/sq. cm., and by improving the porosity of the compact. Other metal sulphides (Zn, Fe, Cu) in the flotation concentrate also oxidize and release metal ions to the electrolyte, which would need to be purified prior to deposition of lead.Power requirement in a laboratory cell was about 0·24 kWh per lb of lead extracted.

Jan 1, 1972

By Koch D. F A

An electrochemical study of the dissolution of gold in aerated cyanide solution shows that the dissolution rate is determined by the nature of the surface. A passive film (probably a basic cyanide) formed at -0·6 V vs a saturated calomel electrode inhibits both the anodic dissolution of gold and the cathodic reduction of oxygen. Thallium has an "activating" effect on these reaction mechanisms.INTRODUCTIONIn general the dissolution rate of a solid in a liquid is studied by weight loss measurements or analysis of the solution at known time intervals. However, if the solid is a metallic conductor, on which a redox reaction can occur at the metal-solution interface, it can be studied by electrochemical techniques. Electrochemical measurements have the advantage that the overall reaction can be separated into its components, in some cases by electrical measurements alone, and in others by a combination of these with analytical determinations.The overall equation for the dissolution of gold in aerated cyanide solution may be written4Au + 02+ 8CN- + 2H20 = 4Au (CN)2- + 40H- ..... (1)which can be separated into the two electrochemical componentsrepresenting the oxidation of gold and the reductionof ogygen respectively.4Au + 8CN-,= 4Au(CN)2- + 4eand O2+ 2H20 + 4e = 40H- These two reactions occur at the gold-electrolyte interface and it is the purpose of this paper to discuss the mechanism...

Jan 1, 1964

By Madgwick J. C

A novel, microaerobic, heterotrophic microbial leaching system for manganese dioxide is described. Mixed natural microflora were enriched by addition of sugars and crude plant nutrItIOnal supplements. Between 84 and 104 mg MnH was solubilized per gram sugar at I per cent (w/v) concentrations for fructose and galactose in the presence of 0.03 per cent w/v dried microalgae (a Chlamydomonas species). Some 42 per cent soluble manganese was dissolved by microbial action on MnO, (3 per cent (w/v) pulp density) using a 10 per cent (w/v) molasses growth substrate.

Jan 1, 1983

By T P. Fischer, S N. Williams, N C. Sturchio

Chemical and isotopic analyses of fumarole gases from Galeras Volcano, Colombia provide significant insights into the origin of volatile components in the magma. Water comprises a mixture of magmatic (rock-equilibrated at high temperature) and meteoric, with the relative proportions varying through time. Carbon isotopes are most consistent with a mantle source, although other contributions to the system cannot be ruled out. Nitrogen most likely originated from organic matter in subducted oceanic sediments. All of these 'primary' components have been modified somewhat by interaction or exchange with other reservoirs during ascent to the surface. In addition to information on the origin of the gases, these data show significant temporal variations precursory to eruptive events. Chemical ratios change and water becomes more meteoric in character prior to eruptions.

Jan 1, 1995

In his account of the Thackaringa sulphide ore, Lawrence (p. 92) reports the presence of valleriite as a minor accessory in chalcopyrite. He says that it is readily identified by its intensive anisotrophy and reflection pleochroism. He also gives (p. 87) the chemical formula as Cu8Fe4S7 Recent work by Evans et al. (1964), Chamberlain and Delabio (1965), and Schidlowski and Otteman (1966) suggests that most material identified as valleriite is, in fact, mackinawite. Mackinawite and valleriite are both extremely anisotropic and strongly reflection pleochroic, and so with fine-grained material it would be hard to distinguish them optically.Mackinawite, according to Evans et al. (p. D66) is a copper-free iron sulphide (PeS) with small quantities of nickel, whereas valleriite is considered to be a dimorph of chalcopyrite (CuPeS2 ). They found that mackinawite is much more widespread than valleriite, and, unlike the latter mineral, is commonly included in other sulphides, particularly chalcopyrite.Lawrence (p. 92) states that the valleriite inclusions are up to 0·01 mm long, so that an electron probe microanalysis should be quite feasible and provide a ready answer to this query.L. J. Lawrence: I thank Dr Townend for his comments on the occurrence of a valleriite-like mineral embedded in chalcopyrite from Thackaringa. The problem of distinguishing valleriite from mackina...

Jan 1, 1967

By Holmes R. J, Miles J. G

An on-stream shale detector has been developed that is suitable for use on conveyor belts carrying iron ore of up to 200 mm particle size. The shale is distinguished from high-grade iron ore by means of the substantially greater total natural gamma-ray activity (400-3000 keV) of the shale, which is due to the presence of uranium (214Bi), thorium (208TI) and potassium (40K). Other gangue materials, sueh as highly siliceous cherts and jaspilites, escape detection because of their low activity. A dynamic trial of the shale detector at Mt Whaleback, using a large-volume (100 × 100 400 mm long) "Polyscin" Nal(TI) gamma-ray detector, demonstrated that as little as 20 tonne of ore on a conveyor belt (equivalent to partial truckloads of ore) can readily be monitored for shale content.

Jan 1, 1981

Igneous phosphate deposits located in tropical and subtropical regions of the world are characterised by low P2O5 grades, very diversified mineralogy and intensive lateral plus vertical variations in chemical composition, mineralogical assembly, rock texture, grain size, slimes content and morphology plus surface coating of apatite particles. Those variations characterise ore types with distinct behaviour in mineral processing plants. This work reviews the main challenges to concentrate those ores, addressing developed solutions. Apatite concentration requires long chain anionic collectors to conduct reverse flotation of barite (alkyl sulfate) before direct flotation of apatite (sarcosinate, sulfosuccinate and oleate) at basic medium. The long chain collectors decrease the surface tension of the flotation solution (?LV) and a particular value of 51 mN/m was found to promote the most selective separation between apatite/gangue at a particular plant. Effective control of the magnitude of ?LV in industrial plants has not been realistic because there is a lack of reliable instruments to carry out in-situ measurements. Regarding gangue depression by starch, a review is made on possible mechanisms that promote its remarkable ability to depress some minerals (Fe-oxides, ortho plus meta silicates and calcite) vis-a-vis others (quartz, apatite and layered silicates). Flotation recovery (R) of fine (20<R<30 per cent) and coarse (30<R<60 per cent) apatite particles is considerably lower than the recovery of the intermediate ones (R>70 per cent) and inadequate hydrodynamic conditions within flotation cells are the main reasons for that behavior. Once the best hydrodynamic conditions to float coarse particles are not the same to float the fine ones, flotation of coarse and fine particles is commonly conducted at separated circuits in industrial scale. Entrainment of very fine gangue particles is an important source of contamination of apatite concentrates, mainly when weathered ores are treated. In most cases, adjustments in flotation equipment (columns or mechanical cells) promote considerable improvement in the quality of final concentrates.

Sep 13, 2010

Lead isotope measurements indicate that lead from the A.B.H. Consols mine and two transgressive veins in Browne's shaft is "Thackaringa" type and not "Broken Hill" type.

Jan 1, 1970

By Lyman G. J

Magnetic separator operation in the recovery of magnetite in heavy medium circuits was studied by conducting a series of tests on a full size wet drum magnetic separator.The significant operational variables were found to be feed pan clearance, squeeze pan clearance, dry magnetite feed rate and separator drum speed. With feed and squeeze pan clearances optimized for typical primary separator operation, magnetite loss and concentrate specific gravity were found to be related in a regular manner to dry magnetite feed rate at any drum speed.A two component model of magnetite loss is proposed, relating total magnetite loss to losses in the feed pan and squeeze pan zones. The experimental curves relating magnetite loss to dry magnetite feed rate at the various drum speeds are interpreted in terms of the two component model.The magnetite particle size distribution in the separator was also examined. The tailings magnetite was found to be finer than the feed magnetite only when the dry magnetite feed rate was low, or immediately following the introduction of a new charge of magnetite when the separator was fed at an intermediate dry magnetite feed rate.

Jan 1, 1983

By Turnbull A. G

Thermodynamic calculations based on the ideal system NiO - Fe203 - H2(g) - CO(g) - H2O(g) - CO2(g) show that it is not possible to selectively reduce nickel oxide in the presence of hematite because ferronickel is the zero-valent nickel-containing product. The calculations show that temperature, reducing atmosphere composition and oxide to reductant ratio have a marked effect on the nickel content of the ferronickel alloy and the amount of nickel reporting jn the alloy. The general trends based on the calculations correlltte well with experimental observations.

Jan 1, 1980

The state of sampling practice in South African minerals industry must be appreciated in the context of the changes that the country has recently lived through. These include the entrepreneurship and investment in local mineral developments, the 'unbundling of mineral assets', the rising foreign control of South Africa's mineral assets, developments in the regulatory environment, and the training that has come to the industry through international experts. In terms of their financial contribution the five most important commodities in the South African mining industry include gold, platinum, non-ferrous metals, ferrous metals, and coal. International Organization for Standardization (ISO) standards form the basis for international trade in two of these commodities and have had a steadying influence on sampling practice over the years. The contribution of the minerals industry to the economic well-being of South Africa and the strong financial links between good sampling practice and minerals marketing, has acted as a driver for improved sampling practice. The state of sampling practice at mining operations of the five main commodities was found to be quite widely divergent with 'pockets of sampling excellence' in each of the commodities. The stages of mineral development at mines for each of the commodities have been described in terms of the sampling sites, sampling mechanisms, and associated problems at different stages of mineral production from exploration, through face sampling, blasthole sampling, and in-mine grade control, ore processing and handling, metallurgical subsampling, process subsampling, and subsampling in the laboratory. In each case the principal errors were identified and their potential impact on value erosion was estimated. In the final analysis it is evident that the greatest problems arise in the early stage of mineral development, and that ferrous metals and coal are particularly susceptible to loss of value through poor sampling practice. Several examples of value lost through poor sampling at the marketing stage that arise from breaching contract specifications, either in terms of grade or sizing, are described. The conclusion is that larger companies with larger ore deposits and greater human, financial, and capital resources are probably more compliant with 'best sampling practice' than smaller companies. Finally the message that equipment suppliers are sending is that business is booming and the awakened awareness of the issues around sampling practice are translating into rands and cents spent on improved sampling equipment.

Mar 1, 2010

The Torington Lodes.This paper describes investigations into mineralogical, petrological and structural associations of cassiterite in occurrences of economic value. The work was carried out in three mines, during a number of years, in the hope that the, data collected might be helpfulin the search for occurrences of cassiterite in stanniferous zones and reduce unproductive exploratory work. Tin-bearing lodes at Torington, in the New England district, New South Wales, have been worked since 1880, and the locality is the most important centre of lode tin occurrence in the, State. The township of Torington is 18 miles westerly from Deepwater, a station on the Great Northern Railway, 446 miles from Sydney.The tinfield forms part of the Mole Tableland, which has an altitude between 3,000 ft. and 4,000 ft. above sea level. The country rock, in which the stanniferous lodes occur, is granite of Permian age, which has been intruded into Carboniferous claystone. An inlierof claystone is exposed to the west of the stanniferous area, 'and in it, close to' its contact with the enclosing granite, are occurrences of wolfram and bismuth. These minerals, together with chalcopyrite, pyrite, arsenopyrite, zinc blende, galena and molybdenite, in very small quantity, are associated with cassiterite in lodes traversing the granite, particularly near the claystone, but decrease in amount with distance from the contact. Their occurrence is sporadic and usually causes no trouble in dressing the cassiterite concentrate, the final product containing less than 1% of total penalisable impurity. In Curnow's mine, pyrite, galena and molybdenite have been observed at 700 ft. from surface.The granite is of the biotite type and its structure may be divided into four groups, or classifications, arising from segregation processes during cooling and consolidation. The four classes, in order of extent of distribution, are:1. Granite of normal type, in which the constituent quartz felspar and mica, have a particle size approximately 14, in.2. Porphyritic granite, in which the felspar particles are from % in. to lin. in size, that of the quartz and mica approximately % in.3. A fine-grained granite, in which the particles of all constituent minerals are % in. or less in size;4. A rock of highly siliceous ground mass containing very small particles of felspar and almost negligible biotite.Type 1 is the general type of granite in all mines of the field; types 2 and 3 may occur as large masses, or all three classes may merge rapidly one into the other, and all be disclosed in 20 ft. of driving. Usually, however, the segregations are massive, but are not associated with any special horizon. For instance, a mass of porphyiiticgranite extends from surface to a depth of 440 ft., for a distance of 350 ft. south from the New England main shaft. The upper levels of Curnow's mine, 14" mile south-east, are in normal granite, but a restricted mass 'of porphyritic granite occurred between Nos. 5 and 6 levels, or 400-500 ft. below the collar of the shaft...

Jan 1, 1950

By M I. Pownceby, N A. S Webster, R Pattel, N Ware

The decrease in quality of Australian iron ore, coupled with the demand for more efficient energy use, necessitates closer monitoring and optimisation of process conditions for iron ore sinter production. Sinter properties are correlated with the mineralogical content and traditionally, quality control has relied on methods such as Fe2+ determination, point counting via optical microscopy and automated electron beam techniques such as QEMSCAN; these can have a typical turnaround time of several days between sampling and reporting of results. As well, there is additional uncertainty around quantifying some of the complex mineralogical phases in sinter such as silico-ferrite of calcium and aluminium (SFCA) and SFCA-I, which are key phases influencing sinter quality. There is a clear need for the implementation of alternative analytical methodologies that offer a turnaround in hours or even minutes to provide information to accurately maintain production conditions.X-ray diffraction (XRD) is a technique widely used for the quantitative mineralogical phase analysis (QPA) of ores and processed materials in the iron ore industry. Partial least squares regression (PLSR) analysis methods have recently been suggested as an indicator of sinter Fe2+ content from powder XRD data without the need for rigorous and often time-consuming mineralogical analysis by the operator. In this study we provide results from the implementation of PLSR to predict the strength of iron ore sinter. Results are sourced from a well-constrained data set comprising strength properties (as quantified by a tumble index (TI)) from a series of compact laboratory test samples. Using this data set it was determined that PLSR has potential to be used as a method for calculation of sinter TI from powder XRD data and, therefore, process control.CITATION:Webster, N A S, Pownceby, M I, Ware, N and Pattel, R, 2017. Predicting iron ore sinter strength through X-ray diffraction analysis, in Proceedings Iron Ore 2017, pp 331–334 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Jul 24, 2017

In 1801, Hatchett discovered the oxide of a new element while he was examining a heavy black mineral from Haddam, America, which had been presented to the British Museum. He named the mineral Columbite and the metal Columbium. A year later a Swedish investigator, Eckeberg, while working with a new mineral resembling columbite-yttrotantalite discovered a new element, to which he gave the name of tantalum.In 1844, Rose announced the existence of two new elements in a sample of columbite from Bodenmais. One was similar to Eckeberg's tantalum and the other was called Niobium. Wollaston, in 1909, attempted to show experimentally that columbium and the tantalum were identical, but it was soon proved that they were two separate elements. Subsequently a number of investigatiors found other "new elements" in tantalum and columbium bearing ores, but these were shown to be mixtures of the twin metals.Since Hatchett's Columbium and Rose's Niobium are identical, it is more fitting to use the name assigned to this element by the earlier investigator. However, the term niobium is still retained in Germany and England.The separation of tantalum and columbium has been one of the major problems for analytical chemists since the discovery of the two elements and it is only in recent years that tantalum of a high degree of purity has been produced. High purity columbium metal is probably still to be obtained.Until 1903 tantalum was merely a laboratory curiosity, but about this time its use in incandescent electric light bulb filaments was discovered. Eight years later it was replaced by tungsten, which was better and cheaper. For a time the electrolytic valve action of tantalum was used in the charging of storage batteries, but the chief uses of the metals' to-day are for corrosion resistant apparatus, in vacuum tubes and as carbides in the manufacture of hard cutting and shaping tools.Most of the recent work on the metals has been done by C. W. Balke, in America. Tantalum has been studied more than columbium. OCCURRENCE OF TANTALUM AND COLUMBIUMI. NATURE OF ORESTantalum and columbium occur, nearly always together, as the oxides in combination with other metallic oxides. The only known exception to this is the reported occurrence of metallic tantalum with gold in the Ural Mountains. Unlike most other metals, no compounds with sulphur are known to occur in nature. Tantalum and columbium minerals occur usually...

Jan 1, 1943

Alluvial gold mining in New Zealand and in particular on the West Coast underwent a renaissance during the early 1980's. This was based on two main factors: a) the sharp rise in the gold price b) the advent of the hydraulic excavator with which to mine the auriferous gravels. The scale of operations varies considerably. A small operation typically consisting of one or two people using an excavator to feed either a land based or pontoon mounted (floating) gold recovery unit, which will typically feature a rotary screen with or without a tailings stacker with the screened fines < 10 - 15 mm) fed over riflle tables. The largest screening plants of this type are operated by DML Resources in their Arahura and Mikonui mining operations where the plants have the capacity to screen up to 250 m3 of gravel per hour. These plants usually employ jigs and centrifugal concentrators for gold recovery. Unique amongst current operations is the Grey River Dredge, a bucket ladder dredge capable of processing 700 m3 per hour similar to those that operated earlier this century. Gold concentration is by a series of jigs. The number of plants operating in recent times peaked at around 130 in 1987 (coinciding with a nominal average gold price of $751) and since that time has steadily declined to 80 units in 1993 and about 45 - 50 units today. This cycle of rapid expansion and then steady decline has been repeated on the West Coast since the original gold discoveries in the mid nineteenth century.

Jan 1, 1996