Search Documents

By W. Dan Hausel

More than 220,000 diamonds have been found in the United States that range from microdiamonds to 40.42 carats in weight (Table 1). Some reports suggest diamonds weighing more than 100 carats have also been found in this country, although these reports have not been verified. The source of most of the diamonds in the United States is a group of kimberlite pipes, dikes and blows in the Colorado-Wyoming State Line district and a group of lamproite pipes in the Murfreesburo, AR area. Other areas where diamonds have been found include the Great Lakes region, the Pacific Coast and the southern Appalachian Mountains. The latter two regions are generally not considered favorable for diamond deposits. This is based on exploration models. They suggest commercial diamond deposits will be restricted to kimberlites emplaced in Archean cratons and to lamproites emplaced in Proterozoic mobile belts accreted to the cratons.

Jan 1, 1995

By H Peiming, W Sakurai, Y Matsuhisa, H Shimazaki, Y Fukahori

Oxygen and carbon isotope mapping of hydrothermally altered wall rocks was applied to the blind ore prospecting of skarn Pb-Zn deposits in the Kamioka mining district, centralJapan, where the wall rocks consist of heterogeneous rock units. The results (d18OSMOW of -1.1 to +17.3ë and d13CPDB of -5.0 to +4.8ë for limestones, and d18OSMOW of -0.8 to +12.5ë for silicate rocks) show isotope zonations of the wall rocks with depletions in heavy isotopes (18O and 13C) toward the centre of mineralization. The isotope zonations likely formed by interaction of migrating thermal waters with the wall rocks during the mineralisation. The heavy-isotope depleted zone, which indicates higher temperature or higher water-to-rock ratio of the system, occurs in the footwall of a fault, indicating that the fault played the role of main conduit of the ore-forming fluids. A heavy-isotope depleted zone was also found in an area, where significant mineralisation had yet to be identified. Recent drilling in this area has intercepted a zone with a grade of 16.0% Zn, 0.8% Pb, and 35g Ag per metric tonne at a depth of about 360 m.

Jan 1, 1995

By Y. J. Zhang

This paper presents the results of a recent study on the development of an in-situ testing method for determining the coal strength in mine pillars. The strength testing is performed in the horizontal holes drilled into the coal rib using the penetrometer. It is a hydraulically powered device that uses an indenter to press the surface of the borehole until the load reaches the failure point of coal. The pressure and the penetration of the loading pin are recorded by the data acquisition unit. The results collected from several coal mines indicate that this testing method is ideally suited for the realistic coal strength measurements. The new testing system not only improves the accuracy of pillar strength determination, but it is more cost efficient than the conventional method.

Jan 1, 1995

By C. L. Glorioso

As an introduction and credentials for our talk, let me take a moment to introduce you to Wimpey Worldwide and specifically to Wimpey Minerals, USA, Inc.

Jan 1, 1995

By R. M. Lamya

A considerable part of gold encapsulated in some refractory sulphides can be dissolved during sulphide matrix decomposition in non-complexing aqueous media. This has been concluded from laboratory results of gold extraction from pyritic and arsenopyritic concentrate containing 172.8ppm Au, into 10% hydrochloric, sulphuric and nitric acids. The degree of sulphides decomposition in the non-oxidative acids limits the gold extraction from this specific concentrate to about 10%. In the case of nitric acid, the gold dissolution follows the rapid decomposition of sulphides, reaching 32 to 42% of extraction at almost total decomposition of the sulphidic minerals. Accumulation of gold in this acidic and non-complexing solution to several ppm was not affected by the soluble products of the sulphides decomposition. An attempt to explain this gold behaviour is undertaken and some practical implications are discussed. In the investigated concentrate the "soluble gold" is mostly associated with arsenopyrite. This is consistent with the latest findings of other authors that gold disseminated in arsenopyrite can occur as cation Au3+. In a comparative test the gold extraction into highly concentrated chloride solution, in the same oxidative conditions, amounted to 92.34%.

Jan 1, 1995

By P M. Black, T Itaya

Arc-type volcanic activity has been a major feature of the late Cenozoic geological evolution of the Pacific-Australian plate boundary in the northernNew Zealand region. The time/space distribution patterns of arc-type rocks reflect changes in the nature of the plate boundary as it has evolved. Inception of volcanism began in the north during earliest Miocene times (26 Ma) following ophiolite emplacement and during the next 10 Ma developed into a volcanic arc segment extending south to the present position of the central North Island. The northern part of the arc became extinct about 15Ma ago, but activity continued in the central (Coromandel) part of the arc until about 5 Ma ago. In late Pliocene and Quaternary times volcanic activity has become concentrated in a northeast trending zone which is collinear with the Tonga-Kermadec Arc.   These time/space patterns can be interpreted in two ways. Firstly, if relative plate motion was essentially transform in the region prior to 26 Ma then a small change in the relative plate vectors may have initiated subduction and the associated volcanism. Southward evolution of the arc can then be seen as the straightening and extension of a single arc. Alternatively, the distribution of volcanic rocks can be interpreted as the capture of a relatively short lived arc by southward migration of the dominant Tonga-Kermadec subduction system.

Jan 1, 1995

Commodities statistics and other information are available through the following three systems: Internet Access: To acccess the USBM Gopher server on Internet, connect to: gopher.usbm.gov port 70, using gopher client software. Or contact the Systems Administrator for technical questions: Matt Klevemann, 412-892-6499 (klevemmc@miner.usbm.gov) These USBM materials are available on Internet, with more to come in the future: Publications and other research findings, including this bimonthly publications list, newsletters, tech transfer updates, enviromental technology research, health and safety information. USBM software products Media materials and speeches. Mines Data: Computer bulletin board. Log on via modem through 202/501-0373, or call 202/501-0406 for technical assistance. Mines FaxBack. Return fax service. 1. Use the touch-tone handset attached to your fax machine's telephone jack. (ISDN [digital] telephones cannot be used with fax machines.) 2. Dial 202/219-3644. 3. Listen to the menu options and punch in the number of your selection, using the touch-tone telephone. 4. After completing your selection, press the start button on your fax machine.

Jan 1, 1995

By J. Davitt McAteer

Improvements in the control of coal mine dust have occurred over the past 25 years. Simultaneously, dramatic improvements in coal mine production technology have occurred, particularly at mines where longwall systems are in use. Production rates of longwall systems have quadrupled since 1978, and dust generation on these systems has significantly increased. However, technology to address control of the resulting increased levels of respirable dust has not kept pace, and many high-production longwall sections continue to experience problems with respirable dust control. This paper seeks to enlist the entire mining community in the effort to resolve the health problems presented by excessive levels of respirable coal mine dust, particularly on longwall systems. These systems are expected to increase in number and length, and to generate increasingly higher levels of dust as production rates of these systems continue to rise.

Jan 1, 1995

By D. W. Fuerstenau, P. C. Kapur

A new method is proposed for measuring the grindability of solids and the energy efficiency of grinding mills. The method is based on a similarity law of fracture mechanics for elastic materials in conjunction with the self- preserving character of the size distributions formed when feed particles are comminuted individually in compression. The single-particle grindability index reflects more closely the inherent resistance of a solid to size reduction than the grindability indices currently in vogue. The performance of grinding mills is evaluated in comparison with the single-particle breakage mode, which has the highest energy utilization or efficiency of all mechanical grinding routes. For a given set of stipulations, the performance index of the commonly used ball mill when grinding dolomite is found m he only 21%. compared to 44% for the high-pressure roll mill, a newly-invented energy-efficient mill. The difference is also reflected in the energy consumed per unit mass offines produced when closed-loop grinding circuits are simulated, which shows that the high-pressure roll mill circuit is invariably more energy-efficient than the ball mill circuit, provided the energy input per pass through the mill does not exceed a prescribed limit. Theoretical upper bounds on the yield of the ground product in a desired size range and lower bounds on the energy investment are calculated from the single-particle breakage mode which has, apart from the highest energy utilization efficiency, a minimum dispersion in size of the comminuted particles. Data on theoretical hounds for seven minerals/ores are presented. It is argued that it is unlikely that a grinding mill can be designed and a comminution circuit constructed whose performance exceeds these bench-mark values.

Jan 1, 1995

By A B. Christie

Stream sediment geochemical analyses from 140 mining company exploration surveys of the Coromandel, Northwest Nelson, Marlborough and Westland regions of New Zealand have been compiled from open file reports and entered on a database called REGCHEM (for Regional Exploration GeoCHEMistry). REGCHEM is supported by GEOEXPL, a custom-written computer software application which operates under the Microsoft Access relational database management system on a PC-type computer, The data are plotted as maps using the MapInfo geographic information system (GIS) package. Thresholds separating anomalous values from background values are determined using the PROBPLOT uni-variant statistical package, which enables the discrimination and modelling of several populations. Relevant REGCHEM data are being incorporated into a GIS for the Coromandel region as part of ongoing studies to develop mineral deposit and metallogenic models for the region, and to assess regional prospectivity.

Jan 1, 1995

By J. G. Pippenger

The Coteau Properties Company's Freedom Mine is a surface lignite mining operation located near Beulah, North Dakota. Coteau, a wholly owned subsidiary of The North American Coal Corporation, produces more than 13.5 million tonnes per year, ranking it among the largest mines in North America. In 1993, Coteau ranked seventh in terms of annual coal sales, becoming the largest mine outside the Powder River Basin of northeastern Wyoming. However, for the Freedom Mine to stand among these Powder River heavyweights, Coteau over-came several obstacles, including a coal seam with three quarters the heating value and overburden to coal ratios three times higher than the Powder River mines. Coteau's ability to compete has been built around a program of continuous productivity improvement, cost efficiency, and innovation. This report highlights a few of the major improvements implemented in recent years.

Jan 1, 1995

By Dr. -Ing. Wolfgang Pietsch

Many primary metallurgical wastes are in the sh.-ape of turnings; borings, chips, dusts, and other particuletes. While such. materials can be easily segregated and/or upgraded to yield high quality ?secondary metals with often known composition for easy recirculation into the production? process; their particulate nature results in various problems. Roller presses are used in an ever increasing number for size enlargement and densification of primary particulate metallurgical wastes for secondary metal recycling. The paper will mention the areas of application from A(luminum) to Z(ink), also including Brass; Cast Iron, lead, and Stainless Steel, as well as dusts from all types of melting and ladle furnaces or converters. Typical systems using roller presses for size enlargement. will be discussed.

Jan 1, 1995

By Darrell L. Beddard

The Loma Prieta earthquake occurred on October 17, 1989 at 5:04 PM, Pacific Daylight Time. It measured 7.1 in magnitude and was felt from Los Angeles north to the Oregon state line and east to western Nevada. The epicenter of the earthquake was located along the San Andreas Fault, in the Southern Santa Cruz Mountains just north of the City of Watsonville, California. It was the largest earthquake to occur in the San Francisco Bay area since the 1906 earthquake. A total of 87 aftershocks of Magnitude 3.0 or larger were recorded within 21 days after the seismic event. As a result of this earthquake thirteen State-owned and five locally-owned bridges were closed to traffic. Three of the most publicized State-owned structures closed to traffic were Struve Slough Bridge, San Francisco-Oakland Bay Bridge and Cypress Street Viaduct. Struve Slough Bridge was twin slab bridges that collapsed and were rebuilt and open to traffic three months after the earthquake. San Francisco-Oakland Bay Bridge experienced collapse of upper and lower spans at Pier 9 and was reopened to traffic one month after the earthquake. The Cypress Street Viaduct experienced collapse of 48 bents that resulted in the upper roadway falling onto the lower roadway.

Jan 1, 1995

By M K. de Brodtkorb

Late Cenozoic ore deposits æPrecious and base metal-richÆ and æSn-Ag-polymetallicÆ ofNorthern Puna are associated either with Middle Miocene dacitic volcanic-dome complexes or with Upper Miocene dacitic-andesitic volcanic centres.   Ore mineralogies and their related volcanism are discussed. A strong link is observed between the Cenozoic Metallogenetic Province of Northern Puna and the Southern Bolivia Tin and polymetallic provinces.  

Jan 1, 1995

By R. Hurman Eric

The flow of molten slag and matte and the estimation of the mass of settled matte in electric furnaces is discussed and the action of the electric furnace as a settler is considered. Particle size of the concentrate and its grade are major contributors to settler dimensions. Discussion on the production capacity of the electric furnace with reference to operating current, power input, furnace dimensions such as width and length and electrode diameter are given. These discussions are based on considerations derived mainly from experimentally determined properties of the slag such as its resistivity, viscosity and liquidus temperature. Extension of the design and operating considerations to circular furnace symmetry is also discussed.

Jan 1, 1995

By E Heath, F McDermott, S Turner, P van Calsteren

U-Th isotope studies of recent volcanic rocks may be used to evaluate the depths and rates of melt generation beneath midocean ridges and ocean islands. Island arc rocks exhibit a similar range in Th isotopes to that seen in both MORB and om, but while those with high (230Th/232Th) tend to have excess 238U, many of those with low (230Th/232Th) are in isotope equilibrium. The excess 238U is attributed to the preferential mobilisation of U in hydrous fluids from the subducted slab within the last 100 000 years, whereas the low Th isotope ratios in the higher-K arcs are primarily due to the introduction of subducted sediment. Preliminary results from mineral separates from lavas on Soufriere on the island of St. Vincent indicate that some of the minerals crystallised up to 50 000 years prior to eruption.

Jan 1, 1995

By David A. Newman

Barrier pillars created during mine development provide additional reserves during mine closure and can be the sole access to reserves previously abandoned because of mining height or out of seam dilution limitations. The safe and successful mining of barrier pillars requires a thorough knowledge of the magnitude and distribution of the vertical stress field in addition to the strength and physical properties of the coal, immediate roof, and immediate floor strata. Quantifying the vertical stress field is complicated by the combination of the overburden stress, abutment stresses transferred onto the barrier pillar from adjacent gob areas, and in multiple seam mining, stresses transferred from abandoned subjacent mines. These variables make the extraction and development of barrier pillar reserves a site specific consideration. Three case histories are presented in which barrier pillar development and extraction provides; i) an access point and the means to ventilate a large area of low seam reserves left by a prior operator after developing only the thicker areas of a multiple split reserve, ii) the means to increase total recovery of a deep, thick seam longwall reserve, and iii) opportunities for the development and retreat mining of a two mile long barrier pillar in thick seam reserves.

Jan 1, 1995

The geological features of the destructive plate margins of the southwest Pacific favour the formation of porphyry type deposits. The major controlling factor favouring the formation of gold-rich deposits of this type is a dense crust. This results in shallow intrusion of large magma bodies beneath andesite stratovolcanoes. Where the intrusives have near vertical sides, the comparatively low temperature of transfer of volatiles from the solidifying melt favours the partitioning of gold into bubbles of volatiles. They reduce the viscosity and density of the melt, resulting in the upward movement of a mineralised melt. With the drop of pressure with upward movement the bubbles separate into a two phase mixture, triggering freezing of the mineralised melt and the formation of a gold mineralised stockwork.

Jan 1, 1995

By K. Kim

A site characterization strategy formulated for the probabilistic design of underground structures is presented, which takes into account the variability of rock properties and the stochastic nature of the rock failure process. It entails determining probability distributions of rock properties by utilizing the theory of extremes and Monte Carlo simulation. Laboratory and field test data are analyzed and presented to illustrate the methodology. The results indicate that the distributive characteristics of rock mass properties can be estimated using empirical relationships currently available. This results of analyses can be incorporated into the design of underground structures, and then, the relative uncertainties of the failure probability of the structures can be assessed.

Jan 1, 1995

By R. Wensel

A composite material consists of two or more dissimilar components. These dissimilar components retain their identities when mixed together and contribute to the properties of the composite. A composite usually consists of two parts, the matrix and the reinforcing material, or extender that is added to the matrix. Typical materials used as the matrix in composites are metals, polymers, and ceramics. The reinforcing material can be added for any number of reasons. For instance, if the matrix is expensive, an inexpensive filler may be added to the matrix to lower the cost of the product without sacrificing properties. An example of this is calcite (CaC03) which is added to systems such as polyvinyl chloride (PVC), to lower the cost. Reinforcements may also serve as a means to enhance the properties of the matrix material. For example, carbon fibers are frequently added to plastics to increase their strength, and glass spheres may be added to plastics to improve flexural strength. Mg(OH)z is added to polymers to give the property of fire retardancy. The latter is the focus of this paper.

Jan 1, 1995