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By Gary Raymond

The Similkameen Division of Newmont Mines Limited operates a low-grade open-pit copper mine in southern British Columbia. The copper mineralization is extremely erratic.As a result, over seven years of operation, ore control methods have evolved from standard flagging based on blast-hole assays, through an in-pit grab sampling system, to the present method of mining to computer-determined ore-waste boundaries based on geostatistics. These improved ore control methods have resulted in a gain of at least 25% on ore tonnage miffed, at the same grade as obtained using earlier techniques. At the same time, geostatistical ore reserve estimates are now within 7% of milled ore tonnage and grade on a yearly basis, whereas the original polygon ore reserves under-estimated mineable ore tonnage by 15% to 45% and over-estimated grade by about 20%. The author discusses why traditional ore estimation methods give poor results in erratically mineralized orebodies and describes the geostatistical approach to the problem.

Jan 1, 1979

By Antoine Fournier, Karen St. Seymour

"Responding to market developments, interest in locating deposits of wollastonite has grown significantly. The many occurrences in the Grenville marbles of Quebec can be grouped into disseminated type and discordant vein-type deposits. Disseminated-type wollastonite occurrences have the disadvantages of lower grade, the need for more elaborate mineral separation, and failure to meet ""long grain"" technical specifications. Vein-type deposits have better grades and technical characteristics; exploration should be focussed on these and in the contact-metamorphic aureoles of fluid-rich intrusions. SOMMAIREEn réactions aux nouveaux de bouches, l'interet pour les gîtes de wollastonite a crû de façon marquée. Les nombreux indices sis dans la province de Grenville au Québec peuvent être regroupes en deux catégories soit: les indices dissémines et les indices en veines discordantes. Les indices dissémines ont le désavantage d'être moins enrichis, de requérir une séparation minéralogique plus complexe, et leur wollastonite ne possède pas le degré d'acicularité voulu. Les indices en veines sont plus enrichis en plus de rencontrer les critêres techniques; les efforts d'exploration devraient se porter vers ce type de gites et aussi, sur les auréoles métamorphiques de contact d’intrusions riches en fluides."

Jan 1, 1992

By Derek E. McBride

"Exploration work in the Beardmore - Tashota area has led to a reinterpretation of relationships between gold mineralization and the geologicalevolution ofthe area. The host volcanic rocks form two parallel volcanic belts connected by a much smaller cross-cutting band, which trends northeast from Beardmore to Tashota. This connecting band consists of mafic and felsic volcanic, plus clastic sedimentary rocks in a sequence conformable with the contact of the Onaman Lake Batholith to the east. A metamorphic aureole or incorporated xenoliths are lacking along this major contact and the only intrusive relationship observed is chilling between mafic dikes and a cold host granite in proximity to it. Granite dikes are totally lacking in the volcanic succession, or the overlying sedimentary sequence. A major part of this sequence consists of a polymict basal conglomerate with granite boulders and volcanic clasts in a fine-grained chloritic matrix. The nearest source for these boulders is the Onaman Lake batholith.Gold mineralization in significant economic quantities (+ 50 000 ounces) occurs at the top or above the mafic volcanic succession as quartz bands. In the Northern Empire Mine, the band is conformable, folded, boudinaged and the mineral zoning shows a relationship to variations in the host rock, not structural overprinting. In general, where the host mafic tuff is medium-grained, the quartz band is thicker, up to a metre wide, and carries economic grade; where the tuff is finer-grained and bedded, the band is very thin and low grade.Stratigraphic control and metal zoning shows that the mineralization formed as part of the development of the volcano- sedimentary succession. The other major deposits in the area (Leitch and Sturgeon River) have a similar structural history suggesting that they are also syngenetic in terms of the stratigraphic development. Regionally, these deposits were emplaced prior to any significant tectonic deformation but at the end of the mafic volcanic event.The mafic activity was related 10 a period ofextension which caused the volcanic troughs to develop along centres of rifting. The hiatus at the end of this event permitted extensive hydrothermal systems to form and deposit siliceous exhalite bands within sedimentary or terminal mafic volcanic environments in promixity to major thermal sources. Subsequently, the Kenoran Orogeny deformed the entire succession into upright folds with a sub-vertical schistosity. Evidence shows that the deposits have remained intact since deposition."

Jan 1, 1987

By Victor Maksaev, Marcos Zentilli

"Abstract - The Domeyko Cordillera, the westernmost uplifted crustal block of the composite High Andes of northern Chile (20°S to 28°S) hosts a narrow N-S trending belt of Late Eocene-Oligocene giant porphyry copper deposits, which include Collahuasi, Chuquicamata, El Abra, La Escondida, and El Salvador. These deposits are spatially and genetically associated with the closing igneous activity along this range, prior to a 30 km eastward jump of the magmatic front in response to tectonic plate interaction. The porphyry copper deposits are also spatially associated with a major intraarc strike-slip shear system, the Domeyko Fault System. Although the tectonic uplift of the 3000 m to 5000 m high range has generally been assumed to be mostly Miocene in age, field relationships suggest that the Domeyko Fault System and tectonic uplift were active as early as the Eocene, coinciding with porphyry copper emplacement between 41 Ma and 30 Ma. Apatite fission track (FT) thermochronology provides both age data and a time-temperature history for rocks since they cooled below a temperature of ca. 125°C (equivalent to a depth of 4 km to 5 km under normal geothermal gradients) on their way to the surface during exhumation, or after a heating event. Apatite FT data from the Paleozoic crystalline basement of the Domeyko Cordillera indicate that at least 4 km to 5 km of rocks were eroded during exhumation of this tectonic block between ca. 50 Ma to 30 Ma (Middle Eocene to Early Oligocene), a time that immediately precedes and overlaps with the emplacement of giant porphyry copper deposits. The FT data constrain the age and duration of a period of crustal thickening and extensive erosion known as the Incaic compression, an event recognized in the Andes of Chile and Peru. Assuming that cooling was due to denudation alone, modelling of the FT data allow estimation of denudation rates between 200 and 100 m/My during this period. In contrast, exposures of pre-porphyry Paleocene intrusives on the western edge of the Domeyko Cordillera reveal apatite FT ages that are concordant with biotite 40Ar-39Ar dates, indicating shallow emplacement, fast cooling and negligible exhumation. The apatite FT ages of the Chuquicamata and El Abra porphyry copper deposits are only marginally younger than their 40Ar-39Ar dates, implying fast cooling/exhumation of shallow mineralizing systems (ca. 2 km to 3 km). Their FT time-temperature history is compatible with extremely low rates of exhumation (ca. 50 m/My) since about 30 Ma (Early Oligocene). This limited extent of erosion, due to exceptional aridity, has greatly contributed to the preservation of rich supergene enriched blankets above some of the porphyry copper deposits.A model is proposed, which is consistent with field relationships, geochemical, and geochronological data. High relative velocities and intermediate-angle oblique convergence between the Nazca and South American plates in the Eocene-Early Oligocene led to intra-arc transpression partitioned into reverse faults of opposing vergence and a right-lateral shear system (the Domeyko Shear System) localized along the magmatic front. Crustal shortening and thickening led to uplift within the Domeyko Fault System and to erosion of at least 4 km to 5 km. Crustal thickening deepened the zone of magma generation, and magmas accumulated in the lower crust, allowing small volumes to rise rapidly to shallow levels along transtensional domains of the regional shear system. A similar model of crustal thickening and shortening before and during porphyry copper emplacement in zones of transtension may also apply to the Miocene-Pliocene giant porphyry copper province of central Chile."

Jan 1, 1999

By J. J. Emery

There is an increasing awareness on an international level of the potential for utilizing many wastes and by-products as construction materials. This has resulted in recent major reviews by OECD and RILEM to supplement national activities by agencies such as CANMET, USBM, BRE and LCPC. Much of this international and national work was reported during an International Conference on the use of by-products and wastes in civil engineering held in November 1978 in Paris. Based on participation in the OECD and RILEM reviews, and the Paris conference, developments in waste and byproduct utilization of particular concern to Canada are described. The major technical and economic constraints involved are also reviewed, so that the over-all evaluation reflects important factors such as inherent variability and transport logistics. Areas requiring further research and development are also identified. Particular emphasis is placed on slags, salvaged asphaltic concrete and surplus sulphur. "Due to the rapid development of China's industry, more than a hundred million tons of industrial wastes are generated annually, which occupy lots of land for storing, bring about environmental pollution, obstruct industrial and agricultural production and cause harm to people 's living. Therefore we must utilize them effectively to create wealth for our country. "

Jan 1, 1979

By Nural Kuyucak, Graham A. Johnson

Noranda's Geco Division is presently developing a comprehensive reclamation plan for closure of its Manitouwadge, Ontario operation, expected late in 1995. As part of a program of progressive rehabilation, the existing 20 year old water treatment plant will be upgraded from a low density to a high density lime neutralization process treating AMD and excess site water. The process, developed by the Noranda Technology Centre, utilizes a two step rather than a single step process to bring the Ph to the required neutralization level, a variation which contributes to improved sludge properties such as higher density and decreased viscosity. This is of particular significance where sludge pumping or storage volumes may present a concerns.

Jan 1, 1995

By R. A. Sproule

Abitibi Greenstone Belt ? ~2.745 to 2.680 Ga (U-Pb zircon ages) ? Several lithologically-distinct and structurally repeated(?) assemblages ? Represents: ? Allochthonous terranes? ? Single autochthonous terrane? ? Combination of the above Volcanology of Komatiites in the Abitibi greenstone belt ? Tisdale and Kidd-Munro assemblages have channelized flows ? Pacaud and Stoughton-Roquemaure assemblages DO NOT have channelized flows ? Only Tisdale and Kidd-Munro assemblages are mineralized

May 1, 2003

Building off the enthusiasm generated by Credit Suisse First Boston on the potential benefits of an industry-wide effort to market gold jewellery, a group of gold producers, led by Randall Oliphant, CEO of Barrick Gold, decides to investigate the long -term impact of launching a significant marketing initiative to stimulate demand for gold jewellery. The group establishes an international Steering Committee representing producers of differing sizes, members and non-members of the World Gold Council. This Committee comprises representatives from AngloGold, Barrick, Gympie Gold, Kinross, Randgold, and Sons of Gwalia and is supported by the Australian Gold Council, the Gold and Silver Institute, and CSFB. The Steering Committee hires McKinsey to assist it by conducting an independent review of whether investment in a large-scale marketing initiative for gold jewellery would be effective in increasing demand for gold, and ultimately of significant economic benefit of the gold industry. Based on McKinsey?s review* and its own discussions with key stakeholders across the industry, the Steering Committee strongly believes the case for marketing is compelling and sufficiently proven to persuade the industry to undertake a major initiative over the next few years. The Steering Committee, supported by McKinsey, conducts briefings with a range of gold producers to share its findings and to understand producers? perspectives on an industry-funded marketing campaign. The Steering Committee is asking for your support to bring this industry-funded marketing campaign to life.

May 1, 2002

By B. R. Neufeld, M. G. Heximer, S. J. Roach, W. G. Van Luven, D. G. Reynolds

"Teck Metals Ltd. operates an integrated zinc-lead complex at Trail, British Columbia, Canada. In the zinc plant, zinc concentrates are roasted to produce calcine, which is then processed in the leaching circuit for zinc recovery. Calcine is first dissolved at pH <2.0. This is followed by a precipitation stage using additional calcine that removes iron as a ferric hydroxide precipitate which is recycled to the lead operations. It is during this pH adjustment that impurities, notably arsenic, antimony and germanium, are co-precipitated with ferric hydroxide which is the primary removal phase for these elements. Recent processing conditions have resulted in lower iron inputs to the leaching circuit and this has resulted in insufficient co-precipitation of key impurities. This paper summarizes the influence of iron concentrations on impurity removal using laboratory and plant scale surveys, and emphasizes key specification limits for the primary iron removal circuit.INTRODUCTIONThe Sulphide Leach Plant (SLP) at Teck Metals Ltd., Trail Operations produces a zinc sulphate electrolyte from three different zinc bearing streams:1. Calcine, primarily containing zinc oxide, from the roasters2. Zinc electrolyte from the Oxide Leach Plant (OLP) is produced by leaching zinc oxide fume from the pyrometallurgical lead smelting operations3. Zinc slurry from the Zinc Pressure Leach (ZPL) autoclaveThe feed sources are blended and introduced to the iron removal process to precipitate iron in the form of a hydrolyzed iron precipitate which is recycled to the lead smelting operation. Along with iron, impurities, namely arsenic, antimony, germanium and silica, also co-precipitate; this process acts as the primary removal step for these elements. Following iron removal, the electrolyte is further processed in the zinc dust purification circuits. Cadmium and copper are removed in the cold stage circuit via zinc dust cementation, followed by cobalt cementation in the hot stage circuit."

Jan 1, 2016

By H. M. Roscoe

Fifty per cent. increased lump coal production and at the same time a reduction of from 20 to 40 per cent. in explosive consumption. These results were achieved by the use of the rock-dust cartridge method of stemming shots in the Ruhr coal mines, as announced by Dr. Alfred Stettbacher in the September 1923 number of the Explosive Engineer. Immediately, the explosive and the coal-mining industries on this continent became intensely interested. Considerable experimental work has been carried on in a number of American and Canadian mines during the past year, by which our engineers have become assured that the use of rock-dust cart-ridges for stemming shots increases the efficiency of explosives and promotes safety in coal mines.

Jan 1, 1924

By R. Erpelding

High Pressure Grinding Rolls (HPGR) are said to be a good option in new grinding circuits. Due to their low energy consumption compared to other grinding systems such as SAG milling, there is a strong incentive to check their behaviour for certain ore types. With rising energy prices the operation will become even more beneficial in future. As published in the past, the HPGR is claimed to increase the liberation of the valuable mineral component within an ore. An investigation at the research centre of Polysius was conducted to substantiate the liberation behaviour in finish grinding operations. A pilot scale HPGR test unit (POLYCOM MAGRO) with a roll diameter of 950 mm and processing capacity of 80 t/h was used. Grinding tests were completed with a screen and a two stage drying air separation process (SEPOL). Analysis included standard screen analysis, fractional XRF and modern MLA technology. A technique was developed to oppose the amount of produced slimes (resuming bad flotation) against the positive effect of producing high mass fraction at flotation-easy sizes. The paper shows how effective the use of a dry grinding separation could be in terms of production of mass of liberated metal.

Jan 1, 2012

By H. Lorenzen, D. Buyze

"Magnesium-bearing salts occur in the Zechstein Basin in the north eastern part of the Netherlands at a depth of 1500m to 2500 m.Exploration results justified the exploitation of these salts by means of solution mining for the production of refractory grade magnesium- oxide. This paper discusses design criteria, operation parameters, and methods used for magnesium chloride solution mining by Noordelijke Zoutwinning B. V. (NOZO), an operating company of Billiton International Metals B. V.A unique variation of widely used solution mining techniques has been developed for mining the complex Veendam salt deposit. NOZO extracts salt through deviated wells with a set of parallel, suspended tubings. Cavern fluid pressure is maintained close to that of overburden rock. These special design features provide an answer for the complex characteristics of the minerals, halite, carnallite, bischofite and kieserite, from which a solution rich in magnesium is generated. Dissolution chemistry and rock mechanical behaviour of these salts are unlike those of rock salt. Throughout a few years, NOZO demonstrated adequate process control while they continue improving the operation.IntroductionBilliton, a wholly owned company of the Royal Dutch /Shell Group of Companies, is active in metal exploration, mining, processing, and trading, mainly in the non-ferrous sector.Since 1972 Billiton has been active with solution mining magnesium salts in the northeastern part of the Netherlands, in the Province of Groningen, near the city of Veendam . Solution mining techniques were developed in conjunction with the ""Koninklijke/Shell Laboratorium Amsterdam"" (KSLA) and the ""Koninklijke/Shell Exploration and Production Laboratorium"" (KSEPL), Rijswijk. The company&apos;s objective is to produce magnesia sinter (MgO) from magnesium-chloride brine (MgClz) and de-carbonated dolomite (CaO.MgO). The magnesia produced serves as base material for the manufacturing of refractory bricks, used for furnace linings in the steel industry."

Jan 1, 1986

By C. R. van Staal, L. R. Fyffe, J. A. Winchester

"The geochemical characteristics of Early Paleozoic volcanic rocks are used to establish the following tectono-volcanic regimes in the northeastern mainland Appalachians: foreland continental rift volcanics; fore-arc ophiolites; island-arc volcanics; back-arc ophiolites; and back-arc ensialic volcanics. A Late Precambrian calc-alkaline magmatic arc bounds the Early Paleozoic volcanics to the southeast.The Precambrian calc-alkaline volcanics of the Caledonian Highlands, primitive Cambro-Ordovician oceanic-arc volcanics of the Eastern Townships, and within-plate Ordovician volcanics of the Miramichi Highlands can be related to a progressive change in mode of subduction from compressional Andean-type in the Late Proterozoic to extensional Western Pacific-type in the Early Paleozoic. The development of marginal oceanic basins in the Paleozoic created a more favourable environment for volcanogenic sulphide deposition, particularly in back-arc areas.The influence of tectonic setting is reflected in the distribution, size and metal ratios of the massive sulphide deposits associated with the volcanic rocks; deposits within intra-oceanic arcs in the Eastern Townships of Quebec average two million tonnes grading 2.2% Cu, 4.0% Zn, and 0.6% Pb, whereas those within a back-arc, ensialic rift near Bathurst, New Brunswick, average nine million tonnes grading 0.6% Cu, 5.4% Zn, and 2.2% Pb."

Jan 1, 1990

By J. Dubinski

Poland is the ninth largest coal producer in the world and Coal Company is the largest Polish producer of this deposit yielding 51% of national production. Coal Company mines are located in the Upper Silesian Coal Basin. These are underground mines which operate in difficult geological conditions at great depths of up to 1200 m. One of the most common natural hazards in Polish mines are rockburst and seismic hazard impacting on surface infrastructure and habitants. In the assessment of the rockburst and seismic hazard methods of mining seismology play a leading role. This paper presents progress that was achieved in the last few years in the implementation of new seismic monitoring systems and seismic interpretation methods for the prevention of hazard deriving from seismicity induced by mining. This progress in assessment of rockburst hazard and impact of seismic activity on the surface infrastructure was achieved through close cooperation between the Coal Company and Central Mining Institute ? the country?s leading scientific institute in the field of mining seismology. New theoretical solutions are in the process of testing in mines of Coal Company where seismic monitoring systems are installed and controlled by well-qualified personnel. Some directions of further joint actions to increase the effectiveness of rockburst and seismic hazard assessment are presented as well.

Aug 1, 2013

By P. J. Deveaux, M. J. Kreczmer

The Tartan Lake deposit 12 km northeast of Flin Flon, Manitoba, was discovered in March of 1984 during the investigation of a VLF anomaly. The discovery hole intersected 16.83 metres averaging 21 gm Au/tonne. Since then, 99 holes, over 21,000 metres, have been drilled outlining 419,491 tonnes grading 11.79 gm Au/tonne in a di stinct group of mineralized lenses referred to as the Main Zone. Three other mineralized zones are known on the property but no reserves have been allowed for these, the South , East and Base Line zones. The Main Zone consists of seven parallel, east-west striking, quartz lenses. The lenses dip steeply to the north and plunge steeply west to northwest. Lens number l strikes for 160m and extends vertically to the 90m level. Within th is lens there is a 60m interval of low grade materi al. Lens number 2 is located about 5-l0m within the foo twall of Lens number I. Near surface this lens stri kes for 40m, at 200m level the strike length increases to 200 m. To date the deepest mineralization on Lens 2 has been intersected at a vert ical depth of 527m. Five smaller lenses are located in the foo twall of Lens 2 , containing gold va lues similar to the two main lenses but erratically distributed. The South Zone, located 100m south of the Main Zone, strikes east-west fo r l00m above the 100m level. The East Zone, found 230m east of the Main Zone, strikes fo r 80m and may possibly be an ex tension of the South Zone. The Base Line Zone is located 275m south of the Main Zone. Sporadic, but very significant gold intersections have been obta ined1n all three zones. Gold-bearing quartz-ca rbonate tourmaline veins are generally confined to a silicous crystal tu ff hori zon. The crystal tuff exhi bits fi ve distinct types of alteration: silicification, sericitization, ca rbonati zation, chlorit ization and pyritization. Silicification has resulted in a perasive addition of silica and the generation of multiple quartz veins. Sericiti zation is restricted to the crystal tuff. All rock units are pervasively carbonatized . Ankerite is abundant in the quartz veins and within the shear zones, particul arly those located along the chilled margin of a nearby gabbro-diorite intrusion. Chloritization is widely spread th roughout the rock units, while pyritization fo rms a broad alteration halo associated with the orebody. Sulphide accesso ry minerals associated with the gold , whi ch fo r the most part appears to be free, are chalcopyrite and pyrite. Generally, the higher grade gold values are related to an increased content of sulphides in the qua rtz veins. The average sulphide content in the deposit is less than 5 %.

Jan 1, 1986

By H. A. Wentworth

The Central Manitoba mining district, as spoken of in Manitoba, refers to that portion of the country lying between lake Winnipeg on the west and the Ontario boundary on the east, and from a few miles north of the Wanipigow (or Hole) river on the north, to a few miles south of the Bird river. It embraces two known mineralized belts; the gold area in the north, extending from the shores of lake Winnipeg almost to the Ontario boundary (in a slightly southeasterly direction) a zone some eight or ten miles wide and perhaps forty miles long, lying (for the most part) north, and parallel to, the Manigotagan river; and the Bird river copper and coppernickel deposits, an area not yet clearly defined, but in general lying immediately north of, and roughly parallel to, the Bird river. To the south of the Bird river is a somewhat differently mineralized area, numerous pegmatite dykes being found, with minerals typically associated with such rocks. More specifically, the known gold deposits are largely segregated in the above belt; (1) around English brook; (2) along the north shore of Wanipigow lake; the Rice lake area (about midway of the belt); and the Long Lake-Bulldog lake area, at the eastern end of the belt. Not all the known gold occurences are found in these segregations, but the density of claim staking in these areas is perhaps an indication of the greater number of prospects found in them to date.

Jan 1, 1925

Innovation ? In economics a change must increase overall value, customer value, or producer value. ? Innovations are intended to make someone better off, and the succession of many innovations adds growth to the whole economy. ? Innovation may refer to both radical and incremental changes to either products, processes or services.

Feb 1, 2008

By P. Henry

Hydrometal provides an adequate technology to reintroduce lost metal units into the industrial cycle, thereby saving resources and energy, while keeping the environment cleaner. Two case studies of our integrated hydrometallurgical recycling facility (Hydrométal) in Belgium are discussed.(I) Recycling of Cu-Co cement generated at the second purification step of zinc sulfate solution. (II) Recycling of lead complex materials from the lead industry.

Jan 1, 2008

By Ainul Akhtar, Steve Cockcroft, Daan Maijer, Carl Reilly, Lu Yao, Jun Ou

A coupled thermal-fluid flow-multicomponent model has been developed to aid in understanding the melting/dissolution behavior of solid Al-rich Material (TiAl) in a liquid titanium pool produced by an electron beam. The model is capable of accounting for the significant phenomena present affecting heat, mass and momentum transport including thermal and compositional buoyancy and Marangoni forces, Darcy attenuation, evaporation, melting and solidification, and the associated release or absorption of latent heat. The modeling results have indicated the compositional fluid flow driving forces (compositional buoyancy and Marangoni forces) contribute significantly to the melting/dissolution process. The model is waiting for validation against empirical data.

Jan 1, 2014

By Peter A. Bevan

"In massive and semi-massive deposits, it is essential to take samples continuously for specific gravity determination as well as for assays of the various metals of interest. Failure to use specific gravity in mixed deposits of high density contrast will result in incorrect determinations of the average grade.Graphs and linear equations of specific gravity against the grade of one metal are not considered accurate enough in a multi-mineral deposit, but appear to be very satisfactory in a theoretical one-sulphide/gangue mix. Specific gravities based on base-metal values only in massive sulphide deposits will result in underestimating the effect of low grade areas on the average grade and also cause a reduction in the calculated tonnage.IntroductionBy definition, a base-metal assay is the percentage of metal by weight contained in a sample, whether it be a soil or silt sample, a grab, chip or channel sample, or a section of diamond drill core.In those cases where there is a series of contiguous samples, say in diamond-drill core, the average grade may be required of the various assays that are above a desired or required cutoff grade.Internal low grade or waste can be included if the assays one side or the other are used to ""carry"" the lower grade material and the resulting average grade is above the cut-off grade. To determine the average grade, the individual assays are ""weighted"" by multiplying these values by their respective lengths, adding up the cumulative assay times length and dividing by the total length. This method is essentially a weighting by volume, because the core diameter is constant and is common to all the samples being considered for estimating the average grade . If some of the core samples have much higher specific gravities than others, and the specific gravities are not used in the weighting process, the resultant average grade will be over-estimated or underestimated, depending on the grade of the heavier sections. When core assays of approximately the same specific gravity or weight are averaged by length, the error in the weighted estimation of the average grade is negligible."

Jan 1, 1993