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I I I I I I I I I I Ore Geology and Industrial Minerals An Introduction GEOSCIENCE TEXTS SERIES EDITOR A. HALLAM Lapworth Professor of Geology University of Birmingham Engineering Geology F.C. BEAVIS Ore Geology and Industrial Minerals: An Introduction A.M. EVANS An Introduction to Geophysical Expl...

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I I I I I I I I I I

Ore Geology and Industrial Minerals An Introduction

GEOSCIENCE TEXTS SERIES EDITOR

A. HALLAM Lapworth Professor of Geology University of Birmingham

Engineering Geology F.C. BEAVIS

Ore Geology and Industrial Minerals: An Introduction A.M. EVANS

An Introduction to Geophysical Exploration P. KEAREY AND M. BROOKS

Principles of Mineral Behaviour A. PUTNIS AND J.D.C. MCCONNELL

The Continental Crust S.R. TAYLOR AND S.M. MCLENNAN

Sedimentary Petrology: an Introduction M.E. TUCKER

GEOSCIENCE TEXTS

Ore Geology and Industrial Minerals An Introduction ANTHONY M. EVANS BSc, PhD, MIMM, FGS Fonnerly Senior Lecturer in Mining Geology University of Leicester

THIRD EDITION

A Blackwell II Publishing

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© 1980,1987,1993 by Blackwell Science Ltd a Blackwell Publishing company BLACKWELL PUBLISHING

350 Main Street, Malden, MA 02148-5020, USA 9600 Garsington Road, Oxford OX4 2DQ UK 550 Swanston Street, Carlton, Victoria 3053, Australia The right of the Author to be identified as the Author of this Work has been asserted in accordance with the UK Copyright, Designs, and Patents Act 1988. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by the UK Copyright, Designs, and Patents Act 1988, \vithout the prior permission of the publisher. First published 1980 under the title An Introduction to Ore Geology Chinese edition 1985 Second edition 1987 German edition 1992 Malaysian edition 1989 Japanese edition 1989 Third edition 1993 15

2009

Library of Congress Calaloging-in-PulJlication Data Evans, Anthony M. Ore geology and industrial minerals/Anthony M. Evans, - 3rd ed. p. em. - (Geoscience texts) Rev. ed. of: An Introduction to ore geology, 2nd ed. 1987. Includes index. ISBN 978-0-632-02953-2 1. Ore deposits. 2. Industrial minerals. 1. Evans, Anthony M. Introduction to ore geology. II. Title III. Series. QE390.E92 1993 553'.1 - 30 kt), has led to a trend towards the large scale mining of low grade orebodies. As far as shape is concerned, orebodies of regular shape can generally be mined more cheaply than those of irregular shape particularly when they include barren zones. For an open pit mine the shape and attitude of the orebody will also determine how much waste has to be removed during mining, which is quoted as the waste-to-ore or stripping ratio. The waste will often include not only overburden (waste rock above the orebody) but waste rock around and in the orebody, which has to be cut back to maintain a safe overall slope to the sides of the pit.

As can be seen from Fig. 1.15, a time comes during exploitation when the waste-to-ore ratio becomes too high for profitable working; for low grade ores this is around 2 : 1 and the mine then must be abandoned or converted into an underground operation. Many small mines start as small, cheaply worked open pits in supergene-enriched ore (Chapter 19), and then develop into underground operations (Fig. 1.16). Haulage always used to be by narrow gauge, electrically operated railways, but now, if the orebody size permits, rubber-tyred equipment is used to produce larger tonnages more economically and shafts are then gentle spiral declines up which ore can be hauled out by diesel trucks (trackless mining). Various factors limit the depth to which under-

Fig. 1.16 Mining terminology. Ore was first mined at the outcrop from an open pit; then an adit was driven into the hillside to intersect and mme the ore at a lower level. An inclined shaft was sunk later to mine at even deeper levels and, eventual1y, a vertical shaft was sunk to serve operations to two orebodies more efficiently. Ore is mined by driving two haulage drifts at different levels and connecting them by raises which are then connected by sublevels. Ore is mined upwards from the lower sublevel to form a stope. Broken ore can be left in the stope to form a working platform and to support its walls (shrinkage stoping), or withdrawn and waste from the mill pumped in (cut-and-fill stoping). Ore between haulage and sublevel is left as supporting pillars until the level is abandoned. A shaft pillar is also left unmined. (After Barnes, 1988, Ores and Minerals, Open University Press, with permission).

MINERAL ECONOMICS

ground mining can penetrate and the present record (1989), about 3810 m below surface, is held by the Western Deep Levels Mine, RSA.

Ore character A loose unconsolidated beach sand deposit can be mined cheaply by dredging and does not require crushing. Hard compact ore must be drilled, blasted and crushed. In hard-rock mining operations a related aspect is the strength of the country rocks. If these are badly sheared or fractured they will be weak and require roof supports in underground working, and in open pitting a gentler slope to the pit sides will be required, which in tum will affect the waste-to-ore ratio adversely.

Cost ofcapital Big mining operations have now reached the stage, thanks to inflation, where they require enormous initial capital investments. For example, to develop the 450+ Mt eu-U-Au Roxby Downs Project in South Australia, Western Mining Corporation and British Petroleum have estimated that a capital investment of A$1200 million will be necessary, and for the 77 Mt Ag-Pb-Zn deposit of Red Dog, in northern Alaska, US$300-500 million will be required; grades there are 17% Zn, 5% Pb, 61. 7 g t- 1 Ag. This means that the stage has been reached where few companies can afford to develop a mine with their own financial resources. They must borrow the capital from banks and elsewhere, capital

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which has to be repaid with interest. Thus the revenue from the mining operation must cover the running costs, the payment of taxes, royalties, the repayment of capital plus interest on it, and provide a profit to shareholders who have risked their capital to set up or invest in the company (Fig. 1.13). The order of magnitude of capital costs for industrial mineral operations in the USA is given in Table 1.8. The models quoted in the table represent shallow underground mining in competent rock and opencast mines in hard rock with moderate stripping ratios and short to medium haulage distances. They are thus typical for a variety of industrial mineral operations. The investment for small- scale sand and gravel operations will be much lower and, by contrast, investment costs for industrial minerals produced on a large scale, such as bauxite, phosphate and soda ash, will be several hundred million US dollars. The cost of infrastructural installations discussed in the next section are not included in the above table.

Location Geographical factors may determine whether or not an orebody is economically viable. In a remote location there may be no electric power supply, roads, railways, houses, schools, hospitals, etc. All or some ofthese infrastructural elements will have to be built, the cost of transporting the mine product to its markets may be very high and wages will have to be high to attract skilled workers.

Table 1.8 Order of magnitude capital costs for model mechanized mines extracting industrial minerals in the USA. (Most data from Noetstaller, 1988) Capital cost range (10 6 1984 US$)

Production capacity (t p.d.)

Type of operation

Undergroud 100

Adit access or shallow shaft, shrinkage stope

Mining operation 2.0-4.0

Model flotation mill 2.5-3.2

1000

Adit access or shallow shaft, cut and fill stoping

10.0-12.0

9.7-10.7

5000

Adit access, room and pillar mining

18.0-20.0

25.2-27.2

4.0-5.0

6.5-7.5

Stripping ratio I: 1 to 2: 1, 750 m hauls, hard rock

9.0-12.0

25.2-27.2

Stripping ratio 1 : 1 to 2 : 1, 2000 m hauls, hard rock

16.0-22.0

41.0-43.5

Open pit 500 5000 10 000

Stripping ratio 1 : 1 to 2 : 1, 400 m hauls, hard rock

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CHAPTER 1

Environmental considerations New mines bring prosperity to the areas in which they are established but they are bound to have an environmental impact. The new mine at NevesCorvo in southern Portugal will raise that country's copper output by 93000% and tin production by 9900%! The total labour force will be about 900. When it is remembered that one mine job creates about three indirect jobs in the community in service and construction industries, the impact clearly is considerable. Impacts of this and even much smaller size have led to conflicts over land use and opposition to the exploitation of mineral deposits by environmentalists, particularly in the more populous of the developed countries. The resolution of such conflicts may involve the payment of compensation and the eventual cost of rehabilitating mined out areas, or the abandonment of projects; '... whilst political risk has been cited as a barrier to investment in some countries, environmental risk is as much of a barrier, if not a greater in others.' (Select Committee 1982). Opposition by environmentalists to exploration and mining was partially responsible for the abandonment of a major copper mining project in the UK in 1973. In its report in 1987, Our Common Future, the United Nations World Commission on Environment and Development, headed by Mrs Brundtland, Norway's Prime Minister, pointed out that the world manufactures seven times more goods today than it did in 1950. The Commission proposed 'sustainable development', a marriage of economy and ecology, as the only practical solution, i.e. growth without damage to the environment. White (1989) quoted James Stevenson of RTZ Corporation as admitting that sustainable growth is an awkward concept for the extractive industry. 'How does mining fit in? How can you regard a copper mine as a sustainable development?' remembering that all mines have a finite life, some of 20 years or even less. White wrote that mine operators must now take a longer term view of their operations. Feasibility studies must look at the closure costs as well as the opening and running costs. The running and closure costs must put something back into the community. The question 'What will be left behind, in physical and human tcrmsT must be faccd squarely and adequately responded to. A number of mining companies are already engaged in environmental impact analyses, but for many companies 'the idea of predicting the effects ofclosure twenty to

forty years ahead is still fairly novel'. However, much thought has been given to the matter and a useful reference on environmental protection during mining operations is Arndt & Luttig (1987); whilst Smith (1989) gives a good summary ofthe legislative controls in a number of developed and developing countries. Noetstaller (1988) has pointed out that whereas industrial mineral operations have the same general environmental impacts on land and ground water disturbance as metalliferous or coal mining, the impact is generally less marked since the mines are usually smaller and shallower, and normally less waste is produced because in most cases ore grades are higher than in metal mining. Pollution hazards owing to heavy metals or acid waters are low or non-existent and atmospheric pollution, caused by the burning of coal or thc smelting of metallic ores, is much less serious or absent. The excavations created by industrial mineral operations are often close to conurbations, in which case these holes in the ground may be of great value as landfill sites for city waste. A British brick company recently sold such a site for £30 million!

Taxation Greedy governments may demand so much tax that mining companies cannot make a reasonable profit. On the other hand, some governments have encouraged mineral development with taxation incentives, such as a waiver on tax during the early years of a mining operation. This proved to be a great attraction to mining companies in the Irish Republic in the 1960s and brought considerable economic gains to that country. When a company only operates one mine, then it is particularly true that dividends to shareholders should represent in part a return of capital, for once an orebody is under exploitation it has become a wasting asset and one day there will be no ore, no mine and no further cash flow. The company will be wound up and its shares will have no value. In other words, all mines have a limited life and for this reason should not be taxed in the same manner as other commercial undertakings. When this fact is allowed for in the taxation structure of a country, it can be seen to be an important incentive to investment in mining in that country.

Political/actors Many large mining houses will not now invest in

MINERAL ECONOMICS

politically unstable countries. Fear of nationalization with perhaps very inadequate or even no compensation is perhaps the main factor. Nations with a history of natIOnalization generally have poorly developed mining industries. Possible political turmoil, civil strife and currency controls may all combine to increase greatly the financial risks of investing in certain countries. Periodical reviews of political risks in various countries are prepared and published by specialized companies and references to these can be found in Noetstaller (1988). Useful articles on the subject are Anon. (1985c) and Anon. (1985d).

Ore reserve classification

In dclineating and working an orebody the mining geologist often has to classify his ore reserves into three classes: proved, probable and possible; frequently used synonyms are: measured, indicated and inferred. Proved ore has been sampled so thoroughly that we can be certain of its outline, tonnage and average grade, within certain limits. Elsewhere in the orebody, sampling from drilling and development workings may not have been so thorough, but there may be enough information to be reasonably sure of its tonnage and grade; this is

+----- Diamond pipe--+ LOW 1 ppm Alluvial Au ._+

30 ppm 0.1

Stratiform PGM «-Disseminated-' ..- Stratiform -. Au _ _ Vein Au Au ..... Porphyry Mo-. +-- Porphyry Cu ----.

..... Skarn W .....

_ _ RedBed ..- Stratiform Sn -+ C • . Unconformity U -+ U ..- Stratiform N,_.

1'}o

Approximate proportion of ore mineral 10%

~

+----- Mississippi Valley ------. type Pb·Zn·fluor·bar Stratiform volcaniC + - - - hosted Cu.Zn-Pb+Au,Ag-

+-

30%

Stratiform sediment· _ _ hosted Pb-Zn:': Ag

...... Sedimentary --+ Mn «- Phosphorites...

« - - - - - - - - - - - tJauxltes Iron formation ........ Stratiform baryte -----+ 60%

+ - - - Podlform Cr

__

Gypsum ---.------.. Stratiform ..... Cr Coal------_. HIGH 100% HIGH

Nature ot mineralized ground (schematic)

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--------C3eoiogicDI cuntinuity LOVi!

=

Jt

" - _..

the south-western USA. At the Nacimiento Mine in New Mexico a deposit of 11 Mt averaging 0.65% copper is being worked by open pit methods. Like other red bed coppers, this deposit has a high metal/sulphur ratio as the principal mineral is chalcocite. This yields a copper concentrate low in sulphur, which is very acceptable to present day custom smelters faced with stringent anti-pollution legislation. Similar deposits are very important in China where they make up nearly 21 % of that country's total stratiform copper reserves (Chen 1988). Copper is not the only base metal that occurs in such deposits. Similar lead ores are known in Germany and silver deposits in Utah. Another important class of pore-filling deposits are the uranium-vanadium deposits of Colorado Plateau or Wcstern States-type, which occur mainly in sandstones of continental origin but also in some siltstones and conglomerates. The orebodies are very variable in form, and pods and irregularly shaped deposits occur, although large concordant sheets up to 3 m thick are also present. The orebodies follow sedimentary structures and depo sitional features. Many mechanical accumulations of high density minerals, such as magnetite, ilmenite, rutile and zircon, occur in arenaceous hosts, usually taking the form of layers rich in heavy minerals in Pleistocene and Holocene sands. As the sands are usually unlithified, the deposits are easily worked and no costly crushing of the ore is required. These orebodies belong to the group called placer deposits-

beach sand placers are a good example (Fig. 2.17). Beach placers supply much of the world's titanium. zirconium, thorium, cerium and yttrium. They occur along present day beaches or ancient beaches where longshore drift is well developed and frequent storms occur. Economic grades can be very low and sands running as little as 0.6% heavy minerals are worked along Australia's eastern coast. The deposits usually show a topographical control, the shapes of bays and the position of headlands often being very important; thus in exploring for buried orebodies a reconstruction of the palaeogeography is invaluable. Rudaceous hosts Alluvial gravels and conglomerates also form important recent and ancient placer deposits. Alluvial gold deposits are often marked by 'white runs' of vein quarLz pebbles, as in the White Channels of the Yukon, the White Bars of California and the White Leads of Australia. Such deposits form one of the few types of economic placer deposits in fully lithified rocks, and indeed the majority of the world's gold is won from Precambrian deposits of this type in South Africa. Figure 2.18 shows the distribution of the gold orebodies in the East Rand Basin where the vein quartz pebble conglomerates occur in quartzites of the Upper Witwatersrand System. Their fan-shaped distribution strongly suggests that they occupy distributary channels. Uranium is recovered as a by-product ofthe working of the Witwatersrand goldfields. In the very similar Blind River area of Ontario uranium is the only

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MORPHOLOGY AND TYPES OF ORE DEPOSIT

N

+ km

~ Present beach and

~ foredune sands

Mined areas

Gl

Outer barrier sands

Inner barrier sands with ridges

Inner barrier sands with no ridges

Bedrock

Fig. 2.17 Geology and mining areas of the beach sand deposits of Crowdy Head, New South Wales, Australia. (After Winward 1975.)

metal produced. In this field the conglomeratic orebodies lie in elongate south-easterly trending sheets (Fig. 2.19) that are composed of layers of braided, interfingering channels and beds. These ore sheets, carrying the individual orebodies, have dimensions measured in kilometres (Robertson 1962, Theis 1979). Similar mineralized conglomerates occur elsewhere in the Precambrian.

Chemical sediments Sedimentary iron, manganese, evaporite and phosphorite formations occur scattered throughout the stratigraphical column, forming very extensive beds conformable with the stratigraphy. They are described in Chapters 18 & 20. Igneous host rocks

Volcanic hosts There are two principal types of deposit to be found in volcanic rocks, vesicular filling deposits and

Ridges Paludal (swamp) and fluviatile sediment in part overlying inner barrier sands

volcanic-associated massive sulphide deposits. The first deposit type is not very important but the second type is a widespread and important producer of base metals often with silver and gold as byproducts. The first type forms in the permeable vesicular t...