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Some observations on copper yields and ore grades
Resources Policy 37 (2012) 59–72
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Some observations on copper yields and ore grades Phillip Crowson n Centre for Energy, Petroleum & Mineral Law & Policy, University of Dundee, Scotland, United Kingdom
a r t i c l e i n f o
abstract
Article history: Received 6 August 2011 Received in revised form 8 December 2011 Accepted 8 December 2011 Available online 9 January 2012
The paper examines trends in the average copper content of mined ores over the years. It has tended to decline over the long term, but by no means evenly. US averages are not typical of global averages, at least in the past four decades. Those have been both higher, and less volatile than in the US. One reason for falling averages is a change in the type of deposit mined, with a rise in the share of relatively low grade porphyry deposits. The different nature of their deposits is reflected in marked differences in grades between the different continents. African and Australian average grades are higher than the global average, and changes in the share of Central Africa in global output have affected the global average grade. Yields are have been consistently lower in North America than elsewhere, and Latin American average grades have trended downwards, reflecting both the ageing of mines and the rising share of production from porphyry deposits. Typically the yield of mines declines over time as mining proceeds. The average copper content of ore deposits is usually below the average yield of the ore accessed in the early years of production. The initial grades of new mines have not declined over the past forty years, and there has been no perceptible tendency for the average grade of porphyry deposits brought into production to decline over time. There is no apparent correlation between average grade and deposit size, but mine operators tend to exploit economies of scale to offset low grades. The relationship between the annual percentage yields (the head grade) and the reserve grades of deposits is not static. In recent years head grades have fallen closer to reserve grades. The relationship may be affected by movements in metal prices. Although the evidence about the influence of prices is not clearcut, it does suggest that prices and cut-off grades may be inversely related. As many ores contain other valuable metals besides copper, copper yields will sometimes be subordinated to the extraction of these other metals. Copper equivalent grades have not moved in the same way as copper grades alone. & 2011 Elsevier Ltd. All rights reserved.
Jel classification: L72 N5 Q30 Q39 Keywords: Copper Ore grades Yields Reserves Prices
Introduction There is a general presumption that the average percentage copper content of ores mined has steadily declined over the years. The higher grade ores that are most accessible and easily treated were mined first. Advances in the technology of exploration, extraction and processing gradually allowed the economic mining of lower grade, less accessible, and more metallurgically complex ores. These technological changes, coupled with economies of scale, more than offset the higher costs of mining lower grade ores and held down the price of copper metal relative to the general price level for extended periods. In practice the development of copper mining has been more complex than this outline suggests. This paper examines the evidence by drawing together data on the metal content of copper
n
Correspondence address: 3 Albany Close, Reigate, Surrey, RH2 9PP, United Kingdom. Tel.: þ44 0 1737248586; fax: þ 44 0 1737 249643. E-mail addresses: [email protected], [email protected] 0301-4207/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.resourpol.2011.12.004
ores from a wide range of sources. It examines various aspects of the historical trends, including differences in the copper content of different types of deposit, which are partly reflected in variations in regional averages. It then looks at how yields of copper alter over the life of mines and at how head grades vary with mine size. The relationship between the percentage grade of ore reserves and of ore treated and how the grades of ore treated might move with metal prices are examined in the next section. One complication in any discussion of ore grades is that deposits may contain several different metals apart from copper. Looking just at the copper content of ore may give only a partial picture, as discussed in the paper’s final section.
Trends in copper ore grades over time Very little statistical data are publicly available on a regular basis about trends in the average copper content of mined copper ores outside the United States. The Appendix on historical trends draws together information, including a figure showing annual copper yields in the US back into the nineteenth century, to
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P. Crowson / Resources Policy 37 (2012) 59–72
1.2 1.1 1 0.9
Western World World
0.8 0.7 0.6
United States
0.5 0.4 0.3 1970 1973 1976 1979 1982 1985 1988 1991 1994 1997 2000 2003 2006 2009 Fig. 1. Weighted average percentage head grades of copper mines, 1970 onwards. Sources: McMahon (1965), Butler and Burbank (1929), US Geological Survey (1932–2009), Rio Tinto (2003), CRU International (2009). (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)
provide some guidance on long term movements. The declining relative importance of US mines means that trends in their average ore grades do not adequately reflect what happened globally in recent decades. By 2009 the US share of global output had fallen below 8% from well over 50% in the first quarter of the twentieth century. Fig. 1 compares global estimates covering the full range of types of copper deposit and mining techniques with the US averages from 1970 onwards. For 1970 to 1993 the figure shows the weighted average percentage yield of copper from mines in the Western World (i.e. world excluding the former Soviet Union, Comecon, China and other communist countries). For the period from 1987 onwards the weighted average yield embraces the entire world. Global average grades fluctuated in a narrow range from the late 1970s to the early 2000s, with no obvious trend, but with less volatility than US grades. The average in 2002 was very similar to that of 1980. The global average has, however, fallen since 2003 at a similar rate to that of the 1970s. The recent decline helps explain why mine output has fallen well short of estimated capacity in recent years (Crowson, 2009). Analysts apparently failed to allow sufficiently for falling ore grades in their estimates. Weighted average yields merely show the broad picture and do not explain the underlying dynamics. The population of mines is continuously changing, with ore deposits being depleted on one hand and new deposits being opened up or mines expanded on the other. Typically the rate of ore extraction from a deposit rises over time as companies seek to exploit economies of scale. Such increases in scale may be accompanied by changes in cut-off grade, the boundary between ore that is processed and material that is sent to waste dumps. Technological progress and rising productivity tend to allow the extraction of ever leaner ores, but rising costs of all types tend in the other direction. There is seldom an outright winner in this perpetual tug of war. Nor is there a universal tendency for new mines to exploit lower grade ores than their predecessors.
Types of ore deposit and regional shifts An important reason for average grades falling over time is a change in the type of deposit being exploited. In the nineteenth century, and well into the twentieth, a substantial share of copper
Table 1 Percentage shares of global copper mine output. Source: ICSG (2008), WBMS (2011).
Europe Africa Middle East and Asia N America C and S America Oceania World (Mt)
1987
1997
2008
20.8 14.7 10.6 23.8 24.9 5.2
14.8 5.4 12.8 22.5 38.1 5.8
9.7 6.4 18.9 12.2 46.1 6.7
8.6
11.5
15.7
mined came from massive sulphides, copper skarns, polymetallic veins and replacement deposits and sediment-hosted copper. With the rundown in African, West European and East Canadian output, such deposits accounted for a falling share of total mine output. Although many, such as the Polish deposits, remain important, and Central African production is reviving, an increasing share of copper has come from porphyry deposits. The varying copper content of different types of ore deposit is inevitably reflected in differences in the average yields of ore mined in different countries and regions. As background to the more recent changes in deposit types Table 1 shows the regional distribution of copper mine production, by continent from 1987. The shares of Europe, Africa and North America have shrunk compared with those of other regions, although Africa has recently regained share. Within Asia, the Chinese share has risen, but inadequate data are available on the percentage yields of its mines. That gap in the data aside, Fig. 2 shows broad regional trends since 1987 in the weighted average copper head grades (the percentage yield of copper in mined ore) of each continent. Grades have been consistently lower in North America than in the other continents, largely reflecting the predominance of porphyry deposits in the United States and Western Canada. Australian grades have normally been much higher than the global average, as have African. The revival of production from the sediment-hosted deposits of Zambia and the Democratic Republic of Congo (DRC) has led to a consequent increase in weighted average African grades and has also partly offset declining average grades elsewhere. Grades in
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2.5
2.0
1.5
1.0
0.5
1987
1989 World
1991
1993
Africa
1995
Asia
1997
Australasia
1999 Europe
2001
2003
NAmerica N. America
2005
2007
S&CAmerica S. & C. America
Fig. 2. Regional weighted average copper head grades 1987 onwards. Source: CRU International (2009).
Table 2 Percentage copper content of different types of copper deposit. Source: Cox and Singer (1996 and 1998). Number of deposits
Porphyry copper 208 Sediment-hosted 57 Cu skarn 64 Polymetallic replacement 52 Polymetallic veins 75 Cyprus massive sulphides 49 Besshi massive sulphides 44 Kuroko massive sulphides 432 Synorganic–synvolcanic 32 ni–cu
Lowest decile (% cu)
Median Top decile (% cu) (% cu)
0.31 1 0.7 n.a. n.a. 0.63 0.64 0.45 0.13
0.54 2.1 1.7 0.094 n.a. 1.7 1.5 1.3 0.47
0.94 4.5 4 0.87 0.89 3.9 3.3 3.5 1.3
Central and South America have trended downwards towards the global average, reflecting both a rise in the share of porphyry-based production and the ageing of many mines. As Table 2 illustrates, porphyry deposits typically have much lower copper ore grades than most other types. The table compares the median copper content of various deposit types from data produced by the US Geological Survey. Where there is sufficient information the grades of the top and bottom deciles of mines ranked in ascending order of copper grades are also shown. By no means all new copper mines are based on porphyry deposits, but most new mines in the South American Andean region, the most rapidly expanding center of production, are of that type. In 1975 porphyry deposits accounted for about 34% of global copper mine output (Raw Materials Group, 2011). Their share rose to 47% in 1988 and 62% in 1998. In 2009, when many mines were closed down, or had lowered their output, especially in the United States, the share of global mine output coming from porphyry deposits had dipped to 55%. Over most of the period since 1975, however, and probably for much longer a rising share of copper production from porphyry deposits has contributed to falling weighted average yields of copper.
Trends in percentage yields of individual ore deposits The percentage copper content of the upper portions of porphyry copper deposits is typically higher than the deposit’s
average as a result of weathering and supergene enrichment. That results from the copper being dissolved, percolating down below the water table and precipitating. One consequence is that the grade of ore mined will inevitably fall below the average of the initial production as mining proceeds. The average percentage copper content of the entire ore deposit will be less than the average of the ore accessed in the early years of mining. Mining companies will exploit this characteristic of ore deposits to maximize their output in a mine’s early years in order to pay off debt finance. Few mines have had a sufficiently long life to compare their percentage yields over an extended period with national or global averages. Fig. 3 does, however, show such data for five mines, Bingham Canyon, Chino and Ray in the United States and Chuquicamata and El Teniente in Chile. The ownership, mining methods and scale of these mines have changed during their lives, but they have each consistently worked the same ore deposits. In each case the average yield has trended downwards, but by no means steadily. Chuquicamata’s yields increased during the early 1930s in contrast to those of El Teniente, presumably as the operators reacted differently to the depressed prices of that period. The boost in Chuquicamata’s grades between 1970 and the early 1980s largely reflected managerial problems and changes associated with its nationalization and the political turmoil of the period. The tendency for grades to fall from the averages of the early years of production is illustrated for a wider range of mines in Fig. 4 which shows index numbers of the head grades (the percentage copper content of ore milled) at successive five yearly dates from initial production. For each mine the average of the first four years’ grades is taken as 100. Chuquicamata was the exception to the general tendency for head grades to decline during the first thirty years of operation. In most cases the rate of decline slowed considerably after the first ten years’ mining. One possible implication is that weighted average head grades might be related to the start-up of new mines, as opposed to expansions of existing operations. When there is little development of new mines, weighted average head grades will naturally reflect the underlying tendency for grades to decline with depth. That tendency will be reinforced where existing mines are expanding, as their increased throughput typically entails lowering cut-off grades: higher output is accompanied by lowered head grades. This is a general phenomenon of the mining industry rather than a specific characteristic of copper mining (McCarthy, 2011).
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3.0 Chino
% copper in ore milled
2.5
El Teniente Chuquicamata
2.0 1.5 Ray
1.0 0.5
Bingham Canyon
0.0 1905
1915
1925
1935
1945
1955
1965
1975
1985
1995
2005
Fig. 3. Average percentage copper content of ore milled at five mines, 1905 onwards. Sources: Long (2010), CRU International (2009), Raw Materials Group (2011).
120 100 80 60 40 20 0 First four
10th
15th
20th
25th
30th
Bethlehem
Bingham Canyon
Cananaea
Candelaria
Chuquicamata
Cuajone
El Abra
Escondida
Gibraltar
Granisle
Ertsberg/Grasberg
Pima
Ray
Toquepala
Fig. 4. Head grades of selected mines over thirty years from initial production. Sources: Long (2010), CRU International (2009), Raw Materials Group (2011).
Where new mines are in the ascendant, their initially high head grades might temporarily offset the tendency for grades of existing mines to decline. Such an offset will only exist where the initial head grades of new mines exceed the average head grades of existing operations. That has been the general pattern, as shown by Fig. 5. This compares the global weighted average head grades of Fig. 1 (the red lines) with the average grades during their first four years of operation of mines which started up in the years shown. These were of different sizes and types, including some mines that are not based on porphyry deposits. In nearly all cases these new mines had higher initial grades than the global average. The clustering of mine openings in the 1990s helps explain why global average grades remained fairly stable during that period. By the mid 2000s, when there were relatively few new mine openings, the average grades of the previous decade’s start-ups were declining. Improvements in productivity, usually based on technological innovations, and increases in the scale of mines have enabled the mining industry to offset much of the potential impact of declining ore grades on the cost of production of copper metal. An earlier paper (Crowson, 2003) showed that there has been a statistically significant inverse relationship between copper-equivalent head
grades and mine size, based on the tonnage milled. The effect of lower head grades on the costs of metal production has been largely offset by economies of scale. One corollary is that there is little correlation between head grades and copper produced, as demonstrated by Fig. 6 which relates copper head grades in 2008 to copper output in 2009 for all the mines for which data are available. Not only has technological change allowed the exploitation of economies of scale, but it has also enabled the economic exploitation of ever leaner ores. In that regard the most obvious instance is the development and spread of solvent extraction–electrowinning (SX–EW), although that is not included in the US data on grades underlying Fig. 1.
Reserve grades One message of Fig. 5 is that the initial head grades of new mines have not displayed a general tendency to decline over the past forty years. This message is reinforced by information on porphyry copper deposits in a large database maintained by the US Geological Survey. This contains data on 690 porphyry copper deposits spread throughout the world, including estimates of their size, grades and dates of
P. Crowson / Resources Policy 37 (2012) 59–72
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12 11 % copper content of ore milled
10 9 8 7 6 5 4 3 2 1 0 1971 1974 1977 1980 1983 1986 1989 1992 1995 1998 2001 2004 2007 2010 Fig. 5. Global average copper head grades compared with the head grades of new mines in their first four years of operation. Sources: Long (2010), CRU International (2009), Raw Materials Group (2011).
Copper output in 2009 '000 tonnes cu
10 9 8 7 6 5 4 3 2 1 0 0
100
200
300
400
500
600
700
800
900
1000
Head grade % copper in 2008 Fig. 6. Copper head grades in 2008 and mine output in 2009. Source: Raw Materials Group (2011).
1.4
% copper content of ore
1.2 1 0.8 0.6 0.4 0.2 0 1858 1868 1878 1888 1898 1908 1918 1928 1938 1948 1958 1968 1978 1988 1998 2008 Start up date Fig. 7. Percentage copper content of ore deposits by date of start up of mining. Note: Where the stated date of start-up is approximate the data have been allocated to the nearest appropriate year. Source: Singer et al. (2008).
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P. Crowson / Resources Policy 37 (2012) 59–72
Grade % copper
10
1
0.1
0.01 1
10
100
1000
10000
100000
Ore tonnage (million tonnes) Fig. 8. Percentage copper content and deposit size for porphyry coppers. Note: The data are plotted on a double logarithmic scale to bring out any relationship more clearly than on an arithmetic scale. Source: Singer et al. (2008).
22000 20000 Tonnage of ore (Million tonnes)
18000 16000 14000 12000 10000 8000 6000 4000 2000 0 1845 1855 1865 1875 1885 1895 1905 1915 1925 1935 1945 1955 1965 1975 1985 1995 2005 Date of discovery 1.8 1.6
% copper content of ore
1.4 1.2 1 0.8 0.6 0.4 0.2 0 1845 1855 1865 1875 1885 1895 1905 1915 1925 1935 1945 1955 1965 1975 1985 1995 2005 Date of discovery Fig. 9. Copper content and size by date of discovery for porphyry copper deposits. (a) Deposit Size and (b) percentage copper content. Note: Where the stated date of discovery is approximate the data have been allocated to the nearest appropriate year, or to the middle of the relevant decade. Source: Singer et al. (2008).
P. Crowson / Resources Policy 37 (2012) 59–72
discovery (Singer et al., 2008). Where they are known, the years in which mining commenced are also included. Fig. 7 relates the average ore grades for the 132 deposits for which the data are available to the dates at which their exploitation first started. In many instances these dates were many years after the deposits were first discovered. There has clearly been no tendency for the average grades of porphyry copper deposits brought into production to decline over time. One possibility might be that higher grade deposits are always smaller than the lower grade, and that there has been a trend towards the mining of the larger deposits. There would then be a tendency for average grades to decline. To test that hypothesis Fig. 8 relates percentage copper content to the tonnage of ore for 423 porphyry copper deposits. There is no correlation in this much larger sample of deposits from the USGS database. To round out the picture Fig. 9 shows tonnages and percentage copper contents by date of discovery for the 269 porphyry deposits for which the data are available. Discoveries were not spread evenly over the period, but were clustered, with the greatest concentration between the mid 1950s and mid 1970s. That reflected an improved understanding of the genesis of porphyry copper. Again there is no evidence of trends in either the size of porphyry ore deposits discovered or in their copper contents. The deposits for which start-up dates are given, as in Fig. 7, do not appear to be a biased sample from the deposits in Fig. 9. In summary, the observed decline in the average yield of copper ore milled has come from a combination of the exhaustion
65
of other types of copper deposits with higher average grades than porphyry coppers and from the tendency, already noted, for head grades to fall towards a deposit’s ore grade as mining proceeds.
Head grades, reserve grades and copper prices How head grades have compared with the average grade of copper deposits during the past decade is examined in Fig. 10. It shows the distribution of the ratios between head grades and ore reserve grades in 2000 and 2008. The head grade equaled or fell short of ore grades in just under 32% of mines in 2000, and in 14% of mines head grades were at least double the ore reserve grades. By 2008 the share of mines whose head grades were at least double their ore grades had fallen to almost 10% and the mines whose head grades were the same as, or less than, their ore grades had risen to 50% of the total. The trend since 1998 in the weighted average ratio for all mines is summarized in Fig. 11. The ratios were even higher in 1998–99 than in 2000 since when the decline has been erratic. The average ratio rose in 2001 and again in 2009. Unfortunately the period covered is too brief to enable firm conclusions about possible reasons, but one might be movements in copper prices. In many mines there is limited, if any scope, to vary annual head grades in response to changing product prices. The rate of extraction is governed by the deposit’s geology and metallurgy,
30
Percentage of mines
25 20 15 10 5 0 0
0.25
0.5
0.75 1 1.25 1.5 1.75 2 Ratio of head to reserve grade
2.25
2.5
M More
0
0.25
0.5
0.75
2.25
2.5
More
45
Percentage of mines
40 35 30 25 20 15 10 5 0 1
1.25
1.5
1.75
Ratio of head to reserve grade Fig. 10. Ratios between copper head grades and ore grades in 2000 and 2008. (a) 2000 and (b) 2008. Source: Raw Materials Group (2011).
2
66
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1.6
Ratio of head to ore grade
1.5
1.4
1.3
1.2
1.1
1 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 Fig. 11. Weighted average ratio of head grades to ore grades for copper mines 1998–2009. Source: Raw Materials Group (2011).
1.1
Head grade % copper
1.0 0.9 0.8 0.7 0.6 y = -0.002x + 1.154 R² = 0.2881
0.5 0.4 100
120
140
160
180
200
220
240
260
280
300
Copper price in 2010 terms cents/lb 0.75
Head grade % copper
0.70 0.65 0.60 y = -0.0002x + 0.6631 R² = 0.1263
0.55 0.50 0.45 0
50
100
150
200
250
300
350
400
Copper price in 2010 terms cents/lb Fig. 12. Head grades and copper prices at Bingham Canyon. (a) 1946–1983 and (b) 1989–2009. Sources: Long (2010), CRU International(2009), Raw Materials Group (2011).
and by the design of the mine. Changing cut-off grades (the metal content of ore that is just economic) normally cannot be done quickly without altering the configuration of the mine, or without
capital expenditure. The stated objective is usually to maximize the extraction of ore over the projected life of the mine, rather than to change operating conditions with volatile metal prices
P. Crowson / Resources Policy 37 (2012) 59–72
and costs of extraction. Yet there have been clear instances, as noted in an earlier section, of mine operators tailoring their annual production to maximize its value. There are different schools of thought about the appropriate response to changing metal prices. One is that companies should concentrate on extracting higher grade ore when prices are high in order to maximize the net present value of their ore deposits. In practice such a policy might shorten the life of a mine by rendering the lower grade ore less accessible or too costly to extract, thereby lowering its net present value. Thus the alternative view is that mine operators should ignore short-term price fluctuations and develop mine plans that maximize the economic output from the deposit over its projected life. This might involve mining lower grade ores when prices are high. Conversely companies might be prompted to extract higher grade ore, to the extent possible, when prices weaken in order to sustain cash flow and cover heavy fixed costs. In some countries the aim has been merely to extract all the useable products from an ore deposit, often regardless of costs. That was the policy followed in the Soviet Union, and still followed to some extent in China. The observed trends in weighted average ore grades over long periods result from too many influences to draw precise conclusions. To complicate matters changes in prices are likely to have had a lagged rather than instantaneous impact on decisions about production. The data are too sparse to separate out the influence of the different factors involved, although average US copper yields did rise in the early 1930s when prices slumped. The grades for individual mines shown in Fig. 3 were affected by political factors in the cases of Chuquicamata and El Teniente, and by changes in ownership and production methods for all five. Even so there were weak correlations between prices and grades at Bingham Canyon, as illustrated in Fig. 12. This plots the annual head grades at Bingham against copper prices in 2010 terms for two periods, 1946–83 and 1989–2009. The mining methods were substantially re-organized in the intervening years. For the earlier period grades are plotted against prices of the preceding year, whereas prices of the same year give a higher R2 for the later period. Unfortunately, most of the remaining data presented in this paper cover too short a time span to elicit any meaningful relationships between trends in copper head grades and copper prices. Fig. 13 does, however, offer some clues. It plots the ratio between head and reserve grades against LME copper prices in real terms between 1998 and 2009. Changes in the ratio would reflect changes in cut-off grades within existing mines.
67
The figure does suggest that an inverse relationship exists between prices and the ratio of head grades to ore grades. That would be consistent with miners lowering cut-off grades when prices rise, and increasing them when prices weaken.
Metal content versus copper content Ore deposits often contain other valuable materials in association with copper so that their copper content is not the sole determinant of their viability. At times changes in the prices of the other constituents may influence a mine’s production program. To the extent that it is possible, operators may maximize the output of other products at the expense of copper production. One example was in 2005–06 where copper–molybdenum mines concentrated on molybdenum in response to a surge in its price (Crowson, 2009). The existence of by- and co-products, and changing relative prices of those and copper need to be recognized when looking at trends in copper yields over time, at differences between individual mines and national or regional averages and at the relationship between copper ore grades and copper prices. Fig. 14 compares the copper head grade of milled ore with ‘copper equivalent’ head grades in 2000 and 2009 for individual mines. The copper head grade is shown on the horizontal axis and the ‘copper equivalent’ grades on the vertical. For each mine copper equivalents are calculated by weighting the content of each product in the milled ore by its price relative to that year’s copper price. Thus for a copper–molybdenum mine, the percentage head grade of copper would be increased by adding the percentage of molybdenum in the milled ore multiplied by the ratio between the price of molybdenum and the price of copper. Similar calculations are made for mines containing gold, silver, cobalt, nickel, platinum group metals, lead and zinc as well as copper. The existence of by- and co-products in many mines can clearly affect their profitability, allowing the exploitation of ores containing lower copper grades than might otherwise be the case. Furthermore changes in the relative prices of the different constituents of ore can modify the trend in weighted average copper head grades. This is demonstrated in Fig. 15 which shows trends since 1998 in global copper head grades and in copper equivalent head grades. The latter are given in both money terms, using each year’s prices for weighting, and in 2008 terms, using the relative prices of 2008 for weighting.
Ratio of average head to ore grade
1.6 1.5 1.4 y = -0.0891x + 1.4324 R = 0.66
1.3 1.2 1.1 1 0
05
1
15
2
25
LME copper price $/lb in 2010 terms Fig. 13. LME copper prices and the ratio between head and reserve grades 1998–2009. Sources: Raw Materials Group (2011).
3
35
4
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P. Crowson / Resources Policy 37 (2012) 59–72
Copper equivalent head grade % 2000
12 10 8 6 4 2 0 0
1
2
4
3
5
6
7
8
9
7
8
9
Copper Head Grade % 2000
Copper equivalent head grade % 2009
14 12 10 8 6 4 2 0 0
1
2
3
4
5
6
Copper Head Grade % 2009 Fig. 14. Copper equivalent head grades in 2000 and 2009. (a) 2000 and (b) 2009. Source: Raw Materials Group (2011).
1
Head grades % copper equivalent
Copper equivalent at money prices
0.95 0.9 Copper equivalent at 2008 prices
0.85 0.8 0.75
Copper only
0.7 0.65 0.6 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009
Fig. 15. Copper equivalent head grades 1998–2009. Source: Raw Materials Group (2011).
In money terms weighted average copper equivalent head grades rose between 1998 and 2003 whilst copper head grades alone were falling. Between 2003 and 2009 they fell faster, but
with a temporary jump in 2007. When calculated at constant 2008 relative prices, however, they experienced no overall decline. One implication is that a rising share of the total
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turnover, in 2008 prices, of mines producing copper came from other constituents of the ore.
Concluding comments This paper has ranged widely over many aspects of the copper content of copper ore mined and of copper ore deposits. Largely because of the inadequacies of the available data it has raised as many questions as it has answered. The gaps in data can only be filled by painstaking analysis of the historical records of individual mining companies. That lies well beyond the scope of this paper. Much of the data has undoubtedly been collected and collated by mining companies and consultants, but it is regarded as proprietary and is not in the public domain. Yet data that lies unused is of little or no commercial value, especially where it is of an historical nature. The relationship between head grades and prices is of much more than academic interest, as it is highly relevant for projections of future mine output. The past few years have clearly demonstrated that analysts ignore changes in average ore grades at their peril. This is a fruitful area for detailed further research.
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trends but may mask substantial changes in the shares of each country within each period. Against this background Appendix Table A2 gives a broad snapshot of global shifts in average grades since the 1770s. It confirms that the average percentage yield of copper from mining has tended to decline over the long term, but by no means evenly. Subsequent paragraphs discuss the data in more detail. The United Kingdom was the world’s largest producer until 1850, extracting copper from rich veins, mainly underground. The south-west of England (Cornwall and Devon) was its most important copper mining region. Appendix Fig. A1 shows the trend in the five-yearly average percentage yields of its copper mines from 1770 to 1900. Average grades declined until the early 1860s, but then rose slightly in the remaining decades of the nineteenth century. From the early 1850s the United Kingdom was increasingly overtaken by other producing countries and its output dwindled to insignificance by the end of the nineteenth century. Before the US assumed its dominant position countries like Chile and Spain were important. Within Spain Rio Tinto was the major producer from 1877 onwards. Appendix Fig. A2 compares its annual average percentage yield of copper up to 1939 with the average yields of UK mines to 1905.
Appendix Historical trends in copper ore grades Historical trends in the average grades of copper ore processed over the past two centuries may seem irrelevant to those primarily concerned with immediate policy issues. That is, however, far too myopic a perspective. Longer term trends in ore grades help illuminate discussions about resource depletion and the adequacy of remaining reserves. Parallels with earlier experiences also offer guidance to those considering likely future trends in mine capacity. This appendix therefore draws together data about trends in the average copper content of mined copper ores from a range of sources. It is placed in an appendix, rather than the main body of the paper, for the convenience of readers who are impatient with historical analysis. One of the major difficulties in analyzing historical trends is a paucity of published data for countries other than the United States, and there is always a danger of drawing firm conclusions from incomplete data. Appendix Table A1 shows how shares of global copper output shifted over the past two centuries, with percentage shares at twenty five year intervals. This table effectively contains a series of stills from a fast changing movie. It illustrates the broad
Table A2 Average percentage yields of copper ore. Sources: Barton (1978), Skelton (1934), Hunt (1855 onwards), HMSO (1884–87), HMSO (1988–96), HMSO (1897–1905), Harvey (1981), Butler and Burbank (1929), McMahon (1965), Rio Tinto (2003), CRU International (2009). 1770–75 1800 1820–25 1850 1865–72 1870 1880 1883–88 1890 1900 1906 1915 1925 1930 1950 1970 1980 1990 2000 2008
Cornish average UK average Cornish average UK average Calumet and Hecla (Michigan) average UK average Calumet and Hecla (Michigan) Rio Tinto (Spain) average Calumet and Hecla (Michigan) Calumet and Hecla (Michigan) US average US average US average US average US average Western world average Western world average World average World average World average
12.17 9.27 8.22 7.84 2.76 6.72 4.81 3.06 3.23 2.61 2.51 1.66 1.54 1.43 0.89 1.11 0.88 0.83 0.85 0.77
Table A1 Percentage shares of global copper output at twenty-five year intervals. Sources: Schmitz (1979), ICSG (2008). 1800
1825
1850
1875
1900
1925
1950
1975
2000
Australia Canada Chile Congo Indonesia Poland Russiaa United States Zambia United Kingdom Spain Others
– – 10.5 – – – 21 – – 37.1 1.4 30.1
– – 11.1 – – – 13.6 – – 43.2 0.4 31.7
9 – 21.8 – – – 12.1 1.3 – 23.4 0.8 78.3
9.2 1.1 38.1 – – – 3 15 – 3.8 16.4 13.3
4.7 2.3 5.3 – – – 1.6 55.6 – 0.2 10.9 19.5
0.9 3.6 13.7 6.4 – – 0.5 54.1 0.1 – 3.4 17.4
0.7 9.6 14.6 7.1 – 0 8.8 33.2 11.3 – 0.6 14.1
3.3 10.9 12.3 7.3 0.9 3.4 14.1 19 10.1 – 0.8 17.9
6.3 4.8 34.8 0.2 7.6 3.4 4.4 11 1.9 – 0.2 25.4
Total output (‘000t)
14.3
24.3
53.1
122.4
494.7
1406.9
2488
6735
13212
a
Figures are for the USSR for 1950 and 1975.
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P. Crowson / Resources Policy 37 (2012) 59–72
13 12 11
% copper content
10 9 8 7 6 5 4 3 2 1 1890-95
1880-85
1870-75
1860-65
1850-55
1840-45
1830-35
1820-25
1810-15
1800-05
1790-95
1770-75
1780-85
Note: The output of Devon is included for 1818-81
0
Fig. A1. Average percentage copper content of Cornish & Devonian ore: five-year averages, 1770–1900. Source: Barton (1978).
11 10 United Kingdom
9 8
Cornwall & Devon
7 6 5 4 3
Rio Tinto, Spain
2 1 0 1850 1855 1860 1865 1870 1875 1880 1885 1890 1895 1900 1905 1910 1915 1920 1925 1930 1935 Fig. A2. Average percentage copper ore yields, United Kingdom and Rio Tinto, Spain. Sources: Harvey (1981), Hunt (1855 onwards), HMSO (1884 onwards).
Table A3 Cumulative output of contained copper and average yields of Michigan copper deposits 1845–1925. Source: Butler and Burbank (1929).
Calumet and Hecla conglomerate Kearsage amygdaloid Pewabic amygdaloid Baltic amygdaloid Osceola amygdaloid Isle Royale amygdaloid Atlantic amygdaloid Atlantic conglomerate Evergreen and succeeding lodes Superior amygdaloid Forest (Victoria) amygdaloid Nonesuch lode Lake amygdeloid
Mt of copper
% cu
1.53 0.53 0.4 0.4 0.19 0.1 0.06 0.03 0.03 0.01 0.01 0.01 0.03
2.56 0.96 1.16 1.35 0.84 0.82 0.68 0.56 0.5–1 0.95 0.58 1.03 1.03
Table A4 Output of copper ore and average yields by US state in 1914. Source: McCasky (1914).
Arizona Michigan Montana Nevada New Mexico Utah Others Total
% share of ore output
% cu
21.3 26.4 12.2 8.2 5.7 21.6 4.5 100
2.6 0.89 2.66 1.05 1.47 0.94 – 1.6
Rio Tinto’s average grades peaked in the 1880s, averaging 3.06% between 1883 and 1888, and then gradually declined to 1.21% in 1924–25. They subsequently rose up to 1931, before
P. Crowson / Resources Policy 37 (2012) 59–72
71
5 4.5 Calumet & Hecla, Michigan
4 3.5 3 2.5 2 US average
1.5
Quincy, Michigan
1 0.5 0 1862 1872 1882 1892 1902 1912 1922 1932 1942 1952 1962 1972 1982 1992 2002 Fig. A3. Average % grade of copper ore treated in the United States, 1862 onwards. Sources: McMahon (1965), Butler and Burbank (1929), US Geological Survey (1932–2009).
again easing back. Even at their peak they were under half the average yields of the British mines, but with much higher levels of output. The United States became the dominant producer in the last quarter of the nineteenth century. In 1900 US mines accounted for 55% of global copper output, and in 1925 for 54%. Thus trends in US average grades may be broadly representative of general trends, at least up to the Second World War. The US was still the leading producer in 1950 although its share had then fallen to 33%. US commercial production of copper began in Michigan in 1845 and Michigan was the major US producing state throughout the nineteenth century. Details of the annual copper production and ore grades of its mines were published by the US Geological Survey in 1929 (Butler and Burbank, 1929). Within Michigan copper was extracted from a range of deposit types with varying grades, which are summarized in Appendix Table A3. The Calumet and Hecla mines, based on several deposits, accounted for 46% of Michigan’s cumulative copper output between 1845 and 1925, and the Quincy mine for a further 9%. During the last quarter of the nineteenth century the Michigan mines declined in relative importance, as initially higher grade deposits were opened up in the western states. In 1914 the average yield of copper was lower in Michigan than in other major copper producing states, as shown by Appendix Table A4. The US Geological Survey (through the US Bureau of Mines for most of the period), has published annual estimates of the average yield of concentrated ore for the entire United States from 1906 onwards. Its copper came from a mixture of underground and open pit mining, with extraction from lower grade porphyry deposits increasingly dominating its production. The calculations of average yields include metal in precipitates but exclude copper recovered from leaching by solvent extraction–electrowinning. That became an increasingly important source of mined copper in the United States from the late 1970s onwards, initially from waste dumps and then from low grade ores. Appendix Fig. A3 combines the Michigan and US figures to show how the average yield of copper moved in the United States from the early 1860s.
Average yields fell in Michigan in the last two decades of the nineteenth century and in the entire United States throughout the twentieth century, and into the twenty first. Leaving aside 1918–22 and the first half of the 1930s, the average grade of ore mined fell progressively until the early 1970s. Subsequently it has fluctuated in the United States with no clear trend.
References Barton, D.B., 1978. A History of copper mining in Cornwall & Devon, third ed. D Bradford Barton Ltd, Truro. Butler, B.S. and Burbank, W.S., 1929. The Copper Deposits of Michigan, Professional Paper 144, US Geological Survey, Washington. Cox, Denis P. and Singer, Donald A. ,Eds. (1996, and 1998 on the web), Mineral Deposit Models, US Geological Survey Bulletin 1693 (/http://pubs.usgs.gov/ bul/b1693S). Crowson, Phillip, 2003. Mine size and the structure of costs. Resources Policy 29 (2003), pp15–36. Crowson, Phillip, 2009. Copper production from the late 1990s- A paradox? Copper Studies, December 2009. CRU International Limited, London. CRU International (2009), London private communication. Harvey, Charles E, 1981. The Rio Tinto Company, An Economic History of a Leading International Mining concern. Alison Hodge, Penzance, Cornwall. 1873 –1954. HMSO (1884–87), The Mineral and Mining Statistics of The United Kingdom of Great Britain and Ireland including Lists of Mines and Mineral Works. HMSO, London. (1884–87). HMSO (1888–96), The Mineral Statistics of the United Kingdom of Great Britain and Ireland with the Isle of Man, HMSO London successive issues. HMSO (1897–1905), General Report and Statistics Relating to the Output and Value of the Mineral Raised in the United Kingdom, the Amount and Value of the Minerals Produced and the Exports and Imports of Minerals, HMSO London successive issues. Hunt, Robert, 1855–1881. Mineral Statistics of the United Kingdom of Great Britain & Northern Ireland HMSO London successive issues (1855 to 1881). ICSG, 2008. Statistics of the International Copper Study Group, private communication. Long, Keith R., 2010. US Geological Survey private communication. McCarthy, Peter, 2011. Production Rate Optimization; Avoiding the temptation of tonnage, Keynote Address to ACG, November 2011. McCasky, H.D., 1914. Mineral Resources of the United States, Part 1, Metals. US Geological Survey, Washington. McMahon, A.D., 1965. Copper. A Materials Survey, Information Bulletin 8225. US Bureau of Mines, Washington D.C. Raw Materials Database, 2011. Raw Materials Group. Stockholm, Sweden. Rio Tinto Mine Information System (2003), private communication.
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Schmitz, Christopher J, 1979. World Non-Ferrous Metal Production and Prices 1700-1976. Frank Cass, London. Singer, Donald A., Berger, Vladamir I., Moring, Barry C., 2008. Porphyry Copper Deposits of the World: Database and Grade and Tonnage Models, Open File Report 2008-1155, US Geological Survey, Menlo Park (/http://pubs.usgs.gov/ of/2008.1155S).
Skelton, A., 1934. Chapter 4 on Copper. In: Elliott, W.Y., May, E.S., Rowe, J.W.F., Skelton, A., Wallace, D.H. (Eds.), International Control in the Non-Ferrous Metals. , Macmillan, New York, pp. 1937. US Geological Survey (1932 to 2009), Copper Chapter of Minerals Yearbook, vol. 1. WBMS, 2011. World Metal Statistics, Monthly Bulletin, February 2011, Ware, Herts.
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Some observations on copper yields and ore grades Phillip Crowson n Centre for Energy, Petroleum & Mineral Law & Policy, University of Dundee, Scotland, United Kingdom
a r t i c l e i n f o
abstract
Article history: Received 6 August 2011 Received in revised form 8 December 2011 Accepted 8 December 2011 Available online 9 January 2012
The paper examines trends in the average copper content of mined ores over the years. It has tended to decline over the long term, but by no means evenly. US averages are not typical of global averages, at least in the past four decades. Those have been both higher, and less volatile than in the US. One reason for falling averages is a change in the type of deposit mined, with a rise in the share of relatively low grade porphyry deposits. The different nature of their deposits is reflected in marked differences in grades between the different continents. African and Australian average grades are higher than the global average, and changes in the share of Central Africa in global output have affected the global average grade. Yields are have been consistently lower in North America than elsewhere, and Latin American average grades have trended downwards, reflecting both the ageing of mines and the rising share of production from porphyry deposits. Typically the yield of mines declines over time as mining proceeds. The average copper content of ore deposits is usually below the average yield of the ore accessed in the early years of production. The initial grades of new mines have not declined over the past forty years, and there has been no perceptible tendency for the average grade of porphyry deposits brought into production to decline over time. There is no apparent correlation between average grade and deposit size, but mine operators tend to exploit economies of scale to offset low grades. The relationship between the annual percentage yields (the head grade) and the reserve grades of deposits is not static. In recent years head grades have fallen closer to reserve grades. The relationship may be affected by movements in metal prices. Although the evidence about the influence of prices is not clearcut, it does suggest that prices and cut-off grades may be inversely related. As many ores contain other valuable metals besides copper, copper yields will sometimes be subordinated to the extraction of these other metals. Copper equivalent grades have not moved in the same way as copper grades alone. & 2011 Elsevier Ltd. All rights reserved.
Jel classification: L72 N5 Q30 Q39 Keywords: Copper Ore grades Yields Reserves Prices
Introduction There is a general presumption that the average percentage copper content of ores mined has steadily declined over the years. The higher grade ores that are most accessible and easily treated were mined first. Advances in the technology of exploration, extraction and processing gradually allowed the economic mining of lower grade, less accessible, and more metallurgically complex ores. These technological changes, coupled with economies of scale, more than offset the higher costs of mining lower grade ores and held down the price of copper metal relative to the general price level for extended periods. In practice the development of copper mining has been more complex than this outline suggests. This paper examines the evidence by drawing together data on the metal content of copper
n
Correspondence address: 3 Albany Close, Reigate, Surrey, RH2 9PP, United Kingdom. Tel.: þ44 0 1737248586; fax: þ 44 0 1737 249643. E-mail addresses: [email protected], [email protected] 0301-4207/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.resourpol.2011.12.004
ores from a wide range of sources. It examines various aspects of the historical trends, including differences in the copper content of different types of deposit, which are partly reflected in variations in regional averages. It then looks at how yields of copper alter over the life of mines and at how head grades vary with mine size. The relationship between the percentage grade of ore reserves and of ore treated and how the grades of ore treated might move with metal prices are examined in the next section. One complication in any discussion of ore grades is that deposits may contain several different metals apart from copper. Looking just at the copper content of ore may give only a partial picture, as discussed in the paper’s final section.
Trends in copper ore grades over time Very little statistical data are publicly available on a regular basis about trends in the average copper content of mined copper ores outside the United States. The Appendix on historical trends draws together information, including a figure showing annual copper yields in the US back into the nineteenth century, to
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P. Crowson / Resources Policy 37 (2012) 59–72
1.2 1.1 1 0.9
Western World World
0.8 0.7 0.6
United States
0.5 0.4 0.3 1970 1973 1976 1979 1982 1985 1988 1991 1994 1997 2000 2003 2006 2009 Fig. 1. Weighted average percentage head grades of copper mines, 1970 onwards. Sources: McMahon (1965), Butler and Burbank (1929), US Geological Survey (1932–2009), Rio Tinto (2003), CRU International (2009). (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)
provide some guidance on long term movements. The declining relative importance of US mines means that trends in their average ore grades do not adequately reflect what happened globally in recent decades. By 2009 the US share of global output had fallen below 8% from well over 50% in the first quarter of the twentieth century. Fig. 1 compares global estimates covering the full range of types of copper deposit and mining techniques with the US averages from 1970 onwards. For 1970 to 1993 the figure shows the weighted average percentage yield of copper from mines in the Western World (i.e. world excluding the former Soviet Union, Comecon, China and other communist countries). For the period from 1987 onwards the weighted average yield embraces the entire world. Global average grades fluctuated in a narrow range from the late 1970s to the early 2000s, with no obvious trend, but with less volatility than US grades. The average in 2002 was very similar to that of 1980. The global average has, however, fallen since 2003 at a similar rate to that of the 1970s. The recent decline helps explain why mine output has fallen well short of estimated capacity in recent years (Crowson, 2009). Analysts apparently failed to allow sufficiently for falling ore grades in their estimates. Weighted average yields merely show the broad picture and do not explain the underlying dynamics. The population of mines is continuously changing, with ore deposits being depleted on one hand and new deposits being opened up or mines expanded on the other. Typically the rate of ore extraction from a deposit rises over time as companies seek to exploit economies of scale. Such increases in scale may be accompanied by changes in cut-off grade, the boundary between ore that is processed and material that is sent to waste dumps. Technological progress and rising productivity tend to allow the extraction of ever leaner ores, but rising costs of all types tend in the other direction. There is seldom an outright winner in this perpetual tug of war. Nor is there a universal tendency for new mines to exploit lower grade ores than their predecessors.
Types of ore deposit and regional shifts An important reason for average grades falling over time is a change in the type of deposit being exploited. In the nineteenth century, and well into the twentieth, a substantial share of copper
Table 1 Percentage shares of global copper mine output. Source: ICSG (2008), WBMS (2011).
Europe Africa Middle East and Asia N America C and S America Oceania World (Mt)
1987
1997
2008
20.8 14.7 10.6 23.8 24.9 5.2
14.8 5.4 12.8 22.5 38.1 5.8
9.7 6.4 18.9 12.2 46.1 6.7
8.6
11.5
15.7
mined came from massive sulphides, copper skarns, polymetallic veins and replacement deposits and sediment-hosted copper. With the rundown in African, West European and East Canadian output, such deposits accounted for a falling share of total mine output. Although many, such as the Polish deposits, remain important, and Central African production is reviving, an increasing share of copper has come from porphyry deposits. The varying copper content of different types of ore deposit is inevitably reflected in differences in the average yields of ore mined in different countries and regions. As background to the more recent changes in deposit types Table 1 shows the regional distribution of copper mine production, by continent from 1987. The shares of Europe, Africa and North America have shrunk compared with those of other regions, although Africa has recently regained share. Within Asia, the Chinese share has risen, but inadequate data are available on the percentage yields of its mines. That gap in the data aside, Fig. 2 shows broad regional trends since 1987 in the weighted average copper head grades (the percentage yield of copper in mined ore) of each continent. Grades have been consistently lower in North America than in the other continents, largely reflecting the predominance of porphyry deposits in the United States and Western Canada. Australian grades have normally been much higher than the global average, as have African. The revival of production from the sediment-hosted deposits of Zambia and the Democratic Republic of Congo (DRC) has led to a consequent increase in weighted average African grades and has also partly offset declining average grades elsewhere. Grades in
P. Crowson / Resources Policy 37 (2012) 59–72
61
2.5
2.0
1.5
1.0
0.5
1987
1989 World
1991
1993
Africa
1995
Asia
1997
Australasia
1999 Europe
2001
2003
NAmerica N. America
2005
2007
S&CAmerica S. & C. America
Fig. 2. Regional weighted average copper head grades 1987 onwards. Source: CRU International (2009).
Table 2 Percentage copper content of different types of copper deposit. Source: Cox and Singer (1996 and 1998). Number of deposits
Porphyry copper 208 Sediment-hosted 57 Cu skarn 64 Polymetallic replacement 52 Polymetallic veins 75 Cyprus massive sulphides 49 Besshi massive sulphides 44 Kuroko massive sulphides 432 Synorganic–synvolcanic 32 ni–cu
Lowest decile (% cu)
Median Top decile (% cu) (% cu)
0.31 1 0.7 n.a. n.a. 0.63 0.64 0.45 0.13
0.54 2.1 1.7 0.094 n.a. 1.7 1.5 1.3 0.47
0.94 4.5 4 0.87 0.89 3.9 3.3 3.5 1.3
Central and South America have trended downwards towards the global average, reflecting both a rise in the share of porphyry-based production and the ageing of many mines. As Table 2 illustrates, porphyry deposits typically have much lower copper ore grades than most other types. The table compares the median copper content of various deposit types from data produced by the US Geological Survey. Where there is sufficient information the grades of the top and bottom deciles of mines ranked in ascending order of copper grades are also shown. By no means all new copper mines are based on porphyry deposits, but most new mines in the South American Andean region, the most rapidly expanding center of production, are of that type. In 1975 porphyry deposits accounted for about 34% of global copper mine output (Raw Materials Group, 2011). Their share rose to 47% in 1988 and 62% in 1998. In 2009, when many mines were closed down, or had lowered their output, especially in the United States, the share of global mine output coming from porphyry deposits had dipped to 55%. Over most of the period since 1975, however, and probably for much longer a rising share of copper production from porphyry deposits has contributed to falling weighted average yields of copper.
Trends in percentage yields of individual ore deposits The percentage copper content of the upper portions of porphyry copper deposits is typically higher than the deposit’s
average as a result of weathering and supergene enrichment. That results from the copper being dissolved, percolating down below the water table and precipitating. One consequence is that the grade of ore mined will inevitably fall below the average of the initial production as mining proceeds. The average percentage copper content of the entire ore deposit will be less than the average of the ore accessed in the early years of mining. Mining companies will exploit this characteristic of ore deposits to maximize their output in a mine’s early years in order to pay off debt finance. Few mines have had a sufficiently long life to compare their percentage yields over an extended period with national or global averages. Fig. 3 does, however, show such data for five mines, Bingham Canyon, Chino and Ray in the United States and Chuquicamata and El Teniente in Chile. The ownership, mining methods and scale of these mines have changed during their lives, but they have each consistently worked the same ore deposits. In each case the average yield has trended downwards, but by no means steadily. Chuquicamata’s yields increased during the early 1930s in contrast to those of El Teniente, presumably as the operators reacted differently to the depressed prices of that period. The boost in Chuquicamata’s grades between 1970 and the early 1980s largely reflected managerial problems and changes associated with its nationalization and the political turmoil of the period. The tendency for grades to fall from the averages of the early years of production is illustrated for a wider range of mines in Fig. 4 which shows index numbers of the head grades (the percentage copper content of ore milled) at successive five yearly dates from initial production. For each mine the average of the first four years’ grades is taken as 100. Chuquicamata was the exception to the general tendency for head grades to decline during the first thirty years of operation. In most cases the rate of decline slowed considerably after the first ten years’ mining. One possible implication is that weighted average head grades might be related to the start-up of new mines, as opposed to expansions of existing operations. When there is little development of new mines, weighted average head grades will naturally reflect the underlying tendency for grades to decline with depth. That tendency will be reinforced where existing mines are expanding, as their increased throughput typically entails lowering cut-off grades: higher output is accompanied by lowered head grades. This is a general phenomenon of the mining industry rather than a specific characteristic of copper mining (McCarthy, 2011).
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P. Crowson / Resources Policy 37 (2012) 59–72
3.0 Chino
% copper in ore milled
2.5
El Teniente Chuquicamata
2.0 1.5 Ray
1.0 0.5
Bingham Canyon
0.0 1905
1915
1925
1935
1945
1955
1965
1975
1985
1995
2005
Fig. 3. Average percentage copper content of ore milled at five mines, 1905 onwards. Sources: Long (2010), CRU International (2009), Raw Materials Group (2011).
120 100 80 60 40 20 0 First four
10th
15th
20th
25th
30th
Bethlehem
Bingham Canyon
Cananaea
Candelaria
Chuquicamata
Cuajone
El Abra
Escondida
Gibraltar
Granisle
Ertsberg/Grasberg
Pima
Ray
Toquepala
Fig. 4. Head grades of selected mines over thirty years from initial production. Sources: Long (2010), CRU International (2009), Raw Materials Group (2011).
Where new mines are in the ascendant, their initially high head grades might temporarily offset the tendency for grades of existing mines to decline. Such an offset will only exist where the initial head grades of new mines exceed the average head grades of existing operations. That has been the general pattern, as shown by Fig. 5. This compares the global weighted average head grades of Fig. 1 (the red lines) with the average grades during their first four years of operation of mines which started up in the years shown. These were of different sizes and types, including some mines that are not based on porphyry deposits. In nearly all cases these new mines had higher initial grades than the global average. The clustering of mine openings in the 1990s helps explain why global average grades remained fairly stable during that period. By the mid 2000s, when there were relatively few new mine openings, the average grades of the previous decade’s start-ups were declining. Improvements in productivity, usually based on technological innovations, and increases in the scale of mines have enabled the mining industry to offset much of the potential impact of declining ore grades on the cost of production of copper metal. An earlier paper (Crowson, 2003) showed that there has been a statistically significant inverse relationship between copper-equivalent head
grades and mine size, based on the tonnage milled. The effect of lower head grades on the costs of metal production has been largely offset by economies of scale. One corollary is that there is little correlation between head grades and copper produced, as demonstrated by Fig. 6 which relates copper head grades in 2008 to copper output in 2009 for all the mines for which data are available. Not only has technological change allowed the exploitation of economies of scale, but it has also enabled the economic exploitation of ever leaner ores. In that regard the most obvious instance is the development and spread of solvent extraction–electrowinning (SX–EW), although that is not included in the US data on grades underlying Fig. 1.
Reserve grades One message of Fig. 5 is that the initial head grades of new mines have not displayed a general tendency to decline over the past forty years. This message is reinforced by information on porphyry copper deposits in a large database maintained by the US Geological Survey. This contains data on 690 porphyry copper deposits spread throughout the world, including estimates of their size, grades and dates of
P. Crowson / Resources Policy 37 (2012) 59–72
63
12 11 % copper content of ore milled
10 9 8 7 6 5 4 3 2 1 0 1971 1974 1977 1980 1983 1986 1989 1992 1995 1998 2001 2004 2007 2010 Fig. 5. Global average copper head grades compared with the head grades of new mines in their first four years of operation. Sources: Long (2010), CRU International (2009), Raw Materials Group (2011).
Copper output in 2009 '000 tonnes cu
10 9 8 7 6 5 4 3 2 1 0 0
100
200
300
400
500
600
700
800
900
1000
Head grade % copper in 2008 Fig. 6. Copper head grades in 2008 and mine output in 2009. Source: Raw Materials Group (2011).
1.4
% copper content of ore
1.2 1 0.8 0.6 0.4 0.2 0 1858 1868 1878 1888 1898 1908 1918 1928 1938 1948 1958 1968 1978 1988 1998 2008 Start up date Fig. 7. Percentage copper content of ore deposits by date of start up of mining. Note: Where the stated date of start-up is approximate the data have been allocated to the nearest appropriate year. Source: Singer et al. (2008).
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P. Crowson / Resources Policy 37 (2012) 59–72
Grade % copper
10
1
0.1
0.01 1
10
100
1000
10000
100000
Ore tonnage (million tonnes) Fig. 8. Percentage copper content and deposit size for porphyry coppers. Note: The data are plotted on a double logarithmic scale to bring out any relationship more clearly than on an arithmetic scale. Source: Singer et al. (2008).
22000 20000 Tonnage of ore (Million tonnes)
18000 16000 14000 12000 10000 8000 6000 4000 2000 0 1845 1855 1865 1875 1885 1895 1905 1915 1925 1935 1945 1955 1965 1975 1985 1995 2005 Date of discovery 1.8 1.6
% copper content of ore
1.4 1.2 1 0.8 0.6 0.4 0.2 0 1845 1855 1865 1875 1885 1895 1905 1915 1925 1935 1945 1955 1965 1975 1985 1995 2005 Date of discovery Fig. 9. Copper content and size by date of discovery for porphyry copper deposits. (a) Deposit Size and (b) percentage copper content. Note: Where the stated date of discovery is approximate the data have been allocated to the nearest appropriate year, or to the middle of the relevant decade. Source: Singer et al. (2008).
P. Crowson / Resources Policy 37 (2012) 59–72
discovery (Singer et al., 2008). Where they are known, the years in which mining commenced are also included. Fig. 7 relates the average ore grades for the 132 deposits for which the data are available to the dates at which their exploitation first started. In many instances these dates were many years after the deposits were first discovered. There has clearly been no tendency for the average grades of porphyry copper deposits brought into production to decline over time. One possibility might be that higher grade deposits are always smaller than the lower grade, and that there has been a trend towards the mining of the larger deposits. There would then be a tendency for average grades to decline. To test that hypothesis Fig. 8 relates percentage copper content to the tonnage of ore for 423 porphyry copper deposits. There is no correlation in this much larger sample of deposits from the USGS database. To round out the picture Fig. 9 shows tonnages and percentage copper contents by date of discovery for the 269 porphyry deposits for which the data are available. Discoveries were not spread evenly over the period, but were clustered, with the greatest concentration between the mid 1950s and mid 1970s. That reflected an improved understanding of the genesis of porphyry copper. Again there is no evidence of trends in either the size of porphyry ore deposits discovered or in their copper contents. The deposits for which start-up dates are given, as in Fig. 7, do not appear to be a biased sample from the deposits in Fig. 9. In summary, the observed decline in the average yield of copper ore milled has come from a combination of the exhaustion
65
of other types of copper deposits with higher average grades than porphyry coppers and from the tendency, already noted, for head grades to fall towards a deposit’s ore grade as mining proceeds.
Head grades, reserve grades and copper prices How head grades have compared with the average grade of copper deposits during the past decade is examined in Fig. 10. It shows the distribution of the ratios between head grades and ore reserve grades in 2000 and 2008. The head grade equaled or fell short of ore grades in just under 32% of mines in 2000, and in 14% of mines head grades were at least double the ore reserve grades. By 2008 the share of mines whose head grades were at least double their ore grades had fallen to almost 10% and the mines whose head grades were the same as, or less than, their ore grades had risen to 50% of the total. The trend since 1998 in the weighted average ratio for all mines is summarized in Fig. 11. The ratios were even higher in 1998–99 than in 2000 since when the decline has been erratic. The average ratio rose in 2001 and again in 2009. Unfortunately the period covered is too brief to enable firm conclusions about possible reasons, but one might be movements in copper prices. In many mines there is limited, if any scope, to vary annual head grades in response to changing product prices. The rate of extraction is governed by the deposit’s geology and metallurgy,
30
Percentage of mines
25 20 15 10 5 0 0
0.25
0.5
0.75 1 1.25 1.5 1.75 2 Ratio of head to reserve grade
2.25
2.5
M More
0
0.25
0.5
0.75
2.25
2.5
More
45
Percentage of mines
40 35 30 25 20 15 10 5 0 1
1.25
1.5
1.75
Ratio of head to reserve grade Fig. 10. Ratios between copper head grades and ore grades in 2000 and 2008. (a) 2000 and (b) 2008. Source: Raw Materials Group (2011).
2
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P. Crowson / Resources Policy 37 (2012) 59–72
1.6
Ratio of head to ore grade
1.5
1.4
1.3
1.2
1.1
1 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 Fig. 11. Weighted average ratio of head grades to ore grades for copper mines 1998–2009. Source: Raw Materials Group (2011).
1.1
Head grade % copper
1.0 0.9 0.8 0.7 0.6 y = -0.002x + 1.154 R² = 0.2881
0.5 0.4 100
120
140
160
180
200
220
240
260
280
300
Copper price in 2010 terms cents/lb 0.75
Head grade % copper
0.70 0.65 0.60 y = -0.0002x + 0.6631 R² = 0.1263
0.55 0.50 0.45 0
50
100
150
200
250
300
350
400
Copper price in 2010 terms cents/lb Fig. 12. Head grades and copper prices at Bingham Canyon. (a) 1946–1983 and (b) 1989–2009. Sources: Long (2010), CRU International(2009), Raw Materials Group (2011).
and by the design of the mine. Changing cut-off grades (the metal content of ore that is just economic) normally cannot be done quickly without altering the configuration of the mine, or without
capital expenditure. The stated objective is usually to maximize the extraction of ore over the projected life of the mine, rather than to change operating conditions with volatile metal prices
P. Crowson / Resources Policy 37 (2012) 59–72
and costs of extraction. Yet there have been clear instances, as noted in an earlier section, of mine operators tailoring their annual production to maximize its value. There are different schools of thought about the appropriate response to changing metal prices. One is that companies should concentrate on extracting higher grade ore when prices are high in order to maximize the net present value of their ore deposits. In practice such a policy might shorten the life of a mine by rendering the lower grade ore less accessible or too costly to extract, thereby lowering its net present value. Thus the alternative view is that mine operators should ignore short-term price fluctuations and develop mine plans that maximize the economic output from the deposit over its projected life. This might involve mining lower grade ores when prices are high. Conversely companies might be prompted to extract higher grade ore, to the extent possible, when prices weaken in order to sustain cash flow and cover heavy fixed costs. In some countries the aim has been merely to extract all the useable products from an ore deposit, often regardless of costs. That was the policy followed in the Soviet Union, and still followed to some extent in China. The observed trends in weighted average ore grades over long periods result from too many influences to draw precise conclusions. To complicate matters changes in prices are likely to have had a lagged rather than instantaneous impact on decisions about production. The data are too sparse to separate out the influence of the different factors involved, although average US copper yields did rise in the early 1930s when prices slumped. The grades for individual mines shown in Fig. 3 were affected by political factors in the cases of Chuquicamata and El Teniente, and by changes in ownership and production methods for all five. Even so there were weak correlations between prices and grades at Bingham Canyon, as illustrated in Fig. 12. This plots the annual head grades at Bingham against copper prices in 2010 terms for two periods, 1946–83 and 1989–2009. The mining methods were substantially re-organized in the intervening years. For the earlier period grades are plotted against prices of the preceding year, whereas prices of the same year give a higher R2 for the later period. Unfortunately, most of the remaining data presented in this paper cover too short a time span to elicit any meaningful relationships between trends in copper head grades and copper prices. Fig. 13 does, however, offer some clues. It plots the ratio between head and reserve grades against LME copper prices in real terms between 1998 and 2009. Changes in the ratio would reflect changes in cut-off grades within existing mines.
67
The figure does suggest that an inverse relationship exists between prices and the ratio of head grades to ore grades. That would be consistent with miners lowering cut-off grades when prices rise, and increasing them when prices weaken.
Metal content versus copper content Ore deposits often contain other valuable materials in association with copper so that their copper content is not the sole determinant of their viability. At times changes in the prices of the other constituents may influence a mine’s production program. To the extent that it is possible, operators may maximize the output of other products at the expense of copper production. One example was in 2005–06 where copper–molybdenum mines concentrated on molybdenum in response to a surge in its price (Crowson, 2009). The existence of by- and co-products, and changing relative prices of those and copper need to be recognized when looking at trends in copper yields over time, at differences between individual mines and national or regional averages and at the relationship between copper ore grades and copper prices. Fig. 14 compares the copper head grade of milled ore with ‘copper equivalent’ head grades in 2000 and 2009 for individual mines. The copper head grade is shown on the horizontal axis and the ‘copper equivalent’ grades on the vertical. For each mine copper equivalents are calculated by weighting the content of each product in the milled ore by its price relative to that year’s copper price. Thus for a copper–molybdenum mine, the percentage head grade of copper would be increased by adding the percentage of molybdenum in the milled ore multiplied by the ratio between the price of molybdenum and the price of copper. Similar calculations are made for mines containing gold, silver, cobalt, nickel, platinum group metals, lead and zinc as well as copper. The existence of by- and co-products in many mines can clearly affect their profitability, allowing the exploitation of ores containing lower copper grades than might otherwise be the case. Furthermore changes in the relative prices of the different constituents of ore can modify the trend in weighted average copper head grades. This is demonstrated in Fig. 15 which shows trends since 1998 in global copper head grades and in copper equivalent head grades. The latter are given in both money terms, using each year’s prices for weighting, and in 2008 terms, using the relative prices of 2008 for weighting.
Ratio of average head to ore grade
1.6 1.5 1.4 y = -0.0891x + 1.4324 R = 0.66
1.3 1.2 1.1 1 0
05
1
15
2
25
LME copper price $/lb in 2010 terms Fig. 13. LME copper prices and the ratio between head and reserve grades 1998–2009. Sources: Raw Materials Group (2011).
3
35
4
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P. Crowson / Resources Policy 37 (2012) 59–72
Copper equivalent head grade % 2000
12 10 8 6 4 2 0 0
1
2
4
3
5
6
7
8
9
7
8
9
Copper Head Grade % 2000
Copper equivalent head grade % 2009
14 12 10 8 6 4 2 0 0
1
2
3
4
5
6
Copper Head Grade % 2009 Fig. 14. Copper equivalent head grades in 2000 and 2009. (a) 2000 and (b) 2009. Source: Raw Materials Group (2011).
1
Head grades % copper equivalent
Copper equivalent at money prices
0.95 0.9 Copper equivalent at 2008 prices
0.85 0.8 0.75
Copper only
0.7 0.65 0.6 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009
Fig. 15. Copper equivalent head grades 1998–2009. Source: Raw Materials Group (2011).
In money terms weighted average copper equivalent head grades rose between 1998 and 2003 whilst copper head grades alone were falling. Between 2003 and 2009 they fell faster, but
with a temporary jump in 2007. When calculated at constant 2008 relative prices, however, they experienced no overall decline. One implication is that a rising share of the total
P. Crowson / Resources Policy 37 (2012) 59–72
turnover, in 2008 prices, of mines producing copper came from other constituents of the ore.
Concluding comments This paper has ranged widely over many aspects of the copper content of copper ore mined and of copper ore deposits. Largely because of the inadequacies of the available data it has raised as many questions as it has answered. The gaps in data can only be filled by painstaking analysis of the historical records of individual mining companies. That lies well beyond the scope of this paper. Much of the data has undoubtedly been collected and collated by mining companies and consultants, but it is regarded as proprietary and is not in the public domain. Yet data that lies unused is of little or no commercial value, especially where it is of an historical nature. The relationship between head grades and prices is of much more than academic interest, as it is highly relevant for projections of future mine output. The past few years have clearly demonstrated that analysts ignore changes in average ore grades at their peril. This is a fruitful area for detailed further research.
69
trends but may mask substantial changes in the shares of each country within each period. Against this background Appendix Table A2 gives a broad snapshot of global shifts in average grades since the 1770s. It confirms that the average percentage yield of copper from mining has tended to decline over the long term, but by no means evenly. Subsequent paragraphs discuss the data in more detail. The United Kingdom was the world’s largest producer until 1850, extracting copper from rich veins, mainly underground. The south-west of England (Cornwall and Devon) was its most important copper mining region. Appendix Fig. A1 shows the trend in the five-yearly average percentage yields of its copper mines from 1770 to 1900. Average grades declined until the early 1860s, but then rose slightly in the remaining decades of the nineteenth century. From the early 1850s the United Kingdom was increasingly overtaken by other producing countries and its output dwindled to insignificance by the end of the nineteenth century. Before the US assumed its dominant position countries like Chile and Spain were important. Within Spain Rio Tinto was the major producer from 1877 onwards. Appendix Fig. A2 compares its annual average percentage yield of copper up to 1939 with the average yields of UK mines to 1905.
Appendix Historical trends in copper ore grades Historical trends in the average grades of copper ore processed over the past two centuries may seem irrelevant to those primarily concerned with immediate policy issues. That is, however, far too myopic a perspective. Longer term trends in ore grades help illuminate discussions about resource depletion and the adequacy of remaining reserves. Parallels with earlier experiences also offer guidance to those considering likely future trends in mine capacity. This appendix therefore draws together data about trends in the average copper content of mined copper ores from a range of sources. It is placed in an appendix, rather than the main body of the paper, for the convenience of readers who are impatient with historical analysis. One of the major difficulties in analyzing historical trends is a paucity of published data for countries other than the United States, and there is always a danger of drawing firm conclusions from incomplete data. Appendix Table A1 shows how shares of global copper output shifted over the past two centuries, with percentage shares at twenty five year intervals. This table effectively contains a series of stills from a fast changing movie. It illustrates the broad
Table A2 Average percentage yields of copper ore. Sources: Barton (1978), Skelton (1934), Hunt (1855 onwards), HMSO (1884–87), HMSO (1988–96), HMSO (1897–1905), Harvey (1981), Butler and Burbank (1929), McMahon (1965), Rio Tinto (2003), CRU International (2009). 1770–75 1800 1820–25 1850 1865–72 1870 1880 1883–88 1890 1900 1906 1915 1925 1930 1950 1970 1980 1990 2000 2008
Cornish average UK average Cornish average UK average Calumet and Hecla (Michigan) average UK average Calumet and Hecla (Michigan) Rio Tinto (Spain) average Calumet and Hecla (Michigan) Calumet and Hecla (Michigan) US average US average US average US average US average Western world average Western world average World average World average World average
12.17 9.27 8.22 7.84 2.76 6.72 4.81 3.06 3.23 2.61 2.51 1.66 1.54 1.43 0.89 1.11 0.88 0.83 0.85 0.77
Table A1 Percentage shares of global copper output at twenty-five year intervals. Sources: Schmitz (1979), ICSG (2008). 1800
1825
1850
1875
1900
1925
1950
1975
2000
Australia Canada Chile Congo Indonesia Poland Russiaa United States Zambia United Kingdom Spain Others
– – 10.5 – – – 21 – – 37.1 1.4 30.1
– – 11.1 – – – 13.6 – – 43.2 0.4 31.7
9 – 21.8 – – – 12.1 1.3 – 23.4 0.8 78.3
9.2 1.1 38.1 – – – 3 15 – 3.8 16.4 13.3
4.7 2.3 5.3 – – – 1.6 55.6 – 0.2 10.9 19.5
0.9 3.6 13.7 6.4 – – 0.5 54.1 0.1 – 3.4 17.4
0.7 9.6 14.6 7.1 – 0 8.8 33.2 11.3 – 0.6 14.1
3.3 10.9 12.3 7.3 0.9 3.4 14.1 19 10.1 – 0.8 17.9
6.3 4.8 34.8 0.2 7.6 3.4 4.4 11 1.9 – 0.2 25.4
Total output (‘000t)
14.3
24.3
53.1
122.4
494.7
1406.9
2488
6735
13212
a
Figures are for the USSR for 1950 and 1975.
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P. Crowson / Resources Policy 37 (2012) 59–72
13 12 11
% copper content
10 9 8 7 6 5 4 3 2 1 1890-95
1880-85
1870-75
1860-65
1850-55
1840-45
1830-35
1820-25
1810-15
1800-05
1790-95
1770-75
1780-85
Note: The output of Devon is included for 1818-81
0
Fig. A1. Average percentage copper content of Cornish & Devonian ore: five-year averages, 1770–1900. Source: Barton (1978).
11 10 United Kingdom
9 8
Cornwall & Devon
7 6 5 4 3
Rio Tinto, Spain
2 1 0 1850 1855 1860 1865 1870 1875 1880 1885 1890 1895 1900 1905 1910 1915 1920 1925 1930 1935 Fig. A2. Average percentage copper ore yields, United Kingdom and Rio Tinto, Spain. Sources: Harvey (1981), Hunt (1855 onwards), HMSO (1884 onwards).
Table A3 Cumulative output of contained copper and average yields of Michigan copper deposits 1845–1925. Source: Butler and Burbank (1929).
Calumet and Hecla conglomerate Kearsage amygdaloid Pewabic amygdaloid Baltic amygdaloid Osceola amygdaloid Isle Royale amygdaloid Atlantic amygdaloid Atlantic conglomerate Evergreen and succeeding lodes Superior amygdaloid Forest (Victoria) amygdaloid Nonesuch lode Lake amygdeloid
Mt of copper
% cu
1.53 0.53 0.4 0.4 0.19 0.1 0.06 0.03 0.03 0.01 0.01 0.01 0.03
2.56 0.96 1.16 1.35 0.84 0.82 0.68 0.56 0.5–1 0.95 0.58 1.03 1.03
Table A4 Output of copper ore and average yields by US state in 1914. Source: McCasky (1914).
Arizona Michigan Montana Nevada New Mexico Utah Others Total
% share of ore output
% cu
21.3 26.4 12.2 8.2 5.7 21.6 4.5 100
2.6 0.89 2.66 1.05 1.47 0.94 – 1.6
Rio Tinto’s average grades peaked in the 1880s, averaging 3.06% between 1883 and 1888, and then gradually declined to 1.21% in 1924–25. They subsequently rose up to 1931, before
P. Crowson / Resources Policy 37 (2012) 59–72
71
5 4.5 Calumet & Hecla, Michigan
4 3.5 3 2.5 2 US average
1.5
Quincy, Michigan
1 0.5 0 1862 1872 1882 1892 1902 1912 1922 1932 1942 1952 1962 1972 1982 1992 2002 Fig. A3. Average % grade of copper ore treated in the United States, 1862 onwards. Sources: McMahon (1965), Butler and Burbank (1929), US Geological Survey (1932–2009).
again easing back. Even at their peak they were under half the average yields of the British mines, but with much higher levels of output. The United States became the dominant producer in the last quarter of the nineteenth century. In 1900 US mines accounted for 55% of global copper output, and in 1925 for 54%. Thus trends in US average grades may be broadly representative of general trends, at least up to the Second World War. The US was still the leading producer in 1950 although its share had then fallen to 33%. US commercial production of copper began in Michigan in 1845 and Michigan was the major US producing state throughout the nineteenth century. Details of the annual copper production and ore grades of its mines were published by the US Geological Survey in 1929 (Butler and Burbank, 1929). Within Michigan copper was extracted from a range of deposit types with varying grades, which are summarized in Appendix Table A3. The Calumet and Hecla mines, based on several deposits, accounted for 46% of Michigan’s cumulative copper output between 1845 and 1925, and the Quincy mine for a further 9%. During the last quarter of the nineteenth century the Michigan mines declined in relative importance, as initially higher grade deposits were opened up in the western states. In 1914 the average yield of copper was lower in Michigan than in other major copper producing states, as shown by Appendix Table A4. The US Geological Survey (through the US Bureau of Mines for most of the period), has published annual estimates of the average yield of concentrated ore for the entire United States from 1906 onwards. Its copper came from a mixture of underground and open pit mining, with extraction from lower grade porphyry deposits increasingly dominating its production. The calculations of average yields include metal in precipitates but exclude copper recovered from leaching by solvent extraction–electrowinning. That became an increasingly important source of mined copper in the United States from the late 1970s onwards, initially from waste dumps and then from low grade ores. Appendix Fig. A3 combines the Michigan and US figures to show how the average yield of copper moved in the United States from the early 1860s.
Average yields fell in Michigan in the last two decades of the nineteenth century and in the entire United States throughout the twentieth century, and into the twenty first. Leaving aside 1918–22 and the first half of the 1930s, the average grade of ore mined fell progressively until the early 1970s. Subsequently it has fluctuated in the United States with no clear trend.
References Barton, D.B., 1978. A History of copper mining in Cornwall & Devon, third ed. D Bradford Barton Ltd, Truro. Butler, B.S. and Burbank, W.S., 1929. The Copper Deposits of Michigan, Professional Paper 144, US Geological Survey, Washington. Cox, Denis P. and Singer, Donald A. ,Eds. (1996, and 1998 on the web), Mineral Deposit Models, US Geological Survey Bulletin 1693 (/http://pubs.usgs.gov/ bul/b1693S). Crowson, Phillip, 2003. Mine size and the structure of costs. Resources Policy 29 (2003), pp15–36. Crowson, Phillip, 2009. Copper production from the late 1990s- A paradox? Copper Studies, December 2009. CRU International Limited, London. CRU International (2009), London private communication. Harvey, Charles E, 1981. The Rio Tinto Company, An Economic History of a Leading International Mining concern. Alison Hodge, Penzance, Cornwall. 1873 –1954. HMSO (1884–87), The Mineral and Mining Statistics of The United Kingdom of Great Britain and Ireland including Lists of Mines and Mineral Works. HMSO, London. (1884–87). HMSO (1888–96), The Mineral Statistics of the United Kingdom of Great Britain and Ireland with the Isle of Man, HMSO London successive issues. HMSO (1897–1905), General Report and Statistics Relating to the Output and Value of the Mineral Raised in the United Kingdom, the Amount and Value of the Minerals Produced and the Exports and Imports of Minerals, HMSO London successive issues. Hunt, Robert, 1855–1881. Mineral Statistics of the United Kingdom of Great Britain & Northern Ireland HMSO London successive issues (1855 to 1881). ICSG, 2008. Statistics of the International Copper Study Group, private communication. Long, Keith R., 2010. US Geological Survey private communication. McCarthy, Peter, 2011. Production Rate Optimization; Avoiding the temptation of tonnage, Keynote Address to ACG, November 2011. McCasky, H.D., 1914. Mineral Resources of the United States, Part 1, Metals. US Geological Survey, Washington. McMahon, A.D., 1965. Copper. A Materials Survey, Information Bulletin 8225. US Bureau of Mines, Washington D.C. Raw Materials Database, 2011. Raw Materials Group. Stockholm, Sweden. Rio Tinto Mine Information System (2003), private communication.
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Schmitz, Christopher J, 1979. World Non-Ferrous Metal Production and Prices 1700-1976. Frank Cass, London. Singer, Donald A., Berger, Vladamir I., Moring, Barry C., 2008. Porphyry Copper Deposits of the World: Database and Grade and Tonnage Models, Open File Report 2008-1155, US Geological Survey, Menlo Park (/http://pubs.usgs.gov/ of/2008.1155S).
Skelton, A., 1934. Chapter 4 on Copper. In: Elliott, W.Y., May, E.S., Rowe, J.W.F., Skelton, A., Wallace, D.H. (Eds.), International Control in the Non-Ferrous Metals. , Macmillan, New York, pp. 1937. US Geological Survey (1932 to 2009), Copper Chapter of Minerals Yearbook, vol. 1. WBMS, 2011. World Metal Statistics, Monthly Bulletin, February 2011, Ware, Herts.