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Decreasing Metal Ore Grades: Are They Really Being Driven by the Depletion of High-Grade Deposits?

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Abstract

A shift to lower grade ores over time can indeed be observed for many minerals; however, this shift has been driven not by depletion of high-grade deposits but rather by dilution of ore grades through a combination of other factors, including: a) major improvements in metallurgical technologies, which have converted previously sub-economic mineral concentrations into valuable ores; b) a movement toward high-volume and lower cost extraction technologies, characterized by lower ore selectivity in the mining process; c) the economic advantages of extending the life of older mines over finding and establishing new mines.
This is the pre-peer reviewed version of the following article: West, J. 2011.
Decreasing Metal Ore Grades. Journal of Industrial Ecology 15(2): 165-168, which
has been published in final form at http://dx.doi.org/10.1111/j.1530-
9290.2011.00334.x
Decreasing metal ore grades: Are they really being driven by the depletion of
high grade deposits?
James West CSIRO Ecosystem Sciences Division
A common view within the industrial ecology community might be summarised
roughly as follows:
1. The grade and quality of mineral deposits being exploited, and of new deposits
being discovered, is decreasing rapidly.
2. This situation has arisen mainly because the best high grade deposits have already
been mined, and so we are being forced to mine ever lower grade deposits.
This view is probably justified in the case of conventional petroleum, judging from
the relative paucity of onshore discoveries in recent decades, and the ongoing shift to
exploration in extremely high cost environments. It is not, however, well justified for
most other mineral resources. A shift to lower grades ores over time can indeed be
observed for many minerals, however this hasn’t been driven by depletion of high
grade deposits, but rather by dilution of ore grades via a combination of other factors,
including:
Major improvements in metallurgical technologies, which have converted
previously sub-economic mineral concentrations into valuable ores.
A movement to high volume and lower cost extraction technologies,
characterized by lower ore selectivity in the mining process.
The economic advantages of extending the life of older mines over finding and
establishing new mines.
The main examples I use to support this assertion come from copper mining, as
copper has a long history of being actively explored for and mined, on a large scale, in
the modern era. I also touch on lead and zinc, taking advantage of an excellent
historical database for Australian mining contained in (Mudd 2007).
Drivers behind decreasing copper grades.
Our ability to profitably extract metals from low grade ores improves over time. This
will surprise nobody, however it is often assumed that this ongoing technological
improvement is largely driven by a necessity to use lower grade ores as high grade
resources run out. In this narrative, the “necessity” for developing improved
technologies is presumably signalled via increasing prices for the depleting
commodity. While conceptually simple and plausible, this view is not well supported
by the data.
In the case of copper, it is instructive to look at the period when the most significant
change in extractive technologies took place. Combining data in (Lankton 2010) and
(Kelly and Matos 2010) we see copper prices at the turn of the 20th century returning
to levels slightly above that of 1890, after being depressed for most of the 1890s.
Prior to 1900 the lowest viable ore grades of copper only deposits were around 3 - 4%.
Copper prices from 1900 1914 were generally below 1890 levels, however it was
during this period, that production targeting the huge but low grade resources of a
porphyry copper system was introduced for the first time (at Bingham Canyon, Utah).
By 1914 the average grade of ore being treated at this enormously profitable operation
was only 1.14% Cu (Strack 2010). The effect of unlocking the potential of huge, low
grade porphyry copper deposits (which continue to provide the bulk of copper mined
today) can be seen in the decrease in copper prices, to average less than 60% of the
1900 1914 price level for the subsequent four decades, while world production
increased from 938,000 tonnes in 1914 to 2,640,000 tonnes in 1954.
The point here is that this massive decrease in copper ore grades was not driven by
depletion of higher grade deposits with resulting higher copper prices. It was instead a
direct result of innovation which converted massive supplies of previously worthless
waste rock into valuable ore. This technological leap was not a fortuitous one-off
event. Improvements in extractive technologies are ongoing, and continue to render
ever lower grades of ore economically viable. This being the case, when demand
continues to increase we would expect average grades to decrease, regardless of
whether high grade resources are depleted or not.
Another key point is that high grade deposits of most metals have never contained a
large proportion of total metal, relative to low grade deposits. Furthermore, those high
grade deposits are frequently just small zones within much larger low grade resources.
Combined with the rise of bulk mining techniques, this provides another powerful
driver of declining average ore grades. Giant porphyry copper deposits such as
Chuquicamata, Morenci, and Bingham Canyon were often preceded by mining
operations which exploited relatively small zones of high grade ore within them. If
discovered today, these high grade lenses would be subsumed within the main low
grade deposit.
To illustrate this point, had a deposit similar to Prominent Hill (copper-gold, South
Australia) been discovered at the end of the 19th Century, it would probably have been
viewed as a deposit totalling several million tonnes. It would have been worked by
underground mining techniques, yielding copper grades above 3% per tonne, with
gold a welcome but relatively unimportant by-product. As Prominent Hill was
discovered at the beginning of the 21st Century, it will actually be exploited by large
scale open cutting, with resources defined at 283Mt grading 0.9% copper and 0.8 g/t
gold (OZ Minerals 2008). The gold content is a crucial component of the project.
Also driving decreasing ore grades are the favourable economics of continuing to
operate existing mines, even after their highest grade resources are depleted. Major
advantages that extending the life of an existing mine offer over starting a new mine
include:
Expensive capital plant and other infrastructure already in place and paid for.
Uncertainties associated with operating a new mine, both technical (ground
conditions, metallurgical qualities of the ore), and administrative (gaining
environmental and operating permits, community consent, etc.) are largely
avoided.
Costs associated with closing a mine are delayed. Postponing final site
rehabilitation costs will usually reduce the final real cost to a company.
It is noteworthy that many of the earliest porphyry copper deposits to be exploited are
still in operation today, in some cases around one century after commencement.
Further evidence that grade dilution rather than grade depletion drives decreasing
grades comes from data on Australian mining of copper, lead, and zinc contained in
(Mudd 2007).
Table 1 is a selection of statistics derived from this database. The final date of 2005
was chosen here as this is the last year prior to the rapid increases in prices of the
recent commodities boom. The lowering of grades to that point thus took place
despite a decrease in metal prices, and so demonstrably contrary to price signals.
1953
1965
1975
1985
1995
2005
World Pb Production (kt)
1,870
2,784
3,570
3,248
2,750
3,298
World Zn Production (kt)
2,360
4,352
6,176
6,651
6,982
10,114
World Cu Production (kt)
2,600
5,070
7,185
8,365
10,180
15,076
Australian Pb Reserves / World Production
3.27
1.57
3.89
4.45
6.62
7.22
Australian Zn Reserves / World Production
2.24
1.00
3.13
3.19
5.56
4.13
Australian Cu Reserves / World Production
0.30
0.41
0.84
1.92
2.36
2.75
Australian Pb Production indexed (1953 = 100%)
100%
134%
149%
181%
166%
277%
Australian Zn Production indexed (1953 = 100%)
100%
134%
193%
287%
354%
513%
Australian Cu Production indexed (1953 = 100%)
100%
241%
575%
683%
996%
2412%
Avg. grade Australian Pb ore mined (volume weighted)
10.6%
10.5%
7.0%
6.2%
4.6%
4.6%
Avg. grade Australian Zn ore mined (volume weighted)
10.7%
10.6%
8.5%
9.1%
8.6%
8.3%
Avg. grade Australian Cu ore mined (volume weighted)
1.2%
1.6%
1.6%
2.2%
2.5%
1.1%
Table 1 Statistics on Australian mining of lead, zinc, and copper derived from (Mudd 2007)
Some features to note here include:
Lead shows the most rapid grade decrease over the 1953 2005 time period. It
is also the metal for which world production increased least (and actually fell
from 1975), and for which price fell the most, to only 62% of the 1953 price
on a constant year 1998 U.S. dollar basis, according to data in (Kelly and
Matos 2010). If lead grades decreased due to depletion of high grade reserves,
one would expect to see a price rise. This data seems more indicative of a
situation where sufficient lead was produced as a co-product of other mining
(i.e. for zinc and silver) to meet demand. Decreasing grades would be
expected if lead were no longer sufficiently valuable to target in its own right.
Despite a major increase in world demand, average grades for zinc decreased
only around 23%, and were roughly stable for the final three decades. Prices in
2005 were only 85% of 1953 levels. Again, this runs contrary to the idea that
lower grade ores are being mined due to price signals.
World production of copper grew enormously over the period. Australia’s
share of that production also grew very strongly. Nevertheless, the average
copper grades being mined were roughly the same in 2005 as 1953, while
prices fell to 82% of 1953 levels. Average copper grades mined in Australia
increased over most of this period, while the rapid fall in copper grades from
1995 to 2005 is largely explained by the addition of two new high tonnage and
low grade copper-gold deposits (Cadia Hill and Telfer), rather than depletion
of high grade deposits.
The detailed Australian data on copper in (Mudd 2007) has other features of interest
in the depletion Vs dilution context. For example, even after it moved to lower grade
and higher volume production, by 2005 Olympic Dam alone produced over three
times Australia’s total copper ore production for 1953, at a grade almost twice as high.
It is also instructive to look at how the stated resources of the Olympic Dam deposit
have evolved over time. In 1992 it had resources of 612 Mt grading 2.14% Cu,
0.062% U3O8, and 0.70 g/t Au. By 2000 it was viewed as a 2,510 Mt deposit grading
1.29% Cu, 0.040% U3O8, and 0.47 g/t Au, still to be mined by underground mining
methods. By 2007, it was being viewed as a potential open cut mine. In that context, it
had a resource of 7,738 Mt of ore grading 0.87% Cu, 0.029% U3O8, and 0.30 g/t Au.
Over the course of 15 years, and well before any significant depletion of the 1992
resource had occurred, we see this major deposit have its apparent copper grade
reduced by 60%, and its total contained metal resource increased by over 400%. This
further illustrates how advances in mining technology (and maturing knowledge on
specific deposits) drive the apparent declines in ore grades.
Conclusion
None of the above is meant to call into question the self evident fact that ongoing
exploitation of non-renewable resources must lead to serious depletion at some point.
Nor is it to discount the view that the drive to continually increase rates of extraction
will likely encounter serious constraints over the medium to longer term, especially
limits imposed by energy demands and emissions limitations, as raised by (Mudd and
Ward 2008). The point of this commentary is merely to highlight that what we see
now, in terms of decreasing ore grades, is much more a manifestation of improving
extractive technologies than of depletion of high grade deposits. My motivation for
pointing this out is to help refocus attention away from issues which I believe will
remain very low order problems for a long time, and back onto problems which will
severely constrain our societies in a much shorter time frame. The sort of low order
problems I am referring to are typified by the current rash of “Peak Commodity-X”
discussions, where those discussions assume the impending depletion of deposits of
Commodity-X.
References
Kelly, T. D. and G. R. Matos (2010). Historical Statistics for Mineral and Material
Commodities in the United States, United States Geological Survey.
Lankton, L. D. (2010). Hollowed ground: copper mining and community building on
Lake Superior, 1840s - 1990s. Detroit, Wayne State University Press.
Mudd, G. M. (2007). Master spreadsheets compiling Australian mining production
data, Department of Civil Engineering, Monash University and Mineral Policy
Institute.
Mudd, G. M. and J. D. Ward (2008). Will Sustainability Constraints Cause ‘Peak
Minerals’. 3rd International Conference on Sustainability Engineering &
Science : Blueprints for Sustainable Infrastructure, Auckland, New Zealand.
OZ Minerals. (2008). Prominent Hill Resource Statement: June 30th 2008. Retrieved
November 2010, 2010, from
www.ozminerals.com/Media/docs/ASX_2008.09.24_ProminentHillJune2008
ResourceStatementExplanatory%20Notes-fe1f4c9d-7719-408b-81de-
8caa1db5303c-0.pdf.
Strack, D. (2010). To Move A Mountain: Railroads and mining in Utah's Bingham
Canyon Early Copper Era, 1900-1914. Retrieved 15 November 2010, 2010,
from www.utahrails.net/bingham/bingham-1900-1914.php.
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To move A mountain: Railroads and mining in Utah's Bingham Canyon early copper era
  • D Strack
Strack, D. (2010). To Move A Mountain: Railroads and mining in Utah's Bingham Canyon Early Copper Era, 1900-1914. Retrieved 15 November 2010, 2010, from www.utahrails.net/bingham/bingham-1900-1914.php.
Master spreadsheets compiling Australian mining production data
  • G M Mudd
Mudd, G. M. (2007). Master spreadsheets compiling Australian mining production data, Department of Civil Engineering, Monash University and Mineral Policy Institute.
Prominent Hill Resource Statement
  • Oz Minerals
OZ Minerals. (2008). Prominent Hill Resource Statement: June 30th 2008. Retrieved November 2010, 2010, from www.ozminerals.com/Media/docs/ASX_2008.09.24_ProminentHillJune2008 ResourceStatementExplanatory%20Notes-fe1f4c9d-7719-408b-81de-8caa1db5303c-0.pdf.
Historical Statistics for Mineral and Material Commodities in the United States, United States Geological Survey
  • T D Kelly
  • G R Matos
Kelly, T. D. and G. R. Matos (2010). Historical Statistics for Mineral and Material Commodities in the United States, United States Geological Survey.
Will sustainability constraints cause
  • G M Mudd
  • J D Ward
Mudd, G. M. and J. D. Ward. 2008. Will sustainability constraints cause "peak minerals?" Paper presented at the third International Conference on Sustainability Engineering & Science: Blueprints for Sustainable Infrastructure, 9-12 December, Auckland, New Zealand.