Research
Nonrenewable copper resources growing scarce

Unlike forests, fisheries and other resources that can be renewed relatively rapidly through natural processes, mineral deposits form so slowly that they're classified as nonrenewable.

But just how nonrenewable are they, and is it possible to put a number on their nonrenewability?

For one mineral — copper — researchers at U-M and Syracuse University have done just that. Their results, reported in the March issue of the journal Geology, show that humans use about 18,000 times more copper each year than Earth manages to put into new deposits.

In the past, estimates of the world's mineral resources have relied on two basic approaches: geologic and economic, says Stephen Kesler, professor of geological sciences and a co-author on the paper. Geologic estimates compile data on mineral deposits in well-explored areas and extrapolate into less-explored regions. Economic estimates look at how much of a given mineral resource already has been found, assume that's a representative sample and then use statistical methods to figure out what proportion of Earth's total reserves the sample represents.

But for most resources, both of these methods provide estimates to depths of only about a kilometer (0.6 mile), ignoring large parts of Earth's 50-kilometer thick crust that might contain important deposits.

He and collaborator Bruce Wilkinson of Syracuse University accomplished this by developing a model that simulates the migration of deposits through Earth's crust over time. To check the accuracy of their model, they compared its estimates for the upper 1 kilometer of crust in the United States to a U.S. Geological Survey estimate of the same area that represented hundreds of person-years of geologic studies. The estimates agreed, "so that made us feel that the model works," Kesler says.

The researchers estimated how soon the amount of copper in deposits above 3.3 kilometers — the likely limit of future mining — will be depleted. The answer — 5,500 years — may sound like a long time, but to geologists it's alarmingly short, Kesler says.

Next, he and Wilkinson want to use their model to estimate the world's gold and oil deposits.