How do geologists find ore deposits

Ore deposits: how are the giants formed?

Large ore deposits are rare and difficult to find. One reason for this: How these mega-deposits are created is still unclear and therefore you don't know exactly where to look. Two geologists have now explored the formation of the ore giants more closely - but come to very different conclusions. The search recommendations for the mining corporations published in “Nature Geooscience” are correspondingly different.

The global demand for raw materials is constantly increasing. But the search for worthwhile deposits is tedious and time-consuming. Those who want to discover them are literally looking for a needle in a haystack. "Only one out of a thousand potential deposits companies are exploring will eventually develop into a mine," said Jamie Wilkinson of Imperial College London. The global hunger for raw materials is growing noticeably. There is growing concern that there could be bottlenecks - especially for those metals that are indispensable for modern technologies.

Ore accumulations at plate boundaries

It is all the more important to make the search for ore deposits more efficient. To do this, it is important to know the geological peculiarities that lead to the emergence of large deposits. Wilkinson and his colleague Jeremy Richards from the University of Alberta both studied the origin of what are known as porphyry ores. These usually arise where tectonic plates push one another.

The ore crystals are finely distributed in the host rock. A large part of the copper, silver, gold and molybdenum mined now comes from such deposits. Wilkinson and Richards focus on particularly large deposits - those that contain more than two megatons of copper or more than 100 tons of gold, for example. They are particularly rewarding - and particularly rare.

Is sulfur the key ingredient?

Wilkinson suspects that such giants can only form under the ore deposits if four conditions are met. According to this, the process begins at a subduction zone, where one tectonic plate pushes itself under the other. Under the pressure, hot, mineral-rich water rises from the lower plate and melts the mantle rock above, which now rises as magma.

The first condition for the formation of a deposit is that this melt is enriched with water and dissolved metals over time. In the second step, it absorbs large amounts of sulphides. The sulfur salts ensure that the metals collect in small, highly concentrated areas. From there, in the third step, they migrate into hydrothermal solutions, very hot, high-pressure accumulations of water in the rock. If the water rises and cools down, the dissolved compounds precipitate in the last step and form ore deposits.

Wilkinson is convinced that the enrichment of the magma with sulfur is the key condition for the formation of rich deposits of porphyry ores - and that mining companies should keep an eye on the sulphide saturation of the rock when exploring.

Or a favorable meeting of normal processes?

Richards, however, sees things differently: In his opinion, no special process is required to turn a measly ore deposit into a giant among the deposits. Instead, it is the “random convergence of processes that run as efficiently as possible and in the best possible sequence”. Many of the huge deposits that Richards examined show that, while normal processes were involved in their formation, they were much more pronounced than normal.

In some places the tectonic conditions were particularly favorable, in other cases the surrounding rock was particularly suitable for the deposition of the ore minerals. Richard's tip for mining companies is therefore to look for special forms of everyday life. The dispute over the emergence of particularly rich deposits of metal ores continues to smolder.

However, the two authors agree that we absolutely have to make the search for ores more efficient - both to satisfy our hunger for raw materials and to avoid unnecessarily polluting the environment. Whoever gives the better tips will ultimately decide in practice. (Nature Geoscience, 2013;

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(Nature Geoscience, October 14, 2013 - NSC)

October 14, 2013