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IT鈥橲 good news for gold-diggers. There鈥檚 now an easier way for mining companies to spot rocks that contain ores richly laced with the precious metal.

The world鈥檚 richest sources of ores such as copper lie in porphyry deposits, which form when magma chambers deep in the Earth鈥檚 crust release steam-rich gases and salty fluids. As these rise and cool, they deposit a rich cargo of metals such as copper and gold. The levels of gold in a deposit are very important to mining companies. 鈥淕old as a by-product significantly increases the value of the ore, and could make the difference between an economic and an uneconomic deposit,鈥� says Werner Halter of the Swiss Federal Institute of Technology in Z眉rich.

That鈥檚 why copper-mining companies as well as scientists are keen to know what determines the ratios of different metals in these deposits, and how to measure them. At the moment, it鈥檚 only possible to work out how much gold is present at a potential mining site by analysing large amounts of rock from drill holes. But now there may be another way.

The secret seems to lie in droplets of molten sulphides, just a few micrometres across, that get swept up from the magma chamber by the hot fluids. Halter鈥檚 team used state-of-the-art mass spectrometry techniques to analyse some of these specks of rock that were taken from a copper mine in Argentina.

They found that the levels of copper and gold in the solidified sulphide globules are directly related to their overall amounts in the surrounding rock. It could be an easy way for mining companies to spot gold-rich deposits, since mass spectrometry of samples picked up on the ground is much quicker and simpler than drilling.

The results also give geologists a clue as to why the richness of the deposits varies. The sulphide blobs scavenge all the gold and copper in the magma, so they contain minerals at levels up to 10,000 times those in the rest of the magma. The researchers say that the best gold deposits probably form when hot fluids dissolve metal-rich sulphides and sweep them up.