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An introduction to geology
An introduction to geology

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2.18 Black smokers

If the hot hydrothermal fluids make it back to the sea floor, the elements dissolved come out of solution very rapidly, forming plumes of black material which look like smoke appearing from a chimney, giving these features the name ‘black smokers’. The video below shows an example.

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Video 2.3 Black smokers
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Their existence was only discovered in the late 1970s when we had developed the technology to send cameras down to the sea floor. The water emerging is still super-heated – it can reach temperatures greater than 300 °C. So hot, in fact, that there is a (perhaps apocryphal) story that the first temperature probes put into the plumes melted!

The material that forms the plume ‘rains’ down on the sea floor around the chimney, and is deposited as a metal-rich sediment. The chimneys themselves are formed of the same sorts of material, and eventually these will collapse, adding further to a mound of metal-rich material. It’s this mound, preserved and then uplifted, that, along with the filled fissures and cracks, forms the ore body at Parys Mountain.

Gangue and ore minerals

Where do the metals come from? Well, in a black smoker-type hydrothermal system, they come from the surrounding oceanic crust. The water percolates through cracks and fissures formed when the igneous oceanic crust forms at the sea floor volcanoes. The rocks that form have copper, silver, zinc and gold in them, but only in very low concentrations. The hydrothermal fluids take those metals out from a very large volume of rocks, and then redeposit them in a much smaller volume of rock, and so at much higher concentration. This geological concentration of elements makes for very good ores.