Massive sulphides in smoky depths
Sulphides Sulphides are chemical compounds of sulphur and metal. The components of massive sulphides include iron sulphide (pyrite), copper sulphide (chalcopyrite), and zinc sulphide (sphalerite), as well as sulphides of other metals such as gold and silver. The relatively high content of precious metals makes the massive sulphides promising for ocean mining.
A very hot stream of waterBeside manganese nodules and cobalt crusts, a third type of metal-bearing mineral resource is found in the sea: massive sulphides. They consist of sulphur compounds, sulphides, which form massive deposits on the sea floor similar to cobalt crusts – thus the name. Massive sulphides originate at hot vents in the ocean where sulphide-enriched water flows out of the seabed. These sites of escaping hot water are called hydrothermal vents. They are found along plate boundaries and at active undersea volcanoes, where the exchange of heat and elements between the crustal rocks and the ocean takes place due to interactions between the volcanic activity and seawater. Seawater penetrates several thousand metres into the bottom through fissures in the sea floor. At these depths the seawater is heated to temperatures of around 400 degrees Celsius by volcanic activity, whereupon it dissolves metals and sulphur from the ambient volcanic rocks. The heated water is less dense than the cooler water above, so it rises quickly and flows back into the sea. In the ocean, the plume of hot water cools again rapidly. This causes the dissolved metals to bind into minute sulphide particles and sink as fine precipitants to the bottom.
- 2.26 > Distribution of hydrothermal vents by depth and type of origin.
- At many hydrothermal vents around the world the sulphides have accumulated to form tall chimney-like structures on the sea floor. Water shoots out of the fissures into the sea like a fountain. More and more material is gradually deposited on the sides of the openings and the tower continues to grow. Because of their appearance, these structures are also called smokers. As the escaping water is usually black-coloured by the minerals it contains, they are also called black smokers. The first black smokers were discovered in 1979 during an expedition to the East Pacific Rise. They caused a sensation not only for eologists, but also for biologists because they were found to be populated by a diverse animal community. Scientists had not expected to find so much life in the deep sea. At that time it was considered to be a bleak and empty landscape. Hydrothermal vents have now been found in all oceans. They usually form in water depths between 1000 and 4000 metres.
- 2.27 > Smokers form in magmatically active ocean regions. Water seeps through fractures in the sea bottom to depths of several thousand metres. Near magma chambers it is warmed to around 400 degrees Celsius and dissolves minerals from the rocks. Because of its low density it rises again and shoots out of the smoker back into the sea. Reaction with the cold seawater causes the formation of mineral particles that are then deposited onto the chimneys of the smoker or the nearby sea floor.
- Massive sulphides occur around the world at plate boundaries. Geologists distinguish 4 different typical areas of origin for hydrothermal vents and the associated massive sulphides: AT MID-OCEAN RIDGES: Mid-ocean ridges are mountain ranges in the ocean that circle the globe like the seam on a baseball. This is where the oceanic plates are drifting apart. The separation produces fractures in the seabed through which water sinks to great depths to be heated at magma chambers. AT ISLAND-ARC VOLCANOES: Island-arc volcanoes are formed when one oceanic plate is forced beneath another one under the sea. The subducted rocks melt at great depths and then rise as magma. Over time a large volcano grows. As long as the volcano does not reach the sea surface, it is called a seamount. Hydrothermal vents can form near the crowns of these underwater volcanoes. Many islands in the southwest Pacific have formed by this kind of subduction of oceanic plates and the rising of magma. There are usually a number of these volcanoes lined up in an arc along the subducting plate boundary because of the spherical form of the Earth. They are then called island arcs. VOLCANOES BEHIND ISLAND ARCS (back-arc basins): When one plate submerges beneath another, tension is produced in the overlying plate. Subduction of the sinking plate causes the overlying plate to thin and pull apart, until it finally splits open. In many cases this kind of tension occurs several dozen kilometres behind the active island-arc volcanoes. This area is therefore referred to as the back-arc basin. AT INTRAPLATE VOLCANOES: In addition to plate boundaries and subduction zones, volcanoes also form in the plate interiors. In these cases magma rises through fissures, burning its way through the Earth’s crust like a blowtorch. Because they form at individual sites or points, they are called hotspots. Single, isolated hydrothermal vents can also be found at these hotspots. The Hawaiian island group is an example of intraplate volcanoes. It was formed as the oceanic plate slowly moved across the hotspot. At various points magma has erupted to build up the islands.
Uncounted hydrothermal ventsTo date, expeditions have discovered around 187 active hydrothermal vents with massive sulphides. An additional 80 known hydrothermal vents are no longer active, but massive sulphides are found here that were deposited in the past. Furthermore, 30 sites are known where high-temperature hydrothermal solutions flow out of the seabed but no massive sulphides have formed. There could, however, be sulphide deposits below the surface here. So there are a total of around 300 hydrothermal vents or massive sulphide deposits known today. 58 per cent of these are located at mid-ocean ridges, 26 per cent at the back-arc spreading zones, 16 per cent at island-arc volcanoes, and one per cent on intraplate volcanoes. Researchers assume that the worldwide number of hydrothermal vents, and thus of massive sulphides, is much larger. This is based on estimates of the geothermal heat flux of the Earth. The amount of heat generated in the Earth’s interior and that released by magmatism and volcanism is accurately known today. This heat amounts to 1.8 trillion watts, equivalent to the output of one million nuclear power plants. According to the estimates, a portion of the heat is released through hydrothermal vents. Based on the calculations, some researchers reckon that there is one hydrothermal vent for every kilometre of mid-ocean ridge or back-arc spreading zone. Considering that the mid-ocean ridges and back-arc spreading zones together have a total length of about 67,000 kilometres, and the island-arc volcanoes a length of around 22,000 kilometres, there could be around 90,000 hydrothermal vents worldwide. Researchers assume that large areas may be found every 50 to 100 kilometres that contain up to 100 black smokers. It is predicted that there are around 500 to 1000 sites around the world with large massive sulphide deposits.
- 2.28 > The number of hydrothermal vents is difficult to determine because they are dispersed around the world. 187 active and 80 inactive hydrothermal vents where massive sulphides have formed are known to exist.
- 2.29 > Hydrothermal vents develop in different kinds of magmatically active areas where water penetrates to great depths and is heated. These areas include island-arc volcanoes, for example, which are formed when rocks plunging far below the sea floor are melted. Behind the island arcs, the seabed ruptures due to the spreading motion of the Earth’s crust, allowing magma to rise. Mid-ocean ridges form when oceanic plates drift apart. Intraplate volcanoes, on the other hand, originate at weak points in the crust.
- The size and metal content of massive sulphides, however, are difficult to measure. This is because the hot-water plume escaping from active smokers disperses rapidly and the sulphides, in part, are carried away by the currents. Massive sulphide areas extending 10 to hundreds of metres can thus be formed that contain several million tonnes of massive sulphides. At a single glance, however, it is not possible to tell how large an occurrence is; this requires bottom samples or drill cores. This costly sampling process is also necessary to determine the metal content. Based on the analyses of many bottom samples carried out in recent decades, researchers believe that massive sulphide deposits containing valuable metals such as copper and gold that are actually large enough for economic mining occur at relatively few hydrothermal vents. Moreover, many of the regions are in rough terrain that is unsuitable for the mining equipment. >