Sedimentary exhalative deposits
In sedimentary-exhalative (abbreviated SEDEX) or volcano-exhalative deposits is deposits caused by the escape of hot, mineral-containing hydrothermal solutions are formed on the seabed. Here, fine-grain ore minerals (mainly sulfides ) are chemically precipitated in seawater and deposited in the form of layered, often banded, ore bodies (exhalites), which are alternately stored with marine sedimentary rocks . That is why they are also known, somewhat inconveniently, as “stratiform sulphide deposits of sedimentary affiliation”. The transition from volcanic-sedimentary massive sulphide deposits to purely volcanogenic massive sulphide deposits is fluid. However, the latter are almost exclusively connected to volcanic rocks . In more recent publications, the term SEDEX is replaced by SHMS ( Sediment Hosted Massive Sulphides - sediment- bound massive sulphides) in order to avoid rash genetic interpretations.
SEDEX deposits consist mainly of pyrite (iron disulfide). Nevertheless, they are the most important source of lead , zinc and barite today and cover an important part of the global demand for silver , copper , gold , bismuth and tungsten . Well-known examples are the European copper shale , the huge deposits of the Zambian - Congolese Copper Belt , the Sullivan Mine in British Columbia , Red Dog in Alaska, and Mount Isa and Broken Hill in Australia. The historic Rammelsberg mine in the Harz Mountains is also assigned to this type of deposit today.
Mineralization
The ore bodies, which are concordantly embedded in marine sediments, are usually lenticular or layered. The ratio of the thickness to the lateral extent is at least one in ten. Individual layers of massive sulphide are usually between a few millimeters and a few meters thick. The well-known copper slate even consists of a single seam that stretches over large parts of Central Europe, but is rarely thicker than a meter. The host rock is mostly fine clay stones or slate , pelite and argillite . The ore zones are often rich in organic matter. In the lead-zinc deposit at Sullivan, however, the sediments are somewhat sandy , with occasional interferences of impure quartzites and coarser conglomerates . In the Zambian-Congolese copper belt, too, the deposits are sometimes embedded in sandy psammites and arkoses . The feed channels from which the mineralizing solutions emerged are seldom exposed , but their existence is assumed. Where they were discovered, they usually show a hydrothermal rock transformation and brecciation . Frequent ore minerals are bornite , chalcocene , chalcopyrite , galena , sphalerite, etc. With the distance from the volcanic vents, the ore content of the sediments also decreases.
Within tectonically disturbed sequences, SEDEX mineralization develops similarly to other massive sulphide deposits, as it is a flexible ( incompetent ) layer with low shear strength within sedimentary rocks made of more rigid silicates . That is why there are many examples of boudinage structures, dikes and veins made of sulphides, as well as hydrothermally remobilized and enriched sections.
Emergence
Until the 1950s, this type of ore was assigned to the hydrothermal displacement deposits, which only penetrate ( epigenetically ) into the surrounding sediments after they have formed. Only then was it recognized that they are for the most part deposited simultaneously ( syngenetically ) with their host rock in sea basins. Since the 1980s it has been assumed that volcanic exhalative deposits arise under conditions that can still be observed today in submarine volcanic chimneys (the so-called black smokers ), but not in the deep sea, but during or after the transgression from relatively shallow seas to continental crust. In the case of the Zambian copper belt, a simultaneous subsidence of tectonic rift systems and basins is assumed . The resulting faults and rift breaks would then have been the scene of volcanic processes and at the same time the feed channels for the mineralizing solutions. Many volcanic-exhalative lead-zinc deposits are located in a milieu similar to the Gulf of California, where today the deposition of sulfides and barite is actually observed in enclosed sea basins. However, in many individual cases it is controversial whether the entry of weathering products and mineral solutions from the neighboring mainland is also important, or whether originally sedimentary mineral formations were later epigenetically influenced.
The source of the metals and mineralizing solutions for SEDEX deposits can be brines from the depths that are in contact with sedimentary rocks. These brines are saline to hypersaline waters that are released during the solidification of sediments ( diagenesis ). Traces of lead, copper and zinc can be found in all sediments. The metals are adsorbed from the sea water and are only weakly bound to the edges of the crystals of hydrous clay minerals or in the crystal lattice of carbonates . The salt was already trapped in the pore water during the deposition of the seabed mud. In the course of the diagenesis, the pore water is pressed out of the sediments. While the sinking of the basin continues and the heat increases to around 150 ° to 350 ° C, the clay minerals begin to recrystallize under the pressure and release their water of crystallization, along with the salt and the incompatible metals. It is estimated that the resulting hydrothermal fluids have a salinity of up to 35% NaCl, with metal concentrations of 5 to 15 ppm zinc, copper and lead, and up to 100 ppm barium and iron. As a rule, these stratified waters also contain considerable amounts of sulfur.
On the other hand, the metals can also have their origin in igneous and hydrothermal fluids from magma chambers below the ocean floor. This process is important in the vicinity of volcanic chains at mid-ocean ridges and island arches .
These hot brines are transported through layered storage rocks towards the geological faults, which subdivide the layer packages into individual sedimentation basins . The brines seep into the pool edge faults, migrate upwards due to the thermal buoyancy and pressure of the underlying reservoir, and are released into the ocean water. The exit zones can be broken volcanic breakthrough tubes ( diatrems ), simple fumaroles , or so-called seepage mounds made of silica schist and sulphides.
However, the deposition occurs on lower areas of the ocean floor, where the heavy, hot brines flow and mix with colder sea water. This leads to the metals dissolved in the brine and the sulfur precipitating out of the solution as solid metal sulfide ores and being deposited as layers of sulfidic sediments.
See also
literature
Anthony M. Evans: Mineral Deposition Studies. Ferdinand Enke Verlag, Stuttgart 1992, ISBN 3-432-99801-5 .