Sedimentary exhalative deposits

In sedimentary - exhalative ( abbreviated as SEDEX ) or volcano- exhalative deposits are deposits that are formed by the escape of hot, mineral-rich hydrothermal solutions on the seabed. This fine-grained ore minerals are (mostly sulphides) in sea water chemically precipitated and layered in shape, often banded, deposited ore body ( Exhalite ), which are alternately stored with marine sedimentary rocks. Therefore, they are also quite cumbersome, referred to as " stratiform sulfide deposits of sedimentary affiliation." The transition from volcano- sedimentary massive sulphide deposits to pure volcanogenic massive sulphide deposits here is fluent. The latter, however, are almost exclusively associated with volcanic rocks. In recent publications, the term is replaced by SHMS SEDEX ( sedimentary Hosted Massive Sulfide - sediment- bound massive sulphides) to avoid premature genetic interpretations.

SEDEX deposits consist mainly of pyrite ( iron disulfide ). However, today they are the primary source of lead, zinc and barite and cover an important part of the world's supply of silver, copper, gold, bismuth and tungsten. Well-known examples are the European Kupferschiefer, the huge deposits of the Zambian - Congolese Copperbelts that. Sullivan mine in British Columbia, Red Dog in Alaska, and Mount Isa and Broken Hill in Australia The historical Mines of Rammelsberg in the Harz is now attributed to this type of deposit.

Mineralization

The concordant embedded in marine sediments ore bodies are lenticular or layered in the rule. The ratio of the lateral dimension to thickness is at least one in ten. Individual massive sulphide layers are powerful usually between a few millimeters and a few meters. The known copper shale is even from only a single seam which extends over large parts of central Europe, but this rarely more powerful than a meter is. When the host rock is mostly fine clays or shales, pelites and argillites. The ore zones are often rich in organic matter. In the lead- zinc deposit of Sullivan, the sediments are somewhat sandy, with occasional intercalations of impure quartzites and coarse conglomerates. Also in the Zambian - Congolese copper belt which deposits are sometimes embedded in sandy Psammite and arkoses. The supply channels to which the leaked mineral isere use solutions are rarely open-minded, their existence is but a prerequisite. There, where they were discovered, they usually have a hydrothermal rock alteration and brecciation. Common ore minerals are bornite, chalcocite, chalcopyrite, galena, sphalerite, etc. With the removal of the volcanic vents also decreases the ore content of the sediments.

Within tectonically disturbed sequences, a SEDEX mineralization followed a similar trend as other massive sulphide deposits, as it is location with low shear strength within sedimentary rocks of rigid silicates to a compliant ( incompetent ). Therefore, there are many examples of boudinage structures, corridors and veins of sulfides and hydrothermally wiedermobilisierte and enriched sections.

Formation

Until the 1950s this type of ore bodies have been attributed to the hydrothermal replacement deposits, entering only after the formation of the surrounding sediments ( epigenetic ) in this. Only after it was realized that they are deposited for the most part the same ( syngeneic ) with their host rocks in ocean basins. Since the 80s, it is assumed that volcano- exhalative deposits occur under conditions such as can still be seen, ( the so-called black smokers ) observed in undersea volcanic vents, they, but not in the deep sea, but during or after the transgression by relatively shallow seas over continental crust. In the case of the Zambian copper belt, one starts from a concurrent depression of tectonic Riftsystemen and pelvis. The resulted disorders and grave breaches would then have been the scene of volcanic processes and at the same time, the supply channels of the mineralizing solutions. Many volcano- exhalative lead-zinc deposits are similar to the Gulf of California rooted in a milieu where today actually the deposition of sulfides and barite is observed in enclosed sea basin. Whether next to it also the entry of products of weathering and mineral solutions from the adjacent mainlands of importance, or original sedimentary mineral formations were overprinted later epigenetically, but is controversial in many individual cases.

The source of metals and mineralizing solutions for SEDEX deposits can be brines from the deep, which are in contact with sedimentary rocks. These brines are saline to hypersaline waters that are released during solidification of sediments ( diagenesis ). Traces of lead, copper and zinc are found in all sediments. The metals are adsorbed from seawater 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 included during the deposition of the seabed sludge in the pore water. In the course of diagenesis, the pore water is squeezed from the sediments out. During the depression of the basin continues and the heat increases to about 150 ° to 350 ° C, the clay minerals begin to recrystallize under the pressure and release their water of crystallization, together 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 of zinc, copper and lead, and up to 100 ppm of barium and iron. Typically this layer waters contain substantial amounts of sulfur.

On the other hand, the metals may also have their origin in magmatic and hydrothermal fluids from magma chambers beneath the ocean floor. This process is much in the area of volcanic chains at mid-ocean ridges and island arcs.

The transport of these hot brines made ​​at by stratified reservoir rocks on the geological faults, which divide the layer packets into individual sedimentation basins. The brines seep into the basin margin faults, migrate due to thermal buoyancy and the pressure of the underlying reservoir upwards, and are released into the ocean water. The exit zones can be breckziierte volcanic breakdown tubes ( diatremes ), simple fumaroles, or so-called seepage mounds of chert and sulphides.

The deposition takes place, however, on deeper areas of the ocean floor, where the heavy, hot brines flow and mix with colder seawater. This means that dissolved in the brine metals and sulfur precipitate as solid metal sulfide ores from the solution and deposited as layers of sulfidic sediments.