Young diamonds(300 Ma) created without cratons and at depths of 80 km,...blasphemy according to some (gggggggggggg)
http://www.minerals.nsw.gov.au/minfacts/41.htm
Geologists from New South Wales have come up with a new theory about the origin of diamonds that could see major new diamond exploration in the State.
The geologists, from the Geological Survey of NSW, have turned conventional wisdom about the formation of diamonds on its head, saying that the model that applies to South Africa and Western Australia is not relevant for NSW.
They say the new theory may have solved the long standing riddle of the origin of the alluvial diamonds found in eastern NSW, especially in the New England region.
The conventional view is that diamonds can only form at pressures and temperatures which are found at depths of more than 150km below the surface of the Earth.
Diamond explorers have until now only searched for diamonds in the rocks, kimberlites or lamproites, which have come from these depths.
The theory they have based their exploration on requires the crust to be thick enough to provide the high pressures required to form the diamonds.
Thick, rigid, cool crust is also necessary to allow the rapid passage of the kimberlite or lamproite to the surface.
A consequence of this is that the prospective crust and the diamonds must be ancient, 1100 million years old or older.
This traditional view is valid for the formation of diamond in a static geological environment. Most of the world's diamond occurrences fit this model, but NSW is a major exception.
Eastern NSW has numerous occurrences of alluvial diamonds but, contrary to the traditional theory, these areas are characterised by thin, relatively young crust and have no kimberlites or lamproites.
The new model developed by the Geological Survey, in collaboration with geologists from the Australian Museum, appears to have solved the riddle of the alluvial diamonds by describing a way in which diamonds can be formed in a dynamic environment at much shallower depths and carried to the surface by rocks other than kimberlite or lamproite.
The new model starts with the sinking or subduction of carbon-bearing marine sediments in a collision zone between two tectonic plates.
If the subducted slab is thick enough, it will remain comparatively cool as it sinks and diamond will form dynamically during subduction when sufficiently high pressure is reached.
Calculations show that diamonds can form at depths as shallow as 80km, which means that rocks other than kimberlite or lamproite can bring them to the surface.
Nephelinites, which are common in eastern Australia, were thought to be suitable rocks. Alkali basalt, also common, was another possible host.
Several of the volcanic pipes in NSW in which diamonds have been found consist of nephelinitic or alkali basaltic rocks.
The model also allows for the formation of diamonds of virtually any age, depending on the time of subduction, which accords with the findings in NSW.
The only diamond from the State to be dated was found to be only about 300 million years old, much younger than the 1100 million years or older age of diamonds formed under the traditional model.
Many parts of eastern NSW which have already been explored without success using the traditional exploration concepts and techniques are now prospective for the new type of diamond deposit described by the Geological Survey's model.
The model may also be applicable to other parts of the world where diamonds are known from alluvial deposits, but where no thick ancient crust or kimberlites and lamproites occur.
The new model also had profound implications for indicator mineral searching, which relies on the location of various minerals which are normally associated with diamond.
The model provides a new range of indicator minerals, and makes specific predictions about the trace element compositions of certain minerals which can be used as an exploration tool.
A detailed report on the new model can be found in 'Quarterly Notes 94', published by the Department of Mineral Resources and available for $3, plus $2.50 for postage and handling, from the Information Counter, Department of Mineral Resources, 29-57 Christie St, St Leonards 2065, (PO Box 536, St Leonards 1590). Phone, (02) 9901 8269. Fax, (02) 9901 8247. |
