Subject: Diamonds

What reconnaissance techniques are used by diamond mining firms to successfully search for productuve kimberlite pipes? And after discovery what techniques are used to exploit the deposits? Could you provide references, and/or sources of information?

    Natural diamonds are found in at least three environments.

    One, of rare occurrence and I believe without any commercial mining, in areas where extreme uplift and extreme erosion has laid bare levels of Earth's upper mantle formed more than 100 km below the surface. The Dabie Shan area of ultrametamorphism in Central China is the principal example under investigation today (Nie and others, Geology, vol 22, November 1994).

    Two, which is to answer your question: diamonds are found in kimberlites and somewhat similar lamproites. These rocks are the volcanic breccias that filled steep narrow throats (pipes, or "diatremes") of gas-rich eruptions from depths greater than 100 km. As the pipes narrow with depth, breccia gives way to igneous kimberlite. Kimberlites are uncommon but widely distributed and mined in Central and Southern Africa (Kimberly discovery 1871), Eastern Siberia (Yakutiya discovery 1954), and Northwest Australia (Fitzroy, etc. discoveries 1977).India and Brazil had some importance before the diamond rush of South Africa, and the most recent interest has been in Canada, New South Wales, and along part of the Wyoming-Colorado state line. Diamonds are a very tiny fraction of the mass of kimberlite or lamproite. For example, several hundred African kimberlites have been discovered, but less than 3 dozen contain any diamonds at all. Of those with diamonds, about half are too lean to be mined economically. Even the very richest kimberlites contain very low diamond contents [about 1 caret, or 0.2 grams, for each three metric tons of kimberlite; Skinner, 1976, Earth Resources, Prentice-Hall]. According to Evans, 1993 (Ore Geology and Industrial Minerals, Blackwell) the pipe at Kimberly itself yielded only 3 tonnes of diamonds out of 24 million tonnes of kimberlite mined between discovery in 1871 and the end of mining from the open pit, >1 km deep, in 1908. General references about the rocks are Kornprobst, ed., 1984 (Kimberlites and Related Rocks; Elsevier), and Rock, 1991 (Lamprophyres; Blackie).
    The pipes may be of almost any age, down to about 20 million years ago. Pipes intruding the very oldest rocks (the 3.2 billion year-old parts of the "Archean" cratons; although Archean may range as young as 2.5 billion years) are very much more likely to bear diamonds than ones cutting younger rocks (see illustrations and comments in Evans, with references to South African and Australian literature that I don't have here). Prospecting in the past has been to start with a search for placer diamonds (section 3 below) or other such indicator minerals of kimberlites and lamproites as pyrope (red-brown or purple-red garnets), chrome diopside (a chromium-bearing magnesium- and calcium-rich pyroxene), magnesian ilmenite (a magnesium-bearing iron and titanium oxide), and perovskite (a magnesium and iron silicate with a dense arrangement of ions in its crystal lattice). The stream sands are panned just as a prospector pans for gold, gently washing away the less dense quartz and feldspar sand and concentrating the dense grains. Descriptions of indicator minerals is by Mosig and by Nixon in a 1980 volume edited by John Glover (Kimberlites and Diamonds) published by the University of Western Australia. Having found a stream with indicator minerals, the prospector searches upstream for shallow lakes or depressions, or masses of yellowish soil. The surface areas of kimberlite pipes are small, generally less than 1 sq km. In Africa and Australia, there may be crater-like shallow surface depressions called maars. They indicate some kimberlites. Younger African maars may be circled by a low ring of volcanic ejecta (tuff) of kimberlite composition. They commonly are filled with fine sediments that formed on the bed of lakes that filled the maars in past times when the climate was wetter. Some kimberlites have no special crater shape or depression to aid prospecting, but the kimberlite weathers to a yellowish soil. Where excavated, the "yellow ground" gives way at depth to fresher "blue ground", and deeper still, to the unaltered dark greenish-brown kimberlite.
    Recent glaciers have scraped away any maar features and weathered soil from the tops of pipes in Canada and Siberia, and deep glacial deposits may cover the tops and adjacent countryside. I have heard that aeromagnetic lines have to be flown at a few 100-m spacing or closer to be able to record the little changes in Earth's magnetic field associated with a diamond pipe. Perhaps one of our recent graduates, Dr.Glenn Brown at University of Toronto may be able to give you a reference on geophysical exploration in Canada, as he has followed that effort (
    Diamond pipes have the shape of carrots, or of inverted teardrops. They do not have the shape of Nieman-Marcus cookies. Some spread out at the surface like the bell of a trumpet. So mining commonly starts as open pit, and may change to underground mining as the cost of open-pit mining increases with depth. Guilbert and Park have a fair description of mining, as well as of ways to concentrate the diamonds from the gangue after mining (1986, The Geology of Ore Deposits; W H Freeman)..

    Three, placers, the most important source of natural industrial diamonds, and a fairly important source of natural gem diamonds. Actually, all but a few percent of diamonds are mined from stream and beach placers. Diamonds are dense, insoluble, and the hardest mineral, and so even though they can be split ("cleavage"), they are preserved when kimberlite weathers and diamond grains are released to be carried away by running water.

   Summary. Work in Archean terrane, the older the better. Pan streams; work upstream. If you discover a kimberlite pipe, mine a lot of it to see what its diamond content is. Good luck.

Dr. Ralph Moberly, Professor
Department of Geology and Geophysics
University of Hawaii, Honolulu, HI 96822

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