The mining and processing of diamond ore is relatively simple and extremely profitable.. Economic deposits of diamonds, however, are difficult and expensive to locate. The process of exploring for and evaluating diamond deposits can cost millions of dollars and take several years. Mine construction and commissioning can also be very expensive. Mines with significant through-put can cost several hundreds of millions of dollars to build.
The diamonds that are mined today were formed more than 3 billion years ago within the earth’s mantle under specific temperature and pressure conditions where carbon is stable in the form of diamond rather than as graphite. This is known as the diamond stability field. The diamond stability field is located at depths of over 150 kilometers within the roots of the earth’s cratons. The cratons are composed of the oldest rocks in the world and form the continental nuclei. With a few notable exceptions, all economic diamond deposits are found in cratons.
The source rock of economic diamonds, with only one known exception (the Argyle Mine in Australia), is a volcanic rock known as kimberlite. Kimberlites are volcanic eruptions that originate below the diamond stability field. Kimberlites may or may not capture diamonds as they ascend through the earth’s mantle on their way to the earth’s surface. At the earth’s surface kimberlites form pipes, steep walled volcanic vents that range in size from a few tenths of a hectare to several tens of hectares. Kimberlites also occur as dykes ranging in size from a few centimeters in width to tens of meters in width and several kilometers long. Kimberlites are generally found in clusters with an aerial extent of hundreds of square kilometers. Typically, there are tens of kimberlites within a single cluster.
Kimberlites have been discovered on every major craton on earth. Globally, there are approximately 6,000 known kimberlites, 800 of these are known to be diamondiferous with only 400 having measurable grade. Currently there are less than 50 kimberlites being mined.
Diamond Exploration Methodologies
Exploring for and evaluating diamond deposits require specialized expertise that exists only thinly outside of the major diamond companies. Diamond exploration is a highly secretive business with interpretive techniques considered proprietary and tightly guarded.
There are four distinct phases in the identification and evaluation of a diamond deposit. These phases are illustrated in the flow chart below.
Target identification is generally carried out with a combination of techniques that include heavy mineral sampling, airborne and ground geophysical surveys. Heavy mineral sampling is conducted to capture a suite of minerals that are specific to and shed from kimberlites. These are known as Kimberlite Indicator Minerals (KIM’s) and occur in much greater concentrations within the kimberlite than do diamonds. Research has shown that those KIM’s that occur as inclusions within diamonds, and hence must have grown within the diamond stability field, have a specific chemical composition. Thus, KIM’s identified in a heavy mineral sample are analyzed with an electron micro-probe to determine their specific chemical make-up.
Unbiased information on Diamonds provided by Bruce Counts of Indicator Minerals Inc.: