Kimberlite is a rare igneous rock that originates deep within the Earth’s mantle and is the primary source for diamonds. This rock forms pipe-like structures that erupt rapidly to the surface, often carrying fragments of the deep mantle, including diamonds and associated minerals. Finding solid kimberlite rock on the surface is unusual because it weathers quickly into a softer material. Therefore, exploration relies on identifying its highly resistant, weathered components found in surface soil and sediment.
Physical Appearance of Weathered Kimberlite Soil
Kimberlite weathers rapidly upon exposure, transforming into a soft, fine-grained material known historically as “yellow ground.” This distinctive color results from the oxidation of iron-bearing minerals, like olivine and serpentine, into brownish-yellow limonite and other iron oxides. Below this oxidized layer, less-weathered material, often called “blue ground,” is typically a darker blue-green or gray due to the presence of serpentine.
The texture of the weathered soil is typically clay-rich and crumbly, distinguishing it from the surrounding host rock. This clay content comes from the alteration of primary kimberlite minerals, particularly olivine, which is unstable at surface conditions. Within this soft matrix, one may observe larger fragments of rock called xenoliths, which are pieces of the surrounding crust or deep mantle carried up during the kimberlite’s ascent. These fragments can include dense peridotite or eclogite, offering a clue to the rock’s deep origin.
Identifying Kimberlite Indicator Minerals
Since the kimberlite matrix breaks down quickly, the most reliable identification method involves searching for highly resistant Kimberlite Indicator Minerals (KIMs). These minerals crystallized deep in the mantle alongside diamonds, surviving weathering and transport, and acting as signposts for a nearby kimberlite pipe. The three primary KIMs are pyrope garnet, magnesian ilmenite, and chromite.
Pyrope garnets are visually striking, presenting as distinctive purple, magenta, or deep red grains, often rounded due to abrasion. These grains may exhibit an “orange-peel” surface texture or a dark green-grey alteration rind, called kelyphite. Magnesian ilmenite, a dense, black iron-titanium oxide, is another common indicator. It appears as rounded, highly polished grains that may have a dull, white-to-grey coating of leucoxene, obscuring its metallic black surface.
Chromite, a chromium-rich spinel, is found as a black, opaque mineral, often occurring as small octahedral crystals. Unlike ilmenite, which is slightly magnetic, chromite is often non-magnetic, aiding in separation. Another important KIM is chrome-diopside, recognized by its bright emerald-green color and blocky crystal shape. The presence of these minerals, particularly the distinctive purple pyrope, strongly indicates a kimberlite source.
Practical Field Sampling and Concentration Techniques
The initial step in kimberlite exploration is systematically collecting a large volume of soil or sediment to ensure a representative sample of indicator minerals. Stream sediment sampling is an effective field method, focusing on drainages where heavy minerals accumulate, such as behind large boulders or in meander bends. Samples are typically 10 to 20 kilograms and must be carefully labeled with coordinates, date, and observations.
Once collected, the bulk sample is processed to isolate the heavy mineral concentrate (HMC) containing the KIMs. Processing begins with wet or dry sieving to isolate the target fraction (usually 0.25 mm to 2.0 mm). Lighter soil components are then removed using concentration techniques like panning or sluicing, which exploit the high density of the indicator minerals.
For thorough separation, the concentrate is sent to a laboratory for heavy liquid separation, often using a high-density solution to float off remaining lighter minerals. Further refinement involves magnetic separation to isolate paramagnetic minerals, such as magnesian ilmenite, from the non-magnetic fraction (pyrope garnet and chromite). This multi-step concentration significantly reduces the sample volume, making visual picking feasible.
Interpreting Geological Context and Indicator Trains
Identifying indicator minerals is the first step; the next is using their distribution to pinpoint the kimberlite source. KIMs disperse from their source, creating a traceable pattern known as an “indicator train” or dispersion halo in the surrounding soil or sediment. The shape of this train depends on local geomorphology, such as glacial ice movement or stream flow.
To locate the source, one must systematically sample and map the concentration of KIMs, following the increasing abundance or coarseness of the grains. The train narrows, and the mineral grains become larger and less abraded as sampling progresses upstream or upslope toward the original pipe. Kimberlite intrusions are most commonly found in the ancient, stable cores of continents called cratons, which provides a regional context for exploration. By plotting the mineral counts, one can vector the search area to where the indicator train terminates, revealing the likely location of the buried kimberlite pipe.