Kimberlites

The generally accepted primary deposit type for economic quantities of diamonds is kimberlite intrusions. Kimberlite is one of two types of rock mined for diamonds in the world; the other is lamproite, which is rarely mined (Argyle, Australia). Kimberlites are rare, most commonly small (1 to 10 ha but can be 200 ha) alkaline ultrabasic intrusions. The kimberlitic magma rises approximately 200 km through the upper mantle and crust, picking up many other rocks and minerals on its way to surface.

The kimberlite magma must be derived from deep within the mantle of the earth, where pressures and temperatures are such that diamonds are stable. The Slave Craton is a large region of ancient, stable and cool crust, which increases the likelihood that the kimberlites tap deep into the mantle, increasing the likelihood that the intrusions will contain a higher concentration of diamonds.

Kimberlites have a distinctly inequigranular texture resulting from the presence of macrocrysts and phenocrysts set in a finer grained matrix. Typically, the matrix consists of olivine phenocrysts and several other minerals of which the most common are carbonates (commonly calcite), serpentine and ilmenite. The macrocrysts and phenocrysts are ferromagnesian minerals such as olivine, various garnets, clinopyroxene and orthopyroxene. The macrocrysts and relatively early formed matrix minerals are commonly altered by serpentinization and carbonatization. Kimberlite usually contains inclusions of upper mantle-derived ultramafic rocks (i.e., fragments of material sampled from depths of 150 to 200 km below the earth's surface) and variable quantities of crustal rock fragments.

Diamonds are absorbed into kimberlitic magma as it passes through the upper mantle. As the magma rises from the lower mantle, it picks up xenoliths and xenocrysts from all other rocks it passes through including olivine (most common), ilmenite-magnetite, chrome diposite, garnets and chromites derived from peridotites, eclogites and other common upper mantle rocks. Where carbon is present, the pressures and temperatures of cratonic mantle rocks make diamond the likely form of the carbon, and under selected conditions, the diamonds can remain significantly unmodified as they are transported to surface. As the magma passes through the crust, it also absorbs xenoliths of crustal rocks including the local rocks currently found nearby (and in some cases "above") the kimberlite.

Diamond content and quality depends on many factors. Content is limited by the many factors including carbon content of upper mantle rocks, pressure and temperature conditions of upper mantle rocks, amount of incorporation of upper mantle material into the magma, oxidation state of the magma (preservation) and contamination by country rock (barren), likely along with other factors that the author is unaware of.

The kimberlite magmas are emplaced from a great depth and must rise through the upper mantle and crust. As the kimberlite magma (along with its xenoliths and xenocrysts) rises, the confining pressure drops rapidly with ascent. Volatiles in the magma form gaseous phases at the "top" of the ascending column and expand rapidly. This causes crack propagation and brecciation, which creates further room for ascent of the magma. As the magma nears the surface of the earth, this brecciation extends to the surface and the pressure drops extremely rapidly, causing a huge amount of degassing, brecciating a huge column of rock which forms the diatreme/crater of the kimberlite intrusion. The surface explosion widens the crater which typically is approximately 100m to 500m across and may extend to a depth of a kilometre or more. The crater and diatreme are typically carrot shaped. Although kimberlite magmas are generally thought to be small volumes, a tuff ring likely forms around the crater and surface material mixes with this material and may or may not slough significant volumes of material back into the crater. In some cases kimberlites may not extend to surface and may present as dykes or sills which also occur in the root zones of the craters.

Typical kimberlite classification is by texture relating to emplacement regime. The kimberlites have historically been divided between crater, diatreme and root zone (hypabyssal) facies representing near surface, middle depths and deeper deposition (or intrusion) respectively. Newer classification schemes focus on the texture modification based upon the method of magma extrusion (or intrusion). Kimberlites are first divided into fragmental or non-fragmental types. The fragmental type is termed volcaniclastic ("VK") where the exact depositional environment is not known or more specifically pyroclastic ("PK") or re-sedimented volcaniclastic ("RVK") when the materials depositional history is supported by physical evidence. Non-fragmental deposits are normally termed coherent ("CK") or hypabyssal ("HK") kimberlite.

As noted above, VK and CK are textural descriptions and do not imply a genetic history. PK deposits are deposited from primary pyroclastic processes such as airfall, ignimbrites and basal flows and may result in sorting of kimberlite clasts or losses of finer material. RVK deposits occur during mixing of surficial material with crater deposits which are remobilized back into the crater after deposition on the crater rim, likely by mass slumping. TK deposits are interpreted as deposition through fluidization of the magma as pressure drops during exposure to surface and incorporates significant crustal xenolith contamination, often as large blocks.

PK, VK and RVK types are typically associated with crater facies. TK and PK types are associated with diatreme facies. CK or HK types are typically associated with root zone facies.

Kimberlite vent emplacement shape appears to be largely related to country rock geology and confining pressure, although other factors such as volatile content affect the final shape and size as well. The shapes are generally divided into class 1, 2 and 3. Class 1 is large carat shaped diatremes infilled with TK or TK breccias. This is a south African model where the confining pressure of the geotechnically competent Karoo basalts limited the drop in confining pressure to a very sudden drop as it reached surface, resulting in a "topdown" fragmentation process and a large cross section extending to depth, without evidence of PK units. Class 2 is the Fort a la Corne ("FALC") model where the soft upper sediments allowed for an extremely quick dissipation of energy resulting in a wide but thin crater in the shape of a champagne glass. Class 3 are Lac de Gras style models which include diatremes of similar shape to the Class 1, with craters being steep sided and extending to great depth. Infill material is dominantly RVK along with associated kimberlitic and non-kimberlitic sediments, potentially transitioning to less sedimentary associated VK at deeper depths.

Source: Shear Diamonds Technical Report on the Jericho Property by SRK Consulting, July 2010

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