What Happens to a Kimberlite Over Time?
Alteration of these pipes is a major guide for exploration, however, it is not widely discussed. The ultramafic minerals associated with kimberlite pipes are highly susceptible to weathering and in the breccia environment, fluid flow is not restricted. Olivine in the groundmass or crysts will alter to serpentine and this will show compositional effects of the weathering.
Calcite replacement also occurs. Phlogopite will weather. Products will be chlorites and vermiculites. Serpentine and calcite occur as deuteric replacement products of the montecellite and phlogopite.
The main types of alteration are serpentinization, calcification and chloritization.
At the surface, kimberlite weathers to a soft, oxidized rock, "yellow ground, which grades into "blue ground", towards the source or into the pipe. This occurs vertically but can also form haloes around a pipe. Olivine weathers to serpentine, which weathers to saponite and smectite, oxidizing iron in the process to produce the yellow coloration..
Exploration methods include geophysical surveys (magnetic, gravity), remote sensing first with Landsat and then with hyperspectral, airborne sensors, indicator mineral identification of associated heavy minerals(Cr-pyrope, garnet, high Cr-Mg chromite, high Na-Ti pyrope-almandine garnet and geochemical sampling.
Kimberlites are extremely complicated systems and much that is known about them is still proprietary to companies.
Bryson
Burke Diamond Corporation
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What are Kimberlites?
Kimberlites are a group of volatile(CO2) rich, potassic ultrabasic rocks with variable composition megacrysts in a fine grained groundmass. The megacrysts can contain ilmentite, pyrope garnet, olivine clinopyroxene, phlogopite, enstatite and chromite. The groundmass or matrix can contain olivine, phlogopite, perovskite, spinel and diopside.
There are two main compositional types of kimberlite. Type I has an inequigranular texture with macro and even megacrysts in a fine grained groundmass, is olivine rich with monticellite, serpentine and calcite. Type 2 is micaceous. Kimberlites are classified on their groundmass mineralogy.
The most common alteration minerals seen in kimberlites include serpentine, calcite, magnesite, phlogopite, biotite, chamesite, clinochlore, saponite, Mg-silicate, vermiculite, and amphibole.
SERPENTINES
Serpentine is one of the most common and diagnostic alteration minerals associated with kimberlites. It shows compositional variations which can be contoured.
CHLORITES
and VERMICULITE
Chlorite is also a common mineral associated with kimberlites and can be a product of weathering of serpentines. Chlorites will form alteration haloes around the kimberlite pipes. Vermiculite is a weathering product of chlorite. Note the large interlayer water feature with the vermiculite [A].
BIOTITE, PHLOGOPITE, AMPHIBOLE, VERMICULITE
and SAPONITE
These minerals and Fe/Mg micas can be very common in kimberlite pipes.
Ultramafic Rocks
Ultramafic rocks are coarse-grained igneous or metamorphic rocks in which magnesian olivine (FO90±04) ± enstatite (EN85±05) ± Chromian diopside comprise £ 90% by volume. Ca-, and Al-bearing accessory minerals may include plagioclase (P<10Kb), spinel (10<P<20Kb), or pyrope-rich garnet (P>20Kb). Olivine-rich (40-100%) varieties are called peridotite. The most abundant varieties of peridotite are dunite (³ 90% olivine), harzburgite (olivine + enstatite), and lherzolite (olivine + enstatite + diopside + an aluminum-rich phase, which may be either plagioclase, spinel or pyrope-rich garnet). Mantle peridotites occur in tectonically-emplaced slices of oceanic lithosphere, referred to as ophiolite complexes, and as xenoliths in alkali olivine basalt (spinel lherzolite) and kimberlite (garnet lherzolite).
