PIMA™ and KIMBERLITES

(Much of the following is abstracted from Mitchell, 1993 and Kjarsgaard, 1995)

PIMA™ is used effectively, worldwide, by many major diamond companies, including DeBeers and Kennecott, for the exploration and characterization of 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.

Kimberlites are restricted to continental shield areas within Archean cratons. They occur as clusters of pipes within fields within provinces, probably along major crustal fracture zones.

Kimberlite and diamond formation is still not well understood. It is currently hypothesized that the roots of continental cratons contain diamond bearing horizons. As Kimberlites are emplaced (some explosively) from the mantle, they pass through these zones and in so doing entrain diamonds, which remain in xenocrysts or eclogites of their original host rock. The development of the diatreme involves extreme brecciation. There is explosive decompression with accompanying fluidization of the magma and subsequent alteration from the downward percolating fluids

Kimberlite diatremes are roughly circular with a pipelike or cone shape (Figure 1) and appear to decrease in diameter with depth. Three textural genetic groups of kimberlites are recognized: 1) crater facies ( may have high concentrations of diamonds, but are not well preserved), 2) diatreme or tuffisitic facies (volcanoclastic breccias), which are the dominate phse and 3) hypabyssal facies, which may be the best characterized petrographically, although they are poor diamond sources.

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.

The following section summarizes the spectral signatures of some of the infrared active minerals found associated with kimberlites.

COMMON ALTERATION MINERALS - KIMBERLITES

The most common alteration minerals seen in kimberlites include [A] serpentine, [B] calcite, [C] magnesite, [D] phlogopite, [E] biotite, [F] chamesite, [G] clinochlore, [H] saponite, [I] Mg-silicate, [J] vermiculite, and [K] amphibole.

SERPENTINES

Serpentine is one of the most common and diagnostic alteration minerals associated with kimberlites. It shows compositional variations which can be contoured.

Figure 2 shows these different species of serpentine: [A] antigorite, [B] cernolite [C] chyrsotile, and [D] williamsite.

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].

Figure 3. These spectra include [A] vermiculite, [B] chamesite - Fe-chlorite, [C] ripidolite - Fe>Mg chlorite, [D] chlinochlore = Mg-chlorite, [E] sheridanite - Mg>Fe chlorite.

BIOTITE, PHLOGOPITE, AMPHIBOLE, VERMICULITE and SAPONITE

These minerals are plotted together to show their similarities. Fe/Mg micas can be very common in kimberlite pipes.

Figure 4 includes [A] biotite, [B] phlogopite, [C] Fe-amphibole, [D] saponite, [E] Fe-saponite, and [F] vermiculite.

Figure 4A This expanded view is in the same order as Figure 4: [A] biotite, [B] phlogopite, [C] Fe-amphibole, [D] saponite, [E] Fe-saponite, and [F] vermiculite.

SELECTED REFERENCES