MEASUREMENT OF KIMBERLITE WEATHERING

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LITERATURE STUDY

KIMBERLITE MINERALOGY

The diamond bearing ore is referred to as kimberlite and is a rare type of volcanic rock. The kimberlite volcanoes are known as pipes due to their inverted-cone shape (Harlow, 1998). Itmis suggested that kimberlite pipes form by viscous magma traveling upward through cracks in the rock. The magma continues to dilate and fracture the crust whilst gases dissolved in the kimberlite dissociate from the matrix and expand, thus driving the magma and entrained rocks and diamonds up to the surface at ultrahigh velocities. The swirling fluid consisting of rock, mineral fragments and expanding gases breaks through the surface and forms the characteristic shape called a diatreme. At the top of the kimberlite pipe a crater zone is formed which is quickly stripped away by rain and weathering (Harlow, 1998).
Kimberlites are diverse and complex hybrid rocks consisting of crystals originating from mantle-derived xenoliths, the discrete nodule suite and primary phases crystallizing from the kimberlite magma as described by Mitchell (1986). These volatile rich (CO2 predominantly) potassic ultrabasic igneous rocks can occur as small volcanic pipes, dykes and sills. Kimberlite is further defined by Mitchell (1986) as inequigranular alkalic peridotites containing rounded and corroded megacrysts, which can include olivine, phlogopite, magnesian ilmenite and pyrope. See table 1 for the chemical composition of these minerals. These macrocrysts  (0.5 – 10 mm) are relatively large compared to the fine-grained groundmass and cause the inequigranular nature of kimberlites. These macrocrysts are set in a fine-grained matrix (groundmass) of second generation primary olivine and / or phlogopite together with serpentine, perovskite, calcite and / or dolomite (carbonates) and spinels. Other minerals include diopside, rutile, apatite, monticellite and nickeliferous sulphides. Weathering  processes such as serpentization and carbonation commonly alters the early-formed matrix minerals as discussed under section 2.2. This causes replacement of early-formed olivine, phlogopite, monticellite and apatite by serpentine, calcite and chlorite as shown in figure 3. Highly weathered kimberlite can be composed of essentially calcite, serpentine, chlorite, smectite and magnetite together with minor phlogopite, apatite and perovskite. The changes in the crystal structure when clay minerals form, are discussed in section 2.2.5. Clay minerals formed during the weathering process are classified under the phyllosilicates  mineral group (see section 2.2.4.5) and consist of extended sheets of SiO4 4- tetrahedra. The members of this mineralogical group are soft and flaky and have a relatively low specific  gravity. Smectite (eg montmorillonite and saponite) and vermiculite are swelling clays commonly present in kimberlite.These clay minerals can generate internal pressures due to  their swelling nature therefore decreasing the overall strength of the rock. The nature of clay minerals is discussed further in section 2.2.4. Anhaeusser, 1998) .Mineralogical classification of kimberlites has been attempted by different authors and is reviewed below.
Wilson and Anhaeusser (1998) proposed classification into two groups. Group I is the olivine rich monticellite-serpentine-calcite kimberlites (<5% mica), which corresponds to the basaltic  kimberlites. Group II is the micaceous kimberlites (> 50% mica) corresponding to the micaceous lamprophyric kimberlites.
Mitchell (1986) rather suggested three groups based upon the predominance of olivine, phlogopite and calcite eg kimberlite (equivalent to basaltic kimberlite), micaceous kimberlite  (equivalent to lamprophyric kimberlite), and calcite or calcareous kimberlite. Skinner and Clement (1979) derived five varieties of kimberlite based upon the predominance of diopside, monticellite, phlogopite, calcite and serpentine. For this study the approach is to identify the mineralogy of kimberlites and relate it to the  mechanism and rate of weathering in the simplest possible way. The mineralogy is therefore rather described in terms of the main mineral groups with little focus on subgroups, cation substitution ratios etc.The focus is also not on the lesser constituents of kimberlites (eg garnets, spinels etc.) and further reading, if required, is directed to the work of Mitchell (1986). The mineral phases commonly present in kimberlites are discussed in the next section.

Olivine

Olivine ((Mg, Fe) 2SiO4) is originally the commonest and most characteristic mineral in kimberlite, according to Mitchell (1986). As groundmass constitute olivine occurs as single crystals smaller than 0.5 mm with the color ranging from pale green to pale yellowish-brown with increasing iron (Mitchell, 1986). Magnesian ilmenite, chrome spinel and rutile can be present in some olivine groundmass as inclusions. Primary olivines richer in iron than Fo85 are not characteristic of kimberlites.Forsterite (“Fo”) refers to the end member of olivine Mg2SiO4, thus Fo100 means no substitution of the Mg2 + by Fe2 + has occurred. Therefore in kimberlites the highest substitution of iron for magnesium in forsterite is 15%, but is seldom this high.
Due to its high weatherability, olivine is commonly altered along margins and fractures to several varieties (pseudomorphs) of serpentine with or without magnetite (see figure 3). The formation of magnetite when olivine is weathered to serpentine is discussed in section 2.2.2.7. Secondary processes can locally form less prominent alteration minerals eg brucite, serpentine, chlorite, calcite, talc, pyrite and other clay minerals (Mitchell, 1986).
Megacrystal olivines, single crystals of 2 – 5 cm in diameter, are present in a few localities but are very rare. Macrocrystal olivines are 0.5 – 1 cm in diameter and occur as round to elliptical single crystals. The ratio of macrocrystal to groundmass olivines can vary widely. Macrocrysts (fragmented megacrysts) are commonly intergrown with enstatite, Cr-pyrope, ilmenite, diopside, chromite, phlogopite, Cu-Ni sulphides and rutile. Macrocrysts can  commonly be identified by CO2 fluid inclusions.

1 BACKGROUND
1.1 PROJECT DEFINITION
1.2 DIAMOND PROCESSING
2 LITERATURE STUDY 
2.1 KIMBERLITE MINERALOGY 
2.1.1 Olivine
2.1.2 Phlogopite / Micas
2.1.3 Serpentine
2.1.4 Monticellite
2.1.5 Perovskite
2.1.6 Carbonates (calcite and dolomite)
2.1.7 Chlorite
2.1 .8 Clay minerals
2.1.9 Spinel
2.1.10 Magnesian Ilmenite
2.1.11 Pyroxene
2.1.12 Apatite
2.1.13 Garnet
2.1.14 Conclusion
2.2 WEATHERING PRINCIPLES
2.3 QUANTIFICATION  / MEASUREMENT OF KIMBERLITE WEATHERING
2.4 PARAMETERS INFLUENCE KIMBERLITE WEATHERING
3. MECHANISMS CONTROLLING KIMBERLITE WEATHERING
4. PROBLEM STATEMENT
5. EXPERIMENTAL PROCEDURE 
6. EXPERIMENTAL RESULTS AND DISCUSSION
7 INDUSTRIAL APPLICATION
8 POSSIBLE FUTURE WORK 
9 CONCLUSION
10 REFERENCES 
APPENDIX A: SPECIFICATION ON XRD WORK DONE PRE AT  AND UNIVERSSION

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