kimberlite.pptx

Kimberlite its mineralogy,morphology,petrology,occurrence, economic importance

1. KIMBERLITES Pritijita Naik Roll No-22GEOL001

2. Contents y Introduction Mineralogy Morphology Petrology 5.OCCUrRENCES 6.ECONOMIC IMPORTANCE 7.CONCLUSION 8.references

3. Introduction The Name kimberlite was coined by Henry Carwell Lewis (1884) to a Diamondiferous mica peridotite from Kimberley area of south Africa. Kimberlites are found in the stable intracratonic plate and no mountain building activity. Kimberlite is a volatile-rich, ultrapotassic, ultrabasic , igneous rock which occurs as small volcanic pipes, dykes, and sills. Inequigranular texture consisting of large crystal or rock fragments (xenoliths) enclosed in a much finer grained matrix.

4. Mineralogy It is mainly consists of olivine, phlogopite, ilmenite, garnet, pyroxenes and groundmass. Olivine crystals(o) are Serpentinised , and have started to loose their outlines Cross-polar photomicrograph of dyke encountered in drill core Olivine crystals(O) have been completely replaced by serpentine (dark grey) and are surrounded by a zone of amorphous serpentine which partially replaces adjacent primary calcite crystals (cc) Fig : a photomicrograph of kimberlite dyke in K2 East

5. Modal composition and texture The modal composition of the rocks of the kimberlite vary greatly. olivine is usually the most abundant mineral , but it may be partly, or completely, replaced by secondary minerals (for example, serpentine group minerals). The abundance of phlogopite and carbonate minerals is also highly variable Kimberlitic rocks normally have porphyritic and/or pyroclastic textures . The fragmental appearance of many of these rocks is enhanced by the occurrence of xenoliths. Most rocks contain megacrysts that are set in a finer grained groundmass that tends to contain microphenocryst . The megacrysts are both xenocrysts and phenocryst, they typically consist of olivine, phlogopite, garnet and pyroxene. They are usually set in a groundmass of serpentine group minerals and carbonate minerals ,together with microphenocrysts of Fe- Ti oxides, micas, spinels , monticellite and apatite .

6. MORPHOLOGY Kimberlites occur as carrot shaped, vertical intrusions termed “pipes”. This classic carrot shaped is formed due to a complex intrusive process of kimberlitic magma which inherits a large proportion of both CO 2 and H 2 O. The morphology of kimberlite pipes and the classical carrot shape, is the result of explosive diatreme volcanism from very deep mantle-derived source.

7. Continued… Three main zones are recognised : ( i ) CRATER ZONE – removed by erosion (ii) DIATREME ZONE – (1-2km) & highly brecciated in nature, hydrothermally altered zone: rich in clay and serpentine (iii) ROOT ZONE – dyke like in nature: hard & compact in nature, most suitable for petrological studies

8. PETROLOGY Historically, kimberlites have been subdivided into two distinct varieties termed basaltic and micaceous (Wagner, 1994). It was re-named by smith(1983) as Group I and Group II based on the isotopic affinities of these rocks. Mitchell (1995), showed that group II kimberlites actually shows closer affinities to lamproites than group I kimberlites.

9. Group I kimberlite Group I kimberlites are of co 2 -rich ultramafic potassic igneous rocks dominated by a primary mineral assemblage of forsterite olivine, ilmenite, Cr-pyrope, phlogopite, enstatite. Group-I kimberlites exhibit a distinctive ineqigranular texture caused by the presence of rounded, anhedral, and fragmented macrocryst (a non-genetic term for 0.5 to 10mm diameter crystal). They are derived from sources depleted in LREE (Light Rare Earth Element).

10. Group II kimberlite Group II kimberlites are ultrapotassic, peralkaline and H 2 O-rich . Phlogopite is the dominant macrocryst and groundmass phase. Derived from sources enriched in LREE. Group I and II kimberlites are distinctive isotopically. Mitchell and Bergman (1991) concluded that the two groups must represent different magma types . Group II kimberlites should be separated from group I kimberlite and be renamed as orangeites . Mineralogically group II kimberlite are similar to lamproites, but have sufficient petrological differences by which they be considered separately from these rocks as well.

11. Where do kimberlites occur? Kimberlite do not erupt in all areas of earth. Globally ,kimberlites all occur below the oldest parts of continents , known as cratons. Cratons have thick lithospheric roots that extend down to atleast 150-200 km, and kimberlite generation in the mantle is probably associated with the physical barrier to mantle upwelling provided by these deep continental roots. Regardless of how kimberlite form, the association of these eruptions with deep continental roots is another of the wonderful mysteries about how kimberlite deliver diamonds .

12. Kimberlite-Diamond field relationships For several decades, diamond was considered to be a phenocryst in kimberlite. But dating of diamond inclusions showed that diamond formation pre-dates kimberlite eruption. Diamonds are found to be much older than kimberlite. For eg : Diamond of Archean age are found in cretaceous kimberlites. Diamonds are hence XENOCRYSTS in kimberlite.

13. Indian occurrence Kimberlites occur in two spatially separate groups: Mahbubnagar cluster that was emplaced at 1400ma and the predominantly diamondiferous Anantapur cluster, emplaced at ~1100 Ma. The well known Wajrakarur diamondiferous field in Anantapur district and area of 250 sqkm , out of 12 pipes identified , pipes 2 and 5 are considered as micaceous kimberlite and the rest are typical kimberlite , these kimberlites are diamondiferrous . Maddur -Mahbub nagar sector, covers an area of about 400 sq km.

14. Economic Importance Kimberlites are the most important source of primary diamonds. Many kimberlites pipes also produce rich alluvial diamond placer deposits. Only about 1 in 200 kimberlites pipes contain gem quality diamonds. Kimberlites are very significant rock group both academically and economically.

15. Conclusion Kimberlite magmas are rich in carbondioxide and water which brings the magma quickly and violently to the mantle . Kimberlite is a gas rich potassic ultramafic igneous rock. Australia is currently the world’s largest producer of diamonds are low quality and used for industrial purposes. The crater facies kimberlite is recognised by sedimentary features. The diatreme facies are recognised by pelletal lapilli . The hypabyssal facies is commonly recognised by the presence of abundant calcite.

16. References Books; JOHN D. WINTER,P;443-445 Mihir K. Bose, igneous petrology WEBSITES; www.geology.com www.Wikipedia.com

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