COURSE TITLE: GEOLOGY PAPER TITLE: ECONOMIC GEOLOGY & MINERAL RESOURCES OF INDIA

Transcription

1 COURSE TITLE: GEOLOGY PAPER TITLE: ECONOMIC GEOLOGY & MINERAL RESOURCES OF INDIA TOPIC OF LESSON: ORE DEPOSITS RELATED TO MAFIC AND ULTRAMAFIC ROCKS. PART 2: KIMBERLITES AND LAMPROITES, ANORTHOSITES, CABBONATITES AND ASSOCIATED MINERALIZATION Contents:- 1. KIMBERLITES AND LAMPROITES 1.1 Introduction 1.2 Mode of occurrence 1.3 Mineralogy 1.4 Mode of formation of diamonds 1.5 Some examples 2 CARBONATITES 2.1 Mode of occurrence 2.2 Economic aspects 2.3 Some occurrences Palabora Igneous Complex, South Africa Mountain Pass occurrences, California Kola Peninsula-Northern Karelia Alkaline Province, Russia 3 ANORTHOSITES 3.1 Mineralogy and mode of formation 3.2 Occurrence and distribution 3.3 Some examples Lac Tio ilmenite deposit, Allard Lake district, Canada Tellnes ilmenite deposit, Norway 4 SUMMARY Objectives: To understand: (a) Mode of occurrence and mineralogy of kimberlites and lamproites and formation of diamond with important examples. (b) Mode of occurrence, mineralogy and economic significance of carbonatites with important examples. (c) Mode of occurrence, distribution and economic mineralization in carbonatites with important examples. 1. KIMBERLITES AND LAMPROITES 1.1 Introduction About 20% of the world production of diamonds is derived from kimberlites and lamproites and the rest is extracted from beach and alluvial placers deposits containing diamonds, the latter released from weathering of the igneous source rocks (Kimberlites and Lamproites). 1

2 Not all kimberlites and lamproites contain diamonds and when present, they are present only in minute concentrations. For example, in the Kimberley mine, South Africa, 24 million tons of kimberlite yielded only 3 tons of diamond which amounts to an ore grade of ppm. The proportion of gem quality and industrial grade diamonds extracted from kimberlite and lamproites vary from one mine to another. In 1989, the total world production of natural diamonds from both bed rock sources and placer deposits was about 97 Mct, of which about half are of industrial grade (Evans 1993). Leading countries involved in diamond extraction are: Australia, Zaire, Botswana, Russia, South Africa, Angola, Namibia, Sierra Leone and Lesotho and a few countries in South America. Fig 1 provides locations of the major occurrences of kimberlite and lamproite fields, bulk of which are confined to cratonic regions of the world. 1.2 Mode of occurrence Fig.1 Distribution of diamondiferous and non-diamondiferous kimberlites and lamproites (After Fergusion, 1980; Groves et al., 1987). Most of the near surface diamond bearing kimberlites and lamproites are small pipe like bodies, generally less than 1 km 2 in plan view. In sectional view, they appear in the shape of inverted tear drop/carrot and become narrower and irregular at depth, some ultimately assuming a dike like form. Most kimberlite occurrences involve multiple intrusive events which form a cluster of a few discrete intrusions (Fig 2). 2

3 Fig.2 Schematic diagram of a kimberlite diatreme pipe and maar (volcanic crater below Ground level and surrounded by a low tuff rim). The maar can be up to 2 km across (After Nixon, 1980) According to Clement et al (1984), a typical uneroded kimberlite contains the following three texturally distinctive facies from bottom upwards: hypabyssal, diatreme and crater facies. Hypabyssal facies kimberlites are prophyritic and result from crystallization of kimberlite magma. Diatreme facies, which may exceed 2 km in depth, exhibit an explosive surface expression and contain mantle and crustal derived rock fragments, kimberlite lapilli and kimberlite fragments. In the upper levels the diatreme facies is usually in the form of so called agglomerate (tufficitic breccia with rounded and embayed fragments in a fine grained matrix). The rounded fragments constitute xenoliths of garnet peridotite or eclogite from upper mantle and metamorphic rocks from deeper crust. The rounded nature of the fragments is attributed to a gas fluidized origin (Dawson 1971). Diatremes often terminate at the surface in maars which may contain diamondbearing maar ring base surge accumulations. Crater facies kimberlites contain pyroclastic fall back breccias or epiclastic water lain material. The lacustrine sediments of the craters are sometimes affected by subsidence, perhaps of a cauldron nature. 1.3 Mineralogy Kimberlites may be defined as a potassic ultrabasic hybrid igneous rock containing large crystals (magacrysts) of olivine (monlicellite), enstatite, Cr-rich diopside, phlogopite, pyropealmandine and Mg-rich ilmenite set in a fine-grained matrix containing several of the following 3

4 minerals as prominent constituents: olivine, phlogopite, calcite, perovskite and ilmenite (Clement et al. 1984). Some of these minerals are used as indicator minerals in stream sediment and soil samples in the search for kimberlites, e.g., red-brown pyrope, purple-red chromium pyrope, Mg-rich ilmenite, Cr-rich diopside (Nixon 1980). Lamproites are potash and magnesia rich lamprophyric rocks of volcanic or hypabyssal origin with mineral assemblages containing a few of the following primary phenocrystal and/or groundmass phases: leucite, Ti-rich phlogopite, clinopyroxene, amphibole (Ti-rich potassic richterite), olivine and sanidine. Accessories may include priderite, apatite, nepheline, spinel, perovskite, wadeite and ilmenite (Rock 1991). Glass may be an important constituent of rapidly chilled lamproites. Xenoliths and xenocrysts including olivine, pyroxene, garnet and spinel of upper mantle origin, may be present. Diamond occurs as an accessory. Fertile lamproites appear to be the silica saturated orendites and madupites which carry sanidine rather than leucite (Gold 1984). Diamondiferous lamproites and other lamprophyres with diamonds have now been discovered in Western Australia, Quebec (Canada), India, Ivory Coast, South Africa, Sweden, Russia and Zambia. 1.4 Mode of formation of diamonds Kimberlites and lamproites are generally regarded as having been intruded upwards through a series of deep seated tension fractures, often in areas of regional doming and rifting in which the magmas started to consolidate as dykes. Subsequently, highly gas (CO 2 -CO-H 2 -H 2 O) charged magma broke through explosively to the surface at points of weakness, such as cross cutting fractures, to form the explosion vent which was filled with fluidized fragmented kimberlite or lamproite and xenoliths of country rock. This was followed by quite emplacement of magma (Fig. 2). Extremely high temperature (1400 to 1600 C) and pressure (50 to 70 kbar) are required to form diamond from pure carbon in areas of 60 km thick continental crust. The approximate depth range for diamond genesis is km (Evans 1993). For many decades there has been a very active debate as to whether diamonds crystallized from the magmas which cooled to form igneous rocks (Kimberlites/lamproites) in which diamonds are now found (as phenocrysts), or whether they were picked up by these magmas as exotic fragments (xenocrysts) from diamond stability fields in the upper mantle(i.e., Passenger-Locomotive relationship). The greater abundance of diamonds in eclogite xenoliths than in the surrounding kimberlite suggests that they have been derived from disaggregated eclogite (Robinson 1978). In South Africa, kimberlites erupted within the confines of the Archaean craton are diamondiferous, while those in adjacent younger orogenic belts are barren (Nixon et al. 1983). According to Gurney (1990) the most prospective areas for diamond exploration are on cratons stabilized by 3.2 Ga. Diamond bearing zones in kimberlites dwindle away from such core zones. 4

5 Radiometric ages of the inclusions in diamonds suggest that in many cases, the xenocrysts of diamonds are much older than the magmas that transported them to the surface (Richardson et. al. 1984). Dating of garnet inclusions in diamonds in two ~90Ma old kimberlites of South Africa indicated that the diamonds are older than 3000Ma suggesting that diamond hosts in upper mantle are also older. Geologic ages of kimberlite and lamproites range from middle Precambrian through Mesozoic. Available data on the inclusions in diamonds clearly indicate two main periods of diamond formation: the first about 3300 Ma ago and the second about Ma ago. The dated inclusions in the first group of diamonds were periodotitic minerals and in the second group were eclogitic minerals. These data indicate that diamonds grew stably within the upper mantle in eclogite and ultramafic rocks. The growth of diamonds occurred in a layer between 132 and 208 km depth beneath continents and 121 to 197 km beneath oceans, provided that carbon was present. Thus any magma that samples a diamondiferous zone of the upper mantle may bring diamonds to the surface if it moves swiftly; the speed of ascent of such magmas has been calculated to be around 70 km per hour (Meyer 1985). Slow ascent could allow time for the resorption of diamonds by transporting magmas as the pressure decreases. δ 13 C values of eclogitic diamonds indicate recycling of crustal carbon back into the mantle by subduction. Gurney (1990) suggested that the subducted material was underplated during the Proterozoic as eclogite on to the existing diamondiferous Archaean peridotitic keels of cratonic areas in which peridotitic diamonds had formed 3.2Ga ago. Diamondiferous layers in the upper mantle (in lithosphere at the boundary between lithosphere and asthenosphere) are probably discontinuous and absence of the layers may account for the absence of diamonds in many kimberlites. Kimberlite magmas generated at shallow levels also do not carry diamonds (eg. kimberlites in New South Wales, Australia, generated at km depth). 1.5 Some examples In Western Australia, diamondiferous kimberlites and lamproites numbering over 100 are encountered in 3 distinct provinces of Kimberley craton. In Lesotho, two kimberlitic pipes in a cluster of non-productive kimberlites are the main source of large and gem quality diamonds. In Siberia (Russia), kimberlites exhibit a regional zoning- a central zone of diamondiferous kimberlites is surrounded by a zone of pyrope and low diamond values, followed by a zone barren of diamonds with pyrope and finally an outer zone in which neither of these high pressure minerals is present in the kimberlites. These kimberlites range in age from Precambrian through Mesozoic. Of them the Palaeozoic kimberlites are most productive (Fig. 3). 5

6 Fig. 3 Distribution of kimberlites in East Siberian craton showing different minerlogical facies. A= diamond, B= diamond-pyrope, C= pyrope, D= other kimberlites (modified after Dawson, 1980). 2. CARBONATITES: Carbonatite complexes consist of intrusive magmatic carbonates and associated alkaline igneous rocks. They range in age mainly from Proterozoic to Recent and belong to alkaline igneous provinces. 2.1 Mode of occurrence Carbonatities are generally found in stable cratonic regions sometimes with major rift faulting (e.g. East African Rift Valley, and St. Laurence River Graben). There is however exceptions where carbonatite complexes are not associated directly with any alkalic rocks (e.g. Sangu Complex, Tanzania, and Kaluve, Zambia). Further, not all alkalic rock provinces and complexes have associated carbonatites. Among the known carbonatites (about 330) less than half occur in rift valleys, the remainder being spatially related to major faults traversing cratonic regions and some in fault intersections (e.g. Northern Karelia Alkaline Province, Russia). Carbonatites often form clusters or provinces within which there may have been several episodes of activity, e.g., the East African Rift Valleys, where late Proterozoic, early Palaeozoic and cretaceous carbonatites occur. Ore bearing carbonatites are 6

7 encountered as volcanic and plutonic bodies in dilatant fractures at places replacing earlier carbonatites and silicate rocks. Carbonatites are normally emplaced later than the bulk of alkalic silicate rocks with which they are genetically related. Many carbonatite occurrences are cylindrical, pipe-like and prominently concentrically zoned, such as Palabora Igneous Complex (South Africa). A few are irregularly shaped unzoned bodies like that at Mountain Pass (California). Some carbonatite complexes appear to be highly differentiated rocks occurring near and with syenites and other alkalic lithologies, but others appear to be undifferentiated, occurring with curious diopside-olivine pyroxenite-harzburgite intrusives. Some carbonatites are associated with barren kimberlites, generally within rifted cratonic interiors suggesting deep crustal or upper mantle processes involving carbonatitic liquid separation and injection or some other form of mantle degassing. 2.2 Economic aspects Carbonatites are exploited for phosphorus (from apatite), magnetite, niobium (pyrochlore), zirconia and rare earth elements (monzanite, bastnasite). To date, only one carbonatite complex is a major producer of copper (Palabora, South Africa). Other economic minerals in carbonatites include fluorite, baryte and strontianite. Carbonatites supply most of the world s niobium (Brazil, Canada) REE (USA, China) and vermiculite (South Africa). Till 1993, about 22 mines were in operation in 19 different carbonatites in 14 countries. The African carbonatite complexes at Sukulu (Uganda), Dorowa (Zimbabwe), Chilwa Island (Malavi), Glenover (South Africa), Mirma Hill (Kenya) and Okurusu (Namibia) contain large resources whose potentials await further appreciation. In India, the Nb-rich pyrochlores in the Savattur and Jogipatti districts of Tamil Nadu constitute potential resources. 2.3 Some occurrences Palabora Igneous Complex, South Africa: The Palabora Igneous Complex (C.2047Ma) lies in the Archaean terrain of the Transvaal. It resulted from alkaline intrusive activity involving initial emplacement of a large body (6km x 2.5km) of apatite-rich phlogopite-pyroxenite. It is kindney-shaped in outcrop (but forming a pipe at depth) and is surrounded by a narrow (~30m wide) envelope of feldspathic pyroxenite and an outer discontinuous ~40m wide zone of fenite (K-rich orthoclastites, a hydrothermal alteration assemblage) (Fig. 4). The northern part of the apatite rich pyroxenite body is intruded by apatite-poor phlogopite-pyroxenite, phlogopite-pyroxene-pegmatoid and serpentine-phlogopite pegmatoid (No.2, Fig4). The central and southern parts of the apatite-rich pyroxenite are invaded by phlogopite-pyroxenite-apatite-pegmatoid. The latter, in the central part of the main apatite-rich phlogopite-pyroxenite body, is further invaded by foskerite (magnetite-olivineapatite rock) and banded carbonatite, resulting in the formation of Loolekop carbonatite-phoskorite pipe (No.3, Fig.4). Fracturing of this pipe led to the intrusion of a dyke-like body of transgressive 7

8 carbonatite (Fig.5) and development of a stockwork of sulfide-rich carbonatite veinlets. These near vertical veinlets occur in parallel trending zone up to 10m wide, although individually the veinlets are usually less than 1cm wide and do not continue for more than 1m. The sulfide-rich veinlet zone extends vertically and continues beyond 1km below the surface. The chalcopyrite and cubanitebearing carbonatite veinlets, according to Aldous (1986) and Erikson (1989), were crystallized possibly from sulfide-rich fluids separated from pyroxenite through the process of liquid immiscibility (liquation). Evidences include: (1) presence of Cu sulphides interstitial between large carbonate grains and (2) sulfurization of older magnetite into Pyrrohtite ± Pyrite by late stage action of Cu(+Au+Ag+REE) and S-bearing fluids. Fig. 4 Geological map of Palabora alkaline complex, RSA. (1) Open pit operation of Foskor which produces phosphate, (2) The vermiculite open pit, (3) The palabora open pit at Loolckoop, which produces copper, magnetite, etc. 8

9 Fig. 5 Geology of the 122m level, Loolckop Carbonatite Complex, Palabora, RSA (Modified after Jacobsen, 1975). There are three separate large mining pits at Palabora (Pit Nos. 1, 2 and 3, Fig. 4). Pit no. 1 (Fosker s pit), which covers only part of the apatite-rich pyroxenite contains large reserves of apatite. In 1984 Fosker s pit produced 2.6million tons of phosphate (apatite) concentrate (36.5%P 2 O 5 ), nearly 9000 tons of baddeleyite (ZrO 2 ) concentrate and tons of copper concentrate (35%Cu). The neighbouring pit no. 2 constitutes the second largest vermiculite mine in the world. In 1990, it produced nearly 2,18,000 tons of concentrate (90% crude vermiculite). The ores from pit nos.1 and 2 are also providing the following minerals as co-products: magnetite, uranium in uranothorite, cobalt in linnaeite and zirconium hafnium in baddeleyite and trace amounts of nickel, gold, silver and PGM. In the Loolekop pipe (pit no. 3) the carbonatite and phoskorite are worked for copper with byproducts of magnetite, apatite, gold, silver, PGM, baddeleyite, uranium, nickel, sulfate and sulfuric acid Mountain Pass occurrences, California: The mountain pass carbonatite (1400Ma) occurs within the Precambrian Rocky Mountain area. The deposits lie in a belt about 10km long and 2.5km wide. The metamorphic country rocks have been intruded by potash rich igneous rocks (granites, syenites and shonkinites). The REE bearing carbonatites of the Mountain Pass region are related spatially, and probably genetically to these Potash-rich igneous rocks (Olson et.al., 1954). In the 9

10 Mountain Pass region, a shonkinite-syenite boey (known as the sulfide Queen Carbonate body (730m long and 200 m wide) carries a large concentration of rare earth elements (Fig 6). Fig. 6 Simplified map of the Mountain Pass region the sulfide Queen Carbonate body, Mountain Pass District, California (After Olson & Pray, 1954). The rare earth elements are carried by bastnasite and parasite; these minerals being in veins that are most abundant in and near the shonkinite-syenite body. Most of the 200 veins that have been mapped are less than 2m thick. Carbonate minerals make up about 60% of the veins and carbonatite bodies; they are chiefly calcite, dolomite, ankerite and siderite. The other constituents are barite, bastnasite, parisite, quartz and variable small quantities of 23 other minerals. The REE content of much of the ore body is 5-15%. The sulfide queen carbonate body is the largest ore body of rare earth minerals in the world at present under exploitation Kola Peninsula-Northern Karelia Alkaline Province, Russia: Alkaline igneous complexes and their associated carbonatites show a broad spatial relationship to areas of hot spot activity, which may be accompanied by doming and fracturing, the Kola peninsula being an excellent example. The upper Palaeozoic alkaline igneous and carbonatite complexes of this region host a number of extremely large ore bodies, of which the most important are Khibina, Kovdor and Sokli. 10

11 Khibina is a ring complex about 40 km across with inward dipping, layered intrusion of various alkaline rock types. One apatite-nepheline ore body forms an arcuate irregular lens-shaped mass with a strike length of 11 km and a proved dip extension of 2 km. The thickness ranges from 10 to 200m (av. =100m) and at least 2.7billion tons of ore averaging 18% P 2 O 5 are present. The apatite concentrates contain significant SrO and Re 2 O 3 values. Nephelene concentrates are also produced for the manufacture of alumina. Russia is the world s second largest producer of phosphate rock with much of its production coming form Khibina. The Kovdor carbonatite complex (Fig. 7) consists of different alkaline and basic rocks (Turjaite and melilitite, Ijotite, melteigite and pyroxene-olivine rock, pyroxenite and nephelenepyroxenite). Apatite-forsterite rocks and magnetite ore body were emplaced at a later stage. Ores on an average contain 36% Fe and 6.6%P 2 O 5. Baddeleyite forms a byproduct. Vermiculate is also produced. In its mineralization and rock types, this complex has affinities with Palabora carbonatite complex described earlier. 11

12 Fig. 7 Sketch map of part of the western section of the Kovdor complex showing position of the magnetite ore body (after Rimskaya-Korsakova, 1964). Sokli complex is located in Finland across the border of the Kola Peninsula. With an areal extent of about 18km 2, it is one of the world s largest carbonatites and it is remarkable for its partial cover of residual ferruginous phosphate rock, forming an apatite-francolite regolith of a type hitherto regarded as being of tropical origin. Proved reserves are over 50million tons averaging 19%P 2 O 5. The residual cover vary from a few meters to 70m in thickness. Pyrochlore with both U-Ta and Th enrichments occur in the carbonatites and apatite-magnetite mineralization is encountered in metaphoskorites. Kimberlitic dykes are also present but no diamonds have been found. The carbonatite intrusion tapers downwards from a diameter of 6km at the surface to about 1km at a depth of 5km. This feature also suggests that at great depths carbonatites, like Kimberlites, become dyke- 12

13 like bodies within the deep fracture zones that have tapped the levels in the mantle from which these alkaline rocks and their rare earth and associated elements have probably been derived (Samoilov and Plyusnin 1982). 3. ANORTHOSITES Anorthosite is practically a monomineralic rock composed of 90% or more of intermediate to calcic plagioclase. 3.1 Mineralogy and mode of formation Two kinds of the nearly monomineralic plagioclase rocks of An 35 or more are encountered: (1) The layered rocks near the upper portions of some layered igneous complexes (e.g. Bushveld Complex) which formed after the mafic minerals had crystallized and sunk, or by the floating of plagioclase crystals within the magma chamber and (2) the so-called anorthosite massifs, plutons typically containing plagioclase that is andesine or labradorite (An ). The anorthosites of layered igneous complexes developed by gravity stratification in ultramafic to mafic complexes, are characterized by rhythmic layering, show many cumulate textures, are free of megacrysts and are composed mostly plagioclase in An range. Anorthosite massifs are of economic significance, as they are the major sources of titanium bearing minerals viz., ilmenite (about 52% TiO 2 ) and rutile (>95%TiO 2 ), along with the fluvial and marine placer deposits derived from them. Widespread accessory ilmenite and commercial concentrations of ilmenite in the anorthosite massifs indicate a genetic relationship between ilmenite and anorthosite. Dykes and veinlets of ilmenite in anorthosite, as well as inclusions of anorthosite in the ore bodies, attest to the younger age of the ore. Dearden (1958) suggested that the ilmenite and the anorthosite are differentiates of the same parent magma, the large deposits of commercial grade ilmenite ore representing a late state segregation. 3.2 Occurrence and distribution Available data indicate that anorthosite massifs are geochronologically and tectonically constrained. These mafic calcium-rich rocks are all Proterozoic in age at 1.3± 0.2 Ga and are confined to terrains of anorogenic intracontinental rifting or incipient rifting tectonic setting in a broad belt across North America, Britain and Scandinavia. Fig. 8 shows the locations of the major occurrences of predominantly ilmenite-bearing and a few rutile-bearing anorthosite massifs of the northern hemisphere. 13

14 Fig. 8 Anorthosites of the northern hemisphere plotted on Bullard s North American reconstruction. Anorthosites are: 1. Korosten, Ukraine; 2. Korsun-Novomir-gorod, Ukraine; 3. Suwalki, Poland; 4. Utsjoki, Finland; 5. Southern Norway; 6. South Harris, Outer Hebrides; 7. Gardar, Greenland; 8. Kiglapait-Nain, Labrador; 9. Michigamau, Labrador; 10. Lac St. Jean, Quebec, including the Allard Lake district and its Lac Tio occurrence; 11. Lake Sanford, Adirondacks, New York; 12. Honeybrook, Pennslyvania; 13. Roseland, Virginia; 14. Duluth, Minnesota; 15. Cambridge Arch, Nebraska; 16. Laramie Range, Wyoming; 17. Bitterroot Range, Montana; 18. Boehls Butte, St. Joe, Idaho; 19. San Garbriel Range, California; 20. Orocopia Range, California; 21.Pluma Hidalgo, Oaxaca, Mexico; and 22. Sierra de Santa Marta, Colombia. (From Herz, 1969). Anorthosite massifs are divisible into two groups (Herz 1976) based on their plagioclase and oxide compositions; they are: (1) Labradorite anorthosite massifs characterized by plagioclase of An composition and either titano-magnetite or its oxidized equivalent magnetite ilmenite (e.g., Michigamau anorthosite, Labrador; Duluth Gabbro complex, Minnesota), and (2) Andesine anorthosite massifs containing plagioclase An48-25, hemo-ilmenite and En:An (i.e., enstatite content of pyroxene: anorthosite content of plagioclase) greater than 1. The latter is associated with the world s principal ilmenite deposits (e.g., anorthosite massifs of Adirondack Mountains in New York; Allard Lake region in Quebec, Canada and Rosland in Virginia, USA). Titanium-rich minerals crystallize late in the magmatic history of anorthosite complex. The behaviour of titanium during crystallization of magma is controlled by several factors including the initial abundance of titanium, the changing chemical activities of silicon, aluminium and iron, the variable partial pressures of oxygen, the possible formation of titanium-rich immiscible liquids and temperatures of crystallization (Verhoogen, 1962). Studies carried out on the magnetite-ilmenite deposits near Lake Sanford, New York, indicate that these ores are differentiates of an anorthosite gabbro magma. The ores are thought to have formed from segregations of a magnetite-ilmenite liquid, part of which was trapped and crystallized in 14

15 interstices between crystals of plagioclase and augite, but much of which was tectonically squeezed through the process of filter pressing. This Fe-Ti- Oxide liquid was injected into adjacent locally stillplastic gabbros and anorthosites, locally forming high grade Fe-Ti-Oxide ores. 3.3 Some examples Lac Tio ilmenite deposit, Allard Lake district, Canada: The Lac Tio deposit contains about 125 million tons of ilmenite ore (32-35%TiO 2 ) and was considered as the largest body of titanium ore till the discovery of another still larger deposit in Norway. In the Lac Tio deposit, ore bodies form irregular lenses, narrow dykes, large sill-like masses and various combinations of these forms. Some of these clearly cut the anorthosite and appear to be later in age. The Lac Tio ore contains crystal aggregates of thick, tabular ilmenite grains upto 10mm across and 2mm thick. Minor amounts, generally about 5%, of plagioclase, pyroxene, biotite, pyrite, pyrrhotite, and chalcopyrite make up the interstitial material. Ilmenite grains contain exsolved hematite and the latter ranges in size from fine grains to blebs upto 10mm in diameter. The ilmenite contains upto 25% of intergrown hematite and the ilmenite hematite mixture is so fine grained that grinding cannot effectively separate the two minerals Tellnes ilmenite deposit, Norway: The world s largest ilmenite ore body is located at Tellnes in the anorthosite belt of Southern Norway about 120 km south of Stavanger. The deposit is boat-shaped, 2.3km long, 400m wide and about 350m deep (Fig. 9). The ilmeno-norite ore occurs in the base of norite. The ilmeno-norite along with the associated norite intrudes the main body of anorthosite (See cross section CD in Fig.5) with which it has sharp contacts. 15

16 Fig. 9 Map and sections of the Tellnes titanium ore body (After Dybdhal, 1960) The ilmeno-norite intrusions shape is apparently an original feature of the intrusion. Ilmenite carries upto 12% hematite as exsolution lamellae (Fig. 10). Proven reserves are 300million tons. The ore contains 18%TiO 2, 2Vol% magnetite and 0.25vol% sulfide (pyrite and Cu-Ni sulfides). Fig. 10 Exsolution bodies of hematite-rich material in an ilmentite-rich base (x263), Tellnes, Norway. 16

17 SUMMARY About 20% of the world production of diamonds is derived from kimberlites and lamproites and the rest is extracted from beach and alluvial placers deposits containing diamonds. Kimberlites and lamproites are carrot shaped and are generally regarded as having been intruded upwards into cratonic areas through a series of deep seated tension fractures, often in terrains of regional doming and rifting. Two main periods of diamond formation: the first about 3300 Ma ago and the second about Ma ago. Inclusions found in diamonds indicate two periods of diamond formation: the first about 3300 Ma ago and the second about Ma ago. Peridotite and eclogitic minerals found as inclusions indicate that diamonds grew stably in eclogites and ultramfic rocks confined to discontinuous diamondiferous zone (diamond stability field) of the upper mantle. Geological ages of 13 kimberlites and lamproites are much younger than the diamonds they conatain and this feature indicates their role as a carrier of diamonds from 100 to 300 km deep upper mantle to near surface environment. Carbonatites are generally found in: (1) stable cratonic regions along major faults and fault intersections and (2) rift valleys. Carbonatites are exploited for phosphorus (from apatite), magnetite, niobium (from pyrochlore), zirconia, Rare earth elements (from monazite and bastanasite), barite, strontianite and vermiculite. Only one carbonatite body (Palabora complex, South Africa) is known for large reserves of copper ore. Anorthosite massifs are essentially Proterozoic in age and confined to terrains of anorogenic intercontinental rifting or incipient rifting tectonic setting in a broad belt across North America, Britain and Scandinavia. Anorthosite massifs are divisible into two groups (Herz 1976) based on their plagioclase and oxide compositions; they are: (1) Labradorite anorthosite massifs (e.g., Michigamau anorthosite, Labrador; Duluth Gabbro complex, Minnesota), and (2) Andesine anorthosite massifs (e.g., anorthosite massifs of Adirondack Mountains in New York; Allard Lake region in Quebec, Canada and Rosland in Virginia, USA). Anorthosite massifs are the major source of titanium minerals (ilmenite and rutile). 17