rocks and minerals in kenya

kenya: mining, minerals and fuel resources

Kenya is located in Eastern Africa between Tanzania and Somalia. The total area of the country is 581,309 km2 and has a population of roughly 48 million. The countrys climate varies from tropical along the coast to arid in the interior regions. The GDP of the country was $76.07 billion in 2012.

Kenya is the economic, transport and financial of East Africa. Since 2014, Kenya has been ranked as a lower middle-income country, as its per capita GDP crossed a World Bank threshold. Previously, The World Bank and IMF restricted aid to the country previously due to rampant corruption. Kenya has a growing entrepreneurial middle class and steady growth, but its economic development has been impaired by weak governance, political instability, corruption and violent clashes between rival parties since gaining independence in 1963.

The natural resources of Kenya include oil, gas, limestone, gypsum, soda ash, diatomite, gemstones, fluorspar, zinc, wildlife, and hydropower. The country is gearing up to move from an agriculture and tourism based-economy to one that is based on mineral resources. Most of the mines and cement plants are privately owned.

Mineral resources in Kenya include gold, iron ore, talc, soda ash, some rare earth minerals, and gemstones. Gold is mostly restricted to the westernmost part of the country, while areas around Mombasa host limestone, niobium, iron ore, gemstones, and salt.

The mining and quarrying sector in Kenya accounts for less than 1 percent of gross domestic product, with the majority being contributed by the soda ash operation at Lake Magadi in south-central Kenya. In 2010, Kenyas share of the worlds soda ash production amounted to 4%. Cement, fluorspar, and petroleum refining were the other mining and mineral processing activities undertaken by the mining sector.

In 2010, Aviva Corporation Ltd. of Australia signed a joint-venture agreement with Lonmin plc of UK to explore for copper, gold, silver, and zinc in the Ndori Greenstone Belt. In the same year, Base Resources Ltd. of Australia acquired the Kwale mineral sand reserves from Vaaldiam Mining Inc. of Canada.

There are five cement producers in Kenya with a combined capacity of 5.7 Mt/yr. The countrys cement production increased to 3.71 Mt in 2010 from 3.32 Mt in 2009. The cement plant owned by Athi River Mining Ltd. (ARM) at Kaloleni has a capacity of 300,000 t/yr. The company has also started operating another plant with a capacity of 730,000 t/yr at Athi River since December 2010. Mombasa Cement produced 300,000 t of cement in 2010 compared to 250,000 t in 2009.

Cement production is set to increase as the existing plants and the newly constructed plants are likely to increase their capacity. The demand for cement in other African countries such as Tanzania, Burundi, Rwanda, and Uganda will also have an impact on cement exports from Kenya.

The mining sector in Kenya is all set to expand to a greater level in the coming years as the recent increase in global demand for minerals has seen several Australian and other mining companies seeking permission from the Kenyan government to explore its mineral reserves.

All these developments are very promising for the future of Kenyas mining sector. It is now up to the government to sort out the internal political issues and pave the way for economic growth by strengthening its mining sector.

Kwale Mineral Sands is located in Kenya, approximately 40km south of Mombasa. The mines development is estimated to cost $298m, and construction began in 2011. The mine began production of ilmenite and rutile in December 2013, with the production of zircon commencing in February 2014. Annual production is projected to be 330,000t of ilmenite, 80,000t of rutile and 40,000t of zircon.

The project is predicted to be one of the top producers of ilmenite and rutile in the world, with production amounting to 14% of the global supply of these minerals. It is expected to generate $1bn in revenues in the mineral sands market over 13 years.

The 2016 Mining Act replaced the former 1940 mining act. The previous act contained out-of-date laws and mining data, which meant that the country had missed out on potential mineral exploration. The new legislation includes more details and provisions about the principles of land policy, public land, regulation of land use and property, environmental obligations and agreements relating to natural resources.

Mining Minister Dan Kazungu wrote a new strategy that spans over the 20 years, with the hopes that 10% of GDP will come from mining by 2030. The new strategy includes plans for up to 20 new mines and aims to have 20 more operators situated in the country over the next 15 years.

east africa's great rift valley: a complex rift system

Figure 1: Colored Digital Elevation Model showing tectonic plate boundaries, outlines of the elevation highs demonstrating the thermal bulges and large lakes of East Africa. Click to Enlarge. The basemap is a Space Shuttle radar topography image by NASA.

The East African Rift System (EARS) is one the geologic wonders of the world, a place where the earth's tectonic forces are presently trying to create new plates by splitting apart old ones. In simple terms, a rift can be thought of as a fracture in the earth's surface that widens over time, or more technically, as an elongate basin bounded by opposed steeply dipping normal faults.

Geologists are still debating exactly how rifting comes about, but the process is so well displayed in East Africa (Ethiopia-Kenya-Uganda-Tanzania) that geologists have attached a name to the new plate-to-be; the Nubian Plate makes up most of Africa, while the smaller plate that is pulling away has been named the Somalian Plate (Figure 1). These two plates are moving away form each other and also away from the Arabian plate to the north.

The point where these three plates meet in the Afar region of Ethiopia forms what is called a triple-junction. However, all the rifting in East Africa is not confined to the Horn of Africa; there is a lot of rifting activity further south as well, extending into Kenya and Tanzania and Great Lakes region of Africa. The purpose of this paper is to discuss the general geology of these rifts are and highlight the geologic processes involved in their formation.

Figure 2: Rift segment names for the East African Rift System. Smaller segments are sometimes given their own names, and the names given to the main rift segments change depending on the source. Click to Enlarge. The basemap is a Space Shuttle radar topography image by NASA.

The oldest and best defined rift occurs in the Afar region of Ethiopia and this rift is usually referred to as the Ethiopian Rift. Further to the South a series of rifts occur which include a Western branch, the "Lake Albert Rift" or "Albertine Rift" which contains the East African Great Lakes, and an Eastern branch that roughly bisects Kenya north-to-south on a line slightly west of Nairobi (Figure 2).

These two branches together have been termed the East African Rift (EAR), while parts of the Eastern branch have been variously termed the Kenya Rift or the Gregory Rift (after the geologist who first mapped it in the early 1900's). The two EAR branches are often grouped with the Ethiopian Rift to form the East Africa Rift System (EARS).

The complete rift system therefore extends 1000's of kilometers in Africa alone and several 1000 more if we include the Red Sea and Gulf of Aden as extensions. In addition there are several well-defined but definitely smaller structures, called grabens, that have rift-like character and are clearly associated geologically with the major rifts. Some of these have been given names reflecting this such as the Nyanza Rift in Western Kenya near Lake Victoria. Thus, what people might assume to be a single rift somewhere in East Africa is really a series of distinct rift basins which are all related and produce the distinctive geology and topography of East Africa.

Figure 3: "Textbook" horst and graben formation (left) compared with actual rift terrain (upper right) and topography (lower right). Notice how the width taken up by the trapezoidal areas undergoing normal faulting and horst and graben formation increases from top to bottom in the left panel. Rifts are considered extensional features (continental plates are pulling apart) and so often display this type of structure. Click to Enlarge.

The exact mechanism of rift formation is an on-going debate among geologists and geophysicists. One popular model for the EARS assumes that elevated heat flow from the mantle (strictly the asthenosphere) is causing a pair of thermal "bulges" in central Kenya and the Afar region of north-central Ethiopia. These bulges can be easily seen as elevated highlands on any topographic map of the area (Figure 1).

As these bulges form, they stretch and fracture the outer brittle crust into a series of normal faults forming the classic horst and graben structure of rift valleys (Figure 3). Most current geological thinking holds that bulges are initiated by mantle plumes under the continent heating the overlying crust and causing it to expand and fracture.

Ideally the dominant fractures created occur in a pattern consisting of three fractures or fracture zones radiating from a point with an angular separation of 120 degrees. The point from which the three branches radiate is called a "triple junction" and is well illustrated in the Afar region of Ethiopia (Figure 4), where two branches are occupied by the Red Sea and Gulf of Aden, and the third rift branch runs to the south through Ethiopia.

The stretching process associated with rift formation is often preceded by huge volcanic eruptions which flow over large areas and are usually preserved/exposed on the flanks of the rift. These eruptions are considered by some geologists to be "flood basalts" - the lava is erupted along fractures (rather than at individual volcanoes) and runs over the land in sheets like water during a flood.

Such eruptions can cover massive areas of land and develop enormous thicknesses (the Deccan Traps of India and the Siberian Traps are examples). If the stretching of the crust continues, it forms a "stretched zone" of thinned crust consisting of a mix of basaltic and continental rocks which eventually drops below sea level, as has happened in the Red Sea and Gulf of Aden. Further stretching leads to the formation of oceanic crust and the birth of a new ocean basin.

Figure 4: Triple Junction in the Afar region of Ethiopia. Image shows areas of stretched and oceanic crust as well as areas of exposed flood basalts that preceded rifting. Areas unshaded or covered by flood basalts represent normal continental crust. As the crust is pulled apart you end up with thinned crust with a complex mixture of continental and volcanic rock. Eventually the crust thins to the point where oceanic-type basalts are erupted which is the signal that new ocean crust is being formed. This can be seen in the Gulf of Aden as well as a small sliver within the Red Sea. The original extent of the flood basalts would have been greater, but large areas have been buried within the rift valley by other volcanic eruptions and sediments. Click to Enlarge.

If the rifting process described occurs in a continental setting, then we have a situation similar to what is now occurring in Kenya where the East African/Gregory Rift is forming. In this case it is referred to as "continental rifting" (for obvious reasons) and provides a glimpse into what may have been the early development of the Ethiopian Rift.

As mentioned in Part I, the rifting of East Africa is complicated by the fact that two branches have developed, one to the west which hosts the African Great Lakes (where the rift filled with water) and another nearly parallel rift about 600 kilometers to the east which nearly bisects Kenya north-to-south before entering Tanzania where it seems to die out (Figure 2).

Lake Victoria sits between these two branches. It is thought that these rifts are generally following old sutures between ancient continental masses that collided billions of years ago to form the African craton and that the split around the Lake Victoria region occurred due to the presence of a small core of ancient metamorphic rock, the Tanzania craton, that was too hard for the rift to tear through. Because the rift could not go straight through this area, it instead diverged around it leading to the two branches that can be seen today.

As is the case in Ethiopia, a hot spot seems to be situated under central Kenya, as evidenced by the elevated topographic dome there (Figure 1). This is almost exactly analogous to the rift Ethiopia, and in fact, some geologists have suggested that the Kenya dome is the same hotspot or plume that gave rise to the initial Ethiopian rifting. Whatever the cause, it is clear that we have two rifts that are separated enough to justify giving them different names, but near enough to suggest that they are genetically related.

Baringo scarps: This image shows several fault scarps that are progressively farther away. Essentially we are looking at the edges of several horst blocks from within a graben that contains Lake Baringo. Image copyright Alex Guth. Click to Enlarge.

What else can we say about the Ethiopian and Kenya Rifts? Quite a lot actually; even though the Eastern and Western branches were developed by the same processes they have very different characters. The Eastern Branch is characterized by greater volcanic activity while the Western Branch is characterized by much deeper basins that contain large lakes and lots of sediment (including Lakes Tanganyika, the 2nd deepest lake in the world, and Malawi).

Recently, basalt eruptions and active crevice formation have been observed in the Ethiopian Rift which permits us to directly observe the initial formation of ocean basins on land. This is one of the reasons why the East African Rift System is so interesting to scientists. Most rifts in other parts of the world have progressed to the point that they are now either under water or have been filled in with sediments and are thus hard to study directly. The East African Rift System however, is an excellent field laboratory to study a modern, actively developing rift system.

This region is also important for understanding the roots of human evolution. Many hominid fossil finds occur within the rift, and it is currently thought that the rift's evolution may have played an integral role in shaping our development. The structure and evolution of the rift may have made East Africa more sensitive to climate changes which lead to many alternations between wet and arid periods. This environmental pressure could have been the drive needed for our ancestors to become bipedal and more brainy as they attempted to adapt to these shifting climates (see Geotimes 2008 articles: Rocking the Cradle of Humanity by Beth Christensen and Mark Maslin, and Tectonic Hypotheses of Human Evolution by M. Royhan Gani and Nahid DS Gani).

Igneous dike in Njorowa Gorge: This was taken at the Njorowa Gorge in Hell's Gate National Park. The gorge was carved by water, and is quite spectacular in many regards, but here we have an igneous dike cutting through the wall of the canyon, with Dr. Wood and one of our guides for scale. Image copyright Alex Guth. Click to Enlarge.

The East African Rift System is a complicated system of rift segments which provide a modern analog to help us understand how continents break apart. It is also a great example of how many natural systems can be intertwined - this unique geological setting may have altered the local climate which may have in turn caused our ancestors to develop the skills necessary to walk upright, develop culture and ponder how such a rift came to be. Just like the Grand Canyon, the East African Rift System should be high on any geologist's list of geologic marvels to visit.

James Wood has a PhD from Johns Hopkins University and is currently Professor of Geology at the Michigan Technological University in Houghton, Michigan where he teaches Earth History, Geochemistry, Remote Mapping and conducts a field course every spring in East Africa. His main research interests are energy deposits, mainly gas and oil, and doing field work in rift valleys. More information on the East Africa field course can be found at www.geo-kenya.com.

Alex Guth is currently a PhD candidate at Michigan Tech and is looking at the effects of climate on desert varnish on the exposed flows and alluvium in the East African Rift Valley. She assists Dr. Wood with the geology field camp. She recently produced a geologic map of the southern half of the Kenya Rift which may be found at www.geo-kenya.com. Her website can be viewed at: pages.mtu.edu/~alguth/.

general geology of kenya - sciencedirect

The oldest supracrustal rocks in Kenya are the Archaean Nyanzian meta-volcanics and the Kavirondian meta-sediments. These rocks are found to the west of the country in the areas adjacent to Lake Victoria. The Neo-Proterozoic Mozambique belt rocks occupy the central parts of Kenya. These are in most parts separated from the Archaean rocks by the Tertiary volcanics associated with the East African Rift System. The eastern parts of Kenya from the north to the south are dominated by sedimentary rock sequences ranging in age from the Jurassic to Recent. Large volumes of sediments are also found within the rift floor. Faulting and rifting characterizes the Mesozoic and Quaternary rocks and sediments. Sedimentary deposits of the Permo-Triassic are as a consequence of faulting and subsequent rifting during the break-up of Gondwanaland leading to the distribution of Karoo-like sediments in an intracratonic basin to the east along the Kenyan coast. These sediments are extensively exposed in the south-eastern coastal region and are locally referred to as the Duruma Group, while the small exposures to the northeast are referred to as the Mansa Guda Formation. Notably, Jurassic shales and limestones associated with shallow to deep marine environments are present alongside the Permo-Triassic sediments. The development of the East African Rift System led to the distribution of the Quaternary volcanics and sediments on the floor of the tectonic rift valley trough. Evidence of the Cenozoic history that is characterized by relict erosion surfaces is seen on certain areas of the coastal zone. Quaternary sediments are widely distributed in the country with extensive deposits in the eastern region (east of the Rift Valley) with limited exposures to the northwest.

petroleum potential of nw-kenya rift basins: a synopsis of evidence and issues - exploration & production geology

ABSTRACT: The petroleum synoptic research work gives an overview subsurface stratigraphy of northwestern Kenya rift basins. The basins evolved through extension tectonics that brought out continental rifting as a part of the major Gondwanaland breakup in the Late Paleozoic time, and continued in the Mesozoic and Tertiary. This movement was accompanied by a stupendous outpouring of the lava flows. The gravity anomaly maps and seismic profiles were most useful for the interpretations incorporated in this paper which revealed the presence of several horsts and grabens structural systems. It was also revealed that the basins attracted potential petroliferous sedimentary piles (~2000 5000 m thick) which were deposited on basement rocks of Precambrian age and later got covered by basaltic flows of mainly Miocene age. The drill core lithologs were available pertaining to wells: LT-1 and LT-2 in the Lokichar and North Kerio-Turkana basin systems (Tertiary) and C1, C2 and C3 in the Chalbi basin (Cretaceous). The northwestern Lotikipi basin (Cretaceous?) has not yet been drilled. Comparing the lithologs from these wells, the strata in which there was oil and/or gas indications was further characterised in the light of the organic matter and other sedimentological parameters in order to understand the sourcereservoirseal rocks which are favourable targets for future petroleum exploration.

Author:male; PhD; main research fields: Hydrocarbons and Environmental Reduction and Risk Management; Department of Mining and Mineral Processing Engineering, Jomo Kenyatta University of Agriculture and Technology, Taita Taveta Campus, Private Bag, Voi, Kenya.

The geology of Kenya is generally known for the coastal terrigenous clastic sediments of the Karroo system, the Kenya-Kilimanjaro volcanics belonging to the Tertiary volcanic activity and the Quaternary archaeological sites of the early man (Fig.1). It is also known for the spectacular landscape resulting out of the great East African Rift System and the chain of rift lakes from which some salt deposits are being exploited [1].

A large part of the Kenyan geology also consists of the Precambrian basement rocks and the Tertiary volcanics that have covered many of sedimentary basins, which are now considered to be potential basins for oil exploration. These basins are known to have evolved through extension tectonics (Figs. 2and3) that brought out continental rifting as a part of the major Gondwanaland breakup in the Late Paleozoic time and continued in the Mesozoic and Tertiary [2, 3, 4]. The region underwent uplift and subsidence, intermittently, along major boundary faults of these basins even in the Miocene period. This movement was accompanied by the stupendous outpouring of the lava flows.

The basins seem to have evolved consequent to a complex tectonic activity related to continental rifting and block faulting of the Lamu-Anza-Abu Gabra and Central African Rift Systems (Fig.3) .

The physiographical features such as the basement ranges and inliers, Tertiary volcanic plateaus, Lake Turkana and lowlands with alluvial plains as well as drainage systems are present in the study area (Fig.2). The entire area is volcanically and seismically active, even throughout the Quaternary period, shown by the thick alluvial cover concealing the entire eroded surface of the older sedimentary sequences as well as the basement rocks [5, 6]. The landscape consisting of flat alluvial plains and high plateaus and ranges intervening them indicates the control of block faulting. The drainage follows most recent strikes of faulting, north-south direction, but the tilts are asymmetric giving rise to rivers flowing in opposite directions. The small rivers flow perpendicular to the ranges and terminate to join the main rivers flowing north-south [7].

The existing information on the geology and stratigraphic succession of the four major sedimentary basins has been highlighted [8, 9]. It is suspected, and is the very basis and premise of this present synoptic work, that the sediments of the Anza Basin to the southeast and the sediments of the Abu Gabra and Sharaf Formations (Cretaceous) in Sudan to the northwest have counterparts in the intervening areas of northern Kenya (Fig.3). The Jurassic-Cretaceous sediments of the Anza Basin and those of the Sudanese Abu Gabra Basin [10] are said to have been deposited in rift basins, with possible encroachments of the sea from both directions, depositing sediments immediately overlying the basement rocks of these basins [7, 11, 12, 13].

Efforts are now being made by National Oil Corporation of Kenya (NOCK) to explore for hydrocarbon reserves in the sedimentary basins belonging to the Jurassic/Cretaceous-Tertiary age; although most wells so far drilled in these basins did not prove any oil reserves. The present work is mainly a synoptic overview of the subsurface stratigraphy in the northwestern Kenya Rift basins with the help of geophysical and geochemical data.

The study is devoted to the collection and interpretation of all the data; structural, geomorphologic, seismic and gravity as well as gamma ray data, pertaining to drill core well profiles obtained from the three wells (C1, C2 and C3), drilled in Chalbi Cretaceous sequence and the two wells, LT-1 (Loperot-1) and LT-2 (Eliye Springs-1), drilled in Lokichar-Kerio/Turkana sub-basins which penetrated chiefly the Palaeocene or younger strata. Future drilling and fossil finds will also provide additional stratigraphic attributes to the seismically defined subsurface Formations in the Lotikipi basin [7].

From the palynological presence of flora and faunal assemblages found in these sedimentary sections the drilling companies (AMOCO and SEPK) have reported that the sediments were deposited in a fluvial-deltaic and marine-lacustrine environments. The sedimentation in these intracratonic rift basin was controlled by intrabasinal and marginal faults, some of them reaching also the basement. While examining the subsurface stratigraphy, it is intended also to assess the prognostic oil and/or gas potential of these basins and extrapolate the information to other parts of the northwest unexplored and yet to be drilled Lotikipi basin.

During the Cretaceous as well as Tertiary times the sedimentation in these rift-related basins, extending NW-SE, was mostly fluvial, gradually becoming brackish and marine in the peripheral regions towards the open seas (Fig.3). From the paleogeographic and dinosaurian fossil evidence, the Cenomanian period had paleoclimatic conditions (warm humid climate and heavy precipitation) predicted conducive for luxuriant thick vegetation [14], which accompanied the fluvial sedimentation of that period in the rift basins under study.

The detailed tectonic structures of the subsurface basins on the basis of basement depth and gravity anomalies (Bouguer anomalies) obtained from the regional contour map [15] covering the study area has been examined (Figs.4and5). The gravity surveys help in limiting the depths of the basins as well as the basement, scanning the lithosphere and the upper mantle mainly according to the relative densities (Table 1).

The positive gravity anomalies (Fig.4) define the limits of the horst structures where the basement is overlain mostly by the lava flows. The Bouguer gravity contours over the rift basins show distinct north-south strike, indicating that the Tertiary rift tectonics affected even the crust-mantle interface. In the present case, it has been found to be quite effective since the basement rocks as well as the upper cover of volcanics have distinctively higher densities than the infilled sedimentary sections within the basins (Table 1).

The basaltic cover has hidden under it a thick succession of Mesozoic and Tertiary sediments. These basins are bounded by major fault systems; the gravity profiles of the present terrain indicate that within the basins are sub-basins, which are asymmetric half-grabens bounded only on one side by a major fault and on the other side by a set of faults (Figs.6and7). The asymmetric rifts or half-grabens are intracontinental and the gravity profiles reveal that they characteristically occur over the crests of regional arches of the basement and the mantle or only on the continental crust with a trough-like mantle profile. The landscape and subsurface basin structures generally indicate that the tectonic activity, rifting and block faulting process, which was initiated in the Cretaceous time or prior, continued in pulses during the Tertiary and even during the Quaternary. It has been possible to demarcate within the basins the various horst and graben-like structures.

For example, in the case of Lake Turkana basin, the basement contours (Fig.5) indicate a deeper half-graben structure towards the northwestern edge of the lake (depth contours increasing from -3 km to -5 km). It implies that the sediment fill progressively deepens not only eastwards under points R and S but also northwards (Fig.7). The gravity picture of the Lake Turkana basin (Fig.4) is in sharp contrast with that towards the immediate eastern and western region. In the Lotikipi plains to the west, the gravity anomalies showed hardly any variations along this latitude 4oN, while to the east under the Koobi Fora region, highly positive anomalies are seen. Thus the faults in the Turkana basin seem to be even mantle-reaching, bringing about a downwarp of the mantle. In comparison, the mantle has been upwarped under the Lotikipi plains and has been sharply thrown upwards under the Koobi Fora (area under points R and S in Figure7).

The stratigraphic succession was ascertained by interpretation of the seismic data from these subsurface basins [7] which have also been correlated with the description of the stratigraphic lithologs obtained from the drilled wells (Figs.9and10). The lithologs have been examined in the light of gamma ray data in order to build up a clear understanding of the subsurface stratigraphic formations with respect to seismic profiles.

These rocks were further distinguished by the gamma-ray logs to demarcate black shales with organic matter, coaly beds and sediments with radioactive elements. The analysis helped in determining the proportion and frequency of the shale horizons within the otherwise sandy sections, as well as the variations in grain size within the sandstone beds.

Marine sediments with higher gamma ray (uranium content) values are of course considered to be better source rocks than those deposited in lacustrine and freshwater conditions. Lacustrine sediments like the present ones have typically low gamma ray radioactivity. The radioactive heat [17] from organic matter (OM) rich sediments adds further to the temperature gradient which is otherwise also higher than the normal geothermal gradient in the intracratonic rift basins [18]. Black shales rich in carbon (2 percent weight TOC) as well as syngenetic uranium (up to 400 ppm) though more common to marine sediments, can also be deposited in other (lacustrine) environments which are biologically productive and anoxic. Speedy sedimentation along with basin subsidence prevents the oxidation of organic matter and preserves it for possible hydrocarbon generation. In India, there has been a recent discovery of a huge gas field in the near shore regions of the Godavari Basin which contains sufficient humic organic matter (shales and associated coals), deposited in intermediate environment [19].

Examples of intracratonic basins are present throughout the northern Africa continent, including areas in Sudan and Libya, as well as Egypt. Their infilled sediments are predominantly non-marine but it is possible that there is some marine influence during the initial sediments filling of the Chalbi Basin (Anza Graben). The geothermal gradients of the Cretaceous and those of the Tertiary should have been different, consequent to the non-uniform mantle upwarping as revealed by the gravity anomaly profiles. Intracratonic basins of these types are poor prospects for the hydrocarbon exploration, but they contain adequate potential reservoir rocks, which can trap whatever hydrocarbons that were generated by the chiefly continental organic matter buried with the sediments. There are a few examples (1.5 percent of worlds proven reserves) of hydrocarbon generating intracratonic basins of this type [20].

The characteristics of identified subsurface strata showing oil and/or gas indications present conducive environments and implications for the generation of hydrocarbons [7]. Hydrocarbons are usually generated during burial and diagenesis of the organic matter (OM) they contain. In such intracontinental basins, like in the present case, the temperature gradient is often higher than the normal because of the process of formation of these basins. It may sometimes reach a gradient of 3033oC/km. Higher temperatures are also reached by the co-precipitations of radioactive elements along the organic matter. Thus field of crude oil generation expands and reaches a maximum between 2 and 3 km depth. In many cases where there is less generation of crude oil, there is still a possibility of finding gas. The most promising depths for gas, however, are beyond 2.8 km depth [21].

Total organic carbon (TOC) is a measure of carbon present in a rock in the form of kerogen and bitumen. The organic matter is usually converted into kerogen and the type of kerogen depends upon the kind of organic matter that gets buried. Not all the organic matter in the sedimentary rocks are convertible into petroleum hydrocarbons. For example, intracratonic basins like the ones under study do not attract marine organic matter. These basins have had fluvial and lacustrine environments of deposition. Therefore, the organic matter brought from the vegetation on the higher lands of the time (?Jurassic-Cretaceous and Early Tertiary) was buried along with the sediments in the mostly lacustrine and fluvial environments [7].

The studies by the drilling company (AMOCO) in the Chalbi Basin did reveal the presence of good reservoirs and source rocks mainly in the Upper Cretaceous. Based on Bouguer gravity anomalies [7] it was possible to visualise the structural configuration of the part of Chalbi basin in which the three wells were drilled (Figs.8and9). The location of C1 well has been taken such that it reached the deepest part of the sedimentary section, immediately to the west of the Kargi fault. The sections with source rocks identified in C1 are represented by 1500 1800 m and 2300 2390 m depths. These rocks have a high gamma ray values (up to 75 API units) and p-wave velocity values of 3.3 to 3.9 km/s. The porosity of the upper source rock interval is good (27%) while the lower one is relatively low (10 15%). The depth at which they occur would make the organic matter in these sediments (with TOC >5%) undergo changes to produce hydrocarbons since the temperature gradient would be a contribution by the igneous intrusive activity of a later date [7].

The section with reported oil and gas shows in well C1 would similarly constituted of good reservoir rocks with good to fair porosity (30%). Since the source and reservoir rocks are not much separated vertically, there is a possibility that the oil and gas have not migrated much in this well section. Both the source rocks as well as the reservoir rocks occur only within the Upper Cretaceous stratigraphic section. The Lower Cretaceous sequence, the entire younger section of the Upper Cretaceous as well as the Tertiary sections, have no potential rocks. However, in the areas west and east of this well C1 (Fig.12), there could be a migrated oil show, taking into consideration the various intrabasinal faults.

The potential rocks towards the north could be judged by the section of C2 drilled immediately to the north of C1. This well has also revealed depths of the order of 3500 m. The source rocks have been represented in the interval depth 2230 m to 3400 m. They are rich in organic matter with up to 2 percent weight TOC content. This section has been characterised by high gamma peaks ranging from 75 to 90 API units and p-wave velocities of 3.8 4.1 km/s. Some good to fair porosity reservoir rocks are also present within this source rock section at depths 2730 m to 3370 m. The reservoirs have same gamma ray and p-wave velocity characteristics as source rocks.

The source rock section, if extended further into the eastern deeper areas of C2, would probably have better oil and gas potential (Fig.12). The entire section containing source rocks is mostly confined to the Upper Cretaceous stratigraphic part. The higher gamma ray peaks indicate the presence of radioactive elements with organic matter which enhance the possibility of reaching temperatures conducive for hydrocarbon generation. The areas to the east of C2 have experienced faster subsidence as they are near the Kargi fault [7]. In the western section there is less possibility of potential source rocks but migration of oil from east to west along tilted faults cannot be ruled out. It will depend upon the channels, on the porosity, and on the intercalations of shales with sandstones.

Unlike the C1 and C2, the well C3 was drilled near the Chalbi fault towards the western part of the basin [7]. It can be seen that it was again drilled in the deeper part of the Chalbi basin but much to the north. Between locations C1, C2 to the south and C3 to the north, a few faults have been interpreted striking WNW ESE to the north of Mount Kulal. The deepening of the basin towards the west in this section is a departure from what is shown in C1 and C2 wells. Therefore it seems logical to visualise that this criss-cutting faults should have also contributed to the subsidence of the basin. The source rocks in the depth range 2300 m to 3100 m is comparable with the depths reached in C1 and C2. The source rocks show high gamma ray (60 API units) and p-wave values of 3.6 to 4.5 km/s.

The LT-1 well showed good seals provided by lacustrine shales at depths where the oil shows have been found. The black shales have high gamma ray log values (120 API units) and low porosity (<5%). They intermittently were encountered in LT-1 well at 850900 m, 9801057 m and 13601420 m depths [7]. The sections of source rocks in LT-1 extending to the deeper regions towards west, could have achieved the temperature realms of 60150oC. Their maturity and cooking at that depth would make these OM-rich sediments more potential for hydrocarbon generation. The extension of the section with hydrocarbon-shows in the well LT-1 would also provide better reservoir rocks. Thus, the areas in the basin west of LT-1 could be projected as the prognostic areas for future exploration targets. It can be visualized to some extent also from the seismic profiles. Northwards also, the Lokichar basin deepens with intermediate horst-like structures separating the North Lokichar (Lodwar) from the South Lokichar.

The reservoir rocks at C3 well are at shallow depths (1660 1700 m and 1860 1960 m), and are characterised by low gamma ray (36 40 API units) and p-wave values 2.9 to 3.2 km/s. Gas shows encountered in C3 are within a section which is younger (Upper Cretaceous). However, both the source rocks as well as the reservoir rocks in which gas shows are found are within the Upper and Lower Cretaceous stratigraphic section. This is a departure from the wells C1 and C2 where the source rocks as well as the reservoir rocks are within the Upper Cretaceous section only. Immediately to the east of well C3, there is a possibility that the Lower Cretaceous section containing these potential rocks reaches deeper parts of the basin, which will have higher temperatures and possible radioactive elements (Fig.12).

The source rocks of well LT-1 identified above belonged to Oligocene to Lower Miocene age. These source rocks range from depths 8001797 m. However, the interval section which proved to have effective source rocks with sufficient organic matter (OM) that could generate potential oil and gas, is considered to be at depths 16501800 m (considering the normal temperatures gradients). The porosity at shallow depths (800 -1760 m) is high (12 - 40%) but decreases with depth (5 10%) at 1760 -1800 m. No wells have so far been drilled in the North Lokichar and South Kerio basins. Future drilling in the deeper parts of both the Lokichar and Kerio-Turkana basins could indicate potential areas for hydrocarbons (presuming that the source rocks could have reached the natural temperatures gradient in these parts). The oldest strata of Paleocene or younger age in LT-1 has a thickness of 360 m (26002960 m), indicating a thicker sedimentary sequence with the exception only seen in the section of Lower Miocene age, which anyway has neither the source rocks nor the oil shows. The sections in which oil and gas indications in LT-1 occur belong to the Upper Oligocene-Lower Miocene and to the lower part of the Lower-Middle Miocene [7].

The other potential area for future exploration would be to the east of Lodwar fault in the North Lokichar basin. These two areas could also be decided after giving due consideration to the east-west lineaments and/or faults controlling the drainage (Turkwell River). It must be recalled that the well LT-2 (in the North Kerio basin) to the east of the basement ridge went dry with no location of either the potential source rocks or the reservoir rocks at depth. There is, however, a chance also to get some rocks with potential hydrocarbons at a deeper level in the area to the east of LT-2. The gravity and seismic data (gamma and sonic ray logs) further give clue to the porosity of the rocks.

The nature of the Lake Turkana subsurface basin (Figs.2-7) is like half-graben, which deepens and becomes more complex from south to north [7]. There seems to be variations in the depth of the basement from lats. 2o30N to 4o30N. It can be seen that between lat. 2o50N and 3oN the basement shallows and is exposed to the surface. This is a horst-like structure which seems to be controlled by E-W faults. This region coincides with the Kajong/Porr area, east of Lake Turkana. The basement gradually deepens towards the east under the Moiti area, which also shows the presence of E-W trending minor faults which have controlled the river courses also. Beyond latitude 3o30N, the basement deepens suddenly; it becomes deepest (-3 to -5.2km) between lats. 4oN to 4o15N.

The nature of the Lake Turkana subsurface basin (Figs.2-7) is like half-graben, which deepens and becomes more complex from south to north [7]. There seems to be variations in the depth of the basement from lats. 2o30N to 4o30N. It can be seen that between lat. 2o50N and 3oN the basement shallows and is exposed to the surface. This is a horst-like structure which seems to be controlled by E-W faults. This region coincides with the Kajong/Porr area, east of Lake Turkana. The basement gradually deepens towards the east under the Moiti area, which also shows the presence of E-W trending minor faults which have controlled the river courses also. Beyond latitude 3o30N, the basement deepens suddenly; it becomes deepest (-3 to -5.2km) between lats. 4oN to 4o15N.

Some minor faults have also been interpreted based on the changes in the slope of the basement profile. The subsurface structure between latitudes 2o50N to 3o45N, based on gravity profiles and basement depth, showed that the basin also deepens towards the east. Thus the Lake Turkana Basin seems to have been initiated prior to the N-S faulting; deepest towards the north between latitudes 3o30N to 4o30N (Fig.6 a). It is in this region one ought to expect the subsurface continuation of the Lapurr Range outcrops.

A cross-section of the basement depth from North Kerio basin to the northern Lake Turkana basin, between lats. 2o30N to 4o30N and longs. 35o45E and 36o15E, shows that though the basement gradually deepens towards north, there is no distinct deep trough existing between lats. 3o30N to 4o30N (Fig.6 b). The horst structure seen under the Moiti area is also not distinctly demarcated in this profile. Thus the subsurface structure is therefore very complicated, characterised by irregular downthrows and upthrows related first to the E-W faulting and later to the N-S faulting in the basin. From the above gravity and basement depth cross-sections, it can be concluded that the entire Lake Turkana subsurface basin is bounded by N-S-striking faults and there exists intrabasinal faults also [7]. The thickness of the sediments seems to be maximum to the north and less to the south, since the basin seems to be deepening northwards.

From the seismic profiles along Lines TVK-4, TVK-5, TVK-6 and TVK-7 (Figs.4,11and12), in the Lotikipi basin, it was possible to identify two sub-basins, between longitudes 34o30E and 35o00E and latitudes 4o15N and 4o45N, which have been named after the nearest river systems as a) the Anam-Natira Formation and b) the Tarach-Nakalale Formation [7].

The anticipated best-developed subsurface sedimentary section, identified on the basis of seismic and gravity studies, under the channels of the Anam and Natira rivers (Figs.2,4and11), has been named as the Anam-Natira Formation. The 1050 m thick sequence (between long 34o30E and 35o00 N), showing P-wave velocity (Vp) between 3.0 and 4.0 km/s on TVK-4 line profile, is interpreted to consist chiefly of sandstones and shales [7]. The lower 350 m section, between 1900 m to 2250 m depth, should contain compact sandstones with frequent thick clay/shale layers, for which the Vp range is between 3.5 to 4.0 km/s.

The upper 700 m section, between 1200 m - 1900 m depths, should be mainly fine-grained sandstones with minor clay/shale layers, followed upward by mainly coarse-grained sandstones. The upper section is characterised by a lower Vp range between 3.0 and 3.5 km/s, but the intercalated clay/shale or less porous, more compact sandstones layers are marked by higher Vp (3.5 km/s).

The best-developed (Tertiary ?) section (1420 m thick) located beneath the Tarach and Nakalale river systems, characterized by Vp between 2.0 and 3.0 km/s, has been named as the Tarach-Nakalale Formation [7]. Deduced along TVK-6 the area covered under Longs. 34o45E and 35o03E and Lats. 4o24 N and 4o42 N shows a better development of this formation, which seems to be constituted of less consolidated sands, gravels, silts and clays which increasingly become more compact towards the basal part of the section (1560-2060m depth). The sub-basin on the TVK-6 line bounded by lats. 4o24N and 4o42 N and longs. 34o45E and 35o03E can be considered as representing the type section of the Tarach-Nakalale Formation. The type Tarach-Nakalale Formation sequence shows well-developed representation of both the members, which range to about 800 m thick each.

Future drilling and fossil finds will provide additional stratigraphic attributes to these seismically defined Formations in the Lotikipi basin. However, at this juncture, the section cannot be assigned a definite stratigraphic age, but occurring in similar tectonic and stratigraphic setup, it is suspected that the Anam-Natira Formation might be homotaxial to the Sharaf and Abu Gabra Formations (Neocomian or Albian-Aptian in age, [10] of southern Sudan. Although future drilling alone would enable assigning additional lithological attributes to these two subdivisions, in the absence of any other criteria to assign a stratigraphic age, it might help to consider Tarach-Nakalale Formation as coeval to the Kordofan Group of southern Sudan (early Tertiary in age, [10].

Additionally, one might also point out that the sub-basins in which the thicker Anam-Natira Formation sequences (Upper Cretaceous?) are suspected are different than the sub-basins in which a greater thickness of Tarach-Nakalale Formation (Lower Tertiary?) is anticipated [7]. Since there has been no exploratory wells drilled in the Lotikipi basin, most of the prognostic evaluation of the basin (Fig. 12) would depend upon the evaluation done on rocks of equivalent age belonging to the other basins (along the NW-SE-trending Anza and Abu Gabra Rifts Fig.3).

From the palynological presence of flora and faunal assemblages found in these sedimentary sections, the drilling companies (AMOCO and SEPK) have reported that the sediments were deposited in marine and deltaic and/or fluvial-lacustrine environments. The sedimentation in these intracratonic rift basins was controlled by intrabasinal and marginal faults, some of them reaching also the basement. While examining the subsurface stratigraphy and the drilled well cores, it is intended also to assess the prognostic oil and gas potential of these sedimentary basins and extrapolate the information to other parts of these basins, as well as to the northwestern unexplored areas of the Lotikipi basin. Their infilled sediments are predominantly non-marine but it is possible that there is some marine influence during the initial sediments filling of the Chalbi Basin (Cretaceous) in the north Anza graben. The geothermal gradients of the Cretaceous basins and those of the Tertiary should have been different, consequent to the non-uniform mantle upwarping as revealed by the gravity anomaly profiles.

Intracratonic basins of these types are poor prospects for the hydrocarbon exploration, but they contain adequate potential reservoir rocks, which can trap whatever hydrocarbons that were generated by the chiefly continental organic matter buried with the sediments. There are a few examples of hydrocarbon generating intracratonic basins of this type. Speedy sedimentation along with basins subsidence prevent the oxidation of organic matter and preserves it for possible hydrocarbons generation. Thus, the examined salient structural features of sourcereservoirseal associations revealed favourable prognostic targets for future petroleum exploration in the northwest Kenya rift basins.

This paper is published with the permission of the Managing Director, National Oil Corporation of Kenya (NOCK). The fieldwork, data analyses and interpretation formed part of the PhD research-related thesis work submitted to the University of Pune. I am grateful to Prof. Patwardhan for his invaluable comments and discussions during my research period.

[1] Barker, B.H.,1986, Tectonics and volcanics of southern Kenya Rift Valley and its influence on rift sedimentation, In: Frostick, L.E. et al (Ed), Sedimentation in the African Rifts, Geological Society Special Publication No. 25, pp.45-57. [2] Girdler, R.W.,1983, Processing of planetary rifting as seen in the rifting and breaking up of Africa, Tectonophysics, Vol. 94, pp. 241-252. [3] Green, L.C.; Richards, D.R. and Johnson, R.A., 1991, Crustal structure and tectonic evolution of the Anza rift, northern Kenya, Tectonophysics, Vol. 197, pp. 203-211. [4] Davidson, A. and Rex, D.C., 1980, Age of volcanism and rifting in southwestern Ethiopia, Nature, Vol. 283, pp. 657-658. [5] Barker, B.H., Morh, P.A. and Williams, L.A.J., 1972, Geology of the Eastern Rift systems of Africa, Geological Society of American Special Paper 136, 67p. [6] Rop, B.K., 2002, Subsurface geology of Kenyan Rift Basins adjacent to Lake Turkana based on gravity anomalies, In the abstracts of International Seminar on Sedimentation and Tectonics in Space and Time, Dept. of Civil Engineering, S.D.M. College of Eng. and Tech., Dharward 580 003, India, 16th 18th April, 2002; pp 83-85. [7] Rop, B.K., 2003, Subsurface stratigraphical studies of Cretaceous-Tertiary basins of northwest Kenya, Ph.D. thesis, University of Pune, India. [8] Walsh, J. and Dodson, R.G., 1969, Geology of Northern Turkana. Geological Survey of Kenya, In: Bishop, W.W. (Eds.), Scottish Academic Press, Edinbergh, pp. 395-414. [9] Rop, B.K., 1990, Stratigraphic and sedimentological study of Mesozoic-Tertiary strata in Loiyangalani area, Lake Turkana district, NW Kenya, M.Sc. Thesis, University of Windsor, Canada. [10] Schull, T.T., 1988, Rift Basins of interior Sudan: Petroleum Exploration and Discovery,The AAPG Bulletin, Vol. 72, pp. 1128-1142.

[11] Morley, C.K., Wescott, W.A., Stone, D.M., Harper, R.M., Wigger, S.T. and Karanja, F.M., 1992, Tectonic evolution of the northern Kenya Rift, Journal of the Geol. Soc., London, Vol. 149, pp. 333-348. [12] Key, R.M., Rop, B.P. and Rundle, C.C., 1987, The development of the Late Cenozoic alkali basaltic Marsabit Shield Volcano, northern Kenya, Journal of African Earth Sciences, Vol. 6, pp. 475-491. [13] Winn, R.D., Steinmetz, J.C. and Kerekgyarto, W.L., 1993, Stratigraphy and Rift History of Mesozoic-Cenozoic Anza Rift, Kenya, The AAPG Bulletin, Vol. 77, pp. 1989-2005. [14] Bloom, A.L.,2002, Geomorphology, Third Edition, Prentice-Hall India Pvt. Ltd., New Delhi. [15] BEICIP, 1984, Petroleum potential of Kenya 1984 follow-up, Ministry of Energy and Regional Development. [16] Sharma, P.V., 1976, Geophysical Methods in Geology, W.H. Freeman and Company, New York. [17] Durrance, E.M., 1986, Radioactivity in Geology: Principles and Applications, Ellis Horwood Limited Publishers, Chichester. [18] Patwardhan, A.M., 1999, The Dynamic Earth System, Prentice-Hall of India. [19] Minax Pal, Venkatesh, V., Balyan, A.K. and Sarkar, A., 1992, Hydrocarbon Prospects of Gondwana Basins in India; Source Rock Studies of Kamthi Sub-Basins of Pranhita-Godavari Graben, Journal of Geological Society of India, Vol. 40, pp. 207-215. [20] Selly, C.R., 1985, Elements of Petroleum Geology, W.H. Freeman and Co. New York. [21] Tissot, B.P. and Welte, D.H., 1984, Petroleum Formation and Occurrence, Second Revised and Enlarged Edition, Springer-Verlag Berlin Heidelberg New York Tokyo. [22] North, F.K., 1985, Petroleum Geology, Boston Unwin Hyman Publishers.

carbonatite rare earth elements deposits | geology for investors

Carbonatites are a special group of carbonate-rich igneous rocks and the worlds primary source of rare earth elements (REE), niobium, zirconium, and phosphate oxide. They contain more than 50% primary carbonate minerals, less than 20% silicate minerals (pyroxene, amphibole, and olivine), and few phosphate minerals. Carbonatite deposits exist around the world, primarily in continental rift settings. In most cases, carbonatites are intrusive or subvolcanic, forming cone sheets, volcanic necks, dykes, sills, breccias, and veins. A notable exception is the Oldoinyo Lengai Mountain in Northern Tanzania, which is the only known active carbonatite volcano on Earth. Preservation of carbonatite lava flows is rare, because the carbonate minerals weather so easily.

Rare Earth Elements (REE) are a group of elements which have similar geochemical properties and occur together in a deposit. They belong to a chemical group called Lanthanides, which occupies atomic numbers 57 to 71. The elements include lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium. Yttrium and Scandium are also considered REE because they occur in the same ore deposits as the lanthanides and have similar chemical properties. REE-bearing ore minerals include bastnsite, monazite, and xenotime.

Rare Earth Elements are not particularly rare in fact, they are more abundant than gold. However, they rarely occur in high concentrations, and similar physical and geochemical properties make it difficult to separate rare earth elements from each other. Separation is achieved through expensive processing technology, which makes many low-grade REE carbonatites uneconomical to mine. Rare earth elements have many uses, including the manufacturing of electric car motors, lithium-ion batteries, computer hard drives, solar panels, ceramic, and magnets.

Carbonatites have the highest known concentration of REEs in any igneous rock, making them an attractive mining target. While early REE mining was mainly in placer deposits in India and Brazil. Carbonatite REE mining took off in the 1950s with the discovery of the Precambrian Mountain Pass deposit in California, USA. Mountain Pass was the worlds main supplier of REEs until the early 2000s when environmental concerns and competition from China caused the mine to close. Three carbonatite deposits in China (Bayan Obo, Maoniuping, and Dalucao) provide the majority of the worlds REEs. Mining at Mountain Pass mine resumed in 2018 in response to increased demand of REEs and geopolitical forces.

Growing demand for REEs has prompted further exploration for REE deposits in carbonatites around the world. The African continent, particularly the East Africa Rift Valley, is home to two prominent carbonatite REE deposits, which are discussed in detail below.

Mrima Hill is a niobium and REE deposit located in Kwale County, about 70 km southwest of the Kenyan coastal city of Mombasa. The Cretaceous deposit consists of four separate alkaline intrusions: Kiriku, Mrima, Nguluku, and Jombo. These intrusions cover a surface extent of approximately 2 by 1.3 km and are thought to be joined at depth. The alkaline intrusions consist of carbonatite, agglomerate, and fenitized sediments. The complex intruded into Jurassic sandstones and is covered by soil and laterite with poor exposures, although the Mrima intrusion is exposed at the surface as a small (1000 x 600 m) hill. Weathering is extensive, going as deep as 100 meters in some places, with the weathered section showing a high concentration of minor resistant minerals. The main rare-earth minerals present are monazite ((Ce, La, Th) PO4) and gorceixite (barium-aluminium phosphate with rare-earths).

The deposit was discovered in the early 1950s and has undergone substantial historical exploration. The Kenya Mines and Geology Department carried out a test shaft and 81 test pits in the 1950s, followed by over 400 test pits and 2,000 m of drilling carried out by Anglo American from 1955-1957 and Pechiney from 1971-1973. By 2010, explorers had sampled more than 3000 m from drilling, 9000 m from test shafts and 37 tonnes from bulk sampling. Historic drilling indicated high mineralization for rare earth elements, niobium, tantalum, phosphate, and manganese up to 150 meters depth.

In 2010, Cortec Mining Kenya Ltd (a Kenyan subsidiary of the Canada-based mineral exploration company Pacific Wildcat Resources Corp) undertook an extensive exploration program, performing a 974 m (31 holes) reverse circulation drilling over the Mrima hilltop. It was followed by 1700 m diamond drilling on the enriched weathered zone to estimate its vertical extent. The results confirmed test pits and historical shaft results, positioning Mrima Hill as a world-class resource for niobium and REE. The indicated mineral resource for niobium was 12.0 million tonnes at 1.21% NbO and an inferred resource of 105 million tonnes at 0.65% NbO. The results also showed potential for a world-class REE deposit with initial target resource of 10-20 Mt at 3-5% total rare earth oxides (TREO).

The government of Kenya revoked the mining license issued to Cortec Mining Kenya Ltd in March 2013, stopping further exploration and mine development. The license issue has not been resolved to date, but with rising global demand for niobium and REE, the Mrima Hill deposit remains one of the largest undeveloped niobium-REE deposits in the world.

The Ngualla carbonatite is located on the western edge of the East Africa Rift Valley in southwest Tanzania, 147 km from the city of Mybea. The exact age of the carbonatite complex is unknown, but it is thought to be around 1 billion years old, and was intruded into Proterozoic gneisses, quartzites, and rhyodacitic volcanic rocks. The intrusion has a pipe-like form, consisting of an inner body of iron-rich dolomite that intruded a plug of calcitic carbonatite. The carbonatite intrusion has a diameter of approximately 4 km, and is encompassed by a 1 km wide fenite alteration zone, which forms a ring of low hills.

Mineralization is enriched in weathered and altered zones of the carbonatite, which can be attributed to weathering of the carbonate minerals and oxidation of the iron carbonate component present in dolomite carbonatite. Removal of carbonate through weathering also caused significant mass reduction and contributed to the formation of a layer of unconsolidated solid material rich in goethite. The main REE-bearing mineral is bastnsite (carbonate-fluoride), which is present in the center ferroan dolomite zone.

Current developers of the deposit, Australia-based Peak Resources, aim to mine the weathered zone of the carbonatite from the surface to a depth of over 140 m. This zone contains high-grade REE mineralization within the mineral bastnsite and can be extracted using a low strip ratio open pit technique. Low levels of thorium, uranium, and phosphate simplify the required ore processing and concentration procedures. Exploration work done on this deposit includes a three-phase drilling program comprising over 38,700 m of drilling in 649 drill holes. This work was followed by an additional 30,139 m infill reverse-circulation (RC) and 3,106 m of diamond drilling, with geochemical analysis completed on 20,403 samples.

In 2016, Peak Resources announced a JORC (Australasian Joint Ore Reserves Committee) compliant mineral resource estimate of 214.4 million tonnes at 2.15% REO (rare earth oxides), for 4,620,000 tonnes of contained REO (above 1% REO cut-off). This represented a 10% increase in tonnes, 8% increase in grade and 5% in contained REO over their 2013 estimate. The mineral resource estimate for above 3% cut-off was 19.9 million tonnes at 4.90% REO, for 980,000 tonnes of contained REO. Mineral reserve for the weathered bastnsite zone at a 1% REO lower grade cut-off was estimated to be 21.3 million tonnes at 4.75% REO for 1,010,000 tonnes contained REO. Within this, the proved reserve was 18.5 million tonnes at 4.80% REO for 887,000 tonnes of contained REO while the probable reserve was 1.7 million tonnes at 5.14% REO for 90,000 tonnes.

The Ngualla deposit is one of the largest and highest grade rare earth deposits in the world, especially for neodymium and praseodymium. The REE mine under development is estimated to have an initial production life of 26 years, aiming to export 32,700 tonnes per year of rare earth concentrate.

As global demand for electronics, batteries, and clean energy increases, so will the demand for REEs. Carbonatites are among the worlds richest sources of REEs and there are promising deposits in the US, Brazil, East Africa, Western Australia, and China. The untapped potential of these unique igneous rocks will play a crucial role in the future of REE mining.

what are the gemstones of africa - howard fensterman minerals

Besides its cultural diversity, breathtaking landscapes and thriving wildlife, Africa is also thriving in an abundance of valuable minerals and stones. Some of the worlds largest gold and diamond mines are located here. In fact, many African economies rely heavily on large deposits of precious minerals.

There is a widespread misconception that Africa is only home to diamonds, copper and gold reserves. However, in reality, the continent possesses a diverse range of precious gemstones inside its vast geological planes.

Ruby is the most precious and sought-after gemstone from the corundum family alongside sapphires. This red stone has a rich history and has always been popular in ornamental items across different cultures and civilizations. Even though it is excavated everywhere in the world, rubies from Africa have become quite popular lately.

Burma rubies that are excavated in Myanmar are considered the best among all the ruby varieties. However, Burma Rubies were not legally available in the US gem market for quite some time due to trade restrictions. To compensate for the demand for clear ruby specimens, the supply from Mozambique came in really handy. Kenya, Tanzania, and Madagascar are also some of the noteworthy African Ruby exporters to many countries in the world.

Garnet belongs to the silicate mineral group and has many varied uses. The uninspiring and impure garnet specimens are used as abrasives in the industries, thanks to its high grading on the Mohs Hardness Scale. On the other hand, the fine and pure garnet specimens that usually have reddish appearance are faceted as jewelry stones.

Tanzania produces the worlds finest and rare garnet specimens. Technically, they are called Rhodolite Garnet because they are the combination of two minerals Almandine and Pyrope. This unique blend gives the specimen an extraordinary crimson luster along with raspberry and violet undertones. Right now, such Garnet composition is rarely mined from any other part of the world. This makes Tanzania the sole exporter of Rhodolite garnets.

Tourmaline is another silicate mineral with Boron traces. It comes in a range of different colors. However, one of its variant that was only discovered in 1980 has become all the rage among gem lovers. Going by the name Paraiba Tourmaline, this variant was first discovered in the Brazilian hills.

Due to its extraordinary aquatic blue luminescence, it quickly gained traction as the precious gem option for engagement rings. Before its discovery in Mozambique and Nigeria in the last decade, Paraiba Tourmaline was so rare that one single specimen was mined for every 10,000 diamonds.

At present, Mozambique is the largest exporter of this exceptional variant of Tourmaline. It is important to mention here that Paraiba Tourmaline is still a rare gemstone even after its discovery in several African countries. In order to make sure that each and every Paraiba Tourmaline is excavated, miners use manual methods to unearth them. A carat of a fine faceted piece of Paraiba Tourmaline is priced around $16,000.

Due to its chemical composition, tsavorite is regarded as one of the peculiar gemstones. Its actually a garnet but with heavy bonding of aluminum and calcium molecules. Garnet usually exhibits a red hue while some rare specimens are available in other colors as well. Tsavorite naturally offers extremely fine clarity. That characteristic is also substantiated by its higher reflective index.

Tsavorite has a strong green color exhibition, which makes it an exceptional choice for jewelry items and as a substitute for emeralds. Eastern African countries like Kenya and Tanzania are gifted with sizable deposits of this unique garnet. Some of the tsavorite specimens excavated in Kenya can go up to $6,000 per carat.

Topaz is another silicate mineral but with the addition of aluminum and fluorine impurities in it. It is the hardest stone among all the silicates. Clear and lustrous topaz stones are considered ideal for jewelry items because of their extended durability and resistance against scratches.

The majority of topaz specimens are found in vitreous blue and gray colors. However, brown, yellow, red and pink rare topaz specimens are also available in the gem market. Nigeria is the home of some fine yellow topaz specimens that are usually priced around $600.

Named after the origin of its discovery, Tanzania, this gemstone is a relatively newer mineral specimen. The stone is exclusively excavated in the mines in Northern Tanzania. Apart from its strong presence, tanzanite also stands apart among other gemstones because of its trichroism exhibition.

A single tanzanite specimen can put up a colorful display when observed from different planes. Blue, burgundy and violet hues are often displayed by tanzanite when they are observed under different lights. High-quality tanzanite is still only found in Tanzania and can cost up to $700 per carat. Well-Faceted tanzanite is used as a primary stone in necklaces.

Zircon is one of the nesosilicates that is formed when underground silicate deposits dissolve with some rare elements. The gem-grade zircons are available with different chromatic attributes. The fine colorless zircon pieces are used as cheap diamond alternatives. In addition, brown, green, blue and yellowish golden variants are also faceted as gemstones. Like a diamond, Zircon also has a protracted crystallization age. On average, a natural zircon specimen is around four billion years old.

Some high-quality colorless zircons are mined at the Limpopo Belt, South Africa. So, South Africa is not just responsible for the large supply of diamonds worldwide, it is also supplying high-quality diamond substitutes in the form of zircons. Apart from white or colorless zircons, its well-saturated variant is also popular among gem aficionados. It is indeed eight times more expensive than transparent diamond substitutes.

visit the real-life inspiration for the lion king at hells gate national park in kenya

The animators of Disneys The Lion King traveled to a park with dramatic landscapes in Kenyas Great Rift Valley to get a feel for, as Mufasa puts it, the great circle of life. When visitors to Hells Gate National Park reach the sweeping cliffs carved by a prehistoric lake that gives the park its name, theyll discover that Pride Rocks real-life inspiration is no less majestic.

The fact that you wont actually see Simba (which means lion in Swahili) is the parks secret advantage: the lack of predators means you can take in savannas and come face to face with giraffes, zebras, antelopes, and warthogs on your own two feetor better yet, on bicycle wheels. Hells Gate is one of the only places in East Africa where you can hike the totality of a park. You can even go on a biking safaria far cry from peeping animals through a safari vehicles roof. (Walt Disney Company is majority owner of National Geographic Partners.)

Roughly 26 square miles in size, the park is situated in the Great Rift Valley, a jagged tectonic tear that stretches from Lebanon to Mozambique and cracks the continent in two by a few millimeters every year. This is what makes Hells Gate a geological marvel worth visiting: towers of rock that provide rock climbing opportunities for adrenaline junkies and hiking trails that wind around a gorge formed by ancient, raging waters. Heres how to see it all in Hells Gate.

Hells Gate is smaller and less packed with animals than Kenyas other parks, but theres nothing like pedaling your bike past a herd of zebras or trying to keep pace with a galloping giraffe. The best bike ride stretches four miles from the main entrance of the park, Elsa Gate, to the picnic site and ranger station near the entrance to the Ol Njorowa Gorge. (Here are some tips for responsible wildlife tourism.)

Set out on the dirt road in the early morning or late afternoon to escape the harsh equatorial; youre likely to see animals all day, such as families of warthogs dashing across the road or herds of gazelles frolicking through acacias. The park proves a popular spot for birdwatching, with more than a hundred species. Give the water buffalo plenty of distance; theyre more dangerous than they look. (Find more destinations around the world to see unusual birds.)

Keep an eye out for Fischers Tower, a 75-foot-high rock formation made from molten volcanic lava forced up through a tear in the earthor, if you prefer local lore, a rebellious girl who was turned to stone after defying her family before her wedding. This site is one of several places for rock climbing in the park. Call Climb BlueSky, a Nairobi rock-climbing gym, for more details.

Bring a packed lunchand a cold Tusker, Kenyas favorite lagerto picnic at the Rangers Station picnic area. Be warned, vervet monkeys and baboons abound and unlike Simbas mandrill friend, Rafiki (which means friend in Swahili), these monkeys are more likely to steal your food than offer sage advice.

From here, you can hire a guidethe area is community-run by Maasai peopleto take you on a hike through the Ol Njorowa Gorge. The trail is not for the faint of heart; youll ford streams, scramble over boulders, and scale short rock faces, but the impressive views are well worth the effort.

At times, the walls of the gorge tower so high above you, and hug so tightly together, they almost squeeze out the light from the sky above. If the setting looks familiar, it might be because The Lion King isnt Hells Gates only silver screen cameoLara Croft: Tomb Raider also filmed here. Toward the end, see billowing steam from the parks geysers and dip your hands in bubbling springs that are hot enough to boil eggs.

Kenya was the first African country to tap geothermal energy and Hells Gate is a leader: The parks geysers and hot springs are used to harvest geothermal energy and fuel almost half of Kenyas electricity. Environmentalists and safari enthusiasts worry about threats to the wildlife and natural beauty. This may be true, but harvesting that kind of power is nothing to sniff at in a country whose population doubled in 25 years and where over 60 percent are still without electricity. Visit the parks newly constructed Olkaria Spa and bathe in the mineral-rich waters produced from the condensed steam to get a hint at how the park may meld tourism and development in the future.

Hells Gate is located about nine miles from the turnoff from the Nairobi-Naivasha highway. The two hour drive from Nairobi is a trip in itself, especially with the Escarpment Roads stunning views of the Great Rift Valley. If youre prepared to bargain, there are opportunities to buy Kenyan tchotchkes from vendors along the way. The road was constructed by Italian prisoners of war during World War II and the tiny church they built is worth a stop. (Here are the top sites to see while visiting Nairobi.)

Hells Gate is situated near two volcanoes, Longonot and Suswa, both of which make great day hikes. Its also near Nakuru National Park, where you can catch the rhinos youll miss at Hells Gate, and Lake Naivasha, where you can go boating. The brilliant pink strips at the edge of the lake? Yes, those are flocks of flamingos.

Hells Gate has three camping sites: Endachata, Naiburta, and Oldubai, where youll fall asleep to the eerie sound of a hyenas cackle and wake up early for a sunrise game drive. Daybreak is when youll see the most animals and when the park will be drenched in golden light.

If youre looking for a little more infrastructure, Camp Carnellys is a 15-minute drive from the parks entrance. There, you can pitch a tent along the shores of Lake Naivasha or stay in a cabin tucked under fever trees. Either way, unwind at the Lazy Bones restaurant with a gin and tonic and one of their inventive samosas. Next-door Fishermans Camp offers similar amenitiesand excellent brick oven pizza.