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Kenticha |
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Ethiopia |
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Kenticha is a large lithium-cesium-tantalum rare earth pegmatite in the Neoproterozoic Adola Belt of southern Ethiopia, located 400 km south of Addis Ababa (#Location: 5° 27' 12"N, 39° 1' 6"E).
The Kenticha tantalum deposit was discovered in the 1980s in the course of a joint Ethiopian-Soviet program exploring for gold in the Adola Belt of southern Ethiopia (Kozyrev et al., 1982). Several other rare-element pegmatites were also discovered and evaluated (Emelyanov et al., 1986; Poletayev et al., 1991). The Kenticha pegmatite, which was the largest, has been worked since 1990, with mining restricted to the deeply weathered regolith. Initially, no testing was undertaken to establish the grade and tonnage of the underlying hardrock pegmatite. Five test holes were drilled into the unweathered pegmatite in 1999, followed by a program of 91 drill holes in a concerted exploration program. Analysis of this program in 2014 led to the estimation of a Soviet style C2 tantalum resource. In late 2017, data from 68 drill holes, including 61 new holes drilled in 2017, were used to estimate a C2 resource for lithium. Later that same year, mining was suspended due to capital and technical inadequacies. Production over the 27 year mine life had been of the order of ~100 to 200 tonnes of ~50 to 70% Ta2O5 tantalum concentrate per annum from deeply weathered pegmatite Küster et al., 2009), which was near 14% of global supply. During this period, there had been a short pause in operations during 2012 due to declining prices and complications with radioactive by-products. In December 2020, following a formal bidding process, Abyssinian Metals Limited was awarded an exploration licence over the deposit by the Ethiopian Ministry of Mines and Petroleum, and began a drilling program to establish JORC compliant resources. As of June 2023, Abyssinian Metals Limited was the joint venture manager and 51% owner of Kenticha Mining PLC, with the regional State owned Oromia Mining Share Company holding the remaining 49%. In late 2023, a dispute arose between the two partners, delaying the proposed new mine start up in 2024.
Regional Setting
The Kenticha pegmatite lies within the southern Arabian-Nubian Shield, which is largely composed of Neoproterozoic juvenile crust formed by accretion of oceanic arc terranes between the East and West Gondwana cratonic blocks. For detail of the regional setting of the shield and the location of Kenticha within it, see the separate Arabian-Nubian Shield record.
The Neoproterozoic rocks of southern Ethiopia are divided into two principal lithofacies, namely (after Yibas et al., 2002):
i). granite-gneiss complexes, composed of high grade, amphibolite to granulite facies quartzo-feldspathic, partly migmatitic, para- and ortho-gneisses with subordinate amphibolites, and sillimanite-kyanite bearing schists. The orthogneisses and deformed batholiths of dioritic to granitic composition are abundant, and are locally the dominant lithology. Yibas et al. (2002) differentiated four separate granite-gneiss complexes, separated by major regional shear zones, one of which is the Adola granite-gneiss complex.
ii). ophiolitic fold and thrust belts composed of north-south to NNW-SSE trending mafic and ultramafic metavolcanic and metasedimentary rocks. These belts have only been subjected to low-to-medium grade, lower amphibolite facies metamorphism. The contact with adjacent granite-gneiss complexes comprise zones of multiple deformation, including both thrusting and strike-slip shear faulting. On the basis of the occurrence of boninitic rocks, and contrasting proportions of ultramafic and metasedimentary rocks, Yibas et al. (2002) distinguished four of these belts, one of which is the Kenticha ophiolitic fold and thrust belt, located within the Adola granite-gneiss complex.
Isotopic dating has yielded Neoproterozoic ages for both the granite-gneiss complexes and ophiolitic fold and thrust belts. U-Pb zircon geochronology of granitoid gneisses yielded ages between 880 and 700 Ma for the former (Abraham et al., 1992; Genzebu et al., 1994; Worku 1996; Teklay et al., 1998; Yibas et al., 2002), whilst mafic magmatism in the ophiolitic fold and thrust belts has been dated at 790 to 700 Ma (Worku 1996; Teklay et al., 1998). Both were probably deposited during to an extensional event marking the opening of the Mozambique Belt.
These rocks are intruded by what are interpreted to be subduction-related to syncollisional granitoid magmatism, with associated metamorphism, thrusting and strike-slip shearing, bracketed between 680 and 550 Ma (Beraki et al., 1989; Worku 1996; Worku and Schandelmeier 1996; Yibas et al., 2002; Yihunie 2002; Tsige and Abdelsalam 2005).
Undeformed biotite granite plutons were emplaced between 550 and 520 Ma (Gichile 1991; Genzebu et al., 1994; Worku 1996; Yibas et al., 2002). These granites have been interpreted to be mainly I-type (Worku 1996).
The broader, 50 to 75 km wide Adola granite-gneiss complex encloses two narrow north-south ophiolitic fold and thrust belts, the Kenticha and the Megado belts juxtaposed across north-south striking thrust and shear zones. The Adola granite-gneiss complex is composed of amphibolite facies quartzo-feldspathic gneisses and biotite-hornblende gneisses as well as deformed metagranitoids. The two ophiolitic fold and thrust belts are each <5 to >10 km wide and are composed of greenschist to lower amphibolite facies metasedimentary rocks, mainly garnet-mica schists, graphite schists, quartzites, marbles, and metamorphosed mafic-ultramafic rocks, represented by amphibolites, talc-tremolite schists and serpentinites (Worku and Schandelmeier 1996; Yihunie 2002; Yibas et al., 2003). In contrast to the Megado belt, the Kenticha belt is characterised by the dominance of ultramafic over mafic rocks. The composite Adola granite-gneiss complex and the two ophiolitic fold and thrust belts together define the Adola Belt.
Metamorphic conditions are estimated to have reached 600 to 650°C and 6 to 7 kb in the Adola granite-gneiss complex (Yihunie et al., 2004; Tsige 2006) and 520 to 580°C and 4 to 5 kb in the Kenticha ophiolitic fold and thrust belts (Yihunie et al., 2004). Deformation and metamorphism of the Adola Belt is inferred to have taken place between 610 and 554 Ma in an east-vergent compressional setting (Worku and Schandelmeier 1996; Yihunie 2002), followed by dextral strike slip displacement along the thrust shear zones (Yihunie 2002). Both the granite-gneiss complex and the ophiolitic associations are intruded by post-orogenic biotite granite plutons that also cut the thrust shear zones. Two of these plutons have been dated, one at 554 ±23 (U-Pb zircon; Robelie granite; Genzebu et al., 1994) and the other at 550 ±18 Ma (Lega Dima granite; Worku 1996). These ages mark the end of compressive deformation in the Adola Belt and the beginning of postorogenic crustal extension and uplift. The age of the Kenticha pegmatites is ~530 Ma (see below).
Deposit Geology and Mineralisation
Whilst granitic pegmatites occur in all lithofacies of the Adola Belt, a distinct cluster, the Kenticha Pegmatite Field is found in the Kenticha ophiolitic fold and thrust belt to the southeast of the town of Shakisso, distributed over an area of ~2500 km2. All belong to the REL-Li subclass, based on the classification scheme of Černý and Ercit (2005). These pegmatites, which mostly strike north-south to NNE-SSW, were emplaced into greenschist to lower amphibolite facies talc-tremolite schists, chromite-bearing serpentinites, and pelitic to graphitic mica schists of the Kenticha ophiolitic fold and thrust belt. However, the individual pegmatites exhibit considerable differences in size, varying from a few tens of metres to >1 km in length, with shapes that range from steeply dipping dykes to almost flat lying sheets. Individual pegmatites also have variations in internal zoning, mineralogy and geochemistry (Poletayev et al., 1991). These differences are the basis of a grouping into a number of types.
These different pegmatite types of the Kenticha Pegmatite Field are spatially zoned around the elongated Kilta Shanbeli biotite to biotite-muscovite granite pluton which is located in the southeastern part of the pegmatite field, directly east of the Kenticha thrust shear zone. Outcropping pegmatites are predominantly found to the west and north of the granite, displaying a progressive mineralogical and geochemical differentiation with increasing distance from the pluton (Zerihun et al., 1995). The zonation of the different types is summarised as folllows:
• Zone A - barren feldspar-muscovite pegmatites, proximal to the granite pluton;
• Zone B - quartz-feldspar-muscovite pegmatites of the beryl-columbite subtype;
• Zone C - variably zoned and partly albitised quartz-feldspar-muscovite pegmatites of the beryl-columbite sub-type, which exhibit a higher degree of geochemical differentiation than zone B pegmatites (Zerihun et al., 1995) and contain Mn-tantalite (Tadesse and Zerihun 1996);
• Zone D - which embraces the Main Kenticha pegmatite, a large spodumene-bearing quartz-feldspar-muscovite pegmatite with a complex internal zonation and occurrence of Mn-tantalite in its upper part (Tadesse and Zerihun 1996). U-Pb dating of Mn-tantalite from two sections of the Kenticha pegmatite gave ages of 530.2 ±1.3 and 530.0 ±2.3 Ma;
• Zone E - the most distal to the Kilta Shanbeli granite, comprising another highly differentiated spodumene- and Mn-tantalite-bearing pegmatite, known as the Bupo pegmatite. Mn-tantalite from this pegmatite, which is 9 km north of Kenticha, has an age of 529.2±4.1 Ma,.
The Kenticha rare-element pegmatite is exposed over a >2 km length and 400 to 700 m width on the western flank of a north-south ridge. It was intruded into a steeply dipping sequence of talc-tremolite and biotite schists and massive serpentinite, and constitutes a flat-lying, north- to NNE-striking, moderately east- to SE-dipping sheet-like dyke. This body pinches out to the south, whilst its northern termination is concealed under thick alluvial cover (Poletayev et al., 1991). It's thickness is very variable, ranging from 40 to 100 m. In the northeast, smaller pegmatite veins or apophyses emanate upwards from the roof of the main intrusion into the country rock. A separate pegmatite body occurs below and to the west of the main Kenticha pegmatite (Poletayev et al., 1991). Also to the NE, it contains large angular xenoliths, mainly composed of serpentinite. These xenoliths are mostly 50 to 100 m long, but may be as much as 500 m, and a few metres to 25 m, and occasionally as much as 100 m thick.
The Kenticha rare-element pegmatite has an asymmetric textural and mineral zonation. It can be divided into three horizontally continuous layers:
• Lower Zone, which is the basal layer of the pegmatite, with an alaskitic composition of fine- to medium-grained muscovite-albite granite and aplitic albite-quartz layers with variable proportions of albite, quartz, microcline and muscovite. It is characterised by an assemblage of accessory and rare minerals that include green tourmaline, garnet, magnetite, pyrite and ilmenite;
• Intermediate Zone, that consists of medium- to coarse-grained muscovite-quartz-albite-microcline pegmatite, with tabular to columnar muscovite. It is characterised by an assemblage of accessory and rare minerals that include pyrite, ilmenite, magnetite and arsenopyrite;
• Upper Zone, that constitutes >50% of the entire pegmatite body. It is chiefly composed of spodumene-bearing pegmatite containing lenses that are mostly discontinuous, composed of blocky microcline, saccharoidal albite, or quartz-rich assemblages, reflecting a compositionally differentiation into spodumene, albite, K feldspar and quartz units.
- The spodumene unit is effectively continuous over the length of the pegmatite, and is composed of a coarse-grained albite-quartz-spodumene-muscovite-microcline pegmatite assemblage. The spodumene is greenish-white and characteristically occurs as giant wedge-shaped crystals up to 4 m in length, whilst albite has variable grain sizes, and microcline may be amazonitic in places. Accessory and rare minerals include amazonite, apatite, amblygonite, beryl, Li-muscovite, topaz, kunzite, cassiterite and possible petalite.
- The albite unit has a high proportion of fine-grained saccharoidal albite and is found as small (<25 m diameter), sometimes layered, but discontinuous spherical bodies throughout the spodumene-bearing pegmatite.
- The K feldspar unit also occurs as discontinuous lenses and consists of giant blocky microcline crystals together with some quartz, muscovite and minor spodumene. These lenses may reach up to 50 m in thickness, although they are commonly thinner (Poletayev et al., 1991).
- The quartz unit which has a variable crystal size and comprises coarse monomineralic quartz crystals overprinted by finer quartz–lepidolite-zinnwaldite greisen. It forms discontinuous bodies, that are up to 20 to 100 m in diameter, within the spodumene bearing pegmatite.
Sections of the upper zone contain high MgO contents of up to 5% as a result of post-magmatic hydrothermal alteration and contamination
by hanging wall serpentinite.
• Exomorphic Zone - alteration reactions along the contact with host rocks over much of the thickness of the pegmatite, and with large xenoliths, generally produced <1 m thick zones of an exomorphic assemblage described as 'glimmerite' (Zerihun, 1991). However, such a border zone is almost completely absent over much of the hanging wall contact, where a thin pegmatitic quartz-microcline-muscovite unit is occasionally developed. The exception is when the wall-rock is an ultramafic, and an exomorphic zone characterised by 'glimmerite' is developed. The mineral assemblage of this exomorphic zone includes holmquistite, talc, tremolite, actinolite, phlogopite and quartz.
The Late- to post-magmatic alteration of the pegmatite includes the development of albite, amazonite, sericite and greisen (Zerihun et al., 1995), whilst kaolinitisation by deep weathering, which is particularly prominent in spodumene-rich sections of the pegmatite mined from the open pit.
The composition and proportion of columbite [(Fe, Mn)Nb2O6] and tantalite [(Fe,Mn)(Ta,Nb)2O6] changes over the thickness of the pegmatite. The Lower Zone is characterised by columbite, and as such Nb is >Ta. The Intermediate Zone contains Fe- and Mn-columbite, again with Nb>Ta, whilst the Upper Zone contains Mn-tantalite, ixiolite [Ta,Mn,Nb)O2] / wodginite [Mn(Sn,Ta)Ta2O8 ] with Ta>Nb.
Unlike some other significant rare-element pegmatites, the Kenticha pegmatite rarely carries cassiterite and has no known pollucite [(Cs,Na)2Al2Si4O12•2H2O].
The bottom-to-top differentiation of the Kenticha pegmatite and the Ta mineralisation in its upper part are interpreted to principally be the result of upward in situ fractionation of a residual leucogranitic to pegmatitic melt, largely under closed system conditions. Geochemical compositions of individual units are interpreted to suggest the transitions between them are gradational. Muscovite chemistry from the three mineralogically differentiated Upper Zone units formed at largely similar levels of fractionation, taken to suggest crystallisation occurred rapidly, prohibiting large scale separation of the consolidating crystal mush into distinct continuous layers (Küster et al., 2009).
Resources, Reserves and Production
The tailings from past production are estimated to total 5.4 Mt (Abyssinian Metals Annual Report, June 2023).
Remaining recoverable metal in the weathered zone - 2223 t of Ta2O5 and 2065 t of Nb2O5 (Abyssinian Metals Website, viewed March 2024).
JORC compliant Lithium Inferred Resource - 87.7 Mt @ 0.78% Li2O (Abyssinian Metals Website, viewed March 2024).
NOTE: An Abyssinian Metals investors presentation (2023) indicate this resource is expected to increase and that it includes a cohesive resource of >1% Li2O. This includes the Southern starter pit with an Inferred Resource of 15 Mt @ 1.40% Li2O (Abyssinian Metals Annual Report, June 2023).
The geological information in this record has been drawn from Küster et al., 2009, as cited below.
The most recent source geological information used to prepare this decription was dated: 2009.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Kenticha
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Selected References:
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Kuster, D., Romer, R.L., Tolessa, D., Zerihun, D., Bheemalingeswara, K., Melcher, F. and Oberthur, T., 2009 - The Kenticha rare-element pegmatite, Ethiopia: internal differentiation, U-Pb age and Ta mineralization: in Mineralium Deposita v.44, pp. 723-750. DOI 10.1007/s00126-009-0240-8.
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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