Mineralium Deposita (1997) 32: 491±504
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Springer-Verlag 1997
ARTICLE
A.Y. Billay á A.F.M. Kisters Kisters á F.M. Meyer á J. Schneider
The geology of the Lega Dembi gold deposit, southern Ethiopia: implications for Pan-African gold exploration
Received Received:: 26 July 1996 / Accepted: Accepted: 8 January January 1997
Dembi deposi depositt is the largest largest gold gold Abstract The Lega Dembi producer in Ethiopia. It is situated in late-Precambrian metamorphosed sediments of the N-S trending, volcanosedimentary Megado belt, which forms part of the lateProterozoic Adola granite-greenstone terrane in southern Ethiopia. The lode-gold mineralization occurs in a N-S trending, steep westerly dipping quartz-vein system that follows the structural structural contact contact between between underlyi underlying ng feldsp feldspath athic ic gneiss gneisses es and and the volcan volcanose osedim diment entary ary sequence of the Megado belt. This contact also marks the northern northernmost most extension extension of the regional-s regional-scale, cale, sinistral sinistral strike-slip strike-slip Lega Dembi-A¯a Dembi-A¯ata ta shear shear zone. zone. Mineraliza Mineraliza-tion tion and intens intensee quart quartz-v z-vein eining ing is best best develo developed ped in graphite-r graphite-rich ich sediments sediments within an area not more than 80 m away away from from this this tecton tectonic ic contac contact. t. Hydrot Hydrother hermal mal wall-rock alteration includes actinolite/tremolite-biotitecalcite-sericite and chlorite-calcite-epidote assemblages. Gold occurs preferentially in the sericite alteration zone, where it is closely associated and intergrown with galena. The variable deformation of the gold-quartz veins suggests suggests a syn-kinem syn-kinematic atic timing for the gold mineralmineralization during transcurrent shearing in a dilational segment ment of the shear zone. zone. In additi addition on to the structur structural al control, lithological control on gold deposition is indicated cated by the almost almost exclus exclusive ive occurr occurrenc encee of the gold gold minera mineraliza lizatio tion n in gra graphi phitete-ric rich h metase metasedim diment ents. s. This This close relationship relationship suggests suggests that gold precipita precipitation tion was the result of chemical reduction of regional ore-bearing ¯uids. ¯uids. Temperatu Temperature re conditions conditions of mineraliza mineralization tion are constrain constrained ed by the actinoliteactinolite-biot biotite ite alteration alteration assemassemblage and by arsenopyrite chemistry, which indicate that Editorial handling: P.G. Eriksson A.Y. Billay (8) á A.F.M. Kisters á F.M. Meyer Institut fu È È r Mineralogie und Lagersta È È ttenlehre, RWTH Aachen, Wu È llnerstr. 2, 52056 Aachen, Germany (e-mail:
[email protected]) J. Schneider Institut fu È È renforschung, È È r Geowissenschaften und Lithospha Justus-Liebig Universita È È t Giessen, Senckenbergstr. 3, 35390 Giessen, Germany
ore deposition occurred at or close to peak metamorphic conditions conditions at upper-gr upper-greens eenschist chist to lower-amp lower-amphibo hibolite lite metamorphic grades. Rb-Sr dating of sericite indicates an age age of about about 545 545 Ma. for hydrot hydrother hermal mal altera alteratio tion n and, and, thus, thus, for gold gold minera mineraliza lizatio tion. n. The style style of gold gold mineraliza mineralization, tion, structural structural pattern pattern and lithologic lithological al assemblages at Lega Dembi are very similar to lode-gold depo deposit sitss most most comm common only ly repo report rted ed from from Arch Archae aean an granite-greenstone terranes. These similarities may open new new pers perspe pect ctiv ives es for for the the expl explor orat atio ion n of lode lode-g -gol old d deposi deposits, ts, which which has has previo previousl usly y primar primarily ily focuse focused d on Archaean Archaean greenstone greenstone belts rather than Proterozoic Proterozoic or even Phanerozoic meta-volcanosedimentary belts.
Introduction Shear-zone hosted, mesothermal lode-gold deposits are a major major source source of world world gold gold produc productio tion n (Wood (Woodall all 1988). 198 8). The vas vastt majori majority ty of these these goldgold-bea bearin ring g vein vein systems is spatially closely associated with the tectonometamorphic evolution of predominantly late-Archaean greenstone belts, that are an intricate feature of many Archaean cratons (e.g. Anhaeusser 1976; Colvine et al. 1984, 198 4, 1988; 1988; Robert Robert and and Brown Brown 198 1986; 6; Kerri Kerrich ch 198 1986; 6; Foster 1989; Groves et al. 1989; de Ronde et al. 1992). In addition addition to their commonly commonly late-Arch late-Archaean aean age, these these deposits deposits bear numerous numerous similarities similarities worldwide, worldwide, which led to the term `Archaean lode-gold deposits', that was coined to describe the styles of gold mineralization, associated sociated alteration alteration patterns and structura structurall controls controls of gold deposits deposits in late-Arch late-Archaean aean granite-g granite-greens reenstone tone terranes ranes (compreh (comprehensiv ensivee reviews reviews are given by, inter alia, Groves Groves and Foster 1991 1991,, Colvine Colvine 1989 1989,, Groves Groves 1993 1993,, and Kerrich and Cassidy 1994) In recent recent years, years, howev however, er, an increa increasin sing g number number of workers have questioned the restrictive use of the term `greenstone belt' for linear to irregularly shaped metavolcanos volcanosedime edimentary ntary rocks rocks of predomina predominantly ntly Archaean Archaean age, since similar lithological associations and tectonic
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styles are also observed in Proterozoic and Phanerozoic supracrustal belts (De Wit and Ashwal 1995, and references therein). The recognition of greenstone-belt type sequences and similar tectonic styles in Proterozoic and Phanerozoic orogens implies that younger Archaeantype mesothermal lode-gold deposits could be far more common than has previously been documented (Nesbitt 1991). This, in turn, opens new perspectives for gold exploration, that has traditionally focused on Archaean granite-greenstone terranes, and which has possibly underestimated the potential for large-scale gold deposits in younger supracrustal belts. The Lega Dembi gold mine in the Sidamo region of southern Ethiopia is the largest gold producer in the country (Sutton-Pratt 1996). The deposit is situated some 500 km south of Addis Ababa, within the lateProterozoic meta-volcanosedimentary Megado belt of the Adola granite-greenstone terrane, which forms the southernmost extension of the Neoproterozoic ArabianNubian Shield (Kazmin et al. 1978; Worku and Schandelmeier 1996) (Fig. 1). Placer gold in the Megado belt was ®rst discovered in 1936 and, since then, more than 55 tons of placer gold has been produced. Primary gold occurrences, including the Lega Dembi deposit, were identi®ed during an exploration campaign in the late 1970s by the Adola Gold Exploration Project (AGEP) of the Ethiopian Mineral Resources Development Corporation (EMRDC). The Lega Dembi deposit is currently being mined by the EMRDC in an open cast operation. The gold production averages 3 t/a (Sutton-Pratt 1996) and reserves of some 60 tons of gold at an average grade of 6 g/t have been identi®ed up to 200 m below surface (Moudrov et al. 1991).
Various models pertaining to the lithostratigraphic and structural evolution of the region have been proposed. However, because of the relatively recent discovery of the Lega Dembi gold deposit, little work on the geological setting, on the style, timing, and controls of the mineralization and on wall-rock alteration is currently available in the literature. The petrochemistry of the igneous rocks of the Adola region has been described by Bisrat (1993), Gichile and Fyson (1993), Beraki (1995), and Worku and Schandelmeier (1996). Ore paragenetic, wall-rock alteration and trace-element geochemical studies are found in Fiori et al. (1987), Tadesse (1990), and Getaneh (1994), and in reports which accompanied the exploration campaign by the AGEP (Morin and Oliver 1986; V/O Tecnoexport 1986; Emelyanov et al. 1987; Moudrov et al. 1991). Fluid inclusion studies related to the gold mineralization at Lega Dembi were carried out by Tadesse (1990). Genetic models for the gold mineralization at Lega Dembi include syngenetic gold mineralization in the metasediments and metavolcanics of the Megado belt, followed by a redistribution and concentration of the gold mineralization during subsequent metamorphic events (Fiori et al. 1987), and a model of a structurally controlled, epigenetic mineralization in a strike-slip shear zone system (Emelyanov et al. 1987; Ghebreab et al. 1992; Worku 1993). This study presents a brief description of the regional geology of the Adola granite-greenstone terrane and the geological setting of the Lega Dembi deposit in particular. The main aim is to focus on the multi-stage quartzveining and associated gold mineralization and wallrock alteration at Lega Dembi. Finally, the formation of the gold-quartz mineralization is assessed in relation to regional deformational and metamorphic events.
Regional geology
Fig. 1 Location map and regional geological setting of the Adola granite-greenstone terrane in southern Ethiopia (modi®ed after Beraki 1995)
The Adola granite-greenstone terrane covers an area of approximately 5000 km2 in southern Ethiopia. It is characterized by two linear, closely spaced, N-S trending belts of metamorphosed supracrustal rocks, namely the Megado volcanosedimentary belt in the west and the Kenticha ultrama®c belt in the east (Fig. 1). The former consists of ultrama®c and tholeiitic basic volcanics and intrusives which are intercalated with sediments made up predominantly of arkoses, feldspathic quartzites, quartzites, and pelites, together with subordinate polymictic conglomerates and graywackes (Gilboy 1970; Chater 1971; Bisrat 1993; Ghenzebu et al. 1994; Worku and Schandelmeier 1996) (Fig. 2). Small pod- or lens-like bodies of mainly tonalitic composition are intrusive into the basic rocks of the Megado belt. In contrast, the Kenticha belt is dominated by ultrama®c rocks, with subordinate amphibolites and sedimentary rocks, the latter comprising biotite schists and minor graphitic schists and marbles (Gilboy 1970; Chater 1971). The two volcanosedimentary belts are surrounded and separated by a gneissic terrane which comprises para- and orthogneisses, including monotonous quartzo-feldspathic biotite gneisses with subordinate muscovite-quartz schists, staurolite-garnet-biotite schists, impure marbles, and amphibolites (Gilboy 1970; Chater 1971; Kozyrev et al. 1985; Ghebreab 1989; Worku and Yifa 1989). Large tonalite bodies also intrude the gneissic terrane. Gneissose granites are con®ned to the gneissic terrain and post-
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Metamorphism Two main metamorphic events (M1-M2) have been recognized for the Adola granite- greenstone terrane (Gilboy 1970; Chater 1971). Evidence of an early M1 event is only represented by relict mineral parageneses containing cordierite, that have been largely overprinted by the subsequent M2 event which has pervasively aected the rocks of the region (Gilboy 1970; Chater 1971). In the sedimentary rocks of the Megado belt, mineral assemblages typically comprise quartz, plagioclase, biotite, muscovite, and accessory rutile, chlorite and epidote. In the basic rocks, actinolite/hornblende, plagioclase (albite-oligoclase), epidote and chlorite are common (Gilboy 1970, Chater 1971). In places, however, kyanite-bearing rocks that occur within the greenschist-facies sequence point to locally higher-grade metamorphic conditions. This juxtaposition of units of dierent metamorphic grades has been interpreted by Worku (1993) to be the result of an imbrication of thrust slices. The rocks of the gneissic terrane and the Kenticha belt have been aected by amphibolite±facies metamorphism of the staurolite-almandine and kyanite-almandine-muscovite subfacies during the M2 metamorphism (Gilboy 1970, Chater 1971). The metamorphic grade increases from lower- and mid-amphibolite facies in the NW to upper-amphibolite and lower-granulite facies in the SE of the gneissic terrane (Gilboy 1970). However, no determinations of P-T conditions of the metamorphism exist for the region.
Structural geology
Fig. 2 Simpli®ed geological map of the Lega Dembi area illustrating the location of the gold deposit along the contact between the greenstone sequence of the Megado belt (in the west) and the gneissic basement (in the east) (modi®ed after Ayalew 1990)
tectonic granites occur marginally to the greenstone belts and in the gneissic terrane (Fig. 1). The eastern contact between the Megado belt and the gneissic terrane is tectonic and is regionally referred to as the Lega DembiA¯ata shear zone (Worku and Yifa 1989; Worku and Schandelmeier 1996). The western margin of the Megado belt is, however, marked by the development of gneissose tonalite which shows primary intrusive contacts with the supracrustal assemblages.
Five main phases of deformation have been distinguished for the Megado belt and its surrounding gneissic terrane. These include: (1) an early gneissosity-forming D1 event in the gneissic terrane, which is expressed by early folds in the Megado belt (Gilboy 1970); the D1 event was related by Beraki et al. (1989) and Worku and Schandelmeier (1996) to an early subduction-related thrust event. (2) N-S trending regional-scale, upright D2 folds that dominate the structural pattern of the region (Gilboy 1970; Chater 1971; Gebreab 1989; Worku and Yifa 1989; Beraki 1995) were associated with an E-W directed collisional event (Worku and Schandelmeier 1996). (3) Strike-slip shearing (D3) along the contacts between the greenstones and the gneissic terrane, due to a NW-SE directed transpressional event (e.g. Beraki 1995; Worku and Schandelmeier 1996). (4) E-W trending, upright, moderate-to-steep easterly and/or westerly plunging folds, referred to by Gilboy (1970), Chater (1971) and Ghebreab (1989) as D3 folds; Worku and Schandelmeier (1996) interpret this folding to be the result of D3 transpressional shearing, but Beraki (1995) refers to these folds as D4 folds. (5) Late, brittle NW-SE and NE-SW to E-W trending faults which disrupt the N-S trending granite-greenstone terrane. The tectonic evolution of the Adola granite-greenstone terrane and the origin of the supracrustal sequences has been a matter of controversy among dierent workers. Three dierent models are proposed for the disposition of the N-S trending linear greenstone belts which overlie the gneissic terrane. The ®rst model proposes an origin of the greenstone sequences as ophiolites, that were thrust onto the gneissic terrane. The ophiolites were later refolded by major N-S trending folds and subsequently modi®ed by strike-slip shearing (Kazmin 1976; De Wit and Chewata 1981; Beraki et al. 1989; Beraki 1995; Worku and Schandelmeier 1996). The second model suggests intra-continental rifting, followed by thick-skinned tectonics, whereby both the greenstones and the gneisses were thrusted in an eastward direction (Worku and Yifa 1989; Ghebreab 1989). The third genetic model proposes intra-continental rifting and multiphase strike-slip shearing (Amenti et al. 1992).
Geochronology Age determinations on the rocks in the Adola region are scarce. The Megado belt supracrustal series has been correlated with
494 similar assemblages of the Upper Proterozoic Arabian-Nubian Shield (Beraki 1995; Teklay et al. 1996; Worku and Schandelmeier 1996). A tonalite body west of the Megado belt yielded an U-Pb age of 765 Ma (EIGS, quoted by Gichile and Fyson 1993), providing a minimum age constraint for the volcano-sedimentary sequence. Upper Proterzoic ages are also inferred for the gneissic basement which is correlated with gneisses of the Mozambique belt (Kazmin et al. 1978; Gass 1977, and references therein; Vail 1976, 1983). Rb-Sr whole rock ages of 630±680 Ma for the gneisses and syntectonic granites are interpreted to represent the age of the M2 metamorphism (Gilboy 1970; Chater 1971). Post-tectonic granites yielded whole-rock Rb-Sr ages of 500±550 Ma (Gilboy 1970; Chater 1971).
The geology of the Lega Dembi gold deposit The Lega Dembi deposit has been divided into four interconnected open pit operations that are locally referred to (from south to north) as Southern, Central, Northern and Upper Lega Dembi (SLD, CLD, NLD and ULD, respectively) (Fig. 2). Outcrop is largely restricted to the mine area and some resistant lithologies that form N-S trending ridges which dominate the geomorphology of the deeply weathered and densely forested area. The steep westerly-dipping lithostratigraphic sequence at Lega Dembi can be subdivided into (1) a series of quartzo-feldspathic and biotite gneisses and amphibolites belonging to the gneissic terrane in the east, and (2) the volcanosedimentary sequence of the Megado belt in the west. In detail, the lithological succession at Lega Dembi comprises ultrama®c schists and various meta-sedimentary lithologies (Fig 2). The ultrama®c talc schists are commonly developed along the contact between the gneissic terrain and the supracrustal sequence. The moderate-to-steep (40±70 °) westerly dipping contact is marked by the development of mylonitic fabrics (see below) and, as such, is clearly tectonic and generally sharp, although intercalations of bands of talc schists within the gneisses are observed locally. The width of the talc schists at NLD is less than 5 m, locally pinching out, but it progressively increases towards the south and attains a maximum thickness of about 180 m south of Reji (Fig. 2). The succession of meta-sedimentary rocks can be subdivided into a lower leucocratic muscovite-quartzplagioclase schist, which is overlain by laminated, darkgreyish, graphite-rich, locally graded feldspathic arenites and quartz wackes. The mineral assemblage of the sediments consists of quartz, biotite, muscovite and plagioclase, together with accessory rutile, epidote, graphite, tourmaline and chlorite. The latter overgrows the main foliation. Kyanite is present locally in proximity to quartz veins. The sedimentary succession attains a maximum thickness of about 280 m at CLD, progressively decreasing to less than 20 m at ULD, where it is buttressed between a massive meta-gabbro in the west and quartzo-feldspathic gneisses in the east (Figs. 2, 3). The massive meta-gabbro, together with minor amphibolites, forms the western margin of the Lega Dembi
deposit. The ma®c units form a N-S trending ridge, parallel to the structural grain of the Megado belt. The ma®c units are locally intruded by stringer- and pod-like tonalite bodies which display penetrative planar and linear fabrics, and which are folded together with the amphibolites on a metre scale.
Structural geology of the Lega Dembi mine The polyphase tectonism of the Adola granite-greenstone terrain is, on the scale of the Lega Dembi deposit, expressed by several generations of folds and faults, together with the development of composite planar and linear fabrics. Tight to isoclinal, cm-to-dm scale intrafolial folds of the bedding (S0) provide evidence of an early deformation event (D1). The present attitude of the folds, which show moderate-to-steep westerly plunges, is the result of the subsequent D2 and D3 deformations. The Lega Dembi mine is situated in a large, N-S trending fold structure which is part of the regional pattern of large-scale F2 folds in the Megado belt. The fold closure is located south of the Lega Dembi deposit where the ultrama®c talc schists attain their maximum thickness (Fig. 2). The lack of unambiguous indicators of the younging direction and parasitic folds hampers the determination of the synformal or antiformal nature of the fold structure. Although F2 folds show, on a regional scale, predominantly shallow northerly and southerly plunges, a steep westerly plunge of the main F2 fold at Lega Dembi is suggested by the westerly plunge of mineral and mineral stretching lineations (see below), together with mesoscale fold axes. A regionally developed, upright, N-S trending S2 fabric is axial planar to the F2 folds (Fig. 3a). The S2 fabric is intensi®ed to pervasively developed mylonitic foliations towards the eastern margin of the Megado belt, where the greenstone succession is juxtaposed against the gneissic terrane. The development of mylonitic fabrics underscores the tectonic nature of this contact. Field and microstructural evidence (e.g. intrafolial folds of S2 in microlithons) suggest, that this fabric is composite and the result of a transposition of the coplanar S2 foliation into the mylonitic shear foliation (S3), so that this fabric is henceforth referred to as S2/S3. Macroscopically, the S2/S3 fabric is further accentuated by foliation-parallel quartz veins (see below). Shear sense indicators in the meta-sediments at Lega Dembi yield ambiguous results (i.e. both a normal and a reverse dip-slip component), but with a consistent sinistral strike-slip component. The orientation of an oblique shear foliation de®ned by biotite, together with the ubiquitous occurrence of asymmetric, S-shaped, steep westerly plunging folds along the contact between the gneissic basement and the volcano-sedimentary sequence, also suggest a prominent sinistral strike-slip component. The cm- to dm-scale folds refold the S2/S3 fabric, together with the foliation-parallel quartz veins,
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but can locally be seen to be transected by the N-S trending S2/S3 fabric, indicating the progressive nature of the fabric-forming event. The component of sinistral strike-slip shearing could possibly also explain the unusually steep westerly plunge of the large-scale F2 fold at Lega Dembi, as a result of the re-orientation and drag of the fold along the shear zone. The development of the ductile shear fabrics along the contact between the gneissic basement and the greenstone succession, show-
Fig. 3 Simpli®ed geological map of the North Lega Dembi (NLD) open pit. Fabric diagrams of a poles to the S2 and S2/S3 foliation from the meta-sediments and meta-volcanics of the Megado belt at Lega Dembi; the great circle distribution of the poles is a result of the D4 open folding about moderate-to-steep easterly plunging axes; b orientation of prominent quartz-sul®de veins at Lega Dembi, subparallel to the S2/S3 foliation; c summary plot of mineral stretching, intersection and mineral lineations (undierentiated) at Lega Dembi (all plots are lower-hemisphere, equal-area projections)
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ing evidence of predominantly sinistral strike-slip kinematics, indicates that the shear zone forms the northern extension of the Lega Dembi-A¯ata shear zone (e.g. Worku and Yifa 1989). All rock units have been aected by open, upright, E-W trending folds which show moderate westerly plunges (Fig. 3a). Wavelengths range from 100 m. These F4 folds refold the composite S2/S3 fabric and the large N-S trending folds, as well as the asymmetric S-shaped folds, resulting in an open type three interference pattern. Steep-to-moderate westerly plunging, E-W trending folds are also observed on a mm- to cm-scale, crenulating the composite S2/S3 transposition fabric. An axial-planar foliation (S4) is expressed by the growth of biotite, which is at a high angle to the N-S trending S2/S3 transposition fabric. A set of NW-SE and E-W trending brittle and brittleductile faults (D5) aects the Lega Dembi deposit in its northern and southern parts, disrupting the continuous extent of the mineralization (Fig. 2). Both the footwall gneisses and the volcano-sedimentary assemblages of the greenstone belt contain a variety of linear fabrics that invariably show moderate to steep westerly plunges, so that their timing with respect to the deformation events described already remains somewhat speculative (Fig. 3c). The various types of lineation include (1) a mineral stretching lineation, formed by stretched quartzfeldspar aggregates in the footwall gneisses, (2) a mineral lineation de®ned by the preferred growth of, for example, amphiboles and tourmaline in the supracrustals, together with kyanite in proximity to quartz veins, and (3) an intersection lineation between the bedding (S 0), and the S2 and S3 foliations in the greenstones, respectively. In addition, the mylonitic foliations close to the contact between the basement gneisses and the greenstones are characterized by a steep westerly plunging mineral stretching lineation and by quartz-rodding. Worku and Yifa (1989) and Ghebreab et al. (1992) have described a set of subhorizontal lineations which they related to the D3 strike-slip shearing along the contact between the Megado belt and the gneissic terrain. The presence of this lineation could not be con®rmed in this study.
by a continuous development of mineralized quartz veins, the various parts of the deposit are separated from each other by barren ground as a result of osets along E-W trending D5 faults. The main mineralization is con®ned to within 80 m of the contact between the basement gneisses and the volcano-sedimentary succession. The mineralized zone is characterized by three main, up to 10 m wide, composite quartz vein systems, which are referred to as the eastern, central and western veins. Thin foliation-parallel quartz veinlets, however, can be observed throughout the metasedimentary succession. The main vein system at NLD shows a strike extent of about 250 m and exploration drilling has delineated a semicontinuous down-dip extent of >350 m (Zemene 1995, personal communication). The eastern vein is hosted in a muscovite-quartzplagioclase schist and the central and the western veins are situated in laminated, graphite-bearing arkoses and quartz wackes (Fig. 3). The graphite content of the meta-sediments increases from £ 0.1±0.4 wt.% in proximity to the main vein system, up to 1.5 wt.% in some of the less altered wall rocks outside the mineralized zone. Based on the orientation and morphology of the quartz veins, together with their relation to the dominant host-rock fabric, four sets of quartz veins are identi®ed, including S2/S3-parallel, intensely deformed, massive to laminated quartz veins which are the main hosts of the gold mineralization ( type 1), veins that are discordant to S2/S3 and which are variably folded (type 2), breccia quartz veins ( type 3), and largely undeformed veins which cut the S2/S3 fabric (type 4). Type 1 veins are the most abundant. They occur parallel to the S2/S3 foliation, or form tight-to-isoclinal folds with the S2/S3 foliation being axial planar (Figs. 3b and 4). Boudinage and pinch-and-swell of the
Gold mineralization at Lega Dembi The gold mineralization at Lega Dembi is situated along the sheared contact between the quartzo-feldspathic gneisses of the gneissic terrain and the volcano-sedimentary sequence of the Megado belt, in strongly foliated meta-sediments (Fig. 3). The N-S strike extent of the mineralization is approximately 2 km, parallel to the S2/S3 fabric and lithological layering. The maximum width of the mineralized zone is approximately 140 m in the central parts of the deposit (i.e. at NLD), but gradually decreases to
