Kimberlite discoveries in NW Liberia

Journal of Geochemical Exploration 173 (2017) 99–109

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Kimberlite discoveries in NW Liberia: Tropical exploration & preliminary results Stephen E. Haggerty Earth & Environment, Florida International University, Miami, FL 33155, USA

a r t i c l e

i n f o

Article history: Received 21 April 2016 Revised 23 November 2016 Accepted 2 December 2016 Available online 05 December 2016 Keywords: Kimberlite Liberia Dikes Pipe Tectonic-control Diamonds

a b s t r a c t This report is brief in context and rich in unexpected discovery. With N 2 km of erosion, kimberlite models predict the near-complete removal of pipes with exposures to the pipe-root-zones of dikes. Exploration in NW Liberia has, indeed, uncovered eight kimberlite dikes (~10 m wide) but also an en echelon pipe, comparable in size to the Kimberley pipe and De Beers' pipe in South Africa. Discoveries are in a narrow 200–300 m wide valley of extraordinary thick bush, undergrowth, and organic overburden. Ilmenite and co-existing leucoxene were used as diagnostic tracers for detecting hard rock kimberlite in this tropical terrane. Micro-diamonds show that the redox state of ilmenite is a potentially useful proxy as an index for macro-diamond preservation. The tectonic control of kimberlites is complex, with diverse lithologies. Discoveries include a well-defined regional trend for kimberlite dikes along paleo-fracture zones, Precambrian in age (Liberia Trend), coupled with kimberlite dikes on the craton that are traced to Mesozoic oceanic transform faults (the Sierra Leone Trend). Although long predicted, this is the first report of kimberlite dike-trends in Liberia that are similar in orientation to those in Sierra Leone. An explosive blow on a Liberia-Trend dike demonstrates a similarity to the dynamics attendant in rich (50–500 cpht) diamond-bearing dikes in Sierra Leone, and in South Africa of comparable age. The potentially high grade dikes, along with the pipe (~500 × 50 m), now more reasonably accounts for the enormous number of alluvial diamonds (blood and non-conflict), recovered over more than seven decades, downstream from the discovery cluster. A neglected region since the classic work by Bardet (1974), and with few contributions on Liberia since then, an update is considered timely, particularly in the context of discoveries of diamond-bearing kimberlite. © 2016 Elsevier B.V. All rights reserved.

1. Introduction West Africa is classically cratonic and diamond-bearing, having supplied at least 10% of the world's diamonds in the 1970s, quite remarkably by artisanal diggings only. Stretching from Mali in the N to adjacent Guinea, Sierra Leone, Liberia, and Ivory Coast (Fig. 1a), the region falls into three age Provinces (Leonian ~ 3 Ga in the W; Liberian, ~ 2.7 Ga in the center; and Eburnian ~ 2 Ga to the E); it is only in the Liberian unit of the Man Shield, however, that bona fide kimberlites have been recognized (Fig. 1a). Those in Ivory Coast are metakimberlites (Bardet, 1974); primary diamond sources have not been found in Ghana, Sino County (Liberia), and E Guinea; the diamonds in Nimba County, North-Central Liberia stem from graphitic schists (Force, 1983). Crustal control on kimberlite intrusions is reasonably well established (Haggerty, 1992) as outlined in Fig. 1b–c, and discussed below.

E-mail address: haggerty@fiu.edu.

http://dx.doi.org/10.1016/j.gexplo.2016.12.004 0375-6742/© 2016 Elsevier B.V. All rights reserved.

With periodic uplift of the West African Craton, a minimum of 1.5 km of erosion, to expose the Man Shield, are considered necessary to account for the offshore accumulation of sediments (Tysdal and Thorman, 1983), placed at over 5 km from oil exploration (Schmid, 2013), suggesting that 2.5 km is a reasonable estimate for erosion over a period of ~100 Ma. At these levels, existing pipes would be drastically reduced in surface diameters or levelled to the point that the pipe-rootzones of feeder-dike complexes would ultimately be exposed (Hawthorne, 1975; Kjarsgaard, 2007). Sill complexes are also typically present as described by Tappe et al. (2014). Brief research visits to Liberia were initiated in 1977 and terminated by the coup d'état on 12th April 1980; internal conflicts lasted until 2003. Field work now dedicated to exploration was resumed in 2010. Results from the early work are limited (Haggerty, 1982, 1992), with a supplement only by Skinner et al. (2004). Exploration has produced some interesting results with the discovery of a kimberlite pipe and the recognition of a botanical indicator for kimberlite (Haggerty, 2015). This contribution will provide the details of discovery in the tropical environment of impenetrable bush in NW Liberia. The Gola Forest, comparable in many respects to Amazonia, is one of the last tracts of

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S.E. Haggerty / Journal of Geochemical Exploration 173 (2017) 99–109

Fig. 1. (a) The Man Shield (West African Craton) in relation to other cratons on the continent showing the distribution of known and inferred kimberlites from Mali to Ghana. Boundaries to the three age provinces (AP) are schematic: Leonian (LeAP-3.0 Ga); Liberian (LiAP-2.7 Ga); and Eburnian (EbAP-2.7). Filled diamond symbols are known kimberlites; crossed diamond symbols in Ivory Coast are meta-kimberlites; and open diamond symbols in Guinea, Liberia, and Ghana are alluvial with no known, or doubtful parent source. Modified from Haggerty (1992). (b) Simplified drainage pattern map of West Africa in the region of the Leo Uplift. Major diamond locations and kimberlite dike trends (1–4) are illustrated with the dominant fracture patterns that were reactivated (Precambrian PԐ to Mesozoic MZ), and along which kimberlites were intruded (~ 140 Ma and 90–120 Ma). Con Conakry (Guinea); Fre Freetown (Sierra Leone); Mon Monrovia (Liberia); and Abi Abidjan (Ivory Coast). Modified from Haggerty (1992). (c) Generalized geological map of the Bopolu Quadrangle, NW Liberia (Wallace, 1977), illustrating the three dominant kimberlite dike trends (A–B, C–D, and E–F) that cut the earlier (180 Ma) dolerite dike swarm; these intrusions parallel the coast-line with injection along stress fractures created by the breakup of Gondwana. Known diamond-bearing kimberlite pipes are at Mona Godua and Camp Alpha (Fig. 2). Current exploration has focused on the C-D Kimberlite Trend. Modified from Haggerty (1992).

equatorial forest, and so environmental concerns are paramount and duly practiced by exploration companies under the watchful eye of the Ministry of Lands, Mines and Energy. 2. Logistics Geological mapping by France and the UK in West Africa has not advanced since colonial independence was obtained. But Liberia, with its Lone Star affiliation to the USA, enjoyed a mapping bonanza between 1965 and 1972, in which the entire country was flown, and in which excellent maps were produced (topographic, magnetic, and radiometric) at a scale of 1:250,000 (Tysdal and Thorman, 1983). Photogrammetry was followed up with ground-truth expeditions to the interior by USGS and Liberian Geological Survey personnel. The end result covers 110 mkm2 in 10 quadrangles. Drafted from the Bopolu Quadrangle (Wallace, 1977), the map in Fig. 1c of NW Liberia is an overview of the

regional geology that is limited in accuracy to stream bed exposures as the area is totally obscured by thick equatorial, tropical vegetation. Access to the diamond district is limited to a pot-holed-laterite road to Kumgbo (Fig. 1c). Further access in any direction is via machetehacked bush paths as the country remains a no-fly zone to all except the UN Peace Keeping Force. With a wet and dry season calendar, field work is limited with a monsoon (July–November) that registers 200– 500 mm of rain annually, log bridges are routinely replaced (not maintained), enormous amounts of rock, gravel, and sand are transported yearly, and tropical weathering is deep with significant alteration of kimberlites and kimberlitic indicator minerals (KIMs). 3. Regional geology The crustal cratonic basement throughout the Regional Province is dominated by granite, granitic gneiss, and members of the TTG suite,


greenstone belts, amphibolites, itabirites, and rare anorthosites (Tysdal and Thorman, 1983; Wright, 1985). Gondwana breakup of West Africa from South America in the Mesozoic produced an en echelon fracture system parallel to the coast along which dolerite dikes were intruded. An impressive swarm of NW-trending dikes (Fig. 1c), similar in age (~ 180 Ma), and magnetic signature to those in NE South America (Haggerty, 1982 and refs therein), dominate the landscape topographically and aeromagnetically. Oceanic fracture zones, trending ENE extend to the interior of the craton (Fig. 1b). Other major fracture systems (Fig. 1b) are Precambrian (NE-SW); Proterozoic (N-S); PanAfrican (NW-SE); and Mesozoic (ENE-WSW). Kimberlite dikes in NW Liberia are along Precambrian fractures, and those in adjacent countries are shown in Fig. 1b (Haggerty, 1992). It is along these reactivated faults that kimberlites were intruded at ~140 Ma and between 90 and 120 Ma (Haggerty, 1982; Skinner et al., 2004), similar in age to the Kimberlite Provinces of Angola, Congo, and those of southern Africa (Haggerty, 1994 and refs. therein). In Cape Mount County (Liberia), kimberlites (registered on maps but mostly inferred from artisanal workings, Wallace, 1977), lie along three sub-parallel Precambrian fracturelineaments, termed the Border Trend (close to the Sierra Leone boundary and the Mano River), the Central or Mano Godua-Kumgbo Trend, and the Wuese-Wuasua Trend (close to the Lofa River). Designated as A-B (multiple dikes), C-D (multiple dikes and two pipes) and E-F (one dike and six pipes), current data show that most of the dikes are diamond-bearing based on the proximal activity of artisanal mining. The six pipes along the Lofa River (E-F), are distinct in age (~800 Ma) and barren (Skinner et al., 2004, and Mano River Company Reports 2007–2010). Kimberlite pipes along the C-D Trend (Fig. 1c) are diamond-bearing at Mano Godua in the S, and in the N at Camp Alpha (Haggerty, 2015). From 2010, exploration has focused on the central C-D kimberlite dike Trend, with a concentration on the area around Camp Alpha

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(Fig. 1c circled and Fig. 2), that from earlier work (KIMs, kimberlite, and satellite image data, Haggerty, 1982), showed considerable promise, and proved to be the case with a vibrant increase in the alluvial artisanal mining of conflict gems (i.e. “Blood Diamonds”), during the revolutionary period (1980–2003). 4. Exploration methods Satellite-derived lineament data has been useful but magnetic anomaly mapping (aero and ground-based) far less so because susceptibility contrasts between kimberlite and the basement are small, and because the residual overburden is indurated in laterite. With deep erosion of the Man Shield, and following the kimberlite model paradigm—of limited or no pipes and abundant dikes—the first priority was to locate the mapped (read inferred) dikes for two main reasons: (1) kimberlites have to be the source of alluvial diamonds found locally, and with reports of large stones (up to 400 ct) in small creeks, the distances travelled could not have been more than a few km. (2) Kimberlite dikes are typically high grade deposits (50–500 cpht) as comprehensively outlined by Gurney and Kirkley (1996) for mines in narrow dikes (b 1 to 2 m) from South Africa, and with grades of 100–250 cpht reported from kimberlite dikes in Sierra Leone and Guinea (Stellar Diamonds in web-based company presentations). An entire field season was devoted to tracking the “mapped” kimberlites recorded by Wallace (1977) (marked in green on Fig. 2); digitized GPS provided accurate locations for siting pits and trenches along NWSE lines in an effort to intersect the NE-SW trending kimberlite dikes (Fig. 1b–c). The effort was singularly unsuccessful. Large numbers (N100) of artisanal pits and soil piles were inspected in the region of Camp Alpha (Fig. 2). Raked and jigged concentrates yielded abundant corundum (Fig. 3a), or no heavy minerals at all. In the second field season, using drainage pattern maps and grid-sampling in streams, pits and

Fig. 2. Field sampling and exploration grid in the Camp Alpha area with a schematic of the newly discovered kimberlite dikes and pipe. More than 100 artisanal mining pits on the claims marked were examined and 82 sites (triangles and X) were sampled for heavy mineral concentrates. A selection of positive kimberlite intersections are located at the purple triangles. The USGS inferred dike sites (Wallace, 1977) are marked in green (filled squares); none are kimberlites. Seven of the eight kimberlite dikes discovered have the Liberia-Trend (N20°E) and one is characteristic of dike trends in Sierra Leone (N60°E). The kimberlite pipe parallels the Liberia dike trend and is a significant drainage channel in the discovery valley that is 200–300 m wide. UTM: WGS 84, Grid Zone 29 N.

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(a)

(b)

(c) (c)

(d)

2 cm

(e)

5 mm

Fig. 3. (a–b) Jigged samples of heavy mineral concentrates with abundant corundum, and ilmenite, respectively. (c) Dusty-cream leucoxene alteration coatings on ilmenite directly from a jig found only in the eluvium above solid kimberlite or very close to the kimberlite. (d) Polished section in reflected light of a white leucoxene reaction rim on ilmenite that is delicate in texture and is readily removed in stream transport. (e) Unusually large ilmenite macrocrysts are found only in the immediate vicinity of kimberlite pipe.

trenches (Fig. 2), followed by jigging (2 mm sieve size) for heavy mineral concentrates, an initially sparse then higher concentrations of ilmenite were discovered in a matter of weeks (3b). Critically, as shown

in Fig. 4: (i) The grain size and abundance of ilmenite increases as the kimberlite parent is approached. (ii) Small grains are abraded and rounded whereas larger grains are typically angular. (iii) It is only at

Ilmenite grain sizes 2 -5 mm

Distal 3-4 km

SC8

SC7

SC9 SC10

ILMENITE: 1 – 1.25 cm Camp Alpha SC12A 12B [mount =2.5 cm diam.]

12C

1.5 – 1.6 cm SC4A

4B

4C

1.5 – 2.5 cm SC5A

*Leucoxene-coated ilmenite is only present in & adjacent to the parent kimberlite. (Disc diam. = 2.5 cm)

5B 5C

~1.5 cm SC6A 6B

6C

*Leucoxene coated 1 – 1.5 cm *Leucoxene = anatase + (Mt-Hem)

Polished sections of ilmenite from the Camp Alpha region showing the variation in grain size as the kimberlite body (bottom) is approached.

Kimberlite SC-LA

LB

LC

Fig. 4. A schematic of ilmenite (fine 2–5 mm and rounded, to large 1–1.5 cm and angular) distributions found in stream sediments along an N-S sampling grid 3–4 km from the kimberlite discoveries. Leucoxene-coated (e.g. SC6A-C) ilmenite (Fig. 3c) is found only a few m from the kimberlite and in the eluvium.


the site of kimberlite, although buried below 1 m or more of overburden, that delicate alteration rims of “leucoxene” (anatase + magnetite → hematite) on ilmenite, are observed (Fig. 3c). The leucoxene-coated ilmenites are in the eluvium above the kimberlite and travel no N 10–15 m from the kimberlite. The grains are optically distinctive but are also magnetic, an added physical property that leaves no doubt to a positive identification in the field. The metamorphic basement abounds in garnet but the tempting, purple G10 diamond-indicators (Gurney and Zweistra, 1995) recovered in jigs and panning is spessartine not Cr-pyrope. Although garnets and pyroxene are common in the eluvium horizons of the dikes and the pipe (Fig. 5a–d) stream abrasion and tropical weathering has ensured destruction of peridotitic and eclogite garnets, pyroxene and olivine. Spinel was not recognized but zircon was useful and is distinctive being pale pink in kimberlite, and red or yellow from the basement (Fig. 5e). The eluvium in some cases was not above hard rock kimberlite as expected but had undergone monsoonal-transport. A dike, 10 m in width for example, can have a dispersion halo of eluvial ilmenite of 50 m or more. Recognizing this anomalous effect we improvised by using a 2.5 m length of rebar (of the type to reinforce concrete structures), with a welded T-handle. By turning and drilling into the soft texture of the swamps in our target areas, and vertically extracting it, an excellent sample of adhering clay, in the crenulations of the rebar, are obtained. This readily mobile and simple augur cut time-consuming efforts in hit-and-miss, pit and trench exploration. The distinguishing features between kimberlite eluvium (grey to green), and the white to grey, but mica-rich clay, of decomposed country rock was quickly learned. A selection of these positive kimberlite intersections (purple triangles) is shown in Fig. 2.

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5. Exploration results 5.1. Dikes Using the kimberlite lineament analysis of dike directions (Fig. 1b), in an area of progressively increasing ilmenite (in grain size and content Figs. 2, 3 and 4), we sank two pits ~ 100 m N of the first discovery. We uncovered an aphanitic dike, 11 m in width with glassy chilled margins and coarser grained crystalline interiors (Fig. 6). A few xenoliths, dominated by garnet-bearing lower crustal lithologies and dolerites up to 20 cm in diameter were encountered. Upper mantle xenoliths and xenocrysts consist of pervasively present ilmenite with coated leucoxene rims, peridotitic and eclogitic garnets, clinopyroxene, lamellar ilmenite-clinopyroxene intergrowths (Fig. 5), and highly altered fragments of harzburgite. This is characteristic of the assemblages found in the Koidu kimberlite, Sierra Leone (Tompkins and Haggerty, 1984), and other kimberlites globally. The northern section of the pit-exposure is an explosive blow (Fig. 2), polymict in nature (Fig. 6a), and reminiscent of crater to diatreme facies kimberlite in pipes; a pipe is not inferred but a gas-charged eruption in the dike is implied as outlined in the descriptions of kimberlite dikes in South Africa (Gurney and Kirkley, 1996; Damarupurshad, 2006; Field et al., 2008), and at Koidu (Tompkins and Haggerty, 1984). The contact between the dike and the blow is beautifully exposed and the rocks are intensely sheared indicating that the blow is temporally later than the dike with clear testament to the forceful nature of explosive injection (Fig. 6a). The blow is dominated by coarsely crystalline lower crustal and upper mantle xenoliths, similar to those observed in the companion dike.

KIMBERLITIC INDICATOR MINERALS (a)

(c)

(d)

1 mm 0.5 mm

1 mm

(b)

(e)

1 mm

Fig. 5. Mantle minerals recovered from jigged kimberlite eluvium are purple peridotitic and orange eclogitic garnets (a–b), symplectic intergrowth of ilmenite and clinopyroxene, (c) plus euhedral perovskite, and (d) pink zircon with a reaction coating of baddeleyite (e).

104


(a)

(b)

Liberia-Trend

(c) Liberia-Trend

2.5 cm

Blow

2.5 cm

Sierra LeoneTrend

2m

Fig. 6. Petrologically diverse dike samples (Liberia-Trend and Sierra Leone-Trend), illustrating the extreme variations in grainsize (chilled contacts to interiors), olivine content, and the polymict nature of the blow (SC11-B6).

The dike and blow are extremely tough and all efforts to extract fresh material failed. Finally resorting to jackhammers, four pits were sunk across the dike to a depth of ~ 1 m which provided a representative cross-section for analytical evaluation (petrography, chemistry, microdiamonds). In addition, as a prelude to bulk sampling, we extracted and stockpiled ~ 25 T of material in large blocks having separated the eluvium which was stacked adjacent to the pit. In continued pitting to the N we sank two pits with the extraordinary good fortune of finding not one but two dikes (Fig. 6b). The pits were expanded and the intervening wall removed. The dikes differ in the following respects: One dike strikes N20°E has a vertical dip, is blue-grey in colour (Fig. 6b), with abundant, rounded serpentinized olivine in a fine grained groundmass (Fig. 6c), has a pit-exposed strike dimension of 30 m and is 7.8 m wide; the second dike strikes N60°E, dips 35° to the NW, has a rusty-brown patina (Fig. 6c), is aphanitic with finely dispersed anhedral olivine, and has a pit-exposed thickness of 1.8 m (Fig. 6c). The contact is perfectly exposed indicating that the former is older and the latter temporally later. An unusual feature of the N60°E dike is the large (15 cm diameter) lower crustal xenolith suite found at the dike contact; unusual in the second sense that the xenoliths contain ~ 2.5 cm diameter clusters of polycrystalline garnet, a feature not previously recognized in our lower crustal xenoliths studies from Liberia (Toft et al., 1989). The significance of the multiply oriented dikes is several fold: (1) the N20°E orientation conforms to the regional Liberia lineament with kimberlites emplaced along reactivated Precambrian fracture zones (Fig. 1b–c). (2) The N65°E strike orientation is that of dikes at the Koidu kimberlite complex in Sierra Leone (Fig. 1b). (3) The Mesozoic intrusive event for kimberlites in Africa is dominantly 80–120 Ma; we can now show that the Liberia N20°E Trend post-dated intrusions along the Sierra Leone transform fault system; the intrusions not only differ in age (the SL dikes are dated at ~145 Ma), but took advantage of vastly different fractures, one Mesozoic and initiated by rifting, and the second contemporaneous with drifting. (4) This increases the potential source of alluvial diamonds at Camp Alpha, and increases the potential grade because the SL kimberlites (Haggerty, 1992), have historically been more productive. (5) If the complexities in tectonic control, and the diverse lithologies present are symptomatic of the entire province, the inevitable conclusion is that only a small fraction of dikes that were mapped in the USGS-Liberia Co-operative program were located. As a byline, it is relevant to note that their mission was not resource-mapping, and at a scale of 1:250,000 only the most prominent dikes were recorded, by inference to proximal artisanal mining. It is, in addition, surprising that although mapped, there are no recorded samples of kimberlites in either in the Geological Survey of Liberia, or in the rock achieves of the USGS.

In a third pit ~20 m s of the blow-kimberlite, we found the possible source of transported kimberlite that appeared sporadically in our offtarget pits. The kimberlite is best described as duricrust (Fig. 2) is easily disaggregated and has an elongate dispersion halo downstream from the exposed outcrop. Offset from the Liberia-Trend, the kimberlite dike and sparse xenoliths are fractured and severely altered. Kimberlite in a fourth successful pit to the N is on strike with the Liberia Trend (N20°E, Fig. 2). The organic overburden is ~ 1 m, the eluvium is ~40 cm in thickness similar to kimberlites in other pits but with the marked difference that it is significantly less robust to the extent that hand excavation through the eluvium and into the crumbling underlying kimberlite was possible down of ~ 2.5 m. The xenoliths suite consists of lower crustal granulites, eclogites, larger than usual ilmenite (5 cm in diameter), peridotitic and eclogitic garnet, and pink zircon with a reaction coating of baddeleyite (Fig. 5). With the dike orientations fairly well established, the leucoxeneindicator unfailingly positive, and rebar probing, additional dikes were located (Fig. 2), firmly establishing that the dikes are en echelon as described elsewhere (e.g. Kjarsgaard, 2007; Field et al., 2008). One significant difference is that the kimberlite dikes at Camp Alpha are substantially wider (~10 m) than dikes and sills (a few cm to ~2 m) in other provinces (op cit., and Tappe et al., 2014). From differences or similarities in petrography the preliminary conclusion is that some of the dikes are off-set by faulting whereas others are from distinct mantle source regions.

5.2. Kimberlite pipe Extremely large (10–15 cm) ilmenites (Fig. 3d) were encountered to the E of the Liberia Dike Trend, and to the NE of the pit with intersecting kimberlite dikes described above (Fig. 2). This was followed by rebar probing with repeated, positive results but always in and around the thick and thorny aerial roots of Pandanus candelabrum (Fig. 7), as described more fully elsewhere (Haggerty, 2015). Avoided throughout in our field work, and undoubtedly by others (i.e. major mining companies), the impenetrable grove, concealing a bonanza, was necessarily and deliberately skirted. The successful discovery of the kimberlite pipe at Camp Alpha (Fig. 2), long suspected (Haggerty, 1982), and elusive, is now a reality. Approximately parallel to the dike system, the pipe is elongated (~500 × 50 m), with 1–1.5 m of organic-rich overburden, and with a comparable thickness of eluvium to hard rock kimberlite (Fig. 8a–c). The xenolith suite recovered to date is dominated by eclogites, ilmenite, discrete garnet, pyroxene, garnet-kyanitecorundum, and equally unusual perovskite-bearing ilmenite-pyroxene intergrowths (Fig. 5d).


(a)

105

(b)

(c)

Fig. 7. Pandanus candelabrum with thorn – encrusted aerial roots; the mangrove entangled root and vine ensemble is typical of the impenetrable vegetation present on the kimberlite pipe at Camp Alpha and on other kimberlite bodies in the area. The plant has a clear preference for fragmented kimberlite pipe material, and is not found on the adjacent hard and aphanitic kimberlite dikes.

Pandanus candelabrum has been found in two other prospects ~1 km E of Camp Alpha; both test positively for kimberlite. Given the areal extent of rebar probing at least two pipes are present. With mounting evidence and systematic exploration, additional pipes will undoubtedly be found, consistent with the clustering of kimberlite pipes globally.

5.3. Kimberlite petrography The kimberlites at Camp Alpha are diverse in nature, ranging from aphanitic to coarser grained varieties in the dikes (Fig. 6), and to distinctly hypabyssal facies kimberlite in the pipe (Fig. 8d). Some samples

(a)

(b)

(c)

(d)

Cut slab of pipe kimb. ~25 cm across

Fig. 8. (a) An exposure at Site 1 (Fig. 2) of the newly discovered kimberlite pipe extending to the grove of Pandanus candelabrum. (b) Massive autoliths of kimberlite from the pipe, 1–2 m in diameter Site 2 (Fig. 2). (c) Stacked eluvium in preparation for washing and jigging Site 1. (d) A cut slab of surprisingly fresh pipe kimberlite with relatively fresh olivine, and peridotitic garnet.


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of the pipe, as well as the blow in one of the dikes, are distinctly polymict (Figs. 5b and 8d). Polished-thin sections (Fig. 9) show that the kimberlites are Group 1 and olivine-rich, with varying concentrations of ilmenite, serpentine, chlorite, phlogopite, and xenocrystic garnet with kelyphitic rims, typical of kimberlites from Koidu, Sierra Leone (Tompkins and Haggerty, 1984). 5.4. Mineral chemistry Xenocrystic ilmenite compositions were obtained by electron microbeam analyses (EMA) on polished sections from samples collected in the primary exploration program (Table 1). Analytical conditions and standards are detailed in Fung and Haggerty (1995). Eight elements (Fe, Ti, Mg, Mn, Cr, Al, Ni, Nb) were routinely determined, with three analyses on the smaller grains (e.g. SC-8, SC-10 Fig. 4), and five analyses on the larger grains (e.g. Fig. 4, SC6a - SC6c); total number of analyses = 1285. Ferric and ferrous iron contents were calculated following the method by Droop (1987), with internal checks from the extensive data base of Haggerty (1991). Four critically determinative plots were employed in the exploration program (Gurney and Zweistra, 1995; Wyatt et al., 2004; Haggerty, 1992). The first (MgO vs TiO2), delineates the fields of kimberlitic vs non-kimberlitic ilmenite (Fig. 10a); the second defines levels of kimberlite prospectivity, modified in accord with current data and an earlier (Haggerty, 1982) survey at Camp Alpha (Fig. 10b); the third (MgO vs Fe2O3) maps regions of diamond preservation (Fig. 10c); and the fourth provides a comprehensive evaluation of redox in terms of isostructural solid solution end-members (ternary ilmenite FeTiO3-geikielite-MgTiO3-hematite Fe2O3 (Fig. 10d). A relatively wide range in MgO (6–17 wt%) was determined that is bimodal with peaks at ~ 9 and ~ 15 wt% MgO (Table 1, Fig. 10). The lower MgO value has correspondingly lower Cr2O3 (0.1–1.5 wt%) contents (Fig. 10b) but higher Fe2O3 (~14 wt% Fig. 10c), equivalent to the ternary cluster at ~20 mol% in Fig. 10d. High MgO ilmenites have larger Cr2O3 (~1–6 wt%) contents and are lower in Fe2O3 (~7 wt%) which is equivalent to ~10 mol% hematite (Fig. 10d). The bimodality in ilmenite in unrelated to sample location, grain size variations, or zoning. It must be due to differences in mineral chemistry among the kimberlite dikes, or in variations between the dikes and the pipe. This requires

(a)

Table 1 Representative EMPA Ilmenite data: exploration sampling. Cr2O3

MgO

MnO

FeO

Fe2O3

Total

High MgO group 1 0.83 52.79 2 0.67 54.00 3 0.58 54.14 4 0.92 52.11 5 0.57 53.28 6 0.45 56.18 7 0.39 54.16 8 0.34 57.49 9 0.68 54.50 10 0.68 53.52

Al2O3

TiO2

2.28 2.15 1.69 2.98 2.03 1.37 1.60 1.08 2.13 2.80

15.47 15.49 15.51 14.95 15.55 15.31 15.40 15.22 15.36 15.10

0.00 0.01 0.00 0.00 0.00 0.03 0.04 0.05 0.02 0.02

19.90 21.85 21.04 20.21 20.21 23.20 19.41 24.53 21.60 21.19

8.56 6.29 6.90 8.49 7.72 4.15 8.20 1.40 5.57 7.07

99.82 100.45 99.86 99.67 99.36 100.68 99.21 100.10 99.87 100.38

Low MgO group 11 0.62 47.82 12 0.53 47.23 13 1.47 46.18 14 0.80 47.28 15 1.42 47.54 16 0.69 46.78 17 0.55 48.28 18 0.74 49.19 19 0.75 47.27 20 0.44 47.99

0.77 0.43 0.67 0.60 0.54 0.53 0.60 0.58 0.63 0.41

8.54 8.48 9.24 8.65 9.21 8.64 8.48 8.83 8.75 8.41

0.00 0.01 0.03 0.00 0.06 0.04 0.02 0.02 0.01 0.05

27.78 27.34 24.12 27.99 26.28 26.63 28.28 28.47 27.80 28.11

15.31 15.85 18.28 14.85 14.38 16.86 13.76 13.13 15.11 15.32

100.84 99.86 99.99 100.16 99.45 100.16 99.97 100.95 100.32 100.73

confirmation, importantly because it bears on the potential preservation of diamond (Gurney and Zweistra, 1995). The high Mg-Cr ilmenites are refractory and geochemically depleted and grew in an environment of low oxygen potential (Fig. 10d). This is consistent with the prediction that co-existing diamonds would have an excellent diamond preservation index as shown in Fig. 10c. Correspondingly, the low Mg-Cr ilmenites are variably oxidized and the diamonds predictability would have a marginal to intermediate preservation index (Fig. 10c). In summary, N95% of the ilmenites are kimberlitic (Fig. 10a), the vast majority indicate positive prospectivity for kimberlites (Fig. 10b), and the oxidation state of ilmenites predict two populations of diamond: one well preserved, the other less so (Fig. 10c–d). The source of ilmenite bimodality as well as a study of mantle and crustal xenoliths will be undertaken at the time of bulk sampling.

(b)

(d)

(c)

(e)

(f)

5 mm

2 mm

Fig. 9. (a) Densely packed euhedral to subhedral olivine in a fine-grained matrix of ilmenite, serpentine, and chlorite typical of the interior of dikes and of some samples from the pipe kimberlite. (b–e) Xenocrystic garnet with kelyphite reaction rims in kimberlite matrices of olivine, serpentine and ilmenite in two dike samples. (d) An unusually-rich chlorite kimberlite from the pipe. Polished-thin-sections in transmitted light.


107

Fig. 10. Compositional plots of ilmenite data from heavy mineral, exploration jigged samples. (a) Discrimination plot for kimberlitic vs non-kimberlitic ilmenite. (b) In a modified Cr2O3MgO plot, the regions outlined define degrees of kimberlite prospectivity. (c) A simplified plot outlining the approximate fields expected from the redox of ilmenite as it might affect diamonds transported in kimberlite. (d) Ternary plot of isostructural endmembers with data plotted as mol%, and with insets showing the ranges in composition in greater detail. This represents a redox plot from reduced conditions at the base to oxidized conditions at the apex. See text for references and discussion.

5.5. Bulk chemistry About 50 kg of the freshest kimberlites were crushed and prepared for XRF analyses according to the procedures described in Taylor et al. (1994). The results are very similar to those previously published for West Africa (op cit.) and are succinctly summarized in Tables 2 and 3 and in selective plots in Fig. 11a (major elements), and Fig. 11b (trace elements). The similarity extends to the fact that MgO ≈ SiO2 for both the dikes and the pipe; the pipe kimberlites, however, are uniformly lower in SiO2 and MgO (av. 31 wt%), relative to the dikes that have an average 34 wt% for each of these oxides; CaO (Fig. 11a) and BaO are

significantly different, with an average of 3.9 wt% and 1100 ppm in the dikes, and 0.96 wt%, and 574 ppm in the pipe, respectively. Titanium (4–5 wt%), P, Na, and K (each b1 wt%), the LREE (50–300 ppm), as well as Cr, Ni, Zr, and Nb contents are very similar in the dikes and the pipe (Fig. 11b). The differences, where present imply variations in the mantle source region of proto-kimberlite, and in the subsequent degree of mixing and melt assimilation of lithospheric horizons (e.g. Haggerty, 1989). Crustal-contaminated samples from the edges of dikes and the blow have higher than average SiO2 (48 wt%), and CaO (12 wt%), and significantly lower MgO (15 wt%). To broaden the scope of the comparison, additional compositional data for kimberlites from South Africa

Table 2 Bulk chemistry major elements. Sample

SiO2

TiO2

Al2O3

Camp Alpha dikes SC11 A1 34.25 3.61 3.77 SC11 A2 35.68 4.71 1.76 SC11 A3 34.62 4.88 1.79 SC11 A4 31.33 6.36 2.78 SC11 A6 39.42 2.56 0.84 SC11 B2 33.25 5.86 2.37 SC11 B3 33.68 6.22 2.64 SC11 B4 35.42 4.93 4.14 SC11 B5 33.80 6.54 2.33 SC11 B6 33.70 5.81 2.35 Notes: Units are wt% and Fe2O3 is TOTAL Fe. Camp Alpha pipe PCA-1 27.97 PCA-2 27.81 PCA-3 33.77 PCA-4 31.34 PCA-5 30.99 PCA-6 29.47 PCA-7 34.81 PCA-8 35.08

7.13 7.66 2.13 3.2 5.43 6.52 2.29 1.67

1.32 1.41 0.81 1.92 1.4 1.69 0.94 0.86

Fe2O⁎3

MnO

MgO

CaO

Na2O

K2O

P2O5

Total

LOI

14.58 18.39 20.28 23.17 12.31 15.26 17.06 14.33 17.72 15.87

0.23 0.43 0.40 0.47 0.16 0.26 0.28 0.23 0.30 0.24

35.58 36.92 35.38 32.54 40.29 32.60 32.11 32.81 32.66 33.10

5.64 0.80 0.71 1.09 2.71 7.14 5.62 5.53 3.51 5.96

0.06 0.26 0.24 0.50 0.00 0.09 0.11 0.10 0.06 0.03

0.80 0.08 0.04 0.06 0.09 0.94 0.92 0.95 1.02 0.85

0.23 0.06 0.07 0.09 0.17 0.59 0.51 0.51 0.12 0.67

98.75 99.09 98.41 98.39 98.55 98.36 99.15 98.95 98.06 98.58

12.83 12.33 13.29 11.66 11.74 12.43 10.74 11.81 12.11 11.92

15.96 15.78 13.13 13.97 15.31 17.39 13.5 11.63

0.25 0.27 0.17 2.82 0.28 1.27 0.14 0.12

28.23 28.96 33.05 29.57 30.96 28.67 33.22 34.18

1.63 0.76 0.46 2.09 0.81 0.84 0.49 0.61

0.26 0.13 0.06 0.07 0.05 0.05 0.05 0.07

0.16 0.29 0.07 0.41 0.06 0.1 0.04 0.12

0.3 0.25 0.23 0.48 0.38 0.37 0.26 0.36

99.57 99.08 99.91 99.01 100.13 98.61 99.87 99.32

16.36 15.76 16.03 12.74 14.46 12.24 14.13 14.62

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Table 3 Bulk chemistry trace element data. Sample

Nb

Camp Alpha dikes Units are in ppm SC11 A1 473 SC11 A2 128.7 SC11 A3 136.9 SC11 A4 276.4 SC11 A6 123.0 SC11 B1 92.8 SC11 B2 277.3 SC11 B3 253.7 SC11 B4 199.5 SC11 B5 300.1 SC11 B6 129.2 Camp Alpha pipe Units are in ppm PCA-1 201 PCA-2 198 PCA-3 166 PCA-4 228 PCA-5 152 PCA-6 210 PCA-7 207 PCA-8 333

Zr

Y

Sr

U

Rb

Th

Pb

Ga

Zn

Ni

Cr

V

Ce

Ba

La

221 416 405 685 161 214 323 325 303 334 276

15.3 6.7 7.7 10.4 1.9 13.6 16.9 18.7 12.6 18.3 11.8

370 68 58 97 137 392 553 468 528 258 775

2 2 2 6 0 1 0 1 0 2 0

47.9 2.7 0.9 1.5 3.4 109.3 59.2 56.7 52.9 67.3 128.4

25 5 7 24 12 8 13 13 12 13 16

4 3 4 5 3 7 4 8 7 4 6

6 6 7 9 3 14 8 7 8 8 11

54 48 65 114 44 68 48 89 51 51 64

1030 1892 2153 1761 1966 549 1070 1146 986 1134 569

1588 1941 2148 2736 1025 824 1282 1168 1026 1456 644

99 139 145 229 79 189 156 190 170 169 195

388 76 103 154 32 121 227 256 152 355 186

1905 266 35 337 145 1482 1321 2128 1411 1649 1845

172 45 61 94 16 67 115 146 80 179 92

304 307 237 289 380 275 327 238

12 11 10 7 9 8 11 5

30 30 120 171 99 38 55 142

Not determined

84 77 69 82 68 70 88 74

1788 1523 1165 1575 1673 1377 2100 1782

1130 1049 957 1027 1896 1004 1144 865

232 222 168 295 88 123 262 110

93 84 69 86 292 78 88 72

417 382 573 607 866 660 613 473

50 46 38 45 161 41 46 39

(Becker and Le Roex, 2006) and Canada (Kjarsgaard et al., 2009), show that although there are overlaps, the kimberlites from Liberia are most similar to Group 1 kimberlites from Sierra Leone, Guinea and South Africa (Fig. 11a–b). A notable feature, however, is that the pipe kimberlite is depleted in CaO (Fig. 11b), possibly due to alteration (Taylor et al., 1994). 5.6. Diamonds Preliminary hand jigging has recovered several diamonds in the 1– 2 ct range, and two diamonds of ~20 ct from terrace gravels on the margin of the discovery valley. These are similar in size and quality to those routinely recovered by local artisanal miners. An exceptional 18.2 ct Type II D-flawless diamond (Fig. 12) was recovered directly from the eluvium of the intersecting dikes to the W of the kimberlite pipe (Sierra Leone-Trend site in Fig. 2). This is a first from Liberia and the first from a kimberlite dike. A total of 167 micro-diamonds (i.e. b 0.5 mm) were recovered from 646.54 kg of kimberlite dike material. The largest stone was a brown,

(a)

semi-transparent broken octahedral crystal. The diamonds are predominantly white/colourless (99%) clear transparent stones displaying good clarity with very little surface corrosion. Minor etch features, such as, ruts and tiny etch pits are on b 30% of the population, and minute inclusions were observed in only 9 of the stones recovered (5% of the population). The micro-diamond population is dominated by octahedral crystals (49%) and composite octahedral crystal aggregates (33%). Surprisingly, very few dodecahedral crystals (10%) are present: “This is unusual and clearly indicates the diamonds have not experienced severe dissolution/resorption during kimberlite emplacement, which is commonly the case in most kimberlites from southern Africa (Garvie, 2013).” That micro-diamonds are present in all of the samples submitted is highly encouraging because macro-diamonds must be present (indeed confirmed) if these co-existed with smaller stones in the upper mantle. The argument is based on surface to volume ratios that increase with decreasing grain size. In the presence of corrosive fluids, microdiamonds in the b0.5 mm size range would simply dissolve as macrodiamonds undergo mere surface etching. The micro-diamond population supports the application of redox-sensitive-ilmenite as a useful

(b)

Liberia dikes

Liberia pipe

Fig. 11. Representative plots of major (a) and trace (b) element compositions for the dike and pipe kimberlites from Camp Alpha showing that these are very similar to Group 1 kimberlites from Sierra Leone, Guinea, and South Africa. Full squares are for kimberlites from Liberia, separated into pipe and dike fields for major elements but are not distinguished for trace elements because of extensive overlap. Full dots are for kimberlite dike and pipe samples from Koidu, Sierra Leone and the partially open squares are for pipe samples from Banankoro, Guinea. The ellipsoidal fields are from Taylor et al. (1994), and the dashed rectangular field is from the data bases of Becker and Le Roex (2006) for South Africa, and Kjarsgaard et al. (2009), (major elements only) for Canada.


109

reviewers (S. Tappe and anon) for their constructive improvements, I express my sincere appreciation. References

Fig. 12. An 18.2 ct Type II diamond recovered from the intersecting dike eluvium at Camp Alpha (Fig. 2). The embroidered logo is the famous 186 ct Koh-i-Nur from India.

proxy in predicting macro-diamond preservation as illustrated in Fig. 10c. This underscores the essential need to relate specific ilmenite compositional modes to specific kimberlites. And in the context of dikes versus pipes, the discovery of the 18.2 ct stone bodes well, because Type II diamonds are generally large, and because kimberlite dikes generally have higher grades than coexisting kimberlite pipes (Gurney and Kirkley, 1996). 6. Conclusions The successful discovery of primary kimberlite in the Camp Alpha region of NW Liberia using ilmenite and associated leucoxene as a pathfinder is complimented by positive analytical results indicating that the field is highly prospective and diamond-bearing. Bulk chemistry on kimberlites show a remarkable similarity to Group 1 kimberlites from South Africa, Sierra Leone and Guinea in major and trace elements. Tectonically, the oldest kimberlite dikes (Sierra Leone Trend ~140 Ma) occupy the most recent (Mesozoic) fracture zones (Gondwana breakup), whereas the later (90–120 Ma Liberia Trend) intrusives filled fractures that were episodically activated since the Precambrian. With known dike orientations, transverse-trenching to locate targets of 10 m or less are better refined, and because these are relatively abundant and can have extraordinarily high diamond grades, the traditional exploration paradigm of pipes as sole targets requires a diligent reassessment. The micro-diamond data along with the recovery of macrodiamonds, and the botanical recognition of Pandanus candelabrum point to a very strong future for the mining of diamonds directly from kimberlites, the first in Liberia. Acknowledgements Field work was supported by the Youssef Diamond Mining Company and H-10 Geo-Consultants. Analytical work was supported by the latter and undertaken at the University of Massachusetts (J.M Rhodes, XRF), and at Florida International University (T. Beasley EMPA). To the companies for permission and support, the Mine Ministry of Liberia, the Ateam of local field assistants, to A. Potra for lab assistance, and the

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