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The Sheahan Diamond Literature Reference Compilation - Technical Articles based on Major Region - Sierra Leone
The Sheahan Diamond Literature Reference Compilation is compiled by Patricia Sheahan who publishes on a monthly basis a list of new scientific articles related to diamonds as well as media coverage and corporate announcements called the Sheahan Diamond Literature Service that is distributed as a free pdf to a list of followers. Pat has kindly agreed to allow her work to be made available as an online digital resource at Kaiser Research Online so that a broader community interested in diamonds and related geology can benefit. The references are for personal use information purposes only; when available a link is provided to an online location where the full article can be accessed or purchased directly. Reproduction of this compilation in part or in whole without permission from the Sheahan Diamond Literature Service is strictly prohibited. Return to Diamond Region Index
Sheahan Diamond Literature Reference Compilation - Scientific Articles by Author for all years
Each article reference in the SDLRC is tagged with one or more key words assigned by Pat Sheahan to highlight the main topics of the article. In addition most references have been tagged with one or more region words. In an effort to make it easier for users to track down articles related to a specific region, KRO has extracted these region words and developed a list of major region words presented in the Major Region Index to which individual region words used in the article reference have been assigned. Each individual Region Report contains in chronological order all the references with a region word associated with the Major Region word. Depending on the total for each reference type - technical, media and corporate - the references will be either in their own technical, media or corporate Region Report, or combined in a single report. Where there is a significant number of technical references there will be a technical report dedicated to the technical articles while the media and corporate references are combined in a separate region report. References that were added in the most recent monthly update are highlighted in yellow within the Region Report. The Major Region words have been defined by a scale system of "general", "continent", "country", "state or province" and "regional". Major Region words at the smaller scales have been created only when there are enough references to make isolating them worthwhile. References not tagged with a Region are excluded, and articles with a region word not matched with a Major Region show up in the "Unknown" report.
Kimberlite - diamondiferous
Lamproite - diamondiferous
Lamprophyre - diamondiferous
Other - diamondiferous
Kimberlite - non diamondiferous
Lamproite - non diamondiferous
Lamprophyre - non diamondiferous
Other - non diamondiferous
Kimberlite - unknown
Lamproite - unknown
Lamprophyre - unknown
Other - unknown
Future Mine
Current Mine
Former Mine
Click on icon for details about each occurrence. Works best with Google Chrome.
CITATION: Faure, S, 2010, World Kimberlites CONSOREM Database (Version 3), Consortium de Recherche en Exploration Minérale CONSOREM, Université du Québec à Montréal, Numerical Database on consorem.ca. NOTE: This publicly available database results of a compilation of other public databases, scientific and governmental publications and maps, and various data from exploration companies reports or Web sites, If you notice errors, have additional kimberlite localizations that should be included in this database, or have any comments and suggestions, please contact the author specifying the ID of the kimberlite: [email protected]
Determination of the Ages of West African Kimberlites and An Interpretation from the Dates of the Different Diamondifero united States Events in the World.
International Symposium AFR. GEOL. 3RD., CGLU, Report No. 6660, 88P.
Sierra Leone, West Africa, Guinea, Central African Republic
Paleogeomorphology and the Occurrence of Diamondiferous Deposits in Koidu Sierra Leone.
Paper Presented Ins. Min. Met. And British Geomorphological Re, Nov. 15TH. LONDON, ABSTRACT IN Institute of Mining and Metallurgy (IMM) Bulletin. No. 934, P. 10.
Report on a Joint Meeting on Placer Deposits Organized by The Institution of Mining and Metallurgy and the British Geomorphological Research Group. Held November 14, 1984.
Institute of Mining and Metallurgy. Transactions, Vol. 94, SECT. B, PP. B46-48.
Bibliography of the geology and mineral resources of Liberia and Sierra Leone and the adjacent Archean terrains of Guinea and Cote d'Ivoire, West Africa.
Economic Geology Research Institute, EGRU Wits, Information Circular, No. 342, 67p.
Geology and evaluation of the K2 kimberlite, Koidu mine, Sierra Leone, West Africa.
Proceedings of the 10th. International Kimberlite Conference, Vol. 2, Special Issue of the Journal of the Geological Society of India,, Vol. 2, pp. 191-208.
Abstract: The Mg isotopic compositions of garnet and clinopyroxene mineral separates and whole rocks from 21 xenolithic eclogites (11 low-MgO eclogites and 10 high-MgO eclogites) from the Koidu kimberlite complex, erupted within the Archean Man Shield, Sierra Leone, West Africa, provide new evidence bearing on the origin of cratonic eclogites. Garnet and clinopyroxene in both low-MgO and high-MgO eclogites generally record equilibrium inter-mineral Mg isotope partitioning, with ?26Mg varying from -2.15‰ to -0.46‰ in garnets and from -0.49‰ to +0.35‰ in clinopyroxenes. Bulk ?26Mg values (-1.38‰ to +0.05‰), constructed from garnet and clinopyroxene data, are similar to results from rock powders (-1.60‰ to +0.17‰), suggesting that kimberlite infiltration has had negligible influence on the Mg isotopic compositions of the xenoliths. The ?26Mg values of low-MgO eclogites (-0.80‰ to +0.05‰) exceed the range of mantle peridotite xenoliths (-0.25‰ ± 0.04‰), consistent with the eclogite’s derivation from recycled altered oceanic crust. Similarly variable ?26Mg values in high-MgO eclogites (-0.95‰ to -0.13‰), together with their high MgO and low FeO contents, suggest that high-MgO eclogites were produced by Mg-Fe exchange between partially molten low-MgO eclogites and surrounding peridotites. Our study shows that cratonic xenolithic eclogites preserve a record of Mg isotopic compositions produced by low-pressure, surficial isotope fractionations. The recycling of oceanic crust therefore increases the Mg isotope heterogeneity of the mantle.
Abstract: The Mg isotopic compositions of garnet and clinopyroxene mineral separates and whole rocks from 21 xenolithic eclogites (11 low-MgO eclogites and 10 high-MgO eclogites) from the Koidu kimberlite complex, erupted within the Archean Man Shield, Sierra Leone, West Africa, provide new evidence bearing on the origin of cratonic eclogites. Garnet and clinopyroxene in both low-MgO and high-MgO eclogites generally record equilibrium inter-mineral Mg isotope partitioning, with ?26Mg varying from -2.15‰ to -0.46‰ in garnets and from -0.49‰ to +0.35‰ in clinopyroxenes. Bulk ?26Mg values (-1.38‰-to +0.05-), constructed from garnet and clinopyroxene data, are similar to results from rock powders (-1.60‰ to +0.17‰), suggesting that kimberlite infiltration has had negligible influence on the Mg isotopic compositions of the xenoliths. The ?26Mg values of low-MgO eclogites (-0.80‰ to +0.05‰) exceed the range of mantle peridotite xenoliths (-0.25- ± 0.04-), consistent with the eclogite’s derivation from recycled altered oceanic crust. Similarly variable ?26Mg values in high-MgO eclogites (-0.95‰ to -0.13‰), together with their high MgO and low FeO contents, suggest that high-MgO eclogites were produced by Mg-Fe exchange between partially molten low-MgO eclogites and surrounding peridotites. Our study shows that cratonic xenolithic eclogites preserve a record of Mg isotopic compositions produced by low-pressure, surficial isotope fractionations. The recycling of oceanic crust therefore increases the Mg isotope heterogeneity of the mantle.-
Abstract: This article briefly outlines the history of the colonial diamond industry of Sierra Leone from 1930 to 1961, highlighting its contingent aspects and the bonds guiding the decisions and actions taken by local social actors in different contexts and at different times. By drawing on colonial documents and memoirs of colonial officers, it shows how the colonial government of Sierra Leone and the mining company that exercised a monopoly on diamond extraction collaborated on the establishment of a series of legislative and disciplinary devices that encompassed forms of biopolitical expertise.
Historical Research, Vol. 89, no 243, pp. 136-157.
Africa, Sierra Leone
History
Abstract: This article briefly outlines the history of the colonial diamond industry of Sierra Leone from 1930 to 1961, highlighting its contingent aspects and the bonds guiding the decisions and actions taken by local social actors in different contexts and at different times. By drawing on colonial documents and memoirs of colonial officers, it shows how the colonial government of Sierra Leone and the mining company that exercised a monopoly on diamond extraction collaborated on the establishment of a series of legislative and disciplinary devices that encompassed forms of biopolitical expertise.
Abstract: This article contributes to evolving debates on Sierra Leone’s post-war “crisis of youth” by providing an extended analysis of the role that young boys and girls assume in negotiating household poverty and enhancing their livelihood opportunities in small-scale mining communities. Child miners – or “half shovels” as they are locally known – are both directly and indirectly involved in small-scale gold extraction in Kono District, Sierra Leone’s main diamond-producing area. But the implications of their involvement are often far more nuanced and complex than international children’s rights advocates understand them to be. Drawing upon recent fieldwork carried out in and around the Kono mining village of Bandafayie, the article argues that children’s participation in the rural economy not only generates much-needed household income, but in many cases is the only way in which they can earn the monies needed to attend school. A blind and uncritical acceptance of international codes and agreements on child labor could have an adverse impact on children and, by extension, poor communities in rural Sierra Leone. Western notions of “progress” and development, as encapsulated in the post-conflict reconstruction programing of international NGOs and donor organizations, often do not match up with the complex realities or competing visions of local people.
Abstract: Since the end of World War II, there have been 259 armed conflicts in 159 locations (1). Sierra Leone's civil war began 25 years ago, at a time when roughly 25% of all countries worldwide were experiencing civil war (2). How can individuals and groups recover from such violent conflicts? On page 787 of this issue, Cilliers et al. (3) provide rigorous evidence on the efficacy of one postwar reconciliation strategy that was implemented in 100 communities in Sierra Leone (4).
Abstract: Type Ib diamonds contain abundant isolated nitrogen and account for less than 0.1% of natural diamonds. This rarity is because over geological time and at temperatures typical of the cratonic lithosphere, isolated nitrogen aggregates to A centres (N pairs) and B centres (4N around a vacancy). Preservation of isolated nitrogen in natural diamonds requires either short mantle residency times or storage at temperatures <700 °C, whereas most cratonic diamonds are billions of years old and resided >1000 °C. Eclogitic sulphides in Zimmi Ib diamonds (Sierra Leone) have 650 Ma Re-Os ages, whose origin is best explained by rapid tectonic exhumation after continental collision to shallower depths in the keel prior to kimberlite eruption (Smit et al., 2016). Here we present SIMS and spectroscopic data for a new suite of Zimmi sulphide-bearing diamonds. These new data allow us to evaluate the speciation of diamond source fluids, and the defects in these diamonds that are responsible for their distinctive canary yellow colours. The combined dataset establishes the link between the spectroscopic features in Zimmi Ib diamonds and their unusual history. Low ?13C values below -15 ‰ in 3 diamonds result from source fluids that originated as organic carbon in the oceanic crust, and were recycled into the lithosphere during subduction. In particular, a trend of decreasing ?13C and N content from core to rim in a single diamond indicates closed-system growth from CH4-rich subducted fluids. Recycled origins for organic carbon supports the temporal link of Zimmi diamond formation with deep underthrusting during Neoproterozoic continental assembly, recorded in the Rokelide orogen. CL imaging of Zimmi diamonds reveals irregular growth patterns with abundant deformation lines, associated with tectonic exhumation of the diamonds. Vacancies formed by deformation were subsequently annealed to form NV centres, where the negative charge state (637 nm) is more abundant due to isolated nitrogen donating an electron. Fancy brownish-yellow to greenish-yellow colours observed in Zimmi Ib diamonds result from a combination of isolated nitrogen, deformation-related amber centres and NV centres. These colour-forming defects can all be attributed to the unique geological history of Zimmi Ib diamonds and their rapid exhumation after formation.
Abstract: The Man Craton region of West Africa has a rich history of diamonds since they were first discovered in the 1930’s.They are primarily alluvial in source with currently only one kimberlite mine in operation at Koidu in Sierra Leone. The total diamond production from Guinea, Liberia and Sierra Leone over the past 10 years is recorded by the Kimberley Process at around 12.2 million carats with a value of $1.9 billion. The two main producing countries during this period are Guinea, which has yielded 6.7 million carats at an average of $52 per carat, and Sierra Leone where production has reached 5 million carats at a higher value of $277 per carat. Liberia is the smallest producer with 0.4 million carats but these have a high average value of $383 per carat. There are two known age provenances of kimberlites in the Man Craton. The larger, Jurassic age provenance comprises six main clusters of small (generally 10 ha) kimberlite pipes and dykes ranging from the older Bounoudou kimberlites in Guinea, at 153 Ma, through to the younger Tongo kimberlites in Sierra Leone dated at 140 Ma. A single, neo-Proterozoic cluster is known in the Weasua area in Liberia and is dated at 800 Ma. The Jurassic age kimberlites are classified as phlogopite-rich kimberlites with abundant groundmass opaque minerals. The older Weasua kimberlites typically contain less phlogopite and groundmass opaque minerals. Although remnants of diatreme facies are present in some pipes, notably the Banankoro, Koidu and Weasua kimberlites, hypabyssal and transitional facies tend to predominate which would indicate that these kimberlites have been eroded down to the interface between the root and diatreme zones. This suggests potential erosion of up to 2 km over the Man Craton; however geomorphological evidence suggests a lesser amount of erosion has taken place (Skinner et al., 2004). Alluvial diamonds are prevalent throughout the Man Craton and are not restricted to the known kimberlite clusters. This would argue for a wide dispersion of diamonds in the alluvial system as a result of significant landmass uplift and weathering since the time of intrusion. It could also indicate that there are diamondiferous kimberlites yet to be discovered, which is supported by the limited exploration data. It is therefore concluded that there are certain areas of the Man Craton which remain highly prospective for diamondiferous kimberlites.
European Geosciences Union General Assembly 2017, Vienna April 23-28, 1p. 9187 Abstract
Africa, Zimbabwe, Sierra Leone
Deposit - Marange, Zimmi
Abstract: Traditional models for diamond formation within the lithospheric mantle invoke either carbonate reduction or methane oxidation. Both these mechanisms require some oxygen exchange with the surrounding wall-rock at the site of diamond precipitation. However, peridotite does not have sufficient buffering capacity to allow for diamond formation via these traditional models and instead peridotitic diamonds may form through isochemical cooling of H 2 O-rich CHO fluids [1]. Marange mixed-habit diamonds from eastern Zimbabwe provide the first natural confirmation of this new diamond growth model [2]. Although Marange diamonds do not contain any silicate or sulphide inclusions, they contain Ni-N-vacancy complexes detected through photoluminescence (PL) spectroscopy that suggest the source fluids equilibrated in the Ni-rich depleted peridotitic lithosphere. Cuboid sectors also contain abundant micro-inclusions of CH 4 , the first direct observation of reduced CH 4-rich fluids that are thought to percolate through the lithospheric mantle [2]. In fluid inclusion-free diamonds, core-to-rim trends in ? 13 C and N content are used to infer the speciation of the diamond-forming fluid. Core to rim trends of increasing ? 13 C with decreasing N content are interpreted as diamond growth from oxidized CO 2-or carbonate-bearing fluids. Diamond growth from reduced species should show the opposite trends-decreasing ? 13 C from core to rim with decreasing N content. Within the CH 4-bearing growth sectors of Marange diamonds, however, such a 'reduced' trend is not observed. Rather, ? 13 C increases from core to rim within a homogeneously grown zone [2]. These contradictory observations can be explained through either mixing between CH 4-and CO 2-rich end-members of hydrous fluids [2] or through closed system precipitation from an already mixed CH 4-CO 2 H 2 O-maximum fluid with XCO 2 (CO 2 /[CO 2 +CH 4 ]) between 0.3 and 0.7 [3]. These results demonstrate that Marange diamonds precipitated from cooling CH 4-CO 2-bearing hydrous fluids rather than through redox buffering. As this growth mechanism applies to both the fluid-rich cuboid and gem-like octahedral sectors of Marange diamonds, a non-redox model for diamond formation from mixed CH 4-CO 2 fluids is indicated for a wider range of gem-quality peridotitic diamonds. Indeed, at the redox conditions of global diamond-bearing lithospheric mantle (FMQ-2 to-4; [4]), CHO fluids are strongly water-dominated and contain both CH 4 and CO 2 as dominant carbon species [5]. By contrast diamond formation in eclogitic assemblages, through either redox buffering or cooling of carbon-bearing fluids, is not as well constrained. Zimmi diamonds from the West African craton have eclogitic sulphide inclusions (with low Ni and high Re/Os) and formed at 650 Ma, overlapping with the timing of subduction [6]. In one Zimmi diamond, a core to rim trend of decreasing ? 13 C (-23.4 to-24.5 %¸) and N content is indicative of formation from reduced C 2 H 6 /CH 4-rich fluids, likely derived from oceanic crust recycled during Neoproterozoic subduction. Unlike mixed CH 4-CO 2 fluids near the water maximum, isochemical cooling or ascent of such reduced CHO fluids is not effficient at diamond precipitation. Furthermore, measurable carbon isotopic variations in diamond are not predicted in this model and therefore cannot be reconciled with the ?1 internal variation seen. Consequently, this Zimmi eclogitic diamond likely formed through redox buffering of reduced subduction-related fluids, infiltrating into sulphide-bearing eclogite.
Mineralogy and Petrology, doi.org/10.1007/ s00710-018- 0587-6 16p.
Africa, Sierra Leone
deposit - Zimmi
Abstract: Zimmi diamonds (Sierra Leone) have 500 million year mantle residency times whose origin is best explained by rapid tectonic exhumation to shallower depths in the mantle, associated with continental collision but prior to kimberlite eruption. Here we present spectroscopic data for a new suite of Zimmi sulphide-bearing diamonds that allow us to evaluate the link between their spectroscopic features and their unusual geological history. Cathodoluminesence (CL) imaging of these diamonds revealed irregular patterns with abundant deformation lamellae, associated with the diamonds' tectonic exhumation. Vacancies formed during deformation were subsequently naturally annealed to form vacancy clusters, NV0/- centres and H3 (NVN0). The brownish-yellow to greenish-yellow colours observed in Zimmi Ib-IaA diamonds result from visible absorption by a combination of isolated substitutional nitrogen ( {N}S^0 ) and deformation-related vacancy clusters. Colour-forming centres and other spectroscopic features can all be attributed to the unique geological history of Zimmi Ib-IaA diamonds and their rapid exhumation after formation.
Mineralogy and Petrology, doi.org/10.1007/s00710-018-0620-9 11p.
Africa, Sierra Leone
deposit - Koidu
Abstract: This paper documents the application of a microdiamond-based approach to the estimation of diamond grade in the Pipe 1 kimberlite at the Koidu mine in Sierra Leone. A geological model of Pipe 1 was constructed to represent the distribution and volume of the dominant kimberlite units within the pipe. Bulk samples, along with representative microdiamond samples, were collected from these units at surface and were used to define the ratio between microdiamond stone frequency (+212 ?m stones per kilogram) and recoverable macrodiamond grade (+1.2 mm carats per tonne; 1 carat?=?0.2 g). These ratios were applied to a comprehensive, spatially representative microdiamond sample dataset and were combined with a spatial model of country-rock xenolith dilution within the pipe to estimate +1.2 mm recoverable grades. The resource estimate was reconciled with subsequent production results in the elevation range 160 to 100 m above sea level. Production results for each of the six 10 m benches covering this elevation range were compared to the estimated average grades for these zones in the pipe. For the five cases where most of the kimberlite mass on a given bench is represented in the production data, the results show a maximum discrepancy of 6% between predicted and reported production grade with no indication of any consistent bias. This indicates that, when supported by a sound geological model and suitable microdiamond and macrodiamond data, the microdiamond-based estimation approach can provide reliable constraints on macrodiamond grade, even in the case of geologically complex bodies such as Koidu Pipe 1.
Journal of Rural Studies, Vol. 61, 1, pp. 110-122.
Africa, Sierra Leone
alluvials
Abstract: This paper reflects critically on the transformational impacts the recent Ebola epidemic has had in diamond-rich areas of rural Sierra Leone. It focuses specifically on the country's ‘diggers’, a sizable group of labourers who occupy the bottom of the country's artisanal diamond mine production pyramid. Based upon research conducted in the diamond-producing localities of Kenema and Kono, the paper argues how, in sharp contrast to the gloomy picture painted in the literature about their existences and struggles, diggers exhibited considerable resilience during the Ebola crisis. Their diversified livelihood portfolios proved to be effective survival strategies and buffers against the shocks and stresses brought about by lengthy periods of quarantine, and during times when mobility was restricted by the government in a bid to prevent the spreading of the disease. Drawing inspiration from the resilience literature, the paper captures the essence of these survival strategies, which should be viewed as latest reshuffling and expansion of diggers' rural livelihood portfolios. Policymakers and donors have yet to embrace fully these changes in a country where the Ebola recovery period promises to be lengthy and at a time when fresh, locally-informed rural development solutions are in short supply.
Conflict, Security and Develoment, Vol. 18, 6, pp. 463-492.
Africa, Sierra Leone
economics
Abstract: Shortcomings in natural resource governance leading to economic mismanagement, political clientelism, underdevelopment and civil conflict, have caused an increase in global norms of ‘good governance’ of natural resource sectors. As a result, a growing number of global governance initiatives are targeting conflict-prone natural resource sectors. Whether these regulatory efforts stand a chance of being successful depends on their implementation in producer countries. As the transnational regulatory framework aimed at curbing the trade in conflict minerals is expanding, this article investigates the local translation of global norms of resource governance. Drawing on the ‘local-to-global’ research perspective developed in this special issue and norm diffusion theories, the article examines one of the most prominent cases of governance reform targeting conflict-affected natural resource sectors: The Sierra Leonean diamond market. Based on extensive field research, the article analyses the implementation of KPCS requirements on the national and subnational level of governance institutions. It evaluates the accomplishments, the challenges and the local adaption to and (formal and informal) interpretation of KPCS norms.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 272-3.
Africa, Sierra Leone
geochronology
Abstract: Diamond ages are obtained from radiogenic isotopic analysis (Rb-Sr, Sm-Nd, Re-Os, and Ar-Ar) of mineral inclusions (garnet, pyroxene, and sulfide). As diamonds are xenocrysts that cannot be dated directly, the ages obtained on mineral inclusions provide a unique set of interpretive challenges to assure accuracy and account for preexisting history. A primary source of geological/mineralogical uncertainty on diamond ages is any process affecting protogenetic mineral inclusions before encapsulation in the diamond, especially if it occurred long before diamond formation. In practical application, the isotopic systems discussed above also carry with them inherent systemic uncertainties. Isotopic equilibrium is the essential condition required for the generation of a statistically robust isochron. Thus, isochron ages from multiple diamonds will record a valid and accurate age when the diamond-forming fluid promotes a large degree of isotopic equilibrium across grain scales, even for preexisting (“protogenetic”) minerals. This clearly can and does occur. Furthermore, it can be analytically tested for, and has multiple analogues in the field of dating metamorphic rocks. In cases where an age might be suspect, an age will be valid if its regression uncertainties can encompass a known and plausible geological event (especially one for which an association exists between that event and the source of diamond-forming fluids) and petrogenetic links can be established between inclusions on the isochron. Diamonds can be dated in six basic ways: 1. model ages 2. radiogenic daughter Os ages (common-Os-free) 3. single-diamond mineral isochrons 4. core to rim ages 5. multiple single-diamond isochron/array ages 6. composite isochron/array ages Model ages (1) are produced by the intersection between the evolution line for the inclusion and a reference reservoir such as the mantle. The most accurate single-diamond age is determined on a diamond with multiple inclusions (3). In this case an internal isochron can be obtained that not only establishes equilibrium among the multiple grains but also unequivocally dates the time of diamond growth. With extreme luck in obtaining the right diamond, concentric diamond growth zones visible in UV fluorescence or cathodoluminescence can sometimes be shown to constrain inclusions to occur in the core of the diamond and in the exterior at the rim. These single grains can be extracted to give a minimum growth time (4) for the diamond. In optimal situations, multiple inclusions are present within single growth zones, in single diamonds, allowing internal isochrons to be constructed for individual growth zones in single diamonds. If enough diamonds with inclusions can be obtained for study, valid ages for diamond populations can be obtained on multiple single-diamond ages that agree (5) or on composited, mineralogically similar inclusions to give an average age (6).
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, 1p. Abstract p. 307.
Africa, Sierra Leone
deposit - zimmi
Abstract: Type Ib diamonds from Zimmi, Sierra Leone, have 500 My mantle residency times whose origin is best explained by rapid tectonic exhumation after continental collision to shallower depths in the mantle prior to kimberlite eruption (Smit et al., 2016). Here we present spectroscopic data for a new suite of Zimmi sulfide-bearing type Ib diamonds that allow us to evaluate the link between their rare Fancy yellow colors, the distribution of their spectroscopic features, and their unusual geological history. Cathodoluminesence (CL) imaging revealed irregular patterns with abundant deformation lamellae, associated with the diamonds’ tectonic exhumation (Smit et al., 2018). Vacancies formed during deformation were subsequently naturally annealed to form vacancy clusters, NV0/? centers, and H3 (NVN0). The brownish yellow to greenish yellow colors observed in Zimmi type Ib diamonds result from visible absorption by a combination of isolated nitrogen and deformation-related vacancy clusters (Smit et al., 2018). Color-forming centers and other spectroscopic features can all be attributed to the unique geological history of Zimmi type Ib diamonds and their rapid exhumation after formation.
Gems & Gemology, Sixth International Gemological Symposium Vol. 54, 3, Fall 1p. Abstract p. 262-3
Africa, Sierra Leone
deposit - Zimmi
Abstract: Isolated nitrogen is one of the major defects in producing yellow color in natural diamonds. In regular type Ib yellow diamonds, isolated nitrogen is normally the dominant form, with limited aggregations in A centers (nitrogen pairs). Type Ib diamonds normally experienced strong plastic deformations. In addition to vacancy clusters, many other optic centers were introduced during annealing over their long geological history, such as GR1, NV, and H3 centers. Diamonds from the Zimmi area of West Africa are a typical example (Smit et al., 2016). As a result, clear brownish and greenish hues are common among these diamonds, so most do not possess true “canary” yellow color. Here we studied more than 2,000 diamonds with real canary yellow color. Their color origin and relationship with type Ib diamonds were explored. Sizes of the studied diamonds ranged from 0.01 to about 1.0 ct. They showed pure yellow color, with grades of Fancy Intense or Fancy Vivid yellow. Infrared absorption analysis showed that they were all type IaA with very high nitrogen concentrations, but a very weak absorption from isolated nitrogen at 1344 cm–1 was detected in all samples. Concentration of isolated nitrogen was estimated at ~2–3 ppm. This isolated nitrogen created smooth absorption in the ultraviolet-visible (UV-Vis) region, increasing gradually to the high-energy side. No other defects were detected using UV-Vis absorption spectroscopy, which explained the pure yellow color we observed. Fluorescence imaging revealed multiple nucleation centers with dominant green color, which was attributed to the S3 defects confirmed through photoluminescence analysis. Compared with natural type Ib diamonds, an outstanding feature of the studied samples is the absence of plastic deformation. For this reason, other vacancy-related defects were not introduced to these diamond lattices over the geological period after their formation. Sulfide inclusions are common in type Ib diamonds, but they were not observed in these canary stones. Instead, some calcite inclusions were observed. All the observations from this study indicated that the canary diamond samples were formed in a different geological environment than type Ib diamonds.
Abstract: Strontium and Pb isotopic compositions of clinopyroxene (cpx) in selected samples from three well-characterised eclogite suites with oceanic crustal protoliths (Lace/Kaapvaal craton, Orapa/Zimbabwe craton and Koidu/West African craton) were acquired by high-precision isotope dilution thermal ionisation mass spectrometry (ID-TIMS) and in situ multicollector-laser ablation-inductively-coupled plasma mass spectrometry (MC-LA-ICPMS). The aims of this study are twofold: (1) assess their utility to obtain formation or resetting age constraints and identify elemental signatures that enhance the chances of successful age dating, and (2) to confirm the veracity and utility of results obtained by novel MC-LA-ICPMS techniques. Strontium-Pb isotope systematics of eclogitic cpx measured in this study are decoupled and may reflect addition of unsupported radiogenic Sr during seawater alteration or interaction with oceanic sediments in subduction mélanges, and/or disturbance due to mantle metasomatism, to which the more incompatible Pb is more susceptible. Despite a complex history, subsets of samples yield meaningful model dates. Clinopyroxene fractions from Lace with high Pb contents (3-6?ppm), unradiogenic Pb isotopic compositions (206Pb/204Pb?=?13.57-13.52) and low 238U/204Pb (1.0-1.5) give single-stage model Pb dates of 2.90-2.84?Ga. In contrast, samples from Orapa plot to the right of the Geochron and do not yield meaningful Pb model ages. However, these data do define secondary isochrons that can be modelled to yield minimum age constraints on major events affecting the cratonic lithosphere. Within the uncertainties, the resultant 2.18?±?0.45?Ga age obtained for Koidu eclogites reflect disturbance of the Pb isotope system due to subduction beneath the craton linked to the Eburnean orogeny, while they retained their unradiogenic 87Sr/86Sr (0.7016). Similarly, the age for samples from Orapa (2.20?±?0.54?Ga) is interpreted as an overprint age related to Palaeoproterozoic accretion at the western craton margin. Gabbroic eclogites (Eu/Eu*?>?1) with plagioclase-rich protoliths having low time-integrated Rb/Sr and U/Pb retain the least radiogenic Sr and, in part, Pb. High model ? (9.0 to 9.1) for several eclogites from Lace with elevated LREE, Th and Pb abundances reflects ca. 3.0?Ga addition of a sedimentary component, possibly derived from reworking of a high-? basaltic protocrust, as observed on other cratons. We suggest that sample targeting can be usefully guided by fast-throughput in situ LA-ICPMS techniques, which largely yield results identical to ID-TIMS, albeit at lower precision, and which can further help identify kimberlite contamination in the mineral separates used for solution work.
Chemical Geology, doi.org/10,1016/j.chem geo.2019.04.014 37p.
Africa, Sierra Leone
deposit - Zimmi
Abstract: Here we present SIMS data for a suite of Zimmi sulphide-bearing diamonds that allow us to evaluate the origin and redox-controlled speciation of diamond-forming fluids for these Neoproterozoic eclogitic diamonds. Low ?13C values below ?15‰ in three diamonds result from fluids that originated as carbon in the oceanic crust, and was recycled into the diamond-stable subcratonic lithospheric mantle beneath Zimmi during subduction. ?13C values between ?6.7 and ?8.3‰ in two diamonds are within the range for mantle-derived carbon and could reflect input from mantle fluids, serpentinised peridotite, or homogenised abiogenic and/or biogenic carbon (low ?13C values) and carbonates (high ?13C values) in the oceanic crust. Diamond formation processes in eclogitic assemblages are not well constrained and could occur through redox exchange reactions with the host rock, cooling/depressurisation of CHO fluids or during H2O-loss from CHO fluids. In one Zimmi diamond studied here, a core to rim trend of decreasing ?13C (?23.4 to ?24.5‰) and decreasing [N] is indicative of formation from reduced CH4-bearing fluids. Unlike mixed CH4-CO2 fluids near the water maximum, isochemical diamond precipitation from such reduced CHO fluids will only occur during depressurisation (ascent) and should not produce coherent fractionation trends in single diamonds that reside at constant depth (pressure). Furthermore, due to a low relative proportion of the total carbon in the fluid being precipitated, measurable carbon isotopic variations in diamond are not predicted in this model and therefore cannot be reconciled with the 1‰ internal core-to- rim variation. Consequently, this Zimmi eclogitic diamond showing a coherent trend in ?13C and [N] likely formed through oxidation of methane by the host eclogite, although the mineralogical evidence for this process is currently lacking.
Abstract: Bimineralic eclogites and pyroxenites (n?=?75?±?accessory rutile, ilmenite, sulphide, apatite) from the Koidu kimberlite (West African Craton) were investigated for mineral major and trace elements and mineral Sr-Nd isotope compositions to constrain (1) the nature and age of their basaltic to picritic protoliths, and (2) the effect, timing and source of mantle metasomatism. Consistent with published work, samples are grouped into low-Mg eclogites with Mg# from 0•49 to 0•73 (median 0•59; n?=?40) and high-Mg eclogites with Mg# from 0•60 to 0•88 (median 0•75; n?=?14), plus pyroxenites [clinopyroxene Na/(Na + Ca) <0•2; n?=?8] and gabbroic eclogites and pyroxenite (Eu/Eu* of reconstructed bulk-rocks >1•05; n?=?8), with five unclassifiable samples. Reconstructed low-Mg and gabbroic eclogites have major and trace element systematics (Eu/Eu*-heavy rare earth elements-Y) indicating crustal protolith crystallisation, confirming an origin as subducted oceanic crust. Their high FeO contents at MgO >?10?wt % require an Fe-rich source, the high melt productivity of which led to the formation of thicker crust, perhaps in a plateau-like setting. This is consistent with SiO2-MgO relationships indicating differentiation at ?0•5?GPa. Unradiogenic Sr in some clinopyroxene (87Sr/86Sr of 0•7010-0•7015), combined with light rare earth element (LREE) depletion relative to normal mid-ocean ridge basalt (N-MORB) for the majority of samples (average N-MORB-normalised Nd/Yb of unmetasomatised samples = 0•51), suggests eclogitisation and partial melt loss in the Neoarchaean, possibly coeval with and parental to 2•7?Ga overlying continental crust. Most reconstructed high-Mg eclogites and some pyroxenites formed by metasomatic overprinting of low-Mg eclogites and gabbroic eclogites, as indicated by the preservation of positive Eu anomalies in some samples, and by the Mg-poorer composition of included versus matrix minerals. Coupled enrichment in MgO, SiO2 and Cr2O3 and in incompatible elements (Sr, LREE, Pb, Th and U) is ascribed to metasomatism by a kimberlite-like, small-volume, carbonated ultramafic melt, mediated by addition of clinopyroxene from the melt (i.e. stealth metasomatism). Strontium-Nd isotope systematics suggest a Neoproterozoic age for this metasomatic event, possibly linked to Rodinia break-up, which facilitated intrusion of asthenospheric carbonated melts with an ocean island basalt-like 87Sr/86Sri of ?0•7035. Cretaceous kimberlite magmatism (including Koidu), with more radiogenic 87Sr/86Sr (?0•7065, intermediate between Kaapvaal kimberlites and orangeites), may have been partially sourced from associated Neoproterozoic metasomes. The presence of diamonds in low-Mg eclogites, but absence in high-Mg eclogites, indicates the diamond-destructive nature of this event. Nevertheless, the moderate proportion of affected eclogites (?35%) suggests preservation of a sizeable diamond-friendly mantle eclogite reservoir beneath Koidu.
Africa, Sierra Leone, Democratic Republic of Congo, Angola
legal
Abstract: This article investigates the impact of the world price of a “lootable,” labor-intensive natural resource on intensity of violent conflict. Results suggest that a price increase can have opposite effects at different geographical levels of analysis: a decrease in conflict intensity overall in resource-rich countries, but an increase in conflict intensity in resource-rich subnational regions. The article argues that intensity of violence decreases overall due to rising opportunity costs of rebellion but that violence concentrates in resource-rich areas as returns to looting rise. The article introduces a new measure of diamond propensity based on geological characteristics, which is arguably exogenous to conflict and can capture small-scale labor-intensive production better than existing measures. The stated effects are found for secondary diamonds, which are lootable and related to opportunity costs of fighting, but not for primary diamonds, which are neither.
Review of African Political Economy, Vol. 45, no. 158, pp. 522-540.
Africa, Sierra Leone
history
Abstract: This article explores the relationship between the 2014-2016 Ebola outbreak and the political economy of diamond mining in Kono District, Sierra Leone. The authors argue that foreign companies have recycled colonial strategies of indirect rule to facilitate the illicit flow of resources out of Sierra Leone. Drawing on field research conducted during the outbreak and in its aftermath, they show how this ‘indirect rule redux’ undermines democratic governance and the development of revenue-generation institutions. Finally, they consider the linkages between indirect rule and the Ebola outbreak, vis-à-vis the consequences of the region’s intentionally underdeveloped health care infrastructure and the scaffolding of outbreak containment onto the paramount chieftaincy system.
Abstract: Diamondiferous rock types worldwide are broadly divided into kimberlite and lamproite, the latter of which have unique characteristics in different regions and include carbonate-rich varieties (formerly orangeites/Group II kimberlites). Diamondiferous rocks in West Africa are typically micaceous and share petrographic, mineralogical, and geochemical characteristics with both kimberlites and lamproites. To further constrain the classification and petrogenesis of diamondiferous rocks worldwide and their variability between different cratonic regions, in this study we combine detailed petrographic observations with olivine, phlogopite, and spinel chemistry for hypabyssal samples from the Jurassic Tongo dike (Sierra Leone) and the Neoproterozoic Weasua cluster (Liberia). The Tongo dike contains macrocrysts of olivine and phlogopite in a groundmass of olivine, abundant phlogopite, spinel, perovskite, and apatite with a base of calcite, dolomite, and lesser serpentine. The phlogopite is characterised by concurrent FeO and Al2O3 enrichment, which is typical of kimberlites and unlike lamproites. These features and the kimberlite-like spinel compositions allow us to classify the Tongo samples as micaceous kimberlites. The Weasua rocks comprise macrocrysts of olivine in a groundmass of olivine, phlogopite, diopside (zoned towards aegirine-rich rims), spinel, perovskite, and apatite with a base of serpentine and less common calcite. The composition of Weasua phlogopite trends to significant FeO enrichment and Al2O3 depletion, i.e. towards tetraferriphlogopite. The enrichment in mica, phlogopite chemistry and presence of magmatic diopside indicates that these rocks are olivine lamproites. The populations of olivine macrocrysts and microcrysts at Tongo and Weasua are similar and characterised by distinct core and rim zones. Two distinct olivine core populations are observed. 1) forsterite-rich (Fo?>?90) olivine interpreted to reflect xenocrysts from typical mantle peridotites. Al-in-olivine thermometry suggests that these cores have P-T equilibration within diamond stability at Weasua and Tongo. 2) Al-, Ca- and Na- rich cores with P-T formation conditions extending beyond the mantle adiabat. These cores are interpreted to reflect metasomatic and thermal perturbation linked with the infiltration of kimberlite/lamproite melts in the deep lithosphere shortly before entrainment in the ascending magma. The olivine rims at Tongo and Weasua show limited variations in Fo contents at similar values of 88.9?±?0.8 for Tongo and 89.6?±?1.2 for Weasua, as well as similar minor and trace element concentrations. Thus, whereas the Tongo and Weasua rock types are classified as kimberlite and olivine lamproite, respectively, the olivine chemistry suggests a similar petrogenetic evolution.
Geochimica et Cosmochimica Acta, Vol. 323, pp. 20-39.
Africa, Sierra Leone
deposit - Koidu
Abstract: Inclusion-bearing diamonds from the Koidu kimberlite complex, Sierra Leone (West African Craton) were analyzed in situ for carbon and nitrogen isotope compositions, nitrogen concentrations and nitrogen aggregation states. In a suite of 105 diamonds, 78% contain eclogitic mineral inclusions, 17% contain peridotitic mineral inclusions, and 5% - an unusually high proportion - contain co-occurring eclogitic and peridotitic mineral inclusions indicating a mixed paragenesis. Major and trace element compositions of mineral inclusions from two mixed paragenesis diamonds (one with omphacite + Mg-chromite, the other with eclogitic garnet + forsteritic olivine) were determined. The presence of positive Eu anomalies in centrally located omphacite and eclogitic garnet inclusions indicates derivation from subducted protoliths, formed as igneous cumulates in lower oceanic crust. Mg-chromite (Cr# 85.5; Mg# 65.2) and olivine (Mg# 94.5) inclusions, located in outer portions of the mixed paragenesis diamonds, have compositions indicative of derivation from strongly depleted cratonic peridotites. Given that the olivine Mg# of 94.5 is the highest reported to date for the West African Craton, the eclogitic and peridotitic inclusions in these mixed paragenesis diamonds cannot have precipitated during infiltration of peridotitic substrates by eclogite-derived fluids, as the consequent fluid-rock interaction should lead to Mg# lower than that for the original peridotitic diamond substrate. The different origins of eclogitic and peridotitic inclusions could be explained by physical transport of their host diamonds from eclogitic into peridotitic substrates, possibly along high-strain shear zones, before renewed diamond growth. Based on the ?¹³C-?¹?N systematics of the entire inclusion-bearing diamond suite from Koidu, three major compositional clusters are identified. Cluster 1 (eclogitic diamond cores; ?¹³C = -33.2 to -14.4 ‰ and ?¹?N = -5.3 to +10.1 ‰) bears the isotopic signature of recycled crustal material (± a mantle component). Cluster 2 (peridotitic diamonds and including the core of a diamond containing omphacite + Mg-chromite; ?¹³C = -6.0 to -1.1 ‰ and ?¹?N = -4.2 to +9.7 ‰) likely involves mixing of carbon and nitrogen from subducted and mantle sources. Cluster 3 (rims of eclogitic diamonds and including the eclogitic garnet + olivine included diamond and the rim of the omphacite + Mg-chromite included diamond; ?¹³C = -7.8 to -3.6 ‰ and ?¹?N = -7.9 to -2.1 ‰) matches convecting mantle-derived fluids/melts. The distinct isotopic signatures of the three diamond clusters, together with differences in nitrogen aggregation and cathodoluminescence response between diamond cores and rims, suggest episodic diamond growth during multiple fluid/melt pulses.
