Diamond Potential Research Articles

Kimberlites and lamproites of the East European Platform: Petrology and geochemistry

Several episodes of kimberlite magmatism occurred in the East European Province (EEP) during a long (about 1.5 Gyr) time period, from the Late Paleoproterozoic (ca. 1.8 Ga) in the Archean Ukrainian and Baltic shields to the Middle Paleozoic (ca. 0.36 Ga) mainly in the Arkhangelsk, Timan, and adjacent regions. Based on the analysis of data on 16 kimberlite occurrences and four lamproite occurrences within the EEP, five time stages can be distinguished; one of them, the Middle Paleozoic stage (Middle Ordovician and Devonian), is the most productive epoch for diamond in the northern hemisphere (EEP, Siberian Craton, and part of the China Craton). The analysis of petrological and geochemical characteristics of kimberlites (lamproites were studied less thoroughly) revealed variations in rock composition and their correlation with a number of factors, including the spatial confinement to the northern or southern Archean blocks of the craton, time of formation of the source of kimberlite melts, contents of volatiles and autoliths, etc. Three petrogeochemical types of kimberlites were distinguished: high-, medium-, and low-Ti (TiO2 > 3 wt %, 1–3 wt %, and <1 wt %, respectively). There are two time intervals of the formation of kimberlite and lamproite sources in the EEP, corresponding to TNd(DM) values of about 2 Ga (up to 2.9 Ga in the Por’ya Guba occurrence) and 1 Ga. The latter interval includes two groups of occurrences with model source ages of about 1 Ga (low-and medium-Ti kimberlites of the Zolotitsa and Verkhotina occurrences) and about 0.8 Ga (high-Ti kimberlites of the Kepino and a number of other occurrences); i.e., there seems to be an evolutionary trend in the composition of kimberlites. Concentric zoning patterns were recognized. The role of the crust in kimberlite sources is discussed; it is assumed that buried remnants of the oceanic lithosphere (megaliths) may underlie whole continents. A unique feature of the composition of low-Ti kimberlites, for instance, kimberlites of the Zolotitsa occurrence (to a smaller extent, medium-Ti kimberlites of the V. Grib pipe) is the distinct depletion of highly charged elements and pronounced negative anomalies of Ti, Zr, Th, U, Nb, and Ta in trace-element distribution patterns, which indicates a contribution of crustal material to the source of these kimberlites. It was shown that autoliths exert a significant influence on the differentiation of kimberlite material, resulting in the enrichment of rocks in the whole spectrum of incompatible elements. It was argued that geochemical criteria can be used together with traditional criteria (including those based on indicator minerals) for the assessment of diamond potential in EEP occurrences. We hope that such a combined approach will yield important outcomes in the future.

Discovery of microdiamonds and associated minerals in the Makhtesh Ramon Canyon (Negev Desert, Israel)

The Makhtesh Ramon erosional‐tectonic depression (canyon), 40 km long and approximately 8 km wide, is situated in the Negev Desert (southern Israel), 65 km southwest of the Dead Sea. The canyon rim and center are located at an altitude of 800‐1000 and 400‐ 600 m, respectively. In terms of tectonics, this structure represents the southern margin of the Syrian-Sinai [1] block‐fold belt. A comprehensive analysis of the available regional geophysical data revealed a collisionrelated origin of the belt [2]. Its formation terminated approximately 132 Ma ago, i.e., during the Levantine tectonic phase [3].The subsequent origination of the Levantine marginal volcanic suture [4] promoted the accumulation of postcollisional trap rocks (alkaline basaltic, alkaline gabbroic, and kimberlitic associations). The kimberlitic association, which was first discovered in western Syria, is represented in the study region by diatremes filled with kimberlite-type rocks containing eclogitic xenoliths [5]. Maximal magmatism in the region occurred approximately 120 Ma ago, which corresponds to the epoch of global tectonothermal activation (the Middle Cretaceous maximum in the development of upper mantle hot spots [7]). In the Makhtesh Ramon area, this stage is reflected in central-type volcanoes composed of lava flows, eruptive breccia, and tuffs that suggest wide development of explosive volcanism in the Middle Cretaceous [8]. Vent facies of the central-type Mahale Khahatsmaut Depression contain peridotite xenoliths with spinel-bearing lherzolites [9]. The presence of such rocks indicates significant depths of explosive volcanism. Magmatic rocks of the western Makhtesh Ramon Canyon are represented by picritic and olivine basalts and alkaline picrites. It should be noted that the development of these rocks does not rule out the presence of kimberlites in the study area. Detailed analysis of the magnetic (including quantitative interpretation and 3D modeling) and gravity fields indicated the possibility of discovery of parental diamondiferous bodies in the Makhtesh Ramon area. The low heat flow (~35 mW/m 2 ) in southern Israel [11] is an additional indicator of diamond potential in the area. The first diamond microcrystal discovered in the Negev Desert 30 km north of Makhtesh Ramon (Israel) in the Cretaceous/Paleogene boundary iridium layer was attributed to a meteorite fall. However, the habit, size, and characteristic impurities in the microcrystal suggest that it was washed out of diamondiferous bedrocks.

Minerals of the Kimberlitic Assemblage in Terrigenous Sediments of Latvia and Perspectives of Its Diamond Potential

Four intermediate collectors of minerals of the kimberlitic association (MKA) are identified in the Latvian territory. They are confined to Upper Devonian (Gauya, Ogre, and Ketleri formations) and Quaternary sediments. Each intermediate collector is characterized by a certain assemblage of minerals. The present communication considers specific features of the MKA distribution within each collector, as well as chemical composition and grain surface character of separate MKAs. Based on the study of typomorphism of minerals and peculiarities of sedimentation in different geological epochs, scheme of the localization of bedrock KMA sources is proposed. It is supposed that the Latvian territory incorporates three bedrock (kimberlite-type) sources with different prospects for diamond potential. The diamond potential of one source located in northeastern Latvia is rather high.

Investigating the diamond potential of southern West Greenland

Southern West Greenland hosts a province of ultramafic alkaline rocks, including swarms of dykes traditionally described as kimberlites and lamproites (Larsen 1991; Jensen et al. 2002). Since the mid-1990s, commercial diamond exploration has been focused on the Sarfartoq region and the region south-east of Maniitsoq (Fig. 1), and has resulted in numerous reports of diamond-favourable indicator minerals from till sampling, finds of kimberlitic dykes, and recovery of diamonds from kimberlitic rocks. A new digital compilation of company data released from confidential status (Jensen et al. 2003a) presents a comprehensive overview of exploration activities and results that have emerged since the Survey’s first compilation of occurrences of kimberlitic and related rocks (Larsen 1991). The new compilation in a GIS (geographic information system) environment allows for refined assessment of the distribution, structural control and possible spatial and petrogenetic relationships that characterise the kimberlitic occurrences. In 2003, the Geological Survey of Denmark and Greenland (GEUS) and the Government of Greenland’s Bureau of Minerals and Petroleum (BMP) went further than has been customary in investigating the economic potential of specific sites. Four areas were temporarily closed to application for exploration licences, pending sampling and testing for diamond content of large samples of more than one tonne each from significant kimberlitic occurrences. Additional characterisation and research initiated on these and other occurrences include magnetic mapping, detailed petrography and studies of mantle xenoliths, as well as indicator mineral chemistry. An extensive programme to determine the ages of kimberlitic and related rocks was also initiated in 2003.

Mapping the mantle lithosphere for diamond potential using teleseismic methods

Recent developments in seismic, magnetotelluric and geochemical analytical techniques have significantly increased our capacity to explore the mantle lithosphere to depths of several hundred kilometres, to map its structures, and through geological interpretations, to assess its potential as a diamond reservoir. Several independent teleseismic techniques provide a synergistic approach in which one technique compensates for inadequacies in another. Shear wave anisotropy and discontinuity studies using single seismic stations define vertical mantle stratigraphic columns. For example, beneath the central Slave craton seismic discontinuities at depths of 38, 110, 140 and 190 km appear to bound two distinct anisotropic layers. Tomographic (3-D) inversions of seismic wave travel-times and 2-D inversions of surface or scattered waves use arrays of stations and provide lateral coverage. In combination, and by correlation with electrical conductivity and xenolith petrology studies, these techniques provide maps of key physical properties within parts of the cratons known to host diamonds. Beneath the Slave craton, the discontinuity at 38 km is the base of the crust; the boundaries at 110 and 140 km appear to bound a layer of depleted harzburgite that is interpreted to contain graphite. To date, only some of these techniques have been applied to the Slave and Kaapvaal cratons so that the origin and geological history of the currently mapped mantle structures are not, as yet, generally agreed.

Petrogenesis of the Late Cretaceous northern Alberta kimberlite province

At present, 48 Late Cretaceous (ca. 70–88 Ma) kimberlitic pipes have been discovered in three separate areas of the northern Alberta: the Mountain Lake cluster, the Buffalo Head Hills field and the Birch Mountains field. The regions can be distinguished from one another by their non-archetypal kimberlite signature (Mountain Lake) or, in the case of kimberlite fields, primitive (Buffalo Head Hills) to evolved (Birch Mountains) magmatic signatures.The dominant process of magmatic differentiation is crystal fractionation and accumulation of olivine, which acts as the main criteria to distinguish between primitive and evolved Group I-type kimberlite fields in the northern Alberta. This is important from the viewpoint of diamond exploration because the majority (about 80%) of the more primitive Buffalo Head Hills kimberlites are diamondiferous, whereas the more evolved Birch Mountains pipes are barren of diamonds for the most part. Petrographically, the Buffalo Head Hills samples are distinct from the Birch Mountains samples in that they contain less carbonate, have a smaller modal abundance of late-stage minerals such as phlogopite and ilmenite, and have a higher amount of fresh, coarse macrocrystal (>0.5 mm) olivine. Consequently, samples from the Buffalo Head Hills have the highest values of MgO, Cr and Ni, and have chemistries similar to those of primitive hypabyssal kimberlite in the Northwest Territories. Based on whole-rock isotopic data, the Buffalo Head Hills K6 kimberlite has 87Sr/86Sr and εNd values similar to those of South African Group I kimberlites, whereas the Birch Mountains Legend and Phoenix kimberlites have similar εNd values (between 0 and +1.9), but distinctly higher 87Sr/86Sr values (0.7051–0.7063).The lack of whole-rock geochemical overlap between kimberlite and the freshest, least contaminated Mountain Lake South pipe rocks reflects significant mineralogical differences and Mountain Lake is similar geochemically to olivine alkali basalt and/or basanite. Intra-field geochemical variations are also evident. The K4 pipe (Buffalo Head Hills), and Xena and Kendu pipes (Birch Mountains) are characterized by anomalous concentrations of incompatible elements relative to other northern Alberta kimberlite pipes, including chondrite-normalized rare-earth element distribution patterns that are less fractionated than the other kimberlite samples from the Buffalo Head Hills and Birch Mountains. The Xena pipe has similar major element chemical signatures and high-Al clinopyroxene similar to, or trending towards, the Mountain Lake pipes. In addition, K4 and Kendu have higher 87Sr/86Sr and lower εNd than Bulk Earth and plot in the bottom right quadrant of the Nd–Sr diagram. We suggest, therefore, that the K4 and Kendu pipes contain a contribution from old, LREE-enriched (low Sm/Nd) lithosphere that is absent from the other kimberlites, are affected by crustal contamination, or both.Based on xenocryst populations, the northern Alberta kimberlite province mantle is dominated by carbonate-saturated lherzolitic mantle. Higher levels of melt depletion characterize the Buffalo Head Hills mantle sample. Despite high diamondiferous to barren pipe ratios in the Buffalo Head Hills pipes, mineral indicators of high diamond potential, such as G10 garnet, diamond inclusion composition chrome spinels and high-sodium eclogitic garnet, are rare.

Macrocrystal phlogopite Rb–Sr dates for the Ekati property kimberlites, Slave Province, Canada: evidence for multiple intrusive episodes in the Paleocene and Eocene

New Rb–Sr age determinations using macrocrystal phlogopite are presented for 27 kimberlites from the Ekati property of the Lac de Gras region, Slave Province, Canada. These new data show that kimberlite magmatism at Ekati ranges in age from at least Late Paleocene (∼61 Ma) to Middle Eocene time (∼45 Ma). Older, perovskite-bearing kimberlites from Ekati extend this age range to Late Cretaceous time (∼74 Ma). Within this age range, emplacement episodes at ∼48, 51–53, 55–56 and 59–61 Ma can be recognized. Middle Eocene kimberlite magmatism of the previously dated Mark kimberlite (∼47.5 Ma) is shown to include four other pipes from the east-central Ekati property. A single kimberlite (Aaron) may be younger than the 47.5 Ma Mark kimberlite. The economically important Panda kimberlite is precisely dated in this study to be 53.3±0.6 Ma using the phlogopite isochron method, and up to six additional kimberlites from the central Ekati property have Early Eocene ages indistinguishable from that of Panda, including the Koala and Koala North occurrences. Late Paleocene 55–56 Ma kimberlite magmatism, represented by the Diavik kimberlite pipes adjacent to the southeastern Ekati property, is shown to extend onto the southeastern Ekati property and includes three, and possibly four, kimberlites. A precise eight-point phlogopite isochron for the Cobra South kimberlite yields an emplacement age of 59.7±0.4 Ma; eight other kimberlites from across the Ekati property have similar Late Paleocene Rb–Sr model ages. The addition of 27 new emplacement ages for kimberlites from the Ekati property confirms that kimberlite magmatism from the central Slave Province is geologically young, despite ages ranging back to Cambrian time from elsewhere in the Slave Province. With the available geochronologic database, Lac de Gras kimberlites with the highest diamond potential are currently restricted to the 51–53 and 55–56 Ma periods of kimberlite magmatism.

Concepts for diamond exploration in “on/off craton” areas—British Columbia, Canada

The tectonic setting of British Columbia (BC) differs from classic diamond-bearing intracratonic regions such as the Northwest Territories and South Africa. Nevertheless, several diamond occurrences have been reported in BC. It is also known that parts of the province are underlain by Proterozoic and possibly Archean basement. Because the continents of today are composites of fragments of ancient continents, it is possible that some of the regions underlain by old crystalline basement in eastern British Columbia were associated with a deep crustal keel. The keel may have predated the break-up of the early Neoproterozoic supercontinent called Rodinia and was preserved possibly until the Triassic. Some of these old continental fragments may have been displaced relative to their position of origin and dissociated from their keel, or the keel may have since been destroyed. Such fragments represent favourable exploration grounds in terms of the “Diamondiferous Mantle Root” model (DMR model) if they were intersected by kimberlites or lamproites prior to displacement or destruction of their underlying deep keel. Therefore, extrapolation of fragments of the diamond-bearing Precambrian basement from the Northwest Territories or Alberta to BC provides a sufficient reason for initiating reconnaissance indicator mineral surveys. The “Eclogite Subduction Zone” model (ES model) predicts formation of diamonds at lower pressure (i.e., depth) than required by the DMR model in convergent tectonic settings. Although not proven, this model is supported by thermal modeling of cold subduction zones and recent discoveries of diamonds in areas characterized by convergent tectonic settings. If the ES model is correct, then the parts of BC with a geological history similar to today's “cold” subduction zones, such as Honshu (Japan), or to continental collision zones, such as Kokchetav massif (Kazakhstan) and the Dabie–Sulu Terrane (east central China), may be diamondiferous. The terranes where geological evidences suggest an ultrahigh pressure (UHP) metamorphic event followed by rapid tectonic exhumation (which could have prevented complete resorption of diamonds on their journey to the surface) are worth investigating. If UHP rocks were intercepted at depth by syn- or post-subduction diamond elevators, such as kimberlites, lamproites, lamprophyres, nephelinites or other alkali volcanic rocks of deep-seated origin, the diamond potential of the area would be even higher.

Assessing the diamond potential of kimberlites from discovery to evaluation bulk sampling

The economic evaluation of diamond-bearing kimberlites is usually carried out in four stages. Expenditure tends to increase by an order of magnitude at each successive stage. At the end of each stage, the sample results should be critically appraised before deciding to proceed to the next phase. In the first stage, even before individual kimberlite bodies have been discovered, the indicator mineral geochemistry will give a first rough idea of the diamond potential. The relative abundance of harzburgitic pyropes (subcalcic chrome-rich) is often directly correlated with the diamond grade. In the next stage, when the kimberlite body has been discovered, a relatively small sample of a few hundred kilograms will be enough to recover sufficient microdiamonds to allow an extrapolation of the size distribution towards the commercial-sized diamonds and a rough estimate of their grade. If positive, the third stage should be a limited bulk sampling programme (order of 200 tonnes) to determine the commercial-sized diamond grade, expressed as carats per tonne. The aim of the final stage is to obtain a parcel of the order of 1,000 carats to estimate the average commercial value of the diamonds. The robustness and reliability of the grade and value estimates can be verified with extreme value analysis and by obtaining the confidence limits with bootstrapping.

Diamond potential estimation based on kimberlite major element chemistry

A strong correlation between the chemical composition of kimberlites and their diamond grade has been established using statistical methods. The investigation is based upon major element analyses of kimberlite samples, from which xenoliths have been removed. The samples represent highly diamondiferous, moderately diamondiferous, slightly diamondiferous and diamond-free rocks from diatremes of the Yakutian Kimberlite Province. In total, the investigation comprises 3792 whole-rock analyses and 1176 determinations of diamond grade. By means of correlation analysis, the kimberlite chemical analyses were subdivided into 113 clusters. Regression analysis was carried out between mean major element oxide concentrations of the clusters and the corresponding average values of diamond contents. The combined use of the regression method with a break-point and generalized nonlinear discrimination analysis has allowed to correctly estimate the average diamond grade of kimberlite clusters in more than 85–90% of the cases. The established statistical relations are convincingly justified from a petrological point of view. The reliable petrochemical description of kimberlite variety, based on averaging 25–30 whole-rock analyses or on analyzing of a sample obtained as a result of sequential quartering of a much larger sample, allows to estimate safely its diamond grade using major oxide contents. The method has worldwide application potential.

THE PRESSURES AND TEMPERATURES OF FORMATION OF DIAMOND BASED ON THERMOBAROMETRY OF CHROMIAN DIOPSIDE INCLUSIONS

To date, the evaluation of the temperature ( T) and pressure (P) of formation of natural diamond has generally relied on thermobarometry of rare polymineralic inclusions containing appropriate assemblages of minerals. The results are commonly ambiguous, because of potential re-equilibration of touching minerals after diamond growth and possible disequilibrium between non-touching minerals. Here, I calculate T and P for over one-hundred inclusions of chromian diopside (mostly isolated) in diamond crystals containing inclusions of peridotitic material, and for peridotite xenoliths from worldwide occurrences, using single-clinopyroxene thermobarometers. The results provide constraints on the conditions and relative timing of diamond genesis . Inclusions in diamond and xenoliths from the same source commonly yield similar P‐T values, suggesting that diamond crystals formed when the lithospheric mantle had already attained a conductive thermal regime comparable to or even colder than that extant at the time of emplacement of the host kimberlite or lamproite. Some inclusions record thermal or metasomatic events, which can be ascribed to the ascent of hot C-rich fluids from which the diamond precipitated. In a few cases, secular cooling o f the cratonic lithosphere is believed to be a possible source of scatter in T estimates. In general, there is no evidence for occurrences of diamond being concentrated at particular levels in the lithosphere. Where significant gaps occur in the distribution of diam ond crystals with included lherzolitic material, they are associated with a scarcity of lherzolitic material in the mantle and do n ot necessarily correspond to a real absence of diamond crystals. The results support the use of chromian diopside thermobarometry as a complementary tool for assessment of diamond potential in exploration programs.

Diamond for Electronics: Future Prospects of Diamond SAW Devices

From a commercialization standpoint, electronic applications have been particularly elusive for diamond. Market estimates of $560 million per year by the year 2000 indicate the original enthusiasm in this area. Now such projections seem unreasonably optimistic, and even a niche commercial application in the area of electronics would be considered a success. However when taken in a broader context, this extended time frame for commercialization is not at all unusual for new technologies, and many new advances have continued to bolster the enthusiasm of diamond electronics research groups. Diamond has such an extensive list of exceptional properties that it continues to be a candidate for numerous electronic applications from heat spreaders to detectors to microvacuum tubes.A variety of theoretical calculations established diamond's potential in the early years of CVD-diamond research. As shown in Table I, figures of merit indicated that diamond's potential far exceeded the potential of more common semiconductors such as silicon and gallium arsenide for certain applications. Typically these applications revolved around high power or high temperature. However more accurate assessments of diamond's capability after further development uncovered several issues with many electronic applications of interest: (1) n-Type doping: Although scattered reports of n-type doping have been made a high-quality, low-resistance n-type material is not available, limiting the potential applications for diamond.(2) Deep activation of p-type carriers: At reasonable dopant levels, the activation energy for p-type carriers is approximately 0.3 eV, causing diamond to be highly resistive and sensitive to temperature variations at normal operating temperatures.

Evaluation of diamond potential from the composition of peridotitic chromian diopside

Evaluation of diamond potential from the composition of peridotitic chromian diopside

Subduction model for the origin of some diamonds in the Phanerozoic of eastern New South Wales

Eastern New South Wales has produced over 500 000 carats of diamonds, mostly from the Copeton‐Bingara area in the north. A local derivation is indicated by their distinct tribal character and lack of surface damage, while carbon isotopic values and composition of mineral inclusions are unlike those in diamonds from conventional diamond‐producing areas, for example Yakutia (Siberia), Kapvaal (South Africa), or Argyle (Western Australia). The eastern New South Wales tectonic setting is also unlike that for ‘conventional’ diamonds, involving a Phanerozoic sequence of accreted subduction terranes, with relatively thin hot crust. A subduction diamond model is developed to explain the origin and geology of eastern New South Wales diamonds. This model involves prolonged subduction of mature oceanic crust, resulting in the development of a low‐temperature metamorphic window into the diamond stability field within the downgoing slab at half the depth required by conventional models. The diamonds are preserved at depth by termination of subduction, and brought to the surface by obduction or by entrainment in suitable magmas. The type of diamond formed depends on the original source rock (diamond affinity is shown in brackets): carbonaceous‐rich and carbonaceous‐poor trench sediments (CaFeMgMn‐silicate, eclogitic), graphite‐poor basalt and gabbro (eclogitic) and lherzolite (peridotitic). The model predicts that the New South Wales diamonds are young (Phanerozoic) and that their features, age, associated minerals and geographic distribution relate to New South Wales tectonic provinces. The subduction diamond model extends the range of indicator minerals to include macro‐diamond, corundum, Na‐garnet, with a new series of carrier magmas (basanite, nephelinite, leucitite). Unsuccessful previous exploration in New South Wales, based on conventional models, has produced some positive results when reinterpreted in terms of the subduction diamond model. A re‐evaluation of diamond potential of eastern New South Wales is warranted.

Diamond exploration and mantle structure imaging using PIXE microanalysis

The compositions of garnets, sampled throughout the earth's upper mantle and carried to the surface in volcanic eruptions, show PIXE trace element signatures that reveal details of the chemistry, thermal structure, and history of fluid alteration, of their source rocks. Trace Ni concentration (5–150 ppm) is strongly temperature dependent, providing a useful geothermometer ( T Ni). Cr concentration can be used to estimate pressure ( P Cr) in Cr-saturated rocks. Trace Zn in chromite also provides a measure of temperature ( T Zn), and shows the extent of Cr-saturation (chromite present). For those garnets from a particular volcanic pipe originating in Cr-saturated rocks in the upper-mantle, T Ni, P Cr maps out the paleogeotherm (the Garnet Geotherm), that is the relationship between T and P (and depth) prevailing at the time of eruption. Ti and trace Zr, Y, and Ga show the effects of metasomatic fluids and deep mantle-derived melts, and show clearly the depth extent of depleted ( Y < 10 ppm) rocks which helps to define the effective base of the lithosphere, the earth's solid outer shell. The Garnet Geotherm and the base of the lithosphere then define the range of depths within the diamond stability field traversed by the erupting magma, and the histogram of T Ni provides a direct measure of the sampling in this range and hence the diamond potential of the pipe. The result is both a powerful diamond exploration tool and a method to map the composition and evolution of the earth's mantle.

The role of petrography and lithogeochemistry in exploration for diamondiferous rocks

Rock samples obtained during an exploration program may be identified and assessed as to their diamond potential by a combination of transmitted light petrography, back scattered electron imagery and energy dispersive X-ray spectrometry. Fresh and slightly altered potentially-diamondiferous rocks such as kimberlite, orangeite and lamproite may be distinguished from petrographically superficially similar un-economic lamprophyres using mineralogical genetic classifications of the former. Highly altered rocks or diatreme and crater facies rocks may be identified from relict textures and oxide minerals or volcanogenic clasts respectively, by similar methods. Whole rock major and trace element compositions are of little use in distinguishing between various groups of diamondiferous rocks but are useful in distinguishing these from highly altered un-economic rocks. Mineralogical criteria for the identification of diverse alkaline ultramafic rocks are presented.

The interpretation of the major element compositions of mantle minerals in diamond exploration

Interpretation of the major element compositions of the mantle macrocryst minerals recovered during stream sediment or soil sampling should be one of the corner-stones of any modern exploration programme for diamonds. Studies must be implemented in such a way as to elucidate the maximum information about the lithospheric mantle underlying the exploration area of interest. This can provide a qualitative or at best semi-quantitative estimate of diamond potential. The composition of a suite of peridotitic garnets from a region or locality is particularly useful in differentiating between diamond bearing and non-diamondiferous mantle samples. Assessments of macrocryst composition are a means of prioritising exploration efforts useful over an extensive period of an active exploration programme, and in a variety of different sets of circumstances. Differences in mantle mineral suite compositions may also be used to identify and isolate specific exploration targets for further attention in areas where numerous potential diamond sources exist.

Lithosphere Evolution in Rifted, Craton, and Mobile-Belt Environments from Zimbabwe and Implications for Diamond Potential

Three major components with different tectonic settings illustrate the diamond potential of Zimbabwe. These are regions that kimberlites have intruded: the stable Archaean craton, the Archaean to Proterozoic Limpopo belt, and the Mesozoic rift basin of the Zambezi Valley. Geotectonic considerations, supported by the presence of kimberlite mantle minerals, lead to the prediction that the Archaean craton should have a thick mantle root with some diamond potential. This mantle lithosphere is less diamondiferous and is characterized by garnets that are less Ca-depleted than those from beneath other Archaean cratons, such as the Kaapvaal craton, suggesting the presence of a correspondingly less-depleted peridotite. Kimberlite mantle minerals show that the Limpopo belt has a thick, diamondiferous (and probably ancient) mantle root containing highly Ca-depleted peridotite. This is considered to have formed prior to, and been preserved beneath, the Limpopo belt. As predicted by the rifting history, the Zambezi Va...

A spectrum of potentially diamondiferous lamproites and minettes from the Jharia coalfield, eastern India

Abundant Cretaceous (≈ 110 Ma) lamprophyric sills in the Gondwana coalfields show a comagmatic continuum from olivine-lamproites to lamproites and minettes. Four sills in the Jharia coalfield have exotic mineralogy, dominated by olivine (Fo 85–92) and phlogopite (5–10% TiO 2, 5–13% Al 2O 3), with subordinate potassium (titanian) richterite and/or arfvedsonite, and spectacularly zoned Sr-rich apatite (up to 20% SrO). Accessory to minor phases include clinopyroxene (titanian diopside to aegirine-augite), K-feldspar (Or > 96), variable spinel (up to 47% Cr 2O 3), perovskite, niobian ilmenite, niobian rutile, sphene and Ba-poor priderite. Complex primary carbonate assemblages involve breunnerite, ferroan dolomite, witherite, alstonite and ?strontianite. The whole rocks show low Al 2O 3 (5–7%) combined with high K 2O (4–5%), MgO (up to 30%) and Cr (up to 1500 ppm), plus extreme TiO 2 (up to 8.5%), Sr, Zr (up to 6000 ppm each), Ba (up to 8000 ppm), and LREE (Ce up to 3000 ppm). In many rocks, K/Al (molecular) > 0.7, (Na + K)/Al > 1, and K/(Na + K) > 0.6. Radiogenic isotopes ( ϵ Sr + 7 to + 21 and ϵ Nd − 1.5 to − 3.1) imply derivation from a metasomatized lithosphere mantle source with time-integrated Rb/Sr and Sm/Nd slightly higher than Bulk Earth. Intensive carbonation (up to 14% CO 2) in some rocks may reflect fluids derived from the magmas themselves, from carbonatite-like fluids, and perhaps from coking of the invaded coal seams. Minor crystal fractionation within sills as little as ≈ 2.5 m thick has yielded complementary olivine-lamproites and feldspathic lamproites; the latter grade ultimately into minettes via increase of Al 2O 3 and decrease of MgO. Positive indicators of diamond potential in these rocks include their bulk compositional similarity to diamondiferous olivine-lamproites elsewhere (e.g., Argyle), their Cr-rich spinels, and perhaps their tectonic position; negative factors include their quiet emplacement, the noted differentiation, and the associated coking of the coal seams.

Olivine: a monitor of magma evolutionary paths in kimberlites and olivine melilitites

Petrographic and chemical criteria indicate that the overwhelming majority of olivines in kimberlites are probably cognate phenocrysts. The implied low volume of xenocryst olivines requires that primitive kimberlite magmas are highly ultrabasic liquids. Two chemically distinctive olivine populations are present in all of the kimberlites studied. The dominant olivine population, which includes large rounded olivines and smaller euhedral crystals, is Mg-rich relative to late-stage rim compositions. It is characterized by a range in 100 Mg/(Mg + Fe) and uniform Ni concentration, reflecting Rayleigh-type crystallization during magma evolution. The most Mg-rich of these olivines are considered to be similiar to those in the mantle source rocks. The second compositional population, generally very subordinate, though markedly more abundant in the megacrystrich Monastery kimberlite, is Fe-rich relative to rim compositions. This group of olivines crystallized from evolved liquids in equilibrium with iron-rich megacrysts, both entrained by the kimberlite magma during ascent. Differences between the chemical fields of Fe-rich olivines in Group I and Group II kimberlites point to relatively deeper derivation of the latter suite. Olivine chemistry can be used to characterize kimberlite magma sub-types, and may prove to be a useful tool for evaluating the diamond potential of kimberlites.