MODELS OF REFLECTION OF KIMBERLITE PIPES OF NORTH-EAST OF BOTSWANA IN EOLIAN HALOES OF DISPERSION

MODELS OF REFLECTION OF KIMBERLITE PIPES OF NORTH-EAST OF BOTSWANA IN EOLIAN HALOES OF DISPERSION

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  MODELS OF REFLECTIONOF KIMBERLITE PIPES OF NORTH-EAST OF BOTSWANA IN EOLIAN HALOES OF DISPERSION V. Ustinov1 , B. Mosigi2 , I. Kukui1 , E. Nikolaeva1 , J. Campbell2 , Yu. Stegnitskiy3 , M. Antashchuk1 1 Diamond Department, Central Research & Exploration Institute (CNIGRI), Saint-Petersburg, Russia, [email protected], [email protected] 2 Botswana Diamonds plc, London, UK, [email protected], [email protected] 3 ALROSA PJSC, Saint-Petersburg, Russia, [email protected] Abstract This paper considers the structure of eolian haloes over kimberlite pipes overlain by Kalahari sands of up to 20 m in thickness. It studies the change of the content, morphology and abrasion of kimberlite indicator minerals (KIMs) with distance from the primary sources in the Orapa kimberlite field in the North-East of Botswana. Based on the data of heavy mineral sampling of surface sediments models of reflection of Late Cretaceous kimberlite pipes of the Orapa field in eolian haloes of KIMs dispersion have been created. The results of the research can be used in diamond prospecting in various regions with similar environments. Keywords Kimberlite indicator minerals; pipe; eolian halo of KIMs; degree of abrasion; Botswana. Introduction The group of eolian haloes of dispersion of KIMs (pyrope, picroilmenite and others) is widely represented on the territories with arid climate. Its present examples primarily being Kalahari, Namib and Sahara sand deserts. An important relief-forming agent is wind, its activity being the major factor of haloes formation in dry tropics. It has been established that the buried kimberlite bodies can manifest themselves in the overlaying eolian sands as dispersion haloes (Baumgartner, Neuhoff, 1998 and other). The denudation of kimberlites due to erosion, rainwash and deflation of surface formations results in KIMs release and concentrating on the weathered surface, which is a major factor of their subsequent redeposition into Kalahari sands. One of the ways of delivery of kimberlite minerals to the surface should be considered is termites activity. Another factor of KIMs grains input into the overlaying deposits is the low-contrast relief of the surface which results in their redeposition into the sand section parts of the relevant hypsometric level. The third factor is considered to be the eolian process which may result both in concentration of KIMs in sands and their downward and upward redeposition within the forms of the sand relief as a result of mixing. The grain relocation amplitude is likely to be commensurate with the amplitude of the existing accumulative relief, which amounts to average 10-20 m. This will result both in abrasion type of mechanical spalling (pitting) on the surfaces of kimberlite minerals and “candy” sculptures. Two categories are identified among sandy formations of aridic areas based on their mobility degree: stationary, localized within the areas of development of the sources of the fragmentary material, and mobile, formed by the sands brought from remote sources. Kimberlite minerals and diamonds identified in eolian deposits in tropical arid climate conditions can be transported in varying distances from the primary sources (Ustinov 2015). Specific features of formation of eolian haloes of dispersion of kimberlite indicator minerals In most cases KIMs dispersion trains consist of eolian haloes of varying travel formed as a result of the denudation of kimberlite bodies located at various distances from the area of accumulation (Fig. 1). As a consequence, the haloes contain pyropes, picroilmenites and other kimberlite minerals of different degrees (indexes or classes) of abrasion: I - unabraded, II - slightly, III - moderately, IV – extensively, V - very extensively abraded (Ustinov et al. 2015). Eolian haloes of long distance travel are represented by grains of pyropes and picroilmenites of IV-V degrees of abrasion. They reveal mineralogical zonation manifested by an increased share of pyropes through reduction of
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  picroilmenites which havea higher density and deposit earlier in terrigenous sediments under equal transport conditions. Eolian haloes of moderate travel are mostly characterized by small (-1+0.5 mm, -0.5 mm) single grains per 20-liter sample of pyropes of I and II degrees of abrasion and rare grains of picroilmenites of III degree of abrasion. Small (- 0.5 mm) extensively rounded chrome-diopsides can occur sometimes. Eolian haloes of short travel represented by KIMs mostly of I-II abrasion indexes show a proximity of kimberlite bodies. Their identification among kimberlite minerals dispersion trains is the most important task in prospecting for primary diamond deposits. A typical example of eolian haloes of dispersion is Orapa kimberlite field (Fig. 2). Fig. 1 Joint presence of kimberlite minerals (picroilmenites and pyropes) of various degrees of abrasion (I-V) in eolian surface sediments of North-Eastern Botswana The sediments of the surface complex of Kalahari Group mostly represented by eolian formations which were formed as a results of the reworking of illuvial, talus, proluvial, deltaic and beach sands, gravels and conglomerates. In some places these sediments are intensively calcretized and silcretized. The basal horizon formations intersected by drill holes are likely to refer to talus or proluvial facies which have also undergone wind reworking and subsequent hypergentetic alterations. In the North-East of Botswana the Kalahari Group formations overlaying kimberlite pipes and Jurassic basalts of Karoo Supergroup are composed of two rock varieties. The lower part of the section is represented by calcretes from 5 to 17 m thickness while the upper part was made up of silty, inequigranular oligomictic quartz-feldspathic sands of eolian origin often containing fragments and debris of the underlying calcrete. The thickness of the sands varies from 3 to 5 m. Kimberlite-enclosing basalts of Stormberg Group have variable thickness - from 10 to 140 m on the territory of Orapa field. They create a strong barrier preventing the upward movement of kimberlitic magma. With regard to basalts the kimberlite bodies are divided into exposed (“opened”), semi-exposed (“semi-opened”) and embryos (“blind”) bodies. Among 85 bodies of Orapa field only few pipes (АК 1, АК 2, АК 3 and some others) are “opened” and have comparatively well-preserved crater facies. The majority of the bodies are “semi-opened” and represented by kimberlites of diatreme and hypabyssal facies. Based on the study of a set of heavy mineral concentrate sampling data (more than 8 000 samples) on the territory of Orapa kimberlite field it was found out that the haloes of KIMs from exposed and semi-exposed (partially breaking through the rocks of kimberlite-containing basement) kimberlite pipes are expressed in different ways (Table 1). Pipes-embryos, not breaking through, but only brecciate or injected basalts of the Karoo, are not shown by the haloes of KIMs in the surface sediments. Haloes of dispersion in the area of the Orapa field are characterized by constant presence of extensively abraded pyropes and (or) picroilmenites (up to 10 gr./20 l) which form a general mineralogical blanket. They are accumulated also in geomorphological traps, where there is a higher content of minerals (20-30 or more gr./20 l). Higher contents of KIMs are located close to pipes. Contents of KIMs decline rapidly at a distance of several hundreds of meters.
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  Fig. 2 Eolian haloesof dispersion of KIMs in the area of Orapa kimberlite field 1-7 – forms and elements of the Neogene-Quaternary relief: megaforms (1 – subhorizontal low accumulative plain, 2 – gently-sloping denudation-accumulative plain, 3 – slightly elevated denudation-accumulative plain); macroforms (4 – depression: Makgadikgadi, M-G; 5 – elevation: Letlhakane, LH); 6 – boundaries of macroforms; mesoforms (7 –river valleys and paleovalleys); 8 – isopaches of the Neogene-Quaternary sediments of Kalahari Group (a – reliable, b – supposed); 9-11 – eolian haloes (more than 10 gr./20 l) of dispersion of KIMs formed due to wind reworking of: 9 – talus and proluvial sediments, 10 – deltaic, 11 – continental and basinal of different genesis; 12-14 – directions of transportation of kimberlite minerals: 12 – water streams, 13 – wind, 14 – waves; 15 – standing levels of an inland basin (a – minimal, b – maximal); 16 – the Early Cretaceous kimberlite pipes (a – diatreme shaped, b – pear shaped and pipes-embryos). Table 1 Specific features of haloes of dispersion of KIMs from “opened” and “semi-opened” kimberlite pipes Main features Haloes from “opened bodies” Haloes from “semi-opened” bodies Size, km2 up to 24 0.03 – 4 Shape Isometric, irregular Elougated, crescent, irregular The internal structure Mosaic Spotted, mosaic Orientation relatively to kimberlite pipe in different directions from the pipe (rarely in one direction) More often in one direction from the pipe The distance of travel Short (hundreds of meters – first kilometers) Short (hundreds of meters) The composition of KIMs Crdi-Prp-Pilm-Crsp Crdi-Prp-Pilm-Crsp ThecontentofKIMs, gr./20l Prp 10-24 Rarely Often Prp 25-49 Often Rarely Prp >50 Rarely No Pilm 10-24 Rarely Often Pilm 25-49 Often Rarely Pilm >50 Often Rarely Crdi 1 Often Rarely Crdi 2-8 Often No
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  Research methodology Sampling ofsurface sediments with collection of 20-50 l heavy concentrate samples was carried out on a grid 200 х 200 m (АК10 pipe), 500 x 500 m and 250-300 m intervals on some of the profiles (AK22 and AK23 pipes). Samples were collected by hand from the near-surface layer up to 0.1 m thickness of loose Quaternary sediments. Definition of coordinates of the samples was performed with the use of GPS navigators. The collected sample was subjected to wet screening on sieves with separation into fractions +4 mm, -4 mm. Grains of +4 mm size were identified in the field visually. Fraction -4 mm was washed by hands using Siberian pans. Heavy concentrates were dried and delivered to the field laboratory. Heavy concentrate and hard rock samples preparation (separation of heavy fraction in heavy liquid, particle size analysis and separation by magnetic properties) was performed in the field by mineralogists. The samples were sieved into fractions: +2 mm, -2+1 mm, -1+0.5 mm, -0.5+0.35 mm; -0.35+0.25 mm; -0.25 mm. Mineralogical analysis of heavy concentrate samples' heavy fraction with selection and study of all KIMs was performed by mineralogists with the use of stereomicroscopes. During the research of the samples mineralogical composition of the heavy fraction, initial diagnostics with picking of kimberlite minerals were performed. Data about the composition of the heavy fraction and quantity of KIMs were recorded in mineralogical tables. Further, mineralogical research of selected minerals was carried out. Their quantity in a sample was determined, as well as size, color, integrity, morphology, type and degree of abrasion of initial surfaces, availability and type of corrosion of grains. Picked KIMs were prepared for photographing and further microprobe analysis in the chemical laboratory of All-Russia Geological Research Institute (Saint-Petersburg, Russia). Eolian haloes over kimberlite pipes in North-Eastern Botswana Examples of known kimberlite pipes of Orapa field (АК10, АК22 and АК23) eolian haloes of short distance of travel have been studied in detail. The halo of KIMs in the area of the kimberlite pipe АК10 is characterized by the joint presence in heavy concentrate samples of minerals of different degrees of abrasion (Fig. 3a). The figure shows the areal change of the contents of picroilmenites and pyropes of +0.35 mm size relative to the position of АК10 kimberlite body and the distribution of minerals of all degrees of abrasion (I-V) intensively (IV-V), moderately (III) abraded, slightly and unabraded (I-II) grains. The changing of the distributions of degrees of abrasion of pyropes and picroilmenites at different distances from the pipe in western, southern, northern and eastern directions are shown on histograms (Fig. 4a). Directly above the body slightly and unabraded minerals predominate. As the distance from the source content of minerals of I and II degrees are gradually reduced and at a distance of 400 m (pyropes) and 600 m (picroilmenites) they disappear completely. The proportion of moderately and extensively abraded grains increases. A halo of short travel (400х800 m) from the pipe АК10 is elongated in a western direction and consists of KIMs of I and II degrees of abrasion. As the distance from the body the number of unabraded and slightly abraded grains is not only reducing but also decreasing in size (Fig. 5a). So, over the body pyropes and picroilmenites of I and II degrees are identified in all fractions, including +2 mm size. As the distance from the body in the direction of the main transport of KIMs increases, the number of large grains of picroilmenites (+1 mm) decreases up to complete disappearing already in 300 m from the body, and are present only in fractions -1+0.5 mm and -0.5 mm. At a distance of 500-600 m picroilmenites of I and II degrees found only in a fine size (-0.5 mm). Pyropes of I and II degrees of +1 mm size recorded in the surface sediments within the location of the kimberlite body. At a distance of 100 m they were distinguished only in the fraction -0.5 mm. It should be noted that the degree of abrasion of KIMs changes smoothly even within a group of slightly abraded grains with the distance from the source. This is shown more clearly on the grains of picroilmenites (Fig. 6). There is a maximum amount of unabraded grains over the АК10 kimberlite body. The size of the part of halo with the highest content of KIMs of I degree of abrasion is approximately 50x100 m. Indicator minerals are represented by pyropes (11-86 gr./20 l), picroilmenites (from the hundreds to the first thousands gr./20 l) and chrome-diopsides (from single grains to first dozens of gr./20 l) and the single fine grains of chrome-spinels. Pyropes and
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  picroilmenites are foundin all fractions, including +1 mm, but they predominate in the small size. Pyropes are represented by all color varieties with a lot of fractured grains, grains with protoshears and relicts of kelyphitic rims. Fig. 3 Haloes of dispersion of pyropes (red) and picroilmenites (blue) of +0.35 mm size in area of AK10 (a) and AK22, AK23 (b) pipes 1 – contour of kimberlite pipe in enclosing rocks (basalts): a – under basalts, b – on the surface of basalts; 2 – occurrences of chrome-spinels; 3 – occurrences of chrome-diopsides; 4 – heavy concentrate samples; 5 – lines showing degrees of abrasion and size distribution of KIMs on histograms on Fig. 4 and Fig. 5. There are growths of pyropes with chrome-diopsides. Picroilmenites are presented as whole grains, rounded and angular-rounded shape with the primary rough or finely spinous surfaces. The leucoxene cover or its relics remain on many grains. There are a large number of fragments with protoshears and fresh shears. The majority of the grains have a monolithic structure, but aggregate grains are also present (see Fig. 6). Chrome-diopsides are distinguished in the fractions -1+0.5 mm and -0.5 mm. They are represented as whole grains, rounded and angular shape with the primary roughened surface with the relics of light-colored cover. Chrome-spinels are found only in -0.5 mm size and represented by octahedrons with rounded edges and a uniformly matted surface.
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  Fig. 4 Distribution ofpyropes (red) and picroilmenites (blue) according to degrees of abrasion in eolian halo of kimberlite minerals from AK10 pipe (a) and AK22, AK23 (b) pipes along lines shown on Fig. 3 Fig. 5 Size distribution of pyropes (red) and picroilmenites (blue) of I-II degrees of abrasion in eolian halo of kimberlite minerals from AK10 pipe (a) and AK22, AK23 (b) pipes along W-E line Fig. 6 Change in the morphological features and degrees of abrasion (I-III) of picroilmenite (black) and pyrope (violet) grains at different distances from AK10 kimberlite pipe
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  Over the kimberlitebody, in the composition of the heavy fraction, olivine is present and its content can reach 62 %. Minerals are represented by transparent and translucent grains angular rounded and angular shapes. Euhedral crystals with matted surfaces were identified. At a distance of 100-200 m from the АК10 pipe in the direction of the main western transport of KIMs pyropes and picroilmenites are also recorded, but their content reduces sharply: to 1-2 gr./20 l for pyropes and to 10-24 gr./20 l for picroilmenites. The content of large grains is also reduced. Rare picroilmenites in the fraction of +1 mm are noted in some samples, but pyropes are found only in the fraction of -0.5 mm. Chrome-diopsides and chrome- spinels are not found but their presence is possible with increasing of volume of the samples. The majority of the grains of picroilmenites and pyropes are slightly abraded but some of them are unabraded. Picroilmenites of II degree of abrasion differ from the unabraded grains by the larger amount of debris, attrited sides of whole grains and slightly abraded primary spinouse surface. There have remained leucoxene and hydroxide adhesions in pits. Slightly abraded pyropes are represented by whole grains, rounded and angular-rounded shapes with a small fresh chipped and slightly attrited primary surfaces, as well as by fragments with fresh chips and relics of the primary surfaces and kelyphitic rims. Unabraded KIMs at a distance of 300-400 m from the АК10 pipe were not found. The content of picroilmenites of II degree is 1-6 gr./20 l, pyropes - 1-2 gr./20 l, chrome-diopsides and chrome-spinels were not identified. Pyropes were distinguished only in fraction -0.5 mm, picroilmenites - in fractions -1+0.5 mm and -0.5 mm. Grains of picroilmenites have fresh chips with slightly attrited edges. Primary spinous surface is slightly abraded, leucoxene and hydroxide adhesions are practically absent. There are many chips among pyropes, but the initial surface is attrited. At a distance of 500-600 m from the body there are grains of pyropes only with moderate and extensive degrees of abrasion. The last ones are the part of mineralogical blanket. Picroilmenites of II - III degrees are represented by a content of 1-4 gr./20 l in fraction -0.5 mm. At a distance of 700-800 m from the pipe there are single grains of picroilmenites (1-4 gr./20 l) s of III degree of abrasion and pyropes (1-2 gr./20 l) of II and III classes. KIMs (see Fig. 6) are of angular-rounded shapes with numerous matted and fresh chips, the primary surface is abraded. At a distance of over 800 m from the АК10 pipe the KIMs association from the body is completely mixed with the mineralogical blanket in terms of the specific features of abrasion of the grain surfaces. AK22 and AK23 kimberlite bodies are approximately 200-250 m apart and oriented in north-western direction. Due to such proximity of the bodies the indicator minerals in surface sediments form a single halo of dispersion. Both pipes break through the basalts of the Karoo Supergroup not completely and have a pear-like shape typical for many bodies of the Orapa field rather than the classical diatreme shape. The halo of dispersion of KIMs of close transport in the area of semi-exposed AK22 and AK23 kimberlite pipes is characterized by the joint presence of minerals of different degrees of abrasion in heavy concentrate samples. Figure 3b shows the areal distribution of picroilmenites and pyropes of +0.35 mm size. This figure shows the distribution by all degrees of abrasion and separately that of extensively, moderately and slightly abraded minerals in the surface eolian sediments. The histograms of distribution of pyropes and picroilmenites by indexes of abrasion trace the change in correlation of grains of different abrasion with the distance from АК22 and АК23 bodies in the north, south, west and east directions (Fig. 4b). The highest content of unabraded KIMs is discovered directly over the АК22 and АК23 bodies where, according to the drilling data, kimberlites break through basalts and are overlain by sediments of the Kalahari Group of 10-11 m thickness. The size of the halo with the maximum content of unabraded minerals is approximately 50х300 m. A typical geomorphological feature of the halo from the АК22 and АК23 pipes is the position within the low- contrast negative form of the relief. As a result, KIMs in surface sediments go into a cone-type environment, and minerals rotate and transport in closed space under the influence of winds and temporary water flows. Grains are abrading without travel from the pipe, which results in the presence of mineralls of I, II and III degrees over the source. In this case the grains attrition does not reflect KIMs transport distance from the source. The correlation of grains of picroilmenites of I, II and III classes of abrasion is approximately 4:6:1. Over the kimberlite bodies in the surface sediments there are pyropes and picroilmenites of all sizes, including +2 mm fraction. Picroilmenites of I and II degrees of abrasion are present as hundreds and thousands gr./20 l in
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  fractions -1+0.5 mmand -0.5+0.35 mm. In +2 mm size their content is dozens of gr./20 l. The content of picroilmenites of III degree varies from dozens and first hundreds of grains per 20 l in fractions -1+0.5 mm and - 0.5+0.35 mm and as single grains of +2 mm size. Unabraded or slightly abraded pyropes have been identified at the content from 2 to 21 gr./20 l in all fractions, including +2 mm size. Pyropes of III degree of abrasion have been found in the fractions -1+0.5 mm and - 0.5+0.355 mm in amount of 1-2 gr./20 l. At 100 m westwards from the pipes pyropes of I-II degrees of abrasion have been found as single grains and moderately abraded pyropes have not being identified. The content of picroilmenites of I-II degrees decrease sharply down to single signs and dozens of gr./20 l. They are present only in fractions of -1+0.5 mm and -0.5+0.355 mm. Picroilmenites of a moderate degree of abrasion are recorded in the same quantities and fractions as over the pipes. Eastwards from the pipe fewer samples have been collected. We have data only for the distance of 1 km from the pipes. No grains of I-II degrees of abrasion have been found here in surface sediments. There are only single grains of pyropes and picroilmenites of III and IV-V degrees. At 100 m southwards, pyropes of I-II degrees of abrasion have not been discovered, and single grains of picroilmenites have been identified (Fig. 7). Fig. 7 Change in the morphological features and degrees of abrasion (I-III) of picroilmenite (black) and pyrope (red) grains at different distances from AK22 and AK23 kimberlite pipes At 500 m from the bodies the content of fine size grains (-0.5 mm) increases substantially. Picroilmenites of I and II degrees have been identified as single grains and first dozens of gr. /20 l but only westwards from the pipes. Content of moderately abraded picroilmenites decrease down to dozens of gr./20 l, and pyropes of II degree have been identified as single grains. Moderately abraded pyropes have not been found. There are single grains of KIMs of IV- V degrees that are likely to be transit ones (see Fig. 7). At a distance of about 1 km from the kimberlite pipes the content of fine grains (-0.5 mm) increases substantially (Fig. 5b). The correlation of medium and fine grains is 1:10. This is probably due to the fact that the halo has two contiguous sources. In different directions from the source in the surface sediments no unabraded or slightly abraded picroilmenites have been found. There are only picroilmenites of III and IV-V degrees of abrasion. Pyropes of I-II degrees have been identified as single grains in fractions +1 mm and -1+0.5 mm. At a distance of over 1 km in the northwest as well as in southern and eastern directions from the pipes pyropes of moderate abrasion are recorded. Conclusion The study of haloes of dispersion of short distance of travel from the kimberlite pipes of the North-East of Botswana has shown how the grain size as well as the content of unabraded and slightly abraded minerals decrease with the distance from the primary sources. It has been established that the reliable identification of the haloes of short travel demands taking into account the availability of pyropes and picroilmenites of I-II degrees of abrasion of +0.35 mm fraction. The maximum content of unabraded and slightly abraded pyropes and picroilmenites (hundreds and thousands gr./20 l) are recorded over the pipes within small areas of about 50x100-300 m. The identification of such areas with anomalous KIMs content by sampling on the grid of 1x1 km or 500x500 m is extremely difficult and in most cases impossible. To identify kimberlite bodies by heavy concentrate sampling method in similar environments it is necessary to carry out sampling on a more detailed grid: 100x100 m or 200x200 m. The optimal sampling
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  volume with thethickness of overlying sediments of up to 10-20 m is 20-50 liters. For areas with a higher thickness of overlying sediments it is recommended to increase the sampling volume to 50-100 liters (thickness: 20-30 m) and 250 l (thickness: 30-80 m). It should be noted that even single grains of unabraded or slightly abraded KIMs found in eolian deposits are of high prospecting significance. References Baumgartner MC, Neuhoff L (1998) The vertical distribution of indicator minerals within Kalahari cover overlying a kimberlite pipe. Extended abstracts: 7th International Kimberlite Conference, Cape Town, pp 55-57 Ustinov VN (2015) Terrigenous diamond-bearing rocks of the Siberian, East-European and African platforms, SPb.: Nauka. 531 p. Ustinov VN, Mosigi B, Kukui IM, Nikolaeva EV (2015) Eolian haloes of kimberlite indicator minerals. Placers and deposits in weathering crusts: study, development, ecology. Perm: Publishing House of Perm University, pp 228- 229