This document summarizes research on modeling eolian haloes of kimberlite indicator minerals (KIMs) dispersed from kimberlite pipes in the Orapa kimberlite field in northeast Botswana. Wind erosion of kimberlites has released KIMs like pyrope and picroilmenite into overlying Kalahari sands, forming haloes around pipe sources. Haloes contain KIMs of varying abrasion degrees, with more abraded minerals found farther from pipes. Sampling of surface sediments around pipes identified KIM contents and helped create models of dispersion from both exposed and partially exposed pipes in the field.

1. MODELS OF REFLECTION OF KIMBERLITE PIPES OF NORTH-EAST
OF BOTSWANA IN EOLIAN HALOES OF DISPERSION
V. Ustinov1
, B. Mosigi2
, I. Kukui1
, E. Nikolaeva1
, J. Campbell2
, Yu. Stegnitskyi1
,
M. Antashchuk1
1
ALROSA, Saint-Petersburg, Russia, UstinovVN@alrosa.ru, KukuyIM@alrosa.ru
2
Botswana Diamonds, London, UK, Bmosigi@gmail.com, James@botswanadiamonds.co.uk
Introduction
The paper considers the structure of eolian haloes over kimberlite pipes overlain by
Kalahari sands of up to 20 m thickness. It studies the change of the content, morphology and
abrasion of kimberlite indicator minerals (KIMs) with distance from the primary sources in the
area of Orapa kimberlite field in the North-East of Botswana. Based on the data of heavy mineral
sampling of surface sediments the models of reflection of Late Cretaceous kimberlite pipes of
Orapa field in eolian haloes of KIMs dispersion have been created.
The research has been carried out using the prospecting data of ALROSA (Russia) and
Botswana Diamonds (England) companies.
1. Specific features of formation of eolian haloes of dispersion of kimberlite indicator
minerals
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 (Fig. 1).
Figure 1: Eolian formations in North-East of Botswana

2. 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 concentration on the weathered surface, which is to be 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 to be 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 commensurable
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 to different distances from the primary sources.
In most cases KIMs dispersion trains consist of haloes of different travel formed as a result
denudation of kimberlite bodies located at various distances from the area of accumulation (Fig.
2). 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).
Figure 2: Joint presence of kimberlite minerals (picroilmenites and pyropes) of various degrees
of abrasion (I-V) in eolian surface sediments of North-Eastern Botswana
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 picroilmenites which have a 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.

3. 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. 3).
Figure 3: Eolian haloes of 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).
The sediments of the surface complex of Kalahari Group mostly represented by eolian
formations which were formed as a results of reworking of illuvial, deluvial, 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 deluvial
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 Karroo Supergroup are composed by 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 by 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.

4. 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). 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 is higher
content of minerals (20-30 or more gr./20 l). Higher contents of KIMs are located close to pipes.
Contents of KIMs are declined rapidly at a distance of several hundreds of meters and first
kilometers.
Table
Specific features of haloes of dispersion of KIMs from “opened” and “semi-opened” kimberlite
pipes
2. Methods of the research
Sampling of surface sediments with collection of 20-50 l heavy concentrate samples was
carried out by grid 200 х 200 m (АК10 pipe), 500 x 500 m and 250-300 m intervals on some
profiles (AK22 and AK23 pipes).
Samples were collected by hands from 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.
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

5. 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 research of the samples mineralogical composition of heavy fraction was
determined, initial diagnostics with picking of kimberlite minerals were performed. Data about
composition of 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).
3. Eolian haloes over kimberlite pipes in North-Eastern Botswana
On 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. 4). 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,
the distribution of minerals of all degrees of abrasion (I-V), intensively (IV-V), moderately (III)
abraded, slightly and unabraded (I-II) grains.
Changing of 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. 5). 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. 6).
So, over the body pyropes and picroilmenites of I and II degrees identified in all fractions,
including +2 mm size. As the distance from the body in the direction of the main transport of
KIMs, the number of large grains of picroilmenites (+1 mm) decreases up to complete
disappearance 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 fraction -
0.5 mm

6. Figure 4: Haloes of dispersion of pyropes (red) and picroilmenites (blue) in area of AK10 pipe
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.

7. Figure 5: Distribution of pyropes (red) and picroilmenites (blue) according to degrees of abrasion
in eolian halo of kimberlite minerals from AK10 pipe along W-E, S-N and E-W lines
Figure 6: Size distribution of pyropes (red) and picroilmenites (blue) of I-II degrees of abrasion
in eolian halo of kimberlite minerals from AK10 pipe along W-E line
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 more clearly can be
shown on the grains of picroilmenites (Fig. 7).

8. Figure 7: Change in the morphological features and degrees of abrasion (I-III) of picroilmenite
and pyrope grains at different distances from AK10 kimberlite pipe
There is a maximal amount of unabraded grains over 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 picroilmenites are found in
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. 8).
Figure 8: Unabraded kimberlite minerals from eolian surface sediments above AK10 kimberlite
pipe: a – picroilmenites, b – pyropes, c – chrome-diopsides, d – olivines
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 has a monolithic structure,
but aggregate grains are also present (see Fig. 8). 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 (see Fig. 8).

9. Chrome-spinels are found only in -0.5 mm size and represented by octahedrons with rounded edges
and a uniformly matted surface.
Over the kimberlite body in the composition of the heavy fraction olivine is present, its
content reaches 62 %. Minerals are represented by transparent and translucent grains angular
rounded and angular shapes. Euhedral crystals with matted surfaces were identified (see Fig. 8).
At a distance of 100-200 m from 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 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. Majority of grains of
picroilmenites and pyropes are slightly abraded (Fig. 9) but some of them are unabraded.
Figure 9: Picroilmenites (left) and pyropes (right) of II degree of abrasion from surface eolian
sediments in 100 m from AK10 kimberlite pipe
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 (see Fig. 9).
Unabraded KIMs at a distance of 300-400 m from the АК10 pipe were not found. The
content of picroilmenites of II degree (Fig. 10) 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 (Fig. 11) 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 (Fig. 12)
are of angular-rounded shapes with numerous matted and fresh chips the primary surface is
abraded.

10. Figure 10: Picroilmenites (black) and pyrope (violet) of II degree of abrasion from surface eolian
sediments in 300-400 m from AK10 kimberlite pipe
Figure 11: Picroilmenites (black) of II-III degrees of abrasion and pyrope (violet) of II degree
of abrasion from surface eolian sediments in 500-600 m from AK10 kimberlite pipe
Figure 12: Picroilmenite (black) of III degree of abrasion and pyropes (red) of II-III degrees from
surface eolian sediments in 700-800 m from AK10 kimberlite pipe

11. At a distance of over 800 m from the АК10 pipe the KIMs association from the body is
completely mixing 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 degree of
abrasion in heavy concentrate samples. The figure 13 shows the areal distribution of picroilmenites
and pyropes of +0.35 mm size. It shows distribution by all degrees of abrasion and separately that
of extensively, moderately and slightly abraded minerals in the surface eolian sediments.
Figure 13: Haloes of dispersion of pyropes (red) and picroilmenites (blue) in area of AK22,
AK23 pipes
1 – contour of kimberlite pipe in enclosing rocks (basalts); 2 – occurrences of chrome-spinels; 3 – heavy concentrate
samples.

12. 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. 14).
Figure 14: Size distribution of pyropes (red) and picroilmenites (blue) in eolian halo of
kimberlite minerals from AK22 and AK23 pipes
The highest content of unabraded KIMs is discovered directly over 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 maximal
content of unabraded minerals is approximately 50х300 m.
A typical geomorphological feature of the halo from АК22 and АК23 pipes is the position
within the low-contrast negative form of the relief. As a result, KIMs in surface sediments get 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 (Fig. 15). Picroilmenites of I and II degrees of abrasion are
present as hundreds and thousands gr./20 l in fractions -1+0.5 mm and -0.5+0.35 mm. In +2 mm
size their content is dozens of gr./20 l. 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.

13. Figure 15: Unabraded kimberlite minerals from eolian sediments above AK22 and AK23 pipes:
pyropes (a, b), chrome-diopsides (c), chrome-spinel (d), picroilmenites (e, f)
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, moderately abraded pyropes not been 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 moderate degree are recorded in the
same quantities and fractions as over the pipes. East words from the pipe less amount of 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 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.
16).
Figure 16: Unabraded pyropes (a, b), picroilmenites (c, d) of II and III degrees of abrasion and
chrome-spinel (e) from eolian sediments in 100 m from AK22 and AK23 pipes

14. At 500 m from the bodies the share 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 have been identified single grains of KIMs of IV-V
degrees that are likely to be transit ones (Fig. 17).
Figure 17: Pyrope (a) of II degree of abrasion, picroilmenites (b, c) of II and III degrees from
eolian sediments in 500 m from AK22 and AK23 pipes
At a distance of about 1 km from the kimberlite pipes the share of fine grains (- 0.5 mm)
increases substantially (Fig. 18). 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.
Figure 18: Size distribution of pyropes (red) and picroilmenites (blue) of I-II degrees of abrasion
in eolian halo of kimberlite minerals from AK22 and AK23 pipes along W-E line

15. CONCLUSION
The study of haloes of dispersion of short distance of travel from kimberlite pipes of North-
East of Botswana has shown how the grain size as well as the share of unabraded and slightly
abraded minerals decrease with the distance from the primary sources. It has been established that
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 maximal 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. Identification of such areas
with anomalous KIMs content by sampling in 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 in a more detailed
grid: 100x100 m or 200x200 m. The optimal sampling volume with the thickness of overlying
sediments of up to 10-20 m is 20-50 liters. For areas with a greater thickness of overlying
sediments it is necessary to increase the sampling volume to 50-100 liters (thickness: 20-30 m)
and 250 l (thickness: 30-80 m).
It is necessary to take into account that even single grains of unabraded or slightly abraded
KIMs found in eolian deposits are of high prospecting significance. These findings are an
evidence of proximity of primary sources and more detailed sampling in these areas in necessary.