Eolian indicator mineral dispersion haloes from the Orapa kimberlite

1. ORIGINAL PAPER
Eolian indicator mineral dispersion haloes from the Orapa kimberlite
cluster, Botswana
Victor N. Ustinov1
& Benjamin Mosigi2
& Irina M. Kukui1
& Evelina V. Nikolaeva1
& James A. H. Campbell2
&
Yury B. Stegnitskiy3
& Michael G. Antashchuk1
Received: 15 November 2017 /Accepted: 16 August 2018
# Springer-Verlag GmbH Austria, part of Springer Nature 2018
Abstract
This paper presents the results of an investigation into the structure of eolian kimberlite indicator minerals (KIMs) haloes present
within Quaternary Kalahari Group sediments (up to 20 m thick) overlying the Late Cretaceous kimberlites in the Orapa field in
North-East Botswana. A database of more than 8000 samples shows that kimberlites create a general mineralogical blanket of
KIMs of various distances of transportation from primary sources in the Orapa area. Models of the reflection and dispersion
patterns of KIMs derived from kimberlite pipes including AK10/ AK22/AK23 have been revealed based on 200 selected heavy
mineral samples collected during diamond prospecting activities in Botswana from 2014 to 2017. Short distance eolian haloes
situated close to kimberlite bodies cover gentle slopes within plains up to 500 × 1000 m in size. They have regularly have oval or
conical shapes and are characterized by the presence mainly of unabraded or only slightly abraded KIMs. A sharp reduction of
their concentration from hundreds and thousands of grains / 20 l immediately above kimberlites toto 10 grains/20 l at a distance of
only 100–200 m from the pipes is a standard feature of these haloes. The variation of concentration, morphology and abrasion of
specific KIMs with increasing distance from the primary sources has been investigated and presented herein. Sample volumes
recommended for pipes present within a similar setting as those studied, with different depth of sedimentary cover are as follows:
up to 10–20 m cover at 20–50 l, 20–30 m cover at 50–100 l and 30–80 m cover at 250 l. It is important to appreciate that the
discovery of even single grains of unabraded or slightly abraded KIMs in eolian haloes are of high prospecting significance in this
area. The results of the research can be applied to in diamond prospecting programs in various regions with similar environments.
Keywords Kimberlite indicator minerals . Degree of abrasion . Garnet . Ilmenite . Chrome-diopside . Diamond
Introduction
Kimberlite indicator minerals (KIMs) are mantle-derived min-
erals that are disaggregated and entrained by kimberlites dur-
ing ascent to surface. The most important KIMs from an ex-
ploration perspective are garnet, ilmenite, spinel, chrome-
diopside and olivine. Several of these minerals display unique
compositional and visual characteristics that distinguish them
from crustal minerals (Nowicki et al. 2007) and they can there-
fore be used as pathfinders for kimberlite or kimberlite-related
rocks following their liberation and dispersion (through
weathering and erosion) into the secondary environment.
The dispersion of KIMs in the secondary environment is
controlled by climatic, tectonic, facial-dynamic, geomorpho-
logical conditions and time. In arid, desert environments
where sand accumulates over the paleorelief surface KIMs
are dispersed into the overlying sands to form haloes (for
instance Baumgartner and Neuhoff 1998) that reflect
higher concentrations of grains and of grains with primary
(unabraded) features directly over the kimberlites. The forma-
tion of these haloes is controlled by a number of processes
(Ustinov et al. 2015). 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
Editorial handling: C. M. Hetman
* Victor N. Ustinov
ustinov@tsnigri.ru; agddiamond@mail.ru
1
Diamond Department, Central Research & Exploration Institute
(TsNIGRI), 24/1 Odoevskogo St., Saint-Petersburg 199155, Russia
2
Botswana Diamonds plc, 20-22 Bedford Row, London WC1R 4JS,
UK
3
ALROSA PJSC, 128A Nevsky Pr., Saint-Petersburg 191036, Russia
Mineralogy and Petrology
https://doi.org/10.1007/s00710-018-0627-2

2. Kalahari sands. One of the ways of delivery of kimberlite
minerals to the surface occurs is by termite activity. Another
process responsible for the dispersion of of KIMs grains into
the overlaying sediments is the presence of a 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 related to eolian processes which may
result both in concentration of KIMs in sands and their down-
ward and upward redeposition within the forms of the sand
relief as a result of mixing. Kimberlite minerals and dia-
monds identified in these sediments in tropical arid climate
conditions are transported by varying distances from the
primary sources.
Eolian haloes of dispersion where multiple kimberlites are
present form a general mineralogical blanket (background).
The locally-derived KIMs haloes connected with pipes typi-
cally display larger grains with primary surface features rela-
tive to the larger scale background dispersions. The structure
and extent of these haloes vary, and are dependent on the
abundance of grains in the primary kimberlite source, sedi-
ment thickness, local relief, direction of wind and other fac-
tors. This paper presents the results of an investigation of the
distribution of KIMs present within Quaternary-aged Kalahari
cover sediments (up to 20 m thick) as eolian haloes over the
AK10, AK22 and AK23 kimberlite pipes in the Orapa kim-
berlite field in North-East Botswana.
Fig. 1 Eolian haloes of dispersion of KIMs in the area of Orapa
kimberlite field. Signatures used: 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: M-G, Makgadikgadi; 5 = elevation: LH, Letlhakane);
6 = boundaries of macroforms; mesoforms (7 = river valleys and
paleovalleys)]; 8a-8b = isopaches of the Neogene-Quaternary sediments
of Kalahari Group (8a, reliable; 8b, supposed); 9–11 = eolian haloes
(more than 10 grains/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); 15a-15b = standing levels of an inland basin (15a,
minimal; 15b, maximal); 16a-16b = the Early Cretaceous kimberlite
pipes (16a, diatreme shaped; 16b, pear shaped and pipes-embryos)
V. N. Ustinov et al.

3. Local geology and dispersion haloes
The AK10, AK22 and AK23 kimberlite pipes on which this
study is based form part of the larger Orapa field in North-East
Botswana. These kimberlites are covered by sediments of the
Kalahari Group, which in this particular location comprise
eolian formations formed through reworking of alluvial, talus,
proluvial, deltaic and beach sands, gravels and conglomerates.
These sediments are locally intensively calcretized and
silcretized. At the sites studied the local basal horizon forma-
tions are likely a talus or proluvial facies which have also
undergone wind reworking and subsequent hypergenetic al-
teration. The lower portion of the Kalahari Group is represent-
ed by calcrete ranging from 5 to 17 m in thickness. The upper
portion comprises silty, inequigranular oligomictic quartz-
feldspathic sands of eolian origin often containing fragments
and debris of the underlying calcrete. These sands range in
thickness from 3 to 5 m.
The pre-Kalahari surface comprises Stormberg Group ba-
salts, which range in thickness from 10 to 140 m within the
Orapa kimberlite field. They present a strong barrier to the
upward movement of kimberlitic magma and have impacted
the morphology of certain pipes by hampering they formation
of typical Bdiatreme^ shapes. Kimberlites can therefore be
present as exposed (Bopened^), semi-exposed (Bsemi-
opened^) and embryos (Bblind^) bodies.
Of the 85 kimberlites discovered to date within the Orapa
field, only few pipes (АK1, АK2, АK3 and some others) are
Bopened^ and have comparatively well-preserved crater fa-
cies. The majority of the bodies (AK8, AK9, AK10 and
others) are considered Bsemi-opened^ and therefore present
limited surface areas that can increase significantly with depth
below the basalt Bcapping^ horizon.
The Orapa kimberlite field has generated large scale back-
ground and numerous locally-derived KIMs dispersion haloes
within Kalahari cover sediments (Fig. 1). These haloes com-
prise predominantly pyrope and picroilmenite (with minor
content of other KIMs) with different degrees of abrasion
(Fig. 2). The abrasion classification of Ustinov (2015) is
adopted here for descriptive purposes: I, unabraded; II, slight-
ly abraded; III, moderately abraded; IV, extensively abraded;
V, very extensively abraded.
Large scale background eolian haloes (reflecting various
distances of KIMs travel and mixing from multiple sources)
are typically represented by grains of pyrope and
picroilmenite displaying abrasion ratings of IV-V. They reflect
indicator mineral zonation with an increase in the relative
proportion of pyrope (< 0.5 mm) with distance, due to the
reduced relative mobility of ilmenite in the secondary envi-
ronment due to its higher specific gravity.
Locally-derived eolian haloes of short travel distances are
represented by KIMs displaying abrasion features rated as I-
II, although these can be mixed with abraded grains from
background KIMs dispersions. These haloes are more
likely to reflect varying mineralogy and can contain
olivine and chrome-diopside (these minerals typically break
down very rapidly in an arid environment). Study of sampling
results (> 8000 samples) within the Orapa kimberlite field
demonstrate that KIM dispersion haloes from opened and
semi-opened kimberlite pipes are expressed in different ways
(Table 1). Blind bodies that have only brecciated or injected
dykes into the basalt horizons do not generate associated KIM
dispersion haloes.
KIM dispersion haloes in the Orapa field are characterized
by a background dispersion of extensively abraded pyropes
and (or) picroilmenites (up to 10 grains / 20 l). These can be
concentrated in heavy mineral trapsites (e.g. coarse deflation
surfaces, 20–30 or more grains/20 l). KIMs are more highly
concentrated in locally-derived haloes over kimberlite bodies,
but decrease in concentration rapidly with distances on the
Fig. 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 indicator mineral dispersion haloes from the Orapa kimberlite cluster, Botswana

4. order of several hundred meters. The presence of unabraded
grains appears to be a key diagnostic feature of locally-derived
KIMs dispersions and therefore are extremely important.
Research methodology
A dataset of more than 170 heavy mineral samples comprising
20–50 l of loose Quaternary sediments were collected from
the top 10 cm of the deflated sand profile over the AK10,
AK22 and AK23 kimberlites. These samples were collected
on grid spacings varying from 200 × 200 m (АK10 pipe) to
500 × 500 m and 250 × 300 m (AK22 and AK23 pipes).
Sample positions were recorded with handheld GPS units.
All samples were subjected to a single sample process meth-
odology, as follows. The collected sample was wet or dry
screened into plus and minus 4 mm size fractions. Visual ob-
servations were recorded for the >4 mm size fraction. The
<4 mm fraction was preconcentrated by panning in the field
and the resulting samples were dried and transported to a field
laboratory for further processing. Samples were passed through
LST (2.95 g/cm3
) heavy liquid to generate heavy mineral con-
centrates that were further subjected to magnetic separation. It
should be noted that panning by hand by experienced field
assistants resulted in very high recovery of KIMs comparable
(or even better) to mechanic jigs or DMS concentrators.
The resulting demagnetized heavy mineral concentrates
were sieved into fractions: > 2 mm, 1–2 mm, 0.5–1 mm,
0.35–0.5 mm, 0.25–0.35 mm; < 0.25 mm. Concentrates were
stripped of KIMs by mineralogists with the aid of stereomi-
croscopes. Relevant sample process information was captured
throughout the procedure, as well as the number, size, color,
integrity, morphology, type and degree of abrasion of initial
surfaces, availability and type of corrosion of grains of each
KIM species. Picked KIMs were prepared for photographing
and further microprobe analysis. It should be noted that only
kimberlite minerals confirmed by microprobe analysis were
taken into account in this study.
Results
AK10 KIMs dispersion halo
The KIMs dispersion halo over AK10 is constrained by a total
of 120 heavy mineral samples within a 2 × 2 km square cen-
tered over the body. The average thickness of Kalahari sedi-
ments overlying the body is 10–12 m. The halo of KIMs
overlying АK10 is characterized by the presence of minerals
displaying different degrees of abrasion (Fig. 3a). The figure
shows the sampled concentration of picroilmenites and py-
ropes of >0.35 mm size relative to the position of АK10 and
the distribution of minerals by degree of abrasion.
The degree of abrasion of pyropes and picroilmenites at
different distances from the pipe in the cardinal directions
are shown on histograms (Fig. 4a). Directly above the body
slightly abraded and unabraded minerals predominate.
Minerals with abrasion ratings of I and II decline with
Table 1 Specific features of haloes of dispersion of KIMs from Bopened^ and Bsemi-opened^ kimberlite pipes
Main features Haloes from Bopened bodies^ Haloes from Bsemi-opened^ bodies
Size (km2
) up to 24 0.03–4
Shape Isometric, irregular Elougated, crescent, irregular
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
Distance of travel Short (hundreds of meters – first kilometers) Short (hundreds of meters)
Composition of KIMs Crdi-Prp-Pilm-Crsp Crdi-Prp-Pilm-Crsp
Content of KIMs (grains/20 l) 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
„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.
Signatures used: 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 Figs. 4 and 5
V. N. Ustinov et al.

5. Eolian indicator mineral dispersion haloes from the Orapa kimberlite cluster, Botswana

6. increasing distance from source, and disappear completely at a
distance of 400 m (pyropes) and 600 m (picroilmenites).
Usually ilmenite does not travel farther than pyrope due to
density but in AK10 kimberlites concentrations of ilmenites
are more than10 times higher than pyropes. The proportion of
moderately and extensively abraded grains increases with dis-
tance from AK10.
A locally-derived KIM halo (400 × 800 m) over AK10 is
elongated westwards, reflecting an overall surface transport
related to the prevailing wind direction. The KIM halo
comprises KIMs with a rating of I and II abrasion. As
distance from the body increases the proportion of un-
abraded and slightly abraded grains reduces and grains
also decrease in size (Fig. 5a). Directly over the body pyropes
Fig. 4 Distribution of pyropes (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
V. N. Ustinov et al.

7. and picroilmenites of I and II abrasion are identified in all size
fractions, including the >2 mm fraction. As distance from the
body (in the dominant transport direction) increases, the num-
ber of large picroilmenites (> 1 mm) decreases to 300 m from
the body where they disappear and are only present as smaller
than 1 mm. At a distance of 500–600 m picroilmenites of I and
II abrasion are found only in smaller size fractions (< 0.5 mm).
Pyropes of I and II abrasion of >1 mm size are present directly
above the kimberlite body. At a distance of 100 m these are
only found in the <0.5 mm and smaller fractions.
It should be noted that the degree of abrasion of KIMs can be
observed to change in a uniform manner even within a group of
only slightly abraded grains increasing distance from the source.
This is best observed on the surfaces of picroilmenites (Fig. 6).
The highest concentration of unabraded grains identified
lies directly over the АK10 kimberlite body. The part of halo
with the highest content of KIMs rated with an I abrasion
define an area of approximately 50 × 100 m. The concentra-
tion of KIMs present in this part of the halo include pyrope
(11–86 grains / 20 l), picroilmenite (from the hundreds to the
first thousands grains / 20 l) and chrome-diopside (from single
grains to first dozens of grains/20 l) as well as single fine
grains of chrome-spinels. Pyropes and picroilmenites are
found in all fractions, including >1 mm, but they predominate
in the smaller size fractions. Pyropes vary in colour and are
commonly fractured. Protoshears and relicts of kelyphitic rims
are also present. Picroilmenites are predominantly whole
grains, rounded to subangular in shape with primary rough
or finely spinous surfaces. Leucoxene remnants remain on
many grains. A large number of fragments are present 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 present in the fractions less than
1 mm. They are present as whole grains, rounded and angular
in shape with primary roughened surfaces and relics of light-
colored cover. Chrome-spinels are found only in the <0.5 mm
and small size fractions and present octahedral shapes with
rounded edges and uniformly matted surfaces. Olivine is pres-
ent in samples directly over the kimberlite and can comprise
up to 62 % of total amount of grains in the heavy mineral
concentrate. Olivines are predominantly transparent and trans-
lucent, with angular and subangular shapes. Euhedral crystals
with matted surfaces were identified.
At a distance of 100–200 m (in the dominant transport
direction) pyropes and picroilmenites are present, but their
concentration becomes significantly reduced to 1–2 grains /
20 l for pyrope and to 10–24 grains / 20 l for picroilmenite.
Grain sizes are also reduced. Rare >1 mm picroilmenites are
present, band pyropes are smaller than 0.5 mm. Chrome-
diopsides and chrome-spinels are typically not present (but
could be in larger samples). Picroilmenites and pyropes can
be unabraded, but are predominantly slightly abraded. The
features of slightly abraded picroilmenite include attrited
sides, remnant spinous surfaces, remnants of leucoxene and
hydroxide adhesions in pits. The features of slightly abraded
pyrope include rounded to subangular shapes, small fresh
chips and slightly attrited primary surfaces, as well as frag-
ments and relics of primary surfaces and kelyphitic rims. No
unabraded KIMs were recovered at distances greater than
300–400 m from the body. At this distance the concentration
of picroilmenites of abrasion II is 1–6 grains / 20 l, the con-
centration of pyropes is 1–2 grains / 20 l, and chrome-
diopsides and chrome-spinels were not identified. Pyropes
were less than 0.5 mm in size, picroilmenites were less than
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
Eolian indicator mineral dispersion haloes from the Orapa kimberlite cluster, Botswana

8. 1 mm in size. Picroilmenites have fresh chips with slightly
attrited edges. Primary spinous surfaces are 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 all
pyropes display moderate and extensive degrees of abrasion.
Pyropes with extensive abrasion are considered to be part of
the larger KIM background dispersion of the Orapa field.
Picroilmenites of abrasion II - III and smaller than 0.5 mm
in size are present in concentrations of 1–4 grains / 20 l. At a
distance of 700–800 m from the pipe the concentration of
picroilmenite decreases to single grains (1–4 grains / 20 l) of
abrasion rating III. The concentration of pyropes also de-
creases to single grains (1–2 grains/20 l) of abrasion rating II
and III. KIMs (see Fig. 6) are of subangular shapes with nu-
merous matted and fresh chips with primary surface abrasion.
At a distance of over 800 m from the body all locally-derived
KIMs are indistinguishable from those related to the back-
ground KIM dispersion in terms of abrasion features.
AK22 and AK23 KIMs dispersion halo
The AK22 and AK23 kimberlites are approximately 200–250 m
apart with AK22 lying to the north-west of AK23. Due their
close proximity the KIMs derived from these bodies form a
single locally-derived (merged) dispersion halo. Both pipes are
semi-opened with incomplete breakage through the Karoo ba-
salts during emplacement. They have a pear-like shape typical
for many bodies of the Orapa field, increasing in diameter below
the basalt horizon. The KIM dispersion halo over AK22/AK23
is constrained by a total of 50 heavy mineral samples within a
2 × 2 km square centered over the two kimberlites.
The locally-derived KIM dispersion halo is characterized
by presence of minerals of different degrees of abrasion.
Figure 3b shows the concentration of picroilmenite and py-
ropes (> 0.35 mm) by abrasion category. Histograms showing
the proportion of pyrope and picroilmenite by abrasion cate-
gory with distance (in the cardinal directions) from АK22 and
АK23 are shown in Fig. 4b.
The highest proportion of unabraded KIMs lies directly
over the bodies where kimberlites are overlain by only 10–
11 m of Kalahari sediments. The size of the halo with the
highest proportion of unabraded minerals is approximately
50 × 300 m. A higher proportion of abraded grains is present
directly over these bodies in comparison with AK10.
The KIMs dispersion halo of AK22/AK23 is thought to
have been influenced by the local geomorphology. The area
directly overlying the bodies is a topographical low, and lib-
erated KIMs dispersed up through the cover sediments are
released into a closed system where they are subject to local
reworking and abrasion without travelling away from the pipe.
Grains with abrasion ratings of I, II and III are therefore pres-
ent over the source. The correlation of grains of picroilmenite
of I, II and III abrasion rating is approximately 4:6:1. It is also
likely that the geomorphologic low has acted as a trapping and
concentrating environment for background dispersion KIMs –
the increased proportion of abraded grains over these bodies is
thought to result from a combination of these two factors. In
this case the grains displaying abrasion ratings up to III do not
reflect simply KIM transport distance from the source.
Pyropes and picroilmenites up to 2 mm and larger in size
are present directly over the bodies. Picroilmenites of abrasion
I and II are present in concentrations of hundreds to thousands
of grains / 20 l in the 0.5–1.0 mm and 0.35–0.5 mm fractions.
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
V. N. Ustinov et al.

9. In the >2 mm fraction their concentration is tens of grains /
20 l. The concentration of picroilmenites of abrasion III varies
from tens to hundreds of grains per 20 l in the 0.5–1.0 mm and
0.35–0.5 mm fractions. Single >2 mm grains are present.
Unabraded or slightly abraded pyropes are present in concen-
trations ranging from 2 to 21 grains / 20 l in all fractions
(including >2 mm). Pyropes of abrasion III are present in the
0.5–1.0 mm and 0.35–0.5 mm fractions in concentrations of
1–2 grains/20 l.
At a distance of 100 m west from the pipes pyropes of
abrasion rating I-II have been found as single grains and mod-
erately abraded pyropes have not been identified. The content
of picroilmenites with abrasion ratings of I-II decreases sharp-
ly down to single and tens of grains / 20 l. They are present
only in the 0.5–1.0 mm and 0.35–0.5 mm fractions.
Picroilmenites with moderate abrasion are recorded in the
same concentrations and size fractions as over the pipes.
Fewer samples have been collected to the east of these pipes
and sample coverage extends only 1 km east of their location.
No grains of abrasion rating I-II have been found to the east;
only single pyropes and picroilmenites of abrasion III and IV-
V have been recovered. At a distance of 100 m to the south
there are no unabraded pyropes (I-II) and only single
picroilmenites (Fig. 7). At 500 m from the bodies the propor-
tion of fine grains (< 0.5 mm) increases substantially.
Picroilmenites of abrasion rating I and II are present in con-
centrations of single to tens of grains / 20 l, but only to the
west of the bodies. The concentration of moderately abraded
picroilmenites decrease to tens of grains/20 l, and single py-
ropes of abrasion II are present. Moderately abraded pyropes
are not present at this distance from the bodies. Single kim-
berlite minerals with abrasion IV-V are present but these are
likely related to the background KIMs dispersion (see Fig. 7).
At a distance of about 1 km from the kimberlite pipes the
proportion of fine grains (< 0.5 mm) increases substantially
(Fig. 5b). The ratio of 0.5–1.0 mm and 0.35–0.5 mm grains is
1:10. No unabraded or slightly abraded picroilmenites have
been found at this distance (only abrasion III and IV-V are
present). Single pyropes of abrasion I-II are present in the 1–
2 mm and 0.5–1.0 mm fractions. Pyropes of moderate abra-
sion of present at distances of over 1 km to the northwest,
south and east of the pipes.
Conclusions
This study provides insight into the distance that locally-
derived KIMs can travel, degree of grain size sorting and
modification of minerals surface textures with distance from
source in locally-derived KIMs dispersions for the AK10 and
the AK22/AK23 kimberlites of the Orapa kimberlite field.
Both of the study locations presented are characterized by
relatively thin (< 20 m) Kalahari Group sedimentary cover.
The results of this investigation show a decrease in grain size
and the relative content of unabraded and slightly abraded
minerals with distance from the kimberlite source. The reli-
able identification of similar locally-derived KIM dispersion
haloes would be based on the recovery of pyropes and
picroilmenites of abrasion rating I-II in the size fractions larger
than 0.35 mm. Maximum concentrations of unabraded and
slightly abraded pyropes and picroilmenites (hundreds to
thousands of grains/20 l) are recorded directly over the pipes
within small areas of about 50 × 100–300 m. Discovery of
similar dispersion haloes through sampling on a grid of 1 ×
1 km or 500 × 500 m would therefore be challenging, and in
some cases may be impossible. To identify kimberlite bodies
by heavy mineral sampling in similar environments it is nec-
essary to carry out sampling on a more detailed grid: 100 ×
100 m or 200 × 200 m. The optimal sampling volume in this
environment varies based on the thickness of the overlying
sediments. Sample volumes for similar conditions are recom-
mended as follows: up to 10–20 m cover at 20–50 l, 20–30 m
cover at 50–100 l and 30–80 m cover at 250 l. It should be
noted that even single grains of unabraded or slightly abraded
KIMs found in areas with eolian cover sediments are of high
prospecting significance.
Acknowledgements It is a pleasure to thank Natalia V. Antonova and
Irina S. Galushkina for their assistance during the preparation of the
manuscript. We are grateful to Casey M. Hetman and to Gareth Garlick
for constructive comments and detailed review which considerably im-
prove the manuscript.
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
Nowicki TE, Moore RO, Gurney JJ, Baumgartner MC (2007) Diamonds
and associated heavy minerals in kimberlite: a review of key con-
cepts and applications. Dev Sedimentol 58:1235–1267
Ustinov VN (2015) Terrigenous diamond-bearing rocks of the Siberian,
East-European and African Platforms (in Russian). Nauka, Saint-
Petersburg, 531 pp
Ustinov VN, Mosigi B, Kukui IM, Nikolaeva EV (2015) Eolian haloes of
kimberlite indicator minerals (in Russian). Extended abstracts: XV
International meeting on the placers geology and weathering crust
deposits BPlacers and deposits in weathering crusts: study, develop-
ment, ecology^, Perm, pp 228–229
Eolian indicator mineral dispersion haloes from the Orapa kimberlite cluster, Botswana