A possible new alluvial diamond field related to the Klipspringer kimberlite swarm, South Africa. TS MCarthy

1. T.S. MCCARTHY AND J.G. ALLAN
SOUTH AFRICAN JOURNAL OF GEOLOGY, 2007, VOLUME 110 PAGE 503-510
503
Introduction
The Marsfontein (M-1) kimberlite pipe (Figure 1) was
discovered in February 1997 by Mike Scott and
Associates (MSA) during follow-up investigations on
positive regional reconnaissance samples collected early
in 1993 (Davies, 2000). At the time of the discovery, MSA
were contracted to Southern Era Resources. The pipe
was relatively small (40 m x 90 m), but the grade was
exceptional. The surficial deposits capping the pipe
carried a grade of 4.5 carats per tonne, and yielded a
value of US$130 per carat. At depth, grade decreased to
3 carats per tonne, with a value of US$142 per carat
(Ruffini, 1998). The open pit established to mine the
pipe had a projected life of two years, and is now
closed. This operation was remarkable in that the
payback period was only one week! (Davies, 2000).
The M-1 pipe is one of several on the farm Marsfontein
91 KS, which form part of what appears to be an
extensive kimberlite pipe and fissure system that has
become known as the Klipspringer swarm.
The kimberlite swarm lies within the Olifants River
drainage basin. The Olifants River flows to the southeast
of the area, whence it makes its way down the
escarpment onto the Lowveld. The river and its
tributaries were extensively prospected in the latter half
of the 19th century, particularly in the escarpment area,
in the search for gold. Many of those early prospectors
would have had experience of alluvial diamond
workings (they included Herbert Rhodes, brother of
Cecil Rhodes: Cartwright, 1962), and yet the Olifants
River system yielded no alluvial diamonds. The possible
reasons for, and the implications of, the apparent dearth
of alluvial diamonds, especially from the Klipspringer
swarm, are explored in this paper.
Regional Geology
Basement geology
The Marsfontein kimberlite intrudes basement granites
and gneisses which form the core of a broad,
southwesterly plunging anticline overlain by Wolkberg
Group and Transvaal Supergroup rocks, which dip to
the south and west (Figure 2). The Transvaal Supergroup
has been intruded by ultramafic and mafic rocks of the
Bushveld Complex, which is locally unconformably
overlain by the Karoo Supergroup.
The southern flank of the basement anticline has
been structurally disturbed by a major tectonic zone, the
Thabazimbi-Murchison Lineament, which has a long
history of activity (e.g. du Plessis, 1991). Post-Karoo
activity along this zone is reflected in the Zebediela fault,
which has brought Karoo Supergroup in juxtaposition
with rocks of the Transvaal Supergroup and Bushveld
Complex (Figure 2). The Zebediela fault strikes east-
west in the Marsfontein area, but changes strike in the
west to a southwesterly orientation, where it borders
the Nylsvley flood plain system. Further to the west are
several splays that include the Wonderfontein fault,
which is associated with the thermal springs in the
Warmbad area. Temperley (1975) has suggested that this
fault system may be neo-tectonically active, and
McCarthy and Hancox (2000) attributed the existence of
the Nylsvley wetland to neo-tectonic activity on the fault.
Several boreholes have been drilled through the
Karoo in this area (Visser and van der Merwe, 1959).
A possible new alluvial diamond field related to the Klipspringer
kimberlite swarm, South Africa
T.S. McCarthy
School of Geosciences, University of the Witwatersrand, Private Bag 3, WITS 2050
e-mail: terence.mccarthy@wits.ac.za
J.G. Allan
Allan Hochreiter (Pty) Limited, P.O.Box 411130, Hyde Park 2196
e-mail: james@allanhochreiter.co.za
© 2007 December Geological Society of South Africa
ABSTRACT
The Marsfontein M-1 kimberlite pipe in the Mokopane (Potgietersrus) area is the highest grade kimberlite ever mined in southern
Africa, and forms part of an extensive east-northeast striking pipe and fissure swarm. The kimberlites are of late Jurassic age and
were emplaced into basement and formerly overlying Karoo Supergroup rocks. The area hosting the kimberlites was subject to
erosion, which has removed the upper portions of the kimberlites and their Karoo host rocks. This phase of erosion commenced
in the Pliocene as a result of the onset of subsidence of the Bushveld Basin. Subsidence was accommodated in part by the
Zebediela fault, which passes immediately south of the kimberlite swarm. Material eroded from the vicinity of the swarm was
deposited south of the fault, burying down-faulted Karoo Supergroup rocks and forming an extensive bahada, which includes up
to 30 m of gravel in proximity to the fault. Alluvial deposits consisting of sand and gravel extend as much as 20 km south of the
fault. It is estimated that some 9 billion tonnes of gravel occur in the section of the deposit immediately south of the kimberlite
swarm with an average grade possibly exceeding 0.2 c/100 tonnes, and it is possible that economically viable diamond
accumulations are locally developed within this gravel sheet.
doi:10.2113/gssajg.110.4.503

2. In the area immediately south of the Zebediela fault, the
maximum thickness of the Karoo is in the region of
370 m. The Dwyka Group is only very sporadically
developed in this area, and the Ecca Group varies from
0 to 30 m thick. Maximum thickness of Beaufort Group
correlatives is about 10 m, while combined thickness of
the Irrigasie (Molteno) and Clarence Formations varies
from 70 to 140 m. The basalts of the Letaba Formation
vary from 45 to 195 m, but the top is erosional.
The minimum vertical displacement on the Zebediela
fault is 155 m, based on the thickness of the Karoo
Supergroup on the farm Zebediela 101 KS (borehole 11,
Visser and van der Merwe, 1959), but appears to vary
along strike, and may exceed 370 m in places.
The Karoo Supergroup thickens towards the south, and
the lower formations, particularly the Ecca Group,
become more prominent, suggesting that the area to the
north of the Zebediela fault was a topographic high
during Karoo times.
Geological mapping in the region has revealed the
presence of numerous northeasterly striking photo-
lineaments, particularly in the basement and Proterozoic
rocks, some of which represent dykes of a variety
of rock types (Geol. Survey, 1978). However, many are
of unknown origin.
Exploration in the Marsfontein area between 1995
and 1997 revealed the presence of several kimberlite
pipes and northeasterly striking fissures (Figure 1;
Davies, 2000). In addition to the eight bodies illustrated
in Figure 1, Ruffini (1998) reported further discoveries:
on the farm Rusland, the Kudu pipe (30 m x 20 m),
200 m north of the Leopard fissure; the Eland fissure,
120 m north of the Kudu pipe, striking parallel to the
Leopard fissure; and a further fissure (Hartebeest) to
the north of this; on Meinhardskraal, to the north of
Marsfontein, a fissure, which probably represents an
extension of the Leopard fissure; and on Marsfontein,
the M3, M4 and M5 bodies, as well as several
prospective heavy mineral anomalies. Collectively, these
finds indicate the presence of a major kimberlite fissure
and pipe swarm. Petrographic work indicates that the
kimberlites are of Group II type, and represent
hypabyssal facies (Vorster, 1999). The fissures on Rusland
have been dated at 148 ± 4 Ma (Westerlund, 2000).
Surficial deposits
The drilling programme carried out by the Geological
Survey on the Springbok Flats (Visser and van der
Merwe, 1959) revealed the presence of extensive
surficial deposits as well as deep weathering of the
underlying Karoo strata. Boreholes drilled in the vicinity
of the Zebediela fault indicated the presence of a thick
gravel wedge overlying the basalt. The underlying basalt
is decomposed to depths of over 30 m. Elsewhere on the
Flats, accumulations of attapulgite up to 12 m thick were
recorded.
The surficial deposits on the farm Zebediela 101 KS
were investigated in considerable detail by Pretorius
(1970) as part of a geohydrological investigation of the
property. Over 300 boreholes were drilled on the farm
in the course of exploration for water to meet
agricultural requirements. These boreholes were drilled
using simple jumper drills and bailers, and drillers kept
logs which Pretorius used to compile a stratigraphy of
the surficial deposits on the farm. Although relatively
crude, these logs led him to deduce the presence of an
extensive alluvial and colluvial blanket across the farm,
and moreover enabled him to construct isopachs of the
SOUTH AFRICAN JOURNAL OF GEOLOGY
A POSSIBLE NEW ALLUVIAL DIAMOND FIELD504
Figure 1. Map showing the locations of some of the kimberlites of the Klipspringer swarm (Davies, 2000), boreholes drilled by the
Geological Survey (Visser and van der Merwe, 1959), and various farms mentioned in the text.

3. gravel (termed “scree” by Pretorius) and sand
accumulations across the property. Figure 3 shows the
total thickness of alluvial deposit on the property
(“alluvium plus scree”), and Figures 4 and 5 show the
thicknesses of gravel (“scree”) and sand respectively.
These isopach maps indicate that the southern portion
of the farm is underlain by more than 15 m of gravel,
and that there are three channel-like deposits of
gravel across the property in which gravel thickness
exceeds 30 m. It should be noted that Pretorius was
evidently unaware of the existence of the Zebediela
fault, and makes no reference to it in his work.
Detailed information exists only for the Zebediela
area, which is small in relation to the total extent of the
Zebediela fault. Nevertheless, it provides some insight
into the nature of surficial deposits which may be
present elsewhere along the fault, as well as more
distally to the south. The lateral continuity of the gravel
deposits along the strike of the Zebediela fault is
confirmed by the drilling programme of the Geological
Survey (boreholes 12, 13 and 14, Figure 1; Visser and
van der Merwe, 1959), which revealed gravel overlying
bedrock.
Regional Geomorphology
The area is transected by a major watershed,
separating the Olifants River system to the south from
the Magalakweng and Sand (Limpopo) drainage
system to the north (Figure 6). Regional geology
influences drainage patterns and topography, but only
locally. The resistant quartzites of the Wolkberg Group
and Transvaal Supergroup form the high ranges
of the Buffelshoekberge, Highlands Mountains and
Strydpoortberge (Figure 6), while to the southwest,
and south of the Zebediela fault, are the Springbok Flats,
underlain by rocks of the Karoo Supergroup.
There are several unusual features of drainages in the
area. On pre-Karoo rocks, drainages are dendritic and
well developed. East-northeast striking quartzites of the
Wolkberg Group and Black Reef Formation on
the southern limb of the anticline form the prominent
Strydpoortberge range. Larger drainages originating on
relatively low lying basement granite to the north of this
range (Hlakaro, Chuniespoort, and Nkupi or Gompies
Rivers and Dooringrivier) cut almost perpendicularly
across these resistant rocks in deep gorges, and drain
towards the Olifants River to the south. However, as
these drainages cross the Zebediela fault onto the
Springbok Flats they undergo a radical change in
character: they experience a decrease in gradient (Hugo
and Hattingh, 1971), and several simply die out (“flood
out” in the terminology of Tooth, 1999). Only the larger
drainages traverse the Springbok Flats to reach the
Olifants River, and even moderate drainages, such as
the Rooisloot, terminate before reaching the Olifants
River (Figure 6). Numerous abandoned or defunct
channels are present on the Flats as far as 20 km south
of the Zebediela fault (Figure 6), presumably reflecting
former positions of drainages emanating from the
highlands to the north. The behaviour of these rivers
indicates that the terrain to the south of the
Zebediela fault is an environment of active sediment
accumulation.
The Springbok Flats is a geomorphologically unusual
feature in the southern African landscape, which has
attracted comment from numerous researchers over
many decades (e.g. Wagner, 1927; du Toit, 1933; King,
1942; Partridge and Maud, 1987; 2000). Its most striking
T.S. MCCARTHY AND J.G. ALLAN
SOUTH AFRICAN JOURNAL OF GEOLOGY
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Figure 2. Simplified geological map of the area around the Klipspringer kimberlite swarm (after Geological Survey, 1978).

4. SOUTH AFRICAN JOURNAL OF GEOLOGY
A POSSIBLE NEW ALLUVIAL DIAMOND FIELD506
features are its lack of relief and the almost total absence
of a coherent drainage pattern. Smaller rivers which
discharge onto the Flats from the higher lying
surrounding areas flood out and disappear, notably
along the Nyl River (McCarthy and Hancox, 2000; Tooth
et al., 2002) and south of the Zebediela fault as
described above, while on the Flats themselves, there is
no coherent drainage system, and surface water arising
from exceptional rainfall is dissipated by sheet flooding.
The Flats are also host to the Nylsvley wetland, a large
area along the Nyl River where there is no defined
channel and which is prone to extensive seasonal sheet
flooding (McCarthy and Hancox, 2000; Tooth et al., 2002).
Discussion
Pre-Karoo topography
The deposition of the Karoo Supergroup was preceded
by extensive, continental glaciation, the erosion
products of which constitute the Dwyka Formation.
The topography at that time was irregular, and Visser
(1987) has identified a prominent mountainous zone
extending across southern Africa from the region of the
lower Orange River to Mpumalanga in the northeast,
which he termed the Cargonian Highlands. Dwyka
Formation tends to be thin or absent over this highland
area, and stratigraphically overlying formations of the
Karoo Supergroup often exhibit onlapping relationships
against the highland. The terminal phase of the Karoo
cycle of sedimentation was marked by the eruption of
the Drakensberg basalts and its stratigraphic equivalents
at 184 Ma, which probably covered most of the
subcontinent. Subsequent denudation has removed
much of the Karoo Supergroup from the Cargonian
Highlands, exposing a broad belt of Precambrian rocks
across southern Africa. Only isolated outliers of Karoo
remain within this zone, one of which underlies the
Springbok Flats.
Based on observations made along the Vaal River,
Helgren (1979) has argued that there has been very little
erosion of the pre-Dwyka basement over large areas to
the north of the river. Moreover, King (1942) suggested
that many of the prominent quartzitic ranges of the
northern provinces such as the Magaliesberg, Daspoort
and Witwatersrand are pre-Karoo in age, as they show
evidence of superimposed drainage in the form of small
streams and rivers which transect the resistant quartzites
in deep gorges. Recently, Cawthorn (2001) has argued
that the topography of the eastern Bushveld, and the
Leolo Mountains in particular, is also pre-Karoo in age,
a view also shared by King (1942).
Thinning of the lower units of Karoo Supergroup
towards the north in the Springbok Flats, as revealed by
drilling, suggests that the mountainous area to the north
of the Zebediela fault represents a pre-Karoo
topographic high. It is therefore suggested that the
Buffelshoekberge and the Strydpoortberge are also pre-
Karoo features. In support of this is the fact that the
drainage in the area is superimposed, and minor streams
Figure 3. Total thickness of alluvial deposits on the farm Zebediela 101KS (reproduced from Pretorius, 1970).

5. T.S. MCCARTHY AND J.G. ALLAN
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such as the Hlakaro, Chunies, Nkupi and Doring Rivers
perpendicularly transect the quartzites of the
Strydpoortberge (Figure 6).
The age and evolution of the Springbok Flats
The oldest land surface in southern Africa, the African
surface, is characterized by the development of deep
weathering and kaolinitization. Remnants of this surface
are widely scattered over southern Africa (Partridge and
Maud, 1987; 2000). This was followed by incision during
the Miocene, forming the Post-African I surface, which is
believed to be widely preserved over southern Africa.
According to Partridge and Maud (2000), advanced
planation was only achieved in a few areas, one being
the Springbok Flats. They believe that Post African
I landscape development was limited to the removal of
the deep weathering mantle of the African surface in
many areas.
During the late Neogene (Pliocene), the Bushveld
Basin experienced subsidence, a phenomenon first
recognized by du Toit (1933), who speculated that the
interior had subsided by as much as 300 m.
The southern margin of the Basin also experienced
uplift, along what du Toit termed the Griqualand-
Transvaal Axis. Subsidence along the northern margin of
the Basin was largely accommodated by displacement
along the Thabazimbi-Murchison Lineament, and
especially the Zebediela and associated faults.
The preservation of Letaba (Drakensberg) Formation
volcanics in the Springbok Flats area indicates that there
has been relatively little erosional denudation in the area
since the Cretaceous, and the land surface would be
similar in age to the Lesotho plateau. The lack of a
coherent drainage indicates a supermature peneplain,
and the deep weathering profiles encountered in
boreholes in the area suggest that the African surface
may be preserved over large areas of the Flats, and
certainly in the area south of the Zebediela fault. It is
probable that the Karoo Supergroup and the African
Surface extended to the north of the Zebediela fault
prior to its activation in the Pliocene.
Subsidence of the Bushveld basin activated the
Zebediela fault and down-cutting of Karoo strata to
the north of the fault would have commenced. Material
shed from this high ground appears to have collected as
a wedge against the fault (Figures 3 to 5), burying the
down-faulted African surface to the south. As the Karoo
was gradually stripped from the area to the north of
the fault, drainages became superimposed on the
underlying pre-Karoo rocks. Today, many of the streams
rising north of the fault deposit their sediment load and
flood out on the Flats to the south, indicating that the
area remains an environment of net aggradation.
In effect, the area south of the Zebediela fault is
essentially an alluvial apron, or more correctly a
bahada.
Figure 4. Thickness of the gravel (“scree”) component of the alluvial deposit on the farm Zebediela 101KS (reproduced from Pretorius,
1970).

6. Certain of the larger streams traverse the Flats to the
Olifants River, while the smaller streams flood out on
the flats. During wetter periods, the smaller streams may
have extended further onto the Flats. Aggradation
associated with river channels would have resulted in
frequent channel avulsion, spreading the apron of gravel
widely across the area to the south. Abandoned
channels are evident more than 20 km south of the fault
(Figure 6), indicating the presence of a very extensive
alluvial apron.
Depth of erosion of the Klipspringer kimberlites
The Klipspringer kimberlites are hypabyssal facies, and
have experienced post emplacement erosion. Southern
Era staff have speculated that between 500 and 1000 m
of material has been eroded from the M-1 pipe (Vorster,
1999). It is possible to make some estimate of the likely
extent of this erosion by reconstructing the geology of
the area in the Cretaceous. It is evident from the
superimposition of drainages that the area to the north
of the Zebediela fault was once buried beneath the
Karoo sequence, although it probably contained local
topographic highs, as discussed above. The foot-slopes
of these highs are preserved to the south of the
Zebediela fault, where the lower formations of
the Karoo Supergroup have been found to thin by onlap
towards the fault.
In order to obtain a minimum estimate of erosion, it
is assumed that minimal erosion of pre-Karoo formations
has occurred. The elevation of the kimberlite outcrops
ranges from around 1130 m for the M-1 pipe to around
1500 m for the fissures and pipes on Rusland. Borehole
13 of Visser and van der Merwe (1959) intersected
350 m of Karoo. Assuming that this represents the total
fault displacement (i.e. no erosion of pre-Karoo units),
and that the fault outcrops at 1100m, the elevation of the
Karoo palaeosurface would have been in the region of
1450 m, using present elevation datum. On this basis,
high ground on Rusland would have projected through
the Karoo cover, while the Marsfontein pipes would
have lost the upper 320 m. This calculation is very
approximate, and clearly underestimates the extent of
erosion of the higher lying kimberlites, but it does
suggest that the extent of erosion is relatively modest,
particularly for the kimberlites which outcrop in the
more elevated terrain. It is suggested that perhaps no
more than about 100 m has been removed from the
more elevated kimberlites, and about 500 m from
kimberlites outcropping in basement.
Economic implications
Prior to the subsidence of the Bushveld Basin, it is likely
that the area north of the Zebediela fault was covered by
Karoo Supergroup. There had probably been minimal
erosion of the Karoo lavas since their emplacement, and
crater facies may well have been preserved above the
kimberlites. The onset of subsidence of the Bushveld
Basin activated the Zebediela fault, and a drainage
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Figure 5. Thickness of the sand component of the alluvial deposit on the farm Zebediela 101KS (reproduced from Pretorius, 1970).

7. T.S. MCCARTHY AND J.G. ALLAN
SOUTH AFRICAN JOURNAL OF GEOLOGY
509
network began to develop on the Karoo rocks to the
north of the fault, which stripped these strata, exposing
basement and the pre-Karoo topography. Slow
subsidence and denudation superimposed the Karoo
drainage net onto the pre-Karoo basement.
Sediments derived from the north were deposited in
the subsiding basin south of the fault due to the
pronounced decrease in gradient. It is likely that only
very fine material, essentially silt and clay, would have
been able to be transported across the Flats. During
more arid periods, even this fine-grained sediment
would have been deposited on the Flats. Subsidence
and erosion were probably slow, controlled by regional
epeirogenic subsidence, and it is possible that most of
the Karoo host rocks to the kimberlites would have been
destroyed by weathering, leaving only more resistant
lithologies in the gravel wedge. Geomorphological
evidence suggests that this gravel wedge extends more
than 20 km south of the fault. The area hosting the
intrusives was denuded by a number of streams, which
crossed the fault over virtually the entire known strike
length of the swarm. The diamonds released during
erosion of the kimberlites would have been trapped in
the alluvial apron deposited by these streams. Drilling
on the farm Zebediela suggests that gravel constitutes a
significant proportion of the alluvial deposit, but the
proportion of gravel probably decreases to the south.
The potential tonnage can be estimated: known
strike length of the fissure system is about 30 km
(Figure 1), and gravels have been dispersed to a distance
of at least 20 km south of the fault. Average gravel
thickness on the farm Zebediela is about 15 m, and we
assume it thins out linearly over the 20 km. Average
thickness is thus 7.5 m. Using these estimates and a
density of 2000 kg/m3
for in situ gravel, the probable
tonnage of gravel amounts to 9 x 109
tonnes.
The quantity of diamonds delivered to the deposit is
difficult to estimate, but the M-1 pipe alone could have
released some 21 million carats (assuming 500 m of
erosion, a diameter of 80 m, a grade of 3 c/tonne, and a
density of 2800 kg/tonne, based on the average of three
samples from the Marsfontein pit). If this were uniformly
distributed in the gravel apron, the average grade would
be 0.23 c/100 tonnes. There are many other pipes and
fissures in the swarm that would also have contributed
diamonds, which would increase the average grade.
The grade is unlikely to be uniform, as certain of the
streams would have transported more kimberlitic
material than others. Moreover, sedimentological
processes would have locally concentrated diamonds.
The Mogoto and Nkupi streams in particular seem to
have most of the presently known kimberlites within
their catchments (Figure 6), and hence the eastern
portion of the delineated block is more prospective.
However, prior to the stripping of the Karoo cover, other
streams may have drained the area to the north, which
have since been cannibalized by the present network,
and thus zones of enrichment may be widespread
through the gravel apron.
The rate of subsidence of the area south of the fault
relative to the rate of denudation to the north would also
influence local concentration of diamonds in the gravel.
Rapid down-faulting, coupled with rapid denudation,
would result in very low grades in the gravel apron.
In contrast, slow subsidence would enhance reworking
of the gravel, resulting in higher grades. Variation in
Figure 6. Drainage patterns in the vicinity of the Klipspringer kimberlite swarm.

8. subsidence rate would therefore be reflected in channel-
like zones of higher grade within the gravel apron.
On average, subsidence rates were probably very low as
only some 15 m of sediment has accumulated in about
5 Ma (this is about the same accumulation rate as
the Kalahari Basin), and it is therefore suggested that the
deposits in the alluvial apron have been subjected to
fairly intensive reworking.
Conclusions
An extensive late Jurassic kimberlite fissure and pipe
swarm is developed immediately north of the Springbok
Flats. Although currently hosted in Transvaal Supergroup
and older rocks, the area was previously overlain by
Karoo Supergroup, through which the kimberlites
intruded. Geomorphological analysis suggests that the
area experienced little erosion until the Pliocene, when
subsidence of the Bushveld Basin commenced.
This subsidence was partially accommodated along the
Zebediela fault, which passes immediately south of
the kimberlite swarm. Subsidence along this fault
resulted in erosion of the Karoo strata from the area of
the swarm, and the eroded material, including
diamonds, was deposited in an extensive clastic wedge
south of the fault. This area is still one of active
aggradation.
Extensive drilling on the farm Zebediela has revealed
the presence of a gravel sheet at least 15 m thick over
the southern half of the farm. There are indications that
this gravel layer is developed along much of the fault.
Surface geomorphology indicates that the gravel
probably extends at least 20 km south of the fault.
The apparent absence of alluvial diamonds in the
Olifants River may well be a result of the entrapment
of the eroded diamonds in the gravels south of
the Zebediela fault. Rough estimates suggest that in the
region of 9 billions tonnes of gravel containing in excess
of 20 million carats are preserved in the clastic wedge
immediately south of the kimberlite swarm, and it is
possible that zones of economic grade are developed
within this gravel deposit.
Acknowledgements
Thanks to Wayne Colliston and Tanya Marshall for
constructive comments on this manuscript.
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A POSSIBLE NEW ALLUVIAL DIAMOND FIELD510