The document discusses the formation of land-based alluvial diamond deposits in Africa by examining lessons learned from African examples. It describes the principal types of secondary diamond deposits that form in Africa, including retained placers that remain on the craton near the primary kimberlite sources, and transient placers that form as diamonds are transported by river systems across the craton and eventually to the ocean. Key factors that control diamond concentration in these secondary placer deposits include the presence of oversized clasts, increases in average clast size, and areas of increased turbulence such as river bends, rapids, and areas with irregular bedrock geology.

1. FORMATION OF LAND-BASED,
ALLUVIAL DIAMOND DEPOSITS:
LESSONS FROM AFRICA
John Ward
October 2012
DIAMONDS AND THEIR PRIMARY & SECONDARY SOURCES
University of Pretoria

2. PLACER
MECHANICAL CONCENTRATION OF HEAVY MINERALS BY SEDIMENTOLOGICAL PROCESSES
DIAMOND SG
3.52

3. Source: Kimberley Process, 2011
AFRICA – 2011 GLOBAL RANKING
• 7 in top 10 Global producers by Value;
• 15 of 22 Global producers (68%)
• 57% of Global carats (volume);
• 62% of Global value (US$)
• 8 of top 10 US$/ct, with Lesotho at $1,602/ct
• “Alluvials” associated with most of the African producers
1
4
7
5
6
10
8
AFRICAN OCCURRENCES

4. SOME ALLUVIAL DIAMOND MEASURES
1. DIAMOND: Hardness 10; SG 3.52; 2. WEIGHT: 1 CARAT (CT) = 0.2 GRAMS
3. GRADE PER UNIT VOLUME: cts/ m3 (or per 100/ m3 ); 4. GRADE PER UNIT WEIGHT: cts/ ton, cpt (or per 100 ton, cpht)
5. SIZE: CARATS PER STONE (cts/stn)
6. DIAMOND CONTENT USUALLY PARTS PER BILLION - THEREFORE DIFFICULT TO EVALUATE DIAMOND PLACERS!

5. ALLUVIAL DIAMONDS = PEBBLES: ABRADED & UNABRADED
+1ct STONES – VAAL RIVER, RSA

6. Photo: Jana Jacob
WITH TRANSPORT DISTANCE – SIZE DECREASES
DIAMONDS: BASICALLY JUST ANOTHER PEBBLE - SG = 3.52; DURABLE!
Photo: D. Bowen

7. • DIAMONDS TEND TO BE RARE PEBBLES
• ASSOCIATION WITH DISTINCTIVE CLAST
ASSEMBLAGEs

8. TROPICAL TO DESERT CLIMATES
ELUVIAL THROUGH ALLUVIAL TO MARINE
AFRICA DIAMOND PLACERS IN A RANGE OF CLIMATES AND SETTINGS

9. FLUVIAL MARINE
AEOLIAN
GLACIAL
PRINCIPAL SEDIMENTARY PROCESSES TRANSPORTING &/OR CONCENTRATING
DIAMONDS

10. ……‘BUT IN AFRICA THE ACTION IS IN THE RIVERS!
TSHIKAPA RIVER, DRC
Photo: Raymond Kisenge

11. APPROX. DISTRIBUTION OF ALLUVIAL DIAMOND DEPOSITS ( )
& KIMBERLITES IN SUB-SAHARAN AFRICA ( )

12. CRATON = SOURCE
OCEAN / SEA = SINK
AFRICAN CRATON (SUBDUED RELIEF)
PRIMARY SOURCES = KIMBERLITES (1)
SECONDARY SOURCES = RETAINED & TRANSIENT PLACERS (2, 3, 4, 5, 6)
CARTOON OF THE PRINCIPAL TYPES OF ALLUVIAL DIAMOND PLACERS DEVELOPED IN AFRICA
COASTAL PLAIN
LITTORAL
OCEAN / SEA
SECONDARY SOURCE = TERMINAL PLACERS (8)
1
1
2
3
5
4
6
7
8
7
PLACER TYPES LEGEND
RETAINED PLACERS (ON CRATON):
2. EPICLASTIC CRATER FACIES WITHIN KIMBERLITE PIPE (PRESERVATION FACTOR)
3. OLDER INTRA-CRATONIC BASIN
4. KARST
5. INTERNAL DRAINAGE
TRANSIENT PLACERS (CRATON & MARGIN):
6. RIVER TERRACE ON CRATON
7. RIVER TERRACE THROUGH CRATON MARGIN & ON TO COASTAL PLAIN
TERMINAL PLACERS (OCEAN OR SEA):
8: LITTORAL, PREDOMINANTLY MARINE WITH SUBORDINATE AEOLIAN
(BOTH SUB-AERIAL & SUB-MARINE SETTINGS BECAUSE OF SEA LEVEL FLUCTUATIONS)
MARGIN
SECONDARY SOURCES =
TRANSIENT PLACERS (7)
MOUNTAINS
& HILLS
Adapted from: Bluck et al., 2005
RIVER

13. CRATON = SOURCE
OCEAN / SEA = SINK
AFRICAN CRATON (SUBDUED RELIEF)
PRIMARY SOURCES = KIMBERLITES (1)
SECONDARY SOURCES = RETAINED & TRANSIENT PLACERS (2, 3, 4, 5, 6)
CARTOON OF THE PRINCIPAL TYPES OF ALLUVIAL DIAMOND PLACERS DEVELOPED IN AFRICA
COASTAL PLAIN
LITTORAL
OCEAN / SEA
SECONDARY SOURCE = TERMINAL PLACERS (8)
1
1
2
3
5
4
6
7
8
7
MARGIN
SECONDARY SOURCES =
TRANSIENT PLACERS (7)
MOUNTAINS
& HILLS
Adapted from: Bluck et al., 2005
RIVER
RETAINED PLACERS ON CRATON:
2. EPICLASTIC CRATER FACIES WITHIN KIMBERLITE PIPE (PRESERVATION FACTOR)
3. OLDER INTRA-CRATONIC BASIN
4. KARST
5. INTERNAL DRAINAGE

14. VOLCANIC EMPLACEMENT: WEATHERING, REWORKING & TRANSPORT AT SOURCE
KIMBERLITE CRATER ERODED: INTERNAL & EXTERNAL
KIMBERLITE EPICLASTIC CRATER FACIES: INTERNALDe Beers Poster Series

15. RETAINED PLACER: WITHIN KIMBERLITE CRATER
De Beers Poster Series
COARSE MARGINAL DEPOSITS
FINE BASINAL DEPOSITS

16. ANGOLA: ALLUVIAL UPGRADING AWAY FROM PRIMARY SOURCES = 10-20 km DISTANCE?
CATOCA KIMBERLITE, ANGOLA, 2007
???

17. MAIN PIPE
ca. 17 ha
SATELLITE PIPE
ca. 5 ha
LETŠENG DIAMOND MINE, LESOTHO: 2 KIMBERLITE PIPES WITH PROXIMAL ALLUVIALS
PROXIMAL
ALLUVIALS
PROXIMAL
ALLUVIALS 171 ct

18. $ COMPARISON -SATELLITE PIPE VS THE ALLUVIALS
0
100
200
300
400
500
600
700
'+5 '+7 '+9 '+11 3grn 4grn 5grn 6grn 8grn 10grn 3ct 4ct
STONE SIZE
DOLLARS
Letseng
Diamonds
Alluvial
Ventures
(Source: Whitelock, Ward & Smith, 2004)
LETŠENG DIAMOND MINE – LESOTHO:
RAPID UPGRADING IN PROXIMAL ALLUVIALS < 1 KILOMETRE

19. CRATON = SOURCE
OCEAN / SEA = SINK
AFRICAN CRATON (SUBDUED RELIEF)
PRIMARY SOURCES = KIMBERLITES (1)
SECONDARY SOURCES = RETAINED & TRANSIENT PLACERS (2, 3, 4, 5, 6)
CARTOON OF THE PRINCIPAL TYPES OF ALLUVIAL DIAMOND PLACERS DEVELOPED IN AFRICA
COASTAL PLAIN
LITTORAL
OCEAN / SEA
SECONDARY SOURCE = TERMINAL PLACERS (8)
1
1
2
3
5
4
6
7
8
7
MARGIN
SECONDARY SOURCES =
TRANSIENT PLACERS (7)
MOUNTAINS
& HILLS
Adapted from: Bluck et al., 2005
RIVER
RETAINED PLACERS ON CRATON:
2. EPICLASTIC CRATER FACIES WITHIN KIMBERLITE PIPE (PRESERVATION FACTOR)
3. OLDER INTRA-CRATONIC BASIN
4. KARST
5. INTERNAL DRAINAGE

20. TAUODENNI BASIN - MAURITANIA
RETAINED PLACER: OLDER, PROTEROZOIC INTRA-CRATONIC BASINS
BASEMENT
TAUODENNI BASIN SEDIMENTS

21. CALONDA BASIN - ANGOLA
WEATHERED BASEMENT
KALAHARI COVER
CALONDA FORMATION
RETAINED PLACER: OLDER, MESOZOIC INTRA-CRATONIC BASINS

22. OVERBANK FINES (BARREN)
CHANNEL & BAR SANDS WITH GRAVEL
(POORLY DIAMONDIFEROUS TO BARREN)
BASAL COBBLE GRAVEL
(DIAMONDIFEROUS PLACER)
RETAINED PLACER: MESOZOIC CALONDA FORMATION
• FINING - UPWARD SEQUENCE = FLUVIAL, MEANDER BELT
• SCALED TO RIVER SIZE

23. RETAINED PLACER: INTERNAL DRAINAGE (POOR PLACER DEVELOPMENT)
CHANNEL “FLATS”

24. RETAINED PLACER: IN SITU WEATHERING IN CALONDA FORMATION
• “DURABLES” versus “NON-DURABLES”

25. RETAINED PLACER: TERRACE + WEATHERING PROFILE
CHEMICAL & PHYSICAL WEATHERING OF TERRACES = ROOIKOPPIE GRAVELS
UPGRADE DIAMOND CONCENTRATION IN SURFACE LAG BY X10!

26. RETAINED PLACER: KARST TERRAIN DEVELOPED IN FLAT-LYING CARBONATES ON
ARCHAEAN FOOTWALL
PIENAAR’S POTHOLE, LICTENBURG, RSA:
MINED SINKHOLE WITH DIAMOND-BEARING
RUBBLE & ALLUVIUM THAT YIELDED > 0.5M carats

27. RETAINED PLACER: KARST FILL
DOLOMITE HOST
DOLOMITE HOST
GRASFONTEIN, LICHTENBURG, RSA:
UN-MINED SINKHOLE WITH DIAMOND-
BEARING RUBBLE & ALLUVIUM

28. Source:
kimberlite pipes
with eluvial halo in
Calonda/Kwango
Terrace/flat diggings:
proximal population
Terrace/flat diggings:
distal transported population
Majimba hole diggings:
in Calonda/Kwango
Tributary diggings:
proximal population
close to source/Kwango
River diggings:
transported population
in fixed trap sites
Kalahari Group
Gres polymorph
Kwango / Calonda Fm
Kimberlite: source
Basement
High level diggings:
on source + eluvial halo
Fan delta diggings:
proximal population
close to source/Kwango
SIMPLIFIED MODELS FOR NE ANGOLA & SOUTHERN DRC OF:
MESOZOIC INTRA-CRATONIC RETAINED PLACER (CALONDA / KWANGO)
AND
GEOLOGICALLY YOUNG, INCIPIENT TRANSIENT PLACERS SOURCE: Spaggiari & Ward, 2007

29. CRATON = SOURCE
OCEAN / SEA = SINK
AFRICAN CRATON (SUBDUED RELIEF)
PRIMARY SOURCES = KIMBERLITES (1)
SECONDARY SOURCES = RETAINED & TRANSIENT PLACERS (2, 3, 4, 5, 6)
CARTOON OF THE PRINCIPAL TYPES OF ALLUVIAL DIAMOND PLACERS DEVELOPED IN AFRICA
COASTAL PLAIN
LITTORAL
OCEAN / SEA
SECONDARY SOURCE = TERMINAL PLACERS (8)
1
1
2
3
5
4
6
7
8
7
MARGIN
SECONDARY SOURCES =
TRANSIENT PLACERS (7)
MOUNTAINS
& HILLS
Adapted from: Bluck et al., 2005
RIVER
TRANSIENT PLACERS ON CRATON:
6. RIVERS & RIVER TERRACES ON CRATON

30. TRANSIENT PLACERS ON CRATON: FLUVIAL SYSTEMS WITH THIN TERRACE GRAVELS
VAAL & RIET RIVERS, RSA

31. RIVERS IN ALLUVIUM RIVERS IN BEDROCK
TRANSIENT PLACERS: TWO MAIN STYLES OF AFRICAN ALLUVIAL PLACERS

32. TRANSIENT PLACER: KWANGO MEANDER BELT UPSTREAM FROM KNICK POINT
• LOW GRADE (<10 cpht) BUT CHEMICALLY ACTIVE WEATHERING INCREASING RESISTATE
CONCENTRATIONS DOWNSTREAM
• MOBILE, POORLY DEVELOPED PLACER
FLOW

33. TRANSIENT PLACER: KWANGO VALLEY NICK POINT BELOW MEANDER BELT
• HIGH GRADES (up to >2,000 cpht) IN SITES OF FIXED TURBULENCE = WATERFALLS, RAPIDS, ETC
FLOW
MINED OUT WATERFALL

34. TRANSIENT PLACERS: RIVERS IN ALLLUVIUM
•MEANDER BELT WITH KNICK POINT CONTROL
• USUALLY LITHOLOGICALLY (MORE RESISTANT) OR
STRUCTURALLY CONTROLLED (FAULT)
MEANDER BELT
KNICK POINT
TSHIKAPA RIVER CATOCA FALLS - ANGOLA

35. SIMPLIFIED MODEL OF TRANSIENT PLACER FORMATION IN RIVERS FLOWING IN
THEIR OWN ALLUVIUM (MEANDER BELT), CENTRAL & WEST AFRICA

36. POOL REACH
SLOOT
SCOUR
RIVERS IN BEDROCK: FIXED SITES OF TURBULENCE

37. TRANSIENT PLACERS: BEDROCK-INCISED FLUVIAL SYSTEMS
SUPERIMPOSED BEDROCK RIVERS AND EXHUMED, PRE-EXISTING VALLEYS

38. BEDROCK - INCISED RIVER: KASAI RIVER, DRC

39. SO WHERE ARE THE DIAMONDS IN A BEDROCK-INCISED SYSTEM…..?
1.2 carat
0.8 carat

40. CURRENT
DIRECTION
SCOUR
PUSH BAR
BEDROCK HIGH
PROTEROZOIC
BEDROCK
FIXED SITES OF TURBULENCE IN BEDROCK-BASED RIVERS
FROM Jacob et. al, 1999

41. SCOUR FILL & PUSH BAR
GRADES: 10 to 100 X Increase
AVE STONE SIZE: 2 to 4 X Increase
FROM Jacob et. al, 1999
SCOUR FILL
PUSH BAR

42. 2m
SCOURS: BREAK THE TURBULENCE WITH OVERSIZE CLASTS

43. NEED THE OVERSIZE CLASTS TO BREAK THE PRIMARY TURBULENCE….
RICHTERSVELD, LOWER ORANGE RIVER, RSA-NAMIBIAN BORDER

44. INPUT OF OVERSIZE CLASTS FROM STEEP TRIBUTARIES: LOWER ORANGE RIVER, RSA
FLOW

45. INCREASE IN LOCAL CLAST SIZE IN AXIAL DRAINAGE
LOWER ORANGE RIVER

46. BEDROCK HIGHS IN RIVER
COURSE
DYKES; QUARTZ VEINS, ETC.
GRADE: 2 to 10 X Increase
AVE STONE SIZE: 1.5 to 2 X Increase
FROM Jacob et. al, 1999

47. GRAVEL FABRIC
FROM Jacob et. al, 1999 TERTIARY – QUATERNARY FABRIC
PRIMARY - SECONDARY FABRIC

48. CURRENT
DIRECTION
Bar Tail
Platform/Riffle
Bar Head
. . . . . . .
.
..
. . . . . . .
. .
. . . . . . .. . . . . . .
.. ..
.
.
. . . . . . .
. . . . . . .
. . . . . . .
. . . . . . .. . . . . . .. . . . . . ... ..
.
..
. . . . . .
. . . .
. . . . . . .
. . . . . . .
PROTEROZOIC
BEDROCK
MOBILE SITES OF TURBULENCE IN SEDIMENT-BASED RIVER
FROM Jacob et. al, 1999

49. TRANSIENT (MOSTLY FLUVIAL) PLACER DIAMOND CONCENTRATION
WHERE:-
- GRAVELS LIE ON AN IRREGULAR BEDROCK CONTACT
- WHERE OVERSIZE IS PRESENT OR THERE IS AN INCREASE IN AVERAGE CLAST SIZE
- PACKING DENSITY OF GRAVELS IS HIGH

50. CRATON = SOURCE
OCEAN / SEA = SINK
AFRICAN CRATON (SUBDUED RELIEF)
PRIMARY SOURCES = KIMBERLITES (1)
SECONDARY SOURCES = RETAINED & TRANSIENT PLACERS (2, 3, 4, 5, 6)
CARTOON OF THE PRINCIPAL TYPES OF ALLUVIAL DIAMOND PLACERS DEVELOPED IN AFRICA
COASTAL PLAIN
LITTORAL
OCEAN / SEA
SECONDARY SOURCE = TERMINAL PLACERS (8)
1
1
2
3
5
4
6
7
8
7
MARGIN
SECONDARY SOURCES =
TRANSIENT PLACERS (7)
MOUNTAINS
& HILLS
Adapted from: Bluck et al., 2005
RIVER
TRANSIENT PLACERS THROUGH CRATON MARGIN:
7. RIVERS & RIVER TERRACES THROUGH CRATON MARGIN
TRANSIENT PLACERS ON TO COASTAL PLAIN:
7. RIVERS & RIVER TERRACES ONTO COASTAL PLAIN

51. TRANSIENT PLACERS: VAAL-ORANGE DRAINAGE OFF KAAPVAAL CRATON THROUGH
Image: D. Apter, 2004
ORANGE
ORANGE
VAAL
LIMPOPO
ZAMBEZI
KUNENE
TUGELA
OLIFANTS
ATLANTIC OCEAN

52. TRANSIENT PLACER: BEDROCK - INCISED RIVER THROUGH MOBILE BELT
ORANGE RIVER BELOW AUGRABIES FALLS (POTENTIAL NUGGET EFFECT)
NATURAL BALLMILL:
UPGRADING THE DIAMOND QUALITY

53. TRANSIENT PLACER: FLUVIAL TRUNK (AXIAL) DRAINAGE THROUGH MOBILE BELT
ORANGE RIVER THROUGH THE RICHTERSVELD

54. TRANSIENT PLACERS: ORANGE RIVER TERRACES: LONG GEOLOGICAL HISTORY
• INTERIOR GRAVELS DELIVERED TO ATLANTIC OCEAN FOR AT LEAST 42 Ma
TERRACES YOUNGING
Eocene
Marine Fossils
RSA Interior Gravels in Marine
Deposits

55. TRANSIENT PLACERS - FLUVIAL SYSTEMS WITH AGGRADED TERRACE GRAVELS
LOWER ORANGE RIVER,
NAMIBIA/RSA BORDER

56. TRANSIENT PLACER – COASTAL PLAIN – TERMINAL PLACER INTERFACE
ORANGE RIVER MOUTH

57. CRATON = SOURCE
OCEAN / SEA = SINK
AFRICAN CRATON (SUBDUED RELIEF)
PRIMARY SOURCES = KIMBERLITES (1)
SECONDARY SOURCES = RETAINED & TRANSIENT PLACERS (2, 3, 4, 5, 6)
CARTOON OF THE PRINCIPAL TYPES OF ALLUVIAL DIAMOND PLACERS DEVELOPED IN AFRICA
COASTAL PLAIN
LITTORAL
OCEAN / SEA
SECONDARY SOURCE = TERMINAL PLACERS (8)
1
1
2
3
5
4
6
7
8
7
PLACER TYPES LEGEND
RETAINED PLACERS (ON CRATON):
2. EPICLASTIC CRATER FACIES WITHIN KIMBERLITE PIPE (PRESERVATION FACTOR)
3. OLDER INTRA-CRATONIC BASIN
4. KARST
5. INTERNAL DRAINAGE
TRANSIENT PLACERS (CRATON & MARGIN):
6. RIVER TERRACE ON CRATON
7. RIVER TERRACE THROUGH CRATON MARGIN & ON TO COASTAL PLAIN
TERMINAL PLACERS (OCEAN OR SEA):
8: LITTORAL, PREDOMINANTLY MARINE WITH SUBORDINATE AEOLIAN
(BOTH SUB-AERIAL & SUB-MARINE SETTINGS BECAUSE OF SEA LEVEL FLUCTUATIONS)
MARGIN
SECONDARY SOURCES =
TRANSIENT PLACERS (7)
MOUNTAINS
& HILLS
Adapted from: Bluck et al., 2005
RIVER