Sorting of Alluvial Diamonds
Sorting of Alluvial Diamonds
Sorting of Alluvial Diamonds
AND THE
Abstract
Oa61-O128/S2/SS/161a-852.50 1618
1614 DONALD G. SUTHERLAND
20øE
nshoso
ZAIRE
ANGOLA
de Corvoiho
I 100km
almostall the mininghasbeenof deposits relatedto during the Cretaceous.The concentrationof dia-
the presentdrainagenetwork,but as hasbeenmost mondsin the Cretaceous conglomerates is not suffi-
fully documented by Fieremans(1955,1961),the ini- ciently high to be of economicinterest,but the sub-
tial distribution of diamonds acrossthe area was ef- sequentreworkingand reconcentrationof the dia-
fectedduring the Cretaceous, when a massiveflu- mondsduringincisionof the presentriver system,
viodeltaicdepositwas built out into the seathat was alliedto a certainamountof transportfrom the kim-
thenslowlyrecedingfrom the Zairebasin.Fieremans berlitesby theserivers,hasresultedin the formation
(1961) was able to demonstratethat the diamonds of economically viabledepositsas much as 600 km
containedwithin the Cretaceous conglomeratic ho- from the source.
rizonsfocused
on an areain northernAngola(Fig. In Sierra Leone, which has limited areas of known
1) where numerouskimberlite bodieshave been dis- kimberlites,
no intermediate hostrockshaveplayed
covered(Friere de Andrade,1952;Reis, 1972). As anyrolein thedistributionof thediamonds alongthe
discussed
later in this paper,the diamondsdiminish presentriversor their formercourses (Fig. 2). The
systematicallyin size away from thesekimberlites; presentAtlantic-directeddrainagehasprobably been
thisobservation,
in additionto thegeographic
focus evolvingsincetheinitiationof Atlanticriftingin this
of the alluvialdiamonddeposits
on the kimberlites, areaabout185m.y. ago(Dalrympleet al., 1975)and
establishes
thismajordiamondfield asthe product musthavebeena well-developed riversystem by the
ofessentially
oneperiod
oferosion
anddeltabuilding timeof kimberliteintrusion about92 m.y. ago(Bar-
TRANSPORT AND SORTING OF DIAMONDS 1615
g-
GUINEA
,gema
Ton(
ooo
8ON
0 km
ATLANTIC
12 11øW
I
ZO
• Nomibson coast
'" • N'Go•r•
0.5 i
650 km
0.0
0 50 ,oo km
zoo ,;o 350 '
20
I///X•\\- n=/.90Oct Upper Sewa
Middle
Lower
Sewa
Sewa
58
66
77
88
80
20
4
4
cleavagestones,translucentor coateddiamonds,etc.
, i I I I "l'-•
004 1 2 3 & 5 6 7 8 9 10 11 12 (Cotty and Wilks, 1971; Harris et al., 1975; 1979).
c! Transportof diamondsapparentlyaffectsthe original
populationby preferentiallydestroyingcertaintypes
and more easilytransportingothers.
50 lb)
Hall (1968)hasgivenfiguresthat relateto the per-
centagesof clear,coated,and bort diamondsat cer-
tain localitiesalong the River Sewa in Sierra Leone,
including the area around the kimberlites at Yen-
/"orl•\ .... n:3/,0Oct gema. Table 2 summarizesthese statistics.Quite
clearlythere is a preferentiallossof bort and coated
30
stonesduring transport. It seemsprobable that the
bort is lost by breakageinto tiny particles;in an ex-
periment with a ball mill, Linari-Linholm (1978)
20
showedthat only six hoursof milling was necessary
to reducebort diamondsfrom M'buji-Mayi in Zaire
to lessthan 60-mesh size (i.e., < 0.001 carat), while
10 after 950 hoursof milling, gem quality diamonds
from the coastaldepositsof Namibia lost only 0.01
percentof their weight. The consistentreductionin
0.04 1 2 3 /, 5 6 7 8 9 10 11 12
the proportion of coated diamondsdown the Sewa
ct may be due to the effectsof surfaceattrition, since
60 the greencoatingson the SierraLeonediamondsare
frequentlyrather shallowand are particularlybrittle
(Grantham and Allen, 1960). The proportions of
50 (c) cleavageand broken diamonds (in the sizesrecov-
ered) also decreaseswith transport; the Namibian
coastaldiamonds,for example,are more than 95 per-
centcomposed of wholecrystalforms(Hallam, 1964).
The dominant crystalforms of diamond in kim-
40r
*•30
I n:t,700ct berlite are the octahedron
(Harris et al., 1979). Transportappearsto favor the
dodecahedron as the depositsalongthe westcoastof
and the dodecahedron
1960; Harris et al., 1975). This factor, togetherwith Thisanomalyof verylargeaverage-sized diamonds
the apparentlyvariableresponse of differentdiamond nearthemouthof theOrangeRivercanbe explained
typesto abrasion,suggests that there may be consid- if (1) the generaltransportmodeldoesnot holdfor
erable differencesin the degreeof developmentof the OrangeRiver, (2) the averagediamondsizein
thesesurfacefeaturesbetweenseparatedepositional the sourcearea is very high, or ($) there is a source
trainsthat are independentof the distancetraveled. (or sources) for a significantproportionof the dia-
In summary,it is apparentthat transporttendsto mondsmuchnearerthan the Kimberleyarea. There
increasethe quality of the diamonds,preferentially is no independentevidencefor the first possibility,
removingbort, mechanicallyweakerdiamonds,and while the secondis contradictedby the size of the
cleavagestones.The greater easewith which more diamonds that were recovered from the River Vaal
rounded forms are entrained also contributes to the workingsin the BarklyWestarea.The information
generalincreasein quality becausesuchdiamonds on diamondcharacteristics pointsto the third possi-
are likelyto sufferlessweightlossduringcutting.The bility, whichhasbeensuggested
elsewhere(Keyser,
absolutevalue (dollarsper carat) of a diamond pop- 1972)from an independentline of reasoning. The
ulation from a particular part of a transportcurve is lack of kimberlite satellite minerals associatedwith
therefore a balance between the reduction in size and thesediamonddeposits arguesagainsta nearbypri-
the increasein quality that are complementaryto the mary source,and it may be that the diamondswere
transport process. hostedin Karroo or older rocks and subsequently
freedby erosionassociated
with the downcutting
of
Discussion the Orange River.
Size diminutionduring transportationcan result
In the precedingsectionsthe generalfeaturesof from eitherthe preferentialcarryingof smallerpar-
the variationsin diamond populationshave been out- ticlesor breakageand attrition of larger particles.
lined. The data for thesegeneralizations have been Both theseeffectsoperateduring the transportation
takenfrom a variety of localities,includingexamples of diamonds,breakageand attritionbeingindicated
of both alluvial and coastaldeposits.An anomaly is by the disappearance of inferior quality diamonds
revealed,however,by the data on size diminution. and the roundingof the cornersof more resistant
This concernsthe coastaldepositsof Namibia. ones,while preferentialcarry is indicated by the
The decrease in diamond size northward from the changingproportions of crystalforms.The Namibian
mouth of the Orange River depicted in Figure $ is coastaldeposits are of particularinterestbecausethe
similar to the variations in size northward from the diamondsthere almostentirely exhibitcrystalforms
mouthsof the Buffels, Swartlientjes,and Olifants riv- and are not brokenor cleavagestones.Breakagecan-
ers along the west coastof South Africa (Hallam, not, therefore, be important in producingthe size
1964). These relationshipsare most easily explained diminutionalong the coast,which must be domi-
by the fact that the rivers are the meansof intro- nantly a sortingprocess.
duction of the diamonds to the coastal zone, after
whichalongshore waveactiontransports and sortsthe Conclusions
diamondsin a northerly direction accordingto the
dominant fetch along this coast.Certain diamonds This paperhasdemonstrated that diamondsfrom
relativelyrestrictedsourceareascanbe transported
recovered from the coast near the mouth of the Or-
ange River have been recognizedas very probably for manyhundreds of kilometers
anddistributedover
havingoriginatedin theKimberleyarea(Wagnerand tensof thousands of squarekilometerseitherby for-
Merensky,1928, p. 20), which implies a transport mer riversthat depositedintermediatehostrocksor
distance for some of the diamonds of at least 1,600 by rivercapturesrelatedto the evolutionof the pres-
km (seealsoWilliams, 1952). The averagediamond ent drainagesystem.Littoral processes are alsoef-
sizenear the mouth of the Orange River is, however, fectivein transporting
diamonds for hundreds of kilo-
over 1 carat per stone(Stocken,1962; Hallam, 1964), meters.
which is in marked contrastto the predicted small Duringtransportation
a varietyof effectshavebeen
diamondsizesthat shouldprevailafter severalhundred indicated.The averagediamondsizediminishesac-
kilometersof transport,accordingto the model pre- cordingto a modifiedexponentialrule, and thissize
sentedearlier and supportedby the size variation diminution is accompaniedby better sorting,an in-
along the coastnorth of the Orange River mouth. creasein the kurtosis,and a decreasein the skewness
Furthermore, the diamonds recovered from the ter- of the diamond size distributions.In general, poorer
racesthat flank the Orange River for over 50 km qualitydiamondssuchasbort,diamonds with inclu-
inland of its mouth also contain diamonds that av-
sions,
andcleavage stones
arepreferentially
destroyed
erageover 1 earat per stone(de BeersGeol. Dept., during transport,while the more roundedcrystal
1976). forms(suchasthe dodecahedra) appearto be more
1620 DONALD G. SUTHERLAND
easily transported.The general quality of the dia- du Toit, A. L., 1951, The diamondiferousgravelsof Lichtenburg:
mondsis thereforeimproved. SouthAfrica Geol. Survey Mere 44, 38 p.
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to their source. The converse, however, need not be OverseasGeol. Mineral Resources,
v. 8, p. 5-25.
true, as the primary sourcemay be a high-quality Hall, P. K., 1968, The diamond fields of Sierra Leone. Sierra Leone
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Hallam, C. D., 1964,The geologyof the coastaldiamonddeposits
diamonds.If the qualityand the sizeof the diamonds
of southernAfrica (1959),in Haughton,S. H., ed., The geology
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averagediamondsize and/or a high proportionof E., 1975,A classification
schemefor diamondand a comparative
study of South African diamondcharacteristics: Physicsand
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from somesouthernAfrican kimberlites,in Boyd, IV. R., and
Meyer, H. O. A., eds., Kimberlites,diatremes,and diamonds:
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