sorting of alluvial diamonds

ECONOMIC GEOLOGY AND THE

BULLETIN OF THE SOCIETY OF ECONOMIC'GEOLOGISTS

VOL. 77 NOVEMBER, 1982 No. 7

The Transport and Sorting of Diamonds by _Fluvial and Marine Processes DONAI•D G. SUTHERI•AND

Placer Analysis Ltd., 2 London Street, Edinburgh Eft3 6NA, Scotland

Abstract

This paper describes the systematic variations in size and quality of diamonds that result from transport by fluvial and marine processes. It is shown that diamonds originating in rather restricted source areas can be spread across many tens of thousands of square kilometers. Downstream or alongshore transport can be followed for several hundred kilometers, with a systematic reduction in average diamond size accompanying such transport. The diamond size distribution is progressively modified with increasing travel distance, and the quality of the diamonds improves as inferior types are destroyed. The accompanying variations in crystal form are outlined. Certain implications for exploration programs are •nentioned.

Introduction

1N the late 1940s, production of dia•nonds from alluvial sources represented about 40 percent of the world total. Thirty years later, alluvial diamond production had more than doubled, and despite the development of major new kimberlite mines, it still represented more than $0 percent of total natural diamond production. The economic importance of alluvial diamonds is thus considerable, and it is fur- ther emphasized by the fact that alluvial diamonds are of consistently higher quality than diamonds recovered from the source kimberlites and also because

certain countries (e.g., Sierra Leone, Central African Republic) are economically largely dependent on the production of alluvial diamonds. Furthermore, while it is generally considered that kimberlite pipes represent the more promising longterm mining target, it is usually through the presence of alluvial diamonds that kimberlite source rocks of economic interest are first suspected.

The wide distribution and economic importance of alluvial diamonds have given rise to a large lit- erature, but the great majority of the studies pub- lished are descriptive and tend to concentrate on the particular characteristics of the local deposits. This concern with the unique aspects of the deposits has led to the continuation of the old adage that "diamonds are where you find them," and many mineral exploration strategies are based on the blanket sample coverage approach that this viewpoint implies. It is the purpose of this paper to demonstrate that alluvial diamonds are as amenable to systematic analysis as any other geologic phenomenon and that it is possible

to produce general statements that are capable of quantifieation and valid for all alluvial diamond deposits wherever they occur.

The Extent of Dimnond Distribution

The hardness of diamond when subjected to a grinding action, its stability at relatively high tem- peratures and pressures, and its chemical inertness all suggest that once diamond is released from its primary source rock it should tend to persist in the sed- imentary record. The brittleness of diamond, which leads to shattering under a sharp impact, will only act to reduce its size. Moreover, as has happened in Ghana (Junner, 1948), diamonds are able to undergo metamorphic events that alter the surrounding rock types and to be released to the fluvial system at a later date.

This ability of diamond to withstand most surface and near-surface processes is reflected in the large size of many diamond fields. The fields of Sierra Leone, for example, cover about 20,000 km s (Hall, 1968) and those astride the Zaire-Angola border, over 60,000 km s (Bardet, 1974). The wide distribution of alluvial diamonds in certain regions can be attributed in part to the occurrence of multiple primary sources, as is the case along the River Vaal in South Africa (Wagner, 1914) and in Guinea (Bardet, 1974), as well as to alluvial dispersion. Consideration, however, of the Zaire-Angola and Sierra Leone examples establishes that diamonds can be widely distributed from relatively limited source areas.

The diamond fields that straddle the Zaire-Angola border are illustrated in Figure 1; the M'buji-Mayi deposits in Zaire are not considered here. In this area

Oa61-O128/S2/SS/161a-852.50 1618

1614 DONALD G. SUTHERLAND

i

20øE

nshoso

I 100km

ZAIRE

ANGOLA

de Corvoiho

FlC;. 1. The diamond fields of the Zaire-Angola border. Symbols: 1, alluvial diamond workings; 2, kimberlite localities; $, limit of occurrence of diamonds in Cretaceous host rocks; 4, crystalline basement rocks; 5, international boundary. Henrique de Carvalho has been renamed Saurimo. Sources: Bardet (1974); Fieremans (1961, 1977); Reis (1972).

almost all the mining has been of deposits related to the present drainage network, but as has been most fully documented by Fieremans (1955, 1961), the initial distribution of diamonds across the area was ef- fected during the Cretaceous, when a massive flu- viodeltaic deposit was built out into the sea that was then slowly receding from the Zaire basin. Fieremans (1961) was able to demonstrate that the diamonds contained within the Cretaceous conglomeratic ho- rizons focused on an area in northern Angola (Fig. 1) where numerous kimberlite bodies have been dis- covered (Friere de Andrade, 1952; Reis, 1972). As discussed later in this paper, the diamonds diminish systematically in size away from these kimberlites; this observation, in addition to the geographic focus of the alluvial diamond deposits on the kimberlites, establishes this major diamond field as the product of essentially one period of erosion and delta building

during the Cretaceous. The concentration of diamonds in the Cretaceous conglomerates is not suffi- ciently high to be of economic interest, but the sub- sequent reworking and reconcentration of the diamonds during incision of the present river system, allied to a certain amount of transport from the kimberlites by these rivers, has resulted in the formation of economically viable deposits as much as 600 km from the source.

In Sierra Leone, which has limited areas of known kimberlites, no intermediate host rocks have played any role in the distribution of the diamonds along the present rivers or their former courses (Fig. 2). The present Atlantic-directed drainage has probably been evolving since the initiation of Atlantic rifting in this area about 185 m.y. ago (Dalrymple et al., 1975) and must have been a well-developed river system by the time of kimberlite intrusion about 92 m.y. ago (Bar-

TRANSPORT AND SORTING OF DIAMONDS 1615

Ton(

,gema

GUINEA

ooo

g-

8ON

ATLANTIC 0 km

12 11øW I

FIe;. 2. The diamond fields of Sierra Leone. Symbols: 1, areas of extensive alluvial diamond workings; 2, localized alluvial diamond workings; $, kimberlite; 4, Bullom Group coastal sediments; 5, international boundary. Sources: Hall (1968) and unpub. repts., Sierra Leone Geological Survey.

det, 1974). The regional tectonic situation is that of a slowly subsiding continental margin allied to a steadily uplifting watershed area, and this has pro- duced an incising river network on the resulting regional slope. This incision (perhaps accelerated by Quaternary climatic and sea-level fluctuations) has given rise to an apparently superimposed drainage network cutting across the maior rock formations and shows close correspondence with more minor structural features at a local level.

River capture has been relatively frequent in such circumstances, as the incising river network has ad- justed to the variations in the underlying structural framework, and this has resulted in a rather wide

distribution of diamond deposits in some areas away from the trunk streams, the Sewa and the Moa. Hall (1968) has argued that an unknown type of diamond source rock is responsible for the wide distribution of diamonds in Sierra Leone. The systematic changes in the size and characteristics of diamonds along the rivers away from the known kimberlites (see below) indicate, however, that while undiscovered sources may exist, they are subsidiary to the known sources as suppliers of diamonds to the fluvial system. It is therefore clear that since the Late Cretaceous dia-

monds have been transported from the known source areas as far as the coastal Bullom Group sediments, a distance of over 200 km.

1616

ZO

DONALD G. SUTHERLAND

• Nomibson coast

'" .... Kasoi

"• Kwango ß

'" • N'Go•r•

0.5 i

0.0 0 50

650 km

,oo zoo ,;o ' 350 km

FI(;. $. The variation in average diamond size with travel distanceß ct/st = carat per stone.

Sorting During Transport

During transport diamonds are sorted by size such that the farther from the source, the smaller the average size of the diamonds. In more detail the size distribution of the diamonds may also be expected to change with transport, while the morphologic characteristics of the diamond population should also vary systematically. These three effects are examined in turn.

Size sorting in the direction of transport In Figure $ the variations in the mean size of dia-

monds along the direction of transport for a number of deposits have been plotted. Three of the deposits represent fluvial sorting processes: Kasai (Fieremans, 1961), the Kwango River (Fieremans, 1977), and the N'Go•r• in the Central African Republic (Berthoum- ieux and Delany, 1956). The other deposit represents alongshore transport by marine processes in the west coast of Namibia (Stocken, 1962). The N'Go•r• deposit has data for only about 50 km of transport; the others have all been traced for hundreds of kilome-

ters. The irregularities on the graphs are likely to be the result of nonuniform sampling, of local sorting effects, and most probably, of the introduction of diamonds from secondary sources such as in the northern part of the Namibian deposits (Hallam, 1964). The term "transport curve" is proposed for

this variation in size with distance downriver or alongshore.

In general the transport curves have a similar form, a rather steeply declining initial portion followed by a more gently declining tail. The final value, toward which all the curves tend, is due in part to the min- imum recovery size of the mining operations, but in some deposits it is claimed that few diamonds occur at smaller sizes (e.g., Hallam, 1964, p. 716). The rapid initial diminution in average diamond size is too steep for the data to be represented by a simple exponential curve, and instead a good fit was found with the fol- lowing modified exponential model:

y = a. exp(-b. xU2), where y is the average diamond size expressed in carats per stone, x is the distance from source mea- sured in kilometers, and a and b are parameters de- termined by a least squares method of curve fitting. In geologic terms, a is equal to the average diamond size in the source area (i.e., at x -- 0 km) and b is the decay constant that relates to the rapidity with which the average size diminishes with transport. Curves of this type have been fitted to the data in Figure g as well as to certain other unpublished data and give good agreement with the observations. The theoretical curve for the Namibian deposit is shown in Figure g, and the values of the relevant parameters 'for all the deposits are given in Table 1.


As shown in Table I there is close agreement between the values of the decay constants, and although there are relatively few deposits considered here, this agreement suggests that values of b may be similar in other deposits. The value of the parameter a is clearly specific to a given source area. The model can be applied to a given river system if the mean diamond size in the source area is known. Thus along the River Sewa in Sierra Leone, if the average diamond size recovered from the alluvial deposits im- mediately adjacent to the kimberlites at Yengema (Fig. 2) is assumed to be about 1 carat per stone (a not unrealistic figure given the frequent recovery of large diamonds in this area) and if the Sewa is assumed to be analogous to the River Kwango in its transport characteristics, the predicted average size of diamonds about 170 km downstream is 0.18 carat

per stone, a figure close to the approximately 0.2 carat per stone average size recovered in prospecting operations in this area.

A final point should be made about the implications of this general model: if the average diamond size in the source area is small, the rapid initial diminution in size suggests that diamonds large enough to be of economic interest may only be found within several tens of kilometers of their source, rather than the hundreds of kilometers exemplified by the above deposits.

Variation in size distributions

The above arguments are based on the use of a single measure to provide an estimate of diamond size at a particular locality. Inevitably such a measure is rather imprecise, and it is more common in sedi- mentological work to consider the size distribution of the particles under study, as this distribution may depart significantly from normal and hence diminish the utility of the mean (or any other single statistic) as a summary of the population size. This matter has an added importance in the consideration of diamonds because there is a nonlinear relationship between the size of a diamond and its value; a small percentage of large diamonds may represent a very significant proportion of the mining income.

It is therefore of both geologic and economic interest to note that there are systematic variations along the direction of transport in the size distributions of diamond populations. These variations are depicted in Figure 4. In Figure 4a two size distribution curves are shown for deposits closely related to their source rocks. Figure 4b depicts two deposits that are from the middle portion of the transport curve, while Figure 4c represents a deposit in the lower portion of the transport curve. The size distributions are based on the weight percent of diamonds occurring in the given class intervals rather than the

T^BLE 1. Parameters for the Theoretical Size Diminution

of Diamonds with Transport

Deposit a b

Namibian coast 1.93 0.16 Kasai 0.92 0.15

Kwango 0.83 0.13 N'Go•r• 0.69 0.18

a = average diamond size in carats per stone in the source area b = decay constant

more frequently reported number percent. This has the advantage of being in line with usual sedimen- tologic practice and of being of greater relevance to the economic assessment of the deposit. There is an artificial lower limit (40.01 carat) to the <0.04-carat class owing to the nonrecovery of very small diamonds during mining. With the exception of the far- thest downtransport deposit in which there is a significant percentage of diamonds in the lowest class, the weight percentages in this class (which accord with the small number percentages for similar class sizes reported from other deposits, e.g., Sichel, 1975, table $) suggest that there •nay not in fact be many diamonds below this size occurring in these deposits, although problems of recovery and identification of very small diamonds make this statement difficult to verify.

A number of syste•natic variations are apparent in the size distributions. With increased transport it is apparent that, together with a shift toward smaller diamonds, the distributions become much more peaked and the tail of coarse dia•nonds that is very prominent in the near-source deposits is progressively diminished. In conventional statistical terms the deposits show an increase in sorting and kurtosis with a decrease in skewness as the mean size diminishes with greater transport.

These sorting effects have obvious implications for the prospecting of deposits that are located at different parts of the transport curve as well as for ultimate economic returns. The greater frequency of large diamonds near the source areas means not only a higher average value (dollars per carat) for the diamonds but also necessitates that prospecting samples be larger and of greater frequency than those in deposits that have resulted from greater transport (cf. Applin, 1972).

Variations in other diamond characteristics

A diamond population is not just composed of diamonds of differing sizes but also of varying colors, crystal forms, and diamond types. Diamond populations from different sources thus are composed of varying percentages of bort, octahedra, dodecahedra,


4O TABLE 2. Diamond Types along the River Sewa, Sierra Leone

Locality Clear Coated Bort (wt %)

(o)

•0 /'-'x._ - - n = 1700 ct

20 I///X•\\ - n = /.90Oct

, i I I I "l'-• 004 1 2 3 & 5 6 7 8 9 10 11 12

c!

50 lb)

/"orl •\ .... n: 3/,0Oct 30

20

10

0.04 1 2 3 /, 5 6 7 8 9 10 11 12 ct

60

50 (c)

40r I n: t, 700ct *•30

20

10

OJ3/. 1 2 3 4 5 6 7 8 9 10 11 12 C!

FIG. 4. Weight percentage diamond size distributions from different parts of the transport curve. a. near source. b. middle part of curve. c. lower curve. ct = carat.

Yengema 50 45 5 Upper Sewa 58 88 4 Middle Sewa 66 80 4

Lower Sewa 77 20

cleavage stones, translucent or coated diamonds, etc. (Cotty and Wilks, 1971; Harris et al., 1975; 1979). Transport of diamonds apparently affects the original population by preferentially destroying certain types and more easily transporting others.

Hall (1968) has given figures that relate to the percentages of clear, coated, and bort diamonds at certain localities along the River Sewa in Sierra Leone, including the area around the kimberlites at Yen- gema. Table 2 summarizes these statistics. Quite clearly there is a preferential loss of bort and coated stones during transport. It seems probable that the bort is lost by breakage into tiny particles; in an ex- periment with a ball mill, Linari-Linholm (1978) showed that only six hours of milling was necessary to reduce bort diamonds from M'buji-Mayi in Zaire to less than 60-mesh size (i.e., < 0.001 carat), while after 950 hours of milling, gem quality diamonds from the coastal deposits of Namibia lost only 0.01 percent of their weight. The consistent reduction in the proportion of coated diamonds down the Sewa may be due to the effects of surface attrition, since the green coatings on the Sierra Leone diamonds are frequently rather shallow and are particularly brittle (Grantham and Allen, 1960). The proportions of cleavage and broken diamonds (in the sizes recovered) also decreases with transport; the Namibian coastal diamonds, for example, are more than 95 percent composed of whole crystal forms (Hallam, 1964).

The dominant crystal forms of diamond in kimberlite are the octahedron and the dodecahedron

(Harris et al., 1979). Transport appears to favor the dodecahedron as the deposits along the west coast of South Africa have disproportionately high percentages of this form. This effect is understandable in terms of the mechanics of initiation of particle move- ment; the more nearly spherical dodecahedral crystals would become entrained more easily than the more elongate octahedra. Dodecahedra will therefore be subjected, on average, to a greater number of transport events and will also be in motion longer during any one such event.

Surface abrasion features such as percussion mark- ings and rounded corners may be expected to result from transport, but abraded diamonds are also widely reported from kimberlites (Grantham and Allen,


1960; Harris et al., 1975). This factor, together with the apparently variable response of different diamond types to abrasion, suggests that there may be considerable differences in the degree of development of these surface features between separate depositional trains that are independent of the distance traveled.

In summary, it is apparent that transport tends to increase the quality of the diamonds, preferentially removing bort, mechanically weaker diamonds, and cleavage stones. The greater ease with which more rounded forms are entrained also contributes to the

general increase in quality because such diamonds are likely to suffer less weight loss during cutting. The absolute value (dollars per carat) of a diamond population from a particular part of a transport curve is therefore a balance between the reduction in size and

the increase in quality that are complementary to the transport process.

Discussion

In the preceding sections the general features of the variations in diamond populations have been outlined. The data for these generalizations have been taken from a variety of localities, including examples of both alluvial and coastal deposits. An anomaly is revealed, however, by the data on size diminution. This concerns the coastal deposits of Namibia.

The decrease in diamond size northward from the

mouth of the Orange River depicted in Figure $ is similar to the variations in size northward from the

mouths of the Buff els, Swartlientjes, and Olifants rivers along the west coast of South Africa (Hallam, 1964). These relationships are most easily explained by the fact that the rivers are the means of introduction of the diamonds to the coastal zone, after which alongshore wave action transports and sorts the diamonds in a northerly direction according to the dominant fetch along this coast. Certain diamonds recovered from the coast near the mouth of the Or-

ange River have been recognized as very probably having originated in the Kimberley area (Wagner and Merensky, 1928, p. 20), which implies a transport distance for some of the diamonds of at least 1,600 km (see also Williams, 1952). The average diamond size near the mouth of the Orange River is, however, over 1 carat per stone (Stocken, 1962; Hallam, 1964), which is in marked contrast to the predicted small diamond sizes that should prevail after several hundred kilometers of transport, according to the model pre- sented earlier and supported by the size variation along the coast north of the Orange River mouth. Furthermore, the diamonds recovered from the ter- races that flank the Orange River for over 50 km inland of its mouth also contain diamonds that av-

erage over 1 earat per stone (de Beers Geol. Dept., 1976).

This anomaly of very large average-sized diamonds near the mouth of the Orange River can be explained if (1) the general transport model does not hold for the Orange River, (2) the average diamond size in the source area is very high, or ($) there is a source (or sources) for a significant proportion of the diamonds much nearer than the Kimberley area. There is no independent evidence for the first possibility, while the second is contradicted by the size of the diamonds that were recovered from the River Vaal workings in the Barkly West area. The information on diamond characteristics points to the third possibility, which has been suggested elsewhere (Keyser, 1972) from an independent line of reasoning. The lack of kimberlite satellite minerals associated with these diamond deposits argues against a nearby primary source, and it may be that the diamonds were hosted in Karroo or older rocks and subsequently freed by erosion associated with the downcutting of the Orange River.

Size diminution during transportation can result from either the preferential carrying of smaller particles or breakage and attrition of larger particles. Both these effects operate during the transportation of diamonds, breakage and attrition being indicated by the disappearance of inferior quality diamonds and the rounding of the corners of more resistant ones, while preferential carry is indicated by the changing proportions of crystal forms. The Namibian coastal deposits are of particular interest because the diamonds there almost entirely exhibit crystal forms and are not broken or cleavage stones. Breakage can- not, therefore, be important in producing the size diminution along the coast, which must be domi- nantly a sorting process.

Conclusions

This paper has demonstrated that diamonds from relatively restricted source areas can be transported for many hundreds of kilometers and distributed over tens of thousands of square kilometers either by former rivers that deposited intermediate host rocks or by river captures related to the evolution of the present drainage system. Littoral processes are also ef- fective in transporting diamonds for hundreds of kilometers.

During transportation a variety of effects have been indicated. The average diamond size diminishes according to a modified exponential rule, and this size diminution is accompanied by better sorting, an increase in the kurtosis, and a decrease in the skewness of the diamond size distributions. In general, poorer quality diamonds such as bort, diamonds with inclu- sions, and cleavage stones are preferentially destroyed during transport, while the more rounded crystal forms (such as the dodecahedra) appear to be more


easily transported. The general quality of the diamonds is therefore improved.

The above effects are of considerable relevance for

diamond exploration (Lampietti and Sutherland, 1978), both at the regional scale in identifying primary sources or large volume alluvial targets and at the small scale in deciding on sampling patterns for alluvial deposits. It may be presumed, for example, that deposits with a large proportion of inferior diamonds and bort (e.g., the Lichtenburg deposits in South Africa, du Toit, 1951) have resulted from relatively slight transportation and are relatively close to their source. The converse, however, need not be true, as the primary source may be a high-quality producer with only a small proportion of inferior diamonds. If the quality and the size of the diamonds in a source area are known, the likelihood of there being extensive placer deposits downstream of this zone can be assessed. Thus a source exhibiting low average diamond size and/or a high proportion of inferior quality diamonds is less likely to produce placer deposits of economic interest for more than several tens of kilometers downstream.

Acknowledgments

Thanks are due to M. Dale, Placer Analysis Ltd., and D. Hodgson, Department of Geography, Uni- versity of Edinburgh, for comments on the initial draft of this paper.

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sorting of alluvial diamonds

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