Diamond-Bearing Gravels Along the Lower Kwango River DRC - Geology and Resource Potential of the Congo Basin - page 347

Fig. 16.21 Grade required to Break-Even (cpht) analysis for Second Splay target (strip ratio of 23:1)

For example, anecdotal diamond production figures
provided by artisanal diggers at the K56 target, combined
with estimates made by BRC staff of gravel volume
processed, enabled calculation of a

In addition, there is a steady decrease in the diamond
sizes downstream in the river basin, supporting a
transported model.
Importantly the Guiliame and Francois-Joseph Falls
near Tembo have a negative impact on the diamond size
distribution directly downstream of the Falls. Here a high
level of breakages of the diamonds is observed, diamond
size is significantly lowered, there is a high proportion of
diamond fragments and the diamonds have a high level of
abrasion features.
Finally, there is also a significant decrease in diamond
concentration in the lower part of the Kwango where the
footwall is composed of aeolian facies of the Kwango
Group, which does not facilitate diamond concentration
compared to the more favourable basement footwall
found upstream of the Falls.
These factors coupled with a small average stone size
(0.07 to 0.1 cts/stn) that earns a lower carat value and the
high stripping ratios and mining costs, presents an
unfavourable economic model at today

back of the envelope

'
grade figure ranging from 20 cpht to 50 cpht. Notwithstand-
ing concerns with relying on production figures provided
by the artisanal diggers these grades coincide well with
diamond grades cited in the historical literature (see Histori-
cal Background) and grades reported at the Midamines oper-
ation which have been estimated to be 25 cpht going up to
between 50 and 100 cpht, where gravel has been deposited
on basement. Sadly, almost no basement occurs in the BRC
project area resulting in less efficient diamond traps. As a
result, BRC concluded that a trial mining exercise would not
be warranted.

'

Conclusions
The stone tool artefacts that have been found within the
terraces along the lower Kwango indicate that these
deposits are younger than Early to Middle Pleistocene.
Hence the oldest recorded diamond occurrences
introduced into these terraces are relatively recent. It is
suggested that the small diamonds along the international
Kwango form part of the advancing diamond front
stretching from Angola. Stream sediment sampling for
kimberlitic heavy minerals at exploration sites along the
international Kwango and its main tributaries confirms
that no other diamond input points exist at least in the
Congo. These kimberlitic minerals also show a high
degree of abrasion to support a transported population.

s diamond prices.

'

Ackowledgements The authors wish to thank the company BRC Pty
Ltd (now Delrand Resources Pty Ltd ) for permission to publish these
exploration data. Renato Spaggiari and Pat Sheahan are thanked for
their comments which significantly improved the manuscript, John
Ward for his geological observations on site and Bastien Linol for his
help with one of the diagrams. Thanks must also go to the various field
personnel that have been involved in the drilling and pitting operations.
Reviewers John Bristow, Tania Marshall and Judith Kinnaird are
thanked for their valuable input and edits.