By Petronella MohaleRockwell Diamonds, Douglas, Northern Cape

petronellam@rockwelldiamonds.com

The impact of mining methodsON grade control at

Northern CapeNiewejaarskraal Mine

Geological setting The Niewejaarskraal mine is located along the south bank of the Orange River between the towns of Douglas and Prieska (figure 1).

Figure 1: Map showing the location of the Niewejaarskraal mine

Types of gravelsRooikoppie

• Deflatedgravel

• Rooikoppiegravelisanunconsolidatedgravelpackage clasts of BIF, Quartzite, Chert, Quartz, Agate, Jasper and Ventersdorp Lava in an oxidised reddish sand matrix (Marshall 2004, McCarthy, 1983)

Figure 2: Rooikoppie gravel and Makondos

Palaeo gravel• Thepalaeogravelwasdepositedbybraided

channels.

• The bars consist of heads and tails of bars that become reworked during the transportation stage (figure 3)

Gravel description• Thegravelispoorlysortedinasandytogritty

calcretised matrix

• Packagesubdividedintoupper,middlingsandbasal gravel (figure 4)

• Middlings/uppergravelisarematrixsupportedwith a dominance of pebble to cobble sized clasts

• Thebasalgraveliscoarser,comprisingofpebble to boulder sized clasts, and has a compound fabric. Gravel more enriched compared to middlings

Figure 4: Upper Orange River gravel (A1), underlain by middlings and a coarse basal gravel (A2), refer to Figure 5

Diamond distribution• Diamondshavebeendescribedasoccurring

as uneven or non-uniformly distributed clusters that are randomly distributed in space (Rombouts, 1987)

Importance of mining method• Selectiveminingisadifficultmethodwith

positives and negatives

• Positives:

– High grading

• Negatives

– High strip ratio

– Thin layers, some reworked, risk of stripping high grade areas

– Reduction of resource volume (life of mine)

Application of mining methods at NJK• Miningisdoneusingaslopingminingmethod

(figure 5)

• Liberation,blendingoflowandhighgradeareas

Figure 5: A view of the gravel sequence and how they are mined

Grade• Gradevariationexample(figure6)

• A,Thickpackageofmiddlings+basalgravelonbedrockhigh–0,44ct/100m3,B-upper/middlingsthicker–0,22ct/100m3, C-Basal gravelbelowB–0,85ct/100m3

• Resultantgrade0,54ct/100m3

Figure 6: Grade variation

15 May 2014 9 June 2014

0

2,50

2,00

1,50

1,00

0,50

Grad

e (c

phm

3 )

Block NN 31 grade

REFERENCEGresse, P. G. (2003). The preservation of alluvial diamond deposits in abandoned meanders of the middle Orange River. Diamonds: Source to Use Colloquim October 29-30, 2003. Saxonwold: SA National Museum of Military History.

Jacob, R. J. (2005). The erosional and Cainozoic depositional history of the lower Orange River, southwestern Africa. Glasgow: Unpubl PhD thesis. University of Glasgow.

Marshall, T. R. (2004, September). Rooikoppie Deposits of South Africa. Rough Diamond Review, 21-25.

Marshall, T. R., Norton. A. G (2010). Technical report on the Niewejaarskraal property. Company technical report, 1-117

McCarthy, T. C. (1983). Evidence for the former existence of a major, southerly flowing river in Griqualand West. Transactions of the Geological Society of South Africa, 86(1), 37-49.

Rombouts, L. (1987). Evaluation of low grade/high value diamond deposits. Mining Magazine, 217-220.

Uncovering the facets of our potential…ACKNOWLEDGEMENTI would like to thank God and the Rockwell team for providing an environment conducive to learning and growth and the opportunity to share the work we do to add value to the Diamond Industry at large, I am truly grateful.

Figure 3: A view of the bar aggradation process

420 x 594 PETRONELLA.indd 1 2014/09/02 1:44 PM