By Penelope MohaleRockwell Diamonds, Douglas, Northern Cape

penelopem@rockwelldiamonds.com

gravel sources on gradeand diamond quality case study:

Northern CapeSaxendrift Hill Complex Mine

The influence of

Introduction Saxendrift Hill Complex is situated along the south bank of the middle Orange River between Douglas and Prieska in the Northern Cape Province of South Africa (Figure 1). In order to determine grades achievable on certain portions of the property it is important to identify what type of source was dominant during gravel deposition as well as other factors including gravel type and different trapsites which influence the grade. This study will focus on the two dominant sources on the property, the different gravel types and trapsites which influence grades achieved thus far. Mapping is of great importance in order to identify the different sources The Kimberlites around the MOR region include the Kimberley and Finch Kimberlites and the Orange River could have brought diamonds from that region (Figure 2).

Picture 1: MOR feed type containing high calcrete content

Palaeo gravel, river sources and trapsitesThe Palaeo gravel is divided into different gravel units which may be interbedded by sand units throughout the sequence. The upper/intermediate gravel is generally finer in a sandy matrix and sees a high zeolitic content throughout the property. The basal gravel is generally coarser in a matrix that varies and ranges from sandy to pebbly and this is also observed throughout the property. The overall sequence has been observed to be fining upwards. The basal package is enriched with diamonds of various sizes while the Upper/Intermediate gravel is enriched in small diamonds and can sometimes be barren depending on the amount of weathering or reworking. These different layers vary in thicknesses ranging from one to six metres. Indicator minerals have been identified and used based on their dominance to determine the source. Basalts, zeolites, granite have been identified as Orange River indicator minerals (Figure 3A). Banded Iron Formation, quartzite, chert, agate, Riebeckite are some of the dominant indicator minerals from the Vaal River (Figure 3B). Individual gravel can have either an Orange River source, Vaal River source or an influence of both.

Through mining and mapping of different sites of the SHC property it is important to identify areas where the optimal concentration of diamonds occurs. These areas can be identified using drilling data (drilled boreholes) and through mapping.

Figure 2: Map of different kimberlites sources

Figure 3: A: Orange River dominated source and B: A Vaal dominated source

A B

There are two basic types of diamond trapsites which are, fixed and mobile trapsites and these are important to identify. Fixed trapsites are known to have a high concentration of diamonds and are created by turbulences in the vicinity of the bedrock. Mobile trapsites are generally associated with aggradation sequences such as alluvial bar features (Jacob et al. 1999). Bedrock depressions, bedrock highs and push bar features have been identified on the property and are seen to act as important fixed trapsites for diamonds. Mobile trapsites which are defined by “head” and “tail” features have been identified within the gravel sequences mentioned above. The head of a bar is termed as an area where coarsest gravel is deposited and the area most favourable for diamond concentration. The tail of a bar is an area defined by fine gravel and deposit of sand formed in lower energy conditions and is generally barren or very low grade. These bars can be Orange or Vaal River influenced and as mentioned above indicator minerals are used to determine this.

GradesGrades have been seen to vary throughout the property due to the mentioned above. The head of a gravel bar is normally enriched with diamonds and can have both an Orange River source and/or Vaal River influenced source. The tail of a bar however can be barren or low grade due to lower energy. The Orange River source is known to enrich the gravel with good quality stones, at a low grade while the Vaal River influenced source is associated with high grade and can have low quality stones (Figure 3).

Figure 5: Grade plot showing different sources associated with grades

Diamond quality and valueThe quality of a diamond is defined by the following: size, shape, colour, cut and clarity. This is used to value a diamond during the evaluation process. The two to 10 sieve classes were used to compare the different values of diamonds per quarter and this data was then related to whether the diamonds recovered were from an Orange River or a Vaal River source (See figure below). There is a difference in sizes between the two sources with the Vaal River influenced gravel seen to have a finer distribution as compared to the Orange River. The Vaal has a better assortment (quality, colour, cut) and tends to have a better dollar per carat especially on the one to 2.5 carat sieve class. The Orange River is seen to have a better assortment on the large stones from +5 to +10 carat sieve class shown by Q3 and Q4 (see Graph 1). The Alana is one of the good quality diamonds recovered from the SHC property in September 2013.

Figure 6: Graph showing dollar per carat values for three quarters

Figure 7: A view of a 169 carat diamond from Orange River gravel cut to a 109 carat

ConclusionThis study shows that grade can be influenced by different factors including the source and different trapsites as mentioned above, it is important to identify the indicator minerals which link to the source and understand whether mining is in an area of diamond concentration or barren area which then relates to the grade achievable. The two sources identified have been seen to have different grade distribution and different diamond assortment with the Vaal having higher value 1-2.5 carat sieve class and the Orange River has higher value in the +5 to +10 carat sieve class from dataset used in this study.

Uncovering the facets of our potential…REREFENCEJacob, R. J., Bluck B. J., Ward J. D 1999. Tertiary-Age Diamondiferous Fluvial Deposits of the Lower Orange River Valley. Economic Geology,vol.94. pp.749-758.Jacob, R. J. 2005. The erosional and Cainozoic depositional history of the lower Orange River, South-Western Africa. Glasgow: Unpubl PhD thesis. University of Glasgow.Marshall, T.R. and Norton, G.A. 2012. Technical report on the Saxendrift alluvial diamond mine (incorporating the Saxendrift, Kwartelspan and Kransfontein properties), Hay district, Republic of South Africa, For Rockwell Diamonds Inc.Diamond Geology, www.debeersgroup.com

Figure 4: Mobile sedimentary trapsites

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