A study of the variation in the Rooikoppie gravels in the Lower Vaal area
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1 A study of the variation in the Rooikoppie gravels in the Lower Vaal area A study of the variation in the Rooikoppie gravels in the Lower Vaal area M. Ndwammbi Rockwell Diamonds Inc The Rooikoppie gravel deposits in the Lower Vaal region are deflated gravels, derived from primary fluvial gravels that are Miocene in age; owing to their remobilized state the Rooikoppie gravels cannot be constrained to a specific age. These primary fluvial gravels are deflated from their original thicknesses and have been reworked by eluvial and colluvial processes to form the Rooikoppie gravels. The Rooikoppie gravels are so termed due to their reddish colour, which has resulted from iron staining of the entire package during oxidation. The colluvial gravels on the Ditaimaneng property are uncemented and consist of pebble- to boulder-size resistant clasts in a fine sandy matrix. This package is overlain by Hutton Kalahari topsoil of varying thickness. The gravel package overlies Ventersdorp lava bedrock that has been weathered to varying degrees. Two sub-varieties of the Type 2 Rooikoppie gravels have been identified on the property; with the key difference between them being the sizes of the clasts. The mineralogy of these gravels is similar, with both consisting of transported quartzites and chert with locally derived quartz-vein material. Emphasis was placed on the degree of bedrock weathering and the influence of fracturing and the resulting quartz veins on the subsequent deposition of the diamondiferous gravel. The bedrock weathering features create trap sites and also allow for the downward migration of the denser clasts into the crevices in the bedrock. The relationship and similarity between the type 2A and 2B Rooikoppie gravels will be explored with emphasis on the average stone size, the recovered grade, and the mineralogy of each of the packages. In recent years there has been emphasis on the quantification of these colluvial gravels, with the aim of reaching a better understanding of these assemblages to aid grade forecasting and mine planning. Introduction The Rooikoppie gravels of the Lower Vaal are aereally extensive deflated gravels that have been mined since the 1900s (Marshall, 2004). These gravels are derived from primary fluvial deposits that are believed to be Miocene of age; however, owing to their remobilized state the Rooikoppie gravels cannot be constrained to a specific depositional period (Marshall, 2004). The name Rooikoppie is local generic name and does not refer to the locatlity the gravels are so named due to the iron that has stained the entire gravel package during oxidation processes (Marshall and Norton, 2012). The Rooikoppie gravels found at the Ditaimaneng property are colluvial and eluvial in nature and have been remobilized at different periods. Two sub-varieties of these gravels have been identified in this region; where gravels are deposited on flat highly weathered bedrock, and 193
2 Diamonds Source to Use 2013 where the bedrock has been weathered to form core stones around which the diamondiferous gravels are deposited (Marshall and Norton, 2012). Depositional model for the Rooikoppie gravels The Rooikoppie gravels are derived from primary alluvial gravels that were deposited 60 m above the current level of the Vaal River (Marshall, 2004). The bedrock comprises million year old Ventersdorp lava. During the Palaeozoic/Mesozoic, the pre-karoo river cut a network of channels that facilitated the deposition and preservation of the current gravel packages (Patridge and Maud, 1987). During the Dwyka period (ca million years) these channels were filled with shales and tillites. The late Karoo River incised these formations and deposited the gravel packages above the Karoo sediments in various cycles (Marshall and Norton, 2012). The main driving forces behind the formation of these colluvial gravels are ferricrete and laterite formation, and slope downwearing and backwearing (Marshall and Norton, 2012). During the African cycle of tectonic uplift and sea level lowering the landscape was weathered and deflation of the originally thick alluvial gravel packages occurred. In some areas these primary gravels were eroded completely. The African cycle of erosion was brought to the end at the beginning of the Miocene age, and it is during that period that both the alluvial and the derived gravels were remobilized and reworked by a post-karoo Vaal River (Marshall, 2004). The original gravels were deflated and re-distributed to cover extensive areas; they are generally lateral, thin, and capped by varying thickness of Hutton Kalahari topsoil. Similar deposits have been identified in surrounding areas as far as 25 km from the Ditaimaneng property. The sheet geometry of these gravels and the absence of obvious river channels lead to the assumption that deposition occurred in wide shallow depression (Marshall and Norton, 2012). The poorly sorted nature of these matrix- and clast- supported gravels leads to the further assumption that re-deposition occurred through sheet-wash and hill-wash processes (Marshall and Norton, 2012). 194
3 A study of the variation in the Rooikoppie gravels in the Lower Vaal area Figure 1. A stratigraphic representation of the gravels of the Lower Vaal River basin (Marshall, 2004) Colluvial gravels The colluvial Rooikoppie gravels on the Ditaimaneng property consists of two sub-varieties; Type 2A and Type 2B. These names are locally derived and do not apply to gravels on other properties. These colluvial gravels consists of uncemented pebble- to boulder-sized resistate clasts contained within a fine sandy matrix. These deflated gravels vary in thickness ranging from 0.2 m to 2 m, and are overlain by Hutton Kalahari topsoil of varying thickness; in some areas the topsoil completely obscures the gravel. The bedrock consists of >2,700 million year old Ventersdorp lava that has been weathered to varying degrees. The extent of bedrock weathering is influenced by fracturing during the periods of tectonic uplift and sea-level weathering of the African continent. The fractures acted as weak zones that facilitated the percolation of water around the bedrock, which resulted in extensive weathering of the originally continuous bedrock. The gravel package is extensive in area and comprises both transported and locally derived clasts. The transported clasts consist of sub-rounded to rounded quartzites and cherts; rounded due to their long distance travel in a fluvial environment. The locally derived clasts comprise weathered Ventersdorp lava and sub-angular to angular vein quartz. The key differences between the two sub-varieties of Type 2 Rooikoppie gravels that have been identified on the property are the sizes of the clasts and the degree of weathering of the bedrock. The mineralogy of these gravels is similar, with both consisting of transported quartzites and chert, together with locally derived vein quartz. 195
4 Diamonds Source to Use 2013 Type 2A Rooikoppie gravels The Type 2A gravels are thin and laterally extensive with an average thickness of 0.2 m to 1 m and comprised of unsorted pebble to boulder-sized clasts up to 150 mm in size. The Ventersdorp lava is highly weathered, thus the gravel package is deposited on fairly flat bedrock that undulates slightly. The locally derived vein quartz facilitates the entrapment of diamondiferous gravels during the continuous remobilization process. The undulating bedrock results in the formation of small pockets with an average depth of 0.2 m to 0.4 m. Figure 2A depicts the general stratigraphy of these gravels; note the thinness of the gravel and the closely packed transported and locally derived clasts. In some areas the small depressions or pockets in the bedrock can be identified within short distances from each other, and the presence of vein quartz facilitates the deposition of these diamondiferous gravels. The grade of the Type 2A Rooikoppie gravels is higher than that of Type 2B, owing to the fact that that the 2A gravels are diluted by a relatively thick cap of topsoil. Figure 2. A. General stratigraphy of the Type 2A Rooikoppie gravels. B. Close-up of Type 2A closely packed gravels Type 2B Rooikoppie gravels The Type 2B Rooikoppie gravels are also extensive, and usually contain patehces of Type 2A gravels. In areas where the bedrock was fractured the degree of weathering is more intense; hence the presence of Type 2A Rooikoppie gravel. The Type 2B gravels comprise pebble- to bouldersized clasts +150 mm in size. The Ventersdorp lava boulders are partially weathered to form large core stones towards the base of the gravel package. These core stones act as trap sites and also allow the downwards migration of denser clasts (Figure 3A). The gravel package is an average thickness of 1 m to 3 m and is capped by Hutton Kalahari topsoil with few clasts. These gravels are mined using an excavator, and an experienced operator is required to ensure that the bedrock is cleaned effectively as well as to manoeuvre around the large boulders (Figure 3B). The recovered grade of these gravel is on average lower than from Type 2A gravels due to dilution from the Ventersdorp lava boulders that acted as trap sites. 196
5 A study of the variation in the Rooikoppie gravels in the Lower Vaal area Figure 3. A. Close-up of gravel clasts that have migrated down the crevices created by the partial weathering of the bedrock. B. Boulders left in the pit after mining of Type 2B Rooikoppie gravels Comparison The main difference between the two sub varieties is the recovered average stone size and the grade. This key difference between the two gravels can be directly linked to the constituent of the bedrock. The Type 2A Rooikoppie is deposited on a weathered and relatively flat bedrock surface. The 2B gravel is deposited on and around weathered Ventersdorp lava corestones. The bedrock on the 2B acts as a trapsite and facilitates the downward migration of the clasts. The issue that arises from the bedrock core stones is the resultant dilution of gravel; the boulders are considered to be part of the package mined. The weathering profile of the gravel processed has an influence on what is recovered. Data obtained from areas mined previously show that the 2A Rooikoppie has a higher grade and average stone size is smaller in these gravels. Factors to consider in this comparison are the origin of the gravel, the percentage of localy derived and transported material. The average stone size is smaller in the Type 2A and this is associated with the finer transported quartzites and cherts sizes; the clasts are comparatively coarser in the Type 2B. Understanding the key differences between the two gravels allows for better planning during the mining process. Summary The two sub-varieties of deflated colluvial gravels have been formed by similar processes that resulted in the down-wearing of the original primary gravels. The mineralogy of the Rooikoppie gravels consists of resistant siliceous clasts, with the main variation being the sizes of the clasts. The Hutton Kalahari topsoil that caps the colluvial gravels is generally thinner on the Type 2B gravels than on Type 2A. Due to the resistant nature of the Ventersdorp lava bedrock, fewer depressions are encountered on the Type 2B Rooikoppie gravels and when encountered the pockets go deep into the weathered bedrock. A better quantification of these gravels is required in order to improve understanding of the differences on a local scale as well as provide information for future mine planning and projects. 197
6 Diamonds Source to Use 2013 References Marshall, T Rooikoppie deposits of South Africa. Rough Diamond Review. Un-numbered volume. pp Marshall, T. and Norton, G.A Technical Report: Geological Report on the Klipdam/Holpan Alluvial Diamond Mine (Incorporating the Klipdam and Holpan Mines as well as ERF 1 and ERF 2004 Prospecting Properties), Barkly West District, Northern Cape Province, Republic of South Africa for Rockwell Diamonds Inc. Patridge, T.C. and Maud, R.R Geomorphic evolution of Southern Africa since the Mesozoic. South Africa Journal of Geology, vol. 90. pp The Author Ndwammbi Mulalo, Mine Geologist, Rockwell Diamonds Inc. I am currently employed at Rockwell Diamonds as Mine Geologist; I have been in current position for eighteen months. I was previously employed as Diamond sorter trainee at State Diamond Trader; I held this position for three months. 198
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