History of leases
The Witwatersrand Basin Project (WBP) comprises two historic mining centres known as the Durban Roodepoort Deep and the Rand Leases on the Northern Edge of the Witwatersrand Basin in the Central Rand Goldfield immediately southwest of the city of Johannesburg (Figure 1). Mining has been taking place within these areas since discovery of the goldfields in 1886. During that time up to seven different gold bearing conglomerate horizons have been mined from surface down to a depth of 3,100 metres and total production for the combined Durban Roodepoort Deep and Rand Leases area was over 30 million ounces of gold at a grade in excess of 5 g/t (ASX: West Wits Mining Limited Prospectus 15/11/2007).
Figure 1. The boundary marks the extent of the WWI tenements in the area.
Historical JORC Mineral Resource Estimate
This historical estimate was compiled by the previous owners of the WBP leases Durban Roodeport Deep Limited (DRD) which was listed on the Australian Stock Exchange between May-1999 to July 2006 and is presented here directly from the DRD Annual Report of 2000 which also reported the closure of the mining operations. The historical estimates used categories of mineralisation as defined by the JORC Code 1997 which are substantially the same as the current JORC Code.
Table 1: Historical Mineral Resource Estimate – DRD and Rand Leases
|Category||Tonnes (millions)||Grade g/t||Ounces AU|
|Measured & Indicated||56.5||4.43||8,050,000|
The Mineral Resource statement from the DRD Annual Report of 2000 and quoted above was reported against an earlier version of the JORC Code (1997) and is not reported in accordance with the current JORC Code of 2012. At this stage no competent person has done sufficient work to classify these historical estimates in accordance with the JORC Code (2012). It is uncertain that following evaluation and further work that the “historical estimates” will be able to be reported as mineral resources in accordance with the JORC Code 2012.
The historical estimates are considered relevant and material to WWI as they encompass the area covered by the recently reinstated WBP’s Prospecting Right.
The process to reinstate the WBP Prospecting Right provided the impetus for an aggregation and full review of all the data. This also involved discussions with individuals who worked on the lease as an operating mine in the late 1990’s and who provided valuable information concerning those operations and their unrealised potential. The release of these historical resource estimates represents a significant step on the road to the redevelopment of the WBP. The next step will involve the formulation of Exploration Targets and commencement of a new exploration program designed around our upgraded knowledge of the area.
The gold mineralisation of the WBP deposits form part of the Central Rand Goldfield hosted by the Witwatersrand Super-group sediments. The Central Rand Goldfield is situated immediately to the south of Johannesburg and has been host to one of the most extensive gold reserves in the world (Figure 2). The reefs have been mined continuously on strike for approximately 32 km in an east/west direction, bounded by DRD in the west, and down-dip, to the south, for about 6 km from its outcrop position, to vertical depths of approximately 3 km.
Between 1897 and 1984, approximately 9,000 tons (approx. 290 Moz Au) of gold were extracted from the Central Rand Goldfield. The gold orebodies occur in stacked, channelized, quartz pebble conglomerate horizons referred to as “reefs”. The major orebodies mined in the Central Rand Goldfield are the Main Reef, Main Reef Leader, South Reef, Bird reefs and Kimberley reefs (Figure 3).
Figure 2. The mines of the Central Rand Goldfield. West Wits WBP comprises two historic leases, namely the DRD lease and Rand Leases.
Figure 3. Geology of the Central Rand Goldfield underlying the WBP lease.
GEOLOGY OF LEASES
The Central Rand Group of sediments host all the mineralised reefs located on the West Rand Goldfield. Sedimentary deposition across the stable granite-gneiss basement commenced around 3 billion years ago. In strati-graphic terms the Witwatersrand sequence is divided into two divisions, the lower dominantly marine, slate rich West Rand Group and the upper dominantly alluvial sandstone rich Central Rand Group. Combined, these units make up a sedimentary package up to 7km thick.
Sedimentary deposition ended approximately 2,700 million years ago when widespread faulting resulted in extensive lavas of the Ventersdorp supergroup covering the Witwatersrand Basin. Reef groups located on the West Rand in order of oldest to youngest are the Main, South, Johnstone, Livingstone, Bird, Kimberley, Elsburg and Ventersdorp Contact Reefs.
An overview of each reef group follows:
The Bird Reef Group
The Bird Reef Group consists of up to five individual reefs; the Upper Monarch Zone 4 (Alpha), the Upper Monarch Zone 2 (Beta), the Monarch, the Footwall Monarch (middle) and the White Reefs. The Bird Reef Group is comprised of coarse quartz conglomerate horizons measuring up to 1.5m thick and containing numerous sulphides, gold and the uranium mineral uranite. Ore body geometry of the Bird Reef Group is classified as continuous through the West Rand Goldfield. The Bird Reef Group dips generally 30-40 degrees to the south, and strikes in a general east-west orientation.
The Kimberley Reef Group
The Kimberley Reef Group extends across the majority of the Company’s Project Area. The Company believes the Kimberly Reef Group holds the potential for shallow mineralisation and possible underground mining. The Kimberley Reef Group consists of two dominant individual reef horizons; the Boulder (Lindum) and the Battery (Horsham) reefs. For many years these reefs provided the main source of gold on the West Rand Goldfield. The Kimberley Reef Group was known to have good continuity, with reef thicknesses varying from as little as half a metre to as much as 15 metres. The Kimberley Reef Group generally dips 30-40 degrees to the south and strikes in a general east- west orientation.
The Main and South Reef Groups
The gold bearing Main and South Reefs are the most extensively mined reefs on the Company’s Project Area. These reef groups have a width ranging from a few centimetres up to a few metres and were mined to depths in excess of 2,000m in some sections of the Project Area. The Main and South Reef Groups were the first reefs in the Witwatersrand to be mined and were historically of a higher grade.
The Elsburg Reef Group
The Elsburg Reef Group is located on the DRD and Rand Lease, is strati-graphically younger than the Kimberley Reef Group and was not extensively explored or mined as it was regarded as uneconomic.
The Elsburg Reef Group is currently being successfully mined by other mining companies on the Witwatersrand for gold with minor uranium. The Company has identified this reef as a potential target, warranting further attention.
The Ventersdorp Contact Reef Group
Ventersdorp Contact Reef Group (VCR) is found on the unconformable boundary between the sediments of the Central Rand Group and the basalt of the Ventersdorp supergroup. The gold bearing VCR received minor exploration attention by the previous owners with moderate success. Original exploration drilling results are unavailable, however the Company believes this reef warrants investigation.
WWI announced an Updated Mineral Resource Estimate for the WBP under the JORC 2012 Code (WWI ASX release 18 Dec 2017 Competent Person Hermanus Swart). The new estimate was based on a detailed technical review completed on the extensive database generated by the previous leaseholders, Durban Roodeport Deep Limited (“DRD”). This database formed the basis of the previous Mineral Resources Estimate (under an earlier version of the JORC code) released by DRD to the ASX in 2000 (refer WWI ASX release 28 August 2015).
Importantly the new Mineral Resource Estimate is derived from the historic DRD resource but only includes ounces to a 1,500m depth from surface and does not include deep underground material previously considered in the DRD estimation. The chosen headline cut-off grade of 2 g/t is based on historic figures and should ensure that the bulk of the resources can be reasonably expected to be exploited.
Table: Updated Global Mre For The Witwatersrand Basin Project At 2.0g/T Cut-Off
|Category||Ore Tonnes (Mt)||Grade g/t Au||Ounces Au|
|Measured & Indicated||21.8||3.51||2,450,000|
Notes: The Global MRE set at a 2.0 g/t Au cut-off. Reported in accordance the JORC Code of 2012. Number differences may occur due to rounding errors.
WWI is not aware of any new information or data that materially affects the information included in the announcement and confirms that all material assumptions and technical parameters underpinning the estimates above continue to apply and have not materially changed. The form and context in which the Competent Persons findings are presented have not materially changed.
Furthermore there are also additional areas of interest identified by WWI that sit outside the announced the JORC resource. Some of these areas of interest formed part of the previously announced Exploration Target but were not taken into the original DRD Mineral Resource Estimate due to the presence of mine infrastructure on the surface and crown pillars. These old mills have been removed and many crown pillars are largely redundant due to the resource limit of 400m depth in these areas.
In addition, WWI is in the process of an assessment of the historic Bird Reef data base and the exploration previously undertaken by WWI (2008 and 2015) and have delineated a substantial zone of unclassified target mineralisation in a previously unmined area that is excluded from the JORC resource or Exploration Target. We expect the results of this study to be available shortly.