South Deep Gold Mine

 mine, production, gold mine

The South Deep Project is owned and operated by Goldfields and is situated some 45 kilometers south-west of Johannesburg, in the districts of Westonaria and Vanderbijlpark in the Gauteng province of South Africa. South Deep, which is a mine in development, was acquired by Gold Fields in December 2006. With some 78.2 million ounces of Mineral Resources (including Uncle Harry’s prospecting right area) and 29.3 million ounces of Mineral Reserves, South Deep is one of the greatest undeveloped ore bodies in the world.

South Deep is a long-life, developing deep-level gold mine operating at between 2,000 and 3,000 meters below surface and production at the mine began in 1961 and South Deep has a projected mine life of 70 years. The property covers an area of 2,469 hectares of surface (freehold) land and 3,563 hectares of continuous mineral rights. . Located on the north-western rim of the Witwatersrand Basin, South Deep predominantly exploits auriferous palaeoplacer reefs or conglomerates that comprise the Upper Elsburg Reefs of the Mondeor Formation and, to a lesser degree, the Ventersdorp Contact Reef (VCR).

South Deep gold mine is an intermediate-to-deep-level gold mine comprising two shaft systems that mine various aurifer- ous conglomerates from open ground and pillars that occur at depths between 1 575 m and 3 500 m below surface, says Gold Fields in a technical report.

The shaft systems comprise the older South Shaft complex – with its main shaft and two subvertical shafts – and the newer flagship complex, known as Twin Shafts.

Ore is processed at a central metallurgical plant. The mining rights comprise a total of 3 566 ha.

The key milestones for South Deep in the next three years include:
• the commissioning of the new tailings facility in 2011;
• the equipping of the ventilation shaft in the next 18 months, with the concurrent deepening (240 m) of the shaft to a final depth of 2 995 m; and
• the commissioning of the ventilation shaft for hoisting in 2012.

These milestones, says Gold Fields, are necessary to facilitate the build-up to full production by December 2014.

At full production, South Deep will produce between 750 000 oz/y and 800 000 oz/y.

Gold Fields’ technical report reflects the latest life-of-mine plan, coupled to an updated mineral resource and mineral reserve state- ment, as at June 30.

Gold Fields manages all the mineral resource and mineral reserve figures reported and mineral resources are inclusive of the mineral reserves.

The company reports that “enhanced” orebody modelling has resulted in improved mining layout and planning.

It says that South Deep is being positioned as a “highly efficient, safe, low-cost, fully mechanised extraction of a world-class orebody”.

The focus remains on developing the South Deep mine to achieve steady-state sustainable production in the next five years from its mine- ral resource base of 63,8-milion ounces, exclu- ding 14,6-million ounces in Uncle Harry’s.

Proven reserves total 29,5-million ounces of gold and a current life-of-mine of 43 years to 2042.

South Deep is located in the West Rand goldfields in the Witwatersrand basin, which remains the most significant gold depository in the history of mining.

The discovery of the payable Ventersdorp contact reef (VCR) and Upper Elsburg conglomerates in the early 1950s, in the southern part of the goldfields, led to the establishment of the Elsburg Gold Mining Company in 1965, which was later consolidated to form Western Areas gold mine.

Continued exploration in the late 1970s confirmed earlier indications that the target reef horizons extended well to the south in the area, which has become known as the South Deep project area.

By 1980, it was recognised that the South Deep orebody had reefs that could be mined by conventional mining methods, and also wide-orebody reef that lent itself to mechanised mining.

In the years to follow, surface and underground drilling, together with detailed seismic surveys, enhanced the grade distribution and geological models, which led to the establish- ment of the South Deep gold mine in 1990, under the stewardship of Johannesburg Conso- lidated Investments.

Gold Fields acquired South Deep in December 2006 from Barrick, which owned half of the shares, and in April 2007 from Western Areas, which owned the other half.

Local Geology

In the western half of the mining authorisation, the VCR occurs as a single reef horizon that overlies the footwall lithologies of the Upper Elsburg reefs.

The Upper Elsburg reefs subcrop below the VCR in a north-north-east trend and comprise multiple stacked reef horizons that form part of an easterly divergent clastic wedge.

This wedge is 130 m thick in the vicinity of the eastern boundary of the mining authorisation.

The structure at South Deep is dominated by north-south-trending fault systems, which include the West Rand, Panvlakte and Water- pan faults.

The stratigraphy and the West Rand/Panvlakte faults are further offset by younger east-west-trending local wrench faults that generally have a dextral displacement.

The West Rand/Panvlakte/Waterpan horst block is situated between Gold Fields’ Kloof gold mine and its South Deep gold mine and regionally demarcates the western boundary of South Deep.

A series of dykes also straddles the mine lease, orientates roughly north-south, and can be 35 m thick.

The younger, post-Black Reef dykes separate the overlying dolomite into water compartments.

The Upper Elsburg reefs constitute 93% of the mineral reserves and accommodate mechanised mining, while the VCR constitutes the remaining 7%.

Ventersdorp Contact Reef

The VCR represents the final phase of sedimentation prior to the extrusion of the lavas of the Klipriviersberg group and unconformably overlays the Upper Elsburg reefs and strata.

The VCR structure and facies model has been updated using the latest available infor- mation, which will facilitate three-dimensional (3-D) evaluation and more efficient mine design and scheduling.

The VCR sedimentation is independent of the Upper Elsburg reefs and is characterised by quartzite-dominated fades types.

Two fades types are recognised. The Sandy 1 fades type is defined as always having a basal conglomerate and the Sandy 2 type is predomi- nantly of quartzite.

By implication, the Sandy fades is the higher-grade prospective target.

The VCR strikes roughly east-west.

Upper Elsburg Reefs

A paradigm improvement in the stratigraphic and sedimentological resolution within the Upper Elsburg reef units has been developed over the past two years, which will facilitate 3-D evaluation and fit-for-purpose mine design and scheduling.

Updated models are being generated using improved resource modelling techniques.

Importantly, accurate grade and tonnage factors are now defined for mining of the Upper Elsburg reefs.

The Upper Elsburg reefs represent a broad upward coarsening cycle of conglomerates and interbedded quartzites, within which several smaller upward fining cycles occur.

The discrete stratigraphic units, of which currently 16 are described, are the product of these smaller upward fining cycles.

The bases of each of the conglomeratic units are unconfor- mity-bounded surfaces with nine such unconformity-bounded sequences distinguished.

With each of the unconformity-bounded sequences, economic gold mineralisation shows a strong positive spatial correlation with the coarse conglomerate units, while the matrix-supported, sandy conglomerate units generally host sub- to uneconomical gold concentrations.

Based on these associations, gold distribution within each of the stratigraphic units is being remodelled.

The Upper Elsburg reefs have an average dip of some 12º south-south-east, but this is effected by a low amplitude, high wavelength regional syncline that trends north-south through the centre of the mine. This feature is more pronounced towards the south of the property.

Exploration and Drilling

The tempo of exploration has significantly increased year-on-year and is multidisciplined with a combination of surface and underground drilling as well as seismic remodelling to test future mining extensions, structural complexity and economic potential.

JCI and Randgold & Exploration (R&E) have agree to relinquish certain rights which they have to the Uncle Harry’s ground, subject to the approval of the shareholder representa- tion of at east 50% of the shares entitled to vote at general meetings of both companies.

Uncle Harry’s equates to the Kalbasfontein prospecting rights and the contiguous ground immediately to the east of South Deep contains an inferred mineral resource of 14,6-million ounces at a cutoff grade of 3 g/t, ground that will be accessed through the existing South Deep infrastructure.

An extensive exploration programme was initiated in 2007 to facilitate improved planning and optimisation to ensure robust geological and evaluation models.

Results of this programme will underpin the resource definition of the VCR and the evaluation of the Upper Elsburg reefs.

The programme will provide clarification on the orebody potential below infrastructure as well as an assessment of the regional potential of contiguous prospecting rights, which include the Uncle Harry’s ground.


Underground Drilling

Gold Fields reports that a major capital long inclined borehole (LIB) drilling programme is under way at South Deep on the 95 and 110 levels.

This drilling programme is designed to provide geological structure, facies and grade information in current mining areas, in zones below current infrastructure and south of the Wrench fault, which duplicates 110 level further south. More that 17 km of drilling is being carried out at an estimated cost of R15,3-million.

Surface Drilling

The surface-drilling programme that targets the Upper Elsburg reefs and VCR in the phase II ground below 110 level, started in the first quarter of the 2008 financial year.

Fourteen boreholes will be sunk at a cost of R200-million between now and the end of Gold Fields 2011 financial year.

Of the nine rigs currently in the field, four will focus on intersecting the comparatively thin but closely stacked proximal Upper Elsburg succession immediately east of the subcrop.

The remaining rigs are targeting the thicker, more distal Upper Elsburg reefs towards the east in the Uncle Harry’s area.

After successful completion of this phase of drilling, the rigs will be moved to drill three holes for the Upper Elsburg reefs in the east of the mine lease area.

A further three holes are planned to test the VCR west of the Elsburg subcrop.

Seismic Survey

A 3-D survey – originally acquired and processed in 2004 as a split data set over the deeper portions below the current infrastructure of the Kloof and South Deep gold mines – was merged into one project after Gold Fields acquired South Deep, in order to facilitate improved structural modelling across one contiguous area.

The South Deep 3-D seismic survey provides high-quality data and gives an excellent image of the VCR reflector and structure disrupting it.

The merged data set will also be used to produce holistic regional tectonic model for the West Wits area.

Mechanised Mining

Both the stoping and the development at South Deep are mechanised operations.

The focus in the immediate future will be on expediting development accessing the orebody, completing the Twin Shaft infrastructure and increasing the rate of mechanised destress mining

Mining Methods

The VCR will be mined by a low-profile mecha- nised mining method, whereas the Upper Elsburg reefs will be mined by a variety of mining methods ranging from mechanised drift and fill or modified drift and bench to longhole stoping.

Trackless twin-boom drills are used to drill the orebody, which is then blasted and taken by load-haul dumpers to internal orepasses for transport to hoisting shafts.

The main South Deep target horizon is predomi- nantly in the Upper Elsburg Massives zone.

In order to extract this ground at depths ranging from 2 500 m to 3 500 m below surface – currently on average at a depth of 2 693 m below surface – the in situ rock stresses need to be reduced from 80 MPa to between 30 MPa and 40 MPa.

This allows large excavations to be mined at close proximity.

To achieve this, the main target horizon needs to be destressed by mining a 2-m slice through the package in an optimal position to ensure a destressed window of 50 m to 60 m above or below the associated stope.

Historically, this was done through conventional mining techniques, but owing to low face advances achieved, destress mining is now also done using mechanised mining techniques.

Backfill is an integral part of the mining method, and is essential to promote both local and regional stability.

The Upper Elsburg Massives are the main target horizon and mechanised mining equipment will be used in their extraction, after completion of the destress mining.

Various mining methods are adopted and are applied in terms of the thickness of the reef target as well as in terms of the payability of the reef.

Mine Planning and Scheduling

Mine design and scheduling at South Deep are done using the software programs Mine 2 4D and Earthworks Protect Scheduler on a Datamine platform.

All designs are based on 3-D resource models.

Owing to the depth of the orebody, mining of destressed horizons have to be scheduled in such a way that sufficient reserves are made available for massive mining extraction.

Large mechanised targets are then sequentially scheduled for extraction within the destress envelopes.

Backfill scheduling is also incorporated in the extraction sequence, both for the destress horizons and the massive horizons.

All designs and schedules are completed in consultation with production and all technical personnel to ensure safe and efficient layouts and plans.

Production efficiencies are based on past experience and production models describing activities for the different fleet combinations.

Mineral reserve development will continue to be a key performance indicator in the 2010 financial year and will be accelerated in appropriate areas.

Projects

Several capital-intensive projects are ongoing to facilitate the build-up to full production by December 2014. Capital expenditure of R8,5-billion is being invested from now through to July 2014 to build the mine to full production of 330 000 t/m of reef and 750 000 oz/y of gold.

Projects on critical path include the deepening of the ventilation shaft. In this ventilation shaft deepening project, the installation of a brattice wall from surface to 94 level has been completed.

The equipping of the shaft to 100A level started in July 2009 and will be complete by December 2010.

The sinking, lining and equipping of the remaining 240 m from 100A level to shaft bottom will follow.

A new rock winder to enable the ventilation shaft to hoist 195 000 t/m will be complete by December 2011 and the shaft is planned to be commissioned by July 2012.

New mine development is under way to develop the mine on 100, 105, 110 and 110A levels in order to ensure access and ore transport and ventilation to the working areas north and south of the Wrench fault.

Three additional refrigeration plants on 94 level are planned to be commissioned by January 2010 to provide cool air to the current workings and the new mine development.

This includes new bulk air-cooling facilities on 95 and 100 levels.

An ice plant will be built on surface to further enhance the mine cooling facilities during 2013.

All return airways are required to be complete by 2014.

Four vertical silo facilities, 9 m in diameter and 56 m long, are to be established for surge capacity by July 2012 in addition to three settler dams with pumping facilities, as part of the shaft bottom infrastructure project.

Backfill
A new full tailings backfill plant is to be erected at South shaft with a pipe distribution network of 24 pipes down South shaft to the workings.

Mineral Resources/Reserves

Gold Fields’ South Deep June 30, 2008, attributable mineral resources and mineral reserves were primarily based on the preacquisition figures as declared for December 2005.

These were audited by an independent review panel of consultants for the Barrick Gold Western Areas Joint Venture between Barrick Gold South Africa – formerly Placer Dome South Africa and Western Areas – for the first and second phases of the old mine, but updated by Gold Fields to June 30, 2008, for mining depletions.

Gold Fields remodelled and re-evaluated the upper Elsburg reefs in the current mine, from 87 to 110 level, in accordance with Gold Fields standards and procedures.

Reference for information:
www.goldfields.co.za/ops_south_deep.php
www.miningweekly.com/article/descending-3-500m-below-su...

www.goldfields.co.za/pdfs/technical_short_forms_09/Sout...

www.miningweekly.com/article/goldmining-delves-to-great...