Overview

Deposit type

• Magmatic

Summary:

The Smokey Hills project is located in the central portion of the eastern limb of the Bushveld complex and is underlain by rocks of the upper critical zone, which hosts two of the more important PGE-bearing units in the Rustenburg Layered Suite: the Merensky Reef and the UG2. In the Smokey Hills Project Area, the Merensky Reef is expected to occur approximately 300 m above the uG2. However, a minimum of approximately 140 m of the Merensky Reef footwall units have been removed by erosion and therefore no Merensky Reef is preserved at the Smokey Hills project. In the central portion of the eastern limb of the Bushveld complex, five chromitite layers have been identified in the Upper Critical Zone, each of which forms the base of a cyclic unit. The cyclic units are termed, from oldest to youngest the UG1, UG2, UG3 UG3A and UG3B. Each of these chromitites carries varying concentrations of PGEs. However, concentrations in the UG1 chromitite layer in all areas of the RLS have been found to be subeconomic. The UG3, UG3A and UG3B chromitite layers are generally too thin and contain insufficient concentrations of PGE mineralisation to have any reasonable expectation to support economic extraction. The UG1 chromitite layer occurs over 100 m below the UG2. The UG3 chromitite layer is approximately 0.25 m thick and occurs approximately 15 m above the UG2. The UG3A and UG3B chromitite layers are approximately 0.1 m thick, occur approximately 5 m above the UG3 and are separated by approximately 0.3 m of pyroxenite. Individual layers in the Upper Critical Zone can generally be correlated over several kilometers and as such, detailed local stratigraphic nomenclature can be established.

Dykes in this area are primarily composed of dolerite, which is generally fine-grained and of good competence, with associated areas of dense jointing and alteration. Underground dyke intersections are generally less than 10 m in width. An airborne magnetic survey has identified several swarms of northeast-striking dolerite dykes. No serious problems were encountered during mining through these features, and no significant displacements have been associated with them.

Potholes are defined as areas where normal reef characteristics are destroyed and hence, pothole areas are believed to be un-mineable and considered a geological loss. The position and size of potholes are typically unpredictable. In most instances, the potholes affect only one of the economic units. However, in minor instances, the underlying or overlying economic unit may be affected by the development of the pothole.

The Bushveld complex stratigraphy is occasionally replaced by late-stage pegmatite bodies. These pegmatite bodies have a range of compositions, from highly ultramafic to felsic.

Rolling reef and small scale undulations of the UG2 do not create geological losses but can contribute towards mining difficulties. Undulations and highly dipping (up to 50°) have been observed at Smokey Hills in the outcrop and during mining. Miners have tried to minimize difficulties associated with the mining of these areas by using up-dip methods. An area of rolling reef has been encountered during underground mining centred on the 4.4 Raise. The UG2 elevation, in relation to adit 4, was reduced by up to 40 m. This has made it difficult to mine because of the proximity of the mining lease boundary.

Mining Methods

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Comminution

Crushers and Mills

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Processing

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Production

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