3.6.1 |
Glen Douglas
Glen Douglas Mine exploits two of the three dolomitic formations of the Malmani Subgroup of the pre-Cambrian Transvaal Sequence, namely the Lyttelton and Monte Christo Formations. The Monte Christo Formation represents a widely deposited oolitic carbonate 300m to 500m thick, which is overlain by the Lyttelton Formation. The Lyttelton Formation consists of a dark, chert-poor, fine-grained dolomicrite 100m to 200m thick. The contact of the Lyttelton Formation with the overlying Eccles Formation is transitional and indistinctly marked by the increased presence of chert bands.
The Eccles, Lyttelton and Monte Christo Formations subcrop beneath soil within the Glen Douglas Mine lease area. Sedimentary bedding in the area dips to the west-northwest at approximately 70º resulting in subcropping of these three formations beneath the soil cover as broad northeast trending bands. The uppermost Eccles Formation exists only in the northwest portion of the property beneath the process plant facility and is not encountered in the LoM pit designs. The Lyttelton Formation subcrops in a broad 800m to 1,000m exposure within which the current and planned pits lie. The Monte Christo Formation subcrops over the remaining area of the property to the east and is encountered only at depth in the pits. Over most of the property bedrock is overlain by a varying thickness of soil (up to 30m) containing a combination of scattered and stacked dolomite boulders representing eroded remnants of karst towers, and matrix-supported dolomite cobbles and chert rubble. In the north of the property the Lyttelton Formation is intruded by a syenodiorite sill which has a similar dip as the sedimentary strata but a strike that is slightly more to the east. The sill gradually transgresses upwards to the south where it eventually forms the hangingwall of the Lyttelton Formation in the southwest of the property.
Also present are diabase dykes that are deleterious in the metallurgical grade material and are classified as waste along with any dolomite significantly contaminated by them. Due to weathering, alteration and mineralogy the dyke material is also unacceptable for aggregate use. The southwest area of the mine is transected by an east-west striking graben structure, which effectively sterilises the dolomite for an additional bench over a width of some 100m due to the increased depth of weathering. Initial recovery of the metallurgical dolomite is affected by the presence of “mud pockets”. These are small depressions and voids in the dolomite produced by deep weathering into which groundwater has introduced an influx of mud, soil and clay contaminants. These pockets are often small enough to be missed by the blasthole drilling and so can be difficult to predict. The only significant potential at Glen Douglas Mine to add to the current LoM metallurgical-grade reserves lies beneath the process plant and workshops. In addition to the metallurgical-grade dolomite, there is a substantial volume of aggregate-grade material currently demonstrated by the mine’ management to be uneconomic if mined independently. The Lyttelton Formation is a dark, homogeneous, finely crystalline and massive dolomicrite with an average
SiO2 content of 1.15%, AI2O3
content of 0.20% and K2O content of 0.05%. The lower layers of the Lyttelton Formation near the contact with
the Monte Christo Formation show an increase in chert partings and siliceous detritus,
which raise the SiO2 content above acceptable limits for metallurgical uses. This higher-silica material, along with the upper layers of the Monte Christo Formation, is mined for construction aggregate.
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