The Vlakfontein Tin Project comprises four farms centred on the old Vlakfontein Tin Mine, situated in the Limpopo Province of South Africa, some 10 km NNW of the town of Marble Hall and 140 km NE of Pretoria. The tin mineralization is exogenic, and is localised in structures within a sequence of sedimentary rocks that abut against Bushveld (Nebo) Granites. These granites are thought to be the source of the mineralizing fluids. Mineralisation occurs in a well-defined breccia zone located within an arkosic sequence, forming part of a gently plunging syncline.
The mineralization occurs in a fault-bounded sliver of dominantly arkoses and quartzites of the Makeckaan Formation (upper Pretoria Group, Transvaal Supergroup) on the southern edge of the Stavoren Fragment (defined by the major Wonderkop Fault), of the Bushveld Complex. This fragment is separated from the Marble Hall Fragment by a NE-SW trending inlier of Nebo or Bushveld Granite from which the mineralisation has been derived and channeled into its present position, in a breccia, likely via the adjacent Wonderkop Fault and associated 2nd order structures. (Fig. 1) The Wonderkop Fault is a major NE-SW trending structure, as shown on regional geological maps, and is clearly seen on regional magnetic maps.
The main breccia zone that contains most of the tin mineralisation in the project area occurs at the contact between quartzite and feldpathic sandstone and crops out on the northwestern and southeasten side of a large hill that straddles both the Vlakfontein and Rietfontein farms. The outcrop pattern and previous drilling shows that the breccia zone forms part of a synclinal structure, which is accentuated by the topographic effect of the hill (Fig. 2).
The tin mine was sporadically worked in the 1920’s and closed because of elimination of the lode by flat faulting. In 1940 various tests by the Mute Fides Company produced a concentrate of about 15% tin and the property was actively explored and partly developed by the Department of Mines between 1949-51. A number of payshoots were recorded, with the A and D lodes averaging 5.7% tin. The mine was abandoned in 1947 and acquired by the African Mining Trust in 1951. Phelps Dodge sampled the slimes dams in the mid 1980’s and reported grades of approximately 1% tin.
Cassiterite and related minerals were probably deposited from fluid moving along the breccia zone and associated faults. The main ore mineral is cassiterite, but chalcopyrite and wolframite are also present in small amounts. The breccia is composed of an aggregate of fine chlorite with quartz, tourmaline, iron oxide, haematite and specularite, and the cassiterite is very fine grained. The iron oxides are reported to be a conspicuous feature of the ore.
The Vlakfontein deposit was prospected by Gold Fields in the 1980’s and was the subject of a preliminary feasibility study, which concluded that the deposit would not be viable for a large company. A total of 60 boreholes were drilled by Gold Fields on the farm Vlakfontein 723 KS (Figure 2). In addition, a number of government boreholes were drilled on adjacent farms. This borehole data is available and has been compiled – most of these boreholes did not intersect significant mineralisation, with a few exceptions (e.g. 0.19% SnO2/1m in HP15 and, 0.47% SnO2/4m in HP6). However, a series of deeper boreholes were drilled roughly along the ‘axis’ of the synclinal structure, in a NE-SW line (boreholes HP87, HP 114, HD1, HP115, HP 116, and HP 117). In almost all of these boreholes, significant mineralisation was intersected (e.g. 1.27% SnO2/5m in HP115, 0.47% SnO2/2m in HP114, 0.53% SnO2/3m in HP105). These boreholes define a NE-SW trend of higher-grade mineralisation which appears to be an extension of the mineralisation mined at the old Vlakfontein mine.
A geological section constructed at right angles to the synclinal axis through boreholes HP86, HP3, HP3/2, HP 87 and HP 114 demonstrates the asymmetry of the syncline reflected by the more steeply dipping south eastern limb, which is also associated with faulting probably related to the major Wonderkop Fault (Sections A-B; Fig. 3).
Section C-D (Fig. 4), roughly along the axis of the syncline, has been constructed through boreholes HP 114, HD1, HP 115, HP116, HP117, and HP105. This section demonstrates the gentle plunge of the syncline to the south west, steepening somewhat to the north east in the area of the old Vlakfontein mine workings.
Reinterpretation of data obtained from the old Vlakfontein tin mine in Limpopo Province and from a number of previous exploration programmes in the vicinity, suggests that a significant tin resources could still be present on the Vlakfontein farm. Modelling of previous data which includes the use of tin grade and thickness distribution maps together with a tin content distribution map suggests that the highest grade and thickest tin mineralisation occurs along or close to the axis of a prominent syncline in Transvaal Supergroup rocks.
The tin mineralisation appears to be associated with an intraformational breccia zone within the gently plunging syncline of arkoses and sandstones lying adjacent to a major fault which, together with associated is faults thought to have acted as the feeder for mineralizing solutions. Offshoots from the synclinal axial zone to the surface outcrop of the breccia zone on the northern limb of the synclie could also exist.
[EXTRACTED FROM A REPORT BY VILJOEN AND LONGRIDGE, VM INVESTMENTS, 2012]