Abstract:
The Areachap Group represents a mid-Proterozoic fossil island arc environment consisting of amphibolite, hornblende gneiss, quartz-feldspathic gneiss, calcsilicates and pelitic schists. Chemical compositions of these highly deformed upper amphibolite/granulite grade metamorphosed rocks indicate protoliths ranging from rhyolite/rhyodacite, calc-alkaline basalt, tholeiite to ultramafic igneous rocks and sediments. The above-mentioned assemblage is typical of an island arc environment. Island arc environments are ideal hosts for volcanic hosted massive sulphide (VHMS) type deposits and may successfully be explored by using the VHMS lithogeochemical alteration model. VHMS deposits not only yield strategic base metals such as zinc (Zn), copper (Cu) and lead (Pb), but significant grades of gold (Au) and silver (Ag) are associated with these deposits. The Areachap Group presents a metallogenic province containing one economic deposit, the Prieska Zn-Cu mine, as well as several sub-economic deposits, including the Areachap mine and other lesser prospects at Boksputs, Kantienpan, Jacomynspan and Rokoptel. The Prieska Zn-Cu mine is the most significant VHMS deposit of the Areachap Group and occurs within the Copperton volcanic centre. This abandoned mine delivered 47 Mt sulphide ore at 1,7 % Cu and 3,8 % Zn with traces of Ag and Au. Four volcanic centres were previously identified in the Areachap Group, namely Upington, Klein Begin, Boksputs and Copperton. Exploration activities were loosely subdivided into the same regions. Regional lithogeochemical sampling campaigns were conducted for the four subproject areas and approximately 5 000 rock samples were analysed for the twelve major oxides and ten trace elements. The region of interest, the Boksputs Subvolcanic area, with a well-established infrastructure, is situated near Groblershoop (50 km east) and Marydale (30 km southeast) in the Northern Cape province and is part of the geological Areachap Group. Several high copper anomalies and the tholeiitic lithological composition of the Boksputs Subproject resulted in this area being selected as the main target region. It was attempted to discriminate between different trace element populations using probability plots, but this was not successful. The complexity of the probability plots was attributed to the large variation in different rock types included in the data set. Corrections were made by determining threshold values for each rock type, but this refinement proved unsuccessful, indicating that the rock classification used was incorrect. Option areas were finally selected, based primarily on absolute Cu values. These areas were mapped in . more detail prior to ground electromagnetic (EM) surveys and drilling. To test the target selection, a proto-lithological map of the area, based on cluster analyses of the lithogeochemical dataset, was drawn. The proto-lithological maps formed the basis of the follow-up work and the application of the VHMS conceptual model. A conductor in the Kantienpan target area was located with a time domain electromagnetic (TDEM) survey and this was drilled. The drilling intersected a massive sulphide body with a tonnage of approximately 5 Mt and an average grade of 4.09 % Zn, 0.49 % Cu and traces of Au and Ag. The orebody was evaluated financially and it was found to be uneconomic as a stand-alone operation. However, if the Kantienpan deposit is considered as an alternative to imported concentrate for the Zincor smelter, this study suggests that the project may be economically feasible. Furthermore, it must be stated that the Areachap Group remains only partly explored and that a world class VHMS deposit may be discovered within the next few years.
