MESOTHERMAL-TYPE MINERALIZATION IN THE SABIE-PILGRIMS REST GOLD FIELD, SOUTH-AFRICA

We describe here a different class of mesothermal gold deposit at Sabi e-Pilgrim's Rest which is probably associated with the Bushveld igneou s event in South Africa. Pressure and temperature estimates indicate t hat the ore fluids of the Sable-Pilgrim's Rest gold field, which occur s within the Early Proterozoic Transvaal Supergroup, were similar to t hose of mesothermal gold deposits. However, a number of features disti nguish the Sable-Pilgrim's Rest ores from typical mesothermal deposits : (1) the presence of ubiquitous Cu and Ri in addition to Au, Ag, hs, and Sb; (2) high salinities (mean approximate to 12 wt % NaCl equiv); (3) low mean bulk CO2 content (approximate to 5 mole %); (4) veins lar gely hosted within a platform carbonate sequence; and (5) mineralizati on along both vertical and horizontal (stratiform) veins, showing a cl ear genetic link. Characteristic features of the Sable-Pilgrim's Rest gold field that correspond to those within the above spectrum of mesot hermal gold deposits include: (1) comparable light stable isotope (del ta D, delta(18)O. delta(34)S) ranges. together with a lack of isotopic evidence for meteoric water involvement; (2) high Au/Ag ratios; (3) p resence of ore shoot structures, which are highly characteristic of me sothermal gold deposits; (4) variably deformed structures within indiv idual sheeted veins which point toward cyclic opening and quartz accre tion events separated by episodes of deformation; and (5) fluid inclus ion properties which indicate high fluid pressures during entrapment. Bushveld magmatism is thought to have been the main heat source respon sible for generating the hydrothermal cell that resulted in the format ion of the Sable-Pilgrim's Rest gold field. Mineralization involved an originally pristine fluid transporting metals that were probably deri ved from the same source, as well as a measure of connate waters from the granitic basement and the overlying sedimentary rocks. The fluid w as relatively acidic and anoxic and equally efficient in transporting gold either as a bisulfide complex or a chloride complex. Gold precipi tation took place as a result of (1) a decrease in sulfur activity wit hin the ore-bearing fluid because of sulfide precipitation, and (2) ch anges in the fluid chemistry as a result of fluid immiscibility.