Mesothermal-type mineralization in the Sabie-Pilgrim's Rest gold field, South Africa

Abstract

We describe here a different class of mesothermal gold deposit at Sabie-Pilgrim's Rest which is probably associated with the Bushveld igneous event in South Africa. Pressure and temperature estimates indicate that the ore fluids of the Sable-Pilgrim's Rest gold field, which occurs within the Early Proterozoic Transvaal Supergroup, were similar to those of mesothermal gold deposits. However, a number of features distinguish 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 largely hosted within a platform carbonate sequence; and (5) mineralization along both vertical and horizontal (stratiform) veins, showing a clear genetic link. Characteristic features of the Sable-Pilgrim's Rest gold field that correspond to those within the above spectrum of mesothermal gold deposits include: (1) comparable light stable isotope (delta 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) presence of ore shoot structures, which are highly characteristic of mesothermal gold deposits; (4) variably deformed structures within individual sheeted veins which point toward cyclic opening and quartz accretion events separated by episodes of deformation; and (5) fluid inclusion properties which indicate high fluid pressures during entrapment. Bushveld magmatism is thought to have been the main heat source responsible for generating the hydrothermal cell that resulted in the formation of the Sable-Pilgrim's Rest gold field. Mineralization involved an originally pristine fluid transporting metals that were probably derived from the same source, as well as a measure of connate waters from the granitic basement and the overlying sedimentary rocks. The fluid was relatively acidic and anoxic and equally efficient in transporting gold either as a bisulfide complex or a chloride complex. Gold precipitation took place as a result of (1) a decrease in sulfur activity within the ore-bearing fluid because of sulfide precipitation, and (2) changes in the fluid chemistry as a result of fluid immiscibility.