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.