The Transvaal lode-gold deposit is hosted in ultramafic schists and graphite-bearing pelites that are metamorphosed to lower and mid-amphibolite facies assemblages. The ore body is structurally controlled by the Transvaal shear zone which overprinted and reactivated the penetrative regional fabric. The main tabular ore body is formed of a massive quartz-sulfide vein and related splays, and adjacent hydrothermally altered wallrock. Prograde alteration in the ultramafic rocks consists of a proximal diopside-actinolite-qd by ultramafic schists, whereas high-Ag electrum is at the ultramafic schist-graphitic-pelite contact. It is proposed that fluid interaction with reducing pelites produced Ag-rich electrum through destabilisation of gold-bisulfide and silver-chloride complexes, while fluid that reacted with more oxidized ultramafic rocks formed Au-rich electrum as silver-chloride complexes were not destabilised.
The main phase of hydrothermal alteration, which included the deposition of prograde silicates, pyrrhotite, and loellingite was syntectonic with respect to the Transvaal shear zone and at approximately peak metamorphic conditions. Electrum in equilibrium with pyrrhotite and/or loellingite was thus deposited during syn-peak metamorphic conditions. However, the majority of electrum grains coexist with arsenopyrite 1 and composite loellingite-arsenopyrite 1 grains, and are therefore interpreted to have formed during the "high temperature" retrogressive replacement of loellingite by arsenopyrite, but is considered to be part of the main gold-bearing hydrothermal event.
Retrograde low-temperature, post metamorphic silicate and sulfide minerals such as chlorite, sericite, pyrite, and cubanite are evidence for processes that occurred after the main, gold-depositing hydrothermal activity took place. Renewed flux of hydrothermal fluids during the protracted metamorphic history of the terrain could have produced the "low-temperature retrogression"; the occurrence of low-temperature sulfides is likely the result of in situ re-equilibration during cooling of the rocks.