The geochemistry of pseudotachylyte and its host rocks from the West Rand Goldfield, Witwatersrand Basin, South Africa: implications for pseudotachylyte genesis

The chemical composition of the pseudotachylyte in the West Rand Goldfield of the Witwatersrand Basin, South Africa, is closely related to the composition of the host rocks and this is reflected in the colour of the pseudotachylyte. Grey pseudotachylyte is generally hosted by and similar in composition to quartzites of the Witatersrand Supergroup, whereas maroon pseudotachylyte has a similar relationship to the mafic lava of the Ventersdorp Supergroup. In some instances, the composition of the pseudotachylyte is intermediate between these two host rock types and a mixing process is proposed.

A study of the ferrous to ferric iron ratio provides limited evidence that pseudotachylyte is slightly more reduced than the rocks from which they have been derived.

The only elements that are consistently enriched in the pseudotachylyte, irrespective of host rock composition, are S, Pb and Au. It is speculated that this indicates the existence of a sulphide-bearing fluid phase along the fault zone either prior to or during pseudotachylyte formation.

Geochemical and petrographic evidence favour an origin by frictional fusion rather than ultracomminution for the pseudotachylyte from the West Rand Goldfield.