Gold in the quartz-pebble conglomerates of the late Archean
Witwatersrand Basin, South Africa, is often intimately associated with
carbonaceous matter of organic/biogenic origin which occurs in the form
of stratiform carbon seams and paragenetically late bitumen nodules.
Both carbon forms are believed to be formed by solidification of
migrating hydrocarbons. This paper presents bulk and molecular chemical
and stable carbon isotope data for the carbonaceous matter, all of which
are used to provide a clue to the source of the hydrocarbons. These data
are compared with those from intra-basinal shales and overlying
dolostone of the Transvaal Supergroup. The delta C-13 values of the
extracts from the Witwatersrand carbonaceous material show small
differences (up to 2.4 parts per thousand) compared to the associated
insoluble organic matter. This suggests that the auriferous rocks were
stained by mobile hydrocarbons produced by thermal and oxidative
alteration of indigenous bitumens, a contribution from hydrocarbons
derived from intra-basinal Witwatersrand shales cannot be excluded.
Individual aliphatic hydrocarbons of the various carbonaceous materials
were subjected to compound specific isotope analysis using on-line gas
chromatography/combustion/stable isotope ratio mass spectrometry
(GC/C/IRMS). The limited variability of the molecular parameters and
uniform delta C-13 values of individual n-alkanes (-31.1 +/- 1.7 parts
per thousand) and isoprenoids (-30.7 +/- 1.1 parts per thousand) in the
Witwatersrand samples exclude the mixing of oils from different sources.
Carbonaceous matter in the dolostones shows distinctly different bulk
and molecular isotope characteristics and thus cannot have been the
source of the hydrocarbons in the Witwatersrand deposits. All the
various forms of Witwatersrand carbon appear indigenous to the
Witwatersrand Basin, and the differences between them are explained by
variable, in general probably short (centimeter- to meter-scale)
hydrocarbon migration during diagenesis and subsequent hydrothermal
infiltration. (C) 2001 Elsevier Science B.V. All rights reserved.