Fluid inclusions in quartz crystals from South-West Africa

Abstract

Quartz crystals from calcite veins of unknown age in Precambrian metasedimentary rocks at Geiaus No. 6 and Aukam farms in South-West Africa contain both primary and secondary inclusions filled with one or a variable combination of: organic liquid, moderately saline aqueous liquid, dark-colored solid, and vapor. Analysis of these materials by microscopy and by gas chromatography and mass spectrometry shows the presence of constituents of both low and high molecular weights. The former include CH 4, C 2H 6, C 3H 8 and possibly C 4H 10 as well as CO, CO 2, H 2O, N 2 and H 2. High molecular weight components are dominantly n-alkanes and isoprenoid hydrocarbons. The n-alkanes range from at least n-C 10 to n-C 33. Concentrations of n-alkanes larger than n-C 17 decrease regularly with increasing carbon number. An homologous series of isoprenoid hydrocarbons ranging from at least C 14 to C 20 is present in unusually high concentrations. Pristane (C 19) is most abundant, and C 17 isoprenoid is least abundant. The molecular composition and distribution of hydrocarbons suggest biological precursors for these components. Consideration of data provided by freezing, crushing and heating experiments suggests that the pressures at the time these in part supercritical fluids were trapped probably exceeded 30–40 atm, and the minimum trapping temperature was about 120–160°C. Both primary and secondary inclusions apparently containing only organic materials were trapped by the growth of the host quartz from aqueous solution. The data obtained neither prove nor preclude Precambrian, Paleozoic or younger sources for the organic materials.