MAKING QUARTZ ARENITES IN THE ARCHEAN: FROM WEATHERING TO BURIAL

In the Phanerozoic world, pure quartz sandstones are made dominantly by erosion of older sandstones and to a limited extent by intense weathering on stable cratons in the tropics. In the Archean, the volume of pre-existing sandstone to erode was limited. Furthermore, higher temperatures and mobile Fe in soil profiles favored total dissolution of both quartz and feldspar in tropical soils. We propose that generally moderate source-area weathering followed by extensive groundwater alteration after deposition accounts for the high proportion of quartz arenites in Archean basins like the Witwatersrand.

To test this hypothesis, we studied four Archean paleosols from Canada and South Africa. These show a small but significant mass-independent fractionation of S isotopes, indicating that they were exposed to an oxygen-free atmosphere and that they are indeed paleosols. Fe is leached from all of these, but quartz and feldspar grains remain in the framework to act as sand sources. Q/F ratios are about 1.9 in granite-hosted profiles.

We measured chemistry and petrography of 160 quartzite samples from the Witwatersrand Basin, both from the edge and from the Vredefort uplift in its center. These quartzites have little feldspar, but contain considerable sericitic matrix. Whole-rock Al2O3 is about 10%, and this Al must have been deposited as feldspar grains. Recalculating the Al2O3 as albite gives an original Q/F of about 2.3 for both the West Rand and the Central Rand. Basin-edge quartzites average Na2O/Al2O3 of 0.05 and CaO/Al2O3 of 0.04 compared with basin-center averages of 0.13 and 0.10. Intrusion of meteoric water from the outcrop towards the basin center leached the alkalis from the feldspars and left a residue of kaolinite (now pyrophyllite and muscovite). This effect can be seen today in the Paraná Basin of Brazil, a close hydrologic analog to the Witwatersrand that has a thick basalt cap overlying porous sandstones that are transmissive aquifers. In the Paraná Basin, leaching of feldspar extends 30 km from the outcrop; calcite is leached for 140 km from the outcrop. We believe higher surface and subsurface temperatures in the Archean would have produced even more extensive meteoric leaching in the Witwatersrand Basin.