A QUANTITATIVE ASSESSMENT OF THE EFFECTS OF MAGMATISM ON THE THERMAL HISTORY OF THE KAROO SEDIMENTARY SEQUENCE

Simple 1-D numerical and analytical thermal models are used to quantif y thermal effects of magmatic processes affecting the upper approximat ely 5km of the crust. The model results suggest that maximum palaeotem perature estimates, derived from vitrinite reflectance profiles and zi rcon fission track data from the Karoo basin, are not easily explained by increased burial in the past or by heating caused by high level in trusions if the excess heat was dissipated only by conduction. Heavy m ineral deposits associated with uranium mineralization in the karoo ba sin show signs of significant hydrothermal alteration, similar to that documented from other uranium provinces. New fission track data from detrital zircons from two sedimentary-hosted uranium ore deposits (Rie tkuil and DR-3 Laingsburg) implies that these deposits experienced max imum palaeotemperatures of at least 250+/-50-degrees-C subsequent to d eposition (for heating times of the order of 10(6) yr). Moreover, the distribution of fission track ages for single zircon grains suggests t hat peak temperatures were reached during the period of karoo magmatis m at approximately 190+/-10 Ma. Taken together, these observations pro vide strong evidence for the existence of magmatically driven hydrothe rmal systems within the Karoo basin and their probable generic associa tion with uranium mineralization.