- © 2000 Geological Society of South Africa
The 87Sr/86Sr ratio of Orange River water increases from 0.7081 at its headwaters in the basalt of the Drakensberg mountains, to a value of 0.7126 before its confluence with the Vaal River tributary. The concomitant increase in Sr concentration is from 0.08 to 1.28 μM. The Vaal River, which drains Precambrian and Permian/Triassic rocks, has higher Sr concentrations, from 0.95 to 6.69 μM, and more radiogenic 87Sr/86Sr values, from 0.713 to 0.731. The geochemistry of the dissolved load of the Orange and Vaal rivers corresponds with that of the dominant catchment lithologies: Jurassic basalt, Permian/Triassic Karoo sediments and Proterozoic/Archaean para- and ortho-gneisses/granitoids. Chemical weathering rates in the Orange River system are at least a factor of 3 lower than the global average value of 36 ton/km2/yr. The highest chemical and physical weathering rates occur in the Upper Orange River, associated with high runoff and relief. In the low-relief Vaal River, chemical weathering predominates over physical weathering.