THE INDUS RIVER SYSTEM (INDIA-PAKISTAN) - MAJOR-ION CHEMISTRY, URANIUM AND STRONTIUM ISOTOPES

The Indus River is one of the large river systems draining the Himalaya. We report in this paper the major-ion chemistry, Sr and U isotope systematics of the Indus system, particularly its headwaters. The results show that: (1) on an average about a third of the cations in the waters can be from silicate weathering; however, most of the (Ca + Mg) is likely to be from the weathering of carbonates and evaporites; (2) the Sr-87/Sr-86 of the waters ranges between 0.7085 and 0.7595, the higher values (> 0.72) are typical of the tributaries draining the Precambrian granite/gneissic terrains. The Sr-87/Sr-86 of the Indus main channel, throughout its entire stretch, shows only minor variations, 0.7104-0.7116. The Indus transports approximately 8.8.10(6) mol Sr to the Arabian Sea annually with a Sr-87/Sr-86 of 0.7111 if our results of Sr concentration and Sr-87/Sr-86 measured at Thatta can be considered typical of the Indus throughout the year; and (3) the U concentration in the Indus and its tributaries is generally high, 0.37-10.3 mug 1(-1). The source for the high uranium can be the weathering of granites, zones of uranium mineralisation and black shales. The Indus results when compared with our earlier data on the Ganga-Brahmaputra show that in all these three river systems carbonate weathering is the dominant source of (Ca + Mg) and HCO3 and that U concentrations are high. In the case of Sr-87/Sr-86, the Indus waters are less radiogenic; however, its tributaries draining the Precambrian granites/gneises have Sr-87/Sr-86 in excess of 0.72, similiar to that in the rivers of the Ganga system. The low Sr-87/Sr-86 of the Indus, 0.7111, suggests that its contribution to the Sr isotope evolution of oceans since the Cenozoic is less significant than that of the Ganga-Brahmaputra.