Water and sediment chemistry of Higher Himalayan lakes in the Spiti Valley: control on weathering,provenance and tectonic setting of the basin

The geochemical study of the Dankar, Thinam and Gete lakes of the Spiti Valley has revealed that these lakes are characterized by varying contents of major ions, i.e. Ca, Mg, HCO₃, Na, K, Cl, SO₄, SiO₂ and Sr as trace element. The concentration of these elements is significant, as they indicate the nature of the lithology and the type of weathering at the source. The sediment chemistry data have also been employed to quantify weathering intensity and to elucidate the provenance and basin tectonic setting where terrigenous sediment is deposited.Dankar Lake is located on the limestone-dolomite-rich Lilang Group of rocks (Triassic), and dissolution of carbonate is the prime source of ionic concentration in this lake. The high (Ca+Mg):HCO₃ equivalent ratio of 6.94 indicates carbonate weathering, and the very low (Na+K):T_Z⁺ ratio of 0.07, which is used as an indicator of silicate weathering, shows insignificant silica dissolution in this lake. On the other hand, in Lake Thinam a relatively low (Ca+Mg):HCO₃ equivalent ratio of 2.09, a (Na+K):T_Z⁺ ratio of 0.12 and other parameters indicate that carbonate is derived from calcareous nodules and thin intercalations of limestone in the Spiti shales (Jurassic), and also some contribution from silicate lithology is evident. Mixing of groundwater cannot be ruled out, as springs are observed in this lake. In Lake Gete, the (Ca+Mg):HCO₃ equivalent ratio is again high at 5.04, and the (Na+K):T_Z⁺ ratio is 0.15, indicating dissolution of both carbonate and silicate rocks in the basin. This is consistent with the corresponding lithology in the lakes, and their denudation. Very high Sr contents of 2,331 µg/l in Dankar Lake, 715 µg/l in Gete Lake and 160 µg/l in Thinam Lake are significant and support dissolution of carbonate rocks, as the silicate rocks contribute less Sr although its isotopic ratio is high. It is also reflected that mechanical erosion and chemical weathering are perhaps the effective processes in this region. The former exposes fresh mineral surface for dissolution. The chemical index of alteration (CIA), with an average value of 78.79 in Dankar and 81.06 in Gete, indicates high weathering conditions. The K₂O–Fe₂O₃–Al₂O₃ triangular plots of the samples demonstrate residual clay formation, indicating intense weathering at the source. The clay mineralogical data corroborate the above observation.The sediment chemistry data document depletion in SiO₂ and Al₂O₃, as they are enriched in carbonates and depleted in Na₂O, K₂O, MnO, and TiO₂, as compared to PAAS and UCC which are related to strong weathering at the source. The positive linear correlation between K and Rb suggests that they are contained in the illitic phase, and high positive correlation of Zr and Y with SiO₂ indicates their association with coarser-grain, quartz-rich sandstone. The high phyllosilicates and low feldspar and major element chemistry indicate recycling and mineral maturity of sediments deposited in the Tethyan basin in a passive margin setting. This also indicates older sedimentary-metasedimentary rocks which are ideally exposed in the Spiti Valley. The tectonic discriminant plots portray a passive margin tectonic setting of the detritus in these lakes.