The Use Of Elemental And B & Cl Stable Isotope Geochemistry To Unravel The Formation Of Salt Rocks,Saline Lakes And Thermal Springs In Pakistan

Evaporites,saline lakes and thermal springs have been extensively studied to improve ourknowledge about their source areas,sedimentation rates and patterns,the regional tectonics and prevailing paleoclimate conditions during their formation and evolution.In Pakistan,evaporites are distributed within two basins;the Salt Range?SR?of the Potwar Basin?PB?and the Kohat Basin?KB?.Both result of the collision between the Eurasian and Indian plates.Regardless of their small sizes,both basins display important geological features bridging from the Eocambrian to the Recent periods.The major and older rock salt deposits and saline lakes are located in the Salt Range area.They are renowned for their thickness?slats:?2000m?,Permo-Triassic boundary and the presence of several regional unconformities.The Kohat basin consists of multiple plates archived the history of the Himalayan collision.Although the thermal spring in Pakistan are spread from north to south,most of them and the hottest?surface temp.?100°C?ones are located within the Himalayan Collison Zone?HCZ:region northward of MMT?along Main Mantle Thrust?MMT?and Main Karakarum Thrust?MKT?.In this work,I aimed at studying the salt rocks,saline springs and thermal spring of Northern Pakistan,using an approach coupling chemistry with Cl,B and water stable isotope systematics,in order to better constrain their conditions of formation and evolution.I also implemented this approach to document the climate change impact on the evolution of saline lakes.The chemistry of the halite in the Kohat and Salt Range basins revealed their marine origin,but underwent redeposition and recrystallisation resulting from multiple?both marine and non-marine?water entries in the basins.Previous studies and laboratory experiments have proved that?³⁷Cl is a good proxy for evaluating evaporation rates in saline and salt lakes,and it can be used to characterise the different stages of the brine evolution.Low?³⁷Cl??0.7‰?values have been proved to reflect the presence of commercially important salt?K-Mg?resources within any evaporite sequence.Here,the Cl stable isotope compositions(?³⁷Cl)from the KB and SR halite samples varied between-1.04 and 1.07‰,an isotope range larger than the one reported for marine halite:0±0.5‰.Our low?³⁷Cl data prove that this region possesses promising K-Mg deposits,in agreement with the conclusions drawn from previous geophysical and geological studies.On the other hand,the highly positive Cl isotope compositions recorded both entries of non-marine waters in the basin and halite recrystallisation.The Collison between the Eurasian and Indian Plates not only threw up the Himalayas but also generated thermal springs in Pakistan.The hot springs in northern Pakistan are of the SO₄,mixed and mainly HCO₃ types,with Na as the main cation.The analysis of the water stable isotope compositions(?D:-88.2‰to-73.3‰and?¹⁸O:-12.9‰to-10.8‰)revealed that the hydrothermal waters are of meteoric origin,mostly from the snow uphill.The corresponding tritium concentrations,between 0-60?TU?,show that the waters are only?50 years old.The low boron?B?content and extremely low boron stable isotope compositions(?¹¹B:?-14‰)indicated an igneous and metamorphic environment and resulted from the mixing of cold and hot waters at variable depths.Saline lakes represent special ecosystems and serve as major regional sources of water for humans as well as a habitat for water birds.Due to overburden and the effects of climate change,the surface of saline lakes are shrinking at a threatening rate worldwide.To test this hypothesis,I studied three saline lake systems forming the Uchhali Complex?Ramsar Site?in the Salt Range.The study of their hydrogeochemical characteristics validates that the changes in the salinity are due to high evaporation,even though rock weathering should not be neglected.?D and?¹⁸O follow a high-slope evaporation line that suggests that evaporation is the governing mechanism.Chlorine isotope compositions are explained by the mixing of several water types.Above all,human activities,such as water pumping for agriculture and other domestic purposes,also contribute to the increase observed in the total dissolved solids?TDS?concentrations.Finally,we coupled elemental and boron isotope geochemistry on the older members of the Salt Range Formation to characterize the paleo-salinity and the evolution of the regional paleoclimate.It indicated that during the deposition of the ancient marine halite the water chemistry changed from?Na-K-Mg-Ca-Cl?Ca-rich brine towards a?Na-K-Mg-Cl-SO₄?SO₄-rich type.Ion relationships revealed that these elements originate from multiple sources and identified the corresponding end members as seawater,B desorption from clays and meteoric precipitations.Halite samples of the area under study displayed Na-HCO₃-type to Ca-Cl type characteristic of sedimentary basin deposition.Coupled to the molar B/Cl ratios,?¹¹B revealed that for some samples desorption from clay minerals explained the added B.This diagenetic B desorption from clay minerals is the consequence of stress-driven mechanism,generated in the proximity of a decollement zone.Results confirmed that B stable isotopes slightly fractionate between the brine and the halite(?¹¹B?1‰).Finally,the relationship existing between ion concentrations and the?¹¹B revealed that a high salinity and drier paleoclimate conditions prevailed in the salt range and adjoining areas during the Neoproterozoic.This represented the very first isotope study in this region.My results greatly improved our understanding of the conditions of formation of regional salt lakes,thermal springs and salt rock deposits as well as their evolution,opening the door for further investigations specifically focusing on regional paleoclimate,tectonics and basin deposition conditions.