Canadian Journal of Earth Sciences Chlorine isotopes unravel conditions of formation of the Neoproterozoic rock salts from the Salt Range Formation, Pakistan Journal: Canadian Journal of Earth Sciences Manuscript ID cjes-2019-0149.R1 Manuscript Type: Article Date Submitted by the 25-Nov-2019 Author: Complete List of Authors: Hussain, Syed; CAS ISL, Han, Feng; CAS ISL Han, jibin; CAS ISL Khan, Hawas;Draft Karakoram International University, Department of Earth Sciences Widory, David; Université du Québec à Montréal, Centre GEOTOP Keyword: Halite, Neoproterozoic, Sylvite, δ37Cl, The Salt Range Formation Is the invited manuscript for consideration in a Special Not applicable (regular submission) Issue? : https://mc06.manuscriptcentral.com/cjes-pubs Page 1 of 31 Canadian Journal of Earth Sciences 1 Chlorine isotopes unravel conditions of formation of the Neoproterozoic rock salts from the 2 Salt Range Formation, Pakistan 3 Syed Asim Hussain1, 2, 3,*, Han Feng-qing1, 2, Han Hibin1, 2, Hawas Khan4, David Widory5 4 1) Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt lake Resources, Qinghai Institute of 5 Salt lakes, Chinese Academy of Science, , Xining,810008, China 6 2) Qinghai Provincial Key Laboratory of Geology and Environment of Salt lakes, Xining, China, 810008 7 3) University of Chinese Academy of Science, Beijing, China, 100049 8 4) Department of Earth Sciences, Karakoram International University, 15100, Gilgit, Pakistan. 9 5) GEOTOP/Earth and Atmosphere Sciences Department, UQAM, Montréal, Canada 10 *corresponding author’s E-mail addresses: [email protected] 11 12 Abstract 13 During the late Neoproterozoic,Draft the Salt Range in Pakistan was one of the regions where 14 the Tethys truncated and marine strata developed. The numerous transgressions and regressions 15 that occurred during that period provided enough initial material for the development of marine 16 evaporites. The geology of the Salt Range is characterized by the presence of dense salt layers and 17 the existence of four regional and local scale unconformities. These thick salt deposits geologically 18 favor potash formation. Here we coupled chloride isotope geochemistry and classical chemistry of 19 local halite samples in order to assess the extent of brine evaporation that ultimately formed the 20 salt deposits. Our results indicate that evaporites in the Salt Range area are Br-rich and precipitated 21 from seawater under arid climate conditions. The corresponding δ37Cl values vary from -1.04 to 22 +1.07‰, with an average of -0.25±0.52‰, consistent with the isotope range values reported for 23 other evaporites worldwide. The positive δ37Cl values we obtained indicate the addition of non- 24 marine Cl, may be from reworking of older evaporites, the influx of dilute seawater, the mixing of 25 meteoric and seawater, and the influence of gypsum-dehydration water. The negative Cl isotope 26 compositions (δ37Cl <-1‰) indicate that brines reached the last stages of salt deposition during the https://mc06.manuscriptcentral.com/cjes-pubs Canadian Journal of Earth Sciences Page 2 of 31 27 Late Neoproterozoic. We conclude that the Salt Range Formation could be promising for K-Mg 28 salts. 29 Keywords: Halite; Neoproterozoic; Sylvite; δ37Cl; The Salt Range Formation 30 31 32 33 34 35 Draft 36 37 38 39 40 41 42 43 44 45 https://mc06.manuscriptcentral.com/cjes-pubs Page 3 of 31 Canadian Journal of Earth Sciences 46 1. Introduction 47 Chlorine (Cl) is an ubiquitous element associated with numerous geochemical processes 48 (Luo et al. 2012). Its uncommon geochemical properties include: highly solubility in water, no 49 bonding to most silicates, and it is usually a trace and incompatible element in nature (Sharp and 50 Draper, 2013). Cl is one of the most abundant elements in many geofluids and one of the major 51 volatile constituents on Earth (e.g. Bureau et al., 2000; Bonifacie et al., 2008). Its global surface 52 average abundance is ~0.5% (Nakamura et al., 2009). Magenheim et al. (1995) estimated that 53 approximately 60% of the Earth Cl is contained within the mantle with the remaining 40% stored 54 in crustal reservoirs (Banks et al., 2000). Studies have shown that Cl is enriched in crustal materials 55 with crustal rocks averaging ~170 ppmDraft of Cl (Rieder et al., 2004). As Cl is evaporative, 56 incompatible (throughout silicates melting) and water soluble, geological processes such as partial 57 melting, magma degassing, hydrothermal activities and weathering concentrate Cl at the Earth 58 surface. In particular, Cl is highly concentrated in marine water (seawater has a Cl concentration 59 of ~20 g/L; Peterson, 2007), saline minerals and terrestrial brines (Bonifacie et al., 2008). 60 Cl has two stable isotopes, 37Cl and 35Cl, whose relative abundances are 24.24:75.76 in 61 nature, respectively (Laeter et al., 2003; Laube, 2010). Due to their relatively large mass difference 62 (5.7%) chlorine stable isotopes display measurable isotope fractionations in the different 63 geological reservoirs (e.g. Tan et al., 2009). For example, salt deposits and saline hydrothermal 64 springs tend to be enriched in 37Cl with respect to seawater (e.g. Kaufmann, 1984). Studies have 65 shown that evaporation and formation of salt minerals favor the heavy 37Cl isotope in the salt 66 deposits with surface reservoirs (evaporites, brines and the oceans) yielding an average δ37Cl of 67 0.05±0.5‰ (e.g. Eggenkamp et al., 1995; Godon et al., 2004; Eastoe et al., 2007). Brines from 68 salt lakes show a δ37Cl range from -2.05 to +1.01‰ or slightly higher (e.g. Liu, 1997; Luo et al., https://mc06.manuscriptcentral.com/cjes-pubs Canadian Journal of Earth Sciences Page 4 of 31 69 2012). Most of the physical processes induce Cl isotope fractionation, among which ion filtration 70 (=1.001 to 1.006; Phillips and Bentley, 1987; Agrinier et., 2019), salt precipitation (=1.00055 71 at 28°C; Luo et al., 2012), ion-exchange (=1.0003±0.00006 at 25°C; Musashi et al., 2004) and 72 diffusion (=1.00192±0.00015 at 80°C; Eggenkamp and Coleman, 2009; Du et al., 2016) are 73 inducing the largest chlorine isotope fractionations. Over their history, the Earth’s Cl reservoirs 74 have shown a δ37Cl consistency around the marine water value of 0‰ (Sharp et al., 2007) with 75 most of Cl-rich samples having their δ37Cl nearing this isotope composition (e.g. Eastoe et 76 al.,2007; Bonifacie et al., 2008). One of the most intriguing properties of chlorine isotope 77 compositions is that they can be used to evaluate the evaporation rate at the time of salt formation 78 (Luo et al., 2016). They can thus be used as tracers for characterizing the formation of potash and 79 Mg-salts. Draft 80 The Neoproterozoic period is geologically significant as it has widely been studied for 81 characterizing a large number of geologic processes: e.g. supercontinents collisions and their 82 movements and subduction (Bowring et al., 2007); volcanism (Allen, 2007) high sedimentation 83 rates (Kaufman and Knoll, 1995); evaporite formation and the characterization of paleoclimate 84 environments (Warren, 1999 and 2010). The investigation of ancient evaporative systems is one 85 of the best tools to study local tectonics, basin sedimentation, oxic or anoxic redox and marine or 86 non-marine conditions (Farooqi et al., 2019). The Salt Range (SR) Formation (Fig. 1), an ancient 87 evaporite in Pakistan, deposited during the late Neoproterozoic/Early Cambrian period and is 88 considered the southern border of the western Himalaya (Ghazi et al., 2015), an active frontal 89 thrust zone of the Himalaya in Pakistan (Baker et al., 1988, Iaremchuk et al., 2017). It results of a 90 tectonic collision between the Indian and the Eurasian plates (Grelaud et al., 2002). The distinct 91 features of the SR are i) its dense salt layers, ii) the existence of four regional and local scale https://mc06.manuscriptcentral.com/cjes-pubs Page 5 of 31 Canadian Journal of Earth Sciences 92 unconformities (Fig. 2A) of Precambrian to Pleistocene ages (Gee and Gee, 1989), and iii) its 93 Permian-Triassic marine belt. For these reasons, the area has been studied for more than a century 94 by geologists and paleontologists, mostly focusing on its tectonics, geological structure, 95 paleontology and petroleum potential but, to our knowledge, there is only a few studies about its 96 geochemistry. 97 With that in mind we carried out this work aiming at the following objectives: (1) to 98 precisely characterize the Cl isotope behavior in halite samples from the Salt Range formation in 99 order to (2) constrain the brine evaporation stages and ultimately (3) to assess the regional potential 100 for potash deposits. 101 2. Geological settings Draft 102 The Salt Range (32°15′–33°0′ N and 71°34′–73°45′ E ) is located in northern Pakistan between 103 the Main Himalayan Fold-Thrust (MHFT) in south and the Main Boundary Trust (MBT) towards 104 north (Fig. 1). It represents the youngest frontal fold and thrust belt with an age of about 67 Ma, 105 which occurred in the advanced stages of the Himalayan orogeny (Powell et al., 1988). The 106 existence of the thick, incompetent SR Fm. at the base of the sedimentary sequence has greatly 107 affected the regional geology and structure (Gee and Gee, 1989). Due to the existence of the 108 evaporitic SR Formation the deformation style of the Potwar Basin is distinct between its southern 109 and northern parts (Lillie et al., 1987; Richards et al., 2015). The SR presents a complicated salt 110 anticlinorium with a chain of salt anticlines, having its maximum thickness in its central part 111 (Farooqi et al., 2019).