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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 Author: 25-Nov-2019

Complete List of Authors: Hussain, Syed; CAS ISL, Han, Feng; CAS ISLHan, jibin; CAS ISLKhan, Hawas; Karakoram International University, Department of Earth SciencesWidory, 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

Issue? :Not applicable (regular submission)

https://mc06.manuscriptcentral.com/cjes-pubs

Canadian Journal of Earth Sciences

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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, China6 2) Qinghai Provincial Key Laboratory of Geology and Environment of Salt lakes, Xining, China, 8100087 3) University of Chinese Academy of Science, Beijing, China, 1000498 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, 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

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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

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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 ppm 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.,

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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.

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

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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

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). Fatimi (1973) evaluated the thickness of the SR Fm. between 800 and 2000

112 m. The SR itself emerged following the movement of the Potwar Plateau towards south, supported

113 by an E-W striking and down to the north basement normal fault (Richards et al., 2015).

114 Stratigraphically, the SR displays from the Neoproterozoic to Paleogene rocks of the SR Fm.

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115 (Jhelum, Nilawahan and Chharat groups) up to the Eocene (Fig. 2). The overall exposed strata is

116 fairly complete in the SR. Rocks older than the Eocambrian are generally not exposed but a few

117 outcrops can be found in the Kirana and Sangla hills of the Punjab plains (southeast of the SR;

118 Krishnan et al., 1966). Gee and Gee (1989) divided the SR into three main sections: i) The Eastern

119 SR (from Jogi Tilla to Khewra), ii) the Central SR (from Khewra to Warchha) and iii) the Western

120 SR (from Sakesar to Kalabagh). The SR is wider in its middle section (between Khewra and

121 Warchha) where it makes up for one of the best successions of the Eocambrian period.

122 The SR Formation is mainly formed of halite, anhydrite, dolomite, gypsum that may extend

123 up to the Hazara District (Latif, 1970). Combined with the coeval similarly restricted marine

124 deposits of the Ara Formation in Oman, the Hurmuz Formation in Iran, and the salt deposits in

125 India, it suggests a widespread evaporite deposition across this area (Smith, 2012). Within the SR

126 area, the SR Formation is divided into three members, based on their location and occurrence: i)

127 the Billianwala Salt Member, ii) the Bandarkas Gypsum Member and iii) the Sahwal Marl Member

128 (Fig. 2A). These three units outcrop in the Khewra Gorge (Fig. 1). The Billianwala Salt Member

129 incorporates marl, thick seams of halite and potash seams. The Bandarkas Gypsum Member

130 displays marl with important deposits of gypsum that range from crystalline to non-crystalline.

131 The Sahwal Marl Member consists of marl and thick seams of halite (~2-4m). The Billianwala

132 Salt Member is observed in the Khewra salt mine and is divided into seven seams, three in the

133 upper group called the Buggy complex and four in the lower group called the Pharwala complex

134 (Fig. 2C). The Khewra area is an asymmetrical dome cut by the Khewra stream that exposes a

135 significant stratigraphic part of this area. A complete sequence of the SR Fm. is exposed about one

136 km from the mouth of the Khewra salt mine (Table 1). All exposed strata in the Khewra salt mines

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137 are part of the lower or main saline marls. Faults in their southern and western parts limit the main

138 mine extension.

139 3. Material and methods

140 1.1 Sampling strategy

141 We collected a total of 28 halite samples in the Kherawa salt mine (Salt Range Formation;

142 Fig. 2). Details about the sample locations are reported in Table 2. Fresh halite samples were

143 collected in order to obtain the most representative samples and to avoid any potential bias

144 resulting from secondary processes.

145 3.2 Elemental concentrations

146 For each sample, we determined K+, Na+, Ca2+, B3+, Mg2+, SO42-, Br-, Cl-, NO3

-

147 concentrations and corresponding Cl stable isotope compositions (δ37Cl). K+, Ca2+, Mg2+, and B3+

148 concentrations were measured by ICP-OES (ICAP6500DUO, USA) with a precision better than

149 5%. Br-, NO3- and SO4

2- were analyzed by Ion Chromatography (IC-5000+, Thermo Fisher USA)

150 and Cl- was measured by chemical mercurimetry, with an accuracy higher than 0.3% (ISL, CAS,

151 1988). δ37Cl was analyzed using high-accuracy Positive Thermal Ionization Mass Spectrometry

152 (TIMS-TRITON, U.S.A) with a detection limit of 1-320 a.m.u. All measurements were made at

153 the Salt Lakes Analytical and Testing Department, Qinghai Institute of Salt Lakes, Chinese

154 Academy of Sciences.

155 3.3 Chlorine isotope compositions analysis

156 All halite samples were dissolved in pure (at least 4 times deionized) water. The ensuing

157 purification process followed the two-steps resin method described by Xiao et al. (1995). Briefly,

158 a polyethylene ion-exchange column (diameter of 0.5 cm) was filled with 2 cm of a reborn H-

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159 cation exchange resin (200-400 mesh, resin type: Dowex 50WX8). Then, 1.6 cm of regenerated

160 Cs-cation exchange resin was inserted into a second polyethylene ion-exchange column (diameter

161 of 0.4 cm). Samples were first eluted through the H-cation exchange resin column and then through

162 the Cs one. During the process, the pH of the solution was maintained between 3 and 6, without

163 the aid of a buffer solution. Ultimately, samples were collected for TIMS analysis. As SO42-/Cl-

164 and NO3-/Cl-molar ratios were all below 2.5 and 0.5 in our samples, respectively, no further

165 purification was needed as interferences by SO42- and NO3

- can then be considered as negligible

166 (Luo et al., 2016). Here, we used NaCl (ISL-354 NaCl) as the Cl isotope standard reference

167 material. This seawater standard was collected at coordinates 4018/ N, 101008/ E and is considered

168 the Standard Mean Ocean Chloride (SMOC) (Xiao et al., 2002).

169 For Cl isotope analysis, solutions were prepared to ultimately contain approximatively 5 mg/ml

170 Cl. A tantalum (Ta) filament was heated under vacuum for one hour (using a current of 2-3A)

171 before being covered with 2.5 µL of a graphite slurry that contained at least 80% of ethanol plus

172 80 µg of graphite. About 2.5 µL of the sample solution, containing at least 10 µg of Cl as CsCl

173 was deposited onto the filament, which was then dried using a current of 1A for <3 minutes.

174 Samples were finally placed into the source of the TIMS mass spectrometer until a vacuum of

175 around 2.5×10-7 mbars was reached. During the analysis, the Cs2Cl+ ion current was kept at 4 ×10-

176 12 A by adjusting the TIMS source current. Data were simultaneously collected on Faraday cup

177 “C” and “H1” by using the ion flows from mass numbers 301 (133Cs235Cl+) and 303 (133Cs2

37Cl+).

178 The 37Cl/35Cl ratio we obtained for the international IAEA ISL-354 NaCl standard was

179 0.319028±0.000058 (n=12), in agreement with Hussain et al. (2019) and with the certified value

180 of Xiao et al. (2002) of 0.31964±0.00092. Precision for the 37Cl determination was >±0.3‰.

181 We reported the Cl stable isotope compositions using the classical “δ” (delta) notation:

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182 (1)𝛿37𝐶𝑙 = ⌈37𝐶𝑙35𝐶𝑙𝑠𝑎𝑚𝑝𝑙𝑒

37𝐶𝑙35𝐶𝑙𝐼𝑆𝐿 ― 354

― 1⌉ × 103

183

184 4. Results and discussion

185 The Cl isotope and elemental compositions of our halite samples are given in Table 2.

186 Results show that Na+ and Cl- are the major ions, followed by SO42–. Additionally, trace elements

187 such as B3+ and Br- are also detected. The δ37C1 of the halite samples range from -1.04 to +1.07‰

188 with an average value of -0.25±0.52‰. Most of the Cl isotope compositions fall between -0.5 and

189 +0.5‰.

190 4.1 Origin of the halite

191 Geochemistry has been widely used to identify the sources of salts (e.g. Schreiber and

192 Tabakh, 2000), discriminating them into three distinct types: terrigenous, hydrothermal and marine

193 (e.g. Guo et al., 2017). Previous studies suggested that there is no evidence of evaporite

194 precipitation from non-marine or continental brines in the SR (e.g. Hardie, 1984). Hydrothermal

195 fluids migrating along the deep large faults are also improbable sources for these salts as: (1) the

196 δ37Cl we measured in the SR hydrothermal waters (about ±1‰) are distinct from those reported

197 worldwide, ranging from negative isotope compositions to as high as +3‰ (Guo et al., 2017). (2)

198 Hydrothermal depositions are usually accompanied by extremely high Mn and Fe concentrations

199 (Hardie, 1990). For the SR, their concentrations are on average extremely low, 0.06 and 0.97 ppm,

200 respectively (Sharif et al., 2007). But ultimately these two elements may be controlled by the pH

201 of the fluids as well as the presence of complexating anions such as Cl- and F-. (3) Kovalevych et

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202 al. (2006) reported an approximate homogenization temperature of the saline inclusions in the

203 halite of 50-1000C, which contradicts a deep hydrothermal origin.

204 The lithology of the SR Formation indicates a marine nature: at an earlier stage, it had been

205 isolated from the main Tethys Sea during a regression event that ultimately formed a lake (Latif,

206 1970; Ghazi et al., 2015). Its depositional environment is considered as a shallow marine under

207 arid conditions in which evaporite succession developed (Latif 1970; Farooqi et al., 2019). The

208 concentrations we measured for elements such as B and Br (Table 2) also support this marine

209 nature, although most of our Br values are between 50-100 ppm, in the lower range of those

210 reported for halite precipitated from marine water (65–270 ppm; Holser, 1979). These low Br

211 concentrations may result from the reworking of older evaporites, the influx of dilute seawater, the

212 mixing of meteoric and seawater, and the influence of gypsum-dehydration water (Kovalevych et

213 al., 2006). Eggenkamp et al. (2019) demonstrated that Br is entering less easily the halite

214 crystalline structure compared to Cl, making the impact of this process less significant on the Br

215 concentrations. Shurui et al. (2019) recently hypothesized that Br contents are more useful to

216 reveal the stage of brine evaporation. Still, together, C1 isotope geochemistry of evaporites, their

217 mineralogy and the sedimentological evidences (i.e. depositional environments) confirm that the

218 SR was formed under marine conditions.

219 4.2 Chlorine isotopes and Cl/Br ratios in the halite

220 Sedimentary basins created by evaporated seawater display negative δ37C1 (≤0.0‰;

221 Eggenkamp et al., 1995; Eastoe et al., 2001). This again comforts our conclusion of a marine origin

222 as the δ 37Cl we obtained for 6 of the 8 salt seams (Fig. 1) are strictly negative, ranging from -1.04

223 to -0.01‰ (average of -0.46±0.30‰). The other 2 samples, collected in the Buggy and North

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224 Buggy sections, unexpectedly, show positive Cl isotope compositions, from 0.02 to 1.07‰, with

225 an average of 0.53±0.36‰).

226 The Cl/Br ratio is one of the most effective and sensitive indexes when determining the

227 geological environment, the degree of concentration and the depositional phase associated to the

228 brine evaporation process (Han et al., 2019). This ratio is mainly controlled by the sequential

229 formation of salt minerals during the evaporation of marine water, resulting from their possible

230 substitution, having similar ion radii. The average Cl/Br (molar) ratio of present seawater is ~300

231 and has remained constant through geological history (Kaufmann 1999a; Sakellariou-

232 Makrantonakie, 2008; Yechieli, 2009). While seawater evaporation increases Br concentrations,

233 dilution will decrease them (Han, et al., 2018). The Cl/Br ratio of evaporated seawater remains

234 constant until halite starts precipitating at a NaCl concentration near 6.2 mol/L (Alcala and

235 Custodio, 2008). During the evaporation process the first minerals formed are Ca salts, followed

236 by SO4 salts, NaCl (Halite), KCl (Sylvite), KMgCl.6H2O (Carnallite) and finally MgCl2.6H2O

237 (Bischofite) (Warren, 2010 and references therein). In general, the later evaporation phases induce

238 a Br- depletion (Wilson and Long, 1993). In our study, the Cl/Br ratios ranged between 1.32×103

239 to 8.47×103, in agreement with the 103-105 ratios generally considered for NaCl minerals

240 (Kloppmann et al., 2001; Cartwright et al., 2006).

241 The relationships existing between Cl/Br and δ37Cl in Figure 3 give information about the

242 halite dissolution or recrystallization processes. Previous studies have proved that dissolution and

243 recrystallization do not affect both Cl/Br and δ37Cl (e.g. Banks et al., 2000), but if continental

244 waters are mixed with brine before evaporation occurs, the Cl/Br ratio of the solution will be

245 distinct from that of the seawater (Han et al., 2019). Here, all Cl/Br ratios are very high compared

246 to the ~300 seawater (Fig. 3), indicating that in the SR seawater was mixed with continental water

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247 and/or that water-rock interaction (WRI; Farooqi et al., 2019) occurred. Figure 4 reports the

248 classical Na/Br vs Cl/Br diagram. When seawater undergoes evaporation and crystallization, Br

249 will preferentially concentrate in the remaining seawater, resulting from its difficulty to enter the

250 halite mineral lattice (e.g. Liu et al., 2016). This results in a decrease of the corresponding Na/Br

251 and Cl/Br molar ratios in the residual seawater (e.g. Liu et al., 2016). On the other hand, brines,

252 formed by dissolution of precipitated halite by fresh water, yield higher Na/Br and Cl/Br molar

253 ratios compared to seawater (e.g. Liu et al., 2016). When plotted into the diagram (Figure 4), the

254 ratios that we obtained for our halite samples also hint at multiple sources for the halite as well as

255 the occurrence of recrystallization and WRI. Moreover, all samples plot above the seawater halite

256 precipitation line defined by Kesler et al. (1995), again indicating multiple WRI processes that

257 explain that all our Na/Br and Cl/Br ratios are greater than those of seawater (Fig. 4).

258 δ37C1 variations in the halite samples are slightly larger than those reported from other

259 sedimentary basins (0.0±0.9‰: Eastoe et al., 1999). This also confirms that, beside a high

260 evaporation rate, continental/non-marine waters influenced the salt formation (e.g. Xiao et al,

261 2000). The evaporation process concentrates brines and continuously decreases the corresponding

262 37Cl (e.g. Luo et al., 2014). Here, as reported for other sedimentary basins (e.g. Tan et al. 2005),

263 the δ37C1 of the potash seam are more negative than those of common salt (Table 2), consistent

264 with previous laboratory experiments (Eastoe et al., 2007).

265 4.3 Inputs of non-Marine Chloride

266 Evaporites forming in basins having no or limited connections with the open sea are expected

267 to episodically get waters and solutes from non-marine sources (Hardie, 1984). Previous studies

268 have shown that the geology deposition cycle in the SR begins with marine evaporite facies,

269 followed by an irregular marine and non-marine sedimentation sequence and finally settled with a

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270 second evaporite facies, i.e. recrystallization (Ghauri, 1979; Kovalevych et al., 2006). In an

271 evaporite basin, the input of non-marine Cl may depend on its shape and geometry (Warren, 1999).

272 As thus, smaller basins, like the Salt Range, are more susceptible to yield δ37C1 significantly

273 distinct from the ones of the corresponding marine waters, as a consequence of those non-marine

274 contributions. Since the SR Fm. is a remnant of the Tethys Sea (Latif, 1970; Ghazi et al., 2015)

275 and chemically demonstrates a marine origin (see section 4.1), the corresponding δ37Cl should be

276 centered around 0±0.5‰ (Eatoe et al., 2001 and 2007). Here, our results show that the 37Cl of the

277 SR halite samples can be separated into two different families (Figure 3): 1) Samples that with the

278 low 37Cl that show a high concentration of ancient brines, i.e. they reached the late phase of

279 deposition. 2) The samples with the positive δ37Cl that indicate that the brines have either been

280 mixed with non-marine Cl (e.g. Eastoe et al., 2007; Farooqi et al., 2019) and/or underwent

281 recrystallization (Kovalevych et al. 2006; Warren, 2016), that resulted in chlorine isotope

282 compositions distinct from the marine evaporites.

283 4.4 Economical deposits and δ37C1

284 Luo et al. (2012, 2014) experimentally proposed that 37Cl constantly decrease during the

285 initial phase of brine deposition. Consequently, when the halite precipitates during the late

286 deposition phase, it yields more negative 37Cl. This 37Cl-depletion continues until the

287 crystallization of K-Mg salts (Luo et al., 2016). Hence, the study of the Cl isotope compositions

288 gives information on the evaporation stage, and thus changes in the 37Cl can be related to the

289 evaporation cycles of saline lakes.

290 Most of our 37Cl are within the range of 0 to -0.5‰ with some values below, which indicates

291 that during salt deposition the evaporation rate was high in the SR area. Reported δ37Cl values for

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292 salt deposits worldwide are usually ≤-0.60‰ for K-salts and <-1‰ for carnallite (Table 3). Our

293 study therefore shows that the Salt Range Formation was developed in the later stage of brine

294 deposition, which is promising in terms of potential for sylvinite and carnallite deposits, in

295 agreement with previous geological studies (Table 2 & 3).

296 4.5 Comparison of the Cl isotope compositions of the Salt Range with other basins

297 It has now been well documented that the Cl stable isotope compositions of terrestrial

298 reservoirs are distinct from the δ37C1 of present-day seawater (Guo et al., 2017; Figure 5): δ37C1

299 around 0‰ are representative of a marine source; terrestrial materials such as basalt (MORB) show

300 highly enriched δ37C1 (7- 8‰; Sharp et al., 2007) and waters from subduction zones have δ37C1

301 around -7.5 ‰ (Ransom et al., 1995; Wei et al., 2008). K-forming basins have δ37C1 ranging from

302 0 to ±1‰ (Tan et al., 2005; Eastoe et al., 2007), isotope compositions varying with the tectonic

303 settings, the Cl sources and the formation environment. Eastoe et al. (2007) reported a -1 to +1‰

304 range for chlorine isotope compositions of K-salts during the study of halite samples from 5

305 different basins. Tan et al. (2005) reported δ37C1 of -2.41 and 0.44‰ for the Navarra and Catalonia

306 Basins (Spain), respectively, while the sylvite from the Mahai Salt Lake (China) was about -0.90‰

307 (Table 3). In the Salt Range the δ37C1values range between -1.04 to +1.07‰, similar to the Mahai

308 Salt Lake (China), the Khorat Basin (Thailand) and the Nepa Basin (Russia) (Table 3). These data

309 indicate that most of the δ37C1 for Mg-K salts lay between -1 and +1‰, confirming that the SR

310 formation is promising for economical K and Mg salts deposits (Figure 5). The proven reserves

311 though are mainly located in the Khewra mine (Figure 1C), but their extraction is rendered difficult

312 due to the fact that gravity (Ahmad et al., 1979) and airborne (OGDCL, 1962, 1963: Drewes et al.,

313 2007) studies have shown that they are situated at shallow depths.

314 5. Conclusions

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315 1) The thick bedded halite in the Salt Range formation is strongly associated to its marine

316 source, demonstrated by the range of their δ37C1, from -1.04 to +1.07‰ (average of -

317 0.25±0.52‰)

318 2) δ 37Cl of the salt rock samples show two dissimilar patterns. Of the eight seams studied,

319 six yielded negative isotope compositions and the other two positive ones. The overall

320 halite isotope variations are slightly larger than the marine water Cl (δ37C1 = 0.0‰). This

321 is explained by the input of non-marine Cl and incongruent solution (which results in the

322 partitioning of Br into saturated NaCl solution, inducing a Cl isotope fractionation; Eastoe

323 et al., 2001). The Potash seam shows lower δ37C1, which is consistent with values

324 previously reported for i) other basins (Tan et al., 2005 reported 37Cl <0.33‰ for salts

325 collected above and below potash strata, <-0.6‰ in sylvite, and <-1‰ for carnallite) and

326 ii) laboratory experiments (e.g. -0.9<37Cl<0‰; Eastoe et al, 1999; Luo et al., 2014).

327 3) The δ37C1 presented here, in conjunction with previous geological studies, suggest that the

328 salts have reached their final stage of precipitation and possibly the stability fields of K-

329 salts. This indicates that the SR area is favorable for sylvite deposits.

330 4) To date, the characterization of the δ37C1 isotope compositions of the Salt Range area is

331 still scarce rending the predictions of the exact salt precipitation stage very difficult.

332 Further studies, comparing δ37C1 with those of known sylvite deposits, particularly with

333 evaporite basins of similar origins (e.g. India, Iran and Oman) are needed to more precisely

334 outline the terminal stages of the brines.

335

336

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337 References

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1 Table1: Detailed stratigraphy of the Salt Range Formation. Thickness and the lithology of the 2 exposed rock from the Khewra Salt Mine are presented.

Stratigraphy sequence(s)

New division Old division

Subdivision/Description Thickness (m)

Upper gypsum dolomite Crystalline to non-crystalline 0-20Sahwal Marl

Upper saline marl a) Bright red marl + minor salt seams

b) Dull red marl + gypsum (3m at top)

>80

50

Bandrakas Gypsum Middle gypsum Crystalline to non-crystalline 50

Lower or main saline marl Halite + sometimes intermixed with

gypsum beds

>650Billianwala Salt

Lower gypsum dolomite Base is not exposed >100

3

4

5

6

7

8

9

10

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11 Table 2: δ37Cl and ion compositions of the halite samples. SPW: South Pharwala, KS: Potash 12 Seam, MPW: Middle Pharwala, NPW: North Pharwala, TS: Thin Seam, S: Sujowal, B: Buggy, 13 NB: North Buggy.

Sample Location Cl-

(Wt. %)

SO42-

(Wt. %)

Na+

(Wt. %)

K+

(Wt. %)

Ca2+

(Wt. %)

Mg2+ (Wt. %)

B3+

(Wt. %)

Br -

(Wt. %)

NO3-

(Wt. %)δ37Cl

(‰)

KH17-1 NLLT 51.05 6.96 36.41 0.32 0.23 0.13 0.0034 0.0080 0.0375 -0.62

KH17-2 SPW 59.43 1.36 38.71 0.22 0.21 0.06 0.0003 0.0105 0.1262 -1.04

KH17-3 SPW 59.35 1.47 38.50 0.32 0.24 0.13 0.0005 0.0089 0.1366 -0.91

KH17-4 KS 36.68 27.21 24.21 7.55 0.03 4.31 0.0052 0.0097 0.1117 -0.58

KH17-5 KS 44.85 0.31 22.30 9.41 0.01 0.06 0.0003 0.0340 0.0948 -0.74

KH17-6 MPW 60.28 0.16 39.00 0.15 0.02 0.02 0.0012 0.0092 0.1534 -0.28

KH17-7 KS 47.61 8.10 31.91 1.44 1.35 0.65 0.0106 0.0064 0.0326 -0.49

KH17-8 KS 40.33 7.71 19.14 9.73 0.03 1.38 0.0014 0.0281 0.0904 -0.73

KH17-9 NPW 54.14 3.81 35.40 0.77 0.52 0.45 0.0072 0.0093 0.0239 -0.81

KH17-10 NPW 60.45 0.12 39.18 0.06 0.04 0.01 0.0000 0.0074 0.1795 -0.13

KH17-11 NPW 60.16 0.29 38.94 0.13 0.10 0.08 0.0000 0.0082 0.0658 -0.34

KH17-12 NPW 45.65 6.73 30.43 1.30 1.18 0.49 0.0082 0.0054 0.0300 -0.01

KH17-13 TS 60.30 0.20 39.05 0.10 0.04 0.02 0.0001 0.0100 0.1863 -0.46

KH17-14 TS 49.69 12.67 31.31 3.80 0.02 2.5 0.0022 0.0240 0.1391 -0.49

KH17-15 TS 57.85 3.15 38.74 0.10 0.07 0.08 0.0001 0.0111 0.0687 -0.15

KH17-16 TS 58.63 2.21 38.62 0.27 0.25 0.02 0.0001 0.0076 0.0992 -0.84

KH17-17 MPW 60.49 0.18 39.21 0.05 0.02 0.04 0.0003 0.0243 0.1752 -0.83

KH17-18 MPW 58.27 2.69 38.02 0.55 0.19 0.28 0.0005 0.0072 0.1231 -0.15

KH17-19 MPW 60.19 0.52 39.09 0.10 0.07 0.03 0.0003 0.0096 0.0993 -0.16

KH17-20 MPW 53.49 0.40 31.01 2.29 0.02 1.35 0.0005 0.0085 0.0734 -0.13

KH17-21 S 49.30 12.68 37.90 0.04 0.02 0.06 0.0009 0.0183 0.1451 -0.18

KH17-23 S 59.67 1.11 39.03 0.08 0.05 0.05 0.0000 0.0095 0.1314 -0.20

KH17-22 S 60.22 0.47 39.04 0.14 0.08 0.04 0.0001 0.0103 0.0848 0.73

KH17-24 B 59.69 1.13 39.03 0.04 0.01 0.11 0.0005 0.0075 0.1175 0.02

KH17-25 B 59.37 1.54 38.56 0.16 0.11 0.25 0.0004 0.0091 0.1158 0.08

KH17-26 NB 59.33 1.48 38.52 0.29 0.27 0.11 0.0020 0.0078 0.0972 0.67

KH17-27 NB 56.31 4.83 36.54 0.20 0.13 0.05 0.0014 0.0076 0.0290 1.07

KH17-28 NB 54.08 7.42 38.26 0.05 0.008 0.18 0.0007 0.0229 0.1530 0.61

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14 Table 3: Comparison of the chlorine stable isotope compositions of worldwide potash promising 15 basins. Sources: Qarhan Salt Lake, Mengye Yunnan, Navarra Basin, Catalonia Basin, Spain, 16 Sitaluobin and Mahai Salt Lake from Tan et al. (2005); East Siberia & Holbrook Basin from Eastoe 17 et al. (2007); Gulf of Mexico from Eastoe et al. (2001); Salt Range from this study.

18

Region Sample 37Cl/35Cl ± (2σ) δ37Cl (‰)

Qarhan Salt Lake, China Salt (0.7― 0.8m depth) Modern carnallite

0.31912±0.000040.31882±0.00006

-0.44-1.38

Mengye Yunnan, China White salt China Sage green sylvinite Caesious sylvinite

0.31914±0.00004 0.31904±0.00006 0.31898±0.00004

-0.38-0.69-0.88

Navarra Basin, Spain Pink sylvinite Red sylvinite Carmine sylvinite

0.31898±0.00016 0.31849±0.00011 0.31902±0.00003

-0.88-2.41-0.74

Catalonia Basin, Spain Salt (lower zone) White salt Pink salt Red sylvinite Carnallite

0.31958±0.00005 0.31941±0.00008 0.31945±0.00006 0.31913±0.00005 0.31887±0.00006

1.00 0.470.60-0.41-1.22

Sitaluobin, Belarus Salt (S-84) Sylvinite (S-83) Sylvinite (S-80)

0.31924±0.00003 0.31880±0.00017 0.31855±0.00004 0.31843±0.00017

-0.06-1.44-2.22-2.60

Mahai Salt Lake, China Salt Early sylvinite deposit Early carnallite deposit

0.31913±0.00003 0.31891±0.00003 0.31864±0.00002

-0.21

-0.90-1.88

East Siberia With sylvite (Angara Fm.) With sylvite (Usolye Fm.) With sylvite (Belsk Fm.) With Carnallite (Angara Fm) Above sylvite (Angara Fm.)

-0.8 -0.9 -0.6 0.9 -0.4 to +0.1

Holbrook Basin, Arizona Potash facies (Supai Fm.) With sylvite (Supai Fm.)

-

-

-0.4 -0.9

Gulf of Mexico Louann & Haynesille salt

Potash - -0.5

Salt Range, Pakistan

Halite (pink) Halite (white) Sylvite

-

-

-0.15 -0.13 -0.49 to -0.73

19

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Figures

Fig. 1. Location and geological maps of the sampling area. (A) Location map of the study area. (B) Geographical map of the Salt Range. (C) Working area and distribution of the informal sub-units of the Salt Range Formation in the Khewra Mine (after Richards, 2015).

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Fig.2. (A) Stratigraphic column (after Grelaud et al., 2002). (B) The Salt Range Member subdivisions (after Ghazi et al., 2015).

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Fig.3. Variations of the δ37Cl (‰) and Cl/Br ratios of halite from the Salt Range formation. The seawater evaporation pathway is also reported: the black line is taken from Eggenkamp et al., 1995 and the blue one from Eastoe et al., 1999). Note that the seawater evaporation process proceeds from right to left.

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Fig. 4. Na-Cl-Br systematics in the Salt Range halite. Square symbol: seawater. Blue line: seawater evaporated past halite precipitation and solid (black) line: theoretical chemical compositions resulting from the dissolution/recrystallization of halite.

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Fig. 5. Range of 37Cl for the Salt Range Formation compared to those of promising potash basins worldwide (data taken from the literature and available in Table 3).

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