Chemical Geology xxx (xxxx) xxx–xxx

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Chemical Geology

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Sr isotope and major ion compositional evidence for formation of Qarhan , western ⁎ QiShun Fana,b, , Tim K. Lowensteinc, HaiCheng Weia,b, Qin Yuana,b,d, ZhanJie Qina,b,d, FaShou Shana,b, HaiZhou Maa,b a Key Laboratory of Comprehensive and Highly Efficient Utilization of Resources, Institute of Salt , Chinese Academy of Sciences, Xining 810008, China b Key Laboratory of Salt Lake Geology and Environment of Qinghai Province, Xining 810008, China c Department of Geological Sciences and Environmental Studies, Binghamton University, Binghamton, NY 13902, USA d University of Chinese Academy University, Beijing 100085, China

ARTICLE INFO ABSTRACT

Editor: E. B Michael Qarhan Salt Lake (QSL) is the largest deposit in the continental (QB) of western Keywords: China. The origins of the Qarhan , whether from migration of Qaidam mega-paleolakes, paleoriver cap- Strontium isotopes ture, or mixing of modern river water and Ca-Cl spring water, are controversial. Details about the origin (hy- Brine evolution drothermal, diagenetic or both) of the Ca-Cl inflow waters entering QSL in the QB are unclear. Here we show Ca-Cl brines from the strontium (Sr) isotope ratios of river waters, intercrystalline brines, spring waters, , gypsum, and Qarhan Salt Lake Corbicula shells from Qarhan playa, that 87Sr/86Sr increases from Bieletan (western QSL) to Huobuxun (eastern Qaidam Basin QSL). This trend verifies that river water-spring water mixing has been operating at QSL for the past Western China 50,000 years. Contributions of Ca-Cl inflow waters to the QB have profoundly influenced the brine evolution and mineralogy of salt deposits. Mixing between low Sr2+ concentration, low 87Sr/86Sr value of river waters and high Sr2+ concentration, high 87Sr/86Sr value of Ca-Cl spring inflow waters has produced various intercrystalline brines and salt deposits at QSL. Ca-Cl inflow waters and brines are enriched in Sr2+ and Ca2+ and depleted in 2+ 2− Mg and SO4 . These characteristics suggest brine-sedimentary rock (silicate) interactions at burial diagenetic or hydrothermal conditions. Ca-Cl inflow waters and brines have a similar Sr isotopic range as oilfield brines and celestite deposits in the QB, which also suggests a diagenetic-hydrothermal origin. Ca-Cl brines discharge as springs or seeps along fault zones in the QB. They are interpreted to reach the surface of the northern margin of Qarhan playa by topographically driven circulation.

1. Introduction intermontane basin on the northeastern Tibetan Plateau. It contains large nonmarine evaporite deposits formed during the Pliocene- Evaporites are chemical sedimentary rocks formed by precipitation Quaternary (Zhang, 1987). Qarhan Salt Lake (QSL), in the eastern QB from hypersaline waters at the earth's surface. In some tectonically (Fig. 1a), is the youngest (~50 ka) and largest modern potash deposit in active, closed basins (e.g., Qaidam Basin, China; Death Valley and the world (Zhang, 1987). The reserves of solid potash and liquid Bristol Dry Lake, California, USA; Salar de Atacama, Chile), Ca-Cl-rich brines in QSL are 2.96 × 108 t and 2.44 × 108 t, respectively (Zhang, inflow waters have influenced brine evolution in terms of major ion 1987; Yuan et al., 1995; Cao and Wu, 2004), which makes it the largest chemistries and mineral precipitation sequences (Lowenstein and potash resource in China. Risacher, 2009). Ca-Cl-rich inflow waters produce brines of the Na-Ca- Over the past three decades, significant research efforts have been Cl type, chemically distinct from seawater and other common surface directed toward investigating halite and brine resources and the origin and near-surface waters (Na-Cl-SO4, Ca-HCO3, or Na-CO3 types; Drever, and evolutionary history of QSL in the QB (Chen and Bowler, 1986; 1988) in their high Ca2+ concentrations, which exceed the combined Zhang, 1987; Lowenstein et al., 1989; Huang and Chen, 1990; Spencer 2− − 2– concentrations of SO4 , HCO3 , and CO3 (Lowenstein et al., 2003). et al., 1990; Zhu et al., 1990; Casas et al., 1992; Yang et al., 1993; Yuan The Qaidam Basin (QB), western China, is the largest Cenozoic et al., 1995; Zhang et al., 1993; Fan et al., 2014a, 2015; Wei et al.,

⁎ Corresponding author. E-mail address: [email protected] (Q. Fan). https://doi.org/10.1016/j.chemgeo.2018.09.001 Received 20 July 2017; Received in revised form 31 August 2018; Accepted 2 September 2018 0009-2541/ © 2018 Published by Elsevier B.V.

Please cite this article as: Fan, Q., Chemical Geology, https://doi.org/10.1016/j.chemgeo.2018.09.001 Q. Fan et al. Chemical Geology xxx (xxxx) xxx–xxx

94 E

Jiahu Lake Altyn Tagh Fault (JH,147) Xiaoliangshan Buzhenhe Shizigou Jiandingshan Dayantan (DYT,149) Th (SZG,198) (SQQ,150) ru Kunteyi Shuangqiquan st Fa Youquanzi Dalangtan(DLT) ul (151) (KTY) Niulang Lake t Nanyishan Chahansilatu Thrust Fault Dezunmahai Youdunzi(153-163) (NL,146) Lake(DZMH) Dafenshan (CHSLT) b) Jianshan Mahai(MH) (DFS,196) Tuolingou Yiliping Balunmahai Kunlun Fault (TLG,197) (YLP) Lake(BLMH,145) 37 N Qaidam B Hot spring Western (34-36) 94 E 37 N Qaidam (XT) (DT) XiaoQaidam Lake Basin Basin Dongling Lake(DL) A ISL1A Xiezuo Lake(XZ) Middle NorthHuobuxun Qaidam NHBX Lake(NHBX) Basin (QSL)

Shell Bar (190-193) Eastern Xiangride River CaCl Lake Qaidam Basin 94 E 97 E a)

Fig. 1. a) Map showing the location of Qarhan Salt Lake (QSL) and other salt lakes in the Qaidam Basin (QB). Rectangle A shows the location of QSL. Detailed information about this salt lake is shown on Fig. 3a; Rectangle B shows the location of Dongtai and Xitai salt lakes. Detailed information about those salt lakes is shown on Fig. 3b; The gray stars in rectangle A represent two sediment cores (ISL1A and NHBX) from QSL. Numbers in parentheses show the numbers of samples listed in Tables 2–7; abbreviations of salt lakes, waters and locations are shown here and in Table 1. b) Map showing the tectonic background of the QB (modified from Tan et al., 2012) and major fault systems (Altyn Tagh Fault, Kunlun Fault and Thrust Fault).

2015; Miao et al., 2016). The origin of the brines and evaporite deposits Dongtai salt lakes are the highest in the QB (Zhu et al., 1990) and Li at QSL, however, is still controversial (Chen and Bowler, 1986; resources are the greatest in salt lakes of western China (Tan et al., Lowenstein et al., 1989; Spencer et al., 1990; Zhang et al., 1993; Zhu 2011; Yu et al., 2013). Hydrochemical water types of every sub-basin in et al., 1990, 1994; Yuan et al., 1995; Liu et al., 2002). the QB are controlled by modern inflow waters, not migrated paleo- There are currently three models for QSL formation. Chen and brines. The model of paleoriver capture under neotectonic movement is Bowler (1985, 1986) and Chen et al. (1990) proposed that QSL formed problematic because the timing of neotectonic movement (~30 ka) by the migration of Qaidam “mega-paleolake” from west to east, fol- does not coincide with the formation period of salt deposits (~50 ka) lowed by concentration of Qarhan lake waters under arid climatic (Fan et al., 2014a, b). In addition, terrace sequences formed in thick conditions. This interpretation was based on geomorphic evidence of Quaternary valley fills along the northern slopes of the eastern Kunlun paleoshorelines, stratigraphic and chronological comparisons [espe- Mountains under neotectonic movement. Optically stimulated lumi- cially salt-bearing layers from Dalangtan, Chahansilatu, Kunteyi, Yi- nescence dating of these terraces indicates that the change from ag- liping and Qarhan playas], the elevation of each sub-basin or playa in gradation to incision occurred between 21.9 ± 2.7 and 16 ± 2.2 ka the QB (high in the west and low in the east), and the sub-basin mi- (Wang et al., 2009). Finally, the paleoriver capture model cannot ex- neralogy (abundant sulphate salts precipitated in Dalangtan, Cha- plain the two different brine types (Cl-SO4 and Ca-Cl types) at QSL. hansilatu, Kunteyi, and Yiliping playas in the western and middle QB, A mixing model involving river waters and Ca-Cl spring waters can and salts in Qarhan playa in the eastern QB). However, Zhu more reasonably explain the relationship between river waters, spring et al. (1990, 1994) concluded that there were a series of paleolakes waters and surface lake waters (brines) in the QSL (Lowenstein et al., (such as paleoKunlun, paleoHuolan, paleoAilake, paleoToson, and pa- 1989; Spencer et al., 1990; Zhang et al., 1993). The model is supported leoXiugou) that existed in the eastern ~30,000 years by the sylvite in surface salt deposits along the north shore of Dabuxun ago. Due to neotectonic movement, those paleolakes were captured by Lake (Yuan et al., 1995) and simulation results of mixing river and the Nalinggele, , and Xiangride paleorivers, which produced spring water using the Pitzer model (Liu et al., 2002). These studies brackish waters that then flowed into Qarhan paleolake. Qarhan pa- focused on surface brines, mineral analyses of surface salt deposits, and leolake later evolved into the nearly desiccated QSL by long-term computer simulations. However, questions remain about the origin of evaporation under arid climate conditions. In addition, Lowenstein brines and salt deposits in the QSL. First, the origin of the Ca-Cl spring et al. (1989), Spencer et al. (1990), and Zhang et al. (1993) argued that waters at the northern margin of QSL is not known. Second, little is QSL was formed by the mixing of modern river waters from the known about the geochemical characteristics of groundwater brines southern QSL with deep-sourced Ca-Cl spring waters discharging at the (here called intercrystalline brines) in many areas of Qarhan Salt Lake northern margins of QSL. They proposed that the mixing process pro- (QSL), including Bieletan, Dabuxun, Qarhan and Huobuxun (Figs. 1–3). duced the various lake water chemistries and mineral sequences, in- Here, Strontium (Sr) isotope compositions of different waters (river cluding calcite, gypsum, halite, and . waters, surface brines from lakes, intercrystalline brines and spring Several problems exist with the mega-paleolake migration model. waters) are used to interpret brine source waters in the QB. The fol- Ion concentrations of brines in QSL in the eastern QB would be in- lowing waters were collected and analyzed: river waters from the herited from those in Yiliping, Xitai and Dongtai salt lakes in the middle Golmud, Tuolahai, Zaohuo and Wutumeiren rivers in southern QSL, QB. However, Li concentrations in brines from Yiliping, Xitai and intercrystalline brines from the Bieletan, Dabuxun, Qarhan and

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