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Origin of secondary potash deposits; a case from Miocene evaporites of NW Central Iran
Journal of Asian Earth Sciences 25 (2005) 157–166 www.elsevier.com/locate/jaes
Origin of secondary potash deposits; a case from Miocene evaporites of NW Central Iran H. Rahimpour-Bonab*, Z. Kalantarzadeh Department of Geology, Faculty of Science, University of Tehran, P.O. Box 14155-6455, Tehran, Iran Received 12 May 2003; revised 13 January 2004; accepted 4 February 2004
Abstract In early Miocene times, an extensive carbonate shelf developed in Central Iran and during several cycles of sea-level fluctuations, evaporite-bearing carbonate sequences of the Qom Formation were deposited. However, in the early-middle Miocene, development of restricted marine conditions led to a facies change from shelf carbonates of the Qom Formation to the evaporite series of the M1 member of the overlying Lower Red Formation. This member is a facies mosaic of lagoonal and salina evaporites (mainly halite beds) admixed with wadi siliciclastics. The purpose of this study, which focuses on two salt mines in the northwestern portion of Central Iran in the Zanjan province, was to reveal the origin, sedimentary environment, and diagenesis of these potash-bearing evaporite sequences. Petrographic examination revealed the following mineral assemblage: halite, gypsum, anhydrite and carnallite as primary precipitates, and langbeinite and aphthitalite as secondary metamorphic potash salts. In the Iljaq mine, distorted halite beds are dominated by burial and deformational textures and a great deal of secondary potash salts. In the Qarah-Aghaje mine, however, the bedded halite shows pristine primary textures and is devoid of the secondary potash salts. High bromine content of most evaporite minerals suggests their marine origin, and confirms the absence of the extensive meteoric alterations and subsequent bromine depletions. Potash salts are mainly secondary, and resulted from diagenetic replacements of distorted halite beds during thermal and dynamic metamorphism in a burial setting. q 2004 Elsevier Ltd. All rights reserved. Keywords: Miocene; Evaporite; Iran; Bromine; Potash
1. Introduction Despite extensive occurrences of evaporites in the Middle East, and especially in Iran, there are very few studies, especially regarding their environment of formation and diagenesis. In Iran, evaporites are present in sedimentary rocks of three major horizons including: (1) Hormoz series of Precambrian age; (2) Upper Jurassic deposits and (3) Tertiary evaporites (e.g. Nabavi, 1976; Darvishzadeh, 1991). In many cases in southern Iran and the Persian Gulf region these evaporites play an important role in forming salt-induced halokinetic structures (domes and folds) for hydrocarbon entrapment and reservoir seals. The Tertiary evaporites are found mainly in the Zagros Basin and in * Corresponding author. E-mail address: [email protected] (H. Rahimpour-Bonab). 1367-9120/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2004.02.004
the Central Iran Tectonic Zone, being Eocene to Miocene in age (Kalhor, 1961; Stocklin, 1968). In the Zagros Basin, these impermeable deposits are important cap rocks for some famous reservoirs such as the Asmari Formation. Likewise, their equivalent deposits in the Central Iran Tectonic Zone, which are the subject of this study, are distributed over thousands of square kilometers. Nevertheless, despite the broad extent of these deposits, the nature of their sedimentary environments, diagenesis, and economic values have been the subjects of only a few studies (e.g. Sadedin and Samimi, 1990; Rahimpour-Bonab and Alijani, 2003). The present study focuses on the potashbearing evaporites of the Zanjan area, in the northwestern domain of Central Iran, and its purpose was to understand the depositional environment, origin, and diagenetic alterations of these evaporite deposits.
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2. Geological setting and stratigraphy Tertiary evaporites of Central Iran are situated in two stratigraphic horizons that include the late Eocene-early Oligocene (Lower Red Formation) and the middle to late Miocene (Upper Red Formation) deposits (Jackson et al., 1991; Darvishzadeh, 1991). In the study area, which is situated in the northwestern region of the Central Iran Tectonic Zone (Fig. 1), evaporite deposits are exclusively present in the Upper Red Formation. This formation extends over an area of about 400,000 km2 being up to 3000 m in thickness, and consists of sandstones, marls, and interlayered evaporites, mostly in the lower parts. The Upper Red Formation, according to its lithology, has been subdivided into three members (Figs. 2 and 3). A relative sea-level rise in the late Oligocene-early Miocene established open marine conditions in Central
Iran, where an extensive carbonate/evaporite-bearing basin developed in Miocene times. This extensive shallow, open-marine succession is more than 500 m thick, and includes shallow-marine limestones and fossiliferous marl, with subordinate evaporites, known as Qom Formation, which extends throughout Central Iran. Evaporites of the M1 member (middle Miocene) of the Upper Red Formation formed in the inner parts of the platform (in hypersaline lagoons and supratidal sabkhas) when open marine circulation was restricted. This member, more than 200 m thick, conformably and transitionally overlies the Qom Formation. In fact, this evaporite succession is a facies mosaic of lagoonal and salina evaporites (mainly halite beds) admixed with wadi siliciclastics. Because most of these evaporites developed in a vast expanse made up by a discontinuous mosaic of marine-fed shallow lagoons, salt pans, and mud flats, tens
Fig. 1. Tectonic zones of Iran and location of the studied area.
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thickness), from bottom to top, displays the following sequence: (a) siliciclastic beds, including thin layers of fine-grained sandstone and thick marlstone beds; (b) interlayers of rounded gypsum crystal clasts. These relatively insoluble salts, appear as thin interlayers of eroded, corroded, and rounded clasts within thick halite beds; (c) mud-free, thick but distorted, halite beds with sporadic interlayers of gypsum and siliciclastic sediment, grading into thin layers of muddy halite. Generally, halite beds comprise almost 80% of the succession. Potash minerals are abundant in the distorted and deformed halite intervals, representing more than 50% of the units in some horizons. The Qarah-Aghaje section, located 30 km southeast of Iljaq and up to 40 m thick, almost shows similar features. However, the bedded halite units are not distorted, having much thicker muddy halite beds in the upper parts of the sequence. Halite beds comprise more than 90% of the succession and bedded halite contains much less potash salts (up to 5%).
3. Methods
Fig. 2. Stratigraphic column of Tertiary sedimentary units in the studied area in the Zanjan territory. Vertical thickness is proportional to the measured thickness.
of thousands of square kilometers wide, they may have been diachronous. Samples were collected from outcrops of two main salt pits, the Iljaq and Qarah-Aghaje mines, in the northwestern area of the Zanjan province. The Iljaq section (about 50 m in
Fig. 3. Iljaq salt mine (lower part of the photograph) and the Upper Red Formation (top).
About 120 samples were systematically collected at regular intervals (half-meter) from outcrops (salt pits) at the Iljaq mine and 80 more samples were also collected from the Qarah-Aghaje salt mine. Samples were slabbed and thin sections prepared with the aid of a water-free diamond wire saw with vegetable oil as a lubricant. After cleaning, samples were polished using sandpaper of progressively finer grit in order to remove any remaining saw marks (Benison, 1995). Petrographic study was carried out by optical microscopy. After polishing and coating, 15 samples were studied with an SEM (DSM-960A model). The mineralogy of 40 samples was determined by XRD (Siemens V-5000). In order to identify potash horizons in the outcrops, dipicrylamine indicator (Hexanitrodiphenylamin with C12N7H5O12 formula) was used (Schwehr and Conan, 1960). Potassium content of some samples was measured by using a Corning 405-flame photometer. Prior to analysis, a blank and several standard samples were measured for instrumental calibration. The bromine content of halite is commonly measured by ion chromatography, X-ray fluorescence (XRF), or wet-chemical titration analysis (bromometry). All samples were analyzed for their bromine content by titration (Harris, 2001), however, 20 samples were also analyzed by XRF for comparison. The results did not show significant discrepancies (less than 10 ppm), considering precision for the bromine titration of about ^ 5 ppm.
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4. Petrography and diagenesis The mineral assemblage of these evaporites, determined by optical and electron microscopy and X-ray diffraction, includes halite, carnallite, gypsum, anhydrite, langbeinite, and aphthitalite. The bedded halite units of the QarahAghaje salt pit mainly contain carnallite as the potassium mineral, whereas langbeinite and aphthitalite are mainly hosted by distorted halite beds in the Iljaq mine. As a general rule, where halite beds show well-preserved bottom-nucleated crystals and syntaxial overgrowths such as chevrons, cuboids, cornets, and syndepositional dissolution features, they only contain sparse carnallite crystals and are devoid of secondary potash minerals. On the other hand, in the deformed halite beds of the Iljaq salt pit, halite crystals associated with langbeinite and aphthitalite lack any syndepositional features and rather display diagenetic textures such as inclusion-free equigranular mosaic crystals with curved boundaries (Fig. 4). Due to variations in the rate of crystallization (due to variations in physiochemical factors such as temperature and saturation) the vertically oriented bottom-nucleated chevron halite crystals display a texture with alternating bands of cloudy, fluid inclusion-rich material (Fig. 5). Alternations develop only where the brine experiences rapid variations in halite concentration, or in temperature, a situation only possible in shallow brine depths (Handford, 1991). Most of these primary inclusions in the halite crystals contain only liquid, are cubic in shape, and range in size from 1 to 160 mm (Fig. 6). In the chevron texture, halite crystals, which are composed of a series of depositional layers, separated from each other by flat dissolution surfaces. Each truncated surface is further penetrated by dissolution cavity (micro-karst) that extends down into underlying layer (Fig. 5). The presence of both bottomnucleated crystals and cumulate crystals in the same bed is a significant indication of a shallow water deposition
Fig. 4. Burial texture in the distorted halite bed of Iljaq salt pit. Note the concentrations of secondary potash minerals along the curved crystal boundaries of halite. Primary depositional texture has been completely eradicated. Polarized light and gypsum plate.
Fig. 5. Bottom-nucleated and vertically oriented halite chevron, displaying compromise boundaries with its neighboring crystals, from Qarah-Aghaje salt pit. Note the alternating bands of clear halite and fluid inclusion-rich halite. White areas are clear inclusion-poor halite, usually present as rims on chevrons. Traces of dissolution and subsequent cementation in chevron crystal are evidence (arrows). Rounding of euhedral crystal is also presented (big arrow). Plane polarized light.
(Lowenstein and Hardie, 1985; Losey and Benison, 2000; Schreiber and El Tabakh, 2000). Progressive evaporation in the brine leads to the evaporative drawdown and generation of pore brines in the shallower parts of the evaporative basin. In this stage, intrasedimentary growths of the evaporite minerals develop, displaying and including terrigenous muddy sediments. Displacive halite is common in the Qarah-Aghaje salt pit (Fig. 7), showing a chaotic fabric of subhedral crystals separated by pockets of mud. This style of crystal growth evidences early post-depositional growth within soft, brinesaturated, shallow muddy sediment. Nevertheless, when euhedral crystals form, they usually are indicative of nearsurface crystallization in formative salina, as seen in many modern playa and sabkha (Hardie, 1968; Logan et al., 1970;
Fig. 6. Photomicrograph of fluid inclusions outlining primary growth band in chevron halite. Note cloudy inclusion-rich bands and clear inclusionpoor bands. Truncation of banding by dissolution voids (micro karsts in upper right of photograph, arrow) are subsequently filled by clear halite spar. Plane polarized light.
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Fig. 7. Subhedral mud incorporative clear halite crystals with enfacial boundaries, displaying a chaotic fabric (from Qarah-Aghaje salt pit). Plane polarized light.
Gornitz and Schreiber, 1981; Handford, 1991; Warren, 1999). In addition, halite crystals displaying bottomnucleated textures as well as mud inclusions (Fig. 8) are indicative of the early syndepositional crystal growth. Such rapid halite growth indicates a high degree of supersaturation with respect to halite in soft mud at depths of about 2 m (Gornitz and Schreiber, 1981; Handford, 1991; Warren, 1999). The primary subaqueous textures were partially dissolved by intermittent brine freshening or subaerial exposure. This process resulted in the generation of the syndepositional dissolution-reprecipitation features, such as truncated dissolution surfaces (micro-karsts) that crosscut crystal faces (Fig. 5). These backreaction processes that further complicate diagenetic events, is an unequivocal signature of the shallow, unstratified evaporative basins. These vadose micro-karsts, which were filled by clear inclusion-poor halite spar cement, are related to subaerial exposures by some researchers (e.g. Shearman, 1970; Shearman and Orti Cabo, 1978; Warren, 1999). Intensity and frequency of dissolution overprints are largely determined by brine depth, input of
Fig. 8. Mud-incorporative euhedral halite displaying a chevron texture in the center of the crystal. Qarah-Aghaje salt pit, plane polarized light.
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Fig. 9. Patches of syndepositional halite chevron crystals preserved among the mosaic of inclusion-free halite spar. Iljaq salt pit, plane polarized light.
dilute waters and the degree of continuity (perennial vs. ephemeral) of the brine body (Lowenstein and Hardie, 1985). Rounded edges of euhedral crystals, followed by syntaxial rehealing, are other evidences of the fluctuations in the brine saturation (Figs. 5 and 9). Like many other marginal marine salinas, the pervasive early loss of the porosity could be due entirely to syndepositional diagenetic cementation by clear halite (Lowenstein and Hardie, 1985; Casas and Lowenstein, 1989). The internal sediments overlying the syntaxial halite overgrowths suggest syndepositional cementation (Fig. 10), and patchy remnants of chevron structure characterize mature salt pans (Lowenstein and Hardie, 1985). This is a good example of mature halite, which has undergone numerous cycles of dissolution and recementation. However, porosity loss could be in part due to chemical or mechanical compaction. Patches of chevron bearing halite, preserved within the mosaic halite spar (Fig. 9), and inclusion-free halite crystals (Fig. 11), stylolites, and strained and broken crystals (Fig. 12), have been observed
Fig. 10. The remnants of banded chevron halite crystals are embayed and syntaxially rehealed by clear, overgrowth halite cement. Mature halite, which undergone many cycles of dissolution and recementation so that only patches of primary chevron still survive. Iljaq salt pit, plane polarized light.
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Fig. 11. Halite with equigranular mosaic texture from Iljaq salt pit. Note the curved boundaries that meet at triple junctions with angles approaching 1208. Muddy materials are concentrated along crystal boundaries and some inclusion of potash minerals are also shown. Polarized light and gypsum plate.
in distorted, potash-rich halite units of the Iljaq mine. Moreover, in some samples (Fig. 4), fluid and solid inclusions have been purged to grain boundaries so that the depositional textures are completely eradicated. These features must almost certainly result from recrystallization and annealing due to increased temperatures and pressures related to burial (Stanton and Gorman, 1968; Spencer and Lowenstein, 1990; Warren, 1999). Sporadically, the primary crystalline framework of halite has been occluded by, presumably, early diagenetic, poikilotopic gypsum cement (Figs. 13 and 14). Moreover, the presence of solution breccias (Fig. 14), prior to gypsum cementation, implies episodes of brine freshening and subsequent cementation by subsurface brines in the capillary fringe of evaporite basin (e.g. Rouchy et al., 1994). The major mechanism for generation of anhydrite nodules is dehydration of gypsum (above 50 – 60 8C). This conversion may occur after a few meters of burial or on deep
Fig. 12. Strained and broken crystals of halite with stylolites. Iljaq salt pit, polarized light and gypsum blade.
Fig. 13. Coarse poikilotopic gypsum cement in a framework of cuboids and pristine halite crystals (dark). Iljaq salt pit, polarized light.
burial (Spencer and Lowenstein, 1990). However, as shown in Fig. 15, associations of anhydrite crystals with polygonal clear halite mosaic (secondary) suggest their burial origin. In addition, tiny anhydrite laths with random orientation are also present inside the secondary halite crystals.
5. Potash genesis Most potash-magnesium salts are diagenetic replacements of, or additions to, earlier halite or sulfate deposits (Kendall, 1992). On the other hand, formation of secondary potash minerals has been ascribed to thermal, solution or dynamic metamorphism (Braitsch, 1971; Hardie, 1984). Moreover, potash minerals may precipitate in the marine or non-marine settings as synsedimentary minerals, during final stage of brine evaporations as early diagenetic replacements, or during later burial events. The potash minerals in the Iljaq area include langbeinite and aphthitalite as secondary potash salts
Fig. 14. Secondary enlargement of intercrystalline porosity (dissolution breccia) by halite dissolution and subsequent cementation by poikilotopic gypsum. Qarah-Aghaje salt pit, polarized light.
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6. Geochemistry
Fig. 15. Anhydrite crystals with radial texture in polygonal clear halite mosaic. Iljaq salt pit, polarized light.
accommodated by distorted halite beds. However, they never occur as separate beds but particularly are concentrated as inclusions (up to 50%) in recrystallized halite crystals (e.g. along curved crystal boundaries) without primary textures and fluid inclusion banding (Fig. 4). The association of high-temperature langbeinite with another secondary potash mineral (aphthitalite) in burial halite as isolated euhedral crystals, the extensive bedding distortions in the potash-bearing horizons, the absence of void-filling potash cements and other primary features, are indications of secondary origin. It seems they have been generated during episodes of thermal and dynamic metamorphism. The abundance of the secondary higher temperature potash salt, i.e. langbeinite, which is stable only at temperatures greater than 40 – 50 8C (Stewart, 1963) implies that the minimum temperature experienced by potash-bearing halite was above at least 40 – 50 8C, as this salt is the minimum temperature geothermometer. Whereas the primary syndepositional potash minerals are absent in the studied sequences, the secondary potash minerals are abundant (in some horizons up to 50%). This paucity could be ascribed to syndepositional modification of seawater composition by sulfate-reducing bacteria that hindered syndepositional potash formation (Wardlaw, 1972; Sonnenfeld, 1984). Accordingly, traces of some bacteria, algae, and native sulfur have been observed by SEM examinations. Of course, the concentration of these organic materials is low and they never developed as separate large masses. Another explanation for such a dearth of primary potash minerals is syndepositional modification of concentrated seawater by non-marine inflow of different origins (e.g. meteoric or hydrothermal) (Valyashko, 1972; Hardie, 1984). The presence of microkarst texture in some halite crystals (Fig. 5) confirms such dilution episodes.
The marine evaporite deposits with halite, sylvite, and carnallite, and entirely free from the primary Mg-sulfate salts, make up more than 60% of ancient potash deposits (Hardie, 1990, 1991). These types of evaporite deposits have been precipitated from Na – Ca –Mg – K –Cl brines with ionic ratios different from concentrated modern seawater. The origin of these MgSO4-depleted potash deposits has been attributed to the modification of marine evaporites or brines by backreactions and/or subsequent metamorphism during burial episodes (Borchert, 1977; Dean, 1978; Hardie, 1984; Wilson and Long, 1993). Accordingly, several processes such as dolomitization, action of sulfate-reducing bacteria, and brine mixing and freshening, could have altered these seawater-derived brines. Although dolomites are absolutely absent, the existence of such bacteria and resultant native sulfur has been disclosed by SEM examinations of the studied samples. The presence of synsedimentary micro-karsts in many primary samples is also suggests episodes of fresh water influx. Commonly, seawater evaporation generates two types of brine: Na – Cl brines and Mg – Cl – SO4 brines. Na – Cl brines dominate in the salinity range 35 – 330‰, whereas at higher salinites, as a result of the massive removal of Na by halite precipitation, the brine become progressively rich in Mg –Cl –SO4 and bittern salts begin to precipitate. However, in the studied area, seawater-derived brines rarely reached the bittern stage. In fact, leakage from the basin floor, brine reflux, backreactions, and brine renewal or freshening along the whole evaporation cycle inhibited extensive bittern salt precipitation. Marine and non-marine evaporites have been successfully differentiated by their bromine contents (Hardie, 1984; Hanor, 1987; Garrett, 1996; Taberner et al., 2000). Nevertheless, Zak (1997) in his study on the Dead Sea evaporites pointed out the possibility of Br reworking by repeated solution and reprecipitation from brines. Accordingly, this process may give rise to extremely high bromine values both in the water and in precipitated salts. In spite of this observation, bromine contents of evaporites have been successfully employed as an exploration tool in potash deposits prospecting, as a paleosalinity marker in the ancient evaporite basins, and to estimate brine temperatures (Holser, 1969; Raup and Hite, 1978; Hardie, 1984; Ayora et al., 1994; Garrett, 1996; Taberner et al., 2000; Rahimpour-Bonab and Alijani, 2003). With this purpose, bromine content was determined in samples along the evaporite series in the Iljaq and Qarah-Aghaje salt pits, with a stratigraphic resolution of a half meter (Figs. 16 and 17). Due to the mild to intense deformation in some horizons of the Iljaq salt pit, its bromine profile displays disturbed trends. Therefore, only the bromine profile of the Qarah-Aghaje salt pit has been appraised in this work.
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Fig. 16. Bromine profile of the Qarah-Aghaje salt pit.
The range of bromine variations in the smoothed profile of the Qarah-Aghaje salt pit is between 40 and 242 ppm which implies a marine bromine signature for most part of the sequence. In this profile from A (depth 40 m) to B (depth 35 m) there is a sharp decrease in the bromine content (from 242 to 122 ppm) interpreted as a positive hydrologic regime (seawater influx, lowered evaporation rates or bittern reflux). However, from B to C and then C1, there are two major cycles of the progressive bromine increase and the maximum bromine value of the whole profile is attained. This pattern implies dominance of a negative hydrologic regime (cessation of bittern reflux) and even ephemeral basin desiccation that is associated with an increase in primary carnallite. This interpretation is also supported by some subaerial structures such as desiccation cracks, found in these horizons. In the interval from C1 to D, a general decrease in the bromine content, with some fluctuations, implies the recovery of the washed-up positive hydrologic regime. From D (depth 18 m) to D1 (depth 9 m) bromine content remains low with small variations. Maintaining a nearly constant salinity and a stable hydrologic regime during deposition of 9 m of halite requires not only flow of the seawater into the basin, but also reflux of bittern brines from the basin. Upward, to the top of the evaporite sequence
Fig. 17. Bromine profile of the Iljaq salt pit.
(from D1 to E), the general trend is toward progressive bromine decrease (down to 40 ppm) that signals dwindling and imminent termination of the evaporative regime and the dominance of siliciclastic environments. Apparently, throughout the sequence, bromine signatures even in the pristine halite beds, barely reached the minimum threshold (approximately 250 ppm) for the primary potash precipitation. Early diagenesis within the basin floor is able to redistribute bromine of early-formed minerals by recrystallization (Dean and Schreiber, 1978). Thus, the early diagenetic void-filling halite spars (inclusion-free) have been microsampled and analyzed for its Br-content by XRF. Their bromine contents were close to their pristine halite framework, suggesting good preservation of the bromine signatures. Some samples with evidence of burial diagenesis (e.g. equigranular clear halite crystals with curved boundaries, Fig. 11) were also analyzed for their Br content. Their average Br content is close to the well-preserved pristine halite crystals. This finding suggests that burial diagenetic fluids (parental brines of langbeinite and aphthitalite) had composition similar to the primary marine-derived brines and were probably originated within the evaporite sequences.
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Extensive late diagenesis and recrystallization by meteoric waters were not significant here, as they commonly produce an unvarying bromine profile (i.e. redistribution of Br), intense bromine leaching, and possible bromine depletion. To compare Br and K variations in different minerals, K-contents of samples were also measured. Theoretically, with progressive increase in the salinity of the brine, the precipitating mineral phases will enrich in K and Br. Therefore, in primary potash deposits, potassium variations should closely be correlated with their bromine signatures. However, in our samples Br and K variations are largely inconsistent due to secondary origin of the most potash salts, and to a lesser extent, the higher mobility of the potassium in diagenetic environments.
7. Depositional model The studied evaporites are laterally, but discontinuously, persistent over large areas of northwestern Central Iran, with only minor changes in mineralogy and facies. Apparently, these sediments formed in the marginal marine environments that were periodically flooded both by meteoric water and seawater resulting in the partial dissolution of the older and more-soluble evaporites. The relatively insoluble salts, such as gypsum, were eroded, corroded, and redeposited as rounded clasts, which appear as thin interlayers within thick halite beds. In addition, finegrained siliciclastic input, brought in by the floods, were deposited over the evaporite crusts as a blanketing layers of mud. These deposits with 5 – 80 m thickness and evidences of subaerial exposures, formed in a siliciclastic-rich ‘wadi’ environment, landward of a mixed lagoonal and salina evaporite setting, that passes seaward into a shallow-water high stand carbonate (HST) facies of the Qom Formation. These carbonate-evaporite transitions are chronologically equivalent to the well-known Asmari Limestone (HST) and its overlying cap rock, the Gachsarran evaporites, in the Zagros Basin.
8. Conclusions Studied evaporites developed as lagoonal and sabkha facies that occurred over shallow, open marine platform carbonate of the Qom Formation Apparently, these extensive, thick, shelf-margin sequences that form mixed carbonate/siliciclastic/evaporite units have been deposited in an intracratonic basin where some form of barrier developed to isolate them from the oceans. Petrographic textures formed under shallow brines, subaerial features, and high bromine content support a marginal, marine-fed, platform evaporite model.
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Although high bromine content of pristine halite crystals suggests their marine origin, the values barely reach the minimum threshold for primary potash precipitation. Presumably, activities of sulfate-reducing bacteria and sporadic meteoric inflows hindered primary syndepositional potash deposition. The lack of continental evaporite paragenesis (including mirabilite, thenardite and trona), the high bromine content and its mineralogical assemblage, suggest a parental marine brine of the Na – K – Mg – SO4 – Cl type that was faintly modified by non-marine inflows.
Acknowledgements This work is supported by a facilities provided by Tehran University (Grant No. 512/2/585) which we are sincerely grateful. Thanks are also due to B.C. Schreiber for her careful review and invaluable comments.
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Hardie, L.A., 1984. Evaporites: marine or non-marine? American Journal of Science 284, 193–241. Hardie, L.A., 1990. The roles of rifting and hydrothermal CaCl2 brines in the origin of potash evaporites: an hypothesis. American Journal of Science 290, 43–106. Hardie, L.A., 1991. The significance of evaporites. Annual Review of Earth and Planetary Sciences 19, 131–168. Harris, D.C., 2001. Exploring Chemical Analysis, Freeman, New York, 607 pp. Holser, W.T., 1969. Bromide geochemistry of salt rocks, In: Second Symposium on Salt: Northern Ohio Geological Society, pp. 248 –275. Jackson, M.P.A., Cornelius, R.N., Craig, C.H., Gansser, A., Stocklin, J., Talbot, J.C., 1991. Salt diapirs of the Great Kavir Central Iran. Geological Society of America Memoir 177, 140. Kalhor, R., 1961. Geology of Neogene formation on in Varamin-Garmsar area and evaluation of Abardej Nose, Unpublished National Iranian Oil Company, Geological Report No. 233, 17 pp. Kendall, A.C., 1992. Evaporites. In: Walker, R.G., James, N.P. (Eds.), Facies Models, Response to Sea-level Changes, second edition, Geoscience Canada Reprint Series 1, pp. 259–296. Logan, B.W., Davies, G.R., Read, J.R., Cebulski, D.F., 1970. Carbonate sedimentation and environments, Shark Bay, Western Australia. American Association of Petroleum Geologists Memoir 13, 223. Losey, A.B., Benison, K.C., 2000. Silurian Paleoclimate data from fluid inclusions in the salina group halite, Michigan Basin. Carbonates and Evaporites 15, 28 –36. Lowenstein, T.K., Hardie, L.A., 1985. Criteria for the recognition of saltpan evaporites. Sedimentology 32, 627–644. Nabavi, M.G., 1976. An introductory to the Geology of Iran. Geological Survey of Iran, 109(in Persian). Rahimpour-Bonab, H., Alijani, N., 2003. Petrography, Diagenesis and Depositional model for potash deposits of the Central Iran-Garmsar Area and use of bromine geochemistry as a prospecting tool. Carbonates and Evaporites 18, 19–28. Raup, O.B., Hite, R.J., 1978. Bromine distribution in marine halite rocks. In: Dean, W.E., Schreiber, B.C. (Eds.), Marine Evaporites, Society of Economic Paleontologist and Mineralogists Short Course Notes 4, pp. 74–85. Rouchy, J.M., Bernet-Ronald, M.C., Maurin, A.F., 1994. Descriptive petrography of evaporites, application in the field, subsurface and laboratory. In: Magithia, M., (Ed.), Geological methods, vol. 1. French Oil and Gas Industry Association Technical Committee, p. 214.
Sadedin, N., Samimi, M., 1990. Potash deposits in the Garmsar area. Geological Survey of Iran, unpublished report, 55(in Persian). Schreiber, B.C., El Tabakh, M., 2000. Deposition and early alteration of evaporites. Sedimentology 47, 215–238. Schwehr, E.W., Conan, H.R., 1960. Determination of Potassium, In: Fertilizer Analysis: a Decade of Collaborative Investigation, The International Fertilizer Society—Proceeding, p. 62. Shearman, D.J., 1970. Recent Halite Rock, Baja, California, Mexico. Mineralogical and Metallurgical Transaction, Section B 79, B155–B162. Shearman, D.J., Orti Cabo, F., 1978. Upper Miocene gypsum: San Miguel de Salinas, S.E. Spain. Memorie della Societa Geologica Italiana 16, 327 –339. Sonnenfeld, P., 1984. Brines and Evaporites, Academic Press, New York, 435 pp. Spencer, R.J., Lowenstein, T.K., 1990. Evaporites. In: Mcllreath, I.A., Morrow, D.W. (Eds.), Diagenesis II, Geoscience Canada Reprint Series 4, pp. 141– 164. Stanton, R.L., Gorman, H., 1968. A phenomenological study of grain boundary migration in some common sulfides. Economic Geology 63, 907 –923. Stewart, F.H., 1963. Marine Evaporites, US Geological Survey, Professional Paper, 440-Y, 52 pp. Stocklin, J., 1968. Salt deposits of the Middle East. Geological Society of America. IWC Special Paper 88, 157–181. Taberner, C., Cendon, D.I., Pueyo, J.J., Ayora, C., 2000. The use of environmental markers to distinguish marine vs. continental deposition and to quantify the significance of recycling in evaporite basin. Sedimentary Geology 137, 213–240. Valyashko, M.G., 1972. Playa lakes—a necessary stage in the development of a salt-bearing basin. In: Richter-Bernburg, G., (Ed.), Geology of Saline Deposits, UNESCO, Earth Science Series Paris 7, pp. 14–51. Wardlaw, N.C., 1972. Unusual marine evaporites with salts of calcium and magnesium chloride in Cretaceous basins of Sergipe, Brazil. Economic Geology 67, 1273–1294. Warren, J.K., 1999. Evaporites: Their Evolution and Economics, Blackwell Science, Oxford, 438 pp. Wilson, T.P., Long, D.T., 1993. Geochemistry and isotope chemistry of Ca –Na– Cl brines in Silurian Strata, Michigan Basin, USA. Applied Geochemistry 8, 507 –524. Zak, I., 1997. Evolution of the Dead Sea brines. In: Niemi, T.M., BenAbraham, Z., Gat, J.R. (Eds.), The Dead Sea: The lake and its Setting, Oxford University Press, Oxford, pp. 133 –144.
Origin of secondary potash deposits; a case from Miocene evaporites of NW Central Iran H. Rahimpour-Bonab*, Z. Kalantarzadeh Department of Geology, Faculty of Science, University of Tehran, P.O. Box 14155-6455, Tehran, Iran Received 12 May 2003; revised 13 January 2004; accepted 4 February 2004
Abstract In early Miocene times, an extensive carbonate shelf developed in Central Iran and during several cycles of sea-level fluctuations, evaporite-bearing carbonate sequences of the Qom Formation were deposited. However, in the early-middle Miocene, development of restricted marine conditions led to a facies change from shelf carbonates of the Qom Formation to the evaporite series of the M1 member of the overlying Lower Red Formation. This member is a facies mosaic of lagoonal and salina evaporites (mainly halite beds) admixed with wadi siliciclastics. The purpose of this study, which focuses on two salt mines in the northwestern portion of Central Iran in the Zanjan province, was to reveal the origin, sedimentary environment, and diagenesis of these potash-bearing evaporite sequences. Petrographic examination revealed the following mineral assemblage: halite, gypsum, anhydrite and carnallite as primary precipitates, and langbeinite and aphthitalite as secondary metamorphic potash salts. In the Iljaq mine, distorted halite beds are dominated by burial and deformational textures and a great deal of secondary potash salts. In the Qarah-Aghaje mine, however, the bedded halite shows pristine primary textures and is devoid of the secondary potash salts. High bromine content of most evaporite minerals suggests their marine origin, and confirms the absence of the extensive meteoric alterations and subsequent bromine depletions. Potash salts are mainly secondary, and resulted from diagenetic replacements of distorted halite beds during thermal and dynamic metamorphism in a burial setting. q 2004 Elsevier Ltd. All rights reserved. Keywords: Miocene; Evaporite; Iran; Bromine; Potash
1. Introduction Despite extensive occurrences of evaporites in the Middle East, and especially in Iran, there are very few studies, especially regarding their environment of formation and diagenesis. In Iran, evaporites are present in sedimentary rocks of three major horizons including: (1) Hormoz series of Precambrian age; (2) Upper Jurassic deposits and (3) Tertiary evaporites (e.g. Nabavi, 1976; Darvishzadeh, 1991). In many cases in southern Iran and the Persian Gulf region these evaporites play an important role in forming salt-induced halokinetic structures (domes and folds) for hydrocarbon entrapment and reservoir seals. The Tertiary evaporites are found mainly in the Zagros Basin and in * Corresponding author. E-mail address: [email protected] (H. Rahimpour-Bonab). 1367-9120/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2004.02.004
the Central Iran Tectonic Zone, being Eocene to Miocene in age (Kalhor, 1961; Stocklin, 1968). In the Zagros Basin, these impermeable deposits are important cap rocks for some famous reservoirs such as the Asmari Formation. Likewise, their equivalent deposits in the Central Iran Tectonic Zone, which are the subject of this study, are distributed over thousands of square kilometers. Nevertheless, despite the broad extent of these deposits, the nature of their sedimentary environments, diagenesis, and economic values have been the subjects of only a few studies (e.g. Sadedin and Samimi, 1990; Rahimpour-Bonab and Alijani, 2003). The present study focuses on the potashbearing evaporites of the Zanjan area, in the northwestern domain of Central Iran, and its purpose was to understand the depositional environment, origin, and diagenetic alterations of these evaporite deposits.
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2. Geological setting and stratigraphy Tertiary evaporites of Central Iran are situated in two stratigraphic horizons that include the late Eocene-early Oligocene (Lower Red Formation) and the middle to late Miocene (Upper Red Formation) deposits (Jackson et al., 1991; Darvishzadeh, 1991). In the study area, which is situated in the northwestern region of the Central Iran Tectonic Zone (Fig. 1), evaporite deposits are exclusively present in the Upper Red Formation. This formation extends over an area of about 400,000 km2 being up to 3000 m in thickness, and consists of sandstones, marls, and interlayered evaporites, mostly in the lower parts. The Upper Red Formation, according to its lithology, has been subdivided into three members (Figs. 2 and 3). A relative sea-level rise in the late Oligocene-early Miocene established open marine conditions in Central
Iran, where an extensive carbonate/evaporite-bearing basin developed in Miocene times. This extensive shallow, open-marine succession is more than 500 m thick, and includes shallow-marine limestones and fossiliferous marl, with subordinate evaporites, known as Qom Formation, which extends throughout Central Iran. Evaporites of the M1 member (middle Miocene) of the Upper Red Formation formed in the inner parts of the platform (in hypersaline lagoons and supratidal sabkhas) when open marine circulation was restricted. This member, more than 200 m thick, conformably and transitionally overlies the Qom Formation. In fact, this evaporite succession is a facies mosaic of lagoonal and salina evaporites (mainly halite beds) admixed with wadi siliciclastics. Because most of these evaporites developed in a vast expanse made up by a discontinuous mosaic of marine-fed shallow lagoons, salt pans, and mud flats, tens
Fig. 1. Tectonic zones of Iran and location of the studied area.
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thickness), from bottom to top, displays the following sequence: (a) siliciclastic beds, including thin layers of fine-grained sandstone and thick marlstone beds; (b) interlayers of rounded gypsum crystal clasts. These relatively insoluble salts, appear as thin interlayers of eroded, corroded, and rounded clasts within thick halite beds; (c) mud-free, thick but distorted, halite beds with sporadic interlayers of gypsum and siliciclastic sediment, grading into thin layers of muddy halite. Generally, halite beds comprise almost 80% of the succession. Potash minerals are abundant in the distorted and deformed halite intervals, representing more than 50% of the units in some horizons. The Qarah-Aghaje section, located 30 km southeast of Iljaq and up to 40 m thick, almost shows similar features. However, the bedded halite units are not distorted, having much thicker muddy halite beds in the upper parts of the sequence. Halite beds comprise more than 90% of the succession and bedded halite contains much less potash salts (up to 5%).
3. Methods
Fig. 2. Stratigraphic column of Tertiary sedimentary units in the studied area in the Zanjan territory. Vertical thickness is proportional to the measured thickness.
of thousands of square kilometers wide, they may have been diachronous. Samples were collected from outcrops of two main salt pits, the Iljaq and Qarah-Aghaje mines, in the northwestern area of the Zanjan province. The Iljaq section (about 50 m in
Fig. 3. Iljaq salt mine (lower part of the photograph) and the Upper Red Formation (top).
About 120 samples were systematically collected at regular intervals (half-meter) from outcrops (salt pits) at the Iljaq mine and 80 more samples were also collected from the Qarah-Aghaje salt mine. Samples were slabbed and thin sections prepared with the aid of a water-free diamond wire saw with vegetable oil as a lubricant. After cleaning, samples were polished using sandpaper of progressively finer grit in order to remove any remaining saw marks (Benison, 1995). Petrographic study was carried out by optical microscopy. After polishing and coating, 15 samples were studied with an SEM (DSM-960A model). The mineralogy of 40 samples was determined by XRD (Siemens V-5000). In order to identify potash horizons in the outcrops, dipicrylamine indicator (Hexanitrodiphenylamin with C12N7H5O12 formula) was used (Schwehr and Conan, 1960). Potassium content of some samples was measured by using a Corning 405-flame photometer. Prior to analysis, a blank and several standard samples were measured for instrumental calibration. The bromine content of halite is commonly measured by ion chromatography, X-ray fluorescence (XRF), or wet-chemical titration analysis (bromometry). All samples were analyzed for their bromine content by titration (Harris, 2001), however, 20 samples were also analyzed by XRF for comparison. The results did not show significant discrepancies (less than 10 ppm), considering precision for the bromine titration of about ^ 5 ppm.
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4. Petrography and diagenesis The mineral assemblage of these evaporites, determined by optical and electron microscopy and X-ray diffraction, includes halite, carnallite, gypsum, anhydrite, langbeinite, and aphthitalite. The bedded halite units of the QarahAghaje salt pit mainly contain carnallite as the potassium mineral, whereas langbeinite and aphthitalite are mainly hosted by distorted halite beds in the Iljaq mine. As a general rule, where halite beds show well-preserved bottom-nucleated crystals and syntaxial overgrowths such as chevrons, cuboids, cornets, and syndepositional dissolution features, they only contain sparse carnallite crystals and are devoid of secondary potash minerals. On the other hand, in the deformed halite beds of the Iljaq salt pit, halite crystals associated with langbeinite and aphthitalite lack any syndepositional features and rather display diagenetic textures such as inclusion-free equigranular mosaic crystals with curved boundaries (Fig. 4). Due to variations in the rate of crystallization (due to variations in physiochemical factors such as temperature and saturation) the vertically oriented bottom-nucleated chevron halite crystals display a texture with alternating bands of cloudy, fluid inclusion-rich material (Fig. 5). Alternations develop only where the brine experiences rapid variations in halite concentration, or in temperature, a situation only possible in shallow brine depths (Handford, 1991). Most of these primary inclusions in the halite crystals contain only liquid, are cubic in shape, and range in size from 1 to 160 mm (Fig. 6). In the chevron texture, halite crystals, which are composed of a series of depositional layers, separated from each other by flat dissolution surfaces. Each truncated surface is further penetrated by dissolution cavity (micro-karst) that extends down into underlying layer (Fig. 5). The presence of both bottomnucleated crystals and cumulate crystals in the same bed is a significant indication of a shallow water deposition
Fig. 4. Burial texture in the distorted halite bed of Iljaq salt pit. Note the concentrations of secondary potash minerals along the curved crystal boundaries of halite. Primary depositional texture has been completely eradicated. Polarized light and gypsum plate.
Fig. 5. Bottom-nucleated and vertically oriented halite chevron, displaying compromise boundaries with its neighboring crystals, from Qarah-Aghaje salt pit. Note the alternating bands of clear halite and fluid inclusion-rich halite. White areas are clear inclusion-poor halite, usually present as rims on chevrons. Traces of dissolution and subsequent cementation in chevron crystal are evidence (arrows). Rounding of euhedral crystal is also presented (big arrow). Plane polarized light.
(Lowenstein and Hardie, 1985; Losey and Benison, 2000; Schreiber and El Tabakh, 2000). Progressive evaporation in the brine leads to the evaporative drawdown and generation of pore brines in the shallower parts of the evaporative basin. In this stage, intrasedimentary growths of the evaporite minerals develop, displaying and including terrigenous muddy sediments. Displacive halite is common in the Qarah-Aghaje salt pit (Fig. 7), showing a chaotic fabric of subhedral crystals separated by pockets of mud. This style of crystal growth evidences early post-depositional growth within soft, brinesaturated, shallow muddy sediment. Nevertheless, when euhedral crystals form, they usually are indicative of nearsurface crystallization in formative salina, as seen in many modern playa and sabkha (Hardie, 1968; Logan et al., 1970;
Fig. 6. Photomicrograph of fluid inclusions outlining primary growth band in chevron halite. Note cloudy inclusion-rich bands and clear inclusionpoor bands. Truncation of banding by dissolution voids (micro karsts in upper right of photograph, arrow) are subsequently filled by clear halite spar. Plane polarized light.
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Fig. 7. Subhedral mud incorporative clear halite crystals with enfacial boundaries, displaying a chaotic fabric (from Qarah-Aghaje salt pit). Plane polarized light.
Gornitz and Schreiber, 1981; Handford, 1991; Warren, 1999). In addition, halite crystals displaying bottomnucleated textures as well as mud inclusions (Fig. 8) are indicative of the early syndepositional crystal growth. Such rapid halite growth indicates a high degree of supersaturation with respect to halite in soft mud at depths of about 2 m (Gornitz and Schreiber, 1981; Handford, 1991; Warren, 1999). The primary subaqueous textures were partially dissolved by intermittent brine freshening or subaerial exposure. This process resulted in the generation of the syndepositional dissolution-reprecipitation features, such as truncated dissolution surfaces (micro-karsts) that crosscut crystal faces (Fig. 5). These backreaction processes that further complicate diagenetic events, is an unequivocal signature of the shallow, unstratified evaporative basins. These vadose micro-karsts, which were filled by clear inclusion-poor halite spar cement, are related to subaerial exposures by some researchers (e.g. Shearman, 1970; Shearman and Orti Cabo, 1978; Warren, 1999). Intensity and frequency of dissolution overprints are largely determined by brine depth, input of
Fig. 8. Mud-incorporative euhedral halite displaying a chevron texture in the center of the crystal. Qarah-Aghaje salt pit, plane polarized light.
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Fig. 9. Patches of syndepositional halite chevron crystals preserved among the mosaic of inclusion-free halite spar. Iljaq salt pit, plane polarized light.
dilute waters and the degree of continuity (perennial vs. ephemeral) of the brine body (Lowenstein and Hardie, 1985). Rounded edges of euhedral crystals, followed by syntaxial rehealing, are other evidences of the fluctuations in the brine saturation (Figs. 5 and 9). Like many other marginal marine salinas, the pervasive early loss of the porosity could be due entirely to syndepositional diagenetic cementation by clear halite (Lowenstein and Hardie, 1985; Casas and Lowenstein, 1989). The internal sediments overlying the syntaxial halite overgrowths suggest syndepositional cementation (Fig. 10), and patchy remnants of chevron structure characterize mature salt pans (Lowenstein and Hardie, 1985). This is a good example of mature halite, which has undergone numerous cycles of dissolution and recementation. However, porosity loss could be in part due to chemical or mechanical compaction. Patches of chevron bearing halite, preserved within the mosaic halite spar (Fig. 9), and inclusion-free halite crystals (Fig. 11), stylolites, and strained and broken crystals (Fig. 12), have been observed
Fig. 10. The remnants of banded chevron halite crystals are embayed and syntaxially rehealed by clear, overgrowth halite cement. Mature halite, which undergone many cycles of dissolution and recementation so that only patches of primary chevron still survive. Iljaq salt pit, plane polarized light.
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Fig. 11. Halite with equigranular mosaic texture from Iljaq salt pit. Note the curved boundaries that meet at triple junctions with angles approaching 1208. Muddy materials are concentrated along crystal boundaries and some inclusion of potash minerals are also shown. Polarized light and gypsum plate.
in distorted, potash-rich halite units of the Iljaq mine. Moreover, in some samples (Fig. 4), fluid and solid inclusions have been purged to grain boundaries so that the depositional textures are completely eradicated. These features must almost certainly result from recrystallization and annealing due to increased temperatures and pressures related to burial (Stanton and Gorman, 1968; Spencer and Lowenstein, 1990; Warren, 1999). Sporadically, the primary crystalline framework of halite has been occluded by, presumably, early diagenetic, poikilotopic gypsum cement (Figs. 13 and 14). Moreover, the presence of solution breccias (Fig. 14), prior to gypsum cementation, implies episodes of brine freshening and subsequent cementation by subsurface brines in the capillary fringe of evaporite basin (e.g. Rouchy et al., 1994). The major mechanism for generation of anhydrite nodules is dehydration of gypsum (above 50 – 60 8C). This conversion may occur after a few meters of burial or on deep
Fig. 12. Strained and broken crystals of halite with stylolites. Iljaq salt pit, polarized light and gypsum blade.
Fig. 13. Coarse poikilotopic gypsum cement in a framework of cuboids and pristine halite crystals (dark). Iljaq salt pit, polarized light.
burial (Spencer and Lowenstein, 1990). However, as shown in Fig. 15, associations of anhydrite crystals with polygonal clear halite mosaic (secondary) suggest their burial origin. In addition, tiny anhydrite laths with random orientation are also present inside the secondary halite crystals.
5. Potash genesis Most potash-magnesium salts are diagenetic replacements of, or additions to, earlier halite or sulfate deposits (Kendall, 1992). On the other hand, formation of secondary potash minerals has been ascribed to thermal, solution or dynamic metamorphism (Braitsch, 1971; Hardie, 1984). Moreover, potash minerals may precipitate in the marine or non-marine settings as synsedimentary minerals, during final stage of brine evaporations as early diagenetic replacements, or during later burial events. The potash minerals in the Iljaq area include langbeinite and aphthitalite as secondary potash salts
Fig. 14. Secondary enlargement of intercrystalline porosity (dissolution breccia) by halite dissolution and subsequent cementation by poikilotopic gypsum. Qarah-Aghaje salt pit, polarized light.
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6. Geochemistry
Fig. 15. Anhydrite crystals with radial texture in polygonal clear halite mosaic. Iljaq salt pit, polarized light.
accommodated by distorted halite beds. However, they never occur as separate beds but particularly are concentrated as inclusions (up to 50%) in recrystallized halite crystals (e.g. along curved crystal boundaries) without primary textures and fluid inclusion banding (Fig. 4). The association of high-temperature langbeinite with another secondary potash mineral (aphthitalite) in burial halite as isolated euhedral crystals, the extensive bedding distortions in the potash-bearing horizons, the absence of void-filling potash cements and other primary features, are indications of secondary origin. It seems they have been generated during episodes of thermal and dynamic metamorphism. The abundance of the secondary higher temperature potash salt, i.e. langbeinite, which is stable only at temperatures greater than 40 – 50 8C (Stewart, 1963) implies that the minimum temperature experienced by potash-bearing halite was above at least 40 – 50 8C, as this salt is the minimum temperature geothermometer. Whereas the primary syndepositional potash minerals are absent in the studied sequences, the secondary potash minerals are abundant (in some horizons up to 50%). This paucity could be ascribed to syndepositional modification of seawater composition by sulfate-reducing bacteria that hindered syndepositional potash formation (Wardlaw, 1972; Sonnenfeld, 1984). Accordingly, traces of some bacteria, algae, and native sulfur have been observed by SEM examinations. Of course, the concentration of these organic materials is low and they never developed as separate large masses. Another explanation for such a dearth of primary potash minerals is syndepositional modification of concentrated seawater by non-marine inflow of different origins (e.g. meteoric or hydrothermal) (Valyashko, 1972; Hardie, 1984). The presence of microkarst texture in some halite crystals (Fig. 5) confirms such dilution episodes.
The marine evaporite deposits with halite, sylvite, and carnallite, and entirely free from the primary Mg-sulfate salts, make up more than 60% of ancient potash deposits (Hardie, 1990, 1991). These types of evaporite deposits have been precipitated from Na – Ca –Mg – K –Cl brines with ionic ratios different from concentrated modern seawater. The origin of these MgSO4-depleted potash deposits has been attributed to the modification of marine evaporites or brines by backreactions and/or subsequent metamorphism during burial episodes (Borchert, 1977; Dean, 1978; Hardie, 1984; Wilson and Long, 1993). Accordingly, several processes such as dolomitization, action of sulfate-reducing bacteria, and brine mixing and freshening, could have altered these seawater-derived brines. Although dolomites are absolutely absent, the existence of such bacteria and resultant native sulfur has been disclosed by SEM examinations of the studied samples. The presence of synsedimentary micro-karsts in many primary samples is also suggests episodes of fresh water influx. Commonly, seawater evaporation generates two types of brine: Na – Cl brines and Mg – Cl – SO4 brines. Na – Cl brines dominate in the salinity range 35 – 330‰, whereas at higher salinites, as a result of the massive removal of Na by halite precipitation, the brine become progressively rich in Mg –Cl –SO4 and bittern salts begin to precipitate. However, in the studied area, seawater-derived brines rarely reached the bittern stage. In fact, leakage from the basin floor, brine reflux, backreactions, and brine renewal or freshening along the whole evaporation cycle inhibited extensive bittern salt precipitation. Marine and non-marine evaporites have been successfully differentiated by their bromine contents (Hardie, 1984; Hanor, 1987; Garrett, 1996; Taberner et al., 2000). Nevertheless, Zak (1997) in his study on the Dead Sea evaporites pointed out the possibility of Br reworking by repeated solution and reprecipitation from brines. Accordingly, this process may give rise to extremely high bromine values both in the water and in precipitated salts. In spite of this observation, bromine contents of evaporites have been successfully employed as an exploration tool in potash deposits prospecting, as a paleosalinity marker in the ancient evaporite basins, and to estimate brine temperatures (Holser, 1969; Raup and Hite, 1978; Hardie, 1984; Ayora et al., 1994; Garrett, 1996; Taberner et al., 2000; Rahimpour-Bonab and Alijani, 2003). With this purpose, bromine content was determined in samples along the evaporite series in the Iljaq and Qarah-Aghaje salt pits, with a stratigraphic resolution of a half meter (Figs. 16 and 17). Due to the mild to intense deformation in some horizons of the Iljaq salt pit, its bromine profile displays disturbed trends. Therefore, only the bromine profile of the Qarah-Aghaje salt pit has been appraised in this work.
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Fig. 16. Bromine profile of the Qarah-Aghaje salt pit.
The range of bromine variations in the smoothed profile of the Qarah-Aghaje salt pit is between 40 and 242 ppm which implies a marine bromine signature for most part of the sequence. In this profile from A (depth 40 m) to B (depth 35 m) there is a sharp decrease in the bromine content (from 242 to 122 ppm) interpreted as a positive hydrologic regime (seawater influx, lowered evaporation rates or bittern reflux). However, from B to C and then C1, there are two major cycles of the progressive bromine increase and the maximum bromine value of the whole profile is attained. This pattern implies dominance of a negative hydrologic regime (cessation of bittern reflux) and even ephemeral basin desiccation that is associated with an increase in primary carnallite. This interpretation is also supported by some subaerial structures such as desiccation cracks, found in these horizons. In the interval from C1 to D, a general decrease in the bromine content, with some fluctuations, implies the recovery of the washed-up positive hydrologic regime. From D (depth 18 m) to D1 (depth 9 m) bromine content remains low with small variations. Maintaining a nearly constant salinity and a stable hydrologic regime during deposition of 9 m of halite requires not only flow of the seawater into the basin, but also reflux of bittern brines from the basin. Upward, to the top of the evaporite sequence
Fig. 17. Bromine profile of the Iljaq salt pit.
(from D1 to E), the general trend is toward progressive bromine decrease (down to 40 ppm) that signals dwindling and imminent termination of the evaporative regime and the dominance of siliciclastic environments. Apparently, throughout the sequence, bromine signatures even in the pristine halite beds, barely reached the minimum threshold (approximately 250 ppm) for the primary potash precipitation. Early diagenesis within the basin floor is able to redistribute bromine of early-formed minerals by recrystallization (Dean and Schreiber, 1978). Thus, the early diagenetic void-filling halite spars (inclusion-free) have been microsampled and analyzed for its Br-content by XRF. Their bromine contents were close to their pristine halite framework, suggesting good preservation of the bromine signatures. Some samples with evidence of burial diagenesis (e.g. equigranular clear halite crystals with curved boundaries, Fig. 11) were also analyzed for their Br content. Their average Br content is close to the well-preserved pristine halite crystals. This finding suggests that burial diagenetic fluids (parental brines of langbeinite and aphthitalite) had composition similar to the primary marine-derived brines and were probably originated within the evaporite sequences.
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Extensive late diagenesis and recrystallization by meteoric waters were not significant here, as they commonly produce an unvarying bromine profile (i.e. redistribution of Br), intense bromine leaching, and possible bromine depletion. To compare Br and K variations in different minerals, K-contents of samples were also measured. Theoretically, with progressive increase in the salinity of the brine, the precipitating mineral phases will enrich in K and Br. Therefore, in primary potash deposits, potassium variations should closely be correlated with their bromine signatures. However, in our samples Br and K variations are largely inconsistent due to secondary origin of the most potash salts, and to a lesser extent, the higher mobility of the potassium in diagenetic environments.
7. Depositional model The studied evaporites are laterally, but discontinuously, persistent over large areas of northwestern Central Iran, with only minor changes in mineralogy and facies. Apparently, these sediments formed in the marginal marine environments that were periodically flooded both by meteoric water and seawater resulting in the partial dissolution of the older and more-soluble evaporites. The relatively insoluble salts, such as gypsum, were eroded, corroded, and redeposited as rounded clasts, which appear as thin interlayers within thick halite beds. In addition, finegrained siliciclastic input, brought in by the floods, were deposited over the evaporite crusts as a blanketing layers of mud. These deposits with 5 – 80 m thickness and evidences of subaerial exposures, formed in a siliciclastic-rich ‘wadi’ environment, landward of a mixed lagoonal and salina evaporite setting, that passes seaward into a shallow-water high stand carbonate (HST) facies of the Qom Formation. These carbonate-evaporite transitions are chronologically equivalent to the well-known Asmari Limestone (HST) and its overlying cap rock, the Gachsarran evaporites, in the Zagros Basin.
8. Conclusions Studied evaporites developed as lagoonal and sabkha facies that occurred over shallow, open marine platform carbonate of the Qom Formation Apparently, these extensive, thick, shelf-margin sequences that form mixed carbonate/siliciclastic/evaporite units have been deposited in an intracratonic basin where some form of barrier developed to isolate them from the oceans. Petrographic textures formed under shallow brines, subaerial features, and high bromine content support a marginal, marine-fed, platform evaporite model.
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Although high bromine content of pristine halite crystals suggests their marine origin, the values barely reach the minimum threshold for primary potash precipitation. Presumably, activities of sulfate-reducing bacteria and sporadic meteoric inflows hindered primary syndepositional potash deposition. The lack of continental evaporite paragenesis (including mirabilite, thenardite and trona), the high bromine content and its mineralogical assemblage, suggest a parental marine brine of the Na – K – Mg – SO4 – Cl type that was faintly modified by non-marine inflows.
Acknowledgements This work is supported by a facilities provided by Tehran University (Grant No. 512/2/585) which we are sincerely grateful. Thanks are also due to B.C. Schreiber for her careful review and invaluable comments.
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