Zeolites of the North Shore of the Minas Basin

1 The earliest Jurassic (Hettangian) North Mountain Basalt of the Bay of Fundy has long been known for its rich assemblage of zeolite minerals in amygdales, vugs, and fractures (Aumento 1966). Some authors have compared the zeolite assemblages to those resulting from burial metamorphism in Iceland (Adams 1980), but more recent work on North Mountain shows them to result from hydrothermal circulation (De Wet and Hubert 1989; Pe-Piper 2000). The Minas Basin, at the eastern end of the Bay of Fundy, is a half-graben with the master fault on the north side showing strong left-lateral movement synchronous with basalt extrusion (Withjack et al. 1995). This area thus provides the opportunity to determine the relative timing of zeolite mineral formation, in relationship to synchronous faulting.

2 The North Shore of the Minas Basin is part of the Cobequid-Chedabucto fault system that was active during the Triassic and Jurassic (Stevens 1987; Withjack et al. 1995). Basalt exposed on the North Shore shows the same stratigraphic succession as the main North Mountain outcrop on the south side of the Bay of Fundy, but tends to be more tectonized and altered (Greenough et al. 1989). The type section of the North Mountain basalt is overlain by the lacustrine Scots Bay Formation, whereas the basalt on the North Shore is overlain by siliciclastic rocks of the McCoy Brook Formation. The basalt of the North Shore hosts zeolites in pipes, joints, veins, fractures, cavities, and fault zones, and in the amygdales and groundmass of the basalt. The study areas of the North Shore include localities known as Five Islands, Two Islands, Wasson Bluff, Partridge Island, Cape Sharp, Horseshoe Cove, and western Cape d'Or (Fig. 1).

Previous Work

3 Previous work on the zeolites of the North Shore of the Minas Basin has been reported by Jackson and Alger (1828), Gesner (1836), How (1868), Gilpin (1881), Walker and Parsons (1922), Sabina (1965), Aumento (1966), Donohoe et al. (1992), and Booth (1994). Most of the studies of zeolite minerals on the North Shore were conducted before the development of analytical techniques such as X-ray diffraction and electron microprobe chemical analyses, and even the more recent studies have not included electron microprobe chemical analyses. Considerable discrepancy exists in the zeolite mineral assemblages in each area as reported by different authors.

Purpose

4 The purpose of this paper is: 1) to accurately identify zeolite minerals of the North Shore of the Minas Basin using up-to-date analytical techniques, thus establishing both their stratigraphic and regional distribution; 2) to determine the order of crystallization of zeolite minerals in each area and hence the events of zeolite formation; 3) to examine the chemical mineralogy of zeolite minerals of representative samples and in particular zeolite species from these areas with no published chemical analyses; 4) to establish the differences in chemistry of some identified zeolite species between the North Shore and the Arlington Quarry (a representative area of the type North Mountain basalt in Kings County) (Fig. 1). Both the field and analytical data in this study have been substantially updated since a preliminary report by Miller (1997).

Methods

5 Zeolite identification is based on hand-specimen identification, electron microprobe analysis, and X-ray diffraction (XRD) analysis. Two-hundred samples from the North Shore of the Minas Basin and seventy samples from Kings County have been studied.

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6 Eighty-seven representative samples from the North Shore and eleven samples from Kings County were selected for X-ray diffraction. Potential zeolite minerals were extracted from each sample based on colour and crystal habit. Careful extraction of minerals helped increase the recovery of monomineralic samples and a microscope was employed to ensure purity of the mineral extracts. The extracts were then powdered using an agate mortar and pestle. A slurry of each of the powdered samples was created before mounting of the sample on a glass disk. Methanol was used in the slurry because it evaporates quickly and it does not react with zeolite minerals. The mixture of methanol and sample was then mounted on a glass slide using a micro-pipette and allowed to dry. The samples were then analysed using a Siemens Cristaloflex diffractometer, CoKa radiation, and a scan from 2° to 42° 20 at a speed of 0.25° 20/min. Peak positions were refined using the algorithm of Appleman and Evans (1973).

7 Thirty polished thin sections of representative specimens from the North Shore of the Minas Basin were prepared for detailed study by electron microprobe. These representative spots were analysed by a JEOL-733 with four wavelength spectrometers and a Tracor Northern 145-eV energy dispersive system detector at the Department of Earth Sciences, Dalhousie University. The beam operated with 15kV and 15nA and a beam diameter of 10 µm. An internal standard (scolecite) was used in addition to the standard regularly used. Because of the difficulty of discriminating accurately heulandite from clinoptilolite, we use the term "heulandite" to include both minerals.

8 Sequence of crystallisation of zeolites and other secondary minerals is based on two types of evidence. In individual veins, amygdales, and vugs, the sequence of minerals from rim to core is interpreted as a time sequence (Pe-Piper 2000). Where faulting and sedimentation was synchronous with zeolite formation, as at Wasson Bluff, geological field evidence can be used to determine the sequence of formation of secondary minerals.

Introduction

9 Study areas on the North Shore of the Minas Basin include Wasson Bluff, Cape Sharp, Five Islands, Partridge Island, Horseshoe Cove, Western Cape d'Or, and Two Islands (Fig. 1). Greenough et al. (1989) showed that basalt at Five Islands, Partridge Island, and Cape d'Or conformably overlying the Blomidon Formation is stratigraphically equivalent to the lower lava unit on North Mountain. They suggested that at McKay Head (not sampled in this study), a thick flow equivalent to the upper lava unit on North Mountain is overlain by several thinner flows and then by the McCoy Brook Formation. These thin flows are not recognised on North Mountain and appear to be correlative with the flows at Wasson Bluff.

Wasson Bluff

10 The Wasson Bluff section is the best area on the North Shore to study the geological events related to the formation of the zeolite-minerals (Figs. 2, 3). Most basalt in the area is massive, but considerable amounts are amygdaloidal. Zeolites at Wasson Bluff commonly occur in basalt, conglomerate with basalt clasts, and less commonly in sandstone, but on average make up no more than 3% of the overall rock. The zeolites are mainly found in veins, less commonly in cavities and the amygdales and groundmass of basalt, and only in a few fault zones. Sinistral strike-slip movement along the Portapique Fault, a few hundred metres north of the coastal outcrops, formed small transtensional grabens in which McCoy Brook Formation clastic sediments accumulated. The predominant lithology in the lower part of the McCoy Brook Formation is eolian dune sandstone, but lacustrine mudstone and talus slope debris-flow breccia of matrix-supported basalt clasts (Tanner and Hubert 1991) are also present. The upper part of the formation consists of red fluvial sandstone. Within the McCoy Brook Formation, slickensides in red sandstone and mudstone were interpreted by Schlische and Olsen (1989) as hydroplastic, having formed in incompletely lithified sediment.

11 The basalt has well developed columnar joints, some of which appear to have been rotated by local fault movement (Schlische and Olsen 1989). Neptunian dykes of fine red sandstone and mudstone occupy joints between cooling columns and sinuous fractures in the basalt (Figs. 2a, 2c, 2e; Fig. 3). They were interpreted by Schlische and Olsen (1989) to represent active tectonic extension of the North Mountain Basalt and filling of fractures by sediment from above. The widest neptunian dykes are orthogonal to the regional extension direction. Schlische and Olsen (1989) also described extensive outcrops of "rubblized" basalt (Fig. 3), which is cataclastic basalt brecciated along strike-slip fault zones. They also described irregularities in the top of one basalt flow developed before the subsequent flow was deposited that they ascribed to faulting during extrusion of the basalt succession.

12 Others have made passing observations on the distribution of zeolites in the Wasson Bluff section. Tanner and Hubert (1991) noted that the talus slope breccia included zeolite-filled fractures cutting both clasts and matrix. Schlische and Olsen (1989) noted that narrow neptunian dykes are commonly silicified, and that cataclastic basalt contains chabazite, but some faults that they interpreted as almost syn-depositional lack zeolite mineralisation.

13 New field observations clarify the occurrence of zeolites in the Wasson Bluff section (Fig. 3). Amygdaloidal basalt flows have amygdale fillings principally of celadonite and silica minerals, with minor amounts of zeolite. Clasts in the talus-slope and debris-flow deposits (Fig. 2b) are similar, principally with celadonite and silica minerals in amygdales. Slickensides in the basalt cataclasite are developed in quartz and celadonite.

14 Where columnar joints in basalt have been progressively widened, they commonly filled with neptunian dykes of red sandstone and mudstone (Fig. 2a). In the basalt, zeolites are common in some thinner joints with neptunian dykes (Fig. 2e). Zeolites fill vugs in the basalt. A few zeolite veins cross-cut both the basalt and neptunian dykes, but are much thinner in the dyke sediments (Fig. 2k), presumably reflecting the greater brittleness of the basalt. Some of the thinner neptunian dykes appear to be silicified (Schlische and Olsen 1989). Zeolites also cement zones where basalt has been brecciated (Fig. 2d). Veins of zeolite cutting basalt are also recognised in basalt clasts in the talus-slope deposits (Fig. 2l).

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15 Zeolites occur only locally in the McCoy Brook Formation. Zeolite veins cut the talus-slope deposits and zeolites also fill vugs in the matrix. Zeolites are also present as irregular veins in the pug of a few major faults. Zeolite rosettes 5—20 mm in diameter appear to have precipitated in situ in the muddy matrix of the debris-flow deposits within a few metres of these faults. Some eolian sandstone has abundant patches of concretions 5—10 mm in diameter that consist of sandstone cemented by stellerite. In the easternmost McCoy Brook Formation basin, an irregular silicified body (5 m in length, 1 m thick) appears to have replaced permeable polymictic coarse litharenite. (The outcrops are inaccessible in the cliff, but have been sampled from modern talus).

16 The fault zone cutting sandstone of the McCoy Brook Formation that shows hydroplastic slickensides described by Schlische and Olsen (1989) also has sub-parallel zeolite veins and veins of silicified sediment. Thin silica veins have been found in other faults cutting sandstone of the McCoy Brook Formation.

17 In summary, the work of Schlische and Olsen (1989) showed that extrusion of North Mountain Basalt and deposition of the immediately overlying sediments of the McCoy Brook Formation took place during active strike-slip faulting and the formation of a small transtensional graben (Fig. 3). The oldest basaltic clasts in the McCoy Brook Formation contain sparse zeolite in amygdales even where matrix zeolite is absent, suggesting that the oldest zeolitization predated erosion of the clasts at fault scarps in the basalt. However, the predominant zeolitization appears to affect the McCoy Brook Formation and the North Mountain Basalt equally. In the North Mountain Basalt, the most prominent zeolitization is in the columnar-jointed basalt and irregular veins and vugs, which is a response to the progressive deformation of the basalt in a strike-slip, pull-apart setting. Within the McCoy Brook Formation, zeolitization is generally restricted to a few discrete zones, including major faults and near the contact of talus-slope deposits with basalt (again, presumably related to active faulting). Nevertheless, significant silicic alteration affects the upper part of the McCoy Brook Formation containing polymictic pebbly litharenite. Rocks younger than the McCoy Brook Formation are not present in the region, so that the age of the youngest zeolitization cannot be determined.

Other North Shore localities

18 Other North Shore localities are described from east to west. At Five Islands, the North Mountain Basalt is highly faulted (Greenough et al. 1989; Stevens 1987; Olsen 1989). Basaltic cataclasite contains only rare zeolites, as at Wasson Bluff. Basaltic breccia with a red mudstone matrix, probably a faulted equivalent of the talus slope breccia seen at Wasson Bluff, contains zeolites, principally in veins, although some occur in vugs in the matrix and in amygdales in the basalt clasts.

19 At Two Islands, the larger island is entirely basalt and has common gmelinite and analcime. Massive basalt at Partridge Island, corresponding to the lower lava unit of North Mountain, conformably overlies the Blomidon Formation. Zeolites in the basalt are mostly in veins and less commonly in cavities.

20 The vesicular and jointed basalt at Cape Sharp also overlies red sandstone of the Blomidon Formation. Zeolite samples were collected on the east side of the headland and are common in joints, veins, and vugs.

21 Zeolites from basalt at Cape d'Or were collected from both Horseshoe Cove in the east and the western part of Cape d'Or near the lighthouse. The host rock is mostly locally vesicular basalt (some joints filled with red sandstone) overlain by massive basalt. Zeolites are common in veins and at western Cape d'Or, also in joints.

Introduction

36 The zeolite mineral assemblages vary in each study locality on the North Shore of the Minas Basin. Wasson Bluff and Cape d'Or display the largest variety of zeolite minerals, whereas the other localities have only two to five zeolite minerals. Minerals associated with the zeolite minerals include quartz and other forms of silica (e.g., chalcedony, amethyst, agate, jaspar, opal), malachite, mica, barite, and calcite.

Wasson Bluff

37 The zeolites we identified at this locality are the same as those reported by Booth (1994) and Donohoe et al. (1992), except that we did not identify scolecite and epistilbite. Chabazite is the dominant zeolite, occurring most commonly in veins. The term "acadialite" has been applied by previous workers to the pink variety of chabazite. Stilbite, "heulandite", and analcime are common, natrolite and thomsonite are rare. The associated minerals include quartz, calcite, and barite.

38 Based on textural relationships between minerals in individual hand specimens, the following sequences of crystallization are recognised, from early to late:

• Silica minerals including quartz to "heulandite"
• Chabazite + "heulandite" + quartz to stilbite,
• Analcime to analcime + "heulandite" to natrolite to calcite.
• Chabazite to gmelinite to barite

39 On geological criteria, the relative timing of crystallisation is much less clear. Celadonite, silica minerals, "heulandite," analcime, and chabazite predominate in amygdales in the basalt flows and in basalt clasts in the McCoy Brook Formation, suggesting that these minerals were "early" (Fig. 3). The McCoy Brook Formation sedimentary rocks are host to, or cut by veins of, "heulandite", chabazite, stellerite, and silica minerals. Late fractures in the basalt flows predominantly contain chabazite.

Five Islands

40 The zeolites identified in the Five Island area are chabazite, "heulandite", barrerite, and stilbite. Walker and Parsons (1922) reported analcime, natrolite, thomsonite, and gmelinite, but we have not identified those minerals in our samples. Calcite, silica minerals, and barite, the last of which occurs alone or with quartz, are commonly associated with the zeolite minerals.

Two Islands

41 The most abundant zeolite on the larger island is analcime, whereas gmelinite and chabazite are common and stilbite and natrolite are rare (I. Booth, personal communication, 1999). We have analysed gmelinite and analcime, with gmelinite having been the first to crystallize.

Partridge Island

42 Chabazite and stilbite are abundant among our studied samples; "heulandite", barrerite, epistilbite, and natrolite are rare. We have not identified thomsonite, reported by Booth (1994). Associated minerals are quartz, chert, other forms of silica, mica, chlorite, magnetite, and galena. The overall order of crystallization from earliest to latest is: chabazite to quartz + stilbite to calcite.

Cape Sharp

43 Stilbite is abundant and chabazite is common in the area, with minor "heulandite" and barrerite. Gesner (1836) reported that a mass of stilbite of over 200 lbs fell from a cliff in 1834. Gmelinite was reported by Gilpin (1881) and natrolite by Walker and Parsons (1922), but we did not identify those zeolite minerals. The overall order of crystallization from earliest to latest is: mica to quartz + amethyst + other forms of silica to chabazite to stilbite to calcite.

Horseshoe Cove

44 Although "heulandite" occurrences were not suggested in earlier studies, this zeolite is abundant in the samples examined. Stilbite is common and associated minerals are quartz and calcite. The overall order of crystallization is: quartz to stilbite + quartz to stilbite to "heulandite" to calcite. The precipitation of "heulandite", a high-temperature zeolite, after stilbite, a lower temperature zeolite, is noteworthy.

Cape d'Or

45 Analcime and natrolite are abundant, mesolite and stilbite are common, and thomsonite, chabazite, laumontite, and "heulandite" are rare. The "heulandite" is present in the form of rosettes. Tetranatrolite, stellerite, barrerite, scolecite, and wairakite have also been identified. The associated minerals include quartz, other forms of silica, malachite and magnetite. Data are insufficient to determine an overall order of crystallization, but sequences of quartz to stilbite, quartz to "heulandite", stilbite to laumontite, and mesolite to quartz are seen.

Table 1. Electron microprobe analyses of representative zeolite minerals.

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Table 1. (Cont)

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Table 2. Electron microprobe analyses of representative zeolite minerals from Kings County

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Table 2. (Cont)

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Introduction

48 Arlington Quarry (Fig. 1) was selected as a representative zeolite-bearing locality from the North Mountain Basalt of Kings County. Sixty-three zeolite samples from the quarry were studied in hand specimen (Fig. 7) and by XRD and electron microprobe. Nine types of zeolite were identified: epistilbite, scolecite, stilbite, stellerite, heulandite (sensu stricto), clinoptilolite, mesolite, thomsonite, and rare analcime (Table 2). Minerals associated with the zeolites include green celadonite, malachite, quartz and other forms of silica, barite, calcite, and fluoroapophyllite. The following sequences of crystallization are recognised, from early to late:

• Heulandite (sensu stricto) to clinoptilolite to stilbite
• Heulandite (sensu stricto) to stilbite to clinoptilolite
• "Heulandite" to silica minerals to "heulandite" to stilbite
• Thomsonite and/or epistilbite to mesolite
• Stellerite to "heulandite"
• Scolecite to stilbite

Mineral chemistry: North Shore vs. Arlington Quarry

49 Chemical analyses of zeolite minerals from Arlington Quarry are given in Table 2. "Heulandite", mesolite, stilbite, thomsonite, natrolite, tetranatrolite, and epistilbite have been analysed from both the North Shore and Arlington Quarry and can therefore be compared.

50 "Heulandite" from Arlington Quarry is generally pink, earthy, and massive, and shows three clusters in terms of Si/Al ratio at ~4.5, ~3.1, and ~2.8 (Fig. 6b). In individual crystals, Si content decreases from core to rim, whereas Al and K increase, and Ca first increases and then decreases. The "heulandite" from the North Shore shows the same clusters of Si/Al ratio (Fig. 6a), less Ca, and more K and Na; minor Mg was also detected. As a result, average (Ca+Mg)/(Na+K) is lower (Fig. 6a) than at Arlington Quarry. Within individual crystals, Ca and K generally decrease from core to rim, whereas Na and Si increase.

51 Mesolite of Arlington Quarry is white and dull, resembling that of the North Shore in hand specimen. Within individual crystals Si, Al, Ca, Na, and H2O decrease from core to rim. In contrast to Arlington Quarry, within individual crystals of mesolite from the North Shore, Si, Al, and Ca increase from core to rim and K was detected, but (Ca+Mg)/(Na+K) from the two areas is similar.

52 Stilbite from Arlington Quarry is generally sheaf-like, vitreous, and colourless, resembling that of the North Shore. Analyses of stil-bite from the two areas shows similar Si/Al and (Ca+Mg)/(Na+K) ratios (Fig. 6). Within individual crystals at Arlington Quarry, Si, Na, and K decrease from core to rim, whereas Al and Ca increase. In contrast, individual crystals from the North Shore show Si and Ca increasing from core to rim, and Al and Na decreasing.

53 Epistilbite is not easy to identify in hand specimen and most of the epistilbite identifications are from XRD and electron micro-probe chemical analyses. The Si/Al ratios of epistilbite from both areas are similar, but epistilbite from the North Shore has lower Al and Ca and higher Na. The natrolite and tetranatrolite from Arlington Quarry in hand specimen do not differ from those from North Shore and their chemical characteristics are also similar (Fig. 6).

54 Thomsonite from Arlington Quarry is generally white, dull, and massive with Si/Al average of 1.10 (Fig. 6a). Within individual crystals, Si, Al, Ca, and Na generally decrease from core to rim, whereas Fe and Mg decrease. Thomsonite from the North Shore has a higher Si/Al ratio average of 1.25 (Fig. 6a), lower Fe, Mg, and Ca, and Na increases from core to rim of individual crystals. Fe, Mg, and K were not detected.

55 In both Figs. 5 and 6, zeolites from Arlington Quarry cluster much more tightly than those from the North Shore. This effect is not simply the result of a greater geographic range on the North Shore, as analyses from the Morden area reported by Pe-Piper (2000), from localities up to 30 km apart, show tight clusters simi lar to those from Arlington Quarry. At both Arlington Quarry and the Morden area (Pe-Piper 2000), few zeolites fall in the field in Figure 5 with either Ca+Mg < 70% or Na < 70%. In contrast, on the North Shore, all epistilbite, thomsonite, and mesolite analyses lie in this field, together with wairakite and many chabazite analyses, two minerals not analyzed from Kings County.

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Minerals present

56 All zeolite minerals previously reported from the North Mountain Basalt, except gyrolite, levyne, and mordenite, have also been identified along the North Shore of the Minas Basin. In addition, wairakite, barrerite, and tetranatrolite, not previously reported, have been identified. Scolecite has been identified only by XRD and some massive samples described as scolecite on the basis of hand specimen character were identified as mesolite by XRD and electron microprobe.

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57 The zeolite minerals found on the North Shore of the Minas Basin differ from those in Kings County and elsewhere on the south coast of the Bay of Fundy in the presence of wairakite and barrerite, and in the abundance of chabazite, which is rare on the south coast. Chabazite was reported by Sabina (1965) from Mink Cove and Cape Split, but we have found no samples at Arlington Quarry and Morden (Pe-Piper 2000). Mordenite, which is common in Kings County, has not been found on the North Shore of the Minas Basin.

Hydrothermal origin of zeolites

58 Previous studies suggested that the zeolite formation in the North Mountain Basalt was the result of burial metamorphism (e.g. Aumento 1966; Adams 1980). New petrographic evidence in Pe-Piper (2000), such as the repetitive zonation in some amygdales and the amount of metal precipitates, suggest a hydro-thermal origin. The difference in chemical composition of zeolite minerals from the North Shore of the Minas Basin and from Kings County may be the result of a difference in temperature of formation and/or different fluid composition.

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Temperatures of zeolite formation

59 Recent experimental data on the formation temperatures of natural zeolites are limited. Barth-Wirshing and Holler (1989) suggested that chabazite forms from basaltic glass at temperatures of 50—150°C, whereas heulandite forms from rhyolitic glass at temperatures of 150—225°C. The dominance of "heulandite" in Kings County and of chabazite on the North Shore suggests that formation temperatures were higher in Kings County. This differentce is also supported by the presence of mordenite in some areas of Kings County (Pe-Piper 2000; Kontak 2000).

60 In general, Ca ions can crystallize out of aqueous solution at much higher temperatures than Na and K (Kotz and Purcell 1987, p. 756—765). High-temperature solutions are thus more likely to precipitate Ca-rich zeolites than Na-rich zeolites. The first zeolite minerals to crystallize in Kings County samples are rich in Ca (i.e. "heulandite") and we infer that Na was not accommodated in the crystal structure of the crystallizing zeolites because fluid temperature was too high. As the temperatures decreased and the circulating fluids became depleted in Ca, lower temperature, Na-rich zeolites (natrolite group) began to crystallize. In contrast, on the North Shore the chemical data presented in this paper indicate that Na was readily accommodated in the crystal structure of crystallizing zeolites, which we interpret as a result of crystallization at lower temperatures. As a result, only limited crystallization of Na-rich zeolites (natrolite group) took place in the North Shore occurrences, and the observed zeolites show a broad scatter in Na, Ca, Mg, and K (Fig. 5).

61 Chemical differences between zeolites of the North Shore and Kings County may also result from different chemistry of crystallizing solutions. Pe-Piper (2000) proposed that the abundance of mordenite in Kings County was in part related to extraction of Na from evaporites of the Blomidon Formation. According to Ackermann et al. (1995), the Blomidon Formation contains a chaotic mudstone which can be traced for 35 km in the Fundy Rift Basin. Ackermann et al. (1995) inferred the unit was formed by subsidence and collapse of a metre thick evaporite-rich bed by groundwater. On the North Shore of the Minas Basin, Mertz and Hubert (1990) described only gypsum precipitation. The variation in evaporite distribution in the Blomidon Formation, from gypsum at the basin margin to halite in the basin centre, may have influenced the chemistry of later hydrothermal zeolites.