Multistage hydrothermal dolomites in the Middle Devonian (Givetian) carbonates from the Guilin area, South China

Pervasive dolomites occur preferentially in the stromatoporoid biostromal (or reefal) facies in the basal Devonian (Givetian) carbonate rocks in the Guilin area, South China. The amount of dolomites, however, decreases sharply in the overlying Frasnian carbonate rocks. Dolostones are dominated by replacement dolomites with minor dolomite cements. Replacement dolomites include: (1) fine to medium, planar‐e floating dolomite rhombs (Rd1); (2) medium to coarse, planar‐s patchy/mosaic dolomites (Rd2); and (3) medium to very coarse non‐planar anhedral mosaic dolomites (Rd3). They post‐date early submarine cements and overlap with stylolites. Two types of dolomite cements were identified: planar coarse euhedral dolomite cements (Cd1) and non‐planar (saddle) dolomite cements (Cd2); they post‐date replacement dolomites and predate late‐stage calcite cements that line mouldic vugs and fractures. The replacement dolomites have δ¹⁸O values from ?13·7 to ?9·7‰ VPDB, δ¹³C values from ?2·7 to + 1·5‰ VPDB and ⁸⁷Sr/⁸⁶Sr ratios from 0·7082 to 0·7114. Fluid inclusion data of Rd3 dolomites yield homogenization temperatures (T_h) of 136–149 °C and salinities of 7·2–11·2 wt% NaCl equivalent. These data suggest that the replacive dolomitization could have occurred from slightly modified sea water and/or saline basinal fluids at relatively high temperatures, probably related to hydrothermal activities during the latest Givetian–middle Fammenian and Early Carboniferous times. Compared with replacement dolomites, Cd2 cements yield lower δ¹⁸O values (?14·2 to ?9·3‰ VPDB), lower δ¹³C values (?3·0 to ?0·7‰ VPDB), higher ⁸⁷Sr/⁸⁶Sr ratios (≈ 0·7100) and higher T_h values (171–209 °C), which correspond to trapping temperatures (T_r) between 260 and 300 °C after pressure corrections. These data suggest that the dolomite cements precipitated from higher temperature hydrothermal fluids, derived from underlying siliciclastic deposits, and were associated with more intense hydrothermal events during Permian–Early Triassic time, when the host dolostones were deeply buried. The petrographic similarities between some replacement dolomites and Cd2 dolomite cements and the partial overlap in ⁸⁷Sr/⁸⁶Sr and δ¹⁸O values suggest neomorphism of early formed replacement dolomites that were exposed to later dolomitizing fluids. However, the dolomitization was finally stopped through invasion of meteoric water as a result of basin uplift induced by the Indosinian Orogeny from the early Middle Triassic, as indicated by the decrease in salinities in the dolomite cements in veins (5·1–0·4 wt% NaCl equivalent). Calcite cements generally yield the lowest δ¹⁸O values (?18·5 to ?14·3‰ VPDB), variable δ¹³C values (?11·3 to ?1·2‰ VPDB) and high T_h values (145–170 °C) and low salinities (0–0·2 wt% NaCl equivalent), indicating an origin of high‐temperature, dilute fluids recharged by meteoric water in the course of basin uplift during the Indosinian Orogeny. Faults were probably important conduits that channelled dolomitizing fluids from the deeply buried siliciclastic sediments into the basal carbonates, leading to intense dolomitization (i.e. Rd3, Cd1 and Cd2).     

Dedolomitization and calcite cementation in the Middle Devonian Winnipegosis Formation in Central Saskatchewan,Canada

Limestone consisting of finely to medium crystalline calcite mosaics is present in the upper part of the Winnipegosis Formation on the east‐central margin of the Elk Point Basin where the overlying Prairie Evaporite deposits have been removed. This type of crystalline limestone is interpreted as dedolomite, based on petrographic observations. The δ¹⁸O and δ¹³C values of the Winnipegosis dedolomite vary from ?12·8‰ to ?11·9‰ VPDB (Vienna Pee Dee Belemnite) and from ?0·5‰ to +1·7‰ VPDB, respectively; both values are significantly lower than those for the corresponding dolomite. The ⁸⁷Sr/⁸⁶Sr ratios of the dedolomite are significantly higher, between 0·7082 and 0·7087. The spatial distribution and geochemical data of the Winnipegosis dedolomite suggest that dedolomitization was related to an influx of fresh groundwater and dissolution of the Prairie Evaporite anhydrite during the latest Mississippian to the Early Cretaceous when the basin was subjected to uplift and erosion. The Winnipegosis dedolomite displays a series of replacement fabrics showing progressive calcitization of dolomite, including the occurrence of dedolomite restricted along fractures and adjacent areas, dolomite patches ‘floating’ in the dedolomite masses and massive dedolomite with sparsely scattered dolomite relicts. However, the characteristic fabrics resulting from dedolomitization documented in the literature have not been observed in the Winnipegosis dedolomite. Coarsely to very coarsely crystalline, subhedral to euhedral calcite cement is restricted in the dedolomite. The petrographic features, isotopic compositions and homogenization temperatures, coupled with the burial history of the Winnipegosis Formation, constrain the precipitation of the calcite cement from a mixing of basinal brines and fresh groundwater during Late Cretaceous to Neogene time. The more negative C‐isotopic signatures of the calcite cement (?5·3‰ to ?2·3‰ VPDB) probably reflect a hydrocarbon‐derived carbon.     

Petrography and geochemistry of early-stage, fine- and medium-crystalline dolomites in the Middle Devonian Presqu'ile Barrier at Pine Point, Canada

The petrography and geochemistry of fine- and medium-crystalline dolomites of the Middle Devonian Presqu’ile barrier at Pine Point (Western Canada Sedimentary Basin) are different from those of previously published coarse-crystalline and saddle dolomites that are associated with late-stage hydrothermal fluids. Fine-crystalline dolomite consists of subhedral to euhedral crystals, ranging from 5 to 25 μm (mean 8 μm). The dolomite interbedded with evaporitic anhydrites that occur in the back-barrier facies in the Elk Point Basin. Fine-crystalline dolomite has δ¹⁸Ο values between ?1·6 to –3·8‰ PDB and ⁸⁷Sr/⁸⁶Sr ratios from 0·7079–0·7081, consistent with derivation from Middle Devonian seawater. Its Sr concentrations (55–225 p.p.m., mean 105 p.p.m.) follow a similar trend to modern Little Bahama seawater dolomites. Its rare earth element (REE) patterns are similar to those of the limestone precursors. These data suggest that this fine-crystalline dolomite formed from Middle Devonian seawater at or just below the sea floor. Medium-crystalline dolomite in the Presqu’ile barrier is composed of anhedral to subhedral crystals (150–250 μm, mean 200 μm), some of which have clear rims toward the pore centres. This dolomite occurs mostly in the southern lower part of the barrier. Medium-crystalline dolomite has δ¹⁸O values between ?3·7 to ?9·4‰ PDB (mean ?5·9‰ PDB) and ⁸⁷Sr/⁸⁶Sr ratios from 0·7081–0·7087 (mean 0·7084); Sr concentrations from 30 to 79 p.p.m. (mean 50 p.p.m.) and Mn content from 50 to 253 p.p.m. (mean 161 p.p.m.); and negative Ce anomalies compared with those of marine limestones. The medium-crystalline dolomite may have formed either (1) during shallow burial at slightly elevated temperatures (35–40 °C) from fluids derived from burial compaction, or, more likely (2) soon after deposition of the precursor sediments by Middle Devonian seawater derived from the Elk Point Basin. These results indicate that dolomitization in the Middle Devonian Presqu’ile barrier occurred in at least two stages during evolution of the Western Canada Sedimentary Basin. The geochemistry of earlier formed dolomites may have been modified if the earlier formed dolomites were porous and permeable and water/rock ratios were large during neomorphism.     

Fault-controlled dolomitization at Swan Hills Simonette oil field (Devonian), deep basin west-central Alberta, Canada

The partly dolomitized Swan Hills Formation (Middle‐Upper Devonian) in the Simonette oil field of west‐central Alberta underwent a complex diagenetic history, which occurred in environments ranging from near surface to deep (>2500 m) burial. Five petrographically and geochemically distinct dolomites that include both cementing and replacive varieties post‐date stylolites in limestones (depths >500 m). These include early planar varieties and later saddle dolomites. Fluid inclusion data from saddle dolomite cements (T_h=137–190 °C) suggest that some precipitated at burial temperatures higher than the temperatures indicated by reflectance data (T_peak=160 °C). Thus, at least some dolomitizing fluids were ‘hydrothermal’. Fluorescence microscopy identified three populations of primary hydrocarbon‐bearing fluid inclusions and confirms that saddle dolomitization overlapped with Upper Cretaceous oil migration. The source of early dolomitizing fluids probably was Devonian or Mississippian seawater that was mixed with a more ⁸⁷Sr‐rich fluid. Fabric‐destructive and fabric‐preserving dolostones are over 35 m thick in the Swan Hills buildup and basal platform adjacent to faults, thinning to less than 10 cm thick in the buildup between 5 and 8 km away from the faults. This ‘plume‐like’ geometry suggests that early and late dolomitization events were fault controlled. Late diagenetic fluids were, in part, derived from the crystalline basement or Palaeozoic siliciclastic aquifers, based on ⁸⁷Sr/⁸⁶Sr values up to 0·7370 from saddle dolomite, calcite and sphalerite cements, and ²⁰⁶Pb/²⁰⁴Pb of 22·86 from galena samples. Flow of dolomitizing and mineralizing fluids occurred during burial greater than 500 m, both vertically along reactivated faults and laterally in the buildup along units that retained primary and/or secondary porosity.     

Dolomitization of the Waulsortian Limestone (Lower Carboniferous) in the Irish Midlands

The Waulsortian Limestone (Lower Carboniferous) of the southern Irish Midlands is dolomitized pervasively over a much larger region than previous studies have documented. This study indicates a complex, multistage, multiple fluid history for regional dolomitization. Partially and completely dolomitized sections of Waulsortian Limestones are characterized by finely crystalline (0·01–0·3 mm) planar dolomite. Planar replacive dolomite is commonly followed by coarse (≥0·5 mm) nonplanar replacive dolomite, and pervasive void‐filling saddle dolomite cement is frequently associated with Zn–Pb mineralization. Planar dolomite has average δ¹⁸O and δ¹³C values (‰ PDB) of –4·8 and 3·9 respectively. These are lower oxygen and slightly higher carbon isotope values than averages for marine limestones in the Waulsortian (δ¹⁸O=–2·2, δ¹³C=3·7). Mean C and O isotope values of planar replacive dolomite are also distinct from those of nonplanar and saddle dolomite cement (–7·0 and 3·3; –7·4 and 2·4 respectively). Fluid inclusions indicate a complex history involving at least three chemically and thermally distinct fluids during dolomite cementation. The petrography and geochemistry of planar dolomites are consistent with an early diagenetic origin, possibly in equilibrium with modified Carboniferous sea water. Where the Waulsortian was exposed to hydrothermal fluids (70–280 °C), planar dolomite underwent a neomorphic recrystallization to a coarser crystalline, planar and nonplanar dolomite characterized by lower δ¹⁸O values. Void‐filling dolomite cement is isotopically similar to nonplanar, replacive dolomite and reflects a similar origin from hydrothermal fluids. This history of multiple stages of dolomitization is significantly more complex than earlier models proposed for the Irish Midlands and provides a framework upon which to test competing models of regional vs. localized fluid flow.     

Hydrothermal Dolomite in the Upper Sinian (Upper Proterozoic) Dengying Formation,East Sichuan Basin,China

Hydrothermal Dolomite （HTD） is present in the Upper Sinian （Upper Proterozoic） Dengying Formation, east Sichuan Basin, China. The strata are comprised by primary dolomite. The HTD has various textures, including zebra dolomite, subhorizontal sheet-like cavities filled by saddle dolomite and breccias cemented by saddle dolomites as well occur as a fill of veins and fractures. Also co-occur MVT type lead-zinc ores in the study area. The δ13C and δ18O isotopes of HTD in the Upper Sinian Dengying Formation are lighter than those of the host rocks, while STSr/86Sr is higher. The apparent difference in carbon, oxygen and strontium isotopes, especially the large difference in S7Sr/S6Sr isotopes ratio indicate crystallization from hot basinal and/or hydrothermal fluids. Saddle dolomite was precipitated at temperatures of 270-320℃. The diagenetic parasequences of mineral assemblage deposited in the Dengying Formation are： （1） dolomite host rock →sphalerite-galena-barite-fluorite; （2） dolomite host rock →saddle dolomite →quartz; （3） dolomite host rock →saddle dolomite→bitumen; （4） dolomite host rock →saddle dolomite →barite. The mean chemical composition of the host dolomite matrix and HTD didn＇t change much during hydrothermal process. The fluids forming the HTDs in the Dengying Formation were mixtures of freshwater from the unconformity at the top of Sinian, fluids from diagenetic compaction and hydrocarbon generation ＆ expulsion from the Lower Cambrian Niutitang Formation mudstones or the Doushantuo Formation silty mudstones, and hydrothermal fluids from the basement. The hydrocarbon reservoirs associated with the HTD were mostly controlled by the basement faults and fractures and karsting processes at the unconformity separating Sinian and Cambrian strata. The hydrocarbon storage spaces of HTD included dissolved cavities and intercrystalline pores. Dissolution cavities are extensive at the top of Dengying Formation, up to about 46m below the unconformity between Sinian an     

Dolomitization of the Devonian Swan Hills Formation, Rosevear Field, Alberta, Canada

The Swan Hills Formation (Middle-Upper Devonian) of the Western Canada Basin is host to several NW-SE-trending gas fields developed in massive replacement dolostone. One of these, the Rosevear Field, contains two major dolostone trends along opposing margins of a marine channel that penetrates into a platform-reef complex. Dolostones consist predominantly of branching and bulbous strdmatoporoid floatstones and rudstones with well-developed moldic and vuggy porosity. Replacement dolomite is coarsely crystalline (100-600 μm), inclusion-rich, composed of euhedral through anhedral crystals and has a blotchy to homogeneous red cathodoluminescence. Geochemically, replacement dolomite is characterized by (i) nearly stoichiometric composition (50.1-51.1 mol% CaCO₃), (ii) negative δ¹⁸O values (mean=-7.5‰, PDB) and (iii) variable ⁸⁷Sr/⁸⁶Sr ratios ranging from values similar to Late Devonian-Early Mississippian seawater (～0.7082) to radiogenic compositions comparable to saddle dolomite cements (>0.7100). Dolomitization began after widespread precipitation of early, equant calcite spar and after the onset of pressure solution, implying that replacement dolomite formed in a burial environment. Oxygen isotope data suggest that dolomite formed at 35-75°C, temperatures reached during burial in Late Devonian through Jurassic time, at minimum depths of 450 m. The linear NW-SE orientation of most dolomite fields in the Swan Hills Formation is suggestive of fault control on fluid circulation. Two models are proposed for fault-controlled circulation of dolomitizing fluids at the Rosevear Field. In the first, compaction-driven, updip fluid migration occurred in response to basin tilting commencing in the Late Palaeozoic. Deep basinal fluids migrating updip were focused into channel-margin sediments along fault conduits. The second model calls upon fault-controlled convective circulation of (i) warm Devonian-Mississippian seawater or (ii) Middle Devonian residual evaporitic brines. The overlap in ⁸⁷Sr/⁸⁶Sr and δ¹⁸O compositions, and similar cathodoluminescence properties between replacement and saddle dolomites provide evidence for neomorphism of some replacement dolomite. Quantitative modelling of Sr and O isotopes and Sr abundances suggests partial equilibration of some replacement dolomite with hot radiogenic brines derived during deep burial of the Swan Hills Formation in the Late Cretaceous-Palaeocene. Interaction of replacement dolomite with deep brines led to enrichment in ⁸⁷Sr while leaving δ¹⁸O similar to pre-neomorphism values.     

Zebra dolomitization as a result of focused fluid flow in the Rocky Mountains Fold and Thrust Belt, Canada

Discordant zebra dolomite bodies occur locally in the Middle Cambrian Cathedral and Eldon Formations of the Main Ranges of the Canadian Rocky Mountains Fold and Thrust Belt. They are characterized by alternating dark grey (a) and white (b) bands, forming an ‘abba’ diagenetic cyclicity. These bands developed parallel to both bedding and cleavage. Dark grey (a) bands consist of fine (< 300 μm) non-planar crystalline impure dolomite. The white (b) bands are composed of coarse (up to several millimetres) milky-white pure saddle dolomites (b1) which are often covered by pore-lining zoned dolomite (b2). The b phases often possess a saddle-shaped morphology. In contrast to the replacement origin of the a dolomite, the zoned b2 dolomite rims are interpreted as a cement formed in open cavities. The b1 dolomite is interpreted as the result of recrystallization with diagenetic leaching of non-carbonate components. All the zebra dolomites studied are (nearly) stoichiometric and are characterized by enriched Na and depleted Sr concentrations. Fe and Mn concentrations in these dolomites differ depending on the sample locality. Fluid inclusion data indicate that the dolomites formed from relatively hot (T_H = 130–200 °C), saline (20–23 wt% CaCl₂ eq.) fluids. A diagenetic high temperature origin is also supported by depleted δ¹⁸O values (−20 to −14‰ VPDB). A contribution of ⁸⁷Sr-enriched fluids is reflected in the ⁸⁷Sr/⁸⁶Sr values (0·7091–0·7123). Zebra dolomite development is explained by focused fluid flow, which exploited areas of structural weaknesses (e.g. basin-platform, rim areas, faults, etc.). Expulsion of hot basinal brines in a tectonically active regime generated overpressures, which explains the development of secondary porosity during zebra dolomitization as well as the intra-zebra fracturing at decimetre to micrometre scale.     

Meteoric diagenesis and dedolomite fabrics in precursor primary dolomicrite in a mixed carbonate–evaporite system

Dedolomitization is a common diagenetic process in shallow burial environments and is often associated with sulphates in mixed carbonate‐evaporite successions. In these settings, elevated Ca²⁺/Mg²⁺ ratios necessary for dedolomitization result from the dissolution of sulphate phases by the incursion of undersaturated groundwater. Reported dedolomite textures from other studies are varied, but the most prevalent is a rhombic texture interpreted to result from the partial to complete pseudomorphic replacement of secondary dolomite rhombs formed in the burial diagenetic realm. In this study of primary cryptocrystalline to finely crystalline dolomicrites in the Prairie Evaporite Formation of north‐eastern Alberta, dedolomitization has resulted in sutured to loosely packed mosaics of dedolomite that range from subhedral to distinctly euhedral (rhombic) crystal fabrics; however, no prior aggrading neomorphism producing dolomite rhombs is evident in the precursor dolomicrites. Non‐pseudomorphic dedolomitization of the dolomicrites results in textures that include rhombic dedolomite crystals with cloudy cores comprising remnant dolomicrite and clear rims. These textures are similar to those observed in the pseudomorphic dedolomitization of secondary dolomite rhombs. The Prairie Evaporite Formation of north‐eastern Alberta has experienced extensive karstification near the erosional margin of the sedimentary succession. Dedolomitization of dolomicrites occurs in marker beds within the Prairie Evaporite succession associated with evaporite karstification. Along with stratigraphic and petrographic considerations, stable isotope results support the interpretation of a shallow dedolomitization event influenced by meteoric waters derived from the basin margin. Negative δ ¹⁸O and low δ ¹³C values (averages of −13·6‰_VPDB and 0·5‰_VPDB, respectively) of the dedolomite, compared with those of the primary dolomicrite (averages of −6·0‰_VPDB and 1·2‰_VPDB, respectively), point to isotopically light diagenetic fluids. These results show that rhombic dedolomite textures can form through shallow, non‐pseudomorphic dedolomitization of dolomicrites by meteoric fluids in the presence of sulphates, with resulting textures that are similar to the pseudomorphic dedolomitization of secondary dolomite rhombs.     

Petrographic and geochemical evidence for the formation of primary, bacterially induced lacustrine dolomite: La Roda 'white earth' (Pliocene, central Spain)

Upper Pliocene dolomites (‘white earth’) from La Roda, Spain, offer a good opportunity to evaluate the process of dolomite formation in lakes. The relatively young nature of the deposits could allow a link between dolomites precipitated in modern lake systems and those present in older lacustrine formations. The La Roda Mg‐carbonates (dolomite unit) occur as a 3·5‐ to 4‐m‐thick package of poorly indurated, white, massive dolomite beds with interbedded thin deposits of porous carbonate displaying root and desiccation traces as well as local lenticular gypsum moulds. The massive dolomite beds consist mainly of loosely packed 1‐ to 2‐μm‐sized aggregates of dolomite crystals exhibiting poorly developed faces, which usually results in a subrounded morphology of the crystals. Minute rhombs of dolomite are sparse within the aggregates. Both knobbly textures and clumps of spherical bodies covering the crystal surfaces indicate that bacteria were involved in the formation of the dolomites. In addition, aggregates of euhedral dolomite crystals are usually present in some more clayey (sepiolite) interbeds. The thin porous carbonate (mostly dolomite) beds exhibit both euhedral and subrounded, bacterially induced dolomite crystals. The carbonate is mainly Ca‐dolomite (51–54 mol% CaCO₃), showing a low degree of ordering (degree of ordering ranges from 0·27 to 0·48). Calcite is present as a subordinate mineral in some samples. Sr, Mn and Fe contents show very low correlation coefficients with Mg/Ca ratios, whereas SiO₂ and K contents are highly correlated. δ¹⁸O‐ and δ¹³C‐values in dolomites range from ?3·07‰ to 5·40‰ PDB (mean=0·06, σ=1·75) and from ?6·34‰ to ?0·39‰ PDB (mean=?3·55, σ=1·33) respectively. Samples containing significant amounts of both dolomite and calcite do not in general show significant enrichment or depletion in ¹⁸O and ¹³C between the two minerals. The correlation coefficient between δ¹⁸O and δ¹³C for dolomite is extremely low and negative (r=?0·05), whereas it is higher and positive (r=0·47) for calcite. The lacustrine dolomite deposit from La Roda is interpreted mainly as a result of primary precipitation of dolomite in a shallow, hydrologically closed perennial lake. The lake was supplied by highly saturated HCO₃^?/CO₃^2? groundwater that leached dolomitic Mesozoic formations. Precipitation of dolomite from alkaline lake waters took place under a semi‐arid to arid climate. However, according to our isotopic data, strong evaporative conditions were not required for the formation of the La Roda dolomite. A significant contribution by bacteria to the formation of the dolomites is assumed in view of both petrographic and geochemical evidence.     

Reflux stratabound dolostone and hydrothermal volcanism-associated dolostone: a two-stage dolomitization model (Jurassic, Lebanon)

The Kesrouane Formation, which is characterized by pervasive dolomitization, has a stratigraphic thickness that exceeds 1000 m. It is part of a broad carbonate platform deposited in the Levant region and represents 60% of the Lebanese Jurassic rocks. Two genetically distinct dolostones are recognized within this unit: (1) fine‐to‐medium crystalline non‐planar grey dolostone; and (2) coarse‐crystalline planar beige dolostone. The former is stratabound and of Early Jurassic age (⁸⁷Sr/⁸⁶Sr = 0·707455). This dolostone locally exhi‐bits pseudomorphs of evaporite nodules, pointing towards seepage‐reflux dolomitization by hypersaline‐ to marine‐related fluids. Exposures of the coarse‐crystalline dolostone are associated with regional pre‐Cretaceous faults, along which Late Jurassic volcanics also occur. Sedimentological and diagenetic considerations coupled with microthermometry support a hydrothermal origin for this dolostone, with T_H values of primary inclusions between 50 and 80 °C. The related dolomitizing fluids are mesosaline (3·5–12·0 eq. wt% NaCl), and are believed to result from the mixing of evaporative brines and sea water. Dolomitization is thus believed to have occurred in two stages, whereby fluids invaded the host rocks first by seepage‐reflux, explaining the resulting Early Jurassic stratabound dolostone, and later through fracture flow along the faults associated with the Late Jurassic volcanism, explaining the coarse‐crystalline hydrothemal dolostone.     

The role and impact of freshwater–seawater mixing zones in the maturation of regional dolomite bodies within the proto Floridan Aquifer, USA

Palaeogene dolostones from the sub‐surface of Florida are ideal for the study of dolomite maturation because they record the early stages of a secondary dolomite overprint without destruction by later diagenetic overprints. Two distinct dolomite textures occur in the dolostones of the Upper Eocene Ocala and Lower Oligocene Suwannee limestones in west‐central Florida: a porous and permeable sucrosic dolomite and a less porous and relatively impermeable indurated non‐sucrosic dolomite. In both textures, the initial matrix dolomite is dully luminescent, whereas the secondary overprint is dominantly luminescent cement in the Suwannee and only neomorphic luminescent dolomite in the Ocala. The abundance of luminescent dolomite ranges from 2% to 38%, which translates to 1·6 km³ of material in the Suwannee and 13·5 km³ in the Ocala. Extrapolated trace‐element contents (Sr and Na) and δ¹⁸O values for the matrix and luminescent end‐members indicate a marine origin for the matrix dolomite in both units, and a freshwater–seawater mixing‐zone origin for the secondary luminescent dolomites. The δ¹⁸O values indicate that a saline, middle mixing‐zone environment overprinted the Suwannee but a more dilute mixing zone affected the Ocala. Fluid–fluid mixing models constrained by modern Floridan aquifer hydrochemistry and extrapolated ⁸⁷Sr/⁸⁶Sr values of the luminescent phases indicate that the mixing zones operated during the Late Miocene to Pliocene in the Ocala and affected the Suwannee in the Pliocene. The luminescent Suwannee mixing‐zone cement reduced porosity up to threefold and permeability up to 100‐fold, which converted many sucrosic dolomites to indurated dolomites. By contrast, the neomorphic luminescent Ocala dolomite did not have an appreciable impact on the maturations. Although freshwater–seawater mixing zones were not the sites of the initial dolomitization, the mixing‐zone environment did dramatically overprint and mature the regionally widespread dolomites of the Ocala and Suwannee limestones. This maturation occurred shortly after formation of the proto‐Floridan aquifer; the timing suggests the matrix dolomites were ‘ripe’ for alteration and that the only prerequisite for mixing‐zone dolomite is pre‐existing dolomite substrates to reduce kinetic barriers. In contrast to recent claims, the results of this study demonstrate that mixing zones can be effective in forming regionally significant amounts of secondary dolomite and influencing the petrophysical maturation of dolomite bodies.     

Massive dolomitization of a late Miocene carbonate platform: a case of mixed evaporative brines with meteoric water,Nijar, Spain

Late Miocene platform carbonates from Nijar, Spain, have been extensively dolomitized. Limestones are present in the most landward parts of the platform, in stratigraphically lower units and topographically highest outcrops, suggesting that dolomitizing fluids were derived from the adjacent Nijar Basin. The dolomite crystals range from <10 to ≈100 μm existing as both replacements and cements. Na, Cl and SO₄ concentrations in the dolomites range from 200 to 1700 p.p.m., 250–650 p.p.m., and 600–7000 p.p.m., respectively, comparable with other Tertiary and modern brine dolomite values, and also overlapping values from mixing-zone dolomites. Sr concentrations range between 50 and 300 p.p.m., and the molar Sr/Ca ratios of dolomitizing fluids are estimated to range between 7× seawater brine to freshwater ratios. The δ¹⁸O and δ¹³C of the dolomites range from ?1·0 to +4·2‰ PDB, and ?4·0 to +2·0‰ PDB, respectively. ⁸⁷Sr/⁸⁶Sr values (0·70899–0·70928) of the dolomites range from late Miocene seawater to values greater than modern seawater. Mixtures of freshwater with seawater and evaporative brines probably precipitated the Nijar dolomites. Modelled covariations of molar Sr/Ca vs. δ¹⁸O and Na/Ca vs. δ¹⁸O from these mixtures are consistent with those of the proposed Nijar dolomitizing fluids. Complete or partial dolomite recrystallization is ruled out by well preserved CL zoning, nonstoichiometry and quantitative water–rock interaction modelling of covariations of Na vs. Sr and δ¹⁸O vs. δ¹³C. The possibility of multiple dolomitization events induced by evaporative brines, seawater and freshwater, respectively, is consistent with mineral-mineral mixing modelling. The basin-derived dolomitizing brines probably mixed with freshwater in the Nijar Basin or mixed with fresh groundwater in the platform, and were genetically related either to deposition of the Yesares gypsum or the Feos gypsum. Dolomitization occurred during either the middle Messinian or the early upper Messinian. Nijar dolomitization models may be applicable to dolomitization of other late Miocene platform carbonates of the western Mediterranean. Moreover, the Nijar models may offer an analogue for more ancient evaporite-absent platform carbonates fringing evaporite basins.     

Formation of modern dolomite in hypersaline pans of the Western Cape,South Africa

Authigenic calcite and dolomite and biogenic aragonite occur in Holocene pan sediments in a Mediterranean‐type climate on the western coastal plain of South Africa. Sediment was analysed from a Late Pleistocene coastal pan at Yzerfontein and four Holocene inland pans ranging from brackish to hypersaline. The pans are between 0·08 and 0·14 km² in size. The δ¹⁸O_PDB values of carbonate minerals in the pan sediments range from ?2·41 to 5·56‰ and indicate precipitation from evaporative waters. Covariance of total organic content and percentage carbonate minerals, and the δ¹³C_PDB values of pan carbonate minerals (?8·85 to ?1·54‰) suggest that organic matter degradation is a significant source of carbonate ions. The precipitation of the carbonate minerals, especially dolomite, appears to be mediated by sulphate‐reducing bacteria in the black sulphidic mud zone found in the brine‐type hypersaline pans. The knobbly, sub‐spherical texture of the carbonate minerals suggests that the precipitation of the carbonate minerals, particularly dolomite, is related to microbial processes. The ⁸⁷Sr/⁸⁶Sr ratios of pan carbonate minerals (0·7108 to 0·7116) are slightly higher than modern sea water and indicate a predominantly sea water (marine aerosol) source for calcium (Ca²⁺) ions with relatively minor amounts of Ca²⁺ derived from the chemical weathering of bedrock.     

Dolomitization of the Capitan Formation forereef facies (Permian, west Texas and New Mexico): seepage reflux revisited

Abstract Interpretation of seepage reflux dolomitization is commonly restricted to intervals containing evaporites even though several workers have modelled reflux of mesosaline brines. This study looked at the partially dolomitized forereef facies of the Capitan Formation to test the extent of reflux dolomitization and evaluate the possible role of the near‐backreef mesosaline carbonate lagoon as an alternative source of dolomitizing fluids. The Capitan Formation forereef facies ranges from 10% to 90% dolomite. Most of the dolomite is fabric preserving and formed during early burial after marine cementation, before and/or during evaporite cementation and before stylolitization. Within the forereef facies, dolomite follows depositional units, with debris‐flow and grain‐flow deposits the most dolomitized and turbidity‐current deposits the least. The amount of dolomite increases with stratigraphic age and decreases downslope. Within the reef facies, dolomite is restricted to haloes around fractures and primary cavities except where the reef facies lacks marine cements and, in contrast, is completely dolomitized. This dolomite distribution supports dolomitization by sinking fluids. Oxygen isotopic values for fabric‐preserving dolomite (δ¹⁸O = 0·9 ± 1·0‰, N = 101) support dolomitization by sea water to isotopically enriched sea water. These values are closer to the near‐backreef dolomite (δ¹⁸O = 2·1 ± 0·7‰, N = 48) than the hypersaline backreef dolomite (δ¹⁸O = 3·6 ± 0·9‰, N = 11). Therefore, the fabric‐preserving dolomite is consistent with dolomitization during seepage reflux of mainly mesosaline brines derived from the near‐backreef carbonate lagoon. The occurrence of mesosaline brine reflux in the Capitan Formation has important implications for dolomitization in forereef facies and elsewhere. First, any area with a restricted carbonate lagoon may be dolomitized by refluxing brines even if there are no evaporite facies present. Secondly, such brines may travel significant distances vertically provided permeable pathways (such as fractures) are present. Therefore, the absence of immediately overlying evaporite or restricted facies is not sufficient cause to eliminate reflux dolomitization from consideration.     

Tectonically driven dolomitization of Cambrian to Lower Ordovician carbonates of the Quruqtagh area,north‐eastern flank of Tarim Basin,north‐west China

Dolomites occur extensively in Cambrian to Lower Ordovician carbonates in the Tienshan orogen of the Quruqtagh area, north‐east Tarim Basin, where thick (up to 1 km), dark grey lenticular limestones of semi‐pelagic to pelagic origin are prominent. The dolomites generally occur as beige, anastomosed geobodies that cross‐cut well‐stratified limestones. Based on detailed field investigations and petrographic examination, three types of matrix dolomite are identified: fine crystalline planar‐e (Md1), fine to medium crystalline planar‐s (e) (Md2) and fine to coarse crystalline non‐planar‐a (Md3) dolomites. One type of cement dolomite, the non‐planar saddle dolomite (Cd), is also common. The preferential occurrence of Md1 along low‐amplitude stylolites points to a causal link to pressure dissolution by which minor Mg ions were probably released for replacive dolomitization during shallow burial compaction. Type Md2, Md3 and Cd dolomites, commonly co‐occurring within the fractured zones, have large overlaps in isotopic composition with that of host limestone, implying that dolomitizing fluids inherited their composition from remnant pore fluids or were buffered by the formation water of host limestones through water–rock interaction. However, the lower δ¹⁸O and higher ⁸⁷Sr/⁸⁶Sr ratios of these dolomites also suggest more intense fluid–rock interaction at elevated temperature and inputs of Mg and radiogenic Sr from the host limestones with more argillaceous matter and possibly underlying Neoproterozoic siliciclastic strata. Secondary tensional faults and fractures within a compressional tectonic regime were probably important conduits through which higher‐temperature Mg‐rich fluids that had been expelled from depth were driven by enhanced tectonic compression and heating during block overthrusting, forming irregular networks of dolomitized bodies enclosed within the host limestones. This scenario probably took place during the Late Hercynian orogeny, as the Tarim block collided with Tienshan island arc system to the north and north‐east. Subsequent downward recharges of meteoric fluids into the dolomitizing aquifer probably terminated dolomitization as a result of final closure of the South Tienshan Ocean (or Palaeo‐Asian Ocean) and significant tectonic uplift of the Tienshan orogen. This study demonstrates the constructive role of notably tensional (or transtensional) faulting/fracturing in channelling fluids upward as a result of intense tectonic compression and heating along overthrust planes on the convergent plate margin; however, a relatively short‐lived, low fluid flux may have limited the dolomitization exclusively within the fractured/faulted limestones in the overthrust sheets.     

Dolomitization by hypersaline reflux into dense groundwaters as revealed by vertical trends in strontium and oxygen isotopes: Upper Muschelkalk,Switzerland

The Trigonodus Dolomit is the dolomitized portion of the homoclinal ramp sediments of the Middle Triassic Upper Muschelkalk in the south‐east Central European Basin. Various dolomitizing mechanisms, followed by recrystallization, have been previously invoked to explain the low δ¹⁸O, high ⁸⁷Sr/⁸⁶Sr, extensive spatial distribution and early nature of the replacive matrix dolomites. This study re‐evaluates the origin, timing and characteristics of the dolomitizing fluids by examining petrographic and isotopic trends in the Trigonodus Dolomit at 11 boreholes in northern Switzerland. In each borehole the ca 30 m thick unit displays the same vertical trends with increasing depth: crystal size increase, change from anhedral to euhedral textures, ultraviolet‐fluorescence decrease, δ¹⁸O_VPDB decrease from ?1·0‰ at the top to ?6·7‰ at the base and an ⁸⁷Sr/⁸⁶Sr increase from 0·7080 at the top to 0·7117 at the base. Thus, dolomites at the top of the unit record isotopic values similar to Middle Triassic seawater (δ¹⁸O_VSMOW = 0‰; ⁸⁷Sr/⁸⁶Sr = 0·70775) while dolomites at the base record values similar to meteoric groundwaters from the nearby Vindelician High (δ¹⁸O_VSMOW = ?4·0‰; ⁸⁷Sr/⁸⁶Sr = >0·712). According to water–rock interaction modelling, a single dolomitizing or recrystallizing fluid cannot have produced the observed isotopic trends. Instead, the combined isotopic, geochemical and petrographic data can be explained by dolomitization via seepage‐reflux of hypersaline brines into dense, horizontally‐advecting groundwaters that already had negative δ¹⁸O and high ⁸⁷Sr/⁸⁶Sr values. Evidence for the early groundwaters is found in meteoric calcite cements that preceded dolomitization and in fully recrystallized dolomites with isotopic characteristics identical to the groundwaters following matrix dolomitization. This study demonstrates that early groundwaters can play a decisive role in the formation and recrystallization of massive dolomites and that the isotopic and textural signatures of pre‐existing groundwaters can be preserved during seepage‐reflux dolomitization in low‐angle carbonate ramps.     

Epigenetic dolomitization of the Přaídolí formation (Upper Silurian), the Barrandian basin,Czech Republic: implications for burial history of Lower Paleozoic strata

Stratabound epigenetic dolomite occurs in carbonate facies of the Barrandian basin (Silurian and Devonian), Czech Republic. The most intense dolomitization is developed in bioclastic calcarenites within the transition between micritic limestone and shaledominated Přídolí and Lochkov formations deposited on a carbonate slope. Medium-crystalline (100–400 μm), inclusion-rich, xenotopic matrix dolomite (δ ¹⁸O=−4.64 to −3.40‰ PDB;δ ¹³C=+1.05 to +1.85‰ PDB) which selectively replaced most of the bioclastic precursor is volumetrically the most important dolomite type. Coarse crystalline saddle dolomite (δ ¹⁸O=−8.04 to −5.14‰ PDB;δ ¹⁸C=+0.49 to +1.49 PDB) which precipitated in fractures and vugs within the matrix dolomite represents a later diagenetic dolomitization event. In some vugs, saddle dolomite coprecipitated with petroleum inclusion-rich authigenic quartz crystals and minor sulfides which, in turn, were post-dated by semisolid asphaltic bitumen. The interpretation of the dolomitization remains equivocal. Massive xenotopic dolomite, although generally characteristic of a deeper burial setting, may have been formed by a recrystallization of an earlier, possibly shallow burial dolomite. Deeper burial recrystallization by reactive basinal pore fluids that presumably migrated through the more permeable upper portion of the Přídolí sequence appears as a viable explanation for this dolomitization overprint. Saddle dolomite cement of the matrix dolomite is interpreted as the last dolomitization event that occurred during deep burial at the depth of the oil window zone. The presence of saddle dolomite, the fluid inclusion composition of associated quartz crystals, and vitrinite paleogeothermometry of adjacent sediments imply diagenetic burial temperatures as high as 160°C. Although high geothermal gradients in the past or the involvement of hydrothermally influenced basinal fluids can account for these elevated temperatures, burial heating beneath approximately 3-km-thick sedimentary overburden of presumably post-Givetian strata, no longer preserved in the basin, appears to be the most likely interpretation. This interpretaion may imply that the magnitude of post-Variscan erosion in the Barrandian area was substantially greater than previously thought.     

A lost Tethyan evaporitic basin: Evidence from a Cretaceous hemipelagic meta‐selenite – red chert association in the Eastern Mediterranean realm

Ancient evaporite deposits are geological archives of depositional environments characterized by a long‐term negative precipitation balance and bear evidence for global ocean element mass balance calculations. Here, Cretaceous selenite pseudomorphs from western Anatolia (‘Rosetta Marble’) — characterized by their exceptional morphological preservation — and their ‘marine’ geochemical signatures are described and interpreted in a process‐oriented context. These rocks recorded Late Cretaceous high‐pressure/low‐temperature, subduction‐related metamorphism with peak conditions of 1·0 to 1·2 GPa and 300 to 400°C. Metre‐scale, rock‐forming radiating rods, now present as fibrous calcite marble, clearly point to selenitic gypsum as the precursor mineral. Stratigraphic successions are recorded along a reconstructed proximal to distal transect. The cyclical alternation of selenite beds and radiolarian ribbon‐bedded cherts in the distal portions are interpreted as a two type of seawater system. During arid intervals, shallow marine brines cascaded downward into basinal settings and induced precipitation. During more humid times, upwelling‐induced radiolarian blooms caused the deposition of radiolarite facies. Interestingly, there is no comparable depositional setting known from the Cenozoic world. Meta‐selenite geochemical data (δ¹³C, δ¹⁸O and ⁸⁷Sr/⁸⁶Sr) plot within the range of reconstructed middle Cretaceous seawater signatures. Possible sources for the ¹³C‐enriched (mean 2·2‰) values include methanogenesis, gas hydrates and cold seep fluid exhalation. Spatially resolved component‐specific analysis of a rock slab displays isotopic variances between meta‐selenite crystals (mean δ¹³C 2·2‰) and host matrix (mean δ¹³C 1·3‰). The Cretaceous evaporite‐pseudomorphs of Anatolia represent a basin wide event coeval with the Aptian evaporites of the Proto‐Atlantic and the pseudomorphs share many attributes, including lateral distribution of 600 km and stratigraphic thickness of 1·5 to 2·0 km, with the evaporites formed during the younger Messinian salinity crisis. The Rosetta Marble of Anatolia may represent the best‐preserved selenite pseudomorphs worldwide and have a clear potential to act as a template for the study of meta‐selenite in deep time.     

Stable isotope,petrological, and fluid inclusion studies of minor mineral deposits from the McArthur Basin: Implications for the genesis of some sediment‐hosted base metal mineralization from the Northern Territory

The geology, stable isotopes and fluid inclusions from mineralized and unmineralized Middle Proterozoic sequences of the McArthur Basin, Northern Territory, have been studied at Eastern Creek, Bulman Mines, Beetle Springs, and other localities in the McArthur Basin where disseminated sulphides in unmineralized black shales were available from drill core. At Eastern Creek, galena and minor chalcopyrite (δ³⁴S+3.6 to +11.2%o) occur in an evaporitic sedimentary sequence. Barite (δ³⁴S+18.4 to +24.7%o) also occurs, and saline brines are trapped along healed fractures in the barite. Pressure‐corrected trapping temperatures in the barite (95–138°C), and in vein dolomite (158–168°C) agree with temperature estimates from the degree of maturation of the sedimentary organic matter. The δ¹⁸O and δ¹³C_Co2 values of the mineralizing fluid were calculated to be +3.5 to +4.5%o and ‐2.7%o, respectively. Sedimentary dolomite has restricted δ¹³C and δ¹⁸O ranges, within the reported ranges for non‐mineralized Middle Proterozoic dolomite. An ore formation model developed for Eastern Creek, in which a basinal fluid at about 200°C carrying base metals and sulphide was released from underlying sediments during local fault movement, may be applicable to a number of other deposits. The mineralization deposited from these fluids occurs only below the pre‐Roper Group unconformity, implying that it may be older than the basal Roper Group. The δ³⁴S values of iron sulphides in fine grained black dolostones (not associated with mineral deposits) from the McArthur Basin were assessed in the light of the values found for sulphides in modern organic‐rich sedimentary environments. The data so obtained suggest that the considerable concentration of iron sulphide in the mineral deposits formed, at least in part, from heated basinal waters and that disseminated iron sulphides remote from mineralization also formed from a similar source.