Paragenesis of Cretaceous to Eocene carbonate reservoirs in the Ionian fold and thrust belt (Albania): relation between tectonism and fluid flow

ABSTRACT This paper examines the diagenetic history of dual (i.e. matrix and fracture) porosity reservoir lithologies in Cretaceous to Eocene carbonate turbidites of the Ionian fold and thrust belt, close to the oil‐producing centre of Fier–Ballsh (central Albania). The first major diagenetic event controlling reservoir quality was early cementation by isopachous and syntaxial low‐Mg calcite. These cements formed primarily around crinoid and rudist fragments, which acted as nucleation sites. In sediments in which these bioclasts are the major rock constituent, this cement can make up 30% of the rock volume, resulting in low effective porosity. In strata in which these bioclasts are mixed with reworkedmicrite, isopachous/syntaxial cements stabilized the framework, and matrixporosity is around 15%. The volumetric importance of these cements, their optical and luminescence character (distribution and dull orange luminescence) and stable isotopic signal (δ¹⁸O and δ¹³C averaging respectively; ?0·5‰ VPDB and +2‰ VPDB) all support a marine phreatic origin. Within these turbidites and debris flows, several generations of fractures alternated with episodes of cementation. A detailed reconstruction of this history was based on cross‐cutting relationships of fractures and compactional and layer‐parallel shortening (LPS) stylolites. The prefolding calcite veins possess orange cathodoluminescence similar to that of the host rock. Their stable isotope signatures (δ¹⁸O of ?3·86 to ?0·85‰ VPDB and δ¹³C of – 0·14 to + 2·98‰ VPDB) support a closed diagenetic rock‐buffered system. A similar closed system accounts for the selectively reopened and subsequently calcite‐cemented LPS stylolites (δ¹⁸O of ?1·81 to ?1·14‰ VPDB and δ¹³C of +1·52 to +2·56‰ VPDB). Within the prefolding veins, brecciated host rock fragments and complex textures such as crack and seal features resulted from hydraulic fracturing. They reflect expulsion of overpressured fluids within the footwall of the frontal thrusts. After folding and thrust sheet emplacement, some calcite veins are still rock buffered (δ¹⁸O of ?0·96 to +0·2‰ VPDB and δ¹³C of +0·79 to +1·37‰ VPDB), whereas others reflect external (i.e. extraformational) and thus large‐scale fluid fluxes. Some of these veins are linked to basement‐derived fluid circulation or originated from fluid flow along evaporitic décollement horizons (δ¹⁸O around +3·0‰ VPDB and δ¹³C around +1·5‰ VPDB). Others are related to the maturation of hydrocarbons in the system (δ¹⁸O around ?7·1‰ VPDB and δ¹³C around +9·3‰ VPDB). An open joint system reflecting an extensional stress regime developed during or after the final folding stage. This joint system enhanced vertical connectivity. This open joint network can be explained by the high palaeotopographical position and the folding of the reservoir analogue within the deformational front. The joint system is pre‐Burdigalian in age based upon a dated karstified discordance contact. Sediment‐filled karst cavity development is linked to meteoric water infiltration during emergence of some of the structures. Despite its sediment fill, the karst network is locally an important contributor to reservoir matrix porosity in otherwise tight lithologies. Development of secondary porosity along bed‐parallel and bed‐perpendicular (i.e. layer‐parallel shortening) stylolites is interpreted as a late‐stage diagenetic event associated with migration of acidic fluids during hydrocarbon maturation. Development of porosity along the LPS system enhanced the vertical reservoir connectivity.     

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.     

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).     

Origin and migration of palaeofluids in the Upper Visean of the Campine Basin, northern Belgium

Upper Visean limestones in the Campine Basin of northern Belgium are intensively fractured. The largest and most common fractures are cemented by non-ferroan, dull brown-orange luminescent blocky calcite. First melting temperatures of fluid inclusions in these calcites are around -57°C, suggesting that precipitation of the cements occurred from NaCl-CaCl₂-MgCl₂ fluids. The final melting temperatures (T_m^ice) are between -5 and -33°C. The broad range in the T_m^ice data can be explained by the mixing of high salinity fluids with meteoric waters, but other hypotheses may also be valid. Homogenization temperatures from blocky calcite cements in the shelf limestones are interpreted to have formed between 45 and 75°C. In carbonates which were deposited close to and at the shelf margin, precipitation temperatures were possibly in the range 70-85°C and 72-93°C, respectively. On the shelf, the calcites have a δ¹⁸O around -9.3‰ PDB and they are interpreted to have grown in a fluid with a δ¹⁸O between −3.5 and +1.0‰ SMOW. At the shelf margin, blocky calcites (δ¹⁸O∼ - 13.5‰ PDB) could have precipitated from a fluid with a δ¹⁸O betweenn -4.0 and -1.1‰ SMOW. The highest oxygen isotopic compositions are comparable to those of Late Carboniferous marine fluids (δ¹⁸O= - 1‰ SMOW). The lowest values are more positive than a previously reported composition for Carboniferous meteoric waters (δ¹⁸O= -7‰ SMOW). Precipitation is likely to have occurred in marine-derived fluids, which mixed with meteoric waters sourced from near the Brabant Massif. Fluids with a similar negative oxygen isotopic composition and high salinity are actually present in Palaeozoic formations. The higher temperature range in the limestones near the shelf margin is explained by the upward migration of fluids from the ‘basinal’ area along fractures and faults into the shelf.     

Zoned calcites in Jurassic ammonite chambers: trace elements, isotopes and neomorphic origin

Zoned calcites were found in the phragmacone chambers of three Sonniniid ammonites from marine Middle Jurassic sandstones (Isle of Skye, U.K.). Each ammonite has a unique sequence of up to nine zones of calcite which fill or partially fill the chambers. Zones are defined by changes in the density of minute opaque inclusions and variation in trace-element composition. Proximal (early) calcites have undulose extinction and some exhibit the specific fabrics of fascicular-optic and radiaxial fibrous calcites. Microdolomite inclusions are found in one specimen. Early calcites, interpreted as replacements after a single isopachous fringe of acicular carbonate (probably high magnesium calcite), are succeeded by blocky ferroan calcite cement. In one specimen there are two distinct generations of calcite, in the others there is a continuous mosaic incorporating both early calcites and late cement. Isotopic composition of the early calcite zones demonstrates the initial importance of organic derived carbon (δ¹³C =— 26‰, δ¹⁸O ‰ O). Further cementation and mineralogical stabilization took place at increased temperatures and probably after modification of the pore water isotopic composition (calcites with δ¹³C =— O‰, δ¹⁸O～— 10‰). The distinctive fabrics and zonal patterns probably developed during the replacement of the precursor cement and are not primary growth features. Reversals in isotopic and trace element trends are believed to be related to the rate of neomorphic crystal growth and hence to the degree of exchange with external pore waters. Further increase in temperature, probably during Tertiary igneous activity, gave rise to the extremely light δ¹⁸O values of the late cements in the ammonite which had previously had least contact with external waters (cements with δ¹³C ～ O, δ¹⁸O ～— 20‰).     

Isotopic constraints on growth conditions of multiphase calcite–pyrite–barite concretions in Carboniferous mudstones

Carbonate concretions in the Lower Carboniferous Caton Shale Formation contain diagenetic pyrite, calcite and barite in the concretion matrix or in different generations of septarian fissures. Pyrite was formed by sulphate reduction throughout the sediment before concretionary growth, then continued to form mainly in the concretion centres. The septarian calcites show a continuous isotopic trend from δ¹³C=?28·7‰ PDB and δ¹⁸O=?1·6‰ PDB through to δ¹³C=?6·9‰ PDB and δ¹⁸O=?14·6‰ PDB. This trend arises from (1) a carbonate source initially from sulphate reduction, to which was added increasing contributions of methanogenic carbonate; and (2) burial/temperature effects or the addition of isotopically light oxygen from meteoric water. The concretionary matrix carbonates must have at least partially predated the earliest septarian cements, and thus used the same carbonate sources. Consequently, their isotopic composition (δ¹³C=?12·0 to ?10·1‰ PDB and δ¹⁸O=?5·7 to ?5·6‰ PDB) can only result from mixing a carbonate cement derived from sulphate reduction with cements containing increasing proportions of carbonate from methanogenesis and, directly or indirectly, also from skeletal carbonate. Concretionary growth was therefore pervasive, with cements being added progressively throughout the concretion body during growth. The concretions contain barite in the concretion matrix and in septarian fissures. Barite in the earlier matrix phase has an isotopic composition (δ³⁴S=+24·8‰ CDT and δ¹⁸O=+16·4‰ SMOW), indicating formation from near‐surface, sulphate‐depleted porewaters. Barites in the later septarian phase have unusual isotopic compositions (δ³⁴S=+6 to +11‰ CDT and δ¹⁸O=+8 to +11‰ SMOW), which require the late addition of isotopically light sulphate to the porewaters, either from anoxic sulphide oxidation (using ferric iron) or from sulphate dissolved in meteoric water. Carbon isotope and biomarker data indicate that oil trapped within septarian fissures was derived from the maturation of kerogen in the enclosing sediments.     

The speleothem record of climate variability in Southern Arabia

Uranium-series dated stalagmites from Oman indicate that pluvial conditions prevailed from 6.3 to 10.5, 78 to 82, 120 to 130, 180 to 200 and 300 to 330 kyr B.P.; all of these periods coincide with peak interglacials. Oxygen (δ¹⁸O) and hydrogen (δD) isotope ratios of speleothem calcite and fluid inclusions reveal the source of moisture and provide information on the amount of precipitation, respectively. δ¹⁸O and δD values of stalagmites deposited during peak interglacials vary between ?8 and ?4 ‰ (VPDB) and ?53 and ?20‰ (Vienna Standard Mean Ocean Water [VSMOW]) respectively, whereas modern stalagmites range from ?2.6 to ?1.1‰ in δ¹⁸O (VPDB) and ?7.6 and ?3.3‰ in δD (VSMOW), respectively. The growth and isotopic records indicate that during peak interglacial periods, the limit of the monsoon rainfall was shifted far north of its present location and each pluvial period was coinciding with an interglacial stage of the marine oxygen isotope record.     

Jurassic septarian concretions from NW Scotland record interdependent bacterial, physical and chemical processes of marine mudrock diagenesis

Septarian concretions in the Staffin Shales Formation (Kimmeridgian, Isle of Skye) allow controls on concretion rheology and septarian cracking to be investigated. Stratabound concretions consist of anhedral ferroan calcite microspar enclosing clay and minor pyrite. Intergranular volumes range from 77% to 88%, and calcite δ¹³C and δ¹⁸O values in most concretion bodies range from ?10·0‰ to ?17·3‰ and +0·3‰ to ?0·6‰ respectively, consistent with rapid and pervasive cementation in marine pore fluids. Septarian rupture occurred during incipient cementation, with a sediment volume reduction of up to 43%. Crack‐lining brown fibrous calcite records pore fluid re‐oxygenation during a depositional hiatus, followed by increasing Fe content and δ¹³C related to bacterial methanogenesis. Brown colouration results from an included gel‐like polar organic fraction that probably represents bacterially degraded biomass. A new hypothesis for concretion growth and septarian cracking argues that quasi‐rigid ‘proto‐concretions’ formed via binding of flocculated clays by bacterial extracellular polysaccharide substances (EPS). This provided rheological and chemical conditions for tensional failure, subcritical crack growth, volume contraction, calcite nucleation, and incorporation of degraded products into crack‐lining cements. Bacterial decay of EPS and syneresis of host muds provided internal stresses to initiate rupture at shallow burial. Development of septarian (shrinkage) cracks in muds is envisaged to require pervasive in situ bacterial colonization, and to depend on rates of carbonate precipitation versus EPS degradation and syneresis. Subsequent modification of septarian concretions included envelopment by siderite and calcite microspar, hydraulic fracturing associated with Cretaceous shallow burial or Palaeogene uplift; and cementation by strongly ferroan, yellow sparry calcite that records meteoric water invasion of the host mudrocks. An abundance of fatty acids in these spars indicates aqueous transport of organic breakdown products, and δ¹³C data suggest a predominantly methanogenic bicarbonate source. However, the wide δ¹⁸O range for petrographically identical cement (?1·3‰ to ?15·6‰) is difficult to explain.     

Changes in carbon and oxygen isotope composition during limestone diagenesis

The calcite fossils of the Derbyhaven Beds, Isle of Man, have δ¹³C values (+ 1·8 PDB) similar to modern, shallow-water marine skeletons, but the δ¹⁸O values (?6·1 PDB) are much lighter than modern skeletons. The light oxygen values indicate either re-equilibration with isotopically light water before cementation started, or Carboniferous sea water with δ¹⁸O of ?6‰. Aragonite dissolution was followed by precipitation of zoned calcite cement. In this cement, up to six intracrystalline zones, recognized in stained thin sections, show isotopic variation. Carbon varies from + 3-8 to + 1-2‰. and oxygen from ? 2-6 to ? 12-4‰. with decreasing age of the cement. This trend is attributed to increasing temperature and to isotopic evolution of the pore waters during burial. The zoned calcite is sequentially followed by dolomite and kaolinite cements which continue the trend towards light isotopic values. This trend is continued with younger, fault-controlled dolomite, and is terminated by vein-filling calcite and dolomite. The younger calcite, interpreted as a near-surface precipitate from meteoric waters, is unrelated to the older sequence of carbonates and has distinctly different carbon isotope ratios: δ¹³C ? 6-8‰.     

Carbon and Oxygen Isotope Compositions of Calcite and Rhodochrosite from Geothermal Exploratory Drills TH‐4 and TH‐6 near the Toyoha Deposit,Hokkaido, Japan

We studied calcite and rhodochrosite from exploratory drill cores (TH‐4 and TH‐6) near the Toyoha deposit, southwestern Hokkaido, Japan, from the aspect of stable isotope geochemistry, together with measuring the homogenization temperatures of fluid inclusions. The alteration observed in the drill cores is classified into four zones: ore mineralized zone, mixed‐layer minerals zone, kaolin minerals zone, and propylitic zone. Calcite is widespread in all the zones except for the kaolin minerals zone. The occurrence of rhodochrosite is restricted in the ore mineralized zone associated with Fe, Mn‐rich chlorite and sulfides, the mineral assemblage of which is basically equivalent to that in the Toyoha veins. The measured δ¹⁸O_SMOW and δ¹³C_PDB values of calcite scatter in the relatively narrow ranges from ?2 to 5‰ and from ?9 to ?5‰, respectively; those of rhodochrosite from 3 to 9‰ and from ?9 to ?5‰, excluding some data with large deviations. The variation of the isotopic compositions with temperature and depth could be explained by a mixing process between a heated surface meteoric water (100°C δ¹⁸O =?12‰, δ¹³C =?10‰) and a deep high temperature water (300°C, δ¹⁸O =?5‰, δ¹³C =?4‰). Boiling was less effective in isotopic fractionation than that of mixing. The plots of δ¹⁸O and δ¹³C indicate that the carbonates precipitated from H₂CO₃‐dominated fluids under the conditions of pH = 6–7 and T = 200–300°C. The sequential precipitation from calcite to rhodochrosite in a vein brought about the disequilibrium isotopic fractionation between the two minerals. The hydrothermal fluids circulated during the precipitation of carbonates in TH‐4 and TH‐6 are similar in origin to the ore‐forming fluids pertaining to the formation of veins in the Toyoha deposit.     

Low-Mg calcite marine cement in Cretaceous turbidites: origin, spatial distribution and relationship to seawater chemistry

Lower Cretaceous (Hauterivian) bioclastic sandstone turbidites in the Scapa Member (North Sea Basin) were extensively cemented by low-Mg calcite spars, initially as rim cements and subsequently as concretions. Five petrographically distinct cement stages form a consistent paragenetic sequence across the Scapa Field. The dominant and pervasive second cement stage accounts for the majority of concretions, and is the focus of this study. Stable-isotope characterization of the cement is hampered by the presence of calcitic bioclasts and of later cements in sponge spicule moulds throughout the concretions. Nevertheless, trends from whole-rock data, augmented by cement separates from synlithification fractures, indicate an early calcite δ¹⁸O value of+0·5 to -1·5‰ PDB. As such, the calcite probably precipitated from marine pore fluids shortly after turbidite deposition. Carbon isotopes (δ¹³C=0 to -2‰ PDB) and petrographic data indicate that calcite formed as a consequence of bioclastic aragonite dissolution. Textural integrity of calcitic nannoplankton in the sandstones demonstrates that pore fluids remained at or above calcite saturation, as expected for a mineral-controlled transformation. Electron probe microanalyses demonstrate that early calcite cement contains <2 mol% MgCO₃, despite its marine parentage. Production of this cement is ascribed to a combination of an elevated aragonite saturation depth and a lowered marine Mg²⁺/Ca²⁺ ratio in early Cretaceous ‘calcite seas’, relative to modern oceans. Scapa cement compositions concur with published models in suggesting that Hauterivian ocean water had a Mg²⁺/Ca²⁺ ratio of ≤1. This is also supported by consideration of the spatial distribution of early calcite cement in terms of concretion growth kinetics. In contrast to the dominant early cement, late-stage ferroan, ¹⁸O-depleted calcites were sourced outwith the Scapa Member and precipitated after 1–2 km of burial. Our results emphasize that bioclast dissolution and low-Mg calcite cementation in sandstone reservoirs should not automatically be regarded as evidence for uplift and meteoric diagenesis.     

Mineralisation of amethyst-bearing geodes in Ametista do Sul (Brazil) from low-temperature sedimentary brines: evidence from monophase liquid inclusions and stable isotopes

Fluid inclusion studies in combination with hydrogen, oxygen and sulphur isotope data provide novel insights into the genesis of giant amethyst-bearing geodes in Early Cretaceous Paraná continental flood basalts at Amestita do Sul, Brazil. Monophase liquid inclusions in colourless quartz, amethyst, calcite, barite and gypsum were analysed by microthermometry after stimulating bubble nucleation using single femtosecond laser pulses. The salinity of the fluid inclusions was determined from ice-melting temperatures and a combination of prograde and retrograde homogenisation temperatures via the density maximum of the aqueous solutions. Four mineralisation stages are distinguished. In stage I, celadonite, chalcedony and pyrite formed under reducing conditions in a thermally stable environment. Low δ³⁴S_V-CDT values of pyrite (?25 to ?32?‰) suggest biogenic sulphate reduction by organotrophic bacteria. During the subsequent stages II (amethyst, goethite and anhydrite), III (early subhedral calcite) and IV (barite, late subhedral calcite and gypsum), the oxidation state of the fluid changed towards more oxidising conditions and microbial sulphate reduction ceased. Three distinct modes of fluid salinities around 5.3, 3.4 and 0.3 wt% NaCl-equivalent characterise the mineralisation stages II, III and IV, respectively. The salinity of the stage I fluid is unknown due to lack of fluid inclusions. Variation in homogenisation temperatures and in δ¹⁸O values of amethyst show evidence of repeated pulses of ascending hydrothermal fluids of up to 80–90 °C infiltrating a basaltic host rock of less than 45 °C. Colourless quartz and amethyst formed at temperatures between 40 and 80 °C, while the different calcite generations and late gypsum precipitated at temperatures below 45 °C. Calculated oxygen isotope composition of the amethyst-precipitating fluid in combination with δD values of amethyst-hosted fluid inclusions (?59 to ?51?‰) show a significant ¹⁸O-shift from the meteoric water line. This ¹⁸O-shift, high salinities of the fluid inclusions with chloride-sulphate composition, and high δ³⁴S values of anhydrite and barite (7.5 to 9.9?‰) suggest that sedimentary brines from deeper parts of the Guaraní aquifer system must have been responsible for the amethyst mineralisation.     

Ferroan calcite replacement indicates former magnesian calcite skeletons

The replacement by ferroan calcite with preservation of the original structures can be used as a new criterion for identifying skeletons originally composed of high-magnesian calcite. This applies to bryozoa, rugose corals, echinoderms, many foraminifera, most ostracods, red algae, and serpulids. On the other hand, skeletons originally composed of low-magnesian calcite were never replaced by ferroan calcite, as shown by belemnites, brachiopods, and most of the pelecypods. Using this criterion, an original low-magnesian calcite composition is inferred for Tentaculites and some ostracods and foraminifera, whereas a previous high-magnesian calcite composition is inferred for trilobites, oligostegina and certain ooids. Chemical instability of high-magnesian calcite is suggested to be the driving force of the replacement by ferroan calcite. In most of the thirty-seven samples investigated, of Oligocene to Devonian age, the ferrous iron concentration of the interstitial fluid increased during diagenesis, as shown by well established sequences of cement A and B and fissure fill. This offers a relative time scale for diagenetic processes. Ferroan calcites contain up to 6 mol % FeCO₃ and up to 5 mol % MgCO₃. In this range of concentration, the distribution coefficients for Fe and Mg between calcite and solution at about 25°C are about 1 to 0-03, respectively, according to experiments. Possible sources of iron are iron oxides and hydroxides as well as clay minerals including glauconite. Though a submarine origin below the sediment surface is conceivable for ferroan calcite, there are serious limiting conditions such as low Eh and, at the same time, lack in sulphate-reducing bacteria. On the other hand, ferroan ‘dedolomite’, compositional zonality in individual ferroan calcite overgrowths, low δ¹⁸C and δ¹⁸O values, and low Mg concentrations point more to a meteoric-phreatic origin of many ferroan calcite occurrences.     

Fluid Inclusion, Rare-Earth Element and Stable Isotope Study of Carbonate Minerals from the Pongkor Epithermal Gold–Silver Deposit, West Java, Indonesia

The Pongkor gold–silver deposit is the largest low‐sulfidation epithermal precious metal deposit in Indonesia, and is of Pliocene age. The deposit consists of nine major subparallel quartz–adularia–carbonate veins with very low sulfide content. Vein infill records five paragenetic sequences, dominated by calcite in the early stage and quartz in the later stage of the hydrothermal evolution. Fluid inclusions in hydrothermal calcite and quartz of all stages indicate a temperature ranging from 180 to 220°C and a meteoric water origin (very low salinity close to 0 wt% NaCl equivalent). Carbon isotope data on calcite display a narrow range from ?6.5 to ?3.0‰δ¹³C. The oxygen isotope values have a wider range of +4.6 to +10.1‰δ¹⁸O. The broadly positive correlation of the δ¹³C versus δ¹⁸O plot suggests that the carbon species, which equilibrated during the formation of calcite, is dominated by H₂CO₃ not far from equilibrium with HCO₃^?. The abundance of rare earth and yttrium (REY) in carbonate samples is very low (>REY mostly <2 ppm). However, there is always a positive Eu anomaly, which indicates a deeper fluid reservoir at >250°C.     

C and O stable isotope variability in recent freshwater carbonates (River Krka, Croatia)

Three types of recent carbonate precipitates from the River Krka, Croatia, were analysed: (1) bulk tufa from four main cascades in a 34 km long section of the river flow through the Krka National Park; (2) a laminar stromatolite‐like incrustation formed in the tunnel of a hydroelectric power plant close to the lowest cascade; and (3) recent precipitates collected on artificial substrates during winter, spring and summer periods. Stable isotope compositions of carbon (δ¹³C) and oxygen (δ¹⁸O) in the carbonate and organic carbon (δ¹³C_org) were determined and compared with δ¹⁸O of water and δ¹³C of dissolved inorganic carbon (DIC). The source of DIC, which provides C for tufa precipitation, was determined from the slope of the line ([DIC]/[DIC₀]?1) vs. (δ¹³C‐DIC × ([DIC]/[DIC₀])) ( Sayles & Curry, 1988 ). The δ¹³C value of added DIC was ?13·6‰, corresponding to the dissolution of CO₂ with δ¹³C between ?19·5 and ?23·0‰ Vienna Pee Dee Belemnite (VPDB). The observed difference between the measured and calculated equilibrium temperature of precipitation of bulk tufa barriers indicates that the higher the water temperature, the larger the error in the estimated temperature of precipitation. This implies that the climatic signals may be valid only in tufas precipitated at lower and relatively stable temperatures. The laminar crust comprising a continuous record of the last 40 years of precipitation shows a consistent trend of increasing δ¹³C and decreasing δ¹⁸O. The lack of covariation between δ¹³C and δ¹⁸O indicates that precipitation of calcite was not kinetically controlled for either of the elements. δ¹³C and δ¹⁸O of precipitates collected on different artificial substrates show that surface characteristics both of substrates and colonizing biota play an important role in C and O isotope fractionation during carbonate precipitation.     

Fluid Inclusions and Hydrogen,Oxygen, Sulfur Isotopes of Nuri Cu‐W‐Mo Deposit in the Southern Gangdese,Tibet

The Nuri Cu‐W‐Mo deposit is located in the southern subzone of the Cenozoic Gangdese Cu‐Mo metallogenic belt. The intrusive rocks exposed in the Nuri ore district consist of quartz diorite, granodiorite, monzogranite, granite porphyry, quartz diorite porphyrite and granodiorite porphyry, all of which intrude in the Cretaceous strata of the Bima Group. Owing to the intense metasomatism and hydrothermal alteration, carbonate rocks of the Bima Group form stratiform skarn and hornfels. The mineralization at the Nuri deposit is dominated by skarn, quartz vein and porphyry type. Ore minerals are chalcopyrite, pyrite, molybdenite, scheelite, bornite and tetrahedrite, etc. The oxidized orebodies contain malachite and covellite on the surface. The mineralization of the Nuri deposit is divided into skarn stage, retrograde stage, oxide stage, quartz‐polymetallic sulfide stage and quartz‐carbonate stage. Detailed petrographic observation on the fluid inclusions in garnet, scheelite and quartz from the different stages shows that there are four types of primary fluid inclusions: two‐phase aqueous inclusions, daughter mineral‐bearing multiphase inclusions, CO₂‐rich inclusions and single‐phase inclusions. The homogenization temperature of the fluid inclusions are 280°C–386°C (skarn stage), 200°C–340°C (oxide stage), 140°C–375°C (quartz‐polymetallic sulfide stage) and 160°C–280°C (quartz‐carbonate stage), showing a temperature decreasing trend from the skarn stage to the quartz‐carbonate stage. The salinity of the corresponding stages are 2.9%–49.7 wt% (NaCl) equiv., 2.1%–7.2 wt% (NaCl) equiv._, 2.6%–55.8 wt% (NaCl) equiv. and 1.2%–15.3 wt% (NaCl) equiv., respectively. The analyses of CO₂‐rich inclusions suggest that the ore‐forming pressures are 22.1 M Pa–50.4 M Pa, corresponding to the depth of 0.9 km–2.2 km. The Laser Raman spectrum of the inclusions shows the fluid compositions are dominated in H₂O, with some CO₂ and very little CH₄, N₂, etc. δD values of garnet are between ?114.4‰ and ?108.7‰ and δ¹⁸O_H2O between 5.9‰ and 6.7‰; δD of scheelite range from ?103.2‰ to ?101.29‰ and δ¹⁸O_H2O values between 2.17‰ and 4.09‰; δD of quartz between ?110.2‰ and ?92.5‰ and δ¹⁸O_H2O between ?3.5‰ and 4.3‰. The results indicate that the fluid came from a deep magmatic hydrothermal system, and the proportion of meteoric water increased during the migration of original fluid. The δ³⁴S values of sulfides, concentrated in a rage between ?0.32‰ to 2.5‰, show that the sulfur has a homogeneous source with characteristics of magmatic sulfur. The characters of fluid inclusions, combined with hydrogen‐oxygen and sulfur isotopes data, show that the ore‐forming fluids of the Nuri deposit formed by a relatively high temperature, high salinity fluid originated from magma, which mixed with low temperature, low salinity meteoric water during the evolution. The fluid flow through wall carbonate rocks resulted in the formation of layered skarn and generated CO₂ or other gases. During the reaction, the ore‐forming fluid boiled and produced fractures when the pressure exceeded the overburden pressure. Themeteoric water mixed with the ore‐forming fluid along the fractures. The boiling changed the pressure and temperature, oxygen fugacity, physical and chemical conditions of the whole mineralization system. The escape of CO₂ from the fluid by boiling resulted in scheelite precipitation. The fluid mixing and boiling reduced the solubility of metal sulfides and led the precipitation of chalcopyrite, molybdenite, pyrite and other sulfide.     

The diagenesis of the late Dinantian Derbyshire-East Midland carbonate shelf, central England

Carbonate cements in late Dinantian (Asbian and Brigantian) limestones of the Derbyshire carbonate platform record a diagenetic history starting with early vadose meteoric cementation and finishing with burial and localized mineral and oil emplacement. The sequence is documented using cement petrography, cathodoluminescence, trace element geochemistry and C and O isotopes. The earliest cements (Pre-Zone 1) are locally developed non-luminescent brown sparry calcite below intrastratal palaeokarsts and calcretes. They contain negligible Fe, Mn and Sr but up to 1000 ppm Mg. Their isotopic compositions centre around δ¹⁸O =?8.5‰, δ¹³C=?5.0‰. Calcretes contain less ¹³C. Subsequent cements are widespread as inclusion-free, low-Mg, low-Fe crinoid overgrowths and are described as having a‘dead-bright-dull’cathodoluminescence. The‘dead’cements (Zone 1) are mostly non-luminescent but contain dissolution hiatuses overlain by finely detailed bright subzones that correlate over several kilometres. Across‘dead'/bright subzones there is a clear trend in Mg (500–900 ppm), Mn (100–450 ppm) and Fe (80-230 ppm). Zone 1 cements have isotopic compositions centred around δ¹⁸O =?8.0‰ and δ¹³C=?2.5‰. Zone 2 cement is bright, thin and complexly subzoned. It is geochemically similar to bright subzones of Zone 1 cements. Dull Zone 3 cement pre-dates pressure dissolution and fills 70% or more of the pore space. It generally contains little Mn, Fe and Sr but can have more than 1000 ppm Mg, increasing stratigraphically upwards. The δ¹⁸O compositions range from ?5.5 to ?15‰ and the δ¹³C range is ?1 to + 3.2^0/00. Zone 4 fills veins and stylolite seams in addition to pores. It is synchronous with Pb, Ba, F ore mineralization and oil migration. Zone 4 is ferroan with around 500 ppm Fe, up to 2500 ppm Mg and up to 1500 ppm Mn. Isotopic compositions range widely; δ¹⁵O =?2.7 to ?9‰ and δ¹³C=?3.8 to+2.50‰. Unaltered marine brachiopods suggest a Dinantian seawater composition around δ¹⁵O = 0‰ (SMOW), but vital isotopic effects probably mask the original δ¹³C (PDB) value. Pre-Zone 1 calcites are meteoric vadose cements with light soil-derived δ¹³C and light meteoric δ¹⁸O. An unusually fractionated‘pluvial’δ¹⁵O(SMOW) value of around — 6‰ is indicated for local Dinantian meteoric water. Calcrete δ¹⁸O values are heavier through evaporation. Zone 1 textures and geochemistry indicate a meteoric phreatic environment. Fe and Mn trends in the bright subzones indicate stagnation, and precipitation occurred in increments from widespread cyclically developed shallow meteoric water bodies. Meteoric alteration of the rock body was pervasive by the end of Zone 1 with a general resetting of isotopic values. Zone 3 is volumetrically important and external sources of water and carbonate are required. Emplacement was during the Namurian-early Westphalian by meteoric water sourced at a karst landscape on the uplifted eastern edge of the Derbyshire-East Midland shelf. The light δ¹⁸O values mainly reflect burial temperatures and an unusually high local heat flow, but an input of highly fractionated hinterland-derived meteoric water at the unconformity is also likely. Relatively heavy δ¹³C values reflect the less-altered state of the source carbonate and aquifer. Zone 4 is partly vein fed and spans burial down to 2000 m and the onset of tectonism. Light organic-matter-derived δ¹³C and heavy δ¹⁸O values suggest basin-derived formation water. Combined with textural evidence of geopressures, this relates to local high-temperature ore mineralization and oil migration. Low water-to-rock ratios with host-rock buffering probably affected the final isotopic compositions of Zone 4, masking extremes both of temperature and organic-matter-derived CO₂.     

Ordovician low- to intermediate-Mg calcite marine cements from Sweden: marine alteration and implications for oxygen isotopes in Ordovician seawater

Petrography demonstrates the presence of three types of fibrous calcite cement in buildup deposits of the Kullsberg Limestone (middle Caradoc), central Sweden. Translucent fibrous calcite has intrinsic blue luminescence (CL) indicative of pure calcite. This cement has 2–5 mol% MgCO₃, low Mn and Fe (≤ 100 p.p.m.), and is considered to be slightly altered to unaltered, primary low- to intermediate-Mg calcite. Grey turbid fibrous calcite has variable but generally low MgCO₃ content (most analyses <2 mol%) and variable CL response, with Mn and Fe concentrations up to 1200 and 500 p.p.m., respectively. The heterogeneous characteristics of this variety of fibrous calcite are caused by diagenetic alteration of a translucent fibrous calcite precursor. Light-brown turbid fibrous calcite has low MgCO₃ (near 1 mol%) and variable Mn (up to 800 p.p.m.) and Fe (up to 500 p.p.m.) concentrations, with an abundance of bright luminescent patches, which formed during alteration caused by reducing diagenetic fluids. The δ¹³C and δ¹⁸O values of all fibrous calcite form a tight field (δ¹³C=1·7 to 3·1‰ PDB, δ¹⁸O= ? 2·6 to ? 4·1‰ PDB) compared with fibrous calcite isotope values from other units. Fibrous calcite δ¹⁸O values are larger than adjacent meteoric or burial cements, which have δ¹⁸O δ ? 8‰ PDB. Consequently, most diagenetic alteration of Kullsberg fibrous calcite is interpreted to have occurred in the marine diagenetic realm. First-generation equant and bladed calcite cements, which pre-date fibrous calcite, are interpreted as unaltered, low-Mg calcite marine cements based on δ¹³C and δ¹⁸O data (δ¹³C = 2·3 to 2·7‰ PDB, δ¹⁸O= ? 2·8 to ? 3·5‰ PDB). Unlike fibrous cement, which reflects global sea water chemistry, first-generation equant and bladed calcite are indicators of localized modification of seawater chemistry in restricted settings. Kullsberg abiotic marine cements have larger δ¹⁸O values than most Caradoc marine precipitates from equatorial Laurentia. Positive Kullsberg δ¹⁸O values are attributed to lower seawater temperatures and/or slightly elevated salinity on the Baltic platform relative to seawater from which other marine precipitates formed.     

Evolution of palaeofluids at the Variscan thrust front in eastern Belgium

The geochemical evolution of the fluids migra- ting at the Variscan thrust front in eastern Belgium has been investigated by a petrographic, mineralogical and geoche-mical study of ankerite, quartz and ferroan calcite veins hosted by lower Devonian rocks. Three vein generations have been recognized. The first generation consists of quartz, chlorite and ankerite filling pre- to early Variscan extensional fractures. The second generation is present as shear veins of Variscan age, and contains quartz, chlorite and ferroan calcite. The third generation consists of ankerite filling post-Variscan fractures. The oxygen and carbon isotopic composition of the two ankerite phases and of the ferroan calcites are respectively between –16.4 and –11.4‰ PDB between –17.8 and –1.7‰ PDB. This range is greater than that of calcite nodules in the lower Devonian siliciclastic sediments (δ¹⁸O= –15.6 to –11.1‰ PDB and δ¹³C= –13.4 to –10.2‰ PDB). This suggests precipitation of the carbonate veins from a fluid which was at most only partly isotopically buffered by the calcite nodules in the host rock. The calculated oxygen isotopic composition of the ambient fluid from which the calcite veins formed is between +7.8 and +10.0‰ SMOW. Two main fluid types have been recognized in fluid inclusions in the quartz and carbonates. The first fluid type is present as secondary fluid inclusions in the first and second vein generations. The fluid has a salinity of 0.5–7.2 eq. wt.% NaCl and a high, but variable, homogenization temperature (Th=124–188°C). Two origins can be proposed for this fluid. It could have been expelled from the lower Devonian or could have been derived from the metamorphic zone to the south of the area studied. Taking into account the microthermometric and stable-isotope data, and the regional geological setting, the fluid most likely originated from metamorphic rocks and interacted with the lower Devonian along its migration path. This is in agreement with numerical simulations of the palaeofluid and especially the palaeotempera-ture field, which is based on chlorite geothermometry and vitrinite reflectance data. The second fluid type occurs as secondary inclusions in the shear veins and as fluid inclusions of unknown origin in post-Variscan ankerite veins. Therefore, it has a post-Variscan age. The inclusions are characterized by a high salinity (18.6–22.9 eq. wt.% CaCl₂). The composition of the fluid is similar to that which caused the development of Mississippi Valley-type Pb–Zn deposits in Belgium.     

'Pseudobreccias' revealed as calcrete mottling and bioturbation in the Late Dinantian of the southern Lake District, UK

Two types of ‘pseudobreccia’, one with grey and the other with brown mottle fabrics, occur in shoaling‐upward cycles of the Urswick Limestone Formation of Asbian (Late Dinantian, Carboniferous) age in the southern Lake District, UK. The grey mottle pseudobreccia occurs in cycle‐base packstones and developed after backfilling and abandonment of Thalassinoides burrow systems. Burrow infills consist of a fine to coarse crystalline microspar that has dull brown to moderate orange colours under cathodoluminescence. Mottling formed when an early diagenetic ‘aerobic decay clock’ operating on buried organic material was stopped, and sediment entered the sulphate reduction zone. This probably occurred during progradation of grainstone shoal facies, after which there was initial exposure to meteoric water. Microspar calcites then formed rapidly as a result of aragonite stabilization. The precipitation of the main meteoric cements and aragonite bioclast dissolution post‐date this stabilisation event. The brown mottle pseudobreccia fabrics are intimately associated with rhizocretions and calcrete, which developed beneath palaeokarstic surfaces capping cycle‐top grainstones and post‐date all depositional fabrics, although they may also follow primary depositional heterogeneities such as burrows. They consist of coarse, inclusion‐rich, microspar calcites that are always very dull to non‐luminescent under cathodoluminescence, sometimes with some thin bright zones. These are interpreted as capillary rise and pedogenic calcrete precipitates. The δ¹⁸O values (?5‰ to ?8‰, PDB) and the δ¹³C values (+2‰ to ?3‰, PDB) of the ‘pseudobreccias’ are lower than the estimated δ¹⁸O values (?3‰ to ?1‰ PDB) and δ¹³C values of (+2‰ to +4‰ PDB) of normal marine calcite precipitated from Late Dinantian sea water, reflecting the influence of meteoric waters and the input of organic carbon.