Showing papers on "Ankerite published in 2000"

The Importance of Oxidized Brines for the Formation of Australian Proterozoic Stratiform Sediment-Hosted Pb-Zn (Sedex) Deposits

Abstract: A two-fold subdivision for stratiform sediment-hosted Pb-Zn (sedimentary exhalative, sedex) deposits is proposed, based on fundamental differences in the chemistry of the mineralizing brines. The type of sedimentary basin from which the ore fluids are derived, and the lithologies contained within the basin, control these differences in fluid chemistry. The two discrete brine types capable of transporting Zn and Pb are oxidized brines and reduced, acidic brines. McArthur-type deposits (e.g., McArthur River, Mount Isa, Hilton) precipitate from oxidized (SO42—predominant), acidic to near-neutral brines that evolve from sedimentary basins dominated by carbonates, evaporites, and hematitic sandstones and shales. Selwyn-type deposits (e.g., Sullivan, Rammelsberg, sedex deposits of the Selwyn basin) precipitate from acidic, reduced (H2S-predominant) connate brines that evolved in reduced siliciclastic and shale basins. Temperature decrease and dilution (fluid mixing), addition of H2S, and pH increase can all be effective depositional processes for Zn and Pb from reduced (Selwyn-type) brines. In contrast, sulfate reduction and/or addition of H2S (via fluid mixing or interaction with earlier formed pyrite) may be the important processes for sphalerite and galena deposition from oxidized (McArthur-type) brines. McArthur-type sedex deposits are intimately associated with siderite or ferroan carbonate alteration halos and most likely precipitate from lower temperature brines than Selwyn-type deposits. The redox state of the mineralized brines (sulfate or sulfide predominant) is important for controlling minor element associations in the two classes of sedex deposits. Weakly acidic to weakly alkaline oxidized brines can precipitate siderite but are incapable of carrying significant gold, tin, and barium in solution, and as such, McArthur-type deposits do not contain anomalous concentrations of these elements. Reduced, acid brines can carry high concentrations of barium, explaining the common association with barite in these deposits. If reduced sulfur concentrations were sufficient in the mineralizing brines, individual Selwyn-type deposits may contain anomalous or ore-grade gold. If the brines were highly reduced (pyrrhotite-stable), they may have carried high concentrations of tin (e.g., Sullivan). The lack of sulfide-bearing feeder systems in McArthur-type deposits and their common occurrence in Selwyn-type deposits probably also relate to the redox state of the brines. From a mineral exploration perspective, oxidized sedimentary brines are more likely to produce large tonnage Zn-Pb-Ag deposits that have siderite or ankerite alteration halos and commonly lack barite lenses and vent complexes. By contrast, deposits that form in reduced siliciclastic and shale-dominated basins are more likely to be lower tonnage and to contain barite, vent complexes and may have minor gold or tin credits.

Depositional and diagenetic controls on the reservoir quality of Lower Cretaceous Pendência sandstones, Potiguar rift basin, Brazil

Abstract: The quality of the reservoirs of the Lower Cretaceous Pendencia Formation of the Potiguar basin, northeastern Brazil, is directly controlled by depositional facies-related carbonate cementation and compaction. The study of the interplay of these processes in the reservoirs offers an opportunity to unravel the diagenetic patterns of clastic sequences in interior rifts and, in particular, the role of carbonate cementation in poorly understood continental systems. The Pendencia Formation is a thick sequence of fan-deltaic, fluvial-deltaic, turbiditic, and lacustrine sandstones, conglomerates, and shales deposited during the rift stage of the basin. The sandstones are predominantly arkoses (average Q49F40L11), with subordinate plutonic and volcaniclastic feldspathic litharenites. Compaction and cementation had similar importance in the destruction of porosity, with a dominance of cementation in the turbidites and of compaction in the fluvial deposits. Carbonate cementation in Pendencia reservoirs increases progressively with depth and is facies controlled. Eodiagenetic, nonferroan calcite I (d18OPDB –10.7 to –4.0‰; d13CPDB –17.5 to +8.5‰), mesodiagenetic rhombohedral ferroan dolomite/ankerite (d18OPDB –9.3 to –3.9‰; d13CPDB, –1.7 to +1.1‰), and ferroan calcite II (d18OPDB –17.2 to –6.8‰; d13CPDB –13.6 to +2.3‰) were precipitated at three distinct temperature intervals calculated from the d18O values: 21 to 58°C, 70 to 79°C, and 85 to 150°C, respectively. According to the d13C values, dissolved carbonate for calcite I was derived from oxidation and methanogenic fermentation of organic matter and from methane oxidation. Ferroan mesogenetic cements were derived from thermal decarboxylation of organic matter. The shales were a major source of dissolved (Begin page 1720) carbonate, as indicated by the isotopic similarity between their calcite (d13CPDB –0.2 to +1.8‰; 87Sr/86Sr ~ 0.719) and most cements in the sandstones and by the peripheral cementation along the contacts of interbedded sandstones. As a result of this cementation pattern, thin turbiditic and deltaic sandstone beds are pervasively cemented. The best reservoir quality potential is encountered in the partially cemented fluvial sandstones at moderate depths. Deltaic and turbiditic sandstones are more pervasively cemented by carbonate derived from the interbedded shales. Alluvial-fan conglomerates and sandstones were flushed by telogenetic meteoric waters close to the borders of the basin and to the proximity to the postrift unconformity. However, porosity enhancement was very limited, due to the precipitation of kaolinite and the intense compaction related to the compositionally immature detrital framework.

Ankerite Cementation in Deeply Buried Jurassic Sandstone Reservoirs of the Central North Sea

Abstract: Upper Jurassic Fulmar Formation shelf sandstones of the high-temperature, high-pressure Franklin and Elgin Fields (South Central Graben, North Sea) contain abundant disseminated and concretionary ankerite. In contrast, most Jurassic North Sea reservoirs contain only minor amounts of dispersed ankerite. Disseminated ankerite cement in the Franklin and Elgin Fields has a fairly uniform isotopic composition (18O -10 to -12.5o/oo PDB, 13C -3 to -5o/oo PDB). Ankerite concretions have 18O values similar to disseminated cements but a wider range of 13C values (+1 to -5.5o/oo PDB). They also have highly variable intergranular volumes, which (together with the 13C data) are interpreted as a combination of pore-filling cementation and in situ replacement of comminuted bioclastic debris by ankerite. Fluid-inclusion, 18O, and paragenetic evidence suggests that ankerite formed during deep burial (c. 3.5 to 4.5 km, 140-170°C), after the onset of overpressuring, but before hydrocarbon emplacement in the reservoirs. The regionally consistent 18O data suggest that ankerite formed via a temperature-influenced mechanism, and the relatively uniform 13C cement value indicates that organic matter and marine bioclastic carbonate contributed to the dissolved carbon reservoir in constant proportion. This can be explained by calcite dissolution in response to pH decrease during thermal breakdown of organic acids. Such acids were derived from adjacent mudrocks undergoing hydrocarbon maturation and clay-mineral transformations, and are likely to have been transported in pore fluids with Mg2+ and Fe2+. The presence of these cations in solution upon thermal decarboxylation is inferred to have stabilized ankerite at the expense of calcite. A relative paucity of ankerite in other Fulmar Formation reservoirs may reflect different sedimentological compositions (less bioclastic debris) and/or lower burial temperatures (less advanced decarboxylation).

Geochemical modelling of diagenetic reactions in a sub-arkosic sandstone

Abstract: A reaction path model was constructed in a bid to simulate diagenesis in the Magnus Sandstone, an Upper Jurassic turbidite reservoir in the Northern North Sea. UKCS. The model, involving a flux of source rock-derived CO 2 into an arkosic sandstone, successfully reproduced simultaneous dissolution of detrital K-feldspar and growth of authigenic quartz, ankerite and illite. Generation of CO 2 occurred before and during the main phase of oil generation linking source rock maturation with patterns of diagenesis in arkosic sandstones and limiting this type of diagenesis to the earlier stages of oil charging. Independent corroborative evidence for the model is provided by formation water geochemical data, carbon isotope data from ankerite and produced gas phase CO 2 and the presence of petroleum inclusions within the mineral cements. The model involves a closed system with respect to relatively insoluble species such as SiO 2 and Al 2 O 3 but is an open system with respect to CO 2 . There are up to seven possible rate-controlling steps including: influx of CO 2 , dissolution of K-feldspar, precipitation of quartz, ankerite and illite, diffusive transport of SiO 2 and Al 2 O 3 from the site of dissolution to the site of precipitation and possibly the rate of influx of Mg 2+ and Ca 2+ . Given the large number of possible controls, and contrary to modern popular belief, the rate of quartz precipitation is thus not always rate limiting.

Disseminated 'jigsaw piece' dolomite in Upper Jurassic shelf sandstones, Central North Sea: an example of cement growth during bioturbation?

Abstract: Unusual textural and chemical characteristics of disseminated dolomite in Upper Jurassic shelf sediments of the North Sea have provided the basis for a proposed new interpretation of early diagenetic dolomite authigenesis in highly bioturbated marine sandstones. The dolomite is present throughout the Franklin Sandstone Formation of the Franklin and Elgin Fields as discrete, non-ferroan, generally unzoned, subhedral to highly anhedral 'jigsaw piece' crystals. These are of a similar size to the detrital silicate grains and typically account for =5% of the rock volume. The dolomite crystals are never seen to form polycrystalline aggregates or concretions, or ever to envelop the adjacent silicate grains. They are uniformly dispersed throughout the sandstones, irrespective of detrital grain size or clay content. Dolomite authigenesis predated all the other significant diagenetic events visible in thin section. The dolomite is overgrown by late diagenetic ankerite, and bulk samples display stable isotope compositions that lie on a mixing trend between these components. Extrapolation of this trend suggests that the dolomite has near-marine delta18O values and low, positive delta13C values. The unusual textural and chemical characteristics of this dolomite can all be reconciled if it formed in the near-surface zone of active bioturbation. Sea water provided a plentiful reservoir of Mg and a pore fluid of regionally consistent delta18O. Labile bioclastic debris (e.g. aragonite, Mg-calcite) supplied isotopically positive carbon to the pore fluids during shallow-burial dissolution. Such dissolution took place in response to the ambient 'calcite sea' conditions, but may have been catalysed by organic matter oxidation reactions. Bioturbation not only ensured that the dissolving carbonate was dispersed throughout the sandstones, but also prohibited coalescence of the dolomite crystals and consequent cementation of the grain framework. Continued exchange of Mg2+ and Ca2+ with the sea-water reservoir maintained a sufficient Mg/Ca ratio for dolomite (rather than calcite) to form. Irregular crystal shapes resulted from dissolution, of both the dolomite and the enclosed fine calcitic shell debris, before ankerite precipitation during deep-burial diagenesis.

Détermination des conditions de précipitation des carbonates dans une zone d'altération du granite de Soultz (soubassement du fossé Rhénan, France) : l'enregistrement des inclusions fluides

Abstract: A fluid inclusion study was carried out in quartz and ankerite veins (drillhole EPS-1, depth 1 641 m) of the Soultz-sous-Forets granite (geothermal exchanger). Homogenisation temperatures in quartz and ankerite lie in a narrow similar range and are consistent with the present-day in situ temperature measured in the well. Ankerite and quartz have precipitated in the same conditions, which appear to be general at the site scale in all observed alteration facies. Quartz and ankerite are likely to precipitate at present. The data suggest that the fluid probably has a pulsated mode of circulation.

Isotope Geochemistry of Ankerite-Bearing Veins Associated with the Coeur d’Alene Mining District, Idaho

Abstract: Ankerite-bearing veins are common and widespread throughout and adjacent to the Coeur d’Alene mining district. Oxygen isotope ratios for quartz from the calcite-dominant and ankerite-dominant veins overlap and range from 12 to 18 per mil, and average 16.2 and 15.5 per mil, respectively. Previous studies of quartz from the Ag-producing veins give average ratios of 14.5 to 14.7 per mil. Ankerite oxygen and carbon isotope ratios range from 11.0 to 15.8 per mil and from –11.8 to +0.1 per mil, respectively. All but one of the calcite oxygen and carbon ratios lie in the ranges 13.0 to 16.0 per mil and –6.9 to –2.8 per mil. Quartz-ankerite fractionations lie between 1.1 and 2.9 per mil, except for samples from four massive ankerite veins, suggesting equilibrium at moderate temperatures (perhaps 270° to 320°C). Quartz-calcite oxygen fractionations are scattered, as are the quartz-ankerite fractionations from the massive ankerite veins, and clearly represent disequilibrium. The high oxygen ratios suggest that the hydrothermal fluids were strongly buffered, prior to entering the veins, by isotope exchange with the sedimentary Belt Supergroup rocks that form the vein walls, and perhaps by deeper Archean rocks. The 40Ar/39Ar age of fluorphlogopite from a calcite-dominant vein is Ž1014 Ma while an ankerite-dominant vein yielded a minimum age of 963 Ma; the latter sample shows partial resetting at 100 to 120 Ma. These ages fall within the bimodal distribution of hydrothermal ages previously reported for the Coeur d’Alene district, one Proterozoic age near 1 Ga and a younger age in Late Cretaceous-early Tertiary time. Initial 87Sr/86Sr ratios in the range 0.738 to 0.787 for several of the carbonate minerals require derivation from an old source. Local Belt wall rocks or deeper Archean schists and gneisses are likely possibilities.

Observations on sediment chemistry of the Slave River Delta, Northwest Territories, Canada.

Abstract: A study was conducted to examine sediment chemistry in the outer, mid and apex areas of the Slave River Delta, Northwest Territories, Canada. Delta sediments consisted primarily of quartz (60-82%) with lesser amounts of calcite (2-6%), feldspars (8-15%), micas (3-7%), montmorillonite (2-9%), dolomite/ankerite (2-4%) and kaolinite (3-5%). The mineralogy and major element composition was remarkably similar in the outer, mid and apex areas of the delta. Concentrations of Cu, Cd, Cr, Pb, As and Hg were significantly higher in the mid-delta. Elevated metal concentrations in this section of the delta are related to geomorphic and hydraulic controls, which cause selective sorting and fining of sediment in smaller mid-delta channels where vegetation is important for sediment trapping and metal cycling. Concentrations of Hg, Cd and As in all samples exceeded the Potential Effect Level of the Canadian Sediment Guidelines for the Protection of Aquatic Life.

Geochemistry of gold-bearing carbonates, Beaconsfield gold mine, Tasmania, Australia

Abstract: The major carbonates minerals in the Beaconsfield gold mine are calcite, dolomite and magnesian ankerite. Gold occurs in magnesian ankerite. Ankerite has high Ca, moderate Fe, Mg and Mn and low Na and Sr. The values of Ca and Mg and total amounts of Mg, Fe and Mn, and values of Fe, Mn and Sr are strongly correlated. This is due to the two layered structure of magnesian ankerite comprised of Mg, Fe and Mn carbonites and Ca carbonites. The elemental and isotopic compositions of magnesian ankerite are related to ordering, substitution of elements, salinity, redox potential, dissolution and reprecipitation, temperature, composition of fluids, environmental setting and gold mineralization. Ca, Sr and Na are derived from dissolution of Ordovician limestone. Mg is derived from leaching of ultramafics. Fe and Mn are leached from both ultramafics and clay minerals. Meteoric water was high in Ca, Fe, Mg and Mn concentrations and formed magnesian ankerite in a reducing burial environment. The heat source was possibly deep burial depth, tectonic hot spots and a few granite intrusions east of Beaconsfield. Gold and ankerite were precipitated in alkaline conditions. Gold and magnesium in ankerite are derived from the leaching of Cambrian ultramafic rocks during the Devonian by the passage of meteoric fluids through tectonically affected Ordovician carbonates.

Cementy węglanowe w formacjach piaszczystych dolnego karbonu na Pomorzu Zachodnim

Abstract: Piaskowce dolnego karbonu zawierają rozne spoiwa, m.in. cementy weglanowe: dolomit, kalcyt oraz w mniejszej ilości ankeryt i syderyt. Analizy EDS dolomitu wskazują na male domieszki Fe i Mn. Trwale izotopy wegla i tlenu w cementach dolomitowych w formacji z Gozdu grupują sie w przedziale δ13C od –2,13‰ do 5,52‰ i δ18O od –10,61‰ do –4,15‰ PDB. Przyjmując δ18O SMOW = 0‰, temperatury krystalizacji dolomitu wahają sie od 63°C do113°C. Wyniki badan inkluzji fluidalnych potwierdzają wnioski o podobnym przedziale temperatur wytrącania sie dolomitu podczas pogrzebania. W piaskowcach formacji z Drzewian procesy krystalizacji dolomitu zachodzily w odmiennych warunkach niz w formacji z Gozdu i w nizszych temperaturach. Wśrod kalcytow dominują nisko Fe odmiany oraz spotyka sie Mn–kalcyty. Kalcytyzacja w formacji z Gozdu miala miejsce w temperaturach: 47° – 61°C, podczas gdy kalcyt formacji z Łobzonki krystalizowal w temperaturze 70°C. Wiekszośc ankerytu jest związana z poźnym etapem diagenezy. W formacji z Łobzonki cement ten wytrącal sie w temperaturze 116°C. Syderoplezyty reprezentują wczesne- i poźnodiagenetyczne odmiany syderytu. Wszystkie dane sugerują, ze cementy obecne w formacjach piaskowcow tworzyly sie na roznych stopniach pogrzebania. Wplyw wysokich temperatur na tworzenie cementow weglanowych w najglebiej pogrzebanych skalach formacji z Łobzonki znajduje swoje odbicie w niskich wartościach δ18O ( od –11,01‰ do – 9,56‰ PDB) spowodowanych slabszą inkorporacją tlenu. CARBONATE CEMENTS IN SANDSTONE FORMATIONS OF THE LOWER CARBONIFEROUS IN THE WEST POMERANIA Summary The Lower Carboniferous sandstones contain different carbonate cements including: dolomite, calcite and minor amounts of ankerite and siderite. The EDS analysis of dolomite reveal a small admixture of Fe and Mn. The stable carbon and oxygen isotope determinations in dolomite cements of the Gozd Formation indicate δ13 C from -2.13‰ to 5.52‰and δ18O from -10.61‰ to -4.15‰PDB. Assuming that δ18O SMOW = O‰; the temperatures of dolomite crystallization ranged from 63° to 113°C. The results of the measurements o/fluid inclusions indicate precipitation temperatures of dolomite during burial diagenesis. The crystallization of dolomite in the Drzewiany Formation occurred in different conditions and at lower temperatures compared to the Gozd Formation. The low Fe-calcite dominated and Mn-calcite was also distinguished. The calcitization in theGozd Formation took place at the temperatures of 47°-61 °C, while that of the Łobzonka Formation crystallized at the temperature of 70°C. The majority of ankerite is connected with later diagenesis. This cement precipitated at 116°C in the Łobzonka Formation. Sideroplesites represent early and late diagenetic varieties of siderite. Al! these data suggest, that the cements present in the sandstone formations formed at different stages of burial. The influence of high crystallization temperature of carbonate cementsfound in the deepest buried rocks of the Łobzonka Formation is indicated by the low δ18O values (from -11.01‰ to -9.56‰ PDB), which was result ofweak oxygen incorporation.