Showing papers on "Ankerite published in 1989"

Relationships between sulphate reduction and oxidation of organic compounds to carbonate diagenesis, hydrocarbon accumulations, salt domes, and metal sulphide deposits

Abstract: The association of dissolved sulphate species and organic compounds generally is thermodynamically unstable in diagenetic environments. Unless kinetically inhibited, reduction-oxidation reactions take place between dissolved sulphate and organic compounds. These reactions take place bacterially at temperatures up to about 85°C (equivalent to vitrinite reflectance of approximately 0.2–0.3% RO) and abiologically/thermochemically at temperatures in excess of about 100–135°C (equivalent to vitrinite reflectance of approximately >1.5% RO). Thereby, sulphate is reduced to sulphide (in the form of H2S, HS−, and S2−) and to elemental sulphur, and organic compounds are oxidized to solid bitumen, bicarbonate, and/or carbon dioxide. A number of minerals may precipitate if the respective cations are available in sufficient amounts: these include calcite, dolomite, siderite, ankerite, pyrite, sphalerite, galena, and other minerals. Secondary and tertiary porosity, as well as solution-collapse brecciation, may occur due to the dissolution of solid sulphates and/or carbonates. Products of redox-reactions between sulphate and organic compounds are nearly ubiquitous in nature. In particular, hydrocarbon accumulations, salt domes, and several types of metal sulphide deposits (e.g. Mississippi-Valley-Type, base metal sulphide, and roll-type uranium deposits) are related genetically through these redox-reactions. Most commonly the reaction products occur in close association with carbonate and evaporite rocks that have been invaded by crude oil and/or natural gas. In some cases, however, long-distance migration of sulphate and/or metals is indicated.

Are the iron carbonate minerals, ankerite and ferroan dolomite, like siderite, important in paleomagnetism?

Abstract: Magnetic, Mossbauer effect, and thermal properties have been evaluated for specimens of the mineral ankerite and are contrasted with similar measurements reported for the mineral siderite to determine if ankerite (and ferroan dolomite), like siderite, may be important in paleomagnetism as a producer of secondary, spurious remanent moments as the result of oxidation in air. Our data indicate that ankerite and siderite both have comparable thermal stabilities and that ankerite does break down to form iron oxides at temperatures as low as 250°C during thermal demagnetization. We conclude from our data that any iron carbonate, siderite, ankerite, or ferroan dolomite, may readily oxidize in air to maghemite or hematite thus producing a spurious remanent moment (RM) that can dominate the magnetic properties of the specimen. Thermal demagnetization of ankerite or siderite at lower temperatures (below 250°C) does not appear to produce such a spurious RM. As might be expected, the quantity of secondary iron oxides produced by oxidation is limited by the iron content in the carbonate crystal lattice. Oxidized siderite produces significantly greater amounts of iron oxides than equivalent percentages of ankerite, which in turn produces more iron oxides than equal amounts of ferroan dolomite in a given sample.

A fluid inclusion and stable-isotope study of the Tom Ba-Pb-Zn deposit, Yukon Territory, Canada

Abstract: The Tom Ba-Pb-Zn deposit, Yukon Territory, Canada, is a stratiform, exhalative, massive sulfide deposit, which is hosted by carbonaceous mudstones, turbiditic sand-banded mudstones, and debris-flow, chert-pebble conglomerates of the Upper Devonian Earn Group. The deposit consists of a lens of massive ore (pyrrhotite-galena-sphalerite-ankerite-chalcopyrite) that passes vertically upward and laterally into laminated ore (sphalerite-galena-pyrite-barite-chert). The footwall units host an area of ankerite and quartz veining, which is interpreted to represent the feeder zone for the mineralizing fluids. The ankerite veins are believed to be related to mineralization, whereas fluid inclusion and stable isotope data suggest that the quartz veins formed during one of the postmineralization deformation events.Homogenization temperatures of fluid inclusions from ankerite and quartz range from 157 degrees to 335 degrees C. This range is interpreted to be due to variable leakage from inclusions during deformation and metamorphism and is not representative of temperatures of original entrapment. However, fluid inclusion type and salinity data can be used to distinguish between the mineralizing fluid and the fluid present during later deformation. Ankerite was deposited from an aqueous solution with a salinity of 9.1 + or - 3.4 equiv wt percent NaCl; tectonic quartz was deposited from fluids with high and variable CO 2 (+ or -CH 4 , N 2 ) contents and salinities of 4.8 + or - 1.3 equiv wt percent NaCl. There are distinctive intergrain and intersample variations in the CO 2 contents of inclusions.A lack of variation in the oxygen isotope compositions of mudstones and <2-mu m mineral fractions, plus the absence of an alteration halo around the mineralization, implies that the extent of interaction between the mineralizing fluid and the country rock was small. This may have been the result of a low water/rock ratio, a high fluid-flow rate, and/or a low temperature (<150 degrees C) for the mineralizing fluid.The mineral assemblages and paragenesis suggest that early mineral deposition (fine-grained quartz and ankerite) occurred in a sulfur-deficient environment. Local production of reduced sulfur eventually allowed the precipitation of sulfides. The mineral zonation exhibited by the massive and laminated ore is typical of hot, reduced hydrothermal fluids mixing with cooler, more oxygenated water.

Conditions of gold-bearing arsenopyrite crystallization in the Villeranges Basin, Marche-Combrailles shear zone, France; a mineralogical and fluid inclusion study

Abstract: The Au-As-Sb deposits from the Marche-Combrailles (northwestern part of the French Massif Central district) are spatially associated to the Marche-Combrailles shear zone. Host rocks are either Visean volcano-sedimentary tuffs or formations of the Paleozoic basement presenting a greenschist facies assemblage which resulted from extended hydrothermal metamorphism. The strong macro- and microfracturing that affected the Villeranges basin as well as the surrounding formations is related to late tectonic reactivation of the Marche-Combrailles shear zone around 300-315 Ma which favored intense circulation of hydrothermal fluids. Solubilization, transport, and deposition of Au, As, and Sb occurred together with specific rock alteration along faulted zones on distances ranging from several centimeters to a few decimeters. Wall rocks are affected by complete bleaching resulting from chlorite replacement by phengite (T = 240 degrees + or - 30 degrees C) followed by complete replacement of the previous minerals by an illite-quartz-ankerite-pyrite assemblage. Gold is carried in a combined state with arsenopyrite, occurring either within a diffuse network of quartz + ankerite veinlets or as isolated crystals in the altered tuff. Arsenopyrite crystals are characterized by important enrichments either at their periphery or within microcrack networks affecting their whole mass. Mass balance calculations on fresh and altered rocks show that during the ore stage, hydrothermal fluids contributed H 2 O, CO 2 , and S (Au, Sb) to authigenic minerals and leached almost completely Na, Ba, Sr, Si, Fe, Ca, Mg, Al, Ti, and K exhibit similar contents in fresh and altered rocks and are redistributed among the authigenic minerals.Ore fluids are characterized by an aqueous composition, a rather low salinity, and temperatures ranging from 160 degrees to 200 degrees C. Ore deposition occurred at a low pH, indicated by the complete alteration of the host rocks into a quartz-illite association, and at a low f (sub O 2 ) intermediate between the f (sub O 2 ) fixed by the hematite-magnetite and Ni-NiO oxygen buffers. Gold deposition probably resulted from a destabilization of Au bisulfide or thioarsenide complexes during the sulfide crystallization.Thus, the Au-As-Sb ores in the Villeranges area result from a single hydrothermal stage and occur during a late stage of brittle deformation, significantly posterior to the continental basin formation and the acid volcanism. Ore fluid circulation is linked mostly to the fluid convection produced by the abnormal heat flow characterizing the Marche-Combrailles shear zone.

Diagenesis of the Upper Proterozoic Siliciclastic Sediments of the Taoudeni Basin (West Africa) and Relation to Diabase Emplacement

Abstract: The predominantly siliciclastic Upper Proterozoic sediments of the Taoudeni basin have been buried to depths of 2.5 to 3 km. In a normal geothermal gradient 25°C/km), maximum burial temperatures of the section studied would be no higher than 100°C. However, detailed petrographic and fluid inclusion data of sandstones and shales from three areas in Mauritania and Mall along the western side of the basin indicate two major diagenetic events: an earlier low temperature (< 70°C) quartz cementation of the sandstones and a later high temperature (135-170°C diagenesis that affected both shales and sandstones. These latter temperatures are too high to have occurred simply in response to burial. The sediments are cut by numerous Jurassic diabase intrusions, which crop out es ecially commonly in the south and the northeast. The emplacement of these diabases was related to the opening of the South Atlantic ocean. Contact metamorphism and hydrothermal alteration are visible near intrusive contacts. In western Mall (southern part of basin) where the largest exposed sill in the basin occurs, at 50 to 60 km from any outcropping massive diabase, the late diagenetic paragenesis consists of micas, illite, chlorite, kaolinite, siderite and ankerite. High-temperature diagenesis is also observed in Adrar of Mauritania (northwestern part of basin) where outcropping diabases are extremely rare. There, calcite cement is associated with illite and chlorite. Fluid inclusions indicate formation temperatures of 135°C for the ankerite and 170°C for the calcite. Thus, in spite of a thin sedimentary pile, the Taoudeni basin experienced a high-temperature diagenesis. We propose that this resulted from circulation of hot fluids, generated during the diabase emplacement, over extensive areas within the basin.

Shallow-level, late-stage gold mineralisation in Sawyers Creek, Shotover valley, northwest Otago, New Zealand

Abstract: A set of mainly west and northwest striking normal faults in Sawyers Creek, Shotover valley, crosscuts post-Oligocene Moonlight Generation structures. These faults have provided access for hydrothermal fluids to shallow crustal levels. Silicification and carbonate alteration of breccias and wall rock has occurred, to form reef zones up to 3 m wide. Gold is found with pyrite and arscnopyrite within the siliceous portions of the reef, especially at fault intersections. The hydrothermal fluid was water with minor dissolved CO2 Mineralisation occurred at about 150-200°C at 2–3 km depth. Wall-rock interaction with the fluid caused replacement of chlorite by ankeritic carbonate. The composition of the ankerite is governed by the composition of the chlorite it replaced. Fluid reaction with chlorite caused the fluid to become progressively more alkaline which eventually resulted in precipitation of gold. Similar fluid activity, in similar structural settings, is found throughout northwest Otago.

Geochemistry of the Matchless metamorphosed massive sulfide deposit, South West Africa/Namibia; wall-rock alteration during submarine ore-forming processes

Abstract: Matchless is one of several metamorphosed massive copper-zinc sulfide deposits associated with the Matchless amphibolite belt in the upper Proterozoic Damara orogen (SWA/Namibia). In the footwall of the ore shoots are pyrite-bearing quartz-muscovite-(chlorite) schists with interlayered bands of massive pyrite-quartz rock and a distinctive biotite-rich marker schist. In the hanging wall of the ore shoots the wall rock consists of chloritic schists containing variable proportions of quartz, ankerite, and calcic amphibole, bands of magnetite quartzite, calc-silicate rock, and amphibolite. This rock association is regarded as representing a premetamorphic stratigraphic sequence.The initial deposit was formed in a subsiding trough in which active turbidite sedimentation formed the precursor rocks of the Kuiseb schists. Sedimentation was interrupted by submarine volcanic activity which produced the basaltic protolith of the Matchless amphibolite. Metal sulfides were precipitated from convecting hydrothermal fluids generated by the reaction of seawater with subsea-floor basalts and clastic sediments. This process was accompanied by an intense hydrothermal alteration of the basaltic wall rock, leading to silicification, chloritization, sulfidization, and in part also, to carbonatization. Geochemical changes in the basalts involved depletion in Na, Ca, and Sr and enrichment of Rb, Ba, Fe, S, and the base metals Cu, Zn, and Pb. Among the immobile elements, Y, and to a lesser extent, Ti, seems to be depleted. Compared to the unaffected turbiditic sediments, the altered sedimentary wall rocks of the deposit are depleted in Na, Ca, Sr, Rb, Ni, and the immobile elements Ce, Zr, Ti, V, and Y and enriched in Fe, S, Ba, Cu, and locally, Zn and Pb. These changes can be explained by wall rock-fluid interaction and/or by synsedimentary chemical precipitation of metal sulfides on the sea floor.

The sedimentology and geochemistry of the sub-Clowne cycle (Westphalian B) of north-east Derbyshire, U.K.

Abstract: SUMMARY The upper part of the cycle between the Manton “ Estheria ” Band and the Clowne Marine Band horizon (upper Westphalian B of the British Pennine Basin) contains a variety of noteworthy features, including a local ankerite/dolomite, an acid tonstein, and a seatearth which varies regionally and locally in colour and grain size. The relationships of these are examined, together with coal thickness variations and contemporary channels; variations in the tonstein’s gamma-ray anomaly are assessed in relation to sediment grade and proximity to channels. The geochemistry of the carbonate, tonstein, and seatearth are described; the combination of geochemical analysis and detailed study of field relationships provides a picture of water-table fluctuations in the Clowne sequence, probably eustatically controlled.

Source processes for Archean Au–Ag vein deposits: evidence from lithophile-element systematics of the Hollinger–McIntyre and Buffalo Ankerite deposits, Timmins

Abstract: Potassic alteration domains of Archean greenstone belt lode gold deposits, typified by the Hollinger–McIntyre and Buffalo Ankerite mines, Timmins, are characterized by systematic partitioning between different groups of incompatible elements such that Al, Ga, Th, U, Ti, and V are decoupled from K, Rb, Ba, Li, Cs, and Tl. The former group of elements has concentrations and interelement ratios in alteration domains that reflect host-rock control, implying approximately isochemical behaviour. In contrast, the lithophile elements K, Rb, and Ba are generally coenriched and are linearly correlated over three orders of magnitude in abundance, where K/Rb = 230–380 (r = 0.92–0.99) and K/Ba = 34–97 (r = 0.82–0.99), values close to average crustal ratios of 285 and 36, respectively. K/Rb and K/Ba ratios trend towards higher values with respect to increasing concentrations of Rb and Ba, possibly because of mixing between host rock and hydrothermal reservoirs of the lithophile elements. Cs and Tl contents average 1.4 ...

Diagenetic evolution of Cretaceous-Tertiary turbidite reservoirs, Campos Basin, Brazil

Abstract: Three sandstone turbidite sequences of the Campos basin (offshore, state of Rio de Janeiro, Brazil) were petrologically studied: (1) the Albian-Cenomanian Namorado Sandstone on the Macae Formation, (2) the Upper Cretaceous Carapebus Member of the Campos Formation, and (3) the Eocene Carapebus Member of the Campos Formation. The sequences represent deep marine deposits consisting mostly of massive sandstones rather than classical turbidites, indicating sand-rich submarine fans were the main depositional system of these sequences. The framework composition of the sandstones averages for quartz, feldspar, and lithics are Q60F40Ltr for the Cretaceous rocks and Q71F29Ltr for the Eocene rocks, plotting granitic r ck fragments at the feldspar pole. The main diagenetic phases that affected the sandstones studied were (1) development of a clay matrix due to compaction of rip-up mud clasts, (2) partial replacement of the matrix by opal, (3) precipitation of small pyrite framboids, (4) widespread direct precipitation or replacement of different materials by calcite, (5) intense generation of secondary porosity, (6) localized kaolinite development, (7) minor precipitation of quartz and feldspar overgrowths, (8) development of dolomite, ferroan dolomite, and ankerite, and (9) replacement of different materials with minor direct precipitation of late pyrite. Geologic and geochemical evidence lets us infer the main processes that controlled diagenetic transformations and mass transfer within the sequence studied. The principal source of carbonate cements was pressure solution of the underlying Macae Formation. Most of the diagenetic evolution of the sandstones was apparently controlled by the relative balance between the activity of CO2 and carboxylic acid species in the formation waters, both related to organic matter transformations within adjacent marine shales. The timing of hydrocarbon (generated in deeper lacustrine shales) migration to the reservoirs was an important factor in preserving diagenetically enhanced porosity.

Mössbauer Spectroscopic Study of the Decomposition Mechanism of Ankerite in CO2 Atmosphere

Abstract: Mrssbauer spectroscopy has been used to identify the iron-containing products that are formed during the thermal decomposition of ankerite in a CO 2 atmosphere. The decomposition takes place in three stages and evidence is produced to show that the first stage involves decomposition of ankerite to yield a periclase-wustite solid solution, (Mg,Fe)O, along with calcite and CO2, the periclase-wustite then reacting with CO 2 to produce magnesioferrite (MgFe204) and CO. In the second stage the magnesioferrite and calcite react to produce periclase and dicalcium ferrite. The third stage does not involve reaction of Fe-containing phases and corresponds to the decomposition of calcite to CaO. KEYWOR9S: ankerite, decomposition, Mrssbauer spectroscopy, CO 2 atmosphere.

Mossbauer and chemical investigations of mudrocks

Abstract: The oxidation state and mineralogical distribution of Fe in two different shale sequences have been studied by Mossbauer spectroscopy and chemical methods. Kimmeridge Clay Formation sediments proved to be richer in total Fe than Gulf Coast shales. In such sediments, Fe may be present in clay minerals (Fe(II) in chlorite, Fe(III) in illite and illite/smectite), pyrite, and ferroan carbonates (siderite, dolomite and ankerite). Pyrite accounts for a much greater proportion of the total Fe in the Kimmeridge Clay samples in which it is difficult to reconcile chemical data with Mossbauer data. There is major doublet overlap of Fe(III) in silicates with Fe(II) in pyrite and spectra cannot be satisfactorily deconvoluted. This would appear to be a fundamental limitation for simple applications. In the pyrite-poor Gulf Coast material, however, chemical and spectroscopic evaluations of overall valence state are much more consistent. Confidence in both determinative techniques is generated, and useful information documenting progressive reduction and redistribution of Fe with burial is obtained.

The effects of substitution in the Dolomite-Ferroan Dolomite-Ankerite series as illustrated by FTIR

Abstract: Determination de la composition chimique. Etude par spectrometrie IR de la substitution Fe, Fe+Mn