Ankerite

About: Ankerite is a research topic. Over the lifetime, 859 publications have been published within this topic receiving 23960 citations.

Petrography of the Sokoman Iron Formation in Part of the Central Labrador Trough, Quebec, Canada

Abstract: The sedimentary textures of the Sokoman Iron Formation are similar to those of limestones; therefore the classification of textural elements in limestone (Folk) can be applied to the iron formation. The authors recognized the following textural elements: (a) femicrite (a matrix of iron silicate and carbonate) and matrix chert, both analogous to micrite; (b) cement chert and carbonatic cements; (c) aggregated particles, comparable to Folk9s allochems: pellets, intraclasts, ooliths, and pisolites. Shard textures are derived from ooliths and intraclasts by compaction. Rock types are defined by the combination of textural elements they contain. The iron formation suffered extensive epigenetic alteration. Dessiccation, shrinkage of silica-gel, compaction, and cementation are early diagenetic. Primocrystallization of quartz concludes the early diagenetic stage. It leads through a cryptocrystalline stage to the end phase of micropolygonal quartz. Quartz re-crystallized further during late diagenesis (burial metamorphism?) and again during a synkinematic to postkinematic regional metamorphism. Hematite dust is the oldest iron oxide. Much of the microscopic magnetite and specularite formed during early diagenesis. Migration of iron occurred; iron has been enriched in magnetite- or hematite-rich layers (“metallic” layers) during early diagenesis. Renewed crystallization of iron oxides occurred during the regional metamorphism. Microgranular siderite is perhaps primary. Porphyroblasts and glomero-porphyroblastic concretions of siderite, ankerite, and ferriferous calcite are early diagenetic. Minnesotaite and stilpnomelane are late diagenetic minerals. Riebeckite (and perhaps talc) formed during the regional metamorphism.

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.

Stratabound gold deposits in Archean banded iron-formation, Rhodesia

Abstract: Stratabound mineral deposits, particularly of gold and associated sulfides, are widely distributed within beds of banded iron-formation in the Archean greenstone belts of Rhodesia. The occurrence of gold at the Vubachikwe gold mine has been studied together with samples of mineralized iron-formation from eight other deposits. These are the Beehive, Camperdown, Connemara, Empress, Marvel, Nelly 404, Pickstone, and Sherwood Starr mines.At the Vubachikwe mine the orebodies occur in several thin beds of banded iron-formation which are interlayered with mafic and felsic aquagene tuffs. These rocks form part of a mafic-ultramafic volcanic assemblage characteristic of the Sebakwian Group and are overlain by basaltic and andesitic lavas and tuffs characteristic of the Bulawayan Group. The orebodies are stratiform and are confined to beds of sulfide and mixed sulfide-carbonate facies banded iron-formation, which consist of layers alternately rich in quartz (cherty), arsenopyrite, pyrrhotite, and ankerite. The gold is occluded within arsenopyrite grains and the average ore grade is 11 ppm. The petrographic and structural evidence indicates that the gold and sulfides are premetamorphic and predeformation in age.It is proposed that, for all the deposits studied, the gold, the sulfides, and the iron-formation are consanguineous. Experimental evidence on gold solubility and data from active fumarolic-stage geothermal areas support the view that thermal brines are capable of transporting significant quantities of gold and other metals to the surface as soluble complex ions. A volcanogenic model for the origin of banded iron-formation, the sulfides, and the gold is favored. Such deposits are considered to represent submarine chemical precipitates deposited from solutions extruded from subaqueous (and possibly subaerial) active fumaroles.There is a regional stratigraphic control to the distribution of these Stratabound mineral deposits, most of which occur in iron-formation associated with the lower mafic-ultramafic volcanic assemblage (Sebakwian Group).

Hydrothermal Alteration and Volatile Element Halos for the Rosebery K Lens Volcanic-Hosted Massive Sulfide Deposit, Western Tasmania

Abstract: A detailed study of alteration mineralogy, mineral chemistry, and lithogeochemistry in the host rocks surrounding the A-B and K lenses at the north end of the Rosebery mine has revealed a series of overlapping alteration halos with characteristic mineralogy and geochemistry. The study involved logging and sampling from nine drill holes spaced at varying distances from the A-B and K lenses. The stratiform Zn-Pb-Cu ore lenses have a sheetlike morphology and are hosted by a mixed sequence of rhyolitic to dacitic massive mediumgrained quartz-porphyritic pumice breccia, black mudstone, and crystal-rich volcaniclastic sandstone, overlying a thick homogeneous sequence of rhyolitic pumice breccia. The major alteration minerals at Rosebery are arranged in a complex series of zones passing away from the deposit-quartz-sericite zone, Mn carbonate zone, chlorite zone, and outer sericite zone. The chlorite zone is best developed in the immediate footwall below the copper-pyrite-rich ore lenses, whereas the strongest Mn carbonate alteration occurs in the immediate hanging-wall volcanics or lateral to the ore lenses. The outermost visible sericitic alteration extends about 60 to 100 m into the footwall, 10 to 20 m into the hanging wall, and over 500 m along the upper contact of the footwall pumice breccias. Thallium and antimony form the most extensive trace element halos related to the mineralization. Thallium forms a halo that extends 200 to 300 m into the overlying volcanics and 60 to 100 m into the footwall. Anomalously high thallium also occurs over 500 m along strike marking the contact between the footwall pumice breccias and the overlying volcaniclastic sandstones. Proximal to the ore lenses Tl and Sb values range from 10 to 100 ppm, compared to the halo zone where they vary from around 1 to 10 ppm. Studies of alteration mineral chemistry at Rosebery have revealed some important relationships that may assist exploration. The Mn content of alteration carbonate increases toward ore, both along strike and across strike. Close to ore, alteration carbonates contain >20 mole percent MnCO3 (kutnahorite, rhodochrosite, Mn siderite, and Mn ankerite), whereas at distances of 40 to 60 m across strike the mole percent MnCO3 in carbonate drops to below 10 percent. At greater than 80 m, the carbonates are Mn-poor calcites and are commonly located in synmetamorphic structures. White mica composition changes with stratigraphy and alteration assemblages and may be related to the mineralizing event, although this has not been convincingly demonstrated. Proximal white mica contains minor Ba substituting for octahedral Al. However, except for their Ba content, these phengetic white micas are similar to those found in nonmineralized areas of the Mount Read Volcanics. Sodic white mica with up to 0.35 Na/(Na + K) and a low phengite content (<0.5 Fe + Mg cations) occurs in a zone of volcanic sandstones and black slates overlying the ore deposit. This research has lead to the development of a series of proximal, medial, and distal vectors useful for both regional and mine-scale exploration. The most useful vectors, listed from proximal to distal, include Zn, Ba content of white mica, Na2O, K2O, Ishikawa alteration index (AI), S/Na2O, Ba/Sr, Mn content of carbonate, Tl, and Sb.