Ankerite

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

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.

Depositional Environment of the Paleoproterozoic Yuanjiacun Banded Iron Formation in Shanxi Province, China

Abstract: The Paleoproterozoic (~2.38–2.21 Ga) Yuanjiacun banded iron formation (BIF), located in Shanxi Province, is a Superior-type BIF in the North China craton. This BIF is within a metasedimentary rock succession of the Yuanjiacun Formation, in the lower Luliang Group, which has undergone lower greenschist-facies metamor phism. Iron oxide (magnetite and hematite), carbonate, and silicate facies are all present within the iron-rich layers. The eastward transition from carbonate- into oxide-facies iron formations is accompanied by a change in mineralogical composition from siderite in the west through magnetite-ankerite and magnetite-stilpnomelane assemblages in the transition zone to magnetite and then hematite in the east. These distinct lateral facies are also observed vertically within the BIF, i.e., the iron mineral assemblage changes upsection from sider ite through magnetite into hematite-rich iron formation. The oxide-facies BIF formed near shore, whereas carbonate (siderite)- and silicate-facies assemblages formed in deeper waters. Based on detailed analyses of these variations on a basinal scale, the BIF precipitated during a transgressive event within an environment that ranged from deep waters below storm wave base to relatively shallow waters. The BIF samples display distinctively seawater-like REEs + Y profiles that are characterized by positive La and Y anomalies and HREEs enrichment relative to LREEs in Post-Archean Australian shale-normalized diagrams. Consistently positive Eu anomalies are also observed, which are typical of reduced, high-temperature hydrothermal fluids. In addition, slightly negative to positive Ce anomalies, and a large range in ratios of light to heavy REEs, are present in the oxide-facies BIF. These characteristics, in combination with consistently positive δ 56Fe values, suggest that deposition of the BIF took place along the chemocline where upwelling of deep, anoxic, iron- and silicarich hydrothermal fluids mixed with shallower and slightly oxygenated seawater. The ankerite displays highly depleted δ13C values and the carbonate-rich BIF has a high content of organic carbon, suggesting dissimilatory Fe(III) reduction of a ferric oxyhydroxide precursor during burial of biomass deposited from the water column; that same biomass was likely tied to the original oxidation of dissolved Fe(II). The fact that the more ferric BIF facies formed in shallower waters suggests that river-sourced nutrients would have been minimal, thus limiting primary productivity in the shallow waters and minimizing the organic carbon source necessary for reducing the hematite via dissimilatory Fe(III) reduction. By contrast, in deeper waters more proximal to the hydrothermal vents, nutrients were abundant, and high biomass productivity was coupled to increased carbon burial, leading to the deposition of iron-rich carbonates. The deposition of the Yuanjiacun BIF during the onset of the Great Oxidation Event (GOE; ca. 2.4–2.2 Ga) confirms that deep marine waters during this time period were still episodically ferruginous, but that shallow waters were sufficiently oxygenated that Fe(II) oxidation no longer needed to be tied directly to proximal cyanobacterial activity.

Authigenic carbonates related to active seepage of methane-rich hot brines at the Cheops mud volcano, Menes caldera (Nile deep-sea fan, eastern Mediterranean Sea)

Abstract: On the passive margin of the Nile deep-sea fan, the active Cheops mud volcano (MV; ca. 1,500 m diameter, ~20–30 m above seafloor, 3,010–3,020 m water depth) comprises a crater lake with hot (up to ca. 42 °C) methane-rich muddy brines in places overflowing down the MV flanks. During the Medeco2 cruise in fall 2007, ROV dives enabled detailed sampling of the brine fluid, bottom lake sediments at ca. 450 m lake depth, sub-surface sediments from the MV flanks, and carbonate crusts at the MV foot. Based on mineralogical, elemental and stable isotope analyses, this study aims at exploring the origin of the brine fluid and the key biogeochemical processes controlling the formation of these deep-sea authigenic carbonates. In addition to their patchy occurrence in crusts outcropping at the seafloor, authigenic carbonates occur as small concretions disseminated within sub-seafloor sediments, as well as in the bottom sediments and muddy brine of the crater lake. Aragonite and Mg-calcite dominate in the carbonate crusts and in sub-seafloor concretions at the MV foot, whereas Mg-calcite, dolomite and ankerite dominate in the muddy brine lake and in sub-seafloor concretions near the crater rim. The carbonate crusts and sub-seafloor concretions at the MV foot precipitated in isotopic equilibrium with bottom seawater temperature; their low δ13C values (–42.6 to –24.5‰) indicate that anaerobic oxidation of methane was the main driver of carbonate precipitation. By contrast, carbonates from the muddy lake brine, bottom lake concretions and crater rim concretions display much higher δ13C (up to –5.2‰) and low δ18O values (down to –2.8‰); this is consistent with their formation in warm fluids of deep origin characterized by 13C-rich CO2 and, as confirmed by independent evidence, slightly higher heavy rare earth element signatures, the main driver of carbonate precipitation being methanogenesis. Moreover, the benthic activity within the seafloor sediment enhances aerobic oxidation of methane and of sulphide that promotes carbonate dissolution and gypsum precipitation. These findings imply that the coupling of carbon and sulphur microbial reactions represents the major link for the transfer of elements and for carbon isotope fractionation between fluids and authigenic minerals. A new challenge awaiting future studies in cold seep environments is to expand this work to oxidized and reduced sulphur authigenic minerals.

Carbonates of the magnesite–siderite series from four carbonatite complexes

Abstract: Carbonates of the magnesite-siderite series have been found and analysed in carbonatites from the Lueshe, Newania, Kangankunde, and Chipman Lake complexes. This series has been represented until now only by a few X-ray identifications of magnesite and three published analyses of siderite and breunnerite (magnesian siderite). Most of the siderite identified in carbonatites in the past has proved to be ankerite, but the new data define the complete solid-solution series from magnesite to siderite. They occur together with dolomite and ankerite and in one rock with calcite. The magnesites, ferroan magnesites and some magnesian siderites may be metasomatic/hydrothermal in origin but magnesian siderite from Chipman Lake appears to have crystallized in the two-phase calcite + siderite field in the subsolidus CaCO3-MgCO3-FeCO3 Textural evidence in Newania carbonatites indicates that ferroan magnesite, which co-exists with ankerite, is a primary liquidus phase and it is proposed that the Newania carbonatite evolved directly from a Ca-poor, Mg-rich carbonatitic liquid generated by partial melting of phlogopite-carbonate peridotite in the mantle at pressures >32 kbar.