Ankerite

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

Diagnostic magmatic features in carbonatites: implications for the origins of dolomite- and ankerite-rich carbonatites

Abstract: Carbonatites are especially susceptible to subsolidus modifications, because of the low strength and high ductility of calcite and the solubility of many carbonatite minerals in aqueous fluids. As a result, magmatic textures and mineral assemblages of carbonatites are commonly altered and even obliterated, and then secondary features may be misinterpreted as magmatic. Furthermore, non-magmatic carbonate-rich rocks may be misidentified as carbonatites. Isotope ratios and trace-element concentrations can provide evidence of magmatic origin that may endure through recrystallization and alteration, but such proof by no means implies that all textural and mineralogical features now visible were formed in the presence of a carbonate-rich magmatic liquid

Timing and Chemistry of Fluid-Flow Events in the Lawn Hill Platform, Northern Australia

Abstract: Mudrocks and carbonates of the Isa superbasin in the Lawn Hill platform in northern Australia host major base metal sulfide mineralization, including the giant strata-bound Century Zn-Pb deposit. Mineral paragenesis, stable isotope, and K-Ar dating studies demonstrate that long-lived structures such as the Termite Range fault acted as hot fluid conduits several times during the Paleoproterozoic and Mesoproterozoic in response to major tectonic events. Illite and chlorite crystallinity studies suggest the southern part of the platform has experienced higher temperatures (up to 300 degrees C) than similar stratigraphic horizons in the north. The irregular downhole variation of illite crystallinity values provides further information oil the thermal regime in the basin and shows that clay formation was controlled not only by temperature increase with depth but also by high water/rock ratios along relatively permeable zones. K-Ar dating of illite, in combination with other data, may indicate three major thermal events in the central and northern Lawn Hill platform Lit 1500, 1440 to 1400, and 1250 to 1150 Ma. This study did not detect the earlier Century base metal mineralizing event at 1575 Ma. 1500 Ma ages are recorded only in the south and correspond to the age of the Late Isan orogeny and deposition of the Lower Roper superbasin. They may reflect exhumation of a provenance region. The 1440 to 1300 Ma ages are related to fault reactivation and a thermal pulse at similar to 1440 to 1400 Ma possibly accompanied by fluid flow, with subsequent enhanced cooling possibly due to thermal relaxation or further crustal exhumation. The youngest thermal and/or fluid-flow event at 1250 to 1150 Ma is recorded mainly to the cast of the Tern-lite Range fault and may be related to the assembly of the Rodinian supercontinent. Fluids in equilibrium with illite that formed over a range of temperatures, at different times in different parts of the platform. have relatively uniform oxygen isotope compositions and more variable hydrogen isotope compositions (delta O-18 = 3.5-9.7 parts per thousand V-SMOW; delta D = -94 to -36 parts per thousand V-SMOW). The extent of the 180 enrichment and the variably depleted hydrogen isotope compositions suggest the illite interacted with deep-basin hypersaline brines that were composed of evaporated seawater and/or highly evolved meteoric water. Siderite is the most abundant iron-rich gangue phase in the Century Zn-Pb deposit, which is surrounded by all extensive ferroan carbonate alteration halo. Modeling suggests that the ore siderite formed at temperatures of 120 degrees to 150 degrees C, whereas siderite and ankerite in the alteration halo formed at temperatures of 150 degrees to 180 degrees C. The calculated isotopic compositions of the fluids are consistent with O-18-rich basinal brines and mixed inorganic and organic carbon Sources (6180 = 3-10 parts per thousand V-SMOW, delta C-13 = -7 to -3 parts per thousand V-PDB). in the northeast Lawn Hill platform carbonate-rich rocks preserve marine to early diagenetic carbon and oxygen isotope compositions, whereas ferroan carbonate cements in siltstones and shales in the Desert Creek borehole are O-18 and C-13 depleted relative to the sedimentary carbonates. The good agreement between temperature estimates from illite crystallinity and organic reflectance (160 degrees-270 degrees C) and inverse correlation with carbonate delta O-18 values indicates that organic maturation and carbonate precipitation in the northeast Lawn Hill platform resulted from interaction with the 1250 to 1150 Ma fluids. The calculated isotopic compositions of the fluid are consistent with evolved basinal brine (delta O-18 = 5.1-9.4 parts per thousand V-SMOW; delta C-13 = -13.2 to -3.7 parts per thousand V-PDB) that contained a variable organic carbon component from the oxidation and/or hydrolysis of organic matter in the host sequence. The occurrence of extensive O-18- and C-13-depleted ankerite and siderite alteration in Desert Creek is related to the high temperature of the 1250 to 1150 Ma fluid-flow event in the northeast Lawn Hill platform, in contrast to the lower temperature fluids associated with the earlier Century Zn-Pb deposit in the central Lawn Hill platform.

Carbonate alteration minerals in the Salton Sea geothermal system, California, USA

Abstract: AaSTRACT. Active geothermal systems in fluvial-deltaic sediment of the Salton Trough typically develop a thick, carbonate-cemented sandstone caprock which shows a regular progression of carbonate minerals, mineral reactions, rock-fluid mass transfer, and physical properties on increasing temperature. The Sinclair 3 well contains calcian ankerite at 45 Voo at 250 ~ The most abrupt decrease occurs in the 140 to 170 ~ range where significant compaction of sandstone occurs as carbonate is reduced to < 25 ~, and kaolinite reacts to form chlorite. The overall result is the loss of significant Ca, Fe, Mg, and CO 2 from sandstone to the fluid phase on increasing temperature. Reaction ofcalcian ankerite to ankerite near 175 ~ and of ankerite to calcite and minor dolomite in the 195 to 245~ range, takes place on a constant carbonate-volume basis by direct replacement of one carbonate by another. The latter reaction produces significant chlorite, with A1 derived from solution of detrital feldspar and from the smectite-to-illite transformation. The equilibrium coexistence of calcite with dolomite and ankerite near 200 ~ has allowed construction of an isothermal section in the Ca-Mg-(Fe+Mn) carbonate phase diagram and provided a low-temperature constraint on the calcite limb of the calcite-dolomite solvus.

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.

Geochemistry of a paleosol horizon at the base of the Sausar Group, central India: Implications on atmospheric conditions at the Archean–Paleoproterozoic boundary

Abstract: A paleosol horizon is described from the contact of the Sausar Group (∼2400 Ma) and its basement (Tirodi Gneiss; >2500 Ma) in Central India. Physical evidence of pedogenesis is marked by the development of stress corrosion cracks, soil peds, corestone weathering and nodular rocks. XRD and SEM-EDX data indicate the presence of siderite, ankerite, uraninite, chlorite, alumino-silicate minerals, ilmenite, rutile and magnetite, in addition to quartz, feldspar and mica. The chemical index of alteration, the plagioclase index of alteration, and the chemical index of weathering show an increasing trend from parent rock to the paleosol and indicate a moderate trend of weathering. The A-CN-K plot indicates loss of feldspars, enrichment in Al2O3 and formation of illite. Different major element ratios indicate baseloss through hydrolysis, clay formation, leaching of some elements, and more precipitation with good surface drainage. The paleosol is depleted in HREE in comparison to the parent rock indicating high fluid-rock interaction during weathering. The paleosol samples show flat Ce and Eu anomalies, low ΣREE, and high (La/Yb)N, indicative of a reducing environment of formation. Reducing condition can also be inferred from the concentration of elements such as V, Co, Cu, Pb, and Zn in the paleosol profile. Although enriched in Fe and Mg, the overall geochemical patterns of the paleosol indicate oxygen deficient conditions in the atmosphere and development by weathering and leaching processes associated with high precipitation and good surface drainage at the time of development of this paleosol during the Archean–Paleoproterozoic transition.