Showing papers on "Ankerite published in 1996"

Synthesis, characterization, and energetics of solid solution along the dolomite-ankerite join, and implications for the stability of ordered cafe(co3 )2

Abstract: Samples along the dolomite-ankerite join were synthesized using a piston-cylinder apparatus and the double-capsule method. Some of the ankerite samples may be disordered. Thermal analysis and X-ray diffraction showed that all samples can be completely decomposed to uniquely defined products under calorimetric conditions (770°C, O2), and a wellconstrained thermodynamic cycle was developed to determine the enthalpy of formation. The energetics of ordered and disordered ankerite solid solutions were estimated using data from calorimetry, lattice-energy calculations, and phase equilibria. The enthalpies of formation of ordered dolomite and disordered end-member ankerite from binary carbonates, determined by calorimetry, are -9.29 :t 1.97 and 6.98 :t 2.08 kllmol, respectively. The enthalpy of formation of ordered ankerite appears to become more endothermic with increasing Fe content, whereas the enthalpy of formation of disordered ankerite becomes more exothermic with increasing Fe content. The enthalpy of disordering in dolomite (approximately 25 kllmol) is much larger than that in pure ankerite, CaFe(C03h (approximately 10 kllmol), which may explain the nonexistence of ordered CaFe(C03)2'

Evolution of hydrothermal fluids in the Ashanti gold belt, Ghana; stable isotope geochemistry of carbonates, graphite, and quartz

Abstract: The oxygen and carbon isotope systematics of ankerite, siderite, calcite, quartz, and graphite from the Bogosu and Prestea mining districts of the Ashanti gold belt, Ghana are used to speculate on the processes leading to hydrothermal alteration and gold deposition. The geologic environment of this turbidite- and graywacke-hosted gold system was a sediment-dominated accretionary complex. The gold ores are localized within the Ashanti structural belt which we suggest became a conduit for deep-seated fluids during uplift and dilatancy.Ankerite and siderite in mineralized sedimentary rocks are broadly similar in oxygen and carbon isotope compositions to corresponding minerals in the metasedimentary country-rocks. Evidence of alteration that preceded gold mineralization is best preserved in spatially associated altered mafic dikes. The compositions of fluids responsible for the gold mineralization and associated alteration have been reconstructed using temperatures from carbonate-mineral geothermometry.Alteration of country rocks occurred under rock-dominant, greenschist facies conditions. The isotopic composition of the least altered mafic dikes was influenced by interaction with fluids generated from Birimian greenschist facies metasedimentary rocks. Later, extensive carbonate alteration of mafic dikes adjacent to the structural conduit occurred during slow upward migration of fluids. Lateral diffusion of hydrogen into the structural conduit permitted partial conversion of CO 2 to methane, resulting in 13 C enrichment of both the residual CO 2 and the deposited carbonate minerals.Rapid, even explosive, expulsion of deep-seated fluids and CO 2 -H 2 O phase separation occurred episodically throughout the evolution of the hydrothermal system. This caused precipitation of sulfides, arsenides, and gold at relatively high crustal levels and low ambient pressure and temperature. During mineralization, fluid ascent was too rapid for isotopic and chemical reequilibration between the deep-seated ore fluid and adjacent country rocks. The ore fluid had chemical and isotopic compositions consistent with deep-seated metamorphic fluids that were subsequently modified by preferential partitioning of 18 O and 13 C into a gas phase. The composition of the deep-seated metamorphic fluid determined using chemical equilibria (CO 2 -H 2 O rich; 7-25 mole % CO 2 ) is in approximate agreement with that required to produce the isotopic depletion of the residual fluid via phase separation.

Tectonic hydrothermal gold mineralisation in the outboard zone of the Southern Alps, New Zealand

Abstract: Gold‐bearing veins cut a belt of low‐grade (pumpellyite‐actinolite/greenschist facies) schist in the Ben Ohau Range to the east of the Main Divide, in the outboard zone of the Southern Alps continental collisional zone, New Zealand. The schist has been exposed along the currently active Ostler Fault system, which has had c. 5 km of reverse motion since the Pliocene. The veins consist of quartz, ankerite, calcite, chlorite, and pyrite, with minor chalcopyrite and galena. Hydrothermal chlorite contains about 490 ppm Zn, and the gold contains 3–5 wt% Ag. Hydrothermal alteration of host rock is minor apart from Sr enrichment (up to four times background). Fluid inclusions in quartz are aqueous with minor dissolved CO2 and salts (<4 wt% NaCl equivalent), and homogenise at 236–270°C. The veins formed at 300 ± 20°C and 1000 ± 800 bars fluid pressure, probably under a hydrostatic fluid pressure regime. Oxygen and carbon isotopic data (δ18O = +10 to +14; δ13C = ‐6 to ‐10‰) are similar to data from economi...

Mass transfer and the path of metasomatic reactions in mesothermal gold deposits; an example from Flambeau Lake, Ontario

Abstract: Gold mineralization, in the Flambeau Lake area, is found in and adjacent to quartz-carbonate-albite veins surrounded by halos of intense ankerite-albite alteration. These features are the result of the passage of hydrothermal fluid through tension fractures and shear zones developed in response to regional dextral shearing. Wall-rock alteration is zoned, consisting of four distinct alteration assemblages: distal weak isochemical alteration; a transitional zone where chlorite was gradually removed and replaced by carbonates and albite; a zone of mainly ankerite and albite; and a rare marginal zone of mainly albite and quartz. A detailed analysis of potential mass balance indicators from a single alteration halo in quartz diorite revealed that Zr, Ti, light REE, U, Th, and to some extent, Nb, remained immobile during alteration. A mass balance study of this halo shows that SiO 2 , Al 2 O 3 , Fe 2 O 3 , MgO, and K 2 O were removed from the wall rock and that CaO, CO 2 , S, and Na 2 O were added. These changes are attributed to the breakdown of chlorite, sericite, calcite, and quartz, and their replacement by albite and ankerite. Vein mineral textures show that the veins were open, allowing the passage of the substantial amounts of hydrothermal fluid required to produce the alteration and to remove large amounts of SiO 2 from the rock (up to 30 wt %). The behavior of perfectly mobile elements indicates that the fluid/rock ratio increased toward the vein. A model is proposed in which the observed alteration is explained by the interaction of the wall rock with a quartz-undersaturated, alkaline, CO 2 -bearing fluid. The study emphasizes the importance of quartz undersaturation as a mechanism for the enhancement of hydrothermal alteration through porosity modification. In addition, it demonstrates how fluid-rock reaction paths can be investigated using simple mass balance calculations.

Nb-Rich Baotite in Carbonatites and Fenites at Haast River, New Zealand

Abstract: Baotite occurs as an accessory mineral in carbonatites, fenites, and carbothermal veins associated with a lamprophyre dyke swarm in the Haast River area of south Westland, New Zealand. Carbonatites are petrogenetically evolved, with assemblages dominated by ankerite, siderite and Ba-Sr-REE carbonates. Microprobe analysis indicates baotite compositions more Nb-rich than previously recorded, with compositions close to Ba4[Ti3(Nb,Fe)s]Si4028C1. Ti must be partially replaced in both crystallographically-independent octahedral sites. Compositional zoning, and stoichiometric considerations suggest that the dominant octahedral substitution is the same as that described in rutile, namely 3Ti 4§ ~ 2Nb 5+ + Fe 2+. Contrary to previous suggestions, Fe in the octahedral site should, therefore, be dominated by Fe 2+. The presence of baotite further documents the involvement of halogens in carbonatite magmas. In the New Zealand occurrences it is suggested that the chlorine originates from associated phonolitic magmas and is partitioned into carbonatite during liquid immiscibility.

Petrology and diagenesis of Narrabeen group sandstones, Sydney Basin, New South Wales∗

Abstract: The petrology of fluvial sandstones from the Narrabeen Group was investigated to document sandstone composition and to assess factors controlling sandstone diagenesis. The sandstones are dominated by detrital quartz and lithic grains. The average composition varies in ascending stratigraphic order from R (rock fragments) > Q (quartz) to Q > R. Major diagenetic events in order of occurrence were: (i) formation of hematite, chlorite and mixed‐layer illite/smectite, and first generation of kaolin (I); (ii) precipitation of calcite, siderite and ankerite; (iii) dissolution of labile detrital grains and carbonate cement, and formation of second generation of kaolin (II); (iv) quartz overgrowth and precipitation of illite; and (v) precipitation of calcite and ankerite. These events were controlled both by detrital mineralogy and pore‐water chemistry. Kaolin (I) and chlorite are mutually exclusive. At shallow burial depth different pore‐water chemistry prevailed in different parts of the basin through meteoric w...

Cleat mineralogy of later permian coal measures, Bowen Basin, Queensland, Australia

Abstract: The Bowen Basin is a complex Permo-Triassic, extensional to foreland basin, extending north-south for about 1000 km. Large commercial coal deposits are known from the Bowen Basin, some of which are currently under investigation as a potential source of Coalbed Methane.Borehole samples ranging in depth from 300m to 989m from two major intervals- the Upper Permian Rangal Coal Measures and German Creek Coal Measures - show a range of rank from High-Volatile Bituminous Coal (0.81%Ro) to Anthracite (2.54%Ro). The nature and origin of cleat mineralisation in Upper Permian coals from the Permo- Triassic Bowen Basin have been investigated using a variety of complementary analytical techniques.A total of nineteen authigenic mineral phases have been found in cleats and joints of the Rangal/Baralaba and the German Creek Coal Measures in the study area. These are illite, calcite, kaolinite, ankerite, quartz, chlorite, siderite, barite, pyrite, illite/smectite mixed-layer, strontianite, wollastonite, whewellite, halite, barium phosphate, bjarebyite, analcime, albite, fluorite. However, the overall cleat mineralogy in the studied area is rather simple. The most frequently occurring minerals belong to two groups, (i) clay mineral-dominated by illite found mostly in face cleats and (ii) carbonates-dommoitd by calcite and found mostly in butt cleats. Ankerite dominates the nonsystematic cleat minerals, while kaolinite dominates superimposed cleats.K/Ar ages and stable isotope geochemistry of face cleat-fill illites (d18O(VSMOW) = + 6.9 to + 14.4p, dD = -123 to - 80.1) and one fracture illite/smectite fill indicate three phases of illite formation during the Triassic from deeply circulating meteoric waters as follows: (1) an early phase with ages clustered around 245 Ma B.P., (2) a second phase about 232 Ma B.P. and (3) the latest phase about 219 Ma B. P. Burial history and thermal modelling of selected boreholes from the study area and other geological data indicate that the first phase of illitisation occurred when the coal seams were about 1000m below the surface and at a temperature of between 70dC and 80dC, at a time when the basin was rapidly subsiding. These early illites were stable and retained their IM polytype even after being exposed to temperatures of 150-190dC during maximum burial in the southeastern Bowen Basin. The second phase of illitisation (at 232+3 Ma) occurred at about the time of maximum burial. The latest phase took place between 225p3-212p3 Ma and occurred in a wider temperature range (between 170-100dC) during uplift associated with the Hunter-Bowen Orogeny in the Late Triassic and a subsequent period of post-orogenic extensional subsidence.Face cleat coals from Banana and Moura areas are dominated by illite, while coprecipitation of illite, chlorite and kaolinite assemblages is noted in face cleat coals from Bluff, Blackdown and Dawson River areas. This change in mineralogy reflects a change in the chemistry of the mineralising fluids during the Triassic. This change was most probably caused by depletion and reduction in activity of K, Ca, Mg and Fe cations within the mineralising fluids as they moved away from their source along the orogenic eastern basin margin towards shallower regions of the basin. Superimposed cleat-fill kaolinite (d18O = + 8.2 to + 14.1p, dD = -112 to - 102.5p) from Emerald coals might also be related to this phase of mineralisation.Ankerite (d18O= + 7.7 to + 11.4p, d13C (PDB) = - 7.1 to -1.6p) mineralisation was contemporaneous with non-systematic cleat formation at temperatures between 40 and 100dC during uplift at the latest stage of the Hunter-Bowen Orogeny in Late Triassic times.The Bowen Basin area experienced a second cycle of subsidence that commenced in Early Jurassic with formation of the Surat Basin, which overlies the Bowen Basin and forms part of the Great Artesian Basin system. Regional uplift and erosion in middle Cretaceous times terminated sediment accumulation in the Surat Basin. During the second cycle of burial widespread carbonates, mainly calcite (d18O = + 9.8 to + 16.8p, d13C = - 7.2 to + 14.7p) mineralisation in butt cleats and joints took place. Calcite fluid inclusions and stable isotopes indicate that calcite was precipitated from meteoric water at temperatures between 70 and l100dC. The widespread carbonate mineralisation in butt cleats and the near absence of carbonates in the face cleats is here attributed to permeability anisotropy, caused by a change in the direction of lateral compressive stress during the Jurassic-Cretaceous time relative to that during the Triassic Hunter- Bowen Orogeny.

Phosphorite from the upper continental slope off Kachchh, northwest coast of India

Abstract: Flat slabs and irregular lumps (1 to 3 cm) of brown, grey, black phosphorites and creamy white limestone (2 ta 4 cm) were recovered from 473 and 501 m depths from the upper continental slope off Kachchh. Phosphorites contain phosphatised skeletal fragments of shallow water origin, calcitic patches and phosphatised black peloids set in dark brownish microsphorite. Limestone contains fragments of foraminifera set in micrite. Carbonate fluorapatite, carbonate hydroxylapatite, ankerite, calcite in phosphorites and high magnesium calcite in limestone are the major minerals. Electron microprobe studies indicate the presence of Ca, P, Mg, Si, Al, K, Fe, S and Ti in phosphorites. P 2 O 5 content ofphosphorites and limestone are 29.42% and 11.76% respectively. It is suggested that the phosphatization is an early diagenetic process took place in shallow water condition most likely during the Quaternary period. Subsequently, tectonic changes brought them to the present site and thus occurring as relict phosphorites.

Diagenesis of the Oseberg Sandstone Reservoir (North Sea): An Example of Integration of Core, Formation Fluid and Geochemical Modelling Studies: ABSTRACT

Abstract: A detailed multidisciplinary integrated study of the Middle Jurassic Oseberg reservoir in 20 wells of the Oseberg field, Norwegian North Sea, was carried out in collaboration with Norsk Hydro and Oseberg partners The objectives were to reconstruct the tinting, conditions and spatial variation of diagenetic transformations; to characterize the nature and origin of diagenetic fluids; and to develop a geochemical model of the observed diagenesis The 20-60 m thick Oseberg Formation occurs at depths of 25 to 32 km, and at present temperatures of 100 to 125[degrees]C The detrital assemblage is mainly composed of quartz, K-feldspar, albite, muscovite and lithic clay clasts, and is very homogeneous throughout the field The diagenetic sequence includes: minor siderite and pyrite, K-feldspar rims, ankerite, pervasive feldspar dissolution, abundant vermiform kaolinite, quartz overgrowths, poikilotopic ferroan calcite, and dickite Diagenetic temperatures were derived from fluid inclusions in ankerite, quartz and calcite, and combined with the modelled burial/thermal history to constrain approximate ages and duration of diagenetic events Isotopic compositions of carbonates and kaolinite indicate that meteoric water and seawater were two major constituents of diagenetic fluids Present formation waters are fairly similar chemically and isotopically at reservoir scale and represent mixing of three end members: seawatermore » ([approximately]54%), meteoric water ([approximately]40%) and primary evaporative brine ([approximately]6%) Stability diagrams and chemical geothermometers indicate that formation fluids are close to equilibrium with the host sandstone at present reservoir temperatures« less

Water-Rock Interactions and Chemical Compositional Variations During Ductile Deformation of the NW-Striking Shear Zone in the Jiapigou Gold Belt, China

Abstract: Two kinds of mylonite series rocks, felsic and mafic, have been recognized in the NW-striking shear zone of the Jiapigou gold belt. During ductile deformation, a large amount of fluid interacted intensively with the mylonite series rocks: plagioclases were sericitized and theAn values declined rapidly, finally all of them were transformed to albites; dark minerals were gradually replaced by chlorites (mostly ripidolite). Meanwhile, large-scale and extensive carbonation also took place, and the carbonatization minerals varied from calcite to dolomite and ankerite with the development of deformation. The δ13C values of the carbonates are −3.0‰ – −5.6‰ suggesting a deep source of carbon. The ductile deformation is nearly an iso-volume one (fv≈1). With the enhancement of shear deformation, SiO2 in the two mylonite series rocks was depleted, while volatile components suchs as CO2 and H2O, and some ore-forming elements such as Au and S were obviously enriched. But it is noted that the enrichment of Au in both the mylonite series rocks did not reach the paygrade of gold. The released SiO2 from water-rock interactions occurred in the form of colloids and absorbed gold in the fluid. When brittle structures were formed locally in the ductile shear zone, the ore-forming fluids migrated to the structures along microfractures, and preciptated auriferous quartz because of reduction of pressure and temperature. Fluid inclusion study shows that the temperature and pressure of the ore-forming fluids are 245–292°C and 95.4–131.7 MPa respectively; the salinity is 12.88–16.33wt% NaCl; the fluid-phase is rich in Ca2+, K+, Na+, Mg2+, F− and Cl−, while the gaseous phases are rich in CO2 and CH4. The δD and δ18O, values of the ore-forming fluid are −84.48‰ – −91.73‰ and −0.247‰ – +2.715‰ respectively, suggesting that the fluid is composed predominantly of meteoric water.

Determination and distinction of Ca-dolomites, Fe-dolomites, ankerite or stoichiometric dolomite by X-ray diffraction using a profile fitting

Abstract: The X-ray diffraction study of dolomites from various sedimentary rocks shows that the dolomite is rarely a stoichiometric one. The substitution of some Mg 2+ (ionic radius: 0.66 A) by larger ions like Fe 2+ (0.74 A) or Mn (0.80A) or Ca 2+ (0.99A) increases the size of the unit cell, which leads to an increase of the d 10.4 spacing. The d 10.4 spacing varies from 2.888 A (dolomite) to 2.901 A (ankerite). The distinction of the dolomite from ferroan dolomite and ankerite in carbonate mixtures is obtained by a profile fitting of X-ray patterns which permits the precise localization of individual peaks even in cases where several distinct carbonates are present. The X-ray determination is confirmed by electron microprobe analysis and the structural data of the dolomites is calculated.

a new mineral from KQla peninsula, Russia, and Quebec, Canada

Abstract: Kukharenkoite-(Ce), ideally Ba2Ce(C03hF, is a new rare-earth fluorocarbonate mineral, occurring as an accessory mineral in the late REE-carbonatites and REE-rich carbonate-zeolite rocks of the Khibina massif, Kola peninsula, Russia. It has also been found in other associations within the Khibina complex as well as in the Vuorijarvi massif, Kola peninsula and the related alkaline intrusions of the Mont Saint-Hilaire complex and the Saint-Amable sill, Quebec, Canada. Kukharenkoite-(Ce) occurs in cavities as small (0.01-1.0 mm), prismatic, bladed crystals which are often in dendritic or stellate groups up to 2-3 mm across. Associated minerals are manganoan ankerite, ferroan rhodochrosite, manganoan siderite, natrolite, synchysite-(Ce), orthoclase, barite, pyrite, cordylite-(Ce), mckelveyite-(Y), ewaldite, galena and sphalerite. The mineral is yellow with a white streak; transparent with a vitreous lustre. Cleavage and parting were not observed. The microhardness VHN25 is 280(25), the Mohs hardness is 4.5; density (meas.) is 4.7(1) g/cm3 and density (calc.) is 4.62 g/cm3. The infrared absorption