Showing papers on "Ankerite published in 1999"

High-pressure behaviour and equation of state of calcite, CaCO3

Abstract: The high-pressure response of the cell parameters of calcite, CaCO3, has been investigated by single crystal X-ray diffraction. The unit cell parameters have been refined from 0 to 1.435 GPa, and the linear and volume compressibilities have been measured as β a =2.62(2) × 10−3 GPa−1,β c =7.94(7) × 10−3 GPa−1, β v =13.12 × 10−3 GPa−1. The bulk modulus has been obtained from a fit to the Birch-Murnaghan equation of state, giving K 0=73.46 ± 0.27 GPa and V 0=367.789 ± 0.004 A3 with K′=4. Combined with earlier data for magnesite, ankerite and dolomite, these data suggest that K 0 V 0 is a constant for the Ca-Mg rhombohedral carbonates.

Spatial distribution of carbon and oxygen isotopes in laterally extensive carbonate-cemented layers; implications for mode of growth and subsurface identification

Abstract: Carbonate concretions and laterally extensive cemented layers are common features in the Prairie Canyon Member of the Mancos Shale at its type locality in western Colorado. Stable-isotope and petrographic data were obtained from these cemented zones in an attempt to determine: (1) how the cemented layers form, (2) whether or not they can be chemically correlated, and (3) if it is possible to differentiate them from concretions in the subsurface. The cement in both the layers and concretions consists of ferroan dolomite and lesser amounts of stoichiometric dolomite and ankerite. Samples for stable-isotope analysis were drilled from 10 grids, with a typical grid spacing of 10 cm10 cm. Gridded samples were obtained from two continuous cemented layers at several locations along the layers (2.3 m to 1.5 km apart on the same layer), and from isolated and stratabound concretions. Taken together, all the cements (regardless of spatial position) show a remarkably linear correlation (R2 = 0.98) between 13C and 18O. Systematic center-to-edge decreases in both 13C and 18O occur in each of the cemented zones and are persistent over lateral intervals of at least 1.5 km. The cores of the cemented zones have positive 13C values (up to > 10o/oo PDB), which result from incorporation of 13C-enriched carbon during methanogenesis. The edges of the cemented zones have negative 13C values (down to < -9o/oo PDB), which suggests incorporation of 13C-depleted carbon from thermocatalytic decarboxylation reactions. Oxygen isotope paleothermometry indicates that precipitation may have started at moderate temperatures ( 25°C) and ended at temperatures as high as 92°C. The spatial distribution of the isotopic data in the cemented layers can be explained as resulting from the coalescence of stratabound concretions, followed by growth above and below the coalesced concretions. Such growth may have occurred in a complex manner, in which early cements precipitated throughout the thickness of the layer but were more abundant in the core zone. Given the lateral persistence of the pattern of isotopic variation in the cemented layers, geochemical correlation of the layers may be possible in the subsurface. In addition, because laterally persistent cemented layers in the Prairie Canyon Member exhibit patterns of isotopic variation different from concretions (layers have a more asymmetrical vertical distribution), it should be possible to distinguish between the two styles of cementation from subsurface core data alone.

Assessing the Influence of Reacting Pyrite and Carbonate Minerals on the Geochemistry of Drainage in the Coeur d'Alene Mining District

Abstract: The relative abundance of minerals that react to generate or consume acid in mineralized areas is critical in determining the quality of water draining from such areas. This work examines the fundamental reactions that influence the pH and composition of drainage from mine adits and tailings piles. The authors construct triangle diagrams that predict stoichiometric relationships between concentrations of dissolved SO{sub 4}{sup {minus}}, dissolved Ca and Mg, and either alkalinity or acidity by considering reactions involving the oxidation of pyrite, dissolution of carbonate minerals, and precipitation of iron oxide and iron hydroxysulfate minerals. Drainage data from the Coeur d'Alene mining district are used to test their stoichiometric approach. Comparisons between theoretical predictions and drainage data indicate that the range of pH values in the mining district is due to reacting pyrite to carbonate mineral ratios that range from near 0/1 to 1/1. Calcite and ankerite are the dominant carbonate minerals that buffer the acid produced during pyrite oxidation and ferrihydrite or schwertmannite precipitation.

Routine staining of drill core to determine carbonate mineralogy and distinguish carbonate alteration textures

Abstract: Carbonate staining has long been utilized with thin sections. Similar techniques are also extremely useful for distinguishing the mineralogy and texture of different carbonate minerals in drill cores. The simplest method for routine staining of whole drill core utilizes a dilute hydrochloric acid solution containing both alizarin red S and potassium ferricyanide. This solution is painted on to the drill core to allow for the rapid identification of calcite, ferroan calcite, ferroan dolomite (ankerite), and rhodochrosite. Production-scale staining of whole drill core rapidly provides a wealth of information on carbonate mineralogy and textural relationships in a number of deposit types. Examples from the Irish Zn-Pb district, the McArthur River Zn-Pb-Ag district, the Ruby Creek Cu-Co deposit, and the Kansanshi copper deposit are presented to illustrate the type of information that can be gained through use of this inexpensive and rapid technique.

Evolving fluid circulation within the variscan beja-acebuches ophiolite complex (se, portugal)

Abstract: The Beja-Acebuches Complex is an extremely dismembered ophiolite sequence incorporated in the Variscan South Iberian Suture, representing a small marginal (back-arc) basin formed during subduction under the Iberian Terrane of normal oceanic crust to the south. Widespread textural evidence for hightemperature (800-900 oC) recrystallization of the basal and intermediate sections of the ophiolite sequence under an anisotropic stress field, suggests that these rocks were obducted before their total consolidation. The later history of this Complex reflects mainly Variscan metamorphism, that peaked at amphibolite facies, and its subsequent waning stages (during which the prevalent hydration of peridotites took place). The final stages of the retrograding path occurred at temperatures below 300 oC and are mainly ascribable to large-scale and repeated H2O, CO2 and (subordinate) SiO2 introduction into the system through WNW-ESE left-handed vertical shear zones, reactivated under brittle conditions with a left-lateral thrusting movement. This large volume, extremely well focused, fluid inflow generally induced very intense carbonatization of the adjoining rocks, with almost total destruction of their original mineralogy and textures and deposition of ankerite + dolomite ± siderite ± magnesite in subsidiary distensive structures. When affecting serpentinized peridotites, this metasomatic process leads to deserpentinization with development of strongly silicified carbonate aggregates. Later hydrothermal events are typically related to the precipitation of microcrystalline quartz and/or calcite in late veins and veinlets. Fluids circulating through the shear zones must have been quite pure, slightly acidic H2O-CO2 mixtures, whose very scarcity in metals promoted the hydrolysis of primary minerals and the removal of Al as aqueous-complexes. The origin of these fluids is believed to be related mainly to degassing of the autochthonous carbonate/schist units during Variscan metamorphism and the Beja Igneous Complex intrusion.

Structural, mineralogical and geochemical studies of the Paleoproterozoic Omai gold deposit, Guyana

Abstract: The granitoid- and greenstone-hosted Omai gold deposit is located in the Paleoproterozoic Barama-Mazaruni greenstone belt, Guiana Shield. At regional scale, the emplacement of the deposit was controlled by the Makapa-Kuribrong crustal shear zone. At local scale, the gold-bearing quartz veins are associated mainly with a quartz monzodioritic intrusion (Omai stock, Fennell pit) and subvolcanic quartz-feldspar porphyry and rhyolite dikes, and subordinately, with intermediate volcanic flows and metasedimentary rocks (Wenot pit). Six gold-bearing undeformed subhorizontal and subvertical vein sets can be distinguished in the two ore zones. On the basis of their internal structures and textures, the veins can be classified as crack and seal, laminated, breccia, and open-space-filling veins. The geometrical and textural relationships between the vein sets suggest that they are broadly contemporaneous. The formation of most veins can be summarized by two filling stages and a late fracture-filling stage related to a protracted hydrothermal process. Although the metallic minerals represent less than 1 percent of the vein volume, their mineralogy is complex and consists of various sulfides, together with tungstates, native elements, tellurides, and sulfosalts. The metallic paragenesis is defined by the Au-Ag-Te-W-Bi-Pb-Zn-Cu-Hg-Mo assemblage. The nonopaque gangue includes mainly quart and subordinately carbonates, albite, sericite, chlorite, tourmaline, rutile, and epidote.Wall rocks of gold-bearing veins are affected by synmineralization alteration. The dominant alteration assemblages include carbonates, sericite, silica, chlorite, albite, epidote, argillic minerals, pyrite, and pyrrhotite. There is an obvious association between the alteration assemblages, the vein internal textures, and the chemical nature of the host rocks.Vein-forming scheelite has 87 Sr/ 86 Sr ratios between 0.7019 and 0.7021 and delta 18 O values between 3.8 and 4.3 per mil, which suggest both consistent temperature and isotopic composition of the hydrothermal solutions during its deposition. Oxygen isotopes measured in vein quartz vary between 13.2 and 14 per mil, similar to the delta 18 O values of carbonates (avg 13.8ppm for calcite and 14.4ppm for ankerite). The carbon isotopes of carbonates range between 1.7 and 4.7 per mil. The delta 18 O values of the mineralizing fluids vary between +5.6 and -2.7 per mil and the delta D values between -52 and +18 per mil. The isotopic composition of the hydrothermal fluids plots outside both magmatic and metamorphic water boxes, therefore suggesting a significant component of surface-derived water.The Omai deposit is late orogenic and its emplacement was controlled by the last brittle to brittle-ductile stages of the Trans-Amazonian orogeny. It can be considered as a Paleoproterozoic equivalent of the Archean epizonal orogenic deposits described in the Yilgarn (Wiluna, Racetrack) and Zimbabwe (Shamwa, Commoner) cratons.

Mineral identification in Colombian coals using Mössbauer spectroscopy and X-ray diffraction

Abstract: Minerals were identified in three Colombian coal samples from the Southwest of the country using Mossbauer spectroscopy and X-ray diffraction Original and sink separated coal fractions of specific gravity 140 and 160 with particle size less than 600 µm were used in the study Using Mossbauer spectroscopy, the minerals identified in the original coal samples were pyrite jarosite, ankerite, illite and ferrous sulfate, whereas by means of X-ray diffraction, minerals identified were kaolinite, quartz, pyrite, and jarosite Differences in mineral composition were found in the original and sink separated fractions using both techniques Mossbauer spectra show that the mineral phases in low concentrations such as illite, ankerite and ferrous sulfate do not always appear in the spectra of sink coals, despite of those minerals occurring in the original coal, due to the fact that they are associated with the organic matter and not liberated in the grinding process X-ray results show that the peak intensity grows as the specific gravity is increased indicating that the density separation method could be an effective process to clean coal

Geology and genesis of the George Fisher Zn-Pb-Ag deposit Mount Isa, Australia

Abstract: The George Fisher deposit (107 Mt @ 11.1% Zn, 5.4% Pb, and 93g/t Ag) is the northernmost significant Mount Isa-style deposit hosted by the -1653Ma Urquhart Shale in the Western Mount Isa Inlier. It is distinguished from the Mount Isa and Hilton Mines by a paucity of syn-late tectonic Cu. This has enabled recognition and examination of a previously ·unrecognized, syndiagenetic, hydrothermal Zn-Pb-Ag mineralization-system, despite superposition of intense deformation during the Diamantinan and Isan Orogenies. George Fisher contains eleven west-dipping, stacked, anastomosing, stratabound ore lenses contained within rhythmically laminated pyritic siltstones intercalated with banded mudstones, and separated by barren stylolitic mudstones. The ore-bearing sequence contains abundant carbonate banding including partly coalesced carbonate nodules in pyritic siltstones and planar white carbonate bands in siltstones and banded mudstones. The deposit is zoned from stratigraphically lower Zn-rich to higher Zn+Pb-rich ore bodies. Subeconomic Cu occurrences are coincident with Zn+Pb zones, but are predominantly restricted to upper portions of the sequence. This zonation was developed within two hydrothermal systems separated by 75-125Ma. Emplacement of Zn-Pb-Ag at George Fisher was the culmination of a complex, syndiagenetic, hydrothermal alteration system. Calcite is a major constituent of texturally distinct carbonate banding and its formation post-dates earliest dolomite-ferroan dolomiteankerite alteration. At George Fisher, these carbonates are distinguished by unique stable isotope geochemistry within the Urquhart Shale (δ¹⁸O = 17.1-18.5‰, δ¹³C = 4.1 to -1.7‰) and are enriched in (Fe,Mn)CO₃ relative to paragenetically equivalent carbonates located away from economic Zn-Pb-Ag. These carbonates formed during infiltration of warm, basin-derived fluids into cool sedimentary rocks, prior to stylolitization and at depths of a few hundred metres. Chemical zonation of early carbonates over 10-20km provides vectors for Zn-Pb-Ag-bearing fluid influx zones. Pyritization by thermochemical sulphate reduction of in situ organic material post-dated stylolitization. Infiltrating fluids lacked ore metals at the time. Bedding-parallel carbonate±quartz±celsian-hyalophane-K-feldspar vein development was localized in the immediate vicinity of, preceded and structurally prepared the sequence for, economic Zn-Pb-Ag mineralization. Sphalerite, galena, hydrophlogopite and bitumen were codeposited. Bitumen reflectance data and mesophase textures constrain maximum temperatures of mineralization and peak thermal conditions during burial to 200°C. Metals were transported in slightly oxidized, near-neutral fluids. Sulphur was either sourced from a separate reduced fluid or was transported as sulphate in the same fluid. Metal precipitation occurred via fluid mixing or sulphate reduction. Much sphalerite is preserved in primary depositional sites whilst galena occurs in syn-late tectonic veins and breccias but retains its primary spatial association with sphalerite at deposit scale. Nineteen galena samples, from a range of paragenetic settings, display homogeneous Pb isotope compositions. A Pb-model age of -1653 Ma is interpreted to reflect the timing of Zn-PbAg mineralization. Key characteristics of the syn-late tectonic Cu system include pyrrhotite, biotite-chloritedominant and ferroan dolomite-ferroan ankerite -dominant alteration. Phyllosilicates occur in stratabound lenses superimposed on Ba-K-feldspar alteration zones, whilst ferroan carbonate alteration is pervasive throughout the deposit hanging wall. These carbonates are texturally, chemically and isotopically (δ¹⁸O = 13.0-17.3‰, δ¹³C = 4.6 to -1.3 ‰) distinct from syndiagenetic carbonates. Copper-bearing fluids had temperatures of 250- 300°C based on phyllosilicate mineral stability relationships. The temporal-spatial-temperature zonation of the Zn-Pb-Ag and Cu systems at George Fisher is not preserved at Mount Isa and Hilton. Mineralogical and geochemical characteristics of carbonate and phyllosilicate alteration assemblages throughout Zn-Pbbearing strata at the latter deposits are products of syn-late-tectonic Cu mineralization, reflecting greater influx of Cu-bearing fluids at these sites. Poor preservation of syndiagenetic hydrothermal signatures at Mount Isa and Hilton has undoubtedly contributed significantly to the long-standing controversy surrounding the origin of Mount Isa-style Zn-Pb-Ag deposits.

IRON IN COAL AND SLAGGING The Significance of the High Temperature Behaviour of Siderite Grains During Combustion

Abstract: Accurate prediction of the severity of ash deposition on heat transfer surfaces in pulverised fuel fired furnaces has been the driving force of many studies. These studies initially attempted to develop engineering indices, which could be used to predict the severity of deposition from a particular coal, based on bulk analysis of the coal prior to utilisation [Raask, 1985]. The indices developed were only applicable to certain coal ash chemistries and found little application for other coals. A number of studies were then undertaken to determine the behaviour of specific minerals [Srinivasachar, 1990a, 1990b], which allowed interpretation of experimental and industrial scale data in terms of mineral specific reactions. Iron in coal has long been linked with slagging. Consequently, several engineering indices make some allowance for iron content when predicting deposition severity [Juniper, 1995]. The Northern Hemisphere coals upon which these indices are based generally contained pyrite as the principal form of iron. In Australian coals iron is predominantly in the form of carbonates, hence the engineering indices developed for the Northern Hemisphere coals would only loosely apply to Australian coals. Siderite limestone dolomite where MgO 4055wt.%) and ankerite are the principal carbonate minerals found in Australian coals. The variable compositions found for siderite in Australian coals led Patterson et al. 1994 to classify four main types. Siderite (where is the

Spoiwa węglanowe skał klastycznych czerwonego spągowca w kujawsko-pomorskim segmencie strefy T-T

Abstract: Wypelnienia przestrzeni porowych, w piaskowcach i zlepiencach przez mineraly weglanowe, stanowią jeden z wazniejszych skladnikow spoiw skal czerwonego spągowca. Najpospolitszymi mineralami weglanowymi są kalcyt i dolomit, sporadycznie ankeryt. Analiza katodoluminescencyjna i zastosowanie mikrosondy elektronowej pozwolilo na wyroznienie dwoch odmian kalcytu (ryc. 2–5): czysty — nie wykazujący luminescencji i nie zawierający domieszek innych pierwiastkow oraz kalcyt manganonośny. Wyniki oznaczen izotopowych w tych mineralach pozwolily określic przyblizone temperatury ich krystalizacji:26°–38°C i 40°–62°C. Sugerują one istnienie co najmniej dwoch epizodow cementacji badanych osadow czerwonego spągowca. Rowniez spoiwo dolomitowe (ryc. 6–9) tworzylo sie glownie w dwoch przedzialach temperatur: 1) 39°–44°C i 2) 58°–82°C. Dolomity krystalizujące w wyzszych temperaturach zaliczono do poźnodiagenetycznych, co wydaje sie potwierdzac obecnośc dolomitu barokowego w osadach z otworu wiertniczego Budziszewice IG1. Cement ankerytowy powstawal rowniez w poźnej fazie cementacji weglanowej. CARBONATE CEMENTS OF THE ROTLIEGEND CLASTIC ROCKS IN THE KUJAVIAN-POMERANIAN SEGMENT OF THE T-T ZONE. Summary Carbonate filling of the pore space in sandstones and conglomerates represents one of the most important constituents of cements in the Rotliegend rocks. The following minerals are the most common: calcite and dolomite, sporadically ankerite. Two calcite varieties (Figs. 2-5) have been distinguished as a result of the cathodoluminescence analysis and microprobe studies, namely: pure calcite - nonluminescent and without impurities; Mn-bearing calcite. Isotope determinations allowed to indicate approximate temperatures of mineral crystaltization which were: 26°- 38°C; 40°- 62°C. These facts suggest at least two cementation episodes of the Rotliegend rocks. In general, the dolomite cement (Figs. 6-9) also formed in two temperature ranges: 39°-44°C and 58° -82°C. The dolomites that crystallized in the higher temperatures are late diagenetic. This is confirmed by the presence of a baroque dolomite in the rocks recorded in the Budziszewice IG 1 borehole. The ankerite cement formed in the late phase of carbonate cementation, too.

Geochemical, and stable and radiogenic isotope records in Devonian and Early Carboniferous carbonates from Valle de Tena, central Pyrenees (Spain): evidence for their diagenetic environments

Abstract: Mineralogical, textural and geochemical investigations were made to determine the post-depositional evolution of Devonian and Early Carboniferous carbonates from Valle de Tena. The carbonate association is made up of low-Mg calcite, which occurs as micrite, spar cements, neomorphic patches and spar filling veinlets. Non-stoichiometric dolomite and ankerite occur as cements (dolomite also as replacements) in the Middle Devonian, post-dating calcite types. All these phases pre-date tectonic stylolites, indicating compaction after stabilization of the carbonate minerals. Strontium concentrations indicate that Early Devonian and Early Carboniferous micrites initially precipitated as aragonite; Middle and Late Devonian micrites precipitated as high-Mg calcites. Both precursors were diagenetically stabilized to low-Mg calcites through interaction with meteoric waters in phreatic environments. Trace elements in dolomite and ankerite indicate precipitation from Sr-enriched meteoric water. All studied carbonates, except Middle Devonian limestones, precipitated in reducing environments, which favoured incorporation of Fe and Mn. Late calcite generations precipitated from more saline waters than micrites. Light δ18O values in micrites suggest alteration mainly in meteoric-phreatic environments. The dolomites and ankerites precipitated from more 18O-depleted fluids than the calcites, suggesting a greater contribution from meteoric waters. Variations in δ13C of micrites represent primary secular trends, according to published δ13C variations. The δ13C oscillations within each succession probably relate to sea-level oscillations. Strontium isotopes also point to a meteoric origin of diagenetic fluids. Model calculations suggest that O and Sr isotopes equilibrated between calcites and fluid at relatively low water/rock ratios, whereas C isotopic signatures are inherited from limestones.

Hydrothermal Antimony Deposits of the Hyundong Mine : Geochemical Study

Abstract: The antimony deposits of the Hyundong mine, located in the northeastern part of the Sobaegsan massif, occur as hydrothermal quartz+carbonate veins and stockworks which fill the fault fractures developed in Precambrian metamOlphic rocks (mainly, granitic gneiss). Hydrothermal alteration occurs commonly in the vicinity of mineralized veins and is characterized by sericitization and silicification. A K-Ar age of alteration sericite is 139.2 4.4 Ma, implying the early Cretaceous age of mineralization, possibly in association with intrusion of nearby acidic dikes (mainly, quartz porphyry). The hydrothermal mineralization occurred in five mineralization stages. These are: (I) stage I, characterized by deposition of chalcedonic quartz; (2) stage II, deposition of quartz with base-metal sulfides and stibnite; (3) stage III, deposition of quartz and carbonates (calcite, dolomite, ankerite, rhodochrosite) with various antimony-bearing minerals such as stibnite, polybasite, berthierite, native antimony, gudmundite and ullmannite; (4) stage IV, deposition of calcite with stibnite; and (5) stage V, deposition of barren calcite. Antimony occurs mostly as stibnite within stages II to IV veins, which has various habits including disseminated, veinlets and euhedral coarse crystals. Fluid inclusion studies indicate that hydrothermal mineralization at Hyundong occurred from the fluids with temperature and salinity of C to 120 and 5.3 wI. % equiv. NaCI. The temperature and salinity of ore fluids systematically decreased with elapsed time in the course of mineralization, possibly due to the influx of larger amounts of meteoric groundwater. The deposition of antimony-bearing minerals occurred at low temperatures «C), mainly due to the cooling and dilution of fluids. Based on the evidence of fluid boiling during the early stage II mineralization, the mineralization occurred under low pressure conditions (about 80 bars, corresponding to depths of about 350 m under hydrostatic pressure regime). Thermodynamic considerations of ore . mineral assemblages indicate that antimony deposition also occurred as the results of decreases in temperature and sulfur fugacity of hydrothermal fluids. Calculated sulfur isotope composition of ore fluids (=5.4 to 7.8) indicates an igneous source of sulfur.

Brazilian Carbonatites: Studies of the Fazenda Varela (SC) and Catalão I (GO) Carbonatites and their Alteration Products

Abstract: This paper presents some Brazilian carbonatites case studies: the Fazenda Varela (SC) and the Catalao I (Go) carbonatites. The mineralogical composition of the Fazenda Varela carbonatite is ankerite, Fe-dolomite, dolomite, synchysite and barite. Apatite and monazite are very rare accessories. The rock presents high amounts of REE, Ba, Ca, Sr, CO 2 and SO 3 , significant Th and U, and small amounts of P, Nb and Ta. The weathering dissolves the carbonates, forms goethite and maintains barite in a saprolite facies. The laterite facies is probably related to the tertiary climate. The weathering promote Fe enrichment, followed by Mn, Th and U in the oxide phase. Ba, REE and P are fixed in the younger weathering (saprolite phase) and lost in the older weathering (laterite phase). In Catalao I Massif five hidrotermal events and the following magmatic events were identified: (1) Phoscorite and pyroxenite; (2) Banded carbonatite with alternated calcite and dolomite layers with apatite, magnetite and pyrite; (3) Magnesium carbonatite with pyrite, rare niobozirconolite and strontiamite. Catalao I carbonatites are poor in Al, Mn, Na and K. Cr, Ni, Co, Cu, Li and Zr-richer samples do occur anomalously. Nb content in carbonatitic veins is very low and suggests that these rocks are not the source for the economic concentration of this element. In both calcite and dolomite, Ba content is smaller than Sr content. Sr, Fe and Mn are mostly associated with dolomite carbonatites. The banded carbonatite is relatively REE-poor, but the magnesium carbonatite bands are REE richer than the associated calcium carbonatite bands, which are extremely poor in all REE. The REE signatures of the distinct carbonatites didn’t show anomalies.