Calcite Samples Research Articles

Analysis of repeated-contact electrification curves

Samples of different materials were repeatedly contacted with metal plates in high vacuum. The transferred charge after contact and separation was measured. This resulted in individual contact electrification curves. Different behaviour of the materials was observed. High-conductivity samples such as oxidized metals or graphite show charge saturation after few contacts. Low-conductivity samples like quartz, calcite and Teflon show charge accumulation up to a large number of contacts. Reversal of the net charge polarity with increasing number of contacts was observed for calcite and quartz samples.

The examination of the phase transformation of aragonite into calcite by means of DSC analysis

The X-ray diffraction method was used to determine the content of aragonite and calcite in samples of precipitated calcium carbonate synthesized in the laboratory. The transformation of the aragonite modification present into the stable calcite structure was traced by means of DSC analysis. All the samples were analysed by DSC under non-isothermal conditions in the temperature range 624–772 K at heating rates of 5, 10, 15, 20 and 25 K min −1. A mathematical analysis of experimental DSC curves defined the mechanism and the kinetics of the phase transformation of aragonite into calcite. The kinetic curves α = f(t) and α = f(T) are sigmoidal in shape, and their inflection points and the v m point of the curves v = f(t) and v = f(T) are interrelated and are defined by the concept of a stationary point. The kinetic curves indicate that v m corresponds to (d H/d t) m of the endothermal peak of transformation of aragonite into calcite, which is characteristic of a topochemical reaction.

Dissolution kinetics of natural calcite minerals in CO 2-water systems approaching calcite equilibrium

We have measured dissolution rates of various natural calcite samples, e.g marbles, limestones and marine pelagic sediments, in CO 2H 2O solutions of fixed P CO 2 and temperature during their approach to equilibrium with respect to calcite. The runs have been carried out as batch experiments using the free-drift technique ( P CO 2 : 5·10 −3atm; T: 20°C), where size fractioned particles of 100 μm were kept in suspension by turbulently stirring the solution. In each experiment three natural specimens and for reference one of synthetic NBS Calcite (SRM 915) have been investigated. The dissolution rates observed for all natural samples can be represented by an empirical rate law given as R=α 1(1 − C C s ) n1 at Ca 2+ concentrations C< xC s ( n 1 ≈ 1.5−2.3, α 1 ≈ 1.6·10 −7−2.2·10 −7 mmol cm −2 s −1, x ≈ 0.6−0.8). Above this concentration the empirical reaction order switches to a higher value of n 2 ≈ 3–4. In contrast, NBS Calcite exhibits a linear rate law of R=α(1- C C s ) with α ≈ 1.6·10 −7 mmol cm −2s −1, which closely follows the dissolution rates predicted by the mechanistic dissolution model (PWP model) of Plummer et al. (1978). Experiments on NBS Calcite in a solution containing 10 μM KH 2PO 4 exhibited results very similar to natural specimens, displaying inhibited dissolution with respect to synthetic calcite, when approaching equilibrium. Only minor influence of phosphate was observed on the dissolution rates of natural samples. From this observation we suggest a model in which Ca 2+ ions are adsorbed to lattice sites (kinks) active to dissolution, blocking further dissolution there. By an attractive interaction between these ions and impurities (heavy metals or phosphate) on the surface of natural materials their adsorption enthalpy increases with increasing coverage Θ of physisorbed Ca 2+ ions on the surface. From this we derive a rate law: R EMP (C)=(1−θ)R PWP ; C=[ θ (1−θ) ]ƒ exp [ −(U o+U o′θ) K B T ] where R PWP is the rate predicted by the PWP model; and (1 − Θ) gives the amount of surface sites still active to dissolution, which is represented by the Fowler-Frumkin isotherm. Our results show that the binding enthalpy U o is a property of the pure calcite surface, whereas U o′ is dependent on the origin of the materials. Using this rate equation we have examined a variety of calcite dissolution data published in the literature and, in all cases, have found them to obey closely the suggested rate law. Especially the earlier published data on inhibition of calcite dissolution by heavy-metal ions, e.g. Cu 2+ and Sc 3+, fit well into our model. It reconciles with many apparently conflicting results and explains the dissolution rates from the chemical reactions proposed by L.N. Plummer and coworkers and adsorption processes of the solute, both acting simultaneously.

Bleaching and thermal stability of γ-induced ESR spectra in synthetic calcite

Abstract Centres induced by γ-irradiation have been investigated in synthetic calcite powder with respect to their applicability for archaeological dating. We report the properties of two axial symmetrical centres with g ⊥ / g ∥ = 2.0035/2.0021 and g ⊥ / g ∥ = 2.0052/2.0024, respectively. The observed ESR spectra of both centres increase in a sublinear manner for doses ranging from 100 up to 3000 Gy. Both centres are light sensitive. The wavelength dependence of the optical cross-section exhibits a threshold at an energy of 2.2 eV for both centres. After 90°C annealing of the totally bleached ESR signals these reappear, which indicates that illumination does not “reset” the geochronological clock of buried calcite samples but rather excites metastable states. High temperature annealing up to 630 K causes a decrease of the centre concentrations: one of the centres fades away according to a non-exponential thermal decay.

Fresh-water carbonates from the Lower Freshwater Molasse (Oligocene, western Switzerland): sedimentology and stable isotopes

The Lower Freshwater Molasse of western Switzerland comprises a thick series of continental deposits of Late Oligocene-Early Miocene age. Fresh-water carbonates occur at two main horizons within this succession. Lower Chattian “palustrine” carbonates (Calcaires Inférieurs) show a shallow fresh-water fauna/flora of ostracods and charophytes, but display clear evidence of subaerial exposure in the form of glaebule/pisoid formation, circumgranular cracking, rhizoliths and microkarstic cavities. Repeated desiccation led to locally intense modification by pedogenesis. These horizons record the development of shallow, ephemeral lakes at the northern margin of the Swiss Molasse Basin. Stable isotope analyses were carried out on 46 calcite samples. Data from matrix and intraclasts reveal typical fresh-water compositions ( δ 13C = −5 to −7.5‰; δ 18O = −5.5 to −9‰). A weak co-variant trend suggests deposition in hydrologically closed lakes, although the range of δ 13C values recorded is also consistent with pedogenetic overprinting associated with subaerial exposure. Spar cements and vadose crystal silts display δ 13C compositions from −6 to −7‰, but show light δ 18O values (from −10 to −11‰), recording formation from meteoric waters. Upper Chattian carbonates (Calcaires d'eau douce et dolomie) include algal-ostracodal laminites, interpreted as profundal lake deposits, as well as bioturbated shallow lake charophyte-ostracod mudstones and wackestones, sandy wackestones and white dolomite. Stable isotope analyses (20 calcites, 2 dolomites) show δ 13C values from −2 to −6‰, and δ 18O from −6 to -11‰. Data for laminites and biomicrites plot in different fields; variations in carbon isotopic composition may be attributable to higher organic productivity during high lake stands (the laminites show heavier δ 13C values) or to areal differences in the proportions of biogenic, authigenic and detrital carbonate ( δ 13C values are generally higher in areas of greater detrital carbonate supply). The heavier δ 18O values shown by the dolomites ( δ 13 C = −3‰ = δ 18 O = 0 to −1‰) suggest formation from evolved, evaporitic lake waters. These carbonates record deposition in an extensive, possibly deeper lake complex subject to clastic input from the Alps in the south.

Isotopic composition of mineral precipitates and free gas associated with hydrothermal vents of Piip submarine volcano, Bering Sea.

Stable isotope ratios of C, S, and O have been measured on minerals associated with active hydrothermal vents, plus a free gas sample which was first found and collected from Piip submarine volcano, southern Bering Sea. The active vents were discovered during the 1990 expedition of the R/V “Akademik Mstislav Keldysh” using the deep submersible “MIR”. The free gas sample and a few fragments of anhydrite chimney and surrounding gypsum- and pyrite-bearing precipitates were collected from the high-temperature field of the North Peak of the volcano. The gas is methane-dominant (80.6%, C1/C2+ = 260). CH4 and CO2 carbon isotope ratios (−48.7‰ and −21.9‰, respectively), as well as the spectrum of C1–C4 hydrocarbons indicate a “thermogenic” origin of the gas. The δ34S values of anhydrite and gypsum (about of 22.5‰) from the northern field are typical for many modern seafloor hydrothermal systems. Pyrite-bearing altered rocks with δ34S = −2.4‰ were formed by recent hydrothermal activity of the volcano. Samples of massive calcite and fragments of aragonite chimneys (containing pyrite) were collected from the low-temperature South Peak field. Aragonite chimney material and massive calcite from the southern field with δ13C = −36 to −29‰ and δ18O = 19–21‰ were formed at about 60°C. Barite and pyrite recovered from the southern field were found to be significantly enriched in 34S (up to 39‰ and +9.2‰, respectively). These data indicate that the C and S isotope compositions of vent material from the southern field are controlled by reduction of marine sulfate by organic sediments or hydrocarbons.

The use of dating techniques to constrain the age of fault activity: a case history from north Somerset, United Kingdom

Abstract Attempts were made to obtain absolute ages constraining the most recent episode of fault activity in the North Somerset Coastal Fault Belt as part of the seismic hazard assessment for the proposed Hinkley Point ‘C’ power station. Radiometric (uranium series, 14 C) and palaeomagnetic dating techniques were applied to calcite from vein systems which cut the Hinkley Point Fault and 14 C dating was applied to bone material from undeformed Quaternary gravel deposits overlying faults in Liassic bedrock at Helwell Bay. Detailed field mapping and microscopy indicated that the samples chosen for dating had not undergone any significant fault-related deformation. Dates obtained from the radiometric techniques ranged from c . 33 ka to greater than 300 ka and reflect the influences of isotopic exchange and contamination as well as the inherent age limitations of the techniques. Palaeomagnetic dating techniques identified reverse polarizaton in a calcite sample from the Hinkley Point Fault, indicating an age greater than 730 ka, the time of the last geomagnetic field reversal. Palaeomagnetic vectors suggest a pre-Pliocene (and conceivably an early Jurassic) age for the calcite vein emplacement.

The effect of procedural variables on TG, DTG and DTA curves of magnesite and dolomite

TG, DTG and DTA curves of magnesite are dependent on procedural variables, especially sample mass, heating rate and partial pressure of carbon dioxide, in a similar manner to those of calcite [1], although the magnitude of the effect is less for magnesite. The first stage of the decomposition of dolomite varies with increasing partial pressure of carbon dioxide in an anomalous manner and hence the effects of these procedural variables (except heating rate) are not similar to those observed for magnesite and calcite. The second stage of the decomposition of dolomite is, however, strongly dependent on these procedural variables and behaves in a manner that would be predicted for a sample of calcite diluted with magnesia. A 1∶1 molar mixture of magnesite and calcite also behaves as would be predicted from the behaviour of the single carbonates but differently from that of dolomite.

Decarbonation and recarbonation of calcites heated m CO 2: Part 1. Effect of the thermal regime

Scanning electron micrographs and thermal analysis curves were obtained from a sample of synthetic calcite that was exposed to a series of heating and cooling cycles in CO 2 at just above atmospheric pressure. The heating rate, the maximum temperature of heating and the residence time at maximum temperature were varied. Scanning electron micrographs revealed that the morphology of the sample changed drastically even after a single heating cycle, but subsequent changes were much smaller, whatever the heat treatment. In contrast, very different thermal analysis curves were obtained. In the first heating cycle calcite decomposed within a narrow temperature range, giving a single, sharp endotherm. An exothermic signal on cooling was due to recarbonation, which occurred abruptly, probably on the surfaces of the particles. An additional small exotherm on the cooling curve was attributed to recrystallisation of poorly crystallised calcium carbonate. In subsequent heating cycles recarbonation partly overlapped decomposition of calcite. Some CO 2 was trapped in pores and more calcite formed as the temperature and pressure in the pores were raised. Depending on the heating regime, one, two or three endotherms resulted.

Structure and bonding environments at the calcite surface as observed with X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED)

The pure calcite surface was examined using techniques sensitive to the near-surface (XPS and LEED) immediately after fracture in ultra-high vacuum (10 −10 mbar) and then following exposure to various atmospheres and aqueous solutions that were free of trace metals. These spectroscopic techniques allow molecular-level observations that offer the possibility of gaining more insight into geochemical processes elucidated from macroscopic solution studies. Several absolute electron binding energies for the atoms in calcite were redetermined with XPS using the gold dot method. The results are 290.1 ± 0.1 eV for C 1 s , 347.7 and 351.2 ± 0.15 eV for Ca 2 p sol3 2 and Ca 2 p sol1 2 , respectively, and 531.9 ± 0.15 eV for O 1 s . Photoelectron energy shifts from main peak positions suggest that immediately after fracture in ultra-high vacuum, bond reconfiguration leads to the formation of carbide-like bonds between Ca and C at the surface. Calcite that has been exposed to water, even as vapour from the atmosphere, shows binding energy shifts that indicate the presence of S · CO 3H and S · CaOH where S · represents the calcite surface. Surface hydration is also supported, independently, by the XPS peak intensity ratios and is consistent with adsorption theory derived from macroscopic solution studies. The modified oxygen Auger parameter, α′, (using O 1 s and O(KVV)), was found to be 1043.9 eV for all samples of calcite tested, whether powder or cleaved from Iceland Spar, clean or contaminated by adventitious carbon, freshly fractured, or exposed briefly to water, or in the process of dissolution or precipitation. LEED patterns of the {101} cleavage surface of samples that were freshly fractured in air and that were exposed to dissolving or precipitating solutions showed that the top few atomic layers exhibit long range order. Lattice spacings at the surface are statistically identical to those of bulk calcite, though some surface CO 3 groups may be rotated relative to the bulk structure. This work provides direct, molecular-level evidence for the processes of reconfiguration and hydration at the calcite surface. These results provide a basis for future spectroscopic studies of trace metal adsorption on calcite and subsequent solid-solution formation.

Instrumental neutron activation analysis of limestone and associated calcite samples from Abakaliki, lower Benue Trough, Nigeria

Twenty four elements: Fe, Al, Ca, Mg, K, Na, Sc, Sr, Co, Cr, Zn, V, Hf, Ir, W, REE (La, Ce, Nd, Sm, Eu, Tb, Yb, Lu) and Th were determined in limestone and associated calcite from Abakaliki, South-Eastern Nigeria, by instrumental neutron activation analysis (INAA) technique. The USSG reference materials BCR-1 and AGV-1 were included in the analysis to assure quality control of the accumulated data. Hitherto, there is very little work of this type on the deposits in the region being studied. This study, therefore, provides baseline data for the deposit.

A mineralogical petrographic and geochemical study of samples from wells in the geothermal field of Milos Island (Greece)

This paper presents a study of hydrothermal alteration on Milos island, Greece. Examination of cores and cuttings from the two drill sites, obtained from a depth of about 1100 m in Milos geothermal field, showed that the hydrothermal minerals occurring in the rock include: K-feldspar, albite, chlorite, talc, diopside, epidote, muscovite, tremolite, kaolinite, montmorillonite, alunite, anhydrite, gypsum, calcite, and opaque minerals. The chemical composition of the minerals (104 analyses) was determined with Electron Probe Microanalysis. The composition of the hydrothermal fluid was determined and correlated with the mineralogy. Isotopic ratios of C and O for one calcite sample taken from 341 m depth were determined and used for geochemical calculations. A number of reactions feasible at the P—T conditions of the geothermal field are given to establish the chemical evolution of the hydrothermal fluid. The distribution of the hydrothermal minerals indicates the dilution of the K-, Na-, Cl-rich hydrothermal fluid of the deep reservoir by a Ca-, Mg-rich cold water at a shallower level.

Pleistocene climatic change in Southern Australia and its effect on speleothem deposition in some Nullarbor caves

AbstractActivity ratios of 234U/238U, 230Th/234U, and 230Th/232Th have been determined for calcite, gypsum and halite speleothems from caves of the Nullarbor Plain, mostly in the area N and NW of Mundrabilla Station, for the purpose of U‐series dating. All calcite speleothems contain adequate amounts of uranium for dating, but some show an excess of 230Th. Stratigraphic relationships indicate that there were at least three phases of calcium carbonate deposition in the Nullarbor caves. The calcite samples, with one possible exception, have ages in excess of ca. 400000 yrs BP. This suggests that no significant amounts of calcium carbonate deposition have taken place during the last 400ka.At present, active deposition of speleothems is restricted almost entirely to gypsum and halite. The only gypsum speleothem dated was found to have a finite age of ca. 185 ka. Six dates on a small halite speleothem containing insect and arachnid remains indicate that it formed rapidly during Holocene time.

High-paleolatitude Late Cretaceous paleotemperatures: New data from James Ross Island, Antarctica

Research Article| January 01, 1990 High-paleolatitude Late Cretaceous paleotemperatures: New data from James Ross Island, Antarctica D. Pirrie; D. Pirrie 1British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge CB3 OET, England Search for other works by this author on: GSW Google Scholar J. D. Marshall J. D. Marshall 2Department of Earth Sciences, Jane Herdmann Laboratories, University of Liverpool, Brownlow Street, P.O. Box 147, Liverpool L69 3BX, England Search for other works by this author on: GSW Google Scholar Geology (1990) 18 (1): 31–34. https://doi.org/10.1130/0091-7613(1990)018<0031:HPLCPN>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation D. Pirrie, J. D. Marshall; High-paleolatitude Late Cretaceous paleotemperatures: New data from James Ross Island, Antarctica. Geology 1990;; 18 (1): 31–34. doi: https://doi.org/10.1130/0091-7613(1990)018<0031:HPLCPN>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Oxygen-isotope analysis of well-preserved macrofossils from the Santonian-Campanian of James Ross Island and the Maastrichtian of Vega Island, Antarctica, indicates that cool high-paleolatitude temperatures prevailed during the Late Cretaceous and suggests that cooling occurred between the Santonian-Campanian and the Maastrichtian. Although more than 50% of the material showed diagenetic alteration, 52 unaltered aragonite and calcite samples were analyzed. Mean δ18O and calculated paleotemperature values were -0.23‰ and 13.6 °C, respectively, for the Santonian-Campanian, and 0.66‰ and 11.7 °C, respectively, for the Maastrichtian. In conjunction with recent Late Cretaceous paleoclimatic data from high northern paleolatitudes, these data indicate the presence of cool polar regions with broad climatic zonation during the Late Cretaceous. This may have partly controlled faunal distributions. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Thermoluminescence study of annealing a geological calcite

Annealing of calcite prior to measuring second-glow TL growth curves is an important part of TL dating of calcite. That large changes in TL sensitivity to ionizing radiation are sometimes brought about by annealing is well documented. Annealing studies of two samples of a geological calcite revealed that TL peaks in the violet region of the spectrum (300–450 nm wavelength) and one in the orange region (≈590 nm) grew in sensitivity with annealing temperature up to some limiting temperature, depending on the TL peak. Upon annealing above this limiting temperature the peak disappeared. These effects were not reversible. The presence of an elevated CO 2 partial pressure in the annealing atmosphere enhanced the sensitivity of at least one violet TL peak, at ≈240°C.

Oxygen and carbon isotope ratios of hydrothermal minerals from Yellowstone drill cores

Oxygen and carbon isotope ratios were measured for hydrothermal minerals (silica, clay and calcite) from fractures and vugs in altered rhyolite, located between 28 and 129 m below surface ( in situ temperatures ranging from 81 to 199°C) in Yellowstone drill holes. The purpose of this study was to investigate the mechanism of formation of these minerals. The δ 18 O values of the thirty-two analyzed silica samples (quartz, chalcedony, α-cristobalite, and β-cristobalite) range from −7.5 to +2.8‰ . About one third of the silica 7samples have δ 18 O values that are consistent with isotopic equilibrium with present thermal waters; most of the other silica samples appear to have precipitated from water enriched in 18 O (up to 4.7‰) relative to present thermal water, assuming precipitation at present in situ temperatures. Available data on fluid-inclusion homogenization temperatures in hydrothermal quartz indicate that silica precipitation occurred mostly at temperatures above those measured during drilling and imply that 15 O enrichments in water during silica precipitation were generally larger than those estimated from present conditions. Similarly, clay minerals (celadonite and smectite) have δ 18 O values higher (by 3.5 to 7.9‰) than equilibrium values under present conditions. In contrast, all eight analyzed calcite samples are close to isotopic equilibrium with present thermal waters. The frequent incidence of apparent 18 O enrichment in thermal water from which the hydrothermal minerals precipitated may indicate that a higher proportion of strongly 18 O-enriched deep hydrothermal fluid once circulated through shallow portions of the Yellowstone system, or that a recurring transient 18 O-enrichment effect occurs at shallow depths and is caused either by sudden decompressional boiling or by isotopic exchange at low water/rock ratios in new fractures. The mineralogy and apparent 18 O enrichments of hydrothermal fracture-filling minerals are consistent with deposition during transient boiling or rock-water exchange (fracturing) events.

Ages and stable-isotope compositions of secondary calcite and opal in drill cores from Tertiary volcanic rocks of the Yucca Mountain area, Nevada

Stable-isotope compositions of fracture-and cavity-filling calcite from the unsaturated zone of three drill cores at Yucca Mountain Tertiary volcanic complex indicate that the water from which the minerals precipitated was probably meteoric in origin. A decrease in 18 O in the calcite with depth is interpreted as being due to the increase in temperature in drill holes corresponding to an estimated average geothermal gradient of 34° per kilometer. A few of the calcite samples and all of the opal samples yielded uranium-series ages older than 400,000 yr, although most of the calcite samples yielded ages between 26,000 and 310,000 yr. The stable-isotope and uranium- series dates from precipitated calcite and opal of this reconnaissance study suggest a complex history of fluid movement through the volcanic pile, and episodes of fracture filling predominantly from meteoric water during at least the past 400,000 yr.

Problems of crush—Leach analyses of low-salinity inclusion-poor material

We present analyses which suggest that single or multiple water leaches of low-salinity fluid inclusions are suspect and their application to chemical geothermometry is likely to give erroneous results. Leaching experiments were conducted on 20 quartz and 1 calcite sample from epithermal veins in the Hauraki Goldfield, New Zealand, and 1 calcite drill-cutting sample from the Ngawha geothermal area. Each sample was crushed under vacuum in stainless-steel tubes and leached with a series of solvents. A three-stage leaching technique using water, neutral-pH ammonium nitrate solution and 13% nitric acid solution was finally adopted. This process is designed to leach the fluid inclusions, remove ions adsorbed on the crushed mineral surfaces, and additionally indicate the presence of contamination from the host mineral and contained solid inclusions. Most of our analyses show contamination of Al, Ca, Li, Mg and, where analysed, Zn. A few are also contaminated by Na, K, Cl, Cs and Rb. Least contaminated analyses (11) indicate that the Hauraki epithermal fluids sampled had salinities between 0.3 and 2.4 wt.% with an average value of 1.0 wt.%.

Temporal changes in interstitial water chemistry and calcite recrystallization in marine sediments

Recrystallization processes in marine sediments can alter the extent to which biogenic calcite composition serves as a proxy of oceanic chemical and isotopic history. Models of calcite recrystallization developed to date have resulted in significant insights into these processes, but are not completely adequate to describe the conditions of recrystallization. Marine sediments frequently have concentration gradients in interstitial dissolved calcium, magnesium, and strontium which have probably evolved during sediment accumulation. Realistic, albeit simplified, models of the temporal evolution of interstitial water profiles of Ca, Mg, and Sr were used with several patterns of recrystallization rate variation to predict the composition of recrystallized inorganic calcite. Comparison of predictions with measured Mg/Ca and Sr/Ca ratios in severely altered calcite samples from several Deep Sea Drilling Project sites demonstrates that models incorporating temporal variation in interstitial water composition more successfully predict observed calcite compositions than do models which rely solely on present-day interstitial water chemistry. Temporal changes in interstitial composition are particularly important in interpreting Mg/Ca ratios in conjunction with Sr/Ca ratios. Estimates of Mg distribution coefficients from previous observations in marine sediments, much lower than those in laboratory studies of inorganic calcite, are confirmed by these results. Evaluation of the effects of diagenetic alteration of biogenic calcium carbonate sediment must be a site-specific process, taking into account accumulation history, present interstitial chemistry and its variation in the past, and sample depths and ages.

Evidence for Cu-(Ag) mineralization by magmatic-meteoric fluid mixing in Keweenawan fissure veins, Mamainse Point, Ontario

Locally rich (up to 40% Cu) Cu-(Fe) sulfide mineralization with minor Ag occurs in fault-related fissure veins cutting, but probably penecontemporaneous with, Keweenawan plateau basalt lava flows at Mamainse Point, Ontario. Paragenetic studies of samples from the Coppercorp mine define four stages of mineralization: (1) early pyrite-(chalcopyrite) with strong silicification and sericitic alteration; (2) a transitional stage of chalcopyrite-hematite and bornite-hematite assemblages, with propylitic alteration; (3) main-stage chalcocite-hematite, often replacing earlier sulfides, with propylitic alteration; and (4) late-stage, low-temperature calcite deposition, with local Cu degrees , Ag degrees , Cu arsenides, and cuprite. This paragenetic sequence suggests an increase in f (sub O 2 ) , perhaps accompanied by an increase in pH and a decrease in total sulfur activity.The mineralized fissures crosscut but are coeval with regional-scale low-grade (zeolitic to local epidotic) alteration in the plateau basalt sequence; their mineralogy, particularly the early sericitic wall-rock alteration, is at variance with the regional alteration mineralogy. This contrast is supported by fluid inclusion and light stable isotope data. Temperatures of the regional alteration reached little higher than 300 degrees C at the base of the lava pile, and fluid salinities were generally below approximately 2 equiv wt percent NaCl, suggesting the dominant presence of meteoric water. However, in the fissure veins, fluids varied from or = 450 degrees C and approximately 22 equiv wt percent NaCl. Fluid inclusions representing the hot saline fluids were only found in the presence of copper mineralization, and it is therefore concluded that these fluids were responsible for metal transport. Analysis of decrepitation residues from fluid inclusions in chalcocite indicate that the fluids were dominantly NaCl-CaCl 2 -H 2 O solutions; suppression of the critical curve by the presence of Ca (super +2) permits the interpretation that these saline, high-temperature fluids were near their critical point at approximately 450 degrees C. Their high temperatures compared to the ambient fluids suggest that they were derived from below the Keweenawan lava pile and that they may have been associated with intrusive magmatic activity. Approximately positively correlated arrays of homogenization temperature vs. salinity data from fluid inclusions in ore samples suggest that mixing between the hot saline fluid and the cooler meteoric ground waters was an important mechanism for deposition of Cu-(Fe) sulfides. The combined effects of rapid cooling, dilution, neutralization, and oxidation resulting from this process would cause a significant drop in copper solubility with consequent deposition of ore; this process would also explain the observed paragenetic sequence of reduced to oxidized assemblages.The delta 13 C values of calcite from the fissure veins average -3.9 + or - 0.9 per mil (n = 12), and fluid delta 18 O ratios calculated from seven samples of quartz and epidote intergrown with sulfides range from 10.7 to 2.5 per mil (+ or -2ppm due to temperature uncertainties). These isotopic ratios overlap the ranges for magmatic carbon and water, respectively. Eight samples of late calcite from mineralized veins indicate fluid delta 18 O values ranging from 4.8 to -3.1 per mil (+ or -0.5-4.5ppm). Calcite from barren veins (n = 3) and from veinlets associated with the regional alteration (n = 4) extend this range down to -6.9 per mil; delta 13 C ratios of between -2.7 and +0.7 per mil were obtained from these last four samples. Negative fluid delta 18 O values and delta 13 C values of carbonates near zero per mil suggest the dominant presence of meteoric waters and atmospheric carbon, respectively, in the regional system and the dominance of meteoric water in barren veins in the fissures; this interpretation is in agreement with the fluid inclusion data. The range of delta 18 O from magmatic to lighter values in the mineralized veins suggests that meteoric water was also present during ore deposition and was mixed in varying proportions with a magmatic fluid. The isotopic data, therefore, support a model of meteoric ground water and magmatic (or highly exchanged) fluid interaction in the fissure veins.Comparisons are made to the nearby Keweenawan Jogran Cu-(Mo) porphyry, and the Tribag Cu-(Mo) breccia pipes. Similarities between these deposits and the Mamainse Point fissure veins in mineralogy, fluid properties, and light stable isotope compositions suggest a genetic link. It is proposed that the Coppercorp, Tribag, and Jogran deposits represent successively deeper levels of exposure in Keweenawan porphyry-related hydrothermal systems. Richest ore deposition occurred in the high-level fissure veins at Coppercorp, due to focused mixing of hot metalliferous fluids with meteoric ground waters.