Conodont Alteration Index Research Articles

Multi-proxy record of the Austrian Upper Triassic Polzberg Konservat-Lagerstätte in light of the Carnian Pluvial Episode

We present a multi-proxy investigation of a lower Carnian basinal succession from Polzberg in the Northern Calcareous Alps (Lower Austria). A section comprising a unique Konservat-Lagerstätte was studied based on bio- and chemostratigraphy along with geophysical methods, yielding a detailed and robust stratigraphic calibration of the Polzberg succession. The Polzberg section revealed the paleoceanographic history and helped to identify a global climatic reversal, the Carnian Pluvial Episode. The age of the Upper Triassic Reingraben formation in the Northern Calcareous Alps is refined as the Austrotrachyceras austriacum Zone within the lower Carnian (Julian 2). Ammonoids and conodonts provide a detailed biostratigraphic subdivision that serves as a basis for analyses of the faunal distribution and the paleoenvironmental evolution of the Upper Triassic Reifling Basin. The succession includes lithological and facies changes similar to those of coeval units in the Tethys. The Carnian was characterized by a weak (~ 1‰) positive δ13C trend, punctuated by a negative shift during the lower Carnian corresponding to the initiation of the Carnian Pluvial Episode, a period representing the onset of early/late Carnian transitional global greenhouse conditions. Organic maturity parameters and the conodont alteration index (CAI) show that the thermal overprint of the Polzberg section is low. Biomarker proxies suggest that the organic matter of the uppermost Göstling formation is a mixture of marine and terrestrial material deposited in a dysoxic environment. Within the overlaying Reingraben formation, the amount of marine biomass decreased gradually upwards. Oxygen-depleted conditions, probably due to water-column stratification, continued during deposition of the Reingraben formation. Bacterial sulfate reduction played a major role in organic matter degradation.

Late Cretaceous upper-crustal thermal structure of the Sevier hinterland: Implications for the geodynamics of the Nevadaplano

Abstract Crustal temperature conditions can strongly influence the evolution of deformation during orogenesis. The Sevier hinterland plateau in Nevada and western Utah (“Nevadaplano”) experienced a Late Cretaceous episode of shallow-crustal metamorphism and granitic magmatism. Here, we investigate the thermal history of the Nevadaplano by measuring peak thermal field gradients attained in the upper 10–20 km of the crust along an east-west transect through nine ranges in eastern Nevada and western Utah, by integrating Raman spectroscopy of carbonaceous material thermometry and published conodont alteration indices with reconstructed cross sections. Thermal field gradients of 29 ± 3 °C/km were obtained in the House and Confusion Ranges in westernmost Utah. The Deep Creek, Schell Creek, and Egan Ranges in easternmost Nevada yielded elevated gradients of 49 ± 7 °C/km, 36 ± 3 °C/km, and 32 ± 6 °C/km, respectively. Moving westward, the White Pine, Butte, Pancake, and Fish Creek Ranges exhibit gradients typically between ~20–30 °C/km. The elevated thermal gradients in easternmost Nevada are interpreted to have been attained during ca. 70–90 Ma granitic magmatism and metamorphism and imply possible partial melting at ~18 km depths. Our data are compatible with published interpretations of Late Cretaceous lithospheric mantle delamination under the Sevier hinterland, which triggered lower-crustal anatexis and the resulting rise of granitic melts. The lack of evidence for structures that could have accommodated deep burial of rocks in the nearby Northern Snake Range metamorphic core complex, combined with thermal gradients from adjacent ranges that are ~1.5–3 times higher than those implied by thermobarometry in the Northern Snake Range, further highlights the debate over possible tectonic overpressure in Cordilleran core complexes. Cross-section retro-deformation defines 73.4 ± 4.6 km (76 ± 8%) of extension across eastern Nevada and 15 km of shortening in the Eastern Nevada fold belt.

Oxygen isotope (δ18O) trends measured from Ordovician conodont apatite using secondary ion mass spectrometry (SIMS): Implications for paleo-thermometry studies

Abstract The oxygen isotopic compositions (δ18O) of minimally altered phosphate minerals and fossils, such as conodont elements, are used as a proxy for past ocean temperature. Phosphate is thermally stable under low to moderate burial conditions and is ideal for reconstructing seawater temperatures because the P-O bonds are highly resistant to isotopic exchange during diagenesis. Traditional bulk methods used to measure conodont δ18O include multiple conodont elements, which can reflect different environments and potentially yield an aggregate δ18O value derived from a mixture of different water masses. In situ spot analyses of individual elements using micro-analytical techniques, such as secondary ion mass spectrometry (SIMS), can address these issues. Here we present 108 new δ18O values using SIMS from conodont apatite collected from four Lower to Upper Ordovician stratigraphic successions from North America (Nevada, Oklahoma, and the Cincinnati Arch region of Kentucky and Indiana, USA). The available elements measured had a range of thermal alteration regimes that are categorized based on their conodont alteration index (CAI) as either low (CAI = 1–2) or high (CAI = 3–4). Though individual spot analyses of the same element yield δ18O values that vary by several per mil (‰), most form a normal distribution around a mean value. Isotopic variability of individual spots can be minimized by avoiding surficial heterogeneities like cracks, pits, or near the edge of the element and the precision can be improved with multiple (≥4) spot analyses of the same element. Mean δ18O values from multiple conodonts from the same bed range between 0.0 and 4.3‰ (median 1.0‰), regardless of low or high CAI values. Oxygen isotopic values measured using SIMS in this study reproduce values similar to published trends, namely, δ18O values increase during the Early–Middle Ordovician and plateau by the mid Darriwilian (late Middle Ordovician). Twenty-two of the measured conodonts were from ten sampled beds that had been previously measured using bulk analysis. SIMS-based δ18O values from these samples are more positive by an average of 1.7‰ compared to bulk values, consistent with observations by others who attribute the shift to carbonate- and hydroxyl-related SIMS matrix effects. This offset has implications for paleo-temperature model estimates, which indicate that a 4 °C temperature change corresponds to a 1‰ shift in δ18O (‰). Although this uncertainty precludes precise paleo-temperature reconstructions by SIMS, it is valuable for identifying spatial and stratigraphic trends in temperature that might not have been previously possible with bulk approaches.

Very low-grade metamorphism of the Dezadeash Formation (Jura-Cretaceous): Constraints on the tectonometamorphic history of the Dezadeash flysch basin and implications regarding the tectonic evolution of the Northern Cordillera of Alaska and Yukon

<abstract> <p>Mesozoic convergence of the Wrangellia composite terrane with the western margin of North America resulted in the collapse of intervening flysch basins. One of these basins, the Jurassic-Cretaceous Gravina-Nuzotin belt, comprises from south to north, the Gravina sequence and Gravina belt in southeastern Alaska, the Dezadeash Formation in Yukon, and the Nutzotin Mountains sequence in eastern Alaska. Previous work shows that the Gravina sequence and Gravina belt were underthrust > 20 km beneath the margin of North America in mid-Cretaceous time, culminating in amphibolite facies metamorphism. This tectonometamorphic scenario was subsequently applied to the entire Gravina-Nutzotin belt, despite any detailed studies pertaining to the tectonometamorphic evolution of the Dezadeash Formation. The present analysis of the Dezadeash Formation reveals that metamorphic mineral assemblages in sandstone and tuff document subgreenschist, high temperature zeolite facies metamorphism; Kübler indices of illite and Árkai indices of chlorite in mudstone record diagenetic to high anchizone metapelitic conditions; and the color of organic matter (i.e., the Thermal Alteration Index of palynomorphs and the Conodont Alteration Index) and pyrolysis of organic matter in mudstone and hemipelagite beds document thermal maturation at catagenesis to mesogenesis stages. Collectively, the mineralogic and organic thermal indicators in the Dezadeash Formation suggest that strata experienced maximum pressure-temperature conditions of 2.5 ± 0.5 kbar and 250 ± 25 ℃ in the Early Cretaceous. The inferred tectonometamorphic evolution of the Dezadeash Formation does not support the northern part of the Gravina-Nutzotin belt being underthrust > 20 km beneath the western margin of North America in mid-Cretaceous time, thus contrasting sharply with the Gravina sequence and Gravina belt in the southern part of the Gravina-Nutzotin belt. The diverse tectonometamorphic histories recorded by the southern and northern parts of the Gravina-Nutzotin belt may be a manifestation of oblique collision and diachronous south-to-north accretion of the Wrangellia composite terrane to North America.</p> </abstract>

Earliest Triassic Conodonts of Gua Panjang, Merapoh, Pahang and their Bearing towards Permian-Triassic Mass Extinction in Malaysia

A single sample from the logged section at eastern side of Gua Panjang limestone hill, southwest of Kampung Kubang Rasa Village, Merapoh, has yielded 5 very important conodont species. They are Hindeodus parvus erectus, Hindeodus parvus parvus, Hindeodus latidentatus latidentatus, Hindeodus latidentatus praeparvus, Hindeodus postparvus, Hindeodus eurypge and Isarcicella staeschi. These Early Triassic conodonts were obtained in a bioclastic dolostone sample, located 2.5 m above bioclastic grainstone which yielded Late Permian foraminifera. The conodonts found were given Conodont Alteration Index (CAI) scale of 5, consistent with the heating of Main Range granitoid during Indosinian Orogeny. Limestone harbouring basal Triassic conodonts in Gua Panjang is interpreted to be deposited in an open shallow marine shelf environment.

Shallow-crustal metamorphism during Late Cretaceous anatexis in the Sevier hinterland plateau: Peak temperature conditions from the Grant Range, eastern Nevada, U.S.A.

Documenting spatio-temporal relationships between the thermal and deformation histories of orogenic systems can elucidate their evolution. In the Sevier hinterland plateau in eastern Nevada, an episode of Late Cretaceous magmatism and metamorphism affected mid- and upper-crustal levels, concurrent with late-stage shortening in the Sevier thrust belt. Here, we present quantitative peak temperature data from the Grant Range, a site of localized, Late Cretaceous granitic magmatism and greenschist facies metamorphism. Twenty-two samples of Cambrian to Pennsylvanian metasedimentary and sedimentary rocks were analyzed, utilizing Raman spectroscopy on carbonaceous material, vitrinite reflectance, and Rock-Eval pyrolysis thermometry. A published reconstruction of Cenozoic extension indicates that the samples span pre-extensional depths of 2.5–9 km. Peak temperatures systematically increase with depth, from ∼100 to 300 °C between 2.5 and 4.5 km, ∼400 to 500 °C between 5 and 8 km, and ∼550 °C at 9 km. The data define a metamorphic field gradient of ∼60 °C/km, and are corroborated by quartz recrystallization microstructure and published conodont alteration indices. Metamorphism in the Grant Range is correlated with contemporary, upper-crustal metamorphism and magmatism documented farther east in Nevada, where metamorphic field gradients as high as ∼50 °C/km are estimated. These data have implications for localized but significant thermal weakening of the plateau crust, including attaining temperatures for quartz plasticity at depths of ∼5–6 km, and the potential for partial melting possibly as shallow as ∼12–15 km. Thermal weakening may have contributed to a slowing of shortening rates documented in the Sevier thrust belt at this latitude at ca. 88 Ma, by locally inducing mid- and lower-crustal ductile thickening in the hinterland.

Neoformed magnetic minerals as an indicator of moderate burial: The key example of middle Paleozoic sedimentary rocks, West Virginia

In order to help unravel the thermal history of middle Paleozoic sedimentary rocks in West Virginia, a rock magnetic study was conducted with a focus on the Marcellus Shale. Vitrinite reflectance, fluid inclusions microthermometry and conodont alteration index data yield contradictory burial temperature within the range 150–250°C (302–482°F). The characterization of magnetite and pyrrhotite may be used as an index to track burial temperature around 200°C (392°F). Low-temperature and room-temperature magnetic measurements were performed in order to determine the magnetic assemblage. Three magnetic assemblages were identified that were stratigraphically distributed. The goethite and nanosized magnetite (A1) assemblage is mainly found in the Clinton Group–Oriskany Sandstone stratigraphic interval (Silurian–Lower Devonian). Nanosized fraction of magnetite and probably pyrrhotite (A3) assemblage essentially constitutes the Marcellus Shale–Chemung Formation sequence (Devonian). Microsized pyrrhotite is the typical mineral for A2 that is only identified near the Alleghenian structural front. Overall, the rare occurrence of micron pyrrhotite in our samples suggests that the study area has not experienced burial temperatures higher than 200°C (392°F).

Challenging the sensitivity limits of Paleomagnetism: Magnetostratigraphy of weakly magnetized Guadalupian–Lopingian (Permian) Limestone from Kyushu, Japan

Despite their utility for bio- and chemostratigraphy, many carbonate platform sequences have been difficult to analyze using paleomagnetic techniques due to their extraordinarily weak natural remanent magnetizations (NRMs). However, the physical processes of magnetization imply that stable NRMs can be preserved that are many orders of magnitude below our present measurement abilities. Recent advances in reducing the noise level of superconducting magnetometer systems, particularly the introduction of DC-SQUID sensors and development of a low-noise sample handling system using thin-walled quartz-glass vacuum tubes, have solved many of these instrumentation problems, increasing the effective sensitivity by a factor of nearly 50 over the previous techniques of SQUID moment magnetometry.Here we report the successful isolation of a two-polarity characteristic remanent magnetization from Middle–Late Permian limestone formed in the atoll of a mid-oceanic paleo-seamount, now preserved in the Jurassic accretionary complex in Japan, which had proved difficult to analyze in past studies. Paleothermometric indicators including Conodont Alteration Indices, carbonate petrology, and clumped isotope paleothermometry are consistent with peak burial temperatures close to 130°C, consistent with rock magnetic indicators suggesting fine-grained magnetite and hematite holds the NRM. The magnetic polarity pattern is in broad agreement with previous global magnetostratigraphic summaries from the interval of the Early–Middle Permian Kiaman Reversed Superchron and the Permian–Triassic mixed interval, and ties the Tethyan–Panthalassan fusuline zones to it. Elevated levels of hematite associated with the positive δ13Ccarb of the Kamura event argue for a brief spike in environmental oxygen. The results also place the paleo-seamount at a paleolatitude of ~12° S, in the middle of the Panthalassan Ocean, and imply a N/NW transport toward the Asian margin of Pangea during Triassic and Jurassic times, in accordance with the predicted trajectory from its tectono-sedimentary background. These developments should expand the applicability of magnetostratigraphic techniques to many additional portions of the Geological time scale.

Freshwater shark teeth (Family Lonchidiidae) from the Middle-Upper Triassic (Ladinian-Carnian) Paramillo Formation in the Mendoza Precordillera, Argentina

ABSTRACTA collection of over 550 elasmobranch teeth (Family Lonchidiidae) were obtained from fresh water fluvial deposits of the Middle–Upper Triassic (Ladinian–Carnian) Paramillo Formation, in northwestern Mendoza Province, Argentina. The new hybodontid species, Lonchidion paramilloensis, sp. nov., solely based on teeth, is within a 20-m-thick interval containing black shale interbedded with andesitic sills and intercalated with lensoidal calcarenites. The teeth have a characteristic labial peg with an accessory cusplet and a low gracile crown that is narrower labiolingually and more elongate laterally. The teeth are similar to some North American Late Triassic and Late Cretaceous species of the genus Lonchidion (e.g., L. humblei, L. griffisi, L. selachos), but are distinct in having a low primary cusp, up to seven pairs of lateral cusplets, a rounded-triangular-shaped labial peg with one accessory cusplet, and an absence of other crown ornaments. Color analyses of upper tooth crowns indicate an ichthyolith alteration index (IAI)/conodont alteration index (CAI) value of 6.0–6.5 (360–610°C). Considerable pitting and alteration of specimens indicates thermal alteration to 610°C and (or) chemical alteration.

Thermal History of Lower Paleozoic Rocks on the Peri-Tornquist Margin of the East European Craton (Podolia, Ukraine) Inferred from Combined XRD, K-Ar, and AFT Data

AbstractThe Upper Silurian–Lower Devonian section of the Dniester gorge in Podolia and samples from boreholes located S and N of this area were studied in order to reconstruct the thermal history of Lower Paleozoic sedimentary rocks in the Dniester segment of the Peri-Tornquist margin of the East European Craton which is the most eastern part of a major shale-gas target in Europe. X-ray diffraction data for illite-smectite from shales and carbonates indicate very advanced diagenesis and maximum paleotemperatures of ~200ºC, higher than interpreted from the ‘conodont alteration index’ (CAI) data. Diagenesis of the Devonian section is slightly less advanced than that of the underlying Silurian section, indicating that it is a regional feature and the result of burial. The regional distribution of the diagenetic grade based on illite matches well with the pattern established from the CAI data. K-Ar dating of illite-smectite from Silurian bentonites and shales gave a consistent set of dates ranging from 390 to 312 Ma. To explain such advanced levels of diagenesis and such K-Ar dates, the extension of the Carboniferous foreland basin (which today is only preserved to the NW of L’viv) toward the SE on the craton margin has to be assumed. The diagenetic zonation pattern of the Carboniferous coals supports this hypothesis. The Carboniferous cover may have been either sedimentary or partially tectonic (Variscan intracratonic duplexes) in origin and the thickness, necessary for the observed level of diagenesis, may have been reduced by an elevated heat flow along the major tectonic zone at the edge of the craton (TESZ). The presence of such cover is confirmed by completely reset Cretaceous apatite fission track (AFT) ages of the Silurian bentonites. The AFT dates also imply a Tertiary heating event in the area.The 10 Å clay mineral present in the dolomitic part of the profile (Silurian), both in bentonites and in other rocks, is aluminoceladonite or intermediate between illite and aluminoceladonite, while in the Devonian shale section only illite was documented. Chlorite is also common in the studied rocks and is at least partially authigenic. It is non-expandable in the samples from boreholes, while often expandable to variable extents in the samples from outcrops, which also contain goethite. Such variation in chlorite is attributed to contemporary weathering.

Very low-grade metamorphism of Rheno-Hercynian allochthons (Variscides, Germany): facts and tectonic consequences

The Horre Belt in the SE part of the Rhenish Massif has been regarded as autochthonous because of the apparent lack of metamorphic inversion. We have performed a K-white mica study using powder X-ray diffraction derived Kubler Index (KI) and K-white mica b cell dimension, and the spectrally derived wavelength of the 2,200 nm absorption feature (“2200wvl”) of supposedly allochthonous units and the underlying par-autochthon. KI reveals a slight, but significant inversion of metamorphic grade, which shows that the Horre Belt and the Hessische Schieferserie adjacent to the South were emplaced by thrusting after they had acquired peak metamorphism (M 1) in deeper parts of the orogenic wedge. Only in the westernmost part of the study area, the par-autochthon and the allochthon show the same grade of anchizonal metamorphism. This observation, together with K-white mica b cell dimensions <9.000 A and 2200wvl <2,204 nm, suggests a second, low-pressure metamorphism M 2 subsequent to the emplacement of the allochthon. This is supported by an anomaly of published conodont alteration index data. Our results expand the original width of the Rheno-Hercynian passive margin to at least 580, more probably to 670 or even 750 km. Palinspastic restoration also reveals that the Givetian to Early Carboniferous volcanic belt of the Lahn/Dill area is not only younger than the Emsian MORB-type lavas of the Rheno-Hercynian ocean, but was also separated, from the latter, by a largely non-volcanic belt of at least c. 220 km width. Apparently, the Givetian to Early Carboniferous volcanic episodes were unrelated to the Rheno-Hercynian drift stage.

Another sea area separated from the Panthalassic Ocean in the Norian, the Late Triassic: The lowest Sr isotopic composition of the Ishimaki limestone in central Japan

Mt. Ishimaki is the Jurassic accretionary complex of the Chichibu Belt in Toyohashi City, near Nagoya in central Japan. The Ishimaki limestone is thought to be seamount-type limestone. The P1 elements of the conodonts Norigondolella navicula and Ancyrogondolella quadrata found in the limestone indicate it is of Norian age. The Sr isotopic compositions of 45 Ishimaki limestone samples ranged from 0.7055 to 0.7077. Eighteen of these samples had lower Sr isotopic compositions than the lowest Sr isotopic composition (0.7068) of seawater throughout the Phanerozoic. The Sr isotopic compositions in the limestone block are generally lower at the base of the block and higher at the top. The present Sr isotopic compositions of the Ishimaki limestone are unlikely to have been reduced by post-depositional alteration because most of the limestone samples had a low amount of Mn (<300ppm) or high Sr/Mn ratios (>2) and the conodont elements had low (1–2) CAI (conodont alteration index) values. Additionally, there was little acid-insoluble residue. Thus, the low Sr isotopic compositions are thought to represent the strontium of the past ambient seawater. The low Sr isotopic compositions are in complete disagreement with the generally recognised range of seawater Sr isotopic compositions in the Norian stage of the Late Triassic (0.7075–0.7078) because the depositional environment of the Ishimaki limestone was closed or semi-closed from the Panthalassic Ocean. Therefore, the Sr isotopic composition of the limestone differs from that of the Panthalassic seawater. The low Sr isotopic compositions were probably affected by Sr inflows from mafic oceanic crust by hydrothermal fluid circulation or from hinterlands surrounded by mafic rocks by river water circulation.

K-Ar dating of the Lower Palaeozoic K-bentonites from the Baltic Basin and the Baltic Shield: implications for the role of temperature and time in the illitization of smectite

AbstractMixed-layer illite-smectite samples from the Ordovician and Silurian K-bentonites of the Baltic Basin and the Baltic Shield (Norway, Sweden, Denmark, Poland and Estonia) were dated by K-Ar on several grain fractions and were studied by X-ray diffraction (XRD), both on oriented and random preparations, in order to reveal the conditions of smectite illitization in the area. Authigenic K-feldspar was also dated. The geographic pattern of the degree of illitization (% smectite in illite-smectite measured by XRD) is consistent with other indicators of palaeotemperatures (acritarchs, conodont alteration index, vitrinite reflectance, apatite fission track ages). It reveals the highest maximum palaeotemperatures (up to at least 200ºC) along the Norwegian and the German-Polish branches of the Caledonides and the lowest palaeotemperatures (120ºC) in the central part of the studied area. The distribution of K-Ar ages is not well correlated with this pattern, revealing a zone of older ages (Lower Devonian-Lower Carboniferous) between Denmark and Estonia, and areas of younger ages (Upper Devonian to Carboniferous/Permian boundary) to the north and south of this zone. The zone of older ages is interpreted as the result of illitization induced by a thermal event in front of the Caledonian orogenic belt (migration of hot metamorphic fluids?). The areas of younger ages are considered as representing deep burial illitization under a thick Silurian-Carboniferous sedimentary cover, perhaps augmented by a tectonic load. The K-Ar dates invalidate the hypothesis of a long-lasting low-temperature illitization as the mechanism of formation of the Estonian Palaeozoic illite-smectite. The ammonium content of illite-smectite from the Baltic K-bentonites reflects the proximity of organic-rich source rocks that underwent thermal alteration at the time of illite crystallization.

The Lower Palaeozoic apparent polar wander path for Baltica: palaeomagnetic data from Silurian limestones of Gotland, Sweden

SUMMARY Well-dated and undeformed Silurian (Lower Wenlock) limestones from Gotland, southern Sweden, yield two stable remanence components following detailed thermal and alternating-field demagnetization studies. (1) A low blocking-temperature/coercivity magnetization, termed L, delineated below 250dC/10mT, is oriented parallel to the present Earth's field (Dec. 001, Inc. +67, n= 4 sites, a95= 16d). (2) A higher blocking-temperature/coercivity magnetization, termed H, unblocked between 250-400dC/10-35mT, has a NNE declination and shallow negative inclination (Dec. 025, Inc. -19, n= 5 sites, a95= 5d). This H component direction compares favourably with a previous result from Gotland based upon blanket cleaning. Given a lack of evidence for subsequent geological heating (Conodont Alteration Index = 1-1.5), or pervasive palaeomagnetic overprinting, the H palaeopole is regarded as reliable and primary/early diagenetic in origin (19dS, 352dE, dp/dm 3/5). The only other well-constrained Mid-Silurian pole from Baltica, that from the Ringerike Sandstone of the Oslo district, is in excellent agreement with the Gotland data. These combined poles resolve previous problems regarding the shape and time-calibration of Silurian apparent polar wander relative to Baltica.

Beyond the Obvious Limits of Ore Deposits: The Use of Mineralogical, Geochemical, and Biological Features for the Remote Detection of Mineralization

Far field features of ore deposits include mineralogical, geochemical, or biological attributes that can be recognized beyond the obvious limits of the deposits. They can be primary, if formed in association with mineralization or alteration processes, or secondary, if formed from the interaction of ore deposits with the hydrosphere and biosphere. This paper examines a variety of far field features of different ore deposit types and considers novel applications to exploration and discovery. Primary far field features include mineral and rock chemistry, isotopic or element halos, fluid pathways and thermal anomalies in host-rock sequences. Examples include the use of apatite chemistry to distinguish intrusive rocks permissive for iron oxide copper gold (IOCG) and porphyry deposits; resistate mineral (e.g., rutile, tourmaline) chemistry in exploration for volcanogenic massive sulfide (VMS), orogenic gold, and porphyry deposits; and pyrite chemistry to vector toward sedimentary exhalative (sedex) deposits. Distinctive whole-rock geochemical signatures also can be recognized as a far field feature of porphyry deposits. For example, unique Sr/Y ratios in whole-rock samples, used to distinguish barren versus fertile magmas for Cu mineralization, result from the differentiation of oxidized hydrous melts. Anomalous concentrations of halogen elements (Cl, Br, and I) have been found for distances of up to 200 m away from some mineralized centers. Variations in isotopic composition between ore-bearing and barren intrusions and/or systematic vertical and lateral zonation in sulfur, carbon, or oxygen isotope values have been documented for some deposit types. Owing to the thermal aureole that extends beyond the area of mineralization for some deposits, detection of paleothermal effects through methods such as conodont alteration indices, vitrinite or bitumen reflectance, illite crystallinity, and apatite or zircon thermochronology studies also can be valuable, particularly for deposits with a low-temperature thermal history. A number of newly investigated secondary far field features include the development of reduced columns by electrochemical processes in transported overburden, geochemical dispersion related to the expulsion of groundwater from tectonic and seismic compression, dispersion of vapor above ore deposits, and geochemical dispersion related to biological processes. Redox gradients have been found between underlying reduced and overlying oxidized environments associated with sulfide bodies, which result in mass transfer through electrochemical dispersion. Recent studies have characterized the pH, oxidation-reduction potential (ORP), and self potential (SP) in overburden overlying sulfide-hosted gold and VMS deposits. Lateral migration of metals in groundwater is well understood from normal groundwater flow, but the processes responsible for vertical mass transfer of groundwater and its dissolved components have been recognized only recently. One process, termed cyclical dilatancy pumping, expels groundwater during and after earthquake events, which can cause the redistribution of metals around deposits in some environments. Soil gases are of interest owing to their high degree of mobility through the vadose zone in transported overburden. Numerous soil gas species (CO2, O2, Hg, Rn, He, sulfur compounds, and light hydrocarbons) have been measured and interpreted as diagnostic of some buried mineral deposits, and some evidence suggests a possible link between vapor dispersion and metal enrichment in soil. Geochemical enrichment in plant material and soils through successive growth-death cycles is well established, but the important role of microorganisms is now increasingly evident. Microorganisms significantly enhance the kinetics of sulfide oxidation and influence the distribution of metals around ore deposits. The presence of metal-resistant bacteria and enhanced concentrations of sulfate-reducing bacteria in exotic overburden over buried sulfide deposits suggests that these organisms are attracted to this environment and, therefore, may be important indicators of deposits under cover. Many of these newly recognized far field features have been documented in only a few case studies. Additional research is required to determine the scale of these features, controls on their formation, and the consistency with which they can be measured. Incorporation of methods for detecting far field features into field-based exploration programs may increase the probability of success, especially in covered terrains.

FORMATION OF A PALEOTHERMAL ANOMALY AND DISSEMINATED GOLD DEPOSITS ASSOCIATEDWITH THE BINGHAM CANYON PORPHYRY Cu-Au-Mo SYSTEM, UTAH--A DISCUSSION

Sir: The recent paper by Cunningham et al. (2004) presents reconnaissance apatite and zircon fission track, conodont alteration index, vitrinite and solid bitumen reflectance, and stable isotope data documenting the extent of the paleothermal anomaly associated with the 37 Ma Bingham Canyon porphyry Cu-Au-Mo deposit. Based on these data, the paleothermal anomaly encompasses the Barneys Canyon and Melco sediment-hosted gold deposits. The authors conclude that the gold deposits are genetically related to the Bingham porphyry system and that the maximum sustained temperature was 100° to 140°C at Melco and near 100°C at Barneys Canyon. Cunningham et al. (2004) contend that Barneys Canyon and Melco are, perhaps, distal disseminated Ag-Au deposits rather than Carlin-type deposits. Fluid inclusion data from ore-grade samples at Barneys Canyon (Presnell, 1992; Presnell and Parry, 1996) and Melco (R.D. Presnell, 1991, Constraints on a genetic model for the Melco gold deposit based on mineralogy, alteration, and fluid inclusions, unpub. report for Kennecott Exploration, 12 p.) yield homogenization temperatures in excess of 200°C. We contend that the paleothermal data support our previous conclusion that the temperature at Barneys Canyon during formation of the Bingham system was near ambient (i.e., 70°C, assuming 2 km of cover and a geothermal gradient of 35°C/km), and that the formation of Barneys Canyon was related to a hydrothermal system separate from Bingham. The data presented in Presnell (1992) and Presnell and Parry (1996) are from drill core through the center and periphery of the orebodies at Barneys Canyon, and the data from Melco (R.D. Presnell, 1991, unpub. report for Kennecott Exploration) are also from drill core through the orebody. All of the samples from Barneys Canyon used in the study by Cunningham et al. (2004) were collected from the pit wall after the orebody was mined, and only two samples from their …

Zn-Pb mineralisation in the Silvermines district, Ireland: a product of burial diagenesis

Carbonate-sulphide cement stratigraphic relationships in the host rock and ore have been used to constrain the age of mineralisation at the Silvermines zinc-lead-barium deposit. The base-metal sulphides post-date planar dolomite and replace stylolites. Furthermore, the pre-mineralisation planar dolomites also replace stylolites. These and other diagenetic observations indicate that the base-metal sulphides formed at burial depths greater than 800 m, but probably predate the Variscan deformation (since pressure shadows overgrow base metal sulphides). This indicates that the sulphides are of epigenetic origin, constraining the age of mineralisation to between the late Chadian (≈347 Ma) and the late Westphalian (≈307 Ma). However, the most likely age for mineralisation, (based on widespread macro-stylolite development) is Asbian (≈339 Ma) or younger. No evidence of synsedimentary sulphides (in the form of hydrothermal chimneys, vent faunas, or sulphides intergrown with marine cements) was observed at Silvermines. Mineralised breccias (black matrix breccias), late-stage internal sediments, and dissolution zones within the carbonate cements all appear to be produced by hydrothermal karsting that occurred during the mineralisation process. Fluid inclusion homogenisation temperatures for ore-stage calcites (up to 300 °C) approach the peak temperature estimates derived from regional maturation parameters (270 to 310 °C from conodont alteration indices and vitrinite reflectance). This suggests that homogenisation temperatures represent maximum heating temperatures (probably during Variscan time) rather than mineralisation temperatures.

Organic geochemistry of artificially matured conodonts

Organic geochemical methods for conodont analysis have been developed. Coniform elements of the conodont genus Drepanodus from the Ordovician Emanuel Formation and pectiniform elements of the conodont genus Polygnathus from the Devonian Napier Formation both from the Canning Basin, Australia have been shown to contain different organic matter. Both conodont samples have been artificially matured and the structure of organic matter present related to the conodont alteration index (CAI) and maturation temperature (TM). The carbon 1s X-ray photoelectron spectra (XPS) acquired from both sample sets contain six peaks. These peaks are assigned to highly ordered graphitic like carbon (282.9 eV), sp 3 hybridised carbon bonded to only carbon or hydrogen (284.7 eV), ether (286.1 eV), alcohol (287.0 eV), carbonyl (287.6 eV) and carboxyl (289.0 eV) functional groups. Alkene and aromatic carbon is also probably present but is difficult to delineate. During artificial maturation, XPS analysis monitors the main chemical modifications expressed by the carbon–oxygen functional groups. The first stage of maturation begins with the removal of carboxyl and alcohol groups before a CAI of 2 (TM 450 °C). Between a CAI of 3–6 (TM 450–800 °C) an increase of sp 3 hybridised carbon bonded to only carbon or hydrogen is found, which is accompanied by a decrease in ether and carbonyl functional groups. From a CAI of 6–7 (TM 800–950 °C), sp 3 hybridised carbon bonded to hydrogen or carbon and carbonyl decreases while the development of highly ordered graphitic like carbon is observed. The first order laser Raman spectra recorded for both sample sets show a low degree of structural order from a CAI of 2–3, and from CAI 4–6 show a progressive increase in structural disorder of conodont organic matter. The D/G band (band at approximately 1345–1365/band at 1610 cm −1) line width ratio correlates with CAI. Conodont colour during artificial maturation occurs by migration of compounds to the surface. As the nitrogen compounds are volatilised from the surface, colour disappears.

Sequence and isotope stratigraphy of Late Triassic distal periplatform limestones from the Northern Calcareous Alps (Kälberstein Quarry, Berchtesgaden Hallstatt Zone)

The Kalberstein quarry at Berchtesgaden exposes Carnian-Norian deeper-water Hallstatt limestones. Conodont biostratigraphy, microfacies and stable isotopes of bulk carbonate matrix were investigated. The biostratigraphic results demonstrate a complete succession from the latest Carnian (Tuvalian 3/I) to the late Norian (Sevatian 2). As expected from the periplatform setting of the Hallstatt Zone, calculated mass accumulation rates conform partly to prograding sequences observed on the contemporary Dachstein platform. However, discrepancies exist, mainly for the middle Norian, pointing to an incomplete knowledge of the platform sequences. The sequence stratigraphic framework based on platform data should be complemented with data from the periplatform Hallstatt Zone. Diagenetic alteration of the limestones from Kalberstein quarry is low with a conodont alteration index (CAI)=1.0 throughout the section. Oxygen isotope values ranging from ± 1.2 to + 0.1½ (VPDB) point to stabilization and cementation at very shallow burial depths in contact with seawater in a deeper-water environment. Carbon isotope values display a clear stratigraphic trend with a rapid increase from 3.6 to 4.1½ (VPDB) during the basal Norian (Lacian 1), high values up to 4.2½ during the Lacian 2, and a slow decline starting in Lacian 3 to 2.6½ at the end of the Norian (Sevatian 1±2). These trends are best explained by variations in the global organic carbon/carbonate burial ratio with maximum organic carbon burial during the middle Lacian.

Formula omitted], [formula omitted] and [formula omitted] evolution of Phanerozoic seawater

A total of 2128 calcitic and phosphatic shells, mainly brachiopods with some conodonts and belemnites, were measured for their δ18O, δ13C and 87Sr/86Sr values. The dataset covers the Cambrian to Cretaceous time interval. Where possible, these samples were collected at high temporal resolution, up to 0.7 Ma (one biozone), from the stratotype sections of all continents but Antarctica and from many sedimentary basins. Paleogeographically, the samples are mostly from paleotropical domains. The scanning electron microscopy (SEM), petrography, cathodoluminescence and trace element results of the studied calcitic shells and the conodont alteration index (CAI) data of the phosphatic shells are consistent with an excellent preservation of the ultrastructure of the analyzed material. These datasets are complemented by extensive literature compilations of Phanerozoic low-Mg calcitic, aragonitic and phosphatic isotope data for analogous skeletons. The oxygen isotope signal exhibits a long-term increase of δ18O from a mean value of about −8‰ (PDB) in the Cambrian to a present mean value of about 0‰ (PDB). Superimposed on the general trend are shorter-term oscillations with their apexes coincident with cold episodes and glaciations. The carbon isotope signal shows a similar climb during the Paleozoic, an inflexion in the Permian, followed by an abrupt drop and subsequent fluctuations around the modern value. The 87Sr/86Sr ratios differ from the earlier published curves in their greater detail and in less dispersion of the data. The means of the observed isotope signals for 87Sr/86Sr, δ18O, δ13C and the less complete δ34S (sulfate) are strongly interrelated at any geologically reasonable (1 to 40 Ma) time resolution. All correlations are valid at the 95% level of confidence, with the most valid at the 99% level. Factor analysis indicates that the 87Sr/86Sr, δ18O, δ13C and δ34S isotope systems are driven by three factors. The first factor links oxygen and strontium isotopic evolution, the second 87Sr/86Sr and δ34S, and the third one the δ13C and δ34S. These three factors explain up to 79% of the total variance. We tentatively identify the first two factors as tectonic, and the third one as a (biologically mediated) redox linkage of the sulfur and carbon cycles. On geological timescales (≥1 Ma), we are therefore dealing with a unified exogenic (litho-, hydro-, atmo-, biosphere) system driven by tectonics via its control of (bio)geochemical cycles.