Poul Emsbo

Precise determination of δ88Sr in rocks, minerals, and waters by double-spike TIMS: a powerful tool in the study of geological, hydrological and biological processes

We present strontium isotopic (88Sr/86Sr and 87Sr/86Sr) results obtained by 87Sr–84Sr double spike thermal ionization mass-spectrometry (DS-TIMS) for several standards as well as natural water samples and mineral samples of abiogenic and biogenic origin. The detailed data reduction algorithm and a user-friendly Sr-specific stand-alone computer program used for the spike calibration and the data reduction are also presented. Accuracy and precision of our δ88Sr measurements, calculated as permil (‰) deviations from the NIST SRM-987 standard, were evaluated by analyzing the NASS-6 seawater standard, which yielded δ88Sr = 0.378 ± 0.009‰. The first DS-TIMS data for the NIST SRM-607 potassium feldspar standard and for several US Geological Survey carbonate, phosphate, and silicate standards (EN-1, MAPS-4, MAPS-5, G-3, BCR-2, and BHVO-2) are also reported. Data obtained during this work for Sr-bearing solids and natural waters show a range of δ88Sr values of about 2.4‰, the widest observed so far in terrestrial materials. This range is easily resolvable analytically because the demonstrated external error (±SD, standard deviation) for measured δ88Sr values is typically ≤0.02‰. It is shown that the “true” 87Sr/86Sr value obtained by the DS-TIMS or any other external normalization method combines radiogenic and mass-dependent mass-fractionation effects, which cannot be separated. Therefore, the “true” 87Sr/86Sr and the δ87Sr parameter derived from it are not useful isotope tracers. Data presented in this paper for a wide range of naturally occurring sample types demonstrate the potential of the δ88Sr isotope tracer in combination with the traditional radiogenic 87Sr/86Sr tracer for studying a variety of biological, hydrological, and geological processes.

Hydrothermal Zebra Dolomite in the Great Basin, Nevada—Attributes and Relation to Paleozoic Stratigraphy, Tectonics, and Ore Deposits

In other parts of the world, previous workers have shown that sparry dolomite in carbonate rocks may be produced by the generation and movement of hot basinal brines in response to arid paleoclimates and tectonism, and that some of these brines served as the transport medium for metals fixed in Mississippi Valley-type (MVT) and sedimentary exhalative (Sedex) deposits of Zn, Pb, Ag, Au, or barite. Numerous occurrences of hydrothermal zebra dolomite (HZD), comprised of alternating layers of dark replacement and light void-filling sparry or saddle dolomite, are present in Paleozoic platform and slope carbonate rocks on the eastern side of the Great Basin physiographic province. Locally, it is associated with mineral deposits of barite, Ag-Pb-Zn, and Au. In this paper the spatial distribution of HZD occurrences, their stratigraphic position, morphological characteristics, textures and zoning, and chemical and stable isotopic compositions were determined to improve understanding of their age, origin, and relation to dolostone, ore deposits, and the tectonic evolution of the Great Basin. In northern and central Nevada, HZD is coeval and cogenetic with Late Devonian and Early Mississippian Sedex Au, Zn, and barite deposits and may be related to Late Ordovician Sedex barite deposits. In southern Nevada and southwest California, it is cogenetic with small MVT Ag-Pb-Zn deposits in rocks as young as Early Mississippian. Over Paleozoic time, the Great Basin was at equatorial paleolatitudes with episodes of arid paleoclimates. Several occurrences of HZD are crosscut by Mesozoic or Cenozoic intrusions, and some host younger pluton-related polymetallic replacement and Carlin-type gold deposits. The distribution of HZD in space (carbonate platform, margin, and slope) and stratigraphy (Late Neoproterozoic Ediacaran–Mississippian) roughly parallels that of dolostone and both are prevalent in Devonian strata. Stratabound HZD is best developed in Ediacaran and Cambrian units, whereas discordant HZD is proximal to high-angle structures at the carbonate platform margin, such as strike-slip and growth faults and dilational jogs. Fabric-selective replacement and dissolution features (e.g., collapse breccias, voids with geopetal textures) are common, with remaining void space lined with light-colored dolomite crystals that exhibit zoning under cathodoluminescence. Zoned crystals usually contain tiny ( ∼70 °C. The oxygen isotopic compositions of HZD are consistent with formation temperatures of 50–150 °C requiring brine circulation to depths of 2–5 km, or more. The few HZD occurrences with the highest concentrations of metals (especially Fe, Mn, and Zn) and the largest isotopic shifts are closely associated with Sedex or MVT deposits known to have formed from hotter brines (e.g., Th > 150–250 °C). These relationships permit that HZD formed at about the same time as dolostone, from brines produced by the evaporation of seawater during arid paleoclimates at equatorial paleolatitudes. Both dolostone and HZD may have formed as basinal brines, which migrated seaward from evaporative pans on the platform, with dolostone forming at low temperatures along shallow migration pathways through permeable limestones, and HZD forming at high temperatures along deeper migration pathways through basal aquifers and dilatant high-angle faults. The small MVT deposits were chemical traps where hot brines encountered rocks or fluids containing reduced sulfur. The abundant Sedex deposits mark sites where hot brine discharged at the seafloor in adjacent basins. Thus the distribution of HZD may map deep migration pathways and upflow zones between eastern shallow marine facies, where evaporative brine could have been generated, and western Sedex deposits, where heated brines discharged along faults into platform margin, slope, and basin facies. The small size and scarcity of Pb-Zn deposits and the abundance of barite deposits in the Great Basin suggests the brines were generally reduced, possibly due to reactions with carbonaceous rocks along deep migration pathways. While this scenario may have occurred at several times, the age and abundance of Sedex deposits suggest that such a hydrology was best developed in the Late Ordovician, Late Devonian, and Early Mississippian, possibly in response to episodes of extension and forebulge faults associated with the Antler orogeny. The improved understanding of HZD may aid future exploration for ore deposits in the Great Basin.

Lower cambrian metallogenesis of south China: Interplay between diverse basinal hydrothermal fluids and marine chemistry

The lowermost Cambrian metalliferous black shales of southern China represent a unique metallogenic province. The shales host a wide range of world-class synsedimentary metal deposit types. The diverse metal assemblages in these deposits are best explained by venting of multiple hydrothermal fluids including reduced H2S-rich brines, petroleum, and oxidized brines into the basin. Coinciden formation of shales that are extraordinarity rich in organic carbon and metals suggest a rapid increase in bioproductivity and anoxic/dysoxic conditions during ore formation. We propose that basinal fluids were the source of bioliming nutrients that caused eutrophication and basin-wide anoxia. The dramatic geologic and geochemical changes in this basin demonstrate the possible impacts of hydrothermal systems on the generation and sequestration of organic matter, formation of black metalliferous shales, and chemical changes of seawater.

Source of ore fluids in Carlin-type gold deposits, China: Implications for genetic models

Fluid inclusion and isotopic data on quartz, carbonate, and clay minerals from Carlin-type deposits in two gold grovinces of southwest China suggest the deposits formed at epizonal levels where metamorphic fluids discharged into foreland fold and thrust belts, reacted with sedimentary rocks, and mixed with local meteoric ground waters.

Geochemistry and δ34S of ores and ore stage iron sulfides in Carlin-type gold deposits, Dian-Qian-Gui area, China: Implications for ore genesis

The relative enrichments of Au, trace elements, S, and Fe in gold deposits in the Dian-Qian-Gui area are nearly identical to those of Nevada Carlin-type deposits. Pyrite, marcasite, and arsenopyrite of the ore stage typically occur as disseminations, and locally in open spaces, and have high concentrations of Au, As, Sb, Hg, and Tl, and low Ag and base metals. These elements are soluble in H2S-rich fluids and precipitated by sulfidation of ferroan minerals in the host rocks or Fe in a contrasting fluid. δ34S data indicate reduced S was derived from sedimentary rocks. These data and the absence of subduction-related plutons in the area are consistent with amagmatic models for these deposits.

Reply to comments by T. Oberthür on “Trace element distribution in uraninite from Mesoarchaean Witwatersrand conglomerates (South Africa) supports placer model and magmatogenic source”

In his comments on our paper (Depine et al. 2013) on the chemistry of uraninite grains from the auriferous conglomerates of the Mesoarchaean Witwatersrand Supergroup, Oberthur (2013) highlights a number of points, some of which had not been fully discussed by us due to the limitation on space in a short “letter”. We are therefore grateful for the opportunity to expand on some of the implications of our new data in this discussion here. The abundance of major and minor elements in uraninite from the Witwatersrand has been the subject of several previous studies. We could add to this knowledge, as acknowledged by Oberthur (2013), by presenting the first data on trace element concentrations in Witwatersrand uraninite as obtained by LA-ICPMS. The main message we tried to convey in our letter has been that the mineral chemistry of the uraninite grains, now also including a range of elements hitherto not or only poorly analysed for, provides unequivocal evidence of the detrital nature of these grains. We are satisfied, though not surprised, that Oberthur, an expert with livelong experience with the studied rocks, fully concurs with this principal conclusion. For many readers, this conclusion may not be worth receiving further attention after dozens of papers have been published over the past decades on the topic of the genesis of the world’s largest gold province and the U-ores therein, in which a strong case for a modified palaeoplacer model has been made. Since the last review (Frimmel et al. 2005), the discussion on the genesis of the Witwatersrand ores has been, however, by no means closed, and views opposing to those of Oberthur and many other workers have been presented more recently. The papers by Phillips and Powell (2011), who advocate a postdepositional syn-metamorphic introduction of the gold into the host conglomerates, or by Horscroft et al. (2011), who argue for a syngenetic, microbially mediated chemical precipitation of gold, uraninite and pyrite, may serve as examples. We therefore believe that it remains timely and opportune to continue this discussion on the Witwatersrand metallogeny with new data. Oberthur (2013) dismisses any model involving syngenetic or epigenetic-hydrothermal introduction of U into the host conglomerates because of the lack of oxidising conditions under an Archaean atmosphere. We could not agree more but want to point out that this has been a circular argument for a long time. After all, the evidence for a postulated reducing atmosphere in the Archaean has come primarily from the interpretation of the ore components, specifically rounded massive pyrite and uraninite, in the Witwatersrand goldfields (Krupp et al. 1994; Frimmel 2005). Only in the last decade, largely based on the discovery of mass-independent S isotope fractionation in Archaean rocks, independent evidence in favour of an O2-deficient Archaean atmosphere could be brought forward (Farquhar et al. 2007), thus supporting the original contention of the detrital Witwatersrand mineralogy. As effectively all points raised by Oberthur (2013) are discussed in greater detail in a full paper currently under review elsewhere (Frimmel et al. 2013), in which we present Editorial handling: B. Lehmann

Sequence boundaries and chronostratigraphic gaps in the Llandovery of Ohio and Kentucky: The record of early Silurian paleoceanographic events in east-central North America

New and published data are integrated herein to resolve the age and stratigraphic relationships for problematic strata of the Aeronian and Telychian (Llandovery; Silurian) in Ohio and Kentucky (USA). At least two major depositional sequences were traced along the eastern flank of the Cincinnati Arch; these are separated by a regionally angular unconformity with complex topography. Underlying units are progressively truncated to the northwest while overlying strata change facies, condense, and onlap in the same direction. The basal unit of the upper sequence is the Waco Member of the Alger Shale Formation in Kentucky and southern Ohio and the Dayton Formation in western Ohio. A persistent, positive carbonate carbon isotope (δ13Ccarb) excursion associated with the mid-Telychian Valgu Event is recognized in the upper subunit of the Waco Member; the absence of a comparable signal in the Dayton Formation corroborates interpretations that it is significantly younger. The correlations proposed here can be used to understand the nuanced depositional history and chronostratigraphic completeness of the lower Silurian in eastern North America. This framework can be used to characterize sea-level history and local conditions that prevailed during global paleoenvironmental events.

Chitinozoan biostratigraphy of the Silurian Wenlock–Ludlow boundary succession of the Long Mountain, Powys, Wales

Systematic collecting through the upper Wenlock (upper Homerian) and lower Ludlow (Gorstian and lowermost Ludfordian) Silurian rock succession of the Long Mountain, Powys, Wales, identifies some 48 chitinozoan species that distinguish four biozones, two subzones and an interregnum. Consideration of the chitinozoan biozones together with those of the graptolites enables a local three-fold subdivision of the late Homerian lundgreni graptolite Biozone, and the distinction of lower and upper intervals for the Gorstian incipiens graptolite Biozone. The base of the Ludlow Series in the Long Mountain more or less equates to the base of the Cingulochitina acme chitinozoan Biozone, although no key chitinozoan first or last appearance datums are associated with the series boundary itself. The new graptolite–chitinozoan biozonation allows enhanced correlation between upper Wenlock and lower Ludlow sedimentary deposits of the Lower Palaeozoic Welsh depositional basin and those of the palaeo-shelf in the stratotype Wenlock and Ludlow areas of Shropshire. Chitinozoans seem affected by the phenomena that caused the late Wenlock ‘Mulde extinction’ in graptolites but, with the final disappearance of 9 species and re-appearance of 11 species following an interval of overall low diversity, they seem to have suffered less severely than their macro-zooplanktonic contemporaries.

The Society of Economic Geologists 2004 Awards R.A.F. Penrose Gold Medal for 2004 Citation of Richard W. Hutchinson

Mr. President, Society members, and guests: It is an honor to introduce this year’s winner of the Penrose Gold Medal, Richard W. Hutchinson. I am humbled by introducing a man who has shaped our global community through his research and teaching. As Dick’s last student (for the record, I did not drive him out of academia) I would like to highlight the unique contributions Dick has made through the more than 75 graduate students he supervised from 17 countries on five continents, the countless geologists he has inspired in his classrooms, in short courses around the world, and on the mine-face during his legendary field trips to the great ore deposits of the world. Dick humbly credits his mentors Gordon Suffel and Eugene Cameron for laying the foundation of his scientific philosophy and sparking his fascination with the earth processes that form ore deposits. Dick’s geologic career began in his hometown, London, Ontario, at the University of Western Ontario, under Gordon Suffel, and he graduated at the top of the class in geology (B.Sc. 1950). His passion for the mining industry began during summer field exploration for various Canadian mining companies. Graduate work at the University of Wisconsin (M.Sc, 1951; Ph.D., 1954) under Eugene Cameron combined regional mapping in the Northwest Territories for the Geological Survey of Canada with chemical and mineralogic zoning of pegmatites. This work led to a series of publications that increased understanding of the links between silicate melts and hydrothermal fluids and launched his industry career mapping rare-element pegmatites for Union Carbide in Mozambique. This led to a job with American Metal Company/AMAX, exploring for massive sulfides in the Bathurst district in New Brunswick. Dick quickly climbed the ranks at AMAX and he began managing global exploration for base metal sulfides, carbonatites, pegmatites, and …