Mineralogy and microstructures of carbonaceous gold ores

Summary This paper describes mineralogical and geological aspects of the Owl Creek and Hoyle Pond gold mines, located in Archean metasediments and metavolcanics in the Abitibi greenstone belt in northern Ontario, Canada. The occurrence of invisible gold of Cenozoic age, hosted in structurally-controlled settings in Paleozoic strata of the southwestern U.S.A., has been known and exploited for over thirty years. Petrographic observations are combined with electron microprobe data in order to describe aspects of gold mineralization from these Ontario mines and from selected analogous sites elsewhere. Details of selected exposures of Archean-Proterozoic lithologies, which are carbonaceous but not strongly enriched in gold, are briefly compared with features of the mine geology. Although no mine generates carbonaceous ores alone, the proportion of such ores varies widely from mine to mine, and district to district. Questions remain, particularly with regard to the chemical behaviour of the reduced carbon, but two common factors of prime economic relevance are established. These are the importance of (a) fluid focusing through incompetent carbonaceous units, and (b) localization of high (commonly visible) gold values on vein-wallrock contacts.

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La Minéralogie et micro-structures des minerais carbonifères d'or

Résumé Ce compte rendu s'intéresse au gîtes carbonifères d'or, spécialement dans la ceinture de roches vertes de l'Abitibi au nord de l'Ontario, au Canada. Des observations pétrographiques sont combinées avec des analyses de microsonde électronique afin de décrire quelques aspects de la minéralogie et de la géologie des mines d'or; en particulier Owl Creek et Hoyle Pond, et aussi des emplacements dont la concentration en or est trop faible pour être exploité. Trois générations de pyrite sont observée à Owl Creek, la troisième contenant de l'or. Souvent l'or est situé entre la veine de quartz et l'encaissante carbonifère, fréquemment avec de l'hydromicas sodiques, de la chlorite ferrifère et de la tourmaline. Cette association dont l'importance économique est substantielle, est produite par la concentration de fluides hydrothermales à travers des rocs carbonifères friables.

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Contribution to the Ore Mineralogy Symposium (IMA/COM)at the 14th General Meeting of the International Mineralogical Association, at Stanford, California, in July, 1986.     

Levels of heavy metals along the Libyan coastline

A coastal survey off the east part of the Libyan coastline which has no river inputs was initiated to measure the existing level and distribution of selected heavy metals (Cu, Zn, Cd, Ni, and Mn) in surface sediments along Benghazi Bay. In order to place the finding of this survey in proper perspective and for practical terms a Pollution Load Index (PLI) was used. The results establish that of the sites examined only the harbour site exhibited relatively high levels particularly in the cases of cadmium and copper indicating a detectable anthropogenic input. The range of concentrations were 1.03%–1.96% μg g⁻¹ (dry wt) for organic content and 64.7%–93% for calcium carbonate while the range of heavy metals concentrations (μg g⁻¹ dry wt) were 8.7–42 for Cu, 2.3–27.3 for Zn, 5.7–19 for Ni, 37–76.7 for Mn, and below the detection limit-1.73 for Cd. A PLI score of 2.93 was obtained at the harbour site and a range of 6.11–7.63 for the other sites. The PLI score for the control site was 9.51 and the score for the overall studied area was 7.51 indicating a very clean environment. The index would be, with some modification, a step in the right direction from the management point of view in Libya due to the absence of environmental expertise.     

Pyrolysis-GC-MS and NMR studies of dissolved seawater humic substances and isolates of a marine diatom

¹³C- and ¹H-NMR spectra were obtained for humic substances isolated from a coastal marine environment and also for the intracellular and extracellular extracts of a marine diatom. Phaeodactylum tricornutum Bohlin. Highly branched alkyl chains constitute a large proportion of the structure of the marine humic material, whereas aromatic components are less important. Carbohydrate-type materials, possibly uronic acids, are also present in appreciable amounts. Furans (derived from carbohydrates). pyrroles and nitriles (derived from proteins, nucleic acids and/or tetrapyrroles) and phenols and methylphenols (non-lignin derived) are pyrolysis products derived from pyrolysis-gas chromatography-mass spectrometry of the extracts. The results indicate the similarities in chemical structure of P. tricornutum exudate and dissolved marine humic material.     

Three-dimensional temperature structure of a ridge-transform-ridge system

We have solved the problem of three-dimensional heat transfer by conduction and material transport within a ridge-transform-ridge system. The boundary conditions are those of a finite plate with temperatures fixed at the top and bottom. We do not solve for the pattern of flow, so must arbitrarily describe the material transport. Within triangular regions beneath the spreading centers, flow is assumed to be vertical and at constant velocity, with the volume of upwelling matching the volume of horizontal transport by the plates. The steady state solution is approached by repeated finite steps in time, each involving two stages. First, to represent material transport, temperatures in the grid of points are shifted horizontally and/or vertically to the point at which the associated particle would arrive given the time period and assumed velocity field. Second, a three-dimensional Fourier Transform technique is used to calculate the cooling which would occur in the static case during the same period.

We find that the geometry of flow in the upwelling region, here parameterized by velocity of upwelling or width of the upwelling zone, is more important than spreading rate in determining the temperature structure. The results indicate that conductive cooling cannot alone account for deepening of the median valley towards the fracture zone. For a 10 m.y. offset on the transform fault, conductive cooling across the fracture zone to the older lithosphere affects the upwelling material only to a distance of about 10 km from the fault, in contrast to the deepening of the median valley over several tens of kilometers observed on the Mid-Atlantic Ridge.     

Constraints and Categories of Vadose Zone Monitoring Devices

Traditional monitoring methods using chemical analysis of ground water samples to detect pollutant migration are being superseded or used in conjunction with innovative approaches. A need to detect pollutants before they reach the water table has drawn interest to vadose (unsaturated) zone monitoring and brought together hydrogeologists, soil scientists and agricultural engineers who have been working on this subject for years.
Recent studies have identified over 50 different types of vadose zone monitoring devices and methods that have optimum utility in varying hydrogeologic settings. In general, measurements made in the vadose zone are trying to define storage, transmission of liquid waste in terms of flux and velocity, and pollutant mobility.
Criteria for the selection of alternative vadose zone monitoring methods are important for the development of site-specific systems. These criteria include: type of site; applicability to new, active, and abandoned sites; power requirements; depth limitations; multiple use capability; type of data collection system; reliability and life expectancy; degree of operational complexity; direct versus indirect methods; applicability to alternate media; effect on flow regime; and effect of hazardous waste on sampling or measurements. Application of the selection criteria is discussed in Everett et al. (1982a).     

Post-depositional migration of elements during diagenesis in brown clay and turbidite sequences in the North East Atlantic

Two cores from a NE Atlantic pelagic clay area of low accumulation rate each contain a single turbidite with different age, thickness and composition. Diagenesis following introduction of exotic turbidite material into the pelagic clay sequence has resulted in distinctive colour changes. The diagenetic process is thought to be driven by bacterially-mediated organic oxidation, and compositional differences in the turbidites and clays allow examination of the effects on metal concentrations of this process.In one core, a long turbidite section emplaced 330,000 years ago is overlain by clay. Organic oxidation has apparently proceeded from the turbidite top downwards and has maintained a U concentration peak below the oxidation front marked by a colour change. The U source is believed to be U initially associated with the organic matter in the oxidised section of the turbidite. Vanadium and Cu behave similarly, but upward migration of the redox-sensitive metals Mn and Ni is also seen. In the second core an 8 cm turbidite section was emplaced about 170,000 years ago in a clay column. In this case organic consumption now appears complete, but evidence for diagenetic effects is found in a 16 cm compositional alteration ‘halo’ of the underlying pelagic clay. Fe(II) has been enriched and the hydrogenous component of Mn, Co, Ni and Cu removed from the pelagic clay to form the halo.     

Observations on the fish populations of an east coast Estuary

Monthly samples of fish were collected from the cooling-water intake screens at Kingsnorth Power Station on the Medway Estuary. A total of 26 372 fish, comprising 41 species, was recorded between April 1981 and August 1983. The seasonal distribution of both species numbers and abundance of fish was similar to earlier studies, although the community structure has altered in recent years. The regular occurrence and increased population size of Osmerus eperlanus, the smelt, and Clupea harengus, the herring, were the most notable changes. The results are compared with previous studies, and the indirect effects of enhanced water quality conditions in the tidal Thames and the subsequent recolonization by fish, which were previously absent for many years, are discussed.     

Factors controlling the lengths of channel-fed lava flows

Factors which control lava flow length are still not fully understood. The assumption that flow length as mainly influenced by viscosity was contested by Walker (1973) who proposed that the length of a lava flow was dependent on the mean effusion rate, and by Malin (1980) who concluded that flow length was dependent on erupted volume. Our reanalysis of Malin's data shows that, if short duration and tube-fed flows are eliminated, Malin's Hawaiian flow data are consistent with Walker's assertion. However, the length of a flow can vary, for a given effusion rate, by a factor of 7, and by up to 10 for a given volume. Factors other than effusion rate and volume are therefore clearly important in controlling the lengths of lava flows. We establish the relative importance of the other factors by performing a multivariate analysis of data for recent Hawaiian lava flows. In addition to generating empirical equations relating flow length to other variables, we have developed a non-isothermal Bingham flow model. This computes the channel and levee width of a flow and hence permits the advance rates of flows and their maximum cooling-limited lengths for different gradients and effusion rates to be calculated. Changing rheological properties are taken into account using the ratio of yield strength to viscosity; available field measurements show that this varies systematically from the vent to the front of a lava flow. The model gives reasonable agreement with data from the 1983–1986 Pu'u Oo eruptions and the 1984 eruption of Mauna Loa. The method has also been applied to andesitic and rhyolitic lava flows. It predicts that, while the more silicic lava flows advance at generally slower rates than basaltic flows, their maximum flow lengths, for a given effusion rate, will be greater than for basaltic lava flows.     

Analysis of X-ray observations of the 15 June 1973 flare in active region NOAA 131

Observations and analyses of the 1B/M3 flare of 15 June, 1973 in active region NOAA 131 (McMath 12379) are presented. The X-ray observations, consisting of broadband photographs and proportional counter data from the Skylab/ATM NASA-MSFC/Aerospace S-056 experiment, are used to infer temperatures, emission measures, and densities for the flaring plasma. The peak temperature from the spatially resolved photographs is 25 × 10⁶ K, while the temperature from the full-disk proportional counter data is 15 × 10⁶ K. The density is 3 × 10¹⁰cm^–3. The X-ray flare emission appears to come primarily from two low-lying curvilinear features lying perpendicular to and centered on the line where the photospheric longitudinal magnetic field is zero. Similarities in the preflare and postflare X-ray emission patterns indicate that no large-scale relaxation of the coronal magnetic configuration was observed. Also discussed are H and magnetic field observations of the flare and the active region. Finally, results of numerical calculations, including thermal conduction, radiative loss and chromospheric evaporation, are in qualitative agreement with the decay phase observations.Presently at NASA/Marshall Space Flight Center.     

Editorial 2003

2002 was a highly successful year. The most recent (2001) ISIImpact Factors for journal citations confirm that Journal ofPetrology remains the premier journal in the field of petrology(Journal of Petrology—3.283; Earth & Planetary ScienceLetters—2.700; Geochimica Cosmochimica Acta—2.614;Contributions to Mineralogy & Petrology