The Kabanga Ni sulfide deposit, Tanzania: I. Geology, petrography, silicate rock geochemistry, and sulfur and oxygen isotopes

The Kabanga Ni sulfide deposit represents one of the most significant Ni sulfide discoveries of the last two decades, with current indicated mineral resources of 23.23 Mt at 2.64% Ni and inferred mineral resources of 28.5 Mt at 2.7% Ni (Nov. 2008). The sulfides are hosted by a suite of ∼1.4 Ga ultramafic–mafic, sill-like, and chonolithic intrusions that form part of the approximately 500 km long Kabanga–Musongati–Kapalagulu igneous belt in Tanzania and Burundi. The igneous bodies are up to about 1 km thick and 4 km long. They crystallized from several compositionally distinct magma pulses emplaced into sulfide-bearing pelitic schists. The first magma was a siliceous high-magnesium basalt (approximately 13.3% MgO) that formed a network of fine-grained acicular-textured gabbronoritic and orthopyroxenitic sills (Mg# opx 78–88, An plag 45–88). The magma was highly enriched in incompatible trace elements (LILE, LREE) and had pronounced negative Nb and Ta anomalies and heavy O isotopic signatures (δ¹⁸O +6 to +8). These compositional features are consistent with about 20% contamination of primitive picrite with the sulfidic pelitic schists. Subsequent magma pulses were more magnesian (approximately 14–15% MgO) and less contaminated (e.g., δ¹⁸O +5.1 to +6.6). They injected into the earlier sills, resulting in the formation of medium-grained harzburgites, olivine orthopyroxenites and orthopyroxenites (Fo 83–89, Mg# _opx 86–89), and magmatic breccias consisting of gabbronorite–orthopyroxenite fragments within an olivine-rich matrix. All intrusions in the Kabanga area contain abundant sulfides (pyrrhotite, pentlandite, and minor chalcopyrite and pyrite). In the lower portions and the immediate footwall of two of the intrusions, namely Kabanga North and Kabanga Main, there occur numerous layers, lenses, and veins of massive Ni sulfides reaching a thickness of several meters. The largest amount of high grade, massive sulfide occurs in the smallest intrusion (Kabanga North). The sulfides have heavy S isotopic signatures (δ³⁴S wr = +10 to +24) that broadly overlap with those of the country rock sulfides, consistent with significant assimilation of external sulfur from the Karagwe–Ankolean sedimentary sequence. However, based partly on the relatively homogenous distribution of disseminated sulfides in many of the intrusive rocks, we propose that the Kabanga magmas reached sulfide saturation prior to final emplacement, in staging chambers or feeder conduits, followed by entrainment of the sulfides during continued magma ascent. Oxygen isotope data indicate that the mode of sulfide assimilation changed with time. The heavy δ¹⁸O ratios of the early magmas are consistent with ingestion of the sedimentary country rocks in bulk. The relatively light δ¹⁸O ratios of the later magmas indicate less bulk assimilation of the country rocks, but in addition the magmas selectively assimilated additional S, possibly through devolatization of the country rocks or through cannibalization of magmatic sulfides deposited in the conduits by preceding magma surges. The intrusions were tilted at ca. 1.37 Ga, during the Kibaran orogeny and associated synkinematic granite plutonism. This caused solid-state mobilization of ductile sulfides into shear zones, notably along the base of the intrusions where sulfide-hornfels breccias and lenses and layers of massive sulfides may reach a thickness of >10 m and can extend for several 10 s to >100 m away from the intrusions. These horizons represent an important exploration target for additional nickel sulfide deposits.     

Precambrian–Cambrian boundary interval deposition and the marginal platform of the Avalon microcontinent

Thick terminal Proterozoic–lowest Cambrian successions allow reference of the Saint John, New Brunswick, and MacCodrum Brook, southern Cape Breton Island, areas to the marginal platform of the Avalon microcontinent. Marginal-platform siliciclastic-dominated sequences form a cover on Late Precambrian arc successions from southern New Brunswick to North Wales. Their deposition in fault-bounded basins began with the origin of the Avalon microcontinent and development of a persistent transtensional regime in the latest Precambrian. The terminal Proterozoic–lowest Cambrian on the Avalonian marginal platform consists of three successive lithofacies associations: lower subaerial rift to marginal-marine facies; overlying cool-water, wave-influenced, marine platform sandstones and shales; and higher macrotidal quartz arenites (=Avalonian depositional sequences 1–2). Only the Lower Cambrian macrotidal quartz arenites onlap southeast, where they form the oldest Cambrian unit on the inner platform. These major lithofacies are the Rencontre, Chapel Island, and Random formations, respectively, in Avalonian North America. Southwest thinning of the Rencontre–Chapel Island–Random interval in southern New Brunswick reflects slower subsidence of a fault-bounded area in the city of Saint John. The depositional sequence 1–2 unconformity, which falls in the sub-trilobitic Lower Cambrian Watsonella crosbyi Zone of the Chapel Island Formation, persists for 650 km along the marginal platform from southeastern Newfoundland to southern New Brunswick and, potentially, appears in Cape Breton Island. Latest Precambrian-earliest Cambrian epeirogenic and depositional history was very uniform along the marginal platform, and a unified lithostratigraphic nomenclature is appropriate.     

Scattered ground‐roll attenuation using model‐driven interferometry

Scattered ground roll is a type of noise observed in land seismic data that can be particularly difficult to suppress. Typically, this type of noise cannot be removed using conventional velocity‐based filters. In this paper, we discuss a model‐driven form of seismic interferometry that allows suppression of scattered ground‐roll noise in land seismic data. The conventional cross‐correlate and stack interferometry approach results in scattered noise estimates between two receiver locations (i.e. as if one of the receivers had been replaced by a source). For noise suppression, this requires that each source we wish to attenuate the noise from is co‐located with a receiver. The model‐driven form differs, as the use of a simple model in place of one of the inputs for interferometry allows the scattered noise estimate to be made between a source and a receiver. This allows the method to be more flexible, as co‐location of sources and receivers is not required, and the method can be applied to data sets with a variety of different acquisition geometries. A simple plane‐wave model is used, allowing the method to remain relatively data driven, with weighting factors for the plane waves determined using a least‐squares solution. Using a number of both synthetic and real two‐dimensional (2D) and three‐dimensional (3D) land seismic data sets, we show that this model‐driven approach provides effective results, allowing suppression of scattered ground‐roll noise without having an adverse effect on the underlying signal.     

Rough seas and statistical deconvolution

The rough‐sea reflection‐response varies (1) along the streamer (2) from shot to shot and (3) with time along the seismic trace. The resulting error in seismic data can be important for time‐lapse imaging. One potential way of reducing the rough‐sea receiver error is to use conventional statistical deconvolution, but special care is needed in the choice of the design and application windows. The well‐known deconvolution problem associated with the non‐whiteness of the reflection series is exacerbated by the requirement of an unusually short design window – a requirement that is imposed by the non‐stationary nature of the rough‐sea receiver wavelet. For a synthetic rough‐sea data set, with a white 1D reflection series, the design window needs to be about 1000 ms long, with an application window about 400 ms long, centred within the design window. Although such a short design window allows the deconvolution operator to follow the time‐variation of the rough‐sea wavelet, it is likely to be too short to prevent the non‐whiteness of the geology from corrupting the operator when it is used on real data. If finely spatial‐sampled traces are available from the streamer, the design window can be extended to neighbouring traces, making use of the spatial correlations of the rough‐sea wavelet. For this ‘wave‐following’ approach to be fruitful, the wind (and hence the dominant wave direction) needs to be roughly along the line of the streamer.     

New Data on the Middle–Late Holocene Evolution of Vegetation Cover in the Northern Baikal Region

Doklady Earth Sciences - A continuous peat bog section from a barely accessible mid-mountain area in the northwestern Baikal region has been obtained for the first time. The materials were studied...     

Variations in the Rate of Anaerobic Succinate Accumulation within the Central and Marginal Regions of an Euryoxic Bivalve Mantle

Abstract. Anaerobic metabolism in the central and marginal portions of the mantle of Mercenaria mercenaria was compared. Anaerobic succinate accumulation was more rapid in the central region. This difference may be due to higher phosphoenolpyruvate carboxykinase activity in the central region. Thus, the central region is more specialized for anaerobic metabolism and the marginal region more for net shell growth. The original rate of succinate accumulation in the mantle is similar in isolated mantles and intact clams, suggesting that mantle succinate production does not require translocation of precursors from other tissues. However, in intact clams, the rate of succinate accumulation in the central region of the mantle slows after four hours. The reduced rate is probably caused by reducing the metabolic rate. Succinate accumulation and shell dissolution are slower in freshly collected clams than in clams that had been stored anaerobically. The difference may be due to induction of PEPCK synthesis during storage. Shell derived calcium did not accumulate in the mantle and, therefore did not alter the intracellular calcium concentration in the mantle.     

R. E. (Ted) Munn — Founding editor; a mini-biography

Ted Munn founded Boundary-Layer Meteorology in 1970 and served as Editor for 75 volumes over a 25 year period. This short article briefly reviews Ted's scientific career with the Atmospheric Environment Service (of Canada), the International Institute for Applied Systems Analysis in Austria and with the Institute of Environmental Studies at the University of Toronto, and as editor of this journal.     

Kleinraummeteorologische Studien im Federseegebiet in Strahlungsnächten

Zusammenfassung Die vorliegende Studie befaßt sich mit den meteorologischen Vorgängen in einer extrem stabil aufgebauten Kaltluft, wie sie sich in Strahlungsnächten in einem abgeschlossenen Becken mit ebener, horizontaler Talsohle von einigen Kilometern Ausdehnung und im wesentlichen einheitlichem Untergrund einer Moorlandschaft herausbildet.Untersucht wird in erster Linie das Eigenleben der so abgeschlossenen Luftmasse. Dabei zeigt sich, daß der Zustand eines ruhenden Kaltluftsees zumeist erst im Endstadium des nächtlichen Prozesses näherungsweise erreicht wird. Während des größten Teiles der Nacht kommen vielmehr überraschend lebhafte und weitreichende angenähert horizontale Bewegungen leicht zustande, die sich im thermischen Gefüge wie in der Windstruktur kundtun und in dem Verhalten der Nebel- und Dunstfelder sichtbar werden. Unter der Wirkung von Schwerkraft, Druckgefälle und Reibung entstehen einfache Gleitvorgänge, Schaukelbewegungen, fortschreitende Wellen und Seiches, die in einzelnen Beispielen beschrieben werden. Ähnlich wie in der Großraummeteorologie bilden sich durch das quantenhafte Verhalten der Energieumsätze Luftkörperindividuen, deren Deformation hier einfacheren Gesetzen folgt. Trotz ihrer sehr verschiedenen Größenordnung von einigen Dekametern bis zu mehreren Kilometern Ausdehnung bei geringer vertikaler Mächtigkeit zeigen sie auf ihrer Vorderseite einheitlich die Form des vonW. Schmidt in Modellversuchen gefundenen Böenkopfes.Der Einfluß höherer Luftschichten auf die stabilisierte Bodenkaltluft äußert sich in aufgeprägten Wellen und Frendlufteinschüben. Das Beispiel eines nächtlichen Wärmeeinbruchs aus der Höhe wird eingehend beschrieben.

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Summary The present study deals with the meteorological processes in an extremely stable cold air, formed in clear nights in a confined basin with a plane, horizontal basis extended for some kilometres, and with an essentially uniform ground in a fen-country.In the first place the proper life of the so limited air is studied. It becomes evident, that the state of a resting lake of cold air generally is approximately reached but in the final phase of the night-time process. Moreover, for the greater part of the night surprising active and extended movements of nearly horizontal course easily occur, becoming evident in the structure of wind and temperature stratification and also visible in the behaviour of fog and haze fields. Produced by gravitation, pressure gradient and friction, simple gliding and swinging motions, proceeding waves and seiches result which are described in single examples. Corresponding to the large scale meteorology individual air bodies form by the quantum-like behaviour of energy transformations. Their deformation in this case however, takes course according to simpler laws. In spite of their much various magnitude of the dimensions of some decametres up to several kilometres, while being of small vertical extension they show in their fore part in all cases the shape of a Böenkopf as found byW. Schmidt by model experiments.The influence of upper atmospheric layers on the stable cold air near the ground manifests itself by impressed waves and by influxes of heterogeneous air. An example of a nocturnal invasion of warm air from aloft is described in detail.

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Résumé Cette étude traite des processus météorologiques dans un air froid d'une stratification extrêmement stable, qui se développent pendant des nuits claires dans un bassin fermé, ayant une base plane et horizontale, d'une étendue de quelques km, et recouverte par le sol essentiellement homogène d'une région marécageuse.C'est surtout la «vie propre» de cette masse d'air limitée qui est étudiée. Il se trouve que, le plus souvent, l'état d'un «lac immobile d'air froid» n'est atteint approximativement qu'à la fin du processus nocturne. Pendant la plus grande partie de la nuit, se produisent au contraire facilement des mouvements presque horizontaux étonnamment actifs et étendus, qui se manifestent dans la structure thermique et du vent, et qui deviennent visibles par le comportement des couches de brouillard et de brume. De simples processus de glissement, des oscillations, des ondes progressives et des seiches se forment sous l'influence de la gravitation, du gradient de pression et du frottement; ils sont décrits par des exemples particuliers. Comme cela se produit dans la météorologie à grande échelle, il se forme des corps atmosphériques individuels à la suite du comportement en quanta des transformations d'énergie. Leur déformation a lieu cependant selon des lois plus simples. En dépit de leur étendue très variable allant de quelques decamètres à plusieurs km, et leur épaisseur peu considérable, ils montrent tous à leur partie antérieure la forme d'un «Böenkopf», pareil à celui queW. Schmidt a trouvé à la suite de ses expériences qu'il fit avec des modèles.L'influence des couches supérieures de l'atmosphère sur l'air froid stable au sol se manifeste par l'impression d'ondes et par des apports d'air étranger. On décrit ensuite en détail une invasion nocturne d'air chaud depuis les couches supérieures.

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Mit 13 Textabbildungen.

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Herrn Prof. Dr.W. Peppler, zum 70. Geburtstag.     

Visual classification of very fine-grained sediments: Evaluation through univariate and multivariate statistics

Classifying very fine-grained rocks through fabric elements provides information about depositional environments, but is subject to the biases of visual taxonomy. To evaluate the statistical significance of an empirical classification of very fine-grained rocks, samples from Devonian shales in four cored wells in West Virginia and Virginia were measured for 15 variables: quartz, illite, pyrite and expandable clays determined by X-ray diffraction; total sulfur, organic content, inorganic carbon, matrix density, bulk density, porosity, silt, as well as density, sonic travel time, resistivity, and -ray response measured from well logs. The four lithologic types comprised: (1) sharply banded shale, (2) thinly laminated shale, (3) lenticularly laminated shale, and (4) nonbanded shale. Univariate and multivariate analyses of variance showed that the lithologic classification reflects significant differences for the variables measured, difference that can be detected independently of stratigraphic effects. Little-known statistical methods found useful in this work included: the multivariate analysis of variance with more than one effect, simultaneous plotting of samples and variables on canonical variates, and the use of parametric ANOVA and MANOVA on ranked data.