New igneous zircon Pb/Pb and metamorphic Rb/Sr ages in the Yaounde Group (Cameroon, Central Africa): implications for the Central African fold belt evolution close to the Congo Craton

Three meta-igneous bodies from the Yaounde Group have been analyzed for their petrography, geochemistry, and ²⁰⁷Pb/²⁰⁶Pb zircon ages. According to their geochemical patterns, they represent meta-diorites. The meta-plutonites yielded identical zircon ages with a mean of 624?±?2?Ma interpreted as their intrusion age. This age is in agreement with previously published zircon ages of meta-diorites from the Yaounde Group. The meta-diorites derived mainly from crustal rocks with minor contribution from mantle material. The ⁸⁷Rb/⁸⁶Sr isochron ages of one meta-diorite sample and three meta-sedimentary host rocks are significantly younger than the obtained intrusion age. Therefore, they are not related to igneous processes. ⁸⁷Rb/⁸⁶Sr isochron ages differ from sample to sample (599?±?3, 572?±?4, 554?±?5, 540?±?5?Ma) yielding the oldest Neoproterozoic age (~600?Ma) for a paragneiss sample at a more northern location. The youngest Rb/Sr isochron age (~540?Ma) was obtained for a mica schist sample at a more southern location closer to the border of the Congo Craton. The ⁸⁷Rb/⁸⁶Sr whole rock-biotite ages are interpreted as cooling ages related to transpressional processes during exhumation. Therefore, several discrete metamorphic events related to the exhumation of the Yaounde Group were dated. It could be shown by Rb/Sr dating for the first time that these late tectonic processes occurred earlier at more distant northern locations of the Yaounde Group and lasted at least until early Cambrian (~540?Ma) more closely to the border of the Congo Craton.     

Structural and biological control of the Cenozoic epithermal uranium concentrations from the Sierra Pe?a Blanca, Mexico

Epithermal uranium deposits of the Sierra Pe?a Blanca are classic examples of volcanic-hosted deposits and have been used as natural analogs for radionuclide migration in volcanic settings. We present a new genetic model that incorporates both geochemical and tectonic features of these deposits, including one of the few documented cases of a geochemical signature of biogenic reducing conditions favoring uranium mineralization in an epithermal deposit. Four tectono-magmatic faulting events affected the volcanic pile. Uranium occurrences are associated with breccia zones at the intersection of fault systems. Periodic reactivation of these structures associated with Basin and Range and Rio Grande tectonic events resulted in the mobilization of U and other elements by meteoric fluids heated by geothermal activity. Focused along breccia zones, these fluids precipitated under reducing conditions several generations of pyrite and uraninite together with kaolinite. Oxygen isotopic data indicate a low formation temperature of uraninite, 45–55°C for the uraninite from the ore body and ～20°C for late uraninite hosted by the underlying conglomerate. There is geochemical evidence for biological activity being at the origin of these reducing conditions, as shown by low δ³⁴S values (～?24.5‰) in pyrites and the presence of low δ¹³C (～?24‰) values in microbial patches intimately associated with uraninite. These data show that tectonic activity coupled with microbial activity can play a major role in the formation of epithermal uranium deposits in unusual near-surface environments.     

Stable isotope geochemistry of the Archean Val-d‘Or (Canada) orogenic gold vein field

A systematic study of the auriferous quartz veins of the Val-dOr vein field, Abitibi, Quebec, Canada, demonstrates that the C, O, S isotope composition of silicate, carbonate, borate, oxide, tungstate and sulphide minerals have a range in composition comparable to that previously determined for the whole Superior Province. The oxygen isotope composition of quartz from early quartz–carbonate auriferous veins ranges from 9.4 to 14.4 whereas later quartz-tourmaline-carbonate veins have ¹⁸O_quartz values ranging from 9.2 to 13.8 . Quartz-carbonate veins have carbonate (¹⁸O: 6.9–12.5 ; ¹³C: –6.2– –1.9 ) and pyrite (³⁴S: 1.2 and 1.9 ) isotope compositions comparable to those of quartz-tourmaline-carbonate veins (¹⁸O: 7.9–11.7 ; ¹³C: –8.0 – –2.4 ; ³⁴S: 0.6–6.0 ). ¹⁸O_quartz values in quartz-tourmaline-carbonate veins have a variance comparable to analytical uncertainty at the scale of one locality, irrespective of the type of structure, the texture of the quartz or its position along strike, across strike or down-dip a vein. In contrast, the oxygen isotope composition of quartz in quartz-tourmaline-carbonate veins displays a regional distribution with higher ¹⁸O values in the south-central part of the vein field near the Cadillac Tectonic Zone, and which ¹⁸O values decrease regularly towards the north. Another zone of high ¹⁸O values in the northeast corner of the region and along the trace of the Senneville Fault is separated by a valley of lower ¹⁸O values from the higher values near the Cadillac Tectonic Zone. Oxygen isotope isopleths cut across lithological contacts and tectonic structures. This regional pattern in quartz-tourmaline-carbonate veins is interpreted to be a product of reaction with country rocks and mixing between (1) a deep-seated hydrothermal fluid of metamorphic origin with minimum ¹⁸O=8.5 , ¹³C=0.6 and ³⁴S=–0.4 , and (2) a supracrustal fluid, most likely Archean seawater with a long history of water-rock exchange and with maximum ¹⁸O=3.9 , ¹³ C=–5.6 and ³⁴S=5.0 .     

Mineral resources inventory in the northern part of haiti: A basic rationale for selecting areas of potential mining interest

Agriculture has always been the major source of income of the Republic of Haiti. Only in the late sixties did the Haitian Government decide to move toward industrialization and explotation of natural resources, but no reliable inventory of mineral potential was available. In June 1970 the Resources and Transport Division of the U.N. (now CNRET) undertook a structural analysis of the Caribbean with reference to defining favorable loci for ore disposition. One of the conclusions of this survey was that from the metallogenetic point of view, the Northern part of Haiti was a favorable area for prospecting for habe metal deposits, especially for copper. Those conclusions were acknowleadged with great enthusiams by the Haitian authorities, and they welcomed in July 1971 a UN technical adviser who outlined a first phase of exploration. Following this mission, a “Mineral exploration Project” was created by the Haitian Government with the cooperation of UNDP, and an area of 9000 km² was selected, based on structural, lithological, and metallogenetic reasons, as an exploration target. The most important aspects of this project were:Mineral Exploration—First Phase (1973–1974). The following operations were carried out by a small team: (a) photo interpretation at 1:50,000 and 1:200,000 scale, (b) geological reconnaissance at 1:50,000 scale, (c) geochemical reconnaissance (stream sediment) at 1:50,000 scale. The photo interpretation helped outline zones of particular structural interest and was followed up by ground control studies. Stream sediment geochemical reconnaissance was carried out over half of the project area, following the indications given by the photo geological interpretation. finally, combining all the data, five areas were selected for detailed follow up survey: (1) Terrier Rouge—Perche, (2) Vallieres, (3) Mont-Organise, (4) Limonade, (5) Gros Morne—Le Borgne.Mineral Exploration—Second Phase (1975–1976). The main zones of interest were reduced to a few hundred km² and porphyry copper type deposits appeared to be the most promising target. Detailed geological mapping (1:25,000 to 1:2,000) and soil geochemistry were carried out on the above mentioned five areas, the most promising appearing quickly to be Terrier Rouge-Perges, with porphyry-type copper mineralization. A geophysical survey (IP and Mag) confirmed the geochemical results. At the end of the second phase of exploration (December 1976) two of the five prospects of the Terrier Rouge area have been diamond drilled and appeared to be of potential economic interest. Because of the number and importance of the prospects discovered a third phase of exploration was deemed necessary with the purpose of continuing the detailed prospection work and drilling the other prospects. This third phase started in January 1977 and was expected to last until December 1979. This paper was presented at the International Geological Correlation Program (IGCP) Project 98: “Standards for Computer Applications in Resource Studies” held at Taita Hills, Kenya, November 8–15, 1977.     

Real-time sea-state surveillance with Skywave Radar

To extract sea-state information from the ionospherically distorted echoes received by a skywave radar, we use a signal-processing strategy that permits real-time decisions about the quality of incoming data. This paper explains the need for an on-line processor and describes some of its engineering details. We use an array processor to quickly compute all the spectra required to display ocean waveheight, as well as some indices of data quality, while the radar interrogates an ocean cell. The results are shown in a test using an experimental radar that mapped waveheight over its North Pacific coverage area. Coverage efficiency was 85 percent, and the radar's waveheight estimates averaged 2ft(0.6 m) higher than those forecast by a numerical model.     

Relicts of an intra-oceanic arc in the Sapi-Shergol mélange zone (Ladakh, NW Himalaya, India): implications for the closure of the Neo-Tethys Ocean

In the Ladakh–Zanskar area, relicts of both ophiolites and paleo-accretionary prism have been preserved in the Sapi-Shergol mélange zone. The paleo-accretionary prism, related to the northward subduction of the northern Neo-Tethys beneath the Ladakh Asian margin, mainly consists of tectonic intercalations of sedimentary and blueschist facies rocks. Whole rock chemical composition data provide new constraints on the origin of both the ophiolitic and the blueschist facies rocks. The ophiolitic rocks are interpreted as relicts of the south Ladakh intra-oceanic arc that were incorporated in the accretionary prism during imbrication of the arc. The blueschist facies rocks were previously interpreted as oceanic island basalts (OIB), but our new data suggest that the protolith of some of the blueschists is a calc-alkaline igneous rock that formed in an arc environment. These blueschists most likely originated from the south Ladakh intra-oceanic arc. This arc was accreted to the southern margin of Asia during the Late Cretaceous and the buried portion was metamorphosed under blueschist facies conditions. Following oceanic subduction, the external part of the arc was obducted to form the south Ladakh ophiolites or was incorporated into the Sapi-Shergol mélange zone. The incorporation of the south Ladakh arc into the accretionary prism implies that the complete closure of the Neo-Tethys likely occurred by Eocene time.     

Découverte des roches à affinité ophiolitique dans la chaîne panafricaine au Cameroun : les talcschistes de Ngoung, Lamal Pougue et Bibodi Lamal

Outcrops of talc schists extending over >1 km have been discovered within the garnet- and muscovite-bearing mica schist of the Pan-African belt near Yaoundé (Cameroon). Mineralogical studies show that a metamorphism of the upper greenschist facies was prolonged by hydrothermal reactions. This latter led to the transformation of hornblendites into talc schists. Chemically, talc schists and relicts of hornblendite remind ultrabasic rocks, and REE patterns point to E-MORB and peridotite. It is thus suggested that the talc schists and relicts of hornblendite may correspond to slices of a dismembered Pan-African ophiolite set. To cite this article: C. Nkoumbou et al., C. R. Geoscience 338 (2006).     

Gradual Deformation and Iterative Calibration of Sequential Stochastic Simulations

Based on the algorithm for gradual deformation of Gaussian stochastic models, we propose, in this paper, an extension of this method to gradually deforming realizations generated by sequential, not necessarily Gaussian, simulation. As in the Gaussian case, gradual deformation of a sequential simulation preserves spatial variability of the stochastic model and yields in general a regular objective function that can be minimized by an efficient optimization algorithm (e.g., a gradient-based algorithm). Furthermore, we discuss the local gradual deformation and the gradual deformation with respect to the structural parameters (mean, variance, and variogram range, etc.) of realizations generated by sequential simulation. Local gradual deformation may significantly improve calibration speed in the case where observations are scattered in different zones of a field. Gradual deformation with respect to structural parameters is necessary when these parameters cannot be inferred a priori and need to be determined using an inverse procedure. A synthetic example inspired from a real oil field is presented to illustrate different aspects of this approach. Results from this case study demonstrate the efficiency of the gradual deformation approach for constraining facies models generated by sequential indicator simulation. They also show the potential applicability of the proposed approach to complex real cases.     

Barium and strontium uptake into larval protoconchs and statoliths of the marine neogastropod Kelletia kelletii

The trace elemental composition of calcified larval hard parts may serve as useful tags of natal origin for invertebrate population studies. We examine whether the trace metal barium (Ba) deposits into the calcium carbonate matrix of molluscan larval statolith and protoconch in proportion to seawater Ba concentration at two temperatures (11.5 and 17°C). We also examine strontium (Sr) uptake as a function of temperature. Using encapsulated larvae of the marine gastropod, Kelletia kelletii, reared in the laboratory under controlled conditions, we demonstrate a significant inverse effect of temperature and a positive effect of seawater Ba/Ca ratio on Ba incorporation into larval carbonates. Ba/Ca partition coefficients (D_Ba) in protoconch were 1.13 at 11.4°C and 0.88 at 17.1°C, while D_Ba in larval statolith measured 1.58 at 11.4°C and 1.29 at 17.1°C. Strontium incorporation into statoliths is also inversely affected by temperature, but there was a significant positive effect of temperature on Sr incorporation into protoconch. These data suggest larval statoliths and protoconchs can meaningfully record variation in seawater physical and chemical properties, and, hence, have potential as natural tags of natal origin.     

The seismological picture of the inner core: structure and rotation

This paper presents a review of recent results concerning the structure and the rotation of the inner core, as inferred from seismological observations. We first focus on the apparent hemispherical pattern of the inner core structure, observed in both the anisotropy and the heterogeneity of the uppermost layers, whereas isotropic velocities seem to be homogeneous in a broad wavelength domain. Then we present results on the radial variations of the attenuation and its possible anisotropy, for which viscoelasticity and wave scattering provide complementary explanations. Linked to these results is a discussion about the detection of the PKJKP phase, which propagates as S-wave inside the inner core. Finally, the seismological observations of a possible inner core rotation are discussed; recent results seem to favour rotation rates below 0.2° yr^?1.