Nd and Sr isotope study of hydrothermal scheelite and host rocks at Omai, Guiana Shield: implications for ore fluid source and flow path during the formation of orogenic gold deposits

Omai is a high tonnage, low-grade, world-class gold deposit located in the Paleoproterozoic Guiana Shield. It is the second most important gold deposit in the Guiana Shield (after Las Cristinas, Venezuela), and one of the largest in South America (4.0 million oz.). Sm-Nd and Sr isotope data are presented for host rocks and for scheelite from auriferous quartz-carbonate-scheelite-sulfide-telluride veins from the Omai deposit. Gold-bearing veins are hosted by the Paleoproterozoic Barama-Mazaruni Supergroup, a greenstone belt sequence consisting of mafic volcanic rocks interbedded with sedimentary rocks that are intruded by quartz-feldspar porphyry and rhyolite dikes. This lithologic sequence was folded and metamorphosed to lower greenschist facies during the Paleoproterozoic Trans-Amazonian orogeny. The volcano-sedimentary unit was intruded by a post-tectonic quartz monzodiorite-diorite-hornblendite stock. Initial Nd isotope ratios for the Omai volcanic rocks range from ɛ_Nd=+2.1 to +4.2. These values suggest that this part of the Guiana Shield was a site of new crust formation during the Paleoproterozoic and was not contaminated by older (Archean), reworked continental crust. Initial Nd isotope ratios for the Omai stock range between +0.5 and +2.3, which suggest limited contamination with previously formed continental crust. Although the Nd isotopic ratios of gold-related scheelites overlap with those of the host rocks, particularly the tholeiitic basalts at the interpreted time of vein emplacement, the lack of both isotopic mixing and significant Nd movement during the hydrothermal process suggest that the Nd isotope composition can be used to determine the isotopic characteristics of the ore fluid source area. At Omai, the ore fluid is largely derived from a radiogenic Nd source, represented by mantle or lower crustal reservoirs. Strontium isotope ratios for the scheelites cluster tightly between 0.7019 and 0.7021. The Sr isotope data suggest that unlike Nd, Sr was significantly mobile during the hydrothermal process. The fluids responsible for the Omai deposit may have picked up Sr along the flow path. The constant low Sr isotope values of scheelites probably reflect the key role that the local tholeiitic basalts played as the main source of Sr in the fluids. Whereas Nd isotopes identify the fluid source area, the Sr isotopes map the fluid flow paths. Received: 11 February 1999 / Accepted: 1 November 1999     

Quick Detection System for Planck satellite

Q uick D etection S ystem or qds is a software package that has been developed for detecting point sources in the Planck satellite data as soon as the data become available, a few days after transmission to the Earth. Point sources are detected by filtering the data with a filter defined by the Mexican hat wavelet. An alert is generated on those detections that are found to be interesting, such as prominent flaring, according to the criteria specified to the software. The goal is to detect spectral or flux variability in active galactic nuclei so that instant multifrequency follow-up observations with other instruments could be arranged to study the interesting behaviour.     

Collisional modification of the acapulcoite/lodranite parent body revealed by the iodine‐xenon system in lodranites

Abstract— The I‐Xe system of three lodranites has been investigated. Samples of Gibson yielded no isochrons, and late model ages are attributed to late addition of iodine. Two metal and one silicate separate from the transitional lodranite Graves Nunataks (GRA) 95209 gave ages that are consistent with each other and with the literature I‐Xe age of Acapulco feldspar. These yield a mean closure age 4.19 ± 0.53 Ma after the Shallowater enstatite reference age (4562.3 ± 0.4 Ma). Such identical I‐Xe ages from distinct phases imply that the parent material underwent a period of rapid cooling, the absolute age of this event being 4558.1 ± 0.7 Ma. Such rapid cooling indicates an increase in the rate at which heat could be conducted away, requiring a significant modification of the parent body. We suggest the parent body was modified by an impact at or close to the time recorded by the I‐Xe system. An age of 10.4 ± 2.3 Ma after Shallowater has been determined for one whole‐rock sample of Lewis Cliff (LEW) 88280. Since the release pattern is similar to that of GRA 95209 this hints that the larger grain size of this sample may reflect slower cooling due to deeper post impact burial.     

AGNs,X-ray background and the possible candidates for great attractor

In the five last years, different structures (density excess 1) have been proposed as the direct cause of our infall toward the direction of Hydra—Centaurus with a velocity of 500 km s^–1. The direct effect of the mentioned matter accumulations on the X-ray background (XRB) can be estimated as a function of the geometry of the structures and of the cosmological evolution of the sources emitting in the X-ray band (2–10 keV) for different universes (₀1). If the XRB comes mostly from AGNs with low luminosity (L _X <10⁴³ erg s^–1 and, therefore, they will have a weak cosmological evolution) and we consider the difference between the intensities coming from both hemispheres (that oriented toward the direction of our motion and the opposite one) obtained by means of different satellites, we can conclude that some candidates are highly unlikely.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain     

Comparison of flood hazard assessments on desert piedmonts and playas: A case study in Ivanpah Valley, Nevada

Accurate and realistic characterizations of flood hazards on desert piedmonts and playas are increasingly important given the rapid urbanization of arid regions. Flood behavior in arid fluvial systems differs greatly from that of the perennial rivers upon which most conventional flood hazard assessment methods are based. Additionally, hazard assessments may vary widely between studies or even contradict other maps. This study's chief objective was to compare and evaluate landscape interpretation and hazard assessment between types of maps depicting assessments of flood risk in Ivanpah Valley, NV, as a case study. As a secondary goal, we explain likely causes of discrepancy between data sets to ameliorate confusion for map users. Four maps, including three different flood hazard assessments of Ivanpah Valley, NV, were compared: (i) a regulatory map prepared by FEMA, (ii) a soil survey map prepared by NRCS, (iii) a surficial geologic map, and (iv) a flood hazard map derived from the surficial geologic map, both of which were prepared by NBMG. GIS comparisons revealed that only 3.4% (33.9 km²) of Ivanpah Valley was found to lie within a FEMA floodplain, while the geologic flood hazard map indicated that ~ 44% of Ivanpah Valley runs some risk of flooding (Fig. 2D). Due to differences in mapping methodology and scale, NRCS data could not be quantitatively compared, and other comparisons were complicated by differences in flood hazard class criteria and terminology between maps. Owing to its scale and scope of attribute data, the surficial geologic map provides the most useful information on flood hazards for land-use planning. This research has implications for future soil geomorphic mapping and flood risk mitigation on desert piedmonts and playas. The Ivanpah Valley study area also includes the location of a planned new international airport, thus this study has immediate implications for urban development and land-use planning near Las Vegas, NV.     

Short-term beach–dune sand budgets on the north sea coast of France: Sand supply from shoreface to dunes, and the role of wind and fetch

Three experimental plots, covering the transition from the upper beach to the dune, on the North Sea coast of France were monitored at various intervals over a period of 18–24 months via high resolution terrain surveys in order to determine inter-site sand budget variability, as well as patterns and processes involved in sand exchanges between the upper beach and dune. The wind regime consists of a fairly balanced mix of moderate (80% of winds are below 8 m/s) onshore, offshore and shore-parallel winds. Sustained dune accretion over several years depends on the periodic local onshore welding of shoreface tidal banks that have developed in the storm- and tide-dominated setting of the southern North Sea. The only site where this has occurred in the recent past is Calais, where bank welding has created a wide accreting upper beach sand flat. At this site, significant sand supply from the subtidal sand bank reservoir to the upper beach flat occurred only once over the 18-month survey following a major storm. The bulk of the sand deposited over this large flat is not directly integrated into the adjacent embryo dunes by onshore winds but is progressively reworked in situ into developing dunes or transported alongshore by the balanced wind regime, thus resulting in alongshore stretching of the embryo dune system. The Leffrinckoucke site near Belgium shows moderate beach–dune mobility and accretion, while the Wissant site exhibits significant upper beach bedform mobility controlled by strong longshore currents that result in large beach budget fluctuations with little net budget change, to the detriment of the adjacent dunes. Accretion at these two sites, which are representative of the rest of the North Sea coast of France, is presently constrained by the absence of a shore-attached sand bank supply reservoir, while upper beach–dune sand exchanges are further limited by the narrow wave-affected upper beach, the intertidal morphology of bars and troughs which segments the aeolian fetch, and the moderate wind energy conditions. The balanced wind regime limits net sand mobilisation in favour of either the beach or the dune, and may explain the relatively narrow longshore morphology of the dune ridges bounding this coast.     

Distribution of nickel,copper and zinc in the volcanic series of Erta'Ale,Ethiopia

NI, Cu and Zn were measured polarographically in 34 samples of volcanic rocks from the chain of Erta'Ale, Ethiopia. In this chain all rock types from picritic basalts to alkaline and peralkaline rhyolites can be found. The series is marked by a considerable iron enrichment in the intermediate stages. The distribution of Ni, Cu and Zn in the series emphasizes the efficiency of the crystal-field theory in accounting for the behaviour of the elements during magmatic fractionation. Nickel shows a regular variation throughout the trend. The variation of copper is similar to that of iron, but the relative rates of enrichment and depletion show that they are not necessarily associated with the same mineral phases. The behaviour of Zn is rather irregular.     

Geometry of a convergent zone, Central British Columbia, Canada

The Canadian Cordillera is separable into two major northerly trending tectonic units—the Pacific Orogen and the Columbian Orogen, with the latter further separated into the Omineca Crystalline Belt and the easterly Rocky Mountain Fold and Thrust Belt. Synkinematic metamorphism of Jurassic age within the Omineca Belt is thought to be associated with accretion of westerly terranes of the Pacific Orogen—more specifically the Quesnellia terrane—that was thrust easterly over the Omineca Belt towards the craton. Mylonitic rocks mark the margin between these two belts and this margin is well-exposed near Crooked Lake, central British Columbia.Structural analysis across the zone of convergence between these two terranes indicates that the cratonic basement and the accreted cover sequences have several phases of deformation and metamorphism in common. The initial common phase of deformation, wherein convergence is accomplished, is characterized by easterly verging folds that are superposed by a second common phase having westerly verging folds that deform the zone of convergence and control the present regional map pattern. A final common phase of deformation produced easterly verging folds.Change in vergence direction is interpreted as resulting from change in direction of transport related to subduction process: first obduction of Quesnellia onto the Omineca craton, followed later by easterly subduction of an oceanic Quesnellia below the craton.All evidence of transport direction(s) points to convergence occurring at very high angles to the zone of convergence. There appears to be no evidence of transport parallel with the strike of the zone. If transport has taken place parallel to the strike of the zone, then this transport occurred before convergence or evidence of this motion has been destroyed during the convergence.     

Slump-initiated and controlled syndepositional sandstone remobilization: an example from the Namurian of County Clare, Ireland

Remobilization of sandstones can dramatically reconfigure original depositional geometries and results in very unusually shaped sandstones, which resemble little, if any, of the original geometry. A number of deformational sandstone bodies, dykes and volcanoes from the upper part of the Carboniferous Ross Formation are described, which offer the opportunity to examine a suite of field-scale reconfigured sandstones. These structures are located in close proximity to the Ross Slide, which outcrops along a 2-km section on the northern coast of the Loop Head Peninsula, County Clare, Ireland. Dome- and ridge-shaped deformational sandstone bodies, dykes and volcanoes are interpreted to be the product of remobilization of a turbiditic sandstone. Liquification and remobilization were triggered by translation, cessation and loading of the underlying turbiditic sandstone by the Ross Slide. Deformational sand body, dyke and volcano development occurred in an asynchronous fashion with deformational sand bodies formed during slump translation. Sand dykes and volcanoes developed after the cessation of slump movement. During slump translation, the minimum principal stress (σ₃) was orientated vertically and the slump behaved in a `ductile' manner. After slump arrest, the minimum principal stress was oriented horizontally, and the unit regained shear strength to behave in a `brittle' manner. The relative change in rheological states with changing applied shear stress is indicative of thixotropic-like behaviour within the slump mass. Ridge-shaped deformational sand bodies are aligned parallel to slump folds, and their morphology is inferred to be controlled by compressional slump deformation associated with heterogeneous cessation of slump movement that was initiated by frontal arrest of the translating mass.     

Geologic evolution of the Canarian Islands of Lanzarote, Fuerteventura, Gran Canaria and La Gomera and comparison of landslides at these islands with those at Tenerife, La Palma and El Hierro

In this paper we discuss the results of a swath bathymetric investigation of the Canary archipelago offshore area. These new data indicate that volcanism is pervasive throughout the seafloor in the region, much more that would be suggested by the islands. We have mapped tens of volcanic edifices between Fuerteventura and Gran Canaria and offshore Tenerife, La Gomera, El Hierro and La Palma. Volcanic flows are present between Tenerife and La Gomera and salic necks dominate the eastern insular slope of La Gomera. This bathymetry also supports land geologic studies that indicate that the oceanic archipelago has acquired its present morphology in part by mass wasting, a consequence of the collapse of the volcanic edifices. In the younger islands, Tenerife, La Palma and El Hierro, the Quaternary (1.2 to 0.15 Ma) debris avalanches are readily recognizable and can be traced offshore for distances measured in tens of km. Off the older islands, Lanzarote, Fuerteventura, Gran Canaria and La Gomera (<20 to 3.5 Ma), the avalanches have been obscured by subsequent turbidity current deposition and erosion as well as hemipelagic processes. The failure offshore western Lanzarote is in the form of a ramp at the base of the insular slope bound on the seaward side by a scarp. Its size and the lack of evidence of rotation along its landwards side precludes the possibility that it is a slump. It probably represents a slide whose outer scarp is caused by break-up of the slide. Mounds on the ramp’s surface may represent post-displacement volcanic structures or exotic blocks transported to their present locations by the slide. The failures offshore Fuerteventura are so large that, although they occurred in the Miocene-Pliocene, exotic blocks displaced from upslope are still recognizable in the insular margin morphology. The Canary Island insular margin appears to be a creation of Miocene-Pliocene mass wasting and more recent turbidity current deposition and erosion, and hemilepagic deposition. Failures offshore La Gomera are due to debris flows and/or turbidity currents. These events have obscured earlier mass wasting events. An erratum to this article is available at .