Styles of lode gold mineralization contributing to the placers of the Indian River and Black Hills Creek,Yukon Territory,Canada as deduced from microchemical characterization of placer gold grains

Between 1978 and 2009, approximately 430,000 oz of placer gold were obtained from the Indian River and Black Hills Creek, which equates to roughly 20% of the production for the entire Yukon Territory during that period. The area is unglaciated, exposure is poor, and there are few known lode gold occurrences present. The technique of microchemical characterization of placer gold grains has been applied to illuminate the style(s) of source mineralization and their relationship to placer gold from the Klondike gold district immediately to the north. A total of 2,613 placer gold grains from 22 localities were characterised in terms of the Au, Ag, Cu, and Hg content of their alloy and associated suite of opaque mineral inclusions. A combination of alloy and inclusion mineralogy was used to define gold signatures which augmented the previous classification of orogenic gold in the Klondike. Gold type 3b (8–25% Ag) is the main component of the placers in lower Dominion Creek but is augmented and eventually replaced by type 3a gold (10–40% Ag) in placers in the main Indian River valley, probably through erosion of gold-bearing veins in the valley floor. Type 4 gold exhibits highly variable Ag which may contain Hg to a maximum of 11 wt.%. This gold type also hosts a distinctive inclusion assemblage of complex polymetallic sulphides, tellurides, sulfotellurides, and sulfosalts and has previously been ascribed to local low sulfidation epithermal mineralization. Placer gold in drainages radiating from Eureka Dome exhibits various proportions of types 3 and 4 gold depending on location, but type 3 gold forms the major component in Black Hills Creek and northerly flowing tributaries of the Indian River with the exception of Eureka and Montana creeks. Type 5 gold is found only in placers in the middle and lower Indian River. It is distinguished by slightly elevated (0.05–0.17%) Cu in the gold alloy, together with low (5–9%) Ag contents. Inclusions of Bi minerals, Cr-bearing magnetite and molybdenite within type 5 gold suggest derivation from an intrusion-related source. Candidates for such a source include undiscovered lode occurrences associated with Cretaceous age intrusions to the south of the Indian River, or deformed Cu-Au (−Mo) porphyry occurrences which are known to be present in the same area. This analysis of placer gold has indicated that the contribution of low sulfidation epithermal gold from Eureka Creek to the larger placers of the Indian River is minor. Consequently, the placer gold inventory of the Indian River is primarily orogenic in origin. Similarly, the characterization of placer gold in Blackhills Creek strongly suggests an orogenic source. This study has demonstrated for the first time that orogenic lode gold mineralization extends a considerable distance to the south of the southern Klondike goldfield. This information contributes to the regional models of gold mineralization in an area which is currently the focus of intensive exploration.     

The distribution of zinc and reactive silicate in the Otago Harbour, New Zealand

The horizontal distributions of reactive silicate and zinc in the Otago Harbour, New Zealand, a shallow, vertically well-mixed estuary, have been compared. Within the harbour silicate displayed a linear salinity profile on two sampling occasions in October and early November. Consideration of the silicate budget suggested some removal of freshwater silicate in the headwaters of the estuary. Zinc concentrations, measured using state-of-the-art clean laboratory techniques were orders of magnitude lower than previously published data for coastal waters. Typical values of 5–20 ng l⁻¹ Zn were recorded in the seaward endmember. Zinc-salinity profiles were approximately linear. Relationships between zinc and silicate concentrations for the estuarine and freshwater samples suggest a coupling between the rates of supply of these elements to the ocean and their utilisation by plankton.     

Surveying the sky with the Arcminute MicroKelvin Imager: expected constraints on galaxy cluster evolution and cosmology

We discuss prospects for cluster detection via the Sunyaev–Zel'dovich (SZ) effect in a blank field survey with the interferometer array, the Arcminute MicroKelvin Imager (AMI). Clusters of galaxies selected in the SZ effect probe cosmology and structure formation with little observational bias, because the effect measures integrated gas pressure directly, and does so independently of cluster redshift.
We use hydrodynamical simulations in combination with the Press–Schechter expression to simulate SZ cluster sky maps. These are used with simulations of the observation process to gauge the expected SZ cluster counts. Even with a very conservative choice of parameters we find that AMI will discover at least several tens of clusters every year with     the numbers depend on factors such as the mean matter density, the density fluctuation power spectrum and cluster gas evolution. The AMI survey itself can distinguish between these to some degree, and parameter degeneracies are largely eliminated given optical and X-ray follow-up of these clusters; this will also permit direct investigation of cluster physics and what drives the evolution.     

Impact cratering on porous asteroids

The increasing evidence that many or even most asteroids are rubble piles underscores the need to understand how porous structures respond to impact. Experiments are reported in which craters are formed in porous, crushable, silicate materials by impacts at 2 km/s. Target porosity ranged from 34 to 96%. The experiments were performed at elevated acceleration on a centrifuge to provide similarity conditions that reproduce the physics of the formation of asteroid craters as large as several tens of kilometers in diameter.Crater and ejecta blanket formation in these highly porous materials is found to be markedly different from that observed in typical dry soils of low or moderate porosity. In highly porous materials, the compaction of the target material introduces a new cratering mechanism. The ejection velocities are substantially lower than those for impacts in less porous materials. The experiments imply that, while small craters on porous asteroids should produce ejecta blankets in the usual fashion, large craters form without ejecta blankets. In large impacts, most of the ejected material never escapes the crater. However, a significant crater bowl remains because of the volume created by permanent compaction of the target material. Over time, multiple cratering events can significantly increase the global density of an asteroid.     

Doppler signatures of Hα flares in AE Aquarii

We present high-speed spectrophotometric observations of the Hα emission in the exotic cataclysmic variable AE Aqr. Over 10 000 spectra with a sampling rate of ∼ 6 s were obtained. The well-known 16.5- and 33-s oscillations were not detected in either the continuum or the line, nor were any quasi-periodic oscillations detected. The Hα line shows remarkably complex behaviour, with changes in flux of up to a factor of 3 in a few hours. The line does not act as a single entity — the instantaneous line profile is highly asymmetric and consists of a superposition of emission components that are localized in velocity. We find that during flares the Hα emission is delayed and decays more slowly than the continuum. The Hα variations are not linearly correlated with the continuum variations. Examination of the trailed spectrograms and Doppler tomograms reveals no signatures of an accretion disc, but rather supports the idea that mass is being ejected by a propeller mechanism.