Bias in the analysis of geological materials for gold using current methods

A consistent pattern has been observed in the results obtained for Au in three soil and two porphyry copper ore samples serving as control reference standards in geochemical analyses. The mean reported by wet chemical methods (regardless of the measurement technique) was less than the mean by fire assay-based methods which, in turn, was less than the mean reported by laboratories using direct instrumental neutron activation analysis (INAA). These data have been obtained from 16 laboratories, some employing more than one method. Compared to INAA, values obtained for Au (at the 30–300 ppb level) using aqua regia (AR) dissolution were low by 24–42%, while those reported by fire assay-based methods were low by 14–26%.Studies of these samples and 32 rocks of widely varying composition revealed that the amount of Au remaining in the residue after AR attack ranged from 4% to 59% of the total. Gold dissolved by AR was determined by graphite furnace atomic absorption spectrometry (GFAAS) after extraction into MIBK (methyl isobutyl ketone), whereas Au remaining in the residue was determined by INAA. The slope of the line obtained by plotting Au by INAA vs. the sum of Au by AR/GFAAS and Au in the residue was not significantly different from 1.0.Modifications to the AR procedure such as prior attack by HCl or HF did not improve the recovery of Au in the three reference soils. However, reduction of sample size from 10 g to 1 g while maintaining the volume of AR at about 30 ml did increase results for Au. Furthermore, addition of HF in the attack on 1-g samples yielded results virtually identical to those obtained by INAA. It is thought that the poor extraction efficiency by AR is due to non-wetting of the larger sized sample, a lack of intimate, prolonged contact between the grains of the sample and the acid mixture and the presence of insoluble gangue surrounding Au particles. Constant agitation of the sample during evaporation with AR is desirable.The mean values obtained for Au in the soils by fire assay methods were not significantly different from the results by INAA (low by 14–19%); this was not the case for the two copper ore samples (low by 26%). This probably reflects the difficulties encountered in fire assay by high concentrations of Cu which hinder effective collection and separation of Au into the Pb button. However, the accuracy of the INAA method has not been established and is dependent upon measurement procedures and the degree of certainty associated with the Au values assumed for the reference materials employed for calibration. While estimation of precision does not present a problem, accuracy is difficult to assess in the absence of certified reference materials for Au at concentrations in the 10–300 ppb range. However, it is concluded that methods based upon AR dissolution can lead to low results, the magnitude being dependent upon the sample matrix and the mineralogical association of the Au present.     

Determination of Precise and Accurate 51V/50V Isotope Ratios by Multi‐Collector ICP‐MS,Part 2: Isotopic Composition of Six Reference Materials plus the Allende Chondrite and Verification Tests

We present the first measurements of vanadium (V) stable isotopes for six reference materials – USGS PCC‐1, BHVO‐2, BCR‐2, BIR‐1a, GSP‐2 and AGV‐2 – plus the widely available carbonaceous chondrite Allende. We present standard addition and matrix spiking tests to assess the robustness and reproducibility of our data. Standard addition utilised an enriched ⁵⁰V solution designated VISSOX (Vanadium Isotope Standard Solution OXford). We further assessed the veracity of the method by spiking collected sample matrices with the same amount of a V standard solution, whose isotopic composition was defined as 0‰. Standard addition and matrix spiking tests recorded no appreciable artificial isotope fractionation. We estimate that the best currently attainable long‐term reproducibility of stable ⁵¹V/⁵⁰V isotope measurements in complex matrices is 0.15‰, which is in the same order as the reproducibility achievable with standard solutions. Finally, a large range of ～ 1.2‰ in stable V isotopic composition was documented, with ～ 0.5‰ of that variation in high temperature igneous materials alone. The range and resolving power of V stable isotopes, with respect to igneous material, compared favourably with the magnitude of fractionation reported for other non‐traditional stable isotope systems, which bodes well for the utility of this new system.     

Signature of explosive volcanic eruptions in the sediments of a high-altitude Swiss lake

The analysis of 125 years of well-dated varved sediments in Lake Silvaplana, located at 1,791 m a.s.l. in the Upper Engadine region of south-eastern Switzerland, reveals that 7 out of the 8 climatically relevant explosive volcanic eruptions between A.D. 1880 and 2004 were followed by distinct peaks in median grain-size. Although the underlying mechanisms are yet unclear, an analysis of local meteorological data suggests that this phenomenon is unlikely to be related to any change in air temperature associated with the eruptions, but instead may be related to an increase in autumn precipitation subsequent to the eruptions that led to the erosion and fluvial transport of particles larger than normal.     

斑岩成矿系统多中心复合成矿作用模型——以西藏甲玛超大型矿床为例

青藏高原冈底斯成矿带诸多大型-超大型斑岩成矿系统的勘查和研究成果显示,传统的陆缘弧或岛弧背景的成矿作用模型不能更好的用于指导碰撞造山背景下斑岩成矿系统的勘查实践。文章以甲玛超大型矿床为实例,基于十余年的勘查实践,通过对不同类型矿体蚀变和矿化结构的精细解剖和成岩成矿年代学的系统分析,建立造山背景下斑岩成矿系统多中心复合成矿作用模型。同时,系统总结甲玛矿床地质特征,阐述成矿作用模型中地层、构造、岩浆等基础地质要素,并详细解析斑岩型钼铜矿体、矽卡岩型铜多金属矿体、角岩型铜钼矿体、Manto型富铜铅锌矿体以及独立金矿体所构成的多元矿体结构特征。最后,揭示甲玛斑岩成矿系统多中心复合成矿作用模型与传统模型的差异,提出其对相同构造背景下斑岩成矿系统勘查评价的启示。     

On the response of the neutral temperature at 78°N and 90 km altitude to solar proton events

We are able to determine neutral air temperatures by examining the fading times of meteor trail echoes in the upper mesosphere/lower thermosphere. It has been suggested that solar proton events may stimulate both dynamic and aeronomic changes in the middle atmosphere and we have endeavoured to investigate this. Despite a variety of approaches to determine the background temperature above which we might expect to see enhancements under conditions of strong proton precipitation, we are unable to detect any significant changes. We have repeated the search during selected seasons and also with various proton flux thresholds, similarly to no avail. We conclude, therefore, that at 90 km altitude, 78°N and 16°E, at least, we are unable to detect enhanced neutral temperatures due to solar proton events. At best, any enhancements, predicted to be of the order of a few K only, are likely to be completely masked by the day-to-day variability of the temperature field.     

Estimation of streambed groundwater fluxes associated with coaster brook trout spawning habitat

We hypothesized that the spatial distribution of groundwater inflows through river bottom sediments is a critical factor associated with the distribution of coaster brook trout (a life history variant of Salvelinus fontinalis) spawning redds. An 80-m reach of the Salmon Trout River, in the Huron Mountains of the upper peninsula of Michigan, was selected to test the hypothesis based on long-term documentation of coaster brook trout spawning at this site. A monitoring well system consisting of 22 wells was installed in the riverbed to measure surface and subsurface temperatures over a 13-month period. The array of monitoring wells was positioned to span areas where spawning has and has not been observed. Over 200,000 total temperature measurements were collected from five depths within each monitoring well. Temperatures in the substrate beneath the spawning area were generally less variable than river temperatures, whereas temperatures under the nonspawning area were generally more variable and closely tracked temporal variations in river temperatures. Temperature data were inverted to obtain subsurface groundwater velocities using a numerical approximation of the heat transfer equation. Approximately 45,000 estimates of groundwater velocities were obtained. Estimated groundwater velocities in the spawning area were primarily in the upward direction and were generally greater in magnitude than velocities in the nonspawning area. Both the temperature and velocity results confirm the hypothesis that spawning sites correspond to areas of significant groundwater flux into the river bed.     

Contrasting responses of dimictic and polymictic lakes to environmental change: a spatial and temporal study

Although comparative analyses between dimictic and polymictic lakes have noted the key role of mixing regime in governing water quality, limnologists have historically focused on dimictic lakes, leaving polymictic lakes relatively understudied. In this study, we investigated whether the effects of agricultural development on water quality differed between dimictic and polymictic lakes in a naturally nutrient-rich region of Alberta, Canada. Through a spatial limnological analysis of 36 sites, we found that the relationship between surface water total phosphorus concentration and the percent of agriculture (% Agr) in the catchments differed between dimictic and polymictic lakes, where the proportion of variance explained was much more pronounced in the dimictic (79% explained) than in the polymictic systems (7% explained). Paleolimnological analyses of subfossil chironomids in surface sediment samples (0–1 cm) from 18 of the 36 study lakes, and in sediment core profiles from the dimictic and polymictic basins of a eutrophic lake, showed that water quality differed between mixis groups. We found that the surface sediment chironomid assemblages differed significantly between polymictic and dimictic lakes. Detailed analyses of the sediment cores demonstrated that the two basin types differed in their responses to land-use change through time, as only the dimictic basin showed a parallel increase in anoxia with increasing agricultural development. We suggest that in naturally-fertile landscapes, external nutrient loading exerts a more notable effect on dimictic lakes, whereas internal nutrient loading is more important in polymictic systems.     

Abrupt deglaciation events and Holocene palaeoceanography from high-resolution cores,Cartwright Saddle,Labrador Shelf,Canada

The uppermost Quaternary sediments in Cartwright Saddle, Labrador Shelf, are acoustically laminated, with reflectors that can be traced over long distances. Two piston cores from the saddle record changes in sediment and meltwater delivery from the northeast margin of the Laurentide Ice Sheet (LIS) during deglaciation. Variations in sediment properties indicate a similar history of sediment accumulation over the last 12 kyr. The temporal sampling interval reaches decadal resolution in the last deglacial period 7–9 ka. Analyses of total carbonate content, sediment magnetic variables, foraminiferal species and stable isotope measurements on planktic foraminifers show that abrupt changes occurred ca. 10.9, 9.2, 8.8, 7.9 and 7 ka (with 450 yr correction). There was no distinct change in sediment character during much of the Younger Dryas chronozone. In the δ¹⁸O record, the 8.8 ka event is a dramatic 1‰ shift toward lower values, which we associate with the Noble Inlet glacial event within Hudson Strait. We do not see the pronounced low δ¹⁸O event at 7.1 ka reported off Nova Scotia, but surprisingly, neither the Nova Scotia records nor other records in the Labrador Sea capture the impressive 8.8 ka change. Serious consideration must be given to the final collapse of the LIS as the cause of the 8.2 cal. ka cold event recorded in Greenland and northwest Europe. Copyright © 1999 John Wiley & Sons, Ltd.     

Temporal Interpolation of Spatially Dynamic Polygons

In this study we compare two methods of interpolating simple metrics from polygons that change over time. Conventional methodology involves computing values of the metrics at points in time where the polygon is observed, and interpolating between these observed values to estimate values at other points in time. An alternative method applies interpolation techniques to the locations of vertices composing polygons are interpolated between observations and metrics are computed from the interpolated polygon shapes. We conducted a Monte Carlo simulation of both techniques using simulated polygons composed of varying numbers of vertices, which were allowed to move randomly over varying numbers of discrete time periods. We observed differences in the success of the two methods when applied to three metrics: Area, Perimeter, and Area/Perimeter Ratio. We found the two techniques to perform differently depending upon the metric being interpolated. Full results are presented and possible reasons why the techniques succeeded or failed under various conditions are discussed.