Mobilization of gold into lake sediments from acid and alkaline mineralized environments in the southern Canadian Shield: Gold in lake sediments and natural waters

To be an effective indicator of mineralization in lake sediment surveys within the Canadian Shield, it is desirable that an element migrate in solution or adsorbed on suspensates. Given the low relief and disorganized drainage patterns of this region, dispersal in clastic form in drainage systems is limited and gives rise to erratic distributions. The purpose of this study was to discover whether Au shows significant hydromorphic mobility, which would justify the increasing use that is being made of this element in lake sediments as an indicator for gold mineralization.Waters and lake sediments were collected from Napier Lake, Ontario; PAP Lake, Saskatchewan; and Foster Lake, Manitoba, all of which contain Au-quartz vein mineralization and lie within the glaciated boreal forest zone of the Canadian Shield. In all three areas, profundal lake sediments down-drainage of mineralization contain Au concentrations higher than regional mean concentrations. Significant dissolution and transport of Au was found under oxidizing conditions associated with waters with pH that varied from acid to alkaline. Waters from drill holes penetrating mineralization contain up to 401 ng L⁻¹ Au (note; 1 ng L⁻¹ is equivalent to 1 part per trillion, 10⁻¹²). Surface waters overlying or near mineralization collected from bogs, seeps, ponds and streams contain up to 13 ng L⁻¹. The content of Au in lake waters is lower, with a maximum of 1.1 ng L⁻¹. There is also a detectable quantity of Au present in suspensates. Two samples of particulates (> 1 μm) filtered from lake water have Au equivalent to 0.17 ng L⁻¹ and 0.039 ng L⁻¹. While the contents of Au present in solution or as suspensates in lake and stream water are relatively small, they are sufficient, if precipitated, to generate anomalies in lake sediments. Thus for Reservoir Lake, in the Foster Lake area, water from the principal stream entering the lake carries 0.3 ng L⁻¹ Au. This provides an annual flux which far exceeds that required to generate the 7.3 ppb Au contained in profundal sediments of this lake; a content that is anomalous relative to the regional median content of < 1 ppb Au for lake sediments.Hydrogeochemical prospecting involving analysis for Au is one method for tracing the source of anomalous Au in lake sediments. Collection of 1 L samples without field treatment, followed by extraction of Au into MIBK, then analysis by graphite-furnace atomic absorption spectrophotometry, permits detection levels for Au of 0.5 ng L⁻¹. This is below the contents of Au found in some waters from mineralized areas. A detection limit of 0.3 ng L⁻¹ was obtained using larger water samples.     

The timescale and mechanism of granulite formation at Kurunegala,Sri Lanka

Incipient charnockite formation at Kurunegala in Sri Lanka is characterized by the growth of orthopyroxene at the expense of amphibole and biotite in an originally homogeneous gneiss. Mineral equilibria in the charnockite assemblage record pressure-temperature (P-T) conditions of 738±60° C and 6.9±1.2 kbar at-17.0±1.2 log fO₂ and aH₂O=0.18±0.16. Wholerock trace-element and isotopic measurements show that charnockite formation was accompanied by a systematic depletion of Sm>Rb>Pb>U>Sr>Nd, with a fractionation of Rb/Sr, Sm/Nd and Th/U ratios, and crystallization of the charnockite assemblage at 535±5 Ma. Major element (Fe–Mg–Ca) and Sm–Nd equilibration between minerals occurred at 524±9 Ma, whereas, Pb and Rb–Sr underwent continued exchange to 501±5 Ma and 486±1 Ma, respectively. Trace-element data for both amphibolite and charnockite minerals show that depletion on a whole-rock scale can be accounted for either by changes in mineral modes or trace-element abundances, within the immediate area of dehydration. The fractionation of Sm/Nd on a whole-rock scale is controlled by the breakdown of amphibole, without the growth of a major new host-phase for Sm in the charnockite. Rubidium and Sr are dependent on the relative behaviour of biotite, plagioclase and alkali-feldspar. Modelling of dehydration-melting involving the breakdown of amphibole, biotite, and alkali-feldspar reproduces the observed Sm/Nd and Rb/Sr fractionation, and indicates the loss of small melt fractions, on a cm scale, from the charnockite. These observations suggest that partial melting is the most plausible means of effecting both the dehydration and depletion that accompanies charnockite formation.     

Experimental and finite element studies of the forced vibration response of morrow point dam

Measured accelerations and water pressures obtained during a recent forced vibration test on a large thin arch dam at high water are compared to predictions from a finite element model for which water compressibility is both included and neglected. The numerical model is calibrated using the antisymmetric response data because they are only slightly affected by water compressibility; good agreement is obtained. In the effort to reproduce the symmetric response data, for which water compressibility plays a strong role, the calibrated model shows better correlation when water compressibility is included, but the agreement is still inadequate. A successful isolation of the fundamental water resonance from the experimental data shows significantly different features from those of the numerical water model, indicating possible inaccuracy in the assumed geometry and/or boundary conditions for the reservoir. Some other results at low water level are also included.     

Emigration of juvenile Atlantic menhaden,Brevoortia tyrannus (Pisces: Clupeidae), from the York River estuary

The emigration of juvenile Atlantic menhaden,Brevoortia tyrannus (Latrobe), from the York River Estuary, Virginia, was studied during the years 1981 and 1982. Concurrent observations of menhaden catch, water temperature, and phytoplankton abundance suggested that the migration began in response to environmental events in the estuary. Juvenile menhaden appeared to begin their migration five days after the sustained onset of water temperatures below 24 °C. The temperature change was hypothesized as the proximate factor that initiates the migration. Coincident with the migrations, phytoplankton communities in the estuary bloomed, suggesting that the ultimate factor that initiates the migration may be the occurrence of sufficient food for the emigrating menhaden as they moved down and out of the estuary. Migration timing may have evolved as a mechanism to enhance the survival of migrating juvenile menhaden during a period of physiologic stress.     

Chronology of the principal scientific and governmental actions leading up to the November 13, 1985 eruption of Nevado del Ruiz, Colombia

The virtue of studying the chronology of an event is the insight it affords as to how man views and responds to the forces of nature. This chronology reflects the fumbling nature of mankind (as only appreciated in hindsight) when faced by an impending event of uncertain nature, in this case in a developing nation, but it might have happened anywhere. It is hoped that we as volcanologists might improve our response to such crises knowing how they have evolved previously.     

Direct dating of Plio-Quaternary pumices by Ar/Ar step-heating and single-grain laser fusion methods: the example of the Monts-Dore massif (Massif Central, France)

K-feldspars separated from Plio-Quaternary pumice flows of the Monts-Dore massif (Massif Central, France) give ⁴⁰Ar/³⁹Ar saddle-shaped age spectra. Laser-probe analysis of hand-picked single grains gives ages in agreement with those of overlying and underlying lava flows previously dated by the conventional K-Ar method. The laser-probe ages are lower than the minimum ages measured on the populations of grains by the step-heating method. As previously suggested by Lo Bello and co-workers for the pumice flow of Neschers belonging to the same volcanic massif, the saddle-shaped age spectra are ascribed to the different Ar-release patterns of two populations of K-feldspars: (1) young sanidine phenocrysts of the pumices; and (2) old K-feldspars plucked from the Hercynian basement during the explosive eruption. Measurements of both the granitic rocks from surrounding areas and the granitic xenoliths included in pumices give ages around 330 Ma and show that most of the xenocrysts included in the pumices did not lose significant amounts of argon during the eruption. With the ⁴⁰Ar/³⁹Ar step-heating method, we were able to detect contamination of Quaternary K-feldspars by Hercynian K-feldspars as low as 0.25. Because pumice flows of the Monts-Dore massif crop out over large areas, these new ages will be useful for establishing a precise stratigraphy of the European Plio-Quaternary.     

Application of universal kriging for estimation of earthquake ground motion: Statistical significance of results

Universal kriging is compared with ordinary kriging for estimation of earthquake ground motion. Ordinary kriging is based on a stationary random function model; universal kriging is based on a nonstationary random function model representing first-order drift. Accuracy of universal kriging is compared with that for ordinary kriging; cross-validation is used as the basis for comparison. Hypothesis testing on these results shows that accuracy obtained using universal kriging is not significantly different from accuracy obtained using ordinary kriging. Tests based on normal distribution assumptions are applied to errors measured in the cross-validation procedure;t andF tests reveal no evidence to suggest universal and ordinary kriging are different for estimation of earthquake ground motion. Nonparametric hypothesis tests applied to these errors and jackknife statistics yield the same conclusion: universal and ordinary kriging are not significantly different for this application as determined by a cross-validation procedure. These results are based on application to four independent data sets (four different seismic events).     

Non-linear earthquake response of concrete gravity dams part 2: Behaviour

The earthquake response of the idealized Pine Flat Dam—water—foundation system of Part 1 is investigated with emphasis on the non-linear behaviour associated with concrete cracking and water cavitation. It is shown that water cavitation has little effect on the dam response. On the other hand, concrete cracking plays a significant role, as demonstrated by several analyses on dams with initial cracks or weak lift joints and by a final analysis on a homogeneous dam. The analyses encountered a number of numerical difficulties requiring further mesh refinement, and these are described. A possible failure mode associated with inclined cracks is revealed.     

Platinum group metals in common plants of northern forests: developments in analytical methods, and the application of biogeochemistry to exploration strategies

All platinum-group metals (PGM) in ashed plant tissues, from an area of platinum mineralization in Saskatchewan, have been measured by neutron activation analysis of a NiS fire assay bead. Concentrations of up to 1350 ppb (ng/g) Pd, 880 ppb Pt, 49 ppb Rh, 37 ppb Ru, 24 ppb Ir, and 15 ppb Os occur in ashed twigs of black spruce (Picea mariana), indicating that the ratios of PGM uptake are about the same as those occurring in the bedrock. Plants growing on drift-covered diabase, known to have about 100 ppb PGM, contain up to 77 ppb Pt in ash, demonstrating the potential value of biogeochemical methods in helping to delineate platiniferous zones. Twigs of black spruce, jack pine, and labrador tea appear to be the optimum sample media in this environment.A rapid multi-element neutron activation analysis by direct irradiation of tissue permits measurement of 0.05 ppb Ir in dry material, or 2 ppb Ir in ashed samples. In view of the normally very low concentrations of Ir in plants, any detectable Ir probably indicates enrichment of PGM in the substrate.Comparison of several wet-chemical analytical methods indicates that the optimum (cost-effective) technique for detecting 1–2 ppb Pt, Pd, or Rh in 2-g samples of ash is by Te co-precipitation (following fusion and dissolution), and analysis by ICP-MS with sample introduction by electrothermal vaporization. However, care must be taken to ensure complete dissolution of all PGMs in the ash.