Derivation of gold by oxidative metamorphism of a deep ductile shear zone: Part 1. Conceptual model

Quartz-carbonate gold deposits were emplaced in shear zones at or above the brittle-ductile transition. Some of the largest deposits are known to have formed along major, long-lived, transcurrent shears. Shears of this type widened downwards in the ductile regime, as a result of decreasing rock viscosity with depth; some were as wide as 40 km at depths of granulite facies metamorphism. Ductile shears are permeable and, since the permeability is along microfractures, fluid flow was pervasive, providing the opportunity for extensive chemical reaction. Reaction rates were enhanced by shear heating and by deformation-induced stress gradients in minerals, and reductions in grain size. Fluid flow tended to be upwards, because of pressure drop into the brittle portion of the shear. Given the wedge-shaped profile of ductile shears, fluids that had passed through a large volume of lower crust would have been focused at the brittle-ductile transition. Thus, if processes existed to selectively remove elements during fluid movement through the lower crust, these elements would also have been focused at this transition.One of the most constant features of quartz-carbonate lodes is carbonate alteration, which may extend kilometers out from major deposits. The ¹³C signature of this is consistent with a mantle source for the CO₂. Upward-moving CO₂ vapour of probable mantle origin has been implicated in the dehydration of amphibolite facies rocks to granulites and the concomitant depletion of large ion lithophile elements (LILE). The best documented cases of modification of the lower crust by CO₂ are from major shear zones. CO₂ streaming at depth could only have occurred under conditions more oxidizing than that required for graphite stability. These conditions favour solubility of gold by (a) oxidizing Au⁰ to Au⁺; (b) by dissolving sulphide from the rocks to complex with Au⁺. Recent work has shown that some major Archean gold deposits were derived from relatively oxidized fluids.A conceptual model is outlined for the genesis of at least some quartz-carbonate gold deposits. CO₂ permeating deep ductile shear zones dehydrated amphibolite facies rocks. A relatively oxidized CO₂-H₂O fluid was produced, which dissolved sulphide and gold from large volumes of lower crust. Gold was carried upwards in the narrowing shear, to be focused and precipitated at or above the brittle-ductile transition.     

Si,Al ordering in leucite by high-resolution 27Al MAS NMR spectroscopy

High-resolution ²⁷Al MAS NMR spectra of natural leucite recorded at H ₀=11.7T contain three resolvable resonances at ²⁷Al δ _i = 69.2, 64.7, and 61.0±0.5 ppm. These three resonances are assigned to the three inequivalent framework positions of leucite: T3, T2, and T1, respectively. Fitting the observed spectra yields a Si,Al distribution for leucite in which approximately one-half of the Al is in T1 and one-quarter in each of T2 and T3. This Si,Al distribution differs substantially from those obtained by previous workers using ²⁹Si NMR spectroscopy and X-ray diffraction. New ²⁹Si NMR spectra and revision of previously reported ²⁹Si NMR peak assignments, however, make the ²⁷Al and ²⁹Si NMR results consistent. The ²⁷Al δ _i correlate linearly with the mean T-O-T′ bond angles of the average structure, which allows the peak assignments to be made. However, this correlation lies distinctly toward higher frequency and larger bond angles than correlations for Si,Al ordered aluminosilicates, suggesting that the mean T(Al)-O-T′(Si) bond angle for each site in leucite is smaller than the mean bond angle of the average structure, which is averaged over T(Al)-O-T′(Si) and T(Si)-OT′(Si,Al) angles.     

A short-range interaction model for polytypism and planar defect placement in sapphirine

The concept of short-range interlayer interactions, fundamental to spin-analogue models for polytypism, is examined in the case of sapphirine. Consideration of interactions out to fourth-nearest neighbours provides a rationale for the difference between the polytype suites observed for sapphirine and wollastonite. In each case, the observed long-period structures are consistent with those predicted to be stable by the appropriate mappings onto the axial next-nearest neighbour model. Short-range interaction parameters may also be used to express stacking fault energies. This approach, combined with a simple nucleation-andgrowth model, is used to examine the possibility of metastable generation of complex polytypes in sapphirine. Statistical analysis of defect distributions and frequencies in sapphirine suggests that interactions over several hundred Ångstroms must be considered if the stacking energetics are to be accurately modelled.     

Desertification as a Global environmental issue

Compared to other environmental issues, such as Global Warming/Climatic Change, and the Ozone Hole, Desertification has been neglected by both scientists and funding agencies. The reasons are the complexity of the problem and the poverty and marginality of the arid lands. The author suggests a policy-oriented definition and draws attention to the differences between drought, desiccation and dry-land degradation.     

Carbon fluxes resulting from U.S. private timberland management

A carbon budget model was developed to examine the effects of forest management practices on carbon storage in U.S. private timberlands. The model explicitly incorporates the demand for wood products and its impact on harvesting and other management decisions. Forest carbon is divided into four components: carbon stored in trees, soils, forest litter, and understory vegetation. Changes in the forest carbon inventory result from tree growth and management activities, in particular harvesting. Harvesting of timber for wood products is determined by demand and supply forces. The model then tracks carbon in timber removals through primary and secondary processing and disposal stages. Harvesting also has effects on carbon in soils, forest litter, and understory vegetation. A base-run scenario projects increases in carbon storage in U.S. private timberlands by 2040; however, this increase is offset by carbon emissions resulting from harvesting.     

Flood hydrology and geomorphic effectiveness in the central Appalachians

This paper compares hydrologic records and geomorphic effects of several historic floods in the central Appalachian region of the eastern United States. The most recent of these, occurring in November 1985, was the largest ever recorded in West Virginia, with peak discharges exceeding the estimated 500-year discharge at eight of eleven stations in the South Branch Potomac River and Cheat River basins. Geomorphic effects on valley floors included some of the most severe and widespread floodplain erosion ever documented and exceeded anything seen in previous floods, even though comparable or greater rainfall and unit discharge have been observed several times in the region over the past 50 years. Comparison of discharge-drainage area plots suggests that the intensity and spatial scale of the November 1985 flood were optimal for erosion of valley floors along the three forks of the South Branch Potomac River. However, when a larger geographic area is considered, rainfall totals and discharge-drainage area relationships are insufficient predictors of geomorphic effectiveness for valley floors at drainage areas of 250 to 2500 km². Unit stream power was calculated for the largest recorded flood discharge at 46 stations in the central Appalachians. Maximum values of unit stream power are developed in bedrock canyons, where the boundaries are resistant to erosion and the flow cross-section cannot adjust its width to accommodate extreme discharges. The largest value was 2570 W m^?2; record discharge at most stations was associated with unit stream power values less than 300 W m^?2, but more stations exceeded this value in the November 1985 flood than in the other floods that were analysed. Unit stream power at indirect discharge measurement sites near areas experiencing severe erosion in this and other central Appalachian floods generally exceeded 300 W m^?2; reach-average values of 200-500 W m^?2 were calculated for valleys where erosion damage was most widespread. Despite these general trends, unit stream power is not a reliable predictor of geomorphic change for individual sites. Improved understanding of flood impacts will require more detailed investigation of interactions between local site characteristics and patterns of flood flow over the valley floor.     

Antarctic sea-ice relationships with indices of the atmospheric circulation of the Southern Hemisphere

A link between the Antarctic sea-ice extent and low-frequency atmospheric variations, particularly ENSO, has been suggested by recent modeling and empirical studies. This question is examined here using a high-resolution (by week, by region) data base of Antarctic sea-ice extent for the 1973–1982 period. Although of relatively short duration by Northern Hemisphere standards, such a data base offers an opportunity rare in Southern Hemisphere climate studies. The seaice variations are examined in the context of longer-term indices of the large-scale atmospheric circulation. These are a Southern Oscillation Index (SOI) and an index of sea-level pressure (SLP) wavenumber one in the Southern Hemisphere extratropics. The indices are updated through 1982, and their associations with regional-scale pressure indices in the Australia-New Zealand sector are also examined. The 1973–1982 period is anomalous when compared with the period 1951–1972. Correlation analysis of the monthly sea ice and circulation index values reveals that much of the apparent link between the ice and the SOI suggested in previous studies arises from autocorrelations present in both data sets and the strong annual cycle of sea-ice extent. Removing these effects from the data and re-running the correlations reveals that most of the resulting significant associations between the ice and one or other of the circulation indices can probably be explained on the basis of chance. In order to reconcile these findings with previous studies that show some strong ice-circulation interactions on regional scales, only those months in which significant correlations occur between both largescale circulation indices and the sea ice are examined further. These occur preferentially in the Ross and Weddell sectors, which constitute the regions contributing most to the variability of Antarctic sea ice. The analysis suggests that the sea-ice-extent changes lag the SOI by several months but may precede changes in extratropical SLP wavenumber one. Confirmation of these tentative regional ice extent-circulation teleconnections necessarily awaits the forward extension of the high-resolution sea-ice data base beyond the 10 years available here.This paper is based on material presented at the Conference on Mechanisms of Interannual and Longer-Term Climatic Variations held at the University of Melbourne, Australia: December 8–12, 1986.