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.     

Expert judgment and climate forecasting: A methodological critique of “climate change to the year 2000”

The National Defense University's study of climate change to the year 2000 was based largely on the judgments of the members of two expert panels. Although the study has been widely distributed and apparently read by policy makers in the U.S. and abroad, the method of eliciting and analyzing expert judgment has not been critically reviewed. This paper uses the literature on judgment and subjective probability to evaluate the expert judgment methods used in the study.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Transport and economic development in the less-developed countries: Some reflections on the seaports of Kenya and Tanzania

The significant role of transport in the development of less-developed countries is widely accepted, but the critical importance of seaports is often under-appreciated. Evidence from the Indian Ocean ports of Kenya and Tanzania is used in this paper to illustrate and support the argument that efficient port operation is an essential contributory factor in the development process.     

Comparison of acoustic instruments in an atmospheric turbulent flow over water

In the fall of 1968 an International Intercomparison Experiment was conducted to compare, among other things, acoustic anemometers designed for measurements of atmospheric turbulence. Excellent agreement was obtained in measurements of the vertical component, but an important discrepancy was revealed in measurements of the downwind velocity. The discrepancy proved far more significant in the cospectra than in the spectra themselves. It is evident that great care must be taken in the design of such instruments. An acoustic instrument was shown to have insufficient signal-to-noise ratio to serve as a thermometer under the near-neutral conditions often encountered over water.Comparison of the data-handling and data-analysis techniques showed that although the methods differed widely, the results were essentially identical.     

Rapid neutron capture in supernova explosions

The implications of recent studies of the dynamics of the cores of highly evolved massive stars are considered with regard to the general problems of nucleosynthesis. The typical conditions estimated for these models are shown to be very promising for the process of element synthesis by neutron capture on a fast time scale (ther-process ofBurbidge et al., 1957).     

Evolution of Planetesimal Velocities Based on Three-Body Orbital Integrations and Growth of Protoplanets

We obtain the viscous stirring and dynamical friction rates of planetesimals with a Rayleigh distribution of eccentricities and inclinations, using three-body orbital integration and the procedure described by Ohtsuki (1999, Icarus137, 152), who evaluated these rates for ring particles. We find that these rates based on orbital integrations agree quite well with the analytic results of Stewart and Ida (2000, Icarus 143, 28) in high-velocity cases. In low-velocity cases where Kepler shear dominates the relative velocity, however, the three-body calculations show significant deviation from the formulas of Stewart and Ida, who did not investigate the rates for low velocities in detail but just presented a simple interpolation formula between their high-velocity formula and the numerical results for circular orbits. We calculate evolution of root mean square eccentricities and inclinations using the above stirring rates based on orbital integrations, and find excellent agreement with N-body simulations for both one- and two-component systems, even in the low-velocity cases. We derive semi-analytic formulas for the stirring and dynamical friction rates based on our numerical results, and confirm that they reproduce the results of N-body simulations with sufficient accuracy. Using these formulas, we calculate equilibrium velocities of planetesimals with given size distributions. At a stage before the onset of runaway growth of large bodies, the velocity distribution calculated by our new formulas are found to agree quite well with those obtained by using the formulas of Stewart and Ida or Wetherill and Stewart (1993, Icarus106, 190). However, at later stages, we find that the inclinations of small collisional fragments calculated by our new formulas can be much smaller than those calculated by the previously obtained formulas, so that they are more easily accreted by larger bodies in our case. The results essentially support the previous results such as runaway growth of protoplanets, but they could enhance their growth rate by 10-30% after early runaway growth, where those fragments with low random velocities can significantly contribute to rapid growth of runaway bodies.