Metalliferous asteroids as potential sources of precious metals

201A

GEOLOGY General 955001 Le granite du Sidobre (Tam) (The Sidobre granite, Tam) D. Aissaoui 8~ R. Perrier, Mines & Carrieres, 76(AugustSept), 1994, pp 81-89. The granite range of the Sidobre region, located several kilometers northeast of Castres, has become France’s largest granite basin, with 48.9% of all domestic production. Each year this basin supplies 65 000 m3 of granite blocks. The geology of the area is described. (English summary) 955002 Geological exploration history of the eastern Sahara E. Klitzsch, Geofogische Runakchau, 83(3), 1994, pp 475-483. The major contributions to our knowledge of the geological history of the East Saharan countries are described. Even though progress in geology is vital to the development of Saharan countries, little has been recorded of this progress. Without an understanding of regional geology and without a good knowledge of groundwater, mineral and petroleum potentials, these countries would be in very different situations today. (from Author) 955003 Metalliferous asteroids as potential sources of precious metals J. S. Kargel, Journal ofGeophysicaf Research, 99(ElO), 1994, pp 21,129-21,141. Recent discoveries of near-Earth asteroids (NEAs) and chemical analyses of fragments of asteroids (meteorites) suggest that there may be a gold mine, literally, in nearEarth space. Judged from meteorite analyses, two types of asteroids offer particularly bright prospects for recovery of large quantities of precious metals (defined as Au, Pt, It, OS, Pd, Rh, and Ru), the ordinary LL chondrites, which contain 1.2~5.3% Fe-Ni metal containing SO-220 ppm of precious metals, and metallic asteroids, which consist almost wholly of Fe-Ni phases and contain variable amounts of precious metals up to several hundred ppm. The actual economic and technological impact of asteroidal metals may be considerably greater due to the increased availability and reduced prices of these resources. Despite this great potential, tirstorder technological, scientific, and economic uncertainties remain before the feasibility of exploitation of asteroids for precious metals can be ascertained. (from Author)

before the final stabilization of the Siberian platform (3600 Ma to 1600 Ma) and the comparatively short period of time for the stabilization of the Kaapvaal craton (3600 Ma to 3050 Ma) and 2) the intensive multiple activation of the marginal zones of the Siberian platform and the tectonically-controlled, magmatic sedimentary, intracratonic activity, typical of the Kaapvaal craton. (from Authors)

955805 Temporal relationships of lode gold mineralization to accretion, magmatism, metamorphism and deformation Archean to present: a review R. Kerrich & K. F. Cassidy, Ore Geology Reviews, 9(4), 1994, pp 263-310. Giant lode gold metallogenic provinces formed at three times in Earth history, in the late-Archean (2.7-2.6 Ga), late Paleozoic (450-340 Ma) and Mesozoic-Cenozoic. These times correspond to accretionary tectonic assembly of a supercontinent, in external orogens. Minor gold deposits also formed in space and time with collisional tectonics in internal erogenic belts of the supercontinent cycle. Consequently, the local temporal relationships of lode gold mineralization to collisional tectonics and post-peak metamorphism is also part of the secular development of a larger global geodynamic cycle. (from Authors)

955006 Arc-related gold-copper mineralization, basement domes and cmstal extension A. H. G. Mitchell & J. C. Carlile, Journal ofSoutheast Asian Earth Sciences, lO(l-2), 1994, pp 39-50. In western Pacific magmatic arcs, many of the largest lowsulphidation epithermal gold vein deposits are situated on basement antiforms which often include ophiolite. Porphyry copper (-gold) deposits in the western Pacific are mostly situated away from antiform axes, in shallow stocks, with their tops at stratigraphic levels similar to those of the deeper parts of low-sulphidation epithermal veins. It is suggested that epithermal systems develop where tension fractures on antifotms permit descent of meteoric fluids to depths of at least several km; here, they mix with ascending deep fluid, perhaps heated by magmatic sills rather than by the shallow stocks requited for generation of porphyry-type deposits. The overall field relationships suggest that porphyry-type and low-sulphidation epithermal gold mineralization take place at specific stages in the evolution of a magmatic arc; with continued extension some arcs undergo intra-arc rifting, ‘back-arc’ spreading and eventally renewed subduction. (from Authors)

Tectonic processes and structural geology 955004 Mineral provinces and tectonic regimes: ancient platforms, mobile belts and zones of tectonic-magmatic activation in Russia and South Africa A. L. Sokolov, R. P. Viljoen & A. D. Scheglov, Exploration & Mining Geology, 3(4), 1994, pp 315-328. The distribution of mineralized provinces is analysed on the basis of their genetic link with specific blocks of the earth’s crust including ancient platforms, mobile belts and zones of tectonic-magmatic activation. Mobile and fold belts surround the Siberian platform in Russia and the Kaapvaal craton in South Africa, with decreasing age zoning away from ancient platform margins. The main differences in the geological development of the two cratonic areas are: 1) the long period

955007 Linear stability of a layered fluid with mobile surface plates B. A. Buffett, C. W. Gable & R. J. O’Connell, Journal of Geophysical Research, 99(BlO), 1994, pp 19,885-19,900. The paper develops a general method of calculating the linear stability of a fluid with homogeneous layers that is heated from below. The method employs a propagator technique to obtain expressions for the fluid velocity, stress, and temperature. The principle advantage of the method is the ease with which solutions are adapted to a wide variety of boundary conditions and fluid properties. The utility of the method is demonstrated using three examples which quantify the effects of: 1) rheological layering, 2) mobile plates at the surface, and 3) multiple phase transitions. Each example is presented in the context of Earth’s mantle. (from Authors)