Добрый день, Коллеги. Важное сообщение, просьба принять участие. Музей Ферсмана ищет помощь для реставрационных работ в помещении. Подробности по ссылке
On the basis of textural features and spatial distributions, both supergene and hypogene origins have been proposed for alunite in sediment-hosted micron gold deposits. If alunite is indeed primary, its presence provides important constraints on the chemistry of the ore fluid. Thus, determination of the genesis of alunite is critical to the understanding of the geologic history of these ore deposits.
At the Post deposit in Nevada K/Ar ages of alunite are much younger than the age of mineralization. Sulfur isotope compositions of alunite from several sediment-hosted micron gold deposits are virtually identical to those of sulfides, suggesting that alunite sulfur came from quantitative (closed-system) oxidation of primary sulfide minerals during weathering. Sulfur and oxygen isotope compositions of barite are markedly different from those of alunite. The oxygen and hydrogen isotope compositions of alunite fall in the supergene alunite fields; data for cogenetic kaolinite fall on the kaolinite line, whereas those for hypogene kaolinite do not. From all available petrographic and geochemical data, it would appear that alunite in the micron gold deposits of Nevada is entirely of secondary supergene origin.
Fe-oxide-Cu-Au deposits typically formed in continental arc and intracratonic tectonic settings, predominantly in the Proterozoic, but also during the Phanerozoic. The lack of reported evaporitic sequences in several major Fe-oxide-Cu-Au districts including the Gawler Craton and Stuart Shelf, Tennant Creek, Great Bear Magmatic Zone, and Carajas districts suggests that the presence of evaporites is not a prerequisite for the formation of these deposits.
Fluid inclusion studies of Fe-oxide-Cu-Au deposits typically indicate the presence of coexisting hypersaline and C02-rich fluid inclusions that may have originated by unmixing of an original H,0-C02-salts fluid. At the Lightning Creek prospect in the Cloncurry district, these fluid types were generated during crystallization of granitic sills that are associated with a major magnetite-rich vein system. Stable isotope data from Lightning Creek and a number of Cu-Au deposits in the Cloncurry district and elsewhere are compatible with formation of these deposits principally from magmatic-hydrothermal fluids. Syn- to post-mineralization Na-Ca-rich fluids are present in many deposits, and may represent meteoric and/or connate fluids that mixed with the hypersaline magmatic fluids during or after mineralization.
Hace exactamente 30 anos, en octubre de 1969, se celebro en Salamanca el I Simposio Intemacional de Zooftlogenia. Las Actas correspondientes serian publicadas en 1971 por Rafael Alvarado, Enrique Gadea у Andres De Haro. La introduction se abria con una frase que bien puede considerarse plenamente vigente, aunque los tiempos, metodos у tecnicas han ido. cambiando en gran medida. Decla:
Africa holds extraordinary prospects for the paleontological investigation of mammalian evolution. Largely still inhabited by relic Plio-Pleistocene mammalian faunas, it is populated by animals such as giraffes, rhinos, lions, gazelles, zebras, okapis, and giant forest hogs, as well as descendants of endemics of longer residence such as hyraxes and catarrhine primates, beneficiaries of having been sheltered from the most extreme effects of climate and ecological change due to the position of the landmass astride the equator, the only continent to span both northern and southern temperate zones (Sanders and Werdelin, 2010). Occupying 20% of the world’s landmass, Africa is home to approximately a quarter of all living mammals (Kingdon, 1997; Medellín and Soberón, 1999), encompassing a tremendous range of species and adaptations, fostered by the size and physical and ecological heterogeneity of the continent (Kingdon, 1989; O’Brien and Peters, 1999). <...>
The Precambrian crust of Saudi Arabia, the eastern Sudan and Egypt consists of abundant granites and granodiorites, arc-type volcanics, and well-authenticated ophiolites. The metamorphic grade is generally low. Archaean remnants are absent. In contrast, the Precambrian in the Mozambique belt in Kenya and Tanzania consists of high-grade metamorphic rocks, contains much reworked Archaean basement, and few granitic or granodioritic plutons. It is proposed that there is a transition from crust formed by successive accretion of island arc/back arc complexes in the north to products of collisions between plates carrying Archaean continental crust in the south. The collision led to Himalayan-type crustal thickening, deep erosion and isostatic uplift, exposing deep-crustal rocks. <...>
Abstract—Following a long period of island arc-like orogenic activity, post-orogenic granites of Pan-African age (670-550 Ma) were emplaced throughout the Arabian Shield. Most of these rocks are typical subsolvus calcalkaline biotite monzogranite. The early Pan-African granites are associated with widespread small tholeiitic gabbro intrusions, many of which are layered. The calc-alkaline granites are not evenly distributed and their area of exposure relative to other rock types increases from S.W. to north and N.E. In the north-eastern part of the shield, they are associated spatially with rhyolites of similar composition and with molasse derived from granitic terrane.
Evolution is one of the fundamental underlying concepts of modern science. This powerful theory explains such phenomena as the history of life preserved in the fossil record; the genetic, molecular, and physical similarities and differences among organisms; and the geographic distribution of organisms today and in the past. Indeed, evolution forms the foundation of modern biology and paleontology and is well documented by evidence from a variety of scientific disciplines. <...>
The geological, geochemical and geodynamic processes involved in the formation of oceanic and continental crust during the Phanerozoic are well understood within the frameworks of plate and plume tectonics. However, processes and the mode (steady-state versus episodic) of oceanic and juvenile continental crust generation in the
This volume was stimulated by a Pardee Symposium titled “Evolution of the Early Atmosphere, Hydrosphere, and Biosphere: Constraints from Ore Deposits,” which we convened in 2002 at the national meeting of the Geological Society of America. The history of Earth’s early atmosphere, hydrosphere, and biosphere, from Hadean through Archean and into Proterozoic time, is one of the enduring puzzles in the geological sciences. When did the oceans appear, and did they remain liquid throughout Earth’s history? What was the composition of the early atmosphere, and how did it affect climate? How did the atmosphere and ocean compositions change through time, and why? When, where, and how did life emerge on Earth? When did cyanobacteria, sulfate reducers, methanogens, and eukarya appear, and how did they affect their geologic environments? How did changes in the atmosphere, hydrosphere, and biosphere affect the lithosphere, and vice versa?
Science progresses by continuous testing of theories. Today, the Earth sciences are intertwined with both economic and social policies in a complex world, more so than most of us could ever have imagined. Challenging the first principles of science and developing alternative explanations for observations are inherent in geological science, but seem to have been ignored when addressing concepts that have social and financial repercussions, such as climate change, environmental degradation, and catastrophic Earth events. Floods, volcanic eruptions, earthquakes, and landslides—all are demanding political and policy attention. Sorohktin, Chilingar, and Sorohktin are meeting that challenge with a book that documents the basis of the Earth sciences and prepares the reader to make better informed decisions about Earth-processrelated problems <...>
