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Автор(ы):David A.Johnson, Mark D.Barton
Издание:PGS Publishing, Linden Park, 2002 г., 18 стр.
Alternative brine source for Fe-oxide (-Cu-Au) systems: implications for hydrothermal alteration and metals

No single model satisfactorily accounts for the diverse characteristic of Fe-oxide-rich hydrothennal systems. Consideration of a spectrum of geologically reasonable models gives insight into the origins of variability among these deposits. Key features that need to be rationalized by any model are the abundance of hydrothermal magnetite and/or hematite, the chemically distinct suite of elements (REE-Cu-Co-Au-Ag-U), the variability of associated magmas, the distributions and volumes of associated hydrothennal alteration, and the broader geologic setting(s).

Geologic and geochemical evidence show that the ore-fonning fluids are brines, but the source of the brines is controversial. Multiple sources are possible, indeed likely. The identity and consequences of alternative sources - magmatic and non-magmatic are considered: First, by a review of plausible fluids and general consequences, second, by examination of the system characteristics, third, by specific consideration of the consequences of alternative models, and fourth, by consideration of selected systems where non-magmatic brines must play a major role.

We review some of the key characteristics of different types of hydrothermal Fe-oxide-rich(-Cu-Au) systems. Some are economic; many are only geochemically anomalous. Two end-members and several variants on these end members are proposed. One group is typified by relatively high-temperature mineralization, and relatively high K/Na and Si/Fe in the alteration. We suggest that these features (and others) are distinctive of magmatic fluid sources and that this group overlaps with porphyry Cu-Au and related deposit types. A second, broad group is typified by more oxide-rich, sulfide-poor mineralization, low Si/Fe ratios, and voluminous alkali-rich alteration where sodic types commonly exceed K-rich varieties. We suggest that the key features of this group reflect involvement of non-magmatic brines and that ore grades are less common as the metals are less easily trapped. Hybrid examples, where fluids of both types are involved, are expected (and known).

Conceptual and quantitative models of magmatic and non-magmatic fluid sources yield insight into the expected differences, the source and distribution of metals within these systems, and possible controls on ore deposition. These models highlight the difference between magma-sourced fluids and non-magmatic fluids. The former tend to be focused at the tops of magma chambers and have a built-in depositional mechanism -cooling. The latter require a different type of focusing mechanism - structural or stratigraphic, and different traps - mixing, specialized host rocks, and/or boiling. These models predict consistent differences in mineral assemblages, metal contents, alteration volumes, zoning, paragenesis, and geochemistry. For magmatic fluid sources, the models reproduce the key characteristics of that group, notably the porphyry-related systems. For non-magmatic brine sources, predicted characteristics match well with observations of Fe-oxide-rich systems in environments where these fluids are known to dominate, including mafic igneous systems and modem analogs such as the Salton Sea. These cases show that Fe-oxide (-Cu-Au-REE-Co) enrichments can result from non-magmatic sources. In other environments (e.g., with intermediate to felsic igneous rocks; settings deeper than ~5 km) the relative importance of various fluid sources and their consequences for mineralization remain to be fully explored. Young systems in the American Cordillera and elsewhere can help unravel the many threads that relate these enigmatic mineral deposits.

Автор(ы):Schoch R.R.
Издание:Wiley Blackwell, 2012 г., 293 стр., ISBN: 978-0-470-65658-7
Amphibian evolution: the life of early land vertebrates / Эволюция земноводных: жизнь ранних сухопутных позвоночных

This book focuses on the first vertebrates to conquer the land, and on their long journey to become fully independent from the water. It will trace the origin of tetrapod features and try to explain how and why they transformed into organs that permit life on land. The classic idea of early land vertebrates is that they were similar to modern amphibians. Right or wrong, the vast majority of early tetrapods are therefore classified as amphibians (or more precisely their stem taxa). Accordingly, this book is centered on early amphibian evolution, a topic that effectively includes all early tetrapods, and it will also analyze facts and opinions on the origins of modern amphibians. The major part of the story covers events that occurred over the past 370 million years, but it is far from restricted to paleontology.

My own motivation to study the amphibian fossil record derives in large part from a fascination with the development, ecology, and evolution of their modern representatives. Therefore I consider many topics that can only be covered by examination of extant animals: features of the soft body, functions of organs that mediate breathing, feeding, hearing, and locomotion, the morphogenesis of body parts, larval development, metamorphosis, and ecology.

Выпуск 24
Автор(ы):Childs C., Nicol A., Walsh J.J.
Издание:Pergamon, 2002 г., 7 стр.
An alternative model for the growth of faults

Conventional growth models suggest that faults become larger due to systematic increases in both maximum displacement and length. We propose an alternative growth model where fault lengths are near-constant from an early stage and growth is achieved mainly by increase in cumulative displacement. The model reconciles the scaling properties of faults and earthquakes and predicts a progressive increase in fault displacement to length ratios as a fault system matures. This growth scheme is directly applicable to reactivated fault systems in which fault lengths were inherited from underlying structure and established rapidly; the model may also apply to some non-reactivated fault systems. Near-constant fault lengths during subsequent growth are attributed to retardation of lateral propagation by interaction between fault tips. The model is validated using kinematic constraints from growth strata, which are displaced by a system of reactivated normal faults in the Timor Sea, NW Australia.

Выпуск 92
Автор(ы):Mark D.Barton, Robert P.Ilchik
Издание:Economic geology, 1996 г., 20 стр.
An amagmatic origin of Carlin-type gold deposits

Carlin-type deposits are major sources of gold, yet their origins are enigmatic. Suggested genetic models make connections to magmatism, regional metamorphism, or regional extension. Depositional mechanisms are uncertain as well. We propose on the basis of geologic, physical, and chemical reasoning, a genetic model in which meteoric fluids were circulated by heat released during crustal extension. These fluids interacted at depth with the sedimentary rock pile and scavenged gold. Upon upwelling, these fluids interacted with various lithologies and/or other fluids and produced the characteristic alteration and metal suites of these deposits. To test the viability of this amagmatic model, we have investigated certain physical and chemical constraints implicit to the model.

Выпуск 256
Автор(ы):Bons P.D., Urai J.L.
Издание:Journal Tectonophysics, 1996 г., 20 стр.
An apparatus to experimentally model the dynamics of ductile shear zones

We present a ring-shear apparatus designed to deform soft, ductile rock analogues to arbitrarily high shear strains in simple shear, in experiments attempting to model processes that occur in natural ductile shear zones. The sintered specimen is deformed between two unsupported cylinders by two rotating grips, under a normal and shear stress up to 5 MPa and at temperatures up to 600 K. First results of experiments with octachloropropane (OCP, C3C18) and camphor (C10Hl6O) are presented.

Автор(ы):Fraser A.J., Gawthorpe R.L.
Редактор(ы):Fleet A.J.
Издание:The Geological Society of London, London, 2003 г., 99 стр.
An Atlas of Carboniferous Basin Evolution in Northern England / Атлас эволюции каменноугольных бассейнов в Северной Англии

Why an atlas of the Carboniferous in northern England? There can hardly be a more researched system in the whole of the British Isles, given its widespread distribution at outcrop and annual appearance in numerous PhD theses (including our own). But perhaps all we really know about the Carboniferous is no more than skimming the surface. In this atlas, using modern multifold seismic and borehole data collected by the oil industry in its search for petroleum accumulations, we can start to look beyond the surface exposures and gain some new insights into the structure and stratigraphy of the subsurface (and surface) Carboniferous. The main elements of this atlas are: (i) a series of regional seismic lines crossing all the basinal areas in northern England illustrating the Carboniferous in section, and (ii) a set of palaeofacies maps describing the evolution of the system in map view.

Выпуск 97
Издание:Economic geology, 2002 г., 6 стр.
An evaluation of fluid inclusion microthermometric data for orpiment-realgar-calcite-barite-gold mineralization at the betze and carlin mines, Nevada

Carlin-type deposits contain gold in association with main-stage quartz-pyrite-kaolinite mineralization and late-stage orpiment-realgar-calcite-barite mineralization. Fluid characteristics for main-stage mineralization are well documented by fluid inclusion and stable isotope studies on quartz. In contrast, fluid characteristics for late-stage mineralization are not well constrained because of large ranges in fluid inclusion microthermo-metric data. These ranges could represent real variations in fluids or be a result of the reequilibration of fluid inclusions.

Microthermometric analyses were conducted on fluid inclusions in samples of barite, calcite, realgar, and or-piment from the Betze and Carlin mines, Nevada. Petrographic studies of individual crystals and cleaved sections reveal that fluid inclusions in realgar and barite have negative crystal shapes, in contrast to elongate and rounded inclusions in orpiment and calcite. Point-count data document that one-phase liquid inclusions (type 1) are the dominant type in barite and realgar, relative to two-phase, vapor-poor inclusions (type 2) in calcite and orpiment. Type 2 inclusions in realgar and barite commonly reequilibrate (e.g., stretch) during analysis and exhibit ranges in homogenization temperatures (Th) of 100º to 250ºC and 110º to 300ºC, respectively. In contrast, type 2 inclusions in orpiment and calcite have Th of 108º to 182ºC, which could be repeated to within 1ºC. Based on these results, fluid inclusions in barite and realgar are most susceptible to reequilibration, with Th of ~100º to 110ºC most representative. Fluid salinities for orpiment and calcite are 1.7 to 5.4 wt percent NaCl equiv, relative to 1.1 to 2.9 wt percent NaCl equiv for barite and realgar. The lower Th and salinity for fluid inclusions in barite and realgar suggest fluid cooling and dilution, following the deposition of paragenetically earlier orpiment and calcite.

Автор(ы):Isaaks E.H., Srivastava R.M.
Издание:Oxford university press, Oxford, 1989 г., 577 стр.
An introduction to applied geostatistics

This began as an attempt to write the book that we wish we had read when we were trying to learn geostatistics, a task that turned out to be much more difficult than we originally envisaged. To the many people who provided encouragement, support, and advice throughout the writing of this book, we are very grateful.

We owe a lot to Andre Journel, without whom this book would never have been written. In addition to providing the support necessary for this project, he has been an insightful technical reviewer and an energetic cheerleader.

Автор(ы):Greg Corbett
Издание:2009 г., 13 стр.
Anatomy of porphyry-related Au-Cu-Ag-Mo mineralised systems: Some exploration implications

Analyses of exploration and mining case studies as well as magmatic arc geothermal systems have facilitated an understanding of the implications to explorationists of the anatomy of porphyry related Au-Cu-Mo-Ag mineralising systems. Deeply eroded magmatic source rocks tend to host sub economic mineralisation, which may become focused in the highly prospective in overlying apophyses to spine-like polyphasal intrusions. Buried targets may be identified by analyses of zoned alteration and mineralisation developed by complex overprinting relationships. Mineralised fluids may exit from the magmatic source migrating to higher crustal levels to form epithermal deposits. High sulphidation epithermal Au + Cu + Ag deposits display characteristic alteration and mineralisation zonation which aids target generation and in some instances evolve to host marginal and overprinting lower sulphidation ores which display improved metallurgy and metal grades. Low sulphidation epithermal Au-Ag deposits are categorised as a number of styles, linked on an overall anatomy, which display considerable variation in metal grade, size, form and metallurgy, typically governed by setting and crustal level of formation, as well as controls to vein formation such as: host rock competency, structure, and mechanism of Au deposition. All these controls and zonation pattern vectors provide valuable tools to explorationists in the search for hidden ores.