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Выпуск 342
Автор(ы):Godard G., Kunze K., Mauler A.
Издание:Journal Tectonophysics, 2001 г., 32 стр.
Crystallographic fabrics of omphacite, rutile and quartz in Vendee eclogites (Armorican Massif, France). Consequences for deformation mechanisms and regimes

This study aims at further understanding of the mechanisms how lattice-preferred orientations (LPO) develop during deformation in the main eclogite minerals. Microstructures and textures of deformed eclogites from the Les Essarts complex (Western France) were investigated using optical microscopy and electron backscatter diffraction (EBSD) in the scanning electron microscope. Microfabric analyses of eclogite-facies minerals are used to identify their deformation mechanisms, which define the rheology at high-pressure metamorphic conditions. Mechanisms of intracrystalline deformation by dislocation movement (dislocation creep) result usually in a non-linear flow law (typically power law), while diffusive processes (diffusion creep) correspond to linear flow laws. General microstructural observations may suggest intracrystalline deformation (dislocation creep) of omphacite. The omphacite LPO vary between S- and L-type and correlate with oblate or prolate grain shape fabrics, respectively. Until now, these LPO types have not been understood by plasticity models based on dislocation glide on the known slip systems in clinopyroxene. An alternative interpretation is given in terms of anisotropic growth and dissolution, with grain boundary diffusion as the rate controlling process. There are further indications suggesting diffusion creep with concomitant anisotropic growth and dissolution as a main deformation mechanism in omphacite. In omphacite around a hollow garnet, crystallographic and shape fabrics align with the c[001] axes parallel to the grain elongations defining the mineral lineation, which rotates locally with the inferred flow direction. In this part, the grain sizes of omphacite and rutile are larger than in the surrounding matrix. The geometry of both the shape and crystallographic fabrics is interpreted to represent the local stress regime (directions and ratios of the principal stresses). The LPO of rutile duplicate the LPO of omphacite and a similar distinction between S- and L-type was used. Rutile deformation mechanisms probably involve dislocation creep as well as diffusion creep. Quartz mainly occurs as an interstitial phase with weak LPO patterns interpreted as random. No representative obliquity of the LPO in omphacite nor rutile with respect to foliation and lineation was observed to be used as potential shear sense criteria. However, the rutile LPO was slightly rotated relative to the omphacite LPO consistently in most samples. The results suggest that diffusion processes are strongly involved in the deformation of eclogites. A linear flow law should be taken into account in tectonic models where eclogites are incorporated. 

Выпуск 303
Издание:Journal Tectonophysics, 1999 г., 27 стр.
Crystallographic preferred orientations and misorientations in some olivine rocks deformed by diffusion or dislocation creep

The development of crystallographic preferred orientations (CPO) and grain misorientation distributions (MOD) in fine-grained (0.5–30 µm) olivine rocks, experimentally deformed by diffusion creep and dislocation creep has been investigated. The use of electron back-scattered diffraction (EBSD), in a scanning electron microscope (SEM), has enabled the measurement of CPO in rocks which are too fine-grained to be measured by conventional U-stage methods. Our objective is to study the influence of deformation and recrystallisation mechanisms on the CPO and MOD. The olivine rocks studied were deformed in uni-axial compression, in a gas-medium apparatus, to 17–24% strain at temperatures of 1200–1300ºC and 300 MPa confining pressures. The samples show a trend of weaker CPO with lower flow stress which may be related to an increasing component of grain boundary sliding and diffusion creep. In the diffusion creep regime the CPO and MOD are weak to random, whereas in the dislocation creep regime the CPO and MOD are non-random but the MOD is principally controlled by the CPO. These results confirm the idea, based on studies from metals, that the CPO and MOD in olivine are characteristic of the deformation mechanism. Dynamic recrystallisation during dislocation creep results in the occurrence of more intermediate-angle (10–40º) grain boundaries than expected from the CPO. In local areas of complete recrystallisation the MOD is controlled by the CPO which implies that the statistical MOD retains no signature of the initial recrystallisation process. In the dislocation creep regime small grains have a weaker CPO compared to large grains. This result is consistent with predictions from deformation mechanism maps which indicate that the fine recrystallised grains deform by a combination of dislocation creep and grain boundary sliding. The grain boundaries found in the deformed olivine polycrystals are predominately high-angle boundaries with misorientations between 60 and 117º. No obvious evidence has been found for the occurrence of preferred misorientation, or special, grain boundaries.

Издание:Canadian Securities Administrators, 2003 г., 17 стр.
CSA staff notice 43-302 - FAQs - NI 43-101 Standart of disclosure for mineral projects

NI 43-101 is a rule that governs how issuers disclose scientific and technical information about their mineral projects to the public. To assist mining industry participants and their advisors in understanding and applying NI 43-101, we compiled a summary of questions and CSA staff responses (Frequently Asked Questions or FAQs) that was first published on October 19, 2001 and was revised on February 8, 2002. Attached to this notice is an update to the FAQs.

We update the summary of FAQs from time to time to assist in the interpretation and application of NI 43-101 as new issues arise. We continuously update this summary of FAQs rather than create supplemental summaries in order to retain one repository of interpretative guidance for the rule. We intend to publish another comprehensive update later this year.

Автор(ы):Jeremy P.Richards
Издание:PGS Publishing, Linden Park, 2005 г., 19 стр.
Cumulative Factors in the Generation of Giant Calc-Alkaline Porphyry Cu Deposits

The formation of porphyry Cu deposits in calc-alkaline magmatic arcs is considered to be the cumulative product of a wide range of processes beginning with dehydration of the subducting oceanic slab. No single process is key to the formation of large deposits, but the absence or inefficient operation of any contributory process, or the action of a deleterious process, can stunt or prevent deposit formation.

Выпуск 4
Издание:Ocean Pictures, Москва, 2001 г., 128 стр., ISBN: 5-900395-28-6
Dalnegorsk. Mineralogical almanac / Дальнегорск. Минералогический альманах

This volume of Mineralogical Almanac is the first issue describing one of the World's Greatest Mineral Locality - Dalnegorsk. The issue provides a collector with information on the most important facts of geological structure, mineralogy and genesis of the Polymetallic and Boron deposits. It contains data on all deposits of the Dalnegorsk ore district. A special attention the authors paid to description of minerals of Dalnegorsk deposits that are of particular interest for collectors.

Автор(ы):Kraus M.
Издание:Wien, 1916 г., 226 стр.
Das staatliche Uranpecherzbergbaurevier bei St.Joachimsthal in Bohmen
Издание:Economic geology, 1972 г., 4 стр.
Data on major and minor elements in host rocks and ores, Carlin gold deposit, Nevada

The purpose of this communication is to summarize and make available a large amount of data on the content of major and minor elements in the host rocks and ores of the Carlin gold deposit and to show the changes in the abundance of these elements as a result of hydrothermal mineralization and subsequent oxidation. Other aspects of the study of minor elements in the Carlin deposit, including the correlation between elements in various types of ore and the influence of geologic features on spatial distribution, will be presented in a later paper. The Carlin gold deposit is located about 33 miles northwest of Elko, Nevada (Fig. 1).

The deposit is characterized by large disseminated replacement-type ore bodies in the upper beds of the Silurian Roberts Mountains Formation. Several of these ore bodies are currently exposed in the West, Main, and East Pit areas of the mine. Although detailed information on the depth of gold deposition and the geometry of individual ore bodies cannot be disclosed (by agreement with Newmont Mining Corporation), the host rocks have been hydro-thermally altered in some parts of the deposit to a depth of 800+ feet. Small amounts of gold are scattered throughout this depth, and larger amounts, concentrated in several zones, make up the ore bodies.

The host rocks for the ore bodies are dark- to medium-gray, thin-bedded, siliceous, argillaceous, dolomitic limestones. Mineralogically the rocks are made up of large and widely varying amounts of calcite, dolomite, illite, and quartz, plus minor kaolin, montmorillonite( ?), chlorite, K-feldspar, plagioclase, pyrite, zircon, barite, rutile, sphene, and carbonaceous materials.    Complete chemical analyses of the fresh carbonate rocks are given by Hausen (1967), Hausen and Kerr (1968), and Radtke and Scheiner (1970).

Издание:The University of Chicago, 2009 г., 21 стр.
Dating magmatism in Central Mongolia / Магматизм в Центральной Монголии

Central Mongolia represents a heterogeneous crustal domain of the Central Asian Orogenic Belt and is composed of contrasting lithotectonic units with distinct preorogenic histories. We report single-zircon evaporation and SHRIMP ages for high-grade rocks of the Neoarchean-Paleoproterozoic Baydrag block and for metaigneous rocks of the junction between the late Neoproterozoic Bayankhongor ophiolite zone (BOZ) and the Baydrag block. Zircon ages for metamorphic rocks of the Baydrag block indicate a major tectonothermal event between 1840 and 1826 Ma, coeval with the emplacement of granitic rocks at middle-crustal level dated at 1839 Ma. A granite-gneiss yielded a much younger crystallization age of 1051 Ma, the first Grenvillian age reported for this region. Together with predominantly Mesoproterozoic detrital zircon ages for a quartzite lens from the Burd Gol accretionary complex, these data attest to the heterogeneity and long Precambrian history of the Baydrag block. Crystallization ages for granite-gneisses from the northeastern margin of the Baydrag block indicate prolonged plutonic activity between 579 and 537 Ma, probably related to southward subduction of the Bayankhongor oceanic crust.Asyntectonic granite vein yielded a crystallization age of 519 Ma, probably linked to accretion of the BOZ onto the northeastern active margin of the Baydrag block. Lastly, a felsic metavolcanic rock from the southeastern termination of the BOZ yielded a crystallization age of 472 Ma and suggests that punctuated volcanic centers developed during the early Ordovician in response to protracted convergence.

Том 20, Выпуск 11
Автор(ы):Hippertt J.F., Hong F.D.
Издание:Elsevier, 1998 г., 14 стр.
Deformation mechanisms in the mylonite/ultramylonite transition

The deformation mechanisms and controls that operate in the mylonite/ultramylonite transition are interpreted from microstructural observation. The investigated mylonites and ultramylonites were derived from a granitic protolith which was deformed under greenschist facies conditions, and in the presence of fluid, in a regional-scale shear zone from northwest Argentina. Several deformation mechanisms were recognized to operate simultaneously in different domains of the microstructure at each particular stage of the microstructural evolution. This continuously mobile deformation partitioning, present throughout the microstructural evolution, ceases abruptly in the ultramylonite stage, where a stable-state microstructure is achieved. Domainal quartz c-axis fabrics indicate that quartz deforms by crystal-plastic processes at the initial and intermediate stages of deformation, but solution-transfer processes become predominant in the ultramylonite stage. Plagio-clase is progressively transformed into muscovite through retrograde softening reactions. K-feldspar is progressively transformed into fine-grade aggregates via cataclastic flow and incipient recrystallization. Mica deforms by kinking and basal slip, with progressive development of fine-grained, morphologically oriented aggregates. Plagioclase disappearance as well as the development of intrafolial microfolds characterize the transition between the mylonitic and ultramylonitic domains. Disruption of these microfolds is interpreted to represent the ultimate control on the localization of the ultramylonite bands, с 1998 Elsevier Science Ltd. All rights reserved

Выпуск 93
Издание:Economic geology, 1998 г., 15 стр.
Deposition of gold in carlin-type deposits: the role of sulfidation and decarbonation at Twin Creeks, Nevada

We report here an investigation of the distribution of Au, As, Sb, Hg, carbonates, K-Al silicates, and pyrite in the Twin Creeks Carlin-type gold deposit. The main objective of the study was to determine the nature and degree of correlation among these variables and use them to identify the process(es) that deposited gold. The study focused on deposit-scale variations in these parameters and was based, in part, on data from two large geochemical databases that were prepared by mine staff.

Country rocks at Twin Creeks include Ordovician-age interlayered calcareous shales and mafic igneous rocks, the overlying Leviathan allochthon, and the Pennsylvanian-Permian Etchart Formation that was deposited unconformably over these rocks. Most gold values are found in calcareous shales in the Ordovician sequence and in limestones in the Etchart Formation, although not all layers contain the same amount of gold. Strongest gold mineralization is not adjacent to faults but its general form and distribution suggest that gold-bearing solutions gained access to favorable layers along the faults. In the Ordovician sequence, gold values are highest in shales that have undergone maximum dissolution of carbonate minerals. Petrographic study shows that some gold is associated with adularia, but deposit-scale comparisons do not show a consistent relation between K/Al ratios and gold values. The distribution of antimony is similar to that of gold, whereas mercury is more concentrated than gold, and arsenic is more widely dispersed than gold.

The relation between gold, iron, and sulfide sulfur values shows that mineralization is concentrated in rocks that have gained sulfur, but not iron, to form gold-bearing arsenian pyrite. Thus, these rocks have undergone sulfidation rather than pyritization. The iron that underwent sulfidation came largely from preore, diagenetic(P) ferroan dolomite and was released into solution by decarbonation, a common form of alteration associated with Carlin-type deposits. The results of this study suggest that wall-rock iron content and decarbonation processes which liberate this iron are the most important factors controlling formation of Carlin-type gold deposits. New deposits should be sought where stratigraphic units containing abundant ferroan dolomite are cut by favorable structures.