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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.

Автор(ы):Bischoff G.
Издание:Bochumer, 1998 г., 92 стр.
Der floren - und faunenschnitt an der grenze Barreme/Apt in NW Europa

Der im Grenzbereich Barreme/Apt weltweit zu registrierende Floren- und Faunenschnitt im marinen Milieu wird auf großräumige paläo-ozeanographische und paläogeographische Veränderungen zurückgeführt. Die im Boreal und in der Tethys im tieferen Apt zu beobachtende Homogenisierung der marinen Floren und Faunen ist vermutlich auf einen Meeresspiegelanstieg zurückzuführen, der das Aussterben zahlreicher endemischer (boreaier) Taxa und das Entstehen bzw. die Ausbreitung kosmopolitischer (tethyaler) Taxa begünstigte.

Издание:PGS Publishing, Linden Park, 2005 г., 13 стр.
Dexing Porphyry Copper Deposits in Jiangxi, China

The Dexing porphyry copper field in Jiangxi, China, is defined by three porphyry copper deposits which are, from southeast to northwest, Fujiawu, Tongchang and Zhushahong respectively, and by the Guanmaoshan gold deposit which lies between Fujiawu and Tongchang. Technically, the field lies on the southeastern edge of the Jiangnan Anteclise, and is controlled by the NE-trending, deep-seated, Gandongbei fracture zone. The emplacement of the ore-bearing Fujiawu, Tongchang and Zhushahong granodiorite porphyry intrusions, dated at 184-172 Ma (Zhu et ah, 1983; Zhu et al9 1990), was also controlled by NW-trending structures. Mineralisation and alteration continued from 172 Ma to 100 Ma, and are characterised by symmetric zoning centred on the contacts between granodiorite porphyries and the enclosing country rocks of the Mesoproterozoic Shuangqiaoshan Group jrfryllites.

Издание:22 стр.
Diachronous subduction to diamond- and coesite-facies conditions in the Kokchetav massif

The absolute and relative time of subduction of rocks is crucial information for subduction-exhumation models. We investigated the timing of subduction in one of the oldest ultra-high pressure (UHP) localities worldwide: the Kokchetav massif in Kazakhstan. SHRIMP ion microprobe dating of monazite from coesite-bearing micaschists of the Kulet unit indicates that subduction occurred between ~500-520 Ma. This new data provides evidence that the Kulet unit underwent UHP metamorphism 10-15 Ma later than the diamond-facies rocks in the nearby Kumdy-Kol unit. This time constrain excludes models that argue for a simultaneous evolution of coesite- and diamond-facies rocks, it suggest that subduction continued well after continental crust was involved, and that exhumation was not initiated by a single event such as slab break-off. The dynamic of this UHP massif also indicates that Cambrian tectonic was similar to that of recent orogenic belts.

Издание:Debeersgroup, 31 стр.
Diamond Geology

Diamond deposits can be classified as primary (kimberlites and lamproites) and secondary (alluvial and marine). In 1995, their relative production contributions in South Africa were, primary (kimberlite) 8.63 million carats (89% of total), alluvial 960,000 carats (10%). and marine 90.000 carats (1%). The percentage of gems in kimberlites is highly variable. Published figures suggest approximately 40% for the Kimberley mines and 55% for Premier mine. The proportion of gems in the West Coast marine deposits is over 98%. The MIBA mine in the Congo and the Argyle mine in Australia have an average gem content of only 5%. The dispersal of diamonds from their primary sources into streams and rivers and ultimately to the sea is generally accompanied by an increase in average value per carat.| as flawed stones are progressively destroyed with greater and greater transport.

Издание 3
Автор(ы):Manutchehr-Danai M.
Издание:Springer, Verlag - Berlin - Heidelberg - New York, 2009 г., 1028 стр., ISBN: 978-3-540-72795-8
Dictionary of gems and gemology

This revision (third edition) is being published 4 years after the publication of the second edition. Many entries have been improved and circa 500 new terms have been added. It includes more than 27,000 entries, 1,500 graphic figures, and nearly 50 tables. New entries include notable sapphires (comprising blue sapphires, sapphires of other colors, engraved or carved sapphires), notable rubies (engraved or carved rubies), and corundums.

If you have criticism or suggestions, please feel free to contact the author.

Автор(ы):Kearey P.
Издание:Penguin Books Ltd, London, 2001 г., 320 стр.
Dictionary of Geology


This dictionary contains 7703 entries which cover definitions of terms relevant to Geology. Because of the overlap which exists between the many branches of the Earth Sciences, for example between Geomorphology and Sedimentology or Palaeontology and Biology, some of the entries might be considered to be at the borders of Geology, but are included because of their importance to the subject.

For reasons of economy of space, I have concentrated on producing concise definitions of the entries rather than longer treatments. If further information should be required about any particular entry, it should be sought from the 432 references, arranged by topic, provided in the Bibliography, which lists modern works covering all aspects of Geology.