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Выпуск 93
Издание:Economic geology, 1998 г., 15 стр.
Язык(и)Английский
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.
Язык(и)Немецкий
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.
Язык(и)Английский
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.
Язык(и)Английский
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.
Язык(и)Английский
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.

Источник:www.debeersgroup.com
Издание 3
Автор(ы):Manutchehr-Danai M.
Издание:Springer, Verlag - Berlin - Heidelberg - New York, 2009 г., 1028 стр., ISBN: 978-3-540-72795-8
Язык(и)Английский
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.
Язык(и)Английский
Dictionary of Geology

Introduction

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.

Издание 2
Редактор(ы):Mark D. Licker
Издание:McGraw-Hill Book Company, New York, 2003 г., 420 стр., ISBN: 0-07-141044-9
Язык(и)Английский
Dictionary of geology & mineralogy

The McGraw-Hill Dictionary of Geology and Mineralogy provides a compendium of more than 9000 terms that are central to a broad range of geological sciences and related fields. The coverage in this Second Edition is focused on the areas of geochemistry, geology, geophysics, mineralogy, paleobotany, paleontology, and petrology, with new terms added and others revised as necessary.

Geology deals with the solid earth and the processes that formed and modified it as it evolved Related disciplines include the study of the physics of the earth (geophysics); earth chemistry, composition, and chemical changes (geochemistry); the composition, properties, and structure of minerals (mineralogy); the description, classification, origin, and evolution of rocks (petrology); and the study of ancient life (paleontology).

Автор(ы):Erben H.K.
Издание:Stuttgart, 1964 г., 118 стр.
Язык(и)Английский, Немецкий
Die evolution der altesten Ammonoidea

The theories of the origin of coiled ammonoids are discussed by examining former Statements in favour or against a derivation from coiled nautiloids (Spath) or from bactritids (Schindewolf), respectivcly. Results of additional studies ndicate an ancestorship of the Bactritidae. Evidence is represented by homologies of morphological elements of the shell, by the similarity of the early ontogentical development, by a progressive evolutionary succession of specics and gencra connecting primitive goniatites with the Bactritidae, and by iterative as well as regressive tendencies indicating relationship.

Автор(ы):Шахов Ф.Н.
Издание:Akademie Verlag, Berlin, 1971 г., 11 стр.
Язык(и)Английский, Немецкий
Die geschichte der vorstellungen uber die genese der kontaktlagerstatten

The term of "contact-metamorphic deposit" appears in the literature at the beginning of the 19th century. In the course of nearly two centuries its content and views held on the formation of such deposits have changed considerably, as have the complexes of deposits included by various authors in this group. In the author's opinion the development of views on the genesis of these deposits is closely connected with a change experienced by ideas on the process of metamorphism, in particular of contaetmetamorphism, as well as by ideas on the mechanism of action of hot Solutions.

The deposits are not only observed by geologists working in the field of economic geology, but also by scientists concerned with the metamorphism and formation of magmas. This may be the reason for a great number of contradictory views held at the present state of knowledge on this group of deposits. In discussions of their genesis the following contradictory points are answered, with a neglection of the fact that the process of metasomatism itself is not likewise considered:

1) role of the contact in the genesis and systcmatization of contact deposits; 2) the chronological relation between skarn formation and contaetmetamorphism; 3) the sources of volatiles, the source of metals, and the position of sulphidic ore formation within the process of skarn formation; 4) role of passive rocks in the formation of skarns. These problems are discussed by the author in greatest detail.


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