Выпуск 90
Автор(ы):Robert P.Ilchik
Издание:Economic geology, 1995 г., 3 стр.
Язык(и)Английский
40Ar/39Ar, K/Ar, and fission track geochronology of sediment-hosted dessiminated gold deposits at Post-Betza, Carlin Trend, Northeastern Nevada - a discussion

Arehart et al. (1993) set out to date hydrothermal Au mineralization at the Post-Betze deposit in an attempt to place the formation of this deposit into a metallogenetic setting. Toward this end they have used many modern dating techniques including 40Ar/39Ar, K/Ar, and fission track geochronology. Based on the 39 measurements presented, they conclude that "based upon what we consider to be the most reliable" data this deposit formed at 117 Ma. This date is said to be consistent with similar ages for other intrusions in northeastern Nevada, and although one is not seen at the deposit, it is implied that a major nearby pluton is responsible for the formation of Post-Betze. By extrapolation, it is then suggested that Carlin-type Au systems are all of similar age, and thus, are the result of compressional tectonics operating prior to development of the current extensional environment. Because of the inescapable importance of the inferences and conclusions generated by the assignment of a 117 Ma age to the formation of the Post-Betze deposit, I would like to discuss the data and conclusions presented by Arehart et al. (1993).

ТематикаПолезные ископаемые
Выпуск 90
Автор(ы):Arehart G.B., Foland K.A., Kesler S.E., Naeser C.W.
Издание:Economic geology, 1995 г., 3 стр.
Язык(и)Английский
40Ar/39Ar, K/Ar, and fission track geochronology of sediment-hosted disseminated gold depositsat at Post-Betze, Carlin Trend, Northeastern Nevada - a reply

We appreciate the comments on our paper on the Post-Betze sediment-hosted disseminated gold deposit which indicate that clarification is required. Ilchik suggests alternative interpretations of our findings with regard to two main issues: the nature of the sill that we have described as "postore," and the absolute age of Post-Betze mineralization in the context of the overall spread in the dates presented in our paper. We address these issues in turn.

The postore sill, which we also loosely originally termed "dike," is important in that it brackets the timing of gold mineralization. Although Ilchik suggests that this intrusion predates mineralization, the weight of evidence is to the contrary. Whereas the postore sill is composition-ally similar to preore Goldstrike stock-equivalent rocks, it is texturally quite distinct. In fact, it was distinctive enough for us to map it out in the subsurface from drill core, something which we were unable to do for any of the other sills. Comparatively, the rock is little altered and it is the only one in the ore zone that retains primary bio-tite. Assays shown in figure 5 of Arehart et al. (1993) are the original assays done on 5-ft intervals before the core was logged; in some samples mineralized inclusions are present within the sample interval. More detailed analysis of this rock type, when clear of any inclusions of mineralized sedimentary rock, yields very low values for gold. This is true of samples from several core holes. In sharp contrast, the preore dikes and sills are all significantly altered and mineralized (i.e., well above background in areas of sedimentary rock mineralization), although they are generally of lower grade than the surrounding sedimentary rocks. We are well aware of the erratic nature of and lithologic control on gold mineralization in sediment-hosted disseminated gold deposits that are also seen at Post-Betze. Even taking this into consideration, it is clear that there is effectively no gold in the postore sill in comparison to significant gold in preore dikes and sills.

ТематикаПолезные ископаемые
Выпуск 88
Автор(ы):Brian J.Maher, McKee E.H., Quentin J.Browne
Издание:Economic geology, 1993 г., 10 стр.
Язык(и)Английский
Constraints on the age of gold mineralization and metallogenesis in the Battle Mountain-Eureka mineral belt, Nevada

The Roberts Mountains of north-central Nevada are comprised of Paleozoic sedimentary rocks that host several gold deposits and subeconomic gold resources (Fig. 1). These gold occurrences are within a regional alignment of precious and base metal deposits in north-central Nevada termed the Battle Mountain-Eureka mineral belt (Roberts, 1966). Field relations and radiometric ages in three areas of the Roberts Mountains (Maher et al., 1990) allow assignment of minimum and probable maximum ages for gold mineralization. New radiometric age data from the Roberts Mountains and other precious and base metal deposits within the Battle Mountain-Eureka mineral belt are combined in this report with previously published geologic data to construct a metallo-genic framework for gold and other metallic deposits in north-central Nevada.

ТематикаПолезные ископаемые
МеткиBattle Mountain-Eureka mineral belt, Gold, Gold mineralization, Золото
Выпуск 88
Автор(ы):Arehart G.B., Foland K.A., Kesler S.E., Naeser C.W.
Издание:Economic geology, 1993 г., 25 стр.
Язык(и)Английский
40Ar/39Ar, K/Ar, and Fission Track Geochronology of Sediment-Hosted Disseminated Gold Deposits at Post-Betze, Carlin Trend, Northeastern Nevada

The Post-Betze deposit of Nevada is the largest sediment-hosted disseminated gold deposit presently known, both dimensionally and in terms of contained metal. Ore occurs primarily as submicron-sized gold that is disseminated in altered sedimentary rocks of the Lower Paleozoic Roberts Mountains Formation. However, significant portions of the ore are present in altered monzonite of the Goldstrike stock. Alteration and mineralization were controlled by both structure and stratigraphy. Alteration began with early decarbonatization and was followed by silicification and, finally, argillization. Phyllosilicate mineral zoning grades from proximal kaolinite to kaolinite + sericite to unaltered rock.

ТематикаПолезные ископаемые
Выпуск 69
Автор(ы):Byron R.Berger, Miles L.Silberman, Randolph A.Koski
Издание:Economic geology, 1974 г., 11 стр.
Язык(и)Английский
K-Ar Age Relations of Granodiorite Emplacement and Tungsten and Gold Mineralization near the Getchell Mine, Humboldt County, Nevada

A granodiorite stock intrudes complexly folded and thrust-faulted Paleozoic sedimentary rocks in the Osgood Mountains of eastern Humboldt County, Nevada. Within the metamorphic aureole surrounding the pluton, the sedimentary rocks are converted to cordierite hornfels and marble; tungsten-bearing tactites developed along the contacts of the granodiorite. Cutting the granodiorite and sedimentary rocks is the Getchell fault, along which the disseminated gold ore bodies of the Getchell mine are localized. K-Ar ages of the granodiorite, andesite porphyry, tungsten-bearing tactites, and altered granodiorite from the Getchell mine indicate that emplacement, alteration, and mineralization are all part of a magmatic-thermal episode which took place approximately 90 m.y. ago.

ТематикаПолезные ископаемые
МеткиGetchell Mine, Gold, Gold mineralization, K-Ar Age Relations, Золото

Gold (-Silver) Ores in Epithermal Deposits: Preface

Владелец инбокса: kaptar.j

This Society of Economic Geologists Compilation (no. 4) contains more than 100 years of classic papers on gold (-silver) ores in epithermal deposits from 1905 to 2010. These include the manuscripts published in Economic Geology, Reviews in Economic Geology, Special Publications, Economic Geology Monographs, SEG Newsletters, SEG Field Trip Guides, and Economic Geology Anniversary Volumes. They feature many of the major gold (-silver) deposits, which formed in Mesozoic and younger magmatic arcs of the circum-Pacific, southeast Asia, and the Caribbean. They also include the rare older deposits occurring in Australia, Sweden, and Canada. Papers describing deposits containing predominantly silver ores are presented because they contain significant gold, they have similar geological characteristics, and/or they provide broader insights to epithermal ore-forming processes.

This compilation is divided into 10 sections, beginning with Review Papers and Ore Deposit Models, which comprise the classic and modern review papers on epithermal mineralization. They document key geological features and important genetic processes, regarding mineralogical and textural characteristics of ore mineralization, hydrothermal alteration zonation, classification schemes, local and regional controls on mineralization, and relationship to volcanic settings (Sillitoe and Bonham, 1984; Hayba et al., 1985; Heald et al., 1987; Sillitoe, 1993; White and Hedenquist, 1995; Cooke and Simmons, 2000; Hedenquist et al., 2000; Jensen and Barton, 2000; Sillitoe and Hedenquist, 2003; Simmons et al., 2005).

Прикрепленные файлы
Категория: Полезные ископаемые Метки: Золото,Полезные ископаемые,

Gold Ores in Metamorphic Terranes

Владелец инбокса: kaptar.j

This Society of Economic Geologists Compilation contains more than one hundred years of classic papers on gold deposits in metamorphic terranes, most of which are now commonly referred to as orogenic gold deposits. These include numerous contributions published in more than a century of issues of Economic Geology, as well as others that first appeared in Society of Economic Geologists Guidebooks, Reviews Volumes, Special Publications, Economic Geology Monographs, SEG Newsletters, Meeting Extended Abstract Volumes, and Anniversary Volumes. Among the papers, there are detailed studies on deposits in all of the world’s important gold provinces located in Precambrian cratons and in Phanerozoic accretionary terranes. In addition, there are many timeless papers here that provide comprehensive descriptions of many key aspects of this group of lode gold ores. Papers from mainly the past decade on intrusion-related gold systems, many in the same general regions as the orogenic gold deposits, are also compiled here. The main contributions to the literature on the difficult to classify detachment-fault related gold lodes of the southwestern USA and adjacent Mexico are included within this compilation. The great Witwatersrand gold deposits in South Africa have been long-debated by many workers as to their detrital or hydrothermal origin, and most of the well-known papers documenting the controversy were published by the Society and are contained all together in this volume.

Прикрепленные файлы
Категория: Полезные ископаемые Метки: Gold,Золото,Метаморфические горные породы,Полезные ископаемые,

Carlin-type gold deposits: a compliation

Владелец инбокса: kaptar.j

Preface to SEG Compilation on Carlin-type Gold Deposits

John Muntean

Nevada Bureau of Mines and Geology

University of Nevada Reno

 

Since the discovery of the original Carlin gold deposit in 1961, approximately 200 million ounces of gold in Carlin-type deposits have been mined or identified in northern Nevada, making the United States one of the world’s largest gold producers for the past several decades. Despite their importance, Carlin-type deposits remain enigmatic.   Controversy over their origin mainly stems from the very fine grained character of the ore and gold. Carlin-type gold deposits are largely hydrothermal replacement bodies with visually subtle alteration dominated by decarbonatization and silicification of carbonate-bearing host rocks. Gold occurs in solid solution or as submicron particles in very fine grained disseminated arsenian pyrite or marcasite. To date, over 90% of the gold mined from what most geologists would call Carlin-type deposits has come from northern Nevada, and debate exists whether similar-looking deposits elsewhere share a comparable origin or whether the deposits in Nevada represent a unique occurrence.

Прикрепленные файлы
Категория: Полезные ископаемые Метки: Carlin type gold,Gold,Золото,Полезные ископаемые,
Автор(ы):Gibbins S.L., Kurt C.Friehauf, Spencer R.Titley
Издание:PGS Publishing, Linden Park, 2005 г., 10 стр.
Язык(и)Английский
Porphyry-style mineralisation in the Ertsberg diorite, Gunung Bijih (Ertsberg/Grasberg) district, West Papua, Indonesia

Newly-recognised porphyry-style mineralisation within the Ertsberg intrusion displays significant differences from porphyry mineralisation at the Grasberg porphyry Cu-Au deposit. Stockwork mineralisation in the Ertsberg occurs near the giant East Ertsberg Skarn System, close to the northern margin of the intrusion. Stockwork mineralisation in the diorite is spatially associated with 5-15 m wide, E-striking, dykes of porphyritic hornblende monzonite that cut equigranular Ertsberg diorite. The porphyry dykes strike parallel to major district structures and occur where those structures project into the Ertsberg intrusion. Hornblende abundance greater than biotite, the much greater content of sphene, a paucity of broken phenocrysts, and the aplitic groundmass distinguish the porphyry dykes in the Ertsberg Stockwork Zone from the finer-grained groundmass Kali dykes of the Grasberg deposit.

ТематикаПолезные ископаемые
МеткиErtsberg diorite, Ertsberg district, Gold, Grasberg district, Gunung Bijih district, Porphyry-style mineralisation, West Papua, Золото
Автор(ы):Laznicka P.
Издание:PGS Publishing, Linden Park, 2002 г., 18 стр.
Язык(и)Английский
Wernecke Mountains (Yukon) Breccias and Scattered Ore Occurrences: What Contribution to FeOx-Cu-Au-U Metallogeny? / Горы Вернеке (Юкон) Брекчии и разрозненные рудопроявления: каков вклад в металлогению FeOx-Cu-Au-U?

The Wernecke and Southern Ogilvie Mountains in Yukon are part of an almost east-west trending range in the northern Canadian Cordillera in which several areas of the Palaeo-Mesoproterozoic basement are exposed, enveloped by a Phanerozoic miogeoclinal sequence. The oldest division, the -1.8-1.4 Ga Wernecke Supergroup, is interpreted as a "clastic rift'". It is an up to 15 km thick pile, the bulk of which is a monotonous, well-bedded siltite-quartz rich litharenite-argillite, topped by carbonate-pelite units. Less than 1% of the area consists of small gabbro to diorite intrusions of several, mostly Palaeoproterozoic and Mesoproterozoic, generations. The predominantly brittle deformation regime produced extensive tracts of disrupted and dismembered units grading to tectonic (not subduction !) melange. These have been overprinted by large scale Na, Ca, Mg, Fe, C02 and lesser Si, K metasomatism to produce widespread albitisation, chloritisation, carbonatisation, hematitisation and less extensive sericitisation (with local biotite) of the fractured sedimentary » magmatic rocks as well as tectonic fragmentites. The "Wernecke Breccia" is a metasomatised disaggregated breccia series and it is associated with hundreds of small scattered showings of specular hematite, magnetite and chalcopyrite, several occurrences of U and Co minerals, and anomalous gold. Not even a marginally economic orebody has so far been discovered despite intermittent exploration going back to the 1960s. It appears that we are dealing with a moderately deep (closely above the ductile-brittle interface) level of regional release and displacement of metals from source rocks (Fe, Cu and Co from gabbros; U perhaps from carbonaceous argillites) by metasomatic destruction of the carrier minerals. However, the system lacked sufficient plumbing and the channelling required to produce better metal accumulations at higher levels.

ТематикаПолезные ископаемые
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