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Выпуск 90
Автор(ы):Arthur W.Rose, Carl A.Kuehn
Издание:Economic geology, 1995 г., 20 стр.
Язык(и)Английский
Carlin gold deposits, Nevada: origin in a deep zone of mixing between normally pressured and overpressured fluids

Gold mineralization at Carlin is clearly younger than hydrocarbon maturation (pre-Cretaceous) and felsic dike intrusion (Cretaceous), and older than deep oxidation (late Tertiary). Within the episode of gold mineralization, the main gold ore (MGO) stage and late gold ore (LGO) stage are distinguished paragenetically, wi-th a variety of vein and mineralization types in each. MGO stage fluids contained 5 to 10 mole percent COa, appreciable H2S, and 3 ± 1 wt percent NaCl equiv. At least portions of MGO stage mineralization were characterized by two-phase boiling (COa exsolution) at 215° ± 30°C and 800 ± 400 bars. In contrast, LGO stage fluids were gas poor with salinities <1.5 wt percent NaCl equiv and record only nonboiling conditions. MGO stage fluids had 518Oh2o values of 5 to 9 per mil, whereas LGO stage fluids resembled unevolved meteoric water with 518Oh2o values < —3 per mil.

From the MGO stage to the LGO stage, calcite 5180 values shifted from near whole-rock values of 12 ± 3 per mil to around 0 ± 1 per mil as LGO stage fluids flooded the system. Jasperoids also record a large range (9-22%o) in 518Oh2o values. These data indicate the involvement of two very different fluids in ore deposition. Because MGO and LGO stage features are closely associated spatially with each other and with Au, As, Sb, Hg, and other ore elements, both fluids are believed to have both been present during most stages of ore deposition.

At pressures of 80 to 85 percent lithostatic, depths of 3.8 ± 1.9 km are required to accommodate the 800 ± 400 bars of pressure recorded in MGO stage fluid inclusions. Carlin, therefore, is not an epi-thermal or hot spring deposit. Carbon dioxide in gas-rich MGO stage fluids may have originated either directly from buried intrusions or their contact aureoles, or from low-grade regional metamorphism at depth. The water may have been originally meteoric, and Au may be magmatic or derived from leaching of deep metamorphic or sedimentary rocks. Ore deposition appears to have occurred in zones of throttling at a pressure seal between normally pressured and overpressured regimes, where fluids experienced a change from near-lithostatic to hydrostatic conditions. Such pressure seals are common in deep sedimentary basins and may be a key to highly localized gold deposition. Mixing of two fluids and interaction with host rocks along thin permeable bioclastic horizons are believed to have been the major factors in depositing ore.

 

Издание:Economic geology, 2005 г., 34 стр.
Язык(и)Английский
Carlin-type gold deposits in Nevada: critical geologic characteristics and viable models

Carlin-type Au deposits in Nevada have huge Au endowments that have made the state, and the United States, one of the leading Au producers in the world. Forty years of mining and numerous studies have provided a detailed geologic picture of the deposits, yet a comprehensive and widely accepted genetic model remains elusive. The genesis of the deposits has been difficult to determine owing to difficulties in identifying and analyzing the fine-grained, volumetrically minor, and common ore and gangue minerals, and because of postore weathering and oxidation. In addition, other approximately contemporaneous precious metal deposits have overprinted, or are overprinted by, Carlin-type mineralization.

Выпуск 1
Автор(ы):Mehrtens M.B.
Издание:Economic geology, 1986 г., 5 стр.
Язык(и)Английский
Case history and problem 1: the Tonkin Springs gold mining district, Nevada, U.S.A.

Geochemical techniques played a major role in exploration of the Tonkin Springs district and ultimately led to discovery of economically significant bodies of gold mineralization. Using some of the information obtained during the exploration program, it is possible to review the geochemical environment, secondary dispersion processes and survey techniques used successfully in this part of Nevada.

The Tonkin Springs district is located in west-central Eureka County, Nevada, within the Simpson Park Range approximately 65 km northwest of the town of Eureka (Figure 7.1). Topography is typical of the Basin and Range structural province being characterized by long narrow valleys and north easterly trending mountain ranges with elevations varying between 1,700 and 3,100 m. Precipitation is in the order of 400 mm per year, the major portion of which occurs in the higher elevations during winter and spring. Soils are light brown to brown desert soils of residual origin in locations above the gravel-filled valleys and pediments. Vegetation consists of sagebrush and sparse grass in the valleys with juniper, pinyon and mountain mahogany in the higher country.

Выпуск 1
Автор(ы):Никитин С.Н.
Издание:Геологический комитет, Петроград, 1916 г., 68 стр.
Язык(и)Русский
Cephalopoda Московской юры

Келловейские и оксфордские отложения московской юры по большей части бедны хорошо сохраненными аммонитами. Остатки их, конечно, попадаются нередко, но это или обломки, или молодые, или внутренние обороты взрослых экземпляров, превращенные в колчеданистые ядра. Отсюда только обстоятельное знакомство с прекрасным палеонтологическим материалом из других окружающих местностей России, где развиты юрские образования, позволило мне определить многие формы аммонитов из подмосковного келловея и оксфорда. Только с тем огромным и разнообразным материалом, который был в моем распоряжении, можно было сравнить и сопоставить эти формы с западно-европейскими с достаточною степенью точности и избежать того смешения в московской юре форм различных этажей и зон, которое допускалось в прежних работах моих предшественников по исследованию.

Издание:Economic geology, 2004 г., 22 стр.
Язык(и)Английский
Characteristics and formation of the Jerónimo carbonate-replacement gold deposit, Potrerillos District, Chile

The Jerónimo sedimentary rock-hosted disseminated Au deposit is located within the Potrerillos district of the Atacama region of northern Chile, east of the Potrerillos porphyry Cu-Mo and El Hueso high-sulfidation Au deposits. Prior to development, the Jerónimo deposit contained a resource of approximately 16.5 million metric tons (Mt) at 6.0 g/t Au. Production began in the oxidized, nonrefractory portion of the deposit in 1997 and terminated in 2002. During that time, approximately 1.5 Mt at 6.8 g/t Au was mined by underground room-and-pillar methods, from which a total of approximately 220,000 oz of Au was recovered by heap-leach cyanidation.

Выпуск 83
Издание:Economic geology, 1988 г., 9 стр.
Язык(и)Английский
Characteristics of gold deposits on Nortern Sonora, Mexico: a preliminary report

The geology of northwestern Mexico is complex and is similar in many respects to that of southeastern California and southern Arizona. The region (Fig. 1), typical of the southern basin-and-range physiographic province of which it is a part, is characterized by elongate, northwest-trending ranges separated by wide alluvial valleys. Basement rocks in the area include Precambrian gneisses, metamorphosed andes-ites, and granites. These rocks are overlain by younger Proterozoic quartzites and limestones, Paleozoic and Mesozoic carbonate rocks, and Mesozoic volcanic, clastic, and carbonate sedimentary rocks. Mesozoic plutonic rocks and Tertiary extrusive and intrusive rocks related to volcanic activity of the Sierra Madre Occidental are widely distributed. Broad areas are underlain by plutonic and associated volcanic rocks of the Sonora-Sinaloabatholith of Cretaceous to early Tertiary (Laramide) age. The outcrop areas of the plutonic rocks are smaller in northwestern Sonora, west of Magdalena de Kino where many of the gold deposits are concentrated, than they are farther to the east and south (Fig. 2).

Выпуск 90
Автор(ы):Parry W.T., Paula N.Wilson
Издание:Economic geology, 1995 г., 20 стр.
Язык(и)Английский
Characterization and dating of argillic alteration in the Mercur gold district, Utah

The Mercur gold district of north-central Utah includes several sediment-hosted disseminated gold deposits which are located in the lower member of the Mississippian Great Blue Limestone. Argillic alteration of host limestone consists of illite (R3 illite-smectite <10% S) + kaolinite + quartz ± Fe oxides or pyrite. Argillized limestone has identical clay mineralogy in both oxidized and unoxidized rock. Unlike some other sediment-hosted disseminated gold deposits, variations in the Kubler index and illite/kaolinite ratios show no spatial relationship to faults or to gold distribution within the mineralized areas.

Выпуск 92
Автор(ы):David A.Mako
Издание:Economic geology, 1997 г., 2 стр.
Язык(и)Английский
Characterization and dating of argillic alteration in the Mercur gold district, Utah - a discussion

Wilson and Parry (1995) present data pertaining to clay alteration and K-Ar age dates for samples from the Mercur gold district. Their data record a wide spread of K-Ar ages for illite ranging from 98.4 to 226 Ma. They estimate the age of gold mineralization to be between 140 and 160 Ma and explain the wide range of ages as functions of partial thermal resetting of the clay minerals and the distance from the hydrothermal conduits. Morris and Tooker (1996) in their discussion of this paper, point out that a Mesozoic age for gold mineralization at Mercur is incompatible with several lines of long-standing regional geologic evidence that suggest a Tertiary age. In their reply, Wilson and Parry (1996) defend their position for a Mesozoic age of mineralization in part by relying on new 40Ar/39Ar age data and the fact that none of the 22 age dates is Tertiary. Although the research by Wilson and Parry may represent a good study of samples in the laboratory, there are several tenuous assumptions and contradictions of the geologic observations at Mercur that must be addressed.

Автор(ы):Parry W.T., Paula N.Wilson
Издание:Economic geology, 1996 г., 3 стр.
Язык(и)Английский
Characterization and dating of argillic alteration in the Mercur gold district, Utah—a reply

We would like to extend our appreciation to Morris and Tooker for their comments, discussion, and additional information that they provide pertaining to the geologic environment of the Mercur gold district, Utah. Their review of the characteristics of the Sevier orogenic belt are particularly relevant; however, such characteristics must be interpreted within the context of the additional geologic events of the region, which include the Jurassic compressional event that has been described from northern Utah and western Nevada. For this purpose, we offer the following reply.

Morris and Tooker have two main points of disagreement with our paper. First, they find the range of K-Ar ages we reported as disturbing and indicate that they date neither tectonic, hydrothermal, nor gold mineralization events; and second, they contend that all mineralized structures at Mercur must be younger than Late Cretaceous in age.

Издание:PGS Publishing, Linden Park, 2002 г., 20 стр.
Язык(и)Английский
Chilean Strata-bound Cu- (Ag) Deposits: An Overview

Strata-bound Cu- (Ag) deposits, long known as 'Chilean manto-type', occur along the Coastal Cordillera of northern Chile (22°-30°S) hosted by Jurassic and Lower Cretaceous volcanic and volcano-sedimentary rocks. These deposits are typical of the first stage of Andean evolution characterised by an extensional setting of the arc magmatism along the active margin of South America. Strata-bound Cu- (Ag) deposits were formed during two metallogenic epochs in the Late Jurassic and uppermost Early Cretaceous. The mineralisation took place at the time of structurally controlled emplacement of batholiths within the Mesozoic volcanic and sedimentary strata. The volcanic-hosted strata-bound Cu- (Ag) deposits invariably occur distal, but peripheral to coeval batholiths emplaced within tilted Mesozoic strata. The prevalent view that these deposits have an inherent genetic relationship with hydrothermal fluid derivation from subvolcanic stocks and dykes is contended here, because these minor intrusions are largely barren and this hypothesis does not fit well with Sr, Os and Pb isotopic data that call for crustal contribution of these elements. The strata-bound Cu- (Ag) mineralisation appears to be produced by fluids of mixed origin that were mobilised within penneable levels and structural weakness zones of the Mesozoic arc-related volcano-sedimentary sequence during the emplacement of shallow granodioritic batholiths under transtensional regimes. These hydrothermal fluids deposited copper and subordinate silver when reacted with organic matter, pyrite and/or cooled away from their heat sources. Although strata-bound Cu- (Ag) mineralisation took place during the same Cretaceous metallogenic event that formed the magnetite-apatite bodies, and Fe-oxide-Cu-Au deposits along the present Coastal Cordillera, the conceivable relationships with these other types of deposits are hampered by the inconclusive debate about the origin of the Chilean Fe-oxide deposits. However, the available data strongly suggest that the Fe oxide-rich deposits are metasomatic in origin and genetically related to contact zones of Lower Cretaceous dioritic batholiths, whereas the iron-poor volcanic-hosted Cu-(Ag) stratabound deposits constitute distal mineralisation peripheral to Upper Jurassic of Lower Cretaceous granodioritic batholiths.

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