Автор(ы):Gray D.J., Lintern M.J., Longman G.D.
Издание:Journal of Geochemical Exploration, 1998 г., 17 стр.
Readsorption of gold during selective extraction — observations and potential solutions

This paper describes a soil extraction method developed to investigate the different chemistries of Au in various soils in the Yilgarn Craton. The extraction solution is 1 M sodium bicarbonater0.1 M potassium iodide, saturated with CO2 and adjusted to pH 7.4 with hydrochloric acid. A soil : solution ratio of 1 : 2 Žg : ml. is used. Two different methods were used: Ž1. net iodide-extractable Au, with solutions analysed directly for Au; Ž2. gross iodide-soluble Au, where activated carbon is added to the mixture and the carbon analysed at the end of the extraction, thus providing a measure of all Au dissolved during the extraction Žincluding that readsorbed during the net extraction.. Depending on the extraction conditions, there may be appreciable readsorption of Au, particularly for organic-rich ŽG50%. and Fe-rich lateritic soils Ž)80%.. This readsorption is enhanced by pulverizing to -75 mm. Consequently, for simple extractions longer than 1 day, pulverized soils give lower apparent Au solubility than do unpulverized soils. Unpulverized carbonate-rich soils show high Au solubilities and little Žoften -20%. readsorption, and consequently show high net iodide-solubilities. These readsorption phenomena could affect other methods used in exploration and should be thoroughly investigated before incorrect conclusions are drawn. The readsorption problems are removed by adding activated carbon to the extraction mixtures; the carbon adsorbs Au as it is dissolved from the sample and is subsequently analysed. However, different soil types still show distinctly different Au solubilities, which should be recognized for interpretation of extraction results. Again, this effect should be tested for other extraction techniques. A more intractable problem may be that biological cycling of the Au through plants and other organisms appears to cause high Au solubilities in many soils. This effect may obscure any potential ‘mineralization signature’ that is being tested by selective extractions, and could cause problems for any extraction method, no matter how well designed

ТематикаГорное дело, Полезные ископаемые
МеткиGold, Selective exploration, Золото, Селективная отработка
Автор(ы):Seltmann R., Yakubchuk A.
Издание:138 стр.
Anatomy of giants-gold Tien Shan province / Строение гигантских золоторудных месторождений Тянь-Шаньской провинции
ТематикаПолезные ископаемые
Выпуск 58
Автор(ы):Filho S.R., Maddock J.E.L.
Издание:Journal of Geochemical Exploration, 1997 г., 10 стр.
Mercury pollution in two gold mining areas of the Brazilian Amazon

Gold has been exploited intensively in the Brazilian Amazon during the past fifteen years using garimpo methods (small-scale gold mining). In this study, two gold mining areas were investigated, the municipalities of Pocone and Alta Floresta located in the state of Mato Grosso, Central Brazil. The elemental mercury (Hg) used in amalgamating the gold, the fir.al stage of the ore dressing process, has caused abnormal Hg concentrations in waterways. This has occurred principally in the Amazon region, where most of the ore prospected is alluvial. Background levels of metals were determined by analyzing sediments and soils located upstream of the anthropogenic inputs and unaffected by mining activities. The study aimed to evaluate the pollution level in sediments and soils, taking into account drainage waters directly affected by gold mining. 'Geoaccumulation indexes' (Igeo) of Hg in sediments from both study areas were used to assesses the pollution level in the aquatic environment. The geoaccumulation indexes of Hg in sediments of the Bento Gomes River in Pocone indicate a relatively high degree of pollution at some sites, even reaching class 4 (1.85 mg/kg). However, when the river reaches the Pantanal swamp, Hg concentrations drop considerably to 0.30 mg/kg. This drop seems to be due to accumulation of metals in the sediments of a lake (sampling site PG-24), which retains most of the sediments transported by the Bento Gomes River. Accumulation of metals in the lake also occurred for Cu, Pb, Zn, Fe and Mn. In the region of Alta Floresta, total Hg concentrations in sediments of the Teles Pires River were studied in the grain size fractions < 74 /urn and > 74 /xm. Hg concentrations in bottom sediments of this river were higher than those found in the Pocone region, with increases of 1.5 to 30 times the background, and thus reaching an Igeo up to class 5.

ТематикаПолезные ископаемые
МеткиAmazon, Brazil, Gold, Gold mining, Mercury pollution, Riverine sediments, Бразилия, Золото, Прибрежные осадки, р. Амазонка, Ртутное загрязнение
Выпуск 57
Автор(ы):Fletcher W.K., Hou Z.
Издание:Journal of Geochemical Exploration, 1996 г., 10 стр.
The relations between false gold anomalies, sedimentological processes and landslides in Harris Creek, British Columbia, Canada

Harris Creek is a Au-rich, cobble-gravel bed stream in southern British Columbia, Canada. A preliminary study, based on analysis of < 0.053 mm sediments and heavy mineral concentrates (SG > 3.2) from bulk sediment samples, identified accumulations of Au at breaks-in-slope of the stream gradient. This is consistent with theoretical models of heavy mineral transport by streams (Day and Fletcher, 1991). However, the breaks-in-slope also coincide with active landslides that might, because of the form of the Au anomalies, be interpreted as the source of the Au. To investigate this we have: (1) monitored erosion of the landslides and determined their Au content; and (2) used multi-element geochemistry (with Al, Mg, Na, Ba, Ti, P and Sr) to fingerprint the influence of one of the landslides on the composition of the < 0.053 mm fraction of the stream sediments.

ТематикаГеохимия, Полезные ископаемые
МеткиGeochemical exploration, Gold, Harris Creek, Landslide, Stream, Геохимические аномалии, Золото, Оползень, Седиментология
Автор(ы):Leake M.J., McCaughey S.N., Parris B.W., White N.C.
Издание:Journal of Geochemical Exploration, 1995 г., 50 стр.
Язык(и)Английский (перевод)
Epithermal gold deposits of the southwest Pacific

Since the freeing of the market price of gold in 1971. exploration for epithermal gold deposits in the southwest Pacific region has been intense, and has been encouraged by some major successes. Sufficient discoveries have been made to summarise what has been found to date, and to assess what implications may be drawn for future exploration. Data are tabulated for 137 epithermal precious metal deposits and prospects in Australia (30), Fiji (2). Indonesia (43 ). New Zealand (22), Palau and Yap (2), Papua New Guinea (18), the Philippines (19), and Solomon Islands (1).

Comparison of the character of epithermal deposits in the southwest Pacific with those in other regions (e.g., the northeastern Pacific margin) suggests that they are in most respects similar with, however, three differences: (i) through much of the region (especially the Philippines and Indonesia) low-sulfidation style deposits commonly show characteristics suggestive of formation at deeper levels than is typical elsewhere; (ii) high-sulfidation deposits appear to be more common than along the northeast Pacific margin; (iii) many of the largest deposits show atypical features, particularly those suggestive of a transition to porphyry or mesothermal conditions, or indicating overprinting of an epithermal system on a porphyry environment. These differences can be partly understood in terms of the tectonic setting and evolution of the volcanic arcs of the southwest Pacific, and the consequent hydrological conditions that existed during mineralization.

The results of exploration in the southwest Pacific indicate that, while the principles of formation of epithermal deposits still apply, deposit models developed in other tectonic/volcanic settings should not be rigidly applied. Gold explorers should first consider the character of the geologic environment at the time of mineralization to try to predict the hydrological conditions during mineralization. Once the likely volcanic environment and paleorelief have been established, then the types, distribution, and zoning of hydrothermal alteration, coupled with observations of deposit form, vein textures and mineralogy, will allow a judgement to be made on the level of system that is now exposed, and on the probable controls that localized mineralization.

The very dynamic character of the tectonic and volcanic setting throughout much of the southwest Pacific has resulted in deposits that reflect changes in the environment of the deposit, in some cases even while deposition was occurring. This has produced some deposits with unusual characteristics which could not have been predicted before exploration.

ТематикаПолезные ископаемые, Региональная геология
МеткиEpithermal gold deposits, Gold, Southwest Pacific, Золото, Тихий океан, Эпитермальные месторождения золота, Юго-западная Пацифика
Автор(ы):David V. Lefebure, Derek A. Brown, Gerald E. Ray
Издание:Geological Fieldwork, 1998 г., 14 стр.
The British Columbia sediment-hosted gold project

In 1997 the British Columbia Geological Survey (BCGS) initiated a project to identify prospective areas for sediment-hosted gold mineralization. The inspiration for the project came from presentations and articles by Howard Poulsen of the Geological Survey of Canada (1996a, 1996b). He pointed out that there is potential to find hypogene, sediment-hosted gold mineralization in Canada akin to deposits found in Nevada. He mentioned that if an intrusive association is important to generate the mineralization, then two different geological environments might host this style of mineralization. Accreted terranes with a basement containing carbonate lithologies intruded by Mesozoic or Cenozic plutonism would be a prospective geological setting, specifically the Stikine and Quesnel terranes. The second favourable environment is within the sediments deposited along the continental margin of ancestral North America which have been cut by Mesozoic magmas, such as are found in the Kootenay Arc and Selwyn Basin (Figure 1).

Others have considered British Columbia as prospective territory for sediment-hosted gold. Early work by companies focused on the Insular Belt rocks exposed on Vancouver Island and the Queen Charlottes. These exploration programs were based largely on an epithermal-style model for the mineralization which was in favour at the time. The discovery of the Babe deposit (now called the Specogna or Cinola) in the Queen Charlotte Islands in 1970 increased the interest in this model because it was initially identified as a Carlin-type deposit (Richards et al, 1976; Champigny  and Sinclair,   1982). <...>

ТематикаПолезные ископаемые
МеткиCarlin-type, Cordilleran geology, Economic geology, Gold, Mineral deposits, Sediment-hosted disseminated gold deposits, Британская Колумбия, Золото, Карлинский тип
Автор(ы):Bourne B.T., Brommecker R., Dobak P.J., McEwan C.J., Robert F., Rowe R.R., Zhou X.
Издание:Ore deposit and exploration technology, 2007 г., 21 стр.
Models and Exploration Methods for Major Gold Deposit Types

Gold occurs as primary commodity in a wide range of gold deposit types and settings. In the last decade, significant progress has been made in the classification, definition and understanding of the main gold deposit types. Three main clans of deposits are now broadly defined, each including a range of specific deposit types with common characteristics and tectonic settings. The orogenic clan has been introduced to include vein-type deposits formed during crustal shortening of their host greenstone, BIF or clastic sedimentary rock sequences. Deposits of the new reduced intrusion-related clan share an Au-Bi-Te-As metal signature and an association with moderately reduced equigranular post-orogenic granitic intrusions. Oxidized intrusion-related deposits, including porphyry, skarn, and high-sulfidation epithermal deposits, are associated with high-level, oxidized porphyry stocks in magmatic arcs. Other important deposit types include Carlin, low-sulfidation epithermal, Au-rich VMS and Witwatersrand deposits. The key geology features of the ore-forming environments and the key geologic manifestations of the different deposit types form the footprints of ore systems that are targeted in exploration programs. Important progress has been made in our ability to integrate, process, and visualize increasingly complex datasets in 2D GIS and 3D platforms. For gold exploration, important geophysical advances include airborne gravity, routine 3D inversions of potential field data, and 3D modeling of electrical data. Improved satellite-, airborne- and field-based infrared spectroscopy has significantly improved alteration mapping around gold systems, extending the dimensions of the footprints and enhancing vectoring capabilities. Conventional geochemistry remains very important to gold exploration, while promising new techniques are being tested. Selection of the appropriate exploration methods must be dictated by the characteristics of the targeted model, its geologic setting, and the surficial environment. Both greenfield and brownfield exploration contributed to the discovery of major gold deposits (>2.5 moz Au) in the last decade but the discovery rates have declined significantly. Geologists are now better equipped than ever to face this difficult challenge, but geological understanding and quality field work were important discovery factors and must remain the key underpinnings of exploration programs.

ТематикаГорное дело, Полезные ископаемые
Автор(ы):Marino F.
Издание:University of Nevada, Reno, 2003 г., 27 стр.
Structural and mineralogical features of the deep post gold deposit, Northern Carlin Trend, Nevada







6. Stratigraphy

7. Structures




ТематикаПолезные ископаемые
Автор(ы):Lewis Teal, Mac Jackson
Издание:2002 г., 11 стр.
Geologic overview of the Carlin trend gold deposits

The Carlin trend in northeastern Nevada forms the largest and most productive accumulation of gold deposits in North America. Teal and Jackson (1997b) reported a gold endowment that by the end of 1996 included past production, reserves, resources, and mineral inventory of over 107 million ounces (3,330 t [metric tons]). More than 40 separate deposits have been delineated since disseminated gold mineralization in carbonate host rocks was first discovered by Newmont geologists John Livermore and Alan Coope in 1961. From their original discovery, a classification for this style of gold mineralization has come to be referred to as “Carlin-type” deposits. By early 2002, more than 50 million ounces (1,560 t) of gold had been recovered on the Carlin trend from 26 separate operating or past producing mines. Open-pit mining began in 1965 at the Carlin Mine, and underground mining began in 1993 on the same deposit.

The scope of this paper is to present a regional geologic setting of the Carlin trend. As part of the concluding discussion, a spectrum of Carlin trend deposit types is categorized to illustrate the relative influence of structural and stratigraphic controls on each deposit.

ТематикаПолезные ископаемые
Автор(ы):Groves D., Hall G., Wijns C.
Издание:Barossa Valley, 2004 г., 4 стр.
Compressional tectonics of the carlin gold trend

Pre-existing crustal structures are important in localising strain related to the large-scale evolution of an orogeny. Rheological contrasts between basement blocks will also influence the degree and location of faulting and relative uplift. In northern Nevada, U.S.A., basement architecture in the form of early rifted continental margins, formed during Proterozoic extension, may dictate the subsequent structural geometry of overlying sedimentary sequences during large-scale compression (Figure 1a). Within the region of the Carlin gold trend, specific anticlinal fold and thrust geometries in the sedimentary rocks, involved in various orogenies up until the Laramide, may focus fluid movement and provide effective traps to the system, resulting in the unique gold endowment of the area. Most mineralisation is situated less than 100 m below the Roberts Mountain thrust, which defines the lower boundary of the sequence of deep-water sedimentary rocks that has ridden over both the basement and younger sedimentary layers.

Muntean et al. (2003) argue that the Carlin and Battle Mountain–Eureka (BME) gold trends (Figure 1b) correspond to reactivated normal faults that likely had their origins in Proterozoic rifting. Numerical modelling offers a way to test the basic hypothesis by which “steps”, relics of continental rifting, control the subsequent location of upper crustal faults and anticlinal structures during compression.

ТематикаПолезные ископаемые
Ленты новостей