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Выпуск 84
Издание:Economic geology, 1989 г., 3 стр.
Comparative elemental and oxygen isotope geochemistry of jasperoid in the northern great basin: evidence for distinctive fluide evolution in gold-producting hydrothermal system - a discussion

Discriminating productive jasperoids associated with economic gold mineralization from barren jasperoids, those with no known gold association, has long been an objective of Great Basin explorationists. Holland et al. (1988) use factor analysis of a multielement geochemical data set to compare jasperoids associated with Carlin-type gold mineralization with jasperoids in barren systems. They go on to propose a genetic model for Carlin-type deposits based on their interpretation of the factor analysis results. There are problems with their approach, several of which the authors acknowledge. The geologic control for their samples is inadequate, the analytical technique used is inaccurate for several critical elements, and their statistical analysis of the data could be much simpler, easier to interpret, and useful.

Издание:Economic geology, 1989 г., 6 стр.
Comparative elemental and oxygen isotope geochemistry of jasperoid in the Nothern Great Basin: evidence for distinctive fluid evolution in gold-producing hydrotermal systems - a reply

Brian K. Jones and Richard A. Leveille raise a number of points regarding the way in which our comparative study of jasperoid geochemistry (Holland et al., 1988) was framed and the implications that might be drawn from our results. Their primary concerns are with (a) the lack of geologic control for samples, (b) the accuracy of analyses for several critical elements, (c) the specific interpretation of factor 3 and its subsequent use in a discussion of genetic implications, and (d) the evaluation of analytical data by means of Q-mode factor analysis. We will address each of these concerns in turn.

Том 17, Выпуск 11
Издание:Elsevier, 1993 г., 11 стр.
Comparative rheological behaviour of albite and quartz in siliceous schists revealed by the microboudinage of piedmontite

This paper presents a detailed comparison of the microboudinage of piedmontite in two different mineralogical hosts, a quartz matrix, and albite porphyroblasts in a siliceous schist, with the aim of clarifying the rheological properties of albite in relation to those of quartz. Stress and strain analyses of the microboudinage confirm that the boudinage took place in the retrograde stage of metamorphism during decreasing temperature, and reveal that albite deformed at the same strain rate as quartz above the plastic-brittle transition temperature of albite.

Издание:PGS Publishing, Linden Park, 2002 г., 6 стр.
Comparison of the Geology of Proterozoic Iron Oxide Deposits in the Adirondack and Mid-Atlantic Belt of Pennsylvania, New Jersey and New York

Proterozoic hydrothermal iron oxide deposits occur within two metallogenic belts in the northeastern U.S.: the Adirondack region, and the Mid-Atlantic (Reading Prong) belt. A 175 km wide belt of Palaeozoic cover separates these two regions, although some iron deposits occur in Proterozoic rocks near the unconformity, suggesting a possible continuation beneath the cover. Although potentially part of the same continuous metallogenic province sharing similar mineralogy, host rock composition and hydrothermal alteration, deposits in the two regions differ in degree of deformation. Differences in the degree of metamorphic deformation fuel the debate of the relative timing of mineralisation, igneous activity, and metamorphism. Generally less deformed textures in the Adirondack deposits led workers in the New York deposits to conclude iron ores in the Adirondacks are associated with anorogenic granites that postdate peak metamorphism. Folded iron ores in granitic gneiss of the Mid-Atlantic belt suggest some deposits in eastern Pennsylvania, northern New Jersey, and southern New York predate peak metamorphism. REE-enriched deposits in both belts are characterised by abundant apatite, tourmaline, and manganese concentrations, as well as the presence of hematite-chlorite alteration in addition to magnetite. Unlike deposits hosted exclusively within granite gneisses, deposits within supracrustal rocks commonly contain significant sulphides and so are potential hosts for copper mineralisation.

Выпуск 65
Автор(ы):Juvonen R., Kontas E.
Издание:Journal of Geochemical Exploration, 1999 г., 11 стр.
Comparison of three analytical methods in the determination of gold in six Finnish gold ores, including a study on sample preparation and sampling

We used samples from six Finnish ore deposits to evaluate the efficiency of sample pretreatment procedures — crushing, splitting and grinding — and to compare three analytical methods based on the atomic absorption determination of gold following: (1) classical lead fire assay (FA); (2) the aqua regia leach (AR) followed by Hg coprecipitation of Au; and (3) the sodium cyanide (NaCN) leach. Sample size used for the method comparison is 20 g. The Au deposits and ore types were: Suurikuusikko and Osikonma¨ki, refractory ores in which Au is associated with arsenopyrite and pyrite; Pampalo and Kutemaja¨rvi ores with metallic Au and Au tellurides; and Jokisivu and Pahtavaara ores containing coarse-grained metallic Au. After crushing, the samples were split into three parts, one of which was put aside into storage. Two splits were further divided into two subsamples which were ground to two grades of fineness (<0.03 and <0.06 mm). The four subsamples thus obtained were analysed for Au using the three analytical methods. Each determination was performed five times on each of the four subsamples. According to t-tests on the FA results of the two splits, crushing and splitting produced samples of equal Au content in all six cases. Grinding to a finer grain size gave a significant difference in Au results only for the Pahtavaara ore sample. If the FA results are assumed to represent 100% recovery of Au, we obtained greater than 95% recoveries for all but the Suurikuusikko sample (87% recovery) by the AR leach method. We also obtained recoveries of over 95% by the NaCN leach method for the Pampalo, Kutemaja¨rvi and Pahtavaara samples, whereas recoveries for the other three samples varied between 73 to 92%. The AR leach was also performed on 1-g samples and the NaCN leach on 250-g samples. For three of the ore samples, decreasing sample size from 20 g to 1 g did not cause a significant difference in the variance of the Au results. Increasing the sample size from 20 g to 250 g significantly improves the representativity of only the Pahtavaara sample. For the Kutemaja¨rvi, Pahtavaara and Jokisivu ores, a sample larger than 250 g is needed in order to obtain a precision equivalent to that for reference samples.

Том 46, Выпуск 9
Автор(ы):Buslov M.M., Vovna G.M.
Издание:Journal Geochemistry International, 2008 г., 10 стр.
Composition and Geodynamic Nature of the Protoliths of Diamondiferous Rocks from the Kumdy-Kol Deposit of the Kokchetav Metamorphic Belt, Northern Kazakhstan

The distribution of rare earth elements was analyzed in the Early Cambrian diamondiferous calcsilicate rocks and gneisses, calciphyres, and marbles of the Kumdy-Kol deposit. These data were compared with the lithogeochemical characteristics of the sedimentary assemblages of weakly metamorphosed Late Precambrian graphite-bearing sedimentary rocks of the Kokchetav metamorphic belt. The obtained results allowed us to suppose that the protoliths of the Kumdy-Kol rocks were compositionally similar to the Late Precambrian graphite-bearing terrigenous–carbonate and sand–shale sequences of the continental shelf of the Kokchetav microcontinent, some of which were transformed in a subduction zone into diamondiferous rocks.

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.

Выпуск 5
Автор(ы):Shengyong Y., Xuejing X., Xueqiu W.
Издание:Journal of Geochemical Exploration, 1995 г., 9 стр.
Concepts for geochemical gold exploration based on the abundance and distribution of ultrafine gold

Conventionally, geochemical exploration for gold is based on the assumptions that (1) gold is chemically inert in surficial environments; (2) gold occurs mainly in discrete grains; and (3) gold is transferred by mechanical means to form clastic dispersion halos and dispersion trains. Consequently, the commonly adopted methodology has been (1) to determine gold in heavy mineral concentrates; (2) to use large samples in order to improve the reproducibility of gold analyses; (3) to use high detection limits and thresholds; and (4) to determine total gold contents and pathfinder elements in the samples. However, these methods are not always successful in locating gold deposits, and they have limited application in the search for buried or blind deposits.

In China, studies of the distribution and migration of particulate and ultrafine gold indicated that (1) gold is active and mobile in surficial environments; (2) gold occurs not only as discrete grains, but also as ultrafine particles and other complex forms; and (3) regional low-concentration gold anomalies as well as local anomalies over buried gold deposits originate from ultrafine gold and other complex forms of gold. The methodology developed in China for regional and local geochemical gold exploration is based on this experience. Results of investigations around two gold deposits in China are presented.

Выпуск 88
Издание:Economic geology, 1993 г., 10 стр.
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.

Автор(ы):Park A.F.
Издание:Geological society of America bulletin, 1991 г., 16 стр.
Continental growth by accretion: A tectonostratigraphic terrane analysis of the evolution of the western and central Baltic Shield

Tectonostratigraphic terrane analysis of the western and central Baltic Shield defines a framework for continental growth through the period 2.50-1,75 Ga, Eight discrete terrenes can he defined in this part of the Svecokarelian orogen: the older cratonic Kuhmo and HsaJmi terranes, the hybrid Lapland and Savo province allochthonous ler-ranes, the juvenile island-arc-I ike Skeltefte-Savonlinna and south Finland-central Sweden terranes. (he back-arc or arc-tike Outokumpu nappe, and the ophiolitic (oceanic?) Jormua nappe. Stitching events can also he recngnized. These include the ca- 1.88-1.87 Ca Svionian magmalic arc, the ca. 2.44 Ca Koillismaa intrusions, the Kalevian flysch basin, and the Bolhnian basin. Constraining the age of these stitching events permits the construction of an accretion history in which the "Svecok3relian orogeny" can be resolved as a number of accretion events, deformation episodes related to accretion or mjgmato-tectonic episodes. These events, defined here, include the pre-2.44 Ca Pohjolan accretion of the component parts of the Lapland hybrid terrane. the ca. 1.95 Ca Kyllikian accretion of the Karelian collage, Ihc ca. 1.90 Ga Karelian orogeny marked by the development of the Savo thrust bell in response to the accretion of the Skellefte-Savonlinna terrane to the Karelian collage, and (he ca. 1.88-1.87 Ga Svionian magmalic arc. The whole Svecokaretian collage had assembled by ca. 1.85 Ca, an amalgamation succeeded and stitched by a diverse collection of igneous and thermal events through the period 1.85-1.75 Ca.