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451.
JÜRGEN TITSCHACK CAMPBELL S. NELSON TIM BECK ANDRÉ FREIWALD ULRICH RADTKE 《Sedimentology》2008,55(6):1747-1776
Autochthonous red algal structures known as coralligène de plateau occur in the modern warm‐temperate Mediterranean Sea at water depths from 20 to 120 m, but fossil counterparts are not so well‐known. This study describes, from an uplifted coastal section at Plimiri on the island of Rhodes, a 450 m long by 10 m thick Late Pleistocene red algal reef (Coralligène Facies), interpreted as being a coralligène de plateau, and its associated deposits. The Coralligène Facies, constructed mainly by Lithophyllum and Titanoderma, sits unconformably upon the Plio‐Pleistocene Rhodes Formation and is overlain by a Maerl Facies (2 m), a Mixed Siliciclastic‐Carbonate Facies (0·2 m) and an Aeolian Sand Facies (2·5 m). The three calcareous facies, of Heterozoan character, are correlated with established members in the Lindos Acropolis Formation in the north of the island, while the aeolian facies is assigned to the new Plimiri Aeolianite Formation. The palaeoenvironmental and genetic‐stratigraphic interpretations of these mixed siliciclastic‐carbonate temperate water deposits involved consideration of certain characteristics associated with siliciclastic shelf and tropical carbonate shelf models, such as vertical grain‐size trends and the stratigraphic position of zooxanthellate coral growths. Integration of these results with electron spin resonance dates of bivalve shells indicates that the Coralligène Facies was deposited during Marine Isotope Stage 6 to 5e transgressive event (ca 135 to 120 ka), in water depths of 20 to 50 m, and the overlying Maerl Facies was deposited during regression from Marine Isotope Stage 5e to 5d (ca 120 to 110 ka), at water depths of 25 to 40 m. The capping Aeolian Sand Facies, involving dual terrestrial subunits, is interpreted as having formed during each of the glacial intervals Marine Isotope Stages 4 (71 to 59 ka) and 2 (24 to 12 ka), with soil formation during the subsequent interglacial periods of Marine Isotope Stages 3 and 1, respectively. Accumulation rates of about 0·7 mm year?1 are estimated for the Coralligène Facies and minimum accumulation rates of 0·2 mm year?1 are estimated for the Maerl Facies. The existence of older red algal reefs in the Plimiri region during at least Marine Isotope Stages 7 (245 to 186 ka) and 9 (339 to 303 ka) is inferred from the occurrence of reworked coralligène‐type lithoclasts in the basal part of the section and from the electron spin resonance ages of transported bivalve shells. 相似文献
452.
G. F. Ufimtsev 《Lithology and Mineral Resources》2008,43(1):76-80
Some rare types of small clinoforms found in the latest continental deposits of lowland platform and mountain regions are described. The clinoforms are represented by prodelta deposits of mountain lakes, oblique-bedded horizons of floodplain alluvium of strongly meandering rivers, thick and short lenses of mountain alluvium, and alluvium horizons of great lowland rivers with oblique bedding grading into horizontal bedding. Such structures bear information on paleogeographic, morphological, and lithodynamic features of continental sedimentation. 相似文献
453.
In the northern limb of the 2.06-Ga Bushveld Complex, the Platreef is a platinum group elements (PGE)-, Cu-, and Ni-mineralized
zone of pyroxenite that developed at the intrusion margin. From north to south, the footwall rocks of the Platreef change
from Archaean granite to dolomite, hornfels, and quartzite. Where the footwall is granite, the Sr-isotope system is more strongly
perturbed than where the footwall is Sr-poor dolomite, in which samples show an approximate isochron relationship. The Nd-isotope
system for samples of pyroxenite and hanging wall norite shows an approximate isochron relationship with an implied age of
2.17 ± 0.2 Ga and initial Nd-isotope ratio of 0.5095. Assuming an age of 2.06 Ga, the ɛNd values range from −6.2 to −9.6 (ave.
−7.8, n = 17) and on average are slightly more negative than the Main Zone of the Bushveld. These data are consistent with local
contamination of an already contaminated magma of Main Zone composition. The similarity in isotope composition between the
Platreef pyroxenites and the hanging wall norites suggests a common origin. Where the country rock is dolomite, the Platreef
has generally higher plagioclase and pyroxene δ
18O values, and this indicates assimilation of the immediate footwall. Throughout the Platreef, there is considerable petrographic
evidence for sub-solidus interaction with fluids, and the Δ
plagioclase–pyroxene values range from −2 to +6, which indicates interaction at both high and low temperatures. Whole-rock and mineral δD values suggest that the Platreef interacted with both magmatic and meteoric water, and the lack of disturbance to the Sr-isotope
system suggests that fluid–rock interaction took place soon after emplacement. Where the footwall is granite, less negative
δD values suggest a greater involvement of meteoric water. Consistently higher values of Δ
plagioclase–pyroxene in the Platreef pyroxenites and hanging wall norites in contact with dolomite suggest prolonged interaction with CO2-rich fluid derived from decarbonation of the footwall rocks. The overprint of post crystallization fluid–rock interaction
is the probable cause of the previously documented lack of correlation between PGE and sulfide content on the small scale.
The Platreef in contact with dolomite is the focus of the highest PGE grades, and this suggests that dolomite contamination
played a role in PGE concentration and deposition, but the exact link remains obscure. It is a possibility that the CO2 produced by decarbonation of assimilated dolomite enhanced the process of PGE scavenging by sulfide precipitation. 相似文献
454.
The Mordor Alkaline Igneous Complex (MAIC) is a composite intrusion comprising a body of syenite and a funnel-shaped layered
mafic–ultramafic intrusion of lamprophyric parentage, the Mordor Mafic–Ultramafic Intrusion or MMUI. The MMUI is highly unusual
among intrusions of lamprophyric or potassic parentage in containing primary magmatic platinum-group element (PGE)-enriched
sulfides. The MMUI sequence consists largely of phlogopite-rich pyroxenitic cumulates, with an inward dipping conformable
layer of olivine-bearing cumulates divisible into a number of cyclic units. Stratiform-disseminated sulfide accumulations
are of two types: disseminated layers at the base of cyclic units, with relatively high PGE tenors; and patchy PGE-poor disseminations
within magnetite-bearing upper parts of cyclic units. Sulfide-enriched layers at cycle bases contain anomalous platinum group
element contents with grades up to 1.5 g/t Pt+Pd+Au over 1-m intervals, returning to background values of low parts per billion
(ppb) on a meter scale. They correspond to reversals in normal fractionation trends and are interpreted as the result of new
magma influxes into a continuously replenished magma chamber. Basal layers have decoupled Cu and PGE peaks reflecting increasing
PGE tenors up-section, due to increasing R factors during the replenishment episode, or progressive mixing of between resident PGE-poor magma and more PGE-enriched
replenishing magma. The presence of PGE enriched sulfides in cumulates from a lamprophyric magma implies that low-degree partial
melts do not necessarily leave sulfides and PGEs in the mantle restite during partial melting.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
455.
Combined fluid inclusion microthermometry and microanalysis by laser ablation inductively coupled plasma mass spectrometry
(LA-ICPMS) are used to constrain the hydrothermal processes forming a typical Climax-type porphyry Mo deposit. Molybdenum
mineralisation at Questa occurred in two superimposed hydrothermal stages, a magmatic-hydrothermal breccia and later stockwork
veining. In both stages, texturally earliest fluids were single-phase, of low salinity (~7 wt.% NaClequiv.) and intermediate-density. Upon decompression to ~300 bar, they boiled off a vapour phase, leaving behind a residual brine
(up to 45 wt.% NaClequiv) at temperatures of ~420°C. The highest average Mo concentrations in this hot brine were ~500 μg/g, exceeding the Mo content
of the intermediate-density input fluid by about an order of magnitude and reflecting pre-concentration of Mo by fluid phase
separation prior to MoS2 deposition from the brine. Molybdenum concentrations in brine inclusions, then, decrease down to 5 μg/g, recording Mo precipitation
in response to cooling of the saline liquid to ~360°C. Molybdenite precipitation from a dense, residual and probably sulphide-depleted
brine is proposed to explain the tabular shape of the ore body and the absence of Cu-Fe sulphides in contrast to the more
common Cu-Mo deposits related to porphyry stocks. Cesium and Rb concentrations in the single-phase fluids of the breccia range
from 2 to 8 and from 40 to 65 μg/g, respectively. In the stockwork veins, Cs and Rb concentrations are significantly higher
(45–90 and 110–230 μg/g, respectively). Because Cs and Rb are incompatible and hydrothermally non-reactive elements, the systematic
increase in their concentration requires two distinct pulses of fluid exsolution from a progressively more fractionated magma.
By contrast, major element and ore metal concentrations of these two fluid pulses remain essentially constant. Mass balance
calculations using fluid chemical data from LA-ICPMS suggest that at least 25 km3 of melt and 7 Gt of deep input fluid were necessary to provide the amount of Mo contained in the stockwork vein stage alone.
While the absolute amounts of fluid and melt are uncertain, the well-constrained element ratios in the fluids together with
empirical fluid/melt partition coefficients derived from the inclusion analyses suggest a high water content of the source
melt of ~10%. In line with other circumstantial evidence, these results suggest that initial fluid exsolution may have occurred
at a confining pressure exceeding 5 kbar. The source of the molybdenum-mineralising fluids probably was a particularly large
magma chamber that crystallised and fractionated in the lower crust or at mid-crustal level, well below the shallow intrusions
immediately underlying Questa and other porphyry molybdenum deposits.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
456.
C. Moreno R. Sáez F. González G. Almodóvar M. Toscano G. Playford A. Alansari S. Rziki A. Bajddi 《Mineralium Deposita》2008,43(8):891-911
The Draa Sfar mineralization consists of two main stratabound orebodies, Sidi M’Barek and Tazacourt, located north and south
of the Tensift River (“Oued Tessift”), respectively. Each orebody is comprised by at least two massive sulfide lenses. The
hosting rocks are predominantly black shales, although minor rhyolitic rocks are also present in the footwall to the southern
orebody. Shales, rhyolitic volcanic rocks, and massive sulfides are all included into the Sarhlef Series, which is recognized
as one of the main stratigraphic units of the Moroccan Variscan Meseta. Hydrothermal activity related with an anomalous thermal
gradient, together with a high sedimentation rate in a tectonically driven pull-apart marine basin, favored the accumulation
of organic-rich mud in the deepest parts of the basin and the sedimentary environment suitable for massive sulfide deposition
and preservation. This took place by replacement of the hosting unlithified wet mud below the sediment–water interface. Geochemical
data suggest a sedimentary environment characterized by oxic water column and anoxic sediment pile with the redox boundary
below the sediment–water interface. The low oxygen availability within the sediment pile inhibited oxidation and pyritization
of pyrrhotite. Biostratigraphic analysis, based on the palynological content of the hosting black shales, restricts the age
of the sulfides to the Asbian substage (mid-Mississippian). This age is consistent with earlier geochronological constraints. 相似文献
457.
Anhydrite pseudomorphs and the origin of stratiform Cu–Co ores in the Katangan Copperbelt (Democratic Republic of Congo) 总被引:1,自引:1,他引:0
Ph. Muchez P. Vanderhaeghen H. El Desouky J. Schneider A. Boyce S. Dewaele J. Cailteux 《Mineralium Deposita》2008,43(5):575-589
The stratiform Cu–Co ore mineralisation in the Katangan Copperbelt consists of dispersed sulphides and sulphides in nodules
and lenses, which are often pseudomorphs after evaporites. Two types of pseudomorphs can be distinguished in the nodules and
lenses. In type 1 examples, dolomite precipitated first and was subsequently replaced by Cu–Co sulphides and authigenic quartz,
whereas in type 2 examples, authigenic quartz and Cu–Co sulphides precipitated prior to dolomite and are coarse-grained. The
sulphur isotopic composition of the copper–cobalt sulphides in the type 1 pseudomorphs is between −10.3 and 3.1‰ relative
to the Vienna Canyon Diablo Troilite, indicating that the sulphide component was derived from bacterial sulphate reduction
(BSR). The generation of during this process caused the precipitation and replacement of anhydrite by dolomite. A second product of BSR is the generation
of H2S, resulting in the precipitation of Cu–Co sulphides from the mineralising fluids. Initial sulphide precipitation occurred
along the rim of the pseudomorphs and continued towards the core. Precipitation of authigenic quartz was most likely induced
by a pH decrease during sulphide precipitation. Fluid inclusion data from quartz indicate the presence of a high-salinity
(8–18 eq. wt.% NaCl) fluid, possibly derived from evaporated seawater which migrated through the deep subsurface. 87Sr/86Sr ratios of dolomite in type 1 nodules range between 0.71012 and 0.73576, significantly more radiogenic than the strontium
isotopic composition of Neoproterozoic marine carbonates (87Sr/86Sr = 0.7056–0.7087). This suggests intense interaction with siliciclastic sedimentary rocks and/or the granitic basement.
The low carbon isotopic composition of the dolomite in the pseudomorphs (−7.02 and −9.93‰ relative to the Vienna Pee Dee Belemnite,
V-PDB) compared to the host rock dolomite (−4.90 and +1.31‰ V-PDB) resulted from the oxidation of organic matter during BSR. 相似文献
458.
Mirta Garrido Fernando Barra Eduardo Domínguez Joaquin Ruiz Victor A. Valencia 《Mineralium Deposita》2008,43(5):591-597
The La Voluntad porphyry Cu–Mo deposit in Neuquén, Argentina, is one of several poorly known porphyry-type deposits of Paleozoic
to Early Jurassic age in the central and southern Andes. Mineralization at La Voluntad is related to a tonalite porphyry from
the Chachil Plutonic Complex that intruded metasedimentary units of the Piedra Santa Complex. Five new Re–Os molybdenite ages
from four samples representing three different vein types (i.e., quartz–molybdenite, quartz–sericite–molybdenite and quartz–sericite–molybdenite
± chalcopyrite–pyrite) are identical within error and were formed between ~312 to ~316 Ma. Rhenium and Os concentrations range
between 34 to 183 ppm and 112 to 599 ppb, respectively. The new Re–Os ages indicate that the main mineralization event at
La Voluntad, associated to sericitic alteration, was emplaced during a time span of 1.7 ± 3.2 Ma and that the deposit is Carboniferous
in age, not Permian as previously thought. La Voluntad is the oldest porphyry copper deposit so far recognized in the Andes
and indicates the presence of an active magmatic arc, with associated porphyry style mineralization, at the proto-Pacific
margin of Gondwana during the Early Pennsylvanian. 相似文献
459.
Detrital zircon provenance of Neoproterozoic to Cenozoic deposits in Iran: Implications for chronostratigraphy and collisional tectonics 总被引:2,自引:0,他引:2
B.K. Horton J. Hassanzadeh D.F. Stockli G.J. Axen R.J. Gillis B. Guest A. Amini M.D. Fakhari S.M. Zamanzadeh M. Grove 《Tectonophysics》2008,451(1-4):97-122
Ion-microprobe U–Pb analyses of 589 detrital zircon grains from 14 sandstones of the Alborz mountains, Zagros mountains, and central Iranian plateau provide an initial framework for understanding the Neoproterozoic to Cenozoic provenance history of Iran. The results place improved chronological constraints on the age of earliest sediment accumulation during Neoproterozoic–Cambrian time, the timing of the Mesozoic Iran–Eurasia collision and Cenozoic Arabia–Eurasia collision, and the contribution of various sediment sources of Gondwanan and Eurasian affinity during opening and closure of the Paleotethys and Neotethys oceans. The zircon age populations suggest that deposition of the extensive ~ 1 km-thick clastic sequence at the base of the cover succession commenced in latest Neoproterozoic and terminated by Middle Cambrian time. Comparison of the geochronological data with detrital zircon ages for northern Gondwana reveals that sediment principally derived from the East African orogen covered a vast region encompassing northern Africa and the Middle East. Although most previous studies propose a simple passive-margin setting for Paleozoic Iran, detrital zircon age spectra indicate Late Devonian–Early Permian and Cambrian–Ordovician magmatism. These data suggest that Iran was affiliated with Eurasian magmatic arcs or that rift-related magmatic activity during opening of Paleotethys and Neotethys was more pronounced than thought along the northern Gondwanan passive-margin. For a Triassic–Jurassic clastic overlap assemblage (Shemshak Formation) in the Alborz mountains, U–Pb zircon ages provide chronostratigraphic age control requiring collision of Iran with Eurasia by late Carnian–early Norian time (220–210 Ma). Finally, Cenozoic strata yield abundant zircons of Eocene age, consistent with derivation from arc magmatic rocks related to late-stage subduction and/or breakoff of the Neotethys slab. Together with the timing of foreland basin sedimentation in the Zagros, these detrital zircon ages help bracket the onset of the Arabia–Eurasia collision in Iran between middle Eocene and late Oligocene time. 相似文献
460.