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991.
David L. Huston Jorge M. R. S. Relvas J. Bruce Gemmell Susan Drieberg 《Mineralium Deposita》2011,46(5-6):473-507
Assessment of geological, geochemical and isotopic data indicates that a significant subgroup of volcanic-hosted massive sulphide (VHMS) deposits has a major or dominant magmatic?Chydrothermal source of ore fluids and metals. This group, which is typically characterised by high Cu and Au grades, includes deposits such as those in the Neoarchean Doyon-Bousquet-LaRonde and Cambrian Mount Lyell districts. These deposits are distinguished by aluminous advanced argillic alteration assemblages or metamorphosed equivalents intimately associated with ore zones. In many of these deposits, ??34Ssulphide is low, with a major population below ?3??; ??34Ssulphate differs from coexisting seawater and ??34Ssulphate?Csulphide????20?C30??. These characteristics are interpreted as the consequence of disproportionation of magmatic SO2 as magmatic?Chydrothermal fluids ascended and cooled and as a definitive evidence for a significant magmatic?Chydrothermal contribution. Other characteristics that we consider diagnostic of significant magmatic?Chydrothermal input into VHMS ore fluids include uniformly high (>3 times modern seawater values) salinities or very 18O-enriched (??18O?>?5??) ore fluids. We do not consider other criteria [e.g. variable salinity, moderately high ??18Ofluid (2?C5??), ??34Ssulphide near 0??, metal assemblages or a spatial association with porphyry Cu or other clearly magmatic-hydrothermal deposits] that have been used previously to advocate significant magmatic?Chydrothermal contributions to be diagnostic as they can be produced by non-magmatic processes known to occur in VHMS mineral systems. However, in general, a small magmatic?Chydrothermal contribution cannot be excluded in most VHMS systems considered. Conclusive data that imply minimal magmatic?Chydrothermal contributions are only available in the Paleoarchean Panorama district where coeval seawater-dominated and magmatic?Chydrothermal systems appear to have been physically separated. This district, which is characterised by chloritic and sericitic alteration assemblages and lacks aluminous advanced argillic alteration assemblages, is typical of many VHMS deposits around the world, suggesting that for ??garden variety?? VHMS deposits, a significant magmatic?Chydrothermal contribution is not required. Other than deposits associated with advanced argillic alteration assemblages, the only deposit for which we ascribe a major magmatic?Chydrothermal contribution is the Devonian Neves Corvo deposit. This deposit differs from other deposits in the Iberian Pyrite Belt and around the world in being extremely Sn-rich, with the Sn closely associated with Cu and in having formed from high 18O-rich fluids (??18Ofluid ??8.5??). We consider these characteristics, particularly the last, as diagnostic of a significant magmatic hydrothermal contribution. Our analysis indicates that two subgroups of VHMS deposits have a major magmatic?Chydrothermal contribution: Cu/Au-rich deposits with aluminous alteration assemblages and reduced, very Sn-rich deposits in which Sn was introduced in a high-temperature ore assemblage. Comparison with ??normal?? VHMS deposits suggests that these subgroups of VHMS deposits may form in specialised tectonic environments. The Cu/Au-rich deposits appear to form adjacent to magmatic arcs, an environment conducive to the generation of hydrous, oxidised melts by melting metasomatised mantle in the wedge above the subducting slab. This contrasts with the back-arc setting of ??normal?? VHMS deposits in which relatively dry granites (In this contribution, we use the term granite sensu latto) formed by decompression melting drive seawater-dominated hydrothermal circulation. The tectonic setting of highly Sn-rich VHMS deposits such as Neves Corvo is less clear; however, thick continental crust below the ore-hosting basin may be critical, as it is in other Sn deposits. 相似文献
992.
Heinz-Günter Stosch Rolf L. Romer Farahnaz Daliran Dieter Rhede 《Mineralium Deposita》2011,46(1):9-21
Iron oxide–apatite (IOA) deposits, often referred to as Kiruna-type iron ore deposits, are known to have formed from the Proterozoic
to the Tertiary. They are commonly associated with calc–alkaline volcanic rocks and regional- to deposit-scale metasomatic
alteration. In the Bafq District in east Central Iran, economic iron oxide–apatite deposits occur within felsic volcanic tuffs
and volcanosedimentary sequences of Early Cambrian age. In order to constrain the age of formation of these ores and their
relationship with the Early Cambrian magmatic event, we have determined the U–Pb apatite age for five occurrences in the Bafq
District. In a 206Pb/238U vs. 207Pb/235U diagram, apatite free of or poor in inclusions of other minerals plots along the Concordia between 539 and 527 Ma with four
out of five samples from one deposit clustering at the upper end of this range. For this deposit, we interpret this cluster
to represent the age of apatite formation, whereas the spread towards younger ages may reflect either minor Pb loss or several
events of IOA formation. Apatite with inclusions of monazite (±xenotime) yields disturbed systems with inclusions having developed
after formation of the iron ore–apatite deposits, possibly as late as 130–140 Ma ago. Obtained apatite ages confirms that
(IOA) and the apatite-rich rocks (apatites) of the Bafq district formed coevally with the Early Cambrian magmatic (-metasomatic)
events. 相似文献
993.
Virtually all of the economic Ni–Cu–(platinum group element (PGE)) mineralization in the central part of the Cape Smith Belt
of New Québec is hosted by thick olivine cumulate units in the Katinniq Member of the Raglan Formation at the base of the
1.9 Ga Chukotat Group. These units transgress underlying gabbros and pelitic metasediments, forming 50–200-m deep and 300–1,000-m
wide V-shaped embayments and have been interpreted on the basis of surface geology, deep diamond core drilling, and magnetic
inversion models to represent the remnants of one or more large, long (at least 20 km, possibly ≥50 km), sinuous, komatiitic
basalt lava channels that formed by thermomechanical erosion of their substrates. We have used a mathematical model to test
these hypotheses regarding komatiitic lava emplacement and erosion by lava. Our modeling predicts that an initially 10-m thick
komatiitic basalt flow should have flowed turbulently near the vent and should have thermomechanically eroded unconsolidated
pelitic sediment during emplacement to reach the observed degree of contamination of ≤10% at distances of ~30–60 km downstream
from the source. Furthermore, our models predict that, at these distances downstream, a fully inflated 100-m thick komatiitic
basalt flow would have had thermal erosion rates over consolidated gabbroic substrate of ~0.7–1.5 m/day, requiring ~70–140 days
to incise a 100-m deep channel, depending on the initial temperature of the lava, the paleoslope, and the initial temperature
and solidus temperature of the gabbro. These erosion rates would have been associated with volumetric flow rates of >105–106 m3/s and eruption volumes of >103–104 km3. Although these flow rates are orders of magnitude larger than those of most modern terrestrial basaltic flows, they are
of the same order as those estimated for the largest terrestrial flood basalt flows and with those inferred for some of the
largest extraterrestrial flows. Our predicted flow volumes are also of the same order as those of the largest terrestrial
flood basalt units, consistent with the great thickness and widespread distribution of the Chukotat Group. Our modeling of
thermomechanical erosion of gabbro by komatiitic basalt results in negligible contamination (<1%), and geochemical studies
show that the spatially and petrogenetically related Chukotat basalts are uncontaminated, suggesting that the observed enrichments
in U–Th–Light Rare Earth Elements (REE) > Middle REE–Heavy REE > Nb–Ta–Ti represent contamination by underlying Povungnituk
semipelites. This result is consistent with present models for the genesis of the Ni–Cu–(PGE) mineralization in the Raglan
Formation that involve thermomechanical erosion of unconsolidated, sulfidic semipelitic sediments, and decoupling of the miscible
silicate and immiscible sulfide components. 相似文献
994.
Mourad Essalhi Stanislas Sizaret Luc Barbanson Yan Chen France Lagroix François Demory José M. Nieto Reinaldo Sáez M. Ángeles Capitán 《Mineralium Deposita》2011,46(8):981-999
In the Rio Tinto district of the Iberian Pryrite Belt of South Spain, the weathering of massive sulfide bodies form iron caps,
i.e., true gossans and their subsequent alteration and re-sedimentation has resulted in iron terraces, i.e., displaced gossans.
To study the stucture and evolution of both types of gossans, magnetic investigations have been carried out with two foci:
(1) the characterisation and spatial distribution of magnetic fabrics in different mineralised settings, including massive
sulfides, gossans, and terraces, and (2) paleomagnetic dating. Hematite has been identified as the suceptibility carrier in
all sites and magnetic fabric investigation of four gossans reveals a vertical variation from top to bottom, with: (1) a horizontal
foliation refered to as “mature” fabric in the uppermost part of the primary gossans, (2) highly inclined or vertical foliation
interpreted as “immature” fabric between the uppermost and lowermost parts, and (3) a vertical foliation interpreted to be
inherited from Hercynian deformation in the lowermost part of the profiles. In terraces, a horizontal foliation dominates
and is interpreted to be a “sedimentary” fabric. Rock magnetic studies of gossan samples have identified goethite as the magnetic
remanence carrier for the low-temperature component, showing either a single direction close to the present Earth field (PEF)
direction or random directions. Maghemite, hematite, and occasionally magnetite are the remanence carriers for the stable
high-temperature component that is characterized by non PEF directions with both normal and reversed magnetic polarities.
No reliable conclusion can be yet be drawn on the timing of terrace magnetization due to the small number of samples. In gossans,
the polarity is reversed in the upper part and normal in the lower part. This vertical distribution with a negative reversal
test suggests remanence formation during two distinct periods. Remanence in the upper parts of the gossans is older than in
the lower parts, indicating that the alteration proceeded from top to bottom of the profiles. In the upper part, the older
age and the horizontal “mature” fabric is interpreted to be a high maturation stage of massive sulfides’ alteration. In the
lower part, the age is younger and the inherited “imature” vertical Hercynian fabric indicates a weak maturation stage. These
two distinct periods may reflect changes of paleoclimate, erosion, and/or tectonic motion. 相似文献
995.
Kevin L. Shelton Justin M. Beasley Jay M. Gregg Martin S. Appold Stephen F. Crowley James P. Hendry Ian D. Somerville 《Mineralium Deposita》2011,46(8):859-880
A newly discovered, extensive sphalerite-bearing breccia (~7.5 wt.% Zn) is hosted in dolomitised Carboniferous limestones
overlying Ordovician–Silurian metasedimentary rocks on the Isle of Man. Although base metal sulphide deposits have been mined
historically on the island, they are nearly all quartz vein deposits in the metamorphic basement. This study investigates
the origin of the unusual sphalerite breccia and its relationship to basement-hosted deposits, through a combination of petrographic,
cathodoluminescence, fluid inclusion, stable isotope and hydrogeologic modelling techniques. Breccia mineralisation comprises
four stages, marked by episodes of structural deformation and abrupt changes in fluid temperature and chemistry. In stage
I, high-temperature (T
h > 300°C), high-salinity (20–45 wt.% equiv. NaCl) fluid of likely basement origin deposited a discontinuous quartz vein. This
vein was subsequently dismembered during a major brecciation event. Stages II–IV are dominated by open-space filling sphalerite,
quartz and dolomite, respectively. Fluid inclusions in these minerals record temperatures of ~105–180°C and salinities of
~15–20 wt.% equiv. NaCl. The δ34S values of sphalerite (6.5–6.9‰ Vienna-Canyon Diablo troilite) are nearly identical to those of ore sulphides from mines
in the Lower Palaeozoic metamorphic rocks. The δ18O values for quartz and dolomite indicate two main fluid sources in the breccia’s hydrothermal system, local Carboniferous-hosted
brines (~0.5–6.0‰ Vienna standard mean ocean water) and basement-involved fluids (~5.5–11.5‰). Ore sulphide deposition in
the breccia is compatible with the introduction and cooling of a hot, basement-derived fluid that interacted with local sedimentary
brines. 相似文献
996.
M. V. Mints 《Geotectonics》2011,45(4):267-290
The integral 3D model of the deep structure of the Early Precambrian crust in the East European Craton is based on interpretation
of the 1-EU, 4B, and TATSEIS seismic CDP profiles in Russia and the adjacent territory of Finland (FIRE project). The geological
interpretation of seismic images of the crust is carried out in combination with consideration of geological and geophysical
data on the structure of the Fennoscandian Shield and the basement of the East European platform. The model displays tectonically
delaminated crust with a predominance of low-angle boundaries between the main tectonic units and the complex structure of
the crust-mantle interface, allowing correlation of the deep structure of the Archean Kola, Karelian, and Kursk granite-greenstone
terrane with the Volgo-Uralia granulite-gneiss terrane, as well as the Paleoproterozoic intracontinental collision orogens
(the Lapland-Mid-Russia-South Baltia orogen and the East Voronezh and Ryazan-Saratov orogens) with the Svecofennian accretionary
orogen. The lower crustal “layer” at the base of the Paleoproterozoic orogens and Archean cratons was formed in the Early
Paleoproterozoic as a result of underplating and intraplating by mantle-plume mafic magmas and granulite-facies metamorphism.
The increase in the thickness of this “layer” was related to hummocking of the lower crustal sheets along with reverse and
thrust faulting in the upper crust. The middle crust was distinguished by lower rigidity and affected by ductile deformation.
The crust of the Svecofennian Orogen is composed of tectonic sheets plunging to the northeast and consisting of island-arc,
backarc, and other types of rocks. These sheets are traced in seismic sections to the crust-mantle interface. 相似文献
997.
Postsedimentation alteration and structural assemblies of the Triassic sedimentary complexes of West Chukotka are discussed.
Zoning of the alteration is based on examination of newly formed structural and mineral assemblages, the chemical composition,
and the polytypes of clay minerals. Three zones of postsedimentation transformation of sandstones are distinguished: (1) the
zone of chlorite, illite, and mixed-layer disordered chlorite-smectite; (2) the zone of illite and chlorite; and (3) the zone
of phengite and ferroan chlorite. The grade of postsedimentation transformation and the composition of the newly formed micas
are correlated with the cleavage type. The development of two-three types of cleavage leads to the highest degree of rock
transformation. The assemblages of clay minerals and the crystal chemistry of the authigenic phengite show that the grade
of postsedimentation transformation of the Triassic rocks attains the stage of greenschistfacies metamorphism in the zone
of development of two cleavage types. Where the second cleavage is not documented or poorly developed, the rocks remain unmetamorphosed.
Evidence is given that postsedimentation transformation of terrigenous rocks in the foldbelt is controlled largely by deformation. 相似文献
998.
Osama M. K. Kassem 《Geotectonics》2011,45(3):244-254
Finite-strain was studied in the mylonitic granitic and metasedimentary rocks in the northern thrust in Wadi Mubarak belt
to show a relationship to nappe contacts between the old granitic and metavolcano-sedimentary rocks and to shed light on the
heterogeneous deformation for the northern thrust in Wadi Mubarak belt. We used the Rf/ϕ and Fry methods on feldspar porphyroclasts, quartz and mafic grains from 7 old granitic and 7 metasedimentary samples in
the northern thrust in Wadi Mubarak belt. The finite-strain data shows that old granitic rocks were moderate to highly deformed
and axial ratios in the XZ section range from 3.05 to 7.10 for granitic and metasedimentary rocks. The long axes (X) of the
finite-strain ellipsoids trend W/WNW and E/ENE in the northern thrust in Wadi Mubarak belt. Furthermore, the short axes (Z)
are subvertical associated with a subhorizontal foliation. The value of strain magnitudes mainly constants towards the tectonic
contacts between the mylonitic granite and metavolcano-sedimentary rocks. The data indicate oblate strain symmetry (flattening
strain) in the mylonitic granite rocks. It is suggested that the accumulation of finite strain was formed before or/and during
nappe contacts. The penetrative subhorizontal foliation is subparallel to the tectonic contacts with the overlying nappes
and foliation was formed during nappe thrusting. 相似文献
999.
1000.
V. A. San’kov A. V. Parfeevets A. V. Lukhnev A. I. Miroshnichenko S. V. Ashurkov 《Geotectonics》2011,45(5):378-393
Comprehensive analysis of the parameters characterizing contemporary and neotectonic deformations of the Earth’s crust and
upper mantle developed in the Mongolia-Siberia area is presented. The orientation of the axes of horizontal deformation in
the geodetic network from the data of GPS geodesy is accepted as an indicator of current deformations at the Earth’s surface.
At the level of the middle crust, this is the orientation of the principal axes of the stress-tensors calculated from the
mechanisms of earthquake sources. The orientation of the axes of stress-tensors reconstructed on the basis of structural data
is accepted as an indicator of Late Cenozoic deformations in the upper crust. Data on seismic anisotropy of the upper mantle
derived from published sources on the results of splitting of shear waves from remote earthquakes serve as indicators of deformation
in the mantle. It is shown that the direction of extension (minimum compression) in the studied region coincides with the
direction of anisotropy of the upper mantle, the median value of which is 310–320° NW. Seismic anisotropy is interpreted as
the ordered orientation of olivine crystals induced by strong deformation owing to the flow of mantle matter. The observed
mechanical coupling of the crust and upper mantle of the Mongolia-Siberia mobile area shows that the lithospheric mantle participated
in the formation of neotectonic structural elements and makes it possible to ascertain the main processes determining the
Late Cenozoic tectogenesis in this territory. One of the main mechanisms driving neotectonic and contemporary deformations
in the eastern part of the Mongolia-Siberia area is the long-living and large-scale flow of the upper mantle matter from the
northwest to the southeast, which induces both the movement of the northern part of the continent as a whole and the divergence
of North Eurasia and the Amur Plate with the formation of the Baikal Rift System. In the western part of the region, deformation
of the lithosphere is related to collisional compression, while in the central part, it is due to the dynamic interaction
of these two large-scale processes. 相似文献