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1.
Martin Reich Carlos Palacios Gabriel Vargas Shangde Luo Eion M. Cameron Matthew I. Leybourne Miguel A. Parada Alejandro Zúñiga Cheng-Feng You 《Mineralium Deposita》2009,44(5):497-504
Supergene enrichment of Cu deposits in the Atacama Desert has played a critical role in making this the prime Cu-producing
province of the world. Previously, this has been believed to have occurred exclusively over a long period from the middle
Eocene to the late Miocene, which ended when climatic conditions changed from arid to hyperarid. Here, we report U-series
disequilibrium ages in atacamite-bearing supergene assemblages that provide a new conceptualization on both the supergene
enrichment process and the onset of extreme hyperaridity in the Atacama Desert. 230Th–234U ages of gypsum intergrown with atacamite in supergene veins from Cu deposits cluster at ~240 ka (Chuquicamata), 130 ka (Mantos
Blancos, Spence), and 80 ka (Mantos de la Luna, Michilla). When coupled with previous data, these results indicate that supergene
enrichment of Cu deposits did not cease after the onset of hyperaridity. We propose that supergene enrichment in the Atacama
region developed in two main stages. The main phase, caused by downward circulation of meteoric waters in a semi-arid setting,
was active from 45 until ~9 Ma, with a last pulse ca. 5 Ma in the southern Atacama Desert. During this phase, atacamite-bearing
supergene assemblages were not preserved because atacamite requires saline water for its formation and rapidly dissolves when
contacted by meteoric water. This was followed by a second stage starting at ~2–1.5 Ma and continuing until at least the late
Pleistocene, when deep formation waters derived from the basement passed up through and modified the pre-existing supergene
Cu oxide minerals. Atacamite has then been preserved in the prevailing hyperarid climate. 相似文献
2.
Martin Reich Carlos Palacios Macarena Alvear Eion M. Cameron Matthew I. Leybourne Artur Deditius 《Mineralium Deposita》2009,44(6):719-722
Recent studies have suggested the involvement of highly saline deep formation waters that modified preexisting Cu assemblages
to form atacamite during supergene oxidation of Cu deposits in the Atacama region. In this report, we document the occurrence
of (Ag–I) inclusions hosted by supergene chalcocite from Mantos de la Luna, an argentiferous Upper Jurassic stratabound Cu
deposit in the Coastal Range of northern Chile. The presence of this unusual mineral assemblage indicates that iodargyrite
precipitated from reducing iodine-rich waters, suggesting that the fluids involved in supergene enrichment of Cu deposits
in the Coastal Range were more complex than previously thought. This suggests the prevalence of hyperarid conditions during
the latest stages of supergene enrichment of the Mantos de la Luna Cu deposit in the Atacama region, supporting the notion
that supergene enrichment processes in hyperarid areas are dynamic in nature and do not exclusively require the presence of
meteoric water. 相似文献
3.
Verónica Oliveros Gilbert Féraud Luis Aguirre Luis Ramírez Michel Fornari Carlos Palacios Miguel Parada 《Mineralium Deposita》2008,43(3):281-293
The 40Ar/39Ar geochronological method was applied to date magmatic and hydrothermal alteration events in the Mantos Blancos mining district
in the Coastal Cordillera of northern Chile, allowing the distinction of two separate mineralization events. The Late Jurassic
Mantos Blancos orebody, hosted in Jurassic volcanic rocks, is a magmatic-hydrothermal breccia-style Cu deposit. Two superimposed
mineralization events have been recently proposed. The first event is accompanied by a phyllic hydrothermal alteration affecting
a rhyolitic dome. The second mineralization event is related to the intrusion of bimodal stocks and sills inside the deposit.
Because of the superposition of several magmatic and hydrothermal events, the obtained 40Ar/39Ar age data are complex; however, with a careful interpretation of the age spectra, it is possible to detect complex histories
of successive emplacement, alteration, mineralization, and thermal resetting. The extrusion of Jurassic basic to intermediate
volcanic rocks of the La Negra Formation is dated at 156.3 ± 1.4 Ma (2σ) using plagioclase from an andesitic lava flow. The first mineralization event and associated phyllic alteration affecting
the rhyolitic dome occurred around 155–156 Ma. A younger bimodal intrusive event, supposed to be equivalent to the bimodal
stock and sill system inside the deposit, is probably responsible for the second mineralization event dated at ca. 142 Ma.
Other low-temperature alteration events have been dated on sericitized plagioclase at ca. 145–146, 125, and 101 Ma. This is
the first time that two distinct mineralization events have been documented from radiometric data for a copper deposit in
the metallogenic belt of the Coastal Cordillera of northern Chile.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
4.
We collected groundwaters in and around a large (313 Mt at 1.08% Cu and 0.3% cutoff) undisturbed porphyry copper deposit (Spence) in the hyperarid Atacama Desert of northern Chile, which is buried beneath 30–180 m of Miocene piedmont gravels. Groundwaters within and down-flow of the Spence deposit have elevated Se (up to 800 μg/l), Re (up to 31 μg/l), Mo (up to 475 μg/l) and As (up to 278 μg/l) concentrations compared to up-flow waters (interpreted to represent regional groundwater flow). In contrast, Cu is only elevated (up to 2036 μg/l) in groundwaters recovered from within the deposit; Cu concentrations are low down gradient of the deposit. The differential behavior of the metals/metalloids occurs because the former group dissolves as anions, enhancing their mobility, whereas the base metals dissolve as cations and are lost from solution most likely through adsorption to clay surface exchange sites and through formation of secondary copper chlorides, carbonates, and oxides. Most groundwaters within and down-flow of the deposit have Eh–pH values around the FeII/FeIII phase boundary, limiting the impact of Fe-oxyhydroxides on oxyanions mobility. Se, Re, Mo, and As are all mobile (with filtered/unfiltered samples ~ 1) to the limit of sampling 2 km down gradient from the deposit. The increase in ore-related metals, metalloids, and sulfate and decrease in sulfate–S isotope ratios (from values similar to regional salars, + 4 to + 8‰ δ34SCDT to lower values closer to hypogene sulfides, + 1 to + 4‰ δ34SCDT) is consistent with active water–rock reactions between saline groundwaters and the Spence deposit. It is likely that hypogene and/or supergene sulfides are being oxidized under the present groundwater conditions and mineral saturation calculations suggest that secondary copper minerals (antlerite, atacamite, malachite) may also be actively forming, suggesting that supergene and exotic copper mineralization is possible even under the present hyperarid climate of the Atacama Desert. 相似文献
5.
Pleistocene recycling of copper at a porphyry system,Atacama Desert,Chile: Cu isotope evidence 总被引:2,自引:1,他引:1
Carlos Palacios Olivier Rouxel Martin Reich Eion M. Cameron Matthew I. Leybourne 《Mineralium Deposita》2011,46(1):1-7
We present Cu isotope data of hypogene and supergene minerals from the Late Paleocene Spence Cu-Mo porphyry in the Atacama
Desert of northern Chile. Chalcopyrite displays a restricted range of δ65Cu values within the values reported for primary porphyry Cu sulfides (+ 0.28‰ to + 0.34‰, n = 6). Supergene chalcocite samples
show heavier and remarkably homogeneous δ65Cu values, between + 3.91‰ and + 3.95‰ (n = 6), consistent with previous models of Cu leaching and enrichment in porphyry
systems. Secondary Cu minerals from the oxide zone show a wider range of composition, varying from + 1.28‰ and + 1.37‰ for
chrysocolla (n = 6) to very light Cu isotope signatures reported for atacamite between -5.72‰ to -6.77‰ (n = 17). These data
suggest redox cycling of Cu during supergene enrichment of the Spence Cu deposit, characterized by a first stage of supergene
chalcocite formation from acidic, isotopically-heavy leach fluids of meteoric origin down-flowing in a semi-arid climate (44
to ~ 15-9 Ma). Reworking of the initial supergene copper assemblage, during the Pleistocene, by rising neutral and chlorine-rich
deep formation waters under well-established hyper-arid climate conditions lead to the formation of atacamite with extremely
fractionated Cu compositions. Essentially coeval chrysocolla formed by dissolution of atacamite during short episodes of wetter
climatic conditions occurring in the latest Pleistocene. 相似文献
6.
Jian-Wei Li Paulo Vasconcelos Wei Zhang Xiao-Dong Deng Nurdan Duzgoren-Aydin Dai-Rong Yan Jian-Qiang Zhang Ming-An Hu 《Mineralium Deposita》2007,42(4):361-383
Supergene Mn-oxide deposits are widely distributed in Guangxi, Guangdong, Yunnan, and Hunan Provinces, South China, accounting
for 18% of the total Mn reserves in the country. Direct dating of supergene Mn enrichment, however, is lacking. In this paper,
we present high-resolution 40Ar/39Ar ages of Mn oxides from the Xinrong Mn deposit, western Guangdong, to place numerical constraints on the timing and duration
of supergene Mn enrichment. A total of ten cryptomelane samples, spanning a vertical extent of 67 m, were dated using the
40Ar/39Ar laser incremental heating technique, with seven samples yielding well-defined plateau or pseudo-plateau ages ranging from
23.48 ± 0.91 to 2.06 ± 0.05 Ma (2σ). One sample yields a staircase spectrum that does not reach a plateau; the spectrum, however, indicates the presence of
two or more generations of Mn oxides in the sample, whose ages are best estimated at 22.34 ± 0.31 and 10.2 ± 0.86 Ma, respectively.
The remaining two samples gave meaningless or uninterpretable results due to significant 39Ar recoil and contamination by old phases. The 40Ar/39Ar data thus reveal a protracted history of weathering and supergene Mn enrichment that started at least in the end of the
Oligocene or beginning of Miocene and extending into the latest Pliocene. Staircase-apparent age spectra, resulting from banded
or botryoidal samples, yield an average growth rate of Mn oxides at 0.6–0.7 × 10−3 mm kyr−1. The values indicate that a 1-mm grain of Mn oxides may host minerals precipitated during a time span of ca. 1.5 m.y., and
accumulation of Mn oxides to form economic deposits under weathering environments may take millions of years. The distribution
of weathering ages shows that the oldest Mn oxides occur on the top of the profile, whereas the youngest minerals are found
at the bottom, suggesting downward propagation of weathering fronts. However, two samples located at the intermediate depths
of the profile yield ages comparable with those occurring at the highest elevations. Such a complexity of age distribution
is interpreted in terms of preferential penetration of Mn-rich weathering solutions along more permeable fault zones, or as
a result of multi stages of dissolution and re-precipitation of Mn oxides. A synthesis of geochronological and geological
data suggests that formation of the Xinrong deposit was a consequence of a combination of favorable lithological, climatic,
and structural conditions. Because the climatic and structural conditions are similar among the provinces of South China during
the Cenozoic, the geochronological results obtained at Xinrong may also have implications for the timing of supergene Mn enrichment
throughout South China. 相似文献
7.
National scale evaluation of groundwater chemistry in Korea coastal aquifers: evidences of seawater intrusion 总被引:3,自引:2,他引:1
Youngyun Park Jin-Yong Lee Jeong-Hee Kim Sung-Ho Song 《Environmental Earth Sciences》2012,66(3):707-718
Pollution of groundwater by seawater intrusion poses a threat to sustainable agriculture in the coastal areas of Korea. Therefore, seawater intrusion monitoring stations were installed in eastern, western, and southern coastal areas and have been operated since 1998. In this study, groundwater chemistry data obtained from the seawater intrusion monitoring stations during the period from 2007 to 2009 were analyzed and evaluated. Groundwater was classified into fresh (<1,500 μS/cm), brackish (1,500–3,000 μS/cm), and saline (>3,000 μS/cm) according to EC levels. Among groundwater samples (n = 233), 56, 7, and 37% were classified as the fresh, brackish, and saline, respectively. The major dissolved components of the brackish and saline groundwaters were enriched compared with those of the fresh groundwater. The enrichment of Na+ and Cl− was especially noticeable due to seawater intrusion. Thus, the brackish and saline groundwaters were classified as Ca–Cl and Na–Cl types, while the fresh groundwater was classified as Na–HCO3 and Ca–HCO3 types. The groundwater included in the Na–Cl types indicated the effects of seawater mixing. Ca2+, Mg2+, Na+, K+, SO4 2−, and Br− showed good correlations with Cl− of over r = 0.624. Of these components, the strong correlations of Mg2+, SO4 2−, and Br− with Cl− (r ≥ 0.823) indicated a distinct mixing between fresh groundwater and seawater. The Ca/Cl and HCO3/Cl ratios of the groundwaters gradually decreased and approached those of seawater. The Mg/Cl, Na/Cl, K/Cl, SO4/Cl, and Br/Cl ratios of the groundwaters gradually decreased, and were similar to or lower than those of seawater, indicating that Mg2+, Na+, K+, SO4 2−, and Br−, as well as Cl− in the saline groundwater can be enriched by seawater mixing, while Ca2+ and HCO3 − are mainly released by weathering processes. The influence of seawater intrusion was evaluated using threshold values of Cl− and Br−, which were estimated as 80.5 and 0.54 mg/L, respectively. According to these criteria, 41–50% of the groundwaters were affected by seawater mixing. 相似文献
8.
Metamorphic fluid origins in the Osborne Fe oxide–Cu–Au deposit,Australia: evidence from noble gases and halogens 总被引:1,自引:1,他引:0
The Osborne iron oxide–copper–gold (IOCG) deposit is hosted by amphibolite facies metasedimentary rocks and associated with
pegmatite sheets formed by anatexis during peak metamorphism. Eleven samples of ore-related hydrothermal quartz and two pegmatitic
quartz–feldspar samples contain similarly complex fluid inclusion assemblages that include variably saline (<12–65 wt% salts)
aqueous and liquid carbon dioxide varieties that are typical of IOCG mineralisation. The diverse fluid inclusion types present
in each of these different samples have been investigated by neutron-activated noble gas analysis using a combination of semi-selective
thermal and mechanical decrepitation techniques. Ore-related quartz contains aqueous and carbonic fluid inclusions that have
similar 40Ar/36Ar values of between 300 and 2,200. The highest-salinity fluid inclusions (47–65 wt% salts) have calculated 36Ar concentrations of approximately 1–5 ppb, which are more variable than air-saturated water (ASW = 1.3–2.7 ppb). These fluid
inclusions have extremely variable Br/Cl values of between 3.8 × 10−3 and 0.3 × 10−3, and I/Cl values of between 27 × 10−6 and 2.4 × 10−6 (all ratios are molar). Fluid inclusions in the two pegmatite samples have similar 40Ar/36Ar values of ≤1,700 and an overlapping range of Br/Cl and I/Cl values. High-salinity fluid inclusions in the pegmatite samples
have 2.5–21 ppb 36Ar, that overlap the range determined for ore-related samples in only one case. The fluid inclusions in both sample groups
have 84Kr/36Ar and 129Xe/36Ar ratios that are mainly in the range of air and air-saturated water and are similar to mid-crustal rocks and fluids from
other settings. The uniformly low 40Ar/36Ar values (<2,200) and extremely variable Br/Cl and I/Cl values do not favour a singular or dominant fluid origin from basement-
or mantle-derived magmatic fluids related to A-type magmatism. Instead, the data are compatible with the involvement of metamorphic
fluids that have interacted with anatectic melts to variable extents. The ‘metamorphic’ fluids probably represent a mixture
of (1) inherited sedimentary pore fluids and (2) locally derived metamorphic volatilisation products. The lowest Br/Cl and
I/Cl values and the ultra-high salinities are most easily explained by the dissolution of evaporites. The data demonstrate
that externally derived magmatic fluids are not a ubiquitous component of IOCG ore-forming systems, but are compatible with
models in which IOCG mineralisation is localised at sites of mixing between fluids of different origin.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorised users. 相似文献
9.
Atacamite, a copper hydroxychloride, is an important constituent of supergene oxide zones of copper deposits in northern Chile,
whereas in similar deposits elsewhere, it is rare. In Chile, it has generally been assumed to be a primary constituent of
the supergene zones. There are two difficulties with this supposition. The first is that atacamite requires saline water for
its formation, whereas supergene oxidation was caused by percolating, oxygenated meteoric water, mainly rainwater. The second
is that atacamite dissolves rapidly or undergoes phase change when exposed to fresh water. Supergene enrichment of copper
deposits in northern Chile extended over a long period, 44 to 9 Ma, being terminated by the onset of hyperaridity. During
this period, there was at least intermittent rainfall, exposing previously formed atacamite to dissolution or phase change.
Furthermore, atacamite-bearing oxide zones in several deposits are directly overlain by thick Miocene alluvial gravels; the
stream waters that transported these gravels would have permeated the oxide zones. In some deposits, atacamite-bearing assemblages
occur both in the oxide zones and in contiguous gravels. We suggest that atacamite-bearing oxide assemblages are more likely
to have been a replacement of preexisting oxide phases after the onset of hyperaridity at about 9 Ma. A hyperarid climate
made possible evaporation and concentration of chloride in meteoric waters. In this paper, we discuss another source of saline
waters to modify oxide zones. Dewatering of the Domeyko Basin expelled brines along faults, some of which had earlier guided
the location of porphyry deposits. At the Spence porphyry copper deposit, saline waters, which δD vs δ
18O isotope analyses identify as basinal brines, are presently rising through the deposit, then flowing away along the base
of the covering gravels. Compositions of these waters lie within the stability fields of atacamite and brochantite, the two
minerals that comprise the oxide zone. Evidence is presented for other porphyry deposits, Radomiro Tomic and Gaby Sur, that
basinal brines may have been involved in the late formation of atacamite. 相似文献
10.
Assessment of recharge to groundwater systems in the arid southwestern part of Northern Territory, Australia, using chlorine-36 总被引:1,自引:0,他引:1
Richard Cresswell John Wischusen Gerry Jacobson Keith Fifield 《Hydrogeology Journal》1999,7(4):393-404
The sustainability of community water supplies drawn from shallow aquifers in the arid southwest of the Northern Territory
has been evaluated using the radioactive isotope chlorine-36 (36Cl). These aquifers include fractured sandstones of the Ngalia Basin, fractured metamorphic rocks and Cainozoic sands and
gravels. 36Cl/Cl ratios for these shallow, regional groundwaters exhibit a bimodal distribution with peaks at 205 (±7) and 170 (±7)×10–15. The higher ratio probably represents modern (Holocene) recharge, diluted with windblown salts from local playa lakes, and
occurs mostly around the margin of the basin. The lower ratio corresponds to a 36Cl "age", or mean residence time, of 80–100 ka, implying that the last major recharge occurred during the last interglacial
interval (Oxygen Isotope Stage 5). These values are mainly observed in the interior of the Ngalia Basin. Lower values of the
36Cl/Cl ratio measured near playa lakes are affected by addition of chloride from remobilised salts. Finite carbon-14 (14C) data for the groundwaters are at variance with the 36Cl results, but a depth profile suggests low recharge, allowing diffusion of recent atmospheric carbon to the water table.
The 36Cl results have important implications for groundwater management in this region, with substantial recharge only occurring
during favourable, wet, interglacial climatic regimes; most community water supplies are dependent on these "old" waters.
Received, September 1997 · Revised, August 1998, March 1999 · Accepted, March 1999 相似文献
11.
Mineralization and alteration events at ten Miocene porphyry Cu and porphyry-related epithermal mineral deposits in southern,
central, and northern Ecuador were dated by means of molybdenite Re-Os, biotite and alunite 40Ar/39Ar, and titanite U-Pb geochronology. Most of these hydrothermal events show a spatio-temporal correlation with porphyry intrusion
emplacement as constrained by zircon U-Pb ages. The total age range for these events spans the 23.5–6.1 Ma period, without
displaying systematic along- or across-arc age distribution trends. While epithermal deposits tend to be spatially associated
with volcanic rocks of a similar age, porphyry Cu deposits in Ecuador are frequently spatially associated with deeper-seated
basement units and batholith-scale precursor intrusive systems assembled over ≥5 m.y. time periods. In most cases, formation
of the porphyry Cu deposits is related to the youngest magmatic (-hydrothermal) event in a given area, postdating batholith
construction at a regional scale. The majority of Miocene deposits occurs in southern Ecuador where areally extensive, post-mineralization
(late Miocene to recent) volcanic sequences with the potential to conceal mineralization at depth are lacking. Only few Miocene
deposits occur in northern-central Ecuador, where they mainly crop out in the Western Cordillera, west of the productive present-day
volcanic arc. The surface distribution of post-mineralization arc volcanism reflects along-arc variations in subducting slab
geometry. Porphyry Cu and epithermal deposits in Ecuador define a Miocene metallogenic belt broadly continuous with its coeval
counterpart in northern-central Peru. Although both belt segments were formed in an overall similar tectonomagmatic and metallogenic
setting, their respective metal endowments differ significantly. 相似文献
12.
Iodine enrichment in the Atacama Desert of northern Chile is widespread and varies significantly between reservoirs, including nitrate-rich “caliche” soils, supergene Cu deposits and marine sedimentary rocks. Recent studies have suggested that groundwater has played a key role in the remobilization, transport and deposition of iodine in Atacama over scales of millions-of-years. However, and considering that natural waters are also anomalously enriched in iodine in the region, the relative source contributions of iodine in the waters and its extent of mixing remain unconstrained. In this study we provide new halogen data and isotopic ratios of iodine (129I/I) in shallow seawater, rivers, salt lakes, cold and thermal spring water, rainwater and groundwater that help to constrain the relative influence of meteoric, marine and crustal sources in the Atacama waters. Iodine concentrations in surface and ground waters range between 0.35 μM and 26 μM in the Tarapacá region and between 0.25 μM and 48 μM in the Antofagasta region, and show strong enrichment when compared with seawater concentrations (I = ∼0.4 μM). In contrast, no bromine enrichment is detected (1.3–45.7 μM for Tarapacá and 1.7–87.4 μM for Antofagasta) relative to seawater (Br = ∼600 μM). These data, coupled to the high I/Cl and low Br/Cl ratios are indicative of an organic-rich sedimentary source (related with an “initial” fluid) that interacted with meteoric water to produce a mixed fluid, and preclude an exclusively seawater origin for iodine in Atacama natural waters. Iodine isotopic ratios (129I/I) are consistent with halogen chemistry and confirm that most of the iodine present in natural waters derives from a deep initial fluid source (i.e., groundwater which has interacted with Jurassic marine basement), with variable influence of at least one atmospheric or meteoric source. Samples with the lowest isotopic ratios (129I/I from ∼215 to ∼1000 × 10−15) strongly suggest mixing between the groundwater and iodine storage in organic-rich rocks (with variable influence of volcanic fluids) and pre-anthropogenic meteoric water, while samples with higher values (∼2000–93,700 × 10−15) indicate the input of anthropogenic meteoric fluid. Taking into account the geological, hydrologic and climatic features of the Atacama region, we propose that the mean contribution of anthropogenic 129I is associated with 129I releases during nuclear weapon tests carried out in the central Pacific Ocean until the mid 1990's (129I/I = ∼12,000 × 10−15). This source reflects rapid redistribution of this radioisotope on a global scale. Our results support the notion of a long-lived continental iodine cycle in the hyperarid margin of western South America, which is driven by local hydrological and climate conditions, and confirm that groundwater was a key agent for iodine remobilization and formation of the extensive iodine-rich soils of Atacama. 相似文献
13.
Evaluation of groundwater quality in coastal areas: implications for sustainable agriculture 总被引:4,自引:1,他引:4
Seawater intrusion is a problem in the coastal areas of Korea. Most productive agricultural fields are in the western and
southern coastal areas of the country where irrigation predominantly relies on groundwater. Seawater intrusion has affected
agricultural productivity. To evaluate progressive encroachment of saline water, the Korean government established a seawater
intrusion monitoring well network, especially in the western and southern part of the peninsula. Automatic water levels and
EC monitoring and periodic chemical analysis of groundwater help track salinization. Salinization of fresh groundwater is
highly associated with groundwater withdrawal. A large proportion of the groundwaters are classified as Na–Cl and Ca–Cl types.
The Na–Cl types represent effects of seawater intrusion. The highest EC level was over 1.6 km inland and high Cl values were
observed up to 1.2 km inland. Lower ratios of Na/Cl and SO4/Cl than seawater values indicate the seawater encroachment. A linear relation between Na and Cl represents simple mixing
of the fresh groundwater with the seawater. The saline Na–Cl typed groundwaters showed Br/Cl ratios similar to or less than
seawater values. The Ca–HCO3 type groundwaters had the highest Br/Cl ratios. Substantial proportions of the groundwaters showed potential for salinity
and should be better managed for sustainable agriculture. 相似文献
14.
Nicholas Culshaw Emilia Mosonyi Peter Reynolds 《International Journal of Earth Sciences》2012,101(1):291-306
The Rodna Mountains afford the most internal structural window into the crystalline units of the Eastern Carpathians in Romania.
The Rodna Mountains consist of Variscan metamorphic nappes that were restacked in the Alpine phase of Carpathian development
forming the Subbucovinian and Infrabucovinian nappes. In order to evaluate age of deformation, ten samples were taken from
the zone of greenschist facies mylonitic schist that marks the Alpine tectonic boundary between the Subbucovinian and Infrabucovinian
nappes and 40Ar/39Ar laser single-grain ages determined for schistosity-forming muscovite. Microstructural assessment of quartz and muscovite
distinguished two deformation events. Single-grain ages from the microstructurally most strongly reworked samples (four samples)
give a tight clustering of ages at ca. 95 Ma. The least reworked schists have a broader clustering of ages spanning ca. 200–280 Ma
with a late Permian peak and some samples showing outlier ages in the range 200–100 Ma. The relative development of the outliers,
which correlates with evidence for increased microstructural reworking, is interpreted to mark progressive isotopic resetting.
The ca. 95 Ma ages for the most reworked schists are estimates for the age of the Alpine nappe stacking. The ca. 200–280 Ma
ages are similar to those of magmatism, metamorphism, and sedimentation thought to mark post-Variscan-pre-Alpine rifting and
ocean basin formation in parts of the Alps and may be the thermal imprint of a related event in the Eastern Carpathians. 相似文献
15.
Summary The complexation of aluminium(III) and silicon(IV) was studied in a simplified seawater medium (0.6 M Na(Cl)) at 25 °C. The
measurements were performed as potentiometric titrations using a hydrogen electrode with OH
− ions being generated coulometrically. The total concentrations of Si(IV) and Al(III) respectively [Si
tot
] and [Al
t
ot], and −log[H
+] were varied within the limits 0.3 < [Si
tot
] < 2.5 mM, 0.5 < [Al
tot
] < 2.6 mM, and 2 ≤ -log[H
+] ≤ 4.2. Within these ranges of concentration, evidence is given for the formation of an AlSiO(OH)
3
2+
complex with a formation constant log β1,1-1 = −2.75 ± 0.1 defined by the reaction
Al
3++Si (OH)4 ↔ AlOSi(OH)
3
2+
+H
+
An extrapolation of this value to I=0 gives log β1,1-1 = −2.30. The calculated value of log K (Al
3++SiO(OH)
3
−
↔ AlOSi(OH)
3
2+
) = 6.72 (I=0.6 M) can be compared with corresponding constants for the formation of AlF
2+ and AlOH
2+ , which are equal to 6.16 and 8.20. Obviously, the stability of these Al(III) complexes decreases within the series OH
−>SiO(OH)
3
−
> F
− 相似文献
16.
Several F, Pb, Zn and Ba deposits are located in the province of Zaghouan in north-eastern Tunisia. They are hosted in Lower
Liassic or Upper Jurassic reef limestones, and the overlying condensed Carixian phosphatic limestones and Campanian marls,
respectively. The mineralization occurs in three types of orebodies: stratiform replacement heaps and lenses (Jebel Stah and
Hammam Zriba), breccia fillings and dissolution void fillings (Sidi Taya) and lodes (Jebel Oust). More than one generation
of fluorite is observed in the stratiform deposits. Microthermometric analyses of the inclusion fluids observed in fluorite
and quartz show that the economic concentrations of fluorite have deposited from moderate to highly saline (12–22.5 wt% NaCl
equivalents) hydrothermal (110–160 °C) mineralizing fluids at the center (Jebel Stah, Sidi Taya) and to the east of the province
(Hammam Zriba). Late remobilizations, observed in the stratiform deposits, are related to the circulation of a warmer (up
to 185 °C) but less saline (10 wt% NaCl equivalents) fluid (Jebel Stah) and more saline (12–22 wt% NaCl equivalents) fluid
(Hammam Zriba). The highest temperature (up to 250 °C) and salinity (32–34 wt% NaCl equivalents) are observed to the west
of the province of Zaghouan (Jebel Oust). Less saline (3–6 wt% NaCl equivalents) and moderately hot to hot fluids (up to 220 ± 20 °C)
and rich in gaseous CO2 invade most of the ore deposits in later stages and give rise to the massive quartz within fractures at Jebel Stah. Chemical
analyses of the fluids extracted from the inclusions occuring in fluorite show compositions dominated by the presence of Na+, Ca2+ and Cl− ionic species and allow the mean temperature of the fluids in the source reservoir to be estimated as 275 ± 25 °C. The circulation
of the ore-forming fluids is triggered by a regional tectonic extensional phase which occurs within the post-Jurassic to ante-Miocene
time interval. The deposition of the economic concentrations of fluorite resulted from the decrease in pressure and temperature
of the hydrothermal brines (Jebel Oust), along with the increase in the dissolved calcium activity (Jebel Stah and Sidi Taya),
or a decrease in salinity due to the mixing with a hot, less saline and Na-poor, Ca-rich fluid (Hammam Zriba). The mineralogical
associations (CaF2, PbS, ZnS, BaSO4) hosted within carbonate rocks, the temperatures and the salinities of the fluids that gave rise to the more important ore
deposits (110–160 °C and 12–22.5 wt% NaCl equivalents), their composition (Na, Ca, Cl) and the molar ratios between the major
ionic species, as well as the presence of liquid hydrocarbons in the mineralizing fluids, show that the ore deposits of the
province of Zaghouan belong to the carbonate-hosted F, Pb, Zn, Ba Mississippi Valley-type deposits.
Received: 23 June 1995 / Accepted: 18 November 1996 相似文献
17.
John R. Bowman John W. Valley Noriko T. Kita 《Contributions to Mineralogy and Petrology》2009,157(1):77-93
Oxygen isotope ratios have been measured by ion microprobe and millimeter-scale dental drill along detailed sampling traverses
across the boundary between periclase-bearing (δ18O = 11.8‰) and periclase-free (δ18O = 17.2‰) marble layers in the periclase (Per) zone of the Alta Stock aureole, Utah. These data define a steep, coherent
gradient in δ18O that is displaced a short distance (~4 cm) into the periclase-free (Cal + Fo) layer. SEM and ion microprobe analyses show
two isotopically and texturally distinct types of calcite at the grain scale. Clear (well polished) calcite grains are isotopically
homogeneous (within analytical uncertainty; ±0.27‰, 2SD). More poorly polished (pitted), texturally retrograde ‘turbid’-looking
calcite has lower and more variable δ18O values, and replaces clear calcite along fractures, cleavage traces or grain boundaries. Despite significant lowering of
the δ18O values in calcite throughout both layers during prograde metamorphism, ion microprobe analyses indicate that individual
clear calcite grains are now isotopically homogeneous across the entire gradient in δ18O. Diffusion calculations indicate that conservative time scales required for isotopic homogenization of calcite grains by
volume diffusion, 30,000–62,000 years at 575–600°C, exceed significantly the timescale (~1,250 years) estimated for the prograde
development of the δ18O gradient at the boundary between these two marble layers. The ion microprobe data and these diffusion calculations suggest
instead that surface reaction mechanisms accompanying recrystallization are responsible for the observed oxygen isotope homogeneity
of these calcite grains. Thus, the ion microprobe data are consistent with the formation of calcite in oxygen isotope exchange
equilibrium with infiltrating fluid during prograde reaction and recrystallization.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
18.
The Chengchao and Jinshandian deposits in the southeast Hubei Province are the two largest skarn Fe deposits in the Middle–Lower Yangtze River Valley metallogenic belt (MLYRVMB), China. They are characterized by NW-striking orebodies that are developed along the contacts between the Late Mesozoic granitoid and Triassic carbonate and clastic rocks. New sensitive high-resolution ion microprobe and laser ablation inductively coupled plasma mass spectrometry zircon U–Pb dating of the mineralization-related quartz diorite and granite at Chengchao yield ages of 129 ± 2 and 127 ± 2 Ma, respectively, and those at Jinshandian of 127 ± 2 and 133 ± 1 Ma, respectively. These results are interpreted as the crystallization age of these intrusions. Hydrothermal phlogopite samples from the skarn ores at Chengchao and Jinshandian have the plateau 40Ar–39Ar ages of 132.6 ± 1.4 and 131.6 ± 1.2 Ma, respectively. These results confirm that both intrusions and associated skarn Fe mineralization were formed contemporaneously in the middle Early Cretaceous time. New zircon U–Pb and phlogopite 40Ar–39Ar ages in this study, when combined with available precise geochronological data, demonstrate that there were two discontinuous igneous events, corresponding to two episodes of skarn Fe-bearing mineralization in the southeast Hubei Province: (1) 140–136 Ma diorites and quartz diorites and 141–137 Ma skarn Cu–Fe or Fe–Cu deposits and (2) 133–127 Ma quartz diorites and granites and 133–132 Ma skarn Fe deposits. This scenario is similar to that proposed for the entire MLYRVMB. The intrusions related to skarn Fe deposits show obviously petrological and geochemical differences from those related to skarn Cu–Fe or Fe–Cu deposits. The former are quartz diorite and diorite in petrology and have similar adakitic geochemical signatures and in equilibrium with a garnet-rich residue, whereas the latter are petrologically granite and quartz diorite that are distinguishable from adakitic rocks and in equilibrium with a plagioclase residue. These features indicated that two episodes of magmatism and the formation of skarn Fe-bearing deposits in the southeast Hubei Province, MLYRVMB, might be associated lithosphere thinning induced by asthenosphere upwelling during the Late Mesozoic. 相似文献
19.
William Sidle 《Environmental Geology》2009,57(2):377-388
Recent identification of elevated excess 210Pb (≤302.6 mBq L−1) and 137Cs (≤111.3 mBq L−1) activity in drinking water wells up to 20 m depth indicates some transport of airborne radionuclide fallout beyond soils
in the Shaker Village catchment, Maine. Estimated airborne mass loading 210Pbex fluxes of about 0.9 mBq m−3, canvass this headwater catchment and may be sufficient to pose risks to unprotected shallow wells. Inventories of 210Pbex and 137Cs in pond sediments indicate maximum median activities of 943 mBq g−1 and 40.0 mBq g−1, respectively. Calculated 210Pbex fluxes in the catchment soils range from 0.62–0.78 Bq cm−2 year−1 and yield a mean residence time of near 140 years. Measured 137Cs activity up to 51.1 mBq g−1 occurs in sediments at least to 5 m depth. Assumed particle transport in groundwater with apparent 85Kr ages less than 5 years BP (2005) may explain the correlation between these particle-reactive radionuclides and elevated
activity in some drinking water wells. 相似文献
20.
Koen de Jong Chikao Kurimoto Gilles Ruffet 《International Journal of Earth Sciences》2009,98(6):1529-1556
The 218.4 ± 0.4, 228.8 ± 0.9 and 231.9 ± 0.7 Ma 40Ar/39Ar laser probe pseudo-plateau ages (2σ; 49–63% 39Ar-release) of very low-grade meta-pelitic whole-rocks from the Sakaigawa unit date high-P/T metamorphism. We argue that this
event occurred in a subduction–accretion complex, not along the East Asian continental margin, but on the Pacific side of
the proto-Japan superterrane. Proto-Japan was a Permian magmatic arc, presently dispersed in the Japanese islands, which also
contained older subduction–accretion complexes. The arc system was fringing but not yet part of the Eurasian continent. The
Middle to Late Triassic high-P/T tectono-metamorphic event was partly coeval with proto-Japan’s collision with proto-Eurasia
along the southward extension of the Central Asian Orogenic Belt, causing the main metamorphism in the Hida-Oki terrane. It
is possible that this system continued via the Cathaysia block (China) to Indochina. The Late Permian to Middle Triassic Indosinian
event might stem from docking of Pacific-derived terranes with Southeast Asia’s continental margin. The concept of the proto-Japan
superterrane implies that the Qinling-Dabie-Sulu suture zone joined the Central Asian Orogenic Belt to the east of the North
China craton and did not continue to Japan, as commonly assumed. 相似文献