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1.
Re–Os isotope compositions of mantle-derived magmas are highly sensitive to crustal contamination because the crust and mantle have very different Os isotope compositions. Crustal contamination may trigger S saturation and thus the formation of magmatic Ni–Cu–(PGE) sulfide deposits. The ∼287-Ma Kalatongke norite intrusion of NW China are hosted in carboniferous tuffaceous rocks and contain both disseminated and massive sulfide mineralization. The Re–Os isotope compositions in the intrusion are highly variable. Norite and massive sulfide ores have γ Os values ranging from +59 to +160 and a Re–Os isochron age of 239 ± 51 Ma, whereas disseminated sulfide ores have γ Os values from +117 to +198 and a Re–Os isochron age of 349 ± 34 Ma. The variability of Os isotope compositions can be explained as the emplacement of two distinct magma pulses. Massive sulfide ores and barren norite in the intrusion formed from the same magma pulse, whereas the disseminated sulfide ores with more radiogenic Os isotopes formed from another magma pulse which underwent different degrees of crustal contamination. Re–Os isotopes may not be suitable for dating sulfide-bearing intrusions that underwent variable degrees of crustal contamination to form magmatic sulfide deposits. 相似文献
2.
煤中黄铁矿的铼-锇同位素含量及其地质意义 总被引:2,自引:0,他引:2
采用Carius管溶样方法,通过热电离质谱对淮北煤田煤中黄铁矿样品的Re、Os含量及其同位素进行了测定,得出黄铁矿样品中Re和Os含量分别为1·22~1·29ng/g和0·0046~0·0054ng/g。对两个样品同位素定年测定得出,其年龄值分别为(73·9±3·2)Ma和(33±9)Ma,两个样品的年龄相差约258~286Ma。含量和同位素年龄差值表明两个样品是不同时代形成的黄铁矿。Re-Os同位素体系比值揭示黄铁矿所赋存的地质体受到来自地壳物质的显著混染。另外,γOTs参数也证实了这一点。该参数分别为+17和+18,表明了有富含Re母体的地壳物质的加入,这为煤中Re-Os含量及地质理论研究提供了参考资料。 相似文献
3.
滇东北会泽超大型铅锌矿Re-Os同位素特征及喜山期成矿作用动力学背景探讨 总被引:2,自引:0,他引:2
会泽超大型铅锌矿是滇东北铅锌多金属成矿域中典型的密西西比河谷型(MVT)或会泽型(HZT)矿床,因其独特的成矿系统以及矿床中富锗而被地质学者熟知,由于该类型矿床成矿温度较低且缺少合适的定年矿物,其成矿时代一直存在较大的争议。本文在会泽铅锌矿麒麟厂矿区1584中段0-11号穿脉坑道块状铅锌硫化物矿石中挑选了9件硫化物样品(黄铁矿、方铅矿和闪锌矿),采用负离子热表面电离质谱法进行Re-Os同位素分析,获得Re-Os等时线年龄为40.7±2.6 Ma(n=9),与模式年龄加权平均值40.0±2.6Ma(n=8)在误差内完全一致,闪锌矿和方铅矿模式年龄分别为38.24±0.41 Ma和36.57±0.40 Ma。上述同位素年龄揭示了会泽超大型铅锌矿的成矿时代可能为始新世。结合滇东北铅锌矿集区NE向逆冲断层和冲断褶皱控矿构造区域构造解析以及断裂、矿体构造-岩相蚀变特征,提出会泽超大型铅锌矿经历了燕山期、喜山期两阶段构造-流体贯入的成矿作用模型。 相似文献
4.
K. N. Malitch T. Auge I. Yu. Badanina M. M. Goncharov S. A. Junk E. Pernicka 《Mineralogy and Petrology》2002,76(1-2):121-148
Summary ?Geological, mineralogical and Os isotopic data for detrital PGE-mineralization derived from the Guli and Bor-Uryah ultramafic
massifs, within the Maimecha-Kotui Province (the northern part of the Siberian Platform, Russia), are presented for the first
time. The detrital platinum-group minerals (PGM) are dominated by Os–Ir–(Ru) species, which is typical for ophiolites or Alpine-type
complexes. However, the PGM assemblage in the placers investigated is similar to that derived from zoned platiniferous clinopyroxenite–dunite
massifs (also known as Uralian-, Alaskan-type and Aldan-type massifs). The unique features of the Au-PGE placers at Guli are
(1) the dominance of Os-rich alloys over other PGM and Au, and (2) the considerable predicted resources of noble metals, particularly
osmium.
Dominant chromite, olivine and clinopyroxene inclusions recorded in Os–Ir–(Ru) alloys imply that they were derived from ultramafic
sources (e.g., chromitite, dunite and clinopyroxenite). The first in situ osmium-isotope measurements by laser ablation -
multiple collector - inductively coupled plasma mass spectrometry of different, intimately intergrown, PGM (e.g., laurite
and Os-rich alloys) in various nuggets from Guli have revealed low 187Os/188Os and γOs values. They yield a very narrow range of 187Os/188Os (0.12432 to 0.12472) and γOs (− 2.39 to − 2.07). These values are indicative of a common chondritic or subchondritic mantle
source of PGE. 187Os/188Os and γOs values of Os-rich alloys, derived from the Bor-Uryah massif, are different (i.e., γOs ranges from − 2.67 to − 1.30).
The mineral-isotopic data obtained are consistent with the conclusion that the PGM were derived from parent ultramafic source
rocks. Os-isotope model ages in the range of 495 to 240 Ma constrain the age of ultramafic protoliths in the northern part
of the Siberian Craton. The variation in 187Os/188Os values for detrital PGM, where the provenance source is unknown, is considered to be a useful technique for distinguishing
parent bedrock sources.
Received July 12, 2001; revised version accepted December 27, 2001 相似文献
5.
Seven 187Re-187Os ages were determined for molybdenite and pyrite samples from two well-dated Precambrian intrusions in Fennoscandia to examine
the sustainability of the Re-Os chronometer in a metamorphic and metasomatic setting. Using a new 187Re decay constant (1.666 × 10−11y−1) with a much improved uncertainty (±0.31%), we determined replicate Re-Os ages for molybdenite and pyrite from the Kuittila
and Kivisuo prospects in easternmost Finland and for molybdenite from the Kabeliai prospect in southernmost Lithuania. These
two localities contain some of the oldest and youngest plutonic activity in Fennoscandia and are associated with newly discovered
economic Au mineralization (Ilomantsi, Finland) and a Cu-Mo prospect (Kabeliai, Lithuania). Two Re-Os ages for vein-hosted
Kabeliai molybdenite average 1486 ± 5 Ma, in excellent agreement with a 1505 ± 11 Ma U-Pb zircon age for the hosting Kabeliai
granite pluton. The slightly younger age suggests the introduction of Cu-Mo mineralization by a later phase of the Kabeliai
magmatic system. Mean Re-Os ages of 2778 ± 8 Ma and 2781 ± 8 Ma for Kuittila and Kivisuo molybdenites, respectively, are in
reasonable agreement with a 2753 ± 5 Ma weighted mean U-Pb zircon age for hosting Kuittila tonalite. These Re-Os ages agree
well with less precise ages of 2789 ± 290 Ma for a Rb-Sr whole-rock isochron and 2771 ± 75 Ma for the average of six Sm-Nd
TDM model ages for Kuittila tonalite. Three Re-Os analyses of a single pyrite mineral separate, from the same sample of Kuittila
pluton that yielded a molybdenite separate, provide individual model ages of 2710 ± 27, 2777 ± 28, and 2830 ± 28 Ma (Re = 17.4,
12.1, and 8.4 ppb, respectively), with a mean value of 2770 ± 120 Ma in agreement with the Kuittila molybdenite age. The Re
and 187Os abundances in these three pyrite splits are highly correlated (r = 0.9994), and provide a 187Re-187Os isochron age of 2607 ± 47 Ma with an intercept of 21 ppt 187Os (MSWD = 1.1). It appears that the Re-Os isotopic system in pyrite has been reset on the millimeter scale and that the 21 ppt 187Os intercept reflects the in situ decay of 187Re during the ∼160 to 170 m.y. interval from ∼2778 Ma (time of molybdenite ± pyrite deposition) to ∼2607 Ma (time of pyrite
resetting). When the Re-Os data for molybdenites from the nearby Kivisuo prospect are plotted together with the Kuittila molybdenite
and pyrite data, a well-constrained five-point isochron with an age of 2780 ± 8 Ma and a 187Os intercept (−2.4 ± 3.8 ppt) of essentially zero results (MSWD = 1.5). We suggest that the pyrite isochron age records a
regional metamorphic and/or hydrothermal event, possibly the time of Au mineralization. A proposed Re-Os age of ∼2607 Ma for
Au mineralization is in good agreement with radiometric ages by other methods that address the timing of Archean Au mineralization
in deposits worldwide (so-called “late Au model”). Molybdenite, in contrast, provides a robust Re-Os chronometer, retaining
its original formation age of ∼2780 Ma, despite subsequent metamorphic disturbances in Archean and Proterozoic time.
Received: 25 September 1996 / Accepted: 27 August 1997 相似文献
6.
Re-Os isotopes were used to constrain the source of the ore-forming elements of the Tharsis and Rio Tinto mines of the Iberian
Pyrite Belt, and the timing of mineralization. The pyrite from both mines has simila]r Os and Re concentrations, ranging between
0.05–0.7 and 0.6–66 ppb, respectively. 187Re/188Os ratios range from about 14 to 5161. Pyrite-rich ore samples from the massive ore of Tharsis and two samples of stockwork
ore from Rio Tinto yield an isochron with an age of 346 ± 26 Ma, and an initial 187Os/188Os ratio of about 0.69. Five samples from Tharsis yield an age of 353 ± 44 Ma with an initial 187Os/188Os ratio of about 0.37. A sample of massive sulfide ore from Tharsis and one from Rio Tinto lie well above both isochrons
and could represent Re mobilization after mineralization. The pyrite Re-Os ages agree with the paleontological age of 350 Ma
of the black shales in which the ores are disseminated. Our data do not permit us to determine whether the Re-Os isochron
yields the original age of ore deposition or the age of the Hercynian metamorphism that affected the ores. However, the reasonable
Re-Os age reported here indicates that the complex history of the ores that occurred after the severe metamorphic event that
affected the Iberian Pyrite Belt massive sulfide deposits did not fundamentally disturb the Re-Os geochronologic system. The
highly radiogenic initial Os isotopic ratio agrees with previous Pb isotopic studies. If the initial ratio is recording the
initial and not the metamorphic conditions, then the data indicate that the source of the metals was largely crustal. The
continental margin sediments that underlie the deposits (phyllite-quartzite group) or the volcanic rocks (volcanogenic-sedimentary
complex) in which the ores occur are plausible sources for the ore-forming metals and should constrain the models for the
genesis of these deposits.
Received: 15 March 1999 / Accepted: 26 July 1999 相似文献
7.
Stable isotope geochemistry and diagenetic mineralization associated with the Tono sandstone-type uranium deposit in Japan 总被引:5,自引:0,他引:5
The Tono sandstone-type uranium mine area, middle Honsyu, Japan is composed of Miocene lacustrine sedimentary rocks in the
lower part (18–22 Ma) and marine facies in the upper part (15–16 Ma). Calcite and pyrite occur as dominant diagenetic alteration
products in these Neogene sedimentary rocks. The characteristics of calcite and pyrite differ significantly between lacustrine
and marine facies. Abundant pyrite, calcite, organic matter, and small amounts of marcasite or pyrrhotite occur in the lacustrine
facies, whereas small amounts of calcite and framboidal pyrite, organic matter and no marcasite or pyrrhotite are found within
the marine units. The δ13C values of calcite in the lacustrine deposits are low (−19 to −6‰ PDB) but those in marine formation are high (−11 to +3‰).
This implies that the contribution of marine carbonate is larger in upper marine sedimentary rocks, and carbon in calcite
in the lower lacustrine formation was derived both from oxidation of organic matter and from dissolved marine inorganic carbon.
The δ34S values of framboidal pyrite in the upper marine formation are low (−14 to −8‰ CDT), indicating a small extent of bacterial
seawater sulfate reduction, whereas those of euhedral-subhedral pyrite in the lower lignite-bearing arkose sandstone are high
(+10 to +43‰), implying a large extent of closed-system bacterial seawater sulfate reduction. The δ34S and δ13C data which deviate from a negative correlation line toward higher δ13C values suggest methanogenic CO2 production. During diagenesis of the lacustrine unit, large amounts of euhedral-subhedral pyrite were formed, facilitated
by extensive bacterial reduction of seawater sulfate with concomitant oxidation of organic matter, and by hydrolysis reactions
of organic matter, producing CH4 and CO2. Uranium minerals (coffinite and uraninite) were also formed at this stage by the reduction of U6+ to U4+. The conditions of diagenetic alteration within the lacustrine deposits and uranium mineralization is characterized by low
Eh in which nearly equal concentrations of CH4 and HCO3
− existed and reduced sulfur species (H2S, HS−) are predominant among aqueous sulfur species, whereas diagenetic alteration of the marine formations was characterized by
a predominance of SO4
2− among dissolved sulfur species. Modern groundwater in the lacustrine formation has a low Eh value (−335 mV). Estimated and
measured low Eh values of modern and ancient interstitial waters in lacustrine environments indicate that a reducing environment
in which U4+ is stable has been maintained since precipitation of uranium minerals.
Received: 9 February 1996 / Accepted: 11 April 1997 相似文献
8.
Distribution of platinum-group elements and Os isotopes in chromite ores from Mayarí-Baracoa Ophiolitic Belt (eastern Cuba) 总被引:1,自引:0,他引:1
The Mayarí-Baracoa ophiolitic belt in eastern Cuba hosts abundant chromite deposits of historical economic importance. Among
these deposits, the chemistry of chromite ore is very variable, ranging from high Al (Cr#=0.43–0.55) to high Cr (Cr#=0.60–0.83)
compositions. Platinum-group element (PGE) contents are also variable (from 33 ppb to 1.88 ppm) and correlate positively with
the Cr# of the ore. Bulk PGE abundances correlate negatively with the Pd/Ir ratio showing that chromite concentrates mainly
Os, Ir and Ru which gives rise to the characteristic negatively sloped, chrondrite-normalized PGE patterns in many chromitites.
This is consistent with the mineralogy of PGEs, which is dominated by members of the laurite–erlichmanite solid solution series
(RuS2–OsS2), with minor amounts of irarsite (IrAsS), Os–Ir alloys, Ru–Os–Ir–Fe–Ni alloys, Ni–Rh–As, and sulfides of Ir, Os, Rh, Cu,
Ni, and/or Pd. Measured 187Os/188Os ratios (from 0.1304 to 0.1230) are among the lower values reported for podiform chromitites. The 187Os/188Os ratios decrease with increasing whole-rock PGE contents and Cr# of chromite. Furthermore, γOs values of all but one of
the chromitite samples are negative indicating a subchondiritc mantle source. γOs decrease with increasing bulk Os content
and decreasing 187Re/188Os ratios. These mineralogical and geochemical features are interpreted in terms of chromite crystallization from melts varying
in composition from back-arc basalts (Al-rich chromite) to boninites (Cr-rich chromite) in a suprasubduction zone setting.
Chromite crystallization occurs as a consequence of magma mixing and assimilation of preexisting gabbro sills at the mantle–crust
transition zone. Cr#, PGE abundances, and bulk Os isotopic composition of chromitites are determined by the combined effects
of mantle source heterogeneity, the degree of partial melting, the extent of melt-rock interactions, and the local sulfur
fugacity. Small-scale (μm to cm) chemical and isotopic heterogeneities in the platinum-group minerals are controlled by the
mechanism(s) of chromite crystallization in a heterogeneous environment created by the turbulent regime generated by successive
inputs of different batches of melt. 相似文献
9.
The sulfur isotopic composition of sulfides and barite from hydrothermal deposits at the Valu Fa Ridge back-arc spreading
center in the southern Lau Basin has been investigated. Sulfide samples from the White Church area at the northern Valu Fa
Ridge have δ34S values averaging +3.8‰ (n= 10) for bulk sphalerite-chalcopyrite mineralization and +4.8‰ for pyrite (n= 10). Barite associated with the massive sulfides exhibits an average of +20.7‰ (n= 10). Massive sulfides from the active Vai Lili hydrothermal field at the central Valu Fa Ridge have much higher δ34S ratios averaging +8.0‰ for bulk sphalerite-chalcopyrite mineralization (n= 5), +9.3‰ for pyrite samples (n= 5), and +8.0‰ and +10.9‰ for a chalcopyrite and a sphalerite separate, respectively. The isotopic composition of barite
from the Vai Lili field is similar to that of barite from the White Church area and averages +21.0‰ (n= 8). Sulfide and barite samples from the Hine Hina area at the southern Valu Fa Ridge have δ34S values that are considerably lighter than those observed for samples from the other areas and average −4.9‰ for pyrite (n= 9), −4.0 and −5.7‰ for two samples of sphalerite-chalcopyrite intergrowth, and −3.4‰ for a single chalcopyrite separate.
The total spread in the isotopic composition of sulfides from Vai Lili and Hine Hina is more than 20‰ over a distance of less
than 30 km. The δ34S values of sulfides at Hine Hina are the lowest values so far reported for volcanic-hosted polymetallic massive sulfides
from the modern seafloor. Barite from the Hine Hina field also has unusually light sulfur with δ34S values of +16.1 to +16.7‰ (n= 5). Isotopic compositions of the sulfides at Hine Hina indicate a dramatic decrease in δ34S from ordinary magmatic values and, in the absence of biogenic sulfur and/or boiling, imply a unique 34S-depleted source of probable magmatic origin. Sulfide-barite mineralization in the Hine Hina area is associated with a distinctive
alteration assemblage consisting of cristobalite, pyrophyllite, kaolinite, opal-CT, talc, pyrite, native sulfur, and alunite.
Similar styles of alteration are typically known from high-sulfidation epithermal systems on land. Alunite-bearing, advanced
argillic alteration in the Hine Hina field confirms the role of acidic, volatile-rich fluids, and a δ34S value of +10.4‰ for the sulfur in the alunite is consistent with established kinetic isotope effects which accompany the
disproportionation of magmatic SO2 into H2S and H2SO4. The Hine Hina field occurs near the propagating tip of the Valu Fa back-arc spreading center (i.e., dominated by dike injections
and seafloor eruptions) and therefore may have experienced the largest contribution of magmatic volatiles of the three fields.
The sulfur isotopic ratios of the hydrothermal precipitates and the presence of a distinctive epithermal-like argillic alteration
in the Hine Hina field suggest a direct contribution of magmatic vapor to the hydrothermal system and support the concept
that magmatic volatiles may be an important component of some volcanogenic massive sulfide-forming hydrothermal systems.
Received: 16 January 1997 / Accepted: 28 October 1997 相似文献
10.
The strata-bound Cu−Pb−Zn polymetallic sulfide deposits occur in metamorphic rocks of greenschist phase of the middle-upper
Proterozoic Langshan Group in central Inner Mongolia. δ34S values for sulfides range from −3.1‰ to +37.3‰, and an apparent difference is noticed between vein sulfides and those in
bedded rocks. For example, δ34S values for bedded pyrite range from +10.6‰ to +20.0‰, while those for vein pyrite vary from −3.1‰ to +14.1‰. δ34S of bedded pyrrhotite is in the range +7.9‰–+23.5‰ in comparison with +6.5‰–+17.1‰ for vein pyrrhotite. The wide scatter
of δ34S and the enrichment of heavier sulfur indicate that sulfur may have been derived from H2S as a result of bacterial reduction of sulfates in the sea water. Sulfur isotopic composition also differs from deposit to
deposit in this area because of the difference in environment in which they were formed. The mobilization of bedded sulfides
in response to regional metamorphism and magmatic intrusion led to the formation of vein sulfides.
δ18O and δ13C of ore-bearing rocks and wall rocks are within the range typical of ordinary marine facies, with the exception of lower
values for ore-bearing marble at Huogeqi probably due to diopsidization and tremalitization of carbonate rocks.
Pb isotopic composition is relatively stable and characterized by lower radio-genetic lead. The age of basement rocks was
calculated to be about 23.9 Ma and ore-forming age 7.8 Ma.207Pb/204Pb−206Pb/204Pb and208Pb/204Pb−206Pb/204Pb plots indicate that Pb may probably be derived from the lower crust or upper mantle.
It is believed that the deposits in this region are related to submarine volcanic exhalation superimposed by later regional
metamorphism and magmatic intrusion. 相似文献
11.
The sulfur isotopic composition of the Herrin (No. 6) Coal from several localities in the Illinois Basin was measured. The sediments immediately overlying these coal beds range from marine shales and limestones to non-marine shales. Organic sulfur, disseminated pyrite, and massive pyrite were extracted from hand samples taken in vertical sections.The values from low-sulfur coals (< 0.8% organic sulfur) underlying nonmarine shale were +3.4 to +7.3%0 for organic sulfur, +1.8 to +16.8%0 for massive pyrite, and +3.9 to +23.8%0 for disseminated pyrite. In contrast, the (> 0.8% organic sulfur) underlying marine sediments were more variable: organic sulfur, ?7.7 to +0.5%0, pyrites, ?17.8 to +28.5%0. In both types of coal, organic sulfur is typically enriched in 34S relative to pyritic sulfur.In general, δ 34S values increased from the top to the base of the bed. Vertical and lateral variations in δ 34S are small for organic sulfur but are large for pyritic sulfur. The sulfur content is relatively constant throughout the bed, with organic sulfur content greater than disseminated pyrite content. The results indicate that most of the organic sulfur in high-sulfur coals is derived from post-depositional reactions with a 34S-depleted source. This source is probably related to bacterial reduction of dissolved sulfate in Carboniferous seawater during a marine transgression after peat deposition. The data suggest that sulfate reduction occurred in an open system initially, and then continued in a closed system as sea water penetrated the bed.Organic sulfur in the low-sulfur coals appears to reflect the original plant sulfur, although diagenetic changes in content and isotopic composition of this fraction cannot be ruled out. The wide variability of the δ 34S in pyrite fractions suggests a complex origin involving varying extents of microbial H2S production from sulfate reservoirs of different isotopic compositions. The precipitation of pyrite may have begun soon after deposition and continued throughout the coalification process. 相似文献
12.
The giant chromite deposits at Kempirsai, Urals: constraints from trace element (PGE, REE) and isotope data 总被引:6,自引:0,他引:6
The investigation of stable and radiogenic isotopes and of platinum-group (PGE) and rare earth elements (REE) in chromitites
and associated ultramafic rocks of the Kempirsai Massif, southern Urals, gives strong evidence for a multistage formation
of giant ophiolitic-podiform chromite deposits present in the southeastern part of the massif. The Kempirsai ophiolite massif
is divided by a shear zone into two parts: in the northwestern area, small bodies of Al-rich chromite formed from basaltic
melts between 420 to 400 Ma, according to Sm-Nd mineral isochrons of harzburgite, pyroxenite, websterite and gabbro. Harzburgites
and pyroxenites in this area are enriched in light REE and have ɛNd(400) > +6 and ɛSr(400) ∼ +5. Chromitites have scattered PGE distributions (Pd/Ir, 0.4–7.0), being partly enriched in Pd and Pt. γOs(400) of one chromitite is −4.4. The southeastern part of the Kempirsai Massif, well-known for its world-class deposits of
podiform low-Al magnesiochromite, is characterized by harzburgite and dunite enriched in light REE with very low ɛNd(400) (+4.3 to –17.1) and positive ɛSr(400) (>+10) values. Chromitites are strongly enriched in Ir, Os and Ru and depleted in Pd and Pt. γOs(400) of three chromitites is uniform and approaches C1 and DMM compositions. In veins and pods postdating crystallization
of massive chromite, pargasitic amphibole formed in equilibrium with fluid-inclusion-bearing chromite at temperatures close
to 1000 °C. These amphiboles give 40Ar/39Ar stepwise heating ages of 365 to 385 Ma and are characterized by low ɛNd(400) (+0.6 to −4.6) and general enrichment in REE. The cooling ages correspond to a 379.3 ± 1.6 Ma Rb-Sr mineral isochron
produced from amphibole and phlogopite of a pyroxenite vein in the western part of the massif. From these data it is concluded
that parts of the Kempirsai Massif have been pervasively metasomatized by large amounts of fluids and melts derived from a
subducted slab composed of oceanic crust and sediments. Subduction occurred at least 15–35 Ma after a melting event that produced
a typical ophiolitic sequence in the Paleozoic Sakmara Zone. We conclude that large chromite orebodies formed from second-stage
high-Mg melts that interacted with depleted mantle and fluids on their way upward in a suprasubduction zone regime, and in
a fore-arc position to the Magnitogorsk island arc.
Received: 21 January 1998 / Accepted: 24 August 1998 相似文献
13.
Paul G. Spry Ryan D. Mathur Todd A. Bonsall Panagiotis Ch. Voudouris Vasilios Melfos 《Mineralogy and Petrology》2014,108(4):503-513
The Lavrion ore district contains carbonate-replacement and vein-type Pb–Zn–Ag deposits as well as low-grade porphyry Mo, Cu–Fe skarn, and minor breccia-hosted Pb–Zn–Cu sulfide mineralization. These ore types are spatially related to a Late Miocene granodiorite intrusion (7 to 10 Ma), and various sills and dikes of mafic to felsic composition. Samples of sphalerite and pyrite from the Ilarion carbonate replacement deposit, and galena from Vein 80 (vein-type mineralization) in the Adami deposit show heterogeneous Re–Os values. These values were partially disturbed by hydrothermal activity associated with the formation of hydrothermal veins (e.g., Vein 80). A plot of initial 187Os/188Os versus 1/Oscommon ratios for pyrite and sphalerite from the Ilarion deposit form a mixing line (r2?=?0.78) between high concentration crustal-like and low concentration mantle-like end-members, or two crustal end-members one of which was more radiogenic than the other. Based on the Re–Os systematics and previously published geological and geochemical evidence, the most plausible explanation for the Re–Os isotope data is that ore-forming components were derived from mixed sources, one of which was a radiogenic crustal source from schists and carbonates probably near intrusion centers and the other, intrusive rocks in the district that are less radiogenic. Although the Re and Os concentrations of galena from Vein 80 are above background values they cannot be used as a chronometer. However, the results of the current study suggest that although pyrite, sphalerite, and galena are poor geochronometers in this ore deposit, due to partial open-system behavior, they still yield valuable information on the origin of the source rocks in the formation of bedded replacement and vein mineralization in the Lavrion district. 相似文献
14.
The absolute timing of epigenetic mineralization, including most types of gold deposits, is difficult to resolve due to the
absence of suitable minerals in veins and replacement zones. However, gold is commonly closely associated with pyrite and
arsenopyrite, which may be amenable to Re–Os geochronology, providing sufficient Re and Os are present within them. This short
paper outlines the use of this method to date two gold deposits in Newfoundland using pyrite. Although the Os contents of
the pyrites are extremely low (≪0.1 ppb), the Os is almost exclusively radiogenic 187Os, and data are amenable to model age calculations, as used in Re–Os molybdenite dating. The pyrites from these deposits
correspond to low-level highly radiogenic sulphides, as defined by other studies. The Stog’er Tight and Pine Cove gold deposits
yield mean Re–Os model ages of 411 ± 7 Ma (n = 4) and 420 ± 7 Ma (n = 5), respectively, which agree with isochron regression of 187Os against 187Re. The Re–Os age for Stog’er Tight is within uncertainty of a previous U–Pb age from ‘hydrothermal’ zircon (420 ± 5 Ma) in
spatially related alteration. A latest Silurian–earliest Devonian age for the mineralization is consistent with indirect age
constraints from some other gold deposits in central Newfoundland and suggests a broad temporal link to the mid-Silurian Salinic
Orogeny. However, the gold mineralization appears to be younger than most plutonic activity associated with this event. The
results illustrate the potential value of Re–Os pyrite geochronology in understanding the temporal framework of epigenetic
mineralization, especially if future improvements in analytical precision and reductions in procedural blanks allow wider
application to material with similarly low Re and Os concentrations. 相似文献
15.
Melissa J. Gregory Bruce F. Schaefer Reid R. Keays Andy R. Wilde 《Mineralium Deposita》2008,43(5):553-573
The syn-tectonic breccia-hosted Mount Isa Cu deposit in northwest Queensland is the largest sediment-hosted Cu deposit in
Australia. Whole-rock samples of chalcopyrite-rich Cu ore form an isochron with a Re–Os age of 1,372 ± 41 Ma. This age is
more than 100 Ma younger than the previously accepted age of Cu ore formation, an Ar–Ar mineral age for biotite separated
from the host rocks within the alteration envelope to the Cu orebody. This discrepancy cannot be unequivocally resolved due
to a lack of other absolute geochronological constraints for Cu mineralisation or the deformation event associated with Cu
emplacement. The 1,372 ± 41 Ma date may reflect (a) the time of Cu deposition, (b) the time of a hydrothermal event that reset
the Re–Os signature of the Cu ore or (c) mixing of the Re–Os isotope systematics between the host rocks and Cu-bearing fluids.
However, a range of published Ar–Ar and Rb–Sr dates for potassic alteration associated with Cu mineralisation also records
an event between 1,350 and 1,400 Ma and these are consistent with the 1,372 Ma Re–Os age. The 1.8 Ga Eastern Creek Volcanics
are a series of tholeiitic basalts with a primary magmatic Cu enrichment which occur adjacent to the Mount Isa Cu deposit.
The whole-rock Os isotopic signature of the Eastern Creek Volcanics ranges from mantle-like values for the upper Pickwick
Member, to more radiogenic/crustal values for the lower Cromwell Member. The Re–Os isotope signature of the Cu ores overlaps
with those calculated for the two volcanic members at 1,372 Ma; hence, the Os isotope data are supportive of the concept that
the Os in the Cu ores was sourced from the Eastern Creek Volcanics. By inference, it is therefore postulated that the Eastern
Creek Volcanics are the source of Cu in the Mount Isa deposit, as both Os and Cu are readily transported by oxidised hydrothermal
fluids, such as those that are thought to have formed the Cu orebody. The Pickwick Member yields a Re–Os isochron age of 1,833 ± 51 Ma,
which is within error of previously reported age constraints. The initial 187Os/188Os isotopic ratio of 0.114 ± 0.067 (γOs = −0.7) is slightly subchondritic, and together with other trace element geochemical
constraints, is consistent with a subcontinental lithospheric mantle source. The Pickwick Member records a minimum age of
ca. 1.95 Ga for melt depletion in the subcontinental lithospheric mantle beneath the Mount Isa Inlier prior to the extraction
of the magmas which formed the Eastern Creek Volcanics. This corresponds with the end of subduction-related magmatism along
the eastern margin of the Northern Australian Craton, which included the Mount Isa Inlier. 相似文献
16.
Infrared microthermometric and stable isotopic study of fluid inclusions in wolframite at the Xihuashan tungsten deposit,Jiangxi province,China 总被引:7,自引:0,他引:7
The Xihuashan tungsten deposit, Jiangxi province, China, is a world-class vein-type ore deposit hosted in Cambrian strata and Mesozoic granitic intrusions. There are two major sets of subparallel ore-bearing quartz veins. The ore mineral assemblage includes wolframite and molybdenite, with minor amounts of arsenopyrite, chalcopyrite, and pyrite. There are only two-phase aqueous-rich inclusions in wolframite but at least three major types of inclusions in quartz: two- or three-phase CO2-rich inclusions, two-phase pure CO2 inclusions and two-phase aqueous inclusions, indicating boiling. Fluid inclusions in wolframite have relatively higher homogenization temperatures and salinities (239–380°C, 3.8–13.7 wt.% NaCl equiv) compared with those in quartz (177–329°C, 0.9–8.1 wt.% NaCl equiv). These distinct differences suggest that those conventional microthermometric data from quartz are not adequate to explain the ore formation process. Enthalpy–salinity plot shows a linear relationship, implying mixing of different sources of fluids. Although boiling occurred during vein-type mineralization, it seems negligible for wolframite deposition. Mixing is the dominant mechanism of wolframite precipitation in Xihuashan. δ34S values of the sulfides range from −1.6 to +0.1‰, indicative of a magmatic source of sulfur. δ18O values of wolframite are relatively homogeneous, ranging from +4.8‰ to +6.3‰. Oxygen isotope modeling of boiling and mixing processes also indicates that mixing of two different fluids was an important mechanism in the precipitation of wolframite. 相似文献
17.
Os isotope ratios in pyrrhotite-bearing pelitic rocks of the ∼1.85 Ga Virginia Formation are variable, with perturbations linked to the emplacement of the ∼1.1 Ga Duluth Complex. Pyrrhotite in footwall rocks of the contact aureole show evidence for a mixing event at 1.1 Ga involving a low 187Os/188Os fluid. However, because rocks with perturbed pyrrhotite Os isotope ratios occur 1½ km or more from the Duluth Complex, the fluid is unlikely to have been of magmatic origin. Fluid inclusions in layer-parallel quartz veins provide evidence of the involvement of a boiling fluid at temperatures between ∼300 and 400 °C. Analyses of fluid inclusions via LA-ICP-MS show that the fluids contain up to 1.7 wt% Na, 1.1 wt% K, 4330 ppm Fe, 2275 ppm Zn, and 415 ppm Mg. The veins also contain pyrite or pyrrhotite, plus minor amounts of chalcopyrite, bornite, pentlandite, and sphalerite. The Re-Os isotopic ratios of pyrite from the veins indicate that they crystallized from low 187Os/188Os fluids (<0.2). δ18O values of vein quartz range from 7.7‰ to 9.5‰, consistent with an origin involving fluid with a relatively low δ18O value between 2‰ and 5‰. Meteoric water with such a low δ18O value could have interacted with the igneous rocks of the Complex and would have acquired Os with a low 187Os/188Os ratio. Strongly serpentinized olivine-rich rocks of the Complex are commonly characterized by such low δ18O values and we propose that the fluid involved in serpentinization was also responsible for the perturbation of the Os isotopic system recorded by pyrrhotite in the Virginia Formation. Two important observations are that only pyrrhotite-bearing assemblages in the contact aureole show isotopic perturbation and that intervals showing Os exchange are spatially restricted, and not uniformly distributed. Os exchange and mixing has occurred only where temperatures were sufficient to convert pyrite to pyrrhotite, and where time-integrated water-rock ratios in the aureole were high enough to provide a supply of Os.Troctolitic and gabbroic rocks of the Partridge River Intrusion, Duluth Complex, are characterized by Os isotope ratios that are indicative of variable degrees of crustal contamination (γOs values of ∼0-543). Xenoliths of carbonaceous and sulfidic pelitic rocks of the Virginia Formation found in the igneous rocks provide evidence that Os was released by organic matter and pyrite in the sedimentary rocks and assimilated by mantle-derived magma. However, residual pyrrhotite produced as a result of pyrite breakdown in the xenoliths is characterized by 187Os/188Os ratios that are much lower than anticipated and similar to those of pyrrhotite in the contact aureole. The Os exchange and addition shown by pyrrhotite in the xenoliths highlight an unusual cycle of Re-Os liberation during devolatilization, kerogen maturation, and pyrite to pyrrhotite conversion (processes that contribute to magma contamination), followed by Os uptake by pyrrhotite during back reaction involving magma and/or fluid characterized by a relatively low 187Os/188Os ratio. The extreme Os uptake recorded by pyrrhotite in the xenoliths, as well as the lesser degree of uptake recorded by pyrrhotite in the contact aureole, is in line with the high Os diffusivity in pyrrhotite experimentally determined by Brenan et al. (2000). Our data confirm that Os isotope ratios in pyrrhotite-bearing rocks may be readily perturbed. For this reason caution should be exercised in the interpretation of Os isotope ratios in rocks where pyrrhotite may be the primary host of Os. 相似文献
18.
Ultra-depleted, shallow cratonic mantle beneath West Greenland: dunitic xenoliths from Ubekendt Ejland 总被引:1,自引:0,他引:1
Stefan Bernstein Karen Hanghøj Peter B. Kelemen C. Kent Brooks 《Contributions to Mineralogy and Petrology》2006,152(3):335-347
Dunitic xenoliths from late Palaeogene, alkaline basalt flows on Ubekendt Ejland, West Greenland contain olivine with 100 × Mg/(Mg + Fe), or Mg#, between 92.0 and 93.7. Orthopyroxene has very low Al2O3 and CaO contents (0.024–1.639 and 0.062–0.275 wt%, respectively). Spinel has 100 × Cr/(Cr + Al), or Cr#, between 46.98 and 95.67. Clinopyroxene is absent. The osmium isotopic composition of olivine and spinel mineral separates shows a considerable span of 187Os/188Os values. The most unradiogenic 187Os/188Os value of 0.1046 corresponds to a Re-depletion age of ca. 3.3 Gy, while the most radiogenic value of 0.1336 is higher than present-day chondrite. The Os isotopic composition of the xenoliths is consistent with their origin as restites from a melt extraction event in the Archaean, followed by one or more subsequent metasomatic event(s). The high Cr# in spinel and low modal pyroxene of the Ubekendt Ejland xenoliths are similar to values of some highly depleted mantle peridotites from arc settings. However, highly depleted, arc-related peridotites have higher Cr# in spinel for a given proportion of modal olivine, compared to cratonic xenolith suites from Greenland, which instead form coherent trends with abyssal peridotites, dredged from modern mid-ocean ridges. This suggests that depleted cratonic harzburgites and dunites from shallow lithospheric mantle represent the residue from dry melting in the Archaean. 相似文献
19.
德兴斑岩铜矿田黄铁矿Re-Os同位素定年及其地质意义 总被引:1,自引:1,他引:0
德兴斑岩铜矿田是中国东部最大的斑岩铜矿系统,一直以来,德兴铜矿的成矿过程和成因机制都是矿床学家关注的热点问题。前人在德兴铜矿的围岩蚀变、矿化期次、流体特征、成岩年代等方面取得了较为一致的认识,但其成矿时代仍争议较大。文章首次对矿田内与黄铜矿密切共生的黄铁矿进行了Re-Os同位素定年,10件样品中的4件w(Re)极低,模式年龄变化范围大,且误差较大,故不对其进行讨论;其余6件的w(Re)为10.58×10~(-9)~102.59×10~(-9),普通w(Os)较低(0.0054×10~(-9)~0.0113×10~(-9)),w(~(187)Os)较高(0.019×10~(-9)~0.177×10~(-9)),Re/Os比值较高(4406~73 422),为低含量高放射性Os成因硫化物。研究获得黄铁矿Re-Os同位素加权平均模式年龄为(165.3±2.3)Ma(MSWD=1.04)。因此,德兴铜矿的铜成矿年龄为中侏罗世,综合前人资料,德兴铜矿最可能形成于古太平洋板块俯冲的远程效应影响下的陆内伸展的地质背景,为陆内环境斑岩铜矿。 相似文献
20.
At Sams Creek, a gold-bearing, peralkaline granite porphyry dyke, which has a 7 km strike length and is up to 60 m in thickness, intrudes camptonite lamprophyre dykes and lower greenschist facies metapelites and quartzites of the Late Ordovician Wangapeka formation. The lamprophyre dykes occur as thin (< 3 m) slivers along the contacts of the granite dyke. δ18Omagma values (+5 to +8‰, VSMOW) of the A-type granite suggest derivation from a primitive source, with an insignificant mature crustal contribution. Hydrothermal gold–sulphide mineralisation is confined to the granite and adjacent lamprophyre; metapelite country rocks have only weak hydrothermal alteration. Three stages of hydrothermal alteration have been identified in the granite: Stage I alteration (high fO2) consisting of magnetite–siderite±biotite; Stage II consisting of thin quartz–pyrite veinlets; and Stage III (low fO2) consisting of sulphides, quartz and siderite veins, and pervasive silicification. The lamprophyre is altered to an ankerite–chlorite–sericite assemblage. Stage III sulphide veins are composed of arsenopyrite + pyrite ± galena ± sphalerite ± gold ± chalcopyrite ± pyrrhotite ± rutile ± graphite. Three phases of deformation have affected the area, and the mineralised veins and the granite and lamprophyre dykes have been deformed by two phases of folding, the youngest of which is Early Cretaceous. Locally preserved early-formed fluid inclusions are either carbonic, showing two- or three-phases at room temperature (liquid CO2-CH4 + liquid H2O ± CO2 vapour) or two-phase liquid-rich aqueous inclusions, some of which contain clathrates. Salinities of the aqueous inclusions are in the range of 1.4 to 7.6 wt% NaCl equiv. Final homogenisation temperatures (Th) of the carbonic inclusions indicate minimum trapping temperatures of 320 to 355°C, which are not too different from vein formation temperatures of 340–380°C estimated from quartz–albite stable isotope thermometry. δ18O values of Stage II and III vein quartz range from +12 and +17‰ and have a bimodal distribution (+14.5 and +16‰) with Stage II vein quartz accounting for the lower values. Siderite in Stage III veins have δ18O (+12 to +16‰) and δ13C values (−5‰, relative to VPDB), unlike those from Wangapeka Formation metasediments (δ13Cbulk carbon values of −24 to −19‰) and underlying Arthur Marble marine carbonates (δ18O = +25‰ and δ13C = 0‰). Calculated δ18Owater (+8 to +11‰, at 340°C) and
(−5‰) values from vein quartz and siderite are consistent with a magmatic hydrothermal source, but a metamorphic hydrothermal origin cannot be excluded. δ34S values of sulphides range from +5 to +10‰ (relative to CDT) and also have a bimodal distribution (modes at +6 and +9‰, correlated with Stage II and Stage III mineralisation, respectively). The δ34S values of pyrite from the Arthur Marble marine carbonates (range from +3 to +13‰) and Wangapeka Formation (range from −4 to +9.5‰) indicate that they are potential sources of sulphur for sulphides in the Sams Creek veins. Another possible source of the sulphur is the lithospheric mantle which has positive values up to +14‰. Ages of the granite, lamprophyre, alteration/mineralisation, and deformation in the region are not well constrained, which makes it difficult to identify sources of mineralisation with respect to timing. Our mineralogical and stable isotope data does not exclude a metamorphic source, but we consider that the source of the mineralisation can best be explained by a magmatic hydrothermal source. Assuming that the hydrothermal fluids were sourced from crystallisation of the Sams Creek granite or an underlying magma chamber, then the Sams Creek gold deposit appears to be a hybrid between those described as reduced granite Au–Bi deposits and alkaline intrusive-hosted Au–Mo–Cu deposits. 相似文献