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1.
Mark E. Brandriss Richard J. Nevle Dennis K. Bird James R. O’Neil 《Contributions to Mineralogy and Petrology》1995,121(1):74-86
Hydrogen and oxygen isotope analyses have been made of hydrous minerals in gabbros and basaltic xenoliths from the Eocene
Kap Edvard Holm intrusive complex of East Greenland. The analyzed samples are of three types: (1) primary igneous hornblendes
and phlogopites that crystallized from partial melts of hydrothermally altered basaltic xenoliths, (2) primary igneous hornblendes
that formed during late–magmatic recrystallization of layered gabbroic cumulates, and (3) secondary actinolite, epidote and
chlorite that formed during subsolidus alteration of both xenoliths and gabbros. Secondary actinolite has a δ18O value of −5.8‰ and a δD value of −158‰. These low values reflect subsolidus alteration by low–δ18O, low–δD hydrothermal fluids of meteoric origin. The δD value is lower than the −146 to −112‰ values previously reported
for amphiboles from other early Tertiary meteoric–hydrothermal systems in East Greenland and Scotland, indicating that the
meteoric waters at Kap Edvard Holm were isotopically lighter than typical early Tertiary meteoric waters in the North Atlantic
region. This probably reflects local climatic variations caused by formation of a major topographic dome at about the time
of plutonism and hydrothermal activity. The calculated isotopic composition of the meteoric water is δD=−110 ± 10‰, δ18O ≈−15‰. Igneous hornblendes and phlogopites from pegmatitic pods in hornfelsed basaltic xenoliths have δ18O values between −6.0 and −3.8‰ and δD values between −155 and −140‰. These are both much lower than typical values of fresh
basalts. The oxygen isotope fractionations between pegmatitic hornblendes and surrounding hornfelsic minerals are close to
equilibrium fractionations for magmatic temperatures, indicating that the pegmatites crystallized from low–δ18O partial melts of xenoliths that had been hydrothermally altered and depleted in 18O prior to stoping. The pegmatitic minerals may have crystallized with low primary δD values inherited from the altered country
rocks, but these values were probably overprinted extensively by subsolidus isotopic exchange with low–δD meteoric–hydrothermal
fluids. This exchange was facilitated by rapid self–diffusion of hydrogen through the crystal structures. Primary igneous
hornblendes from the plutonic rocks have δ18O values between +2.0 and +3.2‰ and δD values between −166 and −146‰. The 18O fractionations between hornblendes and coexisting augites are close to equilibrium fractionations for magmatic temperatures,
indicating that the hornblendes crystallized directly from the magma and subsequently underwent little or no oxygen exchange.
The hornblendes may have crystallized with low primary δD values, due to contamination of the magma with altered xenolithic
material, but the final δD values were probably controlled largely by subsolidus isotopic exchange. This inference is based
partly on the observation that coexisting plagioclase has been extensively depleted in 18O via a mineral–fluid exchange reaction that is much slower than the hydrogen exchange reaction in hornblende. It is concluded
that all hydrous minerals in the study area, whether igneous or secondary, have δD values that reflect extensive subsolidus
isotopic equilibration with meteoric–hydrothermal fluids.
Received: 22 March 1994 / Accepted: 26 January 1995 相似文献
2.
Whole rock oxygen isotope data are presented for the Panorama district, in the Archean Pilbara Craton of Western Australia,
where near-perfect exposure reveals a cross section through a complete volcanogenic massive sulfide (VMS) hydrothermal alteration
system. The δ18O values decrease with depth in the volcanic pile, across semi-conformable alteration zones, to values below 6‰ immediately
above a large (180 km2) subvolcanic intrusion. Altered rocks in the upper parts of the subvolcanic intrusion have lower δ18O values (6–8‰) than least altered granite (8‰), apart from sericite–quartz altered zones, which are slightly higher (8–10‰).
Corridors of low δ18O values crosscut this regional zonation, and are coincident with transgressive feldspar-destructive alteration zones, which
underlie VMS mineralization. The whole rock oxygen isotope distribution patterns are interpreted to represent alteration temperature,
where high δ18O values correspond to low temperature alteration and low δ18O values correspond to high temperature alteration. Alteration temperatures, which were calculated using modal alteration
mineral abundances and an assumed fluid δ18O, are consistent with this interpretation. Increasing temperatures with depth in the volcanic pile and high temperatures
in transgressive corridors leading up to VMS deposits, are consistent with a convective hydrothermal model, in which heat
from the subvolcanic intrusion drove seawater through the volcanic pile. Granite-hosted sericite–quartz alteration zones are
18O-enriched, and are tentatively interpreted to have formed from a mixed magmatic-evolved seawater fluid.
Received: 12 April 1999 / Accepted: 6 April 2000 相似文献
3.
I. J. Richards Theodore C. Labotka Robert T. Gregory 《Contributions to Mineralogy and Petrology》1996,123(2):202-221
Late Proterozoic to Cambrian carbonate rocks from Lone Mountain, west central Nevada, record multiple post-depositional events
including: (1) diagenesis, (2) Mesozoic regional metamorphism, (3) Late Cretaceous contact metamorphism, related to the emplacement
of the Lone Mountain granitic pluton and (4) Tertiary hydrothermal alteration associated with extension, uplift and intrusion
of silicic porphyry and lamprophyre dikes. Essentially pure calcite and dolomite marbles have stable isotopic compositions
that can be divided into two groups, one with positive δ13C values from+3.1 to +1.4 ‰ (PDB) and high δ18O values from +21.5 to +15.8 ‰ (SMOW), and the other with negative δ13C values from –3.3 to –3.6‰ and low δ18O values from +16.9 to +11.1‰. Marbles also contain minor amounts of quartz, muscovite and phlogopite. Brown and blue luminescent,
clear, smooth textured quartz grains from orange luminescent calcite marbles have high δ18O values from +23.9 to +18.1‰, while brown luminescent, opaque, rough textured quartz grains from red luminescent dolomite
marbles typically have low δ18O values from +2.0 to +9.3‰. The δ18O values of muscovite and phlogopite from marbles are typical of micas in metamorphic rocks, with values between +10.4 and
+14.4‰, whereas mica δD values are very depleted, varying from −102 to −156‰. No significant lowering of the δ18O values of Lone Mountain carbonates is inferred to have occurred during metamorphism as a result of devolatilization reactions
because of the essentially pure nature of the marbles. Bright luminescence along the edges of fractures, quartz cements and
quartz overgrowths in dolomite marbles, low δD values of micas, negative δ13C values and low δ18O values of calcite and dolomite, and depleted δ18O values of quartz from dolomite marbles all indicate that meteoric fluids interacted with Lone Mountain marbles during the
Tertiary. Partial oxygen isotopic exchange between calcite and low 18O meteoric fluids lowered the δ18O values of calcite, resulting in uniform quartz-calcite fractionations that define an apparent pseudoisotherm. These quartz-calcite
fractionations significantly underestimate both the temperature of metamorphism and the temperature of post-metamorphic alteration.
Partial oxygen isotopic exchange between quartz and meteoric fluids also resulted in 18O depletion of quartz from dolomite marbles. This partial exchange was facilitated by an increase in the surface area of the
quartz as a result of its dissolution by meteoric fluids. The negative δ13C values in carbonates result from the oxidation of organic material by meteoric fluids following metamorphism. Stable isotopic
data from Lone Mountain marbles are consistent with the extensive circulation of meteoric hydrothermal fluids throughout western
Nevada in Tertiary time.
Received: 1 February 1994/Accepted: 12 September 1995 相似文献
4.
Oxygen isotope constraints on the petrogenesis of the Sybille intrusion of the Proterozoic Laramie Anorthosite Complex 总被引:2,自引:0,他引:2
The origin of monzonitic intrusions that are associated with Proterozoic massif-type anorthosite complexes is controversial.
A detailed oxygen isotope study of the Sybille intrusion, a monzonitic intrusion of the Laramie Anorthosite Complex (Wyoming),
indicates that either derivation from a basaltic magma of mantle origin with a metasedimentary component (∼20%) incorporated
early in its magmatic history, or a partial melt of lower crustal rocks is consistent with the data. The oxygen isotope compositions
of plagioclase, pyroxene and zircon from the Sybille monzosyenite, the dominant rock type in the Sybille intrusion, were analyzed
in order to establish the isotopic composition of the source of the magma. Plagioclase δ18O values range from 6.77 to 9.17‰. We interpret the higher plagioclase δ18O values (average 8.69 ± 0.30‰, n = 19) to be magmatic in origin, lower plagioclase δ18O values (average 7.51 ± 0.44‰, n = 22) to be the result of variable subsolidus alteration, and pyroxene δ18O values (average 6.34 ± 0.38‰, n = 19) to be the result of closed-system diffusional exchange during cooling. Low magnetic zircons, which have been shown
to retain magmatic oxygen isotope values despite high grade metamorphism and extensive subsolidus hydrothermal alteration,
have δ18O values (7.40 ± 0.24‰, n = 11) which are consistent with our interpretation of the plagioclase and pyroxene results. Oxygen isotope data from all
three minerals indicate that the magmatic oxygen isotope composition of the Sybille intrusion is enriched in 18O relative to the composition of average or “normal” mantle-derived magmas. This enrichment is approximately twice the oxygen
isotope enrichment that could result from closed-system fractionation, rendering a closed-system, comag- matic petrogenetic
model between the Sybille intrusion and the mantle-derived anorthositic lithologies of the Laramie Anorthosite Complex improbable.
Received: 7 April 1998 / Accepted: 19 January 1999 相似文献
5.
Summary The eastern Pyrenees host a large number of talc-chlorite mineralizations of Albian age (112–97 Ma), the largest of which
occur in the St. Barthelemy massif. There talc develops by hydrothermal replacement of dolostones, which were formed by alteration
of calcite marbles. This alteration is progressive. Unaltered calcite marbles have oxygen isotope composition of about 25‰
(V-SMOW). The δ18O values decrease down to values of 12‰ towards the contact with dolostones. This 18O depletion is accompanied by Mg enrichment, LREE fractionation and systematic shifts in the Sr isotope compositions, which
vary from 87Sr/86Sr = 0.7087–0.7092 in unaltered calcite marbles to slightly more radiogenic compositions with 87Sr/86Sr = 0.7094 near dolomitization fronts. Dolostones have δ18O values (about 9‰) lower than calcitic marbles, higher REE content and more radiogenic Sr isotope composition (87Sr/86Sr = 0.7109 to 0.7130). Hydrothermal calcites have δ18O values close to dolostones but substantially lower δ13C values, down to −6.5‰, which is indicative of the contribution of organic matter. The REE content of hydrothermal calcite
is one order of magnitude higher than that of calcitic marbles. Its highly radiogenic Sr composition with 87Sr/86Sr = 0.7091 to 0.7132 suggests that these elements were derived from silicate rocks, which experienced intense chlorite alteration
during mineralization. The chemical and isotopic compositions of the calcite marbles, the dolostones and the hydrothermal
calcites are interpreted as products of successive stages of fluid-rock interaction with increasing fluid-rock ratios. The
hydrothermal quartz, calcite, talc and chlorite are in global mutual isotopic equilibrium. This allows the calculation of
the O isotope composition of the infiltrating water at 300 °C, which is in the δ18O
= 2–4.5‰ range. Hydrogen isotope compositions of talc and chlorite indicate a δD
= 0 to −20‰. This water probably derived from seawater, with minor contribution of evolved continental water. 相似文献
6.
R. Abart 《Contributions to Mineralogy and Petrology》1995,122(1-2):116-133
Metasomatic garnet-vesuvianite veins occur within the contact metamorphic marble sequence of the Lower Triassic Prezzo formation
in a narrow, 1–5 m wide zone along an intrusive marble-granodiorite contact at the southwestern border of the Tertiary Adamello
batholith. The metasomatic mineral assemblage is comprised of garnet, vesuvianite, clinopyroxene, wollastonite, and pyrrhotite,
which were precipitated from the vein-forming fluid in a preexisting calcite matrix at conditions of about 2800 bars and 630° C.
The veins are enriched in silicon, aluminum, iron, magnesium, titanium and depleted in calcium with respect to the unaltered
contact metamorphic marble. Graphite, which is present in the unaltered Prezzo Marble is absent in the veins. Irregularly
shaped mineralogically distinct zones with different degrees of silicification can be distinguished within the veins. The
isotopic compositions of calcite (cc) in the unaltered marble are about δ18O (SMOW; Standard mean Ocean Water)=21.0‰ and δ13C(PDB; Peedee belemnite)=0.0‰. They are reset to significantly lower values within the veins, where δ18Occ is 15.0 to 16.0‰ and δ13Ccc is −4.5 to −3.5‰. The isotopic front coincides with an abrupt change in the microscopic texture of matrix carbonate which
occurs at the sharp boundary between graphite-bearing and graphite-free material. Within the veins the oxygen isotope fractionation
between calcite and garnet (gar) varies systematically with distance from highly silicified zones. The variations in Δ18Occ-gar are as large as 2‰, on a millimeter scale, indicating garnet-calcite isotopic disequilibrium. Vein formation was due to the
infiltration of a water rich fluid of magmatic provenance into the carbonate country rock along fractures. Removal of graphite
from the wall rock by dissolution through the metasomatic fluid induced recrystallization of matrix calcite. Permeability
was enhanced during calcite recrystallization facilitating material transport into the wall rock and metasomatic alteration.
Vein garnet was precipitated in isotopic equilibrium with the metasomatic fluid. The isotopic composition of preexisting calcite
was initially out of equilibrium with the vein-forming fluid and it was shifted towards equilibrium by surface-reaction controlled
calcite-fluid isotopic exchange during calcite recrystallization. Due to the short lifetime of the metasomatic system, calcite-fluid
isotopic equilibrium was generally not attained. Within the veins, oxygen and carbon transport was fast relative to mineral-fluid
exchange of their isotopes and the geometry of the isotopic pattern is largely controlled by the kinetics of mineral-fluid
exchange.
Received: 16 June 1994/Accepted: 20 May 1995 相似文献
7.
The Géant Dormant gold mine is a sulfide-rich quartz vein gold deposit hosted by a volcano-sedimentary sequence and an associated
felsic endogenous dome and dikes. The auriferous quartz-sulfide veins were preceded by two synvolcanic gold-bearing mineralizing
events: early sulfidic seafloor-related and later disseminated pyrite in the felsic dome. This deposit differs from classical
Archean auriferous quartz vein deposits by the low carbonate and high sulfide contents of the veins and by their formation
prior to ductile penetrative deformation. The δ18O values of quartz associated with seafloor-related auriferous sulfides average 11.9 ± 0.6‰ (n = 3). The seafloor hydrothermal fluids had a δ18O value of 3.2‰ calculated at 250 °C. The oxygen isotope composition of quartz and chlorite from veins average 12.5 ± 0.3‰
(n = 20) and 5.9 ± 1.1‰ (n = 4) respectively. Assuming oxygen isotope equilibrium between quartz and chlorite, the veins formed at a temperature of
∼275 °C, which is consistent with the calculated temperature of 269 ± 10 °C from chlorite chemistry. The gold-bearing fluids
had a δ18O value of 4.7‰ calculated at 275 °C. The δ34S values of sulfides from the three gold events range from 0.6 to 2.8‰ (n = 32) and are close to magmatic values. Sulfur isotope geothermometry constrains the sulfide precipitation in the gold-bearing
veins at a temperature of ∼350 °C. The similarity of the isotope data, the calculated δ18O of the mineralizing fluids and the likely seawater fluid source suggest that the three mineralizing events are genetically
related to a volcanogenic hydrothermal system. The high value of the auriferous fluids (δ18O = 4.7‰) is attributed to a significant magmatic fluid contribution to the evolved seawater-dominated convective hydrothermal
system. The two-stage filling of veins at increasing temperature from quartz-chlorite (275 °C) to sulfides (350 °C) may reflect
the progressive maturation of volcanogenic hydrothermal systems. These results, together with field and geochemical data,
suggest that formation of gold-rich volcanogenic systems require specific conditions that comprise a magmatic fluid contribution
and gold from arc-related felsic rocks, coeval with the mineralizing events. This study shows that some auriferous quartz-vein
orebodies in Archean terranes are formed in volcanogenic rather than mesothermal systems.
Received: 12 December 1998 / Accepted: 5 July 1999 相似文献
8.
A. I. Grabezhev B. G. Pokrovskii F. P. Buslaev V. V. Zaikov G. N. Pshenichnyi 《Lithology and Mineral Resources》2000,35(1):70-75
Substantial differences in isotopic compositions of micas and pyrophyllites from metasomatites related to various stages of
the process that formed the giant Gai massive sulfide deposit have been established. The illite from the earliest and predominant
chlorite-illite-quartz metasomatite is characterized by the least δD values of −(50–85)‰ and δ18O=7–11‰. The pyrophyllite-quartz metasomatite as well as illite and pyrophyllite schists developed locally in the southern
part of the deposit that likely correspond to the site of discharge of late geothermal paleosystem, contain pyrophyllite and
illite with much higher values of δD=−(25–45)‰ and δ18O=4–9‰. Local zones of illite-paragonite schist complete the mineral formation and are characterized by the transitional δD values of −(30–55)‰ and elevated δ18O of 10–11‰. The most plausible model of isotopic evolution in the hydrothermal system, with an initial temperature of mica
formation at 250°C, assumes the mixing of transformed sea water with a magmatic (metamorphic) water at the initial stage when
the background metasomatites and massive sulfide orebodies of the northern lode have been formed. Subsequently, after the
burial of the northern lode beneath basaltic andesite flows, the repeated sea water invasions took place in the southern discharge
site of the system. As a result, the pyrophyllite-quartz metasomatite was formed; the pyrophyllite and illite schists originated
in tectonic compression zones. The interaction of this water with silicate rocks was completed by a formation of illite-paragonite
schist. In general, the substantial contribution of sea water to the formation of metasomatic halo of the deposit casts no
doubt. 相似文献
9.
The source of metasomatic fluids in iron-oxide–copper–gold districts is contentious with models for magmatic and other fluid sources having been proposed. For this study, δ
18O and δ
13C ratios were measured from carbonate mineral separates in the Proterozoic eastern Mt Isa Block of Northwest Queensland, Australia. Isotopic analyses are supported by petrography, mineral chemistry and cathodoluminescence imagery. Marine meta-carbonate rocks (ca. 20.5‰ δ
18O and 0.5‰ δ
13C calcite) and graphitic meta-sedimentary rocks (ca. 14‰ δ
18O and −18‰ δ
13C calcite) are the main supracrustal reservoirs of carbon and oxygen in the district. The isotopic ratios for calcite from the cores of Na–(Ca) alteration systems strongly cluster around 11‰ δ
18O and −7‰ δ
13C, with shifts towards higher δ
18O values and higher and lower δ
13C values, reflecting interaction with different hostrocks. Na–(Ca)-rich assemblages are out of isotopic equilibrium with their metamorphic hostrocks, and isotopic values are consistent with fluids derived from or equilibrated with igneous rocks. However, igneous rocks in the eastern Mt Isa Block contain negligible carbon and are incapable of buffering the δ
13C signatures of CO2-rich metasomatic fluids associated with Na–(Ca) alteration. In contrast, plutons in the eastern Mt Isa Block have been documented as having exsolved saline CO2-rich fluids and represent the most probable fluid source for Na–(Ca) alteration. Intrusion-proximal, skarn-like Cu–Au orebodies that lack significant K and Fe enrichment (e.g. Mt Elliott) display isotopic ratios that cluster around values of 11‰ δ
18O and −7‰ δ
13C (calcite), indicating an isotopically similar fluid source as for Na–(Ca) alteration and that significant fluid–wallrock interaction was not required in the genesis of these deposits. In contrast, K- and Fe-rich, intrusion-distal deposits (e.g. Ernest Henry) record significant shifts in δ
18O and δ
13C towards values characteristic of the broader hostrocks to the deposits, reflecting fluid–wallrock equilibration before mineralisation. Low temperature, low salinity, low δ
18O (<10‰ calcite) and CO2-poor fluids are documented in retrograde metasomatic assemblages, but these fluids are paragenetically late and have not contributed significantly to the mass budgets of Cu–Au mineralisation. 相似文献
10.
Whole rock and mineral stable isotope and microprobe analyses are presented from granitoids of the North Chilean Precordillera.
The Cretaceous to Tertiary plutonic rocks contain important ore deposits and frequently display compositional and textural
evidence of hydrothermal alteration even in barren rocks. Deuteric alteration includes replacement of biotite and amphibole
by chlorite and epidote, sericitization and saussuritization of feldspars, and uralitization of clinopyroxene and/or amphibole.
While whole rock compositions are not significantly affected, compositional variations in amphiboles suggest two types of
hydrothermal alteration. Hornblende with actinolitic patches and rims and tight compositional trends from hornblende to Mg-rich
actinolite indicate increasing oxygen fugacity from magmatic to hydrothermal conditions. Uralitic amphiboles exhibiting irregular
Mg-Fe distribution and variable Al content are interpreted as reflecting subsolidus hydration reactions at low temperatures.
The δD values of hydrous silicates vary from −63 to −105‰. Most δ18O values of whole rocks are in the range of 5.7 to 7.7‰ and are considered normal for igneous rocks in the Andes. These δ18O values also coincide well with the oxygen isotope composition of geochemically similar recent volcanics from the Central
Andean Volcanic Zone (δ18O = 7.0–7.4‰). Only one sample in this study (δ18O = 3.0‰) appears to be depleted by isotope exchange with light meteoric water at high temperatures. The formation of secondary
minerals in all other intrusions is mainly the product of deuteric alteration. This also holds true for the sample from El
Abra, the only pluton associated with mineralization. This indicates the dominant role of a magmatic rather than a meteoric
fluid in the alteration of the Cretaceous and Tertiary granitoids in northern Chile.
Received: 8 July 1998 / Accepted: 15 April 1999 相似文献
11.
Ian Cartwright Ian S. Buick Roland Maas 《Contributions to Mineralogy and Petrology》1997,128(4):335-351
The Jervois region of the Arunta Inlier, central Australia, contains para- and orthogneisses that underwent low-pressure
amphibolite facies metamorphism (P = 200–300 MPa, T = 520–600 °C). Marble layers cut by metre-wide quartz + garnet ± epidote veins comprise calcite, quartz, epidote, clinopyroxene,
grandite garnet, and locally wollastonite. The marbles also contain locally discordant decimetre-thick garnet and epidote
skarn layers. The mineral assemblages imply that the rocks were infiltrated by water-rich fluids (XCO2 = 0.1–0.3) at ∼600 °C. The fluids were probably derived from the quartz-garnet vein systems that represent conduits for fluids
exsolved from crystallizing pegmatites emplaced close to the metamorphic peak. At one locality, the marble has calcite (Cc)
δ18O values of 9–18‰ and garnet (Gnt) δ18O values of 10–14‰. The δ18O(Gnt) values are only poorly correlated with δ18O(Cc), and the δ18O values of some garnet cores are higher than the rims. The isotopic disequilibrium indicates that garnet grew before the
δ18O values of the rock were reset. The marbles contain ≤15% garnet and, for water-rich fluids, garnet-forming reactions are
predicted to propagate faster than O-isotopes are reset. The Sm-Nd and Pb-Pb ages of garnets imply that fluid flow occurred
at 1750–1720 Ma. There are no significant age differences between garnet cores and rims, suggesting that fluid flow was relatively
rapid. Texturally late epidote has δ18O values of 1.5–6.2‰ implying δ18O(H2O) values of 2–7‰. Waters with such low-δ18O values are probably at least partly meteoric in origin, and the epidote may be recording the late influx of meteoric water
into a cooling hydrothermal system.
Received: 29 April 1996 / Accepted: 12 March 1997 相似文献
12.
Alluvial and colluvial gem sapphires are common in the basaltic fields of the French Massif Central (FMC) but sapphire-bearing
xenoliths are very rare, found only in the Menet trachytic cone in Cantal. The O-isotope composition of the sapphires ranges
between 4.4 and 13.9‰. Two distinct groups have been defined: the first with a restricted isotopic range between 4.4 and 6.8‰
(n = 22; mean δ18O = 5.6 ± 0.7‰), falls within the worldwide range defined for blue-green-yellow sapphires related to basaltic gem fields (3.0 < δ18O < 8.2‰, n = 150), and overlaps the ranges defined for magmatic sapphires in syenite (4.4 < δ18O < 8.3‰, n = 29). A second group, with an isotopic range between 7.6 and 13.9‰ (n = 9), suggests a metamorphic sapphire source such as biotite schist in gneisses or skarns. The δ18O values of 4.4–4.5‰ for the blue sapphire-bearing anorthoclasite xenolith from Menet is lower than the δ18O values obtained for anorthoclase (7.7–7.9‰), but suggest that these sapphires were derived from an igneous reservoir in
the subcontinental spinel lherzolitic mantle of the FMC. The presence of inclusions of columbite-group minerals, pyrochlore,
Nb-bearing rutile, and thorite in these sapphires provides an additional argument for a magmatic origin. In the FMC lithospheric
mantle, felsic melts crystallized to form anorthoclasites, the most evolved peraluminous variant of the alkaline basaltic
melt. The O-isotopic compositions of the first group suggests that these sapphires crystallized from felsic magmas under upper
mantle conditions. The second group of isotopic values, typified for example by the Le Bras sapphire with a δ18O of 13.9‰, indicates that metamorphic sapphires from granulites were transported to the surface by basaltic magma. 相似文献
13.
George B. Perkins Zachary D. Sharp Jane Selverstone 《Contributions to Mineralogy and Petrology》2006,151(6):633-650
Spinel lherzolite and pyroxenite xenoliths from the Rio Puerco Volcanic Field, New Mexico, were analyzed for oxygen isotope ratios by laser fluorination. In lherzolites, olivine δ18O values are high (+5.5‰), whereas δ18O values for pyroxenes are low (cpx=+5.1‰; opx=+5.4‰) compared to average mantle values. Pyroxenite δ18O values (cpx=+5.0‰; opx=+5.3‰) are similar to those of the lherzolites and are also lower than typical mantle oxygen isotope compositions. Texturally and chemically primary calcite in pyroxenite xenoliths is far from isotopic equilibrium with other phases, with δ18O values of +21‰. The isotopic characteristics of the pyroxenite xenoliths are consistent with a petrogenetic origin from mixing of lherzolitic mantle with slab-derived silicate and carbonatite melts. The anomalously low δ18O in the pyroxenes reflects metasomatism by a silicate melt from subducted altered oceanic crust, and high δ18O calcite is interpreted to have crystallized from a high δ18O carbonatitic melt derived from subducted ophicarbonate. Similar isotopic signatures of metasomatism are seen throughout the Rio Puerco xenolith suite and at Kilbourne Hole in the southern Rio Grande rift. The discrete metasomatic components likely originated from the subducted Farallon slab but were not mobilized until heating associated with Rio Grande rifting occurred. Oxygen diffusion modeling requires that metasomatism leading to the isotopic disequilibrium between calcite and pyroxene in the pyroxenites occurred immediately prior to entrainment. Melt infiltration into spinel-facies mantle (xenoliths) prior to eruption was thus likely connected to garnet-facies melting that resulted in eruption of the host alkali basalt. 相似文献
14.
Oxygen isotopic composition was studied in the altered host rocks of the Dal’negorsk borosilicate deposit in order to establish
a boron source and the origin of ore-forming fluids responsible for deposition of economic borosilicate ore. The relationships
between oxygen isotopic composition and geochemistry of the altered igneous rocks occupying various structural and temporal
positions in the ore zone were studied, including premineral high-potassium minor intrusions located in the zones of datolite
mineralization; alkali basalt, gabbro, and breccia from the sedimentary framework of the deposit; and postmineral basaltic
andesite, basalt, and dolerite dikes. It was suggested that interaction of aqueous fluid with host rocks brought about not
only variation in oxygen isotopic composition but also shifts in geochemistry of these rocks, especially as concerns the chemical
elements contained in ore-forming fluid. The disturbance of oxygen isotopic system is typical of all studied rocks: δ18O values sharply decrease indicating interaction with aqueous fluid at elevated temperatures. The lowest δ18O (from −2.9 to +0.1‰) is characteristic of the premineral high-potassium and ultrapotassium minor intrusions from skarn-datolite
zone. Igneous rocks from the sedimentary framework of the deposit have δ18O of +2 to −0.9‰ The δ18O of postmineral basaltic andesite, basalt, and dolerite dikes varies from 0 to +7‰ with increasing distance from the ore
zone. The oxygen isotopic composition of aqueous fluid evidences its exogenic origin. The geochemical and isotopic characteristics
of ore-forming fluid show that it could have been deep-seated subsurface water similar to the contemporary water of the Alpine
fold zone, which contain up to 700–1000 mg/l B and is distinguished by high K, Li, Rb, Cs contents and high K/Na ratio. Similar
geochemistry is characteristic of the fluid inclusions in quartz from ore zones. It cannot be ruled out that continental evaporites
were a source of boron as well. The relationships between δ18O, K-Ar age, and geochemical parameters of premineral and postmineral altered intrusive bodies allow us to suggest that the
subsurface B-bearing water discharged through narrow channels controlled by premineral basaltic bodies. The discharge was
probably initiated by emplacement of basalt and dolerite dikes. 相似文献
15.
Oxygen isotope systematics of gem corundum deposits in Madagascar: relevance for their geological origin 总被引:1,自引:1,他引:1
Gaston Giuliani Anthony Fallick Michel Rakotondrazafy Daniel Ohnenstetter Alfred Andriamamonjy Théogène Ralantoarison Saholy Rakotosamizanany Marie Razanatseheno Yohann Offant Virginie Garnier Christian Dunaigre Dietmar Schwarz Alain Mercier Voahangy Ratrimo Bruno Ralison 《Mineralium Deposita》2007,42(3):251-270
The oxygen isotopic composition of gem corundum was measured from 22 deposits and occurrences in Madagascar to provide a gemstone
geological identification and characterization. Primary corundum deposits in Madagascar are hosted in magmatic (syenite and
alkali basalt) and metamorphic rocks (gneiss, cordieritite, mafic and ultramafic rocks, marble, and calc-silicate rocks).
In both domains the circulation of fluids, especially along shear zones for metamorphic deposits, provoked in situ transformation
of the corundum host rocks with the formation of metasomatites such as phlogopite, sakenite, and corundumite. Secondary deposits
(placers) are the most important economically and are contained in detrital basins and karsts. The oxygen isotopic ratios
(18O/16O) of ruby and sapphire from primary deposits are a good indicator of their geological origin and reveal a wide range of δ18O (Vienna Standard Mean Ocean Water) between 1.3 and 15.6‰. Metamorphic rubies are defined by two groups of δ18O values in the range of 1.7 to 2.9‰ (cordieritite) and 3.8 to 6.1‰ (amphibolite). “Magmatic” rubies from pyroxenitic xenoliths
contained in the alkali basalt of Soamiakatra have δ18O values ranging between 1.3 and 4.7‰. Sapphires are classified into two main groups with δ18O in the range of 4.7 to 9.0‰ (pyroxenite and feldspathic gneiss) and 10.7 to 15.6‰ (skarn in marble from Andranondambo).
The δ18O values for gem corundum from secondary deposits have a wide spread between −0.3 and 16.5‰. The ruby and sapphire found in
placers linked to alkali basalt environments in the northern and central regions of Madagascar have consistent δ18O values between 3.5 and 6.9‰. Ruby from the placers of Vatomandry and Andilamena has δ18O values of 5.9‰, and between 0.5 and 4.0‰, respectively. The placers of the Ilakaka area are characterized by a huge variety
of colored sapphires and rubies, with δ18O values between −0.3 and 16.5‰, and their origin is debated. A comparison with oxygen isotope data obtained on gem corundum
from Eastern Africa, India, and Sri Lanka is presented. Giant placer deposits from Sri Lanka, Madagascar, and Tanzania have
a large variety of colored sapphires and rubies with a large variation in δ18O due to mingling of corundum of different origin: mafic and ultramafic rocks for ruby, desilicated pegmatites for blue sapphire,
syenite for yellow, green, and blue sapphire, and skarn in marbles for blue sapphire. 相似文献
16.
Benita Putlitz Alan Matthews John W. Valley 《Contributions to Mineralogy and Petrology》2000,138(2):114-126
Oxygen and hydrogen stable isotope ratios of eclogite-facies metagabbros and metabasalts from the Cycladic archipelago (Greece)
document the scale and timing of fluid–rock interaction in subducted oceanic crust. Close similarities are found between the
isotopic compositions of the high-pressure rocks and their ocean-floor equivalents. High-pressure minerals in metagabbros
have low δ18O values: garnet 2.6 to 5.9‰, glaucophane 4.3 to 7.1‰; omphacite 3.5 to 6.2‰. Precursor actinolite that was formed during
the hydrothermal alteration of the oceanic crust by seawater analyses at 3.7 to 6.3‰. These compositions are in the range
of the δ18O values of unaltered igneous oceanic crust and high-temperature hydrothermally altered oceanic crust. In contrast, high-pressure
metabasalts are characterised by 18O-enriched isotopic compositions (garnet 9.2 to 11.5‰, glaucophane 10.6 to 12.5‰, omphacite 10.2 to 12.8‰), which are consistent
with the precursor basalts having undergone low-temperature alteration by seawater. D/H ratios of glaucophane and actinolite
are also consistent with alteration by seawater. Remarkably constant oxygen isotope fractionations, compatible with isotopic
equilibrium, are observed among high-pressure minerals, with Δglaucophane−garnet = 1.37 ± 0.24‰ and Δomphacite−garnet = 0.72 ± 0.24‰. For the estimated metamorphic temperature of 500 °C, these fractionations yield coefficients in the equation
Δ = A * 106/T
2 (in Kelvin) of Aglaucophane−garnet = 0.87 ± 0.15 and Aomphacite−garnet = 0.72 ± 0.24. A fractionation of Δglaucophane–actinolite = 0.94 ± 0.21‰ is measured in metagabbros, and indicates that isotopic equilibrium was established during the metamorphic
reaction in which glaucophane formed at the expense of actinolite. The preservation of the isotopic compositions of gabbroic
and basaltic oceanic crust and the equilibrium fractionations among minerals shows that high-pressure metamorphism occurred
at low water/rock ratios. The isotopic equilibrium is only observed at hand-specimen scale, at an outcrop scale isotopic compositional
differences occur among adjacent rocks. This heterogeneity reflects metre-scale compositional variations that developed during
hydrothermal alteration by seawater and were subsequently inherited by the high-pressure metamorphic rocks.
Received: 4 January 1999 / Accepted: 7 July 1999 相似文献
17.
The Eastern Iberian Central System has abundant ore showings hosted by a wide variety of hydrothermal rocks; they include
Sn-W, Fe and Zn-(W) calcic and magnesian skarns, shear zone- and episyenite-hosted Cu-Zn-Sn-W orebodies, Cu-W-Sn greisens
and W-(Sn), base metal and fluorite-barite veins. Systematic dating and fluid inclusion studies show that they can be grouped
into several hydrothermal episodes related with the waning Variscan orogeny. The first event was at about 295 Ma followed
by younger pulses associated with Early Alpine rifting and extension and dated near 277, 150 and 100 to 20 Ma, respectively
(events II–IV). The δ18O-δD and δ34S studies of hydrothermal rocks have elucidated the hydrological evolution of these systems. The event I fluids are of mixed
origin. They are metamorphic fluids (H2O-CO2-CH4-NaCl; δ18O=4.7 to 9.3‰; δD ab.−34‰) related to W-(Sn) veins and modified meteoric waters in the deep magnesian Sn-W skarns (H2O-NaCl, 4.5–6.4 wt% NaCl eq.; δ18O=7.3–7.8‰; δD=−77 to −74‰) and epizonal shallow calcic Zn-(W) and Fe skarns (H2O-NaCl, <8 wt% NaCl eq.; δ18O=−0.4 to 3.4‰; δD=−75 to −58‰). They were probably formed by local hydrothermal cells that were spatially and temporally
related to the youngest Variscan granites, the metals precipitating by fluid unmixing and fluid-rock reactions. The minor
influence of magmatic fluids confirms that the intrusion of these granites was essentially water-undersaturated, as most of
the hydrothermal fluids were external to the igneous rocks. The fluids involved in the younger hydrothermal systems (events
II–III) are very similar. The waters involved in the formation of episyenites, chlorite-rich greisens, retrograde skarns and
phyllic and chlorite-rich alterations in the shear zones show no major chemical or isotopic differences. Interaction of the
hydrothermal fluids with the host rocks was the main mechanism of ore formation. The composition (H2O-NaCl fluids with original salinities below 6.2 wt% NaCl eq.) and the δ18O (−4.6 to 6.3‰) and δD (−51 to −40‰) values are consistent with a meteoric origin, with a δ18O-shift caused by the interaction with the, mostly igneous, host rocks. These fluids circulated within regional-scale convective
cells and were then channelled along major crustal discontinuities. In these shear zones the more easily altered minerals
such as feldspars, actinolite and chlorite had their δ18O signatures overprinted by low temperature younger events while the quartz inherited the original signature. In the shallower
portions of the hydrothermal systems, basement-cover fluorite-barite-base metal veins formed by mixing of these deep fluids
with downwards percolating brines. These brines are also interpreted as of meteoric origin (δ18O< ≈ −4‰; δD=−65 to −36‰) that leached the solutes (salinity >14 wt% NaCl eq.) from evaporites hosted in the post-Variscan
sequence. The δD values are very similar to most of those recorded by Kelly and Rye in Panasqueira and confirm that the Upper
Paleozoic meteoric waters in central Iberia had very negative δD values (≤−52‰) whereas those of Early Mesozoic age ranged
between −65 and −36‰.
Received: 9 June 1999 / Accepted: 19 January 2000 相似文献
18.
The Jinwozi gold deposit consists of gold-bearing quartz veins in a biotite granodiorite of Hercynian age (zircon U-Pb age
≈ 335.7 Ma). Ore mineralogy is simple. In addition to native gold, there are only small amounts of sulfides, mainly pyrite
and minor sphalerite, chalcopyrite and galena. δ34S values average 6.69‰, and δ18O 13.99‰ Abundant CO2 is contained in fluid inclusions from quartz. Homogenization temperatures of fluid inclusions are between 186 and 262 °C.
REE distribution patterns indicate that the igneous mass may have been derived from a common initial material of calcareous-argillaceous
sediments and alkali basalts as the country rocks. In other words, the Jinwozi granodiorite is of remelting origin from crustal
material. Isotopic evidence of S, O and Pb shows that the ore-forming material is genetically related to magmatic hydrothermal
activity. 相似文献
19.
Alan J. Wilson David R. Cooke Benjamin J. Harper Cari L. Deyell 《Mineralium Deposita》2007,42(5):465-487
The alkalic porphyry gold–copper deposits of the Cadia district occur in the eastern Lachlan Fold Belt of New South Wales,
Australia. The district comprises four porphyry deposits (Ridgeway, Cadia Quarry, Cadia Hill, and Cadia East) and two iron–copper–gold
skarn deposits (Big Cadia and Little Cadia). Almost 1,000 tonnes of contained gold and more than four million tonnes of copper
have been discovered in these systems, making Cadia the world’s largest known alkalic porphyry district, in terms of contained
gold. Porphyry gold–copper ore at Cadia is associated with quartz monzonite intrusive complexes, and is hosted by central
stockwork and sheeted quartz–sulfide–(carbonate) vein systems. The Cadia porphyry deposits are characterized by cores of potassic
and/or calc–potassic alteration assemblages, and peripheral halos of propylitic alteration, with late-stage phyllic alteration
mostly restricted to fault zones. Hematite dusting is an important component of the propylitic alteration assemblage, and
has produced a distinctive reddening of feldspar minerals in the volcanic wall rocks around the mineralized centers. Sulfide
mineralization is strongly zoned at Ridgeway and Cadia East, with bornite-rich cores surrounded by chalcopyrite-rich halos
and peripheral zones of pyrite mineralization. The Cadia Hill and Cadia Quarry deposits have chalcopyrite-rich cores and pyrite-rich
halos, and Cadia Hill contains a high-level bornite-rich zone. Distinctive sulfur isotopic zonation patterns have been identified
at Ridgeway, Cadia Hill, and Cadia East. The deposit cores are characterized by low δ34Ssulfide values (−10 to −4‰), consistent with sulfide precipitation from an oxidized (sulfate-predominant) magmatic fluid at 450 to
400°C. Pyrite grains that occur in the propylitic alteration halos typically have δ34Ssulfide values near 0‰. There is a gradual increase in δ34Ssulfide values outwards from the deposit cores through the propylitic halos. Water–rock interaction during propylitic alteration
caused magmatic sulfate reduction and concomitant oxidation of ferrous iron-bearing minerals, resulting in enrichment of 34S in pyrite and also producing the distinctive reddened, hematite-rich alteration halos to the Cadia deposits. These results
show that sulfur isotope analyses have potential applications in the exploration of alkalic porphyry-style deposits, with
zones of depleted δ34Ssulfide values most prospective for high-grade mineralization. 相似文献
20.
Ozcan Yigit Albert H. Hofstra Murray W. Hitzman Eric P. Nelson 《Mineralium Deposita》2006,41(6):527-547
Gold Bar is one of several Carlin-type gold mining districts located in the Battle Mountain–Eureka trend, Nevada. It is composed of one main deposit, Gold Bar; five satellite deposits; and four resources that contain 1.6 Moz (50 t) of gold. All of the deposits and resources occur at the intersection of north-northwest- and northeast-trending high-angle faults in slope facies limestones of the Devonian Nevada Group exposed in windows through Ordovician basin facies siliciclastic rocks of the Roberts Mountains allochthon. Igneous intrusions and magnetic anomalies are notably absent. The Gold Bar district contains a variety of discordant and stratabound jasperoid bodies, especially along the Wall Fault zone, that were mapped and studied in some detail to identify the attributes of those most closely associated with gold ore and to constrain genetic models. Four types of jasperoids, J0, J1, J2, and J3, were distinguished on the basis of their geologic and structural settings and appearance. Field relations suggest that J0 formed during an early event. Petrographic observations, geochemistry, and δ18O values of quartz suggest it was overprinted by the hydrothermal event that produced ore-related J1, J2, and J3 jasperoids and associated gold deposits. The greater amount of siliciclastic detritus present in J0 jasperoids caused them to have higher δ18O values than J1,2,3 jasperoids hosted in underlying limestones. Ore-related jasperoids are composed of main-ore-stage replacements and late-ore-stage open-space filling quartz with variable geochemistry and an enormous range of δ18O values (24.5 and −3.7‰). Jasperoids hosted in limestones with the most anomalous Au, Ag, Hg, ±(As, Sb, Tl) concentrations and the highest δ18O values are associated with the largest deposits. The 28‰ range of jasperoid δ18O values is best explained by mixing between an 18O-enriched fluid and an 18O-depleted fluid. The positive correlation between the sizes of gold deposits and the δ18O composition of jasperoids indicates that gold was introduced by the 18O-enriched fluid. The lowest calculated δ18O value for water in equilibrium with late-ore-stage quartz at 200°C (−15‰) and the measured δD value of fluid inclusion water extracted from late-ore-stage orpiment and realgar (−116‰) indicate that the 18O-depleted fluid was composed of relatively unexchanged meteoric water. The source of the 18O-enriched ore fluid is not constrained. The δ34S values of late-ore-stage realgar, orpiment, and stibnite (5.7–15.5‰) and barite (31.5–40.9‰) suggest that H2S and sulfate were derived from sedimentary sources. Likewise, the δ13C and δ18O values of late-stage calcite (−4.8 to 1.5‰ and 11.5 to 17.4‰, respectively) suggest that CO2 was derived from marine limestones. Based on these data and the apparent absence of any Eocene intrusions in the district, Gold Bar may be the product of a nonmagmatic hydrothermal system. 相似文献