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1.
Melt inclusions in pegmatite quartz: complete miscibility between silicate melts and hydrous fluids at low pressure 总被引:10,自引:1,他引:9
Fluorine-, boron- and phosphorus-rich pegmatites of the Variscan Ehrenfriedersdorf complex crystallized over a temperature
range from about 700 to 500 °C at a pressure of about 1 kbar. Pegmatite quartz crystals continuously trapped two different
types of melt inclusions during cooling and growth: a silicate-rich H2O-poor melt and a silicate-poor H2O-rich melt. Both melts were simultaneously trapped on the solvus boundaries of the silicate (+ fluorine + boron + phosphorus) − water
system. The partially crystallized melt inclusions were rehomogenized at 1 kbar between 500 and 712 °C in steps of 50 °C by
conventional rapid-quench hydrothermal experiments. Glasses of completely rehomogenized inclusions were analyzed for H2O by Raman spectroscopy, and for major and some trace elements by EMP (electron microprobe). Both types of melt inclusions
define a solvus boundary in an XH2O–T pseudobinary system. At 500 °C, the silicate-rich melt contains about 2.5 wt% H2O, and the conjugate water-rich melt about 47 wt% H2O. The solvus closes rapidly with increasing temperature. At 650 °C, the water contents are about 10 and 32 wt%, respectively.
Complete miscibility is attained at the critical point: 712 °C and 21.5 wt% H2O. Many pegmatites show high concentrations of F, B, and P, this is particularly true for those pegmatites associated with
highly evolved peraluminous granites. The presence of these elements dramatically reduces the critical pressure for fluid–melt
systems. At shallow intrusion levels, at T ≥ 720 °C, water is infinitely soluble in a F-, B-, and P-rich melt. Simple cooling
induces a separation into two coexisting melts, accompanied with strong element fractionation. On the water-rich side of the
solvus, very volatile-rich melts are produced that have vastly different physical properties as compared to “normal” silicate
melts. The density, viscosity, diffusivity, and mobility of such hyper-aqueous melts under these conditions are more comparable
to an aqueous fluid.
Received: 15 September 1999 / Accepted: 10 December 1999 相似文献
2.
Olivier Bachmann Paul J. Wallace Julie Bourquin 《Contributions to Mineralogy and Petrology》2010,159(2):187-202
The >60 km3 rhyolitic Kos Plateau Tuff provides an exceptional probe into the behavior of volatile components in highly evolved arc magmas:
it is crystal-rich (30–40 vol% crystals), was rapidly quenched by the explosive eruptive process, and contains abundant homogeneous
melt inclusions in large quartz crystals. Several methods for measuring major, trace and volatile element concentrations (SIMS,
FTIR, Raman spectroscopy, electron microprobe, LA–ICPMS) were applied to these melt inclusions. We found a ~2 wt% range of
H2O contents (4.5–6.5 wt% H2O, measured independently by SIMS, FTIR, and Raman spectroscopy) and relatively low CO2 concentrations (15–140 ppm measured by FTIR, with most analyses <100 ppm). No obvious correlations between H2O, CO2, major and trace elements are observed. These observations require a complex, protracted magma evolution in the upper crust
that included: (1) vapor-saturated crystallization in a chamber located between 1.5 and 2.5 kb pressure, (2) closed-system
degassing (with up to 10 vol% exsolved gas) as melts percolated upwards through a vertically extensive mush zone (2–4 km thick),
and (3) periodic gas fluxing from subjacent, more mafic and more CO2-rich magma, which is preserved as andesite bands in pumices. These processes can account for the range of observed H2O and CO2 values and the lack of correlation between volatiles and trace elements in the melt inclusions. 相似文献
3.
Raffaello Cioni 《Contributions to Mineralogy and Petrology》2000,140(1):40-54
The evolution of volatiles in the AD 79 magma chamber at Vesuvius (Italy) was investigated through the study of melt inclusions
(MI) in crystals of different origins. FTIR spectroscopy and EMPA were used to measure H2O, CO2, S and Cl of the different melts. This allowed us to define the volatile content of the most evolved, phonolitic portion
of the magma chamber and of the mafic melts feeding the chamber. MI in sanidine from phonolitic and tephri-phonolitic pumices
show systematic differences in composition and volatile content, which can be explained by resorption of the host mineral
during syn-eruptive mixing. The pre-eruption content of phonolitic magma appears to have been dominated by H2O and Cl (respectively 6.0 to 6.5 wt% and 6700 ppm), while magma chamber refilling occurred through the repeated injection
of H2O, CO2 and S-rich tephritic magmas (respectively 3%, 1500 ppm and 1400 ppm). Strong CO2 degassing probably occurred during the decompressional path of mafic batches towards the magma chamber, while sulphur was
probably released by the magma following crystallization and mixing processes. Water and chlorine strongly accumulated in
the magma and reached their solubility limits only during the eruption. Chlorine solubility appears to have been strongly
compositionally controlled, and Cl release was inhibited by groundmass crystallization of leucite, which shifted the composition
of the residual liquid towards higher Cl solubilities.
Received: 28 October 1999 / Accepted: 21 April 2000 相似文献
4.
The Voisey's Bay nickel–copper deposit and the Pants Lake sulfide occurrences are the most important mineral systems discovered
to date within the Nain Plutonic Suite in northern Labrador. There are many intriguing similarities at both locations. Both
are hosted by relatively small troctolite/gabbro bodies that intrude the sulfide-bearing paragneiss of the Churchill Province,
and these intrusions contain inclusions of the paragneiss. Similar chemical reactions of the gneissic inclusions with the
host magmas are observed at both locations. The reactions resulted in the addition of SiO2, K2O, Na2O and sulfur to the magmas, and are responsible for sulfide-saturation and resultant segregation of immiscible sulfide liquids
from the magmas. The initial sulfide liquids in both cases were relatively poor in metals, containing <2.5 wt% Ni and 2 wt%
Cu. The sulfides at Pants Lake remained poor in metals because of a lack of subsequent interaction with new, chalcophile-undepleted
magma. At Voisey's Bay, the initial sulfides segregated in a dynamic conduit, and were subsequently upgraded in metals to
∼6 wt% Ni and 3 wt% Cu by a new surge of undepleted magma using the same conduit. These sulfides were then concentrated to
form large sulfide bodies in the wider parts of the conduit and its entry to an upper chamber in response to a sudden change
of liquid velocity in these environments. This study confirms three of the most important factors for the formation of magmatic
sulfide deposits in an evolved magma system: (1) contamination of magma with sulfide-bearing country rock to achieve sulfide
saturation; (2) a dynamic magmatic system such as a magma conduit to transport large volume of sulfide liquid and to concentrate
them in limited localities, and (3) upgrading of metals in the sulfide by new, chalcophile-undepleted magma.
Received: 20 February 2000 / Accepted: 14 September 2000 相似文献
5.
Quartz crystals from topaz–zinnwaldite–albite granites from Zinnwald (Erzgebirge, Germany) contain, in addition to primary and secondary fluid inclusions (FIs), abundant crystalline silicate-melt inclusions (MIs) with diameters up to 200 m. These MIs represent various stages of evolution of a highly evolved melt system that finally gave rise to granite-related Sn–W mineralization. The combination of special experimental techniques with confocal laser Raman-microprobe spectroscopy and EMPA permits precise measurement of elevated contents of H2O, F, and B in re-homogenized MIs. The contents of H2O and F were observed to increase from 3 to 30 and 1.9 to 6.4 wt%, respectively, during magma differentiation. However, there is a second MI group, very rich in H2O, with values up to 55 wt% H2O and an F concentration of approximately 3 wt%. Ongoing enrichment of volatiles H2O, F, B, and Cl and of Cs and Rb can be explained in terms of magma differentiation triggered by fractional crystallization and thus, is suggested to reflect elemental abundances in natural magmas, and not boundary-layer melts. Partitioning between melt and cogenetic fluids has further modified the magmatic concentrations of some elements, particularly Sn. The coexistence of two types of MIs with primary FIs indicates fluid saturation early in the history of magma crystallization, connected with a continuous sequestration of Sn, F, and B. The results of this study provide additional evidence for the extraordinary importance of the interplay of H2O, F, and B in the enrichment of Sn during magma differentiation by decreasing the viscosity of and increasing the diffusivity in the melts as well as by the formation of various stable fluoride complexes in the melt and coexisting fluid.
相似文献
Rainer ThomasEmail: Phone: +49-331-2881474 |
6.
The behaviour of boron in a peraluminous granite-pegmatite system and associated hydrothermal solutions: a melt and fluid-inclusion study 总被引:1,自引:0,他引:1
Thomas Rainer Förster Hans-Jürgen Heinrich Wilhelm 《Contributions to Mineralogy and Petrology》2003,144(4):457-472
Detailed analyses of melt and fluid inclusions combined with an electron-microprobe survey of boron-bearing minerals reveal the evolution of boron in a highly evolved peraluminous granite-pegmatite complex and the associated high- and medium-temperature ore-forming hydrothermal fluids (Ehrenfriedersdorf, Erzgebirge, Germany). Melt inclusions in granite represent embryonic pegmatite-forming melts containing about 10 wt% H2O and 1.8 wt% B2O3. These melts are also enriched in F, P, and other incompatible elements such as Be, Sn, Rb, and Cs. Ongoing differentiation and volatile enrichment drove the system into a solvus, where two pegmatite-forming melts coexisted. The critical point is at about 712 °C, 100 MPa, 20 wt% H2O and 4.1 wt% B2O3. Cooling and concomitant fractional crystallisation from 700 to 500 °C induced development of two conjugate melts, an H2O-poor (A-melt) and an H2O-rich melt (B-melt) along the opening solvus. Boron is a major element in both melts and is preferentially partitioned into the H2O-rich melt. Temperature-dependent distribution coefficients DboronB - melt/A - melt D_{{\rm{boron}}}^{{\rm{B - melt/A - melt}}} are 1.3 at 650 °C, 1.5 at 600 °C, and 1.8 at 500 °C. In both melts, boron concentrations decreased during cooling because of exsolution of a boron-rich hypersaline brine throughout the pegmatitic stage. Boromuscovite containing up to 8.5 wt% was another sink for boron at this stage. The end of the melt-dominated pegmatitic stage was attained at a solidus temperature of around 490 °C. Fluid inclusions of the hydrothermal stage reveal trapping temperatures of 480 to 370 °C, along with varying densities and highly variable B2O3 contents ranging from 0.20 to 2.94 wt%. A boiling system evolved, indicating a complex interplay between closed- and open-system behaviour. Pressure switched from lithostatic to hydrostatic and back, generating hydrothermal convection cells where meteoric waters were introduced and mixed with magmatic fluids. Boron-rich solutions originated from magmatic fluids, whereas boron-depleted fluids were mainly of meteoric origin. This highlights the potential of boron for discriminating fluids of different origin. Tin is continuously enriched during the evolution because tin and boron are cross-linked by formation of boron-, fluorine- and tin-fluorine-bearing complexes and is finally deposited within quartz-cassiterite veins during the transition from closed- to open-system behaviour. Boron does not only trace the complex evolution of the Ehrenfriedersdorf complex but exerts, together with H2O, F and P, an important control on the physical and chemical properties of pegmatite-forming melts, and particularly on the formation of a two-melt solvus at low pressure. We discuss this with respect to experimental results on H2O solubility and the critical behaviour of the haplogranite-water system which contained variable concentrations of volatiles. 相似文献
7.
A model of the cooling history of tin-bearing granitic magma forming the Schellerhau granites (Eastern Erzgebirge, Germany)
is shown on the basis of quartz textures. Similar grain size, similar grain habit and correlatable growth textures of phenocrysts
in different granite varieties give proof of a common crystallization history before the melts of the Schellerhau granite
varieties were intruded. Four nucleation events occurred during crystallization in different crustal levels between about
20 and 1 km depth. The parental melt of the Schellerhau granites is interpreted to have contained<2.5 wt% H2O originally. The water content of the melt during the subvolcanic intrusion stage amounted to more than 5 wt% and characterizes
highly evolved residual melts that enable the formation of tin deposits. This study contributes to a better understanding
of the development and behaviour of fractionated tin-bearing granitic melts, and links quartz cathodoluminescence (CL) with
microanalytical studies.
Received: 28 October 1998 / Accepted: 18 August 1999 相似文献
8.
Granites and primary tin mineralization in the Erzgebirge were dated using (1) conventional U–Pb dating of uraninite inclusions
in mica, (2) Rb–Sr dating of inclusions in quartz that represent highly evolved melts, (3) Re–Os dating of magmatic–hydrothermal
molybdenite, and (4) chemical Th–U–Pb dating of uraninite. Conventional isotope dilution and thermal ion mass spectrometry
and chemical Th–U–Pb dating of uraninite in granites from the Ehrenfriedersdorf mining district provide ages of 323.9 ± 3.5
Ma (2σ; Greifenstein granite) and 320.6 ± 1.9 and 319.7 ± 3.4 Ma (2σ, both Sauberg mine), in agreement with U–Pb apatite ages of 323.9 ± 2.9 and 317.3 ± 1.6 Ms (2σ, both Sauberg mine). Rb–Sr analysis of melt inclusions from Zinnwald gives highly radiogenic Sr isotopic compositions that,
with an assumed initial Sr isotopic composition, permit calculation of precise ages from single inclusions. The scatter of
the data indicates that some quartz-hosted melt inclusions have been affected by partial loss of fluid exsolved from the melt
inclusion. Re–Os dating of two molybdenite samples from Altenberg provides ages of 323.9 ± 2.5 and 317.9 ± 2.4 Ma (2σ). Together with age data from the literature, our new ages demonstrate that primary tin mineralization and the emplacement
of the large Sn-specialized granites in the Erzgebirge fall in a narrow range between 318 and 323 Ma. Primary Sn mineralization
occurred within a short interval during post-collisional collapse of the Variscan orogen and was essentially synchronous over
the entire Erzgebirge. In contrast to earlier claims, no systematic age difference between granites of the eastern and western
Erzgebirge was established. Furthermore, our data do not support a large age range for Late-Variscan granites of the Erzgebirge
(330–290 Ma), as has been previously suggested. 相似文献
9.
Summary ?Part of the Mesoproterozoic (1.6 Ga) Gawler Range Volcanics in South Australia is composed of mingled feldspar- quartz- phyric
dacite, rhyodacite and rhyolite lavas. Field relationships suggest that dacite erupted first, locally grading into rhyodacite,
followed by mingled dacite and rhyolite or rhyodacite and rhyolite, and finally in some areas rhyolite, and imply that the
three lithofacies co-existed in a compositionally stratified magma chamber. Data on the bulk rock, groundmass and melt inclusion
compositions suggest that post-eruption alteration has had very little effect on the original rock compositions. Melt inclusions
in quartz from rhyolite and rhyodacite-dacite, respectively, belong to two compositional populations. Inclusions in the rhyolitic
quartz have less evolved compositions with lower SiO2 (72–76.4 wt %) and higher Al2O3 (13.2–15.6 wt%) and Na2O (2.5–4.2 wt%) abundances. In contrast, melt inclusions in quartz from the rhyodacite-dacite are more “evolved” (i.e., 75.5–78.3 wt%
SiO2, 11.2–12.7 wt% Al2O3 and 1.7–2.2 wt% Na2O). The two melt populations define a single compositional trend towards groundmass compositions, which are essentially similar
in all three lithofaci es (77.8–80.5 wt% SiO2, 9.9–11.1 wt% Al2O3 and 2.2–2.4 wt% Na2O). This trend is consistent with the derivation of the groundmass melt from a single precursor melt of rhyolitic composition
by means of crystallisation of dominant plagioclase, K-feldspar and minor quartz. Plagioclase-enriched dacite-rhyodacite magma
comprises a mixture of the residual melt and plagioclase phenocryst s that accumulated in the upper part of the magma chamber
and erupted first. Similar residual melt containing quartz and K-feldspar phenocrysts was present deeper in the magma chamber
and erupted later to form quartz-, K-feldspar-phyric rhyolite.
Received April 1, 1999;/revised version accepted July 27, 1999 相似文献
Zusammenfassung ?Die Bildung von Si-reichem Dacit aus rhyolitischer Schmelze: Evidenz aus Schmelzeinschlüssen in Laven der 1.6 Ga Gawler Range Volcanics, Südaustralien Ein Teil der mesoproterozoischen (1.6 Ga) Gawler Range Volcanics in Südaustralien setzt sich aus “mingled” Feldspat- Quarz-phyrischen dacitischen, rhyodacitischen und rhyolithischen Laven zusammen. Gel?ndebefunde legen nahe, da? die Dacite, die lokal in Rhyodacite übergehen, zuerst eruptierten, gefolgt vom “mingled” Dacit und Rhyolith oder Rhyodacit und Rhyolith. Schlie?lich bildeten sich in einigen Gebieten Rhyolithe. Diese Beobachtungen lassen die Schlu?folgerung zu, da? die drei Lithofazies in einer geschichteten Magmenkammer koexistierten. Die Daten der Gesamtgesteins-, Grundmasse- und Schmelzeinschlu?- Zusammensetzungen zeigen, da? Alterationsvorg?nge nach der Eruption einen sehr minimalen Effekt auf die ursprüngliche Gesteinszusammensetzung hatten. Die Schmelzeinschl üsse in den Rhyolithen und Rhyodaciten geh?ren zwei unterschiedlich en Populationen an. Die Schmelzeinschlüsse in Quarz der Rhyolithe sind weniger deutlich “entwickelt” mit niedrigeren SiO2 (72–76.4 Gew.%) und h?heren Al2O3 (13.2–15.6 Gew.%) und Na2O-(2.5–4. 2 Gew.%) Gehalten. Im Unterschied dazu sind die Einschlüss e in Quarz aus Rhyodacit-Dacit st?rker “entwickelt” (i.e., 75.5–78.3 Gew.% SiO2, 11.2–12.7 G ew.% Al2O3 und 1.7–2.2 Gew.% Na2O). Die beiden Populationen von Schmelzeinschlüssen definieren einen einzigen Entwicklungstrend hin zur Zusammensetzung der Grundmasse, die in allen drei Lithofazies ?hnlich ist (77.8–80.5 Gew.% SiO2, 9.9–11.1 Gew.% Al2O3 und 2.2–2.4 Gew.% Na2O). Dieser Trend ist mit der Herkunft der Grundmasse-bildenden Schmelze aus einer einzigen Ausgangsschmelze rhyolithischer Zusammensetzung infolge der Kristallisation von haupts?chlich Plagioklas, Alkalifeldspat und untergeordnet Quarz konsistent. Dacit-Rhyodacitmagmen, die an Plagioklas angereichert sind, stellen eine Mischung der Residualschmelze mit Plagioklas- Ph?nokristallen, die sich in den oberen Teilen der Magmenkammer akkumuli ert haben, dar; sie eruptierten zuerst. ?hnliche residuale Schmelzen mit Quarz und Akalifeldspat-Ph?nokristallen waren auf die tieferen Teilen der Magmenkammer beschr?nkt; sie eruptierten sp?ter und bildeten die Quarz- und Akalifeldspat-phyrischen Rhyolithe.
Received April 1, 1999;/revised version accepted July 27, 1999 相似文献
10.
To interpret the degassing of F-bearing felsic magmas, the solubilities of H2O, NaCl, and KCl in topaz rhyolite liquids have been investigated experimentally at 2000, 500, and ≈1 bar and 700° to 975 °C.
Chloride solubility in these liquids increases with decreasing H2O activity, increasing pressure, increasing F content of the liquid from 0.2 to 1.2 wt% F, and increasing the molar ratio
of ((Al + Na + Ca + Mg)/Si). Small quantities of Cl− exert a strong influence on the exsolution of magmatic volatile phases (MVPs) from F-bearing topaz rhyolite melts at shallow
crustal pressures. Water- and chloride-bearing volatile phases, such as vapor, brine, or fluid, exsolve from F-enriched silicate
liquids containing as little as 1 wt% H2O and 0.2 to 0.6 wt% Cl at 2000 bar compared with 5 to 6 wt% H2O required for volatile phase exsolution in chloride-free liquids. The maximum solubility of Cl− in H2O-poor silicate liquids at 500 and 2000 bar is not related to the maximum solubility of H2O in chloride-poor liquids by simple linear and negative relationships; there are strong positive deviations from ideality
in the activities of each volatile in both the silicate liquid and the MVP(s). Plots of H2O versus Cl− in rhyolite liquids, for experiments conducted at 500 bar and 910°–930 °C, show a distinct 90° break-in-slope pattern that
is indicative of coexisting vapor and brine under closed-system conditions. The presence of two MVPs buffers the H2O and Cl− concentrations of the silicate liquids. Comparison of these experimentally-determined volatile solubilities with the pre-eruptive
H2O and Cl− concentrations of five North American topaz and tin rhyolite melts, determined from melt inclusion compositions, provides
evidence for the exsolution of MVPs from felsic magmas. One of these, the Cerro el Lobo magma, appears to have exsolved alkali
chloride-bearing vapor plus brine or a single supercritical fluid phase prior to entrapment of the melt inclusions and prior
to eruption.
Received: 6 November 1995 / Accepted: 29 January 1998 相似文献
11.
The Sn-rich Qiguling topaz rhyolite dike intrudes the Qitianling biotite granite of the Nanling Range in southern China; the granite hosts the large Furong Sn deposit. The rhyolite dike is typically peraluminous, volatile-enriched, and highly evolved. Whole-rock F and Sn concentrations attain 1.9 wt.% and 2700 ppm, respectively. The rhyolite consists of a fine-grained matrix formed by quartz, feldspar, mica and topaz, enclosing phenocrysts of quartz, feldspar and mica; it is locally crosscut by quartz veinlets. Lithium-bearing micas in both phenocrysts and the groundmass can be classified as primary zinnwaldite, “Mus-Ann” (intermediate member between annite and muscovite), and secondary Fe-rich muscovite. Topaz is present in the groundmass only; common fluorite occurs in the groundmass and also in a specific cassiterite, rutile and fluorite (Sn–Ti–F) assemblage. Cassiterite and rutile are the only Sn and Ti minerals; both cassiterite and Nb-rich rutile are commonly included in the phenocrysts. The Sn–Ti–F assemblage is pervasive, and contains spongy cassiterite in some cases; cassiterite also occurs in quartz veinlets which cut the groundmass. Electron microprobe and LA-ICP-MS compositions were used to study the magmatic and hydrothermal processes and the role of F in Sn mineralization. The presence of zinnwaldite and “Mus-Ann”, which are respectively representative of early and late mica crystallization during magma differentiation, also suggests a significant decrease in f(HF)/f(H2O) of the system. Cassiterite included in the zinnwaldite phenocrysts is suggested to have crystallized from the primary magma at high temperature. Within the Sn–Ti–F aggregates, rutile crystallized as the earliest mineral, followed by fluorite and cassiterite. Spongy cassiterite containing inclusions of the groundmass minerals indicate a low viscosity of the late fluid. The cassiterite in the quartz veinlets crystallized from low-temperature hydrothermal fluids, which possibly mixed with meteoric water. In general, cassiterite precipitated during both magmatic and hydrothermal stages, and over a range of temperatures. The original fluorine and tin enrichments, f(HF)/f(H2O) change in the residual magma, formation of Ca,Sn,F-rich immiscible fluid, decrease of the f(HF) during groundmass crystallization, and mixing of magma-derived fluids with low-saline meteoric water during the late hydrothermal stage, are all factors independently or together responsible for the Sn mineralization in the Qiguling rhyolite. 相似文献
12.
R. L. Linnen 《Mineralium Deposita》1998,33(5):461-476
The most important tin mineralization in Thailand is associated with the Late Cretaceous to Middle Tertiary western Thai granite
belt. A variety of deposit types are present, in particular pegmatite, vein and greisen styles of mineralization. A feature
common to most of the deposits is that they are associated with granites that were emplaced into the Khang Krachan Group,
which consists of poorly sorted, carbonaceous, pelitic metasediments. Most of the deposits contain low to moderately saline
aqueous fluid inclusions and aqueous-carbonic inclusions with variable CH4/CO2 ratios. Low salinity aqueous inclusions represent trapped magmatic fluid in at least one case, the Nong Sua pegmatite, based
on their occurrence as primary inclusions in magmatic garnet. Aqueous-carbonic inclusions are commonly secondary and neither
the CO2 nor NaCl contents of these inclusions decrease in progressively younger inclusions, implying that they are not magmatic in
origin. Reduced carbon is depleted in the metasediments adjacent to granites and the δD values greisen muscovites are variable,
but are as low as −134 per mil, indicative of fluid interaction with organic (graphitic) material. This suggests that the
aqueous-carbonic fluid inclusions represent fluids that were produced, at least in part, during contact metamorphism-metasomatism.
By comparing the western Thai belt with other Sn-W provinces it is evident that there is a strong correlation between fluid
composition and pressure in general. Low to moderately saline aqueous inclusions and aqueous-carbonic inclusions are characteristic
of mineralization associated with relatively deep plutonic belts. Mineralized pegmatites are also typically of deeper plutonic
belts, and pegmatite-hosted deposits may contain cassiterite that is magmatic (crystallized from granitic melt) or is orthomagmatic-hydrothermal
(crystallized from aqueous or aqueous-carbonic fluids) in origin. The magmatic aqueous fluids (those that were exsolved from
granitic melts) are interpreted to have had low salinities. As a consequence of the low salinities, tin is partitioned in
favour of the melt on vapour saturation. Thus with a high enough degree of fractionation, the crystallization of a magmatic
cassiterite (or different Sn phase such as wodginite) is inevitable. Because tin is not partitioned in favour of the vapour
phase upon water saturation of the granitic melts, it is proposed that relatively deep vein and greisen systems tend to form
by remobilization processes. In addition, many deeper greisen systems are hosted, in part, by carbonaceous pelitic metasediments
and the reduced nature of the metasediments may play a key role in remobilizing tin. Sub-volcanic systems by contrast are
characterized by high temperature-high salinity fluids. Owing to the high chlorinity, tin is strongly partitioned in favour
of the vapour and cassiterite mineralization can form by of orthomagmatic-hydrothermal processes. Similar relationships between
the depth of emplacement and fluid composition also appear to apply to other types of granite-hosted deposits, such as different
types of molybdenum deposits.
Received: 8 September 1997 / Accepted: 28 October 1997 相似文献
13.
N. W. A. Odling D. H. Green B. Harte 《Contributions to Mineralogy and Petrology》1997,129(2-3):209-221
An experimental method of melt inclusion synthesis within olivine crystals has been developed to determine the composition
of the melt present in a partially molten peridotite assemblage. Trace element doped peridotite was equilibrated with 5 wt%
of a C-O-H volatile source at 20 kbar/1175 °C in a piston-cylinder apparatus under buffered oxygen and sulphur fugacity conditions
[log(f
O2) ∼ IW +1 log unit, log (f
S2) ∼ Fe/FeS > +1 log unit]. A single crystal of olivine, which had been cut to a disc shape, was included in the sample capsule.
At run conditions the peridotite charge formed olivine, orthopyroxene, clinopyroxene, Fe-Ni sulphide and a volatile-bearing
melt. The melt phase is preserved as homogeneous glass inclusions up to 50 μm in size, trapped in situ in the olivine disc.
The major element composition of the glass inclusions showed them to be of broadly basaltic character, but with a low Mg/(Mg + ΣFe),
which is associated with precipitation of olivine from the melt inclusion onto the walls of the olivine disc during quenching.
Thus the equilibrium melt composition has been calculated from the glass inclusion composition by addition of olivine component
using the Fe/Mg exchange coefficient of Roeder and Emslie (1970); the desired Mg/(Mg + ΣFe) being determined from the composition
of olivine formed at run conditions in the peridotite section of the charge. The melt composition obtained is close to the
trend for dry melting established by Falloon and Green (1988), and it is evident that although the reduced volatiles in this
case have induced a liquidus depression of some 250 °C, there has been only a small shift in melt composition. Trace element,
carbon and hydrogen contents of thirteen melt inclusions have been determined by secondary ion mass spectrometry (SIMS). The
trace element signature is consistent with ∼29% melting in equilibrium with a lherzolitic assemblage. The equilibrium melt
has a C/H of 0.48 by weight. Carbon solubility in partial melts is thus significant under reducing conditions in the presence
of dissolved “water components” and establishes a major melt fluxing role for carbon in the upper mantle. The ubiquitous presence
of carbon and hydrogen in basaltic magmas underscores the importance of determining both the position of vapour-present solidi
and the composition of melts generated, when developing petrogenetic models.
Received: 1 July 1996 / Accepted: 25 June 1997 相似文献
14.
Chemical composition of tourmaline from the Yunlong tin deposit, Yunnan, China: implications for ore genesis and mineral exploration 总被引:1,自引:0,他引:1
Summary ?The Yunlong tin deposit is located in the northern part of the Lancangjiang metamorphic zone of the Sanjiang Tethys orogen
series in western Yunan province of China. It consists of vein-type cassiterite ores, which are mainly hosted in migmatites
of Caledonian age. Abundant tourmaline is associated with the ores, quartz–tourmaline veins and barren migmatized gneiss and
migmatites. A detailed electron microprobe study has been carried out to document the chemical compositions of tourmaline
from this deposit. The results exhibit a systematic compositional change that might be used as tracer for ore genesis and
in prospecting for tin mineralization. Tourmalines from the ore bodies are dravite with Fe/(Fe + Mg) ratios of 0.09 ∼ 0.31
and Ca/(Ca + Na) ratios of 0.03 ∼ 0.40. These tourmalines are also rich in chromium (up to 0.74 wt% Cr2O3) and tin (up to 0.42 wt% Sn). In contrast, tourmalines from the barren migmatites are mostly schorl with Fe/(Fe + Mg) ratios
of 0.38 ∼ 0.94 and Ca/(Ca + Na) ratios of 0.00 ∼ 0.14. Tourmalines from quartz–tourmaline veins that occur between ore bodies
and the migmatites show intermediate compositions, i.e., Fe/(Fe + Mg) = 0.09 ∼ 0.59, Ca/(Ca + Na) = 0.01 ∼ 0.22.
It is suggested that the Mg-rich nature of the tourmaline can be used as an exploration tool in this region to target tin
mineralization, because the tourmalines show increasing Mg contents and are more dravitic when approaching the ore bodies.
It is likely that the formation of the Yunlong tin deposit was related to migmatitic-hydrothermal processes. The high Mg and
Cr contents in tourmalines from the ore bodies were probably derived from the local meta-sedimentary and meta-volcanic rocks
of the Precambian Chongshan Group rather than from the granites in the region.
Received December 28, 2000; revised version accepted January 25, 2002 相似文献
15.
Tin deposits are often closely associated with granitic intrusions. In this study, we analyzed tin partition coefficients between different fluids and melts (\({\text{D}}_{Sn}^{aq.fl./melt}\)) as well as various crystals and melts \({\text{D}}_{Sn}^{aq.fl./melt}\)(\({\text{D}}_{Sn}^{crystal/melt}\)) from the Furong tin deposit associated with the Qitianling A-type granite. Our experimental results indicate that tin partition behavior is affected by the chemical compositions of fluids, melts, and minerals. Tin is prone to partitioning into the residual magma in fractional crystallization or other differential magmatic processes if the magma originated from crustal sources with high alkali content, high volatile content, and low oxygen fugacity. Highly evolved residual peralkaline granitic magma enriched in tin can lead to tin mineralization in a later stage. Furthermore, the volatiles F and Cl in the magma play important roles in tin partitioning behavior. Low F contents in the melt phase and high Cl content in the aqueous fluid phase are favorable factors for tin partitioning in the aqueous fluid phase. High Cl content in the aqueous fluid catalyzes water–rock interaction and leads to the extraction of tin from tin-bearing minerals. All these findings support a hydrothermal origin for the tin deposits. In light of the geotectonic setting, petrochemical characteristics, and mineralizing physicochemical conditions of the Furong tin deposit, it is inferred that the ore-forming fluid of the Furong tin ore deposit could have derived from the Qitianling peralkaline intrusion. 相似文献
16.
S. Signorelli G. Vaggelli C. Romano M. Carroll 《Contributions to Mineralogy and Petrology》2001,140(5):543-553
The distribution of H2O, F, Cl and S in the Campanian Ignimbrite (CI) magma chamber was investigated through study of primary glass inclusions and matrix glasses from pumices of the Plinian fall deposit. The eruption, fed by trachytic to phono-trachytic magmas, mainly produced a trachytic non-welded to partially welded tuff, underlain by a minor cogenetic fallout deposit. The entire chemical variability of the eruptive products is well represented in the pumices of the Plinian fall deposit, which we divide into a basal Lower Fall Unit (LFU) and an overlying Upper Fall Unit (UFU). Primary glass inclusions were only found in clinopyroxenes associated with the LFU pumice and contain a mean of 1.60ǂ.32 wt% H2O (analysed by FTIR), 0.11ǂ.08 wt% F, 0.37ǂ.03 wt% Cl and 0.08ǂ.04 wt% SO3 (EMP analysis); CO2 concentrations were below the FTIR detection limit (10-20 ppm). The coexisting matrix glasses contain similar amounts of halogens and sulfur but less water (~0.60 wt%). Partially degassed matrix glasses from UFU pumices contain a mean of 0.30ǂ.02 H2O, 0.28ǂ.10 F, 0.04ǂ.02 SO3 and 0.80ǂ.04 wt% Cl. To reconstruct the total amount of volatiles dissolved in the most evolved trachytes we have used experimental solubility data and mass balance calculations concerning the amount of crystal fractionation required to produce the most evolved trachyte from the least evolved trachyte; these yield an estimated pre-eruptive magma volatile content (H2O + Cl + F) of ~5.5 wt% for the most evolved magmas. On the basis of new determinations of Cl solubility limits in hydrous trachytic melts coexisting with an aqueous fluid phase + hydrosaline melt (brine), we suggest that the upper part of the magma chamber which fed the CI eruption was fluid(s) saturated and at a minimum depth of ~2 km. Variations in eruptive style (Plinian fallout, pyroclastic flows) do not appear to be related to significant variations in pre-eruptive volatile contents. 相似文献
17.
James G. Brophy Carla S. Whittington Young-Rok Park 《Contributions to Mineralogy and Petrology》1999,135(2-3):277-290
Several high alumina basalts from the Aleutian volcanic centers of Cold Bay and Kanaga Island contain large (up to 1.5 cm
diameter) megacrysts of sector-zoned augite. The megacrysts are invariably euhedral with well developed {001}, {010} and {111}
forms. All crystals display concentric bands that are rich in mineral and glass inclusions. The sector zonation typically
occurs as well developed (010), (100), (111) and (110) sectors which grew at different rates. A comparison of the width of
synchronous growth bands indicates that following relative growth rates: (111) ≫ (100) ∼ (110) > (010). Compositionally, SiO2 and MgO abundances decrease, and TiO2, Al2O3, FeO and Na2O abundances increase in the different sectors in the order (111), (100) ∼ (110), (010). This order is identical to that deduced
for the relative growth rates, implying that growth rate clearly had a role in the development of the sector zonation. Calculated
pre-eruption H2O contents of the basalts range from 1 to 3 wt% but actual (measured) post-eruption H2O contents range from 0.01 to 0.3 wt%. Deteurium isotopic values are heavily depleted and range from −110 to −141‰ . Together
these indicate significant vapor (H2O) exsolution prior to eruption. Maximum H2O abundances in primitive glass inclusions, thought to be most representative of the host liquid reservoir at the time of
melt entrapment, systematically decrease from the core to the rim of one augite megacryst studied in detail. We conclude that
the presence of sector-zoned augite is due to augite supersaturation and rapid crystallization brought about by magma decompression
and volatile (H2O) exsolution. The calculated pre-eruption H2O contents of 1–3 wt% limit vapor exsolution and basalt crystallization to depths of less than 3 and more likely 1.5 km. Very
rapid crystallization at very shallow depths makes it unlikely that the time scales between initial crystallization and final
eruption are sufficient to permit appreciable amounts of fractional crystallization. Given that high alumina basalt fractionation
is the dominant process for generating more evolved andesite, dacite and rhyolite magmas of the calc-alkaline suite, the inability
of parental high alumina basalt to yield such derivative magmas in the low pressure environment places the likely site of
fractionation in the high pressure environment, at or near the base of the crust.
Received: 1 December 1997 / Accepted: 23 December 1998 相似文献
18.
T. F. D. Nielsen I. P. Solovova I. V. Veksler 《Contributions to Mineralogy and Petrology》1997,126(4):331-344
Perovskite and melilite crystals from melilitolites of the ultramafic alkaline Gardiner complex (East Greenland) contain
crystallised melt inclusions derived from: (1) melilitite; (2) low-alkali carbonatite; (3) natrocarbonatite. The melilitite
inclusion (1) homogenisation temperature of 1060 °C is similar to liquidus temperatures of experimentally investigated natural
melilitites. The compositions are peralkaline, low in MgO (ca.␣5 wt%), Ni and Cr, and they are low-pressure fractionates of
more magnesian larnite-normative ultramafic lamprophyre-type melts of primary mantle origin. Low-alkali carbonatite compositions
(2) homogenise at 1060–1030 °C and are compositionally similar to immiscible calcite carbonatite dykes derived from the melilitolite
magma. Natrocarbonatite inclusions (3) homogenise between 1030 and 900 °C and are compositionally similar to natrocarbonatite
lava from Oldoinyo Lengai. Nephelinitic to phonolitic dykes which are related to the calcite carbonatite dykes, are very Zr-rich
and agpaitic (molecular Na2O + K2O/Al2O3 > 1.2) and resemble nephelinites of Oldoinyo Lengai. The petrographic, geochemical and temporal relationships indicate unmixing
of carbonatite compositions (ca. 10% alkalies) from evolving melilitite melt and continued fractionation of melilitite to
nephelinite. It is suggested that the natrocarbonatite compositions represent degassed supercritical high temperature fluid
formed in a cooling body of strongly larnite-normative nephelinite or evolved melilitite. The Gardiner complex and similar
melilitolite and carbonatite-bearing ultramafic alkaline complexes are believed to represent subvolcanic complexes formed
beneath volcanoes comparable to Oldoinyo Lengai and that the suggested origin of natrocarbonatite may be applied to natrocarbonatites
of Oldoinyo Lengai.
Received: 18 January 1996 / Accepted: 2 September 1996 相似文献
19.
D. M. Ruscitto P. J. Wallace A. J. R. Kent 《Contributions to Mineralogy and Petrology》2011,162(1):109-132
Primitive chemical characteristics of high-Mg andesites (HMA) suggest equilibration with mantle wedge peridotite, and they
may form through either shallow, wet partial melting of the mantle or re-equilibration of slab melts migrating through the
wedge. We have re-examined a well-studied example of HMA from near Mt. Shasta, CA, because petrographic evidence for magma
mixing has stimulated a recent debate over whether HMA magmas have a mantle origin. We examined naturally quenched, glassy,
olivine-hosted (Fo87–94) melt inclusions from this locality and analyzed the samples by FTIR, LA-ICPMS, and electron probe. Compositions (uncorrected
for post-entrapment modification) are highly variable and can be divided into high-CaO (>10 wt%) melts only found in Fo > 91
olivines and low-CaO (<10 wt%) melts in Fo 87–94 olivine hosts. There is evidence for extensive post-entrapment modification
in many inclusions. High-CaO inclusions experienced 1.4–3.5 wt% FeOT loss through diffusive re-equilibration with the host olivine and 13–28 wt% post-entrapment olivine crystallization. Low-CaO
inclusions experienced 1–16 wt% olivine crystallization with <2 wt% FeOT loss experienced by inclusions in Fo > 90 olivines. Restored low-CaO melt inclusions are HMAs (57–61 wt% SiO2; 4.9–10.9 wt% MgO), whereas high-CaO inclusions are primitive basaltic andesites (PBA) (51–56 wt% SiO2; 9.8–15.1 wt% MgO). HMA and PBA inclusions have distinct trace element characteristics. Importantly, both types of inclusions
are volatile-rich, with maximum values in HMA and PBA melt inclusions of 3.5 and 5.6 wt% H2O, 830 and 2,900 ppm S, 1,590 and 2,580 ppm Cl, and 500 and 820 ppm CO2, respectively. PBA melts are comparable to experimental hydrous melts in equilibrium with harzburgite. Two-component mixing
between PBA and dacitic magma (59:41) is able to produce a primitive HMA composition, but the predicted mixture shows some
small but significant major and trace element discrepancies from published whole-rock analyses from the Shasta locality. An
alternative model that involves incorporation of xenocrysts (high-Mg olivine from PBA and pyroxenes from dacite) into a primary
(mantle-derived) HMA magma can explain the phenocryst and melt inclusion compositions but is difficult to evaluate quantitatively
because of the complex crystal populations. Our results suggest that a spectrum of mantle-derived melts, including both PBA
and HMA, may be produced beneath the Shasta region. Compositional similarities between Shasta parental melts and boninites
imply similar magma generation processes related to the presence of refractory harzburgite in the shallow mantle. 相似文献
20.
Daizo ISHIYAMA Ryoji SATO Toshio MIZUTA Yohei ISHIKAWA Jingbin WANG 《Resource Geology》2001,51(4):377-392
Abstract: The Anle Sn‐Cu and Huanggangliang Fe‐Sn deposits have been exploited in the Linxi district, which is located 165 km northwest of Chifeng City in northern China. In this study the formation mechanisms of the tin deposits in the Anle and Huanggangliang mining area were investigated to understand the mechanisms of tin mineralization in northern China. The veins of the Anle deposit are divided into cassiterite–quartz–chlorite veins, chalcopyrite‐bearing quartz veins, cassi–terite–chalcopyrite–bearing quartz veins and sphalerite‐quartz veins. The sequence of mineralization is tin mineralization (stage I), copper mineralization (stage II), and lead‐zinc mineralization (stage III). The Huanggangliang tin deposit consists of magnetite skarn orebodies and many cassiterite‐bearing feldspar–fluorite veins and veinlets cutting the magnetite orebodies. The fluid inclusions in quartz and fluorite in ores from the Anle and Huanggangliang tin deposits are divided into two‐phase fluid inclusions, vapor‐rich fluid inclusions and poly‐phase fluid inclusions. The final homogenization temperatures of fluid inclusions of quartz in the ores of the Anle deposit and fluorite of tin‐bearing feldspar veins in the Huanggangliang tin deposit range from 195 to 425C and from 215 to 450C, respectively. The fluids responsible for the Anle and Huanggangliang tin deposits were of very high temperature and NaCl‐rich ones containing K, Ca, Al, Si, Ti, Fe and Cl in addition to ore metals such as Sn and Cu. The temperature and chemical composition of fluid in fluid inclusions of igneous rocks in the mining area are very similar to those of fluid in fluid inclusions in the ores of these deposits. The fluid for these ore deposits had a close relation with the fluid coexisting with melt of Late Jurassic granitic rocks in this mining area. Salinities of fluid inclusions from these ore deposits and granitic rocks in the mining area were estimated to range from 35 to 50 wt % NaCl equivalent. Based on arsenopy‐rite geothermometry and fluid inclusion studies, a fluid containing 40 wt% NaCl (eq.) could be formed by phase separation of fluid having 6 wt% NaCl (eq.) at a temperature of 420 to 500C and a pressure of 0.3 to 0.4 kb. The temperatures and pressures presented above indicate an NaCl‐rich magmatic fluid derived from granitic melt that had intruded into a shallow level of crust caused the Sn–Fe–Cu mineralization of the mining area. The geological relationship between these ore deposits and granitic bodies around the ore deposits, and the similarity of fluids forming these ore deposits and coexisting with granitic melt, suggest that these ore deposits were formed by the activity of fluid derived from granitic melt in Late Jurassic age. 相似文献