Determination of fluid/melt partition coefficients by LA-ICPMS analysis of co-existing fluid and silicate melt inclusions: Controls on element partitioning

Analyses of co-existing silicate melt and fluid inclusions, entrapped in quartz crystals in volatile saturated magmatic systems, allowed direct quantitative determination of fluid/melt partition coefficients. Investigations of various granitic systems (peralkaline to peraluminous in composition, log fO₂ = NNO−1.7 to NNO+4.5) exsolving fluids with various chlorinities (1-14 mol/kg) allowed us to assess the effect of these variables on the fluid/melt partition coefficients (D). Partition coefficients for Pb, Zn, Ag and Fe show a nearly linear increase with the chlorinity of these fluid (D_Pb ∼ 6 ∗ m_Cl, D_Zn ∼ 8 ∗ m_Cl, D_Ag ∼ 4 ∗ m_Cl, D_Fe ∼ 1.4 ∗ m_Cl, where m_Cl is the molinity of Cl). This suggests that these metals are dissolved primarily as Cl-complexes and neither oxygen fugacity nor the composition of the melt affects significantly their fluid/melt partitioning. By contrast, partition coefficients for Mo, B, As, Sb and Bi are highest in low salinity (1-2 mol/kg Cl) fluids with maximum values of D_Mo ∼ 20, D_B ∼ 15, D_As ∼ 13, D_Sb ∼ 8, D_Bi ∼ 15 indicating dissolution as non-chloride (e.g., hydroxy) complexes. Fluid/melt partition coefficients of copper are highly variable, but highest between vapor like fluids and silicate melt (D_Cu ? 2700), indicating an important role for ligands other than Cl. Partition coefficients for W generally increase with increasing chlorinity, but are exceptionally low in some of the studied brines which may indicate an effect of other parameters. Fluid/melt partition coefficients of Sn show a high variability but likely increase with the chlorinity of the fluid (D_Sn = 0.3-42, D_W = 0.8-60), and decrease with decreasing oxygen fugacity or melt peraluminosity.     

Rb and Sr partitioning between haplogranitic melts and aqueous solutions

Rubidium and strontium partitioning experiments between haplogranitic melts and aqueous fluids (water or 1.16-3.56 m (NaCl + KCl) ± HCl) were conducted at 750-950 °C and 0.2-1.4 GPa to investigate the effects of melt and fluid composition, pressure, and temperature. In addition, we studied if the applied technique (rapid and slow quench, and in-situ determination of trace element concentration in the fluid) has a bearing on the obtained data. There is good agreement of the data from different techniques for chloridic solutions, whereas back reactions between fluid and melt upon cooling have a significant effect on results from the experiments with water.The Rb fluid-melt partition coefficient shows no recognizable dependence on melt composition and temperature.For chloridic solutions, it is ∼0.4, independent of pressure. In experiments with water, it is one to two orders of magnitude lower and increases with pressure. The strontium fluid-melt partition coefficient does not depend on temperature. It increases slightly with pressure in Cl free experiments. In chloridic fluids, there is a sharp increase in the Sr partition coefficient with the alumina saturation index (ASI) from 0.003 at an ASI of 0.8 to a maximum of 0.3 at an ASI of 1.05. At higher ASI, it decreases slightly to 0.2 at an ASI of 1.6. It is one to two orders of magnitude higher in chloridic fluids compared to those found in H₂O experiments. The Rb/Sr ratio in non-chloridic solutions in equilibrium with metaluminous melts increases with pressure, whereas the Rb/Sr ratio in chloridic fluids is independent of pressure and decreases with fluid salinity.The obtained fluid-melt partition coefficients are in good agreement with data from natural cogenetic fluid and melt inclusions. Numerical modeling shows that although the Rb/Sr ratio in the residual melt is particularly sensitive to the degree of fractional crystallization, exsolution of a fluid phase, and associated fluid-melt partitioning is not a significant factor controlling Rb and Sr concentrations in the residual melt during crystallization of most granitoids.     

<Emphasis Type="Italic">P</Emphasis>–<Emphasis Type="Italic">T</Emphasis> and fluid evolution of barren and lithium pegmatites from Vlastějovice,Bohemian Massif,Czech Republic

Fluid inclusions, mineral thermometry and stable isotope data from two types of mineralogically and texturally contrasting pegmatites, barren ones and lithium ones, from the Moldanubian Zone of the Bohemian Massif were studied in order to constrain P–T conditions of their emplacement, subsolidus hydrothermal evolution and to estimate composition of the early exsolved fluid and that of the parental melt. Despite the fact that the lithium pegmatites are abundant throughout the crystalline units of the Bohemian Massif, data similar to this paper have not been published yet. The studied pegmatites are hosted by iron-rich calcic skarn bodies. This specific setting allowed scavenging of calcium, fluorine and some other elements from the host rocks into the pegmatitic melts and post-magmatic fluids. Such contamination process was important namely in the case of barren pegmatites, as can be deduced from the variation in anorthite contents in plagioclase and from the presence of fluorite, hornblende (with F content) or garnet in the contact zones of pegmatite dykes. Fluid inclusions were studied mostly in quartz, but also in fluorite, titanite and apatite. Early aqueous–carbonic and late aqueous fluids were identified in both pegmatite types. The P–T conditions of crystallization as well as the detailed composition of exsolved magmatic fluid, however, particularly differ. The magmatic fluids associated with barren pegmatites correspond to H₂O–CO₂ low salinity fluids, composition of which evolved from 20 to 23 to <5 mol% CO₂, and from 2 to 4–6 mol% NaCl eq. Sudden decrease in the CO₂ content of the post-magmatic fluids (<5 mol% CO₂) seems to coincide with the enrichment of the fluid in calcium (from the contamination process) and resulted in precipitation of calcites (frequently found as trapped solid phases in fluid inclusions). The fluids associated with lithium pegmatites are more complex (H₂O–CO₂/N₂–H₃BO₃–NaCl). The CO₂ content of early exsolved fluid is 26–20 mol% CO₂ and remains the same in the next fluid generation. The main difference between the magmatic and the first post-magmatic fluids is the presence of 7–9 wt% of H₃BO₃ (identified as daughter mineral sassolite) in the former. The second post-magmatic fluids are again CO₂-poor (∼4 mol%) and more saline (∼4 mol% NaCl eq.). The composition of exsolved fluid was further used to constrain volatile composition and content of the parental melts. Finally, P–T conditions of pegmatite crystallization are constrained: 600–640°C and 420–580 MPa for the barren pegmatites and 500–570°C and 310–430 MPa for the lithium pegmatite. While the emplacement of the former occurred in thermal equilibrium with the Moldanubian host rock environment, the emplacement of the later suggests substantial thermal disequilibrium.     

The nature of fluids during pegmatite development in metamorphic terrains: Evidence from Hamadan complex,Sanandaj-Sirjan metamorphic zone,Iran

In the Sanandaj-Sirjan zone of metamorphic belt of Iran, the area south of Hamadan city comprises of metamorphic rocks, granitic batholith with pegmatites and quartz veins. Alvand batholith is emplaced into metasediments of early Mesozoic age. Fluid inclusions have been studied using microthermometry to evaluate the source of fluids from which quartz veins and pegmatites formed to investigate the possible relation between host rocks of pegmatites and the fluid inclusion types. Host minerals of fluid inclusions in pegmatites are quartz, andalusite and tourmaline. Fluid inclusions can be classified into four types. Type 1 inclusions are high salinity aqueous fluids (NaCl_eq >12 wt%). Type 2 inclusions are low to moderate salinity (NaCl_eq <12 wt%) aqueous fluids. Type 3 and 4 inclusions are carbonic and mixed CO₂-H₂O fluid inclusions. The distribution of fluid inclusions indicate that type 1 and type 2 inclusions are present in the pegmatites and quartz veins respectively in the Alvand batholith. This would imply that aqueous magmatic fluids with no detectable CO₂ were present during the crystallization of these pegmatites and quartz veins. Types 3 and 4 inclusions are common in quartz veins and pegmatites in metamorphic rocks and are more abundant in the hornfelses. The distribution of the different types of fluid inclusions suggests that CO₂ fluids generated during metamorphism and metamorphic fluids might also contribute to the formation of quartz veins and pegmatites in metamorphic terrains.     

Pan-African high-grade metamorphism of southern Sinai,Egypt, fluid inclusion evidence

Fluid inclusions in the leucosomes of Wadi Feiran migmatites showed that CO ₂ , H₂O and (H₂O-CO₂) fluids were likely to have been present when partial melting began in these rocks. Low salinity, aqueous fluid, to a lesser extent, CO₂-rich fluids are the most abundant fluids. The present study suggests that high-density CO₂ inclusions were formed at the earliest stage, while H₂O inclusions were formed at the late stage. In an intermediate stage, low-density CO₂ and H₂O, CO₂ inclusions were formed. At the early stage of uplift and during melt crystallization, the CO₂-bearing vapour was trapped at grain boundaries. At the late stage of uplift, H₂O released at the time of crystallization of the melt was trapped as inclusions.     

Fluid and magmatic processes in the formation of the Ary-Bulak ongonite massif (eastern Transbaikalia)

The paper discusses the formation conditions of the Ary-Bulak ongonite massif (eastern Transbaikalia). Studies of melt and fluid inclusions have shown that, along with crystalline phases and a silicate melt, ongonitic magma contained aqueous–saline fluids of different types, fluoride melts compositionally similar to fluorite, sellaite, cryolite, chiolite, and more complex aluminum fluorides as well as silicate melts with abnormal Cs and As contents. An ongonite melt crystallized with the participation of P–Q fluids as vapor solutions, presumably NaF-containing and slightly admixed with chlorides. We studied the properties and composition of brine inclusions from Ca- and F-rich rocks on the margin of the massif. Depending on the thermophysical properties of the host rocks and ongonite melt, the duration of its crystallization has been estimated for a magma chamber of the size and shape of the Ary-Bulak massif. Magma chamber cooling has been modeled, and the density, viscosity, and Rayleigh number of the ongonite melt have been estimated from the composition of silicate glasses in melt inclusions. These data strongly suggest intense convection in the residual magma chamber lasting for centuries. We have calculated possible fluid overpressure during the crystallization and degassing of the ongonite melt in a closed magma chamber.Calcium- and fluorine-rich aphyric and porphyritic rocks on the southwestern margin of the massif might have formed by the following mechanism. Local decompression in the magma chamber quenched an oxygen-containing calcium fluoride melt accumulated at the crystallization front, and then these rocks altered during the interaction with fluids. When penetrating the marginal zone, a P–Q magmatic fluid which coexisted with the melt in the residual chamber cooled and changed its composition and properties. This caused the fluid to boil and segregate into immiscible phases: a vapor solution and a brine extremely rich in Cl, F, K, Cs, Mn, Fe, and Al. The fluoride and silicate liquids were immiscible; the silicate melts had abnormal Cs and As contents; changes in the composition and properties of the magmatic fluids caused them to boil and produce brines. All this is evidence for complex fluid–magma interaction and heterogeneous ongonitic magma during the crystallization of the Ary-Bulak rocks. These processes were favored by the low viscosity and high mobility of the F- and water-rich ongonite melt, intense melt convection in the residual chamber, and rising fluid pressure during its degassing.     

The behavior of trace elements during the chemical evolution of the H2O-, B-, and F-rich granite–pegmatite–hydrothermal system at Ehrenfriedersdorf, Germany: a SXRF study of melt and fluid inclusions

Detailed melt and fluid inclusion studies in quartz hosts from the Variscan Ehrenfriedersdorf complex revealed that ongoing fractional crystallization of the highly evolved H₂O-, B-, and F-rich granite magma produced a pegmatite melt, which started to separate into two immiscible phases at about 720°C, 100 MPa. With cooling and further chemical evolution, the immiscibilty field expanded. Two conjugate melts, a peraluminous one and a peralkaline one, coexisted down to temperatures of about 490°C. Additionally, high-salinity brine exsolved throughout the pegmatitic stage, along with low-density vapor. Towards lower temperatures, a hydrothermal system gradually developed. Boiling processes occurred between 450 and 400°C, increasing the salinities of hydrothermal fluids at this stage. Below, the late hydrothermal stage is dominated by low-salinity fluids. Using a combination of synchrotron radiation-induced X-ray fluorescence analysis and Raman spectroscopy, the concentration of trace elements (Mn, Fe, Zn, As, Sb, Rb, Cs, Sr, Zr, Nb, Ta, Ag, Sn, Ta, W, rare earth elements (REE), and Cu) was determined in 52 melt and 8 fluid inclusions that are representative of distinct stages from 720°C down to 380°C. Homogenization temperatures and water contents of both melt and fluid inclusions are used to estimate trapping temperatures, thus revealing the evolutionary stage during the process. Trace elements are partitioned in different proportions between the two pegmatite melts, high-salinity brines and exsolving vapors. Concentrations are strongly shifted by co ncomitant crystallization and precipitation of ore-forming minerals. For example, pegmatite melts at the initial stage (700°C) have about 1,600 ppm of Sn. Concentrations in both melts decrease towards lower temperatures due to the crystallization of cassiterite between 650 and 550°C. Tin is preferentially fractionated into the peralkaline melt by a factor of 2–3. While the last pegmatite melts are low in Sn (64 ppm at 500°C), early hydrothermal fluids become again enriched with about 800 ppm of Sn at the boiling stage. A sudden drop in late hydrothermal fluids (23 ppm of Sn at 370°C) results from precipitation of another cassiterite generation between 400 and 370°C. Zinc concentrations in peraluminous melts are low (some tens of parts per million) and are not correlated with temperature. In coexisting peralkaline melts and high-T brines, they are higher by a factor of 2–3. Zinc continuously increases in hydrothermal fluids (3,000 ppm at 400°C), where the precipitation of sphalerite starts. The main removal of Zn from the fluid system occurs at lower temperatures. Similarly, melt and fluid inclusion concentrations of many other trace elements directly reflect the crystallization and precipitation history of minerals at distinctive temperatures or temperature windows.     

Microthermometry and geochemistry of fluid inclusions from the Tennant Creek gold-copper deposits: implications for ore deposition and exploration

Gold-copper-bismuth mineralization in the Tennant Creek goldfield of the Northern Territory occurs in pipe-like, ellipsoidal, or lensoidal lodes of magnetite ± hematite ironstones which are hosted in turbiditic sedimentary rocks of Proterozoic age. Fluid inclusion studies have revealed four major inclusion types in quartz associated with mineralized and barren ironstones at Ten nant Creek; (1) liquid-vapour inclusions with low liquid/vapour ratios (Type I), (2) liquid-vapour inclusions with high liquid/vapour ratios or high vapour/liquid ratios and characteristic dark bubbles (Type II), (3) liquid-vapour-halite inclusions (Type III), and (4) liquid-vapour inclusions with variable liquid/vapour ratios (Type V). Type I inclusions are present in the barren ironstones and the unmineralized portions of fertile ironstones, whereas Types II and III inclusions are recognized in fertile ironstones. Trails of Types II and III inclusions cut trails of Type I inclusions. Type I fluid inclusions have homogenization temperatures of 100° to 350 °C with a mode at 200° to 250 °C. Type II inclusions in mineralized ironstones (e.g. Juno, White Devil, Eldorado, TC8 and Gecko K-44 deposits) have homogenization temperatures of 250 °C to 600 °C with a mode of 350 °C. Type I fluid inclusions have a salinity range of 10 to 30 NaCl equiv. wt %. Salinity measurements on fluid inclusions in the mineralized zones gave a range of 10 to 50 NaCl equiv. wt % with a mode of 35 NaCl equiv. wt %. Fluid inclusion studies indicate that the Tennant Creek ironstones were formed from a relatively low temperature and moderately saline fluid, where as gold and copper mineralization was deposited from later hydrothermal fluids of higher temperature and salin ity. Gas analysis indicates the presence of N₂ and CO₂, with very minor CH₄ in Types II inclusions but no N₂ or CH₄ gases in Type I inclusions. Microprobe analysis of the fluid inclusion decrepitates indicates that the inclusions from Tennant Creek contain sodium and calcium as dominant cations and potassium in a subordinate amount. The high temperatures ( 350 °C), high salinities ( 35 NaCl equiv. wt. %) and cation composition of the Tennant Creek ore fluids suggest that the ore fluids were derived from upward migrating heated basinal brines, although contribution from a magmatic source cannot be ruled out. Close association of vapour-rich Type IIb and salt-rich Type III inclusions in the mineralized ironstones (e.g. Juno, White Devil, Eldorado, TC8 and Gecko K-44) indicates heterogeneous trapping of ore fluids. This heterogeneous trapping is interpreted to be due to unmixing (exsolution) of a gas-rich (e.g. N₂) fluid during the upward migration of the metal bearing brines and/or due to degassing caused by reaction of oxidized ore fluids and host ironstones. Fluid inclusion data have important implications regarding the deposition of gold in the ironstones, and may have application in discriminating fertile from barren ironstones.     

Magmatic petrogenesis and the evolution of (F:Cl:OH) fluid composition in barren and tungsten skarn-associated plutons using apatite and biotite compositions: Case studies from the northern Canadian Cordillera

A detailed study of apatite and biotite compositions in multiple intrusive phases from five composite plutons in the northern Canadian Cordillera was undertaken with the aim of determining the composition of magmatic fluids relative to F:Cl:OH for several plutons―both barren and mineralizing―and for specific intrusive phases from each pluton that may be related to nearby tungsten skarns. Magmatic apatite and biotite compositions are consistent with a crustal source of magma, either derived from predominantly supracrustal rocks, and (or) derived from predominantly infracrustal rocks and fractionated to felsic compositions. Increasing MnO (± FeO and XF:XCl) with decreasing CaO in apatite broadly correlates with an increasing degree of magmatic differentiation, although Fe# vs. total aluminum in biotite is a better indicator of inter- and intra-plutonic differentiation. Anomalously iron-rich biotites occur in highly fractionated and (or) wallrock-contaminated phases of plutons associated with tungsten skarns.Estimates of magmatic fluid composition―calculated as the activity ratios log[aHCl / a_HF] and log[a_HOH / a_HF] from apatite and biotite compositions―show several trends with respect to magmatic differentiation. Two barren plutons demonstrate that fluids in silicic magmas become HF enriched relative to HCl with increasing differentiation, although re-equilibration with late-stage sub-solidus or hydrothermal fluids may obscure this trend. The three intrusions with associated tungsten skarn mineralization, including the world-class Cantung deposit, also become HF-enriched with magmatic evolution. However, magmatic apatite and biotite in individual intrusive phases that are the most closely associated with mineralization have equilibrated with compositionally distinct fluids. In these particular intrusive phases, apatite appears to have equilibrated with an earlier HCl- and H₂O-rich magmatic fluid, and biotite appears to have equilibrated with a later HCl- and (a particularly) H₂O-rich magmatic fluid. The fluid in these magmas apparently evolved to a H₂O-rich (or less saline) composition as the temperature of the magma decreased. None of the other intrusive phases, from either the barren or tungsten-associated plutons, have apatite and biotite activity ratios that are suggestive of equilibration with such an HCl/HF- and H₂O/HF-rich fluid. Instead, the activity ratios calculated from apatite and biotite in intrusive phases that are not as closely associated with mineralization are well-coupled and unremarkable (although small variations are common), which would suggest that both minerals equilibrated with similar fluids. These intrusive phases do not appear to have produced a large quantity of saline hydrous fluid capable of seggregating and transporting tungsten. The identification of intrusive phases that did produce magmatic fluids that were anomalously enriched in HCl and H₂O could, therefore, be a predictor of nearby tungsten skarn mineralization.     

Copper mineralization around the Ahar batholith, north of Ahar (NW Iran): Evidence for fluid evolution and the origin of the skarn ore deposit

This investigation presents and interprets fluid inclusion data from different lithological units of the Cu skarn deposits at Mazraeh, north of Ahar, Azarbaijan, NW Iran. The results provide an assessment of the P–T conditions and mineral–fluid evolution and suggest new exploration parameters. Five types of inclusions are recognized from quartz and garnet. The temperature of homogenization of Type I inclusions with daughter minerals halite and sylvite ranges from 312° to 470 °C with total salinity of 52 to 63 wt.% NaCl equiv.; Type II and III inclusions with halite have homogenization temperatures of 230° to 520 °C and salinity of 31 to 50 wt.% NaCl equiv. The salinity of Types IV and V biphase (liquid + vapor) inclusions, based on their final ice melting temperature, varies between 10.2 to 20.8 wt.% NaCl equiv. T_h vs. salinity plots of inclusions show that the salinity of the fluids correlates positively with temperature. The inclusions formed at low pressure. Changes in the temperature and salinity of the fluids can be reconstructed from the inclusions. Highly saline, high-temperature fluids were most abundant during the main chalcopyrite ore-forming phase in the skarn and mineralized quartz veins. Low-salinity aqueous fluids were abundant in barren veins, in which there is no evidence for early hot high-salinity brine, and might have resulted from late-stage dilution and mixing of hydrothermal fluids with meteoric water. Based on petrographic features and fluid-inclusion data, early-stage magnetite deposition is related to boiling of fluid at temperatures of about 500 °C. At a later stage, boiling at temperatures of around 320° to 400 °C favored the deposition of sulfides and Fe mobility was decreased at these lower temperatures. The following inclusion characteristics may be used as exploration parameters in the Mazraeh area. (i) Presence of high-temperature, salt-bearing inclusions, with T_h between 300 and 500 °C; (ii) High-salinity fluid inclusions; and (iii) Inclusions showing evidence of boiling of the fluid. In addition, the presence of magnetite is an important exploration parameter.     

Depth of emplacement, fluid provenance and metallogeny in granitic terranes: a comparison of western Thailand with other tin belts

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     

Metamorphic fluid flow in the northeastern part of the 3.8-3.7 Ga Isua Greenstone Belt (SW Greenland): A re-evalution of fluid inclusion evidence for early Archean seafloor-hydrothermal systems

Fluid inclusions in quartz globules and quartz veins of a 3.8-3.7 Ga old, well-preserved pillow lava breccia in the northeastern Isua Greenstone Belt (IGB) were studied using microthermometry, Raman spectrometry and SEM Cathodoluminescence Imaging. Petrographic study of the different quartz segregations showed that they were affected by variable recrystallization which controlled their fluid inclusion content. The oldest unaltered fluid inclusions found are present in vein crystals that survived dynamic and static recrystallization. These crystals contain a cogenetic, immiscible assemblage of CO₂-rich (+H₂O, +graphite) and brine-rich (+CO₂, +halite, +carbonate) inclusions. The gas-rich inclusions have molar volumes between 44.8 and 47.5 cm³/mol, while the brine inclusions have a salinity of ∼33 eq. wt% NaCl. Modeling equilibrium immiscibility using volumetric and compositional properties of the endmember fluids indicates that fluid unmixing occurred at or near peak-metamorphic conditions of ∼460 °C and ∼4 kbar. Carbonate and graphite were precipitated cogenetically from the physically separated endmember fluids and were trapped in fluid inclusions.In most quartz crystals, however, recrystallization obliterated such early fluid inclusion assemblages and left graphite and carbonate as solid inclusions in recrystallized grains. Intragranular fluid inclusion trails in the recrystallized grains of breccia cementing and crosscutting quartz veins have CO₂-rich assemblages, with distinctly different molar volumes (either between 43.7 and 47.5 cm³/mol or between 53.5 and 74.1 cm³/mol), and immiscible, halite-saturated H₂O-CO₂-NaCl(-other salt) inclusions. Later intergranular trails have CH₄-H₂ (X_H2 up to ∼0.3) inclusions of variable density (ranging from 48.0 to >105.3 cm³/mol) and metastable H₂O-NaCl(-other salt?) brines (∼28 eq. wt% NaCl). Finally, the youngest fluid inclusion assemblages are found in non-luminescent secondary quartz and contain low-density CH₄ (molar volume > 105.33 cm³/mol) and low-salinity H₂O-NaCl (0.2-3.7 eq. wt% NaCl). These successive fluid inclusion assemblages record a retrograde P-T evolution close to a geothermal gradient of ∼30 °C/km, but also indicate fluid pressure variations and the introduction of highly reducing fluids at ∼200-300 °C and 0.5-2 kbar. The quartz globules in the pillow fragments only contain sporadic CH₄(+H₂) and brine inclusions, corresponding with the late generations present in the cementing and crosscutting veins. We argue that due to the large extent of static recrystallization in quartz globules in the pillow breccia fragments, only these relatively late fluid inclusions have been preserved, and that they do not represent remnants of an early, seafloor-hydrothermal system as was previously proposed.Modeling the oxidation state of the fluids indicates a rock buffered system at peak-metamorphic conditions, but suggests a change towards fluid-graphite disequilibrium and a logf_H2/f_H2O above the Quartz-Fayalite-Magnetite buffer during retrograde evolution. Most likely, this indicates a control on redox conditions and on fluid speciation by ultramafic rocks in the IGB.Finally, this study shows that microscopic solid graphite in recrystallized metamorphic rocks from Isua can be deposited inorganically from a fluid phase, adding to the complexity of processes that formed reduced carbon in the oldest, well-preserved supracrustal rocks on Earth.     

Caractéristiques de la phase fluide associée à la genèse des gisements d'étain d'Abbaretz et de La Villeder (Bretagne Meridionale)

The fluid inclusions occurring in quartz of cassiterite-bearing quartz veins from two localities of Southern Brittany have been studied (microthermometry and chemical analysis). In both localities, two sorts of fluids have been recognized: 1. Early fluids, related to the precipitation of cassiterite. Those fluids are closed to a chloride-bearing aqueous solution, with very little CO₂ and hydrocarbons. The salinity is rather low (6 to 9 wt % eq. NaCl). The inclusions homogenize between 150 à 300 °C. The K/Na atomic ratio is about 0,1. From these data and the mineralogical associations (muscovite + kaolinite), the physical and chemical properties of the solution at the time of cassiterite crystallization have been calculated: temperature 350°C, pressure 800 bars; molalities of NaCl, KCl and HCl are, respectively about 1, 0.1 and 0.01 (pH at 25 °C, 1 bar 2, lower than 3 in any case). 2. Late fluids, related to an important kaolinization. They are generally colder, and have a either lower or higher salinity, than the early fluids.     

Coexistence of a Fluid Phase with Granitic Magma: Geochemical Characteristics of REE and Cu in Miyako Granite and in Scheelite-bearing Aplitic Veins at the Yamaguchi Cu-W Skarn Deposit, Iwate, Japan

Abstract: The North granitic body of the Miyako pluton is located in the Northern Kitakami belt, Northeast Japan. The formation of the scheelite–chalcopyrite–magnetite–bearing aplitic veins and scheelite–chalcopyrite–magnetite–bearing Yamaguchi skarn deposit was closely associated with the formation of the Miyako plutons. Petrographic facies of the North granitic body vary from quartz diorite in marginal zone (zone A), to tonalite and granodiorite (zone B), and to granite (zone C) in the central. The large numbers of aplitic veins distributed around the Yamaguchi mining area are divided into two groups: barren and scheelite–mag–netite–chalcopyrite–bearing aplitic veins. The latter cut massive clinopyroxene skarns of the Yamaguchi deposit, and are composed of plagioclase, K‐feldspar and titanite. Some plagioclase crystals have dusty cores with irregularly shaped K‐feldspar flakes, and clear rims of albite. Textures of plagioclase in the mineralized aplitic veins are different from the idiomorphic textures with sharp plagioclase crystal boundaries that occur in the North granitic body and barren aplitic veins. These textural data suggest that the mineralized aplitic veins were formed from hydrothermal fluid. Changes in the contents of major and minor (Rb, Sr, Sc, Co, Th, U) elements in the North Miyako granitic body are similar to those of zoned plutons formed by typical magmatic differentiation processes. On the other hand, concentrations of REE, especially middle to heavy REE, of granitic rocks in zone C and barren aplitic veins are significantly lower than those of granitic rocks in zones A and B. The hypothetical chondrite‐normalized REE patterns, calculated assuming fractional crystallization from zone B granitic melt, suggest that REE concentrations of the residual melt increased with the degree of fractional crystallization, and changed into a pattern with enriched LREE and strongly negative Eu anomaly. However, the REE patterns of granitic rocks in zone C are different from the hypothetical patterns. Moreover, the REE patterns of magnetite–scheelite–chalcopyrite aplitic veins are quite different from those of granitic rocks. The Cu contents of granitic rocks in the North Miyako body increase from zone A (5–26 ppm) to zone B (10–26 ppm), and then clearly decrease to zone C (5–7 ppm) and drastically increase to the barren aplitic veins (39–235 ppm). Concentrations of Cu in the mineralized aplitic veins are also higher than those of the granitic rocks in zone C. The decrease in REE and Cu contents of granitic rocks from zone B to zone C is not a result of simple magmatic fractional differentiation. Fluid inclusions in quartz from mineralized aplitic veins contain 3.3 wt% NaCl equivalent and 5.8 wt% CO₂. It was also demonstrated experimentally that the removal of MREE and HREE by fluid from melt enabled the formation of complexes of REE and ligands of OH^‐ and CO₃^2‐. Based on the possibility that the melt of the granitic rocks of zone C and the mineralized aplitic veins coexisted with CO₂‐bearing fluid, it is thought that REE were extracted from the melt to the CO₂‐bearing fluid, and that the REE in the mineralized aplitic veins were transported by the CO₂‐bearing fluid. It is likely that the low HREE and Cu contents of the granitic rocks in zone C could have been caused by the removal of those elements from the granitic melt by the fluid coexisting with the melt. The expelled materials could have been the sources of scheelite–magnetite–chalcopyrite–bearing aplitic veins and copper mineralization of the Yamaguchi Cu‐W skarn deposit.     

滇西北雪鸡坪铜矿床流体包裹体特征研究及矿床成因讨论

雪鸡坪铜矿床产于印支晚期石英二长闪长玢岩-石英闪长玢岩-石英二长斑岩复式侵入体内,为一斑岩型铜矿床。矿床形成经历了多阶段热液成矿作用,主要有微细脉浸染状黄铁矿±黄铜矿-石英、细脉状辉钼矿±黄铁矿±黄铜矿-石英及微细脉状贫硫化物-石英-方解石等。流体包裹体岩相学、显微测温、激光拉曼及碳、氢、氧同位素综合研究表明,微细脉浸染状黄铁矿±黄铜矿-石英阶段石英中主要发育含Na Cl子矿物三相及气液两相包裹体,与含矿的石英二长斑岩石英中发育的流体包裹体特征相似,表明成矿流体主要为中高温、高盐度Na Cl-H2O体系热液,可能主要来源于印支期石英二长斑岩侵入体;辉钼矿±黄铁矿±黄铜矿-石英中主要发育含CO2三相及气液两相包裹体,成矿流体为中温、低盐度Na Cl-CO2-H2O体系热液,与前者来源明显不同;贫硫化物-石英-方解石石英中主要发育气液两相包裹体,成矿流体为中低温、低盐度Na Cl-H2O体系热液,推测其可能较多来自于大气降水。因此,雪鸡坪铜矿床为不同来源、不同地球化学性质热液叠加成矿作用的结果。     

Segregation of magmatic fluids and their potential in the mobilization of platinum-group elements in the South Kawishiwi Intrusion,Duluth Complex,Minnesota — Evidence from petrography,apatite geochemistry and coexisting fluid and melt inclusions

Pegmatitic and other felsic rock pockets and dike-like intrusions are abundant in the South Kawishiwi Intrusion of the Duluth Complex, including the basal, Cu–Ni–PGE mineralized units. These occurrences are found as pockets, pods or as veins and contain abundant accessory apatite and quartz. Quartz hosts primary fluid inclusions as well as silicate melt inclusions. Combined microthermometry and Raman spectroscopy helped to determine the bulk composition of primary fluid inclusions that are CO₂-rich (95 mol%) and contain small amounts of H₂O (4.5 mol%), CH₄ (0.4 mol%) and trace N₂, respectively. This combined technique also made it possible to measure total homogenization temperatures of the inclusions (Th_tot = ~ 225 ± 10 °C), otherwise not detectable during microthermometry. Silicate melt inclusions have been quenched to produce homogeneous glasses corresponding to the original melt. Composition of the entrapped melt is granitoid, peraluminous and is very poor in mafic components. We interpret the melt as a product of partial melting of the footwall rocks due to the contact effect of the South Kawishiwi Intrusion. The presence of CO₂ in the vapor bubbles of the quenched melt inclusions and petrographic evidence suggest that the fluid and melt inclusion assemblages are coeval. The composition of the fluid and melt phase implies that the fluid originates from the mafic magma of the South Kawishiwi Intrusion and the fluid and melt phases coexisted as a heterogeneous melt–fluid system until entrapment of the inclusions.Coexistence of primary fluid and melt inclusions makes it possible to calculate a minimum entrapment pressure (~ 1.7 kbar) and thus estimate formation depth (~ 5.8 km) for the inclusions. Chlorine is suggested to behave compatibly in the silicate melt phase in the fluid–melt system represented by the inclusions, indicated by the high (up to 0.3%) Cl-concentrations of the silicate melt and CO₂-rich nature of the fluid.Apatite halogen-contents provide further details on the behavior of Cl. Apatite in pegmatitic pockets often has elevated Cl-concentrations compared to troctolitic rocks, suggesting enrichment of Cl with progressive crystallization. Systematic trends of Cl-loss at some differentiated melt pockets suggest that in some places Cl exsolved into a fluid phase and migrated away from its source. The segregation of Cl from the melt is probably inhibited by the presence of CO₂-rich fluids until the last stages of crystallization, increasing the potential for the development of late-stage saline brines.Platinum-group minerals are often present in microcracks in silicate minerals, in late-stage differentiated sulfide veinlets and in association with chlorapatite, indicating the potential role of Cl-bearing fluids in the final distribution of PGEs.     

Synthetic fluid inclusions in natural quartz. III. Determination of phase equilibrium properties in the system H2O-NaCl to 1000°C and 1500 bars

Phase equilibria in the system H₂O-NaCl have been determined to 1000°C and 1500 bars using synthetic fluid inclusions formed by healing fractures in inclusion-free Brazilian quartz in the presence of the two coexisting, immiscible H₂O-NaCl fluids at various temperatures and pressures. Petrographic and microthermometric analyses indicate that the inclusions trapped one or the other of the two fluids present, or mixtures of the two. Salinities of the two coexisting phases were obtained from heating and freezing studies on those inclusions which trapped only a single, homogeneous fluid phase.Results of the present study are consistent with previously published data on the H₂O-NaCl system at lower temperatures and pressures, and indicate that the two-phase field extends well into the P-T range of most shallow magmatic-hydrothermal activity. As a consequence, chloride brines exsolved from many epizonal plutons during the process of “second-boiling” should immediately separate into a high-salinity liquid phase and a lower salinity vapor phase and produce coexisting halite-bearing and vapor-rich fluid inclusions. This observation is consistent with results of numerous fluid inclusion studies of ore deposits associated with shallow intrusions, particularly the porphyry copper deposits, in which halite-bearing and coexisting vapor-rich inclusions are commonly associated with the earliest stages of magmatic-hydrothermal activity.     

Comparative geochemical,magnetic susceptibility,and fluid inclusion studies on the Paleoproterozoic Malanjkhand and Dongargarh granitoids,Central India and implications to metallogeny

The Malanjkhand granodiorite (MG) hosting economic copper mineralization and the hitherto barren Dongargarh granitoids (DG) have subtle differences in their petrographic and bulk geochemical features. The two plutons are contiguous and occur in the northern part of the Bhandara Craton in Central India with intervening volcanosedimentary sequence of the Dongargarh Supergroup amidst older gneisses. The Dongargarh granitoids studied in two smaller units have higher bulk magnetic susceptibility than the Cu-bearing MG; the majority of samples studied from the latter being ilmenite-series rocks. DG crystallized at higher pressures compared to MG. Plagioclase composition ranges from albite to high bytownite in MG, whereas its compositional range is restricted to high andesine in DG. However, both intrusions give identical temperature ranges estimated by binary feldspar thermometry. Biotite in MG shows higher Fe/Mg ratios, as well as a greater range of compositional variation, than that in DG. MG has a moderately fractionated rare earth element distribution pattern without any significant Eu anomaly, showing depletion in mid-range rare earth elements (REE) and no depletion in heavy REE. DG is characterized by a prominent negative Eu anomaly. Geochemical features indicate subtle differences in the nature of source rocks and/or melting processes responsible for the generation of the two granitoids. MG displays more consistent bulk chemical features and is possibly a result of crystallization from a homogeneous granodioritic melt. DG displays a greater diversity and possibly incorporated a significant felsic crustal component that contributed to the parent melt. A fluid inclusion study of quartz grains from the granitoids and barren quartz veins occurring in MG indicates identical low-temperature nature of the fluid in both cases. They differ from the fluid in the mineralized zone in MG in the absence of a high-temperature component and CO₂ in the fluid. Late-stage fluids in DG and associated barren quartz veins compare well with those from MG. The hydrothermal activity following the granite emplacement seems to have operated under identical temperature conditions, and the aqueous fluid at the two occurrences seems to have been broadly similar. In both cases, internal evolution of the exsolved fluid to low temperatures and moderate salinity are visualized. Based on the existing information, the lack of ore potential in DG may be attributed to the metal and volatile (water + halogens) deficient nature of the parental granitic melt.     

Inclusions of magmatic fluids: P-V-T-X properties of aqueous salt solutions of various types and petrological implications

The P-V-T-X properties of H₂O-salt systems are compared depending on the solubility coefficient of compounds contained in these systems and the presence or absence of critical phenomena in the saturated solutions. Data on synthetic and natural inclusions captured in minerals at elevated temperatures and pressures and employed to discuss the principal features of phase diagrams of the H₂O-NaCl system (type I) and H₂O-NaF system (type II or P-Q type). It is demonstrated how characteristics of magmatic fluids of various types are manifested during the development of miarolitic pegmatites (Malkhan field in Transbaikalia) and during the crystallization of F-rich ongonitic melts (Ary-Bulak Massif in eastern Transbaikalia). Characteristics of solutions and gas-rich (gaseous) fluid inclusions in quartz phenocrysts from porphyritic ongonites (disappearance of the liquid regardless of its density and the overall salinity near the critical point of water, distinctive features of the dissolution of the crystalline phase, and the ability of the inclusions to withstand heating to 1400°C without decrepitation), and the richness of the fluid-magmatic system as a whole in F suggest that the ongonite melt crystallized in the presence of low-density NaF-bearing fluids of the P-Q type with a minor admixture of chlorides. It is important to identify the type of solutions in the fluid inclusions, because without knowing this type, it is impossible to accurately enough calculate the pressure at the temperatures of inclusion capture. For example, the unwarranted classification of solutions of type II (P-Q) in inclusions with the chloride system results in a significant overestimation of the calculated fluid pressures. A technique is proposed for studying the high-temperature immiscibility region in P-Q systems based on data obtained on gaseous fluid inclusions.     

Fluid evolution in the Pote Shear Zone Harare-Shamva-Bindura greenstone belt (northeast Zimbabwe)

A fluid inclusion study was completed on syn-deformational quartz veins of the Pote River Shear Zone, which is situated on the border between the Harare-Bindura greenstone belt and the granitoids of the Chinamora Batholith. The fluid inclusions were studied by means of microthermometry and Laser-Raman microspectrometry. The fluid inclusions consist of three major compositional types: (1) H₂OCO₂±N₂±halite inclusions in clusters and trails; (2) H₂OCO₂ inclusions (H₂O = 30–60 vol. %) in trails; and (3) H₂O-halite inclusions in trails. These fluid generations are explained by trapping at different P-T conditions of two different fluids: a high salinity aqueous fluid and a low salinity H₂OCO₂ fluid with XH₂O around 0.8. High salinity aqueous fluid inclusions are characteristic for the granite-greenstone contact and are absent within the Harare-Shamva-Bindura greenstone belt. The high salinity aqueous fluid has, therefore, been interpreted as magmatic in origin. The low salinity H₂OCO₂ fluid is most likely metamorphic in origin.