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.     

The origin of tin deposits in the Mount Wells region,Pine Creek Geosyncline,Northern Territory

Tabular steeply dipping cassiterite‐bearing lodes in the Mount Wells region are hosted by lower greenschist fades metasediment of the Pine Creek Geosyncline within the contact aureole of late orogenic granitoids. The latter are predominantly I‐type, but S‐type phases are developed near the sediment‐granitoid contact.

Quartz, cassiterite, pyrite, arsenopyrite, chalcopyrite and pyrrhotite are the main minerals. Two types of lodes are present: (i) Sn‐quartz lodes containing 5–10 vol% sulphide minerals; and (ii) Sn‐sulphide lodes containing ～ 70 vol% sulphide minerals. At the surface, the former appear as normal quartz veins and the latter as hematite‐quartz breccia resulting from the collapse of original sulphide‐rich lodes as a consequence of volume reduction due to oxidation and leaching.

Two stages of quartz veining are recognized in both types of lodes. Cassiterite is present in stage I while stage II is composed of barren quartz with minor pyrite. Late stage III carbonate veinlets are present in Sn‐sulphide lodes. The lode‐wallrock contact is sharp with weak alteration effects confined to the fringe of the lodes. The alteration minerals include sericite, quartz, tourmaline, chlorite, pyrite and minor K‐feldspar.

Four types of fluid inclusions are present in vein quartz and cassiterite: Type A (CO₂ ± H₂O ± CH₄); Type B (H₂O+～ 20% vapour); Type C (H₂O+ < 15% vapour) and Type D (H₂O+ < 15% vapour + NaCl). Early ‘primary’ inclusions represented by Types A and B are present in stage I only and have a well‐defined temperature mode at ～300°C and a salinity range of 1–20 wt% eq NaCl. Types C and D inclusions are ‘secondary’ in stage I and primary in stage II and have a temperature mode at 120–160°C and salinities from about 1 to more than 26 wt% eq NaCl. Variable H₂O‐CO₂ ratios of Type A inclusions and homogenization in CO₂ or H₂O phase at near identical temperature indicate entrapment at the H₂O‐CO₂ solvus and define a pressure of ～ 100 MPa. The melting sequence of frozen inclusions suggests that the ore fluids were mainly H₂O‐CO₂‐CH₄‐Na‐Ca‐Cl brines. This is also confirmed by Raman Laser Spectrometry.

Oxygen and sulphur isotope data are consistent with a magmatic origin of the ore fluids. The δD values are up to 20%0 higher than those expected for magmatic fluids and probably resulted from interaction of the latter with the carbonaceous strata. This interpretation is supported by δ¹³C data on the fluid inclusion CO₂.

Fluid inclusions, stable isotope and mineralogical data are used to approximate the physico‐chemical parameters of the ore fluids which are as follows: T 300°C, m Cl～2, fO₂ ～ 10^‐35, mSS ～ 0.01, Sn ～ 1 ppm, Cu ～ 1 ppm and pH ～ 5.5.

It is suggested that fluids of granitic parentage interacted with the enclosing sediment and picked up CO₂, CH₄ and possibly Ca. The granitic phases became reduced due to this interaction and developed S‐type characteristics. Tin was probably partitioned into the CH₄‐bearing reduced fluids. At some stage the fluid overpressure exceeded the lithostatic lode enforcing failure of the carapace and the intruded rocks by hydraulic fracturing causing CH₄ and CO₂ loss resulting in the precipitation of the ore minerals.     

Fluorite deposits at Encantada–Buenavista, Mexico: products of Mississippi Valley type processes

Petroleum and aqueous fluid inclusions from the Encantada–Buenavista fluorite mineralized zone in northern Mexico were analyzed by microthermometry, UV fluorescence, Raman Spectroscopy, Fourier Transform Infrared Spectroscopy (FTIR) and Confocal Scanning Laser Microscopy (CSLM) to evaluate the geochemical evolution of the mineralizing fluids. Two-phase (petroleum or brine+vapor) and three-phase (petroleum+brine+vapor) inclusions are described. Aqueous and petroleum-rich inclusions commonly occur in the same plane. Vapor-decrepitated and stretched fluid inclusions are present. A low-salinity methane-saturated fluid and a high salinity-fluid with highly variable methane contents are recognized. H₂S is not quantified but is always detected in close association with methane. Petroleum inclusions are of two types: a low methane petroleum fluid (20 mol%) with low T_h (60 °C) and a petroleum fluid with a methane of content near 30 mol% and a T_h of 90 °C.Pressure and temperature diagrams for the aqueous and petroleum inclusions show three main intersects that allow P–T–X reconstruction of fluid evolution at La Encantada–Buenavista. A CH₄- and H₂S-rich low-salinity brine was mixed with oil that migrated under hydrostatic conditions with a thermal gradient of 70 °C/km. The arrival and mixing of a high-salinity aqueous fluid produced overpressure to 300 bars. A return to hydrostatic conditions was accompanied by an increase in the thermal gradient.The brine related to the fluorite orebodies appears to have a genetic relationship with the brines reported from the Jurassic petroleum basins located west of the fluorite bodies and similarities with reported fluids from Mississippi Valley type deposits. It is interpreted that the fluorine-rich fluids migrated toward the platform margins during the mid-Tertiary (30 to 32 Ma) using extension zones related to Basin and Range tectonism. Mixing of two different brines was responsible for precipitation and mineralization. Heat from magmas, related to tectonic extension, caused decrepitation and changes in the shape of fluid inclusions near the contact zones.     

Compositions of magmatic melts and evolution of mineral-forming fluids in the banska Stiavnica epithermal Au-Ag-Pb-Zn deposit, Slovakia: A study of inclusions in minerals

Melt and fluid inclusions were investigated in minerals from igneous rocks and ore (Au-Ag-Pb-Zn) veins of the Stiavnica ore field in Central Slovakia. High H₂O (7.1–12.0 wt %) and Cl (0.32–0.46 wt %) contents were found in silicate melt inclusions (65–69 wt % SiO₂ and 5.2–5.6 wt % K₂O) in plagioclase phenocrysts (An 68–36) from biotite-homblende andesites of the eastern part of the caldera. Similar high water contents are characteristic of magmatic melts (71–76 wt % SiO₂ and 3.7–5.1 wt % K₂O) forming the sanidine rhyolites of the Vyhne extrusive dome in the northwestern part of the Stiavnica caldera (up to 7.1 wt %) and the rhyolites of the Klotilda dike in the eastern part of the ore field (up to 11.5 wt %). The examination of primary inclusions in quartz and sanidine from the Vyhne rhyolites revealed high concentrations of N₂ and CO₂ in magmatic fluid (8.6 g/kg H₂O and 59 g/kg H₂O, respectively). Fluid pressure was estimated as 5.0 kbar on the basis of primary CO₂ fluid inclusions in plagioclase phenocrysts from the Kalvari basanites. This value corresponds to a depth of 18 km and may be indicative of a deep CO₂ source. Quartz from the granodiorites of the central part of the Stiavnica-Hodrusa complex crystallized from a melt with 4.2–6.1 wt % H₂O and 0.24–0.80 wt % Cl. Magmatic fluid cogenetic with this silicate melt was represented by a chloride brine with a salinity of no less than 77–80 wt % NaCl equiv. Secondary inclusions in quartz of the igneous rocks recorded a continuous trend of temperature, pressure, and solution salinity, from the parameters of magmatic fluids to the conditions of formation of ore veins. The gold mineralization of the Svyatozar vein system was formed from boiling low-salinity fluids (0.3–8.0 wt % NaCl equv.) at temperatures of 365–160°C and pressures of 160–60 bar. The Terezia, Bieber, Viliam, Spitaler, and Rozalia epithermal gold-silver-base metal veins were also formed from heterogeneous low-salinity fluids (0.3–12.1 wt %) at temperatures of 380–58°C and pressures of 240–10 bar. It was found that the salt components of the solutions were dominated by chlorides (high content of fluorine, up to 0.45 mol/kg H₂O, was also detected), and sulfate solutions appeared in the upper levels. The dissolved gas of ore-forming solutions was dominated by CO₂ (0.1–8.4 mol %, averaging 1.3 wt %) and contained minor nitrogen (0.00–0.85 mol %, averaging 0.05 mol %) and negligible methane admixtures (0.00–0.05 mol %, averaging 0.004 mol %). These data allowed us to conclude that the magmatic melts could be sources of H₂O, Cl, CO₂, and N₂. The formation of the epithermal mineralization of the Stiavnica ore field was associated with the mixing of magmatic fluid with low-concentration meteoric waters, and the fluid was in a heterogeneous state.     

Fluid composition and evolution in coesite-bearing rocks (Dora-Maira massif,Western Alps): implications for element recycling during subduction

Fluid inclusions and F, Cl concentration of hydrous minerals were analysed in the coesite-pyrope quartzite, the interlayered jadeite quartzite and their country-rock gneiss from the Dora-Maira massif using a combination of microthermometry, Raman spectrometry, synchrotron X-ray microfiuorescence and electron microprobe analysis. Three populations of fluid inclusions were recognized texturally and can be related to distinct metamorphic stages. A low-salinity aqueous fluid occurs in the retrogressed country gneiss and as late secondary inclusions in jadeite quartzite and chloritized pyrope. An earlier secondary population is found in matrix quartz of the jadeite- and pyro-pe-quartzites. This population can be related to the early decompression and so to incipient breakdown of garnet into phlogopite-bearing assemblages. The inclusion fluid is highly saline (up to 84 wt% equivalent NaCl) and contains Na, Ca, Fe, Cu and Zn as major cations. In pyrope quartzite, additional K was found in these brines, which locally coexist with CO₂-rich inclusions. The oldest fluid inclusions are preserved in kyanite grains included in fresh pyrope and in pyrope itself. In pyrope, all inclusions have decrepitated and contain magnesite, an Mg-phosphate, sheet-silicate(s), a chloride and an opaque phase, with no fluid preser ved. In contrast, the kyanite inclusions in pyrope preserve primary H₂O-CO₂ low-salinity fluid inclusions, probably owing to the low compressibility of the kyanite inclusions and host garnet. In spite of in-situ re-equilibration, these inclusions can be interpreted as relics of the dehydration fluid that attended pyrope growth. These correlations between textural and chemical fluid inclusion data and metamorphic stages are consistent with the fluid composition calculated from the halogen content of different generations of phlogopite and biotite. The preservation of different fluid compositions, both in time and space, is evidence for local control and possibly origin of the fluids, in agreement with isotopic data. These results, in particular the absence of CO₂ in the jadeite quartzite, are best interpreted in terms of a fluid-melt system evolution. With increasing metamorphism, partitioning of H₂O, Na, Ca, Fe and heavy metals into melt (jadeite quartzite) and Mg, Na/K, F, CO₂ and P(?) into a residual aqueous fluid can account for depletion in Na, Ca and Fe of the pyrope quartzite. During the retrograde path, a _{H 2 O} rose as melt crystallized, generating the two populations of hypersaline and water-rich fluids that were highly reactive to pyrope. The process of fluid-melt interaction envisioned here coupled with models of melt extraction in subduction zones provides an attractive opportunity for the instantaneous ( < 1 Ma) and selective transport of elements between a downgoing slab and the overlying mantle wedge.     

Hydrothermal Fluid Evolution Associated with Gold Mineralization at the Wenyu Mine, Xiaoqinling District, China

Abstract: The Wenyu mesothermal gold deposit is located in the Xiaoqinling district about 1000 km southwest of Beijing in central China. It occurs in the Late Archean to Early Proterozoic metamorphosed volcanic and sedimentary rocks. Three distinct stages of veins have been identified: (I) gold‐poor quartz–pyrite veins, (II) gold‐rich sulfide–quartz veins, and (III) gold‐poor carbonate–quartz veins. Stage II can be subdivided into IIa and IIb. Gold typically occurs as fracture‐fillings associated with chalcopyrite and galena. Fluid inclusions were examined in quartz samples from veins of both stage I and II. Three types of fluid inclusions are identified: CO₂–H₂O, CO₂–rich, and aqueous inclusions. The first two types are of primary in origin. The last type occurs in two ways: coexisting with CO₂–H₂O and CO₂–rich inclusions and thus primary in origin; and occurring along late healed fractures and hence secondary in origin. CO₂–H₂O inclusions display progressively decreasing Th and increasing Thco₂, from the highest Th (311–408C) and lowest Thco₂ (average 18C) in stage I quartz through middle Th (284–358C) and ThCO₂(average 25C) in stage IIa quartz to the lowest Th (275–314C) and highest ThCO₂ (average 28C) in stage IIb quartz, indicating an evolving H₂O–CO₂–NaCl fluid system. CO₂–rich and primary aqueous inclusions show consistent ThCO₂ or Th with their coexistent CO₂–H₂O inclusions. Whereas the secondary aqueous inclusions in stage I and IIa quartz have almost the same Th and salinity as the primary aqueous inclusions in stage IIb quartz. Comparing with CO₂–H₂O inclusions, these non–CO₂, low salinity aqueous inclusions may come from different origin, most probably meteoric water. Unlike in both stage I and IIa quartz, fluid inclusions in stage IIb do not show evidence of fluid immiscibility. The fact that most of gold is associated with stage IIa and IIb veins and not with veins of stage I which is the main stage of vein formation suggests that gold deposition occurs at the later stage of fluid immiscibility. The continuing phase separation led to the deposition of large amounts of gold at the Wenyu mine.     

Metallogenesis and hydrothermal evolution of the Tonggou Cu deposit in the Eastern Tianshan: Evidence from fluid inclusions,H-O-S isotopes,and Re-Os geochronology

The Tonggou Cu polymetallic deposit in the Bogda Orogenic Belt, Eastern Tianshan shows evidence for three stages of hydrothermal mineralization: early pyrite veins (Stage 1), polymetallic sulfide ± epidote–quartz (Stage 2), and late-stage pyrite–calcite veins (Stage 3). Fluid inclusion petrography and microthermometry analyses indicate that the liquid-rich aqueous inclusions (L), vapour-rich aqueous inclusions (V), and NaCl daughter mineral–bearing three phase inclusions (S) formed during the main stage of mineralization, and that the ore fluids represent high-temperature and high-salinity H₂O-NaCl hydrothermal fluids that underwent boiling. Stable isotope (H, O) data indicate that the ore fluids of the Tonggou deposit were originally derived from magmatic water in Stage 2 and subsequently mixed with local meteoric water during Stage 3. Sulphur isotope compositions (6.7‰ to 10.9‰) are consistent with the δ³⁴S values of pyrite from the Qijiaojing Formation sandstone, indicating the primary source of the sulphur ore. Furthermore, chalcopyrite grains separated from the chalcopyrite-rich ore samples yield an isochron age of 303 ± 12 Ma (MSWD = 1.2). These results indicate that the Tonggou deposit is a transition between high–sulfidation and porphyry deposits which formed in the Late Carboniferous. It also suggests an increased likelihood for the occurrence of Cu (Au, Mo) in the Bogda Orogenic Belt, especially at locations where the Cu-Zn deposits are thicker; further deep drilling and exploration are encouraged in these areas.     

Fluid types and their genetic meaning for the BIF-hosted iron ores,Krivoy Rog,Ukraine

This paper contributes to the understanding of the genesis of epigenetic, hypogene BIF-hosted iron deposits situated in the eastern part of Ukrainian Shield. It presents new data from the Krivoy Rog iron mining district (Skelevatske–Magnetitove deposit, Frunze underground mine and Balka Severnaya Krasnaya outcrop) and focuses on the investigation of ore genesis through application of fluid inclusion petrography, microthermometry, Raman spectroscopy and baro-acoustic decrepitation of fluid inclusions. The study investigates inclusions preserved in quartz and magnetite associated with the low-grade iron ores (31–37% Fe) and iron-rich quartzites (38–45% Fe) of the Saksaganskaya Suite, as well as magnetite from the locally named high-grade iron ores (52–56% Fe). These high-grade ores resulted from alteration of iron quartzites in the Saksaganskiy thrust footwall (Saksaganskiy tectonic block) and were a precursor to supergene martite, high-grade ores (60–70% Fe). Based on the new data two stages of iron ore formation (metamorphic and metasomatic) are proposed.The metamorphic stage, resulting in formation of quartz veins within the low-grade iron ore and iron-rich quartzites, involved fluids of four different compositions: CO₂-rich, H₂O, H₂O–CO₂(± N₂–CH₄)–NaCl(± NaHCO₃) and H₂O–CO₂(± N₂–CH₄)–NaCl. The salinities of these fluids were relatively low (up to 7 mass% NaCl equiv.) as these fluids were derived from dehydration and decarbonation of the BIF rocks, however the origin of the nahcolite (NaHCO₃) remains unresolved. The minimum P–T conditions for the formation of these veins, inferred from microthermometry are T_min = 219–246 °C and P_min = 130–158 MPa. The baro-acoustic decrepitation analyses of magnetite bands indicated that the low-grade iron ore from the Skelevatske–Magnetitove deposit was metamorphosed at T = ~ 530 °C.The metasomatic stage post-dated and partially overlapped the metamorphic stage and led to the upgrade of iron quartzites to the high-grade iron ores. The genesis of these ores, which are located in the Saksaganskiy tectonic block (Saksaganskiy ore field), and the factors controlling iron ore-forming processes are highly controversial. According to the study of quartz-hosted fluid inclusions from the thrust zone the metasomatic stage involved at least three different episodes of the fluid flow, simultaneous with thrusting and deformation. During the 1^st episode three types of fluids were introduced: CO₂–CH₄–N₂(± C), CO₂(± N₂–CH₄) and low salinity H₂O–N₂–CH₄–NaCl (6.38–7.1 mass% NaCl equiv.). The 2^nd episode included expulsion of the aqueous fluids H₂O–N₂–CH₄–NaCl(± CO₂, ± C) of moderate salinities (15.22–16.76 mass% NaCl equiv.), whereas the 3^rd event involved high salinity fluids H₂O–NaCl(± C) (20–35 mass% NaCl equiv.). The fluids most probably interacted with country rocks (e.g. schists) supplying them with CH₄ and N₂. The high salinity fluids were most likely either magmatic–hydrothermal fluids derived from the Saksaganskiy igneous body or heated basinal brines, and they may have caused pervasive leaching of Fe from metavolcanic and/or the BIF rocks. The baro-acoustic decrepitation analyses of magnetite comprising the high-grade iron ore showed formation T = ~ 430–500 °C. The fluid inclusion data suggest that the upgrade to high-grade Fe ores might be a result of the Krivoy Rog BIF alteration by multiple flows of structurally controlled, metamorphic and magmatic–hydrothermal fluids or heated basinal brines.     

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.     

Fluid inclusion study of the bournac polymetallic (Sb-As-Pb-Zn-Fe-Cu...) vein deposit (montagne noire,France)

Mineralogical studies demonstrate that the Hercynian polymetallic antimony-rich deposit of Bournac can be described by four stages of ore deposition and one of partial ore remobilization. Fluid inclusion data permit calculation of the composition and temperature of the fluids associated with each stage of hydrothermal mineralization and concomitant wall-rock alteration. Stages I and II (Fe-As and Zn) are represented by moderate-salinity H₂O-CO₂-(NaCl) inclusions which correlate closely with early carbonate deposition. Stage III fluids which are responsible for the deposition of Pb-Sb ores are characterized by low-salinity H₂O-(NaCl) inclusions. During the final stage of mineralization (IV), corresponding to the main phase of stibnite deposition, abundant aqueous inclusions confirm the continued involvement of low-salinity fluids and the intense development of potassic clays and secondary silica in the wall rocks. Homogenization temperatures suggest that the whole cycle of mineralization took place during a gradual decrease in fluid temperature of 380°–140°C. Stibnite deposition is restricted to the interval of 230°–140°C thus confirming an essentially epithermal environment. Stage V (partial remobilization) is distinguished by the presence of high-salinity CaCl₂-rich inclusions which are tentatively related to Triassic barite mineralization in the region and therefore postdate the Bournac antimony ores. Homogenization temperatures for this stage range 140°–60°C.     

Geology,mineralization, and fluid inclusion characteristics of the Lermontovskoe reduced-type tungsten (±Cu,Au, Bi) skarn deposit,Sikhote-Alin,Russia

The Lermontovskoe deposit (∼48 Kt WO₃; average 2.6% WO₃, 0.24% Cu, 0.23 g/t Au) is situated in a W-Sn-Au metallogenic belt that formed in a collisional tectonic environment. This tungsten skarn deposit has a W-Au-As-Bi-Te-Sb signature that suggests an affinity with reduced intrusion-related Au deposits. The deposit is associated with an intrusion that is part of the ilmenite-series, high-K peraluminous granitoid (granodiorite to granite) suite. These rocks formed via mantle magma-induced melting of crustal sources.The deposit comprises reduced-type, pyroxene-dominated prograde and retrograde skarns followed by hydrosilicate (amphibole-chlorite-pyrrhotite-scheelite-quartz) and phyllic (muscovite/sericite-carbonate-albite-quartz-scheelite-sulfide, with abundant apatite) alteration assemblages. Fluid inclusions from the skarn assemblages indicate high-temperature (>500 °C), high-pressure (1400–1500 bars) and high-salinity (53–60 wt% NaCl-equiv.) magmatic-hydrothermal fluids. They were post-dated by high-carbonic, methane-dominate, low-salinity fluid at the hydrosilicate alteration stage. These fluids boiled at 360–380 °C and 1300–1400 bars. The subsequent phyllic alteration started again with a high-temperature (>450 °C), high-pressure (1000–1100 bars) and high-salinity (42–47 wt% NaCl-equiv.) fluid, with further incursion of high-carbonic, methane-dominated, low-salinity fluid that boiled at 390–420 °C and 1150–1200 bars. The latest phyllic alteration included the lower-temperature (340–360 °C), lower pressure (370–400 bars) high-carbonic, methane-dominated (but with higher CO₂ fraction), low-salinity fluid, and then the low-temperature (250–300 °C) H₂O-CO₂-CH₄-NaCl fluid, with both fluids boiled at the deposit level. The high-salinity aqueous fluids are interpreted to have come from crystallizing granitoid magma, whereas the reduced high-carbonic fluids probably came from a deeper mafic magma source. Both of these fluids potentially contributed to the W-Au-As-Bi-Te-Sb metal budget. Decreasing temperatures coupled with high aCa²⁺ and fluid boiling promoted scheelite deposition at all post-skarn hydrothermal stages.The deposit is characterized by limited downdip extent of mineralized zones and abundant coarse-grained muscovite-quartz (+apatite, scheelite) aggregates that formed at the phyllic alteration stage. Together with presence of high-temperature, high-pressure and high-salinity fluids directly exsolving from crystallizing magma, this suggests a root level of the mineralized magmatic-hydrothermal system of reduced W skarn deposits.     

Dissolution channels in quartz and the role of pressure changes in gold and sulfide deposition in the Archean, greenstone-hosted, Hutti gold deposit, Karnataka, India

The pressure, temperature and composition of ore fluids that resulted in gold deposition in the Archean, greenstone-hosted Hutti deposit have been studied using fluid inclusions and the compositions of arsenopyrite and chlorite. Five types of fluids have been identified in fluid inclusions in quartz veins associated with mineralization. They are (1) monophase CO ₂-rich fluid; (2) low-salinity (0 to 14 wt% NaCl equivalent) and high-salinity (16 to 23 wt% NaCl equiv.) aqueous fluids; (3) high-salinity (28 to 40 wt% NaCl equiv.), polyphase aqueous fluids; (4) CO ₂–H ₂O–NaCl fluids of low salinity (0–8 wt% NaCl equiv.); and (5) a few carbonic inclusions with halite±nahcolite. The diversity of entrapped fluid composition is explained in terms of changes in fluid pressure and temperature which affect a more or less uniform supply of primary low-salinity CO ₂–H ₂O–NaCl fluid to the shear zone. Geothermobarometric studies indicate that during mineralization temperature ranged between 360 and 240 °C, and fluid pressure between 3,600 and 1,600 bar. The data are interpreted in terms of the cyclic fault-valve mechanism for active shear zones. Deposition of gold and sulfides has been studied on the basis of constraints from the composition of wall-rock chlorite, ore-mineral assemblages, and textural features. Tubular channels, 20 to 100 µm wide and up to 500 µm long that arise from fractures and C-planes in sheared quartz veins are reported for the first time. The channels have pyrrhotite, arsenopyrite, pyrite and gold at their distal ends, with calcite filling up the remaining part. These channels form in response to increases in T and P, by dissolution of quartz grains, guided by dislocations in them. At the P– T conditions of interest, gold and sulfide deposition takes place in the shears and fractures of quartz veins from CO ₂–H ₂O–NaCl ore fluid of low salinity and pH due to changes in phase compositions that occur during the process of shear failure of the enclosing rocks. In the wall rock where pH is buffered, gold deposition takes place from the predominant Au(HS) ₂ ^- species with progressive sulfide deposition and decrease in SS, from 0.01 to 0.001 mol/kg as T falls from 360 to 240 °C.     

Listvenite–lode association at the Barramiya gold mine,Eastern Desert,Egypt

Several occurrences of gold-bearing quartz veins are situated along the east–northeast-trending Barramiya–Um Salatit ophiolitic belt in the central Eastern Desert of Egypt. In the Barramiya mine, gold mineralization within carbonaceous, listvenized serpentinite and adjacent to post-tectonic granite stocks points toward a significant role of listvenitization in the ore genesis. The mineralization is related to quartz and quartz–carbonate lodes in silicified/carbonatized wallrocks. Ore minerals, disseminated in the quartz veins and adjacent wallrocks are mainly arsenopyrite, pyrite and trace amounts of chalcopyrite, sphalerite, tetrahedrite, pyrrhotite, galena, gersdorffite and gold. Partial to complete replacement of arsenopyrite by pyrite and/or marcasite is common. Other secondary phases include covellite and goethite. Native gold and gold–silver alloy occur as tiny grains along micro-fractures in the quartz veins. However, the bulk mineralization can be attributed to auriferous arsenopyrite and arsenic-bearing pyrite (with hundreds of ppms of refractory Au), as evident by electron microprobe and LA-ICP-MS analyses.The mineralized quartz veins are characterized by abundant carbonic (CO₂ ± CH₄ ± H₂O) and aqueous-carbonic (H₂O–NaCl–CO₂ ± CH₄) inclusions along intragranular trails, whereas aqueous inclusions (H₂O–NaCl ± CO₂) are common in secondary sites. Based on the fluid inclusions data combined with thermometry of the auriferous arsenopyrite, the pressure–temperature conditions of the Barramiya gold mineralization range from 1.3 to 2.4 kbar at 325–370 °C, consistent with mesothermal conditions. Based on the measured δ³⁴S values of pyrite and arsenopyrite intimately associated with gold, the calculated δ³⁴S_Σs values suggest that circulating magmatic, dilute aqueous-carbonic fluids leached gold and isotopically light sulfur from the ophiolitic sequence. As the ore fluids infiltrated into the sheared listvenite rocks, a sharp decrease in the fluid fO₂ via interaction with the carbonaceous wallrocks triggered gold deposition in structurally favorable sites.     

Origin of CO2-rich fluid inclusions in synorogenic veins from the Eastern Mount Isa Fold Belt, NW Queensland, and their implications for mineralization

Synorogenic veins from the Proterozoic Eastern Mount Isa Fold Belt contain three different types of fluid inclusions: CO₂-rich, aqueous two-phase and rare multiphase. Inclusions of CO₂ without a visible H₂O phase are particularly common. The close association of CO₂-rich inclusions with aqueous two-phase, and possibly multiphase inclusions suggests that phase separation of low- to -moderate salinity CO₂-rich hydrothermal fluids led to the selective entrapment of the CO₂. Microthermometric results indicate that CO₂-rich inclusions homogenize between –15.5 and +29.9 °C which corresponds to densities of 0.99 to 0.60 g.cm⁻³. The homogenization temperatures of the associated aqueous two-phase inclusions are 127–397 °C, with salinities of 0.5 to 18.1 wt.% NaCl equivalent. The rarely observed multiphase inclusions homogenize between 250 and 350 °C, and have salinities ranging from 34.6 to 41.5 wt.% NaCl equivalent. Evidence used to support the presence of fluid immiscibility in this study is mainly derived from observations of coexisting H₂O-rich and CO₂-rich inclusions in groups and along the same trail. In addition, these two presumably unmixed fluids are also found on adjacent fractures where monophase CO₂-rich inclusions are closely related to H₂O-rich inclusions. Similar CO₂-rich inclusions are widespread in mineral deposits in this region, which are simply metal-enriched synorogenic veins. Therefore, we argue that fluid immiscibility caused volatile species such as CO₂ and H₂S to be lost from liquid, thus triggering ore deposition by increasing the fluid pH and decreasing the availability of complexing ligands. Received: 28 April 1997 / Accepted: 4 January 1999     

Compositions of magmatic hydrothermal fluids determined by LA-ICP-MS of fluid inclusions from the porphyry copper–molybdenum deposit at Butte, MT

We analyzed 85 fluid inclusions from seven samples from the porphyry Cu–Mo deposit in Butte, MT, using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The Butte deposit formed at unusually great depth relative to most porphyry deposits, and fluid inclusions in deep veins trapped a low-salinity, CO₂-bearing, magmatically derived, supercritical fluid as a single aqueous phase. This fluid is interpreted to be the parent fluid that cooled, decompressed, unmixed, and reacted with wall rock to form the gigantic porphyry Cu deposit at Butte. Few previous analyses of such fluids exist.Low-salinity, aqueous fluids from the earliest veins at Butte are trapped in deep veins with biotite-rich alteration envelopes (EDM veins). These veins, and the Butte quartz monzonite surrounding them, host much of the Butte porphyry Cu mineralization. Twenty fluid inclusions in one EDM quartz vein are dominated by Na, K, Fe (from 0.1 to 1 wt.%) and contain up to 1.3 wt.% Cu. These inclusions contain only small amounts (tens of ppm) of Pb, Zn, and Mn, and typically contain Li, B, Ca, As, Mo, Ag, Sn, Sb, Ba, and W in less than detectable quantities. The abundance of Cu in early fluids indicates that a low-salinity, Cu-rich, aqueous ore fluid can be directly produced by aqueous fluid separation from a granitic magma. Similar inclusions (eight) in an early deep quartz–molybdenite vein with a K-feldspar selvage have similar compositions but contain significantly less Cu than most inclusions in the biotite-altered vein. Analyzed inclusions in both veins contain less than detectable concentrations of Mo even though one is molybdenite-bearing.Low-salinity, CO₂-bearing aqueous fluids are also trapped in pyrite–quartz veins with sericitic selvages. These veins cut both of the above vein types and contain inclusions that were trapped at lower pressure and temperature. Thirty-nine inclusions in two such veins have compositions similar to early fluids, but are enriched by up to a factor of 10 in Mn, Pb, and Zn relative to early fluids, and are slightly depleted in Fe. Many of these inclusions contain as much or more Cu than early fluids, although little chalcopyrite is found in or around pyrite–quartz veins.Eighteen halite-bearing inclusions from three veins from both chalcopyrite-bearing and barren veins with both K-silicate and sericitic selvages were analyzed as well. Halite-saturated inclusions are dominated by Na, K, Fe, and in some inclusions Ca. Whereas these inclusions are significantly enriched in Ca, Mn, Fe, Zn, and Pb, fluids in all three veins contain significantly less Cu than early, high temperature, low-salinity inclusions.Analyses of all inclusion types show that whereas bulk-salinity of the hydrothermal fluid must be largely controlled by the magma, fluid–rock interactions have a significant role in controlling fluid compositions and metal ratios. Cu concentrations range over an order of magnitude, more than any other element, in all four samples containing low-salinity inclusions. We infer that variations are the result of fluid trapping after different amounts of fluid–rock reaction and chalcopyrite precipitation. Enrichment, relative to early fluids, of Mn, Pb, and Zn in fluids related to sericitic alteration is also likely the result of fluid–rock reaction, whereby these elements are released from biotite and feldspars as they alter to sericite. In halite-bearing inclusions, concentrations of Sr, Ca, Pb, and Ba are elevated in inclusions from the pyrite–quartz vein with sericitic alteration relative to halite-bearing inclusions from unaltered and potassically altered samples. Such enrichment is likely caused by the breakdown of plagioclase and K-feldspar in the alteration envelope, releasing Sr, Ca, Pb, and Ba.     

http://www.sciencedirect.com/science/article/pii/S1674987114000590

This paper reviews the origin and evolution of fluid inclusions in ultramafic xenoliths,providing a framework for interpreting the chemistry of mantle fluids in the different geodynamic settings.Fluid inclusion data show that in the shallow mantle,at depths below about 100 km,the dominant fluid phase is CO_2±brines,changing to alkali-,carbonate-rich(silicate) melts at higher pressures.Major solutes in aqueous fluids are chlorides,silica and alkalis(saline brines;5-50 wt.%NaCl eq.).Fluid inclusions in peridotites record CO_2 fluxing from reacting metasomatic carbonate-rich melts at high pressures,and suggest significant upper-mantle carbon outgassing over time.Mantle-derived CO_2(±brines) may eventually reach upper-crustal levels,including the atmosphere,independently from,and additionally to magma degassing in active volcanoes.     

Mesothermal Gold Vein Mineralization of the Seolhwa Mine,Asan Mining District,Korea

Gold mineralization of the Seolhwa mine occurs in a single stage of massive quartz veins which filled the north‐east‐trending fault shear zones in the Jurassic granitoid of 161 Ma within the Gyeonggi Massif. The vein quartz contains three main types of fluid inclusions at 25°C: (i) aqueous type I inclusions (0–15 wt.% NaCl) containing small amounts of CO₂; (ii) gas‐rich (more than 70 vol. %), vapor‐homogenizing, aqueous type II inclusions; and (iii) low‐salinity (less than 5 wt.% NaCl), liquid CO₂‐bearing, type III inclusions. The H₂O‐CO₂‐CH₄‐N₂‐NaCl inclusions represent immiscible fluids trapped earlier along the solvus curve in the temperature range 250–430°C at pressures of ～1 kb. Detailed fluid inclusion chronologies suggest a progressive decrease in pressure during the mineralization. Aqueous inclusion fluids represent either later fluids evolved through extensive fluid unmixing from a homogeneous H₂O‐CO₂‐CH₄‐N₂‐NaCl fluid due to decreases in temperature and pressure, or the influence of deep circulated meteoric waters. Initial fluids were homogeneous H₂O‐CO₂‐CH₄‐N₂‐NaCl fluids as follows: 250° to 430°C, 16–62 mol% CO₂, 5–14 mol% CH₄, 0.06–0.31 mol% N₂ and salinities of 0.4–4.9 wt.% NaCl. The T‐X data for the Seolhwa mine suggest that the hydrothermal system has been probably located nearer to the granitic melt, which facilitated the CH₄ formation and resulted in a reduced fluid state indicated by the predominance of pyrrhotite. Measured and calculated isotopic compositions of the hydrothermal fluids [δ¹⁸O = 5.3–6.5‰; δD =?69 to ?84‰] provide evidence of the CH₄‐H₂O equilibria and further indicate that the auriferous fluids were magmatically derived. Both the dominance of δ³⁴S values of sulfides close to the meteoric reference (?0.6–1.4‰; δ³⁴S_ΣS values of 0.3–1.1‰) and the available δ¹³C data (?4‰) are consistent with their deep igneous source. The Seolhwa mine was probably formed by extensive fracturing and veining due to the thermal expansion of water derived from the Jurassic granitoid melt.     

Fluid Inclusions in the Gold—Bearing Quartz Veins at Um Rus Area,Eastern Desert,Egypt

Fluid inclusions in the gold-bearing quartz veins at the Um Rus area are of three types: H₂O, H₂O−CO₂ and CO₂ inclusions. H₂O inclusions are the most abundant, they include two phases which exhibit low and high homogenization temperatures ranging from 150 to 200°C and 175 to 250°C, respectively. The salinity of aqueous inclusions, based on ice melting, varies between 6.1 and 8 equiv. wt% NaCl. On the other hand, H₂O−CO₂ fluid inclusions include three phases. Their total homogenization temperatures range from 270 to 325°C, and their salinity, based on clathrate melting, ranges between 0.8 and 3.8 equiv. wt% NaCl. CO₂ fluid inclusions homogenize to a liquid phase and exhibit a low density range from 0.52 to 0.66 g/cm³. The partial mixing of H₂O−CO₂ and salt H₂O−NaCl fluid inclusions is the main source of fluids from which the other types of inclusions were derived. The gold-bearing quartz veins are believed to be of medium temperature hydrothermal convective origin.     

Carbonic metamorphism of charnockites in the southwestern Indian Shield: A fluid inclusion study

Forming the southwestern segment of the Precambrian granulite facies terrain of the Indian shield, the Kerala region largely comprises charnockites, khondalites and migmatitic gneisses. Fluid inclusions in quartz from the charnockites show distinct distribution patterns consistent with three generations of inclusions. The early monophase type records entrapment of high-density CO₂-rich fluid (0.95–1.0 g cm⁻³). A subsequent monophase type with lower-density CO₂-rich fluid (0.65–0.75 g cm⁻³) coexists with CO₂H₂O inclusions having an average degree of filling of 0.2 (H₂O = 20%; CO₂ = 80%). Late aqueous biphase inclusions show coexistence with a second category of CO₂H₂O inclusions showing a degree of filling of 0.6 (H₂O = 60%; CO₂ = 40%). The CO₂-isochores for early carbonic inclusions yield a pressure range of 4.6–6.1 kbar at granulite facies temperatures of 650–800°C, depicting the entrapment of fluids present during or close to the peak metamorphic stage. A definite sequence of fluid evolution is traceable for the subsequent stages. Thus, the coexisting CO₂ and CO₂H₂O inclusions were entrapped at 510°C and 2.2 kbar, marking the waning of carbonic regime and the beginning of aqueous regime. At 330°C and 0.4 kbar, fluid unmixing occurred, leading to the simultaneous entrapment of mixed CO₂H₂O and H₂O inclusions along rehealed microfractures. The data presented indicate that the metamorphic fluids evolved from early high-density carbonic through mixed carbonic-aqueous to late aqueous types. The dry granulite mineral assemblage of charnockites is a result of metamorphic equilibration under water-deficient and high-P_CO2 conditions.     

苏北青龙山超高压变质榴辉岩流体包裹体特征与流体演化

根据青龙山超高压变质榴辉岩中流体包裹体的化学成分、矿物中的分布特征将岩石中的流体包裹体分为五类,即富N2包裹体、高盐度(22.4-略大于23.2wt%NaCl)的NaCl CaCl2 H2O体系流体包裹体、中高盐度(12.6-16.0wt%NaCl)的含Mg2 或Fe2 的NaCl H2O体系流体包裹体、中等盐度(6.4-10.5wt%NaCl)水溶液包裹体和低盐度(3.3-0.2wt%NaCl)的水溶液包裹体。富N2包裹体形成于超高压变质峰期阶段,高盐度的流体包裹体形成于超高压变质岩折返早期固体出溶体出溶阶段,中高盐度的流体包裹体形成于高压变质重结晶作用阶段,中等盐度的流体包裹体形成于角闪岩相变质重结晶作用阶段,低盐度的流体包裹体形成于折返晚期的绿片岩退变质作用阶段。超高压变质峰期阶段和折返早期的高盐度流体和中高盐度的流体主要来自继承原岩中的流体(如含NH4 矿物分解或片麻岩原岩中的有机质分解,名义上无水矿物中羧基水的出溶),晚期角闪岩相退变质阶段的中等盐度的流体除名义上无水矿物中羟基水的出溶外还有外来流体的加入,绿片岩相退变质作用阶段的流体主要为外来流体。