Salinity Of Inclusions Research Articles

Fluid inclusions, muscovite Ar–Ar age, and fluorite trace elements at the Baiyanghe volcanic Be–U–Mo deposit, Xinjiang, northwest China: Implication for its genesis

The Baiyanghe Be–U–Mo deposit is located in the Late Paleozoic Xuemisitan–Kulankazi island arc of the northwestern margin of the Junggar plate, Northwest China. It is the largest Be deposit (2.2Mtons of ore with grades ranging from 0.2% to 1.4%) in Asia. Orebodies in the deposit occur as fractures along contact zones between the Yangzhuang granite porphyry intrusion and Devonian pyroclastic country rocks and within the porphyry itself. Muscovite–fluorite veins are closely associated with the Be–U–Mo mineralization. A new Ar–Ar dating of the muscovite in this study yields a plateau age of 303.0±1.6Ma, which constrains the timing of the Be–U–Mo mineralization of the deposit. Three stages of fluorite of different colors have been identified at the deposit, with the earliest dark-purple fluorite more closely associated with the mineralization. Microthermometry of fluid inclusions obtained from the three stages of fluorite suggests that the fluorites were precipitated as veins from low temperature (120–150°C) hydrothermal fluids with salinity ranging from 4.7 to 19.7wt.% NaCl eqv. Based on the trace elemental concentrations and REE patterns of the fluorite, the style of veining, and the low salinity and low temperature characters of the fluid inclusions, it is suggested that Be and U were most likely transported as fluoride complexes and Mo as hydroxyl complexes. Pb isotopic compositions of the ores and country rocks, as well as O and H isotopic characters of the ore-related muscovite, indicate mixing between magmatic and meteoric waters; both contributed to formation of the ore-forming fluids. Metallic Be, U, and Mo were most likely leached out from the granite porphyry by the fluids. The fluid mixing led to the reduction of U, Mo, and Be and their precipitation at the deposit.

Mineralisation of amethyst-bearing geodes in Ametista do Sul (Brazil) from low-temperature sedimentary brines: evidence from monophase liquid inclusions and stable isotopes

Fluid inclusion studies in combination with hydrogen, oxygen and sulphur isotope data provide novel insights into the genesis of giant amethyst-bearing geodes in Early Cretaceous Parana continental flood basalts at Amestita do Sul, Brazil. Monophase liquid inclusions in colourless quartz, amethyst, calcite, barite and gypsum were analysed by microthermometry after stimulating bubble nucleation using single femtosecond laser pulses. The salinity of the fluid inclusions was determined from ice-melting temperatures and a combination of prograde and retrograde homogenisation temperatures via the density maximum of the aqueous solutions. Four mineralisation stages are distinguished. In stage I, celadonite, chalcedony and pyrite formed under reducing conditions in a thermally stable environment. Low δ34SV-CDT values of pyrite (−25 to −32 ‰) suggest biogenic sulphate reduction by organotrophic bacteria. During the subsequent stages II (amethyst, goethite and anhydrite), III (early subhedral calcite) and IV (barite, late subhedral calcite and gypsum), the oxidation state of the fluid changed towards more oxidising conditions and microbial sulphate reduction ceased. Three distinct modes of fluid salinities around 5.3, 3.4 and 0.3 wt% NaCl-equivalent characterise the mineralisation stages II, III and IV, respectively. The salinity of the stage I fluid is unknown due to lack of fluid inclusions. Variation in homogenisation temperatures and in δ18O values of amethyst show evidence of repeated pulses of ascending hydrothermal fluids of up to 80–90 °C infiltrating a basaltic host rock of less than 45 °C. Colourless quartz and amethyst formed at temperatures between 40 and 80 °C, while the different calcite generations and late gypsum precipitated at temperatures below 45 °C. Calculated oxygen isotope composition of the amethyst-precipitating fluid in combination with δD values of amethyst-hosted fluid inclusions (−59 to −51 ‰) show a significant 18O-shift from the meteoric water line. This 18O-shift, high salinities of the fluid inclusions with chloride-sulphate composition, and high δ34S values of anhydrite and barite (7.5 to 9.9 ‰) suggest that sedimentary brines from deeper parts of the Guarani aquifer system must have been responsible for the amethyst mineralisation.

Newly-formed illite preserves fluid sources during folding of shale and limestone rocks; an example from the Mexican Fold-Thrust Belt

We combine structural, fluid-inclusion microthermometry, illite-crystallinity, X-Ray Diffraction (XRD) and O and H stable isotope analyses of authigenic illite to determine the source of local fluids interacting with rock during folding in anchizonal shales of the Mexican Fold-Thrust Belt (MFTB). A well-exposed train of mesoscopic, asymmetrical folds in a sequence of Cretaceous limestones interbedded with shale was targeted for this study. We test the hypothesis that syn-folding vein minerals and clay minerals were formed from the same fluids by comparing the δ2H composition of inclusion fluids in calcite and quartz from veins, and from illite concentrates from sheared shale layers, and the sources of that fluid.Five clay size-fractions (<0.05, 0.05–0.2, 0.2–1, 1–2, and <2 μm) were separated from eight shale samples. In the 40 clay grain-size fractions analyzed, illite, calcite, kaolinite, smectite, chlorite and minor quartz were identified by XRD analysis. Most samples show different proportions of various clay minerals, except for the finer fractions in two of the samples (BL3 and BL4) were illite is the only clay phase present. The discriminating potential of δ18O values of clay is generally masked by the abundance of calcite in all samples. In contrast, samples containing chlorite and smectite show very low values in δ2H (−75.9 to −53.9‰), while samples containing illite and kaolinite or pure illite show relatively high δ2H values (−33.1 to −50.1‰). The latter fall within the δ2H range (−39 to −49‰) determined in fluid inclusions of syntectonic veins, indicating isotopic equilibrium between water, veins fillings and illitic clay during deformation, according to fractionation factors at these temperatures (220–250 °C). The δ2H values and fluid inclusion salinities in the sampled rocks indicate that water active during folding was partly marine and partly meteoric and that the amount of such pore-water represented a small fraction of the deformed rock volume. Our study demonstrates that the H-isotopic composition of anchizonal illitic clays, commonly present in exhumed fold-thrust belts, can be used to determine the source(s) of fluids that were active during deformation.

Evidence for exsolution of Au-ore fluids from granites crystallized in the mid-crust, Archaean Louis Lake Batholith, Wyoming

Abstract The Louis Lake Batholith is contemporaneous with and sits adjacent to Archaean lode gold deposits and minor copper stockwork vein systems of the South Pass Greenstone Belt, Wyoming Province, and may thus have contributed fluid into these hydrothermal systems. The batholith is granodiorite to granite, lacks extreme fractionation, and contains evidence for early formation and later replacement of an immiscible early sulphide phase. Mafic enclaves contain anomalous concentrations of gold and copper, suggesting that metals were introduced from in-mixed mafic magma. Apatite SO 3 and Cl values imply relatively early exsolution of sulphur- and chlorine-bearing fluids during fractionation. Exsolution of possibly immiscible high- and low-salinity fluids is implied from fluid inclusion petrography, even though crystallization pressures were greater than 3 kbar. Low salinity inclusions share affinities with those of orogenic gold systems, but contain notably higher Pb and Zn concentrations. High salinity inclusions have similar elemental compositions to brines of intrusion-centred ore systems. It is proposed that sulphide melt separated early during magma fractionation, scavenging Au and Cu, but became later destabilized, possibly because of increased f O 2 , releasing scavenged metals. Metals were transported from the pluton in sulphur- and chlorine-bearing hydrothermal fluids of low and high salinity, with Au being preferentially transported in the low-salinity fluid. Supplementary material: Whole Rock Geochemical data are available at http://www.geolsoc.org.uk/SUP18751 .

Sedimentary-rock-hosted epithermal systems of the Tertiary Eastern Rhodopes, Bulgaria: new constraints from the Stremtsi gold prospect

Abstract Precious metal epithermal, sedimentary-rock-hosted prospects constitute a new class of ore deposits recently described in the Tertiary Eastern Rhodopes of southeastern Bulgaria. The Stremtsi prospect investigated in this contribution is located in a distal location with respect to the main cluster of sedimentary-rock-hosted Ada Tepe and Rosino gold prospects of the Eastern Rhodopes. The Stremtsi prospect is hosted by a Priabonian clastic sedimentary rock sequence, overlying metamorphic rocks of the Central Rhodopean dome. The eastern part of the Stremtsi prospect contains high gold grades, and is characterized by a strongly silicified zone, including adularia and silicified dolomite blades, diagnostic for boiling conditions during ore formation in such low-sulphidation epithermal systems. The western part of the Stremtsi prospect consists of a barite, sphalerite and galena mineralization, associated with silicification, and illite and carbonate alteration. Both parts are underlain by subvertical quartz-carbonate-pyrite veins. Primary and secondary fluid inclusions, respectively, in dolomite and barite yield homogenization temperatures ranging between 90 and 247 °C. The salinity of primary inclusions in dolomite falls between 1.9 and 5.1 wt% NaCl equivalent, whereas the one of secondary fluid inclusions in barite ranges between 0.0 and 3.1 wt% NaCl equivalent. The variable homogenization temperatures reflect post-entrapment re-equilibration of the fluid inclusions, whereas the salinities were preserved and the inclusions in dolomite are interpreted in terms of dilution of a saline fluid in the western part of the Stremtsi prospect. The sulphur isotope compositions of sulphides from Stremtsi range mainly between −4 and +4‰. They are not diagnostic and can be attributed to magmatic, metamorphic, and sedimentary sources. They overlap with the main compositional range of sulphides from other sedimentary-rock-hosted epithermal systems and reveal the existence of hydrothermal fluids with common characteristics during ore formation throughout the Eastern Rhodopes. In addition, at Stremtsi, negative δ 34 S values between −42.6‰ and −8.8‰ combined with framboidal pyrite and elevated δ 34 S values of +7.0‰ to +19.5‰ support locally derived sulphur generated, respectively, by bacterial and thermochemical sulphate reduction. Modelling of O, C, and Sr isotope data of dolomite support the above described ore-forming processes. A positive correlation between δ 18 O (+12.7‰ to +19.7‰ V-SMOW) and δ 13 C (−2.8‰ to +1.5‰ V-PDB) values for dolomite from the eastern, silicified and gold-enriched zone of the Stremtsi prospect is satisfactorily modelled by boiling between 140 and 180 °C of a deeply circulating fluid characterized by δ 18 O and δ 13 C values of +5.5‰ V-SMOW and −1.5‰ V-PDB, respectively, and radiogenic strontium leached from the metamorphic basement rocks or its clastic counterparts in the Priabonian host rocks. By contrast, negative correlations of δ 18 O values (+13.4‰ to +23.3‰ V-SMOW) with δ 13 C values (−0.6‰ to −3.9‰ V-PDB) and 87 Sr/ 86 Sr ratios of dolomite from the western, barite and base metal-rich zone are adequately modelled by a shallow, low temperature (70 °C), intra-formational fluid recharged by meteoric water, which interacted with organic matter, that is, coal layers, and carbonate rocks from the Priabonian host sequence, mixing with a deep, moderate temperature (190 °C), 87 Sr-enriched fluid characterized by δ 18 O and δ 13 C values of +5.5‰ V-SMOW and −1.5‰ V-PDB, respectively. Disequilibrium conditions revealed by sulphur isotope thermometry of two galena-barite pairs yielding discrepant temperatures of 190 and 306 °C are consistent with fluid mixing. A plateau age of 37.57±0.31 Ma obtained by 40 Ar/ 39 Ar dating of adularia from Stremtsi is interpreted as a maximum age because of the saddle-shaped age spectrum. Combined with 40 Ar/ 39 Ar age data from previous studies, it reveals that the sedimentary-rock-hosted epithermal prospects constitute an independent, regional and older ore-forming hydrothermal system, distinct from the younger volcanic-rock-hosted epithermal deposits of the Bulgarian and Greek Eastern Rhodopes.

STRUCTURAL CONTROL, FLUID INCLUSIONS AND CATHODOLUMINESCENCE STUDIES OF BIRIMIAN GOLD-BEARING QUARTZ VEIN SYSTEMS IN THE PALEOPROTEROZOIC MAKO BELT, SOUTHEASTERN SENEGAL

The Paleoproterozoic Mako Belt in eastern Senegal is characterised by gold-bearing quartz veins. These are superimposed on the regional Birimian structural complex consisting of volcano-sedimentary and intrusive rocks in an Early Proterozoic greenstone belt. In the Mako Belt the multi-stage deformed quartz veins and hydrothermally altered protoliths are characterised by distinctive microstructures and CL properties. The high-grade Au-bearing quartz veins contain one or more generations of hydrothermal quartz. Brecciation of the veins is indicated by cemented fragments of zoned quartz. Gold in the mineralised zones occurs either as inclusions in pyrite or as native gold. Mineralisation typically occurs in vein systems which are brecciated, laminated, and/or show crack-seal textures. Microstructures indicate syn- to late-tectonic mineralisation. Au-mineralised veins contain quartz, carbonate, muscovite, fuchsite, tourmaline, and chlorite. Fluid inclusion studies reveal early, highly saline aqueous inclusions followed by main-stage aqueous-carbonic inclusions of low salinity and finally aqueous low-salinity inclusions. Petrographic observation and microthermometric data suggest that fluid inclusions in main stage quartz were trapped after phase separation from a heterogeneous H2O-CO2 fluid. Gold deposition occurs over a temperature range of 220 to 320°C and pressures of 1.4 to 2.75 kbar (~5 to 10 km depth). The late-stage quartz carbonate vein corresponds to pressures of 0.75 to 1.25 kbar and depths of 3.0 to 4.5 km. A fluid pressure drop due to fracture failure is likely to have triggered Au precipitation. The Mako Belt Au deposits are comparable to a large number of orogenic lode gold deposits in the West African Craton.

Acid saline fluid inclusions: examples from modern and Permian extreme lake systems

AbstractSome of the Earth's most extreme aqueous chemistries are found at acid saline lakes and associated shallow groundwaters. Modern lakes and groundwaters in Western Australia have pHs as low as 1.5, total dissolved solids as high as 32%, unusually high concentrations of Al, Si, Fe, As, and Sb, and many other chemical characteristics atypical of natural waters. Analogous Permian red bed and evaporite lake deposits in the US midcontinent also had extreme water compositions, including negative pH. Fluid inclusions in chemical sediments from modern and Permian acid saline lakes record specific physical, chemical, and biological conditions. Primary inclusions in this bedded halite and gypsum are trapped shallow lake water, air bubbles, crystals of other minerals, and microorganisms and organic compounds. Isolated fluid inclusions in early diagenetic phases of halite are remnants of shallow groundwaters and can be analyzed to determine chemical compositions of past groundwaters. A variety of traditional methods, including petrography, freezing–melting microthermometry and laser Raman spectroscopy, and innovative variations, such as homogenization of artificially nucleated vapor bubbles and UV‐vis petrography, produce data detailing environmental conditions. Other traditionally used methods, such as laser ablation ICP‐MS, are not effective due to the inclusions' high and complex salinity, extremely low pH and/or low pressure. Making synthetic solutions that match the complexity of natural acid saline inclusions is another challenge. Fluid inclusions in acid‐precipitated halite have proven to be excellent repositories for past surface chemical, physical, and biological conditions. Although the methods used and data obtained from acid‐precipitated halite and gypsum are not exactly the same as for neutral‐alkaline evaporites, acid inclusions provide high‐resolution environmental data for modern and ancient extreme environments. These extreme fluids may also have similarities to other past extreme environments, to ore mineralizing fluids, and to past liquids on Mars.

On the homogeneity of fluids forming bedding‐parallel veins

AbstractA group of 400–500 m long, bedding‐parallel calcite veins are exposed in the central La Popa Basin of northeastern Mexico. These veins provide a unique opportunity to determine the kilometer‐scale fluid–rock system associated with bedding‐parallel vein formation, and to test for sampling bias in studies that often use one or two samples to constrain the characteristics of regional‐scale paleohydrogeological systems. We use fluid inclusion microthermometry in conjunction with measurements of δ13C, δ18O, and 87Sr/86Sr ratios to constrain the vein‐forming fluid temperatures, compositions and sources, and compare these values along and between the veins to establish the homogeneity of the vein‐forming fluids and fluid–rock system. The δ13C values of the veins are close to those of the host rock, and average – 3.96‰ (PDB). The δ18O values of the veins are typically 1‰ lower than those of the host rocks, and average – 9.54‰ (PDB). Fluid inclusion homogenization temperatures average 137°C and inclusion salinities are all &lt;6 wt% NaCl equivalent. The 87Sr/86Sr ratios of the veins average 0.70731 and are substantially lower than the values expected for the host rock. Calculated fluid δ18O values range from 4 to 10‰ (SMOW). The isotopic and microthermometric data indicate the veins most likely formed at depths of 3–4 km when meteoric water mixed with upward migrating, warm basinal brines. Vein microstructures and field characteristics indicate they formed from multiple slip events that most likely were associated with transport of individual fluid pulses that migrated along bedding planes. The large‐scale homogeneity of vein geochemistry is remarkable and demonstrates that only one or two samples would be sufficient to accurately characterize the kilometer‐scale paleohydrogeological system for these veins.

Characteristics of Mineralizing Fluids of the Darreh‐Zerreshk and Ali‐Abad Porphyry Copper Deposits, Central Iran, Determined by Fluid Inclusion Microthermometry

AbstractThe Darreh‐Zereshk (DZ) and Ali‐Abad (AB) porphyry copper deposits are located in southwest of the Yazd city, central Iran. These deposits occur in granitoid intrusions, ranging in composition from quartz monzodiorite through granodiorite to granite. The ore‐hosting intrusions exhibit intense hydrofracturing that lead to the formation of quartz‐sulfide veinlets. Fluid inclusions in hydrothermal quartz in these deposits are classified as a mono‐phase vapor type (Type I), liquid‐rich two phase (liquid + vapor) type (Type IIA), vapor‐rich two phase (vapor + liquid) type (Type IIB), and multi‐phase (liquid + vapor + halite + sylvite + hematite + chalcopyrite and pyrite) type (Types III). Homogenization temperatures (Th) and salinity data are presented for fluid inclusions from hydrothermal quartz veinlets associated with potassic alteration and other varieties of hypogene mineralization. Ore precipitation occurred between 150° to &gt;600°C from low to very high salinity (1.1–73.9 wt% NaCl equivalent) aqueous fluids. Two stages of hydrothermal activity characterized are recognized; one which shows relatively high Th and lower salinity fluid (Type IIIa; Th(L‐V) &gt; Tm(NaCl)); and one which shows lower Th and higher salinity (Type IIIb; Th(L‐V) &lt; Tm(NaCl)). The high Th(L‐V) and salinities of Type IIIa inclusions are interpreted to represent the initial existence of a dense fluid of magmatic origin. The coexistence of Type IIIb, Type I and Type IIB fluid inclusions suggest that these inclusions resulted either from trapping of boiling fluids and/or represent two immiscible fluids. These processes probably occurred as the result of pressure fluctuations from lithostatic to hydrostatic conditions under a pressure of 200 to 300 bar. Dilution of these early fluids by meteoritic water resulted in lower temperatures and low to moderate salinity (&lt;20 wt% NaCl equiv.) fluids (Type IIA). Fluid inclusion analysis reveals that the hydrothermal fluid, which formed mineralized quartz veinlets in the rocks with potassic alteration, had temperatures of ∼500°C and salinity ∼50 wt% NaCl equiv. Cryogenic SEM‐EDS analyses of frozen and decrepitated ore‐bearing fluids trapped in the inclusions indicate the fluids were dominated with NaCl, and KCl with minor CaCl2.

Thermodynamic modeling of ternary CH4[sbnd]H2O[sbnd]NaCl fluid inclusions

This paper reports the application of thermodynamic models, including equations of state, to ternary CH4H2ONaCl fluid inclusions. A simple equation describing pressure–temperature–salinity relations on the CH4 hydrate-liquid-vapor surface has been developed to calculate the NaCl contents (salinities) of inclusions, where the dissociation pressure of CH4 hydrate coexisting with vapor and liquid at a given temperature is calculated with a pressure equation of pure CH4. The pressure equation is a function of temperature and CH4 Raman peak position shift corrected by Ne lamp. With these relations and the latest CH4 solubility and PVTx models, a new iterative approach is presented to calculate the CH4 contents of CH4H2ONaCl inclusions on the assumption that the bulk molar volume of an inclusion at the melting temperature of CH4 hydrate and at the vapor bubble disappearance (homogenization) temperature are identical. A prominent merit of this method is that the compositions, molar volumes and homogenization pressures of CH4H2ONaCl inclusions can be simultaneously obtained without having to use volume fractions of vapor bubbles at the dissociation temperatures of CH4 hydrates determined based on optical observations or measurements. The homogenization pressures and isochores of CH4H2ONaCl fluid inclusions from updated models are briefly discussed. The code to estimate PVTx properties of inclusions in the ternary system CH4H2ONaCl, based on microthermometric and Raman data, can be obtained from Chemical Geology or the corresponding author (maoshide@163.com).

Genesis and mineralization of gold-bearing quartz veins in the Xiao Qinling area, central China

Gold-bearing quartz veins of the Taihua Group consisting of Archean metavolcanic rocks are a main gold deposit type in the Xiao Qinling area, one of the three biggest gold production areas in China. The quartz veins experienced strong alteration characterized by a typical mesothermal hydrothermal altered mineral assemblage. The grade of gold is affected by the contents of sulphides, e.g. galena, pyrite and chalcopyrite. Results of minor elements analysis for the of gold-bearing quartz veins indicate higher contents of Au and high contents of Ag, Pb, Cu, Cd, W, and Mo. Abundant fluid inclusions were found in the gold-bearing quartz veins. Three types of fluid inclusions were identified: (1) aqueous inclusions; (2) CO2-bearing inclusions; and (3) daughter crystal-bearing fluid inclusions. Homogenization temperatures ranged from 110 to 670°C with low and high peaks appearing at 160–180°C and 280–300°C, respectively. The salinity of aqueous inclusions varies between 1.8 wt% and 38.2 wt% NaCl. The homogenization temperature and salinity show a positive correlation. The H and O isotopes of fluid inclusions in the gold-bearing quartz veins indicate that magmatic solution and metamorphic hydrothermal solution, together with meteoric water, were involved in the formation of gold-bearing fluid. Mesozoic magma activities related to granite intrusions should be the main source of CO2 fluid with higher temperature and salinity.

Determination of Cl and Br concentrations in individual fluid inclusions by combining microthermometry and LA-ICPMS analysis: Implications for the origin of salinity in crustal fluids

The origin of salinity in crustal fluids can be deduced from their Cl, Br and Na concentrations and associated Cl/Br and Na/Br ratios. Here, a protocol for quantitative analysis of Cl, Br and Na concentration in individual fluid inclusions of variable size, bulk salinity, Cl, Br and Na contents is presented. It combines microthermometry and laser ablation — inductively coupled plasma mass spectrometry (LA-ICPMS) analysis.The method is based on: (i) microthermometric measurement of ice melting, hydrate melting, halite dissolution and sylvite dissolution temperatures in fluid inclusions; (ii) calibration of LA-ICPMS signals of Br by analyzing Na–Br–Cl standard solutions loaded in silica glass capillaries; (iii) calculation of solute (Cl, Na, Ca, K, Mg) concentration based on LA-ICPMS intensities and the Pitzer thermodynamic model (for inclusions of salinity below 23wt.% equivalent NaCl), or a charge-balance approach (for higher salinity inclusions) and (iv) calculation of uncertainty on Cl, Br and Na concentration and Cl/Br and Na/Br ratios in the fluid.This method is applied to natural fluid inclusions with variable Cl, Br and Na contents as previously determined by microthermometry and bulk crush-leach coupled with ion chromatography analysis. The studied materials consist of: i) low-salinity inclusions from Alpine quartz veins and ii) high-salinity inclusions from the Trimouns talc deposit and from Athabasca Basin uranium deposits. Molar Cl/Br and Na/Br ratios range from 140±50 to 1200±400 and 17±6 to 880±290 in the analyzed inclusions. The calculated uncertainty on Cl/Br and Na/Br ratios in individual fluid inclusions is between 30 and 38%, regardless of the salinity. It appears that the analysis of individual inclusions coupled with uncertainty calculation allows the identification of significant variation of Cl/Br and Na/Br ratios within individual samples that could not be identified from previous bulk crush-leach and ion chromatography analyses. Because Br is a minor solute (molar Cl/Br is between ~100 and ~10000 in most crustal fluids), and because of its high first ionization energy, the determination of Br concentration in individual inclusion by combining microthermometry and LA-ICPMS analysis is most readily achievable for large (>50μm in diameter) inclusions. For smaller inclusions (10 to 50μm in diameter), the applicability of the present method is more sensitive to the Br concentration.

Highly Oxidized Magma and Fluid Evolution of Miocene Qulong Giant Porphyry Cu‐Mo Deposit, Southern Tibet, China

AbstractThe Miocene Qulong porphyry Cu‐Mo deposit, which is located at the Gangdese orogenic belt of Southern Tibet, is the largest porphyry‐type deposit in China, with confirmed Cu ∼10 Mt and Mo ∼0.5 Mt. It is spatially and temporally associated with multiphase granitic intrusions, which is accompanied by large‐scale hydrothermal alteration and mineralization zones, including abundant hydrothermal anhydrite. In addition to hydrothermal anhydrite, magmatic anhydrite is present as inclusions in plagioclase, interstitial minerals between plagioclase and quartz, and phenocrysts in unaltered granodiorite porphyry, usually in association with clusters of sulfur‐rich apatite in the Qulong deposit. These observations indicate that the Qulong magma‐hydrothermal system was highly oxidized and sulfur‐rich. Three main types of fluid inclusions are observed in the quartz phenocrysts and veins in the porphyry: (i) liquid‐rich; (ii) polyphase high‐salinity; and (iii) vapor‐rich inclusions. Homogenization temperatures and salinities of all type inclusions decrease from the quartz phenocrysts in the porphyry to hydrothermal veins (A, B, D veins). Microthermometric study suggests copper‐bearing sulfides precipitated at about 320–400°C in A and B veins. Fluid boiling is assumed for the early stage of mineralization, and these fluids may have been trapped at about 35–60 Mpa at 460–510°C and 28–42 Mpa at 400–450°C, corresponding to trapping depths of 1.4–2.4 km and 1.1–1.7 km, respectively.

Hydrocarbon- and ore-bearing basinal fluids: a possible link between gold mineralization and hydrocarbon accumulation in the Youjiang basin, South China

The Youjiang basin, which flanks the southwest edge of the Yangtze craton in South China, contains many Carlin-type gold deposits and abundant paleo-oil reservoirs. The gold deposits and paleo-oil reservoirs are restricted to the same tectonic units, commonly at the basinal margins and within the intrabasinal isolated platforms and/or bioherms. The gold deposits are hosted by Permian to Triassic carbonate and siliciclastic rocks that typically contain high contents of organic carbon. Paragenetic relationships indicate that most of the deposits exhibit an early stage of barren quartz ± pyrite (stage I), a main stage of auriferous quartz + arsenian pyrite + arsenopyrite + marcasite (stage II), and a late stage of quartz + calcite + realgar ± orpiment ± native arsenic ± stibnite ± cinnabar ± dolomite (stage III). Bitumen in the gold deposits is commonly present as a migrated hydrocarbon product in mineralized host rocks, particularly close to high grade ores, but is absent in barren sedimentary rocks. Bitumen dispersed in the mineralized rocks is closely associated and/or intergrown with the main stage jasperoidal quartz, arsenian pyrite, and arsenopyrite. Bitumen occurring in hydrothermal veins and veinlets is paragenetically associated with stages II and III mineral assemblages. These observations suggest an intimate relationship between bitumen precipitation and gold mineralization. In the paleo-petroleum reservoirs that typically occur in Permian reef limestones, bitumen is most commonly observed in open spaces, either alone or associated with calcite. Where bitumen occurs with calcite, it is typically concentrated along pore/vein centers as well as along the wall of pores and fractures, indicating approximately coeval precipitation. In the gold deposits, aqueous fluid inclusions are dominant in the early stage barren quartz veins (stage I), with a homogenization temperature range typically of 230°C to 270°C and a salinity range of 2.6 to 7.2 wt% NaCl eq. Fluid inclusions in the main and late-stage quartz and calcite are dominated by aqueous inclusions as well as hydrocarbon- and CO2-rich inclusions. The presence of abundant hydrocarbon fluid inclusions in the gold deposits provides evidence that at least during main periods of the hydrothermal activity responsible for gold mineralization, the ore fluids consisted of an aqueous solution and an immiscible hydrocarbon phase. Aqueous inclusions in the main stage quartz associated with gold mineralization (stage II) typically have a homogenization temperature range of 200–230°C and a modal salinity around 5.3 wt% NaCl eq. Homogenization temperatures and salinities of aqueous inclusions in the late-stage drusy quartz and calcite (stage III) typically range from 120°C to 160°C and from 2.0 to 5.6 wt% NaCl eq., respectively. In the paleo-oil reservoirs, aqueous fluid inclusions with an average homogenization temperature of 80°C are dominant in early diagenetic calcite. Fluid inclusions in late diagenetic pore- and fissure-filling calcite associated with bitumen are dominated by liquid C2H6, vapor CH4, CH4–H2O, and aqueous inclusions, with a typical homogenization temperature range of 90°C to 180°C and a salinity range of 2–8 wt% NaCl eq. It is suggested that the hydrocarbons may have been trapped at relatively low temperatures, while the formation of gold deposits could have occurred under a wider and higher range of temperatures. The timing of gold mineralization in the Youjiang basin is still in dispute and a wide range of ages has been reported for individual deposits. Among the limited isotopic data, the Rb–Sr date of 206 ± 12 Ma for Au-bearing hydrothermal sericite at Jinya as well as the Re–Os date of 193 ± 13 Ma on auriferous arsenian pyrite and 40Ar/39Ar date of 194.6 ± 2 Ma on vein-filling sericite at Lannigou may provide the most reliable age constraints on gold mineralization. This age range is comparable with the estimated petroleum charging age range of 238–185 Ma and the Sm–Nd date of 182 ± 21 Ma for the pore- and fissure-filling calcite associated with bitumen at the Shitouzhai paleo-oil reservoir, corresponding to the late Indosinian to early Yanshanian orogenies in South China. The close association of Carlin-type gold deposits and paleo-oil reservoirs, the paragenetic coexistence of bitumens with ore-stage minerals, the presence of abundant hydrocarbons in the ore fluids, and the temporal coincidence of gold mineralization and hydrocarbon accumulation all support a coeval model in which the gold originated, migrated, and precipitated along with the hydrocarbons in an immiscible, gold- and hydrocarbon-bearing, basinal fluid system.

Combined microthermometric and Raman spectroscopic technique to determine the salinity of H 2O–CO 2–NaCl fluid inclusions based on clathrate melting

Fluid inclusions approximated by the system H 2O–CO 2–NaCl are common in many geologic environments. In order to apply microthermometric data from these inclusions to infer P–T (pressure–temperature) trapping conditions, the composition of the inclusions, including the salinity, must be known. Normally, salinities of aqueous inclusions are determined from ice-melting temperatures obtained during microthermometry. However, when CO 2-bearing aqueous fluid inclusions are cooled they often form a hydrate that incorporates H 2O into the structure, and salinities estimated from ice-melting temperatures are therefore higher than the actual salinity. A technique that combines data from Raman spectroscopic and microthermometric analyses of individual inclusions was developed to determine the salinity of CO 2-bearing aqueous inclusions based on measured clathrate melting temperatures and CO 2 pressures obtained from Raman analyses. In this study, the pressure within inclusions was determined using Raman spectroscopy based on the splitting of the Fermi diad of CO 2, measured at the clathrate melting temperature. The CO 2 densities (and pressures) predicted by the equation developed in this study are in relatively good agreement with previously published equations, except for very low densities and correspondingly low pressures. The combined Raman spectroscopy – microthermometry technique thus provides both the temperature and the pressure in the inclusion at clathrate melting. For inclusions in which the clathrate melts in the presence of CO 2 liquid, the salinity can be determined with a precision of a few tenths of a wt% NaCl, whereas for inclusions in which clathrate melts in the presence of CO 2 vapor the salinity error may be a few wt% NaCl. Applying the method to synthetic fluid inclusions with known salinity suggests that the technique is valid for determining salinity of H 2O–CO 2–NaCl fluid inclusions in which clathrate melts in the presence of liquid CO 2 only or vapor CO 2 only.

Formation of the Vysoká–Zlatno Cu–Au skarn–porphyry deposit, Slovakia

The central zone of the Miocene Stiavnica stratovolcano hosts several occurrences of Cu–Au skarn–porphyry mineralisation, related to granodiorite/quartz–diorite porphyry dyke clusters and stocks. Vysoka–Zlatno is the largest deposit (13.4 Mt at 0.52% Cu), with mineralised Mg–Ca exo- and endoskarns, developed at the prevolcanic basement level. The alteration pattern includes an internal K- and Na–Ca silicate zone, surrounded by phyllic and argillic zones, laterally grading into a propylitic zone. Fluid inclusions in quartz veinlets in the internal zone contain mostly saline brines with 31–70 wt.% NaCl eq. and temperatures of liquid–vapour homogenization (Th) of 186–575°C, indicating fluid heterogenisation. Garnet contains inclusions of variable salinity with 1–31 wt.% NaCl eq. and Th of 320–360°C. Quartz–chalcopyrite veinlets host mostly low-salinity fluid inclusions with 0–3 wt.% NaCl eq. and Th of 323–364°C. Data from sphalerite from the margin of the system indicate mixing with dilute and cooler fluids. The isotopic composition of fluids in equilibrium with K-alteration and most skarn minerals (both prograde and retrograde) indicates predominantly a magmatic origin (δ18Ofluid 2.5–12.3‰) with a minor meteoric component. Corresponding low δDfluid values are probably related to isotopic fractionation during exsolution of the fluid from crystallising magma in an open system. The data suggest the general pattern of a distant source of magmatic fluids that ascended above a zone of hydraulic fracturing below the temperature of ductile–brittle transition. The magma chamber at ∼5–6 km depth exsolved single-phase fluids, whose properties were controlled by changing PT conditions along their fluid paths. During early stages, ascending fluids display liquid–vapour immiscibility, followed by physical separation of both phases. Low-salinity liquid associated with ore veinlets probably represents a single-phase magmatic fluid/magmatic vapour which contracted into liquid upon its ascent.

Trace elements and cathodoluminescence of quartz in stockwork veins of Mongolian porphyry-style deposits

The combination of scanning electron microscope–cathodoluminescence (CL), fluid inclusion analysis and high-resolution electron probe microanalysis of Al, Ti, K and Fe in vein quartz has yielded results permitting a greater understanding of the complex mineralisation of the Central Oyu Tolgoi and Zesen Uul porphyry-style deposits, southern Mongolia. These data demonstrate the relationship between quartz precipitation, dissolution and ore deposition as the mineralising fluid chemistry changed through time. Four major quartz generations are identified in the A-type veins from the stockworks of both the Central Oyu Tolgoi (OTi to OTiv) and Zesen Uul deposits (ZUi to ZUiv). Despite differences in the associated alteration and mineralisation style, the observed CL textures and trace element signatures of the quartz generations are comparable between deposits. The OTi and ZUi stage formed both the primary network of A-type veins and pervasive silicification of the host rock. Using the Ti-in-quartz geothermometer, crystallisation temperatures for OTi and ZUi of between 598°C and 880°C are indicated. The main stage of sulphide mineralisation was accompanied by the dissolution of pre-existing quartz (OTi and ZUi) and precipitation of a weakly luminescent generation of quartz (OTii and ZUii) with a low Ti content, reflected in a calculated temperature drop from approximately 700°C to 340°C in Central Oyu Tolgoi and 445°C in Zesen Uul. OTii and ZUii stage quartzes show high and variable Al concentrations. The next stage of quartz in both deposits (OTiii and ZUiii) forms a fine network of veins in cracks formed in pre-existing quartz. OTiii and ZUiii quartz contain measurable fluid inclusions of moderate salinity (3–17.1 wt.% NaCl eq.), entrapped in the temperature range 256°C to 385°C. OTiii and ZUiii are not related to any sulphide mineralisation. The final OTiv and ZUiv stages are characterised by quartz–calcite micro-breccias that penetrate the A-type veins. Based on the calculated entrapment temperatures, the OTiv/ZUiv stage crystallised between 212°C and 335°C, and the quartz is characterised by elevated but variable Al and Fe contents. The CL and trace element signatures of the OTi to OTiii and ZUi to ZUiii stages of the two Mongolian porphyries show similar features to those observed in porphyry-style deposits from other regions. This suggests that a common sequence of quartz crystallisation occurs during the formation of early veins in many porphyry copper systems.

Hydrothermal recrystallization of the Lower Ordovician dolomite and its significance to reservoir in northern Tarim Basin

Discovered in S15 and some other wells, the Lower Ordovician in the northern Tarim Basin consists mainly of brown gray-dark gray very fine-fine crystalline dolomite, with a minor portion of locally light gray-white medium-coarse crystalline dolomite. Silicification can be observed in the medium-coarse dolomite, and some euhedral drusy quartz can also be found in pores and fractures of the dolomite. The homogenization temperature of the fluid inclusions in the medium-coarse dolomite is between 110 and 200°C with maximum between 140 and 190°C, and the salinity is between 10.7 and 18.5 wt.% NaCl Eq. The homogenization temperature and salinity of the fluid inclusions in the medium-coarse dolomite are similar to those in the drusy quartz. Compared with the very fine-fine dolomite, the medium-coarse phase contains relatively high Fe and Mn. The average concentration of FeO and MnO in the medium-coarse dolomite is 1.917% and 0.323%, respectively. The medium-coarse dolomite has a remarkable negative Eu anomaly, consistent with the REE pattern of the intermediate-felsic igneous rocks in the Tarim Basin. The oxygen isotopic composition of the medium-coarse dolomite is relatively lighter than that of the very fine-fine dolomite. The δ18OPDB values of the medium-coarse dolomite are between −10.35‰ and −7.31‰. The δ18OSMOW values of the fluid associated with the medium-coarse dolomite can be calculated according to homogenization temperature and oxygen isotope fractionation factor between dolomite and fluid, and the calculated values are between +4‰ and +10‰, consistent with those of the hydrothermal fluid. The medium-coarse dolomite has relatively high 87Sr/86Sr ratios as well, indicating an origin associated with intermediate-felsic igneous rock. The homogenization temperature, element composition, REE pattern, oxygen and strontium isotopes demonstrate that the medium-coarse dolomite is the result of recrystallization of very fine-fine dolomite under hydrothermal environment. The hydrothermal dolomite recrystallization is a special event but exists extensively in the Tarim Basin. The recrystallized dolomite becomes well reservoir bed for many intercrystalline pores and dissolution pores are produced in this event, so that more attention should be paid to the altered dolomite during the petroleum exploration in the lower Paleozoic in the Tarim Basin.

Fluids in coesite-bearing rocks of the Tso Morari Complex, NW Himalaya: evidence for entrapment during peak metamorphism and subsequent uplift

AbstractFluid inclusions trapped in coesite-bearing rocks provide important information on the fluid phases present during ultrahigh-pressure metamorphism. The subduction-related coesite-bearing eclogites of the Tso Morari Complex, Himalaya, contain five major types of fluids identified by microthermometry and Raman spectroscopy. These are: (1) high-salinity brine, (2) N2, (3) CH4, (4) CO2and (5) low-salinity aqueous fluids. These fluids were trapped during both deep subduction and exhumation processes. The coesite-bearing rocks are inferred to have been buried to a depth of &gt;120 km, where they experienced ultrahigh-pressure metamorphism. The fluid–rock interaction provides direct evidence for fluid derivation during a deep subduction process as demonstrated by silica–carbonate assemblages in eclogite. High salinity brine, N2and CH4inclusions are remnants of prograde and peak metamorphic fluids, whereas CO2and low-salinity aqueous fluids appear to have been trapped late, during uplift. The high-salinity brine was possibly derived from subducted ancient metasedimentary rocks, whereas the N2and CH4fluids were likely generated through chemical breakdown of NH3-bearing K minerals and graphite. Alternatively, CH4might have been formed by a mixed fluid that was released from calcareous sediments during subduction or supplied through subducted oceanic metabasic rocks. High density CO2is associated with matrix minerals formed during granulite-facies overprinting of the ultrahigh-pressure eclogite. During retrogression to amphibolite-facies conditions, low-salinity fluids were introduced from external sources, probably the enclosing gneisses. This source enhances salinity differences as compared to primary saline inclusions. The subducting Indian lithosphere produced brines prior to achieving maximal depths of &gt;120 km, where fluids were instead dominated by gaseous phases. Subsequently, the Indian lithosphere released CO2-rich fluids during fast exhumation and was then infiltrated by the low-salinity aqueous fluids near the surface through external sources. Elemental modelling may improve quantitative understanding of the complexity of fluids and their reactions.

Fluid–rock interaction at the roots of an Eopaleozoic epithermal system, Sierra Norte de Córdoba, Argentina: Paragenesis, thermometry and fluid sources

A deep epithermal vein system hosted in Late Proterozoic to Cambrian granodiorite has been identified in the Sierra Norte de Córdoba, the easternmost range of the Sierras Pampeanas Orientales of Argentina. The vein swarm extends over an area of 3 km 2 parallel to a mylonitic belt and formed in fractured granodiorite. Thicknesses of veins are less than 0.5 m and their visible strike length is less than 100 m. Veins are either barren or weakly mineralized in base-metal sulfides. Most veins have mineral associations dominated by calcite and quartz with lesser amounts of chlorite, sericite, pyrite, and minor illite. In other less exposed albite-rich, adularia-bearing veins, chalcopyrite, bornite, galena, sphalerite, chalcocite and covellite may occur. The widespread occurrence of bladed calcite without any petrographic or microthermometric evidence of boiling implies that this particular habit of calcite may also develop under sub-near boiling fluid conditions. Thermometric calculations based on fluid inclusion data, chlorite composition and oxygen isotopes in the quartz–calcite pair, constrain the formation of the system between 300 and ∼350 °C, at pressures between 42 and 64 MPa (1.5–2.3 km). Stable isotope data suggest that W/ R interaction might have been the most probable mechanism of alteration, involving the participation of meteoric fluids; nevertheless, the metallic signature of some weakly mineralized veins as well as intermediate fluid inclusion salinities favor a magmatic input and a mixed origin for the fluids. Textures and mineral associations, as well as the absence of evidence of boiling in fluid inclusions, all suggest that the silica–carbonate vein system formed deeper than typically shallow Au and Ag-bearing boiling solutions. A 485 (±25) Ma lamprophyre dike crosscuts some of these veins locally producing metasomatic reactions and skarn formation, which constrains the age of the hydrothermal system to the Cambrian-Early Ordovician time span.