Saline Fluid Inclusions Research Articles

Fluid inclusions and ore precipitation mechanism in the giant Xikuangshan mesothermal antimony deposit, South China: Conventional and infrared microthermometric constraints

The giant Xikuangshan antimony deposit, located in the Xiangzhong (central Hunan) Basin, South China, is the largest antimony deposit in the world. Stibnite is the predominant ore mineral, and gangue minerals consist of quartz, calcite, fluorite and barite. The ore mineral assemblages include early-stage quartz-stibnite, fluorite-quartz-stibnite and barite-quartz-stibnite, and late-stage calcite-stibnite. In spite of numerous research works on the Xikuangshan deposit during the past several decades, the signatures of the ore-forming fluids and the ore precipitation mechanisms are poorly constrained. In order to characterize the fluids responsible for antimony mineralization and ascertain the genetic processes in the Xikuangshan ore district, conventional and infrared microthermometric measurements have been performed in this study on fluid inclusions hosted in transparent minerals (quartz, fluorite and barite) and opaque stibnite, respectively. Four types of fluid inclusions were identified for the early-stage mineralization based on their phase assemblages, including type I (pure liquid inclusions), type II (two-phase, liquid-rich aqueous inclusions), type III (two-phase, vapor-rich aqueous inclusions) and type IV (pure vapor inclusions). Stibnite only contains type I and type II inclusions, while all types of fluid inclusions are common in the gangue minerals. The homogenization temperatures and salinities of fluid inclusions in stibnite and associated gangue minerals in this ore district display significant differences. Fluid inclusions in stibnite exhibit relatively low homogenization temperatures (112–323 °C) and higher salinities (0.2–15.4 wt% NaCl equiv.) than inclusions in coexisting gangue minerals (119–366 °C, 0.2–4.2 wt% NaCl equiv.), usually about 50 °C less than the “intergrown” gangue minerals. According to the petrography of fluid inclusions and the relationship between their homogenization temperature and salinity, it was concluded that stibnite and gangue minerals in the Xikuangshan ore district are not co-genetic, instead, they were deposited from two distinctly different hydrothermal solutions. Fluid mixing was responsible for stibnite precipitation, but the precipitation of gangue minerals resulted from fluid boiling due to an abrupt drop in pressure. The Xikuangshan deposit is a typical example of mesothermal deposits, rather than an epithermal deposit as previously considered.

Multiple Dolomitization and Fluid Flow Events in the Precambrian Dengying Formation of Sichuan Basin, Southwestern China

AbstractThe Precambrian Dengying Formation is a set of large‐scale, extensively dolomitized, carbonate reservoirs occurring within the Sichuan Basin. Petrographic and geochemical studies reveal dolomitization was a direct result of precipitation by chemically distinct fluids occurring at different times and at different intensities. Based on this evidence, dolomitization and multiple fluid flow events are analyzed, and three types of fluid evolution models are proposed. Results of analysis show that Precambrian Dengying Formation carbonates were deposited in a restricted peritidal environment (630–542 Ma). A high temperature and high Mg2+ concentration seawater was a direct result of dolomitization for the micrite matrix, and for fibrous aragonite in primary pores. Geochemical evidence shows low δ18O values of micritic dolomite varying from −1.29± to −4.52± PDB, abundant light rare earth elements (REEs), and low dolomite order degrees. Microbes and meteoric water significantly altered dolomite original chemical signatures, resulting in algal micritic dolomite and the fine‐grained, granular, dolosparite dolomite having very negative δ18O values. Finely crystalline cement dolomite (536.3–280 Ma) and coarsely crystalline cement dolomite have a higher crystallization degree and higher order degree. The diagenetic sequence and fluid inclusion evidence imply a linear correlation between their burial depth and homogenization temperatures, which closely resemble the temperature of generated hydrocarbon. Compared with finely crystalline dolomite, precipitation of coarsely crystalline dolomite was more affected by restricted basinal fluids. In addition, there is a trend toward a more negative δ18O value, higher salinity, higher Fe and Mn concentrations, REE‐rich. Two periods of hydrothermal fluids are identified, as the exceptionally high temperatures as opposed to the temperatures of burial history, in addition to the presence of high salinity fluid inclusions. The early hydrothermal fluid flow event was characterized by hot magnesium‐ and silicon‐rich fluids, as demonstrated by the recrystallized matrix dolomite that is intimately associated with flint, opal, and microcrystalline quartz in intergranular or intercrystalline pores. This event was likely the result of a seafloor hydrothermal chimney eruption during Episode I of the Tongwan Movement (536.3±5.5 Ma). In contrast, later hydrothermal fluids, which caused precipitation of saddle dolomite, were characterized by high salinity (15–16.05wt% NaCl equivalent) and homogenization temperatures (250 to 265°C), δ18O values that were more enriched, and REE signatures. Geochemical data and the paragenetic sequence indicate that this hydrothermal fluid was related to extensive Permian large igneous province activity (360–280 Ma). This study demonstrates the presence of complicated dolomitization processes occurring during various paleoclimates, tectonic cycles, and basinal fluids flow; results are a useful reference for these dolomitized Precambrian carbonates reservoirs.

Genesis of the Nanyangtian scheelite deposit in southeastern Yunnan Province, China: evidence from mineral chemistry, fluid inclusions, and C–O isotopes

The Nanyangtian skarn-type scheelite deposit is an important part of the Laojunshan W–Sn polymetallic metallogenic region in southeastern Yunnan Province, China. The deposit comprises multiple scheelite ore bodies; multilayer skarn-type scheelite ore bodies are dominant, with a small amount of quartz vein-type ore bodies. Skarn minerals include diopside, hedenbergite, grossular, and epidote. Three mineralization stages exist: skarn, quartz–scheelite, and calcite. The homogenization temperatures of fluid inclusions in hydrothermal minerals that formed in different paragenetic phases were measured as follows: 221–423 °C (early skarn stage), 177–260 °C (quartz–scheelite stage), and 173–227 °C (late calcite stage). The measured salinity of fluid inclusions ranged from 0.18% to 16.34% NaCleqv (skarn stage), 0.35%–7.17% NaCleqv (quartz–scheelite stage), and 0.35%–2.24% NaCleqv (late calcite vein stage). Laser Raman spectroscopic studies on fluid inclusions in the three stages showed H2O as the main component, with N2 present in minor amounts. Minor amounts of CH4 were found in the quartz–scheelite stage. It was observed that the homogenization temperature gradually reduced from the early to the late mineralization stages; moreover, δ13CPDB values for ore-bearing skarn in the mineralization period ranged from − 5.7‰ to − 6.9‰ and the corresponding δ18OSMOW values ranged from 5.8‰ to 9.1‰, implying that the ore-forming fluid was mainly sourced from magmatic water with a minor amount of meteoric water. Collectively, the evidence indicates that the formation of the Nanyangtian deposit is related to Laojunshan granitic magmatism.

Understanding Fluid Flow during Tectonic Reactivation: An Example from the Flamborough Head Chalk Outcrop (UK)

Flamborough Head chalks are located at the extremities of E-W and N-S trending fault systems along the Yorkshire coast (UK). Rock deformation is expressed in Selwicks Bay where a normal fault is exposed along with a high density of calcite veins. The fault mineralization is tested using geochemistry. Crosscutting relationships are used to differentiate between three vein generations: a network of parallel veins that are oriented perpendicular to stratigraphy (Group I), hydraulic breccia with typical jigsaw puzzle structure (Group II), and a third generation of calcite veins crosscutting the two previous generations (Group III). Geochemical analyses revealed that all three generations possess the same chemical signature and must reflect successive pulses from the same mineralizing fluid source. Strontium isotope analyses showed that the veins have elevated 87Sr/86Sr ratios, that is, up to 7.110, while ratios of the chalk matrix equal 7.707. The latter value is in agreement with the signature of Late Cretaceous seawater. Consequently, the source of the fluid is external, reflecting an open system. The radiogenic Sr-isotope ratios, combined with low iron concentration, suggest that fluids migrated through sandy deposits. Fluid inclusion salinities range from 0 to 12 eq. wt% NaCl equiv. with a dominance of very low salinity inclusions, reflecting a meteoric signal. This leads to a model where meteoric fluids stored in an underlying confined sandstone aquifer were remobilized. The wide range of salinities could result from mixing of the meteoric fluid with some more saline fluids present in the rock sequence or from the dissolution of salts in the subsurface. In addition to the understanding of the local paragenetic evolution of the veining in Flamborough Head chalks, this study offers an insight into the way how fluid flows and mineralizes along fault zones.

Fluid inclusion constraints on the genesis of the Puladi muscovite deposit in Gongshan County, Yunnan Province

This paper focuses on beryl mines in the Maji region of Yunnan Province, which are characterized by fluid inclusions. Based on petrography theory, mineralogy, and ore-forming geological conditions, beryl can be divided as CO2 and CO2-H2O inclusions. In addition, the characteristics of inclusions in the coordinate of A/B is summarized. The homogenization temperature of fluid inclusions in the coordinate of A ranges from 250 °C to 397 °C, while the salinity of fluid inclusions ranges from 0.18% to 4.27%. By contrast, the homogenization temperature in the coordinate of B ranges from 210 °C to 340 °C, and the salinity is from 0.22% to 5.11%. The pressure of ore-forming fluid in the coordinate of A/B is approximately 83 MPa with densities of 0.8034 g/m3 and 0.8363 g/m3, which are characteristic of mediumtemperature, low-salinity, and medium-density fluids. Based on Raman spectra and different metallogenic depths, the two types of beryl belong to different metallogenic belts. The beryl deposits in Gongshan are of medium-temperature gas–hydrothermal type.

(U-Th)/He thermochronology of metallic ore deposits in the Liaodong Peninsula: Implications for orefield evolution in northeast China

The Liaodong Peninsula is an important region where polymetallic ores are mined in northeast China, but precise thermochronological data needed to effectively constrain the evolution of this orefield are presently lacking. For this study, fifteen samples of granitoid rocks were collected from the Qingchengzi and Wulong orefields located in the Liaodong Peninsula to undertake (U-Th)/He study of zircon and apatite. Single-grain ZHe and AHe ages from the Qingchengzi orefield range from 73 to 166 Ma and from 22 to 48 Ma, respectively, whereas single-grain ZHe and AHe ages from the Wulong orefield range from 42 to 88 Ma and from 21 to 56 Ma, respectively. Some samples yield dispersed ages. Comprehensive analyses indicate that the main causes of the dispersion in the ZHe ages are heterogeneous distribution of U and Th within the crystals, radiation damage and cooling rates, whereas the dispersion in the AHe ages is primarily due to the first two factors. Based on the helium ages, we calculate the average exhumation and cooling rates of the two orefields during different periods and the associated denudation, which are 4.3 km for the Qingchengzi orefield and 4.7 km for the Wulong orefield. The mineralization depths are estimated using homogenization temperature and the salinity of fluid inclusions, and are calculated to be 1.5–3.2 km and 1.0–2.2 km for the Qingchengzi Pb-Zn and Au-Ag deposits, respectively, and 2.1–3.1 km for the Wulong Au deposit. These results are much less than the total denudation, suggesting that the orebodies have been heavily eroded. In conjunction with the distribution of the helium ages, the modeled low-T thermal histories of the two orefields reveal that the Liaodong Peninsula experienced three stages of relatively rapid cooling during the Early Cretaceous (ca. 135–115 Ma), from the end of the Late Cretaceous to the Early Eocene (ca. 70–50 Ma) and from the Late Eocene to the Oligocene (ca. 38–26 Ma). The last stage of cooling affected both orefields and represents a period of relatively intense tectonic activity. Finally, these results are compared with those of previous studies from the adjacent Jiaodong Peninsula. This discussion shows that the cooling experienced by the orefields in the Liaodong Peninsula is largely consistent with that experienced by the Jiaodong Peninsula. These data provide valuable information for further studies of the evolution of orefields in northeast China.

Fluid Inclusion Studies on Quartz Associated with Massive Ores of the Paleoproterozoic Carbonate-hosted Pb-Zn Deposit at Balaria, Rajasthan: Possible Evidence for Fluid-mixing

ABSTRACT Mixing of metal-rich connate waters derived from evaporative seawater with local sulfide-rich water is postulated as the mechanism of ore deposition in many Phanerozoic carbonate-hosted Pb-Zn deposits. The study of the Paleoproterozoic Balaria deposit of the Zawar belt of Rajasthan that hosts structurally-controlled, stratabound Pb-Zn mineralization, provides insight into the nature of fluids in quartz that is closely associated with massive Pb-Zn ore. The present study is focused on the massive-type ore which forms one of the major ore-types at Balaria. Massive-type ore consists predominantly of galena and sphalerite (with minor pyrite and pyrrhotite) that occur in shear-induced dilatational fractures. The ore is associated with clear, rounded to semi-rounded bodies of quartz that are embedded within the ore. The quartz bodies are inferred to be fragments of vein-quartz that underwent shear deformation along with enclosing massive galena-rich ore. Primary-biphase fluid inclusions in quartz belong to H2O-NaCl±CO2, H2O-NaCl-CH4-N2-CO2 and primary-monophase fluid inclusions to CH4-N2-CO2 types, while secondary fluid inclusions are biphase and belong to H2O-NaCl type. The salinity of primary-biphase H2O-NaCl type fluid inclusions range from 2 to 14 wt. percent NaCl equiv while that of secondary-biphase H2O-NaCl type fluid inclusions range from 3 to 5 wt. percent NaCl equiv. Homogenization temperatures for primary-biphase fluid inclusions range from 127 °to 217 °C while that of secondary-biphase fluid inclusions is 131 ° to 151 °C. The molecular proportions of of CH4, N2 and CO2 in primary-biphase (H2O-NaCl-CH4-N2-CO2) and primary-monophase (CH4-N2-CO2) as measured from Raman spectra are widely variable. Variability in molecular proportions of CH4, N2 and CO2 is interpreted as an evidence for mixing of pulses of a regional fluid with a local fluid and reactions with graphitic and carbonate rocks at the site of deposition. The low salinity of fluid inclusions in the quartz and the lack of any sulfur species in the aqueous or gaseous phase of the entrapped fluids indicates that although quartz occurs as entrained bodies within the massive ore, the fluids in them may not represent ore-forming fluids of the main ore-stage of massive-type ore.

Lead and sulfur isotope composition of trace occurrences of Mississippi Valley-type mineralization in the U.S. midcontinent

Mississippi Valley-type (MVT) Zn-Pb mineralization in the U.S. mid-continent has formed in numerous widely dispersed trace occurrences and in a few large ore deposits that have been mined. Based on similarly high fluid inclusion salinities and high homogenization temperatures relative to inferred burial depth, the trace MVT occurrences and MVT ore deposits may be genetically related by having precipitated from the same regional groundwater flow system mobilized by Pennsylvanian-Permian tectonism. The purpose of the present study was to test the robustness of this inferred genetic relationship by characterizing the Pb and S isotope composition of trace MVT occurrences in light of the much more thoroughly studied MVT ore deposits.Lead and sulfur isotope compositions have been obtained for trace MVT occurrences from three broad regions of the U.S. mid-continent: the Forest City basin (FCB), the Illinois basin (ILB), and the Greater Upper Mississippi Valley (GUMV) region. Lead isotope compositions of trace MVT occurrences from each of these three areas define discrete linear trends that are similarly oriented but not strictly collinear with one another or with the Pb isotope trends of MVT ore deposits, suggesting that trace MVT occurrences in each of these three regions derived their Pb from separate pairs of end-member Pb sources. An exception is the GUMV trace MVT occurrences and the Upper Mississippi (UMV) district ores, which appear to have collinear Pb isotope trends and thus may be the most closely related genetically with respect to sources of Pb. All of the trace MVT occurrences have radiogenic Pb suggesting derivation from Precambrian igneous basement rocks and not from the formations that host trace MVT mineralization. Trace MVT occurrences tend to have less radiogenic Pb than the MVT ore deposits, although trace MVT occurrences from the GUMV region are the most heterogeneous and can have Pb that is as radiogenic as the MVT ore deposits. Regionally, Pb isotope compositions of trace MVT occurrences tend to become less radiogenic southwestward and southward, but do not correlate with host rock age, and by proxy, distance from the Precambrian basement.The sulfur isotope compositions of sphalerite and galena from the trace MVT occurrences are light compared to the MVT ore deposits and likely reflect incomplete reduction of marine sulfate reservoirs or derivation from organic matter. The Pb and S isotope compositions of the trace MVT occurrences do not correlate well, suggesting that Pb and S were not transported together in the same fluid.

Complex accumulation and leakage of YC21-1 gas bearing structure in Yanan sag, Qiongdongnan basin, South China Sea

YC21-1 is a gas-bearing structure found within the Yanan sag in the Qiongdongnan Basin, South China Sea. While the structure bears many geological similarities to the nearby YC13-1 gas field, it nevertheless does not contain commercially viable gas volumes. The main reservoirs of the YC21-1 structure contain high overpressures, which is greatly different from those of the YC13-1 structure. The pressure coefficients from drillstem tests, wireline formation tests and mud weights are above 2.1. Based on well-log analysis, illite content and vitrinite reflectance data of mudstones in well YC21-1-2, combining with tectonic and sedimentation characteristics, the timing and causes of overpressure generation are here interpreted. The results indicate the existence of two overpressure segments in the YC21-1 structure. The first overpressure segment resides mainly within the lower and the middle intervals of the Yinggehai Formation, and is interpreted to have been mainly caused by clay diagenesis, while disequilibrium compaction and hydrocarbon generation may also have contributed to overpressure generation. The second overpressure segment comprising the Sanya Formation (Pressure transition zone) and the Lingshui and Yacheng Formations (Hard overpressure zone) is interpreted to owe its presence to kerogen-to-gas cracking. According to petrography, homogenization temperature and salinity of fluid inclusions, two stages of oil-gas charge occurred within the main reservoirs. On the basis of overpressure causes and oil-gas charge history, combining with restored tectonic evolution and fluid inclusion characteristics, a complex accumulation and leakage process in the YC21-1 gas bearing structure has been interpreted. Collective evidence suggests that the first oil charge occurred in the Middle Miocene (circa 16.3–11.2 Ma). Small amount of oil generation and absence of caprocks led to the failure of oil accumulation. Rapid subsidence in the Pliocene and Quaternary gave rise to a sharp increase in geotemperature over a short period of time, leading to prolific gas-generation through pyrolysis and, consequently, overpressure within the main reservoirs (the second overpressure segment). During this period, the second gas charge occurred in the Pliocene and Quaternary (circa 4.5–0.4 Ma). The natural gas migrated in several phases, consisting of free and water soluble phases in a high-pressure environment. Large amounts of free gas are considered to have been consumed due to dissolution within formation water in highly pressured conditions. Water soluble gas could not accumulate in high point of structure. When the pore-fluid pressures in main reservoirs reached the fracture pressure of formation, free gas could leak via opened fractures within cracked caprocks. A repeated fracturing of caprocks may have consumed natural gas stored in formation water and have made water-soluble gas unsaturated. Therefore, the two factors including caprocks fracturing and dissolution of formation water are interpreted to be mainly responsible for the failure of natural gas accumulation in the YC21-1 structure.

Fluid inclusions in quartz from uranium mineralization areas of the Litsa ore cluster (Kola Peninsula)

Fluid inclusions in quartz from rocks of different ages and with different uranium contents were studied by microthermometry, gas chromatography, and Raman spectroscopy in three areas with uranium mineralization (Skal’noe, Polyarnoe, and Dikoe) in the Litsa ore district of the Kola Peninsula. The studies have revealed differences in the homogenization temperatures, composition of water-salt systems, and salinity of primary and secondary fluid inclusions in the rocks of these areas. Primary fluid inclusions in the rocks of the Skal’noe and Dikoe areas are characterized by a water-salt system with K and Mg (seldom, Na) chlorides and salinity of 0.2 to 13.9 wt.% NaCl equiv. Secondary inclusions are of higher salinity, up to 19.5 wt.% NaCl equiv., and contain Ca chlorides. Primary inclusions in the rocks of the Polyarnoe area show a more complex salt composition with K, Mg, Ca, and, probably, Li chlorides, with salinity varying from 1.7 to 10.4 wt.% NaCl equiv. Salinity of secondary inclusions is higher, up to 16.9 wt.% NaCl equiv. The gas component of fluid in the rocks of the Skal’noe and Dikoe areas is almost pure CO2 (99.3-100 mol.%); the presence of N2 (1.1 mol.%) and CH4 (0.4 mol%.) was revealed in few inclusions only. In the Polyarnoe area, only water inclusions with N2 were found in the rocks. Thus, the fluids in the rocks of the three areas have mostly a CO2-H2O composition with the presence of K, Mg, and Ca chlorides and, seldom, a water-salt composition with the presence of Na salts only. The increased salinity of secondary inclusions and their more complex composition (CO2 with N2 and CH4 impurities) indicate that the fluid became more reducing and might be responsible for the formation of uranium mineralization.

Whole rock geochemistry, molybdenite Re-Os geochronology, stable isotope and fluid inclusion investigations of the Siah-Kamar deposit, western Alborz-Azarbayjan: New constrains on the porphyry Mo deposit in Iran

The Siah-Kamar porphyry Mo deposit, located in the western Alborz-Azarbayjan magmatic belt, is the first and largest Mo deposit in the Iran. This deposit is mainly hosted by an I-type, shoshonitic quartz monzonite to monzonite intrusion and also extends in the surrounding lower to middle Eocene volcanic rocks. The geochemical features of the Siah-Kamar intrusion show enrichment in large-ion lithophile elements (LILE) and light rare earth elements (LREE), and significant negative anomalies of Nb, Ta and Ti analogues to the magmas derived from metasomatized sub-continental mantle. Porphyry molybdenum mineralization is associated with potassic, sericitic, argillic, and propylitic alteration zones. Mineralization occurs in disseminated form, in veins/veinlets and in hydrothermal breccias. The main ore minerals comprise molybdenite, chalcopyrite and bornite. The Microthermometric analyses at Siah-Kamar deposit showed that the halite-bearing inclusions contain high salinity (30.9–60.7wt% NaCl eq.) with homogenization temperature ranging from 226°C to 397°C. The homogenization temperature of two phase liquid-rich inclusions range between 224°C and 375°C. The salinity of this type inclusions range from 0.6 to 7.5wt% NaCl equivalent. The two-phase vapor-rich fluid inclusions homogenized at 270°C to 397°C. The salinity of this type fluid inclusions lie within the range of 0.6 to 4.24wt% NaCl equivalent. Coexisting two phase V-rich and L-rich fluid inclusions in quartz associated with molybdenite provide evidence for boiling at 270°C to 400°C. The δ18Owater values of quartz in the molybdenite-bearing veins vary from +2.16 to +4.05‰, suggesting a magmatic origin for the ore-forming fluids. Re-Os isotopic dating of molybdenite indicated a mineralization age of 41.9±3.6Ma. The Re concentration in molybdenite suggests incorporation of mantle derived melt with crustal materials. The late Eocene magmatism along the western Alborz-Azarbayjan magmatic zone resulted from the Neo-Tethys subduction beneath the Iranian plateau. The Siah-Kamar monzonitic intrusion hosting the Mo deposit, could be considered asan example among the late Eocene intrusions within the western Alborz-Azarbayjan magmatic zone for any further exploration in this zone.

Characteristics and Behavior of Hydrothermal Fluids for Gold Mineralization at the Hishikari Deposits, Kyushu, Japan

AbstractThe Hishikari gold–silver deposits in southern Kyushu, Japan, are low‐sulfidation vein‐type epithermal deposits. Bulk composition analysis, fluid inclusion microthermometry and quantitative gas composition analysis on the basis of the crush‐fast scan (CFS) method were conducted to elucidate characteristics and behavior of hydrothermal fluids responsible for the gold mineralization of the Hishikari deposits. Quartz vein samples were collected from widely scattered locations of ore veins: Zuisen‐6, ‐3, ‐1 (Main deposit), Keisen‐2 and Shosen‐5 (Sanjin deposit), and Seisen‐8 (Yamada deposit) between −70 and 64 ML (mine meter level). The samples are composed mainly of quartz and adularia, and ore minerals of electrum, pyrargyrite, pyrite and/or stibnite, having an average bulk Ag/Au weight ratio of 0.54. Fluid inclusions in quartz and adularia are two‐phase liquid–vapor, and their trapping temperatures and salinities are ca. 190–220 °C and 1.5–2.7 wt.% NaCl eq., respectively. The salinity is highest with an average 2.4 wt.% NaCl eq. of the fluid inclusions from the deepest level (Zuisen‐6 vein at −70 ML), while the fluid inclusion salinities from other samples are relatively low less than ca. 2.0 wt.% NaCl eq. Parallel‐ and lattice‐bladed quartz occur in a number of veins, suggesting fluid boiling during the vein formation.Gases of fluid inclusions are composed mainly of H2O (93.4–99.8 mol%) and CO2 (0.15–6.2 mol%), N2 (0.007–0.96 mol%), Ar (0.0001–0.0047 mol%), CH4 (0.002–0.048 mol%), H2S (0.0004–0.0028 mol%), H2 (0–0.074 mol%) and He (0.0002–0.0011 mol%). CO2, N2 and Ar contents of the fluid inclusions are elevated in the samples collected from deep levels, while H2O, H2S and He contents are elevated in those from shallow levels. The Ar/He, CO2/CH4 and N2/Ar ratios of fluid inclusion gas indicate a systematic transition from magmatic to meteoric signatures in order from deep to shallow levels. The CO2/N2 ratio and volatiles contents are positively correlated suggesting condensation in relatively high Au‐grade quartz vein samples (19.5–27.7 ppm Au), while negative correlations suggest boiling with volatile contents less than 1% are indicative of low Au‐grade quartz vein samples (0.14 to 0.18 ppm Au). The elevated CO2 contents observed in the Zuisen veins are signatures of the deeply circulating hydrothermal fluid with a magmatic contribution. Due to intermittent boiling of hydrothermal fluid, CO2 and H2S were partitioned into the vapor phase in deeper portions and condensed into the steam‐heated meteoric water in shallower portions. The signatures of both meteoric and magmatic fluids in fluid inclusion gas compositions were detected in the samples collected near the unconformity between Shimanto Supergroup (shale and sandstone) and Hishikari Lower Andesite. This suggests that a mixing of the deeply circulating hydrothermal fluids with the steam‐heated meteoric water took place near the unconformity, and it was one of the mechanisms to have precipitated gold.

Origin of the saline paleofluids in fault-damage zones of the Jabal Qusaybah Anticline (Adam Foothills, Oman): Constraints from fluid inclusions geochemistry

The Jabal Qusaybah Anticline, in north Oman, is affected by syn-folding strike-slip and extensional fault zones developed during foreland deformation ahead of the Northern Oman Mountains thrust wedge, in Cenozoic times. Migration of fluids in fault-damage zones is recorded in complex calcite vein networks. By employing the microthermometric and compositional microanalysis of the fluid inclusions (crush-leach), two distinct generations of veins have been studied. The aim was to determine the source of elevated salinity in fluids involved in their cementation and explain their compositional evolution through fluid-rock interactions. The ionic ratios (Na/Br and Cl/Br) obtained from crush-leach analysis give supporting evidence that the elevated salinity of fluid inclusions in both vein groups originated from an evaporated seawater beyond the onset of halite precipitation (residual brines). The results reveal a gradual increase in salinity of the fluids, F/Cl molar ratios, as well as Li/Cl molar ratios. These results imply the progressively increasing contribution of evaporitic residual brines and fluids that interacted with, or were derived from siliciclastic rocks. We suggest that the most likely origin of the former fluids is provided by residual brines associated with precipitation of the Ara evaporites (Cambrian). The regional driving mechanism for such a significant fluid migration is believed to be compaction-driven upward flow that was channeled into faults and fractures during major deformational events.

Thermobarometry in the Sarvian Fe-skarn deposit (Central Iran) based on garnet–pyroxene chemistry and fluid inclusion studies

The Sarvian Fe skarn deposit is located in the Urumieh–Dokhtar magmatic arc, western Iran. The Sarvian quartz diorite intruded the surrounding Permian to Tertiary limy formation, culminated in thermal metamorphism as well as skarnification in which the Sarvian deposit formed. Microthermometry studies in the Sarvian skarn deposit reveal two distinct inclusion groups; group A with medium–high temperature and hypersaline and group B with low–medium temperature and low salinity. Group A inclusions which are entrapped during formation of prograde are thought to be derived from the magmatic source. Fluid boiling and subsequent developing of hydraulic fracturing led to inflow and/or mixing of early magmatic fluids (group A) with circulating groundwater culminated in formation of low salinity and low temperature fluid inclusions (group B) during the formation of retrograde assemblage. Fluid inclusion thermometry reveals the formation temperature and the salinity of 300–370 °C and 31–33 wt% NaCl for the prograde stage and 180–230 °C and 1–15 wt% NaCl for the retrograde stage of skarnification at Sarvian skarn rocks. Fe-mineralization as well as hydrothermal minerals occurred during retrograde metasomatism. The estimated depth and pressure of occurrence for prograde stage are 1000–1200 m and 100–150 bars, and for retrograde stage, these are about 200 m and 50 bars, respectively. Garnet and pyroxene, as the main constituent minerals of prograde stage, are the most informative minerals offering a suitable tool to constrain the skarnification conditions. Garnets in the Sarvian deposit are mainly grossular and andradite, showing both normal and inverse zoning as the result of variation in their chemical composition. Such types of zoning represent alternation of high acidity oxidation and low acidity oxidation conditions that were prevailed on skarnification in the Sarvian prograde assemblage. Also, chemical composition of the Sarvian pyroxenes shows an alternation of high oxygen fugacity and low oxygen fugacity conditions for their formation. This is also supported by fluctuation of the ratios of andradite to grossular and diopside to hedenbergite, denoting to an obvious shifting that was prevailed between oxidizing and redox conditions during formation of prograde assemblage in the Sarvian. Garnet–pyroxene thermometry determines the formation temperature of prograde assemblage between 370 and 550 °C at Sarvian skarn rocks which is verified also by fluid inclusion thermometry. Decomposition of limestone by reaction of high-temperature hydrothermal fluids with carbonate host rock resulted in injection of CO2 into the Sarvian system that caused oxidation, changing Fe+2 to Fe+3 culminated in the magnetite deposition.

PIXE and microthermometric analyses of fluid inclusions in hydrothermal quartz from the 2.2 Ga Ongeluk Formation, South Africa: Implications for ancient seawater salinity

The Ongeluk Formation mainly consists of submarine volcanics (pillow lavas and sheet flows) composed of basaltic andesites that are between underlying and overlying glaciogenic deposits (i.e., the Makganyene diamictite and a dropstone layer at the base of the Hotazel Formation, respectively). The stratigraphic position of the Ongeluk Formation indicates that the Ongeluk volcanism occurred during a period of global glaciation. The Ongeluk volcanic rocks are host to subseafloor hydrothermal quartz deposits as drainage cavities and interpillow voids. The hydrothermal quartz contains many primary (Type 1) and secondary (Type 2) liquid–vapor fluid inclusions, as well as inclusions that are randomly distributed without a trace of secondary healed cracks (Type 3). All these fluid inclusions types were individually analyzed with microthermometry and particle-induced X-ray emission (PIXE) methods. The results show that Type 1 fluid inclusions are highly saline, whereas Type 2 fluid inclusions are relatively less saline. Type 3 fluid inclusions have bimodal peaks of salinity corresponding to those of Type 1 and Type 2 fluid inclusions and thus appear to represent mixtures between latter two inclusion types. Among the various fluid inclusion salinities of fluid inclusions, a wide range in Na/Ca values was identified in the high-salinity fluid inclusions (i.e., Type 1 and a subset of Type 3) which are thought to represent subseafloor fluids circulated by the Ongeluk submarine volcanism. The wide range in Na/Ca values can be explained by a mixing process between Na-rich and Ca-rich fluids associated with albitization of the host basaltic andesites. In the albitization process, Na is removed from the fluids and fixed by the host rocks; by contrast Ca is released from the host rocks into the fluids. PIXE analysis also showed two distinct trends (i.e., vertical and horizontal) on variation diagram of Ca/Cl versus Cu/Cl and positive correlations between Mn/Cl, Cu/Cl, Zn/Cl and Pb/Cl. These data are best explained by the presence of other mixing processes between; (1) a Na-rich, Ca- and Cu-poor fluid, (2) a Ca-rich, Na- and Cu-poor fluid, and (3) a Na- and Cu-rich, Ca-poor fluid. The Cu-, Mn-, Zn- and Pb-rich fluid likely represents a high-temperature hydrothermal fluid from a deep reaction zone in the Ongeluk subseafloor hydrothermal system. By contrast, the (1) Na- (and K-) rich, Ca- and Cu-poor endmember for the high-salinity primary inclusions is considered to represent the composition of 2.2Ga Ongeluk seawater. We propose the estimation of 2.2Ga Ongeluk seawater composition is >∼2237mmol/kg of Na, 200–1000mmol/kg of K, <135mmol/kg of Ca, ∼3230mmol/kg of Cl (average values), and ∼400–500 of Cl/Br.

Identification of Fluid Inclusion Solid Phases Using A Focused Ion Beam Scanning Electron Microscope With Energy Dispersive Spectroscopy: Implications For Interpreting Microthermometric Data and Composition of Fluids In SN (-W-Mo) Deposits

Abstract A feature of many granite-related mineral deposits is the presence of highly saline fluid inclusions that contain abundant daughter minerals, the identity and chemistry of which hold important information for the interpretation of the chemistry of mineralizing fluids in such environments. A focused ion beam (FIB) coupled with a scanning electron microscope (SEM) and energy-dispersive spectroscope (EDS) was used to identify daughter minerals within such fluid inclusions in fluorite from the Mount Pleasant Sn (-W-Mo-In) deposits, New Brunswick, Canada. The technique can be used to obtain quantitative compositional information for solid phases in fluid inclusions for which the optical properties have previously been studied, but whose identification remains equivocal. In addition, for solid-rich fluid inclusions, this technique enables identification of solids that are obscured by other solids and are not readily visible using optical microscopy. Methods such as Raman spectroscopy and SEM-EDS analysis of broken surfaces have limited success in mineral identification in these inclusions due to strong fluorescence in fluorite and an inability to correlate solids with their optical properties, respectively. The FIB-EDS analyses of the daughter minerals and precipitates formed by evaporation of the liquid in the inclusions show that the complex inclusions from Mount Pleasant contain brines with high concentrations of Na, Fe, K, Mn, Pb, Zn, and Cl; daughter minerals comprise halite and a variety of Fe-K(-Mn) chloride hydrates, and less frequently, Pb chlorides and Sn-bearing phases. The precipitates contain similar phases, although in general they are more K-rich than the daughter minerals. Unequivocal identification of most solid phases was not possible due to inaccuracies inherent in the FIB-EDS chemical analyses and a lack of Raman spectra for many K-Fe-Mn chloride hydrate minerals. Our data indicate that square (“cubic”) solids in such inclusions are not always halite, and that misidentification of halite could result in erroneous salinity estimates, which could lead to inaccurate interpretations of fluid composition using techniques such as laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS).

Fluid Inclusion and Stable Isotope Study at the Southeastern Martabe Deposit: Purnama, Barani and Horas Ore Bodies, North Sumatra, Indonesia

AbstractThe Martabe Au–Ag deposit, North Sumatra Province, Indonesia, is a high sulfidation epithermal deposit, which is hosted by Neogene sandstone, siltstone, volcanic breccia, and andesite to basaltic andesite of Angkola Formation. The deposit consists of six ore bodies that occurred as silicified massive ore (enargite–luzonite–pyrite–tetrahedrite–tellurides), quartz veins (tetrahedrite–galena–sphalerite–chalcopyrite), banded sulfide veins (pyrite–tetrahedrite–sphalerite–galena) and cavity filling. All ore bodies are controlled by N–S and NW–SE trending faults. The Barani and Horas ore bodies are located in the southeast of the Purnama ore body.Fluid inclusion microthermometry, and alunite‐pyrite and barite‐pyrite pairs sulfur isotopic geothermometry show slightly different formation temperatures among the ore bodies. Formation temperature and salinity of fluid inclusions of the Purnama ore body range from 200 to 260 C and from 6 to 8 wt.% NaCl equivalent, respectively. Formation temperature and salinity of fluid inclusions of the Barani ore body range from 200 to 220 °C and from 0 to 2.5 wt.% NaCl equivalent and those of the Horas ore body range from 240 to 275 °C and from 2 to 3 wt.% NaCl equivalent, respectively. The Barani and Horas ore bodies are less silicified and sulfides are less abundant than the Purnama ore body.A relationship between enthalpy and chloride content indicates mixing of hot saline fluids with cooler dilute fluids during the mineralization of each of the ore bodies. The δ18O values of quartz samples from the southeast ore bodies exhibit a wide range from +4.2 to +12.9‰ with an average value of +7.0‰. The δ18O values of H2O estimated from δ18O values of quartz, barite and calcite confirm the oxygen isotopic shift to near meteoric water trend, which support the incorporation of meteoric water. Salinity of the fluid inclusions decrease from &gt;5 wt.% NaCl equivalent in the Purnama ore body to &lt;3 wt.% NaCl equivalent in the Barani ore body, indicating different fluid systems during mineralization.The δ34S values of sulfide and sulfate in Purnama range from − 4.2 to +5.5‰ and from +1.2 to +26.7‰, those in the Barani range from − 4.3 to +26.4‰ and from +3.9 to +18.5‰ and those in the Horas ore body range from − 11.8 to +3.5‰ and from +1.4 to +25.7‰, respectively. The δ34S of total bulk sulfur in southeastern ore bodies (Σδ34S) was estimated to be approximately +6‰.The estimated sulfur fugacity during formation of the Purnama and Horas ore bodies is relatively high. It was between 10−4.8 and 10−10.8 atm at 220 to 260 °C. Tellurium fugacity was between 10−7.8 and 10−9.5 atm at 260 °C and between 10−9 and 10−10.6 atm at 220 °C in the Purnama ore body. The Barani ore body was formed at lower fS2, lower than about 10−14 atm at 200 to 220 °C based on the presence of arsenopyrite and pyrrhotite in the early stage, and between 10−14 and 10−12 atm based on the existence of enargite and tennantite in the last stage. © 2016 The Society of Resource Geology

Multiple Gold Mineralizing Styles in the Northern Pataz District, Peru

Gold production from the northern Pataz district in the eastern Andean Cordillera of Peru has been sourced mainly from mesothermal quartz-carbonate-sulfide veins hosted by the Mississippian Pataz batholith. Gold is also found in basement-hosted veins underlying the batholith, in the Vijus-Santa Filomena area of the district. Both are located within a central horst; similar vein mineralogy and proximal phengitic white mica alteration are common to both. However, comb-textured quartz, the chemical compositions of bulk ore and sulfide minerals, and the presence of barite and siderite veins suggest that the basement-hosted veins formed at a shallower crustal level. Similar expressions of hydrothermal alteration associated with anomalous gold, As, Sb, and Tl are also present in the adjacent Lavasen graben, where alteration is intimately associated with volcanic processes that deposited the Mississippian Lavasen Volcanics. K-Ar and 40 Ar- 39 Ar ages for hydrothermal illite from all three locations range between Mississippian and Late Triassic but are consistent with a single Mississippian hydrothermal event, if the data record a minimum age for original illite formation. The geologic setting, mineralization styles, and chemical data suggest a range of crustal depths, ranging from mesothermal batholith-hosted veins through shallow to intermediate depths for the Vijus-Santa Filomena area to a near-surface epithermal setting for the Misquichilca area. Telescoping of this 10- to 13-km crustal range into a 3-km topographic section of the Andes is attributed to syn- and postmineralization uplift and erosion. Sulfide-rich high-grade ore shoots and moderately saline fluid inclusions in the batholith-hosted veins are inconsistent with the original orogenic gold model and suggest a magmatic source component for the ore fluid, consistent with stable isotope (O, H, C, and S) compositions of quartz, illite, carbonates, and sulfides. The isotopic data suggest a mixed magmatic-meteoric ore fluid in the basement-hosted deposits of the Vijus-Santa Filomena area and the volcanic-hosted Misquichilca area. Both the Pataz batholith and the Esperanza Subvolcanic Complex are of the same Mississippian age as the hydrothermal alteration and mineralization. The Esperanza Subvolcanic Complex, comagmatic with the Lavasen Volcanics, contains cognate mineral clots from which a subjacent magma chamber can be inferred. It exhibits potassic, calc-silicate, and argillic alteration, and evidence for the evolution of an Fe-rich volatile phase. The Lavasen-Esperanza magma suite is ferroan and weakly alkaline, with A-type affinities. These features provide a conceptual genetic link with hydrothermal alteration associated with gold mineralization, including Fe (±As) sulfides, phengitic white mica, celadonite, Fe-rich carbonates, and less common Fe oxides. An oxidized intrusion-related model is proposed for gold and hydrothermal alteration in the northern Pataz district.

Origin and evolution of ore fluids in the late Mesozoic Naozhi epithermal Au–Cu deposit, Yanbian area, Northeast China: evidence from fluid inclusion and isotopic geochemistry

The Naozhi Au–Cu deposit is located on the continental margin of Northeast China, forming part of the West Pacific porphyry–epithermal gold–copper metallogenic belt. In this paper, we systematically analyzed the compositions, homogenization temperatures, and salinity of fluid inclusions as well as their noble gas isotopic and Pb isotopic compositions from the deposit. These new data show that (1) five types of fluid inclusions were identified as pure gas inclusions (V-type), pure liquid inclusions (L-type), gas–liquid two-phase inclusions (W-type, as the main fluid inclusions (FIs)), CO2-bearing inclusions (C-type), and daughter-mineral-bearing polyphase inclusions (S-type); (2) W-type FIs in quartz crystals of early, main, and late stage are homogenized at temperatures of 324.7–406.7, 230–338.8, and 154.6–308 °C, with salinities of 2.40–7.01 wt% NaCleq, 1.73–9.47 wt% NaCleq, and 6.29 wt% NaCleq, respectively. S-type FIs in quartz crystals of early stage are homogenized at temperatures of 328.6–400 °C, with salinities of 39.96–46.00 wt% NaCleq; (3) Raman analysis results reveal that the vapor compositions of early ore-forming fluids consisted of CO2 and H2O, with H2O gradually increasing and CO2 being absent at the late mineralization stage; (4) fluid inclusions in pyrite and chalcopyrite have 3He/4He ratios of 0.03–0.104 Ra, 20Ne/22Ne ratios of 9.817–9.960, and 40Ar/36Ar ratios of 324–349. These results indicate that the percentage of radiogenic 40Ar* in fluid inclusions varies from 8.8 to 15.5 %, containing 84.5–91.2 % atmospheric 40Ar; (5) the 206Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb ratios of sulfides are 18.1822–18.3979, 15.5215–15.5998, and 38.1313–38.3786, respectively. These data combined with stable isotope data and the chronology of diagenesis and metallogenesis enable us suppose that the ore-forming fluids originated from the melting of the lower crust, caused by the subduction of an oceanic slab, whereas the mineralized fluids were exsolved from the late crystallization stage and subsequently contaminated by crustal materials/fluids during ascent, including meteoric water, and the mineral precipitation occurred at a shallow crustal level.

A salt diapir-related Mississippi Valley-type deposit: the Bou Jaber Pb-Zn-Ba-F deposit, Tunisia: fluid inclusion and isotope study

The Bou Jaber Ba-F-Pb-Zn deposit is located at the edge of the Bou Jaber Triassic salt diapir in the Tunisia Salt Diapir Province. The ores are unconformity and fault-controlled and occur as subvertical column-shaped bodies developed in dissolution-collapse breccias and in cavities within the Late Aptian platform carbonate rocks, which are covered unconformably by impermeable shales and marls of the Fahdene Formation (Late Albian–Cenomanian age). The host rock is hydrothermally altered to ankerite proximal to and within the ore bodies. Quartz, as fine-grained bipyramidal crystals, formed during hydrothermal alteration of the host rocks. The ore mineral assemblage is composed of barite, fluorite, sphalerite, and galena in decreasing abundance. The ore zones outline distinct depositional events: sphalerite-galena, barite-ankerite, and fluorite. Fluid inclusions, commonly oil-rich, have distinct fluid salinities and homogenization temperatures for each of these events: sphalerite-galena (17 to 24 wt% NaCl eq., and Th from 112 to 136 °C); ankerite-barite (11 to 17 wt% NaCl eq., and Th from 100 to 130 °C); fluorite (19 to 21 wt% NaCl eq., Th from 140 to 165 °C). The mean temperature of the ore fluids decreased from sphalerite (125 °C) to barite (115 °C) and increased during fluorite deposition (152 °C); then decreased to ∼110 °C during late calcite precipitation. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of fluid inclusions in fluorite are metal rich (hundreds to thousands ppm Pb, Zn, Cu, Fe) but the inclusions in barite are deficient in Pb, Zn, Cu, Fe. Inclusions in fluorite have Cl/Br and Na/Br ratios of several thousand, consistent with dissolution of halite while the inclusions analysed in barite have values lower than seawater which are indicative of a Br-enriched brine derived from evaporation plus a component of halite dissolution. The salinity of the barite-hosted fluid inclusions is less than obtained simply by the evaporation of seawater to halite saturation and requires a dilution of more than two times by meteoric water. The higher K/Na values in fluid inclusions from barite suggest that the brines interacted with K-rich rocks in the basement or siliciclastic sediments in the basin. Carbonate gangue minerals (ankerite and calcite) have δ13C and δ18O values that are close to the carbonate host rock and indicate fluid equilibrium between carbonate host rocks and hydrothermal brines. The δ34S values for sphalerite and galena fall within a narrow range (1 to 10 ‰) with a bulk value of 7.5 ‰, indicating a homogeneous source of sulfur. The δ34S values of barite are also relatively homogeneous (22 ‰), with 6 ‰ higher than the δ34S of local and regional Triassic evaporites (15 ‰). The latter are believed to be the source of sulfate. Temperature of deposition together with sulfur isotope data indicate that the reduced sulfur in sulfides was derived through thermochemical sulfate reduction of Triassic sulfate via hydrocarbons produced probably from Late Cretaceous source rocks. The 87Sr/86Sr ratio in the Bou Jaber barite (0.709821 to 0.711408) together with the lead isotope values of Bou Jaber galena (206Pb/204Pb = 18.699 to 18.737; 207Pb/204Pb = 15.635 to 15.708 and 208Pb/204Pb = 38.321 to 38.947) show that metals were extracted from homogeneous crustal source(s). The tectonic setting of the Bou Jaber ore deposit, the carbonate nature of the host rocks, the epigenetic style of the mineralization and the mineral associations, together with sulfur and oxygen isotope data and fluid inclusion data show that the Bou Jaber lead-zinc mineralization has the major characteristics of a salt diapir-related Mississippi Valley-type (MVT) deposit with superimposed events of fluorite and of barite deposition. Field relations are consistent with mineral deposition during the Eocene–Miocene Alpine orogeny from multiple hydrothermal events: (1) Zn-Pb sulfides formed by mixing of two fluids: one fluid metal-rich but reduced sulfur-poor and a second fluid reduced sulfur-rich; (2) barite precipitation involved the influx of a meteoric water component that mixed with a barium-rich fluid; and (3) fluorite precipitated from a highly saline fluid with higher temperatures.