Fluid Inclusion Populations Research Articles

Are fluid inclusions in gypsum reliable paleoenvironmental indicators? An assessment of the evidence from the Messinian evaporites

Abstract The paleosalinity of water from which the gypsum precipitated during the Messinian salinity crisis is a controversial issue. Recent microthermometry studies on primary fluid inclusions in gypsum provided very low salinity values not compatible with precipitation from seawater, and suggested strong mixing between seawater and nonmarine waters enriched in calcium sulfate. We applied a new microthermometric protocol on gypsum crystals from nine Mediterranean sections that were experimentally stretched to measure a larger population of fluid inclusions. The results show salinities ranging from 9 to 238 wt‰ NaCl equivalent, largely falling within the evaporation path of normal seawater. The data from previous studies were obtained mostly from those fluid inclusions capable of nucleating a stable bubble after a weak stretching, which probably correspond to those having a lower salinity acquired through post-depositional crack-and-seal processes. Our data suggest instead that the primary gypsum precipitated from a marine brine, later modified by post-trapping processes during tectonics and exhumation.

Geological implications of apatite within the granite-related Jiepai W–(Cu) deposit, Guangxi, South China

The Jiepai W–(Cu) deposit is one of the largest Caledonian granite-related W deposits in the Miaoershan–Yuechengling (MY) pluton of southern China. The deposit consists of skarn-type and altered granite-type mineralization along with an ore shoot hosted by altered arkose. This study presents new petrographic and geochemical data for apatite from fresh biotite granite (ApI), mineralized biotite granite (ApII), and ore shoot (ApIII) samples and uses these data to constrain the magmatic and hydrothermal processes that formed the deposit. ApI is associated with primary biotite and has core–rim textures or oscillatory zoning visible during backscattered electron (BSE) imaging, indicating that this apatite has a magmatic origin. ApII is associated with scheelite and secondary biotite and consists of BSE-bright cores (ApIIa) with oscillatory zoning and rare fluid inclusions surrounded by BSE-dark rims (ApIIb) free of fluid inclusions. These relationships suggest that ApIIa and ApIIb record the effects of slight and significant fluid metasomatism, respectively. ApIII appears homogenous during BSE imaging, is associated with abundant scheelite, sericite and quartz, and contains abundant fluid inclusions, indicative of a hydrothermal origin. ApI has increasing total rare earth elements (REE) (2959–14057 ppm) and Y (1598–9099 ppm) contents from cores to rims, high Na2O (0.05–0.2 wt%) and MnO (0.1–0.32 wt%) and low Sr (71.6–86.1 ppm) contents, and low LaN/YbN values (0.55–1.21). ApIIa contains similar REE and Y contents to ApI, but has higher Sr (194–258 ppm) and lower Na2O (<0.03 wt%) and MnO (0.04–0.11 wt%) contents. ApIIb contains similar Na2O, MnO, and Sr contents to ApIIa, but with much lower REE (1766–2021 ppm) and Y (1128–1512 ppm) contents. ApIII apatite has similar Na2O, MnO, REE, and Y contents to ApIIb, but with slightly lower Sr (103–173 ppm) contents and higher LaN/YbN values (0.92–3.28).The large increase in REE contents from core to rim within ApI is indicative of prolonged crystallization, generating W-enriched residual melts and eventually W mineralization. ApIIa and ApIIb contain similar Na, Mn, and Sr contents to ApIII, but have REE contents that are similar to ApI and ApIII, respectively. This suggests that Na, Mn, and Sr were affected by weak fluid metasomatism, whereas REE can only be mobilized by significant fluid metasomatism. The relatively low REE contents and high LaN/YbN values of ApIII reflect the presence of abundant co-precipitated scheelite. These data indicate that ApI, ApII, and ApIII all have different textural and geochemical (Na, Mn, Sr, Y, REE, and LaN/YbN) characteristics and fluid inclusion populations. All of this suggests that apatite is a potential indicator mineral in mineral exploration for granite-related W–(Cu) mineralization.

Mineralization, fluid inclusion and sulfur isotope studies in The Gardane-Shir Pb-Zn deposit ,Ardestan,Isfahan Province

Gardaneshir carbonate-hosted Pb-Zn deposit on the ground of study area located southwest of Ardestan in Isfahan province. Base on lithostratigraphy,the main structure, besides the small outcrops of Jurassic shales, has been made up of carbonate and detrital rock materials depending on Paleozoic,Triassic and Cretaceous time stages. Dolomitic carbonate which is attributed to Shotori Formation , played the major role of host rock in ore-mineralization. Ore-mineral description obtained from the mineralized zone and caused the following ore-paragenesis in ascending order.of:pyrite,chalcopyrite,galena,sphalerite,malachite,cerussite,smithsonite,iron oxides and gangues of barite, quartz and calcite. Physico-chemical information of ore-solutions performed by entrapped fluid inclusion studies in gangue minerals. Liquid-rich two phase(L+V) inclusions as predominant types were recognized. These type of inclusions are homogenized into liquid state with a range of TH and related salinities between; TH :78 to 183 and 216 to 283°C, Salinity:3.5 to 9.7 and 10.2 to 25 wt% NaCl eq. The microthermometric data reflect the nature of two population of fluid inclusions originating from different sources. The source materials could have been provided by basinal brines, derived during compaction of sediments in a shallow sea environment and by movement into sediments, the stratabound Pb-Zn deposit are formed. Furthermore, the negative delta value ranges(δ34S) from -0.6 ‰ to -20.4 ‰ that have been extracted by galena can be an evidence of bacterial sulfate reduction in a subsidence sedimentary basin. In conclusion, based on field, mineralogical, fluid inclusion and sulfur isotope evidence, we propose that Gardaneshir Pb-Zn deposit is a stratabound carbonate hosted of Mississipi Valley Type(MVT).

Fluid inclusion systematics in porphyry copper deposits: The super-giant Grasberg deposit, Indonesia, as a case study

Fluid inclusion data provide a unique insight into the behaviour of magmatic fluids as they expand from source to surface to form porphyry copper deposits. For example, saline, liquid phase inclusions in combination with vapor phase inclusions help to define the environment of mineralization in terms of large scale, two-phase fluid flow within a magmatic vapor plume. However, the pressure at which ore formation occurs within the plume is controversial because inclusion-based estimates are commonly much higher than those based on geological estimates. Uniquely, the Grasberg supergiant porphyry copper deposit occupies the breccia-filled diatreme underlying the crater of a maar volcano and preserved a lacustrine-volcaniclastic sequence that marks the paleo-surface to within a few hundred meters. Interpreting fluid inclusion data with respect to this paleo-surface shows that porphyry mineralization extended from a paleo-depth of about 500 m to over 1500 m depth, as defined by drilling.The Grasberg deposit has three distinct populations of fluid inclusions (low salinity, hypersaline liquid and multi-solid inclusions) that are strongly clustered with respect to salinity and trapping temperature. Similar clusters of co-existing liquid- and vapor-rich inclusions are also common to porphyry copper deposits worldwide. Applying the principles of energy and mass conservation, a simple model of isenthalpic phase separation by condensation may be derived to account for these data clusters and how they are related. Vapor- and liquid-rich hypersaline inclusions may be shown quantitatively to be the consequence of expansion of a continuum low salinity, low density magmatic vapor phase to a two-phase fluid where the liquid fraction is minor (<2 vol%) and is best considered to be an aerosol or fog. The clustering of the inclusion data is the product of the different ranges of pressures that fluids encountered from the initiation of individual stockwork fractures through to their resealing.The application of mass and energy conservation principles also shows that, even though metal partitioning to the liquid phase may be favored, the proportion of total metal fractionated to the small mass of liquid is low relative to that remaining in the vapor phase. Therefore, while partitioning of metals between liquid and vapor phases may be a contributing process for metal deposition in porphyry copper deposits, other vapor-phase processes are required to efficiently deposit these major metal resources.

Characterization of fluid inclusions from mineralized pegmatites of the Damara Belt, Namibia: insight into late-stage fluid evolution and implications for mineralization

Mineralized NYF and LCT pegmatites occur throughout the northeast-trending Neoproterozoic Damara Belt, Namibia. Mineralization in the pegmatites varies geographically, from the northeast, where they are enriched in Li-Be, to the southwest, where they also contain notable Sn and U. Similar fluid inclusion populations occur throughout the pegmatites, regardless of their respective metal enrichments, and primary fluid inclusion textures were destroyed by continued fluid activity. Pseudosecondary to secondary inclusions are aqueo-carbonic, carbonic, and aqueous in composition, and have been divided into five types. The earliest populations are saline (>26.3 eq. wt.% NaCl), homogenizing at temperatures in excess of 300 °C. Their carbonic phase is composed of CO2, with minor CH4, and micro-elemental mapping indicates they contain trace metals, including Ca, Fe, Zn, Cu, and K. Type 3 inclusions formed later, homogenize at ~325 °C, and are less saline, with a carbonic phase composed of CO2. Type 4 carbonic inclusions are composed of pure CO2, and represent the latest stages of fluid evolution, while Type 5 aqueous inclusions are believed to be unrelated to the crystallization of the pegmatites, and rather the result of regional Cretaceous magmatism, or the ingress of meteoric water. The similarities in fluid inclusion populations observed in the pegmatites suggest that differences in mineralization were driven by magma composition rather than fluid activity alone, however saline fluids facilitated the enrichment and deposition of metals during the late stages of crystallization. Furthermore, the similarities between fluid inclusion populations in different pegmatites suggests they share a similar fluid evolution.

N2-rich fluid in the vein-type Yangjingou scheelite deposit, Yanbian, NE China

Nearly pure N2 fluid inclusions (Th (L) = −151~−168 °C; Th (V) = ~150.3 °C) were identified in W-mineralized quartz veins from the Yangjingou scheelite deposit, in the eastern Yanbian area, NE China. Other fluid inclusion populations include N2-CO2, NaCl-H2O ± N2 and CO2 ± N2-NaCl-H2O, but no hydrocarbons were detected. The host rocks are part of the Wudaogou Group metamorphic series, which mainly consist of Ca-rich mica schist. Subhedral sulfide minerals occur in early disseminated W-mineralized quartz veins, or have partially replaced early scheelite. ThN2 and ThN2-H2O indicate N2 fluid-trapping from 315 °C to 410 °C and from 80 MPa to 350 MPa. Oxygen and hydrogen isotopic data (δD = −74.9‰~−77‰, δ18O = 9.6‰~12‰, V-SMOW) suggest that the mineralizing fluids were composed of mixed magmatic and metamorphic water, N2-rich inclusions (δ15N = −0.5‰ to 1.4‰) indicate fluid-rock interaction with metamorphic rocks. The N2-rich fluid was closely associated with scheelite precipitation. During thermal decomposition under high oxygen fugacity conditions, which occurred synchronously with metamorphism and magmatic activity, large amounts of N2 were liberated from NH4+-micas, which then accumulated in the parent fluid of the quartz scheelite veins.

The synorogenic pegmatitic quartz veins of the Guacha Corral Shear zone (Sierra de Comechingones, Argentina): A textural, chemical, isotopic, cathodoluminescence and fluid inclusion study

The quartz veins and pegmatites of the Sierra de Comechingones (Sierras de Córdoba, NE Argentina) belong to the Comechingones Pegmatite field (CPF). For the quartz veins and the zoned pegmatites related parental granites are missing. The country rock of the quartz veins are mylonitic augengneisses in granulite to upper amphibolite facies. Field relations, microscopy, cathodoluminescence, radiometric age data, fluid inclusion, chemical and isotopic composition and literature define the quartz veins as synorogenic formed during the high-temperature phase of the Famatinian (480–460Ma) event. During the Famatinian up to the Achalian (382–366Ma) event the synorogenic quartz veins were subjected to high temperature ductile deformation documented by folding, boudinage and finally brittle shearing. K-Ar ages of illite from the shear zones of about 166Ma document the final cooling of the Sierras Pampeanas below 100°C. The long lasting thermal and deformational history of the study area is reflected by very different populations of fluid inclusions in vein quartz with remarkably high contents of thermogenic hydrocarbons in the early-formed fluid inclusions. LA–ICP–MS analysis reveals very low lattice-bound trace element contents, i.e. high purity quartz.

Origin and evolution of the ore-forming fluids of the Erdaogou and Xiaobeigou gold deposits, Jiapigou gold province, NE China

The Jiapigou gold province, located in the eastern part of the northern margin of the North China Craton, includes the Erdaogou and Xiaobeigou gold deposits, which are hosted in amphibolites, amphibolitic gneisses, and tonalite-trondhjemite-granodiorite gneisses of the Jiapigou Group. The gold deposits occur as gold-bearing quartz veins and disseminated- and veinlet-type mineralization. Orebodies are strictly controlled by NW–SE -trending secondary faults. Four stages of mineralization have been identified in the Erdaogou gold deposit: (1) milky quartz, (2) pyrite-quartz, (3) native gold-quartz-pyrite, and (4) quartz-carbonate. In the Xiaobeigou gold deposit, five stages of mineralization have been identified: (1) milky quartz, (2) pyrite-sericite-quartz, (3) pyrite-quartz, (4) native gold-quartz-pyrite, and (5) quartz-carbonate. Three fluid inclusion populations have been identified in the deposits: CO2-H2O (C-type), aqueous (W-type), and pure CO2 (PC-type). C-type, W-type, and PC-type fluid inclusions are observed in the Erdaogou gold deposit, whereas C-type and W-type fluid inclusions are dominant in the Xiaobeigou gold deposit. Microthermometric data indicate that the fluid inclusions of the deposits homogenize at 128–370°C (0.18–15.35wt.% NaCl equivalent). Pressures estimated from fluid inclusion data cluster into two populations at 140–165MPa and 60–80MPa, indicating that mineralization occurred at a depth of 5.1–8.0km. The O-isotope ratios (δ18OH2O) of ore-forming fluids in the Xiaobeigou gold deposit range from −1.84‰ to −1.14‰, with δD of −102‰, whereas late-stage fluids have δ18OH2O of −9.40‰ to −9.20‰, and δD of −78‰. Integrated C-H-O-S-Pb-isotope data suggest that the initial ore-forming fluids of the deposits consisted of magmatic water produced by the partial melting of lower crustal material containing a mantle component. Water-rock interaction occurred in the early mineralization stage. Fluid immiscibility resulted in rapid gold deposition. Regional and deposit geology, fluid inclusions, and stable isotopes indicate that the Erdaogou and Xiaobeigou gold deposits are mesothermal, related to subduction of the Paleo-Pacific plate. The Jiapigou gold province has significant potential for deep-seated mineralization, and should be the focus of future exploration.

Petrography, mineral chemistry, fluid inclusion microthermometry and Re–Os geochronology of the Küre volcanogenic massive sulfide deposit (Central Pontides, Northern Turkey)

The Re–Os isotopic system is applied for the first time to the sulfide ores and the overlying black-shales at the Küre volcanogenic massive sulfide deposit of the Central Pontides, Northern Turkey. The ore samples collected include predominantly pyrite, accompanied by chalcopyrite, sphalerite and other species. Massive ore is almost free of gangues, whereas the stockwork ore includes quartz and calcite gangue. The composition of sphalerite is similar to ancient and modern massive sulfide mineralizations globally. Microthermometric studies from quartz from the stockwork ore has shown two populations of two-phase fluid inclusions with vapor/liquid ratios between 4 to 28%, low to intermediate Th (161.5–317.0°C) and low salinities (0.9–5wt.% NaCl equiv.) which are mostly in good agreement with the ranges for volcanogenic massive sulfide mineralizations. These studies also suggest a H2O–CaCl2–KCl–MgCl2 ore-forming fluid system in a shallower subsurface near the seafloor vents. The Re–Os dating of the LLHR sulfides yield a nominal depositional age of upper Toarcian for the massive sulfide mineralization. Two largely different model ages obtained are attributed to other pyrite crystallization events prior to and postdating the main sulfide deposition. Some lower homogenization temperatures (<200°C) from the quartz of the stockwork may also similarly be related to the post-VMS events. It is concluded that a submarine volcanic extrusion episode has continued until upper Toarcian in the Küre Basin, when it has entered a stagnation period that allowed the discharge of hydrothermally circulated sulfide-laden fluids from the seafloor vents. This age data promotes the palaeotectonic models interpreting the Küre Basin as a Permian–early Jurassic marginal back-arc basin of the Devonian-Triassic Karakaya Ocean. The Re–Os data from the overlying black-shale provide a glimpse to the initial Os isotope ratio of the water column at the time of the sedimentation (0.45–0.46 for 180Ma). The lack of common Os from the sulfides does not let us to infer a source of Os and initial 187Os/188Os ratios from the black shale are not statistically robust to make a significant deduction. A further detailed study on the isotopic composition of the black shale strata may help us to make an apporach to the Os source(s) in the deposition environment of the Küre VMS deposit.

Petrography and geochemistry of fault-controlled hydrothermal dolomites in the Riópar area (Prebetic Zone, SE Spain)

The present paper reports the first detailed petrographical and geochemical studies of hydrothermal dolomites related to MVT Zn-(Fe–Pb) deposits in the Riópar area (Mesozoic Prebetic Basin, SE Spain), constraining the nature, origin and evolution of dolomitizing and ore-forming fluids. Mapping and stratigraphic studies revealed two stratabound dolostone geobodies connected by other patchy bodies, which replace carbonate units of Upper Jurassic to Lower Cretaceous ages. These dolostones are associated to the W–E trending San Jorge fault, indicating a main tectonic control for fluid flow. Seven different dolomite types were identified: i) matrix-replacive planar-s (ReD-I); ii) matrix-replacive planar-e (ReD-II); iii) planar-e sucrosic cement (SuD); iv) non-planar grey saddle dolomite cement (SaD-I) pre-dating Zn-(Fe–Pb) sulfides; v) non-planar milky to pinkish saddle dolomite cement (SaD-II) post-dating Zn-(Fe–Pb) ores; vi) ore-replacive planar-e porphyrotopic (PoD); and vii) planar-s cloudy cement (CeD). Meteoric calcite types were also recognized. The different dolomite types are isotopically characterized by: i) depleted δ18O (from +25.1 to +27.6‰ V-SMOW) and δ13C (from −2.3 to +0.9‰ V-PDB) values compared to Upper Jurassic to Lower Cretaceous limestone signature (δ18O: +27.6 to +30.9‰ V-SMOW; δ13C: +0.5 to +3.2‰ V-PDB); and ii) 87Sr/86Sr ratios for the main dolomitization phases (ReD and SuD: 0.70736–0.70773) close to the Jurassic and Cretaceous carbonate values (0.70723–0.70731) whereas more radiogenic values (0.70741–0.70830) for saddle dolomites (SaD) related to the Zn-(Fe–Pb) sulfide mineralization prevailed after fluid interaction with Rb-bearing minerals. Microthermometrical studies on two-phase liquid and vapor fluid inclusion populations in planar and non-planar dolomites and sphalerite show homogenization temperatures between 150 and 250 °C. These data indicate that both planar and non-planar dolomite textures formed at high-temperatures under hydrothermal conditions in deep-burial diagenetic environments. The main dolomitizing phase (ReD-I/ReD-II and SaD-I) shows low to moderate fluid inclusions salinity (5–14 wt.% eq. NaCl), whereas the dolomitization related to ore precipitation (sphalerite and SaD-II) spreads to higher salinity values (5–25 wt.% eq. NaCl). These data may respond to a mixing between a low salinity fluid (fluid A, less than 5 wt.% eq. NaCl) and a more saline brine (fluid B, more than 25 wt.% eq. NaCl) at different fluid proportions.

The chemical behavior of fluids released during deep subduction based on fluid inclusions

This review combines fluid inclusion data from (HP-)UHP rocks with experimental research and thermodynamic models to investigate the chemical and physical properties of fluids released during deep subduction, their solvent and element transport capacity, and the subsequent implications for the element recycling in the mantle wedge. An impressive number of fluid inclusion studies indicate three main populations of fluid inclusions in HP and UHP metamorphic rocks: (1) aqueous and/or non-polar gaseous fluid inclusions (FI); (2) multiphase solid inclusions (MSI); and (3) melt inclusions (MI). Chemical data from preserved fluid inclusions in rocks match with and implement “model” fluids by experiments and thermodynamics, revealing a continuity behind the extreme variations of physico-chemical properties of subduction-zone fluids. From fore-arc to sub-arc depths, fluids released by progressive devolatilization reactions from slab lithologies change from relatively diluted chloridebearing aqueous solutions (±N2), mainly influenced by halide ligands, to (alkali) aluminosilicate-rich aqueous fluids, in which polymerization probably governs the solubility and transport of major (e.g., Si and Al) and trace elements (including C). Fluid inclusion studies point to a reconsideration of the petrological models explaining deep volatile liberation, and their flux into the mantle wedge.

Mineralogical and fluid characteristics of the fluorite-rich Monakoff and E1 Cu–Au deposits, Cloncurry region, Queensland, Australia: Implications for regional F–Ba-rich IOCG mineralisation

Mineralogical and fluid characteristics of the fluorite-rich Monakoff and E1 Cu–Au deposits, Cloncurry region, Queensland, Australia: Implications for regional F–Ba-rich IOCG mineralisation

Concealed Basalt-Matrix Diatremes with Cu-Au-Ag-(Mo)-Mineralized Xenoliths, Santa Cruz Porphyry Cu-(Mo) System, Pinal County, Arizona

The Santa Cruz porphyry Cu-(Mo) system near Casa Grande, Arizona, includes the Sacaton mine deposits and at least five other concealed, mineralized fault blocks with an estimated minimum resource of 1.5 Gt @ 0.6% Cu. The Late Cretaceous-Paleocene system has been dismembered and rotated by Tertiary extension, partially eroded, and covered by Tertiary-Quaternary basin-fill deposits. The mine and mineralized fault blocks, which form an 11 km (~7 miles) by 1.6 km (~1 mile) NE-SW–trending alignment, represent either pieces of one large deposit, several deposits, or pieces of several deposits. The southwestern part of the known system is penetrated by three or more diatremes that consist of heterolithic breccia pipes with basalt and clastic matrices, and subannular tuff ring and maar-fill sedimentary deposits associated with vents. The tephra and maar-fill deposits, which are covered by ~485 to 910 m (~1,600–3,000 ft) of basin fill, lie on a mid-Tertiary erosion surface of Middle Proterozoic granite and Late Cretaceous porphyry, which compose most xenoliths in pipes and are the host rocks of the system. Some igneous xenoliths in the pipes contain bornite-chalcopyrite-covellite assemblages with hypogene grades >1 wt % Cu, 0.01 ounces per ton (oz/t) Au, 0.5 oz/t Ag, and small amounts of Mo (<0.01 wt %). These xenoliths were derived from mineralized rocks that have not been encountered in drill holes, and attest to additional, possibly higher-grade deposits within or subjacent to the known system. The geometry, stratigraphy, and temporal relationships of pipes and tephras, interpreted from drill hole spacing and intercepts, multigenerational breccias and matrices, reequilibrated and partially decomposed sulfide-oxide mineral assemblages, melted xenoliths, and breccia matrix compositions show that the diatremes formed in repeated stages. Initial pulses of basalt magma fractured granite, porphyry, and other crustal rocks during intrusion, transported multi-sized fragments of these rocks upward, and partially melted small fragments. Rapid decompression of magma induced catastrophic devolatilization that ruptured overlying rocks to the surface, and generated fragment-volatile suspensions that abraded conduits into near-vertical cylindrical structures. Fragments entrained in suspensions were milled and sorted, and ejected as basal surge, pyroclastic deposits, and airfall tephra that built tuff rings around vents and filled vent depressions. Comminuted m- to mm-sized fragments of wall rocks in magma and suspensions that remained in conduits solidified as heterolithic breccias. Subsequent pulses of basalt magma ascended through the same conduits, brecciated older heterolithic breccias, devolatilized, and quenched, leaving two or more generations of nested and mingled heterolithic breccias and internal zones of fluidized fragments. Tephra and maar-fill deposits from later eruptions are composed of more hydrous and oxidized minerals than earlier tephras, reflecting a higher proportion of water in transport fluid which, based on fluid inclusion populations in mineralized xenoliths, was saline water and CO2. The large vertical extent (~600 m; ~2,000 ft) of basalt matrix in pipes, near-paleosurface matrix vesiculation, and plastically deformed basalt lapilli indicates that diatreme eruptions were predominantly phreatic. Diatreme xenoliths represent crustal stratigraphy and, as in the Santa Cruz system, provide evidence of concealed mineral resources that can guide exploration drilling through cover. Vectors to the source of bornite-dominant xenoliths containing >1% Cu and significant Au and Ag could be determined by refinement of breccia pipe geometries, by reassembly of mineralized fault blocks using modal, chemical, and temporal characteristics of hydrothermal mineral assemblages and fluid inclusions, and by paleodrainage analysis.

Noble gases (Ar, Kr, Xe) and halogens (Cl, Br, I) in fluid inclusions from the Athabasca Basin (Canada): Implications for unconformity-related U deposits

The formation of unconformity-related uranium deposits in the Proterozoic Athabasca Basin (Canada) involved basin-scale circulation of U-bearing brines during high-grade diagenesis (150–200°C) at ∼1.6–1.5Ga. The UO2 ores occur both sides of the unconformity and are associated with extensive brecciation and illite–sudoite–dravite alteration. Quartz and dolomite cementing veins and breccias are associated with alteration and mineralisation and contain a fairly uniform population of fluid inclusions characterised by variable Na:Ca and salinities of 25–35wt.% salts and high U concentrations of up to 600ppm U. In order to further constrain the origin of these U-rich brines, we analysed the naturally occurring isotopes of Ar, Kr and Xe, together with halogens (Cl, Br and I), K, Ca and U in irradiated quartz and dolomite samples containing representative fluid inclusions. This was achieved by the noble gas method for halogen measurement (extended 40Ar–39Ar methodology) using a combination of noble gas extraction techniques.The fluid inclusions opened by crushing quartz and dolomite samples in vacuum have similar molar Br/Cl ratios of 5.8×10−3 to 10.4×10−3, and molar I/Cl ratios of 1.8×10−6 to 8.2×10−6. These compositions lie over the top half of the modern-day seawater evaporation trajectory, consistent with the fluids deriving the bulk of their salinity by subaerial evaporation of seawater, beyond the point of halite saturation. The I/Cl ratios are much lower than is typical of fluids that have interacted with I-rich organic matter present in many sedimentary basins or fluid inclusions found in Mississippi Valley type (MVT) Pb–Zn ore deposits. This is significant because provided the U-rich fluid inclusions are representative of the ore-stage fluids, the low I/Cl ratios of the fluid inclusions do not favour fluid interaction with organic matter (or hydrocarbons), as a major process for localising U mineralisation.The majority of samples contain fluid inclusions with age-corrected 40Ar/36Ar of between the modern atmospheric value of ∼300 and 450. These values are considered representative of the fluid's initial composition and are typical of upper crustal sedimentary formation waters. The fluid inclusions non-radiogenic 84Kr/36Ar and 129Xe/36Ar ratios are slightly enriched in 129Xe relative to air and the fluid inclusions are estimated to contain 0.5–17.3×10−10molg−136Ar which is up to twenty times the 36Ar concentration of air-saturated seawater. The data are interpreted to reflect acquisition of atmospheric noble gases from sedimentary rocks and suggest acquisition of radiogenic 40Ar within K-rich basement rocks, that would have been an important source of excess 40Ar, was limited by temperatures of less than 200°C.Taken together the halogen and noble gas composition of the U-bearing fluid inclusions are strongly controlled by subaerial evaporation and subsequent interaction with sedimentary rocks, showing that low temperature evaporitic brines dominated the mineralising system. Mineralisation is unlikely to have been triggered by fluid interaction with organic matter, or mixing with voluminous basement-derived fluids; however, the data do not completely preclude a role for volumetrically minor fluid or gas phases introduced by deep-seated basement faults preferentially located at the sites of mineralisation.

Genetic investigation and comparison of Kartaldağ and Madendağ epithermal gold deposits in Çanakkale, NW Turkey

Two epithermal gold deposits (Kartaldağ and Madendağ) located in NW Turkey have been characterized through the detailed examinations involving geologic, mineralogical, fluid inclusion, stable isotope, whole-rock geochemistry, and geochronology data.The Kartaldağ deposit (0.01–17.65ppm Au), hosted by Eocene dacite porphyry, is associated with four main alteration types with characteristic assemblage of: i) chlorite/smectite–illite±kaolinite, ii) quartz–kaolinite, iii) quartz–alunite–pyrophyllite, iv) quartz–pyrite, the last being characterized by three distinct quartz generations comprising massive/vuggy (early), fine–medium grained, vug-lining (early), and banded, colloform, comb (late) textures. Observed sulfide minerals are pyrite, covellite, and sphalerite. Oxygen and sulfur isotope analyses, performed on quartz (δ18O(quartz): 7.93 to 8.95‰ and calculated δ18O(H2O): −7.95 to 1.49‰) and pyrite (δ34S(pyrite): −4.8‰ and calculated δ34S(H2S): −6.08 to −7.20‰) separates, suggest a meteoric water source for water in the hydrothermal fluid, and an igneous source for the sulfur dissolved in ore-related fluids. Microthermometric analyses of primary fluid inclusion assemblages performed on quartz (late quartz generation) yield temperatures (Th) dominantly in the range of 245–285°C, and generally low salinity values at 0 to 1.7wt.% NaCl eq. Based on the quartz textures and the associated base metal concentrations, along with fluid inclusion petrography, the early vug-lining quartz is considered to have been associated with the mineralization possibly through a boiling and a late mixing process at >285°C.The Madendağ deposit (0.27–20.60ppm Au), hosted by Paleozoic mica schists, is associated with two main alteration types: sericite–illite–kaolinite, and quartz–pyrite dominated by two distinct quartz generations i) early colloform, comb and banded quartz and ii) late quartz, forming the cement in hydrothermal breccia. Whereas oxygen isotope analyses of quartz (δ18O(quartz): 9.55 to 18.19‰ and calculated δ18O(H2O): −2.97 to 5.54‰) suggest varying proportions of meteoric and magmatic sources for the ore bearing fluid, sulfur isotope ratios (δ34S(pyrite): −2.2‰ and calculated δ34S(H2S): (−3.63) to (−3.75) ‰) point to an essentially magmatic source for sulfur with or without contribution from sedimentary sources. Microthermometric analysis carried out on primary fluid inclusion populations of a brecciated sample (early quartz), give a temperature (Th) range of 235–255°C and 0.0 to 0.7wt.% NaCl eq. salinity. Based on the textural relationship, base metal and high gold contents, the ore precipitation stage is associated with late stage quartz formation via a possible boiling process.The presence of alunite, pyrophyllite and kaolinite, vuggy quartz and covellite suggest a high-sulfidation type of epithermal deposit for Kartaldağ. On the other hand, Madendağ is identified as an adularia-sericite type owing to the presence of significant sericite, neutral pH clays (mostly illite, chlorite/smectite, and kaolinite), low temperature quartz textures (e.g., colloform, comb, and banded quartz), and limited sulfide minerals.Given the geographical proximity of Kartaldağ and Madendağ deposits, the similar temperature and salinity ranges obtained from their fluid inclusions, and the similar ages of igneous rocks in both deposits (Kartaldağ: 40.80±0.36 to 42.19±0.45Ma, Madendağ: 43.34±0.85Ma) the mineralizing systems in both deposits are considered to be genetically related.

Ore Fluid Properties and Sources from Quartz-Associated Gold at the Betze-Post Carlin-Type Gold Deposit, Nevada, United States

Quartz at the Betze-Post deposit, the largest Carlin-type gold system in the world, was examined to better understand ore fluid properties and sources that generated Carlin-type gold deposits. Detailed petrography and paragenesis investigations distinguished textural relationships, luminescence, various generations of quartz, and fluid inclusion populations; microthermometry, chemical analyses of quartz, and in situ and conventional oxygen isotope analyses of quartz provided information about hydrothermal fluid conditions as well as fluid sources and ore formation processes. Pre-, syn-, late-, and post-ore stages of quartz were distinguished during petrographic studies and confirmed by cathodoluminescence analyses. Ore-stage jasperoid and late-ore drusy quartz are spatially associated with Au-bearing pyrite; both lack luminescence. Two generations of post-ore drusy quartz overgrew ore and late-ore quartz and exhibit bright and multiply zoned luminescence. Electron probe microanalyses determined that nonluminescing ore and late-ore quartz have elevated concentrations of Al, in contrast to post-ore luminescent quartz that contains low trace element concentrations. Fluid inclusion microthermometry determined that ore-stage fluids had low salinities, generally <5 wt % NaCl equiv, and were trapped at 180° to 240°C. The δ 18O values, calculated for inclusion fluids and determined for quartz using in situ ion probe and conventional analyses, indicate that the isotopic composition of hydrothermal fluids became 18O depleted with time, from the ore stage through the late-ore stage to the post-ore stage. The δ 18O composition of each stage of quartz also varies spatially across the Betze-Post deposit, and δ 18OVSMOW values of ore-, late-ore, and post-ore stage quartz became more depleted with increasing distance from the master Post fault. These patterns are interpreted to result from increasing dilution of the 18O-enriched ore fluid by unevolved meteoric fluids over time and with increased ore fluid transport distance to the west, away from the Post fault, the master fault that transmitted hydrothermal fluids to the deposit. Oxygen isotope results are consistent with a low fluid/rock ratio during upward transport of the ore fluid via the Post fault. Below the deposit, some ore fluids were diverted into the JB Series faults, associated with the Betze ore zones, and the Shalosky fault, associated with the Screamer ore zone in the western part of the deposit. Ore fluids were increasingly diluted by meteoric fluids with distance as ore fluids traveled away from the Post fault and over time as different stages of quartz precipitated. Acidic, 180° to 240°C ore fluids accessed the level of the deposits where they reacted with the Devonian Popovich Formation and precipitated nonluminescing, Al-enriched ore-stage jasperoid and late-ore drusy quartz, along with Au and trace element-rich pyrite. Fluid-rock reaction, dissolution, and replacement of silty carbonate host rocks may have produced Al that substituted for Si in non-luminescing jasperoid and late-ore stage quartz. As the hydrothermal ore system began to cool and collapse, late-ore drusy quartz and realgar precipitated in open space as ore-stage pyrite precipitation declined. The hydrothermal system collapsed with meteoric water flooding; post-ore minerals, including luminescent, Al-poor post-ore drusy quartz and later calcite and barite, precipitated either from latest Carlin-related fluids, neutralized by infiltration of meteoric waters, or as part of a later, separate hydrothermal event.

Paleofluid analysis from fracture-fill cements in the Asmari limestones of the Kuh-I-Mond field, SW Zagros, Iran

Kuh-I-Mond field in the Zagros foreland basin is a conventional heavy oil resource and is composed of fractured carbonates whose fractures were filled by calcite, dolomite, and anhydrite cements. Oil inclusions occurred within the fracture-fill cements indicate that fractures were open and played an active role during oil migration and charge. The highest measured values for secondary porosities belong to fractures in Asmari Formation, which is characterized by significant amounts of vug- and fracture-filling cements. Fractures facilitated fluid circulation and subsequently dissolution of allochems and high Mg carbonates. In contrast, fine-grained carbonate facies were less cemented, and thus, porosity enhancement by cement dissolution was insignificant. Temperature profiles of oil inclusions in the dolomite, calcite, and anhydrite minerals characterized by distinct variations in the homogenization temperatures (Th) that are divided into two ranges below 50°C in anhydrites and from 45°C to 125°C in dolomites and calcites. The lower Th ranges for anhydrite suggests that it may have formed at shallower burial depths during early to middle diagenesis. The oil inclusions display trend for increasing temperature downward which conform to Formation geothermal gradient. In other word, the decreasing trend of Th temperatures upward within Asmari Formation that can be observed in Th versus depth plot is consistent with the uplift events at Late Miocene time and later that caused removal of about 1,300 m of the crest of the Kuh-I-Mond anticline. Vitrinite reflectance data from source rock intervals in the field area do not support vertical migration of locally generated hydrocarbons into the Kuh-I-Mond accumulation, and long-distance lateral oil migration and charge from a source kitchen to the southwest is proposed. Vitrinite reflectance data from this dolomite and limestone reservoir suggest low maturation levels corresponding to paleotemperatures less than 50°C. The observed maturation level (<0.5% Ro) does not exceed values for simple burial maturation based on the estimated burial history. Also, homogenization temperatures from fluid inclusion populations in calcite and dolomites show expected good correlation with reflectance-derived temperatures. The Th data represent pore fluids became warmer and more saline during burial. As aqueous fluid inclusions in calcite veins were homogenized between 22°C and 90°C with a decrease in salinity from 22 to 18 eq. wt.% NaCl. The Th values suggest a change in water composition and that dolomite and calcite cements might have precipitated from petroleum-derived fluids. The hydrocarbon fluid inclusions microthermometry data suggest that the reservoir was being filled by heavy black oils in reservoir during Cenozoic. Aqueous fluid inclusions hosted by calcite equant sparry/fossil cavity fills suggest low cementation temperatures (<45°C) and high salinities (19 eq. wt.% NaCl), while those in dolostones are characterized by highly variable homogenization temperature (52°C to 125°C) and salinities (6.5 to 20 eq. wt.% NaCl).

Fluid inclusion study of the Tangjiaping Mo deposit, Dabie Shan, Henan Province: implications for the nature of the porphyry systems of post-collisional tectonic settings

The Tangjiaping porphyry Mo deposit in Shangcheng County is located in the Dabie continental collisional orogenic belt. The hydrothermal alteration and mineralization processes can be divided into three stages: K-feldspar + quartz + pyrite + magnetite + molybdenite in the early stage, quartz + pyrite + molybdenite ± chalcopyrite in the middle stage, and quartz + carbonate ± pyrite veins or carbonate veinlets in the late stage. The middle stage involved important Mo mineralization. Four compositional populations of fluid inclusions (FIs) occur in hydrothermal quartz formed in the early and middle stages, namely: pure CO2, CO2–H2O, daughter mineral-bearing, and NaCl–H2O. The late-stage quartz-carbonate ± pyrite veinlets contain only NaCl–H2O FIs. Homogenization temperatures of the early-stage FIs are mainly above 375°C, with salinities up to 62.10 wt.% NaCl equiv. Haematite daughter minerals, which probably represent an oxidizing environment, together with halite, sylvite, and chalcopyrite, are present in the FIs of this stage. Most of the middle-stage FIs homogenized between 235 and 335°C, with fluid salinities ranging from 1.06 to 45.87 wt.% NaCl equiv. In middle-stage quartz, besides halite and sylvite daughter minerals, abundant chalcopyrite and jamesonite are also present, reflecting a reducing environment. The daughter mineral-bearing FIs coexist with vapour- and liquid-rich FIs with contrasting salinities, and they homogenized in divergent ways at similar temperatures. This feature strongly suggests that fluid boiling occurred during the middle stage, which is well accepted as an important mechanism for the precipitation of ore-forming materials. FIs in late-stage minerals totally homogenized in the range 115–195°C, with low salinities ranging from 1.91 to 9.98 wt.% NaCl equiv. Combined with the published H–O isotope data, we propose that the initial ore fluids were magmatic in origin and were characterized by high temperature, high salinity, high oxygen fugacity, high levels of metallic elements, and high levels of CO2. In the middle stage, the fluids boiled and resulted in CO2 release, oxygen fugacity decrease, and rapid precipitation of ore-forming materials. The late-stage fluids, characterized by low temperatures, low salinities, and low contents of CO2, might have been sourced from meteoric water. Daughter mineral-bearing CO2–H2O FIs, especially multiple species daughter mineral-bearing FIs, are regarded as indicators of porphyry ore systems generated in continental collisional settings. This understanding is validated by recent studies of other porphyry ore systems in Chinese collision orogens.

Evolution of Magmatic Vapor to Gold-Rich Epithermal Liquid: The Porphyry to Epithermal Transition at Nevados de Famatina, Northwest Argentina

Fluid inclusions in the subeconomic porphyry Cu-Mo-Au system at Nevados de Famatina and closely associated high-sulfidation epithermal Cu-Au-(Ag-As-Sb-Te) veins at La Mejicana, northwest Argentina, were studied to reconstruct the evolution of hydrothermal fluids from their deep magmatic source to the shallow epithermal environment. Field geology, vein petrography, fluid inclusion microthermometry, and single-inclusion microanalysis by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) were combined to determine the evolution of pressure, temperature, and ore metal concentrations in the fluids. Cathodoluminescence imaging complements transmitted light petrography to constrain the successive stages of quartz formation and the entrapment sequence of the fluid inclusion populations. Aqueous liquid inclusions of ~5 wt percent NaCl equiv salinity trapped in quartz-sericite-pyrite (QSP) veins between 360° and 325°C contain unusually high concentrations of Cu (400–5,000 ppm), As (~200 ppm), Sb and Te (both up to 100 ppm), and Au (several ppm) along with other ore-forming elements. These veins are paragenetically transitional between subeconomic porphyry copper mineralization exposed in the valley floor, and high-sulfidation epithermal veins with high Au grades, which are preserved along a high ridge adjacent to the porphyry stock. Low-density vapor and hypersaline liquid (i.e., brine) inclusions were trapped in early-formed quartz of the porphyry stockwork veins, between 450° and >600°C. The brine contains lower Cu and Au concentrations than coexisting vapor inclusions and texturally even earlier inclusions of intermediate density that are trapped in phenocrysts and some stockwork veins; the latter may be equivalent to a parental fluid. The low- and intermediate-density fluids compositionally overlap in salinity with the aqueous liquids recorded by the later transitional quartz-sericite-pyrite veins. Fluid inclusions and geologic time relations indicate that the transitional quartz-sericite-pyrite veins were the channelways for metal-rich aqueous liquids that generated the high-sulfidation epithermal Cu-Au deposit. These mineralizing liquids are interpreted to have evolved by continuous density increase (“contraction”) from a low-salinity, S-rich magmatic fluid of low to intermediate density. This vaporlike fluid was produced by exsolution from a magma that existed at greater depth during the late stages of cooling of the magmatic-hydrothermal complex, and probably underwent minor brine separation prior to reaching the present level of exposure. Epithermal mineral precipitation occurred upon dilution of the low-salinity magmatic fluid with meteoric water, which entered the hydrothermal system as it was cooled and successively eroded during continued magmatic fluid ascent.

Geochemical modelling of a Zn–Pb skarn: Constraints from LA–ICP–MS analysis of fluid inclusions

The Bismark deposit (northern Chihuahua, Mexico) is one of several base metal-rich high-temperature, carbonate-replacement deposits hosted in northern Mexico. Previous fluid inclusion studies based on microthermometry and PIXE have shown that the Zn-rich, Pb-poor Bismark deposit formed from a moderate salinity magmatic fluid [Baker, T. and Lang, J.R., 2003. Reconciling fluid inclusion types, fluid processes, and fluid sources in skarns: an example from the Bismark Deposit, Mexico. Mineralium Deposita 38(4), 474–495; Baker, T., van Achterberg, E., Ryan, C.G. and Lang, J.R., 2004. Composition and evolution of ore fluids in a magmatic-hydrothermal skarn deposit. Geology 32(2), 117–120]. The exact precipitation mechanisms are unclear and may have due to cooling, salinity decrease and wall rock reaction. Furthermore, PIXE data suggested that Pb and Zn concentrations were comparable and inconsistent with the Zn-rich nature of the ore. However, Pb was commonly below the limit of detection for PIXE and the data presented by Baker et al. [Baker, T., van Achterberg, E., Ryan, C.G. and Lang, J.R., 2004. Composition and evolution of ore fluids in a magmatic-hydrothermal skarn deposit. Geology 32(2), 117–120] are regarded as the maximum concentrations of Pb in the fluid. In this study new LA ICP MS analysis was carried out on the same fluid inclusion population to compare with the PIXE data in order to constrain the uncertainty related to the Pb data and the new results are used to model possible ore deposition mechanisms. The new laser ablation data reveal overall lower concentrations of Pb in the ore fluid (average value ~ 285 ppm) than previously indicated by PIXE analysis (average value ~ 713 ppm). Chemical modelling using the new laser ablation data tested the following ore deposition processes: 1) cooling; 2) fluid–rock reaction at constant temperature; 3) cooling and simultaneous fluid–rock interaction. Modelling results show that the gangue and ore minerals observed at Bismark are best reproduced by fluid–rock interaction and simultaneous cooling. Results from the simulations strongly indicate that ore deposition was mainly driven by a pH increase due to the neutralization of the acidic ore fluid (pH = 3.9) as the result of the reaction with the limestone. Modelling results also suggest that the deposit likely formed under cooling conditions, but do not support the hypothesis of a temperature decrease as the principal ore-forming process.