Primary Aqueous Inclusions Research Articles

Paleofluid Fingerprint as an Independent Paleogeographic Correlation Tool: An Example from Pennsylvanian Sandstones and Neighboring Crystalline Rocks (Tisia Composite Terrane, S Hungary)

In the basement areas of the southern Pannonian Basin, Central Europe (Tisia Composite Terrane, Hungary), Variscan blocks are essential components. The existing paleogeographic reconstructions, however, are often unclear and contradictory. This paper attempts to give a contribution for paleogeographic correlation of the Tisia using paleohydrological features (e.g., vein mineralization types, inclusion fluid composition and origin) of the Pennsylvanian continental succession and neighboring crystalline complexes. Vein-type mineralization in the studied samples dominantly forms blocky morphological types with inclusion-rich quartz and carbonate crystals. The evolution of hydrothermal mineralization and host rock alteration in the study area comprises three major stages. The first one is characterized by chloritization, epidotization, and sericitization of metamorphic rocks together with subsequent formation of Ca-Al-silicate and quartz-sulfide veins (clinopyroxene-dominant and epidote-dominant mineralization). The related fluid inclusion record consists of high-temperature and low-salinity aqueous inclusions, corresponding to a reduced retrograde-metamorphic fluid phase during the Late Westphalian (~310 Ma). The next mineralization stage can be related to a generally oxidized alkaline fluid phase with a cross-formational character (hematite-rich alkali feldspar-dominant and quartz-dolomite veins). High-salinity primary aqueous inclusions probably were originated from the Upper Permian playa fluids of the region. The parent fluid of the third event (ankerite-hosted inclusions) was derived from a more reductive and low-salinity environment and can represent a post-Variscan fluid system. Fluid evolution data presented in this paper support that the W Tisia (Mecsek–Villány area) belonged to the Central European Variscan belt close to the Bohemian Massif up to the Early Alpine orogenic phases. Its original position is presumably to the northeast from the Bohemian Massif at the Late Paleozoic, north to the Moravo-Silesian Zone. The presented paleofluid evolution refines previous models of the paleogeographic position of the Tisia and puts constraints on the evolution of the Variscan Europe.

The Use of Integrated Fluid Inclusion Studies for Constraining Petroleum Charge History at Parsons Pond, Western Newfoundland, Canada

This study, based on fluid inclusion petrography, microthermometry and ultraviolet microspectroscopy of inclusion oil, investigates the petroleum charge history at Parsons Pond, western Newfoundland. To address this matter, drill core and cuttings samples of allochthonous and autochthonous strata in the Parson’s Pond area were collected from three exploration wells. Fluid inclusions were examined from fragments of calcite and quartz veins, diagenetic cements in sandstone, and in large hydrothermal dolomite and calcite crystals. Primary aqueous inclusions in authigenic sandstone cements indicate that cementation occurred at relatively shallow depths and low temperatures (<50 °C). Hydrocarbon-bearing fluid inclusions (petroleum, wet gas and gas) are generally restricted to calcite and quartz veins, indicating that petroleum and gas migration at Parson’s Pond is fracture-controlled. No hydrocarbons were observed in the diagenetic cements of the essentially tight sandstones. Fluid inclusion microthermometry and ultraviolet microspectroscopy indicate the presence of multiple generations of hydrocarbon fluid, ranging in composition from ~33 API gravity petroleum to pure CH4. Petrographic evidence suggests that hydrocarbons were generated multiple times during progressive burial and heating. In addition, the distribution of hydrocarbon bearing inclusions with depth suggests that deeper levels are gas-prone, with petroleum confined to relatively shallow depths. Although only gas flow was encountered during the drilling of exploration wells at Parson’s Pond, the presence of petroleum-bearing fluid inclusions in calcite and quartz veins indicates that the historical production from shallow wells in the Parsons Pond area likely tapped small reservoirs of fractured petroliferous strata.

Fluid evolution in the Strange Lake granitic pluton, Canada: Implications for HFSE mobilisation

Strange Lake is a mid-Proterozoic peralkaline granite pluton (Québec-Labrador, Canada) that underwent extreme enrichment in high field strength elements (HFSE), including the rare earth elements (REE). The HFSE mineralisation is confined to highly altered pegmatites and the most altered parts of the granites, implying a genetic association between hydrothermal fluids and HFSE enrichment. This study uses analyses of fluid inclusions to investigate the hydrothermal evolution of the Strange Lake pluton and the role of hydrothermal processes in concentrating the HFSE to potentially exploitable levels.Five groups of inclusions were distinguished. From earliest to latest, these groups are: primary aqueous inclusions (~25wt% NaCl eq.) associated with melt inclusions (Group 1); primary aqueous inclusions (13–23wt% NaCl eq.) associated with CH4 inclusions (Groups 2a and b); primary aqueous inclusions (~9wt% NaCl eq.) associated with CO2 inclusions (Group 3); primary aqueous inclusions (~9wt% NaCl eq.), which contain no carbonic component (Group 4); and finally, secondary aqueous inclusions (19wt% NaCl eq.), including inclusions that outline mineral pseudomorphs, also with no detectable carbonic component (Group 5). Most of the inclusions (except those in Group 5) have conspicuous ‘implosion’ haloes, evident as numerous tiny voids. This indicates that the inclusions re-equilibrated, most likely during isobaric cooling.Fluid evolution commenced with the exsolution of a saline aqueous liquid (~25wt% NaCl eq.) and an immiscible CH4+H2 gas from the pegmatitic melt at temperatures of ~450–500°C and a pressure of ~1100bars. During isobaric cooling, the gas component of the fluid was gradually oxidised, evolving from being CH4-dominant to a CH4 fluid with a significant proportion of higher order hydrocarbons (due to oxidative coupling of methane induced by the consumption of O2 through the alteration of arfvedsonite to aegirine; ~325–360°C), and finally to a CO2-dominated fluid at ~300°C. The apparent salinity of the aqueous fluid decreased from ~25 to ~4.5wt% NaCl eq. due to fluid-rock interaction. The latter also caused precipitation of nahcolite (as a result of the reaction of newly formed CO2 with sodium from decomposing minerals), formation of pseudomorphs after primary Na-zirconosilicates and Na-titanosilicates and replacement of primary REE-silicates by bastnäsite-(Ce). Owing to this interaction, the carbonic component of the fluid was consumed which, together with the consumption of H2O to form Al-, K- and Fe-phyllosilicates, contributed to an increase in the fluid salinity (up to ~19wt% NaCl eq.).Light rare earth elements (LREE) were remobilised over 10s to 100s of metres by the high temperature high salinity fluid (preserved as Group 1 and 2a inclusions), whereas heavy rare earth elements (HREE) were remobilised on a much smaller scale by a late, low temperature high salinity fluid (trapped as inclusion Group 5). Fluid preserved as inclusion Group 3 may have been responsible for remobilisation of Zr and Ti.

New insights into the origin of the Evate apatite-iron oxide-carbonate deposit, Northeastern Mozambique, constrained by mineralogy, textures, thermochronometry, and fluid inclusions

The Evate deposit represents the largest resource of apatite in south-east Africa (155 Mt. ore grading 9.3wt.% P2O5) accumulated in up to 100m thick magnetite-carbonate-apatite horizons conformable to the granulitic gneiss of the Monapo Klippe. Baddeleyite and zircon from early iron-oxide (magnetite, geikielite, spinel), apatite- and forsterite-bearing rocks have been dated to 590±6Ma using the LA-ICPMS U-Pb method, whereas monazites from anhydrite-apatite-carbonate rocks show a concordant U-Pb-Th age corresponding to 449±2Ma. Temperatures inferred from calcite-dolomite solvus data and graphite structural ordering span the interval from ≥815 to 276°C. Primary and secondary fluid inclusions in apatite document calciocarbonatite melts associated with early apatite, and CO2-bearing sulfate-chloride brines progressively diluted with low-salinity, probably metoric waters, towards ultimate stages of the deposit formation. The calciocarbonatite melts have initially coexisted with liquid nitrogen and later with sulfate-chloride brines mixed with N2±CO2 gas.Crystallization of spinel around baddeleyite by the mechanism of Ostwald ripening, nucleation of graphite spherules along pyrrhotite-carbonate boundaries, the occurrence of molybdenite, baddeleyite-to-zircon transformation, and high crystallization temperatures inferred from graphite structural ordering and calcite-dolomite thermometry suggest a magmatic origin of the early mineral assemblages. In contrast, microthermometric characteristics of primary aqueous inclusions in the late apatite and the presence of zeolites (thomsonite-Ca, mezolite) is diagnostic of a low-temperature hydrothermal crystallization.Formation of the early magnetite-apatite-forsterite assemblage is thought to be coeval with mafic alkalic intrusions of the Mazerapane Suite superimposed on the granulite facies metamorphism of the Monapo Klippe. The low-temperature, anhydrite-bearing mineralization was associated with the massive circulation of sulfate-rich brines along fractures activated during the Late Cambrian-Ordovician extension. Origin of the sulfate-rich brines may be genetically related either with the magmatic-hydrothermal differentiation, or with the remobilization of crustal evaporites.

Hydrothermal mineralization in the sandstone–hosted Hangjinqi uranium deposit, North Ordos Basin, China

Tabular–type uranium ore deposits (the Hangjinqi and Daying deposits) have recently been found in the Middle Jurassic Zhiluo Formation, north of the Ordos Basin, China. Petrographic observations, the chemical composition of U minerals determined by EMPA and fs–LA–ICP–MS, whole rock geochemistry and the microthermometric study of fluid inclusions have been integrated to characterize the genetic conditions of the U mineralization in the Hangjinqi sandstone–hosted deposit. Two different groups of U minerals have been identified. One group includes coffinite(I) associated with vanadium–rich micas. Coffinite(I) is enriched in vanadium (V) and devoid of iron (Fe) and yttrium (Y) and has a LREE–enriched chondrite–normalized REE pattern. The U minerals of this group are similar to meteoric fluid infiltration related deposits. The second group has coeval coffinite(II) and coarsely crystalline calcite cement. Coffinite(II) is enriched in Y and Fe and depleted in V and is marked by a flat chondrite–normalized REE pattern, which is compatible with typical hydrothermal genetic deposits with high–salinity mineralizing fluids. The temperature and salinity of the primary aqueous inclusions in the ore–stage calcite are 120–180°C and 8.00–16.34% (eq. wt% NaCl), respectively. These mineral assemblages, temperatures and salinities indicate that the Hangjinqi deposit was affected by two distinct types of ore–bearing fluids: low–salinity meteoric waters and high–salinity hydrothermal fluids. The meteoric fluids event began at 97±5Ma with the titling of the northern Ordos Basin and the uplift of the Hetao region to the north. Hydrothermal U mineralization occurred since 39±2Ma with the rifting of the Hetao graben. Thus, the previous biogenic model for the U mineralization should be modified in the uraniferous region of the north Ordos Basin.

Hydrothermal alteration and Cu–Ni–PGE mobilization in the charnockitic rocks of the footwall of the South Kawishiwi intrusion, Duluth Complex, USA

In the Neoarchean (~2.7Ga) contact metamorphosed charnockitic footwall of the Mesoproterosoic (1.1Ga) South Kawishiwi intrusion of the Duluth Complex, the primary metamorphic mineral assemblage and Cu–Ni–PGE sulfide mineralization is overprinted by an actinolite+chlorite+cummingtonite+prehnite+pumpellyite+quartz+calcite hydrothermal mineral assemblage along 2–3cm thick veins. In calcite, hosted by the hydrothermal alteration zones and in a single recrystallized quartz porphyroblast, four different fluid inclusion assemblages are documented; the composition of these fluid inclusions provide p–T conditions of the fluid flow, and helps to define the origin of the fluids and evaluate their role in the remobilization and reprecipitation of the primary metamorphic sulfide assemblage.Pure CO2 fluid inclusions were found as early inclusions in recrystallized quartz porphyroblast. These inclusions may have been trapped during the recrystallization of the quartz during the contact metamorphism of the footwall charnockite in the footwall of the SKI. The estimated trapping pressure (1.6–2.0kbar) and temperature (810–920°C) conditions correspond to estimates based on felsic veins in the basal zones of the South Kawishiwi intrusion.Fluid inclusion assemblages with CO2–H2O–NaCl and CH4–N2–H2O–NaCl compositions found in this study along healed microfractures in the recrystallized quartz porphyroblast establish the heterogeneous state of the fluids during entrapment. The estimated trapping pressure and temperature conditions (240–650bar and 120–150°C for CO2–H2O–NaCl inclusions and 315–360bar and 145–165°C for CH4–N2–H2O–NaCl inclusions) are significantly lower than the p–T conditions (>700°C and 1.6–2kbar) during the contact metamorphism, indicating that this fluid flow might not be related to the cooling of the Duluth Complex and its contact aureole. The presence of chalcopyrite inclusions in these fluid inclusions and in the trails of these fluid inclusion assemblages confirms that at least on local scale these fluids played a role in base metal remobilization. No evidences have been observed for PGE remobilization and transport in the samples. The source of the carbonic phase in the carbonic assemblages (CO2; CH4) could be the graphite, present in the metasedimentary hornfelsed inclusions in the basal zones of the South Kawishiwi intrusion.The hydrothermal veins in the charnockite can be characterized by an actinolite+cummingtonite+chlorite+prehnite+pumpellyite+calcite (I–II)+quartz mineral assemblage. Chlorite thermometry yields temperatures around 276–308°C during the earliest phase of the fluid flow. In the late calcite (II) phase, high salinity (21.6–28.8 NaCl+CaCl2 equiv. wt.%), low temperature (90–160°C), primary aqueous inclusions were found. Chalcopyrite (±sphalerite±millerite), replacing and intersecting the early hydrothermal phases, are associated to the late calcite (II) phase. The composition of the formational fluids in the Canadian Shield is comparable with the composition of the studied fluid inclusions. This suggests that the composition of the fluids did not change in the past 2Ga and base metal remobilization by formational fluids could have taken place any time after the formation of the South Kawishiwi intrusion.Sulfur isotope studies carried out on the primary metamorphic (δ34S=7.4–8.9‰) and the hydrothermal sulfide mineral assemblage (δ34S=5.5–5.7‰) proves, that during the hydrothermal fluid flow the primary metamorphic ores were remobilized.

Paleohydrogeological and thermal events recorded by fluid inclusions and stable isotopes of diagenetic minerals in Lower Cretaceous sandstones, offshore Nova Scotia, Canada

Upper Jurassic–Lower Cretaceous sandstones of the Scotian Basin, offshore eastern Canada, are important gas reservoirs. Previous studies of fluid inclusions in Jurassic limestones and apatite thermochronology of Cretaceous sandstones have suggested a late Mesozoic thermal event. Fluid inclusions in different authigenic cements were analyzed to determine the temperature and composition of basinal fluid at the time of precipitation and the timing of hydrocarbon migration and entrapment. Fifty-one sandstone samples were analyzed for stable isotope composition (18O and 13C) of carbonate cements. Trapping temperatures for primary aqueous inclusions hosted in quartz overgrowths (89–175C) and in late carbonate cements (138–173C) are higher than predicted by two-dimensional modeling from burial alone based on postrift geothermal gradients. These inclusions contain high-salinity fluids (mostly 19–22 wt. % NaCl equivalent). Second, predominantly aqueous inclusions have much lower salinities (5.2–6.7 wt. % NaCl equivalent), and some contain liquid hydrocarbons. Late Fe-calcite cement in Hauterivian sandstone shows negative values of (13.17 to 9.2), whereas cements in deeper and shallower sandstones have higher 13C values. These data indicate that high-temperature, high-salinity fluids preceded hydrocarbon migration. The oldest and youngest rocks studied show less effect of high temperatures than do rocks that were buried to depths of at least 2 km (1.2 mi) after 135 Ma and before 100 Ma. Both fluid inclusions and 13C of carbonate cement suggest that highest temperatures were achieved during burial at 115 to 105 Ma. This Early Cretaceous thermal event had a geothermal gradient of at least 55C/km. It is earlier than the previously reported event based on apatite fission tracks but is consistent with the fission track data. The event is synchronous with regional evidence of volcanism, and its peak coincides with rapid salt-tectonic deformation in the deep basin. Hydrocarbon charge to the outer-shelf wells occurred after this thermal event and thus not before the Late Cretaceous.

Thermal and hydrocarbon-charge history and the relationship between diagenesis and reservoir connectivity: Venture field, offshore Nova Scotia, eastern Canada1Geological Survey of Canada Contribution 20100418.

Fluid inclusions in diagenetic cements in Upper Jurassic – Lower Cretaceous sandstones offshore Nova Scotia provide constraints on the fluid migration history in gas reservoirs of the Scotian basin. Diagenetic minerals from six wells in the Venture field were analysed by optical petrography, scanning electron microscopy (SEM), and electron microprobe. A total of 122 primary and secondary fluid inclusions were analysed from different cements. Primary aqueous inclusions in quartz overgrowths have homogenization temperatures (Th) of 111.8 ± 7.1 °C (1σ) and in later carbonate cements 126.5 ± 2.1 °C; inclusions in both cements are highly saline (16–26.1 wt.% NaCl equivalent). Secondary aqueous and hydrocarbon-bearing inclusion trails crosscutting silica cement and detrital quartz have Th of 121.6 ± 13.6 °C and low salinities (8.7 ± 6.0 wt.%). Secondary carbonic inclusions have CO2 melting temperatures (–56.6 ± 0.1 °C) and Th (–9.3 ± 0.8 °C) indicating a high-density carbonic phase. Late carbonate cements in the same sandstone units vary in chemical composition in different wells, and connected reservoirs show similar late carbonate assemblages, suggesting that the late carbonate cementation may be partly controlled by the reservoir fill and spill sequence. Silica and late carbonate cementation involved highly saline fluid flow, likely at about ∼135 Ma. Hydrocarbon migration postdated silica cementation and was associated with secondary fracturing, suggesting that it corresponded to the onset of overpressure.

Origin of ferroan alabandite and manganoan sphalerite from the Tisovec skarn, Slovakia

Sulphidic skarn in an exo-contact zone between diorite and Triassic carbonates contains the Mn-rich minerals ferroan alabandite (Mn0.84-0.89Fe0.11-0.16S), manganoferroan sphalerite (Zn0.37-0.76Mn0.01-0..36Fe0.22-0.28S), hubnerite, manganoan magnesio horn- blende, and Mn-carbonates. The Mn-content in sphalerite (up to 21.4 wt. %) is the highest ever recorded in hydrothermal settings. Magnetite and sulphides like galena, pyrite, pyrrhotite, arsenopyrite are also present. Anhydrite is ubiquitous as inclusions in sulphide minerals and crystals in drusy cavities, postdating hydrothermal Ca-Mn-Fe carbonates. Primary aqueous inclusions with variable salinity (2.9-44.8 wt. % NaCl eq.) occur in sphalerite and Mn-calcite. The fl uid inclusions locally exhibit textural evidence for heterogeneous entrapment of boiling fl uids. Crystallization temperatures of 300-350 °C were obtained from various mineral geothermometers, δ 34 S values in coexisting galena and sphalerite, and infrared fl uid inclusion microthermometry. The observed anhydrite-chalcopyrite-magnetite-pyrrhotite-arsenopyrite-alabandite assemblage constrains fO2 and fS2values to -34.5 ± 1.5 and -12.5 ± 0.5 log units, respectively, at 300 °C and atmospheric pressure. Together with an assemblage of silicate minerals (chlorite, sericite, K-feldspar, hyalophane) and high-salinity fl uids, the calculated oxygen and sulphur fugacities are consistent with an inter- mediate sulphidation hydrothermal system. The increased δ 34 S values of the ore-forming fl uids, 2-2.5 ‰, result from the disequi- librium isotopic exchange between mantle-derived H2S and marine sulphate.

Hydrocarbon migration in the Porcupine Basin, offshore Ireland: evidence from fluid inclusion studies

ABSTRACT A petrographic and microthermometric study of fluid inclusions in Jurassic and Cretaceous sandstones from the Porcupine Basin, offshore Ireland was integrated with innovative fluorescence lifetime measurements of hydrocarbon-bearing fluid inclusions to determine the compositions of the fluids associated with diagenesis and post-diagenetic fluid migration and the extent of hydrocarbon and aqueous fluid migration pathways. Petrographic analyses indicate that Jurassic strata were the main fluid migration pathways for hydrocarbon fluids and that hydrocarbon migration occurred relatively late in the diagenetic history of these sandstones. UV fluorescence and fluorescence lifetime measurements have recognized at least two chemically distinct hydrocarbon groups (Types 1a and 1b) with dissimilar lifetime-wavelength (τ-λ) profiles, consistent with at least two petroleum charges derived from different sources. Primary aqueous inclusions in authigenic cements show that cementation in Cretaceous sandstones occurred at relatively shallow levels at low temperatures (

Geothermobarometry and fluid inclusions of dioritic rocks in Bangladesh: Implications for emplacement depth and exhumation rate

Application of hornblende thermobarometry and fluid inclusion studies to the Palaeoproterozoic (1.7 Ga) basement rocks from Maddhapara, NW Bangladesh, provide information on the pressure and temperature ( P–T) conditions of crystallization, the emplacement depth and composition of magmatic fluid. The basement rocks are predominantly diorite or quartz diorite with a mineral assemblage of plagioclase, hornblende, biotite, quartz, K-feldspar, titanite, and secondary epidote and chlorite. The calculated P–T conditions of the dioritic rocks are 680–725 °C and 4.9–6.4 kbar, which probably correspond to crystallization conditions. Fluid inclusion studies suggest that low- to medium-salinity (0–6.4 wt.% NaCl eq) H 2O-rich fluids are trapped during the crystallization of quartz and plagioclase. The isochore range calculated for primary aqueous inclusions is consistent with the P–T condition obtained by geothermobarometry. The basement rocks likely crystallized at a depth of ∼17–22 km, with a minimum average exhumation rate of ∼12–15 m/Ma during Palaeoproterozoic to Lopingian time. Such slow exhumation indicates low relief continental shield surface during this period.

Fluid inclusions study of Abukuma metamorphic rocks from the Hanazono district, northeast Japan

A systematic fluid inclusion study on pelitic and granitic gneisses of the Cretaceous Abukuma metamorphic rocks, exposed in the Hanazono district, northeast Japan, identified three categories of fluid inclusions: primary aqueous inclusions in porphyroblastic garnet, primary to pseudosecondary carbonic inclusions in quartz, and secondary aqueous inclusions in quartz. The primary aqueous inclusions in garnet exhibit melting and homogenization temperatures of −2.5 to −0.3 °C and +168.2 to +360.0 °C, respectively. The dominant primary to pseudosecondary fluid inclusions in undeformed quartz display melting temperatures of −64.2 to −56.6 °C, indicating a CO2-rich composition with additional CH4 and/or N2. The carbonic inclusions can be divided into high-density (1.121-1.153 g/cm3) and medium-density (0.683-1.111 g/cm3) types. The fluid densities of the latter inclusions, when translated into isochores, indicate entrapment of CO2 at the peak P-T conditions experienced by rocks in the study area (2.8-5.2 kbar at 770-850 °C). In contrast, the isochores of high-density category in quartz and aqueous inclusions in garnet do not intersect the expected P-T trajectory, yet are consistent with the prograde high-pressure condition of the Abukuma metamorphic rocks in the Takanuki district (8-10 kbar at 700-750 °C). Isochores calculated for carbonic inclusions in granitic gneiss intersect the P-T path at 2-3 kbar at 650 °C, suggesting that the fluids were captured during the retrograde stage. It is inferred that H2O was the dominant fluid species during the prograde stage, while CO2 was more abundant during the peak to retrograde stages of the Abukuma metamorphic belt. The prograde H2O-bearing fluid was probably derived from the break down of hydrous minerals. Although the origin of carbonic fluid is not known, the presence of CO2-rich fluid during peak metamorphism probably lowers the H2O activity in the rocks. The isochores computed for secondary aqueous inclusions in quartz give very low P-T conditions (300 °C and 1.7-3.5 kbar), suggesting that the fluids are of retrograde origin.

Geology, fluid inclusions, isotope geochemistry, and geochronology of the Paishanlou shear zone-hosted Gold Deposit, North China Craton

The Paishanlou gold deposit lies along the northern margin of North China Craton gold province, the third largest Au province in China. The deposit is mainly hosted in Archean metamorphic rocks of the Jianping Group and is structurally controlled by two sets of ductile shear zones. Gold mineralization is closely associated with intense hydrothermal alteration along the ductile shear zones, with a typical greenschist facies alteration assemblage of sericite + chlorite + calcite + biotite + quartz and a distinct alteration zoning of inner pyrite-sericite zone, middle carbonate zone, and outer epidote-biotite-chlorite zone from orebody to wall rock. Fluid inclusion petrography and microthermometric results suggest that four types of fluid inclusions are present at the deposit. Type 1 CO 2-rich inclusions have homogenization temperatures of 22.4–27.9 °C. Type 2 H 2O–CO 2 inclusions show salinities of 4.6–12.3 wt.% NaCl equivalent and Th-tot of 242 to 337 °C. Type 3a primary aqueous inclusions have salinities of 5.9–11.2 wt.% NaCl equivalent and Th-tot of 187–289 °C, while Type 3b secondary aqueous inclusions have salinities of 3.1–8.3 wt.% NaCl equivalent and Th-tot of 128–239 °C. Type 4 multiphase inclusions have salinities ranging from 31.4 to 35.2 wt.% NaCl equivalent and Th-tot of 370–447 °C. The close association of CO 2-rich inclusions and H 2O-rich inclusions in groups and along the same trail suggests the presence of fluid immiscibility. Estimated P– T values range from 1.4 to 1.9 kbar at 322–417 °C. The δ 18O values of the ore-forming fluids are calculated to range from 3.8‰ to 7.4‰ and the δD values from − 87.3 to − 116.2‰, indicating a mixed source of deep-seated magmatic water and shallower meteoric water. The δ 34S values (0.3–7.5‰) of sulfide ore minerals are similar to those of both pyrite from metamorphic country rocks and whole rock sulfur of metamorphic country rocks. The 206Pb/ 204Pb, 207Pb/ 204Pb and 208Pb/ 204Pb of sulfide ores range within 16.40–17.0, 15.21–15.37, and 36.69–37.38, respectively, with the model ages of 397–727 Ma, consistent with those of Late Mesozoic granites and lying between Archean metamorphic rock lead and the Proterozoic dolomitic marble lead. These sulfur and lead features reflect the fact that the ore-forming materials originated mainly from multiple wall-rock sources. The well-defined Ar/Ar plateau age (126.6 ± 1.1 Ma) of biotite from the NNE ductile shear zone provides a possible temporal link between Au mineralization, magmatism and deformation at the Paishanlou deposit. The concordance of the biotite age with other well-constrained mineralization ages for the deposit, and the intrusion age of the granite, suggests that Au mineralization was essentially contemporaneous with late Mesozoic granitic magmatism and the second stage of mylonitization. In terms of its genetic linkage with the subduction process at the leading northern edge of the North China Craton, the Paishanlou deposit can be favored as an orogenic gold deposit. Its formation benefited from the combined action of protracted activity of ductile fault zones, hydrothermal circulation associated with granitoid intrusion and preferred structural setting favoring gold deposition. The major geodynamic switch towards generalized extensional tectonics that occurred during uplift, and thinning of the upper crust after the collisional stages in the northern edge of the North China Craton, may be the key factors controlling Au concentration and enrichment in the Paishanlou deposit.

Fluid inclusions record thermal and fluid evolution in reservoir sandstones, Khatatba Formation, Western Desert, Egypt: A case for fluid injection

A fluid inclusion and petrographic study, focused on quartz overgrowths, was performed in reservoir sandstones from the Jurassic Khatatba Formation (Salam oil field, Egypt's Western Desert). The combination of detailed fluid inclusion petrography and scanning electron microscope (SEM) cathodoluminescence imaging has allowed us to relate individual fluid inclusion assemblages, that is, the most finely discriminated groups of petrographically associated fluid inclusions, to specific growth zones of authigenic quartz, establishing the relative timing of entrapment of the inclusions. After entrapment, fluid inclusions in authigenic quartz have been preserved without reequilibration, as indicated by the narrow ranges of homogenization temperatures (<4-5 degreesC) in most fluid inclusion assemblages. Three main growth zones are distinguished under SEM cathodoluminescence in the quartz overgrowths and are termed Q1, Q2, and Q3 from the oldest to the youngest. Zone Q1 is further subdivided into three subzones and contains abundant primary aqueous inclusions. Their homogenization temperatures range from 162 to 130 degreesC, with the earliest assemblages having the highest temperatures and with some large temperature fluctuations indicated between successive assemblages. Most Q1 inclusions have salinities in the freshwater to seawater range, with a trend toward increasing salinity through time. Zone Q2 contains primary aqueous inclusions with homogenization temperatures (overall range 148-125 degreesC) also recording large temperature fluctuations and cooling events. The Q2 fluid inclusions have high salinities (~5-20 wt. % NaCl equivalent), with salinity increasing through time. Zone Q3 contains both aqueous and oil inclusions of primary origin. The Q3 aqueous inclusions have homogenization temperatures (overall range 134-112 degreesC) recording overall cooling and high salinities (21-24 wt. % NaCl equivalent). In early Q3 subzones, oil inclusions appear to be of medium gravity, undersaturated with respect to gas. In later Q3 subzones, oil inclusions are of gas-saturated lighter oil. Our results indicate that fluid flow, involving drastic changes in temperature and salinity, was responsible for the precipitation of some of the quartz cement. The earliest quartz (Q1) precipitated from freshwater and seawater at temperatures significantly higher than those expected from the burial history and thermal maturity of these rocks. This quartz is interpreted to have precipitated during cooling of injected fluids that originated as hot connate fluids from deeper parts of the basin. The Q2 precipitation is interpreted to have resulted from episodes of injection of hot saline brines from below. Late quartz cement (Q3) precipitated during oil charge, from progressively cooler and more saline brines interpreted to have refluxed from the surface; it preserves a record of increasing oil maturity and gas saturation through time.

Geology and genesis of Variscan porphyry-style gold mineralization, Petr�?kova hora deposit, Bohemian Massif, Czech Republic

A large number of Variscan mesothermal gold deposits are located in the central part of the Bohemian Massif, close to the Central Bohemian Plutonic Complex. The Petrackova hora deposit has many features that distinguish it from other deposits in the region and suggest its mineralization is closely related to the late magmatic processes associated with the Petrackova hora granodiorite. The gold ores occur as sheeted arrays of quartz veins and veinlets hosted by the small Petrackova hora granodiorite stock. Gold is found mainly as free grains of >900 fineness, and is accompanied by abundant pyrrhotite and chalcopyrite, and accessory pyrite, arsenopyrite, loellingite, and molybdenite. Molybdenite from the Petrackova hora deposit has been dated by the Re–Os method at 344.4±2.8 Ma. Hydrothermal alteration in the Petrackova hora deposit exhibits a distinct temporal paragenesis. Selectively pervasive, early K-alteration and silicification are the oldest hydrothermal phases. These were followed by early quartz veins (Q1 to Q4) that contain most of the gold mineralization. Late quartz veins (Q5) and fracture-controlled silicification are gold-poor or barren. Barren calcite veins are the youngest hydrothermal product. Extensive low-temperature, meteoric-water dominated alteration, as is typical of classic porphyry deposits, is absent. However, the lower δ18O whole rock values for Petrackova hora granodiorite and aplite (+2.4 to +5.1‰ SMOW) compared to other intrusions in the region reflect either interaction with isotopically light external fluids or magma assimilation of small volumes of hydrothermally altered country rock. The δ18O isotopic compositions for quartz, scheelite and hornblende (7.7 to 13.4‰ SMOW) and the δ34S compositions for sulfide minerals (–1 to +3.5‰ CDT) from early, gold-rich quartz veins indicate formation at high temperatures (590 to 400 °C) from fluids with a magmatic isotopic signature (δ18OFLUID of 5.7 to 7.2‰). Fluids related to late quartz veins (Q5) suggest the presence of a significant component of non-magmatic water (δ18OFLUID: +2.5 to +4.0‰). The δ34S values of post-Q5 sulfide minerals (–4.5 to –3.5‰) reflect at least partial derivation of late-stage sulfur from a source external to the intrusions. Aqueous, aqueous–carbonic and nitrogen-bearing fluid inclusions were identified in hydrothermal and igneous quartz, with the aqueous inclusions being the most common. In hydrothermal vein quartz, the salinity of primary aqueous inclusions falls into ranges 6 to 23 and 33 to 41 equiv. wt% NaCl; in igneous quartz, populations in salinity were observed between 5 to 16, 35 to 40 and 62 to 70 equiv. wt% NaCl. The salt component of these fluids is best, and minimally, approximated by the NaCl–KCl–CaCl2 system. Low- and high-salinity aqueous–carbonic inclusions are accessory in many of the analyzed samples. Three large successive pulses of fluids are recognized. Each pulse begins with a high-salinity (>30 equiv. wt% NaCl) magmatic fluid and evolves toward a lower salinity (~5 equiv. wt% NaCl) fluid. Data suggest that external (meteoric?) water(s) were significant for only the third fluid pulse, which formed the late Q5 quartz veins and the calcite veins. Polyphase fluid inclusions hosted by igneous quartz of the Petrackova hora granodiorite indicate minimum trapping conditions of about 3 kbar and 550 °C. The gold-rich Q1 to Q4 veins may have formed along a quasi-isobaric cooling path at 2.5 to 1.5 kbar and 590 to 400 °C. This was followed by uplift, and formation of late Q5 quartz veins (0.5 to 1.5 kbar; ~300 °C) and post-ore calcite veins (<0.5 kbar; 100 to 140 °C). The characteristics of the Petrackova hora deposit suggest that it may represent a position intermediate between intrusion-related gold systems (e.g., Fort Knox deposit, Alaska) and gold-rich, copper-poor porphyry deposits (e.g., Maricunga Belt in Chile). As such, the Petrackova hora deposit might be an example of the reduced gold sub-type of porphyry deposit.

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

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

Fluid inclusion characteristics of mesothermal gold deposits in the Xiaoqinling district, Shaanxi and Henan Provinces, People's Republic of China

Fluid inclusions were studied in quartz samples from early (stage I) gold-poor quartz veins and later (stage II) gold- and sulphide-rich quartz veins from the Wenyu, Dongchuang, Qiangma, and Guijiayu mesothermal gold deposits in the Xiaoqinling district, China. Fluid inclusion petrography, microthermometry, and bulk gas analyses show remarkably consistent fluid composition in all studied deposits. Primary inclusions in quartz samples are dominated by mixed CO2-H2O inclusions, which have a wide range in CO2 content and coexist with lesser primary CO2-rich and aqueous inclusions. In addition, a few secondary aqueous inclusions are found along late-healed fractures. Microthermometry and bulk gas analyses suggest hydrothermal fluids with typically 15–30 mol% CO2 in stage I inclusions and 10–20 mol% CO2 in stage II inclusions. Estimates of fluid salinity decrease from 7.4–9.2 equivalent wt.% NaCl to 5.7–7.4 equivalent wt.% NaCl between stage I and II. Primary aqueous inclusions in both stages show consistent salinity with, but slightly lower Th total than, their coexistent CO2-H2O inclusions. The coexisting CO2-rich, CO2-H2O, and primary aqueous inclusions in both stage I and II quartz are interpreted to have been trapped during unmixing of a homogeneous CO2-H2O parent fluid. The homogenisation temperatures of the primary aqueous inclusions give an estimate of trapping temperature of the fluids. Trapping conditions are typically 300–370 °C and 2.2 kbar for stage I fluids and 250–320 °C and 1.6 kbar for stage II fluids. The CO2-H2O stage I and II fluids are probably from a magmatic source, most likely devolatilizing Cretaceous Yanshanian granitoids. The study demonstrates that gold is largely deposited as pressures and temperatures fall accompanying fluid immiscibility in stage II veins.

Thermal History Constraints from Studies of Organic Matter, Clay Minerals, Fluid inclusions, and Apatite Fission Tracks at the Ardeche Paleo-margin (BA1 Drill Hole, GPF Program), France

ABSTRACT The thermal history of the Ardeche paleo-margin (southeastern France) has been reconstructed using various analytical data from minerals and organic matter and conventional interpretation. These methods comprise Tmax and vitrinite reflectance measurements on organic matter, determination of the smectite content in mixed-layer illite/smectite, fluid-inclusion microthermometry in quartz, barite, dolomite, and anhydrite, and fission-track analyses in apatite. The Balazuc (BA1) drill hole, 1730 m deep, intersects mainly Jurassic carbonate rocks, Triassic sandstones, and sulfate-rich claystones and ends in Carboniferous rocks. The present bottom-hole temperature is 70°C. Other drill holes, less than 6 km away, were studied when necessary. One of the main objectives of this paper is to ssess the agreement between the different geothermometric methods, and to check their limitations and convergence when used on carbonate- and sandstone-dominated reservoirs. In the Triassic sandstones, primary aqueous inclusions in anhydrite cement and at the boundary between the detrital quartz grain and the quartz overgrowth are one-phase brine inclusions entrapped below 70°C. The homogenization temperature determined on the later barite cement is about 100°C. Aqueous two-phase inclusions in healed microfractures in anhydrite are present at the base of the drill core near major faults. Temperatures as high as 210°C recorded for these fluid inclusions could be associated with episodic fluid injections from below through the fault zones during the Early Jurassic. The fluid inclusions in dolomite, most of them reequilibrated during burial, indicate a trapping temperature of 130-145°C at 1600 m and a temperature decrease towards the surface. This value is in good agreement with the MPTB (maximum paleotemperature of burial) method on organic matter, which converges to a maximum burial temperature of 130°C at the same depth. The apatite fission-track data agree with these temperatures and indicate that the temperature decreased below 120 ± 10°C during the Eocene. These paleotemperatures, much higher than the present ones, can be explained by the erosion of 1900 m of mainly Cretaceous sediments. This interpretation implies a high average sedimentation rate during the Cretaceous, in agreement with data determined on a regional scale. A time-temperature evolution is proposed for the formations present in the Balazuc drill hole and the eroded sedimentary rocks. The application of the dual reaction model of the transformation of smectite to illite (Velde and Vasseur 1992) shows that the data are in good agreement with the kinetic model. However, the vitrinite reflectance variation trend with depth, predicted from the Burnham and Sweeney model (1989), is different from the data variation. No definitive explanation can be proposed for the failure of aturity assessment through vitrinite reflectance in the BA1 drill hole.

Quartz Veins Record Vertical Flow at a Graben Edge: Fulmar Oil Field, Central North Sea

The Fulmar oil field lies at the faulted western margin of the Central Graben in the central North Sea. Chalcedony and quartz, and later dolomite, occur as diagenetic cements in veins within the Upper Jurassic Fulmar Formation. Quartz in one particular vein contains primary aqueous fluid inclusions with homogenization temperatures ranging from 85 degrees to 140 degrees C, with a mean of 117 degrees C. The measured d18O of the authigenic quartz ranges from +25.1 to +27.1o/oo SMOW. The d18O of the water from which most of the quartz precipitated is calculated to have been around +8o/oo, which is significantly more positive than the measured present-day formation water d18O of +4o/oo. Quartz and chalcedony cements in the vein are postdated texturally by dolomite that has a d18O of +23.5o/oo SMOW. High salinities are recorded in the fluid inclusions in the quartz, the majority ranging from 15 to 19 wt. % NaCl equivalent. Also, the quartz contains inclusions of anhydrite. High-salinity aqueous inclusions, combined with anhydrite inclusions in the quartz and the 18O-enriched calculated water composition, suggest that the quartz precipitated from an evaporite-influenced fluid. The same fluid contributed Mg for dolomite precipitation. More deeply buried sections, including the Permian Zechstein evaporites underlying the Fulmar field, are the most obvious source of an 18O-enriched saline fluid. Mineralization of the vein resulted from cross-formational flow of warm fluids from depth through the Zechstein and into the Jurassic succession over vertical distances of 600-2000 m. Fluids most probably migrated up faults and fractures during episodes of salt movement or overpressure release.

Controls on halogen concentrations in sedimentary formation waters

AbstractChlorine is the most abundant halogen in sedimentary formation waters with concentrations from &lt;100 to &gt;250000 mg/l. Bromine is the second most abundant halogen at &lt;1 mg/l to &gt;6000 mg/l with iodine from &lt;0.1 mg/l to &gt;100 mg/l and fluorine from &lt;0.1 mg/l to 30 mg/l. Chlorine and bromine show a strong systematic covariation suggesting that they are subject to the same controlling mechanisms. Fluorine only shows relatively high concentrations at elevated chlorine and bromine concentrations showing that fluorine, chlorine and bromine are possibly controlled by the same processes. Iodine does not correlate with any of the other halogens indicating that unique processes control iodine.Key geological parameters that influence chlorine and bromine (and possibly fluorine) concentrations are the presence of salt in a basin, the age of the reservoir unit and the kerogen-type within the main hydrocarbon source rock in a basin. The presence of salt in a basin shows that sea water was evaporated to halite saturation producing connate waters with high concentrations of chlorine and bromine. The presence of salt also leads to high salinity waters through water-salt interaction during burial and diagenesis. Tertiary reservoirs typically have much lower chlorine and bromine concentrations than Mesozoic or Palaeozoic reservoirs. The age of the reservoir unit may simply reflect the different amounts of time available for formation water to interact with salt. The dominance of type II marine kerogen in a basin leads to higher bromine concentrations. This may reflect the dominance of a marine influence in a basin which is more likely to lead to salt deposition than terrestrial depositional environments. Iodine concentrations are independent of all these parameters. Other geological parameters such as depth of burial, temperature, basin forming mechanism and reservoir lithology have no influence upon halogen concentrations.Key processes that affect halogen concentrations are sea water evaporation and dilution, water—salt interaction and input from organic sources. Chlorine and bromine data lie close to the experimentally-derived sea water evaporation trend showing that sea water evaporation may be an important general control on halogens. Sea water dilution is probably responsible for most low salinity formation water chlorine and bromine concentrations for the same reason. Sea water dilution can occur either by meteoric invasion during burial, or following uplift and erosion, or by diagenetic dehydration reactions. Water can interact with salt in a variety of ways: halite dissolution by congruent processes, halite recrystallization by incongruent processes, sylvite dissolution or recrystallization and halite fluid inclusion rupture. Halite dissolution will lead to high chlorine and relatively low bromine waters because halite contains little bromine. In contrast, halite recrystallization will lead to bromine-enhanced waters because NaBr dissolves preferentially to NaCl. The occurrence of dissolution or recrystallization will depend on the water rock ratio, greater volumes of water will lead to more dissolution and waters with higher Cl/Br ratios. Sylvite is usually rich in bromine so dissolution will lead to bromine-enhanced waters. Primary aqueous inclusions in halite contain high bromine concentrations so that rupture, during deformation or recrystallization, will lead to bromine-enhanced formation water. A combination of these processes are responsible for the very limited range of Cl/Br ratios although congruent halite dissolution must have a limited role due to the absence of waters with high Cl/Br ratios.Iodine is strongly concentrated in organic materials in the marine environment; oils and organic rich-source rocks have high I/Cl and I/Br ratios relative to sea water or evaporated sea water. All formation waters are enriched in iodine relative to sea water implying that there has been input from organic matter or interaction with oil. However, hydrocarbon source rock type in a basin has no discernible effect upon iodine concentrations.