Fluid Inclusions (fluid + inclusion)

Distribution by Scientific Domains
Distribution within Earth and Environmental Science

Terms modified by Fluid Inclusions

  • fluid inclusion data
  • fluid inclusion studies

  • Selected Abstracts


    Determining Pressure with Daughter Minerals in Fluid Inclusion by Raman Spectroscopy: Sphalerite as an Example

    ACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 3 2009
    Yuping YANG
    Abstract: Raman frequency of some materials, including minerals, molecules and ions, shifts systematically with changing pressure and temperature. This property is often used as a pressure gauge in high pressure experiments with the hydrothermal diamond anvil cell (HDAC). Since the system of fluid inclusion is similar to that of HDAC, it can also be used to determine the internal pressure of fluid inclusions. Sphalerite is a common daughter mineral. In this study, the frequency shift of the 350 cm,1 peak of sphalerite has been studied from 296 to 523 K and from 0.07 to 2.00 GPa using the HDAC. The global slope of the isotherms (,V350/,p)T is 0.0048 in the studied pressure range. No significant variation of the slopes with temperature has been observed. The correlation between the frequency shift of the 350 cm,1 peak of sphalerite and pressure and temperature is constrained as P=208.33(,Vp)350+3.13T,943.75. This relationship may be used to estimate the internal pressure of the sphalerite-bearing fluid inclusions. [source]


    Synthetic Fluid Inclusions in the Systems NaCl-H2O and NaCl-CO2 -H2O: Dissolution Temperatures of Halite

    RESOURCE GEOLOGY, Issue 2 2006
    Hiroki Nagaseki
    Abstract. This study examined the effect of CO2 on NaCl solubility in hydrothermal fluid, with the synthetic fluid inclusion technique. Fluid inclusions of 30,40 wt% NaCl and 5 mol % CO2 were synthesized, and their halite dissolution temperatures, Tm(halite), were measured. The solubilities of NaCl in CO2 -bearing aqueous fluid were obtained at 160,320d,C under vapor-saturated pressures. The Tm(halite) value in aqueous fluid with 5 mol % CO2 obtained in this study agrees with that of Schmidt et al. (1995), showing that 5 mol % CO2 reduces the solubility of NaCl by about 1 wt%. Calculation of magnetite solubility suggests that 5,10 mol % CO2 decreases magnetite solubility by 4.5,8.9 % relative to the magnetite solubility in CO2-free solution. Therefore, an increase of CO2 content in ore-forming solutions may cause deposition of iron minerals and produce ore deposits. [source]


    Magmatic Fluid Inclusions from the Zaldivar Deposit, Northern Chile: The Role of Early Metal-bearing Fluids in a Porphyry Copper System

    RESOURCE GEOLOGY, Issue 1 2006
    Eduardo A. Campos
    Abstract. The occurrence of a distinct type of multi-solid, highly-saline fluid inclusions, hosted in igneous quartz phe-nocrysts from the Llamo porphyry, in the Zaldivar porphyry copper deposit of northern Chile is documented. Total homoge-nization of the multi-solid type inclusions occurs at magmatic temperatures (over 750d,C), well above the typical temperatures of hydro thermal fluids (less than 600d,C) usually recorded in porphyry copper systems. The analysis of this type of fluid inclusions, using a combination of non-destructive microthermometry, Raman and PIXE techniques and the identification of daughter minerals by SEM method, indicates that the trapped fluid was a dense, complex chloride brine in which Cl, Na, K, Fe, Cu, and Mn are dominant. The high chlorine and metal contents indicate that the metals were separated from the crystallizing magma as homogeneous aqueous chloride-rich solutions that represent the primary magmatic fluids exsolved at high temperatures and depth during the crystallization of the parental intrusive. The multi-solid type inclusion illustrates the mechanism by which ore components are sequestered from the crystallizing parental magma and concentrated in the exsolved magmatic aqueous fluids. These fluids are significant with respect to the origin of porphyry copper deposits, as they are responsible for the first enrichment of metals and represent the precursors of metal-bearing hydrothermal fluids in a porphyry copper system. [source]


    Mineral Paragenesis and Fluid Inclusions of Some Pluton-hosted Vein-type Copper Deposits in the Coastal Cordillera, Northern Chile

    RESOURCE GEOLOGY, Issue 1 2003
    Dania Trista
    Abstract. Formation conditions of some vein-type copper deposits of the Tocopilla district (Deseada, San Jose, Santa Rosa) and the Gatico district (Yohanita, Toldo-Velarde, Argentina) in the Coastal Cordillera of northern Chile were inferred from mineral paragenesis and fluid inclusion data, and were compared with those of neighboring stratiform copper deposits. The vein-type copper deposits are hosted in Late Jurassic dioritic to quartz-dioritic plutons intruding extensively an andesite-dominant volcanic pile of the Jurassic La Negra Formation. Primary mineralization is characterized by chalcopyrite + magnetite + pyrite + bornite, and supergene alteration of these minerals produced anilite, covellite, atacamite and chrysocolla. The hypogene mineral assemblage indicates relatively high sulfur fugacity and weakly oxidized conditions, distinct from the stratiform copper deposits formed under low sulfur fugacity and moderately oxidized conditions. Furthermore, the fluid inclusion data of the vein-type deposits indicate high temperature (401,560d,C) and high salinity (39,68 wt% NaCl equiv.) ranges in contrast to the stratiform deposits, suggesting that this type of deposits formed by magma-associated hypersaline ore fluids. [source]


    Mineral Paragenesis, Fluid Inclusions and Sulfur Isotope Systematics of the Lepanto Far Southeast Porphyry Cu-Au Deposit, Mankayan, Philippines

    RESOURCE GEOLOGY, Issue 3 2000
    Akira IMAI
    Abstract: The Lepanto Far Southeast porphyry Cu-Au deposit is located beneath and to the southeast of the Lepanto enargite-luzonite Cu,Au deposit in Mankayan, Benguet Province, Philippines. The principal orebody consists of potassic alteration subjected to partial retrograde chlorite alteration that rims stock-work of quartz-anhydrite veinlets. Fluid inclusions found in stockwork quartz and anhydrite in the biotitized orebody center are dominated by polyphase inclusions that homogenize at temperatures of >500C. Sulfur isotopic thermometry applied to the sulfides-anhydrite pairs suggests around 500C. The principal ore minerals associated with quartz-anhydrite stockworks are chalcopyrite and pyrite with minor bornite and Bi,Te,bearing tennantite, with trace of native gold. Rounded pyrite grains appear fractured and corroded and are interpreted as remnants of primary intermediate solid solution + pyrite assemblage. A breccia pipe truncates the deposit. Mineralization in the breccia pipe is brought by quartz-anhydrite veinlets and infilling in the interstices between clasts. Chalcopyrite-Au mineralization associated with molybdenite is recognized in the deeper zone in the breccia pipe. Fluid inclusion microthermometry on polyphase inclusions in veinlet quartz as well as sulfur isotope thermometry applied for the pair of anhydrite and sulfides suggests >450C. Fluid inclusions in veinlet quartz and anhydrite in the fringe advanced argillic alteration are chiefly composed of coexisting liquid-rich inclusions and gas-rich inclusions, in addition to coexisting polyphase inclusions and gas-rich inclusions. These inclusions exhibit a wide range of homogenization temperatures, suggesting heterogeneous entrapping in the two-fluid unmixing region. Sulfur isotopes of aqueous sulfide and sulfate exhibit a general trend from the smallest fractionation pairs (about 11%) in the biotitized orebody center to the largest fractionation (about 25%) pairs in the fringe advanced argillic alteration, suggesting a simple evolution of hydrothermal system. The slopes of arbitrary regression lines in ,34S versus 34S[SO4 = ,H2S] diagram suggest that the abundance ratio of aqueous sulfate to sulfide in the hydrothermal fluid has been broadly constant at about 1:3 through temperature decrease. The intersection of these two regression lines at the ,34S axis indicates that the bulk ,34S is about +6%. Thus, the Lepanto FSE deposit is a further example which confirms enrichment in 34S in the hydrous intermediate to silicic magmas and associated magmatic hydrothermal deposits in the western Luzon arc. [source]


    A Preliminary Study on Fluid Inclusions and Mineralization of Xitieshan Sedimentary-Exhalative (SEDEX) Lead-Zinc Deposit

    ACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 4 2008
    WANG Lijuan
    Abstract The Xitieshan lead-zinc deposit is located at the northern margin of the Qaidam Basin, Qinghai Province, China, and had developed a complete marine sedimentary-exhalative system. Our preliminary study of ore-forming fluids shows that fluid inclusions in quartz from altered stockwork rocks that represent the pipe facies have a wide range of temperature and salinity. The intense fluid activities are characteristics of the pipe facies of the exhalative system. Fluid inclusions in carbonates near the unstratified ore bodies hosted in the thick-bedded marble which represents vent-proximal facies are large in size and have moderate to high temperatures. They represent unerupted sub-seafloor fluid activity. Fluids in altered stockwork rocks and carbonates have similar H2O-NaCl-CO2 system, both belonging to the sedimentary-exhalative system. The fluids migrate from the pipe facies to the unstratified ore bodies. Boiling of the fluids causes the separation of CO2 vapor and liquid H2O. When the fluids migrate into the unconsolidated thick-bedded marble, the escape of CO2, decreasing temperature and pressure as well as some involvement of seawater into the fluids result in the unmixing of fluids with high and low salinity and deposition of ore-forming materials. The two unmixed fluids were trapped in unconsolidated carbonates and the ore-forming materials were deposited in the unconsolidated carbonates to form the sedimentary-exhalative type unstratified ore bodies. The ore-forming temperature of unstratified ore bodies is up to high temperature indicating that there is a huge ore-forming potential in its deep. [source]


    Hydrothermal alteration of late- to post-tectonic Lyon Mountain Granitic Gneiss, Adirondack Mountains, New York: Origin of quartz,sillimanite segregations, quartz,albite lithologies, and associated Kiruna-type low-Ti Fe-oxide deposits

    JOURNAL OF METAMORPHIC GEOLOGY, Issue 1 2002
    J. Mclelland
    Abstract Quartz,sillimanite segregations, quartz,albite lithologies (Ab95,98), and Kiruna-type low-Ti iron-oxide deposits are associated with late- to post-tectonic (c. 1055 Ma) leucogranites of Lyon Mountain Gneiss (LMG) in the Adirondack Mountains, New York State. Most recent interpretations of these controversial features, which are global in occurrence, favour hydrothermal origins in agreement with results presented here. Field relations document that quartz,sillimanite veins and nodules cut, and therefore post-date, emplacement of host LMG leucogranites. Veins occur in oriented fracture networks, and aligned trains of nodules are interpreted as disrupted early veins. Late dykes of leucogranite cut veins and nodules demonstrating formation prior to terminal magmatism. Veins and nodules consist of sillimanite surrounded by quartz that commonly embays wall-rock feldspar indicating leaching of Na and K from LMG feldspar by acidic hydrothermal fluids. Subsequent, and repeated, ductile flow disrupted earlier veins into nodular fragments but produced little grain shape fabric. Geochemical and petrographic studies of quartz,albite rock indicate that it formed through metasomatic replacement (albitization) of LMG microperthite by sodic hydrothermal fluids that resulted in diagnostic checkerboard albite. Low-Ti iron-oxide ores are commonly associated with the quartz,albite sub-unit, and it is proposed that hydrothermal fluids related to albitization transported Fe as well. The regional extent of sodic alteration suggests large quantities of surface-derived hydrothermal fluids. Fluid inclusion and oxygen isotope data are consistent with high temperature, regionally extensive fluids consisting primarily of evolved surface-derived brines enriched in Na and Cl. Quartz,sillimanite veins and nodules, which are significantly more localised phenomena and require acidic fluids, were most likely formed from local magmatic fluids in the crystallizing carapaces of LMG plutons. [source]


    Geology, Wall-rock Alteration and Vein Paragenesis of the Bilimoia Gold Deposit, Kainantu Metallogenic Region, Papua New Guinea

    RESOURCE GEOLOGY, Issue 3 2007
    Joseph Onglo Espi
    Abstract The Bilimoia deposit (2.23 Mt, 24 g/t Au), located in the eastern Central Mobile Belt of mainland Papua New Guinea, is composed of fault-hosted, NW,NNW-trending Irumafimpa,Kora and Judd,Upper Kora Au-quartz veins hosted by Middle,Late Triassic basement that was metamorphosed to medium-grade greenschist facies between Middle,Late Triassic and Early,Middle Jurassic. Mineralizing fluids were introduced during crustal thickening, rapid uplift, change of plate motions from oblique to orthogonal compression, active faulting and S3 and S4 events in an S1,S4 deformation sequence. The Bilimoia deposit is spatially and temporally related to I-type, early intermediate to felsic and late mafic intrusions emplaced in Late Miocene (9,7 Ma). Hydrothermal alteration and associated mineralization is divided into 10 main paragenetic stages: (1) chlorite,epidote-selvaged quartz,calcite,specularite vein; (2) local quartz,illite,pyrite alteration; (3) quartz,sericite,mariposite,fuchsite,pyrite wall-rock alteration that delimits the bounding shears; (4) finely banded, colloform-, crustiform- and cockade-textured and drusy quartz early wolframite late adularia; (5) hematite; (6) pyrite; (7) quartz amethyst-base metal sulfides; (8) quartz,chalcopyrite,bornite,Sn and Cu sulfides,Au tellurides and Te Bi Ag Cu Pb phases; (9) Fe Mn carbonates; and (10) supergene overprint. Fluid inclusions in stage 4 are characterized by low salinity (0.9,5.4 wt% NaCl equivalent), aqueous,carbonic fluids with total homogenization temperatures ranging from 210 to 330C. Some of the inclusions that homogenized between 285 and 330C host coexisting liquid- and vapor-rich (including carbonic) phases, suggesting phase separation. Fluid inclusions in quartz intergrown with wolframite have low salinity (0.9,1.2 wt% NaCl equivalent), aqueous,carbonic fluids at 240,260C, defining the latter's depositional conditions. The ore fluids were derived from oxidized magmatic source initially contaminated by reduced basement rocks. Wall-rock alteration and involvement of circulating meteoric waters were dominant during the first three stages and early part of stage 4. Stage 5 hematite was deposited as a result of stage 4 phase separation or entrainment of oxygenated groundwater. Gold is associated with Te- and Bi-bearing minerals and mostly precipitated as gold-tellurides during stage 8. Gold deposition occurred below 350C due to a change in the sulfidation and oxidation state of the fluids, depressurization and decreasing temperature and activities of sulfur and tellurium. Bisulfides are considered to be the main Au-transporting complexes. The Bilimoia deposit has affinities that are similar to many gold systems termed epizonal orogenic and intrusion-related. The current data allow us to classify the Bilimoia deposit as a fault-controlled, metamorphic-hosted, intrusion-related mesothermal to low sulfidation epithermal quartz,Au,Te,Bi vein system. [source]


    Genetic Environment of the Intrusion-related Yuryang Au-Te Deposit in the Cheonan Metallogenic Province, Korea

    RESOURCE GEOLOGY, Issue 2 2006
    Sang Joon Pak
    Abstract. The Yuryang gold deposit, comprising a Te-bearing Au-Ag vein mineralization, is located in the Cheonan area of the Republic of Korea. The deposit is hosted in Precambrian gneiss and closely related to pegmatite. The mineralized veins display massive quartz textures, with weak alteration adjacent to the veins. The ore mineralization is simple, with a low Ag/Au ratio of 1.5:1, due to the paucity of Ag-phases. Ore mineralization took place in two different mineral assemblages with paragenetic time; early Fe-sulfide mineralization and late Fe-sulfide and Au-Te mineralization. The early Fe-sulfide mineralization (pyrite + sphalerite) occurred typically along the vein margins, and the subsequent Au-Te mineralization is characterized by fracture fillings of galena, sphalerite, pyrrhotite, Te-bearing minerals (petzite, altaite, hessite and Bi-Te mineral) and electrum. Fluid inclusions characteristically contain CO2 and can be classified into four types (Ia, Ib, IIa and IIb) according to the phase behavior. The pressure corrected temperatures (,500d,C) indicate that the deposit was formed at a distinctively high temperature from fluids with moderate to low salinity (<12 wt% equiv. NaCl) and CH4 (1,22 mole %). The sphalerite geo-barometry yield an estimated pressure about 3.5 ,2.1 kbar. The dominant ore-deposition mechanisms were CO2 effervescence and concomitant H2S volatilization, which triggered sulfidation and gold mineralization. The measured and calculated isotopic compositions of fluids (,18OH2O = 10.3 to 12.4 %o; ,DH2O = -52 to -77 %o) may indicate that the gold deposition originated from S-type magmatic waters. The physicochemical conditions observed in the Yuryang gold deposit indicate that the Jurassic gold deposits in the Cheonan area, including the Yuryang gold deposit are compatible with deposition of the intrusion-related Au-Te veins from deeply sourced fluids generated by the late Jurassic Daebo magmatism. [source]


    Synthetic Fluid Inclusions in the Systems NaCl-H2O and NaCl-CO2 -H2O: Dissolution Temperatures of Halite

    RESOURCE GEOLOGY, Issue 2 2006
    Hiroki Nagaseki
    Abstract. This study examined the effect of CO2 on NaCl solubility in hydrothermal fluid, with the synthetic fluid inclusion technique. Fluid inclusions of 30,40 wt% NaCl and 5 mol % CO2 were synthesized, and their halite dissolution temperatures, Tm(halite), were measured. The solubilities of NaCl in CO2 -bearing aqueous fluid were obtained at 160,320d,C under vapor-saturated pressures. The Tm(halite) value in aqueous fluid with 5 mol % CO2 obtained in this study agrees with that of Schmidt et al. (1995), showing that 5 mol % CO2 reduces the solubility of NaCl by about 1 wt%. Calculation of magnetite solubility suggests that 5,10 mol % CO2 decreases magnetite solubility by 4.5,8.9 % relative to the magnetite solubility in CO2-free solution. Therefore, an increase of CO2 content in ore-forming solutions may cause deposition of iron minerals and produce ore deposits. [source]


    Evolution of Hydrothermal System at the Dizon Porphyry Cu-Au Deposit, Zambales, Philippines

    RESOURCE GEOLOGY, Issue 2 2005
    Akira Imai
    Abstract. Evolution of hydrothermal system from initial porphyry Cu mineralization to overlapping epithermal system at the Dizon porphyry Cu-Au deposit in western central Luzon, Zambales, Philippines, is documented in terms of mineral paragen-esis, fluid inclusion petrography and microthermometry, and sulfur isotope systematics. The paragenetic stages throughout the deposit are summarized as follows; 1) stockwork amethystic quartz veinlets associated with chalcopyrite, bornite, magnetite and Au enveloped by chlorite alteration overprinting biotite alteration, 2) stockwork quartz veinlets with chalcopyrite and pyrite associated with Au and chalcopyrite and pyrite stringers in sericite alteration, 3) stringer quartz veinlets associated with molybdenite in sericite alteration, and 4) WNW-trending quartz veins associated with sphalerite and galena at deeper part, while enargite and stibnite at shallower levels associated with advanced argillic alteration. Chalcopyrite and bornite associated with magnetite in quartz veinlet stockwork (stage 1) have precipitated initially as intermediate solid solution (iss) and bornite solid solution (bnss), respectively. Fluid inclusions in the stockwork veinlet quartz consist of gas-rich inclusions and polyphase inclusions. Halite in polyphase inclusions dissolves at temperatures ranging from 360d,C to >500d,C but liquid (brine) and gas (vapor) do not homogenize at <500d,C. The maximum pressure and minimum temperature during the deposition of iss and bnss with stockwork quartz veinlets are estimated to be 460 bars and 500d,C. Fluid inclusions in veinlet stockwork quartz enveloped in sericite alteration (stage 2) consist mainly of gas-rich inclusions and polyphase inclusions. In addition to the possible presence of saturated NaCl crystals at the time of entrapment of fluid inclusions that exhibit the liquid-vapor homogenization temperatures lower than the halite dissolution temperatures in some samples, wide range of temperatures of halite dissolution and liquid-vapor homogenization of polyphase inclusions from 230d,C to >500d,C and from 270d,C to >500d,C, respectively, suggests heterogeneous entrapment of gaseous vapor and hypersaline brine. The minimum pressure and temperature are estimated to be about 25 bars and 245d,C. Fluid inclusions in veinlet quartz associated with molybdenite (stage 3) are dominated by gas-rich inclusions accompanied with minor liquid-rich inclusions that homogenize at temperatures between 350d,C and 490d,C. Fluid inclusions in vuggy veinlet quartz associated with stibnite (stage 4) consist mainly of gas-rich inclusions with subordinate polyphase inclusions that do not homogenize below 500d,C. Fluid inclusions in veinlet quartz associated with galena and sphalerite (stage 4) are composed of liquid-rich two-phase inclusions, and they homogenize into liquid phase at temperatures ranging widely from 190d,C to 300d,C (suggesting boiling) and the salinity ranges from 1.0 wt% to 3.4 wt% NaCl equivalent. A pressure of about 15 bars is estimated for the dilute aqueous solution of 190d,C from which veinlet quartz associated with galena and sphalerite precipitated. In addition to a change in temperature-pressure regime from lithostatic pressure during the deposition of iss and bnss with stockwork quartz veinlets to hydrostatic pressure during fracture-controlled quartz veinlet associated with galena and sphalerite, a decrease in pressure is supposed to have occurred due to unroofing or removal of the overlying piles during the temperature decrease in the evolution of hydrothermal system. The majority of the sulfur isotopic composition of sulfides ranges from 0 % to +5 %. Sulfur originated from an iso-topically uniform and homogeneous source, and the mineralization occurred in a single hydrothermal system. [source]


    Chemical, Isotopic, and Fluid Inclusion Evidence for the Hydrothermal Alteration of the Footwall Rocks of the BIF-Hosted Iron Ore Deposits in the Hamersley District, Western Australia

    RESOURCE GEOLOGY, Issue 2 2003
    Makoto Haruna
    Abstract. The petrography, chemical, fluid inclusion and isotope analyses (O, Rb-Sr) were conducted for the shale samples of the Mount McRae Shale collected from the Tom Price, Newman, and Paraburdoo mines in the Hamersley Basin, Western Australia. The Mount McRae Shale at these mines occurs as a footwall unit of the secondary, hematite-rich iron ores derived from the Brockman Iron Formation, one of the largest banded iron formations (BIFs) in the world. Unusually low contents of Na, Ca, and Sr in the shales suggest that these elements were leached away from the shale after deposition. The ,18O (SMOW) values fall in the range of + 15.0 to +17.9 per mil and show the positive correlation with calculated quartz/sericite ratios of the shale samples. This suggests that the oxygen isotopic compositions of shale samples were homogenized and equilibrated by postdepositional event. The pyrite nodules hosted by shales are often rimmed by thin layers of silica of varying crystallinity. Fluid inclusions in quartz crystals rimming a pyrite nodule show homogenization temperatures ranging from 100 to 240d,C for 47 inclusions and salinities ranging from 0.4 to 12.3 wt% NaCl equivalent for 18 inclusions. These fluid inclusion data give direct evidence for the hydrothermal activity and are comparable to those of the vein quartz collected from the BIF-derived secondary iron ores (Taylor et al, 2001). The Rb-Sr age for the Mount McRae Shale is 1,952 289 Ma and at least 200 million years younger than the depositional age of the Brockman Iron Formation of , 2.5 Ga in age. All the data obtained in this study are consistent with the suggestion that high temperature hydrothermal fluids were responsible for both the secondary iron ore formation and the alteration of the Mount McRae Shale. [source]


    Mineral Paragenesis, Fluid Inclusions and Sulfur Isotope Systematics of the Lepanto Far Southeast Porphyry Cu-Au Deposit, Mankayan, Philippines

    RESOURCE GEOLOGY, Issue 3 2000
    Akira IMAI
    Abstract: The Lepanto Far Southeast porphyry Cu-Au deposit is located beneath and to the southeast of the Lepanto enargite-luzonite Cu,Au deposit in Mankayan, Benguet Province, Philippines. The principal orebody consists of potassic alteration subjected to partial retrograde chlorite alteration that rims stock-work of quartz-anhydrite veinlets. Fluid inclusions found in stockwork quartz and anhydrite in the biotitized orebody center are dominated by polyphase inclusions that homogenize at temperatures of >500C. Sulfur isotopic thermometry applied to the sulfides-anhydrite pairs suggests around 500C. The principal ore minerals associated with quartz-anhydrite stockworks are chalcopyrite and pyrite with minor bornite and Bi,Te,bearing tennantite, with trace of native gold. Rounded pyrite grains appear fractured and corroded and are interpreted as remnants of primary intermediate solid solution + pyrite assemblage. A breccia pipe truncates the deposit. Mineralization in the breccia pipe is brought by quartz-anhydrite veinlets and infilling in the interstices between clasts. Chalcopyrite-Au mineralization associated with molybdenite is recognized in the deeper zone in the breccia pipe. Fluid inclusion microthermometry on polyphase inclusions in veinlet quartz as well as sulfur isotope thermometry applied for the pair of anhydrite and sulfides suggests >450C. Fluid inclusions in veinlet quartz and anhydrite in the fringe advanced argillic alteration are chiefly composed of coexisting liquid-rich inclusions and gas-rich inclusions, in addition to coexisting polyphase inclusions and gas-rich inclusions. These inclusions exhibit a wide range of homogenization temperatures, suggesting heterogeneous entrapping in the two-fluid unmixing region. Sulfur isotopes of aqueous sulfide and sulfate exhibit a general trend from the smallest fractionation pairs (about 11%) in the biotitized orebody center to the largest fractionation (about 25%) pairs in the fringe advanced argillic alteration, suggesting a simple evolution of hydrothermal system. The slopes of arbitrary regression lines in ,34S versus 34S[SO4 = ,H2S] diagram suggest that the abundance ratio of aqueous sulfate to sulfide in the hydrothermal fluid has been broadly constant at about 1:3 through temperature decrease. The intersection of these two regression lines at the ,34S axis indicates that the bulk ,34S is about +6%. Thus, the Lepanto FSE deposit is a further example which confirms enrichment in 34S in the hydrous intermediate to silicic magmas and associated magmatic hydrothermal deposits in the western Luzon arc. [source]


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

    RESOURCE GEOLOGY, Issue 2 2000
    Neng JIANG
    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. [source]


    Insulating effect of coals and organic rich shales: implications for topography-driven fluid flow, heat transport, and genesis of ore deposits in the Arkoma Basin and Ozark Plateau

    BASIN RESEARCH, Issue 2 2002
    J.A. Nunn
    ABSTRACT Sedimentary rocks rich in organic matter, such as coal and carbonaceous shales, are characterized by remarkably low thermal conductivities in the range of 0.2,1.0 W m,1 C,1, lower by a factor of 2 or more than other common rock types. As a result of this natural insulating effect, temperature gradients in organic rich, fine-grained sediments may become elevated even with a typical continental basal heat flow of 60 mW m,2. Underlying rocks will attain higher temperatures and higher thermal maturities than would otherwise occur. A two-dimensional finite element model of fluid flow and heat transport has been used to study the insulating effect of low thermal conductivity carbonaceous sediments in an uplifted foreland basin. Topography-driven recharge is assumed to be the major driving force for regional groundwater flow. Our model section cuts through the Arkoma Basin to Ozark Plateau and terminates near the Missouri River, west of St. Louis. Fluid inclusions, organic maturation, and fission track evidence show that large areas of upper Cambrian rocks in southern Missouri have experienced high temperatures (100,140 C) at shallow depths (< 1.5 km). Low thermal conductivity sediments, such as coal and organic rich mudstone were deposited over the Arkoma Basin and Ozark Plateau, as well as most of the mid-continent of North America, during the Late Palaeozoic. Much of these Late Palaeozoic sediments were subsequently removed by erosion. Our model results are consistent with high temperatures (100,130 C) in the groundwater discharge region at shallow depths (< 1.5 km) even with a typical continental basal heat flow of 60 mW m,2. Higher heat energy retention in basin sediments and underlying basement rocks prior to basin-scale fluid flow and higher rates of advective heat transport along basal aquifers owing to lower fluid viscosity (more efficient heat transport) contribute to higher temperatures in the discharge region. Thermal insulation by organic rich sediments which traps heat transported by upward fluid advection is the dominant mechanism for elevated temperatures in the discharge region. This suggests localized formation of ore deposits within a basin-scale fluid flow system may be caused by the juxtaposition of upward fluid discharge with overlying areas of insulating organic rich sediments. The additional temperature increment contributed to underlying rocks by this insulating effect may help to explain anomalous thermal maturity of the Arkoma Basin and Ozark Plateau, reducing the need to call upon excessive burial or high basal heat flow (80,100 mW m,2) in the past. After subsequent uplift and erosion remove the insulating carbonaceous layer, the model slowly returns to a normal geothermal gradient of about 30 C km,1. [source]


    A Preliminary Study on Fluid Inclusions and Mineralization of Xitieshan Sedimentary-Exhalative (SEDEX) Lead-Zinc Deposit

    ACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 4 2008
    WANG Lijuan
    Abstract The Xitieshan lead-zinc deposit is located at the northern margin of the Qaidam Basin, Qinghai Province, China, and had developed a complete marine sedimentary-exhalative system. Our preliminary study of ore-forming fluids shows that fluid inclusions in quartz from altered stockwork rocks that represent the pipe facies have a wide range of temperature and salinity. The intense fluid activities are characteristics of the pipe facies of the exhalative system. Fluid inclusions in carbonates near the unstratified ore bodies hosted in the thick-bedded marble which represents vent-proximal facies are large in size and have moderate to high temperatures. They represent unerupted sub-seafloor fluid activity. Fluids in altered stockwork rocks and carbonates have similar H2O-NaCl-CO2 system, both belonging to the sedimentary-exhalative system. The fluids migrate from the pipe facies to the unstratified ore bodies. Boiling of the fluids causes the separation of CO2 vapor and liquid H2O. When the fluids migrate into the unconsolidated thick-bedded marble, the escape of CO2, decreasing temperature and pressure as well as some involvement of seawater into the fluids result in the unmixing of fluids with high and low salinity and deposition of ore-forming materials. The two unmixed fluids were trapped in unconsolidated carbonates and the ore-forming materials were deposited in the unconsolidated carbonates to form the sedimentary-exhalative type unstratified ore bodies. The ore-forming temperature of unstratified ore bodies is up to high temperature indicating that there is a huge ore-forming potential in its deep. [source]


    Effects of host mineral re-equilibration during uplift and cooling on the fidelity of primary hydrothermal fluid inclusions: a theoretical example using Mississippi Valley-type ore fluids

    GEOFLUIDS (ELECTRONIC), Issue 2 2009
    M. A. McKIBBEN
    Abstract At the moment of its trapping as a primary fluid inclusion, a hydrothermal fluid is typically at or near equilibrium with multiple mineral species at depth and temperature. After trapping, however, the isolated inclusion fluid can re-equilibrate only with its own host mineral species during later uplift and cooling to surface conditions. Because the solubility versus temperature behavior is unique for each host mineral species, identical inclusions trapped at the same time within different species may re-equilibrate in a disparate manner upon cooling and become variably less representative of the original trapped fluid once they reach ambient temperature. To test the significance of this effect, a series of theoretical equilibrium reaction models was constructed in which a trapped hydrothermal fluid characteristic of Mississippi Valley-type ore deposits is cooled in contact with silicate, sulfide and carbonate hosts, respectively, from 100 to 25C. Dissolved base metal concentrations are predicted to decline by two to four orders of magnitude in inclusions in all hosts, due to the precipitation of optically undetectable masses of sulfide daughter minerals. Fluids in the calcite host show the greatest decline in dissolved base metals upon cooling, due to its retrograde solubility and consequent shift in the pH and aqueous C speciation of the fluid. ,13C values for CO2 in all hosts become depleted by 2,7, relative to the original trapped fluid, with depletions again being the greatest for the calcite host due to its retrograde dissolution. Analytical techniques that extract and analyze the complete contents of fluid inclusions at room temperature can account for the predicted precipitation of microscopic daughter minerals during cooling, but may not compensate for chemical changes caused by the retrograde dissolution of calcite. Such solubility effects are another reason to be cautious in using carbonate minerals for fluid inclusion studies, in addition to their undesirable physical properties of softness, deformability and perfect cleavage. [source]


    The application of structured-light illumination microscopy to hydrocarbon-bearing fluid inclusions

    GEOFLUIDS (ELECTRONIC), Issue 2 2008
    N. J. F. BLAMEY
    Abstract Structured-light illumination (SLI)-based microscopy offers geologists a new perspective for screening of hydrocarbon-bearing (HCFI) and small aqueous fluid inclusion (AFI) assemblages. This optical-sectioning technique provides rapid, confocal-like imaging, using relatively simple and inexpensive instrumentation. The 3D fluorescent images of HCFI planes and large single HCFIs permit the visualization of the relationships between HCFI assemblages, examination of HCFI morphology, and volume estimates of the fluorescent components within HCFIs. By the use of normal white light illumination, SLI image capture, and varying acquisition time it is also possible to image AFI because of the random movements of vapour bubbles within the inclusions. This allows the near-simultaneous visualization of hydrocarbon and AFI which is of significant importance for the study of sedimentary basins and petroleum reservoirs. SLI is a unique and accessible 3D petrographic tool, with practical advantages over conventional epifluorescence and confocal laser scanning microscopy. [source]


    Contrasting paleofluid systems in the continental basement: a fluid inclusion and stable isotope study of hydrothermal vein mineralization, Schwarzwald district, Germany

    GEOFLUIDS (ELECTRONIC), Issue 2 2007
    B. BAATARTSOGT
    Abstract An integrated fluid inclusion and stable isotope study was carried out on hydrothermal veins (Sb-bearing quartz veins, metal-bearing fluorite,barite,quartz veins) from the Schwarzwald district, Germany. A total number of 106 Variscan (quartz veins related to Variscan orogenic processes) and post-Variscan deposits were studied by microthermometry, Raman spectroscopy, and stable isotope analysis. The fluid inclusions in Variscan quartz veins are of the H2O,NaCl,(KCl) type, have low salinities (0,10 wt.% eqv. NaCl) and high Th values (150,350C). Oxygen isotope data for quartz range from +2.8, to +12.2, and calculated ,18OH2O values of the fluid are between ,12.5, and +4.4,. The ,D values of water extracted from fluid inclusions vary between ,49, and +4,. The geological framework, fluid inclusion and stable isotope characteristics of the Variscan veins suggest an origin from regional metamorphic devolatilization processes. By contrast, the fluid inclusions in post-Variscan fluorite, calcite, barite, quartz, and sphalerite belong to the H2O,NaCl,CaCl2 type, have high salinities (22,25 wt.% eqv. NaCl) and lower Th values of 90,200C. A low-salinity fluid (0,15 wt.% eqv. NaCl) was observed in late-stage fluorite, calcite, and quartz, which was trapped at similar temperatures. The ,18O values of quartz range between +11.1, and +20.9,, which translates into calculated ,18OH2O values between ,11.0, and +4.4,. This range is consistent with ,18OH2O values of fluid inclusion water extracted from fluorite (,11.6, to +1.1,). The ,D values of directly measured fluid inclusion water range between ,29, and ,1,, ,26, and ,15,, and ,63, and +9, for fluorite, quartz, and calcite, respectively. Calculations using the fluid inclusion and isotope data point to formation of the fluorite,barite,quartz veins under near-hydrostatic conditions. The ,18OH2O and ,D data, particularly the observed wide range in ,D, indicate that the mineralization formed through large-scale mixing of a basement-derived saline NaCl,CaCl2 brine with meteoric water. Our comprehensive study provides evidence for two fundamentally different fluid systems in the crystalline basement. The Variscan fluid regime is dominated by fluids generated through metamorphic devolatilization and fluid expulsion driven by compressional nappe tectonics. The onset of post-Variscan extensional tectonics resulted in replacement of the orogenic fluid regime by fluids which have distinct compositional characteristics and are related to a change in the principal fluid sources and the general fluid flow patterns. This younger system shows remarkably persistent geochemical and isotopic features over a prolonged period of more than 100 Ma. [source]


    Principal features of impact-generated hydrothermal circulation systems: mineralogical and geochemical evidence

    GEOFLUIDS (ELECTRONIC), Issue 3 2005
    MIKHAIL V. NAUMOVArticle first published online: 14 JUL 200
    Abstract Any hypervelocity impact generates a hydrothermal circulation system in resulting craters. Common characteristics of hydrothermal fluids mobilized within impact structures are considered, based on mineralogical and geochemical investigations, to date. There is similarity between the hydrothermal mineral associations in the majority of terrestrial craters; an assemblage of clay minerals,zeolites,calcite,pyrite is predominant. Combining mineralogical, geochemical, fluid inclusion, and stable isotope data, the distinctive characteristics of impact-generated hydrothermal fluids can be distinguished as follows: (i) superficial, meteoric and ground water and, possibly, products of dehydration and degassing of minerals under shock are the sources of hot water solutions; (ii) shocked target rocks are sources of the mineral components of the solutions; (iii) flow of fluids occurs mainly in the liquid state; (iv) high rates of flow are likely (10,4 to 10,3 m s,1); (v) fluids are predominantly aqueous and of low salinity; (vi) fluids are weakly alkaline to near-neutral (pH 6,8) and are supersaturated in silica during the entire hydrothermal process because of the strong predominance of shock-disordered aluminosilicates and fusion glasses in the host rocks; and (vii) variations in the properties of the circulating solutions, as well as the spatial distribution of secondary mineral assemblages are controlled by tempera ure gradients within the circulation cell and by a progressive cooling of the impact crater. Products of impact-generated hydrothermal processes are similar to the hydrothermal mineralization in volcanic areas, as well as in modern geothermal systems, but impacts are always characterized by a retrograde sequence of alteration minerals. [source]


    Geothermometry and geobarometry of overpressured environments in Qiongdongnan Basin, South China Sea,

    GEOFLUIDS (ELECTRONIC), Issue 3 2003
    Honghan Chen
    Abstract We demonstrate the use of PVT fluid inclusion modelling in the calculation of palaeofluid formation pressures, using samples from the YC21-1-1 and YC21-1-4 wells in the YC21-1 structural closure, Qiongdongnan Basin, South China Sea. Homogenisation temperatures and gas/liquid ratios were measured in aqueous fluid inclusions, and associated light hydrocarbon/CO2 -bearing inclusions, and their compositions were determined using a crushing technique. The vtflinc software was used to construct P,T phase diagrams that enabled derivation of the minimum trapping pressure for each order of fluid inclusion. Through the projection of average homogenisation temperatures (155, 185.5 and 204.5C) for three orders of fluid inclusion on the thermal-burial history diagram of the Oligocene Yacheng and Lingshui formations, their trapping times were constrained at 4.3, 2.1 and 1.8 Ma, respectively. The formation pressure coefficient, the ratio of fluid pressure/hydrostatic pressure established by PVT modelling coupled with DST data, demonstrates that one and a half cycles of pressure increase,discharge developed in the Yacheng and Lingshui formations for about 4.3 Ma. In comparison, the residual formation pressure determined by 2D numerical modelling in the centre of LeDong depression shows two and a half pressure increase,discharge cycles for about 28 Ma. The two different methods suggest that a high fluid potential in the Oligocene reservoir of the YC21-1 structure developed at two critical stages for regional oil and natural gas migration and accumulation (5.8 and 2.0 Ma, respectively). Natural gas exploration in this area is therefore not advisable. [source]


    CONSTRAINTS ON DIAGENESIS AND RESERVOIR QUALITY IN THE FRACTURED HASDRUBAL FIELD, OFFSHORE TUNISIA

    JOURNAL OF PETROLEUM GEOLOGY, Issue 1 2001
    C. I. Macaulay
    The hydrocarbon reservoir of the Hasdrubal field (offshore Tunisia) lies within the Eocene El Garia Formation. This formation was deposited on a shallow north- to NE-facing ramp in the Early Eocene and is composed of a belt of nummulitic wackestones-grainstones. The nummulitic fades occupies a range of depositional environments from outer to mid ramp. In addition to Hasdrubal, several other producing oil- and gasfields have been discovered in the variably dolomitised El Garia Formation offshore Tunisia. Cores from three Hasdrubal wells were examined. Reservoir quality shows a limited relationship to primary depositional fabric and has been influenced significantly by compaction and later diagenesis. The highest permeabilities are typically developed within a dolomitised zone which occurs near the middle of the reservoir interval across the entire field, and which may follow a primary wackestone lithofabric (typically 20,30% bulk volume dolomite, with porosities of 15,22% and permeabilities of l-30mD). Fractures, particularly in zones surrounding faults, have resulted in enhanced permeabilities. Combined results of isotope (,18 O -5.0 to -7.3%oPDB) and fluid inclusion (Th 80,90d,C) analyses of dolomites from this dolomitised zone indicate that matrix dolomites are burial diagenesis cements. Dolomitisation of the reservoir was a "closed system " event and was not the result of major fluid flow or mixing. Magnesium ions for dolomitisation were derived from the transformation of high-Mg to low-Mg calcite in nummulite tests within the reservoir fades. Our analyses indicate that calcite cements were precipitated at temperatures of up to almost 150d,C in primary and secondary pores and in variably-sealed fractures Fracture lining and filling cements show a range of ,18 O values, which suggests that the fractures acted as fluid conduits over a range of temperatures during burial diagenesis Fracture densities measured in core increase rapidly close to seismically-resolvable faults in the reservoir facies Fracturing probably resulted in the leakage of hydrocarbons through the Compact Micrite Member seal which overlies the accumulation, as well as facilitating the ingress of hot fluids from stratigraphically deeper levels in the basin [source]


    Modeling of a Constrained Porous Material with an Inclusion

    PROCEEDINGS IN APPLIED MATHEMATICS & MECHANICS, Issue 1 2003
    Pasquale Giovine Prof. Dr.
    A recently developed theory allows to describe a porous solid with very large lacunae partially, or totally, filled by a uid inclusion like an atypical immiscible mixture which consists of a solid with an ellipsoidal microstructure and of a classical fluid. Here we investigate the condition of saturation when the fluid inclusion is incompressible. [source]


    Chemical, Isotopic, and Fluid Inclusion Evidence for the Hydrothermal Alteration of the Footwall Rocks of the BIF-Hosted Iron Ore Deposits in the Hamersley District, Western Australia

    RESOURCE GEOLOGY, Issue 2 2003
    Makoto Haruna
    Abstract. The petrography, chemical, fluid inclusion and isotope analyses (O, Rb-Sr) were conducted for the shale samples of the Mount McRae Shale collected from the Tom Price, Newman, and Paraburdoo mines in the Hamersley Basin, Western Australia. The Mount McRae Shale at these mines occurs as a footwall unit of the secondary, hematite-rich iron ores derived from the Brockman Iron Formation, one of the largest banded iron formations (BIFs) in the world. Unusually low contents of Na, Ca, and Sr in the shales suggest that these elements were leached away from the shale after deposition. The ,18O (SMOW) values fall in the range of + 15.0 to +17.9 per mil and show the positive correlation with calculated quartz/sericite ratios of the shale samples. This suggests that the oxygen isotopic compositions of shale samples were homogenized and equilibrated by postdepositional event. The pyrite nodules hosted by shales are often rimmed by thin layers of silica of varying crystallinity. Fluid inclusions in quartz crystals rimming a pyrite nodule show homogenization temperatures ranging from 100 to 240d,C for 47 inclusions and salinities ranging from 0.4 to 12.3 wt% NaCl equivalent for 18 inclusions. These fluid inclusion data give direct evidence for the hydrothermal activity and are comparable to those of the vein quartz collected from the BIF-derived secondary iron ores (Taylor et al, 2001). The Rb-Sr age for the Mount McRae Shale is 1,952 289 Ma and at least 200 million years younger than the depositional age of the Brockman Iron Formation of , 2.5 Ga in age. All the data obtained in this study are consistent with the suggestion that high temperature hydrothermal fluids were responsible for both the secondary iron ore formation and the alteration of the Mount McRae Shale. [source]


    Determining Pressure with Daughter Minerals in Fluid Inclusion by Raman Spectroscopy: Sphalerite as an Example

    ACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 3 2009
    Yuping YANG
    Abstract: Raman frequency of some materials, including minerals, molecules and ions, shifts systematically with changing pressure and temperature. This property is often used as a pressure gauge in high pressure experiments with the hydrothermal diamond anvil cell (HDAC). Since the system of fluid inclusion is similar to that of HDAC, it can also be used to determine the internal pressure of fluid inclusions. Sphalerite is a common daughter mineral. In this study, the frequency shift of the 350 cm,1 peak of sphalerite has been studied from 296 to 523 K and from 0.07 to 2.00 GPa using the HDAC. The global slope of the isotherms (,V350/,p)T is 0.0048 in the studied pressure range. No significant variation of the slopes with temperature has been observed. The correlation between the frequency shift of the 350 cm,1 peak of sphalerite and pressure and temperature is constrained as P=208.33(,Vp)350+3.13T,943.75. This relationship may be used to estimate the internal pressure of the sphalerite-bearing fluid inclusions. [source]


    Analysis of hopanes and steranes in single oil-bearing fluid inclusions using time-of-flight secondary ion mass spectrometry (ToF-SIMS)

    GEOBIOLOGY, Issue 1 2010
    S. SILJESTRM
    Steranes and hopanes are organic biomarkers used as indicators for the first appearance of eukaryotes and cyanobacteria on Earth. Oil-bearing fluid inclusions may provide a contamination-free source of Precambrian biomarkers, as the oil has been secluded from the environment since the formation of the inclusion. However, analysis of biomarkers in single oil-bearing fluid inclusions, which is often necessary due to the presence of different generations of inclusions, has not been possible due to the small size of most inclusions. Here, we have used time-of-flight secondary ion mass spectrometry (ToF-SIMS) to monitor in real time the opening of individual inclusions trapped in hydrothermal veins of fluorite and calcite and containing oil from Ordovician source rocks. Opening of the inclusions was performed by using a focused C60+ ion beam and the in situ content was precisely analysed for C27,C29 steranes and C29,C32 hopanes using Bi3+ as primary ions. The capacity to unambiguously detect these biomarkers in the picoliter amount of crude oil from a single, normal-sized (15,30 ,m in diameter) inclusion makes the approach promising in the search of organic biomarkers for life's early evolution on Earth. [source]


    Generation and accumulation of oil and condensates in the Wenchang A Sag, western Pearl River Mouth Basin, South China Sea

    GEOFLUIDS (ELECTRONIC), Issue 4 2009
    H. J. GAN
    Abstract The Pearl River Mouth (PRM) Basin is one of four Cenozoic basins in the South China Sea, and the Wenchang A Sag is a secondary depression in the western part of the basin. Both the Wenchang and Enping formations contain good source rocks in the western PRM Basin; however, only the latter has been considered a likely source of the discovered oil and gas. New data from fluid inclusions and the analysis of oil,source rock correlations for the WC10-3 oil and gas pools indicate two stages of petroleum charging, the earlier originating from the Wenchang Formation and the later from the Enping Formation. Kinetics of petroleum generation and structural evolution modeling were employed to further investigate the mechanism of formation of the WC10-3 oil and gas pools. It was shown that the crucial condition for the formation of pools is the time of development of the structural trap. The Wenchang Formation source rocks generated oil from 25 to 14 Ma in the possible source area of the WC10-3 oil and gas pools in the Wenchang A Sag, so that only traps formed earlier than this period could capture oil sourced by the Wenchang Formation. The Enping Formation source rock experienced its oil window from 18 Ma to the present with the main stage of oil generation from 15 to 5 Ma. During this period structural traps in the sag continued to form until movements became weak, so that most pools in the Wenchang A Sag originated from the Enping Formation source rock. The likely dissipation of oil and gas from the earlier stage of charging should be taken into account in assessing the oil potential of the Wenchang A Sag. [source]


    Effects of host mineral re-equilibration during uplift and cooling on the fidelity of primary hydrothermal fluid inclusions: a theoretical example using Mississippi Valley-type ore fluids

    GEOFLUIDS (ELECTRONIC), Issue 2 2009
    M. A. McKIBBEN
    Abstract At the moment of its trapping as a primary fluid inclusion, a hydrothermal fluid is typically at or near equilibrium with multiple mineral species at depth and temperature. After trapping, however, the isolated inclusion fluid can re-equilibrate only with its own host mineral species during later uplift and cooling to surface conditions. Because the solubility versus temperature behavior is unique for each host mineral species, identical inclusions trapped at the same time within different species may re-equilibrate in a disparate manner upon cooling and become variably less representative of the original trapped fluid once they reach ambient temperature. To test the significance of this effect, a series of theoretical equilibrium reaction models was constructed in which a trapped hydrothermal fluid characteristic of Mississippi Valley-type ore deposits is cooled in contact with silicate, sulfide and carbonate hosts, respectively, from 100 to 25C. Dissolved base metal concentrations are predicted to decline by two to four orders of magnitude in inclusions in all hosts, due to the precipitation of optically undetectable masses of sulfide daughter minerals. Fluids in the calcite host show the greatest decline in dissolved base metals upon cooling, due to its retrograde solubility and consequent shift in the pH and aqueous C speciation of the fluid. ,13C values for CO2 in all hosts become depleted by 2,7, relative to the original trapped fluid, with depletions again being the greatest for the calcite host due to its retrograde dissolution. Analytical techniques that extract and analyze the complete contents of fluid inclusions at room temperature can account for the predicted precipitation of microscopic daughter minerals during cooling, but may not compensate for chemical changes caused by the retrograde dissolution of calcite. Such solubility effects are another reason to be cautious in using carbonate minerals for fluid inclusion studies, in addition to their undesirable physical properties of softness, deformability and perfect cleavage. [source]


    Composition and evolution of fluids during skarn development in the Monte Capanne thermal aureole, Elba Island, central Italy

    GEOFLUIDS (ELECTRONIC), Issue 3 2008
    F. ROSSETTI
    Abstract We describe the chemistry of the fluids circulating during skarn formation by focusing on fluids trapped in calcsilicate minerals of the inner thermal aureole of the Late Miocene Monte Capanne intrusion of western Elba Island (central Italy). Primary, CH4 -dominant, C-O-H-S-salt fluid inclusions formed during prograde growth of the main skarn-forming mineral phases: grossular/andradite and vesuvianite. The variable phase ratios attest to heterogeneous entrapment of fluid, with co-entrapment of an immiscible hydrocarbon,brine mixture. Chemical elements driving skarn metasomatism such as Na, K, Ca, S and Cl, Fe and Mn were dominantly partitioned into the circulating fluid phase. The high salinity (apparent salinity between 58 and 70 wt% NaCl eq.) and the C-component of the fluids are interpreted as evidence for a composite origin of the skarn-forming fluids that involves both fluids derived from the crystallizing intrusion and contributions from metamorphic devolatilization. Oxidation of a Fe-rich brine in an environment dominated by fluctuation in pressure from lithostatic to hydrostatic conditions (maintained by active crack-sealing) contributed to skarn development. Fluid infiltration conformed to a geothermal gradient of about 100C km,1, embracing the transition from high-temperature contact metamorphism and fluid-assisted skarn formation (at ca 600C) to a barren hydrothermal stage (at ca 200C). [source]


    The application of structured-light illumination microscopy to hydrocarbon-bearing fluid inclusions

    GEOFLUIDS (ELECTRONIC), Issue 2 2008
    N. J. F. BLAMEY
    Abstract Structured-light illumination (SLI)-based microscopy offers geologists a new perspective for screening of hydrocarbon-bearing (HCFI) and small aqueous fluid inclusion (AFI) assemblages. This optical-sectioning technique provides rapid, confocal-like imaging, using relatively simple and inexpensive instrumentation. The 3D fluorescent images of HCFI planes and large single HCFIs permit the visualization of the relationships between HCFI assemblages, examination of HCFI morphology, and volume estimates of the fluorescent components within HCFIs. By the use of normal white light illumination, SLI image capture, and varying acquisition time it is also possible to image AFI because of the random movements of vapour bubbles within the inclusions. This allows the near-simultaneous visualization of hydrocarbon and AFI which is of significant importance for the study of sedimentary basins and petroleum reservoirs. SLI is a unique and accessible 3D petrographic tool, with practical advantages over conventional epifluorescence and confocal laser scanning microscopy. [source]