Inclusion Assemblages Research Articles

Are silicate‐rich inclusions in spodumene crystallized aliquots of boundary layer melt?

AbstractThe constitutional zone refining model for the internal crystallization of granitic pegmatites stipulates that a highly fluxed and incompatible element‐enriched boundary layer melt evolves at the surface of crystal growth fronts in undercooled granitic melts. It has been argued that silicate‐rich inclusions in primary spodumene and petalite from the Tanco pegmatite are the crystallized products of an entrapped boundary layer melt. However, inclusions of identical character and composition to those in primary spodumene are also abundant in the spodumene component of SQI (spodumene + quartz intergrowths) that formed by the isochemical breakdown of petalite. Both textural types of spodumene at Tanco host fluid inclusion assemblages that display highly variable proportions of solid phases (dominantly cookeite, quartz, and zabuyelite) and a low salinity (approximately 6 wt% NaCl equivalent) aqueous fluid. The average major element content of three fluid inclusion assemblages is approximately Li2O (3 wt%), Al2O3 (11 wt%), SiO2 (30 wt%), CO2 (3 wt%) and H2O (53 wt%). The entrapment of these fluid inclusions is temporally and spatially unrelated to the flux‐rich melt that may have developed at the primary crystallization front of the pegmatite. Petrographic evidence and the mineralogical and chemical composition of spodumene‐hosted inclusions suggest that inclusions in spodumene are the products of hydrothermal dissolution of the host and the precipitation of quartz, cookeite, and zabuyelite prior to and during necking.

Hydrothermal Fluid Processes and Evolution of the Giant Serra Norte Jaspilite-Hosted Iron Ore Deposits, Carajas Mineral Province, Brazil

Hydrothermal Fluid Processes and Evolution of the Giant Serra Norte Jaspilite-Hosted Iron Ore Deposits, Carajas Mineral Province, Brazil

Hydrocarbon charge history of the Tazhong Ordovician reservoirs, Tarim Basin as revealed from an integrated fluid inclusion study

Abstract This paper presents an integrated workflow for investigating hydrocarbon charge history using fluid inclusions using the Ordovician reservoirs from the Tazhong Oilfield, Tarim Basin as an example. The work flow involves the delineation of fluid inclusion assemblage (FIA) using fluid inclusion petrography and spectroscopy for microthermometric analysis. The interpretation of microthermometric data is based on data derived from synthetic inclusion experiments and takes consideration of the P-T re-equilibration of fluid inclusions in carbonate and over pressure effect on fluid inclusion trapping. The workflow also emphasizes on the requirements of adequate sample preparation, FIA identification, quantitative spectroscopic data (CIE), coeval petroleum and aqueous fluid inclusions, adequate number of microthermometric measurements and integrated interpretation using borehole temperature. In the study reservoirs in the Tazhong area, fluid inclusion spectroscopy reveals the presence of three groups of oil inclusions of near yellow, near blue and near white fluorescence colours. The fluid inclusion microthermometry data indicate the presence of at least two predominant hydrocarbon fluid inclusion assemblages including: homogenous liquid phase yellow fluorescence inclusions and liquid phase oil and condensate fluid inclusion assemblages with homogenous near blue, near white and near yellow fluorescence colours. The aqueous inclusions from the Ordovician reservoirs in the Tazhong area reveal the presence of low, moderate, high and very high salinities in the reservoirs. The extremely high salinity is believed to be caused by regional brine intrusions relating to regional structural movement and deformation.

Segregation of magmatic fluids and their potential in the mobilization of platinum-group elements in the South Kawishiwi Intrusion, Duluth Complex, Minnesota — Evidence from petrography, apatite geochemistry and coexisting fluid and melt inclusions

Pegmatitic and other felsic rock pockets and dike-like intrusions are abundant in the South Kawishiwi Intrusion of the Duluth Complex, including the basal, Cu–Ni–PGE mineralized units. These occurrences are found as pockets, pods or as veins and contain abundant accessory apatite and quartz. Quartz hosts primary fluid inclusions as well as silicate melt inclusions. Combined microthermometry and Raman spectroscopy helped to determine the bulk composition of primary fluid inclusions that are CO2-rich (95mol%) and contain small amounts of H2O (4.5mol%), CH4 (0.4mol%) and trace N2, respectively. This combined technique also made it possible to measure total homogenization temperatures of the inclusions (Thtot=~225±10°C), otherwise not detectable during microthermometry. Silicate melt inclusions have been quenched to produce homogeneous glasses corresponding to the original melt. Composition of the entrapped melt is granitoid, peraluminous and is very poor in mafic components. We interpret the melt as a product of partial melting of the footwall rocks due to the contact effect of the South Kawishiwi Intrusion. The presence of CO2 in the vapor bubbles of the quenched melt inclusions and petrographic evidence suggest that the fluid and melt inclusion assemblages are coeval. The composition of the fluid and melt phase implies that the fluid originates from the mafic magma of the South Kawishiwi Intrusion and the fluid and melt phases coexisted as a heterogeneous melt–fluid system until entrapment of the inclusions.Coexistence of primary fluid and melt inclusions makes it possible to calculate a minimum entrapment pressure (~1.7kbar) and thus estimate formation depth (~5.8km) for the inclusions. Chlorine is suggested to behave compatibly in the silicate melt phase in the fluid–melt system represented by the inclusions, indicated by the high (up to 0.3%) Cl-concentrations of the silicate melt and CO2-rich nature of the fluid.Apatite halogen-contents provide further details on the behavior of Cl. Apatite in pegmatitic pockets often has elevated Cl-concentrations compared to troctolitic rocks, suggesting enrichment of Cl with progressive crystallization. Systematic trends of Cl-loss at some differentiated melt pockets suggest that in some places Cl exsolved into a fluid phase and migrated away from its source. The segregation of Cl from the melt is probably inhibited by the presence of CO2-rich fluids until the last stages of crystallization, increasing the potential for the development of late-stage saline brines.Platinum-group minerals are often present in microcracks in silicate minerals, in late-stage differentiated sulfide veinlets and in association with chlorapatite, indicating the potential role of Cl-bearing fluids in the final distribution of PGEs.

A Fluid Inclusion and Stable Isotope Study at the Loulo Mining District, Mali, West Africa: Implications for Multifluid Sources in the Generation of Orogenic Gold Deposits

The world-class Loulo mining district (15.5 Moz resource), in the Birimian terrane of western Mali, contains a range of mineralogically diverse styles of orogenic gold mineralization. The district is distinguished by As-rich orebodies as at Yalea, as well as tourmaline-bearing, Fe-rich, orebodies as at Gara. New fluid inclusion and stable isotope data presented here constrain the nature of the hydrothermal fluids responsible for these different types of mineralization, and point towards the role of multifluid sources (metamorphic and magmatic) in the formation of orogenic gold deposits. Microthermometric and laser Raman studies from Yalea Main and two other similar orebodies (Loulo-3 and Baboto) reveal the dominance of coexisting CO 2 -N 2 ±CH 4 (type 1) and H 2 O-NaCl (type 2) fluid inclusions. These inclusions show evidence of fluid unmixing from reduced (quartz-faylite-magnetite, QFM, buffered), low-salinity (≤10 wt % NaCl equiv), CO 2 -rich-H 2 O-NaCl-N 2 ± CH 4 primary ore fluids. The combination of microthermometric data and geothermometry based on ore and alteration assemblages indicate trapping temperatures and pressures of 270° to 340°C at 1.4 to 1.8 kbar. The P-T-X state of the Yalea-style mineralizing fluids, along with the δ 18 O fluid compositions of 8.8 to 10.7‰, is consistent with the derivation of auriferous fluids during greenschist facies regional metamorphism of the host terrane. Similar fluid compositions were previously reported elsewhere in the Birimian crust of West Africa and in other orogenic gold districts worldwide. The precipitation of gold from the H 2 S-rich metamorphic fluid is primarily linked to phase separation of the ore fluid, which is controlled by P-T fluctuations and/or fluid-rock interaction with carbonaceous host sedimentary rocks (confirmed by low δ 13 C values of −21.7 to −15.8‰). Fluid inclusion investigations from Gara and a similar style orebody (Yalea North) indicate the presence of coexisting CO 2 ± N 2 ± CH 4 (type 1) and mixed-salinity (5–21 wt % NaCl equiv) CO 2 -rich-H 2 O-NaCl inclusions (type 3). Inclusion assemblages also contain common oxidized (hematite-magnetite, HM, buffered), high temperature (>400°C), hypersaline (∼35–50 wt % total dissolved solids), metalliferous (Na+Fe+Ca+Cu+Ni+W+ Pb+Zn), multiphase H 2 O-rich-CO 2 -NaCl-FeCl 2 inclusions (type 4). This inclusion type has not been previously reported in other Birimian terranes. The composition of the brines, along with carbon isotope data ( δ 13 C of −14.4 to −4.5‰), suggests a magmatic input to the Gara-style hydrothermal system. The coexistence in the fluid inclusion assemblages of the magmatic brines with Yalea-style, CO 2 -rich, metamorphic fluids and the positive correlation between salinity and homogenization temperatures suggest mineralization was locally controlled by fluid mixing. The interaction of these two chemically contrasting fluids explains the distinctive petrographic characteristics of the Gara-style orebodies. This includes the growth of widespread multi-stage Fe 3+ -rich tourmaline (B-rich granite source) and sodic alteration, and ore assemblages consisting of abundant nickeloan pyrite, (REE)-phosphates, Ni ± Co ± Pb ± Zn minor/trace sulfides, and scheelite. Gold deposition in the Gara-style hydrothermal system is related to physical and chemical changes of the two fluids during mixing (e.g., decreases in f O2 and T in the brines and retrograde boiling of the CO 2 component in the metamorphic fluids, a “salting out effect”).

Origin of primary PGM assemblage in сhromitite from a mantle tectonite at Harold’s Grave (Shetland Ophiolite Complex, Scotland)

In this paper we present textural and mineral chemistry data for a PGM inclusion assemblage and whole-rock platinum-group element (PGE) concentrations of chromitite from Harold’s Grave, which occurrs in a dunite pod in a mantle tectonite at Unst in the Shetland Ophiolite Complex (SOC), Scotland. The study utilized a number of analytical techniques, including acid digestion and isotope dilution (ID) ICP-MS, hydroseparation and electron microprobe analysis. The chromitite contains a pronounced enrichment of refractory PGE (IPGE: Os, Ir and Ru) over less refractory PGE (PPGE: Rh, Pt and Pd), typical of mantle hosted ‘ophiolitic’ chromitites. A ‘primary’ magmatic PGM assemblage is represented by euhedrally shaped (up to 60 μm in size) single and composite inclusions in chromite. Polyphase PGM grains are dominated by laurite and osmian iridium, with subordinate laurite + osmian iridium + iridian osmium and rare laurite + Ir-Rh alloy + Rh-rich sulphide (possibly prassoite). The compositional variability of associated laurite and Os-rich alloys at Harold’s Grave fit the predicted compositions of experiment W-1200-0.37 of Andrews and Brenan (Can Mineral 40: 1705–1716, 2002) providing unequivocal information on conditions of their genesis, with the upper thermal stability of laurite in equilibrium with Os-rich alloys estimated at 1200–1250 °C and f(S2) of 10−0.39–10−0.07.

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

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

Major and trace-element composition and pressure–temperature evolution of rock-buffered fluids in low-grade accretionary-wedge metasediments, Central Alps

The chemical composition of fluid inclusions in quartz crystals from Alpine fissure veins was determined by combination of microthermometry, Raman spectroscopy, and LA-ICPMS analysis. The veins are hosted in carbonate-bearing, organic-rich, low-grade metamorphic metapelites of the Bundnerschiefer of the eastern Central Alps (Switzerland). This strongly deformed tectonic unit is interpreted as a partly subducted accretionary wedge, on the basis of widespread carpholite assemblages that were later overprinted by lower greenschist facies metamorphism. Veins and their host rocks from two locations were studied to compare several indicators for the conditions during metamorphism, including illite crystallinity, graphite thermometry, stability of mineral assemblages, chlorite thermometry, fluid inclusion solute thermometry, and fluid inclusion isochores. Fluid inclusions are aqueous two-phase with 3.7–4.0 wt% equivalent NaCl at Thusis and 1.6–1.7 wt% at Schiers. Reproducible concentrations of Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, B, Al, Mn, Cu, Zn, Pb, As, Sb, Cl, Br, and S could be determined for 97 fluid inclusion assemblages. Fluid and mineral geothermometry consistently indicate temperatures of 320 ± 20 °C for the host rocks at Thusis and of 250 ± 30 °C at Schiers. Combining fluid inclusion isochores with independent geothermometers results in pressure estimates of 2.8–3.8 kbar for Thusis, and of 3.3–3.4 kbar for Schiers. Pressure–temperature estimates are confirmed by pseudosection modeling. Fluid compositions and petrological modeling consistently demonstrate that chemical fluid-rock equilibrium was attained during vein formation, indicating that the fluids originated locally by metamorphic dehydration during near-isothermal decompression in a rock-buffered system.

Temporal and spatial distribution of alteration, mineralization and fluid inclusions in the transitional high-sulfidation epithermal–porphyry copper system at Red Mountain, Arizona

Red Mountain, Arizona, is a Laramide porphyry Cu system (PCD) that has experienced only a modest level of erosion compared to most other similar deposits in the southwestern United States. As a result, the upper portion of the magmatic–hydrothermal system, which represents the transition from shallower high-sulfidation epithermal mineralization to deeper porphyry Cu mineralization, is well preserved.Within the Red Mountain system, alteration, mineralization and fluid inclusion assemblages show a systematic distribution in both time and space. Early-potassic alteration (characterized by the minerals biotite and magnetite) is paragenetically earlier than late-potassic alteration (K-feldspar–anhydrite) and both are followed by later phyllic (sericite–pyrite) alteration. Advanced argillic alteration (pyrophyllite–alunite–other clay minerals) is thought to be coeval with or postdate phyllic alteration. Minerals characteristic of advanced argillic alteration are present in the near surface. Phyllic alteration extends to greater depths compared to advanced argillic alteration. Early-potassic and late-potassic alteration are only observed in the deepest part of the system. Considerable overlap of phyllic alteration with both early-potassic and late-potassic alteration zones is observed.The hypogene mineralization contains 0.4–1.2% Cu and is spatially and temporally related to the late-potassic alteration event. Molybdenum concentration is typically <300ppm but positive anomalies (between 600 and 1200ppm) occur, and typically correlate with the zones of higher Cu grades. Silver and Au range up to 50ppm and 1ppm, respectively, and mostly occur in the deeper parts of the system. Individual assays of up to 18ppm Au and 274ppm Ag in the shallower part of the system are interpreted to be associated with areas of highly focused fluid flow (i.e., breccias and thick veins). A near-surface, discontinuous chalcocite blanket is represented by scattered Cu anomalies within the mixed oxide/sulfide zone and its discontinuous nature may reflect differential permeability along fractures and faults.In the deepest part of the system, an early generation of low-to-moderate density and salinity liquid+vapor inclusions with opaque daughter minerals is followed in time by halite-bearing inclusions that also contain opaque daughter minerals indicating that an early intermediate-density magmatic fluid evolved to a high-density, high-salinity mineralizing fluid. The increase in density and salinity of fluids with time observed in the deeper parts of the system may be the result of immiscibility (“boiling”) of the earlier magmatic fluids or may reflect the compositional evolution of fluids that exsolved from the magma. Trails of inclusions consisting of only vapor-rich inclusions are common in the shallow parts of the system, and are associated with advanced argillic alteration, suggesting that intense boiling (“flashing”) occurred at (or below) this level. Fluid inclusion assemblages consisting of coexisting vapor-rich and halite-bearing inclusions are observed in samples extending from the surface to the upper part of the late-potassic zone, indicating that fluid immiscibility occurred within this depth interval.

Multiple metamorphic events revealed by zircons from the Diancang Shan−Ailao Shan metamorphic complex, southeastern Tibetan Plateau

Analyses of mineral inclusions within complexly zoned zircons, combined with SIMS UPb ages for various domains within the zircons, provide evidence of the origin and multistage metamorphic evolution of the Diancang Shan−Ailao Shan metamorphic complex, southeastern Tibetan Plateau. Zircons separated from paragneisses, amphibolites, garnet pyroxenites, and marbles were subdivided into inherited (magmatic or detrital) and metamorphic zircons based on cathodoluminescence (CL) and Laser Raman spectroscopy. Weakly zoned, inherited detrital zircons from paragneisses yield ages of 700−540Ma, 1000−800Ma, ~1800Ma, and 2550−2450Ma, and contain inclusion assemblages of Qtz+Kfs+Pl+Ap, recording widespread geological events throughout the ancient Gondwana continent. In contrast, clearly zoned, inherited magmatic zircons in amphibolites and garnet pyroxenites yield ages of ~778−773Ma and contain Cpx+Pl±Qtz+Ap inclusions, representing the crystallization age of the mafic protolith. Metamorphic zircons also record discrete Early−Middle Triassic and the Eocene−Oligocene thermal events. Homogeneous metamorphic zircons with low Th/U ratios (<0.1) contain high-pressure (HP)-diagnostic Grt+Ky+Phe/Mus+Kfs+Qtz+Ap, and Grt+Cpx+Pl±Qtz inclusions in paragneisses, and amphibolites and garnet pyroxenites, respectively, suggesting their formation at 650−720°C and ~14kb. These HP zircons yield 206Pb/238U ages of 249−230Ma, indicating an Early−Middle Triassic HP metamorphic event in the Diancang Shan−Ailao Shan complex belt. Two groups of Eocene−Oligocene metamorphic zircons are also present. One of the groups comprises homogenous zircons with low Th/U ratios (<0.1) that contain medium-pressure (MP) amphibolite–granulite facies Grt+Ky+St+Bt+Kfs+Qtz, Grt+Cpx+Hb+Pl, and Dol+Cal+Ap in paragneisses, amphibolites, and marbles. These assemblages are stable at peak pressure−temperature (P−T) conditions of 720°C−760°C and 8.0−9.6kb, and yield consistent Eocene ages (44−36 Ma). The second group is homogeneous and all zircons have similar Th/U ratios, but contain distinct low-pressure (LP) Grt+Sil+Bt+Kfs+Qtz+Ap, Opx±Cpx±Grt+Hb+Pl, Cal±Cpx+Ap inclusions in paragneisses, amphibolites and garnet pyroxenites, and marbles, recording post-peak P−T conditions of 700°C−750°C and 5.0−6.5kb, and yielding younger Oligocene metamorphic ages (32−25Ma). 40Ar/39Ar analyses of biotite, muscovite, and amphibole yield the youngest ages (25−14Ma, Miocene) related to late cooling during retrogressive metamorphism under conditions of 520°C−620°C and 4.0−4.5kb.These new data suggest a clockwise P−T−t path for the Diancang Shan−Ailao Shan metamorphic complex belt, typical of continent−continent collision, indicating collision−subduction tectonism prior to the strong, left-lateral, ductile deformation along the Ailao Shan−Red River (ASRR) shear zone that started at ~32Ma and lasted until 25Ma, causing mid-crustal (18−25km depth) high-temperature metamorphic conditions. Continued uplift, and a slowing of left-lateral ductile shearing occurred at ~25−14Ma at a depth of 10−15km and under relatively low-temperature conditions. This temporal and kinematic link between left-lateral shearing along the ASRR and the opening of the South China Sea supports the occurrence of the block extrusion of Indochina from Eurasia along lithospheric-scale strike-up faults.

Hydrothermal fluid evolution and metal transport in the Kiruna District, Sweden: Contrasting metal behaviour in aqueous and aqueous–carbonic brines

Iron oxide–copper–gold (IOCG) deposits and Fe oxide-apatite deposits from Norrbotten, Sweden, formed in similar settings, and in some cases IOCG mineralisation overprinted Fe oxide-apatite mineralisation. Fluid inclusions in quartz veins cutting Fe oxide-apatite deposits range in salinity from 33 to 37wt.% NaCl eq., and those in IOCG-type deposits from 41 to 54wt.% NaCl eq. Minimum trapping conditions for these inclusions are ∼200–300MPa and 200–300°C in the Fe oxide-apatite bodies, and 250 to >300MPa and 300–500°C in the IOCG deposits. Deformed Cu–Au deposits have similar early fluid characteristics, but contain complex secondary fluid inclusion assemblages including halite saturated (20–30wt.% NaCl eq.), aqueous–carbonic (3–13wt.% NaCl eq.; X(CO2) 0.17–0.29) and CO2-rich fluids. The aqueous–carbonic and carbonic inclusions are consistent with aqueous–carbonic fluid immiscibility at ∼150MPa. A secondary population, with a high Ca-content occurs in all deposit types.The chemical composition of these inclusions has been determined by crush-leach analysis and LA-ICPMS. Halogen contents indicate a range of salinity sources with possible inputs from both magmatic and halite-dissolution brines. Element ratios suggest the alkali content of the fluid exceeded the buffer capacity of the host rocks. Iron and other transition metal contents correlate strongly with Cl concentrations, with secondary controls on solubility from pH, redox and temperature. Copper and Ag contents are higher in lower salinity aqueous–carbonic brines (up to 5000ppm Cu, 900ppm Ag) than in the most saline brines (up to 2297ppm Cu, 837ppm Ag). This may reflect differences in metal source between deposit types, but is also consistent with the complexation of Cu by bisulphide in the lower salinity fluids. Late stage aqueous–carbonic fluid flux through the deformed deposits either introduced additional copper to the deposits, remobilised pre-existing copper or both.

Origin of enclaves in S-type granites of the Lachlan Fold Belt

The more mafic S-type granites of the Lachlan Fold Belt contain a distinctive assemblage of lithic inclusions of deep crustal origin. Two types predominate, the schistose enclaves and the microgranular enclaves. There is also a small proportion of fragments of local country rock. All the features of the schistose enclaves are consistent with an origin as lithic restite fragments from the source. Their ubiquity in the mafic S-type granites and absence from both felsic S-type granites and I-type granites, conform with such an origin. The argument that these enclaves are not in chemical equilibrium with the host granite because they do not complement its composition is not valid, since they represent less fertile parts of the source that had different compositions that, as a result, melted to a lesser extent. Models that derived these enclaves from regions of the crust above the zones of partial melting, so that they are not restites, are complex, unnecessary, do not conform with their ubiquity in the more mafic S-type granites, and are not consistent with their chemical compositions.The origin of the microgranular enclaves is controversial. In most S-type granites these enclaves have been recrystallized to a similar mineral assemblage to the host granite with a higher proportion of biotite. In a few places, such material is seen to be forming by recrystallization of material in the core of the enclave, which has the assemblage quartz+calcic plagioclase+orthopyroxene±cordierite±biotite with accessory ilmenite, sulfide and apatite. All of the microgranular enclave cores show pseudo-doleritic texture in which calcic plagioclase crystals (>An60) and orthopyroxene crystals project into or are enclosed by quartz. Large plagioclase crystals are zoned, with some core compositions as calcic as An94. The low Na contents of the microgranular enclaves are not consistent with an igneous origin. We consider that these enclaves were derived by metamorphism of calcareous mudstones or argillaceous limestones. The pseudo-igneous textures result from the presence of a partial melt during metamorphism. An analysis of a calc-silicate lens of upper amphibolite grade from the Wilsons Group of Victoria Land, Antarctica is remarkably similar in composition to the microgranular enclaves from Lachlan Fold Belt S-type granites. We suggest that rocks equivalent to that Group, which is of appropriate age and general composition, were the source rocks for the S-type granites of the Lachlan Fold Belt.

Fluid evolution of the Yuchiling porphyry Mo deposit, East Qinling, China

The Yuchiling Mo deposit, East Qinling, China, belongs to a typical porphyry Mo system associated with high-K calc-alkaline intrusions. The pure CO2 (PC), CO2-bearing (C), aqueous H2O-NaCl (W), and daughter mineral-bearing (S) fluid inclusions were observed in the hydrothermal quartz. Based on field investigations, petrographic, microthermometric and LA-ICP-MS studies of fluid inclusions, we develop a five-stage fluid evolution model to understand the ore-forming processes of the Yuchiling deposit. The earliest barren quartz±potassic feldspar veins, developed in intensively potassic alteration, were crystallized from carbonic-dominant fluids at high temperature (>416°C) and high pressure (>133MPa). Following the barren quartz±potassic feldspar veins are quartz-pyrite veins occasionally containing minor K-feldspar and molybdenite, which were formed by immiscible fluids at pressures of 47–159MPa and temperatures of 360–400°C. The fluids were characterized by high CO2 contents (approximately 8mol%) and variable salinities, as well as the highest Mo contents that resulted in the development of quartz-molybdenite veins. The quartz-molybdenite veins, accounting for >90% Mo in the orebody, were also formed by immiscible fluids with lower salinity and lower CO2 content of 7mol%, at temperatures of 340–380°C and pressures of 39–137MPa, as constrained by fluid inclusion assemblages. After the main Mo-mineralization, the uneconomic Cu-Pb-Zn mineralization occurred, as represented by quartz-polymetallic sulfides veins consisting of pyrite, molybdenite, chalcopyrite, digenite, galena, sphalerite and quartz. The quartz-polymetallic sulfide veins were formed by fluids containing 5mol% CO2, with minimum pressures of 32–110MPa and temperatures of 260–300°C. Finally, the fluids became dilute (5wt.% NaCl equiv) and CO2-poor, which caused the formation of late barren quartz±carbonate±fluorite veins at 140–180°C and 18–82MPa.It is clear that the fluids became more dilute, CO2-poor, and less fertile, with decreasing temperature and pressure from quartz-pyrite to late barren veins. Molybdenite and other sulfides can only be observed in the middle three stages, i.e., quartz-pyrite, quartz-molybdenite and quartz-polymetallic sulfide veins. These three kinds of veins are generally hosted in potassic altered rocks with remarkable K-feldspathization, but always partly overprinted by phyllic alteration. The traditional porphyry-style potassic–phyllic–propylitic alteration zoning is not conspicuous at Yuchiling, which may be related to, and characteristic of, the CO2-rich fluids derived from the magmas generated in intercontinental collision orogens.Among the fluid inclusions at Yuchiling, only the C-type contains maximum detectable Mo that gradationally decreases from 73ppm in quartz-pyrite veins, through 19ppm in quartz-molybdenite veins, and to 13ppm in quartz-polymetallic sulfide veins, coinciding well with the decreasing CO2 contents from 8mol%, through 7mol%, to 5mol%, respectively. Hence it is suggested that decreasing CO2 possibly results in decreasing Mo concentration in the fluids, as well as the precipitation of molybdenite from the fluids. This direct relationship might be a common characteristic for other porphyry Mo systems in the world.The Yuchiling Mo deposit represents a new type Mo mineralization, with features of collision-related setting, high-K calc-alkaline intrusion, CO2-rich fluid, and unique wall-rock alterations characterized by strong K-feldspathization and fluoritization.

Chemical changes of fluid inclusion oil trapped during the evolution of an oil reservoir: Jabiru-1A case study (Timor Sea, Australia)

This study of the Jabiru oil field, Timor Sea (Australia), shows how the variability of visual properties of fluid inclusions in palaeo-oil zones can be mapped to identify internal structures that result from fluid interactions in the reservoir. Variability in the attributes of oil inclusion assemblages (OIAs) is greater within an inferred palaeo-gas cap (Zone 1) and current residual zone (Zone 3) and less within the current oil zone (Zone 2). In Zone 3, where water imbibed, a higher proportion of OIAs with visual white fluorescing oil inclusions correlates with less mono-aromatic compounds and a higher abundance of polar compounds determined on a bulk extract of inclusion oil using High Performance Liquid Chromatography (HPLC). The CH2/CH3 ratios and water contents, determined by Fourier Transform-Infrared Spectroscopy (FT-IR), are higher. In Zone 1, where gas drained, there is a higher proportion of OIAs with visual yellow fluorescing oil inclusions, and these correlate with low HPLC mono-aromatic and moderate polar contents, and low FT-IR CH2/CH3 ratios and water contents. Interpretation of the grains with oil inclusions (GOI™) and pressure–volume–temperature-composition (PVTx) fluid inclusion data suggests that a gas-saturated blue fluorescing palaeo-oil was present in the three zones at the same Pressure–Temperature fluid inclusion forming event PT1. Lighter gas-saturated blue fluorescing oil was present in Zone 2, but was rarer in Zones 1 and 3 at the later fluid inclusion forming event PT2. The yellow and white inclusion oils, trapped in Zones 1 and 3 respectively, are gas-depleted and interpreted to result from the interaction between non-movable residual oil around grains with the surrounding gas and water, respectively. Comparison between the PVTx data and the published burial curve suggests that most of the fluid inclusions were trapped during two different hot fluid flow events. The full oil column is interpreted to have been present during Early–Mid Tertiary. The palaeo-oil column was reduced during the Late Miocene by imbibition of water in the present residual zone and drainage of gas at the top of the reservoir.

Easily altered minerals and reequilibrated fluid inclusions provide extensive records of fluid and thermal history: gypsum pseudomorphs of the Tera Group, Tithonian-Berriasian, Cameros Basin

Abstract This study reports a complex fluid and thermal history using petrography, electron microprobe, isotopic analysis and fluid inclusions in replacement minerals within gypsum pseudomorphs in Tithonian-Berriasian lacustrine deposits in Northern Spain. Limestones and dolostones, formed in the alkaline lakes, contain lenticularly shaped gypsum pseudomorphs, considered to form in an evaporative lake. The gypsum was replaced by quartz and non-ferroan calcite (Ca-2), which partially replaces the quartz. Quartz contains solid inclusions of a preexisting non-ferroan calcite (Ca-1), anhydrite and celestine. High homogenization temperatures (T h) values and inconsistent thermometric behaviour within secondary fluid inclusion assemblages in quartz (147–351°C) and calcite (108–352°C) indicate high temperatures after precipitation and entrapment of lower temperature FIAs. Th are in the same range as other reequilibrated fluid inclusions from quartz veins in the same area that are related to Cretaceous hydrothermalism. Gypsum was replaced by anhydrite, likely during early burial. Later, anhydrite was partially replaced by Ca-1 associated with intermediate burial temperatures. Afterward, both anhydrite and Ca-1 were partially replaced by quartz and this by Ca-2. All were affected during higher temperature hydrothermalism and a CO2-H2O fluid. Progressive heating and hydrothermal pulses, involving a CO2-H2O fluid, produce the reequilibration of the FIAs, which was followed by uplift and cooling.

Fluid inclusion studies in datolite of low grade metamorphic origin from a Jurassic pillow basalt series in northeastern Hungary

Abstract The Jurassic pillow basalt blocks in the Szarvaskő Unit in the SW-Bükk Mountains are incorporated into an accretionary mélange of Cretaceous age which had been displaced from the Dinarides to NE-Hungary during the Alpine-Carpathian collision. The pillow basalt series is a part of an incomplete remnant of an ophiolite-like sequence from which ultramafic units and sheeted dikes are almost completely absent. At the studied Egerbakta quarry, closely packed pillow, pillow fragmented hyaloclastite breccia and peperitic facies of submarine basaltic volcanism were recognized. Peperitic facies was formed by local admixture of unconsolidated siliciclastic sediment into the basaltic lava. Basaltic pillows contains short and thin prehnite-chlorite-carbonate-quartz veins formed during the interaction of seawater with the cooling lava, as well as cross-cutting datolite-prehnite-quartzchlorite-albite bearing veins. Calcite in the short cooling cracks crystallised from upheated seawater at around 160°C temperature and 0.5–0.6 kbar pressure (water depth of about 5–6 km). Datolite precipitations in veins can be classified into at least three different textural-morphological types but all of them contain methane bearing primary fluid inclusion assemblages, with variable methane/water ratios, suggesting an inhomogeneous entrapment from a heterogeneous aqueous-carbonic fluid. End member primary fluid inclusions contain aqueous liquid with methane rich vapour phase and apparently methane only inclusions characterized by liquid phase on room temperature. A combination of results from fluid inclusion microthermometry, Raman spectroscopy (used not only for phase identification but also for determination of salinities) and temperatures calculated from chlorite compositions, show that the precipitation of datolite had taken place from low salinity (0.2–2 NaCl equiv. mass%) aqueous-methane fluids under increasing temperature and pressure (from 160 to 210°C, and from 0.6 to 1.1 kbar) conditions. The elevated pressures and temperatures in comparison to the submarine hydrothermal processes suggest that the datolite bearing mineral parageneses had been formed during the progression of Alpine very low grade metamorphic processes.

Fluid–rock interaction across the South Tibetan Detachment, Garhwal Himalaya (India): Mineralogical and geochemical evidences

The Malari Leucogranite in the Garhwal Himalaya is cut across by a continental-scale normal fault system called the South Tibetan Detachment (STD). A mineralogical, geochemical and fluid inclusion study of samples from the fault zone of the Malari Granite was performed to reveal the imprints of fluid–rock interaction. Fluid inclusion assemblages observed in the alteration zone indicate the presence of NaCl-dominated aqueous fluids with varied salinity of 6–16 wt.% of NaCl equivalent. Mineralogical changes include the alteration of feldspar to muscovite and muscovite to chlorite. This alteration took place at temperatures of 275°–335°C and pressures between 1.9 and 4.2 kbars as revealed by the application of chlorite thermometry, fluid isochores, and presence of K-feldspar+muscovite+chlorite+quartz mineral assemblage. Geochemical mass-balance estimates predict 32% volume loss during alteration. An estimated fluid/rock ratio of 82 is based on loss of silica during alteration, and reveals presence of a moderately low amount of fluid at the time of faulting.Results of fluid inclusion and alteration mineralogy indicate that the Malari Leucogranites were exhumed due to normal faulting along the STD and erosion from mid-crustal levels. Most of the leucogranites in the Himalayas occur along the STD and possibly a regional-scale fluid flow all along the STD might have caused similar alteration of leucogranites along this tectonic break. Regional fluid flow was probably concentrated along the STD and channelized through mesoscopic fractures, microcracks and grain boundaries.

Mineral composition in cognate inclusions in Late Miocene–Early Pliocene potassic lamprophyres with affinities to lamproites from the Denizli region, Western Anatolia, Turkey: Implications for uppermost mantle processes in a back-arc setting

Mineral assemblages of both cognate inclusions and host-lamprophyres with lamproitic affinity in Kocapınar district (Denizli), Western Anatolia, consist of phlogopitic mica, diopsidic clinopyroxene, K-feldspar, apatite, opaque and carbonates (calcite–dolomite). Two distinct types of cognate inclusions have been defined in host lamprophyres: clinopyroxene-rich (CCI) and clinopyroxene–phlogopite-rich (CPCI). Whole-rock compositions of inclusions show a near-primitive nature with high MgO (7.56–15.1wt.%), Cr (195–2270ppm), Ni (213–335ppm) contents and a potassic character [K2O (2.0–2.8wt.%)>Na2O (0.4–2.2wt.%)]. Data imply that the inclusions crystallized from magmas formed by melting of phlogopite-bearing pyroxenites in a peridotitic mantle source, however, the presence of reverse-zoning in clinopyroxenes with salitic Fe-rich green cores in host-lamprophyres and CPCI inclusions suggest that the Denizli lavas represent mixtures of distinct (probably ultrapotassic and alkali basaltic) magmas. Estimated geobarometric constraints inferred from clinopyroxene compositions in CCI and CPCI inclusions indicate moderate pressures of pyroxene crystallization (ranging between 1.7 and 2.2GPa and corresponding to 53–70km depths) under low pressure magma fractionation. Results reveal that i) the origin of Denizli lamprophyres with transitional (between arc-type and intra-plate-type) geochemical signatures is consistent with a shallow level mantle petrogenesis, rather than a deep-seated origin related to mantle convection, ii) the source was a highly refractory and metasomatized peridotitic mantle present at the base of the lower crust, iii) metasomatic agents that affected the mantle lithosphere beneath Denizli region are distinct from those beneath other western Anatolia orogenic centers, and iv) the transitional character of Kocapınar (Denizli) lamprophyric rocks were probably formed as a result of either underplating or contamination of asthenospheric magma at the base of the mantle lithosphere, or assimilation of delaminated continental edge-lithospheric mantle via ascending asthenosphere. Denizli lamprophyres are interpreted to have formed during the formation of a basin/graben structure in the Latest Miocene–Early Pliocene, just after Late Miocene exhumation of Menderes massif.

Mineral textures and fluid inclusion petrography of the epithermal Ag–Au deposits at Guanajuato, Mexico: Application to exploration

Fluid inclusion petrography and vein mineral textures indicative of boiling have been characterized in 855 samples from epithermal precious metals deposits along the Veta Madre at Guanajuato, Mexico. Mineral textures and fluid inclusions characteristic of fluid immiscibility or boiling, including colloform quartz, plumose/feathery/flamboyant quartz, lattice-bladed calcite and lattice-bladed calcite replaced by quartz, as well as coexisting liquid-rich and vapor-rich fluid inclusions and assemblages of vapor-rich only inclusions, have been identified in mineralized samples from the Veta Madre. Most samples studied were assayed for Au, Ag, Cu, Pb, Zn, As and Sb, and were divided into ore grade and sub-economic samples based on the gold and silver concentrations. For silver, samples containing >100ppm were classified as ore grade, and ore grade gold samples contained >1ppm Au. The feature that is most closely associated with ore grades of both gold and silver is colloform quartz that was originally precipitated as amorphous silica, and this feature also shows the largest difference in average grade between samples that show colloform texture (178.8ppm Ag and 1.1ppm Au) and those that do not exhibit this texture (17.2ppm Ag and 0.2ppm Au). Statistical analysis of the data confirmed the petrographic observations that indicated that colloform quartz is the feature that has the greatest predictive power for distinguishing between ore grade and sub-economic samples. For both Ag and Au, there is no significant difference in average grade of samples containing coexisting liquid-rich and vapor-rich fluid inclusions or assemblages of vapor-only inclusions and those that do not, suggesting that fluid inclusion evidence for boiling is not correlative with ore grades. This result is consistent with the fact that most forms of silica that are precipitated during boiling do not trap useful fluid inclusions. The results of this study suggest that mineral textures and fluid inclusions provide complementary information that should both be used in exploration for epithermal precious metal deposits.Metal grades and boiling intensity of samples collected along a traverse perpendicular to the Veta Madre and above known economic mineralization are both low at relatively short distances away from the vein and increase as the vein is approached. This suggests that mineralogical and fluid inclusion evidence for boiling are restricted to the immediate vicinity of, and increase in the direction of, mineralized veins and may be used in exploration to establish vectors towards vein systems that may host precious metal mineralization. Previous studies of epithermal systems show that the Ag and Au mineralization zone is most often located at or above the bottom of the boiling zone. In this regard, the presence of abundant evidence for boiling that is observed in the deepest levels of the Veta Madre that have been sampled suggests that additional precious metal mineralization may be present beneath the deepest levels that have been explored.

Metamorphic replacement of mineral inclusions in detrital zircon from Jack Hills, Australia: Implications for the Hadean Earth

The Hadean (before 4.0 Ga) crust has long been considered to comprise mainly primitive mafic and ultramafic rocks. However, mineral inclusions in detrital zircons as old as 4.4 Ga from Jack Hills, Australia, have been interpreted to be magmatic and to provide evidence for extensive granitic crust. In situ U-Pb dating of monazite and xenotime inclusions in 4.25–3.35 Ga detrital zircons from Jack Hills shows that these inclusions are not magmatic, but formed during metamorphism at either 2.68 Ga or 0.8 Ga. Monazite-xenotime thermometry of intergrowths in the inclusions and the quartz-muscovite rock matrix constrain temperatures to between 420–475 °C, corresponding with conditions during peak regional metamorphism. Petrography and U-Pb geochronology of zircon inclusions from other localities show that the replacement of primary inclusions may commence in the igneous host rock and continue through weathering, sedimentation, and diagenesis. With increasing metamorphic grade, the inclusion assemblage increasingly reflects the composition of the rock matrix. In Jack Hills, most of the inclusions have the same composition and abundances as the metamorphic matrix, consistent with their formation during metamorphism. The titanium content of quartz inclusions indicates formation temperatures of 350–490 °C, supporting a metamorphic origin. Several lines of evidence indicate that at least some of the muscovite inclusions are also secondary. The lack of apatite inclusions in zircons from Jack Hills, relative to zircon in common granitic rocks, suggests that secondary minerals may have replaced primary apatite. Thus, detrital zircon may not be impermeable to post-depositional fluids, raising doubts about the use of the mineral inclusions they contain to infer initial magma chemistry. These results call for a reassessment of the source melts of the Hadean zircons and the composition of the earliest crust.