Inclusion Assemblages Research Articles

Mineral inclusions in zircons of para- and orthogneiss from pre-pilot drillhole CCSD-PP1, Chinese Continental Scientific Drilling Project

The pre-pilot drillhole CCSD-PP1, Chinese Continental Scientific Drilling Project (CCSD), with depth of 432 m, is located in the Donghai area in the southwestern Sulu terrane. The core samples are mainly comprised of paragneiss, orthogneiss and ultramafic rock with minor intercalated layers of eclogite and phengite-bearing kyanite quartzite. All analyzed paragneiss and orthogneiss samples were overprinted on amphibolite facies retrograde metamorphism. Coesite and coesite-bearing ultrahigh-pressure (UHP) mineral assemblages were identified by Raman spectroscopy and electron microprobe analysis as inclusions in zircons separated from paragneiss, eclogite and phengite-bearing kyanite quartzite samples. In the paragneiss samples, UHP mineral inclusion assemblages mainly consist of Coe+Omp+Grt+Phe, Coe+Jd+Phe+Ap preserved in the mantles (M) and rims (R) of zircons. These UHP mineral inclusion assemblages yield temperatures of 814–852 °C and pressures of ≥28 kbar, presenting the P– T condition of UHP peak metamorphism of these country rocks. According to the mineral inclusions and cathodoluminescence images of zircons, the orthogneisses can be divided into two types: UHP (OG1) and non-UHP (OG2). In OG1 orthogneisses, low-pressure mineral inclusion assemblage, mainly consisting of Qtz+Phe+Ab+Ksp+Ap, were identified in zircon cores (C), while coesite or coesite-bearing UHP mineral inclusions were identified in the mantles (M) and rims (R) of the same zircons. These features suggest that the OG1 orthogneisses, together with the paragneisses, phengite-bearing kyanite quartzite and eclogite experienced widespread UHP metamorphism in the Sulu terrane. However, in the zircons of OG2 orthogneiss samples, no UHP mineral inclusions were found. Inclusions mainly comprised Qtz+Phe+Ap and were identified in cores (C), mantles (M) and rims (R) of OG2 zircons; the cathdoluminescence images of all analyzed zircons showed clear zonings from cores to rims. These features indicate that the OG2 orthogneisses in pre-pilot drillhole CCSD-PP1 did not experience UHP metamorphism. Therefore, we should not rule out the possibility that some orthogneisses in Sulu terrane might represent relatively low-pressure granitic intrusives emplaced after the UHP event.

A fluid inclusion reconnaissance study of the Huanuni tin deposit (Bolivia), using LA-ICP-MS micro-analysis

Fluid inclusions have been studied in selected quartz-cassiterite veins of the Huanuni tin deposit, Bolivia. Fluid inclusion assemblages were found in quartz that bracket at least one stage of cassiterite deposition, and some were found in quartz crystallised during a later sulphide mineralisation stage. All fluid inclusions consisted of two phases only (liquid+vapour), which homogenised to a liquid upon heating and were classified to be of pseudosecondary or secondary in origin. Salinities in fluid inclusions related to the cassiterite deposition were up to 22 wt% NaCl equiv., and homogenisation temperatures of low and high salinity inclusions overlap in the range of 370 to 390 °C. Laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used to determine element concentrations in selected fluid inclusions. The high salinity fluid inclusions show elevated contents of Li, B, Na, K, Mn, Fe, Zn, Ge, As, Rb, Sn, Sb, Cs, La and Pb compared with the lower salinity samples (wt% NaCl equiv. lower than 6%). High concentrations of incompatible lithophile elements including Li, B and Cs are characteristic for fluids derived from highly fractionated granitic systems. Fe, Zn, Pb, and Ge are the most important heavy metals transported with tin in the aqueous fluid. Sn, Fe, Zn, Pb are probably transported as chloride complexes, and the initial fluid-chemical data for successive fluid-inclusion generations indicates that SnO2 precipitation occurred by injection of a hot saline magmatic fluid into heated low-salinity waters of possible meteoric origin.

Neutral coated inclusions in conductivity and anti–plane elasticity

The problem of neutral inclusions for two-dimensional conductivity (or, equivalently, anti-plane elasticity) is considered. Such an inclusion when inserted in a matrix containing a uniform applied electric field does not disturb the field outside the inclusion. Consequently, assemblages of neutral inclusions have certain moduli of their effective conductivity tensor that can be determined exactly. The inclusion is assumed to have a hole (or perfect conductor) at its core surrounded by a thick coating of isotropic material. The whole construction is embedded in a possibly anisotropic matrix. Analytic formulae for the boundaries of the core and coating are found with the use of conformal mapping techniques. The admissible inclusion shapes depend on the applied electric field and on the conductivities of matrix and coating. It is shown that the inclusions can have a variety of shapes and are not just restricted to being coated confocal ellipses. However, coated confocal ellipses are the only inclusions which are neutral to multiple applied fields.

The Bronzewing lode-gold deposit, Western Australia: P– T– X evidence for fluid immiscibility caused by cyclic decompression in gold-bearing quartz-veins

The Bronzewing lode-gold deposit is located in the Yandal greenstone belt in the Yilgarn Craton of Western Australia. Gold mineralization is hosted in tholeiitic basalt that is metamorphosed to greenschist facies. Individual ore bodies are controlled by a complex, gold-bearing quartz vein system that comprises shear and extension veins formed during a progressive D 2 deformation event. Gold is localized in quartz along fractures and deformed grain boundaries, and is interpreted to have formed late in the formation of the veins. Detailed petrography and microthermometry on primary, pseudosecondary and secondary fluid inclusions trapped in gold-bearing shear and extension veins revealed five types of fluid inclusions: (1) CO 2±CH 4–H 2O–NaCl inclusions of variable salinity (0.1 to 17.5 eq. wt.% NaCl) containing between 10 and 99 mol% CO 2 and molar volumes that range from 22 to 76 cm 3; (2) H 2O–NaCl inclusions of variable salinity (0.4 to 22.1 eq. wt.% NaCl); (3) CO 2±CH 4 inclusions with up to 58 mol% CH 4 and molar volumes between 54 and 73 cm 3; (4) CH 4–H 2O inclusions with CH 4 ranging from 80 to 90 mol%; and (5) CH 4 inclusions with low molar volumes of 19 to 23 cm 3. Types 1, 2 and 3 constitute a fluid inclusion assemblage that occurs consistently in primary, pseudosecondary and secondary trails and clusters within the gold-bearing quartz vein system. These co-existing fluids are interpreted to have formed by fluid immiscibility of a low-salinity, homogeneous parent fluid at about 300°C and 1500 bars. Locally, Type 2 fluid inclusions exhibit total homogenization temperatures that are on average 100°C less than the co-genetically trapped Type 1 aqueous-carbonic inclusions. This discrepancy is interpreted to have involved CO 2 effervescence in response to fluid pressure fluctuations. Types 4 and 5 fluid inclusions are rare, and are only present locally in secondary trails and clusters in extension veins. Significant pressure fluctuations, but relatively constant homogenization temperatures for Types 1, 2 and 3 fluid inclusions, suggest that cyclic decompression of the hydrothermal fluids, due to seismic activity along the shear zones that host the gold-bearing veins, triggered fluid immiscibility. The process of fluid immiscibility and subsequent lowering of gold solubility is interpreted to be the most efficient precipitation mechanism for gold in the D 2 shear zone hosted vein system at Bronzewing.

Evidence for a nonmagmatic component in potassic hydrothermal fluids of porphyry cu-Au-Mo systems, Yukon, Canada

Isotopic (H, Sr, Pb, Ar) and fluid inclusion data for hydrothermal fluids associated with potassic alteration from three Late Cretaceous porphyry Cu occurrences, west central Yukon, suggest a nonmagmatic fluid component was present in these hydrothermal fluids. Potassic stage quartz veins contain a dominant assemblage of saline and vapor-rich fluid inclusions that have δD values between −120 and −180‰. Phyllic stage quartz veins are dominated by vapor-rich fluid inclusions and have δD values that overlap with but are, on average, heavier (−117 to −132‰) than those in potassic stage quartz veins. These δD values are significantly lower than those from plutonic quartz phenocrysts (−91 to −113‰), and from values typically reported for primary fluids from porphyry-style mineralization (−40 to −100‰). The initial Sr ( 87Sr/ 86Sr i) isotopic values for the plutons are 0.7055 (Casino), 0.7048 (Mt. Nansen), and 0.7055 (Cash). The 87Sr/ 86Sr i compositions of hydrothermal K-feldspar ranges from magmatic Sr i values to more radiogenic compositions (Casino: 0.70551–0.70834, n = 8; Mt. Nansen: 0.7063–0.7070, n = 4; Cash: 0.7058, n = 1). The fluid inclusion waters from potassic quartz veins have 87Sr/ 86Sr i values that are similar to those of co-existing hydrothermal K-feldspar. The Pb isotopic compositions of hydrothermal K-feldspar show a weak positive correlation with Sr i for identical samples. Fluid inclusion waters of phyllic quartz veins also have Sr i compositions more radiogenic than the plutons. The Pb isotopic composition of pyrite and bornite from phyllic alteration veins are similar to, or more radiogenic than, hydrothermal K-feldspar Pb isotopic values. Hydrothermal K-feldspar samples yield 40Ar/ 39Ar ages (Casino = 71.9 ± 0.7 to 73.4 ± 0.8 Ma; Mt. Nansen = 68.2 ± 0.7 and 69.5 ± 0.6 Ma; Cash = 68.3 ± 0.8 Ma) similar to the U-Pb zircon, K-Ar biotite and Re-Os molybdenite ages of the Late Cretaceous plutons, with the age spectra indicating no excess 40Ar or disturbance. The 40Ar/ 36Ar values (285–292) of the K-feldspar samples are similar to the atmospheric compositions (295 ± 5) during Late Cretaceous time. The H, Sr, Pb, and Ar isotopic compositions of hydrothermal K-feldspar and quartz vein fluid inclusion waters that characterize the potassic hydrothermal fluids show evidence for an exotic component in addition to magmatic water (fluid). This component has a low δD, radiogenic Sr and Pb, and an atmospheric Ar composition. The inheritance of pre-existing isotope compositions from the host rocks, postpotassic alteration isotope exchange, or the replenishment of the magma chamber with magma of different isotopic composition cannot explain the isotope data. We suggest that to generate the observed H, Sr, Pb, and Ar isotope compositions, crustal fluids must be a component (15–94%) of potassic hydrothermal fluids in porphyry mineralization in the deposits studied.

Coesite inclusions and prograde compositional zonation of garnet in eclogite from Zekou in the Su-Lu ultrahigh-pressure terrane, eastern China

In the Su-Lu UHP terrane, eastern China, coesite-bearing eclogites have been reported in five areas (Weihai, Yangkou, Rizhao, Donghai and Rongcheng). The presence of a coesite inclusion (70 μm) in garnet in a strongly retrograded eclogite at Zekou (about 40 km southwest of Rongcheng) shows for the first time that this region also underwent coesite-eclogite facies metamorphism. In addition, the garnet of the Zekou eclogite preserves a distinct compositional zonation. The eclogite consists of garnet (20%), symplectite (70%: amphibole+plagioclase) and other accessory minerals (amphibole, rutile, quartz and ilmenite). Garnet porphyroclasts (0.5-1 mm in diameter) show compositional zonation that can be divided into three parts: core (Alm58-59 Sps0.7-0.9 Prp15-17 Adr0.1-0.2 Grs23-24), mantle (Alm55-58 Sps0.5-0.8 Prp14-15 Adr0.1-0.2 Grs25-28), and rim (Alm53-56 Sps0.4-0.6 Prp17-18 Adr0.1-0.2 Grs25-27). Each part has a characteristic inclusion mineralogy: the core part contains rutile, quartz and no coesite, the mantle part contains coesite, quartz-pseudomorphs after coesite, rutile, omphacite and zircon, and the rim part contains rutile. The coesite inclusion (70 μm) is located in the outer mantle part and is associated with radial fractures in the surrounding garnet. The different inclusion assemblages and the chemical zonation from core to rim in garnet porphyroclasts suggest that the Zekou eclogite preserves the information of the prograde stage during UHP metamorphism.

Mineral chemistry in fenites of the Kalkfeld carbonatite complex and its bearing on the composition of the fenitising fluid

Porphyry-style mineralization is characterized by superposition of multiple generations of quartz growth, punctuated by dissolution and juxtaposed within a single vein. Such superposition inevitably obscures temporal relationships between quartz formation, alteration events, and fluid inclusion assemblages (FIAs), and consequently complicates the interpretation of fluid origin and evolution. Very detailed information of these relationships recorded within hydrothermal quartz from the Taiyangshan porphyry Cu-Mo deposit (NW China) is obtained using integrated cathodoluminescence (CL) imaging, trace element analyses, and fluid inclusion petrography.The CL characteristics range from bright blue-CL quartz (Q1) associated with early high-temperature potassic alteration, to dark blue-CL quartz (Q2) broadly coinciding with metal precipitation, to post-ore dull red-CL fine-grained, anhedral, and recrystallized quartz (Q3) subgrains. Early bright blue-CL quartz (Q1) crystals, generally at the edges of stockwork veinlets close to wallrock, are characterized by oscillatory growth zoning, and exhibit alignment of primary, equant-shaped, brine-bearing FIAs along the banded growth zones. Later dark blue-CL quartz (Q2) crystals are generally euhedral in shape and grow with c-axes pointing inward to the vein center. They exhibit well-defined, but rarely preserved, growth zones truncating Q1 and crystal terminations projecting into pyrite and chalcopyrite in the vein center although they are not spatially associated with sulfides. These vein features clearly indicate that Q2 crystals grow into an open cavity or fracture during the ore precipitation. This is also evident by coexisting, secondary, irregular-shaped, liquid-rich inclusions and vapor-rich inclusions along fractures transecting Q1 oscillatory growth bands. The coexistence of hypersaline and vapor-rich inclusions moreover provides unequivocal evidence for phase separation during mineralization. Dull red-CL quartz (Q3) crystals appear to coincide with or follow sericitic alteration, and cut and encompass preexisting Q1 and Q2 crystals. They spatially associated with sulfides. The Q3 grains contain rare well-defined FIAs, but show scattered, secondary, irregular-shaped, low-salinity aqueous inclusions with small- to medium-sized vapor bubbles (10–30 vol%), as well as complex fluid inclusions that have migrated to the margins of anhedral Q3 subgrains due to quartz dissolution and recrystallization.Formation of early blue-CL Q1 and Q2 crystals at moderate temperatures are consistent with enrichment of Ti in the quartz lattice compared to neighboring younger Q3. The Q2 crystals are typified by elevated concentrations of Al and K concentrations when compared to Q1 and Q3. The substitution of Ti4+ for Si4+ in the tetrahedral sites of quartz leads to an increased CL intensity from Q1, to Q2, and then to Q3 corresponding to a decrease in quartz precipitation temperature. Ore deposition, recorded by integration of CL textures, FIAs petrography, and trace element variations in Q2, in the Taiyangshan porphyry Cu-Mo deposit must have occurred over a significant change in physicochemical conditions. This suggests that phase separation due to fluid cooling/pressure drops during the transition from lithostatic to hydrostatic regimes is critical to porphyry mineralization. We further propose a complex hydrothermal quartz vein formation history involving multiple fluid inclusions under varying pressure and temperature conditions.

Fluid inclusions in sedimentary and diagenetic systems

Some of the major problems in sedimentary geology can be solved by using fluid inclusions in sedimentary and diagenetic minerals. Important fluids in the sedimentary realm include atmospheric gases, fresh water of meteoric origin, lake water, seawater, mixed water, evaporated water, formation waters deep in basins, oil, and natural gas. Preserving a record of the distribution and composition of these fluids from the past should contribute significantly to studies of paleoclimate and global-change research, is essential for improving understanding of diagenetic systems, and provides useful information in petroleum geology. Applications of fluid inclusions to sedimentary systems are not without their complexities. Some fluid inclusions exposed to natural conditions of increasing temperature may be altered by thermal reequilibration, which results in stretching, or leakage and refilling, of some fluid inclusions. Similarly, overheating in the laboratory can also cause reequilibration of fluid inclusions, so fluid inclusions from the sedimentary realm must be handled carefully and protected from overheating. Natural overheating of fluid inclusions must be evaluated through analysis of the most finely discriminated events of fluid inclusion entrapment, fluid inclusion assemblages (FIA). Consistency in homogenization temperatures within a fluid inclusion assemblage, consisting of variably sized and shaped inclusions, is the hallmark of a data set that has not been altered through thermal reequilibration. In contrast, fluid inclusion assemblages yielding variable data may have been altered through thermal reequilibration. If a fluid inclusion assemblage has not been altered by thermal reequilibration, its fluid inclusions may be useful as geothermometers for low- and high-temperature systems, or useful as geobarometers applicable throughout the sedimentary realm. If a fluid inclusion assemblage has been altered partially by thermal reequilibration, techniques for distinguishing between altered and unaltered fluid inclusions may be applied. In studies of global change, fluid inclusions can be used as sensitive indicators of paleotemperature of surface environments. Fluid inclusions also preserve microsamples of ancient seawater and atmosphere, the analysis of which could figure prominently into discussions of past changes in chemistry of the atmosphere and oceans. In petroleum geology, fluid inclusions have proven to be useful indicators of migration pathways of hydrocarbons; they can delineate the evolution of the chemistry of hydrocarbons; and they remain important in understanding the thermal history of basins and relating fluid migration events to evolution of reservoir systems. In studies of diagenesis, fluid inclusions can be the most definitive record. Most diagenetic systems are closely linked to temperature and salinity of the fluid. Thus, fluid inclusions are sensitive indicators of diagenetic environments.

The Application of Microchemical Analysis of Alluvial GoldGrains to the Understanding of Complex Local and Regional GoldMineralization: A Case Study in the Irish and Scottish Caledonides

Terranes of the Appalachian-Caledonian orogen north of the Iapetus suture host gold mineralization in North America, Scotland, and Ireland. To date, commercial exploitation has been confined to the Appalachian terrane of Canada. In Scotland and Ireland, regional analysis of the types of gold mineralization has been handicapped by the relatively small number of bedrock gold occurrences available for study. However, there are a large number of alluvial gold localities. A technique of microchemical characterization of gold grains has been applied to facilitate the classification of alluvial gold localities in terms of the style or styles of source mineralization, thereby permitting a more complete interpretation of regional gold mineralization. The technique of microchemical characterization used in this study classifies populations of gold grains from each locality in terms of the assemblage of opaque mineral inclusions enclosed within the gold grains and the concentration of minor alloying elements in the gold. A total of 2,400 gold grains collected from 44 sites in Ireland and Scotland have been studied. The microchemical signatures of populations of gold grains extracted from bedrock samples correlate closely with those from adjacent alluvial populations, and furthermore, our data are compatible with the descriptions of the vein mineralogy of bedrock mineralization reported by other workers. This result has facilitated informed speculation on the type of source mineralization of other alluvial occurrences where the bedrock source remains undiscovered. Populations of alluvial gold from individual localities have been grouped into larger regional data sets on the basis of similarities in microchemical signature. The signatures of these data sets were analyzed using the same criteria, and 10 classes of gold were identified. Regional analysis of the distribution of the gold classes showed that the occurrence of the four major classes correlates strongly with host terrane, irrespective of geographical location. Gold from the Southern Uplands terrane is characterized by inclusions of base metal sulfides and sulfarsenides, with the silver content of the gold being less than 15 percent. In contrast, populations of alluvial gold from the Grampian and Highland terranes commonly exhibit median silver contents of over 20 percent and frequently contain silver telluride and bismuth telluride inclusions, which are extremely rare in gold from the Southern Uplands terrane. However, in other cases, the style of mineralization is controlled by specific host or source rocks and is independent of the terrane. Two examples are gold mineralization within porphyry systems and gold associated with red-bed sediments. The study has confirmed the value of the technique of gold grain characterization, using information from both concentrations of alloying elements and inclusion assemblages as a powerful tool in interpreting complex local and regional gold mineralization.

The use of microchemical analysis of alluvial gold grains in mineral exploration: experiences in Britain and Ireland

The examination of populations of gold grains by SEM and EPMA generates a microchemical signature based upon the assemblage of opaque mineral inclusions within the gold and the concentration of minor alloying metals. Duplicate samples of alluvial gold from the same locality have yielded the same microchemical signature independent of the field worker who collected the sample, or the date of collection. A study of five separate alluvial localities yielded a microchemical signature consistent with the mineralogy of the adjacent host mineralisation. This result has permitted informed speculation on the styles and complexity of gold mineralisation contributing to the alluvial population of gold grains where the style of source mineralisation is unknown. The technique can yield information about several aspects of the source mineralisation of an alluvial population at an early stage in the exploration process. Distinctive microchemical signatures can be used to evaluate the dispersion of gold either by fluvial or glacial action. The technique can identify gold derived from separate mineralising events either within a region or along a watercourse, thereby allowing speculation on the magnitude of the source mineralisation. It is also possible to establish whether specific types of gold mineralisation of potential economic interest have contributed to an alluvial population. In areas subjected to glaciation, analysis of the microchemical signature of populations of alluvial gold may indicate whether the gold grains are of proximal or distal origin. The volume of information made available through the study of alluvial gold grains has contributed greatly to the understanding of regional gold mineralisation. Interpretation of microchemical signatures of populations of alluvial gold from about 130 localities throughout Great Britain and Ireland has facilitated characterisation of gold in terms of the style of the source mineralisation. This information has augmented that available from about ten bedrock gold localities and has permitted a more complete classification of gold occurrences throughout the region. Quantitative analysis of some minerals within the opaque inclusion assemblage can provide information on the chemistry of the mineralising fluid, which may be related to the capacity for gold transportation. In Devon, South West England, a model for the emplacement of gold mineralisation was developed from interpretation of the microchemical signature of alluvial gold grains. The selenide-rich inclusion suite containing no sulphides suggested that oxidising conditions prevailed during precipitation. The model of transport of gold in solution through the oxidising environment of Permian red beds and its subsequent precipitation in underlying rocks due to reduction in Eh was consistent with the observed distribution of gold in alluvial sediment, residential overburden and drill core. Using this model other targets for gold mineralisation were successfully predicted.

Contrasting P-T Paths in the Eastern Himalaya, Nepal: Inverted Isograds in a Paired Metamorphic Mountain Belt

Petrology and phase equilibria of rocks from two profiles in Eastern Nepal from the Lesser Himalayan Sequences, across the Main Central Thrust Zone and into the Greater Himalayan Sequences reveal a Paired Metamorphic Mountain Belt (PMMB) composed of two thrust-bound metamorphic terranes of contrasting metamorphic style. At the higher structural level, the Greater Himalayan Sequences experienced high-T/moderate-P metamorphism, with an anticlockwise P–T path. Low-P inclusion assemblages of quartz + hercynitic spinel + sillimanite have been overgrown by peak metamorphic garnet + cordierite + sillimanite assemblages that equilibrated at 837 ± 59°C and 6·7 ± 1·0 kbar. Matrix minerals are overprinted by numerous metamorphic reaction textures that document isobaric cooling and re-equilibrated samples preserve evidence of cooling to 600 ± 45°C at 5·7 ±1·1 kbar. Below the Main Central Thrust, the Lesser Himalayan Sequences are a continuous (though inverted) Barrovian sequence of high-P/moderate-T metamorphic rocks. Metamorphic zones upwards from the lowest structural levels in the south are: Zone A: albite + chlorite + muscovite ± biotite; Zone B: albite + chlorite + muscovite + biotite + garnet; Zone C: albite + muscovite + biotite + garnet ± chlorite; Zone D: oligoclase + muscovite + biotite + garnet ± kyanite; Zone E: oligoclase + muscovite + biotite + garnet + staurolite + kyanite; Zone F: bytownite + biotite + garnet + K-feldspar + kyanite ± muscovite; Zone G: bytownite + biotite + garnet + K-feldspar + sillimanite + melt ± kyanite. The Lesser Himalayan Sequences show evidence for a clockwise P–T path. Peak-P conditions from mineral cores average 10·0 ± 1·2 kbar and 557 ± 39°C, and peak-metamorphic conditions from rims average 8·8 ± 1·1 kbar and 609 ± 42°C in Zones D–F. Matrix assemblages are overprinted by decompression reaction textures, and in Zones F and G progress into the sillimanite field. The two terranes were brought into juxtaposition during formation of sillimanite–biotite ± gedrite foliation seams (S3) formed at conditions of 674 ± 33°C and 5·7 ± 1·1 kbar. The contrasting average geothermal gradients and P–T paths of these two metamorphic terranes suggest they make up a PMMB. The upper-plate position of the Greater Himalayan Sequences produced an anticlockwise P–T path, with the high average geothermal gradient being possibly due to high radiogenic element content in this terrane. In contrast, the lower-plate Lesser Himalayan Sequences were deeply buried, metamorphosed in a clockwise P–T path and display inverted isograds as a result of progressive ductile overthrusting of the hot Greater Himalayan Sequences during prograde metamorphism.

Regional variation in gold mineralization in the vicinity of the Glengaber Burn, Scottish Borders

Synopsis The alluvial gold at Glengaber Burn in the valley of the Megget Water is of interest because of its coarse size and historical working. A total of 350 alluvial gold grains from 17 sites in Glengaber Burn and the vicinity have been studied using the technique of microchemical characterization, and two major types of gold have been identified, each with well defined geographical distribution. Type 1a gold contains only pyrite inclusions, and has an Ag content of between 8% and 11%. Type 1b has a similar Ag content, but the inclusion assemblage contains galena together with pyrite. Type 2a gold typically contains between 3% and 7% Ag, and a suite of inclusions comprising chalcopyrite, sphalerite, tetrahedrite, arsenopyrite and gersdorffite. Type 2a gold occurs in a SW–NW trending zone which contains the catchments of the Glengaber and Craigierigg Burns. Type 2b gold is confined to the catchment of the Cramalt Burn and contains an inclusion suite similar to Type 2a, but is more silver-rich. The abundance of alluvial gold of all types falls dramatically to the south of Megget Water, although the occurrence to the north of the Glengaber Burn and Craigierigg Burns has not been established. The distribution of Type 1a gold is mutually exclusive with Type 2a and flanks the zone of Type 2a and 2b to both east and west suggesting zonation in a single mineralizing event. Type 1b gold occurs to the west of Type 1a and is probably unrelated to the other gold types. The mineralization acting as the source for the alluvial gold could be controlled by deep structures roughly perpendicular to the regional strike, revealed by prominent gravity lineaments, and their intersections with other structural features.

Abstract: Fluid inclusion assemblages in quartz overgrowths constrain diagenetic systems in jurassic-cretaceous sandstone reservoirs, Libya and Egypt

Abstract: Fluid inclusion assemblages in quartz overgrowths constrain diagenetic systems in jurassic-cretaceous sandstone reservoirs, Libya and Egypt

Tectono-metamorphic history of Neoproterozoic rocks in eastern Eritrea

The Neoproterozoic juvenile crust of Eritrea lies within the East African Orogen that marks a megasuture along which the East and West Gondwana converged. In eastern Eritrea, repeatedly deformed Neoproterozoic rocks of this orogen consist of two lithotectonic terranes, Ghedem and Nacfa. The Ghedem Terrane consists of amphibolite facies metamorphic rocks with dominantly subhorizontal to moderately dipping fabrics. The structurally overlying Nacfa Terrane consists of greenschist facies metamorphic rocks with steeper fabrics. Three deformation phases are distinguished that can be related to E–W lateral compression and E–W orogenic collapse of Neoproterozoic age. D1 structures are represented by an originally N–S/NNE–SSW steep and penetrative S1 foliation which is axial planar to F1 isoclinal upright folds. D1 structures were refolded by recumbent folds and nappes during D2, when deformation was dominated by subhorizontal shear zones accompanied by recumbent meso-microscopic NNW to NNE trending F2 folds. D3 deformation formed open upright F3 folds co-axial with F2 and a steep S3 spaced axial planar foliation accompanied by strike–slip shear zones. The D1–3 structures and fabrics are interpreted as phases of a long-lived Neoproterozoic lateral compression. Orogenic collapse during D2–3 was characterized by structures that were ductile at depth and semi-brittle at shallow levels. P– T conditions of metamorphism were determined from the mineral chemistry of pelitic–semipelitic rocks in the Ghedem Terrane. Mineral assemblages included within garnets and matrix assemblages in textural equilibrium with garnet rims define two P– T fields. Estimated P– T conditions of metamorphism for the inclusion assemblages in garnet cores range between 3.9 and 5.3 kbar and 516–584°C whereas those for the rim–matrix assemblages are between ∼5–11 kbar and 526–637°C. Microstructural inferences and the P– T conditions obtained from the mineral assemblages within garnets indicate that prograde M1 greenschist facies metamorphism was contemporaneous with D1 deformation. Similarly, the P– T equilibrium conditions constrained for the rim–matrix assemblages relate to late M2 amphibolite facies metamorphism during late D2 deformation. Textural relationships reveal that M3 metamorphism was retrogressive during D3. Inclusion and rim–matrix geothermobarometry depict a family of anticlockwise P– T paths indicating heating and contemporaneous loading during M1/D1 at intermediate crustal levels and nearly isothermal loading during early M2/D2 at deeper crustal levels followed by a return path during late D2 and D3 deformations. These rocks represent a segment of the laterally shortened and subsequently extended mid-lower continental crust of the East African Orogen that was exhumed from as deep as 48 km (12 kbar) during late Neoproterozoic interaction between orogenic collapse, erosion and deep isostatic root recovery. The preliminary interpretation is that the relationships between the P– T conditions of metamorphism and deformation in the Ghedem Terrane rocks of eastern Eritrea indicate a continental orogen related to the late Neoproterozoic terminal convergence between East and West Gondwana.

High‐ and Ultrahigh‐pressure Metamorphism and Retrogressive Textures of Gneiss in the Donghai Area

AbstractIn the gneisses from the drillhole ZK2304 of the Donghai area, there have been preserved high‐ and ultrahigh‐pressure metamorphic mineral assemblages, a series of complicated retrogressive textures and relevant metamorphic reactions. In addition to garnet, jadeititic‐clinopyroxene and rutile, other peak stage (M2) minerals in some gneisses include phengite, aragonite and coesite or quartz pseudomorphs after coesite. The typical peak‐stage mineral assemblages in gneisses are characterized by garnet + jadeitic‐clinopyroxene + rutile + coesite, garnet + jadeitic‐clinopyroxene + phengite + rutile ± coesite and garnet + jadeitic‐clinopyroxene + aragonite + rutile ± coesite. The grossular content (Gro) in garnet is high and may reach 50.1 mol%. The SiO2 content of phengite ranges from 54.37% to 54.84% with 3.54–3.57 p.f.u. Quartz pseudomorphs after coesite occur as inclusions in garnet.The gneisses of the Donghai area have been subjected to multistage recrystallization and exhibit a closewise P‐T evolutional path characterized by the near‐isothermal decompression. The inclusion assemblage (Hb+Ep+Bi+Pl+Qz) within garnet and other minerals has recorded a pre‐peak stage (M1) epidote amphibole fades metamorphic event. High‐ and ultrahigh‐pressure peak metamorphism (M2) took place at T=750–860°C and P>2.7 GPa. The symplectitic assemblages after garnet, jadeitic‐clinopyroxene and rutile imply a near‐isothermal decompression metamorphism (M3, M4) during the rapid exhumation. Several lines of evidence of petrography and metamorphic reactions indicate that both gneisses and eclogites have experienced ultrahigh‐pressure metamorphism in the Donghai area. This research may be of great significance for an in‐depth study of the metamorphism and tectonic evolution in the Su‐Lu ultrahigh‐pressure metamorphic belt.

Petrogenesis of silicate inclusions in the Weekeroo Station IIE iron meteorite: differentiation, remelting, and dynamic mixing

The Weekeroo Station IIE iron meteorite contains a variety of felsic and mafic inclusions enclosed in an FeNi-metal host. Petrographic, EMP, and SIMS data suggest that the petrogenesis of the silicates was complex, and included differentiation, remelting, FeO-reduction, and dynamic mixing of phases. Differentiation produced a variety of olivine-free inclusion assemblages, ranging from pyroxene + plagioclase + tridymite with peritectic compositions, to coarse orthopyroxene, to plagioclase + tridymite and its glassy equivalent. Individual phases have similar trace-element abundances and patterns, despite large variations in inclusion textures, modes, and bulk compositions, probably as a result of mechanical separation of pre-existing phases in an impact event that dynamically mixed silicates with the metallic host. Trace-element data imply that augite and plagioclase grains in different inclusions crystallized from the same precursor melt, characterized by relatively unfractionated REE abundances of ∼20–30 × CI-chondrites except for a negative Eu anomaly. Such a precursor melt could have been produced by ∼2–5% equilibrium partial melting of an H-chondrite silicate protolith, or by higher degrees of partial melting involving subsequent fractional crystallization. Glass appears to have formed by the remelting of pre-existing plagioclase and orthopyroxene, in a process that involved either disequilibrium or substantial melting of these phases. During remelting, silicate melt reacted with the FeNi-metal host, and FeO was reduced to Fe-metal. Following remelting and metal-silicate mixing, inclusions apparently cooled at different rates in a near-surface setting on the parent body; glass- or pigeonite-bearing inclusions cooled more rapidly (≥2.5°C/hr between 1000–850°C) than pigeonite-free, largely crystalline inclusions. The results of this study point to two likely models for forming IIE iron meteorites, both involving collision between an FeNi-metal impactor and either a differentiated or undifferentiated silicate-rich target of H-chondrite affinity. Each model has difficulties and it is possible that both are required to explain the diverse IIE group.

Layered Mantle Lithosphere in the Lac de Gras Area, Slave Craton: Composition, Structure and Origin

from some highly depleted ophiolites from convergent-margin settings, Heavy-mineral concentrates (garnets, chromites) and xenoliths from and may have formed in a similar situation during the accretion of 21 Cretaceous–Tertiary kimberlite intrusions have been used to map the Hackett and Contwoyto terranes (magmatic arc and accretionary the lithospheric mantle beneath the Lac de Gras area in the central prism, respectively) to the ancient continental Anton terrane at part of the Slave Province. Analyses of Nickel Temperature ( TNi) 2·6–2·7 Ga. The deeper layer is interpreted as a plume head, and Zinc Temperature ( TZn) have been used to place garnet and which rose from the lower mantle and underplated the existing chromite xenocrysts, respectively, in depth context. Paleogeotherms lithosphere at 2·6 Ga; evidence includes a high proportion of the derived from both xenoliths and concentrates lie near a 35 mW/ superdeep inclusion assemblage (ferropericlase–perovskite) in the m conductive model at T Ζ 900°C, and near a 38 mW/m diamond population. This event could have provided heat for model at higher T, implying a marked change in conductivity and/ generation of the widespread 2·6 Ga post-tectonic granites. Proteroor a thermal transient. Plots of garnet composition vs TNi also zoic subduction from east and west may have modified the cratonic show a sharp discontinuity in mantle composition at 900°C. root, mainly by introduction of eclogites near its base. Garnets from <145 km depth are ultradepleted in Y, Zr, Ti and Ga, whereas those from greater depths (to [200 km) are similar to garnets from Archean mantle world-wide. Relative abundances of garnet types indicate that the shallow layer consists of ~60% (clinopyroxene-free) harzburgite and 40% lherzolite, whereas the

Layered Mantle Lithosphere in the Lac de Gras Area, Slave Craton: Composition, Structure and Origin

Heavy-mineral concentrates (garnets, chromites) and xenoliths from 21 Cretaceous–Tertiary kimberlite intrusions have been used to map the lithospheric mantle beneath the Lac de Gras area in the central part of the Slave Province. Analyses of Nickel Temperature (TNi) and Zinc Temperature (TZn) have been used to place garnet and chromite xenocrysts, respectively, in depth context. Paleogeotherms derived from both xenoliths and concentrates lie near a 35 mW/m2 conductive model at T ≤ 900°C, and near a 38 mW/m2 model at higher T, implying a marked change in conductivity and/or a thermal transient. Plots of garnet composition vs TNi also show a sharp discontinuity in mantle composition at 900°C. Garnets from <145 km depth are ultradepleted in Y, Zr, Ti and Ga, whereas those from greater depths (to ≥ 200 km) are similar to garnets from Archean mantle world-wide. Relative abundances of garnet types indicate that the shallow layer consists of ∼60% (clinopyroxene-free) harzburgite and 40% lherzolite, whereas the deeper layer contains 15–20% harzburgite and 80–85% lherzolite. T estimates on eclogite xenoliths show that all were derived from the deeper layer. Xenolith data and garnet compositions indicate that the shallow layer is more magnesian (Fo92–94) than the deeper layer (Fo91–92), and both layers are more olivine rich than South African or Siberian Archean peridotite xenoliths. The composition and sharply defined structure of the Lac de Gras lithosphere are unique within our current knowledge of Archean mantle sections. The shallow layer of this lithosphere section is similar to peridotites from some highly depleted ophiolites from convergent-margin settings, and may have formed in a similar situation during the accretion of the Hackett and Contwoyto terranes (magmatic arc and accretionary prism, respectively) to the ancient continental Anton terrane at 2.6–2.7 Ga. The deeper layer is interpreted as a plume head, which rose from the lower mantle and underplated the existing lithosphere at 2.6 Ga; evidence includes a high proportion of the superdeep inclusion assemblage (ferropericlase–perovskite) in the diamond population. This event could have provided heat for generation of the widespread 2.6 Ga post-tectonic granites. Proterozoic subduction from east and west may have modified the cratonic root, mainly by introduction of eclogites near its base.

Permian paleoclimate data from fluid inclusions in halite

This study has yielded surface water paleotemperatures from primary fluid inclusions in mid Permian Nippewalla Group halite from western Kansas. A `cooling nucleation' method is used to generate vapor bubbles in originally all-liquid primary inclusions. Then, surface water paleotemperatures are obtained by measuring temperatures of homogenization to liquid. Homogenization temperatures ranged from 21°C to 50°C and are consistent along individual fluid inclusion assemblages, indicating that the fluid inclusions have not been altered by thermal reequilibration. Homogenization temperatures show a range of up to 26°C from base to top of individual cloudy chevron growth bands. Petrographic and fluid inclusion evidence indicate that no significant pressure correction is needed for the homogenization temperature data. We interpret these homogenization temperatures to represent shallow surface water paleotemperatures. The range in temperatures from base to top of single chevron bands may reflect daily temperatures variations. These Permian surface water temperatures fall within the same range as some modern evaporative surface waters, suggesting that this Permian environment may have been relatively similar to its modern counterparts. Shallow surface water temperatures in evaporative settings correspond closely to local air temperatures. Therefore, the Permian surface water temperatures determined in this study may be considered proxies for local Permian air temperatures.

Pressure-temperature paths from P - T pseudosections and zoned garnets: potential, limitations and examples from the Zanskar Himalaya, NW India

The extraction of P-T histories from metamorphic rocks provides a valuable dataset for the elucidation of the tectonic mechanisms for orogeny. While continued re-equilibration frequently obliterates early information, garnet zonation and inclusion assemblages can often surmount this problem. The task is more difficult in high variance assemblages or if inclusions are not preserved, but one approach is to use pseudosections that are specific to the bulk composition of a given rock. In the latter case, the compositions and abundances of all the minerals are fixed at a given P-T point such that, if the effective bulk composition is known, the garnet composition alone can be used to reconstruct the history. Here, we explore this approach using examples from the Zanskar Himalaya, NW India. Pseudosections have been calculated for four pelitic to semipelitic rocks from the Zanskar Himalaya and have been contoured for garnet composition. The calculations adequately model the mineral assemblages in the rocks and predict the presence of chlorite in the early assemblage where chlorite is found as inclusions within garnet. Moreover, the pseudosections successfully model the garnet core compositions, with all three independent compositional contours overlapping at a single pressure and temperature. This occurs at ∼550 °C and at pressures varying from 3–7 kbar for the four rocks studied. We have been less successful, however, at modelling garnet compositions beyond the cores because fractionation of the effective bulk composition is caused by garnet growth itself. However, in this case, a combination of the␣pseudosection and conventional thermobarometry using␣Fe-Ti inclusions and matrix phases allows us to reconstruct␣the entire P-T history. The resulting P-T paths record burial of 3–5 kbar without significant temperature increase followed by isobaric heating of 50–100 °C. This evolution is consistent with Himalayan collision in the early Tertiary but a combination of the P-T data presented here and published geochronological data suggests renewed thrusting south of the suture zone in the Oligocene. In addition, the data demonstrate that no extra heat source is required to cause melting of the Himalayan crust in the Miocene. While melting could have occurred both by dehydration during decompression or in the presence of a fluid, the lack of garnet resorption does suggest decompression was rapid and followed quickly by cooling. This scenario favours melting by decompression.