The Key Influence of Fluid in the Preserves, Mineral Assemblages, Compositionsand Structures: Study from High-Pressure Eclogite and Its Amphibolization in the Western Dabie Mountains,Central China

Pseudosection modeling for the garnet amphibolite samples from the Western Dabie Mountains show they have experienced similar HP metamorphic evolution with that of the adjected eclogites.The common assemblage of

The Key Influence of Fluid in Metamorphic Rock Preserves, Mineral Assemblages, Compositions and Structures: Study from High-Pressure Eclogite and Its Amphibolization in the Western Dabie Mountains,Central China

The most of high/ultrahigh-pressure(HP/UHP)terranes of the world are characterized by the occurrence of numerous pods,lenses or layered blocks of eclogite and amphibolites(e.g.O’Brien,1997;Elvevold and Gilotti,2000;Zhang et al.,2003;and references there in).Field and petrological features suggest that amphibolites should

Characteristics of heavy minerals composition and distribution in surface sediment from the Xinghua Bay of Fujian,China

The composition, assemblage and distribution characteristics of heavy minerals from 63 to 125 μm grain sizes in surface sediment from the Xinghua Bay (17 stations) are studied. The matter source of silt and the relationship between heavy minerals and sedimentary environment are also discussed. The results show that there are 37 kinds of heavy minerals and the average content of them is 12.08%, which exceeds 6% compared with the Meizhou Bay (5.67%). The dominant minerals are magnetite, hornblende, epidote, ilmenite, hematite, limonite, zircon and so on. Mineral kinds reveal that the sources of silt in this bay are the fluvial input and eroded products of bedrock in circumjacent land and islands of the Xinghua Bay. However, the matter source from outside this bay is less. Four mineral assemblage zones can be divided based on heavy mineral contents and distribution characteristics in the Xinghua Bay, which are not only influenced by matter source, but also controlled by hydrodynamic condition and sedimentary environment in the Xinghua Bay.

Mineral Components, Texture, and Forming Conditions of Hydrothermal Chimney on the East Pacific Rise at 9°-10°N

To characterize the hydrothermal processes of East Pacific rise at 9°-10°N, sulfide mineral compositions, textural, and geochemical features of chimney ores were studied using ore microscope,scanning electron microscope, X-ray diffraction analysis, and electron microprobe techniques. Results show that there are three mineral assemblages for the hydrothermal chimney ores, namely：（i） anhydrite ＋ marcasite ＋ pyrite, （ii） pyrite ＋ sphalerite ＋ chalcopyrite, and （iii） chalcopyrite ＋ bornite ＋ digenite ＋ covellite. Mineral assemblages, zonational features, and geochemical characteristics of the ore minerals indicate that ore fluid temperature changed from low to high then to low with a maximum temperature up to 400 ℃. The chimney is a typical black smoker. The initial structure of the chimney was formed by the precipitation of anhydrites, and later the sulfides began to precipitate in the inner wall.

Study on properties of sinter mineral phases

In this study,the properties of sinter mineral phases were investigated by X-ray diffraction,optical microscopy quantitative observation,electron probe microanalysis,and the nanoindentation technique. The mechanisms that form return fines are discussed with respect to the factors of microregion composition,sintering temperature,and the reactive behavior of pisolite. The study results indicate the following：（ 1） Sinter mineral assemblage mainly comprises hematite,magnetite,calcium ferrite,and glass. In addition,the mineral assemblage of sinter products includes a great deal of calcium ferrite and melt-erosive magnetite,an abundance of secondary hematite,and a small amount of primary iron ore; whereas the mineral assemblage of return fines contains plentiful amounts of euhedral magnetite and secondary hematite,a large amount of relic pisolite with particle sizes less than1 mm,and relatively less calcium ferrite. In particular,some calcium ferrite was found to coexist with relic iron ore in a fiber-like microstructure.（ 2） Dentritic calcium ferrite has less SiO_2 and Al_2O_3,higher basicity（ w_（CaO）/w_（SiO_2））and a mole ratio of Fe_2O_3/CaO,whereas platy and blocky calcium ferrites have more SiO_2 and Al_2O_3,lower basicity and a mole ratio of Fe_2O_3/CaO.（ 3） The hardness of hematite is the highest（ around 18-22 GPa）,those of calcium ferrite and magnetite are relatively lower,and that of glass is the lowest. In terms of the formation mechanism of return fines,because of their weak ability to resist external shocks,these sorts of mineral phases and microstructures-（1） euhedral magnetite and glass formed in microregions with low basicity;（2） SFCA-Ⅰand relic iron ore formed in regions with a relatively low sintering temperature; and（3） relic pisolite and its nearby reaction regions-are inclined to form return fines.

A Reconstruction and Discussion of the Effect of Diagenetic Environment on Hydrocarbon Generation Based on Diagenetic Mineral Assemblage in Mudstone

Inorganic minerals in mudstone are composed of clay minerals,carbonate and detrital minerals.Detrital minerals(such as quartz and feldspar)are mainly original deposit.However,clay minerals(kaolinite,illite,and chlorite)and carbonate(calcite and dolomite)are mostly diagenetic minerals.Furthermore,conversion of the four kinds of clay minerals are common.The formation of clay minerals and carbonate is controlled by temperature,pressure,p H,Eh and type of cations during diagenesis.Therefore mineral assemblage can indicate the characteristics and change of diagenetic environment.In addition to inorganic minerals,there are also organic matter of different sources and chemical properties in mudstone.Traditionally,it is considered that evolution of organic matter is controlled by thermal effect.Now studies show that inorganic and organic matter can interact with each other and form clay-organic complexes.This suggest that attention should be paid to the influence of diagenetic mineral assemblage and diagenetic environment on the evolution of organic matter* Samples of mudstone from 1500-4500m of the Palaeogene in the Dongying Depression,China,were collected to investigate the changes of mudstone diagenetic environment.XRD,thin section and SEM were used to detect diagenetic minerals and assemblage characteristics.Results showed that content of detrital minerals,which are floating in mud matrix or preserved as silt laminae,is basically unchanged from shallow to deep strata.Clay minerals which are gathered as argillaceous matrix or preserved as argillaceous laminae have growth and decline relation to carbonate which mainly appear as micropoikilitic ferriferous calcite and ferriferous dolomite.All these characteristics indicate that detrital minerals are exogenetic,whereas carbonate is diagenetic minerals.Based on the SEM analysis of the clay minerals,it was found that smectite present honeycomb and reticulate structure,while illite present filiform and schistose structure and there are growth and decline relationship between them.Nevertheless,hexagonal tabular and stratified kaolinite has the highest content from 2400m to3300m.Rosette and stratified chlorite shows increase trend when the burial depth is deeper than 3300m.These characteristics indicated that clay minerals are diagenetic minerals and there are conversions among the four types.Therefore form shallow to deep,three diagenetic mineral assemblage zones can be divided based on the characteristics of carbonate and clay minerals in mudstone.Namely,smectite+illite/smectite zone in the depth of 2000-2500m;kaolinite+illite/smectite zone in the depth of 2500-3300m and illite+chlorite+carbonate zone below 3300m.Previous studies showed that kaolinite is stable under acidic conditions,while other clay minerals and carbonate are stable under alkaline conditions.Hence according to mineral assemblages feature,it was inferred that diagenetic environment of mudstonehasundergonethechangeof alkaline-acid-alkaline.For the organic matter with different chemical properties in mudstone,the hydrocarbon generation will be different in the acidic and alkaline diagenetic environment even if the conditions of temperature and pressure are the same.Therefore,for hydrocarbon generation we should not only focus on thermal effect,but also pay more attention to the differences of diagenetic environment which have great significance for the understanding of hydrocarbon generation,hydrocarbon expulsion and reservoir formation in mudstone.

Sequence structure, sedimentary evolution and their controlling factors of the Jurassic Lianggaoshan Formation in the East Sichuan Basin, SW China

Based on the data of outcrops, seismic sections, thin sections, heavy mineral assemblages and detrital zircon U-Pb dating, the sedimentary characteristics, lake level fluctuation and provenance characteristics of the Middle Jurassic Lianggaoshan Formation(J_(2)l) in eastern Sichuan Basin, SW China, were investigated to reveal the control of tectonic movements of the surrounding orogenic belts on the sedimentary systems. The J_(2)lmainly developed a delta–lake sedimentary system, which contained a complete third-order sequence that was subdivided into four lake level up-down cycles(fourth-order sequence).The lake basins of cycles Ⅰ and Ⅱ were mainly distributed in eastern Sichuan Basin, while the lake basins of cycles Ⅲ and Ⅳ migrated to central Sichuan Basin, resulting in the significant difference in sedimentary characteristics between the north and the south of eastern Sichuan Basin. The provenance analysis shows that there were three types of provenances for J_(2)l. Specifically, the parent rocks of Type Ⅰ were mainly acidic igneous rocks and from the proximal northern margin of the Yangtze Plate;the parent rocks of Type Ⅱ were intermediate-acid igneous rocks and metamorphic rocks and from the central parts of the southern and northern Qinling orogenic belts;the parent rocks of Type Ⅲ were mainly metamorphic rocks followed by intermediate–acid igneous rocks, and from the North Daba Mountain area. It is recognized from the changes of sedimentary system and provenance characteristics that the sedimentary evolution of J_(2)lin eastern Sichuan Basin was controlled by the tectonic compression of the Qinling orogenic belt. In the early stage, the lake basin was restricted to the east of the study area, and Type Ⅰ provenance was dominant. With the intensifying north-south compression of the Qinling orogenic belt, the lake basin expanded rapidly and migrated northward, and the supply of Type Ⅱ provenance increased. In the middle and late stages, the uplift of the North Daba Mountain led to the lake basin migration and the gradual increase in the supply of Type Ⅲ provenance.

Low-Pressure Metamorphism of Granulite Facies in an Early Proterozoic Orogenic Event in Central Inner Mongolia

According to the kinds of feldspar and rock associations in the Ai-rich gneisses, the low-pressure metamorphic crust of the Early Proterozoic granulite facies in central Inner Mongolia can be divided into southern and northern belts which are composed of six rock associations. They represent the relevant rock sequences of the layered metamorphic rock series formed under specific metamorphic temperature and pressure conditions as well as tectonic environments. Mineral inclusions and reaction texture have recorded that the medium-temperature high-pressure mineral assemblages are replaced by the high-temperature low-pressure mineral assemblages, thus, giving rise to: garnet+quartz→ hypersthene+plagioclase; kyanite→sillimanite and garnet+kyanite/sillimanite+quartz→cordierite. The deformation fabrics of the rocks, the change of mineral assemblages and the PTt path of metamorphism indicate that the contempranceous high-temperature normal-slip ductile shearing is the main cause of the formation of the low-pressure metamorphic crust of granulite facies. In the orogenic event, the co-action of thrusting and extension resulted in the change of a medium-temperature high-pressure metamorphic environment into the high-temperature low-pressure metamorphic conditions.

Conditions and processes leading to large-scale gold deposition in the Jiaodong province,eastern China

The gold deposits in the Jiaodong Peninsula constitute the largest gold mineralized province in China.The mineralization shows common characteristics in their tectonic setting,ore-forming fluid and metallogenic system.Sulfidation and fluid immiscibility are two important mechanisms controlling gold precipitation,both of which consume sulfur in the oreforming fluids.The escape of H2S from the main ore-forming fluids and the decrease of total sulfur concentration not only lead to the efficient precipitation of gold,but also result in the crystallization of reducing minerals such as pyrrhotite and oxidizing minerals such as magnetite.Quartz solubility shows strong dependence on temperature,pressure,and CO2 content.The dependence of quartz solubility on pressure is weak at low temperatures,and progressively stronger at higher temperatures.Similarly,the temperature dependence of quartz solubility is relatively low at low pressures,but becomes gradually stronger at high pressures.The results of solubility modeling can constrain the dissolution and reprecipitation behavior of quartz in the oreforming veins and the formation mechanism of different types of quartz veins.The multi-stage mineralization fluid activity resulted in the complex dissolution structure of quartz in the Jiaodong gold veins.Pyrite in the main metallogenic period in the Jiaodong gold deposits shows complex microstructure characteristics at single crystal scale.The trace elements(mainly the coupling of As-and Au-rich belt)and sulfur isotope composition also display a certain regularity.The As-rich fluids might have formed by the initial pulse of ore-forming fluids through As-rich metasedimentary strata,while the As-Au oscillation zone at the margin of pyrite grains is related to the pressure fluctuation caused by fault activity and the local phase separation of fluids.There is a temporal and spatial evolution of gold fineness in the Jiaodong gold deposits.Water/rock reaction(sulfidation)was the main ore-forming mechanism of early gold mineralization,forming relatively high fineness gold,while significant pressure drop in the shallow part accompanied by fluid phase separation promoted the late gold mineralization,forming low fineness gold.Under cratonic destruction setting,dehydration of the amphibolite and granulite facies metamorphic lower-crust resulted in the formation of Au-CO2-rich ore-forming fluids,which rose along the deep fault and secondary structure,and formed the largescale fault-controlled gold deposits in Jiaodong.