Uraniferous diagenetic xenotime in northern Australia and its relationship to unconformity-associated uranium mineralisation

Stratabound, uraniferous diagenetic xenotime cements provide a minimum depositional age of 1,632±3 Ma for the sedimentary Birrindudu Group in the Killi Killi Hills, Tanami Region in northern Australia. The age of xenotime formation is broadly coeval with that recently proposed (1,650–1,600 Ma) for uranium mineralisation in the unconformity-associated deposits of the Pine Creek Inlier, northern Australia, and Athabasca Basin, Canada. The geological setting and formation model for the uraniferous xenotime crystals are similar to those widely proposed for unconformity-associated uranium deposits, suggesting a genetic link between the two. However, xenotime formation in the Birrindudu Group occurred during an apparently earlier stage of diagenesis, compared to late diagenetic formation of unconformity-associated uranium deposits. This could be explained by variations in the thickness of sediment cover and diachronous diagenesis across the basin, at the time of the basin-wide uranium mobilisation event, herein dated at ca. 1,630 Ma. In such a scenario, stratabound uraniferous xenotime cements could represent the remote distal zones of a more deeply buried, uranium mineralising system. Alternatively, the xenotime layer represents a precursor to, or a source for, later unconformity-associated ore deposition. In this case, the presence of diagenetic uraniferous xenotime in an area prospective for unconformity-associated uranium mineralisation would be an indication of, and still provide an approximate age for, uranium mobilisation within the cover sequence. Xenotime is a far more robust mineral than uraninite for U–Pb geochronology and can potentially provide a more reliable and precise timeframe for uranium mineralisation and subsequent recrystallisation events if present in the immediate uranium-ore environment.     

Textural and geochemical discrimination between xenotime of different origin in the Archaean Witwatersrand Basin, South Africa

Xenotime (YPO₄) of detrital, diagenetic, and hydrothermal origin within siliciclastic rocks of the Archaean Witwatersrand Basin, South Africa, has been identified on the basis of petrography and in situ ion microprobe (SHRIMP) age data. The chemical composition of xenotime, determined by in situ electron microprobe analysis, can be correlated with its origin. This allows the origin of any xenotime grain to be assessed by a non-destructive microanalytical method prior to ion microprobe geochronology. The main chemical discriminators are MREE-HREE abundance, normalised HREE slope and Eu anomaly, and, in some cases, U and Th contents. Igneous-detrital xenotime (> 2800 Ma) is distinguished from diagenetic (∼2780 Ma) and hydrothermal (< 2780 Ma) xenotime in having lower Eu, Dy, and Gd concentrations and a distinctively lower Gd/Yb ratio. Hydrothermal xenotime has distinctively lower U and Th concentrations when compared to igneous-detrital and diagenetic xenotime. Three separate hydrothermal fluid events and episodes of post-diagenetic xenotime growth are recognised in the geochemical and geochronological data, which correspond in time to the extrusion of the Ventersdorp lavas at ∼2720 Ma, the emplacement of the Bushveld Igneous Complex at ∼2061 Ma, and an event of unknown affinity at ∼2210 Ma. Although geochemical discrimination of the xenotime types from the Witwatersrand Basin, in combination with careful petrography, appears achievable, universal application of these discriminators to xenotime in other sedimentary basins remains untested.     

Response of xenotime to prograde metamorphism

Xenotime is a widespread accessory mineral in lower greenschist to upper amphibolite facies metasedimentary rocks from the Palaeoproterozoic Mount Barren Group, southwestern Australia. Xenotime is closely associated with detrital zircon, commonly forming syntaxial outgrowths, in samples of sandstone, micaceous quartzite, slate, phyllite, garnet-bearing semi-pelites, and in kyanite-, garnet-, and staurolite-bearing mica schists. In situ geochronology of xenotime from lower greenschist sandstones has previously yielded multiple U–Pb ages with peaks at ~2.0, ~1.7, and ~1.65 Ga, interpreted to represent the age of detritus, early diagenesis, and a later thermal event, respectively. New U–Pb dating of xenotime in slate yields a major population at ~1.7 Ga with a minor population at ~1.2 Ga, reflecting diagenetic and metamorphic growth, respectively, whereas xenotime in phyllite forms a minor age population at ~1.7 Ga and a main peak at ~1.2 Ga. Mid-greenschist facies semi-pelitic schists (quartz-muscovite-garnet) contain xenotime that formed before 1.8 Ga and at 1.2 Ga, representing detrital and peak metamorphic ages, respectively. Xenotime in samples of amphibolite facies schist (650°C and ~8 kbars) yields U–Pb ages of ~1.2 Ga, coinciding with the time of peak metamorphism. A single analysis of a xenotime core from an amphibolite facies schist gave an age of ~1.8 Ga, consistent with the presence of detrital xenotime. Our results suggest that detrital xenotime may be preserved under greenschist facies conditions, but is largely replaced during upper amphibolite facies conditions. Detrital xenotime is replaced through dissolution–reprecipitation reactions forming compositionally distinct rims during greenschist and amphibolite facies metamorphism at 1.2 Ga. Diagenetic xenotime is present in lower greenschist facies samples, but was not observed in metasedimentary rocks that had experienced temperatures above mid-greenschist facies metamorphism (450°C). The apparent disappearance of detrital and diagenetic xenotime and appearance of metamorphic xenotime during prograde metamorphism indicates that some of the yttrium, heavy rare earth elements, and phosphorus needed for metamorphic xenotime growth are probably derived from the replacement of detrital and diagenetic xenotime.     

Chemical and structural relations of epitaxial xenotime and zircon substratum in sedimentary and hydrothermal environments: a TEM study

Xenotime overgrowths on detrital zircon in siliciclastic sediments have been reported in numerous studies. However, in natural samples, solid solution of zircon and xenotime is limited to near-end-member compositions. In order to characterize the interface region between both minerals and to draw inferences on the growth mechanisms of authigenic xenotime, we studied xenotime overgrowths on detrital zircon grains from two Phanerozoic sandstone samples with contrasting post-depositional histories. In one sample, the small (≤10 μm), pyramidal xenotime overgrowths are of diagenetic origin and grew without major discontinuity on the detrital zircon grain. The second sample shows up to >50-μm-wide, porous and inclusion-rich, hydrothermal xenotime overgrowths on detrital zircon, whereas the transition zone between both minerals is accompanied by large pore volume. Chemical compositions of the xenotime precipitates from the two samples differ particularly in Y, REE, Th and Sc concentrations, whereas high MREE availability in the diagenetic sample and the presence of Sc in the hydrothermal sample, respectively, appear to have promoted xenotime growth. Transmission electron microscopy on electron-transparent foils cut from the interface region shows that both the diagenetic xenotime and the hydrothermal xenotime are crystalline and grew in optical and crystallographic continuity to their detrital zircon substrata. Only a narrow transition zone (≤90 nm—diagenetic sample, 200–300 nm—hydrothermal sample) between zircon and xenotime is in part made up of nanometre-scale crystalline domains that are slightly distorted and may have formed from dissolution–re-precipitation processes at the zircon rim along with precipitation from the respective fluid.     

SHRIMP U–Pb geochronology of authigenic xenotime and its potential for dating sedimentary basins

SHRIMP (Sensitive High‐Resolution Ion MicroProbe) analytical procedures have been developed to enable dating of the small, early diagenetic xenotime overgrowths that commonly occur on zircons in siliciclastic sedimentary rocks. The method will be particularly useful in Precambrian terranes, where diagenetic xenotime dating could play a role equivalent to biostratigraphic dating in the Phanerozoic. Reliable ²⁰⁷Pb/²⁰⁶Pb data are more readily obtained than ²⁰⁶Pb/²³⁸U, which also favours application to the Precambrian. However, it is demonstrated that ²⁰⁶Pb/²³⁸U dating of larger overgrowths (>10 μm) is also viable and applicable to Phanerozoic samples. SHRIMP Pb/Pb geochronology of authigenic xenotime in an unmetamorphosed Palaeoproterozoic sandstone in the Kimberley Basin has constrained diagenesis to a precision of ± 7 Ma. In contrast, greenschist‐facies metasediments of the Archaean Witwatersrand Basin, South Africa, contain both authigenic and alteration xenotime that record a complex history of growth from early diagenesis to the last major thermal event to affect the basin.     

SHRIMP U–Pb ages of diagenetic and hydrothermal xenotime from the Archaean Witwatersrand Supergroup of South Africa

SHRIMP dating of xenotime overgrowths on detrital zircon grains can constrain maximum durations since diagenesis and therefore provide minimum dates of sediment deposition. Thus, xenotime dating has significant economic application to Precambrian sediment-hosted ore deposits, such as Witwatersrand Au–U, for which there are no precise depositional ages. The growth history of xenotime in the Witwatersrand Supergroup is texturally complex, with several phases evident. The oldest authigenic xenotime ²⁰⁷Pb/²⁰⁶Pb age obtained in sandstone underlying the Vaal Reef is 2764 ± 5 Myr (1 σ), and most likely represents a mixture of diagenetic and hydrothermal growth. Nevertheless, this represents the oldest authigenic mineral age yet recorded in the sequence and provides a minimum age of deposition. Other xenotime data record a spread of ages that correspond to numerous post-diagenetic thermotectonic events (including a Ventersdorp event at ≈ 2720 Ma) up to the ≈2020 Ma Vredefort event.     

Geochronological constraints on uranium mineralization within the Hüttenberg siderite deposit (Eastern Alps,Austria)

A small-scale hydrothermal uranium mineralization hosted within the siderite deposit at Hüttenberg (Eastern Alps, Austria) was re-investigated using modern scanning electron microscope (SEM) and microprobe (EMPA) methods. The uranium mineralization comprises brannerite, coffinite and uraninite, and is spatially associated with Fe-, Ni- and Co-arsenides (loellingite, rammelsbergite, safflorite), bismuth and bismuthinite, as well as rare accessory silver-bearing minerals and gold locally. The U-bearing fluids also carried P, Y and REEs, which precipitated as xenotime and P–Y coffinite. The uranium mineralization paragenetically postdates the metasomatic formation of the host siderite ore.Uraninite allows for precise single spot EMPA ages to be calculated (±2 Ma) due to its high radiogenic lead content. There is an excellent internal consistency in the uraninite data with calculated dates ranging between 77 and 84 Ma. We interpret that these record the crystallization age of the uraninite, and that any influences of lead loss or common Pb are minimal. Brannerite was also analyzed but U-total Pb microprobe data scatter (c.30–80 Ma) with evidence of lead loss, particularly in altered domains. It does, however, provide a minimum constraint on the timing of mineralization that is consistent with the crystallization age of the uraninite. Coffinite proved completely unsuitable for U-total Pb geochronology due to extreme lead loss. However, hydrothermal xenotime yields a U-total Pb age of 78 ± 5 Ma, consistent with the uraninite age. Therefore, the U-total Pb geochronological data support a late Cretaceous age for the U-mineralization of c. 80 Ma. Considering this timing constraint together with other geological and indirect thermochronological aspects, we conclude that the siderite deposit of Hüttenberg is older and formed at c. 90−80 Ma.     

伊通地堑岔路河断陷储层成岩演化史与成岩模式

依据岔路河断陷大量的岩心、薄片、扫描电镜、X衍射等资料, 详细研究了岔路河地区储集层特征及成岩作用类型.按照成岩阶段划分标准, 将成岩阶段分为早成岩A期和B期, 晚成岩A期和B期, 其中晚成岩A期又分为A1和A2两个亚期; 运用BasinMod软件对主要构造部位进行埋藏史和热史的分析, 并结合孔隙演化史的研究, 分析了储层成岩演化过程, 结果显示, 在30Ma以前, 储层以压实和胶结作用为主, 下部的双阳组和奢岭组孔隙度从32%左右下降至15%左右, 上部的永吉组和万昌组孔隙度从32%左右下降至20%左右.在30~10Ma, 各地层都沉降至最大埋深, 较深部的双阳组和奢岭组砂岩中长石大量溶解, 孔隙度增加至18%左右, 为油气储存提供了良好的孔隙空间, 永吉组和万昌组孔隙度变化不大.在充分考虑了岔路河地区的埋藏史、有机质热演化史、成岩作用演化史以及岩石类型与沉积相等影响因素的基础上, 建立了该断陷的3种成岩模式: 冲积扇成岩模式、扇三角洲成岩模式和半深湖-深湖相成岩模式.      

In situ U-Pb dating and element mapping of three generations of monazite: Unravelling cryptic tectonothermal events in low-grade terranes

In situ U-Pb dating of monazite and xenotime in sedimentary rocks from the mid-Archean Soanesville Group in the Pilbara Craton, yields ages for provenance, diagenesis and multiple low-grade metamorphic events. Detrital monazite and xenotime grains give dates >3250 Ma, whereas diagenetic xenotime provides a new minimum age of 3190 ± 10 Ma for deposition of the basal Soanesville Group, previously constrained between ∼3235 Ma and ∼2955 Ma. Metamorphic monazite provides evidence for three episodes of growth: at 2.88, 2.16 and 1.65 Ga. Element mapping of monazite for La, Sm, Y and Th reveals distinct cores and rims in some crystals that were used to guide the placement of analytical spots during in situ U-Pb dating by sensitive high-resolution ion microprobe (SHRIMP). Specifically, La and Sm distributions closely correlate with different generations of monazite. The presence of two generations in single monazite crystals highlights the need for characterizing mineral chemistry prior to geochronology. It also shows the importance of using in situ dating techniques rather than methods that rely on the analysis of entire, potentially multi-aged, crystals. The ages recorded by metamorphic monazite span more than one billion years and are interpreted to record cryptic tectonothermal events within the craton. The 2.88 Ga age coincides with a phase of regional deformation, metamorphism and gold mineralization along a major crustal lineament, whereas the most common monazite age population (at 2.16 Ga) corresponds with the migration of a foreland fold-and-thrust belt across the craton. The youngest age (1.65 Ga) coincides with an episode of tectonic reworking in the Capricorn Orogen along the southern Pilbara margin. The prolonged history of monazite growth may, in part, relate to channelized fluid flow during reactivation of long-lived N- to NE-trending crustal structures that transect the craton. Despite repeated episodes of metamorphism, the isotopic system in each generation of monazite remained unperturbed, yielding precise dates. The ability of monazite to record three separate events, and in some instances two events in a single crystal, distinguishes it from most other low-temperature mineral chronometers, which are readily reset during metamorphic overprinting. Low-temperature monazite geochronology can provide a detailed isotopic history of cryptic thermal events and reveal the temporal and spatial patterns of far-field fluid flow related to tectonic processes. The previously unrecognized history of crustal fluid flow in the Pilbara Craton has implications for chemical, mineralogical and isotopic studies seeking to understand conditions on the early Earth.     

Xenotime–hematite aggregates on opaline filaments: evidence for biomineralization in weathered siliciclastic rocks, Capanema, Quadrilátero Ferrífero of Minas Gerais, Brazil

Dissolution cavities in weathered pebbly quartzite of the ~2.5-Ga Moeda Formation at Capanema, Quadrilátero Ferrífero of Minas Gerais, Brazil, are decorated with suspended filaments of opaline silica. The filaments sustain xenotime–hematite aggregates in the open space. Xenotime occurs as inclusions in buds and botryoidal aggregates of hematite. The filamentous structures consist of strand-forming buds, hypha-like extensions, and thin strands that compose mat-like arrangements. They resemble microbial filaments that were replaced by opaline silica and fossilized. The occurrence of spherical hematite as protuberances on hematite-free opaline hyphae is interpreted as accretion of dissolved iron onto extracellular polymers. Phosphate sites in polymeric substances expelled from the microbial filaments might have adsorbed yttrium and heavy rare-earth elements from groundwater to the iron-accreting polymers. These would have resulted in botryoidal aggregates of hematite with xenotime inclusions. The presence of authigenic xenotime in the weathering zone opens a new possibility to constrain the evolution of lateritic profiles by xenotime geochronology.     

大牛地气田上古生界储层埋藏—成岩演化过程

综合运用铸体薄片、扫描电镜、包裹体测温和拉曼分析等资料和方法,从地层埋藏历史、地温、压力、储层成岩特征、储层流体差异和物性演化等方面对大牛地气田上古生界致密砂岩储层成岩-成藏过程进行细致解剖。研究表明:大牛地气田上古生界储层埋藏成岩过程可划分为强机械压实阶段、复杂成岩演化阶段、储层定型阶段和抬升-弱改造阶段。四者对应的典型特征分别为储层强烈减孔（压实率约22.2%～97.5%）;发生两期油气充注（距今约200 Ma和180 Ma）,同时成岩演化类型多样,深刻影响储层物性;发生第三期油气充注（距今约150 Ma）,并且多数储层达到致密化程度（孔隙度小于10%）;构造大幅度抬升,张裂缝弱发育。油气充注早于储层致密化,微裂缝的发育可能为储层渗透率提升的关键。     

金滩和白马山印支期花岗岩体LA-ICPMS锆石U-Pb定年及其成岩启示

对江西金滩和湖南白马山印支期花岗岩体高精度的LA-ICPMS锆石U-Pb年代学研究表明:金滩黑云母二长花岗岩体给出了(222.1±1.6)Ma的谐和年龄,属印支中晚期,而不是以往所认为的形成于海西期;白马山印支期的黑云母二长花岗岩和黑云母花岗闪长岩分别给出了(223.3±1.4)Ma、(204.5±2.2)Ma的谐和年龄。融合其他年代学数据,湖南白马山印支期花岗质岩浆可能经历了三期侵入活动:其中第一次侵入在印支早期(240Ma左右),其后又经历了220Ma左右和204Ma的另外两幕侵入活动。结合其他相关研究资料进一步表明220Ma左右华南内陆地区可能存在一次广泛的构造-岩浆活动。     

Potential minerals for determining U–Th–Pb chemical age using electron microprobe

After a decade of studies and development, it is now accepted that reliable U–Th–total Pb isochron ages can be calculated for monazite using an electron microprobe at μm scale, either directly on thin sections or on separated grains mounted in polished section. The potential for determining U–Th–Pb chemical ages from other U- and Th-enriched phases has been investigated compared to chemical monazite-dating results for which individual spot-age precisions of 20 to 100 Ma can be achieved from individual spot analyses. Using isochron plots for monazite, the age homogeneity of a given population of data can be assessed and, depending upon the number of analyses (n  50), a precision of 5 to 10 Ma can be obtained. The U content in xenotime widely varies from less than 0.1 wt.% up to 3 wt.%, but Th rarely exceeds 1 wt.%. As a consequence, the amount of radiogenic Pb produced during a given period remains significantly lower for xenotime than for monazite, leading to a lower precision (± 20 Ma) on the mean ages. Xenotime, however, appears to remain as a closed system, but common Pb must be carefully checked. Furthermore, the electron-microprobe technique (EPMA) allows controlling any age discrepancy on xenotime grains as small as 10–20 μm that cannot be dated by other isotopic methods. Such xenotime ages can be useful when studying the monazite–xenotime equilibrium. The electron microprobe is not the most reliable method for dating zircon since U and Th concentrations are generally low and common Pb is not negligible. Nevertheless, the spatial resolution of EPMA coupled with isotope methods allows conclusive in situ studies about radiogenic Pb mobility and metamictization. Thorite does not seem suitable for dating with either isotope methods or EPMA because of continuous radiogenic Pb loss. Conversely, the oxide phases, thorianite and baddeleyite are robust minerals with closed systems. They are rather rare and seem to incorporate negligible common Pb, making EPMA a method of choice for dating them. For thorianite, the precision on the mean age can be similar as that obtained for monazite, or even better, while the precision for baddeleyite cannot be significantly better than 20 to 50 Ma due to the limited amount of U ( 0.1%) and the lack of Th.     

An improved EPMA analytical protocol for U-Th-Pbtotal dating in xenotime: Age constraints from polygenetic Mangalwar Complex,Northwestern India

EPMA U-Th-Pb_total dating in U- and Th bearing minerals (e.g., monazite, zircon, and xenotime) is a low-cost and reliable technique used for retrieving age information from detrital, diagenetic and low to high-T metamorphic, as well as magmatic rocks. Although, the accuracy on measured ages obtained using EPMA is considered to be poor compared to isotopic ages, the superior spatial resolution, ability to integrate textural and age information by in-situ measurement, lack of sample damage and easier and cheaper data generation in EPMA make chemical dating a very valuable tool to decipher diverse petrological processes.This contribution presents an improved analytical protocol to obtain precise estimates of U, Th and Pb concentrations in xenotime. Results were tested on monazite standard (Moacyr pegmatite, Brazil; TIMS age: 487 ± 1 Ma) as the reference material. The proposed analytical protocol has been successfully applied to achieve an analytical uncertainty of less than 10% in U, Th and Pb measurements in xenotime. The protocol was further used to resolve polygenetic xenotime ages (ca. 1.82, 1.28 and 0.93 Ga) in metapelite samples from the Mangalwar Complex, Northwestern India. Monazites in the same samples were also analyzed and found to preserve the two younger ages (i.e., ca. 1.28 and 1.0 Ga). The obtained ages from the xenotime and monazite very well corroborate with the earlier published ages from the area validating the proposed analytical protocol.     

Magmatic-to-hydrothermal crystallization in the W–Sn mineralized Mole Granite (NSW, Australia): Part I: Crystallization of zircon and REE-phosphates over three million years—a geochemical and U–Pb geochronological study

Zircon, monazite and xenotime crystallized over a temperature interval of several hundred degrees at the magmatic to hydrothermal transition of the Sn and W mineralized Mole Granite. Magmatic zircon and monazite, thought to have crystallized from hydrous silicate melt, were dated by conventional U–Pb techniques at an age of 247.6 ± 0.4 and 247.7 ± 0.5 Ma, respectively. Xenotime occurring in hydrothermal quartz is found to be significantly younger at 246.2 ± 0.5 Ma and is interpreted to represent hydrothermal growth. From associated fluid inclusions it is concluded that it precipitated from a hydrothermal brine ≤ 600 °C, which is below the accepted closure temperature for U–Pb in this mineral. These data are compatible with a two-stage crystallization process: precipitation of zircon and monazite as magmatic liquidus phases in deep crustal magma followed by complete crystallization and intimately associated Sn–W mineralization after intrusion of the shallow, sill-like body of the Mole Granite. Later hydrothermal formation of monazite in a biotite–fluorite–topaz reaction rim around a mineralized vein was dated at 244.4 ± 1.4 Ma, which distinctly postdates the Mole Granite and is possibly related to a younger hidden intrusion and its hydrothermal fluid system.

Obtaining precise age data for magmatic and hydrothermal minerals of the Mole Granite is hampered by uncertainties introduced by different corrections required for multiple highly radiogenic minerals crystallising from evolved hydrous granites, including ²³⁰Th disequilibrium due to Th/U fractionation during monazite and possibly xenotime crystallization, variable Th/U ratios of the fluids from which xenotime was precipitating, elevated contents of common lead, and post-crystallization lead loss in zircon, enhanced by the fluid-saturated environment. The data imply that monazite can also survive as a liquidus phase in protracted magmatic systems over periods of 10⁶ years. The outlined model is in agreement with prominent chemical core-rim variation of the zircon.     

碳酸盐岩成岩作用与孔隙演化

在总结我国碳酸盐岩沉积和成岩基本特征的基础上,阐明有利于孔隙形成的白云石化、去膏、去云化、淡水和埋藏溶解等作用,以及破坏性成岩作用的特征和识别标志。成岩地质体是成岩环境的产物,可根据成岩组构、地球化学和发光特征等加以鉴别。成岩模式是成岩组合、成岩特征和孔隙演化的总概括,以滩相成岩模式展示了成岩相与孔隙的关系。     

Garnet and Zircon U-Pb Geochronology and Geochemistry Reveal Genesis of the Dafang Au-Pb-Zn-Ag Deposit, Southern Hunan

Garnet is a primary mineral in skarn deposits and plays a significant role in recording copious mineralization and metallogenic information. This study systematically investigates the geochemistry and geochronology of garnet and zircon in the Dafang Au-Pb-Zn-Ag deposit, which represents prominent gold mineralization in southern Hunan, China. Garnet samples with distinct zoning patterns and compositional variations were identified using various analytical techniques, including Backscattered Electron (BSE) imaging, Cathodoluminescence (CL) response, textural characterization, and analysis of rare-earth elements (REE), major contents, and trace element compositions. The garnet was dated U-Pb dating, which yielded a lower intercept age of 161.06 ± 1.93 Ma. This age is older than the underlying granodiorite porphyry, which has a concordia age of 155.13 ± 0.95 Ma determined by zircon U-Pb dating. These results suggest that the gold mineralization may be related to the concealed granite. Two groups of garnet changed from depleted Al garnet to enriched Al garnet, and the rare earth element (REE) patterns of these groups were converted from light REE (LREE)-enriched and heavy REE (HREE)-depleted with positive europium (Eu) anomalies to medium REE (MREE)-enriched from core to rim zoning. The different REE patterns of garnet in various zones may be attributed to changes in the fluid environment and late superposition alteration. The development of distal skarn in the southern Hunan could be a significant indicator for identifying gold mineralization.     

障壁坝砂体储层特征与成岩孔隙定量演化模式 ——以鄂尔多斯盆地延长探区本溪组为例

为明确海陆变迁背景下障壁坝砂体成岩孔隙演化规律,运用岩石薄片、扫描电镜及配套能谱、阴极发光等资料,对延长探区石炭系本溪组障壁成因砂体储集特征与成岩作用进行综合分析,并结合砂岩初始孔隙度恢复模型与薄片孔隙定量分析技术,开展基于埋藏热演化历程的储层孔隙演化参数定量评价,建立该区本溪组障壁坝砂岩孔隙定量演化模式。结果表明:储层砂岩以石英砂岩、岩屑质石英砂岩及岩屑砂岩为主;孔隙度、渗透率平均值分别为4.72%和1.22×10^(-3 )μm^2;储集空间以溶蚀扩大孔、粒内孔、晶间孔为主;现今储层主体已达到晚成岩阶段;砂岩初始孔隙度平均值为38.1%,埋藏至今共经历了四个典型的成岩增/减孔阶段,即P_1～T_2(285～208 Ma)"低地温、快埋藏"的大幅减孔阶段(-30.7%)、T_2～J_3(208～153 Ma)"高地温、缓沉降"的溶蚀增孔阶段(+3.9%)、J_3～K_1(153～96 Ma)"高地温、稳埋深"的减孔定型阶段(-6.6%)、K_1至今(96 Ma～)"低地温、晚隆升"的弱改造阶段。     

Pre-Variscan evolution of the Western Tatra Mountains: new insights from U-Pb zircon dating

In situ LA-MC-ICP-MS U-Pb zircon geochronology combined with cathodoluminescence imaging were carried out to determine protolith and metamorphic ages of orthogneisses from the Western Tatra Mountains (Central Western Carpathians). The metamorphic complex is subdivided into two units (the Lower Unit and the Upper Unit). Orthogneisses of the Lower Unit are mostly banded, fine- to medium-grained rocks while in the Upper Unit varieties with augen structures predominate. Orthogneisses show a dynamically recrystallised mineral assemblage of Qz?+?Pl?+?Bt?±?Grt with accessory zircon and apatite. They are peraluminous (ASI?=?1.20?C1.27) and interpreted to belong to a high-K calc-alkaline suite of a VAG?Ctype tectonic setting. LA-MC-ICP-MS U-Pb zircon data from samples from both units, from crystals with oscillatory zoning and Th/U?>?0.1, yield similar concordia ages of ca. 534?Ma. This is interpreted to reflect the magmatic crystallization age of igneous precursors. These oldest meta-magmatics so far dated in the Western Tatra Mountains could be linked to the fragmentation of the northern margin of Gondwana. In zircons from a gneiss from the Upper Unit, cores with well-developed oscillatory zoning are surrounded by weakly luminescent, low contrast rims (Th/U?<?0.1). These yield a concordia age of ca. 387?Ma corresponding to a subsequent, Eo-Variscan, high-grade metamorphic event, connected with the formation of crustal-scale nappe structures and collision-related magmatism.     

开江-梁平地区上二叠统长兴组储层岩石学特征

以露头剖面测量、单井岩芯观察和室内薄片鉴定为依据,结合不同结构和成因类型白云岩(或白云石)的n(Mg)/n(Ca)比值,有序度和C、O、Sr同位素特征,对开江-梁平地区长兴组碳酸盐岩储层成岩作用与孔隙发育和演化规律进行了综合研究,确定研究区对储层破坏最大的成岩作用是胶结、压实-压溶、新生变形和次生矿物的充填等作用,而对储层贡献最大的建设性成岩作用主要为埋藏白云岩化、重结晶、溶蚀和破裂等作用,有效地改善了储层的孔渗性,形成以粒间溶孔、粒内溶孔、铸模孔、晶间溶孔、超大溶孔、溶洞和溶裂缝等多种次生孔隙类型组合的礁、滩相储层基本特征。在再造成岩序列和恢复孔隙演化历史的基础上,对储层发育与孔隙演化的关系可得出如下几点结论:①有利的礁、滩相带决定了储层发育的位置和空间展布规律;②礁、滩复合体的多期次埋藏白云岩化是形成储层的基础;③破裂作用和溶蚀作用是改造储层和提高储层质量的关键。