Geologic, Fluid Inclusion and Stable Isotope Constraints on Mechanisms of Ore Deposition at the Datuanshan Copper Deposit, Middle–Lower Yangtze Valley, Eastern China

The Datuanshan deposit is one of the largest and most representative stratabound copper deposits in the Tongling area,the largest ore district in the Middle-Lower Yangtze River metallogenic belt.The location of the orebodies is controlled by the interlayer-slipping faults between the Triassic and Permian strata,and all the orebodies are distributed in stratiform shape around the Mesozoic quartz monzodiorite dikes.Based on field evidence and petrographic observations,four mineralization stages in the Datuanshan deposit have been identified:the skarn,early quartz-sulfide,late quartzsulfide and carbonate stages.Chalcopytite is the main copper mineral and mainly formed at the late quartz-sulfide stage.Fluid inclusions at different stages were studied for petrography,microthermometry,laser Raman spectrometry and stable isotopes.Four types of fluid inclusions,including three-phase fluid inclusions(type 1),liquid-rich fluid inclusions(type 2),vapour-rich fluid inclusions(type 3) and pure vapour fluid inclusions(type 4),were observed.The minerals from the skarn,early and late quartz-sulfide stages contain all fluid inclusion types,but only type 2 fluid inclusions were observed at the carbonate stage.Petrographic observations suggest that most of the inclusions studied in this paper are likely primary.The coexistence of different types of fluid inclusions with contrasting homogenization characteristics(to the liquid and vapour phase,respectively) and similar homogenization temperatures(the modes are 440-480℃,380-400℃ and 280-320℃ for the skarn,early and late quartz-sulfide stages,respectively) in the first three stages,strongly suggests that three episodes of fluid boiling occurred during these stages,which is supported by the hydrogen isotope data.Laser Raman spectra identified CH_4 at the skarn and early quartz-sulfide stages.Combined with other geological features,the early ore-forming fluids were inferred to be under a relatively reduced environment.The CO_2 component has been identified at the late quartz-sulfide and carbonate stages,indicating that the late ore-forming fluids were under a relatively oxidized environment,probably as a result of inflow of and mixing with meteoric water.In addition,microthermometric results of fluid inclusions and H-O isotope data mdicate that the ore forming fluids were dominated by magmatic water in the early stages(skarn and early quartz-sulfide stages) and mixed with meteoric water in the late stages(late quartz-sulfide and carbonate stages).The evidence listed above suggests that the chalcopyrite deposition in the Datuanshan deposit probably resulted from the combination of multiepisode fluid boiling and mixing of magmatic and meteoric water.     

The Ore Mineralogy of the Kedrovskoe Gold Deposit (the Muya Region,the Republic of Buryatia,Russia)

The ore mineralogy of the largest quartz vein, Osinovaya, at the Kedrovskoe gold deposit has been studied. Three stages of mineral formation, namely, marcasite–pyrrhotite–pyrite, gold–polysulfide, and hypergenic stages are identified. Native gold is attributed to the gold–polysulfide stage and is represented by two generations. The earlier high fineness generation (600–870, 780–820 prevails) cements the fragments of pyrite grains or forms inclusions in pyrite, and the later low fineness generation (520–580, 540–580 prevails) is associated with sphalerite–chalcopyrite–galena veinlets in pyrite. The disappearance of arsenious pyrite, the increase in iron content of sphalerite, and the change in pyrite to pyrrhotite with depth is recorded.     

Cooling history and tectonic exhumation stages of the south-central Tibetan Plateau (China): Constrained by 40Ar/39Ar and apatite fission track thermochronology

Between the Qiangtang Block and Yalung-Zangpo Suture Zone in the south-central Tibetan Plateau, the following geological units and suture zones have been identified from south to north: the Gangdese Granitic Belt, the Lhasa Block, the Nyainqentanghla Shear Zone, the Dangxiong–Sangxiong Tectono-granitic Belt and the Bangong–Nujiang Suture Zone. To better constrain the tectonic evolution and cooling histories of these units, ⁴⁰Ar/³⁹Ar muscovite, biotite and K-feldspar, as well as apatite fission track dating and thermochronological analysis have been carried out. The analytical results indicate that the south-central Tibetan Plateau, with the exception of the Nyainqentanghla Shear Zone, provides a record of three cooling stages at 165–150, 130–110 and ∼45–35 Ma. Fission-track data modelling also indicates that the stages of cooling were different in the different tectonic belts or blocks. Very different cooling phases occurred in the south-central Tibetan Plateau, compared with southern Tibet, as well as along the Yalung–Zangpo Suture Zone. There is no thermochronological evidence to indicate that the south-central part of Tibetan Plateau was influenced by the underthrusting of Indian Plate.The three-stage cooling history and the stages of tectonic exhumation were controlled completely by the closure of the Bangong–Nujiang Suture Zone along its eastern segment during Middle–Late Jurassic (165–150 Ma) and its western segment in the Early–Late Cretaceous (130–110 Ma), as well as by the collision between the Indian and Asian plates in the Paleogene (45–35 Ma).     

The role of amphiboles in the metamorphic evolution of the UHP rocks: a case study from the Tso Morari Complex, northwest Himalayas

Amphiboles represent a crucial phase of the ultra-high-pressure (UHP) metamorphic rocks as their solid solution behavior reflects both bulk compositional and P–T changes. Three different types of amphibole have been reported from the UHP metamafic rocks of the Tso Morari Crystalline Complex, NW Himalayas: Na-rich (glaucophane); Na–Ca-rich (barroisite, taramite, winchite) and Ca-rich (tremolite, magnesio-hornblende, pargasite). The Na-amphibole is presented as a core of the zoned amphibole with Na–Ca-rich rim; Na–Ca-amphibole is presented as inclusion in garnets as well as in matrix, and Ca-amphibole is generally found in the matrix. The Na–Ca-amphibole is observed at two different stages of metamorphism. The first is pre-UHP, and the second is post-garnet–omphacite assemblage though with a significant difference in composition. The pressure–temperature estimations of the formation of these two sets of Na–Ca-amphiboles corroborate their textural associations. Ca-rich amphiboles are generally present in the matrix either as symplectite with plagioclase or as a pseudomorph after garnet along with other secondary minerals like chlorite and biotite. Two different types of zoning have been observed in the amphibole grains: (1) core is Na-rich followed by Na–Ca rim and (2) core of Na–Ca-amphibole is followed by Ca-rich rim. The pre-UHP (or the prograde P–T path) and post-UHP stages (or the retrograde P–T path) of Tso Morari eclogites are defined by characteristic amphibole compositions, viz. Na/Na–Ca-amphibole, Na–Ca-amphibole and Ca-amphibole and thus indicate their utility in inferring crustal evolution of this UHP terrain.     

Three Stages of Mesozoic Bimodal Igneous Rocks and Their Tectonic Implications on the Continental Margin of Southeastern China

There are large-scale Mesozoic bimodal igneous rock associations on the continental margin of southeastern China. They aroused extensive attention in the 1980s because of their specific tectonic implications, and have been found frequently during recent geological surveys. This paper reviews the studies of regional Mesozoic bimodal rocks, and concludes that they can be subdivided into three stages, i.e., the Early Jurassic (209-170 Ma, the first (Ⅰ) stage), the Late Jurassic-early Early Cretaceous (154-121 Ma, the second (Ⅱ) stage), and the late Early Cretaceous-Late Cretaceous (115-85 Ma, the third (Ⅲ) stage). These three stages of bimodal rocks were formed in different tectonic settings, and are important indicators for regional Mesozoic tectonic evolution.     

The Muruntau Deposit: Geodynamic Position and a Variant of Genetic Model of the Ore-Forming System

The paper presents the results of original structural studies conducted over a long time within the Kyzylkum ore province and at the Muruntau deposit (Central Asia), including the period when the central part of the deposit, as well as horizons at the eastern and southeastern flanks (abandoned or inaccessible at present), were available to visit. Owing to the geodynamic and structural analysis of the region and the deposit itself, several stages of deformation that created the Muruntau interference structure have been identified. These stages were attributed to the Caledonian (subduction, collision, and transpression regimes), Hercynian (subduction, collision, and transpression regimes), and Cimmerian (transpression regime) cycles of tectonogenesis. The fault–fracture structures of these stages are accompanied by the development of various newly formed (vein–veinlet-metasomatic) mineral assemblages. Based on the averaged characteristics of gold mineralization in various mineral formations, a morphogenetic classification of orebodies was developed and their role in the deposit was revealed. The morphological features of the Main Muruntau ore deposit are considered. A critical review of the results of isochron age dating was made. The subduction–hydrothermal model of the formation of the giant Muruntau gold ore deposit is discussed.     

New Data on the Age of Gold Mineralization in the Southeastern Part of Eastern Sayan

This article presents new data on the age of the largest gold deposits in the southeastern part of Eastern Sayan. The dates have been obtained by Ar–Ar analysis of micas occurring in gold-bearing quartz veins and mineralized zones. The obtained Ar–Ar ages of fuchsite and sericite from the tectonized and mineralized zones of the Zun–Holba deposit (ore body Severnoye-3), range within 353.9–386.4 Ma; a similar result of 352.9 Ma was yielded by Ar–Ar dating of Cr–muscovite from mylonitized listvenite in the veins of the periphery of the Zun–Ospa gold deposit. However, muscovite from the ore-bearing quartz vein of the Pioneer gold–quartz deposit, located near Zun–Holba, has been dated to 421.9 Ma. The obtained new data on isotopic age of the gold–quartz ores and gold–sulphide–quartz deposits allow recognition of the Early Palaeozoic accretion–collision and the Late Palaeozoic shearing stages of formation of gold mineralization in the SE Eastern Sayan.     

Tectonics of the junction region between the East European craton and West Arctic platform

The region of the junction and interaction between the East European Craton (EEC) and the West Arctic Craton (WAC) is regarded as a complexly built zone or assembly of both the volumetric and dividing linear tectonic elements: the Trollfjord–Rybachi–Kanin (TRK) Lineament, the pericratonic subsidence zone of the EEC, the Karpinskii Lineament, the Murmansk Block of the Fennoscandian (Baltic) Shield, and the Kolmozero–Voronya Zone, which are briefly characterized in this paper. Evidences of thrusting have been established not only in the TRK Suture Zone and on the Rybachi Peninsula, which represent a fragment of the Timanides fold–thrust belt, but also to the southwest, in the Upper Riphean and Vendian terrigenous sequences making up the Sredni Peninsula and related to the pericratonic trough of the VEC. Two phases of fold–thrust deformations with elements of left-lateral strike-slip offset pertaining to the activity and evolution of the lineament suture dividing the Sredni and Rybachi peninsulas have been recorded. The variously oriented fault–fold systems within this fault zone are evidence for multistage deformation and can be explained by an at least twostage change in the kinematics that control displacement along the fault. The disintegrated granitic massifs of the Archean crystalline basement tectonically squeezed out in the upper crust as protrusions are localized within TRK Fault Zone. Plagiogranitic bodies, which underwent superposed fault-fold deformations of both kinematic stages, are an evidence of the vigorous tectonic event that predated folding and two-stage strike-slip displacement along the TRK Fault—by thrusting of Riphean sequences from north to south toward the Archean craton. The nappe–thrust regional structure was formed at this stage; elements of it have been recognized in the Sredni, Rybachi, and Kanin peninsulas. The main stages of tectonic evolution in the junction zone between the EEC and the WAP have been revealed and substantiated.     

Late Cenozoic Sedimentary Evolution of Pagri‐Duoqing Co graben,Southern End of Yadong‐Gulu Rift,Southern Tibet

The north trending rifts in southern Tibet represent the E–W extension of the plateau and confirming the initial rifting age is key to the study of mechanics of these rifts. Pagri–Duoqing Co graben is located at southern end of Yadong–Gulu rift, where the late Cenozoic sediments is predominately composed of fluvio-lacustrine and moraine. Based on the sedimentary composition and structures, the fluvio-lacustrine could be divided into three facies, namely, lacustrine, lacustrine fan delta and alluvial fan. The presence of paleo-currents and conglomerate components and the provenance of the strata around the graben indicate that it was Tethys Himalaya and High Himalaya. Electron spin resonance (ESR) dating and paleo-magnetic dating suggest that the age of the strata ranges from ca. 1.2 Ma to ca. 8 Ma. Optically stimulated luminescence (OSL) dating showed that moraine in the graben mainly developed from around 181–109 ka (late Middle Pleistocene). Combining previous data about the Late Cenozoic strata in other basins, it is suggested that 8–15 Ma may be the initial rifting time. Together with sediment distribution and drainage system, the sedimentary evolution of Pagri could be divided into four stages. The graben rifted at around 15–8 Ma due to the eastern graben-boundary fault resulting in the appearance of a paleolake. Following by a geologically quiet period about 8–2.5 Ma, the paleolake expanded from east to west at around 8–6 Ma reaching its maximum at ca. 6 Ma. Then, the graben was broken at about 2.5 Ma. At last, the development of the glacier separated the graben into two parts that were Pagri and Duoqing Co since the later stages of the Middle Pleistocene. The evolution process suggested that the former three stages were related to the tectonic movement, which determined the basement of the graben, while the last stage may have been influenced by glacial activity caused by climate change.     

Mesozoic and Cenozoic structural deformation in the NW Tarim Basin,China: a case study of the Piqiang–Selibuya Fault

The Piqiang–Selibuya Fault is the most significant fault in the NW Tarim Basin, China. It has attracted increasing attention because of the discovery of a series of oil (gas) fields in and around the fault zone. The structural characteristics and evolution of the Piqiang–Selibuya Fault remain controversial. Field geological surveys and seismic data interpretation reveal that the fault has experienced three stages of activity. The thicknesses of the Permian and Miocene strata on opposing sides of the fault are clearly different, and these reveal that the fault has experienced two stages of significant thrusting. The first stage took place at the end of the Triassic and was associated with the Qiangtang Block amalgamated to the south margin of Eurasia. The second stage occurred at the end of the Miocene and might have been caused by the northwards overthrusting of the Pamir. These two stages of thrusting led to the lower–middle Cambrian detachment layer in the eastern part of the Keping thrust belt being 2 km shallower than in the western part. Since the Pliocene, the southern Tien Shan orogenic belt has been reactivated and thrust towards the interior of the Tarim Basin, and a series of ENE–WSW-trending thrust sheets have formed in the Keping thrust belt. Because of the different depth of the detachment layer on the opposing sides of the Piqiang–Selibuya Fault, the number and spacing of thrust sheets formed to the east of the fault differ from those to the west. This dissimilar deformation led to the strike–slip displacement on the Piqiang–Selibuya Fault. The three stages of fault activity record three important tectonic events in the NW Tarim Basin. Qualitative analysis of this activity helps us better understand the influence of the far-field effect of the collisions that occurred on the southern margin of the Eurasia plate on the structural deformation of the NW Tarim Basin.     

Re–Os and U–Pb geochronology of porphyry Mo deposits in central Jilin Province: Mo ore-forming stages in northeast China

Central Jilin Province lies along the eastern edge of the Xing–Meng orogenic belt of northeast China. At least 10 Mo deposits have been discovered in this area, making it the second-richest concentration of Mo resources in China. To better understand the formation and distribution of porphyry Mo deposits in the area, we investigated the geological characteristics of the deposits and applied zircon U–Pb and molybdenite Re–Os isotope dating to constrain the age of mineralization. Our new geochronological data show the following: the Jidetun Mo deposit yields molybdenite Re–Os model ages of 164.6–167.1 Ma, an isochron age of 168 ± 2.5 Ma, and a weighted mean model age of 165.9 ± 1.2 Ma; the Houdaomu Mo deposit yields molybdenite Re–Os model ages of 167.4–167.7 Ma, an isochron age of 168 ± 13 Ma, and a weighted mean model age of 167.5 ± 1.2 Ma; and the Chang’anpu Mo deposit yields a zircon U–Pb age for granodiorite porphyry of 166.9 ± 1.5 Ma (N = 16). These new age data, combined with existing molybdenite Re–Os dates, show that intense porphyry Mo mineralization was coeval with magmatism during the Middle Jurassic (167.8 ± 0.4 Ma, r > 0.999). The geotectonic mechanisms responsible for Mo mineralization were probably related to subduction of the Palaeo-Pacific plate beneath the Eurasian continent. Combining published molybdenite Re–Os and zircon U–Pb ages for northeast China, the Mo deposits are shown to have been formed during multiple events coinciding with periods of magmatic activity. We identified three phases of mineralization, two of which had several stages: the Caledonian (485–480 Ma); the Indosinian comprising the Early–Middle Triassic (248–236 Ma) and Late Triassic (226–208 Ma) stages; and the Yanshanian phase comprising the Early–Middle Jurassic (202–165 Ma), Late Jurassic–early Early Cretaceous (154–129 Ma), and Early Cretaceous (114–111 Ma) stages. Although Mo deposits formed during each phase/stage, most of the mineralization occurred during the Early–Middle Jurassic.     

Transformation of Fe–Ti gabbro to coronite, eclogite and amphibolite in the Baie du Nord segment, Manicouagan Imbricate Zone, eastern Grenville Province

Fe–Ti gabbros from the Baie du Nord Segment of the Manicouagan Imbricate Zone, metamorphosed under high P–T conditions during the Grenvillian orogeny, have been the focus of a detailed micropetrological study. Textures and mineral chemistry suggest that the mineral assemblages represent progressive stages of metamorphic transformation resulting in the formation of coronas, pseudomorphs after igneous phases (transitional) and true, granoblastic eclogites. The transitional and eclogitic samples also have coronas which are developed locally around igneous xenocrysts of plagioclase and olivine. The deformed margins of coronitic Fe–Ti gabbros are transformed to amphibolite and contain clinopyroxene-bearing leucosomes with garnet poikiloblasts that are indicative of high-P–T dehydration melting. Interpretation of garnet zoning and thermobarometry suggest that the highest P–T conditions are recorded by coronas around xenocrysts (c. 720–800 °C at 14–17 kbar) and garnet–clinopyroxene cores in granoblastic assemblages (c. 740–820 °C at 13–17 kbar) in the eclogitic samples. Re-equilibration during the early stages of exhumation at high-T conditions (>700 °C) affected all samples, and is evidenced by the widespread development of pargasite-bearing plagioclase collars in the coronitic and transitional metagabbros and by widespread re-equilibration of the eclogites giving lower P–T estimates at grain boundaries. However, the difference in calculated pressure conditions between coronite and eclogite samples is consistent with increasing pressure (depth) from the coronites (11–13 kbar) to the eclogites (13–17 kbar). The P–T conditions recorded by these rocks define a metamorphic field gradient which suggests high heat flow through the lower crust during the Grenvillian orogeny.     

Reconstructing the stages of orogeny around the Junggar basin from the lithostratigraphy of Late Paleozoic,Mesozoic, and Cenozoic sediments

The Junggar basin contains an almost continuous section of Late Carboniferous–Quaternary terrigenous sedimentary rocks. The maximum thicknesses of the stratigraphic units constituting the basin cover make up a total of ~ 23 km, and the basement under the deepest part of the basin is localized at a depth of ~ 18 km. Both the folded framing and the basin edges have undergone uplifting and erosion during recent activity. These processes have exposed all the structural stages of the basin cover. Considering the completeness and detailed stratigraphic division of the section, we can determine the exact geologic age of intense mountain growth and erosion periods as well as estimate the age of orogenic periods by interpolating the stratigraphic ages. During the Permian orogeny, which included two stages (255–265 and 275–290 Ma), the Junggar, Zaisan, and Turpan–Hami basins made up a whole. During the Triassic orogeny (210–230 Ma), the Junggar and Turpan–Hami basins became completely isolated from each other. During the Jurassic orogeny (135–145 and 160–200 Ma), the sedimentation took place within similar boundaries but over a smaller area. During the Cretaceous orogeny (65–85 and 125–135 Ma), the mountain structures formed mainly at the southern boundaries of the basin and along the Karamaili–Saur line. The Junggar and Zaisan basins were separated at that time. The Early and Middle Paleogene were characterized by relative tectonic quiescence. The fifth orogenic stage began in the Oligocene. The recent activity consists of two main stages: Oligocene (23–33 Ma) and Neogene–Quaternary (1.2–7.6 Ma to the present).     

Geological and Fluid Inclusion Constraints on Gold Deposition Processes of the Dayingezhuang Gold Deposit,Jiaodong Peninsula,China

The Dayingezhuang gold deposit, hosted mainly by Late Jurassic granitoids on Jiaodong Peninsula in eastern China, contains an estimated 170 t of gold and is one of the largest deposits within the Zhaoping fracture zone. The orebodies consist of auriferous altered pyrite–sericite–quartz granites that show Jiaojia-type (i.e., disseminated and veinlet) mineralization. Mineralization and alteration are structurally controlled by the NE- to NNE-striking Linglong detachment fault. The mineralization can be divided into four stages: (K-feldspar)–pyrite–sericite–quartz, quartz–gold–pyrite, quartz–gold–polymetallic sulfide, and quartz–carbonate, with the majority of the gold being produced in the second and third stages. Based on a combination of petrography, microthermometry, and laser Raman spectroscopy, three types of fluid inclusion were identified in the vein minerals: NaCl–H2O (A-type), CO2–H2O–NaCl (AC-type), and pure CO2 (PC-type). Quartz crystals in veinlets that formed during the first stage contain mainly AC-type fluid inclusions, with rare PC-type inclusions. These fluid inclusions homogenize at temperatures of 251°C–403°C and have low salinities of 2.2–9.4 wt% NaCl equivalent. Quartz crystals that formed in the second and third stages contain all three types of fluid inclusions, with total homogenization temperatures of 216°C–339°C and salinities of 1.8–13.8 wt% NaCl equivalent for the second stage and homogenization temperatures of 195°C–321°C and salinities of 1.4–13.3 wt% NaCl equivalent for the third stage. In contrast, quartz crystals that formed in the fourth stage contains mainly A-type fluid inclusions, with minor occurrences of AC-type inclusions; these inclusions have homogenization temperatures of 106°C–287°C and salinities of 0.5–7.7 wt% NaCl equivalent. Gold in the ore-forming fluids may have changed from Au(HS)0 as the dominant species under acidic conditions and at relatively high temperatures and fO2 in the early stages, to Au(HS)2– under neutral-pH conditions at lower temperatures and fO2 in the later stages. The precipitation of gold and other metals is inferred to be caused by a combination of fluid immiscibility and water–rock interaction.     

Modeling of the impact of dolomite and biotite dissolution on vermiculite composition in a gneissic shallow aquifer of the Sila Massif (Calabria,Italy)

Reaction path modeling of water–rock interaction in a gneissic shallow aquifer of the Sila Massif was performed in kinetic (time) mode, under conditions of closed-system with secondary minerals and closed-system with CO₂, to investigate the influence of both dolomite dissolution and biotite dissolution on the chemical characteristics of secondary vermiculites. Magnesium–Al- and calcium–Al-vermiculites are the major components of the vermiculite solid solution precipitated in the early stages of the process, which is dominated by dolomite dissolution. In contrast, Mg–Mg–Fe- and Ca–Mg–Fe vermiculites are important components of the vermiculite solid solution produced in the late stages of the process, where biotite dissolution prevails. Outcomes of this reaction-path-modeling exercise on vermiculite chemistry are fully consistent with the results obtained by Apollaro et al. (in press) through speciation–saturation calculations. In particular, Apollaro et al. (in press) showed that the pH of Mg–Al-vermiculite/Mg–Mg–Fe-vermiculite coexistence is 7.3. This value is virtually equal to the pH of Mg–Al-vermiculite/Mg–Mg–Fe-vermiculite iso-activity, 7.35, which is obtained from the results of reaction-path-modeling runs 3 and 4 carried out in this work.     

Does subduction polarity control metallogeny? The Mediterranean case

The distribution of ore‐deposit types in different sectors of the circum‐Mediterranean realm that have been affected by subduction processes since the Cretaceous varies in space and time. Sectors involved in W‐directed subduction (Sardinia, Apennines–Maghrebides, Internal Betics, Tyrrhenian, Western–Eastern Carpathians) are dominated by relatively low‐sulphidation epithermal (±VMS) deposits. Orogens formed by NE‐directed subduction (Dinarides–Hellenides–Pontides–Anatolides–Taurides; DHPAT) were initially dominated by pluton‐related porphyry–skarn–high‐sulphidation epithermal associations. These distinct metallogenic styles can be related to the systematic tectono‐magmatic asymmetry of E–NE‐ and W‐directed subduction systems and are analogous to the relationship observed in circum‐Pacific belts. Exceptions to this simple pattern occurred in the DHPAT in the Cenozoic, when deposit associations typical of both E‐directed and W‐directed systems were formed. Such exceptions are interpreted to reflect superimposition of contrasting subduction trends and inheritance from earlier metallogenic stages (Apuseni) or the interference of subduction processes with subduction‐unrelated extension (Hellenides, West Anatolia).     

Study of late-Mesozoic magmatic rocks and their related copper-gold-polymetallic deposits in the Guichi ore-cluster district,Lower Yangtze River Metallogenic Belt,East China

ABSTRACT

The Guichi ore-cluster district in the Lower Yangtze River Metallogenic Belt hosts extensive Cu–Au–Mo polymetallic deposits including the Tongshan Cu–Mo, Paodaoling Au, Matou Cu–Mo, Anzishan Cu–Mo, Guilinzheng Mo and Zhaceqiao Au deposits, mostly associated with the late Mesozoic magmatic rocks, which has been drawn to attention of study and exploration. However, the metallogenic relationship between magmatic rocks and the Cu–Au-polymetallic deposits is not well constrained. In this study, we report new zircon U–Pb ages, Hf isotopic, and geochemical data for the ore-bearing intrusions of Guichi region. LA-ICP-MS U–Pb ages for the Anzishan quartz diorite porphyrite is 143.9 ± 1.0 Ma. Integrated with previous geochronological data, these late Mesozoic magmatic rocks can be subdivided into two stages of magmatic activities. The first stage (150–132 Ma) is characterized by high-K calc-alkaline intrusions closely associated with Cu–Au polymetallic ore deposits. Whereas, the second stage (130–125 Ma) produced granites and syenites and is mainly characterized by shoshonite series that are related to Mo–Cu mineralization. The first stage of magmatic rocks is considered to be formed by partial melting of subducted Palaeo-Pacific Plate, assimilated with Yangtze lower crust and remelting Meso-Neoproterozoic crust/sediments. The second stage of magmatism is originated from partial melting of Mesoproterozoic-Neoproterozoic crust, mixed with juvenile crustal materials. The depression cross to the uplift zone of the Jiangnan Ancient Continent forms a gradual transition relation, and the hydrothermal mineralization composite with two stages have certain characteristics along the regional fault (Gaotan Fault). Guichi region results from two episodes of magmatism probably related to tectonic transition from subduction of Palaeo-Pacific Plate to back-arc extensional setting between 150 and 125 Ma, which lead to the Mesozoic large-scale polymetallic mineralization events in southeast China.     

Early Stage in the Evolution of the Paleoasian Ccean at the Western Margin of the Siberian Craton: Geochemical and Geochronological Evidence

The discovery of glaucophane relicts in the high-pressure tectonites of the Yenisei suture zone of the Yenisei Ridge suggests the manifestation of the “Chilean-type” convergent margin on the western Siberian Craton, which was controlled by subduction of oceanic crust beneath the continental margin. These rocks are restricted to the tectonic suture between the craton and the Isakovka ocean-island terrane and experienced two metamorphic stages. Petrogeochemical characteristics of the mafic tectonites indicate that their protoliths are N-MORB and E-MORB basalts. More primitive N-MORB basalts were formed at the initial spreading stages through melting of the upper depleted mantle. Higher Ti basalts were formed by melting of enriched mantle protolith at the later spreading stages. U–Pb zircon age of 701.6 ± 8.4 Ma of the metamorphosed analogues of normal basalts marks the initiation of oceanic crust in the region. Revealed sequence of spreading, subduction (640–620 Ma), and shear deformations (~600 Ma) records the early stages in the evolution of the Paleoasian ocean in its junction zone with the western margin of the Siberian craton: from formation of fragments of oceanic crust to the completion of accretionary–subduction events.     

Palaeoclimatic record in the loess–palaeosol sequence of the Strelitsa type section (Don glaciation area,Russia) deduced from rock magnetic and palynological data

Until now, palaeoclimatic reconstructions for the major stages in the development of the Quaternary loess–palaeosol sequence on the Russian Plain have been based on pedological, palynological and faunal (vertebrates and molluscs) analyses. In order to demonstrate the palaeoclimatic influence on the magnetic properties of this sequence, the magnetic susceptibility signature of the Strelitsa type section in the Upper Don basin is compared with a detailed landscape – climate reconstruction of loess and soil from palynological data. Large amplitude fluctuations of palaeoclimate and palaeoenvironment are reflected clearly in the lithology and in the rock magnetic properties, which usually are enhanced in wet and warm interglacial periods, but stay at low levels during cold glacial epochs. Palynological climate zonation, however, is sometimes in conflict with the pedologic–magnetic record. Strong climate fluctuations, as indicated by changing pollen assemblages, are not always paralleled by corresponding changes in lithology and/or rock magnetic properties. Alternatively, light coloured illuvial horizons with low magnetic signal sometimes appear to have formed during early stages of interglacials, and the top parts of some palaeosols apparently formed during glacial stages. Copyright © 2000 John Wiley & Sons, Ltd.