Solution-collapse breccias of the Minkinfjellet and Wordiekammen Formations, Central Spitsbergen, Svalbard: a large gypsum palaeokarst system

Large volumes of carbonate breccia occur in the late syn-rift and early post-rift deposits of the Billefjorden Trough, Central Spitsbergen. Breccias are developed throughout the Moscovian Minkinfjellet Formation and in basal parts of the Kazimovian Wordiekammen Formation. Breccias can be divided into two categories: (i) thick, cross-cutting breccia-bodies up to 200 m thick that are associated with breccia pipes and large V-structures, and (ii) horizontal stratabound breccia beds interbedded with undeformed carbonate and siliciclastic rocks. The thick breccias occur in the central part of the basin, whereas the stratabound breccia beds have a much wider areal extent towards the basin margins. The breccias were formed by gravitational collapse into cavities formed by dissolution of gypsum and anhydrite beds in the Minkinfjellet Formation. Several dissolution fronts have been discovered, demonstrating the genetic relationship between dissolution of gypsum and brecciation. Textures and structures typical of collapse breccias such as inverse grading, a sharp flat base, breccia pipes (collapse dolines) and V-structures (cave roof collapse) are also observed. The breccias are cemented by calcite cements of pre-compaction, shallow burial origin. Primary fluid inclusions in the calcite are dominantly single phase containing fresh water (final melting points are ca 0 °C), suggesting that breccia diagenesis occurred in meteoric waters. Cathodoluminescence (CL) zoning of the cements shows a consistent pattern of three cement stages, but the abundance of each stage varies stratigraphically and laterally. δ¹⁸O values of breccia cements are more negative relative to marine limestones and meteoric cements developed in unbrecciated Minkinfjellet limestones. There is a clear relationship between δ¹⁸O values and the abundance of the different cement generations detected by CL. Paragenetically, later cements have lower δ¹⁸O values recording increased temperatures during their precipitation. Carbon isotope values of the cements are primarily rock-buffered although a weak trend towards more negative values with increasing burial depth is observed. The timing of gypsum dissolution and brecciation was most likely related to major intervals of exposure of the carbonate platform during Gzhelian and/or Asselian/Sakmarian times. These intervals of exposure occurred shortly after deposition of the brecciated units and before deep burial of the sediments.     

湖南常宁县康家湾铅锌金矿硅化角砾岩岩石地球化学特征

湖南常宁县康家湾铅锌金矿床硅化角砾岩带由未硅化角砾岩—极弱硅化角砾岩—硅化角砾岩—强硅化角砾岩(似硅质岩)组成,产于侏罗系与下伏二叠系间的不整合面附近,角砾成份复杂,充填物和胶结物类型多样。随着硅化程度的加强,硅化角砾岩带SiO₂含量变化大,最高可达95.34%,而Al₂O₃、MgO、FeO、K₂O、Na₂O、CaO、CO₂和P₂O₅含量特别是MgO、CaO、CO₂和P₂O₅含量明显降低,且K₂O>Na₂O,TiO₂的含量显著偏低。硅化角砾岩带LREE/HREE比值为1.95～4.93,Ce(δCe=0.44～0.81)和Eu(δEu=0.58～0.89)均为弱负异常,属轻稀土富集型,但随硅化程度增高稀土元素含量显著减少:未硅化和弱硅化角砾岩稀土元素总量较高,为(176.82～318.93)×10^-10,与当冲组泥质岩配分曲线相似;硅化强烈的角砾岩稀土元素总量低,为(7.71～65.95)×10^-10,与下伏栖霞组灰岩稀土元素配分曲线极为相似。结合微量元素F、Ba、Cl、Cr、Ni、Sr、V研究结果及硅化角砾岩带自底部至顶部特有的下粗上细的韵律性层理构造,认为康家湾铅锌金矿床硅化角砾岩带是在地台体制向地洼体制转变期的大地构造环境下,由于地壳快速隆升,二叠纪灰岩、泥质岩、石英砂岩等岩石剥蚀,在古河流环境下搬运、沉积形成的。此     

Geological Characteristics and Genesis of the Jiamoshan MVT Pb–Zn Deposit in the Sanjiang belt, Tibetan Plateau

The carbonate-hosted Pb–Zn deposits in the Sanjiang metallogenic belt on the Tibetan Plateau are typical of MVT Pb–Zn deposits that form in thrust-fold belts. The Jiamoshan Pb–Zn deposit is located in the Changdu area in the middle part of the Sanjiang belt, and it represents a new style of MVT deposit that was controlled by karst structures in a thrust–fold system. Such a karst-controlled MVT Pb–Zn deposit in thrust settings has not previously been described in detail, and we therefore mapped the geology of the deposit and undertook a detailed study of its genesis. The karst structures that host the Jiamoshan deposit were formed in Triassic limestones along secondary reverse faults, and the orebodies have irregular tubular shapes. The main sulfide minerals are galena, sphalerite, and pyrite that occur in massive and lamellar form. The ore-forming fluids belonged to a Mg2+–Na+–K+–SO2-4–Cl-–F-–NO-3–H2 O system at low temperatures(120–130°C) but with high salinities(19–22% NaCl eq.). We have recognized basinal brine as the source of the ore-forming fluids on the basis of their H–O isotopic compositions(-145‰ to-93‰ for δDV-SMOW and-2.22‰ to 13.00‰ for δ18 Ofluid), the ratios of Cl/Br(14–1196) and Na/Br(16–586) in the hydrothermal fluids, and the C–O isotopic compositions of calcite(-5.0‰ to 3.7‰ for δ13 CV-PDB and 15.1‰ to 22.3‰ for δ18 OV-SMOW). These fluids may have been derived from evaporated seawater trapped in marine strata at depth or from Paleogene–Neogene basins on the surface. The δ34 S values are low in the galena(-3.2‰ to 0.6‰) but high in the barite(27.1‰), indicating that the reduced sulfur came from gypsum in the regional Cenozoic basins and from sulfates in trapped paleo-seawater by bacterial sulfate reduction. The Pb isotopic compositions of the galena samples(18.3270–18.3482 for 206 Pb/204 Pb, 15.6345–15.6390 for 207 Pb/204 Pb, and 38.5503–38.5582 for 208 Pb/204 Pb) are similar to those of the regional Triassic volcanic-arc rocks that formed during the closure of the Paleo-Tethys, indicating these arc rocks were the source of the metals in the deposit. Taking into account our new observations and data, as well as regional Pb–Zn metallogenic processes, we present here a new model for MVT deposits controlled by karst structures in thrust–fold systems.     

Genesis of the Changba–Lijiagou Giant Pb-Zn Deposit, West Qinling, Central China: Constraints from S-Pb-C-O isotopes

The extensive Changba-Lijiagou Pb-Zn deposit is located in the north of the Xihe–Chengxian ore cluster in West Qinling. The ore bodies are mainly hosted in the marble, dolomitic marble and biotite-calcite-quartz schist of the Middle Devonian Anjiacha Formation, and are structurally controlled by the fault and anticline. The ore-forming process can be divided into three main stages, based on field geological features and mineral assemblages. The mineral assemblages of hydrothermal stage I are pale-yellow coarse grain, low Fe sphalerite, pyrite with pits, barite and biotite. The mineral assemblages of hydrothermal stage II are black-brown cryptocrystalline, high Fe shalerite, pyrite without pits, marcasite or arsenopyrite replace the pyrite with pits, K-feldspar. The features of hydrothermal stage III are calcite-quartz-sulfide vein cutting the laminated, banded ore body. Forty-two sulfur isotope analyses, twenty-five lead isotope analyses and nineteen carbon and oxygen isotope analyses were determined on sphalerite, pyrite, galena and calcite. The δ34 S values of stage I(20.3 to 29.0‰) are consistent with the δ34 S of sulfate(barite) in the stratum. Combined with geological feature, inclusion characteristics and EPMA data, we propose that TSR has played a key role in the formation of the sulfides in stage I. The δ34 S values of stage II sphalerite and pyrite(15.1 to 23.0‰) are between sulfides in the host rock, magmatic sulfur and the sulfate(barite) in the stratum. This result suggests that multiple S reservoirs were the sources for S2-in stage II. The δ34 S values of stage III(13.1 to 22‰) combined with the structure of the geological and mineral features suggest a magmatic hydrothermal origin of the mineralization. The lead isotope compositions of the sulfides have 206 Pb/204 Pb ranging from 17.9480 to 17.9782, 207 Pb/204 Pb ranging from 15.611 to 15.622, and 208 Pb/204 Pb ranging from 38.1368 to 38.1691 in the three ore-forming stages. The narrow and symmetric distributions of the lead isotope values reflect homogenization of granite and mantle sources before the Pb-Zn mineralization. The δ13 CPDB and δ18 OSMOW values of stage I range from-0.1 to 2.4‰ and from 18.8 to 21.7‰. The values and inclusion data indicate that the source of fluids in stage I was the dissolution of marine carbonate. The δ13 CPDB and δ18 OSMOW values of stage II range from-4 to 1‰ and from 12.3 to 20.3‰, suggesting multiple C-O reservoirs in the Changba deposit and the addition of mantle-source fluid to the system. The values in stage III are-3.1‰ and 19.7‰, respectively. We infer that the process of mineralization involved evaporitic salt and sedimentary organic-bearing units interacting through thermochemical sulfate reduction through the isotopic, mineralogy and inclusion evidences. Subsequently, the geology feature, mineral assemblages, EPMA data and isotopic values support the conclusion that the ore-forming hydrothermal fluids were mixed with magmatic hydrothermal fluids and forming the massive dark sphalerite, then yielding the calcite-quartz-sulfide vein ore type at the last stage. The genesis of this ore deposit was epigenetic rather than the previously-proposed sedimentary-exhalative(SEDEX) type.     

Discussion: Stratigraphy and structural summary of the Cooma metamorphic complex

Field and petrographic studies on granitic, hematitic and chloritic breccias in the central portion of the Mount Painter Inlier, South Australia, indicate that: (i) breccias and brecciated basement extend to depths exceeding 400 m and have gradational contacts; (ii) clasts are mainly autochthonous and contain fine‐scale hematite, chlorite or quartz veinlets and fractures; (iii) K‐metasomatism preceded hematitisation and chloritisation; (iv) hematitic breccia intrudes a pegmatite dyke correlated with the Ordovician Arkaroola Pegmatite; and (v) U, F and REE‐containing minerals are present in the Proterozoic basement rocks, and concentrated in the breccias.

With a single exception, δ³⁴S values for pyrite from the breccias and brecciated granites fall in the narrow range —2.9% to +3.5%, implying formation from magmatic emanations or reducing fluids that leached sulphide minerals of magmatic derivation. δ³⁴S values for three barite samples are all close to +16%o, and firm conclusions cannot be drawn from these data. Calcites from the same rock‐types as the pyrite have δ¹³C values of — 22.3%o to —4.2%o and δ¹⁸O values of —4.0%o to +23.1%., with an inverse δ¹³C/δ¹⁸O relationship. The more ¹³C‐depleted calcites probably incorporated CO₂ from organic C, and their δ¹⁸O values are compatible with precipitation from magmatic or metamorphic fluids; mixing of such fluids with meteoric waters is implied by the calcites with variably lower δ¹⁸O values.

The above features indicate that the major processes leading to brecciation and associated metasomatism were hydraulic fracturing and hydrothermal activity resulting from ascent of granitic magmas to shallow crustal levels during late stages (late Ordovician‐?Silurian) of the Delamerian Orogeny. Tectonic and sedimentary processes appear to have played relatively minor roles in breccia formation.     

Stable isotope evidence for magmatic fluid input during large-scale Na–Ca alteration in the Cloncurry Fe oxide Cu–Au district, NW Queensland, Australia

Sodic–calcic alteration is common in mineralized hydrothermal systems, yet the relative importance of igneous vs. basinal fluid sources remains controversial. One of the most extensive volumes of sodic–calcic rocks occurs near Cloncurry, NW Queensland, and was formed by overlapping hydrothermal systems that were active synchronously with emplacement of mid‐crustal batholithic granitoids (c. 1.55–1.50 Ga). Altered rocks contain albite–oligoclase, actinolite, diopside, titanite and magnetite. Alteration was localized by: (A) composite veins and breccias containing crystallized magma intimately intergrown with hydrothermal precipitates; (B) intrusions that host setting A veins and breccias; and (C) extensive breccia and vein systems linked to regional fault systems. Isotope analyses of actinolites in settings A and B indicate calculated δ¹⁸O_H2O (+8.2 to +10.6‰) and variably depleted δD_H2O (?130 to ?54‰) compared with typical magmatic fluids, whereas those from setting C typically indicate calculated δ¹⁸O_H2O (+8.0 to +12.8‰) and δD_H2O (?29 to ?99‰). The lowest δD_H2O values are interpreted as representing residual fluids after significant (> 90%) open‐system magmatic degassing. Overall the stable isotope, field, geochronological and geobarometric data suggest that these sodic–calcic alteration systems were formed by the episodic incursion of magmatic fluids that underwent minor isotopic modification as a result of varying degrees of interaction with country rocks.     

Genesis of the Gold Deposit in the Indus-Yarlung Tsangpo Suture Zone, Southern Tibet: Evidence from Geological and Geochemical Data

The Nianzha gold deposit,located in the central section of the Indus-Yarlung Tsangpo suture(IYS) zone in southern Tibet,is a large gold deposit(Au reserves of 25 tons with average grade of 3.08 g/t) controlled by a E-W striking fault that developed during the main stage of Indo-Asian collision(~65-41 Ma).The main orebody is 1760 m long and 5.15 m thick,and occurs in a fracture zone bordered by Cretaceous diorite in the hanging wall to the north and the Renbu tectonic melange in the footwall to the south.High-grade mineralization occurs in a fracture zone between diorite and ultramafic rock in the Renbu tectonic melange.The wall-rock alteration is characterized by silicification in the fracture zone,serpentinization and the formation of talc and magnesite in the ultramafic unit,and chloritization and the formation of epidote and calcite in diorite.Quartz veins associated with Au mineralization can be divided into three stages.Fluid inclusion data indicate that the deposit formed from H_2O-NaCl-organic gas fluids that homogenize at temperatures of 203℃-347℃ and have salinities of 0.35wt%-17.17wt%NaCl equivalent.The quartz veins yield δ~(18)O_(fluid) values of 0.15‰-10.45‰,low δD_(V-SMOW)values(-173‰ to-96‰),and the δ~(13)C values of-17.6‰ to-4.7‰,indicating the ore-forming fluids were a mix of metamorphic and sedimentary orogenic fluids with the addition of some meteoric and mantle-derived fluids.The pyrite within the diorite has δ~(34)S_(V-CDT) values of-2.9‰-1.9‰(average-1.1‰),~(206)Pb/~(204)Pb values of 18.47-18.64,~(207)Pb/~(204)Pb values of 15.64-15.74,and ~(208)Pb/~(204)Pb values of 38.71-39.27,all of which are indicative of the derivation of S and other ore-forming elements from deep in the mantle.The presence of the Nianzha,Bangbu,and Mayum gold deposits within the IYS zone indicates that this area is highly prospective for large orogenic gold deposits.We identified three types of mineralization within the IYS,namely Bangbu-type accretionary,Mayum-type microcontinent,and Nianzha-type ophiolite-associated orogenic Au deposits.The three types formed at different depths in an accretionary orogenic tectonic setting.The Bangbu type was formed at the deepest level and the Nianzha type at the shallowest.     

Genesis of the Giant Late Miocene to Pliocene Copper Deposits of Central Chile in the Context of Andean Magmatic and Tectonic Evolution

Multiple large mineralized breccia pipes (Cu grades up to >10%; individual pipes with >10 × 10⁶ metric tons of Cu) are prominent, if not dominant, features in the three giant Andean Cu deposits of Los Pelambres, Los Bronces-Rio Blanco, and El Teniente of central Chile. At Los Bronces-Rio Blanco, over 90% of the >50^x 10⁶ metric tons of hypogene Cu occurs within the matrix of breccias and/or clasts and wall rock altered in association with the formation of these breccias, while at the other two deposits a lesser but still significant amount of Cu ore also is directly related to breccias. At both Los Pelambres and Los Bronces-Rio Blanco, high-grade (>0.5%) Cu occurs in zones of potassic alteration characterized by stockwork biotite veining and intense biotitization associated spatially, temporally, and genetically with biotite breccias. At Los Bronces-Rio Blanco, high-grade ore also occurs in younger tourmaline breccia pipes, emplaced both within and around the older central biotite breccia complex and potassic alteration zone after a period of uplift and erosion. Potassic alteration, sericitization, silicification, and mineralization of clasts in these tourmaline breccias occurred during their formation. At El Teniente, a significant amount of high-grade Cu ore also occurs in different tourmaline-rich breccias, including the marginal portion of the Braden breccia pipe and a related zone of quartz-sericite alteration that surrounds this pipe. Small, shallow, weakly mineralized or barren silicic porphyry intrusions occur in each of these three deposits, but their main role has been to redistribute rather than emplace mineralization.

The mineralized breccia pipes in each deposit were emplaced into early and middle Miocene volcanic and plutonic rocks during the late Miocene and Pliocene by the expansion of boiling aqueous fluids. Fluid-inclusion and stable-isotope data indicate that the high-temperature, saline, metalrich fluids that produced the brecciation, precipitated the Cu ore in the matrix of the breccias, and generated the associated alteration and mineralization in clasts and wall rock were magmatic in origin. These magmatic fluids were not derived from the early and middle Miocene host plutons, which already were solidified at the time of breccia emplacement. Sr- and Nd-isotopic compositions of breccia-matrix minerals indicate that breccia-forming fluids were exsolved from magmas that were isotopically transitional between older volcanic and plutonic host rocks and younger silicic porphyry stocks, dikes, and extrusives. The fact that the roots of the breccias have not yet been encountered implies that these magmas cooled at depths >3 km to form plutons not yet exposed at the surface.

The generation of the multiple mineralized breccias at each deposit occurred over a relatively short (but still significant) time period of 1 to 3 million years, during the final stages of existence of the long-lived (7gt;15 m.y.) Miocene magmatic belt in central Chile. The decline of magmatic activity in this belt was tectonically triggered, as subduction angle decreased in association with the subduction of the Juan Fernandez Ridge. This caused a decrease in the sub-arc magma supply and subsequently eastward migration of the magmatic arc, as well as crustal thickening, uplift, and erosion, which led to the superposition of younger and shallower alteration and mineralization events on older and deeper events in each deposit.

The giant Cu deposits of central Chile cannot be explained by a static model in which their size is a function of the mass of a single pluton or the longevity of a single hydrothermal convection system. These deposits are giant because they were produced by multistage processes involving the formation, over a period of 1 to 3 million years, of multiple superimposed mineralized breccias and associated alteration zones resulting from the exsolution of metalrich magmatic fluids from independent magma batches cooling at depths >3 km. Neither an unusually large magma supply nor Andean magmas of unusually high Cu content is required to produce the sequence of multiple mineralization     

Identifying the Sources of Solutes in Karst Groundwater in Chongqing,China: a Combined Sulfate and Strontium Isotope Approach

Groundwater from karst subterranean streams is among the world’s most important sources of drinking water supplies, and the hydrochemical characteristics of karst water are impacted by both natural environment and people. Therefore, the study of hydrochemistry and its solutes’ sources is very important to ensure the normal function of life support systems. In this paper, thirty?five representative karst groundwater samples were collected from different aquifers (limestone and dolomite) and various land use types in Chongqing to trace the sources of solutes and relative hydrochemical processes. Hydrogeochemical types of karst groundwater in Chongqing were mainly of the Ca?HCO3 type or Ca (Mg)?HCO3 type. However, some hydrochemical types of karst groundwater were the K+Na+Ca?SO4 type (G25 site) or Ca?HCO3+SO4 type (G26 and G14 site), indicating that the hydrochemistry of these sites might be strongly influenced by anthropogenic activities or unique geological characteristics. The dissolved Sr concentrations of the studied groundwater ranged from 0.57 to 15.06 μmmol/L, and the 87Sr/86Sr varied from 0.70751 to 0.71627. The δ34S?SO42? fell into a range of ?6.8‰?21.5‰, with a mean value of 5.6‰. The variations of both 87Sr/86Sr and Sr values of the groundwater samples indicated that the Sr element was controlled by the weathering of limestone, dolomite and silicate rock. However, the figure of 87Sr/86Sr vs. Sr2+/[K++Na+] showed that the anthropogenic inputs also obviously contributed to the Sr contents. For tracing the detailed anthropogenic effects, we traced the sources of solutes collected karst groundwater samples in Chongqing according to the δ34S value of potential sulfate sources. The variations of both δ34S and 1/SO42? values of the groundwater samples indicated that the atmospheric acid deposition (AAD), dissolution of gypsum (GD), oxidation of sul?de mineral (OS) or anthropogenic inputs (SF: sewage or fertilizer) have contributed to solutes in karst groundwater. The influence of oxidation of sul?de mineral, atmospheric acid deposit and anthropogenic inputs to groundwater in Chongqing karst areas was much widespread.     

Stable Carbon Isotope Variations in Cave Percolation Waters and their Implications in Four Caves of Guizhou, China

Monitoring and sampling of main plants,soil CO2,soil water,bedrock,spring water,drip water and its corresponding speleothem were performed at four cave systems of Guizhou,Southwest China,from April 2003 to May 2004,in order to understand stable carbon isotope ratios variations of dissolved inorganic Carbon(DIC) in cave percolation waters(δ13CDIC) and their implications for paleoclimate.Stable carbon isotopic compositions and ions(Ca,Mg,Sr,SO4,Cl etc.) were measured for all samples.The results indicate that there are significant differences among the δ13CDIC values from inter-cave,even inter-drip of intra-cave in the four caves.The δ13CDIC values from the Liangfeng Cave(LFC) is lightest among the four caves,where vegetation type overlying the cave is primary forest dominated by tall trees with lighter average δ13C value(–29.9‰).And there are remarkable differences in δ13CDIC values of different drip waters in the Qixing Cave(QXC) and Jiangjun Cave(JJC),up to 6.9‰ and 7.8‰,respectively.Further analyses show that the δ13CDIC values in cave drip waters are not only controlled by vegetation biomass overlying the cave,but also hydro-geochemical processes.Therefore,accurate interpreting of δ13C recorded in speleothems cannot be guaranteed if these effects of the above mentioned factors are not taken into consideration.     

Fluidization： An Important Process in the Formation of the Qiyugou Au-bearing Breccia Pipes in Central China

Fluidization processes based on experiments are reviewed to gain some useful insights and comparisons with those that occur in hydrothermal systems. Field and petrographic work, and microscope observation were carried out on samples from the Qiyugou Au-bearing breccia pipes from the East Qinling region, Henan Province. Evidence from macro- and micro-textures suggests that the style of breccias in the Qiyugou area can be grouped into three types： （1） jigsaw fit-stockwork texture, in which the interval between clasts is marked by fractures or filled with calcite or quartz veins; （2） larger breccias that are supported by smaller breccias, rock flour and alteration materials; in this type clasts moved over short distances, creating open spaces; （3） fluidized texture, where the clasts of different lithologies have rounded shapes. These observations are compared with those resulting from experiments on fluidization processes. The results of this comparison suggest that fluidization is an important geological process in the formation of the Qiyugou Au-bearing breccia pipes and gold mineralization. In addition, fluidization processes such as expansion, bubbling, slugging, channeling and spouting must have contributed to the formation of the pipes and were conducive to the development of gold mineralization. In the Qiyugou breccia pipes, gold mineralization occurs as disseminations, in stockwork veins, and open space infills. The ore zones form subparallel sheets that are nearly perpendicular to the walls of the pipes.     

Dolomitization by hypersaline reflux into dense groundwaters as revealed by vertical trends in strontium and oxygen isotopes: Upper Muschelkalk,Switzerland

The Trigonodus Dolomit is the dolomitized portion of the homoclinal ramp sediments of the Middle Triassic Upper Muschelkalk in the south‐east Central European Basin. Various dolomitizing mechanisms, followed by recrystallization, have been previously invoked to explain the low δ¹⁸O, high ⁸⁷Sr/⁸⁶Sr, extensive spatial distribution and early nature of the replacive matrix dolomites. This study re‐evaluates the origin, timing and characteristics of the dolomitizing fluids by examining petrographic and isotopic trends in the Trigonodus Dolomit at 11 boreholes in northern Switzerland. In each borehole the ca 30 m thick unit displays the same vertical trends with increasing depth: crystal size increase, change from anhedral to euhedral textures, ultraviolet‐fluorescence decrease, δ¹⁸O_VPDB decrease from ?1·0‰ at the top to ?6·7‰ at the base and an ⁸⁷Sr/⁸⁶Sr increase from 0·7080 at the top to 0·7117 at the base. Thus, dolomites at the top of the unit record isotopic values similar to Middle Triassic seawater (δ¹⁸O_VSMOW = 0‰; ⁸⁷Sr/⁸⁶Sr = 0·70775) while dolomites at the base record values similar to meteoric groundwaters from the nearby Vindelician High (δ¹⁸O_VSMOW = ?4·0‰; ⁸⁷Sr/⁸⁶Sr = >0·712). According to water–rock interaction modelling, a single dolomitizing or recrystallizing fluid cannot have produced the observed isotopic trends. Instead, the combined isotopic, geochemical and petrographic data can be explained by dolomitization via seepage‐reflux of hypersaline brines into dense, horizontally‐advecting groundwaters that already had negative δ¹⁸O and high ⁸⁷Sr/⁸⁶Sr values. Evidence for the early groundwaters is found in meteoric calcite cements that preceded dolomitization and in fully recrystallized dolomites with isotopic characteristics identical to the groundwaters following matrix dolomitization. This study demonstrates that early groundwaters can play a decisive role in the formation and recrystallization of massive dolomites and that the isotopic and textural signatures of pre‐existing groundwaters can be preserved during seepage‐reflux dolomitization in low‐angle carbonate ramps.     

Chemical and Isotopic Characters of the Water and Suspended Particulate Materials in the Yellow River and Their Geological and Environmental Implications

The chemical and isotopic characteristics of the water and suspended particulate materials(SPM)in the Yellow River were investigated on the samples collected from 29 hydrological monitoring stations in the mainstem and several major tributaries during 2004 to 2007.TheδD andδ~(18)O values of the Yellow River water vary in large ranges from-32‰to-91‰and from-3.1‰to-12.5‰,respectively.The characters of H and O isotope variations indicate that the major sources of the Yellow River water are meteoric water and snow melting water,and water cycle in the Yellow River basin is affected strongly by evaporation process and human activity.The average SPM content(9.635g/L)of the Yellow River is the highest among the world large rivers.Compared with the Yangtze River,the Yellow River SPM has much lower clay content and significantly higher contents of clastic silicates and carbonates.In comparison to the upper crust rocks,the Yellow River SPM contains less SiO_2,CaO,K_2O and Na_2O,but more TFe_2O_3,Co,Ni,Cu,Zn,Pb and Cd.The abnormal high Cd contents found in some sample may be related to local industrial activity.The REE contents and distribution pattern of the Yellow River SPM are very close to the average value of the global shale.The averageδ~(30)Si_(SPM)in the Yellow River(-0.11‰)is slightly higher than the average value(-0.22‰)of the Yangtze River SPM.The major factors controlling theδ~(30)Si_(SPM)of the Yellow River are the soil supply,the isotopic composition of the soil and the climate conditions.The TDS in the Yellow River are the highest among those of world large rivers.Fair correlations are observed among Cl~-,Na~+,K~+,and Mg~(2+)contents of the Yellow River water,indicating the effect of evaporation.The Ca~(2+)and Sr~(2+)concentrations show good correlation to the SO_4~(2-)concentration rather than HCO_3~-concentration,reflecting its origin from evaporates.The NO_3~-contents are affected by farmland fertilization.The Cu,Zn and Cd contents in dissolved load of the Yellow River water are all higher than those of average world large rivers,reflecting the effect of human activity.The dissolved load in the Yellow River water generally shows a REE distribution pattern parallel to those for the Yangtze River and the Xijiang River.Theδ~(30)Si values of the dissolved silicon vary in a range from 0.4‰to 2.9‰,averaging1.34‰.The major processes controlling the D_(Si)andδ~(30)Si_(Diss)of the Yellow River water are the weathering process of silicate rocks,growth of phytolith in plants,evaporation,dissolution of phytolith in soil,growth of fresh water diatom,adsorption and desorption of aqueous monosilicic acid on iron oxide and human activities.The averageδ~(30)Si_(Diss)value of the Yellow River is significantly lower than that of the Nile River,Yangtze River and Siberia rivers,but higher than those of other rivers,reflecting their differences in chemical weathering and biological activity.Theδ~(34)S_(SO4)values of the Yellow River water range from-3.8‰to 14.1‰,averaging 7.97‰.There is some correlation between SO_4~(2-)content andδ~(34)S_(SO4).The factors controlling theδ~(34)S_(SO4)of the Yellow River water are the SO_4 in the meteoric water,the SO_4 from gypsum or anhydrite in evaporite rocks,oxidation and dissolution of sulfides in the mineral deposits,magmatic rocks and sedimentary rocks,the sulfate reduction and precipitation process and the sulfate from fertilizer.The~(87)Sr/~(86)Sr ratios of all samplesrange from 0.71041 to 0.71237,averaging 0.71128.The variations in the~(87)Sr/~(86)Sr ratio and Sr concentration of river water are primarily caused by mixing of waters of various origins with different~(87)Sr/~(86)Sr ratios and Sr contents resulting from water-rock interaction with different rock types.     

Distinct zircon U–Pb and O–Hf–Nd–Sr isotopic behaviour during fluid flow in UHP metamorphic rocks: evidence from metamorphic veins and their host eclogite in the Sulu Orogen,China

Fluid plays a key role in metamorphism and magmatism in subduction zones. Veins in high‐pressure (HP) to ultrahigh‐pressure (UHP) rocks are the products of fluid‐rock interaction, and can thus provide important constraints on fluid processes in subduction zones. This contribution is an integrated study of zircon U–Pb and O–Hf, as well as whole‐rock Nd–Sr isotopic compositions for a quartz vein, a complex vein, and their host eclogite in the Sulu UHP terrane to decipher the timing and source of fluid flow under HP‐UHP metamorphic conditions. The inherited magmatic zircon cores from the host eclogite constrain the protolith age at c. 750 Ma. Their variable ε_Hf(t) values from ?1.11 to 2.54 and low δ¹⁸O values of 0.32–3.40‰ reflect a protolith that formed in a rift setting due to the breakup of the supercontinent Rodinia. The hydrothermal zircon from the quartz and the complex veins shows euhedral shapes, relatively flat HREE pattern, slight or no negative Eu anomaly, low ¹⁷⁶Lu/¹⁷⁷Hf ratios, and low formation temperatures of 660–690 °C, indicating they precipitated from fluids under HP eclogite facies conditions. This zircon yielded similar U–Pb ages of 217 ± 2 and 213 ± 3 Ma within analytical uncertainty, recording the timing of fluid flow during the exhumation of the UHP rock. It is inferred that the fluids might be of internal origin based on the homogeneity of δ¹⁸O values of the hydrothermal zircon from the quartz (?2.41 ± 0.13‰) and complex veins (?2.35 ± 0.12‰), and the metamorphic grown zircon of the host eclogite (?2.23 ± 0.16‰). The similar ε_Nd(t) values of the whole rocks also support such a point. Zircon O and whole‐rock Nd isotopic compositions are therefore useful to identify the source of fluid, for they are major and trace components in minerals involved in metamorphic reactions during HP‐UHP conditions. On the other hand, the hydrothermal zircon from the veins and the metamorphic zircon from the host eclogite exhibit variable ε_Hf(t) values. Model calculation suggests that the Hf was derived from the breakdown of major rock‐forming minerals and recycling of the inherited magmatic zircon. The variable whole‐rock initial ⁸⁷Sr/⁸⁶Sr ratios might be caused by subsequent retrograde metamorphism after the formation of the veins.     

A Fluid Inclusion Study of Vein-type Copper Mineralization in the East Wulasigou Pb–Zn–Cu Deposit, Altay, Northwestern China

The Wulasigou Cu-Pb-Zn deposit,located 15 km northwest of Altay city in Xinjiang,is one of many Cu-Pb-Zn polymetallic deposits in the Devonian Kelan volcanic-sedimentary basin in southern Altaids.Two mineralizing periods can be distinguished:the marine volcanic sedimentary PbZn mineralization period,and the metamorphic hydrothermal Cu mineralization period,which is further divided into an early bedded foliated quartz vein stage(Q1) and a late sulfide-quartz vein stage(Q2) crosscutting the foliation.Four types of fluid inclusions were recognized in the Q1 and Q2 quartz from the east orebodies of the Wulasigou deposit:H_2O-CO_2 inclusions,carbonic fluid inclusions,aqueous fluid inclusions,and daughter mineral-bearing fluid inclusions.Microthermometric studies show that solid CO_2 melting temperatures(T_(m,CO2)) of H_2O-CO_2 inclusions in Ql are from-62.3℃ to-58.5C,clathrate melting temperatures(T_(m,clath)l) are from 0.5 C to 7.5 C,partial homogenization temperatures(T_(h,CO2)) vary from 3.3℃ to 25.9℃(to liquid),and the total homogenization temperatures(T_(h,tot)) vary from 285℃ to 378℃,with the salinities being 4.9%-15.1%NaCl eqv.and the CO_2-phase densities being 0.50-0.86 g/cm~3.H_2O-CO_2 inclusions in Q2 have T_(m,CO_2) from-61.9℃ to-56.9℃,T_(m,clath)from 1.3℃ to 9.5℃,T_(h,CO2) from 3.4℃ to 28.7℃(to liquid),and T_(h,tot) from 242℃ to 388℃,with the salinities being 1.0%-15.5%NaCl eqv.and the CO_2-phase densities being 0.48-0.89 g/cm~3.The minimum trapping pressures of fluid inclusions in Q1 and Q2 are estimated to be 260-360 MPa and180-370 MPa,respectively.The δ~(34)S values of pyrite from the volcanic sedimentary period vary from2.3‰ to 2.8‰(CDT),and those from the sulfide-quartz veins fall in a narrow range of-1.9‰ to 2.6‰(CDT).The δD values of fluid inclusions in Q2 range from-121.0‰ to-100.8‰(SMOW),and theδ~(18)O_(H2O) values calculated from δ~(18)O of quartz range from-0.2‰ to 8.3‰(SMOW).The δD-δ~(18)O_(H2O)data are close to the magmatic and metamorphic fields.The fluid inclusion and stable isotope data documented in this study indicate that the vein-type copper mineralization in the Wulasigou Pb-Zn-Cu deposit took place in an orogenic-metamorphic enviroment.     

Imbrium provenance for the Apollo 16 Descartes terrain: Argon ages and geochemistry of lunar breccias 67016 and 67455

In order to improve our understanding of impact history and surface geology on the Moon, we obtained ⁴⁰Ar-³⁹Ar incremental heating age data and major + trace element compositions of anorthositic and melt breccia clasts from Apollo 16 feldspathic fragmental breccias 67016 and 67455. These breccias represent the Descartes terrain, a regional unit often proposed to be ejecta from the nearby Nectaris basin. The goal of this work is to better constrain the emplacement age and provenance of the Descartes breccias.Four anorthositic clasts from 67016 yielded well-defined ⁴⁰Ar-³⁹Ar plateau ages ranging from 3842 ± 19 to 3875 ± 20 Ma. Replicate analyses of these clasts all agree within measurement error, with only slight evidence for either inheritance or younger disturbance. In contrast, fragment-laden melt breccia clasts from 67016 yielded apparent plateau ages of 4.0-4.2 Ga with indications of even older material (to 4.5 Ga) in the high-T fractions. Argon release spectra of the 67455 clasts are more variable with evidence for reheating at 2.0-2.5 Ga. We obtained plateau ages of 3801 ± 29 to 4012 ± 21 Ma for three anorthositic clasts, and 3987 ± 21 Ma for one melt breccia clast. The anorthositic clasts from these breccias and fragments extracted from North Ray crater regolith (Maurer et al., 1978) define a combined age of 3866 ± 9 Ma, which we interpret as the assembly age of the feldspathic fragmental breccia unit sampled at North Ray crater. Systematic variations in diagnostic trace element ratios (Sr/Ba, Ti/Sm, Sc/Sm) with incompatible element abundances show that ferroan anorthositic rocks and KREEP-bearing lithologies contributed to the clast population.The Descartes breccias likely were deposited as a coherent lithologic unit in a single event. Their regional distribution suggests emplacement as basin ejecta. An assembly age of 3866 ± 9 Ma would be identical with the accepted age of the Imbrium basin, and trace element compositions are consistent with a provenance in the Procellarum-KREEP Terrane. The combination of age and provenance constraints points toward deposition of the Descartes breccias as ejecta from the Imbrium basin rather than Nectaris. Diffusion modeling shows that the older apparent plateau ages of the melt brecia clasts plausibly result from incomplete degassing of ancient crust during emplacement of the Descartes breccias. Heating steps in the melt breccia clasts that approach the primary crystallization ages of lunar anorthosites show that earlier impact events did not completely outgas the upper crust.     

Geochronology and mineralogy of the Weishan carbonatite in Shandong province,eastern China

The Weishan REE deposit is located at the eastern part of North China Craton (NCC), western Shandong Province. The REE-bearing carbonatite occur as veins associated with aegirine syenite. LA-ICP-MS bastnaesite Th-Pb ages (129 Ma) of the Weishan carbonatite show that the carbonatite formed contemporary with the aegirine syenite. Based on the petrographic and geochemical characteristics of calcite, the REE-bearing carbonatite mainly consists of Generation-1 igneous calcite (G-1 calcite) with a small amount of Generation-2 hydrothermal calcite (G-2 calcite). Furthermore, the Weishan apatite is characterized by high Sr, LREE and low Y contents, and the carbonatite is rich in Sr, Ba and LREE contents. The δ¹³C_V-PDB (−6.5‰ to −7.9‰) and δ¹³O_V-SMOW (8.48‰–9.67‰) values are similar to those of primary, mantle-derived carbonatites. The above research supports that the carbonatite of the Weishan REE deposit is igneous carbonatite. Besides, the high Sr/Y, Th/U, Sr and Ba of the apatite indicate that the magma source of the Weishan REE deposit was enriched lithospheric mantle, which have suffered the fluid metasomatism. Taken together with the Mesozoic tectono-magmatic activities, the NW and NWW subduction of Izanagi plate along with lithosphere delamination and thinning of the North China plate support the formation of the Weishan REE deposit. Accordingly, the mineralization model of the Weishan REE deposit was concluded: The spatial-temporal relationships coupled with rare and trace element characteristics for both carbonatite and syenite suggest that the carbonatite melt was separated from the CO₂-rich silicate melt by liquid immiscibility. The G-1 calcites were crystallized from the carbonatite melt, which made the residual melt rich in rare earth elements. Due to the common origin of G-1 and G-2 calcites, the REE-rich magmatic hydrothermal was subsequently separated from the melt. After that, large numbers of rare earth minerals were produced from the magmatic hydrothermal stage.     

Anatomy of a large Ag–Pb–Zn deposit in the Great Xing'an Range,northeast China: metallogeny associated with Early Cretaceous magmatism

The Bairendaba vein-type Ag–Pb–Zn deposit, hosted in a Carboniferous quartz diorite, is one of the largest polymetallic deposits in the southern Great Xing'an Range. Reserves exceeding 8000 tonnes of Ag and 3 million tonnes of Pb?+?Zn with grades of 30 g/t and 4.5% have been estimated. We identify three distinct mineralization stages in this deposit: a barren pre-ore stage (stage 1), a main-ore stage with economic Ag–Pb–Zn mineralization (stage 2), and a post-ore stage with barren mineralization (stage 3). Stage 1 is characterized by abundant arsenopyrite?+?quartz and minor pyrite. Stage 2 is represented by abundant Fe–Zn–Pb–Ag sulphides and is further subdivided into three substages comprising the calcite–polymetallic sulphide stage (substage 1), the fluorite–polymetallic sulphide stage (substage 2), and the quartz–polymetallic sulphide stage (substage 3). Stage 3 involves an assemblage dominated by calcite with variable pyrite, galena, quartz, fluorite, illite, and chlorite. Fluid inclusion analysis and mineral thermometry indicate that the three stages of mineralization were formed at temperatures of 320–350°C, 200–340°C, and 180–240°C, respectively. Stage 1 early mineralization is characterized by low-salinity fluids (5.86–8.81 wt.% NaCl equiv.) with an isotopic signature of magmatic origin (δ¹⁸O_fluid = 10.45–10.65‰). The main ore minerals of stage 2 precipitated from aqueous–carbonic fluids (4.34–8.81 wt.% NaCl equiv.). The calculated and measured oxygen and hydrogen isotopic compositions of the ore-forming aqueous fluids (δ¹⁸O_fluid = 3.31–8.59‰, δD_fluid?=??132.00‰ to??104.00‰) indicate that they were derived from a magmatic source and mixed with meteoric water. Measured and calculated sulphur isotope compositions of hydrothermal fluids (δ³⁴S_∑S?=??1.2–3.8‰) indicate that the ore sulphur was derived mainly from a magmatic source. The calculated carbon isotope compositions of hydrothermal fluids (δ¹³C_fluid?=??26.52‰ to??25.82‰) suggest a possible contribution of carbon sourced from the basement gneisses. The stage 3 late mineralization is dominated (1.40–8.81 wt.% NaCl equiv.) by aqueous fluids. The fluids show lower δ¹⁸O_fluid (?16.06‰ to??0.70‰) and higher δD_fluid (?90.10‰ to??74.50‰) values, indicating a heated meteoric water signature. The calculated carbon isotope compositions (δ¹³C_fluid?=??12.82‰ to??6.62‰) of the hydrothermal fluids in stage 3 also suggest a possible contribution of gneiss-sourced carbon. The isotopic compositions and fluid chemistry indicate that the ore mineralization in the Bairendaba deposit was related to Early Cretaceous magmatism.     

Lithologic controls on mineralization at the Lagunas Norte high-sulfidation epithermal gold deposit, northern Peru

The 13.1-Moz high-sulfidation epithermal gold deposit of Lagunas Norte, Alto Chicama District, northern Peru, is hosted in weakly metamorphosed quartzites of the Upper Jurassic to Lower Cretaceous Chimú Formation and in overlying Miocene volcanic rocks of dacitic to rhyolitic composition. The Dafne and Josefa diatremes crosscut the quartzites and are interpreted to be sources of the pyroclastic volcanic rocks. Hydrothermal activity was centered on the diatremes and four hydrothermal stages have been defined, three of which introduced Au ± Ag mineralization. The first hydrothermal stage is restricted to the quartzites of the Chimú Formation and is characterized by silice parda, a tan-colored aggregate of quartz-auriferous pyrite–rutile ± digenite infilling fractures and faults, partially replacing silty beds and forming cement of small hydraulic breccia bodies. The δ³⁴S values for pyrite (1.7–2.2?‰) and digenite (2.1?‰) indicate a magmatic source for the sulfur. The second hydrothermal stage resulted in the emplacement of diatremes and the related volcanic rocks. The Dafne diatreme features a relatively impermeable core dominated by milled slate from the Chicama Formation, whereas the Josefa diatreme only contains Chimú Formation quartzite clasts. The third hydrothermal stage introduced the bulk of the mineralization and affected the volcanic rocks, the diatremes, and the Chimú Formation. In the volcanic rocks, classic high-sulfidation epithermal alteration zonation exhibiting vuggy quartz surrounded by a quartz–alunite and a quartz–alunite–kaolinite zone is observed. Company data suggest that gold is present in solid solution or micro inclusions in pyrite. In the quartzite, the alteration is subtle and is manifested by the presence of pyrophyllite or kaolinite in the silty beds, the former resulting from relatively high silica activities in the fluid. In the quartzite, gold mineralization is hosted in a fracture network filled with coarse alunite, auriferous pyrite, and enargite. Alteration and mineralization in the breccias were controlled by permeability, which depends on the type and composition of the matrix, cement, and clast abundance. Coarse alunite from the main mineralization stage in textural equilibrium with pyrite and enargite has δ³⁴S values of 24.8–29.4?‰ and $ {\delta^{18 }}{{\mathrm{O}}_{{\mathrm{S}{{\mathrm{O}}_4}}}} $ values of 6.8–13.9?‰, consistent with H₂S as the dominant sulfur species in the mostly magmatic fluid and constraining the fluid composition to low pH (0–2) and logfO₂ of ?28 to ?30. Alunite–pyrite sulfur isotope thermometry records temperatures of 190–260 °C; the highest temperatures corresponding to samples from near the diatremes. Alunite of the third hydrothermal stage has been dated by ⁴⁰Ar/³⁹Ar at 17.0?±?0.22 Ma. The fourth hydrothermal stage introduced only modest amounts of gold and is characterized by the presence of massive alunite–pyrite in fractures, whereas barite, drusy quartz, and native sulfur were deposited in the volcanic rocks. The $ {\delta^{18 }}{{\mathrm{O}}_{{\mathrm{S}{{\mathrm{O}}_4}}}} $ values of stage IV alunite vary between 11.5 and 11.7?‰ and indicate that the fluid was magmatic, an interpretation also supported by the isotopic composition of barite (δ³⁴S?=?27.1 to 33.8?‰ and $ {\delta^{18 }}{{\mathrm{O}}_{{\mathrm{S}{{\mathrm{O}}_4}}}} $ ?=?8.1 to 12.7?‰). The Δ³⁴S_py–alu isotope thermometry records temperatures of 210 to 280 °C with the highest values concentrated around the Josefa diatreme. The Lagunas Norte deposit was oxidized to a depth of about 80 m below the current surface making exploitation by heap leach methods viable.     

Aragonite stromatolitic buildups from Santorini (Aegean Sea,Greece): Geochemical and palaeontological constraints of the caldera palaeoenvironment prior to the Minoan eruption (ca 3600 yr bp)

The pyroclastic deposits of the Minoan eruption (ca 3600 yr bp ) in Santorini contain abundant xenoliths. Most of these deposits are calcareous blocks of laminated‐botryoidal, stromatolite‐like buildups that formed in the shallow waters of the flooded pre‐Minoan caldera; they consist of (i) light laminae, of fibrous aragonite arranged perpendicular to layering, and (ii) dark laminae, with calcified filaments of probable biological origin. These microstructures are absent in the light laminae, suggesting a predominant inorganic precipitation of aragonite on substrates probably colonized by microbes. Internal cavities contain loose skeletal grains (molluscs, ostracods, foraminifera and diatoms) that comprise taxa typical of shallow marine and/or lagoon environments. Most of these forms are typical of warm water environments, although no typical taxa from hydrothermal vents have been observed. Past gasohydrothermal venting is recorded by the occurrence of barite, pyrolusite and pyrite traces. The most striking features of the stable isotopic data set are: (i) an overall wide range in δ¹³C_PDB (0·16 to 12·97‰) with a narrower variation for δ¹⁸O_PDB (?0·23 to 4·33‰); and (ii) a relatively uniform isotopic composition for the fibrous aragonite (δ¹³C = 12·40 ± 0·43‰ and δ¹⁸O = 2·42 ± 0·77‰, n = 21). The δ¹³C and δ¹⁸O values from molluscs and ostracods display a covariant trend, which reflects a mixing between sea water and a fluid influenced by volcano‐hydrothermal activity. Accordingly, ⁸⁷Sr/⁸⁶Sr from the studied carbonates (0·708758 to 0·709011 in fibrous aragonite and 0·708920 to 0·708991 in molluscs) suggests that the aragonite buildups developed in sea water under the influence of a hydrothermal/volcanic source. Significant differences in trace elements have been detected between the fibrous aragonite and modern marine aragonite cements. The caldera water from which the fibrous aragonite crusts formed received an input from a volcano‐hydrothermal system, probably producing diffuse venting of volcanogenic CO₂ gas and of a fluid enriched in Ca, Mn and Ba, and depleted in Mg and probably in Sr.