Hydrothermal Overprint Research Articles

Orogenic-type copper-gold-arsenic-(bismuth) mineralization at Flatschach (Eastern Alps), Austria

Structurally controlled Cu-Au mineralization in the historic Flatschach mining district (Styria, Austria) occurs in a NE–SW to NNE–WSW oriented vein system as multiple steep-dipping calcite-(dolomite)-quartz veins in amphibolite facies metamorphic rocks (banded gneisses/amphibolites, orthogneisses, metagranitoids) of the poly-metamorphosed Austroalpine Silvretta-Seckau nappe. Vein formation postdated ductile deformation events and Eoalpine (Late Cretaceous) peak metamorphism but predated Early to Middle Miocene sediment deposition in the Fohnsdorf pull-apart basin; coal-bearing sediments cover the metamorphic basement plus the mineralized veins at the northern edge of the basin. Three gold-bearing ore stages consist of a stage 1 primary hydrothermal (mesothermal?) ore assemblage dominated by chalcopyrite, pyrite and arsenopyrite. Associated minor minerals include alloclasite, enargite, bornite, sphalerite, galena, bismuth and matildite. Gold in this stage is spatially associated with chalcopyrite occurring as inclusions, along re-healed micro-fractures or along grain boundaries of chalcopyrite with pyrite or arsenopyrite. Sericite-carbonate alteration is developed around the veins. Stage 2 ore minerals formed by the replacement of stage 1 sulfides and include digenite, anilite, “blue-remaining covellite” (spionkopite, yarrowite), bismuth, and the rare copper arsenides domeykite and koutekite. Gold in stage 2 is angular to rounded in shape and occurs primarily in the carbonate (calcite, Fe-dolomite) gangue and less commonly together with digenite, domeykite/koutekite and bismuth. Stage 3 is a strongly oxidized assemblage that includes hematite, cuprite, and various secondary Cu- and Fe-hydroxides and -carbonates. It formed during supergene weathering. Stage 1 and 2 gold consists mostly of electrum (gold fineness 640–860; mean = 725; n = 46), and rare near pure gold (fineness 930–940; n = 6). Gold in stage 3 is Ag-rich electrum (fineness 350–490, n = 12), and has a high Hg content (up to 11 mass %). The Cu-Au deposits in the Flatschach area show similarities with meso- to epizonal orogenic lode gold deposits regarding the geological setting, the structural control of mineralization, the type of alteration, the early (stage 1) sulfide assemblage and composition of gold. Unique about the Flatschach district is the lower-temperature overprint of copper arsenides (domeykite and koutekite) and copper sulfides (djurleite, yarrowite/spionkopite) on earlier formed sulfide mineralization. Based on mineralogical considerations temperature of stage 2 mineralization was between about 70 °C and 160 °C. Gold was locally mobilized during this low-temperature hydrothermal overprint as well as during stage 3 supergene oxidation and cementation processes.

Geochemical stratigraphy, sedimentology, and Mo isotope systematics of the ca. 2.58–2.50 Ga-old Transvaal Supergroup carbonate platform, South Africa

The Neoarchean-Paleoproterozoic Transvaal Supergroup in South Africa contains the well-preserved stromatolitic Campbellrand-Malmani carbonate platform, which was deposited in shallow seawater shortly before the 2.40–2.32Ga Great Oxidation Event (GOE). This platform is composed of alternating stromatolitic carbonates and mudstones and is a prominent candidate for (isotope-) geochemical mapping to investigate the appearance of very small amounts of free oxygen that accumulated in shallow waters preceding the GOE. Mo isotopes in sedimentary archives are widely used as a proxy for redox-changes in modern and ancient environments and recent evidence suggests that oxy-molybdate (MoO42−) is directly transferred from ocean water to inorganic carbonates with negligible fractionation, thus reflecting oceanic Mo isotope signatures.In this study we analyzed major and trace element compositions as well as Mo isotopic compositions of carbonate and mudstone samples from the KMF-5 drill core. Geochemical indicators, such as Fe and Mn concentrations and Fe-to-Mn abundance ratios reveal the preservation of some geochemical indicators despite the widespread silicification and dolomitization of the platform. Heavy δ30Si values of silicified carbonates between 0.53 and 2.35‰ point to Si precipitation from surface water during early diagenesis rather than to a later hydrothermal overprint. This assessment is supported by the frequent observation of rip-up structures of silica (chert) layers within the entire sedimentary succession. The δ98Mo values of whole rock samples throughout the Malmani-Campbellrand platform range between −0.82 and +1.40‰, similar to values reported for deeper slope carbonates from the Griqualand West area, but variations are independent from lithology or depositional water depth. These large variations in δ98Mo values indicate molybdenum redox cycling and thus the presence of free oxygen in the atmosphere-ocean system at that time, in agreement with earlier Mo isotopic studies on Campbellrand carbonates and shales. A similar range in δ98Mo values for carbonates between +0.40 and +0.87‰, however, was also found on the hand specimen scale, indicating the danger of a sample bias on the Mo isotopic stratigraphy of this carbonate platform. Results of previously unpublished adsorption experiments of Mo on CaCO3 clearly indicate that the Mo inventory of Malmani-Campbellrand carbonates was not only influenced by primary adsorption from seawater, but to a much larger degree by secondary processes during early diagenesis, which also affected the Mo isotopic composition of the samples on a local scale. Our results indicate that Mo concentrations and isotopic compositions in ancient stromatolitic carbonates were subject to redox changes within microbial mats and within the soft sediment during early diagenesis and later lithification, and as such cannot be used to quantitatively reconstruct the amount of free atmospheric oxygen or its fluctuations through Earth's history. Nevertheless, we interpret our heavy Mo isotopic signatures from carbonates as a minimum value for Neoarchean seawater and reinforce the assumption that free atmospheric oxygen built up a heavy oceanic Mo reservoir at that time.

Metallogeny of the syenite-related Dongping gold deposit in the northern part of the North China Craton: A review and synthesis

The Dongping gold deposit is located at the northern margin of the North China Craton in Hebei Province, China; it is the largest alkaline pluton-related Au deposit in China. The ore deposit is hosted in the 400–386Ma (LA-ICPMS and SHRIMP zircon U–Pb method) Shuiquangou syenite intrusion, which cuts Archean-aged metamorphic rocks. The ore bodies consist of a set of en echelon lenses and veins controlled by shear zones. The ores can be divided into auriferous quartz vein, stockwork, and disseminated ores. Two stages of gold mineralization can be distinguished, the first stage mineralization consists of high grade (>10g/t) grayish quartz veins enveloped by stockwork and disseminated ores, whereas the second stage mineralization occurs as low grade (<10g/t) milky white quartz veins. The ores are sulfide-poor and enriched in Te. Gold is mainly present as native gold, calaverite, electrum and petzite.Studies of fluid inclusions in quartz indicate that the ore-forming fluids have low salinity (0.7–8.1% NaCl equivalent) in the H2O–CO2–NaCl system and have homogenization temperatures varying from 310 to 350°C. The ore-forming fluids have δ18O values varying from −1.7 to +6.9‰ with an average of 2.9‰ and δD values from −108 to −66.5‰ with an average of −85‰, indicating a magmatic origin with some involvement of meteoric fluid. The ore-forming fluids have initial 87Sr/86Sr385Ma ratios of 0.705–0.706 and 3He/4He ratios of 0.3 to 5.2 Ra (Ra being the atmospheric ratio of 1.0), indicative of a mantle contribution of the fluids. The ores have bulk δ34S values varying from −1 to +2‰, similar to those of the host syenites (δ34S=1.8–3‰) and the Archean metamorphic rocks (δ34S=0.04 to +4.4‰). Lead isotope analyses of sulfides, stock and disseminated ores, quartz veins, fresh host syenites and the Archean metamorphic rocks have a linear correlation of 208Pb/206Pb and 207Pb/206Pb, indicating a genetic link between the gold mineralization and the syenites. Hydrothermal zircons from the high grade ores of the first stage can be distinguished from magmatic zircons based on internal textures on cathodoluminescence images and rare earth element patterns; these zircons have been dated at 389±1.0 to 385±5.7Ma, whereas those from the second stage low-grade auriferous quartz vein were dated at ~140Ma (LA-ICPMS and SIM U–Pb methods).We propose a model for the formation of the Dongping gold deposit involving a late-magmatic hydrothermal process related to a Devonian mantle sourced syenites with a Late Jurassic–Early Cretaceous hydrothermal overprint.

U‐Pb dating of interspersed gabbroic magmatism and hydrothermal metamorphism during lower crustal accretion, Vema lithospheric section, Mid‐Atlantic Ridge

AbstractNew U/Pb analyses of zircon and xenotime constrain the timing of magmatism, magmatic assimilation, and hydrothermal metamorphism during formation of the lower crust at the Mid‐Atlantic Ridge. The studied sample is an altered gabbro from the Vema lithospheric section (11°N). Primary gabbroic minerals have been almost completely replaced by multiple hydrothermal overprints: cummingtonitic amphibole and albite formed during high‐temperature hydration reactions and are overgrown first by kerolite and then prehnite and chlorite. In a previous study, clear inclusion‐free zircons from the sample yielded Th‐corrected 206Pb/238U dates of 13.528 ± 0.101 to 13.353 ± 0.057 Ma. Ti concentrations, reported here, zoning patterns and calculated Th/U of the dated grains are consistent with these zircons having grown during igneous crystallization. To determine the timing of hydrothermal metamorphism, we dated a second population of zircons, with ubiquitous &lt;1–20 µm chlorite inclusions, and xenotimes that postdate formation of metamorphic albite. The textures and inclusions of the inclusion‐rich zircons suggest that they formed by coupled dissolution‐reprecipitation of metastable igneous zircon during or following hydrothermal metamorphism. Th‐corrected 206Pb/238U dates for the inclusion‐rich zircons range from 13.598 ± 0.012 to 13.503 ± 0.018 Ma and predate crystallization of all but one of the inclusion‐free zircons, suggesting that the inclusion‐rich zircons were assimilated from older hydrothermally altered wall rocks. The xenotime dates are sensitive to the Th correction applied, but even using a maximum correction, 206Pb/238U dates range from 13.341 ± 0.162 to 12.993 ± 0.055 Ma and postdate crystallization of both the inclusion‐rich zircons and inclusion‐free igneous zircons, reflecting a second hydrothermal event. The data provide evidence for alternating magmatism and hydrothermal metamorphism at or near the ridge axis during accretion of the lower crust at a ridge‐transform intersection and suggest that hydrothermally altered crust was assimilated into younger gabbroic magmas. The results of this study show that high‐precision U‐Pb dating is a powerful method for studying the timing of magmatic and hydrothermal processes at mid‐ocean ridges.

REPLACEMENT PROCESSES INVOLVING HIGH FIELD STRENGTH ELEMENTS IN THE T ZONE, THOR LAKE RARE-METAL DEPOSIT

The Thor Lake rare-element (Y-REE-Nb-Ta-Zr-Be) deposit, Northwest Territories, Canada, is a world-class deposit hosted by alkaline syenite and granite and has two main mineralized zones, namely the Nechalacho and T Zones deposit. The T Zone is shown to have originally been a pegmatite, but has experienced a strong hydrothermal overprint. Coarse-grained to megacrystic, elongate crystals defining the pegmatitic texture are completely pseudomorphed and are interpreted to have originally been nepheline. The T Zone underwent three main alteration stages, which are (from earliest to latest): (I) silicification of the principal phases of the pegmatite and the associated replacement of igneous aegirine and arfvedsonite by quartz + magnetite + high field strength element (HFSE)-bearing minerals; (II) polylithionite [KLi 2 AlSi 4 O 10 (F,OH) 2 ] alteration (Li metasomatism) that caused pervasive replacement of albite by polylithionite; and (III) phenakite (Be 2 SiO 4 ) alteration (Be metasomatism). Replacement of the primary mineralogy resulted in the formation of abundant pseudomorphs that are characterized by various HFSE minerals [typically, bastnasite-(Ce), zircon, rutile, xenotime-(Y), and columbite] and phenakite (the predominant Be mineral). The habit and mineralogy of the pseudomorphs, as well as textural relationships in partially altered crystals, make aegirine and mica-group minerals the most likely precursors of the HFSE-rich and Be-rich pseudomorphs, respectively. Although aegirine and arfvedsonite from alkaline rocks can contain HFSE, LA-ICP-MS analysis of aegirine and arfvedsonite from the T Zone indicates that the HFSE concentrations in these two minerals are insufficient to account for the bulk HFSE concentrations of the HFSE-rich pseudomorphs. Therefore, HFSE must have been added to the pseudomorphs during fluid-mineral interaction, requiring hydrothermal mobility of HFSE. An open-space filling process may have been involved in the precipitation of HFSE minerals, including zircon, rutile, anatase, and REE fluorcarbonates. Initial replacement of aegirine by quartz + magnetite increased porosity in some of the T Zone rocks, which may have facilitated HFSE precipitation. Our study also demonstrates that most hydrothermal zircon shows no association with fluorite and that the fluorite in REE-rich pseudomorphs commonly postdates the REE fluorcarbonate minerals [ e.g. , bastnasite-(Ce)]. These relationships indicate that HFSE precipitation in the T Zone requires a model that does not rely on the mixing of HFSE-, F-bearing fluids with Ca-rich fluids, as has been proposed for other HFSE deposits hosted by alkaline igneous rocks. As the precipitation of HFSE appears to be restricted to pseudomorphs where aegirine and micas have been replaced, the degree of hydrothermal enrichment of HFSE in the T Zone, and possibly in other deposits containing such minerals, may be dictated by the primary abundance of these minerals.

Skarn Textures of Yongping Copper Deposit, Yanshan, Jiangxi Province

Yongping copper deposit is located at the collisional belt between Yangtze Block and Cathaysia Block. It is a historic mine, and plays an important role in China’s copper production. Owing to the complicated geologic settings, the genesis of this deposit remains controversial for decades; VMS and generalized Skarn have been proposed. According to the VMS model (Gu et al., 2007), Yongping deposit formed in late Paleozoic basins, appearing as strata-bound massive sulfides, and underwent remobilization of ore and magmatic hydrothermal overprinting by Mesozoic magmatism. Nevertheless, Mao et al. (2011) proposed a tectono-genetic model for porphyry-skarn-stratabound Cu-Au-Mo-Fe which is really suitable for Yongping Deposit. However, all these studies focus on regional scale rather than local scale. Therefore, we try to concentrate on small scale, ore textures for instance, to search for evidence of the genesis of this deposit.

LATE CAMBRIAN Au-Pd MINERALIZATION AND Fe ENRICHMENT IN THE ITABIRA DISTRICT, MINAS GERAIS, BRAZIL, AT 496 Ma: CONSTRAINTS FROM U-Pb MONAZITE DATING OF A JACUTINGA LODE

Bonanza-style mineralization of palladiferous gold in specular hematite-rich lodes engendered the most important gold rush in Brazilian history, which took place at the end of the seventeenth century in the province that became known as Minas Gerais. The timing of this itabirite-hosted vein mineralization, locally referred to as jacutinga, is controversial; different views have ascribed jacutinga to one of the two orogenic events recorded in the Quadrilatero Ferrifero of Minas Gerais, the ~2.0 Ga Transamazonian event and the ~0.6 Ga Brasiliano event. This controversy is due mainly to the absence of reliable age data for hydrothermal minerals. Here we report the first U-Pb age for a jacutinga lode, obtained by laser ablation-sector field-inductively coupled plasma-mass spectrometry (LA-SF-ICP-MS) on monazite grains recovered from the Conceicao Fe-ore deposit within the Itabira district. The monazite analyses yield a precise Late Cambrian age of 495.6 ± 2.2 Ma, which is well within the time period between ca. 524 and 444 Ma, suggested by recently published geochronologic data on regionally distributed similar hydrothermal systems. The age of 496 Ma for the jacutinga monazite corroborates an orogen-scale hydrothermal overprint at later stages of the Brasiliano event. This hydrothermal overprint also affected surrounding Neoarchean rocks of the Minas Supergroup, as well as younger rocks of the southern Serra do Espinhaco, along the platiniferous Au-Pd belt of Minas Gerais.

Chemical and U–Sr isotopic variations in stream and source waters of the Strengbach watershed (Vosges mountains, France)

Abstract. This is the first comprehensive study dealing with major and trace element data as well as 87Sr/86Sr isotope and (234U/238U) activity ratios (AR) determined on the totality of springs and brooks of the Strengbach catchment. It shows that the small and more or less monolithic catchment drains different sources and streamlets with very different isotopic and geochemical signatures. Different parameters control the diversity of the source characteristics. Of importance is especially the hydrothermal overprint of the granitic bedrock, which was stronger for the granite from the northern slope; also significant are the different meteoric alteration processes of the bedrock causing the formation of 0.5 to 9 m thick saprolite and above the formation of an up to 1m thick soil system. These processes mainly account for springs and brooks from the northern slope having higher Ca / Na, Mg / Na, and Sr / Na ratios, but lower 87Sr/86Sr isotopic ratios than those from the southern slope. The chemical compositions of the source waters in the Strengbach catchment are only to a small extent the result of alteration of primary bedrock minerals, and rather reflect dissolution/precipitation processes of secondary mineral phases like clay minerals. The (234U/238U) AR, however, are decoupled from the 87Sr/86Sr isotope system, and reflect to some extent the level of altitude of the source and, thus, the degree of alteration of the bedrock. The sources emerging at high altitudes have circulated through already weathered materials (saprolite and fractured bedrock depleted in 234U), implying (234U/238U) AR below 1, which is uncommon for surface waters. Preferential flow paths along constant fractures in the bedrocks might explain the – over time – homogeneous U AR of the different spring waters. However, the geochemical and isotopic variations of stream waters at the outlet of the catchment are controlled by variable contributions of different springs, depending on the hydrological conditions. It appears that the (234U/238U) AR are a very appropriate, important tracer for studying and deciphering the contribution of the different source fluxes at the catchment scale, because this unique geochemical parameter is different for each individual spring and at the same time remains unchanged for each of the springs with changing discharge and fluctuating hydrological conditions. This study further highlights the important impact of different and independent water pathways on fractured granite controlling the different geochemical and isotopic signatures of the waters. Despite the fact that soils and vegetation cover have a great influence on the water cycle balance (evapotranspiration, drainage, runoff), the chemical compositions of waters are strongly modified by processes occurring in deep saprolite and bedrock rather than in soils along the specific water pathways.

In-situ LA-ICPMS trace elements and U–Pb analysis of titanite from the Mesozoic Ruanjiawan W–Cu–Mo skarn deposit, Daye district, China

In this paper, we present U–Pb ages and trace element compositions of titanite from the Ruanjiawan W–Cu–Mo skarn deposit in the Daye district, eastern China to constrain the magmatic and hydrothermal history in this deposit and provide a better understanding of the U–Pb geochronology and trace element geochemistry of titanite that have been subjected to post-crystallization hydrothermal alteration. Titanite from the mineralized skarn, the ore-related quartz diorite stock, and a diabase dike intruding this stock were analyzed using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Titanite grains from the quartz diorite and diabase dike typically coexist with hydrothermal minerals such as epidote, sericite, chlorite, pyrite, and calcite, and display irregular or patchy zoning. These grains have low LREE/HREE and high Th/U and Lu/Hf ratios, coupled with negative Eu and positive Ce anomalies. The textural and compositional data indicate that titanite from the quartz diorite has been overprinted by hydrothermal fluids after being crystallized from magmas. Titanite grains from the mineralized skarn are texturally equilibrated with retrograde skarn minerals including actinolite, quartz, calcite, and epidote, demonstrating that these grains were formed directly from hydrothermal fluids responsible for the mineralization. Compared to the varieties from the quartz diorite stock and diabase dike, titanite grains from the mineralized skarn have much lower REE contents and LREE/HREE, Th/U, and Lu/Hf ratios. They have a weighted mean 206Pb/238U age of 142±2Ma (MSWD=0.7, 2σ), in agreement with a zircon U–Pb age of 144±1Ma (MSWD=0.3, 2σ) of the quartz diorite and thus interpreted as formation age of the Ruanjiawan W–Cu–Mo deposit. Titanite grains from the ore-related quartz diorite have a concordant U–Pb age of 132±2Ma (MSWD=0.5, 2σ), which is 10–12Ma younger than the zircon U–Pb age of the same sample and thus interpreted as the time of a hydrothermal overprint after their crystallization. This hydrothermal overprint was most likely related to the emplacement of the diabase dike that has a zircon U–Pb age of 133±1Ma and a titanite U–Pb age of 131±2Ma. The geochronological results thus reveal two hydrothermal events in the Ruanjiawan deposit: an early one forming the Wu–Cu–Mo ores related to the emplacement of the quartz diorite stock and a later one causing alteration of the quartz diorite and its titanite due to emplacement of diabase dike. It is suggested that titanite is much more susceptible to hydrothermal alteration than zircon. Results from this study also highlight the utilization of trace element compositions in discriminating titanite of magmatic and hydrothermal origins, facilitating a more reasonable interpretation of the titanite U–Pb ages.

From the lavas to the gabbros: 1.25km of geochemical characterization of upper oceanic crust at ODP/IODP Site 1256, eastern equatorial Pacific

Here we present trace element and Sr-Nd-Hf-Pb (double spike) isotopic data covering the entire igneous section of oceanic crust drilled at Ocean Drilling Program (ODP)/Integrated Ocean Drilling Program (IODP) Site 1256 on the Cocos Plate. The penetrated interval extends from the upper lavas through the sheeted dike complex to the gabbroic plutonic rocks, formed during superfast spreading at the mid-Miocene equatorial East Pacific Rise. The data are used to characterize the effects of chemical alteration, resulting from convection of seawater and hydrothermal fluids, on the trace element and isotopic composition of oceanic crust. Compared to normal mid-ocean-ridge basalt, the igneous basement of Site 1256 (Holes 1256C/D) is isotopically slightly enriched but shows only narrow downhole variations in Nd-Hf-Pb isotope ratios: 143Nd/144Nd = 0.513089 ± 0.000028 (2σ), 176Hf/177Hf = 0.283194 ± 0.000033 (2σ), 206Pb/204Pb = 18.61 ± 0.11 (2σ), 207Pb/204Pb = 15.521 ± 0.014 (2σ), 208Pb/204Pb = 38.24 ± 0.15 (2σ). We believe that this minor variability is mainly of primary (magmatic) origin. The Sr isotopic composition shows considerably larger variation and, as expected, serves as sensitive tracer of seawater influence, which is particularly pronounced in the lava-dike transition zone and the sheeted dikes. The seawater influence is most prominent in a highly metal sulfide-enriched breccia layer encountered in the transition zone with 87Sr/86Sr of ~ 0.706, indicating a maximum water-rock mixing ratio of ~ 12. However, compared to the igneous section drilled at Site 504 (Hole 504B), which formed at intermediate, i.e., slower spreading rates at the Galapagos Spreading Center and hosting a much thicker sulfide-rich stockwork zone, the average intensity of water-rock interaction is lower. This is expressed by lesser mobility of base metals, narrower variability of alteration-sensitive incompatible elements, and less radiogenic Sr isotopic compositions on average at Site 1256. The amount of metal sulfide precipitation seems to be positively correlated with the degree of hydrothermal overprint. The less intense alteration of the Site 1256 transition zone, compared to Site 504, most likely reflects the higher rate of spreading, eventually resulting in a shorter period of time of continuous exposure to hydrothermal convection at the ridge crest. The observed seafloor alteration, leading to modified radiogenic parent/daughter ratios in the Site 1256 rocks, is ultimately not sufficient to develop time-integrative high 206Pb/204Pb and moderate 87Sr/86Sr ratios, as being characteristic of the HIMU (high μ = high 238U/204Pb) mantle signature proposed to originate from hydrothermally altered, subducted oceanic crust. Therefore, additional modification during the subduction process must be taken into account.

A review of the genesis of the world class Bayan Obo Fe–REE–Nb deposits, Inner Mongolia, China: Multistage processes and outstanding questions

The Bayan Obo Fe–REE–Nb deposit is the world's largest rare earth element (REE) resource and with the increasing focus on critical metal resources has become a focus of global interest. The deposit is hosted in the Palaeoproterozoic Bayan Obo Group, mainly concentrated in the H8 dolomite marble. The ores consist of light REE enriched monazite and bastnäsite, with a wide array of other REE minerals. Niobium mineralisation is hosted primarily in aeschynite and pyrochlore, although there are a wide range of other Nb-minerals. The origin of the host dolomite and ore bodies has been a subject of intense debate. The host dolomite has been proposed to be both of sedimentary origin and an igneous carbonatite. Carbonatite dykes do occur widely in the area, and consideration of the textural, geochemical and isotopic composition of the dolomite suggests an origin via intrusion of magmatic carbonatite into meta-sedimentary marble, accompanied by metasomatism. The origin of the ore bodies is complex, indicated most strongly by an ~1Ga range in radiometric age determinations. Compilation of available data suggests that the ores were originally formed around 1.3Ga (Sm–Nd isochron ages; Th–Pb ages of zircon), close in time to the intrusion of the carbonatite dykes. The ores were subsequently subjected to several stages of deformation and hydrothermal overprint, culminating in deformation, metamorphism and fluid flow related to the Caledonian subduction of the Mongolian Plate under the North China Craton from ~450 to 420Ma (Th–Pb ages of monazite). This stage resulted in the formation of the strong foliation (‘banding’) of the ore. The presence of undeformed veins with alkali mineral fills, and the overprinting of the foliation by Nb minerals suggest that secondary fluid flow events may also have contributed to the metal endowment of the deposits, as well as remobilising the original Fe and REE mineralisation. The alteration mineralogy and geochemistry of the ores are comparable to those of many REE mineralised carbonatites. Initial Nd isotope ratios at 450Ma, however, suggest crustal sources for the metals. These conflicting lines of evidence can be reconciled if a (at least) two stage isotopic evolution is accepted for the deposits, with an original mantle-sourced, carbonatite-related metal accumulation forming around 1.3Ga with εNd close to 0. The ore was remobilised, with associated re-equilibration of Th–Pb isotope systematics during deformation at ~450Ma. A further stage of alkaline hydrothermal fluid was responsible for Nb mineralisation at this stage. The complex geological history, with multiple stages of alkaline, high field strength element-rich, metasomatic fluid flow, is probably the main reason for the exceptional metal endowment of the Bayan Obo area.

The distribution of halogens (F, Cl, Br) in granitoid rocks

The halogen contents of granitoid rocks and their distribution between apatite, biotite, muscovite and amphibole from two different localities (German part of the Bohemian Massif and Central Aegean, Greece) are investigated in detail. Whole rock F and Cl contents and F/Cl mass ratios vary systematically with the magmatic halogen-bearing mineral assemblage in which biotite and apatite are present in all rock samples: muscovite-bearing granites are F-rich (mostly 1100–2000μg/g, but up to 3100μg/g) and are relatively Cl-poor (50–130μg/g). In contrast, amphibole-bearing rocks reach high Cl contents (50–900μg/g) and are relatively F-poor (500–1400μg/g). Only biotite- and apatite-bearing granites show intermediate F (750–1100μg/g) and Cl (120–350μg/g) contents. Bromine contents of all rock samples are below the detection limit of about 0.6μg/g.In general, F and Cl are preferentially incorporated in apatite over biotite and biotite usually contains higher amounts of F and Cl than muscovite. Similarly, biotite prefers F over amphibole, but the distribution of Cl between biotite and amphibole varies between different rock samples. Bromine contents in muscovite are below the detection limit of 0.4μg/g, but can reach up to 0.8μg/g in biotite and up to 2.5μg/g in apatite and amphibole.Near-solidus effects such as fluid exsolution, diffusive re-equilibration and hydrothermal overprint may change the initial halogen distribution in the rock. Importantly, significant amounts of the whole rock's F (up to 60%) and Cl (up to 90%) may be accommodated in post-magmatic alteration phases (fluorite and Na–K–Ca-bearing sheet silicates) and fluid inclusions. Therefore, great care has to be taken when interpreting halogen contents of whole rocks and the influence of post-magmatic halogen-carriers needs to be taken into account.

U–Pb dating of hydrothermal zircon from the Dongping gold deposit in North China: Constraints on the mineralization processes

The Dongping gold deposit is a large deposit with total gold reserves of >100tons. It is located at the northern margin of the North China Craton, northwestern Hebei province, China. The ore-bodies are hosted by the Devonian Shuiquangou syenite complex and consist mainly of auriferous quartz veins and disseminated ore in the altered and silicified syenite. U–Pb dating of zircon from hornblende syenite on the western margin of the complex yields a crystallization age of 400±3.5Ma (MSWD=0.018). Morphology, cathodoluminescence imaging and geochemical classifications of zircon from the first stage of disseminated ore and gray auriferous quartz veins, and from later stage, low grade quartz veins point to their newly crystallized hydrothermal origin. The hydrothermal zircon from the disseminated ore and auriferous gray quartz vein are dated at 389±1.0Ma and 385±5.7Ma, respectively, which are detectably younger than but close to the crystallization age of the syenite complex and might have been formed by post-magmatic hydrothermal processes. Both types of ore are dominant in the ore deposit, and we propose that the pervasive, post-magmatic hydrothermal alteration is the main ore forming stage. Hydrothermal zircon from a low grade auriferous quartz vein yields a U–Pb age of ~140Ma, interpreted as forming during a younger period of superimposed Yanshanian hydrothermal mineralization. Thus, the ore of the Dongping gold mine represents a post-magmatic Devonian hydrothermal ore deposit with Jurassic–Cretaceous hydrothermal overprinting.

Hydrothermal overprint on Cenozoic sediments in the Quadrilátero Ferrífero of Minas Gerais: implications for precious metals in cratonic terrains

AbstractCenozoic sediments deposited in graben‐like basins in the Quadrilátero Ferrífero, southern São Francisco craton, Brazil, have anomalous contents of Hg and Sb. Two localities have been studied: the topaz deposit of Antônio Pereira, and the remains of the old opencast workings at Cata Preta, one of the earliest Au mines of Minas Gerais that dates back to about 1690. At Cata Preta, a brecciated wad‐like rock exhibits geochemical fingerprints that point to fractionation of Pt and Pd from Ag. This metal fractionation, reflected in the Au ingots produced in the region, genetically links Cata Preta to the near‐surface bonanza ore of Serra Pelada, which formed as a result of a Late‐Cretaceous hydrothermal overprint in the Amazon craton. Here, we propose that anorogenic hydrothermal activity in cratonic terrains prepared the ground for the enrichment in precious metals that gave rise to two of the most important Au rushes in Brazilian history.

The Mesoproterozoic Copperwood Sedimentary Rock-Hosted Stratiform Copper Deposit, Upper Peninsula, Michigan

Copperwood is located in Gogebic County, Upper Peninsula, Michigan, within the Porcupine Mountains copper district. It was discovered in 1956 and, in 2008, Orvana Minerals Corp began exploration and development work with production projected in the near future. Stratiform copper mineralization is hosted by gray to black shales and siltstones of the basal Nonesuch Formation. Copper mineralization is overlain by laminated gray siltstones of the Nonesuch Formation and is underlain by red-bed sandstones, conglomerates, and minor siltstones of the Copper Harbor Formation; these rocks are part of a thick section of clastic sedimentary rocks that comprise the upper fill of the Mesoproterozoic (1.1–1.0 Ga) Midcontinent rift. The copper deposit is a tabular body that averages 2.5 m thick, lacks significant basal undulations, and dips 8° to 12° northward on the southwest limb of the Western syncline, an open fold with a shallow plunge to the northwest. Chalcocite is the only Cu-bearing mineral in the orebody and occurs as concentrations along stratigraphic laminations and as disseminated grains. Within the orebody, there is no mineral or metal zoning; Cu, Ag, and S are strongly enriched, Zn is very slightly enriched and Pb and Co lack enrichment. Gangue minerals in the orebody are quartz, clinochore, muscovite/illite, plagioclase, K-feldspar, calcite, and hematite, which are consistent with temperatures of less than 130°C. Only one significant fault, a shallow NW-dipping reverse fault with displacement of up to 7 m, has disturbed the section. The mineralized zone at Copperwood is stratigraphically equivalent to that at the White Pine mine, but Copperwood lacks the structural complexity and the hydrothermal overprint manifested by the native Cu mineralization at White Pine. Reported Canadian National Instrument 43-101-compliant measured, indicated, and inferred resources are 33 million metric tonnes (Mt) of 1.60% Cu and 4.1 ppm Ag for total contained Cu of about 1.165 billion lb and 4.32 Moz of Ag. The characteristics of Copperwood are consistent with those of reduced-facies, or Kupferschiefer-type, sedimentary rock-hosted stratiform copper deposits. The proposed genetic model is chalcocite replacement of pyrite in unlithified sediments during diagenesis as a result of emanating upward-focused, compaction-driven, Cu-bearing saline basinal waters with Cu leached from the underlying red-bed paleoquifer. The simplicity of the mineralizing event and the undeformed nature of the deposit distinguish Copperwood from most other examples of reduced-facies, or Kupferschiefer-type, sedimentary rock-hosted stratiform copper deposits throughout the world.

SULFIDE Re-Os AND Rb-Sr ISOTOPE DATING OF THE KANGDIAN IOCG METALLOGENIC PROVINCE, SOUTHWEST CHINA: IMPLICATIONS FOR REGIONAL METALLOGENESIS

The Kangdian iron oxide copper-gold (IOCG) metallogenic province contains a number of economically important Fe-Cu deposits in South China and northern Vietnam. Due to the lack of precise mineralization ages, the metallogenesis of this province has long been a matter of debate. In this study, molybdenite Re-Os, pyrite Re-Os, and Rb-Sr isotope methods are used to date the Yinachang and E’touchang Fe-Cu deposits in the central part of this province. Fine-grained (<0.1 mm) molybdenite occurs as veinlets along bandings or fractures of magnetite ores and hydrothermal veins in the Yinachang deposit. Molybdenite separates from magnetite ores have highly reproducible Re-Os model ages of 1654 ± 7 Ma, whereas those from hydrothermal veins have a much younger Re-Os model age of 1451 ± 6 Ma. Euhedral pyrite grains occur locally along foliations of hematite ores and their proximal alteration assemblages in the E’touchang deposit. Pyrite separates have an Re-Os isochron age of 1487 ± 110 Ma and a single-grain Rb-Sr isochron age of 1453 ± 28 Ma, both of which are compatible with the molybdenite Re-Os age of hydrothermal veins from the Yinachang deposit. The molybdenite age of magnetite ores is in good agreement with zircon U-Pb ages of dolerite intrusions in the region, and is considered to represent the timing of a major ore formation event at ~1655 Ma. The younger Re-Os and Rb-Sr ages likely record a secondary hydrothermal overprint event at ~1450 Ma, which has not been recognized before. The mineralization potential and implications of this event need to be investigated in future studies. Our new isotopic data set hence demonstrates that the Kangdian IOCG deposits were likely formed at ~1655 Ma, synchronous with the late Paleoproterozoic intracontinental rifting event of the western Yangtze block.

The lateral boundary of a metamorphic core complex: The Moutsounas shear zone on Naxos, Cyclades, Greece

We describe the structure, microstructures, texture and paleopiezometry of quartz-rich phyllites and marbles along N-trending Moutsounas shear zone at the eastern margin of the Naxos metamorphic core complex (MCC). Fabrics consistently indicate a top-to-the-NNE non-coaxial shear and formed during the main stage of updoming and exhumation between ca. 14 and 11 Ma of the Naxos MCC. The main stage of exhumation postdates the deposition of overlying Miocene sedimentary successions and predates the overlying Upper Miocene/Pliocene conglomerates. Detailed microstructural and textural analysis reveals that the movement along the Moutsounas shear zone is associated with a retrograde greenschist to subgreenschist facies overprint of the early higher-temperature rocks. Paleopiezometry on recrystallized quartz and calcite yields differential stresses of 20–77 MPa and a strain rate of 10−15–10−13 s−1 at 350 °C for quartz and ca. 300 °C for calcite. Chlorite geothermometry of the shear zone yields two temperature regimes, 300–360 °C, and 200–250 °C. The lower temperature group is interpreted to result from late-stage hydrothermal overprint.

A two-stage evolution model for the Amantaytau orogenic-type gold deposit in Uzbekistan

A lithogeochemical, mineral chemical, isotopic, and fluid inclusion study of barren, low-, and high-grade Au-mineralized samples from the shear zone-hosted Amantaytau gold deposit, Uzbekistan, shows that the local host rocks, Late Ordovician–Earlz Silurian carbonacous shales, are likely to have been an important source of Au, As, Ni, and S in the formation of the deposit. Syn-depositional pyrite in these shales contains on average 0.23 ppm Au, 1,083 ppm As, and 861 ppm Ni. The distribution of rare earth elements (REE) indicates a homogeneous source of light REE, whereas the heavy REE distribution reflects most likely primary variations in the sediments. The mineralized zone is marked by a positive Eu anomaly, which supports reducing conditions during the mineralization. A hydrothermal overprint by an aqueous–carbonic fluid is reflected in a high-grade Au-mineralized sample by δ13C values of −13.0 ‰ (V-PDB). The δ34S values in pyrite (−0.13 to +7.30 ‰ CDT) from barren and mineralized samples are consistent with marine sulfate being the principal source of the ore sulfur. Assuming a formation temperature of between 300 and 400 °C for the main stage of mineralization, as indicated by the alteration mineral assemblage, the calculated δ18Ofluid is between 9.5 and 13.4 ‰ V-SMOW, which points at a metamorphic origin of the ore fluid.

X-Ray Crystallography of Uraninites Associated with the Albitite Belt of Western India: Evidence for the High-Temperature Origin of Uranium and Associated Mineralisation

Abstract X-ray diffraction (XRD) studies on the radioactive ore samples from various parts of Rajasthan and Haryana have revealed the presence of several uranium and other atomic mineral occurrences in the albitite belt of western India. The primary uranium minerals (PUMs) are uraninite and brannerite, whereas, the secondary uranium minerals (SUMs) show considerable speciations: phosphate, silicate, hydrous oxide hydrate, and vanadate. Multiple oxides (MOs) are davidite, fergusonite, aeschynite-(Y), microlite, samarskite, euxenite, betafite, and columbite-tantalite. The thorium minerals are huttonite, thorite, uranoan-thorite, thorianite, thorutite, and brabantite. The yttrium and REE-bearing minerals are xenotime, britholite, allanite, chevkinite, tritomite, and monazite. It is noted that the measured unit cell dimension (a0) of the investigated uraninites ranges from 5.4110 Å to 5.4646 Å. The highest unit cell dimension (5.4646 Å) represents a composition (or oxidation grade) of UO2.05, whereas, the lowest one (5.4110 Å) corresponds to a composition of UO2.54. Furthermore, it is also apparent that, with increase in oxidation grade, there is a concomitant decrease in unit cell dimension. As most of the values of ao of uraninites from the albitite belt are high (&amp;gt; 5.45 Å), it may be inferred that the overall temperature of formation of uraninites of the albitite belt was higher (ca. 400°C). However, the low values of a0 in certain localities could be due to the prevalence of relatively low and fluctuating temperature regimes locally (ca. 400° - 100° C). Numerous occurrences of refractory, multiple oxides, and REE minerals, in association with uranium mineralisation, also support a high-temperature origin for the investigated uraninites. Binary data plots of unit cell dimension (a0) versus oxidation grade/composition (UO2+x) of uraninites (n = 36) suggest that the gross uranium mineralisation in the albitite belt of western India is mainly linked to regional metamorphism, anatexis, granitic intrusion, metasomatism, and contact metamorphosed granite-pegmatite aureoles and granite-related vein type with hydrothermal overprints, including redistribution of intrinsic sedimentary uranium and its concentration along suitable structural locales. These interpretations are consistent with the known gross geologic features of the albitite belt. Furthermore, the presence of marialite (calcian) in many places in the albitite belt also supports such a contention, as this mineral is known to be restricted to metamorphic and metasomatic environments. The speciation of secondary uranium minerals could be due to the higher oxidation of U4+ to U6+ in surface to near-surface conditions and its (U6+) remobilisation as uranyl ions. The combination of moving uranyl ions with available cations and anions en route caused re-precipitation of U as diversified assemblages of low-temperature uranyl minerals under suitable physicochemical conditions.

Carboniferous–Permian volcanic evolution in Central Europe—U/Pb ages of volcanic rocks in Saxony (Germany) and northern Bohemia (Czech Republic)

Nine SHRIMP U/Pb ages on zircon and two Pb/Pb single zircon ages have been determined from Late Paleozoic volcanic rocks from Saxony and northern Bohemia. Samples came from the Teplice-Altenberg Volcanic Complex, the Meissen Volcanic Complex, the Chemnitz Basin, the Dohlen Basin, the Brandov-Olbernhau Basin, and the North Saxon Volcanic Complex. The Teplice-Altenberg Volcanic Complex is subdivided into an early Namurian phase (Mikulov Ignimbrite, 326.8 ± 4.3 Ma), thus older than assumed by previous studies, and a late caldera-forming phase (Teplice Ignimbrite, 308.8 ± 4.9 Ma). The age of the latter, however, is not well constrained due to a large population of inherited zircon and possible hydrothermal overprint. The Leutewitz Ignimbrite, product of an early explosive volcanic episode of the Meissen Volcanic Complex yielded an age of 302.9 ± 2.5 Ma (Stephanian A). Volcanic rocks intercalated in the Brandov-Olbernhau Basin (BOB, 302 ± 2.8 Ma), Chemnitz Basin (CB, 296.6 ± 3.0 Ma), Dohlen Basin (DB, 296 ± 3.0 Ma), and the North Saxon Volcanic Complex (NSVC, c. 300–290 Ma) yielded well-constrained Stephanian to Sakmarian ages. The largest Late Paleozoic ignimbrite-forming eruption in Central Europe, the Rochlitz Ignimbrite, has a well-defined middle Asselian age of 294.4 ± 1.8 Ma. Ages of palingenic zircon revealed that the Namurian-Westphalian magmatism assimilated larger amounts of crystalline basement that formed during previous Paleozoic geodynamic phases. The Precambrian inherited ages support the chronostratigraphic structure assumed for the Saxo-Thuringian Zone of the Variscan Orogen. The present results help to improve the chronostratigraphic allocation of the Late Paleozoic volcanic zones in Central Europe. At the same time, the radiometric ages have implications for the interbasinal correlation and for the geodynamic evolution of the Variscan Orogeny.