The anisotropic properties of granites – effects of tectonic emplacement mode on potential crystalline host rocks for nuclear waste deposits in Germany

For permanent nuclear waste disposal sites, crystalline rocks, especially granitic/granodioritic batholiths, are considered an appropriate host rock. Principally, three types of granitic plutons occur in the extra-alpine crystalline basement of Germany that were consolidated during the late Paleozoic Variscan orogeny of Central Europe: (i) Pre-Variscan voluminous granodiorites that are hardly affected by the subsequent continent–continent collision; (ii) voluminous granites in various tectonic settings intruded during the late orogenic stage of the Variscides; (iii) post-orogenic granites related to vast Permian intracontinental extension. Thus, in terms of the syn-intrusive tectonic setting and post-intrusive processes there are significant differences. Although it can be expected that different tectonic environments caused significant differences in the material properties, for Germany, however, there is no systematic study regarding the fabric of such plutonites. In order to find the most suitable “granite” we investigate the primary anisotropy of granites evolved during the emplacement and crystallization of the melt. For this we sample rocks of all three principal types and various syn-intrusive tectonic settings, i.e., compression, extension, strike-slip, transtension, and transpression. By means of combined measurements of the “Anisotropy of the Magnetic Susceptibility” and the “Shape Preferred Orientation” we characterize the syn-intrusive flow pattern, i.e., the magmatic foliation and lineation. The Crystallographic Preferred Orientation is analyzed by a combination of neutron time-of-flight experiments and electron backscatter diffraction measurements at the Frank Laboratory of Neutron Physics at JINR, Dubna, Russia, and the TU Bergakademie Freiberg respectively. Furthermore, special attention is given to the systematic mapping of annealed microcracks evolved during late magmatic fluid escape and/or post-crystallization hydrothermal activity. In a second step we compare the primary anisotropy with the post-magmatic fracture pattern of the particular granites. Those fractures constitute probable fluid pathways and, thus, the first-order risk for a potential permanent nuclear waste disposal. All datasets are organized in a Geological Information System allowing for a complete traceability of the different investigation steps. The results of this study will serve as a basis for a future detailed exploration.

The Perron Gold Deposit, Archean Abitibi Belt, Canada: Exceptionally High-Grade Mineralization Related to Higher Gold-Carrying Capacity of Hydrocarbon-Rich Fluids

The Perron deposit, an Archean orogenic gold deposit located in the Abitibi belt, hosts a quartz vein-type gold-bearing zone, known as the high-grade zone (HGZ). The HGZ is vertically continuous along >1.2 km, and is exceptionally rich in visible gold throughout its vertical extent, with grades ranging from 30 to 500 ppm. Various hypotheses were tested to account for that, such as: (1) efficient precipitating mechanisms; (2) gold remobilization; (3) particular fluids; (4) specific gold sources for saturating the fluids; and (5) a different mineralizing temperature. Host rocks recorded peak metamorphism at ~600 °C based on an amphibole geothermometer. Visible gold is associated with sphalerite (<5%) which precipitated at 370 °C, based on the sphalerite GGIMFis geothermometer, during late exhumation of verticalized host rocks. Pyrite chemistry analyzed by LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) is comparable to classical orogenic gold deposits of the Abitibi belt, without indication of a possible magmatic fluid and gold contribution. Comparison of pyrite trace element signatures for identifying a potential gold source was inconclusive to demonstrate that primary base-metal rich volcanogenic gold mineralization, dispersed in the host rhyolitic dome, could be the source for the later formation of the HGZ. Rather, nodular pyrites in graphitic shales, sharing similar trace element signatures with pyrite of the HGZ, are considered a potential source. The most striking outcome is the lack of water in the mineralizing fluids, implying that gold was not transported under aqueous complexes, even if fugacity of sulfur (−6) and oxygen (−28), and pH (~7) are providing the best conditions at a temperature of 350 °C for solubilizing gold in water. Fluid inclusions, analyzed by solid-probe mass spectrometry, are rather comparable to fossil gas composed mostly of hydrocarbons (methane and ethane and possibly butane and propane and other unidentified organic compounds), rich in CO2, with N2 and trace of Ar, H2S, and He. It is interpreted that gold and zinc were transported as hydrocarbon-metal complexes or as colloidal gold nanoparticles. The exceptional high content of gold and zinc in the HGZ is thus explained by the higher transporting capacity of these unique mineralizing fluids.

Magmatic fluid pathways in the upper crust: insights from dense magnetotelluric observations around the Kuju Volcanoes, Japan

SUMMARY Magnetotelluric (MT) observations have revealed subvertical electrical conductors that extend from shallow depths into the mid-crust at various geothermal zones, active volcanoes and active faults worldwide. These deeply rooted subvertical conductors have typically been interpreted to represent entire zones of dedicated fluid transport through the crust. We estimate the high-resolution 3-D crustal resistivity structure below the Kuju Volcanoes, Japan, using dense observations from 153 broad-band MT measurement sites and 40 telluric measurement sites. The resistivity structure highlights subvertical conductors that merge into a deep conductor to the north of the volcanoes, with deep low-frequency earthquakes occurring near the southeastern edge of this subvertical conductor at 10–30 km depth. This deep conductor branches into several subvertical conductors at 2–10 km depth, coinciding with a shallow zone where tectonic earthquakes rarely occur. The surface expressions of active geothermal areas and past volcanic eruptions are all located above the edges of the conductors at 2–6 km depth. Widespread conductive layers exist around the volcanoes above 2 km depth, and their distribution approximately corresponds to a low-gravity-anomaly zone. We discuss the nature of these subvertical conductors, the potential causes of their complex structure and their relationship to local magmatic fluid transport. These subvertical conductors, a shallow clay-rich layer, developed fracture systems and high-strength solidified magma may all contribute to magmatic fluid transport to the surface at the Kuju Volcanoes. In this study, we add the possibility that the edges of these subvertical conductors act as important magmatic fluid pathways.

Periodically Released Magmatic Fluids Create a Texture of Unidirectional Solidification (UST) in Ore-Forming Granite: A Fluid and Melt Inclusion Study of W-Mo Forming Sannae-Eonyang Granite, Korea

The Upper Cretaceous Sannae-Eonyang granite crystallized approximately 73 Ma and hosted the Sannae W-Mo deposit in the west and the Eonyang amethyst deposit in the east. The granite contained textural zones of miarolitic cavities and unidirectional solidification texture (UST) quartz. The UST rock sampled in the Eonyang amethyst mine consisted of (1) early cavity-bearing aplitic granite, (2) co-crystallization of feldspars and quartz in a granophyric granite, and (3) the latest unidirectional growth of larger quartz crystals with clear zonation patterns. After the UST quartz was deposited, aplite or porphyritic granite was formed, repeating the prior sequence. Fluid and melt inclusions occurring in the UST quartz and quartz phenocrysts were sampled and studied to understand the magmatic-hydrothermal processes controlling UST formation and W-Mo mineralization in the granite. The composition of melt inclusions in the quartz phenocrysts suggested that the UST was formed by fractionated late-stage granite. Some of the melt inclusions occurring in the early-stage UST quartz were associated with aqueous inclusions, indicating fluid exsolution from a granitic melt. Hypersaline brine inclusions allowed the calculation of the minimum trapping pressure of 80–2300 bars. Such a highly fluctuating fluid pressure might be potentially due to a lithostatic-hydrostatic transition of pressure-attending fluid loss during UST formation. Highly fluctuating lithostatic-hydrostatic pressures created by fluid exsolution allowed shifting of the stability field from a quartz-feldspar cotectic to a single-phase quartz. The compositions of brine fluid assemblages hosted in the quartz phenocrysts deviated from the fluids trapped in the UST quartz, especially regarding the Rb/Sr and Fe/Mn ratios and W and Mo concentrations. The study of melt and fluid inclusions in the Eonyang UST sample showed that the exsolution of magmatic fluid was highly periodic. A single pulse of magmatic fluids of variable salinities/densities might have created a single UST sequence, and a new batch of magmatic fluid exsolution would be required to create the next UST sequence.

Textural and geochemical characteristics of garnet from the Luoyang Fe skarn deposit, eastern China: implications for ore-forming fluid evolution and mineralization conditions

The late Palaeozoic Yong’an–Meizhou depression belt is an important iron (Fe) and polymetallic metallogenic belt in southern China. It has undergone a transformation from Tethys to the circum-Pacific tectonic domain. The Luoyang deposit is one of the typical Fe skarn deposits in the Yong’an–Meizhou depression belt of eastern China. Garnet is a characteristic mineral in the deposit. Two generations of garnets are detected in the deposit based on their textural characteristics and trace-element contents, and are represented by Fe-enriched andradite. The first generation of garnets (Grt1) have two types of garnets (Grt1-A and Grt1-B). Type A garnets of the first generation (Grt1-A) (Adr80-88) replaced by massive diopside-magnetite assemblage exhibit distinct oscillatory zonings and display patterns of enriched light rare earth elements (LREE) to weak heavy rare earth elements (HREE), with weak negative to positive Eu anomalies, and highest U, ΣREE and Sn contents. Type B garnets of the first generation (Grt1-B) are irregular zones (Adr94-96) coexisting with magnetite, in which Grt1-A is generally dissolved, and have obviously LREE-enriched and HREE-depleted patterns, with weak negative to positive Eu anomalies, and moderate U, ΣREE and Zn contents. Garnets of the second generation (Grt2) (Adr96-99) that replaced massive magnetite together with sphalerite show unzoned patterns, with a flat REE pattern and pronounced negative Eu anomalies as well as contents of lowest U and ΣREE, and highest W. The substitution of REEs in garnets occurs as [X2+]VIII –1[REE3+]VIII +1[Si4+]IV –1[Z3+]IV +1in an Al-enriched environment. Luoyang hydrothermal fluids shifted from reducing conditions with relatively high-U and -ΣREE characteristics to oxidizing conditions with relatively low-U and -ΣREE characteristics. The reduced siderophile elements and increased fO2 in fluid during Grt1-B formation caused magnetite mineralization and reduced Zn contents during Grt2 formation, causing the deposition of sphalerite. All garnets formed from magmatic fluid and were controlled by infiltrative metasomatism in an opened system.

Mineralogy and Geochemistry of Ocelli in the Damtjernite Dykes and Sills, Chadobets Uplift, Siberian Craton: Evidence of the Fluid–Lamprophyric Magma Interaction

The study reports petrography, mineralogy and carbonate geochemistry and stable isotopy of various types of ocelli (silicate-carbonate globules) observed in the lamprophyres from the Chadobets Uplift, southwestern Siberian craton. The Chadobets lamprophyres are related to the REE-bearing Chuktukon carbonatites. On the basis of their morphology, mineralogy and relation with the surrounding groundmass, we distinguish three types of ocelli: carbonate-silicate, containing carbonate, scapolite, sodalite, potassium feldspar, albite, apatite and minor quartz ocelli (K-Na-CSO); carbonate–silicate ocelli, containing natrolite and sodalite (Na-CSO); and silicate-carbonate, containing potassium feldspar and phlogopite (K-SCO). The K-Na-CSO present in the most evolved damtjernite with irregular and polygonal patches was distributed within the groundmass; the patches consist of minerals identical to minerals in ocelli. Carbonate in the K-Na-CSO are calcite, Fe-dolomite and ankerite with high Sr concentration and igneous-type REE patterns. The Na-CSO present in Na-rich damtjernite with geochemical signature indicates the loss of the carbonate component. Carbonate phases are calcite and Fe-dolomite, and they depleted in LREE. The K-SCO was present in the K-rich least-evolved damtjernite. Calcite in the K-SCO has the highest Ba and the lowest Sr concentration and U-shaped REE pattern. The textural, mineralogical and geochemical features of the ocelli and their host rock can be interpreted as follows: (i) the K-Na-CSO are droplets of an alkali–carbonate melt that separated from residual alkali and carbonate-rich melt in highly evolved damtjernite; (ii) the Na-CSO are droplets of late magmatic fluid that once exsolved from a melt and then began to dissolve; (iii) the K-SCO are bubbles of K-P-CO2 fluid liberated from an almost-crystallised magma during the magmatic–hydrothermal stage. The geochemical signature of the K-SCO carbonate shows that the late fluid could leach REE from the host lamprophyre and provide for REE mobility.

Multistage development of a hydrothermal tungsten deposit during the Variscan late-orogenic evolution: the Puy-les-Vignes W breccia pipe (Massif Central, France)

The Puy-les-Vignes W deposit, located in the northwestern French Massif Central (FMC), is a rare occurrence worldwide of a wolframite-mineralized hydrothermal breccia pipe hosted in high-grade metamorphic gneisses. We present here an integrated study of this deposit aiming to characterize the ore-forming hydrothermal system in link with the Variscan late-orogenic evolution of the FMC. Based on a set of representative samples from the host rocks and mineralization, we describe a detailed paragenetic sequence and we provide the major and trace element geochemistry of the host rocks and W-Nb-Ta-Sn-Ti oxide minerals, in situ U-Pb and 40Ar/39Ar geochronology, and a fluid inclusion study in quartz and wolframite. We demonstrate that the formation of this W-mineralized breccia pipe results from a multistage development between ca. 325 and 300 Ma related to four major episodes during the late Carboniferous. The first episode corresponds to the emplacement of an unexposed peraluminous granite at ca. 325 Ma, which generated microgranite dykes exposed at the present-day surface. The second episode is the formation of the quartz-supported breccia pipe and wolframite mineralization at ca. 318 Ma at a paleodepth of 7 km. The mineralizing fluids have a H2O-NaCl-CO2-CH4-N2 composition, a moderate-salinity (<9 wt.% NaCl eq) and were trapped at high-temperatures (400-450°C) during lithostatic to hydrostatic pressure variations caused by intense hydraulic fracturing of the host rocks. Wolframite deposition is interpreted to result from the mixing between a W-rich intermediate-density magmatic fluid exsolved from an evolved leucogranite and low-salinity volatile-rich metamorphic fluids of distal origin. The third episode corresponds to a magmatic-hydrothermal Nb-Ta mineralization overprinting the W-mineralized system and related to the intrusion at ca. 314 Ma of a rare-metal granite, which is part of a regional peraluminous rare-metal magmatism during the 315-310 Ma period. Finally, the last episode corresponds to the formation of a disseminated Bi±Au-Ag mineralization at ca. 300 Ma, which shares similar textural and mineral features with orogenic gold deposits in the FMC. The Puy-les-Vignes W deposit records, therefore, a multistage and long-lived development that extends over a timespan of 25 million years in a regional setting dominated by protracted peraluminous magmatism and HT-LP metamorphism. Although the local environment of ore deposition is atypical, our results show that the mineral assemblages, alteration styles, and fluid characteristics of the Puy-les-Vignes breccia pipe are similar to those of other peri-granitic W deposits in the FMC.

Origin of the Kyaukmyet Low-Sulfidation Epithermal Gold Prospect, Monywa District, Central Myanmar

The Kyaukmyet prospect is one of the principal epithermal gold prospects in the Monywa District, Central Myanmar; its gold- and base metal-bearing quartz veins contain around 3 g/t gold. Ore minerals are mainly hosted by volcanic and volcaniclastic rocks of the Late Oligocene to Middle Miocene Magyigon Formation. The distribution of magmatic intrusions in the area is controlled by ENE-WSW trending faults; these faults are likely related to ore mineralization. Common ore minerals at the Kyaukmyet prospect include pyrite, sphalerite, galena, chalcopyrite, and electrum. They occur in mineralized crustiform-textured brecciated quartz veins and banded (colloform) and massive quartz veins. Mineralized rock is accompanied by silicification and propylitic and argillic alterations. The alteration mineral assemblages include quartz, adularia, calcite, chlorite, illite/smectite, sericite, and illite. Fluid inclusions in the quartz veins have homogenization temperatures ranging from 148 °C to 304 °C and salinities from 0.35 wt % to 2.75 wt % NaCl equiv. The quartz in the mineralized quartz veins was most likely precipitated at a depth ranges165-256 m below the paleosurface. The precipitation of gold at the Kyaukmyet prospect may have been formed by mixing large amounts of meteoric fluid with small amounts of magmatic fluid. The coexistence of liquid-rich and vapor-rich inclusions and presence of adularia and bladed calcite indicate that fluid boiling is caused the main mechanism of ore formation. The vein textures, ore mineral assemblages, alteration minerals and fluid inclusion data suggest that the Kyaukmyet prospect is a polymetallic low-sulfidation epithermal gold deposit.

Trace Element Signatures in Pyrite and Marcasite From Shallow Marine Island Arc-Related Hydrothermal Vents, Calypso Vents, New Zealand, and Paleochori Bay, Greece

Fluid conditions of shallow marine hydrothermal vent sites (&lt;200 mbsl) in island arcs resemble those of subaerial epithermal systems. This leads to a distinct mineralization-style compared to deeper arc/back-arc (&gt;200 mbsl) and mid-ocean ridge-related environments (&gt;2000 mbsl). At Calypso Vents in the Bay of Plenty and Paleochori Bay at the coast of Milos Island, fluids with temperatures &lt;200°C are emitted through volcaniclastic sediments in water depths &lt;200 mbsl. The hydrothermal mineralization from these fluids is dominated by pyrite and marcasite showing diverse textures, including colloform alternations, semi-massive occurrences surrounding detrital grains, vein-type pyrite, and disseminated fine-grained assemblages. Pyrite and marcasite from Calypso SE show elevated concentrations of volatile elements (e.g., As, Sb, Tl, Hg) implying a vapor-rich fluid phase. By contrast, elements like Zn, Ag, and Pb are enriched in hydrothermal pyrite and marcasite from Calypso SW, indicating a high-Cl liquid-dominated fluid discharge. Hence, vapor-liquid element fractionation induced by fluid boiling is preserved in the seafloor mineralization at Calypso Vents. Hydrothermal mineralization at very shallow vent sites (&lt;10 mbsl), like Paleochori Bay, are affected by wave action causing a seasonal migration of the seawater-fluid interface in the sediment cover. The δ34S composition of native S crusts and crystalline S (0.7–6.7‰) is indicative for host rock leaching and thermochemical reduction of seawater sulphate. By contrast, the highly negative δ34S signature of native S globules in sediments (−7.6 to −9.1‰) is related to microbial sulphate reduction or a subordinate magmatic fluid influx. Alunite-jarosite alteration (Paleochori Bay) and a mineral assemblage consisting of orpiment, realgar, and native S (Calypso Vents) may also suggest a contribution by an oxidised (sulphate-rich) low pH fluid of potential magmatic origin. However, fluid boiling is pervasive at Calypso Vents and Paleochori Bay, and the condensation of vapor-rich fluids in a steam-heated environment may produce a similar alteration and mineralization assemblage without a significant magmatic fluid influx, as known from some subaerial epithermal systems.

Geological position of the gold-sulfide-quartz deposits of the Chilean active margin

Research subject. The gold-sulphide-quartz deposits of Central Chile are typical prospecting objects, having small ore intervals (from the first tens of centimetres to 1 m), intermittent and nested ore distribution and extremely uneven gold contents.Materials and methods. The patterns of ore mineralization distribution are considered against the background of the results of modern geophysical studies of the lithosphere: the Moho surface, density and thermal regime of the upper mantle. Detailed studies were conducted on the Yapin ore field.Results. It was shown that the faults controlling gold-sulphidequartz mineralization are derivatives of shear tectonics under the conditions of a transpression regime along the Chilean active margin. At an early stage, these faults developed in a right-shift environment, which was accompanied by the introduction of diabase dikes into the northeastern faults, and gold-sulfide-quartz mineralization superimposed on the dikes was deposited during the left-shift stage. The geological structure of the deposits in the ore field Yapin was characterized. It was shown that a diverse mineralization is developed in the ore field - copper-porphyry, IOCG-type and gold-sulphide-quartz. According to geochemical data, the latter is characterized by a clear enrichment of chalcophilic elements (Au, As, Ag, Cd, Cu, Bi, Pb, Zn, Te, Co). The marked enrichment of Bi, Te and Co ores indicates the participation of magmatic fluid in ore formation and the similarity of the mineralization of the Escondida deposit with the type of gold deposits associated with granitoid intrusions. According to geochemical features, gold-sulphide-quartz mineralization in the general zoning pattern occupies a boundary position between IOCG-type objects and copper-porphyry deposits.Conclusions. The conclusion is drawn about the independence of gold-sulphide-quartz mineralization and its difference from epithermal gold deposits. It is noted that, in the volcanic belts of the North-East of Russia, the prospects for discovering unconventional gold-sulphidequartz deposits similar to those of Central Chile are rather real.