Active source seismic imaging on near-surface granite body: case study of siting a geological disposal repository for high-level radioactive nuclear waste

In order to research whether it is suitable to set a geological disposal repository for high-level radioactive nuclear waste into one target granite body, two active source seismic profiles were arranged near a small town named Tamusu, Western China. The study area is with complex surface conditions, thus the seismic exploration encountered a variettraveltimey of technical difficulties such as crossing obstacles, de-noising harmful scattered waves, and building complex near-surface velocity models. In order to address those problems, techniques including cross-obstacle seismic geometry design, angle-domain harmful scattered noise removal, and an acoustic wave equation-based inversion method jointly utilizing both the and waveform of first arrival waves were adopted. The final seismic images clearly exhibit the target rock’s unconformable contact boundary and its top interface beneath the sedimentary and weathered layers. On this basis, it could be confirmed that the target rock is not thin or has been transported by geological process from somewhere else, but a native and massive rock. There are a few small size fractures whose space distribution could be revealed by seismic images within the rock. The fractures should be kept away. Based on current research, it could be considered that active source seismic exploration is demanded during the sitting process of the geological disposal repository for nuclear waste. The seismic acquisition and processing techniques proposed in the present paper would offer a good reference value for similar researches in the future.

Triassic magmatism along the Eurasian margin of the Palaeotethys: U-Pb zircon age constraints from the western part of the Sakar-Strandzha Zone, Bulgaria

<p>In the Aegean sector of the Alpine orogen of the Eastern Mediterranean, the Sakar-Strandzha Zone (SSZ) represents a major tectonic unit that straddles the territories of Bulgaria and Turkey. The westernmost part of the SSZ in Bulgaria includes the area along the Maritsa river valley and the St. Iliya Heights, both connected through several small outcrop areas under the Cenozoic sedimentary cover. In Bulgaria, the Triassic felsic magmatism along the Maritsa river valley was inferred by Chatalov (1961) on the basis of the stratigraphy, but only a single U-Pb zircon age revealed Early Triassic (ca. 249 Ma) felsic magmatism in the SSZ of Turkey (Aysal et al., 2018). Here, we constrain the timing of Triassic magmatism using U-Pb LA-ICP-MS zircon geochronology of felsic magmatic bodies in the western part of the SSZ in Bulgaria.</p><p>A sample from a (meta) rhyolite body yielded a concordant age of 237.8 ± 3.4 Ma, which confirmed a crystallization likely concomitant with the deposition of the Triassic clastic rocks in the northern Maritsa river valley. To the east along the valley, a leucocratic granite body located south of the Permian Sakar batholith (ca. 295-296 Ma, Bonev et al., 2019), yielded a concordant age of 242.1 ± 1.8 Ma for the crystallization, having crosscutting relationships with the high-grade metamorphic basement. A leucocratic and K-feldspar porphyric meta-granite bodies yielded concordant ages of 243.3 ± 5.8 Ma and 240.6 ± 2.3 Ma, respectively, for the crystallization within the so-called Harmanli block to the south along the valley. At St. Iliya Heights a sample from the Prochorovo Formation (meta) rhyolite body yielded a concordant age of 245.4 ± 1.5 Ma for the crystallization, which implies an Early Triassic age of the clastic rocks with which it inter-fingers. In the area between the Maritsa river valley and the St. Iliya Heights at the village of Svetlina a leucocratic meta-granite body yielded a concordant age of 229.6 ± 2.4 Ma. The concordantly dated zircons that yielded Triassic ages of the igneous/meta-igneous protoliths all have Th/U ratios compatible with the magmatic process. The major elements of the dated samples reveal calc-alkaline to high-K-alkaline peraluminous felsic compositions similar to the adjacent Late Carboniferous-Permian igneous/meta-igneous rocks of the SSZ.</p><p>The U-Pb zircon ages reveal Early-Middle Triassic magmatic phase (ca. 245-230 Ma) in the western SSZ of Bulgaria. These age data provide a regional-scale temporal link for the Triassic magmatism extending to the easternmost extremity of the SSZ in Turkey. The Triassic continental type felsic magmatism in the western SSZ is interpreted to result from the ongoing Paleotethyan subduction under the Eurasian plate, which magmatism follows the development of a Late Carboniferous-Permian continental magmatic arc of the SSZ (Bonev et al., 2019).</p><p>References</p><p>Aysal, N., Şahin, S.Y., Güngör, Y., Peytcheva, I., Öngen, S., 2018. Journ. Asian Earth Sci., 164, 83-103.</p><p>Bonev, N., Filipov, P., Raicheva, R., Moritz, R., 2019. Int. Geol. Rev., 61, 1957-1979.</p><p>Chatalov, G., 1961. Compt. Rend. Acad. Bulg. Sci., 14, 503-506.</p><p>Acknowledgements: The study was supported by the NSF Bulgaria DN04/6 contract.</p>

REE and Y Mineralogy of the Krudum Granite Body (Saxothuringian Zone)

The Krudum granite body comprises highly fractionated granitic rocks ranging from medium-F biotite granites to high-F, high-P2O5 Li-mica granites. This unique assemblage is an ideal site to continue recent efforts in petrology to characterize the role of zircon, monazite and xenotime as hosts to REEs. The granitic rocks of the Krudum body analysed in this study were found to contain variable concentrations of monazite and zircon, while xenotime was only found in the high-F, high-P2O5 Li-mica granites and in the alkali-feldspar syenites of the Vysoký Kámen stock. For analysed monazites of all magmatic suites cheralite substitution was significant. The highest concentration of cheralite was found in monazites from the high-F, Li-mica granites and from the alkali-feldspar syenites. The proportion of YPO4 in all analysed xenotimes ranges from 71 to 84 mol. %. Some xenotimes were found to be hydrated and the observed water content estimated from analytical data ranged from 5 to 11 wt. % H2O. Analysed xenotimes were commonly enriched in HREEs (9.3– 19.5 wt. % HREE2O3) and thorite-coffinite and cheralite exchange was observed. Analysed zircons from granite suites of the Krudum granite body contained moderate Hf concentrations (1.0–4.7 wt. % HfO2; 0.010–0.047 apfu Hf). The highest concentrations of HfO2 were found in zircons from the high-F, high P2O5 Li-mica granites (1.2–4.7 wt. % HfO2) and from the alkali-feldspar syenites (1.3–4.1 wt. % HfO2). Zircons from the high-F, high-P2O5 Li-mica granites were often hydrated and fluorised. The concentrations of F in zircon from partly greisenised high-F, high-P2O5 Li-mica granites reached up to 1.2 wt. % (0.26 apfu F). In zircons from the alkali-feldspar syenites enrichment in P, which is not associated with a simultaneous enrichment in Y + REE, was also observed. Analysed zircons from the high-F, high P2O5 Li-mica granites were enriched in Y (up to 5.5 wt. % Y2O3; 0.10 apfu Y) and Sc (up to 1.17 wt. % Sc2O3; 0.03 apfu Sc).

Origin and Evolution of the Ore-Forming Fluids in the Liyuan Gold Deposit, Central North China Craton: Constraints from Fluid Inclusions and H-O-C Isotopic Compositions

The Liyuan gold deposit is hosted within Archean basement metamorphic rocks and controlled by the NNE-trending faults in the central North China Craton. The ore-forming processes can be divided into three stages (early, middle, and late). Three types of primary fluid inclusions (FIs) are identified in the Liyuan, including pure carbonic, carbonic-aqueous, and aqueous inclusions. The primary FIs of three stages are mainly homogenized at temperatures of 318–408°C, 201–329°C, and 136–229°C, with salinities of 2.1–8.9, 0.5–12.4, and 0.4–6.3 wt.% NaCl equivalent, respectively. The main Au mineralization is related to the middle stage, and water-rock interaction caused rapid precipitation of gold in this stage. The initial ore-forming fluids were likely magmatic water or metamorphic fluid and mixed with meteoric water at later stages. Due to the lack of granite body at the present mining levels, we speculate that it was magmatic water that might have been exsolved from a concealed granite body at greater depth or it was metamorphic fluid that was directly transported from depth via deep faults. Based on all the available geological and geochemical evidence, we suggest that the Liyuan deposit belongs to orogenic gold deposit that located in the interior North China Craton.