Abstract Epizonal orogenic Au‐Sb deposits are generally Phanerozoic in age, possibly as a consequence of erosion that can entirely remove older mineral deposits in rapidly uplifting orogenic setting. Quantifying post‐mineralization thermotectonic processes is essential for documenting the exhumation and preservation of epizonal deposits, which in turn is critical for constraining regional deformation. This study focuses on the giant Zaozigou deposit, and documents cooling rates using amphibole and biotite Ar‐Ar, zircon U‐Th/He, and apatite fission track dating. Six cooling phases are identified, including Early to Middle Triassic very rapid cooling, Late Triassic rapid cooling, Early Jurassic slow cooling, Middle to Late Jurassic rapid cooling, Cretaceous to Oligocene slow cooling, and Miocene to present rapid cooling. Initial cooling corresponds to thermal exchange between magmatic rocks and wall rocks. Phases two through five are related to a sequence of post‐mineralization compressional, strike‐slip, compressional, and extensional events, pointing to multi‐phase tectonic evolution of the Qinling Orogen since the Late Triassic. Late exhumation is probably related to lateral growth of the Tibetan Plateau and/or to rapid erosion induced by intensification of the Asian monsoon. In total, ∼6.3 km of rock was removed post‐mineralization. We conclude that Cretaceous to Oligocene extension retarded the continuous erosion and thus played an important role in the preservation of Zaozigou. Combined with known orogenic processes, our results demonstrate that a long‐lived extension is a favorable tectonic environment for the preservation of epizonal orogenic Au‐Sb deposits in rapidly uplifting orogenic setting.
The metamorphic model explaining orogenic gold ore formation has become widely accepted. However, there has been extensive debate regarding whether a magmatic-hydrothermal system contributes fluids or metals in the source of orogenic gold deposits. The Yidinan gold deposit is hosted by Triassic quartz diorite in the West Qinling Orogen, China, which is controlled by NNE-trending high-angle brittle-ductile faults. The gold mineralization is characterized by vein and disseminated type ores comprising auriferous pyrite and arsenopyrite. Magmatic apatite U-Pb and magmatic biotite Ar-Ar dating pinpoint the emplacement and the cooling of ore-hosting quartz diorite at 241.8 ± 2.8 Ma and 241.7 ± 0.32 Ma, respectively. In situ U-Pb dating of hydrothermal monazite yield an age of 234.6 ± 2.8 Ma for the gold mineralization. Systematic fluid inclusion investigation suggests that the ore-forming fluids belong to a NaCl-H2O-CO2 ± CH4 system with low salinity (5.76−10.09 wt% NaCl equiv.) and medium temperatures (253−395 °C). During fluid evolution, phase separation occurred, with CO2 and other gases preferentially fractionating into the vapor phase. The sulfur isotope data range from 5.50‰ to 7.85‰ and are higher than those from the nearby magmatic-hydrothermal deposits. Such results support that the gold-bearing fluids were sourced from devolatilization of underlying sedimentary rocks during regional metamorphism. Fluid immiscibility caused by fault-valve processes might be the critical mechanism for the gold deposition. Although the geological and geochronological evidence suggested gold mineralization was spatially and temporally associated with the quartz diorite, the ore-forming fluids are not consistent with a magmatic source; therefore, the Yidinan gold deposit is of an orogenic type. This study reveals that despite orogenic gold mineralization and magmatic activities showing a broad temporal or spatial overlap during orogenesis processes, there is no genetic link between gold mineralization and granitic magmatism in many hydrothermal gold deposits. The low-salinity auriferous metamorphic fluid was released from underlying metasedimentary sequences during orogenesis. The rapid cooling of the granitoid after emplacement further prevented it from contributing to gold-bearing fluid formation or creating the necessary pressure-temperature conditions for gold deposition.
Phanerozoic orogenic gold mineralization at craton margins is related to the metasomatism of the lithospheric mantle by crustal material. Slab subduction transfers Au from the crust to the metasomatized mantle and oxidizes the latter to facilitate the mobilization of Au into mantle melts. The role of volatiles in the mobilization of Au in the mantle is unclear because of the absence of direct geochemical evidence relating the mantle source of Au to Au mineralization in the overlying crust. This study uses lithium isotopes from a large suite of lamprophyres to characterize the mantle beneath the eastern North China Craton, which hosts giant Mesozoic gold deposits. Our results indicate a strong genetic link between carbonate metasomatism in the mantle and Au mineralization in the overlying crust. Although pre-enrichment of Au in the mantle is critical for forming giant Au provinces, the oxidation of the lithospheric mantle controls the mobilization of Au.
Thorium deposit related to alkaline/peralkaline magmatism is the dominant thorium source. Its ore-forming thorium mechanism has long been fascinating. The Younusisayi Th deposit, hosted in early Paleozoic alkali-feldspar granite in the South Altyn Complex, NW China, is an example to explore the Th mineralization processes in magmatic-hydrothermal systems. Zircon U-Pb dating shows that the ore-hosting alkali-feldspar granites are emplaced at 445.7–439.0 ± 1.3 Ma. The apatite that occurs concomitantly with thorite in the mineralized alkali-feldspar granites formed at are 442.0–438.0 ± 14 Ma, indicating Th mineralization temporally related to the magmatism. Four populations of fluorite exhibit distinct occurrences, the fluorite Ia and Ib are in intergrowth with thorite, but the fluorite IIa and IIb are not. Notably, four populations of fluorite are differentiated by their Th + U and HREE concentrations, fluorite Ia and Ib show higher Th + U contents and lower HREE contents than fluorite IIa and IIb. Th was extracted by magma during emplacement. As a strong incompatible element, thorium will not be enriched in rock-forming minerals. In the process of magmatic evolution, thorium is gradually enriched in the post-magmatic fluids which contains abundant volatile components. Thorium can be transported as F-Th complexes in F-rich fluids. As temperature decreased, thorium precipitated in the form of thorite, and fluorite Ia and Ib crystallized with a low HREE content. Fluorine from the decomposition of F-Th complexes, reacts with HREE in the fluids to form F-HREE complexes, which increases the HREE content in the fluid. Therefore, the fluorite IIa and IIb which formed in the late stage of mineralization have obviously higher contents of HREE. We propose that the Younusisayi deposit, with an average grade of 0.4 wt% ThO2, formed in a post-collisional extensional environment during the early Paleozoic. The Younusisayi deposit is best classified as a magmatic-hydrothermal deposit. Our results demonstrate the usefulness of geochemistry of fluorite for tracing the ore-forming process, thus establishing the ore formation model.
Abstract Orogenic gold deposits are generally thought to represent one perhaps protracted event. However, recent research on orogenic gold deposits increasingly offers evidence for some deposits forming through multiple and clearly discreet hydrothermal episodes. The giant Zaozigou orogenic Au-Sb deposit in the Triassic to Cretaceous West Qinling Orogen, central China, includes both steeply dipping and gently dipping orebodies. The two distinct mineralization styles provide a valuable setting for investigating a multiple mineralization model by integrating structural analysis within a robust geochronological framework. Through fieldwork and geochronology, we define a progression of major tectonic events in the area of the Zaozigou deposit. The deposit is hosted within a well-bedded sequence of Early Triassic metasedimentary rocks of the South Qinling Terrane. Pre-mineralization E-W shortening (D1) during subduction of the Mianlue oceanic slab include folding with resulting axial planes striking N-S, emplacement of Triassic ENE-striking and WNW-striking dacite dikes accompanied by Middle Triassic greenschist facies metamorphism. Late Triassic gold-stibnite quartz vein and disseminated mineralization formed along ENE-striking and steeply dipping D2 brittle to ductile sinistral faults. Their orientations suggest a link to the regional NNE-SSW maximum principal stress coinciding with transpression caused by the Late Triassic collision between the South China Block and South Qinling Terrane. Overprinting Early Cretaceous quartz-stibnite veins developed along gently dipping (20° to 40°) brittle D3 normal fault zones, which exhibit a NE-SW minimum principal stress. This younger deformation event is interpreted to be related to the Early Cretaceous tectonic transition from shortening to extension of the West Qinling Orogen. Therefore, the Zaozigou deposit reveals a model of multiple orogenic gold mineralizing events, with migration of hydrothermal fluids during discrete deformation episodes and the resulting formation of a single composite deposit formed along overprinting structures at separate times of orogenesis.
Abstract. We develop a new data assimilative (DA) approach by combining two parallel frameworks: a parallel DA framework (PDAF) and a flexible model coupling framework (ESMF). The new DA system is built on the ESMF at the top level that drives the PDAF and any combination of earth system modeling (ESM) components, to allow maximum flexibility and easy implementation of data assimilation for fully coupled ESM applications. We demonstrate the new DA system using a 3D unstructured-grid ocean model as ESM in this paper. The new system is validated using a simple benchmark and applied to a realistic case of Kuroshio simulation around Taiwan. The new system is demonstrated to significantly improve the model skill for temperature, velocity and surface elevation before, during and after typhoon events. The flexibility and ease of implementation make the new system widely applicable for other coupled ESMs.
Abstract This study presents a petrogeochemical and boron isotope study on tourmaline from the barren Damai, and the contemporaneous but ore-bearing Dewulu and Meiwu intrusions, to better understand the origins, sources, and fluid evolution of magmatic-hydrothermal ore systems and provide ore formation implications for gold, copper, and iron deposits in the Xiahe-Hezuo polymetallic district in the West Qinling, China. Tourmaline from all three intrusions shows similar compositions and encompasses Na-Fe schorl and Na-Mg dravite. Tourmaline at Dewulu is primarily found in tuffaceous breccias and a quartz diorite porphyry. In the tuffaceous breccia body, tourmaline occurs as fine-grained anhedral masses that fill voids and cement fragments of breccia and sickle quartz. Tourmalines in breccia are texturally similar to those formed in typical breccia pipes, which are attributed to explosion or collapse induced by a transition from magmatic to hydrothermal Si- and B-rich fluids. They display element substitutions controlled by Fe2+Mg–1, indicating a reduced environment. Values of δ11B are –6.6 to –4.0‰, representing the primary magmatic-hydrothermal fluids. Tourmaline from the Dewulu quartz diorite porphyry is coarse-grained, euhedral, and found in quartz-sulfide veins. The tourmaline displays oscillatory zoning textures but lacks correlative variations of major elements. The Fe2+Mg–1 and Fe3+Al–1 substitution mechanisms are dominant, demonstrating more oxidized fluids. The δ11B values in the cores, ranging from –7.1 to –5.6‰, suggest that the tourmalines in the quartz veins were inherited from magmatic-hydrothermal fluids that precipitated the fine-grained tourmaline in the tuffaceous breccia body. A large δ11B isotopic fractionation that decreases from cores (–5.6‰) to rims (–10.7‰) indicates significant fractionation during degassing occurred, increasing oxygen fugacity of the residual liquid. The Meiwu locality hosts fine-grained euhedral tourmalines coexisting with magnetite. Their composition is controlled by substitution between Al3+ by Fe3+ and has the lightest δ11B values ranging from –11.4 to –10.0‰. They are interpreted to result from skarn formation under oxidized conditions. In contrast, X☐Al(NaMg)–1 is the dominant substitution mechanism for Damai tourmalines and attributed to (geochemically) reduced fluids with a low salinity. We conclude that tourmalines with low Fe values, substitution mechanisms dominated by Fe3+Al–1, and large shifts of B isotopic composition are potentially an ore-forming indicator in the Xiahe-Hezuo polymetallic district.
Abstract This study presents a low‐temperature thermochronological data set from the Jiaodong Peninsula in eastern China, including new zircon U‐Th/He and apatite fission‐track data from the Sulu Terrane in the eastern side of the Jiaodong Peninsula and published data from the Jiaobei Uplift to the west. These data reveal differential tectonic evolution between the Jiaobei Uplift and the Sulu Terrane, which is responsible for their heterogeneous gold endowment. Thermal history inverse modeling and structural analysis indicate three broad periods of cooling in the Sulu Terrane. These include: (a) slow cooling from 114 to 50 Ma; (b) rapid cooling from 50 to 40 Ma; and (c) tectonic stability after 40 Ma with little cooling. This thermotectonic evolution pattern shows that the local tectonic evolution of Sulu Terrane and Jiaobei Uplift are relatively independent, although they share identical far‐field geodynamic controls. The critical characteristic is the Early Cretaceous extension within the Sulu Terrane was much weaker than that in the Jiaobei Uplift that underwent a rapid period of uplift in the Aptian. The differential tectonic evolution of the two areas induced variation in structural architecture and consequential differences in degree of extension that are important factors in defining the heterogeneous gold endowment rather than simply suggested differences in ore‐forming fluid chemistry, metal precipitation mechanism, and/or exhumation levels. The intense extension in the Jiaobei Uplift was conducive to voluminous fluid migration and gold precipitation. In contrast, the much weaker extensional setting in the Sulu Terrane was not conducive to large‐scale fluid migration from deep.
The Western Qinling has been acknowledged to witness superimposed orogeny including north subduction of Paleotethys ocean and collision between North China and South China blocks; however, the precise timing constraints on transition of tectonic regime are remaining enigmatic. The Wenquan composite batholith comprising five phases and mafic enclaves is an ideal example to unlock this puzzle. The host granitoids are felsic, metaluminous to peraluminous, and high-K calc-alkaline to shoshonitic suite with I-type affinity. The mafic enclaves, however, are intermediate, and high-K calc-alkaline to shoshonitic. Zircon ages of multiple phases indicate an episodic growth lasting nearly 30 million years ranging from 238, 228, 218 to 208 Ma, consistent to Triassic igneous activity recording a transition regime from a subduction setting to a syn-collision setting and a post-collision setting in Western Qinling. Lead isotopes of whole-rock and K-feldspar at Wenquan and Lu-Hf isotopes of zircons separated from biotite monzogranite porphyry, porphyritic monzogranite, monzogranite porphyry, and hosted mafic enclaves suggest that the heat and the hot mafic melt initiated by the break-off of the northward subducting South China block lithosphere triggered partial melting of the Mesoproterozoic subcontinental lithospheric mantle to produce mafic magmas, and the underplated mafic magmas caused partial melting of the shallow subducted Mesoproterozoic lower crust generating granitic magmas at Wenquan. Combined our field observations and petrology study with a holistic review on previous geochronological and geochemical data of Triassic granitoids throughout the Western Qinling, we in this contribution proposed that the Triassic igneous activity in the Western Qinling corresponding to superimposed orogeny evolved from the northward subduction of Palaeotethys ocean (250–235 Ma) through syn-collision (228–215 Ma) to post-collision (215–185 Ma) between the North China and South China blocks.
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