Unveiling the connection between lode gold mineralization and the metamorphic core complex evolution from the large Yangzhaiyu gold deposit, North China Craton
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Abstract The Xiaoqinling gold province, located in the Neoarchean–Paleoproterozoic uplifted footwalls of the Xiaoqinling metamorphic core complex (XMCC), is one of China’s largest gold producers; however, achieving a consensus regarding their metallogenic model remains elusive. Scheelite is an indicator mineral that commonly occurs in lode gold deposits worldwide used to recognize deposit types and understand hydrothermal evolution and the origin of features. Xenotime, monazite, and rutile are common hydrothermal minerals in association with lode gold deposits worldwide. Here, we provide textual, in situ U-Pb geochronology of xenotime, monazite, and rutile, and in situ elemental and Sr-Nd isotopic compositions of scheelite within different stages from the large Yangzhaiyu lode gold deposit, aiming to elucidate its genesis and, for the first time, establish a holistic correlation between the lode gold mineralization and the evolution of the XMCC. Notably low εNd(t) values (−30.7 to −23.7), high 87Sr/86Sr ratios (0.72659–0.75914), and distinct rare earth elements, Sr, Mo, and As contents of scheelite confirm a metamorphic crustal source. Xenotime U-Pb dating and pre-ore (Stage I) scheelite reveal that ore-barren metamorphic fluids at ca. 140 Ma were oxidized with low Bi contents and buffered by greenschist facies metamorphism when the XMCC initiated. Monazite and rutile U-Pb dating combined with ore-stage scheelite geochemistry indicate a compositional shift in the more reduced auriferous metamorphic fluids, which dominated during major gold deposition periods (stages II and III) from 130 Ma to 120 Ma, characterized by significantly depleted Na and increased Bi contents. This resulted from the prograde greenschist-to-amphibolite metamorphism at mid-lower crustal depths as the result of the XMCC isostatic doming and the lithospheric mantle thinning after 130 Ma. This study highlights the crucial role of metamorphic core complexes in governing the timing, locations, and resources of the lode gold metallogenic system.Keywords:
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Metamorphic core complex
Retrogressive deformation and metamorphism are often reported from the main low-angle shear zones and detachments of metamorphic core complexes, but their importance is not sufficiently emphasized for the footwall interior. In order to contribute to a better understanding of exhumation-related retrogression processes within and at the top of metamorphic core complexes, an integrated detailed microstructural, textural, 40Ar/39Ar geochronological, and thermobarometric study on the Naxos metamorphic core complex within the Aegean Sea is presented that provides a new perspective on low-grade retrogression during exhumation through shallow ductile levels. We found variable retrogressive deformation within the Naxos metamorphic core complex, which even pervasively affected significant portions of the migmatite-grade metamorphic core and remnant high-pressure areas of the metamorphic core complex, where retrogression led to pervasive formation of new fabrics within greenschist-facies metamorphic conditions during brittle-ductile transition. Within a continuum of retrogression, 40Ar/39Ar white mica dating allowed us to deduce three retrogressive ages at 16.52 ± 0.39 Ma (within the Naxos metamorphic core complex), 12.6 ± 0.28 Ma (Moutsounas detachment shear zone on the eastern boundary of the metamorphic core complex), and 10.43 ± 0.44 Ma to 8.40 ± 0.76 Ma (last ductile activity along the Naxos-Paros shear zone to the north of the metamorphic core complex). A further stage of retrogression at 12–11 Ma occurred along distinct low-angle normal faults within the middle Miocene Naxos Granite. Retrogressive microstructures, low-temperature calcite fabrics in marbles, and chloritization in metapelites (at temperatures of ∼350–130 °C) in the metamorphic core complex core resulted mainly from late-stage E-W shortening and folding. Late-stage flow of hydrous fluids resulted in resetting of fabrics and enhancement of ductile deformation. The middle–late Miocene retrogression events are also reflected by a similarly aged tectonic collapse basin in the hanging-wall unit above the detachment. The wide temporal range of retrogression within the Naxos metamorphic core complex coincides in age with retrogressive deformation within other metamorphic core complexes of the Aegean Sea. We interpret the long temporal range of retrogression to reflect outward, southwestward retreat of the subduction and sequential activation of major detachment zones.
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Abstract The Danubian window, characterized by diagenetic to low greenschist facies conditions at a high thermal gradient, is evidently of great interest for methodological studies, because high metamorphic thermal gradient conditions during low grade metamorphism have received little attention so far. The general increase in metamorphic grade from SW to NE in the Danubian window is indicated by mineral Parageneses studies, as well as by illite Kübler index (KI) measurements and organic matter reflectance (OMR). For the first time, this study distinguishes between metamorphic conditions related to Jurassic ocean floor, Cretaceous nappe stacking, post-collisional accommodation and syn-kinematic Getic detachment metamorphism and cooling after Oligocene exhumation. The occurrence of the prehnite–pumpellyite facies in the Severin–Cosustea units in the southeastern area is the result of Cretaceous metamorphism. Remnants of ocean floor metamorphism prevailed. The highest pressure is constrained by the upper stability limit of prehnite to be at around 4.0 kbar. The Danubian units situated within the diagenetic zone were not below 200 °C, due to epidote formation. The KI, OMR and mineral data, indicate diagenetic conditions. Assuming temperatures between >200 and <250 °C, pressures between 1.8 and 2.6 kbar were calculated using kinetic and numerical maturity models. Orogenic collisional Cretaceous peak pressure conditions of 4.0±1.0 kbar are found in the Danubian nappes not altered by a subsequent syn-detachment metamorphic overprint. Highest temperatures in chloritoid schists and epidote–hornblende-bearing mylonites have been inferred for samples from the northern border of the Danubian window (between >300 and <400 °C). Along a syn- to post-detachment retrograde pressure path, post-dating the chloritoid formation, the occurrence of clinozoisite+chlorite+quartz suggests temperatures >300 °C in the northwest, while the association andalusite+quartz and biotite+muscovite indicates temperatures between 370 and 400 °C at <3.5 kbar in the northeast. It is demonstrated that the slope of the regression lines between KI and OMR data gives valuable qualitative information about the relative magnitudes of P and T: the slope of the regression line for the Danubian window samples indicates normal heat flow conditions during nappe stacking and hyperthermal conditions during the formation of the Getic detachment. High thermal gradient conditions can easily be explained by partly isothermal decompression during the Getic detachment event, the elevation of the geotherm being caused by crustal thinning and rapid exhumation of the Danubian units. Probably, also a higher heat-flux prevailed at the end of the Getic detachment, at a time when the retrograde chloritoid decomposition reactions took place, documenting late-stage HT greenschist facies metamorphism.
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The Yangzhaiyu gold deposit is one of numerous lode gold deposits in the Xiaoqinling district, southern margin of the North China Craton. Gold mineralization is hosted in Neoarchean to early Paleoproterozoic amphibolite facies metamorphic rocks and consists of auriferous quartz veins and subordinate disseminated ores in the vein-proximal alteration zone. Ore-related hydrothermal alteration is dominated by sericite + quartz + sulfide assemblages close to gold veins, and biotite + quartz + pyrite ± chlorite ± epidote alteration generally distal from mineralization. Pyrite is the predominant sulfide mineral, locally coexisting with minor amounts of chalcopyrite, sphalerite, and galena. Gold occurs mostly as free gold enclosed in or filling microfractures of pyrite and quartz and is also present in equilibrium with Au-bearing tellurides, mainly petzite and calaverite coexisting with hessite, tellurobismuthite, and altaite.
Fluid inclusion studies suggest that gold veins were deposited at intermediate temperatures (175°–313°C) from aqueous or aqueous-carbonic fluids with moderate salinity (5–14 wt % NaCl equiv). δ 34S values of sulfide minerals range mainly from 2.0 to 4.4‰, whereas auriferous quartz vein samples have δ 18O values of 12.4 to 9.6‰, with calculated δ 18OH2O values of 6.0 to 3.2‰. Gold-related pyrite grains yield elevated 3He/4He ratios (1.51-0.32 Ra) relative to crustal reservoirs and mantle-like 20Ne/22Ne and 21Ne/22Ne ratios (9.90-9.68 and 0.029, respectively). The stable and noble gas isotopes thus suggest deep-seated, most likely magmatic and mantle-derived, sources for the ore fluids, sulfur and, by inference, other components in the ore system.
40Ar/39Ar dating of ore-related sericite and biotite separates indicates two episodes of gold genesis at 134.5 to 132.3 and 124.3 to 123.7 Ma. The mineralization ages overlap zircon U-Pb ages of 141.0 ± 1.6 to 125.8 ± 1.4 Ma (2 σ ) for the Wenyu and Niangniangshan monzogranite Plutons and a number of mafic to intermediate dikes intruding these Plutons, all being proximal to the Yangzhaiyu gold deposit. The synchronism of gold genesis and magmatism provides additional weights of evidence for a magmatic derivation of ore fluids and sulfur. The geochronologic data also suggest that gold veining took place billions of years after the stabilization of the North China Craton and associated metamorphism in the Late Archean to early Paleoproterozoic. This contrasts sharply to lode gold deposits in other Precambrian cratons that formed predominantly in Late Archean to Paleoproterozoic, temporarily and genetically related to regional high-grade metamorphism and compressional or transpressional tectonism.
Available data have demonstrated that the North China Craton was reactivated in the late Mesozoic, as marked by voluminous igneous rocks, faulted-basin formation, high crustal heat flow, and widespread metamorphic core complexes in the eastern part of the craton. It is thus suggested that the Yangzhaiyu gold deposit, together with other deposits of similar ages in the Xiaoqinling district, were products of this craton reactivation event. Lithospheric extension and extensive magmatism related to the craton reactivation may have provided sufficient heat energy, fluid, and sulfur required for the formation of the gold deposits.
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Abstract The Charleston Metamorphic Group in the central Paparoa Range, southwest Nelson, New Zealand, comprises metasedimentary and granitoid lithologies that have been metamorphosed to sillimanite‐almandine conditions. Mineral assemblages and thermobarometric calculations indicate metamorphic temperatures and pressures of c. 600 (±50)°C and 4 (±1) kbar, respectively. A K‐feldspar‐sillimanite isograd mapped in pelitic gneisses indicates that metamorphic grade increases to the southwest, though this is not verified by thermobarometry results, which vary little throughout the entire range. A strong foliation is present in all lithologies. This foliation is folded but typically strikes between north and northeast. The gneisses are intruded by several postmetamorphic granitoids, which in places retain their primary magmatic foliation (commonly striking between north and west), are unfolded and unmetamorphosed. These granitoids are distinct from the orthogneisses within Charleston Metamorphic Group. Together, the granitoids and gneisses comprise the Paparoa Metamorphic Core Complex, which was uplifted and unroofed in the mid Cretaceous, when estimated geothermal gradients were 50–90°C/km and uplift rates were 0.6–1.0 mm/a. Keywords: Paparoa RangePaparoa Metamorphic Core ComplexCharleston Metamorphic GroupKaramea SuiteRahu Suitethermobarometrymetamorphismdeformation
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Through this work and comprehensive research on a large amount of previous data, four metamorphic core complexes were identified in the south of Liaodong peninsula, which were respectively Jinzhou metamorphic core complex, Wanfu metamorphic core complex, Xinfang metamorphic core complex and Lizifang metamorphic core complex. Xinfang metamorphic core complex and Lizifang metamorphic core complex were discovered and proposed for the first time. They have kinematic unity, geometric asymmetry, tectonic magma joint activity and the difference in formation time.
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