Abstract Early Cretaceous mafic rocks are first reported in the northern Guangxi region from the western Qin-Hang belt in the interior South China Block. A systematic investigation of zircon U–Pb dating, whole-rock geochemistry, Sm–Nd isotopes and zircon Hf–O isotopes for these mafic rocks reveals their petrogenesis and the mantle composition as well as a new window to reconstruct lithospheric evolution in interior South China Block during Late Mesozoic. Zircon U–Pb dating yielded ages of 131 ± 2 Ma to 136 ± 2 Ma for diabase and gabbro from Baotan area, indicating the first data for Early Cretaceous mafic magmatism in the western Qing-Hang belt. These mafic rocks show calc-alkaline compositions, arc-like trace element distribution patterns, low zircon ε Hf ( t ) of − 9.45 to − 6.17 and high δ 18 O values of + 5.72 to + 8.09‰, as well as low whole-rock ε Nd ( t ) values of − 14.27 to − 9.53. These data suggest that the studied mafic rocks are derived from an ancient lithospheric mantle source that was metasomatized during Neoproterozoic subduction. Thus, the occurrence of these mafic rocks indicates a reactivation of Neoproterozoic subducted materials during an extension setting at Late Mesozoic in the western Qin-Hang belt, an old suture zone that amalgamates the Yangtze and Cathaysia blocks.
Abstract Rare metals including Lithium (Li), Beryllium (Be), Rubidium (Rb), Cesium (Cs), Zirconium (Zr), Hafnium (Hf), Niobium (Nb), Tantalum (Ta), Tungsten (W) and Tin (Sn) are important critical mineral resources. In China, rare metal mineral deposits are spatially distributed mainly in the Altay and Southern Great Xingán Range regions in the Central Asian orogenic belt; in the Middle Qilian, South Qinling and East Qinling mountains regions in the Qilian–Qinling–Dabie orogenic belt; in the Western Sichuan and Bailongshan–Dahongliutan regions in the Kunlun–Songpan–Garze orogenic belt, and in the Northeastern Jiangxi, Northwestern Jiangxi, and Southern Hunan regions in South China. Major ore‐forming epochs include Indosinian (mostly 200–240 Ma, in particular in western China) and the Yanshanian (mostly 120–160 Ma, in particular in South China). In addition, Bayan Obo, Inner Mongolia, northeastern China, with a complex formation history, hosts the largest REE and Nb deposits in China. There are six major rare metal mineral deposit types in China: Highly fractionated granite; Pegmatite; Alkaline granite; Carbonatite and alkaline rock; Volcanic; and Hydrothermal types. Two further types, namely the Leptynite type and Breccia pipe type, have recently been discovered in China, and are represented by the Yushishan Nb–Ta– (Zr–Hf–REE) and the Weilasituo Li–Rb–Sn–W–Zn–Pb deposits. Several most important controlling factors for rare metal mineral deposits are discussed, including geochemical behaviors and sources of the rare metals, highly evolved magmatic fractionation, and structural controls such as the metamorphic core complex setting, with a revised conceptual model for the latter.
Skarn iron deposits, as representative examples of Co-rich magmatic-hydrothermal deposits, are attracting increasing attention due to rising cobalt demand worldwide. However, the specific Co enrichment mechanisms and metallogenic processes in skarn deposits remain elusive. This study presents high-precision in situ U–Pb geochronological data for garnet from skarns, major and trace element analyses of sulfarsenides and sulfides, and X-ray mapping in the Galinge deposit. The Galinge skarn iron deposit formed at 237.1 ± 0.3 Ma (garnet U–Pb dating), during which time the region was in the post-collisional stage. During this period, the crust was thickened, causing delamination of the lithospheric mantle, which further led to the upwelling of asthenospheric materials and partial melting of the lower crust. As the resultant mixed magma ascended, it reacted with carbonate strata to form skarn deposits. Cobalt shows two occurrence modes in the Galinge deposit: independent cobalt minerals such as skutterudite and cobaltite, and isomorphic substitution of Co with other metals in ore minerals (arsenopyrite, pyrrhotite, sphalerite, pyrite, magnetite, and chalcopyrite). Our results revealed that arsenopyrite is the most cobalt-enriched ore mineral in the Galinge deposit, with an average Co content of 34,077 ppm. Other minerals generally contain insignificant Co contents of less than 500 ppm, including sphalerite (471 ppm) > pyrite (194 ppm) > pyrrhotite (145 ppm) > alabandite (∼100 ppm) > magnetite (7 ppm) > chalcopyrite (2 ppm). In arsenopyrite, cobalt and nickel replace iron in accordance with the inverse correlation between the concentrations of cobalt and nickel (wt%) and that of iron. In pyrite and chalcopyrite, a portion of the isomorphic cobalt substitutes for Fe or Cu. The weak correlation between Co (ppm) and Cu or Fe (wt%) indicates that only isomorphic cobalt is carried out by substituting Fe or Cu. The negative correlation between Co + Fe and Zn or Mn suggests that cobalt and iron replace Zn or Mn in sphalerite and alabandite. No evidence of element substitution was observed in pyrrhotite. Our study highlights that during the early mineralization stage, cobalt in the magmatic-hydrothermal fluids migrated in the form of CoCl42-. Subsequently, under the influence of an increase in pH, CoCl42- reacts with H3AsO30 to form CoAs3 (skutterudite). With the continuous precipitation of arsenides and sulfarsenides, the As/S (reduced) ratio decreased, leading to the CoAs3 (skutterudite) changing to CoAsS (cobaltite).
Accurate determination for most major and trace elements in mica with 5% relative deviation and relative standard deviation is achieved by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) without an internal standard.
Highly evolved granitic melts typically experience late-stage melt-melt and fluid-melt immiscibility as well as fluid-melt and fluid-rock interaction. These processes are particularly important in the formation of deposits of the rare metals Nb, Ta, W, and Sn. We document the relation between immiscibility and alteration processes and the partitioning behavior of rare metals for the Zhaojinggou rare-metal deposit of northern China. This deposit shows a systematic change from a magmatic to a hydrothermal system, including the reaction of the exsolved fluid with earlier crystallized granite and the formation of late-stage quartz veins. The magmatic stage (Stage I) includes biotite alkali-feldspar granite (BAG) with moderate Nb-Ta mineralization. Extreme fractional crystallization of BAG eventually resulted in melt-melt immiscibility and the separation of a hydrosaline melt. Fractional crystallization of this hydrosaline albite granite (AG) melt finally exsolved a magmatic fluid. Therefore, the magmatic-hydrothermal transition (Stage II) includes a melt-dominated Stage IIa with strong Nb-Ta-Sn mineralization in AG and a fluid-dominated Stage IIb with minor Nb-Ta-Sn mineralization in muscovite and biotite greisen. Late hydrothermal processes (Stage III) formed quartz veins with important W mineralization. There are several texturally and chemically distinct generations of cassiterite and columbite-group minerals (CGM) in BAG and AG reflecting crystallization from an evolving magma. The porous and patchy-zoned reaction rims of tantalite-(Mn) and wodginite on CGM in AG are the result of fluid-melt interaction. Texture and compositions show that wolframite in AG is hydrothermal and formed through interaction of early exsolved magmatic fluids with the host granite. CGM and cassiterite in the biotite greisen and Ta-rutile in the muscovite greisen, as well as wolframite and scheelite in quartz veins that formed when fluid-rock interaction reduced the availability of H+ or F− or the temperature of the fluid decreased. The distribution and importance of mineralization demonstrate that Nb, Ta, W, and Sn strongly partitioned into the hydrosaline melt during melt-melt immiscibility and that W partitioned into the magmatic fluid during fluid-melt immiscibility. Exsolved magmatic fluids interacted with earlier crystallized rocks mobilizing rare (Nb, Ta, and Sn) and base (Fe and Ti) metals from Li-Fe mica, providing the ore elements for subordinate Nb-Ta-Sn mineralization in AG and in biotite and muscovite greisen. Thus, magmatic processes (with later metal redistribution by magmatic fluids) dominantly control economic Nb-Ta-Sn mineralization, whereas hydrothermal processes mainly control the formation of economic W mineralization.
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