Skarn deposits are significant sources of copper, yet there are different understandings on their fluid evolution and precipitation mechanism due to lack of direct constraints on fluid compositions of mineralization process. Here, we present a detailed reconstruction of fluid evolution history from the pre-ore prograde stage through the syn-ore retrograde stage to the post-ore stage of the Chuankeng deposit in the North Wuyi area, South China, based on fluid inclusion microthermometry and LA-ICP-MS analysis of typical minerals from three stages. The pre-ore fluids trapped in pyroxene have higher homogenization temperatures (450–550 ℃), higher salinities (31.9–51.5 wt% NaCl equiv), but lower concentrations of Cu (∼556 ppm) than syn-ore fluids (338–465 ℃, 31.3–38.9 wt% NaCl equiv, ∼3247 ppm Cu). The post-ore fluids show lower temperatures (139-163℃), salinities (1.1–6.5 wt% NaCl equiv), and low concentrations of Cu (<387 ppm). Fluids of all paragenetic stages have high K/Na and Rb/Na ratios, exhibiting signatures of magmatic hydrothermal fluids. Mixing of the magmatic fluids with meteoric waters of lower temperature and salinity occurred at the final post-ore stage, thus it is not related to Cu mineralization. As the temperature descends from 445 ℃ to 340 ℃, the Cu/(Na + K) ratios of syn-ore fluids diminish exponentially, manifesting fluid cooling within a certain temperature may be an important factor controlling Cu mineralization. Concomitantly, a moderate negative correlation between the Ca/(Na + K) ratios and Cu/(Na + K) ratios of syn-ore fluids suggests that fluid-rock interaction do have some impact on copper contents. Additionally, the Cu/(Na + K) ratios of syn-ore fluids in quartz are an order of magnitude higher than those of pre-ore fluids, and it is proposed that during the initial retrograde stage—the onset of mineralization—the magma chamber released fluid pulse with higher copper contents. Based on these findings, the fluids of pre-ore prograde stage and syn-ore retrograde stage are distinct magmatic fluid pulses, and copper-rich fluids, thermal cooling within a specific temperature range and fluid-rock reaction are suggested to be key factors controlling metal enrichment in skarn copper systems.
The Jiangjunshan and Dakalasu alkali‐feldspar granites are located in the central part of the Chinese Altay orogen. In this paper, we present detailed geochemical, zircon U–Pb, and Hf isotopic data of these granites. The Jiangjunshan and Dakalasu alkali‐feldspar granites show a high content of SiO 2 (72.05–73.27 and 69.55–71.04 wt%, respectively), total alkalis (Na 2 O + K 2 O = 8.41–8.71 and 7.24–8.66 wt%, respectively), and high‐field strength elements (Zr + Nb + Ce + Y = 400.3–482.9 and 156.7–339.3 ppm, respectively), as well as high Ga/Al ratios (10,000 × Ga/Al = 3.46–4.19 and 2.62–3.28, respectively) and depletion in Ba, Nb, Sr, and Ti, showing geochemical characteristics similar to those of A‐type granites. Zircon U–Pb dating of the Jiangjunshan and Dakalasu alkali‐feldspar granites yielded weighted mean dating 206 Pb/ 238 U ages of 268.3 ± 1.9 and 270.4 ± 1.9 Ma, respectively, indicating that these granites intruded during the Permian. The Jiangjunshan and Dakalasu alkali‐feldspar granites show highly variable zircon ε Hf ( t ) values ranging from −7.0 to +5.6, implying that these granites originated from a mixing of mantle‐derived magma with crustal materials. Our data on the Jiangjunshan and Dakalasu alkali‐feldspar granites, coupled with previous studies of Permian magmatism and metamorphism, suggest that the tectonic regime was in a postcollisional extensional environment in the Chinese Altay orogen during the Permian. Therefore, the change in stress from compression to extension and asthenospheric upwelling triggered by slab break‐off plays a significant role in the generation of Jiangjunshan and Dakalasu alkali‐feldspar granites.
Abstract In this paper, zircon U–Pb geochronology, major and trace elements, and Sr–Nd isotope geochemistry of the Baiyanghe dolerites in northern West Junggar of NW China are presented. The U–Pb dating of zircons from the dolerites yielded ages of 272.2±4 Ma and 276.7±6.2 Ma, which indicate the emplacement times. The dolerites are characterized by minor variations in SiO 2 (46.89 to 49.07 wt%), high contents of Al 2 O 3 (13.60 to 13.92 wt%) and total Fe 2 O 3 (11.14 to 11.70 wt%), and low contents of MgO (2.67 to 3.64 wt%) and total alkalis (Na 2 O+K 2 O, 5.1 to 5.97 wt%, K 2 O/Na 2 O = 0.37–0.94), which indicate affinities to metaluminous tholeiite basalt. The REE pattern ((La/Sm) N = 2.25–2.34, (La/Yb) N = 7.42–8.36), V–Ti/1000 and 50*Zr–Ti/50–Sm discrimination diagrams show that these rocks are OIB-type. The high contents of Zr and Ti indicate a within-plate tectonic setting, and samples plot in the ‘plume source’ field shown on the Dy/Yb (N) versus Ce/Yb (N) diagram. The positive εNd(t) values (+7.09 to +7.48), high initial 87 Sr/ 86 Sr ratios (0.70442 to 0.70682) and depletions of Nb and Ta elements in the samples can be explained by the involvement of subducted sediments. In summary, it is possible that the Baiyanghe dolerites were derived from an OIB-like mantle source and associated with a mantle plume tectonic setting. Therefore, our samples provide the youngest evidence for the existence of a mantle plume, which may provide new insights into the Late Palaeozoic tectonic setting of West Junggar.
Abstract Banded iron formations (BIFs) are among the few chemical sedimentary archives that capture the biogeo-chemical evolution of Fe cycling and the redox evolution of the early Earth. Although biologically recycled continental Fe has been previously proposed to be a significant source of Fe in BIFs deposited from a stratified ocean at the onset of the Great Oxidation Event (GOE; ~2.5–2.2 Ga), constraining Fe sources and pathways in Archean BIFs remains challenging. Here we present major and trace element and Fe-Nd-Cr isotope data for the largest BIF (i.e., Dataigou) in China to test whether a benthic Fe shuttle was operative during deposition of pre-GOE BIFs. The absence of true, shale-normalized Ce anomalies, coupled with unfractionated Cr and positive Fe isotope compositions, suggests that BIF deposition occurred in an anoxic water column under reducing atmospheric conditions, whereas positive Eu anomalies indicate a significant input from a high-temperature hydrothermal source. Based on a significant correlation between initial Nd and Fe isotope data, we suggest that two Fe sources were periodically mixed and resulted in deposition of the Dataigou BIF. Here, we suggest the following sources: (1) hydrothermal fluids from sea-floor systems (low εNd(t) and high δ56Fe), derived from the interaction of fluids with underlying, older continental crust, and (2) a benthic Fe flux (high εNd(t) and low δ56Fe), generated by microbial Fe(III) reduction in coastal sediments during weathering of a nearby depleted landmass. Results presented here confirm, for the first time, that a microbially driven Fe shuttle was operational and supplied Fe on a basin-wide scale in the absence of atmospheric oxygen.
The sandstone-type uranium deposit of the Kelulun Depression is the first industrially valuable uranium deposit discovered in the Hailar Basin. This study performed a systematic examination of 17 sandstone samples from the Yimin Formation in the Kelulun Depression based on various analytical techniques. The findings of the current study were synthesized with previous research to investigate the impact of the redox conditions and the tectonic background of the source area, as well as the paleoclimatic evolution of the Yimin Formation on uranium mineralization. The elemental Mo, U/Th, V/Cr, Ni/Co, and V/(V + Ni) ratios indicate that the paleowater was in an oxygen-rich environment during the deposition of the Yimin Formation. Additionally, the C-value, Sr/Cu, Al2O3/MgO, and Rb/Sr ratios indicate that the Yimin Formation was formed in a paleoclimate characterized by arid-to-semi-arid conditions. The geochemical characteristics of the observed elements indicated that the sediment source of the Yimin Formation was mainly felsic rocks from the upper continental crust, the weathering of the rock was weak, and the tectonic background was a passive continental margin. Coffinite is distributed in the form of cementation and stellates within or around pyrite crystals, and uranium-titanium oxide is mostly distributed in an irregular granular distribution in the biotite cleavage fractures of the study area. In summary, the findings of this study reveal that the tectonic settings, provenance, uranium source, paleoclimate, and oxygen-rich paleowater of the Yimin Formation have important geological significance for the large-scale uranium mineralization of the Kelulun Depression.
We conducted petrographic, geochemical, and new U–Pb laser ablation inductively coupled mass spectrometry analyses of detrital zircons in the sandstone samples from the important Mengqiguer sandstone-type uranium deposit located in the Yili basin of the Xishanyao Formation. This formation (Shuixigou Group) is a major uranium-bearing target stratum in the basin. The main purpose of our investigations was to determine the provenance of the host sand units that has remained unclear to date. Our petrographic results suggested that the host sandstone is texturally and chemically immature, indicating superior physicochemical conditions for later uranium mineralisation. The major and trace element data of the detrital sandstone samples point towards the upper continental crust, and the chondrite-normalised rare earth element (REE) plots show enriched light REE and relatively flat heavy-REE distribution, with a negative Eu anomaly. The characteristics of the REEs and the trace elements point to felsic rocks as the provenance of the target strata with a continental arc tectonic setting. The detrital zircons indicate an age span of ∼235–∼2327 Ma, with major age populations of ∼300–∼500 Ma. The increasing amount of zircon grains from the Late Devonian to the Early Carboniferous coinciding with the age of volcanic magmatic activities of the Wusun and Nalati mountains indicates that the rocks formed under the Tianshan collisional orogeny and the subsequent extensional environment.
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