Yawen Bao1, Mingjie Zhang1*, Xinbo Wang1, Liwu Li2,
1. School of Earth Sciences, Lanzhou University, Lanzhou, 730000; 2. *Corresponding author: mjzhang@lzu.edu.cn
Key words: Ore-forming magmatism, C-He-Ne-Ar isotopes, the Langmuri mafic-ultramafic complex in China
1. Introduction
The Paleozoic mafic-ultramafic intrusions in East Kunlun orogenic belt host super-large to medium-sized magmatic Ni-Co-Cu sulfide deposits, such as Langmuri, Xiarihamu, and Shitoukengde. The Xiarihamu mafic-ultramafic intrusion hosted the first global ultra-large magmatic sulfide deposit in the orogenic belt settings. The Langmuri intrusion has the potential to search for the national strategic key metals such as Ni-Co-PGE.
The Langmuri mafic-ultramafic intrusion is located in the eastern part of East Kunlun orogenic belt, western China. The Langmuri intrusion are mainly pyroxene peridotite, pyroxenite, and gabbro, and contains some Ni-Co sulfide ore bodies with independent Pt-Pd ore bodies. The main rock types of the mafic-ultramafic intrusion Co content ranges from 17.06×10⁻⁶ to 185×10⁻⁶, with an average of 105.27×10⁻⁶; Ni content ranges from 37.14×10⁻⁶ to 1591×10⁻⁶, with an average of 591.18×10⁻⁶; Cu content ranges from 5.79×10⁻⁶ to 236×10⁻⁶, with an average of 149.04×10⁻⁶. The Co, Ni, and Cu contents decrease sequentially from pyroxene peridotite to pyroxenite to gabbro.
2. Samples and analytical methods
Samples used in this study were collected from different drill cores, including pyroxene peridotite, pyroxenite, and gabbro. All of the rock samples were examined using microscopy, and the least altered samples based on the microscopic observations were selected for mineral separation. The olivine (Olv), pyroxene (Pyx) and plagioclase (Pl) separates in the pyroxene peridotite, pyroxenite, and gabbro were selected by magnetic separation and followed by hand picking under a binocular microscope.
The analysis of petrogeochemistry, carbon isotope and noble gas isotope, to explore the characteristics of ore-forming magma source, petrogenesis and mineralization of Ni-Co-PGE ore-forming magma. The Carbon isotopes of CO2 and CH4 were analyzed by a GC-C-MS system using a Delta plus XP mass spectrometer. The noble gases from olivine and pyroxene separates were determined using an online Noblesse mass spectrometer connected to a vacuum heating system in Key Lab of Petroleum Resources, CAS.
3. The C-He-Ne-Ar isotopic compositions
The carbon isotopic compositions of CO2 and CH4 in different magmatic minerals of Langmuri intrusion are within the range of crust and mantle. The carbon isotopic composition of CO2 ranges from -25.2‰ to -7.7‰, and the carbon isotopic composition of CH4 ranges from -48.6‰ to -14.5‰. The fluid components of ore-forming magma have the carbon isotopic characteristics of mantle, the crust and the pyrolysis of organic matter.
The He-Ne-Ar isotopic composition of the Langmuri intrusion indicate that the magma fluid has the characteristics of lithospheric mantle, crust and atmospheric components. The 3He/4He values range from 0.01Ra to 3.45Ra, which is located in the 3He/4He characteristic range of the crust and mantle. The 20Ne/22Ne values range from 8.13 to 13.50, and the 21Ne/22Ne values range from 0.019 to 0.062. 40Ar/36Ar values range from 302 to 3020, which are located between atmospheric and crust values.
4. The implications of the mafic magmatism
There may be contributions from deep mantle material. The 3He/4He values of pyroxene minerals in pyroxene rocks are up to 3.8Ra after deducting the radioactive origin, and the distribution characteristics of 21Ne/22Ne-20Ne/22Ne along the L-K line indicate that the source of ore-forming magma is lithospheric mantle. The correlation between Th/Yb-La/Ba and Th/Zr-Nb/Zr indicates that the magma source was metasomatized by the dehydrating of subduction plates. There may be deep source material with rich PGE in the mantle wedge magma source of the orogenic belt.
Sulfide segregation is the main mechanism of Ni-Co-PGE enrichment, and the controlling factors are mainly shell crustal contamianation. The characteristics of 3He/4He and 40Ar/36Ar indicate that the ore-forming magma is contaminated with crustal materials. The characteristics of δ13CCO2 and δ13CCH4 indicate that the magma contains fluid volatilized from crustal sources and the pyrolysis of sedimentary organic matter. The correlation of La/Yb-Ce/Yb and Th/Yb-Nb/La indicates that crustal contamianation occurred during the magma ascent. In addition, there is an obvious covariant relationship between Th/Nb and La/Yb with 3He/4He, 40Ar/36Ar and δ13CCO2 due to crustal contamianation.
The Langmuri mafic magma enriched the platinum group element deposit, indicating that the parent magma was the product of high degree of partial melting of the lithosphere mantle, and the fluid metasomatism of the subduction plate promotes the partial melting degree. The source region may have the material enriched with nickel and cobalt platinum group elements from the deep mantle, and the magma rose to the magma chamber for contamianation, which promoted sulfur saturation and sulfide melting. Forming nickel cobalt platinum sulfide ore body.
5. Conclusion
(1) The magma source of Langmuri intrusion is the lithospheric mantle, and was metasomatized by the dehydration of subduction plates.
(2) The crustal materials contamianated with the magma of Langmuri are mainly derived from the crust and sedimentary organic matter, which occur during the magma ascent process and in the magma chamber.
References
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