Permian Mafic Magmatism And Mineralization In The Pobei Area, Xinjiang, Western China: Constraints From Petrogeochemistry, Chemical And C-He-Ar Isotopic Compositions Of Volatiles

Nickel, copper and platinum group elements(PGE) mainly come from large layered maficultramafic complex, which are associated to lare-scale mafic magmatism. The Pobei, Hongshishan, Xuanwoling, etc., many mafic-ultramafic complexes are distributed in the Beishan area, in the northeast margin of the Tarim block, which is one of the largest mafic-ultramafic layered complex in China. The mafic magmatism and mechanism as well as sulfur saturation for Cu-Ni sulfide mineralization is lack of the constraints from chemical compositions and C-He-Ar isotopic compositions of volatiles, which is helpful to constrain the dynamic setting of the magma evolution combined with the petrochemical compostions. Hence, a study of magmatism and Cu-Ni sulfide mineralization for Pobei mafic-ultramfic complex have been carried out using integrated approaches including mineralogy, petrology, and geochemistry of major, trace elements, noble gas and carbon isotopes. These data from this study are used to reveal source characteristics, the compositions of parental magma, the condition and process of volatiles during magma evolution, the mechanism for sulfur saturation, and provide insight into the dynamic setting of mafic magmatism of Pobei Cu-Ni-PGE sulfide deposit. The most important conclusions from this study are summarized below.1. The Pobei mafic-ultramfic complex shows the systematic variation features of the in-situ crystallization of magma for correlation of whole-rock major oxide contents and trace elements of different types of rocks. The whole-rock major element compositions are mainly controlled by the types and the abundances of the magtic minerals, such as spinel, olivine, pyroxene and plagioclase. The primitive mantle-normalized trace element patterns of different types of rocks are similar patterns, and are characterized by enrichments of large ion lithophile elements of Rh and Th, and depletion of high field strength elements of Nb, Ta, Ti, Zr and Hf. They showed light rare earth elements(LREE) enriched or flat chondrite-normalized rare earth element patterns, which indicated the light fractionation between rare earth elements. Some gabbros showed positive anomaly of Sr and Eu. These are significantly different from flood basalts in Tarim large igneous province and OIB.2. The chemical compositions of volatiles in olivine, pyroxene and plagioclase of Pobei mafic-ultramafic complex were determined. They were released at 200-400 oC, 400-700 oC and 700-900 oC intervals during vacuum stepwise-heating. The volatiles are 3695.98 mm3/g in total contents on average, which is dominated by H2O(av. 55 mol.%), and minor H2(25%) and CO2(13%). The volatiles released at 400-1200 oC intervals are taken as the volatiles from magma process. The volatiles of olivine crystalized at the early stage of magma cystallizaiton is 4709.2 mm3/g in contents, which is dominated by H2O(4114.7), and minor CO2(323.7), H2(150.7) and H2S(39.5). The volatile contents of pyroxene during cystallizaiton fractionation is 2716.6 mm3/g, which is dominated by H2O(1653.0), and minor H2(468.0), CO2(412.3), CO(97.7) and H2S(35.4). The contents of plagioclase during the late stage of crystallization fractionation is 1420.0 mm3/g, which is dominated by H2O(825.6), and minor CO2(480.6), H2(62.5) and H2S(16.0). The high content of H2 indicated that Pobei magma was evolved in relative reduced condition, and volatile escaped during magma evolution.3. The δ13C values of olivine, pyroxene and plagioclase from different types of rocks in Pobei mafic-ultramafic complex showed that mantle-derived magma volatiles were mixed with the crustal components, and thermogenic components of sedimentary organic matter. The δ13CCO2 values ranged from-4.00‰ to ~-33.65‰, and the δ13CCH4 values varied from-10.39‰ to ~-29.14‰. The values are within the range of mantle, crust and thermogenic components. The δ13C values of CH4, C2H6, C3H8, and C4H10 etc. methane homologue volatiles show a normal distribution pattern of carbon isotopic among CH4 to C4H10 with partial reversal. The carbon isotopes of CO2 and methane homologue indicated that magma originated from the mantle, mixed with the crustal components, and thermogenic components of sedimentary organic matter. The crust-derived component was derived from either sedimentary organic matter of subduction in the source or contamination from country rocks in magma chambers.4. The noble gas isotopes of different minerals of Pobei mafic-ultramafic complex imply that the magma could be originated from the deep mantle, with the contribution from the mantle plume. 3He/4He values range from 1.23 Ra to 6.15Ra(Ra is 3He/4He values of air, 1Ra=1.399×10-6), with the average value of 2.90 Ra. 40Ar/36 Ar values ranges from 326.42 to 1004.30 with the average of 535.29. The values of 3He/4He and 40Ar/36 Ar plot between the end-members of atmospheric composition and mantle plume, indicating that the magma of Pobei complex originated from the deep mantle, with the contribution from the mantle plume.5. The parent magma of Pobei mafic-ultramafic complex are high MgO magma formed by high degrees of partial melting, and were contaminated by crustal and sediments from the country rocks. Using the latest geochemical data and the highest Fo value in reference, and olivine-liquid compositional relationship, the composition of parental magma of Pobei complex has been estimated as high MgO content, which is the products of high degrees of partial melting. The plot of(Th/Nb)PM and(Th/Yb)PM verus(La/Gd)PM reveal the depleted mantle source similar to N-MORB. Th/Yb and Nb/Yb ratios indicate the magma may have been modified by slab-derived melts. Weak positive correlation between(Th/Nb)PM plus(Th/Sm)PM and SiO2 contents shows the assimilation and fractional crystallization. The mixing calculation of trace elements reveals about 5-25% of crustal contamination, and the carbon isotopic distribution patterns may indicate the sediment contamination from the country rocks.6. The sulfur saturation and the formation of sulfide deposit in the Pobei mafic- ultramafic complex were discussed based on chemical compositions and carbon isotopes of volatiles. the δ13CCO2 values become light, the δ13CCH4 values become heavy, and the contents of CO2 and H2 S increase with decrease of depth from the deep(2400m) to the surface. The external S-rich and C-rich volatiles have been added to the evolved magma chamber, and the rocks in the 781 m have the highest contents of H2 S, H2 O and H2. H2 S contents showed a weak positive correlation with CO2 and H2 contents, indicating the addition of H2 O, CO2 and H2 is helpful to sulfide segregation. The crustal contamination and addition of H2 O and CO2-rich volatiles are the key factor controlling the sulfur saturation and the formation of sulfide deposit.7. The mantle plume could contributed to the formation of Pobei mafic-ultramafic complex. They have the similar ages with the Tarim mantle plume. The magma source has both the deep mantle and lithospheric mantle material modified by subduction- derived fluid. The high MgO contents in parental magma and high crystallization temperature indicated picritic magma originated from a high degree of partial melting at high temperature. The parental magma composition is similar to the typical picrites related to mantle plume in the world. Helium and argon isotopes reveal mantle plume taking part in the formation of Pobei intrusion. It may be a product of the Permian Tarim mantle plume.