Early Paleozoic magmatism and metallogeny in Northeast China: a record from the Tongshan porphyry Cu deposit

摘要

The Tongshan Cu deposit is located in the northern segment of the Great Xing'an Range and represents one of the few early Paleozoic porphyry Cu deposits in northeastern China. The granitic rocks in the Tongshan Cu deposit include concealed granodiorite and exposed tonalite, which yield LA-ICP-MS zircon U-Pb ages of 478 +/- 3 Ma and 214 +/- 3 Ma, respectively. The granodiorite has relatively high SiO2 (60.5-63.5 wt%) and Sr (596-786 ppm) contents, low Yb (1.21-1.53 ppm) and Y (9.81-13.0 ppm) contents, and initial Sr-87/Sr-86 ratios (0.7038-0.7040), suggesting adakitic affinity. Combined with its positive epsilon(Nd)(t) values (3.5-5.4), low Mg-# values (41-50), and low contents of Cr (18.6-29.0 ppm) and Ni (7.3-9.1 ppm), we propose an origin by partial melting of a juvenile mafic lower crust in a post-collisional setting after the amalgamation of the Erguna and Xing'an Blocks. The tonalite is characterized by high SiO2 (63.1-65.9 wt%) and Al2O3 (16.0-16.3 wt%) contents, low (Sr-87/Sr-86) (i) ratios (0.7041-0.7042), positive epsilon(Nd)(t) values (2.6-3.0), along with LILE and LREE enrichments and Nb-Ta-Ti depletions, suggesting an origin by partial melting of juvenile mafic lower crust, coupled with fractional crystallization, in a post-orogenic setting after the collision between the Xing'an and Songnen Blocks. The delta D values of ore-forming fluids range from -100 to -93 aEuro degrees, and the delta O-18 values calculated from hydrothermal quartz are between -3 and 10 aEuro degrees. The delta S-34 values of sulfides vary from -2.6 to -1.1 aEuro degrees. Field observations, as well as the geochronological and H-O-S isotopic data, suggest that the Cu mineralization at Tongshan was genetically linked with the granodiorite.