Abstract:

Controlled by NW-trending fractures, the Erdaohezi lead-zinc deposit occurs in volcanic rocks and conglomerate of the Tamulangou Formation in the Hailaer-Genhe Jurassic volcanic fault-depression basin. Its ore bodies are mainly of vein type. Based on geological and geochemical investigation of volcanic rocks and ores as well as dating of volcanic rocks, the authors studied the genesis of the ore deposit. An analysis of major elements, trace elements, rare earth elements and Pb, Sr and Nd isotopes has revealed that the content of SiO2 in the lava is 48.01%-56.44% and that of the lithic crystal tuff is 69.09%-71.73%, and that Pb, Zn and Cu values in the former are much lower than those in the latter. The two kinds of rocks have similar REE patterns, characterized by weak Eu negative anomalies. LILE such as  Rb, U and Th are rich in the volcanic rocks, while Sr and such HFSE as Nb, Ta and Ti are relatively depleted. In the volcanic rocks, I_Sr=0.704 925-0.706 632, ε_Nd(t)=0.5~2.0, ε_Sr(t)=8.5-32.8 and t_DM=619-730 Ma. In addition, the composition of Pb isotopes in volcanic rocks is very close to that in sulfide ores, their ²⁰⁶Pb/²⁰⁴Pb,²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios are 18.130-18.475, 15.526-15.567 and 38.078-38.286 respectively, with μ values being 9.32-9.40. With the SHRIMP zircon U-Pb method, the lithic crystal tuff was dated at (164.2±2.3) Ma. These data indicate that the magma that formed the volcanic rocks was derived from the mantle with the mixture of a small amount of crustal materials, and the ore-forming materials came from the magma system. The volcanism had experienced two phases: early magma was mainly intermediate, whereas late magmatism occurred in late Middle Jurassic and formed acid volcanic clastic rocks. Ore-forming material was abundant in the late magma, which might have also formed sub-volcanic rocks related to the mineralization. Ore-forming fluids from late magma differentiation might have formed the ore deposit by filling and metasomatism along the fractures in volcanic rocks and conglomerate, and the mineralization occurred in late Middle Jurassic to Upper Jurassic period.

Key words: lead-zinc deposit, geochemistry, SHRIMP U-Pb dating, genesis, Erdaohezi, Inner Mongolia