Geochemical characteristics of granites in the Suonaga Pb-Zn-Ag deposit of Dong Ujimqin Banner, Inner Mongolia, and their tectonic and ore-forming implications

The ore-bearing pluton in the Suonaga Pb-Zn-Ag deposit is composed of medium-coarse-grained granite,fine-grained biotite granite and pseudoporphyritic K-feldspar granite.The zircon U-Pb dating results show that the medium-coarse-grained granite was formed at 319.6±4.1 Ma,the fine-grained biotite granite was formed at 172.5 ±1.4 Ma,and the pseudoporphyritic K-feldspar granite was formed at 165.5 ±1.8 Ma.The petrogeochemistry of the three rock bodies is characterized by high K,rich alkali and poor magnesium,so they all belong to calcium-basic,quasi-aluminous to weakly peraluminous and highly-differentiated rocks.All of the granites have high REE and negative Eu anomalies,exhibiting relative enrichment of LREE.These rock bodies belong to S-type granites,because they are enriched in U and Th and their Rb/Sr ratios are higher than those of the crustal source.Regional geology and granite geochemistry indicate that the medium-coarse-grained granite was formed in a post-collisional extensional environment in Late Carboniferous;the Middle Jurassic fine-grained biotite granite and pseudoporphyritic K-feldspar granite were formed in an intraplate extensional environment,and they were products of different evolutionary stages of a comagmatic region.The pseudoporphyritic K feldspar granite was probably the ore-forming parent rock,because its diagenetic age is consistent with the molybdenite Re-Os ages(163.4±2.4Ma to 166.6±2.4Ma) and the rocks are enriched in metallogenic elements Pb and Zn.