Abstract:

Located in eastern Inner Mongolia, the Mengentaolegai silver-polymetallic deposit is one of large-scale deposits. Ore bodies occur as vein-like, and are controlled by the east-west trending fault. Geochronology and geochemistry of the main intrusive rocks occurring in and around the Mengentaolegai silver-polymetallic ore district are studied. In order to constrain the emplacement age of intrusive rocks, the zircons in four samples from the intrusive rocks were dated by the LA-MC-ICP-MS method. Four reliable weighted mean ²⁰⁶Pb/²³⁸U ages of (240.5±1.2) Ma（MSWD=0.48）, (234.3±3.2) Ma (MSWD=0.21), (154.5±0.5) Ma (MSWD=0.41), and (127.5±0.7) Ma (MSWD=1.3) were obtained for the biotite plagioclase granite and muscovite plagioclase granite in Mengentaolegai batholiths, biotite K-feldspar granite in Duerji batholiths and andesite porphyry in southern part of the Mengentaolegai deposit, respectively. It is indicated that there were three times of magmatic emplacement in and around the Mengentaolegai silver-polymetallic ore district, during Middle Triassic, Late Jurassic and Early Cretaceous period, respectively. The geochemical analysis of these intrusive rocks show that the biotite plagioclase granite, muscovite plagioclase granite and biotite K-feldspar granite are characterized by the high SiO₂, enriched alkali, meta-aluminum and calc-alkaline, with LREE differentiation obviously. They are enriched in LILE elements, characterized by relatively depleted Ba, Nb, La, Sr, P and Ti, and enriched Rb, Th, K, Ta, Ce, Nd, Hf, Sm, Y and Yb. Combined with the dating results of the intrusive rocks, it can be concluded that the Mengentaolegai batholiths and Duerji batholiths were formed under collisional and extensional settings, respectively. The reported Ar-Ar age of the muscovite associated the mineralization is (179. 0士1. 5) Ma, which is quite different from the emplacement ages of the intrusive rocks studied. It is suggested that these exposed intrusive rocks are not likely related to the silver-polymetallic mineralization in Mengentaolegai area. The intrusive rocks related to the silver-polymetallic mineralization may be buried in the depth, or have been re-melted due to the intruding of the Duerji batholiths.

Key words: zircon LA-MC-ICP-MS dating, geochronology, geochemistry, silver-polymetallic deposit, Mengentaolegai, Inner Mongolia