Magmatism And Gold Metallogeny In Beishan Mt., Northwestern China

The Beishan Mt. spans three provinces-Gansu, Xijiang and Inner Mongolia, having an area of120 000 km². Yectonically, it transverses three different tectonic units, i.e. Siberia, Kazakhstan andTarim plates, respectively, and is composed of nine terrains with widely exposed Precambrian andPaleozoic strata, complex structures, intensive magmatic activities and widespread ore deposits. Itis not only a main part of Tianshan-Yinshan-Great Hinggan metallogenic belt in China, but also akey to understand the evolution of central-Asian orogenic system. Therefore, the studies of basicgeology, ore deposits and mineral exploration have attracted great attention from domestic andabroad geologists.At present, more than 100 gold deposits and prospects have been discovered, explored andmined. Based on the host rocks and geological features, the gold deposits in the study area can bedivided into five groups (or types), (1) porphyry Au deposit, such as Liushashan Mo-Au deposit;(2) volcanic type Au deposit, such as Mazhuangshan, Xinjinchang, Laojinchang and Langwashandeposits; (3) plutonic intrusions-related Au deposit, such as Nanjinshan, Jinwozi, Zhaobishan,Shijinpo, Jinmiaogou, Jingoujing, Mojindong and Mingshujing deposits; (4) hosted byPrecambrian metamorphic rocks, such as Xiaoxigong, Laodonggou and Jiaochagou deposits; and(5) skarn type Au deposit, such as Huaniushan deposit. The origin of all of these gold deposits isclosely related to the magmatism.In order to discuss the relationship between the magmatism and the gold metallogeny, theresearch on petrology, geochemistry, Nd and Pb isotopes and ⁴⁰Ar-³⁹Ar age dating of the granitoidintrusions in the study area was first conducted systematically, and the emplacement setting andages of the major granitoid intrusions as well as their relations to gold deposits were discussed indetail. Although the emplacement ages of the granitoids in the Beishan Mt. area vary fromPrecambrian to Mesozoic Yanshanian, the Hercynian granitoid intrusions are predominant, and arecharacterized by wide distribution, large outcrop area and high concentration of ore-formingelements. Most granitoid intrusions belong to theâ… type of granites intruded along the ancientcontinental margin or collision zone. According to the Nd isotopic analytical results, somegranitoid intrusions bears both positive and negativeεNd (t) values, which indicates that theisotopic system of the intrusion is not homogeneous, and in the other hand, a large amounts ofPrecambrian metamorphic rocks were assimilated by the granitoid intrusions. Due to theproportional difference of the ancient and juvenile crustal materials in the granitic intrusions, theεNd (t) value varies greatly.Four typical gold deposits, which are included in the former four types of gold deposits mentioned above, was discussed in detail in light of the ore-forming setting, ore body features, oretextures and structures, isotopic and elemental geochemistry. It has been suggested that theMazhuangshan and Liushashan deposits are spatially and temporally related to the Hercyniangranitoid intrusions, while the Nanjinshan gold deposit is associated with the Indosinianmagmatism. They all belong to the mesothermal vein type gold deposit. In contrast, theemplacement of the Hercynian granitoid intrusion into the Precambrian metamorphic rocksresulted in the formation of the Xiaoxigong deposit. It belongs to the mixed sources (magmaticand metamorphic hydrothermal) type mesothermal gold deposit.The regional gold deposits in the study area are mainly distributed along the suture zones.The studies on these deposits indicate that the gold metallogeny in the Beishan Mt. area is mainlyconcentrated in three periods, i.e. Precambrian, Hercynian and Indosinian. Among these periods, the late Hercynian gold metallogeny is the most intense ones. Most of gold deposits are formedduring this period. The gold metallogeny at this period is intimately related to the intensemagmatic activities caused by the collisions among Siberia, Kazakhstan and Tarim plates.Therefore, the gold metallogeny is controlled by the orogenic process. The gold deposits formed atthis period could be called syn-orogenic gold deposit. In contrast, the gold deposits formed duringIndosinian are characterized by post-orogenic gold deposit.