青海鄂拉山北段牦牛沟铜金矿辉钼矿Re-Os年龄及其地质意义

摘要

The Ngola Mountain lies at the conjunction between West Qinling and East Kunlun in the west of Central Orogenic Belt. Owing to its low research level, some problems concerned, such as the regional metallogenesis, geological setting and geodynamic process in Indosinian period, remain unclear. In this study, the authors used the method of temporal and spatial distribution characteristics of specific type deposits to deduce the geological background and geodynamic process. Based on a detailed study of geological features and ore-forming geochronology of the Maoniugou copper-gold deposit in northern Ngola Mountain, and combined with the traditional tectonic study and diagenetic evidence, the authors tried to explore the regional ore-forming geological background, the geodynamic process and the assignment of the Ngola Mountain through a comparison with the typical porphyry-skarn deposits in East Kunlun. The results show that the Maoniugou deposit is a small copper-gold skarn deposit closely related to Muleer granodiorite. Mineralizations associated with Cu-Au mineralization are mainly silicification, sericitization, chalcopyritization and pyritization. Ore minerals are mainly composed of chalcopyrite, pyrite and molybdenite. The authors measured Re-Os isotopes of representative molybdenite-bearing samples from the main orebody, and the six yielded model ages range from (237±3.3) Ma to (240.7±3.3) Ma with a well-constrained weighted average model age of (238.7±1.4) Ma. Based on these new age data and ages of regional porphyry-skarn ore deposits, the authors have reached the conclusion that the Ngola Mountain and East Kunlun had similar post-subduction settings and geodynamic processes at least in middle and late Triassic, and they belonged to the continental margin settings with the north subduction of Buqing-Anymaqin Mountain. The East Kunlun Triassic mineralization may have two peak periods: Fe-Cu skarn mineralization at about 237 Ma and Cu-Au porphyry mineralization at about 222 Ma, related respectively to break-off of the subducted slab and lithospheric delamination after crustal thickening due to collision between continental plates in Triassic.%鄂拉山是中央造山带西段之东昆仑和西秦岭的结合部位，由于研究程度不高而影响着大家对其区域印支期成矿作用、成矿地质背景及地球动力学过程的认识。因此，本文从特定矿床类型时空分布特征反演成矿地质背景和地球动力学过程的方法入手，以鄂拉山北段的牦牛沟铜金矿为研究对象，详细研究其矿床地质特征和成矿年代学，将其与区域东昆仑典型斑岩-矽卡岩型矿床进行时空对比研究，并结合传统区域大地构造研究和成岩证据，试图探讨区域成矿地质背景、地球动力学过程以及鄂拉山的归属问题。结果表明，牦牛沟铜金成矿为小型矽卡岩矿床，成矿与木勒儿花岗闪长岩关系密切。与铜金成矿有关的矿化主要为硅化、绢云母化、黄铜矿化和黄铁矿化。矿床矿石矿物有黄铜矿、黄铁矿以及辉钼矿等。首次获得有意义的辉钼矿Re-Os加权平均年龄为(238.7±1.4) Ma。同时，三叠纪鄂拉山与东昆仑具有相似的地质演化特征，属于东昆仑南北向挤压构造背景下的成矿事件。东昆仑三叠纪成矿可能有两个相对峰期：约237 Ma以矽卡岩矿床为主的铁铜成矿期和约222 Ma的以斑岩为主的铜金成矿期，前者可能和俯冲碰撞转换期的板块断离有关，后者则与后碰撞拆沉背景有关。