Triassic magmatism and Mo mineralization in Northeast China: geochronological and isotopic constraints from the Laojiagou porphyry Mo deposit

The Laojiagou Mo deposit is a newly discovered porphyry Mo deposit located in the Xilamulun Mo metallogenic belt, Northeast China. Mo mineralization mainly occurred within the monzogranite and monzogranite porphyry. Re–Os isochron dating of molybdenites indicate a mineralization age of 234.9 ± 3.1 Ma. Zircon LA–ICP–MS U–Pb analysis for monzogranite porphyry and monzogranite yield ²⁰⁶Pb/²³⁸U ages of 238.6 ± 1.8 and 241.3 ± 1.5 Ma, respectively, indicating that Laojiagou Mo mineralization is related to Middle Triassic magmatism. Hf isotopic compositions of zircons from both monzogranite porphyry and monzogranite are characterized by positive ε_Hf(t) values [ε_Hf(t) = 2.9–7.3 and 1.5–7.9, respectively] and young T_DM2 model ages, which implies that the magma was derived from juvenile crust created during accretion of the Central Asian Orogenic Belt (CAOB). Identification of the Laojiagou Mo deposit adds another important example of Triassic Mo mineralization in the Xilamulun Mo metallogenic belt where most Triassic Mo deposits in northeast China cluster around the northern margin of North China Craton. Based on the regional geological setting and geochronological and Hf isotope characteristics, we propose that Triassic Mo deposits and related magmatic rocks in northeast China formed during the last stages of evolution of the CAOB. These deposits formed during post-collisional extension after the closure of the Palaeo-Asian Ocean and amalgamation of the North China–Mongolian Block with the Siberian Craton.     

内蒙古西拉木伦成矿带劳家沟斑岩型钼矿流体 包裹体特征及地质意义

[摘 要]劳家沟钼矿是西拉木伦成矿带东段阿鲁科尔沁旗天山镇新发现的一个斑岩型钼矿床。矿床赋矿岩石为中细粒斑状二长花岗岩,矿化类型以细脉状和浸染状辉钼矿为主。成矿过程包括早阶段(石英-辉钼矿-黄铁矿组合)、中阶段(石英-多金属硫化物组合) 和晚阶段(石英-方解石组合) 三个阶段。早阶段的石英细脉中发育气相水包裹体(PV)、液相水包裹体(PL)、CO₂-H₂O 三相包裹体( C)、富液相水溶液包裹体(L)、富气相水溶液包裹体(V)和含子矿物多相包裹体(S),子矿物包括石盐、硬石 膏及其它不明矿物;中阶段较早阶段除缺少CO₂三相包裹体外,其它相似;而晚阶段的石英-方解石细脉中流体包裹体发育较少,且类型单一,只有L 类和少量V 类。流体包裹体岩相学、显微测温及单个包裹体激光拉曼光谱分析结果表明劳家沟斑岩钼矿的成矿流体是以高温、高盐度、高氧逸度和含CO₂为特征的岩浆热液型流体,从早到晚其演化规律为:早阶段高温(320～>550℃)、高盐度(6～16wt% NaCleqv、52. 89～>66. 75wt% NaCleqv)、高氧逸度、富CO₂→中阶段中高温(280～>550℃)、高盐度(5～>15wt%NaCleqv、>66. 75wt%NaCleqv)、含CO₂→晚阶段中低温(160～250℃)、低盐度(0. 5～5 wt% NaCleqv)、贫CO₂。利用含石盐子晶包裹体的气液均一温度Th_L-V、石盐消失温度Tm_halite 和压力三者之间的关系对其最小捕获压力进行估算,结果显示成矿深度大约为5～7 km。根据区域成矿规律推测劳家沟斑岩钼矿的成矿时间大致为早白垩世,成矿构造背景对应于中国东部岩石圈强烈减薄。本文将劳家沟斑岩钼矿的成矿机制大致概括为:岩石圈强烈减薄诱发岩浆侵入活动,岩浆在侵入晚期发生液态不混溶作用形成高温、高盐度、高氧逸度、含CO₂的含矿气水热液,在地壳快速抬升期间流体多次减压沸腾,导致成矿物质沉淀。