内蒙古正蓝旗丹金地区赋矿火山岩年代学、地球化学特征及铀成矿时代

Chronology, geochemical characteristics of the host rocks and uranium mineralization age in Danjin area, Zhenglan Banner, Inner Mongolia

内蒙古锡林郭勒盟正蓝旗丹金地区近年来取得了较好的铀矿找矿成果,但赋矿火山岩与铀成矿时间关系及成岩成矿背景尚不明确。本文开展了成岩成矿年代学和岩石地球化学研究,表明赋矿围岩流纹质晶屑熔结凝灰岩和流纹斑岩分别形成于143.8±0.5Ma(MSWD=0.61)和141.5±0.5Ma(MSWD=0.40),属于早白垩世早期;铀矿石的铀铅等时线年龄分别为113.0±1.0Ma(MSWD=0.10)和80.6±2.6Ma(MSWD=1.60),分别属于早白垩世晚期和晚白垩中期;赋矿岩石属于过铝质亚碱性系列流纹岩类,来源于陆壳部分熔融,属A型流纹岩。两个铀成矿年龄与赋矿岩石存在约30Ma和63Ma的时差,成矿作用可能与对应同时期火山活动有关。该火山岩是重要的铀源体,同时充足的成岩成矿时差、构造叠加及流纹斑岩等共同控制着该区的铀成矿。

Objectives:Danjin area, Zhenglan Banner, Inner Mongolia is located in the southern part of Great Hinggan magmatic activity belt. Some uranium prospecting achievements have been made in this area in recent years, but the age and background of uranium mineralization are not clear. Based on the study of the host volcanic rocks and uranium mineralization age in Danjin area, this paper discusses the diagenetic mineralization age and metallogenic background.Methods:8 geochemical samples and 2 rock zircon U-Pb age samples were taken in DanJin region, and 11 uranium ore samples were taken from the deep ore body of the borehole.Zircon isotopic datingwas completed in the laboratory of Beijing Yandu Zhongshi Testing Technology Co., Ltd. This test was completed by LA-Q-ICP-MS analysis. The laser ablation system is New Wave UP213, and ICP-MS is Brooke M90. In U-Pb isotopic dating, zircon standard GJ-1 was used as external standard for isotopic fractionation correction. GJ-1 was analyzed twice for every 5 ~ 10 sample points. The content of Microelements in zircon was quantitatively calculated by using SRM610 as external standard and Si as internal standard. According to the micrograph of zircon, the zircon dating area is selected, and the denudation spot diameter is 30 μm.Theisochron age analysis of U-Pb in uranium ore was completed in Analtical Laboratory of BRIUG. After crushing and gravity separation, the sample powder with high purity was selected for experimental test. Isoprobe-T hot surface ionization mass spectrometer was used in the experiment.The determination of major elements, trace elements and rare earth elements in chemical analysis samples was completed in the laboratory ofALS Chemex(Guangzhou) Co Ltd. After the sample is crushed, 300 g is separated and ground to 75 μ m(200 mesh). The content of rare earth trace elements is determined by ME-MS81 fusion inductively coupled plasma mass spectrometry, and the main elements of rocks are precisely analyzed by MEXRF26 X fluorescence spectrometer fusion method.Results: Based on the study of the field characteristics, microscopic characteristics, geochronology and geochemical characteristics of the host volcanic rocks in Danjin region, as well as the uranium metallogenic age, the rock types of the volcanic rocks are identified in detail, the diagenesis and metallogenic age are determined, and the petrogenesis. The zircons collected from the host volcanic rocks have a zonal structure with Th/U values greater than 0. 1, which is consistent with the magmatic zircons. LA-ICP-MS zircon U-Pb isotopic dating shows that the volcanic rocks were formed in the early Cretaceous(143. 8 ± 0. 5 Ma and 141. 5 ± 0. 5 Ma). After U-Pb isotopic dating of uranium ore, the metallogenic ages are respectively the late Early Cretaceous(113. 0 ± 1. 0 Ma) and the late Cretaceous(80. 6 ± 2. 6 Ma). All samples of volcanic rocks are rich in SiO2(78. 65% ~ 80. 26%);K2 O(4. 10% ~ 6. 58%), K2 O/Na2 O value is greater than 2. 34, europium negative anomaly( δ EU = 0. 05 ~ 0. 10). They are enriched in LREEs relative to HREEs. All the samples are relatively enriched in LILEs(e. g., K and Rb) and depleted in HFSEs(e. g., Nb, Ta, P and Ti).Conclusions: The host volcanic rocks in Danjin region were formed in the early Early Cretaceous, belonging to A-type rhyolite. The magma originated from the partial melting of continental crust. The uranium deposits were formed in the late Early Cretaceous and late Cretaceous, which denied that the previous uranium mineralization occurred in the process of rhyolite porphyry emplacement in the late stage of the volcanic activity, and believed that the uranium mineralization may be related to the late Early Cretaceous and late Cretaceous volcanism. The area with deep source structure superimposed near the deep rhyolite porphyry is an important prospecting area.