北祁连车路沟斑岩型金矿床含矿岩体年代学、地球化学与岩石成因

车路沟金矿是北祁连山发现的首例与斑岩有关的金矿床, 矿体产于加里东晚期侵入的中酸性斑岩体及其与围岩接触带内。为了查明含矿斑岩体侵位时代、岩石成因和源区性质, 本文以车路沟一带含矿斑岩体作为研究对象, 对其开展了岩石地球化学、LA-ICP-MS锆石U-Pb同位素定年和Hf同位素特征研究。该岩体主要由花岗闪长斑岩、石英闪长斑岩、英云闪长斑岩组成, 对花岗闪长斑岩、英云闪长斑岩进行了LA-ICP-MS锆石U-Pb同位素定年, 分别获得锆石²⁰⁶Pb/²³⁸U加权平均年龄为438.6±3.8Ma和433.0±3.8Ma, 指示岩浆侵位时代为早志留世晚期。岩石SiO₂含量为61.4%~68.6%, Al₂O₃含量为15.0%~16.7%, MgO含量为0.36%~2.95%, σ值介于1.20~2.03之间, A/CNK值为1.0~1.1之间, 属钙碱性系列准铝质花岗岩类; 具有弱负铕异常-正铕异常(δEu=0.90~1.18), 高Sr(319×10^-6~549×10^-6)含量和Sr/Y(36.6~77.5)比值, 低Y(7.09×10^-6~10.2×10^-6)和Yb(0.69×10^-6~0.89×10^-6)含量, 具有埃达克岩地球化学特征。该岩体的锆石ε_Hf(t)值均为正值, 变化于+12.7~+15.0之间, 两阶段模式年龄t_DM2(Hf)=461~563Ma, 推测岩浆源区可能主要来自新元古代晚期至寒武纪新生洋壳物质。研究认为车路沟斑岩体形成于陆陆碰撞后期, 地壳持续加厚, 诱发中新元古代晚期至寒武纪新生洋壳发生部分熔融, 形成埃达克质岩浆, 在其不断上侵和演化过程中, 形成车路沟斑岩体与其相伴产出的金矿体。

Geochronology,geochemical characteristics and genesis of Chelugou gold deposit ore-bearing porphyry in the North Qilian orogenic belt

The Chelugou deposit is a first discovered porphyry-related gold deposit in the North Qilian orogenic belt. The ore body is hosted in the Late Caledonian intermediate to felsic porphyritic complex and its contact zone with wall rock. In this study, major and trace element geochemistry, LA-ICP-MS zircon U-Pb geochronology and Lu-Hf isotope were conducted to investigate timing of magma emplacement, petrogenesis and magma source of the ore-bearing porphyry. The causative porphyries are dominantly composed of granodiorite porphyry, quartz diorite porphyry and tonalite porphyry. LA-ICP-MS zircon U-Pb dating shows that the 206Pb/238U weighted mean ages are 438.6±3.8Ma and 433.0±3.8Ma for the granodiorite porphyry and quartz diorite porphyry, respectively, indicating the magma emplacement can be constrained to the late Early Silurian. These porphyries have SiO2 content of 61.4%~68.6%, Al2O3 content of 15.0%~16.7%, MgO content of 0.36%~2.95%, σ values of 1.20~2.03, and A/CNK values of 1.0~1.1, indicating that they belonging to low potassium tholeiitic and metaluminous granites. They show geochemical characteristics of weak and positive Eu anomalies (δEu=0.90~1.18), high Sr contents (319×10-6~549×10-6) and Sr/Y ratios (36.6~77.5), low Y (7.09×10-6~10.2×10-6) and Yb (0.69×10-6~0.89×10-6) contents, demonstrating an adakitic affinity. Such porphyries show positive εHf(t) values of +12.7 to +15.0 and yield two-stage model (tDM2) ages of 488~587Ma, suggesting that the magma source is likely derived from the Late Neoproterozoic to Cambrian juvenile crust. Combined with the regional geological background, we proposed that the Chelugou porphyries were formed in the late stage of continent-continent collision during the late Early Silurian. The continued crust thickening induced the partial melting of the Late Mesoproterozoic to the Cambrian juvenile oceanic crust and the formation of the adakitic magma.