青海纳日贡玛斑岩钼(铜)矿床：岩石成因及构造控制

Genesis of porphyries and tectonic controls on the Narigongma porphyry Mo(-Cu)deposit,southern Qinghai.

初步矿产普查评价成果表明,三江北段已初步显示出巨大的成矿潜力。在该区东部,以纳日贡玛-陆日格含矿斑岩体为中心的斑岩-矽卡岩大型成矿系统已初露端倪。纳日贡玛,作为该区的最具代表性的斑岩型矿床,了解其含矿斑岩的性质,查明斑岩的可能源区,厘定其与玉龙铜矿带的关系,具有重要的理论与现实意义。为此,本文对纳日贡玛矿区出露的主要斑岩体开展了详细的年代学、岩石地球化学及 Sr-Nd—Pb 同位素地球化学研究,结果表明:纳日贡玛主含矿斑岩锆石 U-Pb 年龄为43.3±0.5Ma,明显为玉龙斑岩铜矿带的北延;其主含矿斑岩为高钾钙碱性系列,高度演化的斑岩为钾玄岩系列,岩浆源区可能为50～80km 处壳幔过渡带,经历了明显的流体交代;与玉龙铜矿带含矿斑岩相比,纳日贡玛斑岩钾含量偏低,Sr-Nd—Pb同位素组成更向亏损地幔靠拢,反映岩浆源区自 NW 至 SE 地壳组分逐渐增多和/或流体交代逐渐增强。自纳日贡玛至玉龙带,成矿斑岩的结晶年龄逐渐变新,说明斑岩的形成不仅具有统一的源区,可能受控于统一的动力学机制,因印度-亚洲大陆碰撞产生的始新世右行断裂系统,可能是控制区域岩浆上侵及时空分布的动力学机制。纳日贡玛带矿床矿化以 Mo 为主,显著不同于玉龙带的 Cu-Mo(-Au)矿化组合,造成区域上矿化组合的差异即可因深部过程,也可因岩浆就位后的结晶分异过程,更多的证据显示可能受后者控制明显;因此,纳日贡玛矿床可能遭受了较强的剥蚀。区内应加强斑岩侵位较深时形成的斑岩钼矿及夕卡岩型矿床的寻找。

A large porphyry （-skarn） type mineralization belt, located at Southern Qinghai, is manifested by recent preliminary exploration. Narigongma porphyry Mo （-Cu） deposit is the most important deposit in the belt, so characteristics and origin of the porphyries related to the deposit, mainly composed of granite-diorite porphyry, is pivotal to understanding the relationship between the belt and the Yulong porphyry belt. For this purpose, detailed petrological, element geochemical, Sr-Nd-Pb isotopic and radioactive dating data were carried out for the porphyries. The SHRIMP U-Pb zircon dating result defines emplacement of the porphyries occurred at 43.3 Ma ago, which indicates that the deposit is the northern extension of Yulong copper belt. Geochemical data indicate that these ore-bearing porphyries are high K calc-alkaline, with highly evolutional as shoshonitic, and exhibit calc-alkaline composition with high Sr/Y and La/Y coupled with low Y and HREE, showing some affinities with the adakite. Sr-Nd-Pb isotopic data indicates that the porphyries were derived by partial melting of crust-mantle transition belt at a depth between 50 and 80 km. In comparison with porphyries at the Yulong belt, porphyries at Narigongma are charactered by much lower K2O content and more depleted Sr-Nd-Pb isotopic, reflecting more and more addition of the crust constituent in the source from northwest to southeast. Additionally, from Narigongma to Yulong, formation ages of ore-bearing porphyries decrease systematically with a duration of magmatism of about 7 Ma, which indicates that formation of the porphyries at Narigongman, as well at Yulong belt, was controlled by a uniform dynamic mechanism. An Eocene regional-scale dextral strike-slip fault system caused by the India-Asia continental collision may be the key factor triggered melting of the crust-mantle transition belt. Metal assemblages at Narigongma are dominated by Mo-Cu, which is different from Cu-Au assemblages at Yulong belt. This difference of metal assemblages may be caused by deep magma process, as well as shallow crystalline process. The later process is evident by more and more facts. If it is fact, Narigongma should be a deeply denudated deposit, and porphyry Mo and skarn type deposits should be focused in the next exploration in the area.