东秦岭金堆城大型斑岩钼矿床同位素及元素地球化学研究

Isotopic and element geochemical study of Jinduicheng superlarge porphyry Mo deposit in East Qinling area

文章系统研究了金堆城钼矿床的含矿钾长斑岩、围岩、矿石、矿石中的黄铁矿及矿化围岩的地球化学特征,深入探讨了矿区成矿物质的来源。金堆城含矿斑岩的稀土元素分布和特征地球化学参数显示,金堆城含矿斑岩富集LREE(LaN/YbN=5.40～16.84),轻、重稀土元素分馏较明显,Eu异常不明显或无Eu异常(δEu=0.70～0.96)。矿石中黄铁矿富集LREE(LaN/YbN=3.15～26.28),具有弱的Eu负异常,无Ce异常(δEu=0.64～0.81,δCe=0.88～1.03),并与金堆城含矿钾长斑岩和矿石具有一致的球粒陨石标准化配分曲线和特征的地球化学参数,显示金堆城钼矿床的成矿物质来源与钾长斑岩同源。矿床铅、硫、碳和氢-氧同位素地球化学综合研究表明,成矿物质来源于深部,与钾长斑岩同源。围岩在矿化和蚀变过程中元素的迁移计算结果表明,在热液成矿过程中Mo随成矿流体加入到围岩并使围岩发生蚀变和矿化。钼矿床的成矿物质主要来自钾长斑岩,围岩不提供成矿物质。金堆城含矿斑岩和钼矿化的发生处于秦岭造山带在中新生代的挤压-伸展转变期,受板片断离作用和壳幔边界附近发生的基性岩浆底侵作用影响,加厚的华北地块南缘下地壳物质发生熔融形成花岗质岩浆,并沿构造薄弱带上升到浅部侵位,形成金堆城等同熔型斑岩和斑岩型矿床。

This paper made a systemic study of isotopic and element geochemical characteristics of the ore-bearing porphyry, wall rocks, ores, pyrites in the ore and mineralized wall rocks in the Jinduicheng Mo deposit, with a discussion on the origin of the ore-forming materials. The results indicate that the Jinduicheng ore-bearing porphyry is enriched with light rare earth elements （LREE） and has obvious fractionation between LREE and HREE as well as indistinct or no negative Eu anomalies （δEu = 0.70- 0.96）. Four pyrite samples are enriched with LREE and have slightly negative Eu and no Ce （δEu= 0.64-0.81, δCe= 0.88-1.03） anomalies. In addition, their chondrite-normalized REE patterns are similar to those of Jinduicheng porphyry and other types of ores. All these characteristics indicate that the ore-bearing hydrothermal fluids were derived from the porphyry. Lead, sulfur, carbon and hydrogen-oxygen isotopic characteristics show that the ore-forming materials of the Jinduicheng deposit had the same source as the porphyry, and they were all derived from the lower crust and the upper mantle. Immobility isocon diagrams were plotted based on the element transform formula during alteration. The slope of the immobility isocon straight line represents the fundamental ratio of the rock mass before alteration to the rock mass after alteration. These diagrams show that Mo was consistently and significantly added to the country rocks from the Jinduicheng porphyry. All this has led the authors to reach the concludion that the ore-forming materials were mainly derived from the porphyry instead of from the wall rocks. The Jinduicheng porphyry Mo deposit was formed during the transforming stage of the Qinling Orogen from compression to extension in Mesozoic-Cenozoic period. Influenced by the slab breaking off and underplating of basaltic magma from the boundary between the crust and the mantle, the thickening lower crust at the south edge of the North China plate was subjected to partial melting and generated granitic magma. With the magma migrating upward to the upper crust along the structural loci, the ore-forming fluid generated by magmatic condensation and fractionation filled the cracks and replaced the wall rocks, and finally resulted in the formation of the Jin- duicheng Mo deposit.