斑岩钼-热液脉状铅锌银矿成矿系统特征、控制因素及勘查指示

Characteristics,controlling factors and exploration implications of porphyry molybdenum-hydrothermal vein-style lead-zinc-silver metallogenic systems

斑岩钼矿与热液脉状铅锌银矿为两类重要的矿床类型,两者往往分别独立产出,但越来越来的勘查实例揭示二者也可共生产出,构成统一的成矿系统。斑岩钼-热液脉状铅锌银成矿系统,主要分布在北美西部、加拿大西南部、中国秦岭-大别地区、华北北缘及西拉沐伦带、大兴安岭北段-额尔古纳等地区。根据斑岩钼矿与热液脉状铅锌银矿的平面关系,成矿系统可分为近源和远源两类:近源时,两者直接叠置或者平面距离小于2km;而远源时,两类矿化平面距离一般不超过6km。成矿系统空间上表现可为上铅-锌-银、下钼的垂向叠置或者内钼、外铅-锌-银侧向共存的形式。时间上两类矿化一般近同期形成,或者相差通常不超过8Myr。成矿系统岩浆性质多为高演化的钙碱性花岗质岩浆,起源于下地壳且加入了不同比例的地幔物质。成矿系统的蚀变特征一般为斑岩钼矿化蚀变向热液脉状铅锌银矿蚀变的渐变,其中粘土化带与绢英岩化带是两类矿床的叠加区。钼矿化常与钾硅酸盐化或者绢英岩化带内侧密切相关,铅锌银矿化则常与浅部的低温硅化-绢云母-伊利石-水白云母化、碳酸盐化密切相关。基于S、Pb、Sr、Nd等同位素研究成果,钼铅锌银系统中成矿物质主要为岩浆来源,但可能有地层物质的加入。成矿流体主要以岩浆水来源为主,初始流体通常为单相中低密度流体,辉钼矿沉淀往往伴随着减压沸腾、大气水混合、冷却及/或水岩反应的进行,发生大规模钼矿化的温度区间通常在300~450℃。浅部脉状铅锌银矿化则由持续降温的流体在混入较多大气水或流体p H值中和而形成,温度区间在175~320℃。成矿系统空间上钼-铅-锌-银的分带,可能受控于流体演化过程中上述多个过程的综合叠加作用。通过总结对比钼铅锌银成矿系统、单一斑岩钼矿、单一热液脉状铅锌银矿床在勘查历史、构造因素、成矿岩体属性、流体特征、特征矿物、地球物理-地球化学勘查指标等方面的异同,本文提出了指示浅部热液脉状铅锌银矿之下同一成矿系统深部斑岩钼矿的找矿标志,且对该成矿系统形成的岩浆性质、岩浆-热液系统、成矿元素、构造条件、保存条件等多个方面进行了探讨。在前人基础上,本文提出本类成矿系统理论研究展望:1)利用微区原位技术分辨矿物的不同期次及元素的分布状态,进而获得该类型铅锌银矿相对准确的成矿年龄;2)确定斑岩钼-热液脉状铅锌银成矿系统的初始流体成矿元素和相关配位剂元素的含量;3)建立钼铅锌银成矿系统的矿物学指示标志;4)查明成矿系统岩浆过程、元素行为等精细成矿过程,研究其与其他成矿系统的差异。上述问题的深入研究和找矿标志的提出或将提高对斑岩钼-脉状铅锌银成矿系统成矿过程的认识,为该类系统勘查找矿工作提供理论支撑。

Porphyry molybdenum-vein-style lead-zinc-silver mineralization systems are principally distributed in the western part of North America,Qinling-Dabie orogen in China,northern margin of North China Craton,Xilamulun metallogenic belt,and Eerguna district-northern Great Xing’an Range.Based on the alignment relationships between porphyry Mo deposits and vein-style Pb-Zn-Ag deposits,this study divides this kind of mineralization system into proximal and distal types:the proximal type presents those include both types of deposits overlapped or aligned in the distance less than 2 km,and the distal type includes the alignment distance between two types of mineralization from 2 km to 6 km.In the cross-sections,vein-style mineralization can overlap in the upper part or distribute alongside porphyry-type mineralization.The deposit clusters were usually formed in the same period or may remain active for 8 Myr or shorter.The ore-forming magma of the system is tied to highly-evolved calc-alkaline granitic magmatism,and mainly derived from lower crust with different contribution of mantle material.The alteration patterns in the system are featured as the gradual evolution from the porphyry Mo-type to vein-style alteration,and among them,the argillic and phyllic alteration zones represent the transitional,overlapped district.Molybdenite mineralization is characteristic of potassic or inner phyllic zones,and galena+sphalerite+silver-host minerals assemblages often occur in the sericite+illite alteration and low-temperature carbonate alteration in the shallow part.Based on the S,Pb,Sr and Nd isotopic compositions,magma is considered as the predominant source for ore-forming material of the system,and strata may also make some contributions.Hydrothermal fluids causing Mo mineralization are exsolved from magma and showing characteristics of single-phase intermediate-density.This fluid undergoes various processes to precipitation molybdenum,such as decompression,phase separation,cooling,mixing and interaction with host rocks.The temperature range of the major molybdenite precipitation varies from 450℃to 300℃.This same liquid could also cause vein-style Pb-Zn-Ag mineralization in the shallow,peripheral parts of the system,caused by mixing with meteoric water in a great content or neutralization of the fluid acidity(p H value)between 175℃and 320℃.Thus,this combination of a variety of hydrothermal controlling factors in the fluid evolution discussed above could account for the metal zonation in the porphyry Mo-vein-style Pb-Zn-Ag mineralization systems.This study also compiles a dataset of exploration histories,geological features,structural factors,characteristics of causative intrusions,fluid compositions,targeting appraisal for geophysical-geochemical exploration in Mo-Pb-Zn-Ag systems,porphyry Mo deposits,and vein-style Pb-Zn-Ag deposits,to propose the exploration indicators to target the hidden porphyry Mo deposits underlying the vein-style deposits in the shallow.This study also discusses some controlling factors for the system formation,including magma,magmatic-hydrothermal system,ore-forming material,tectonic and preserve conditions.Several key problems needed to be solved in the future research are as follows:(1)to acquire the high-precision geochronologic data of the mineralizing events in the vein-style Pb-Zn-Ag deposits,based on multiple-stage of alteration-mineralization minerals and the element distribution by in situ analytical technology;(2)the determination on the compositions of metal and volatile contents(F,Cl and S)in the primary fluids in the porphyry molybdenum-vein-style lead-zinc-silver mineralization systems;(3)the set-up of mineral indicators for exploration in this system;(4)to understand the mineralization processes such as magma evolution and element behaviors of the systems,and study their differences from other metallogenic systems(i.e.Mo only,Pb-Zn-Ag only deposits).The solving of these problems mentioned above and the advanced discussion of the proposed exploration indicators may help improve the understandings on the Mo-Pb-Zn-Ag mineralization process and provide supports for the exploration of the same mineralization systems.