Characteristics, Controlling Factors And Exploration Implications Of Porphyry Molybdenum-Hydrothermal Vein-Style Lead-Zinc-Silver Metallogenic Systems.

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 2km, and the distal type includes the alignment distance between two types of mineralization from 2km to 6km. 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 8Myr 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 degrees C to 300 degrees C. 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 (pH value) between 175 degrees C and 320 degrees C. 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.