Study On Yanshanian Magmatic To Hydrothermal Mineralization Processes Of Major Tin-Polymetallic Deposit-concentrated Area In South Hunan

A great number of data have been collected and sorted out on the metallogenic geological settings and characteristics as well as magmatism and polymetallic metallogeny of major tin-polymetallic deposit-concentrated areas in South Hunan. Meanwhile, a detailed study has been conducted on the typical deposits and magmatic plutons, especially ore-forming fluids with their activities, including composition and character of ore-forming fluids and their large scale migration mechanism, as well as relationship between ore-forming fluid action and metallogeny. Based on the data and study, a discussion has been made on the Yanshanian granitic magmatic to hydrothermal mineralization processes. The following are the achievements that have been acquired in this study. 1. A summarization on the regional tectonic evolution history and tectonic pattern in South Hunan combined with an analysis on the geochemical background of regional magmatism and metallogeny has been made, leading to inference that the development of different grade faults in NE and NW directions could play an important role in the Yanshanian large scale mineralization. It is suggested that surrounding rocks with assemblages of special lithologies are closely related to regional metallogeny. 2. It is proposed based on the summarization of characteristics of several typical felsic plutons and features of magmatic activities that a large part of materials for mineralization in the area might be derived from the Yanshanian felsic plutons. Analyzing with comparison on geochemical characteristics of the felsic plutons exampled by the Qitianling and Qianlishan plutons gives a clue to better understanding regularities of temporal and spatial evolution of the magmatic activities associated with metallogeny in South Hunan. 3. The polymetallic deposits dominated by tin, tungsten, lead and zinc deposits in the deposit-concentrated areas are classified into hydrothermal, skarn and stratabound-like hydrothermal types of deposits. Investigation on major types of ore deposits, especially the Bailashui tin deposit within the Furong ore field provides new insight into understanding the relationship between regional mineralization and Yanshanian magmatic activities. 4. Fluid inclusion measurement combined with sulfur, lead, hydrogen and oxygen isotopic analysis on ores from the typical deposits leads to preliminary ascertaining characteristics of the Yanshanian ore-forming fluids and derivations of ore-forming materials. The ore-forming fluids are rich in alkalis and volatiles, and totally belong to a NaCl～KCl～CaSO4～H2O type. The mineralization age is measured by using Rb-Sr dating of fluid inclusions to be 177±3 Ma. Sulfur isotopic analyses are consistent with derivation of sulfur from a mixed source in deep crust, and lead isotopic analyses support derivation of lead from multiple sources. 5. A preliminary model for the regional granitic magmatic to hydrothermal mineralization processes is set up on the basis of the researches on tectonic setting, mineralization and alteration, ore characteristics, and influence of physicochemical condition variation on metallogeny combined with the analyses on circulation of ore-forming fluids, enrichment of ore-forming materials in the fluids, and precipitation of metals from the fluids.