A Study On The Geochemistry Of The Rocks And Minerals In The Granite Depression Zones In The Gejiu Tin-polymetallic Orefield

The Gejiu tin-polymetallic orefield is one of the most important production localities of tin in the world. In the depth of the Gejiu Orefield, the so-called granite depression zones, with special features in occurrence, were found in recent years, which were considered to bear close relations with tin-polymetallic mineralization and thus regarded as an important indication for ore prospecting.This thesis, starting from a summary of the spatial distribution and accompanied orebodies of the granite depression zones, investigates the petrology, petrochemistry, mineralogy, fluid inclusions and trace elements geochemistry of the granite depression zones and the relationship to ore mineralization, using modem analytical techniques, including microscope, SEM, ICP-MS, liquid and gas chromatograph etc. Conclusions and viewpoints obtained by this study are as follows:1. It is shown from field survey that the granite depression zones mainly occur around the protrusions of the Laoka rockbody, including East, West, North and Xinshan Depression Zones. The East Zone mainly hosts copper orebodies containing tin. The West Zone hosts tin orebodies containing copper. The North Zone hosts both tin and copper mineralizations. The Xinshan Zone mainly hosts copper orebodied accompanied with tin and polymetals.2. The microscopic studies of the rocks and ores from the depression zones show that pervasive alterations were present in earlier-formed basalts as a result of the infiltration of the ore-forming fluids brought along by granite intrusions, and an overprint of mineralization was exerted on earlier mineralization. The complex paragenetic relashioship among ore minerals suggests that multi-stage mineralizations, in which the ore embryos in basalts might be reformed and superposed to form rich orebodies by and later granite intrusions.3. Pervasive phlogopite alteration of basalt was confirmed from microscopic observation and mineral chemistry analysis, leading to the increase of acidity of the fluid, and in turn facilitating the activation and migration of tin, thus resulting in the association of tin mineralization and metal sulfides.4. The petrochemical and trace dements evidences indicate that the basalt in the depression zones belongs to continental alkali peridotic basalt with slight negative Eu anormaly and the increase in Mg and derease inΣREE occurred due to later hydrothermal alteration. The granite in the depression zones belongs to magmatic granite of continental crust-reformation type, with high degree of differentiation, favoring the mineralization of tin as compared to the granite far from the depression zones.5. The study of trace elements geochemistry shows that Sn-Cu-polymetallic mineraltion in the depression zones is closely related to granite. The trace elements of the meta-volcanic rocks is featured by the hydrothermal reformation of the Yanshanian granite and thus not an important source supplier for tin mineralization. Independent enrichments of Au, W, Mo, As etc in granite, meta-volcanic rocks and skarn may indicate the characteristics of the Yanshanian hydrothermal activites and multi-stages of mineralization.6. It is inferred from this study that the metallogenesis of the granite depression zones underwent four stages: the first stage, as represented by the widespread skarn alteration, was produced from the interaction between the granite and the carbonate country rocks in the process of the granite intrusion; the second, as shown by pervasive phlogopite alteration in basalt, resulted from the fluid related to granite interacting with basalt; the third, as featured by widespred silicification and sericitization of granite, was caused by late hydrothermal fluid altering granite and country rocks; the fourth stage was dominated by the formation of polymetallic orebodies after the precipitation of metal complexes from the fluid under suitable conditions of temperature, pressure and media chemistry in the depression zones.