| 其他摘要 | Previous researches indicated that tin deposit is generally associated with S-type granite in metallogenesis, but in recent years many tin deposits associated with A-type granite were found. So the study on the relationship between tin deposit and A-type granite has been attracting many geologists.
The Furong tin polymetallic deposit is located at the famous Nanling W-Sn polymetallic mineralization belt; this tin deposit occurs within or near the contact zones of the A-type granite. The study on ore-forming and rock-forming chronology revealed that the ore-forming events and the rock-forming events are sequential. The Furong tin polymetallic deposit is a typical deposit closely associated with the Qitianling A-type granite spatially and temporally, and so the Qitianling granite was selected as an object in this dissertation. Based on the detailed field investigation, we have made a systematic study on petrology, petrogenesis, physical and chemical conditions of the rock formation, and the characteristics of the fluid which was extracted from the granitic magma, the characteristics of the volatile in the granite, and the relationship between tin deposit and the granite and so on. We attempt to probe into the mechanism of fluid congregation and its control in mineralization during the formation of the Qitianling A-type granite, and to reveal the essential relationship between the A-type granite and tin deposit. Several conclusions obtained in this study are as follows:
1. By means of the major and trace element geochemistry, and isotopic geochemistry of the Qitianling granite, as well as the trace element geochemistry of the biotite of the Qitianling granite, it was indicated that the Qitianling hornblende biotite granite and the biotite granite both are highly differentiated granite, characterized by high Si, high K, and enrichment in alkali and Al. The granite tended to be Si-enriched and alkali enriched with the magmatic evolution of the Qitianling granite. The fractionation between LREE and HREE in granite is obvious, possessing a mediun-strong negative Eu anomaly. They are enriched in large ion lithophile elements such as Rb, Th, and high field strength elements such as Zr, Hf, depleted in Ba, Nb, Sr, P, Ti. The characteristics of Sr and Nd isotope for two stages of the Qitianling granite are similar each other, and they are congenetic, belonging to A2-type granite of crust mantle mixing
2. The systematic studies on mineralogy, mineral chemistry, and the contents of Sn in granite indicated that the temperatures and the oxygen fugacities（logfO2） of the hornblende biotite granite crystallization are 774~796℃ and -15.30~-15.00, and that of the biotite granite crystallization are 714~784℃ and -17.5~-20.00, respectively. With the magmatic evolution, the temperature and oxygen fugacity decreased from the hornblende biotite granite to the biotite granite. With the progress of magmatic evolution, the contents of Cl in the granite decreased, while the contents of F increased. Cl is favourable to enter into aqueous fluid. With the increasing differentiation degree, the temperature of the granite crystallization decreased, oxygen fugacity decreased, and the contents of Sn decreased. Tin tends to distribute into Cl-enriched fluid. The Cl-rich and Sn-rich fluid can be separated from biotite granitic magma during the period of the magmatic evolution.
3. By studying the inclusions in quartz of the Qitianling granite, it was indicated that the hornblende biotite granite and the biotite granite experienced two stages -magmatic stage and magma-fluid transitional stage. The melt inclusions and fluid-melt inclusions are the immediate evidence, respectively. Combining with the characteristics of the mineral structure, it was suggested that the magmatic fluid can be derived from the Qitianling granite during the magma evolution. The fluid of H2O-CO2-NaCl-KCl-CaCl2 system can be extracted with the characteristics of high temperature(190~ 494℃) and high salinity(32.98~52.04Wt%NaCleq.), under the pressure of 600~800bar. Together with the previous researches, it can be concluded that the Cl-rich, Sn-rich ore-forming fluid was extracted from the Qitianling granite, belonging to the magmatic hydrothermal fluid generated during the period of magmatic crystallization.
4. Based on the studies of the granites and tin deposits, it is confirmed that tin-polymetallic mineralization has a close temporal, spatial and genetic relationship with the granite. Tin deposits occur within or near the contact zones of granitic intrusions. The ore-forming ages are coincided well with the rock-forming ages. With the magmatic evolution, the temperature and oxygen fugacity of the granite crystallization decreased，the contents of Cl in granite decreased, and the contents of F in granite increased. Cl is favourable to enter into aqueous fluid, and tin tends to distribute into this fluid, and predominately presents as Sn-Cl complex. It was realized that Cl-rich and Sn-rich fluid can be extracted from the Qitianling granite. The ore-forming fluid of the Furong tin polymetallic deposit is the magmatic hydrothermal fluid which extracted during the period of magmatic crystallization. |
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