| 其他摘要 | Tin deposit is a typical deposit closely related to granite in spatial, temporal and metallogenic. Most granite associated with tin deposits is peraluminous, K-rich, and Si-rich. Traditionally, it is well known that tin deposits are always associated with S-type granite. But, in recent years, some important economic tin deposits which are closely associated with peralkaline intrusive have been found in many countries. To do research on the metallogeny of this new type tin deposit has been carried out, and it is one research hotspot in mineral deposit geology. Though, there are some important results about the new type tin deposit, but it is still controversial whether peralkaline intrusive could differentiate tin-rich aqueous fluid. Metal mineralization associated with intrusions depends, to a great extent, on partitioning of metallic elements between melt and fluid. The partition behaviour of metallic elements depends on the composition of melt, the composition of fluid derived from the melt and the physicochemical conditions under which the partitioning occurs. The previous experiments on tin partitioning have focused mostly on various aqueous fluid, but less data are available on different melt systems, as well as they were conducted in a single chlorine- or single fluorine-bearing system. Up to now, tin partitioning behaviors in different stage of magma evolving are still not clearly known. In order to understand tin mineralization associated with granite and find valuable information about tin ore formation, experimental study on tin partitioning between granitic silicate melt and coexisting fluid has been conducted. Then, the favorable factors of tin mineralization are concluded based on the geochemical characteristics of peralkaline granite associated with tin deposit, and the distribution behaviors of tin in crystals, melts and aqueous fluid phases. The results provide important new experimental information for investigating on tin metallogenic mechanism, and expanding tin deposit formation theory. The experiment is significance to experimental geochemistry developing.
These experiments were conducted in cold seal rapid quench (RQV) pressure vessels with physical conditions of 850℃, 100MPa and fo2 near NNO at Ore Forming Laboratory in the State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences. Synthesized haplogranitic gels with different compositions and different solutions made from reagent-grade chemicals were adopted as starting melts and starting fluids, respectivly. The experiments include three series as following.
1. Tin partition coefficients between the same K-rich peralkaline melt and different solutions have been determined. Different solution of NaCl, KCl, HCl, HF or deionized water was employed as starting fluid in these experiments.
2. The distribution of tin between different granitic melt (with different ASI or Na/K mole ratio) and coexisting aqueous fluid of 0.1mol/L HCl solution has been determined.
3. Tin distribution in F and Cl coexisting granitic magma has been determined. Starting melt added various quantities of (NaF+KF) mixture and starting fluid with different HCl content were employed in the experiments.
The results are following.
1. Different ligands and their concentration in the fluid phase obviously affect tin partitioning behavior. Tin partition coefficient D Sn increases with fluorine or chlorine existing in aqueous fluids. Especially, tin inclines to partition to aqueous fluid with pH low and high chlorine content; D Sn increases rapidly with increasing HCl content in aqueous fluid, the relationship is described as logD Sn= 2.0247×log[HCl]＋0.6717 ([HCl] unit is M), and SnCl2 is the dominant tin-bearing complex in aqueous fluid. Furthermore, ASI of melt after equilibrium with high HCl concentration in the aqueous fluid phase will increase because alkalis in melt were transported to aqueous fluid phase.
2. Tin distribution coefficients are constrained by the composition of melts. D Sn decreases with increasing alkali content in melt and increases with increasing ASI in melt, respectively. The relationship is described by D Sn=－0.0489×MAlk＋0.4516, R2=0.98 (where MAlk is the Na2O+K2O mole content in melt), and D Sn=0.1886×ASI－0.1256, R2=0.99, respectively. Tin distribution coefficients are correlated negatively with Na/K mole ratio in melt when the alkalinity and other components concentrations are relatively constant in the peraluminous melts, the relationship is described as D Sn=－0.0314×RNa/K＋0.0483, R2=0.82 (where RNa/K is the Na/K mole ratio in melt). This relationship implies that Na-rich melt is a factor favorable to tin partitioning in melt phase, and that K-rich melt is favorable to tin distribution in aqueous fluids.
3. The results of tin distribution in F and Cl coexisting granitic magma system show as follows. ① Cl distribution between melt and coexisting aqueous fluid is obviously effected by F content in melt, and decreasing F content in melt is favorable for tin partitioning to aqueous fluid phase.② D Sn is less than 0.1 with a little variation when F content in melt is more than about 1 wt%, but D Sn is increasing rapidly with F content decreased from about 1 wt%. The results suggest that granitic silicate melt with high F content (more than about 1 wt%) could extract tin and enrich tin in the melt. ③D Sn increases with increase ASI in melt phase and increase HCl content in aqueous fluid phase, which imply that tin tends to partitioning to aqueous fluid phase in peraluminous granitic melt and HCl-rich fluid magma system.
According and analyzing the character of tin mineralization associated with granite and the behavior of tin distribrution between different crystal and melt, it is concluded that tin is incline to residual melt and aqueous fluid phase in crystallizing and differentiating process when the magma is of aluminum, peralkline, high volotiles, and low Ca, Fe and Mg. The highly evolved residual magma could extract tin and enrich tin in the melt which can be served as a tin ore reservoir for late tin deposit formation. tin-rich aqueous fluid could derived from the late magma when pressure, temperature, F content in melt phase decreasing, and water saturation, silica content in melt phase increasing.
Based on the conclusion above, we analyzed and discussed the petrochemistry and the physicochemical conditions of Qi Tian Lining granite which is closely associated with Fu Rong tin deposit in Hu Nan province. It is concluded that mineralized aqueous fluid of Fu Rong tin deposit could derived from the late magma in Qi Tian Ling peralkline intrusive.
In conclusion, 1) Tin tends to partition to granitic silicate melt phase when the magma system is of water undersaturated and with deficient ligands in aqueous fluids phase; 2) Tin inclines to distribute in aqueous fluid phase when the magma system is K-rich peraluminous granitic silicate melts with saturated water and aboundant volatiles; 3) K-rich peraluminous peralkaline intrusive with high volatile contents could differentiate tin-rich aqueous fluid under favorable physicochemical conditions；Qi Tian Ling peralkline intrusive can differentiate mineralized aqueous fluid for the formation of Fu Rong tin deposit. |
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