科研之友
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摘要
The Yangchuling porphyry W-Mo deposit is located in the middle part of the Jiangnan orogenic belt, which is recognized as the first typical porphyry type W-Mo deposit in South china. Reporting 63100t of WO3 with an average grade of 0. 2% and 16900t of Mo metal with a grade range of 0. 03% similar to 0. 06%. The W-Mo mineralization is closely related to the Mesozoic granitic magmatism. Disseminated W mineralization is intimately associated with the late-stage monzogranite porphyry whereas the early-stage granodiorite is barren, it remains obscure as to the relationship between two types of magmatic rocks and what control their contrasting ore-forming potential. In this study, we compared the major and trace elements of biotites from the monzogranite porphyry and granodiorite, respectively, to explore their geochemical differences in magma differentiation, oxygen fugacity and halegon fugacity that may affect their mineralization potential. Major elements compositions of biotites in Yangchuling show that both granodiorite and monzogranite porphyry are derived from crust material, significant different Mg/( Fe + Mg) ratio and Al-VI values from the biotite in S-type granite worldwide indicating they are I-type granite. Trace elements of biotite can used to evaluate the differentiation degree of magma. The lower K/Rb, Nb/Ta ratios and higher Rb, Cs, Nb and Ta contents in biotite from monzogranite porphyry relative to biotite in granodiorite indicate higher differentiation degree of monzogranite porphyry, which is conducive to the enrichment of W. Both of magma lies near the NNO buffer line, but monzogranite porphyry shows more reducing characteristics, and the reducing conditions are conducive to mineralization. The biotite ( type 2) in the monzongranite porphyry has a lower IV ( F) and a higher IV ( F/Cl) value indicating higher F enrichment relative to biotite ( type 1) in the granodiorite, The halogen fugacity log (fH(2)O/fHF)(fluid), log (fH(2)O/ fHCl)(fluid), log(fHF/fHCl)(fluid) of associated magmatic fluid calculated by biotite show type 2 has a lower log (fH(2)O/fHF)(fluid) and log (fH(2)O/fHCl)(fluid) than type 1 , suggesting monzogranite porphyry is richer in HF and HCl. Higher log(fHF/fHCl)(fluid) in type 2 reflecting higher HF/HCl ratio, further indicating higher evolution degree. The Yangchuling monzogranites were derived from fractional crystallization of grandiorites, it is apparent that through differentiation the volatile components F and Cl, and W and Mo metals were gradually transferred from the granodiorite intrusion to the smaller monzogranitic porphyry, where the W-Mo ore system formed. This finding demonstrates that biotites are effective in indicating magma source and magma differentiation and can be used as a robust tool in mineral exploration.
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