| 其他摘要 | The Fanshan alunite deposit is located in the southeast of the Jiangshan-Shaoxing deep fault zone and Hercynian-Indosinian fold belt, at the joint of the southeast tectonic zone of the China tectonic plate and the Pacific plate, belonging to the southeast tectonic zone of the China tectonic plate. This deposit is an ultra-large alunite deposit located in the Fanshan caldera. Vanadium and gallium in this deposit can be also used. But it had been poorly studied, even these is no geochemical study about it. Systematic geochemical studies have been carried out including major elements, trace elements, rare earth elements, sulfur isotope and lead isotpe. Base on these studies, the following achievements have been obtained:
1. The ore-forming materials of this deposit derived from the volcanic rocks. The contents of the ore-forming elements such as K, Al and Na in the volcanic rocks of mining area are higher than those in others areas. And the ore-forming elements represent obviously enrichment trend from country rocks to mineralization rocks and to alunite ores. The evidence that come from REE and lead isotope also indicate the ore-forming materials came from the volcanic rocks.
2. The contents of V and Ga of ores have arrived the comprehensive utilized grade, the average content of vanadium in ores is 211.6 ppm, and the average content of gallium is 16.7 ppm. And we suggested that vanadium entered into alunite crystal lattice by means of isomorphism as V5+ replaced the Al3+, while the content of gallium is correlated with the contents both Al3＋ and Fe3＋.
3. REE geochemical characteristics of alunite are complicated. According to the differences in δEu, they can be divided into three types: weak negative Eu anomaly, weak positive Eu anomaly and remarkable positive Eu anomaly. Main factors controlled the REE distribution patterns are the content of alkaline feldspar in source rocks, the fO2 and temperature during the mineralization period. And the ore texture such as the porosity could also effect it. The ores REE distribution patterns is LREE-enrichment pattern, as same as that of the volcanic rocks.
4. The δ34S of pyrite ranges from 1.9 to 3.2‰, while the alunite is 13.62-16.02‰. We considered that the values of δ34Spy represent the primary value of magmatic sulfur, while the values of δ34Saln represent the sulfur value after isotopic fractionation. The ores lead isotope compositions are 206Pb/204Pb=17.963-18.606, 207Pb/204Pb＝15.439-15.672, 208Pb/204Pb＝38.405-38.796. Compared with the lead isotope compositions of the Mesozoic volcanic rocks and basement metamorphic rocks, the ore lead came from the Mesozoic volcanic rocks and had nothing with the basement metamorphic rocks.
5. According to all the studies on geochemical characteristics of Fanshan alunite deposit, and combining with geological characters and references, the following ore genesis was proposed: the ore-forming environment is supergene, low temperature and oxidized; the mineralization materials came from the country rocks, and the source of sulfur came from magmatic sulfur that formed after isotopic fractionation; the mineralization age is 73-95 Ma, 10-20 Ma later than that of the wall rocks; the ore genesis is volcanic hydrothermal replacement deposit.
The Zhejiang Mesozoic volcanic mineralization system mainly refers a series different types ore deposits assemblages that is controlled by the Mesozoic volcanic structures and magmatic activities in Zhejiang. Jiangshan-Shaoxing deep fault zone and continental volcanism of Mesozoic control this mineralization system. Many studies about single deposit in this system had been done, but the comparative research of the deposits was lacked. In this paper, we have elementarily studied the connection between metallic ore deposits and nonmetallic ore deposits base on the comparism of these two types of deposits. And we have also studied the correlation between the mineralization system and evolution of Mesozoic volcanic rock by connected with the evolution process of Mesozoic volcanism. Base on these, some points were achieved:
1. The distributions of all types of deposits of this system are controlled by deep or large faults such as Jiangshan-Shaoxing deep fault and Wenzhou-Zhenhai large fault. The ore-contol structures (or host structure) of deposits are 2nd ord compress-shear faults and caldera, especially the broken craters are very important in ore-forming processes.
2. All deposits of the mineralization system are typical epithermal deposits.
3. The lead isotope researches indicated the lead of all deposits derived from the Mesozoic volcanic rocks, and have not connection with the basement metamorphic rocks directly. According to the evidence of hydrogen-oxygen isotopes, mineralizing fluid of the deposits came from meteotric water of Mesozoic, while few magmatic water was involved in the ore-forming process.
4. Obvious time differences between mineralization and petrogenesis are existed, for metallic ore deposits is 12.44-45.6 Ma, for Wuyi fluorite ore deposits is 25-75 Ma and for other nonmetallic ore deposits is 10-20 Ma. And the Pb-Zn-Ag (Au) deposits represented obviously the character of two-period ore-forming process. |
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