| 其他摘要 | Abstract
The temporal distribution between the large-scale mineralization and the crustal extension, lithospheres thinning during Mesozoic in South China and their geodynamic setting have attracted considerable attentions in recent years. The alkali-rich intrusion and its associated mineralization were considered as a convincing evidence of the extensional structures in the continental crust. Recent studies showed that the Huashan-Xishan-Xianghualing-Qitianling NE-trending granite belt, characterized by high εNd (t) values and low Nd model ages, was an alkali-rich intrusion belt and accompanied by a large-scale W-Sn polymetallic mineralization. Thus, it is an ideal object for studying the coupling relationship between crustal extension, lithosphere thinning and large-scale W-Sn polymetallic mineralization, and its deep dynamical mechanism. This doctoral dissertation is focused on petrology, elemental geochemistry, isotopic geochemistry and isotopic chronology of the granites and associated Sn-polymetallic deposit in Xianghualing, respectively. Moreover, we discussed the petrogenesis and tectonic setting of the granite in this area, the sources of ore-forming materials and fluids, the time-scale of Sn-polymetallic mineralization, as well as the corresponding metallogenic geodynamic setting. Finally, a preliminary genetic model for the Xianghualing deposit was proposed. The main conclusions are listed as follows:
(1) The studies on the major, trace elements and Sr, Nd isotopes composition of the Laiziling and Jianfengling biotite granites in Xianghualing area, indicated that both intrusions are characterized by high-silica, alkali-rich, LILE-rich, and some HFSE (Zr, Ga). They also present high concentrations of ore-forming elements, but are depleted in Ca, Mg, and Eu. Furthermore, both have high εNd(t) values and low Nd model ages. Thus, both plutons belong to the post-orogenic aluminous A-type granites (A2 type), and are of mantle-crustal mixed origin.
(2) The 40Ar/39Ar stepwise heating dating method was employed on muscovites from the Xianghualing tin polymetallic deposit, Xianghuapu tungsten polymetallic deposit and Jianfengling greisen-type tin polymetallic deposit, respectively. And we also report the first precise TIMS U-Pb isotopic data on cassiterite from the Xianghualing tin polymetallic deposit. The results show that the tungsten-tin polymetallic mineralization are restricted within 150-161 Ma, which coincide well with the K-Ar ages on biotite from the granite in this area, and is consistent with the timing of regional W-Sn mineralization in Southern Hunan.
(3) A systematic study of trace elements, rare earth elements and isotopic geochemistry, combined with previous studies, demonstrated that the Xianghualing tin-polymetallic deposit has a close genetic relationship with the granites in this area. The granites not only provide the thermodynamic conditions, but also provide the main ore-forming materials for mineralization. Besides, the ore-hosted strata probably supplied some ore-forming materials. The ore-forming fluids were mainly derived from the magma at early stage of mineralization. Nevertheless, at the late ore-forming stage, the fluids were mainly derived from a mixture of meteoric and magmatic waters.
(4) The super-enrichment mechanism for tin in the Xianghualing tin-polymetallic deposit is preliminarily discussed. The super-enrichment of tin maybe be ascribed to the following factors. Firstly, the supercritical ore-forming fluids rich in volatile components and ore-forming elements were originated from the highly-evolved granitic magma with high contents of volatile components such as fluorine. The ore-forming elements in host strata were extracted by the supercritical fluid when it flowed through the strata. Then, the ore-forming fluid was highly concentrated by fluid boiling and the consecutive degassing during enclosure pressure decrease. When the fluid became saturated or oversaturated, the deposits formed because of the precipitation of metallic minerals. Finally, the early-formed deposits were further superimposed by later mineralization.
(5) Based on the studies of the granites and their related deposits, we can confirm that tin-polymetallic mineralization has a close temporal, spatial and genetic relationship with the granites in this area. Thus, both of them should be formed under the same tectonic setting. The results obtained in this study, consistent with the published geochronological results from other important deposits in this region, suggest that the large-scale tungsten-tin polymetallic deposits in the central Nanling region were emplaced under the same geodynamic setting which is probably related to the asthenospheric upwelling, crustal extension, lithosphere thinning, and the process of crust-mantle interaction during the transformation of tectonic regimes of the middle-late Jurassic.
(6) Based on the studies above, a preliminary genetic model was proposed. The granitic magma with a mantle-crustal mixed source was formed by the melting of the crustal materials and was accompanied by crust-mantle interaction caused by the ascending asthenosphere under the geodynamic setting of crustal extension and lithosphere thinning. The granitic magma experienced a highly differentiated evolution because of its high content of volatile components. As a result it was fractionated into the tin-bearing granite and has generated the supercritical ore-forming fluid with high-content ore-forming elements and enriched in volatile components. Then, the ascending ore-forming fluid has extracted the ore-forming materials from the granite and the host rocks. Lastly, it was highly concentrated by fluid boiling and consecutive degassing and deposited along favorable structural sites, and superimposed by later mineralization, resulting in the formation of extremely high-grade tin ores. |
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