The Diagenesis And Metallogenesis Research Of Mesozoic Magmatic Rocks In Tongling Region, Anhui Province

The Tongling region of Anhui Province is an important ore district of the famous Lower and Middle Yangtze River Cu-Au-Fe-S metallogenic belt in China, and also a good example of skarn deposits in China. Many researchers have documented that the metal deposits are closely related to the Mesozoic intrusive rocks in the region. Although plentiful and substantial results in the magmatic rocks have been obtained, many questions remain unsolved, the crucial one is the genesis of intrusive rocks and tectonic setting, the key problem is chronology of these intrusive rocks.In this study, mineralogy, petrology, element and isotopic geochemistry, LA-1CP-MS zircon chronology of three-type Mesozoic intrusives (i.e., pyroxene diorite-monzonite, quartz diorite-monzonite and granodiorite assemblies related with ore-forming) were systematically investigated in the Tongling region, respectively. We also firstly obtained Re-Os precise dating of pyrite in Xinqiao ore deposit. The results are as follows:1. The mixing of magmas was likely the main function of granitoid crystallization, on the basis of petrology and mineralogy characteristics of the intrusive rocks.2. Evidences from fluid inclusion characteristics of the intrusive rocks and ore deposits manifest that ore-forming fluids of the Cu-Au deposits are mostly magmatic in origin. The processes of cooling down, decompress and boiling possibly led to enrichments of large amounts of copper and gold during rock-fluid interaction.3. The ore-bearing intrusive rocks are mainly alkaline and sub-alkaline series, high-K calc-alkaline series and shoshonitic series. Some geochemical characteristics are consistent to that of adakite.4. The ore-bearing intrusive rocks aren't single but magma mixture of more than two compositional end-members, i.e., the mixing of mantle-derived and crust-derived magmas, the possible end-member derived from subduction of West Pacific plate. This is possibly the distinguished features of the rock-forming mechanism in the Tongling region.5. The results of high precise zircon U-Pb isotopic ages present the orders of intrusions in Tongling region: granodiorite→pyroxene diorite-pyroxene monzodiorite→quartz diorite-quartz monzodiorite. The zircon U-Pb ages are also identical to other Ar-Ar, Rb-Sr isotopic ages within errors, manifesting the intrusive rocks experienced fast cooling during and/or immediately after crystallization.6. Laser ablation ICP-MS zircon U-Pb dating yielded two groups of ages (c.a. 130-132 Ma and 138-140 Ma) for the intrusive rocks, which are the most important magmatic rocks in Tongling area related to Cu-Au deposits. Combined the high-precision zircon U-Pb datings and pyrite Re-Os age for Xinqiao ore deposit with zircon trace element measurements, the one ore-forming event with high Ce abnormality (molybdenite Re-Os age of 138-140Ma) was closely related to copper and gold ore deposit, another probably ore-forming event with low Ce abnormality (about 126 Ma) closely related to iron and sulfur ore deposits in the Tongling region.7. The results of petrochemistry and Sr-Nd-Pb isotopes clearly indicate that West Pacific subduction was the dominant factor controlled Mesozoic ore-forming intermediate intrusive rocks in Tongling, which is the major tectonic event in eastern China in the Early Cretaceous. The geochemical characteristics of these intrusives were likely formed in magmatic arc in land edge. The subducted slab released high fO₂ aqueous supercritical fluid, melting of the mantle wedge formed mother magmas enriched volatilizations (S and so on) and ore-forming elements (Cu, Au and so on). The mixing of the mother magmas and the crustal matters released Cu, Au-bearing epithermal. These ore metal-bearing epithermal ultimately formed ore deposits in the Tongling region.