| The Middle-Lower Yangtze River Valley metallogenic belt (MLYRB) is one of the most important Fe-Cu-polymetallic metallogenic belts in eastern China, and has been studied extensively in the past. Various hypotheses have been proposed concerning the regional metallogeny, with major ones including the "porphyrite iron deposit ", "stratabound skarn deposits" and the "superimposed metallogenic system’ With the discoveries of large porphyry Cu-Au deposits at Shaxi, Shujiadian and Baiyunshan in the recent years, porphyry deposits are becoming important exploration targets in the MLYB. Since there is little systematic regional study on the MLYB porphyry Cu-Au deposits. In order to clarify the diagenetic and metallogenic epoch, the ore genesis, metallogenic regularity and the relationship between the magmatism and the mineralization many comprehensive research work needs to do. This paper taking the Shaxi porphyry Cu-Au deposit, Shujiadian porphyry Cu deposit and Dongguashan deep-seated porphyry Cu-Au deposit as the subject, based on the summarise of previous studies, by the means of detailed geological investigation, sampling and microscope observation, combine with geochemistry, geochronology, fluid inclusion geochemistry and in-situ laser ablation trace elements analysis methods, aims to achieve a comprehensive understanding of magmatism and mineralization.The Shaxi, Shujiadian and Dongguashan deposit have typical porphyry-style alteration, including potassic, propylitic and phyllic alteration, which have spatial zonality, and potassic, phyllic alteration are close to mineralization. On the basis of petrographic studies, zircon U-Pb dating and molybdenite Re-Os dating, we find the emplacement sequences of the intrusive rocks in Shaxi intrusion are coarse grained diorite porphyry, biotite quartz diorite porphyry, medium quartz diorite porphyry, fine quartz diorite porphyry, mafic rock and diorite porphyry, the ages of which are 130.60 ±0.97Ma,129.30±1.00Ma,127.10±1.50Ma,129.46±0.97Ma,128.8±3.4Ma and 126.7±2.1Ma, and mineralization occurred at 130.0±1.0Ma. The emplacement sequences of the intrusive rocks in Shujiadian intrusion are quartz diorite porphyry, pyroxene diorite, quartz diorite and granite porphyry, the ages of which are 143.7± 1.7Ma,139.3 ±1.8Ma,139.0±1.2 Ma and 128.3±2.4Ma, and mineralization occurred at 140.6±2.0Ma. The Dongguashan quartz diorite crystallize at 140.3±2.0Ma. Combined with previous age data, we The MLYRB porphyritic magmatism and associated mineralization could be subdivided into two stages, i.e., the Early (149-137Ma), and Late (135～127Ma) Stage.Geological characteristics of whole-rock and minerals suggest that the intrusive rocks of Shujiadian intrusion and Dongguashan intrusion are high-K calc-alkaline, and which of Shaxi intrusion are calc-alkaline. All of them are the product of the mixing magma of an enriched mantle-derived mafic magma and a felsic magma generated by partial melting of the thickened lower crust. Mafic magma played a key role and made the parental magma rich in water, sulfur, metals (Cu, Au) and gave it a high oxygen fugacity.Studies of LA-ICP-MS trace elements of ore minerals, fluid inclusions and isotopes suggest that metallic elements in the porphyry deposits of MLYRB were mainly derived from magmatic hydrothermal fluids, and mixed with other fluids (e.g., meteoric water, formation water). The hydrothermal evolution of Shaxi, Shujiadian and Dongguashan deposit can be divided into four episodes:early boiling, overpressuring, second boiling and mixing with other fluid (e.g., meteoric water, formation water). Second boiling is the main factor for metal precipitation, and the changes of oxygen fugacity and sulfur content are also the factors for metal precipitation. Also, Shaxi, Shujiadian and Dongguashan deposit suffered from varying degrees of erosion (about 1km).Compared with porphyry deposits in magmatic arc settings, the MLYRB porphyry deposits contain fundamentally the same geological characteristics as typical magmatic arc-generated porphyry deposits, but shallow advanced argillic lithocaps and shallow high sulfidation alteration is absent. The nature of magma source at the MLYRB are also different from those of typical magmatic arc-generated porphyry deposits. Based on the above comprehensive study results, we discussed the genesis of the main deposits, and propose a metallogenic model of the MLYRB porphyry deposits. |
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