Geological Characteristics And Genesis Of The Ruanjiawan Cu-Mo-W And Yinshan Pb-Zn-Ag Deposits

The E’dong Fe-Cu pollymetallic distinct is located in southeast Hubei Province and west of the well-known middle and lower Yangtze Metallogenic Belt. Ore deposits in this district mainly consist of Fe-Cu deposits, with minor amounts of Mo, Au, Pb, W, and Zn deposits. These deposits are spatially related to the intrusive rocks, and mineralization mostly occurred at the contact between the intrusive rocks and marine carbonate rocks. The Ruanjiawan Cu-Mo-W deposit and Yinshan Pb-Zn-Ag deposit are located in southern margin of the Yangxin pluton, and are the biggest W deposits and Pb-Zn deposits in the E’dong district, respectively. Study of geological characteristics and genesis of these deposits potentially provide insights into metallogenesis and ore-forming processes in this district. In this thesis, the author caiesry out detailed mineralogical, geochemical, and geochronological investigations to provide a better understanding in the genesis of district-wide mineralization. Based on detailed and systematic field work, the ore deposits and related intrusive rocks were characterized by optical microscopy; X-ray fluorescence (XRF) and Inductively coupled plasma mass spectrometry (ICP-MS, zircon and titanite U-Pb dating, and (LA-ICPMS trace element analysis of sulfides; Sr-Nd and S-C-O isotope analysis. These results not only provide the basis for the evaluation of deep ore-bodies, but also provide information for the ore genesis, mineralization processes, and metallogenic system, which has important theoretical and practical significance.LA-ICPMS zircon U-Pb dating indicates that the Ruanjiawan and Xiniushan granodioritic porphyry stocks and granite porphyry dike in the Yinshan mine yield U-Pb ages of143±1Ma,135±1Ma,147±1Ma, and141±1Ma, respectively. LA-ICP-MS U-Pb titanite analyses results demonstrated that the formation of the Ruanjiawan granodiorite, lamprophyre and skarn occurred at132±4Ma,131±2Ma and141±1Ma respectively. The age of hydrothermal titanite form skarn (141±1Ma) is consistent with the molybdenite Re-Os age of mineralization and U-Pb zircon age of granodiorite (143±1Ma), indicating that the geochronology of hydrothermal titanite can represent the true age of mineralization. In addition, the titanite age of Ruanjiawan granodiorite is consistent with zircon and titanite U-Pb ages of lamprophyre, suggesting that the late stage hydrothermal event that is related to numerous intrusion of dikes cause the resetting of U-Pb isotopic system in titanite. The coeval magmatism and mineralization recognized in district regions of the southern Edong and Mid-Lower Yangtze River indicate that they had the same geodynamic setting. Trace elements results show that the altered titanite grains from granodiorite have similar chondrite-normalized REE patterns, and LREE/HREE and Th/U and Lu/Hf ratios with magmatic titanite, but La/Ce ratios are similar to hydrothermal titanite. In contrast, the REE composition, Th/U, Lu/Hf, and La/Ce ratios of altered titanite in lamprophyre are consistent with the classical hydrothermal titanite, demonstrating that the intensities of hydrothermal alteration are different between these titanites. However, the trace elements of zircon grains from the granodiorite and lamprophyre have similar geochemical characterizes with magmatic zircons, indicating that zircon grains are relative stable during the late stage hydrothermal events. These results demonstrated that the late stage hydrothermal alterations are facilitated to change the texture and chemical composition of titanite, thus U-Pb ages and trace elements analyses on altered titanite may provide the accurate time constraints on the hyrothermal events and mineralization.The major elements results show that Ruanjiawan granodiorite and Xiniushan granodioritic porphyry are high-K calc-alkaline. These intrusions display chemical characteristics of enrichment in large ion lithophile elements (eg. Rb, Ba, Th, and La) and relative depletion in high field strength elements (eg. Nb, Ta, and Zr). Geochemical features of Ruanjiawan and Xiniushan intrusions also have similar to adakites, such as high Sr (681×10-6～874×10-6and575×10-6～687×10-6), low Y (12.0×10-6～17.4×10-6and9.7×10-6～14.0×10-6), high Sr/Y ratios (39.13-61.37and46.83-61.73), low Yb (0.90×10-6～1.49×10-6and0.62×10-6～1.01×10-6) and La/Yb ratios (42.15-53.66and47.63-60.06)。The Sr-Nd-Hf isotopic results of these intrusion demonstrate that Sr, Nd, and Hf isotopes of Ruanjiawan granodiorite are (87Sr/86Sr)t=0.70649-0.70651, εNd(t)=-5.9～6.2, and sHf(t)=-12.0～6.8; and of Ruanjiawan lamprophyre are (87Sr/86Sr)t=0.70555-0.70559, εNd(t)=-1.4, and εHf(t)=-14.4～4.5. The Sr and Nd isotope of Xiniushan granodioritic porphyry are (Sr/Sr)t=0.70618-0.70627and εNd(t)=-5.6～6.5. Geochemical and Nd-Sr-Hf isotopic data demonstrate that granodiorites and granodioritic porphyry originated as partial melts of an enriched mantle source that experienced significant contamination of lower crust materials and fractional crystallization during magma ascent. In addition, Ruanjiawan lamprophyre is derived from deep mantle.The S isotope results show that the δ34S values are-0.98‰～2.48‰(Runjiawan W-Cu-Mo deposit),0.81%o～2.709‰(Xiniushan), and-0.50‰～5.70‰(Yinshan Pb-Zn-Ag deposits). These values are close to meteoriric sulfur, indicating that the sulfur maily come from magma and minor part of sulfur in Yinshan may have contamination of sediments sulfur. Mineralization temperature calculated from sulfur isotope geothermometer of chalcopyrite-pyrrhotite and Galena-sphalerite demonstrated that the metallogenic temperatures are292～394℃in Ruanjiawan deposit and295～300℃in Yinshan deposit. The oxygen isotope results show that the818O values of fluids rang from3.5%o to10.4%o, indicating that the mineralizing fluids are mainly magmatic water. The trace elements results of sulfides analyzed by the LA-ICPMS show that chalcopyrite and pyrite in Rujiawan deposit have higher contents of Co, Ni, Se,but relative lower As, Sb, Ag than Yinshan deposit. These results indicate that Rujianwan and Yinshan deposits are related with magmatism, and formed by different mineralization stages.The ore of Ruanjiawan deposit mainly occurred at the contact zone between Ordovician limestone and granodiorite. The occurrences of orebody are bedded, near-bedded and lenticular shapes. The hydrothermal alteration zonations are clear in Ruanjiawan deposit. Skarn minerals include diopside, garnet, tremolite, chlorite and epidote. Geologic characteristics combined with S and O isotopes and trace elements of pyrite demonstrated that Ruanjiawan W-Cu-Mo deposit is a skarn deposit. The occurrences of Yinshan Pb-Zn orebodie are related to the fractures and unconformity surface between Carboniferous and Silurian. These characteristics are different from SEDEX and MVT Pb-Zn deposits. The geological features combined with S isotope and mineralization temperatures suggest that Yinshsan Pb-Zn deposit belongs to epithermal deposit that related to magmatic activity. Based on these resluts, we proposed a metallogenic system model for this district. This metallogenic system is characterized by well spatial zonation of these deposits that decreased with mineralization temperatures (from porphyry Cu-Mo deposit, skarn Cu-Zn deposit, breccia W-Cu-Mo deposit, and vein Pb-Zn-Ag deposit) The building of minerogenetic series provide an important tool for mineralization prospecting.Based on strata, structural and magmatic and geophysical and geochemical characteristics, we divided four prospect targets in this district. Level A-C targets are Yinshan and Xiniushan, eastern Ruanjiawan, and Yinshanguo, respectively. The occurrence of epithermal Pb-Zn-Ag deposit in Yinshanguo target suggests that it is possible to find the porphyry Cu-Mo deposit in deep. The presence of magmatic anhydrite and high contents of volatile component in Xiniushan granodioritic porphyry demonstrated that Xiniushan porphyry has a great potential for finding the porphyry Cu-Mo deposits. This is also supported by the occurrence of porphyry Cu-Mo ore spots in Xiniushan intrusion. The targets of eastern Ruanjiawan and Yinshanguo have two known deposits distributed. The geophysical and geochemical anomalies suggest that these two targets have potential for finding the deposit that is similar to Ruanjiawan and Yinshan.