The Geochemical Characteristics And Genetic Mechanism Of The Kongxintou Fe-Cu-Mo Skarn Deposit In Jiaodong Metallogenic Area

Kongxintou is a typical medium scaled Fe-Cu-Mo polymetallic skarn deposit in the Qixia-Penglai-Fushan metallogenic belt, central Jiaodong, China. This paper chooses the Yuangezhuang pluton and Kongxintou deposit as the research object. Based on systematic study of genetic mineralogy, rock geochemistry, geochronology, isotopic geochemistry and fluid geochemistry, the geochemical characteristics and tectonic setting of Yuangezhuang pluton are analyzed. The genetic mineralogy of pyroxene, garnet and magnetite, molybdenite Re-Os dating, S isotope and fluid inclusions are studied to discuss the origin of ore-forming materials, togather with the genetic mechanism of the Kongxintou deposit.Yuangezhuang pluton consists of monzonitic granite. They are high potassium, alkali, and metaluminous to peraluminous. It is a multiple staged granitic complex which is generated by mantle-crust interaction. The isochron age of molybdenite is 117.8±5.7 Ma, with a model age of 119Ma-117 Ma. It is consistent with the rockforming age(113.4±2.5Ma) and ore-forming ages from other known Cu-Mo deposits of Late Yanshanian age in Jiaodong.The genetic mineralogy of typical skarn minerals shows that pyroxene belongs to diopside and sahlite, which has some resemblance with typical molybdenum skarn deposit. Besides, the garnet belongs to andradite and grossular, indicating a low oreforming pressure which benefits the migration of fluid and hydrothermal replacement. The magnetite mostly belongs to hydrothermal magnetite, and some locate in the BIF region, indicating that the hydrothermal magnetite may be formed by the magmatic hydrothermal replacement of the former BIF magnetite.The fluid inclusions study shows that in the skarn period, the homogeneous temperature focuses on 400°C-500°C, with salinity of 3.55wt%NaCl-13.62wt%NaCl, and the fluid is reduced. In the carbonate period, the homogeneous temperature focuses on 100°C-200°C, with salinity of 0.18wt%Na Cl-9.08wt%NaCl, and the fluid is oxidized. The fluid belongs to H2O-NaCl system, which is medium to high temperature, medium to low salinity. It includes C3H8, SO42-, OH-. SO42- facilitates the forming of magnetite. The ore-forming depth is about 3km. δ34S in ore is 4.0‰-10.5‰, indicating a magmatic origin.In a word, the Kongxintou Fe-Cu-Mo deposit is formed by contact replacement after the Yuangezhuang pluton intruded. SO42- in the magmatic hydrothermal fluid can accelerate Fe activating, transferring and precipitating. Subsequently, Cu and Mo deposited combine with magmatic sulfur.