Petrogenesis Of The Gold-hosting Granitoids And Their Contribution To Mineralization In The Xincheng Gold Deposit, Jiaodong Peninsula

The Jiaodong peninsula is one the most important gold provinces in China, which contains nearly 1/4 of the country’s gold resources. More than 95% of the gold resources are hosted by Mesozoic granites, making Jiaodong one of the largest known granitoid-hosted gold provinces in the world. The Xincheng gold deposit is the largest gold deposit hosted by the earliest Early Cretaceous granitoids in Jiaodong peninsula, which provides an ideal place to research the relationship between the earliest Early Cretaceous granitoids and gold mineralization. This paper focuses on the geological characteristics of Xincheng gold deposit, metallogenic chronology, geological and geochemical characteristics of the gold-hosting granites, the process of alteration and gold mineralization, and contribution of gold-hosting granitoids to the gold mineralization, and the following conclusions were made through detailed field and laboratory research.1. The host rocks of the deposit are mainly made up of the earliest Early Cretaceous granites, the Xincheng granitoids. The orebody, strictly controlled by the Jiaojia fault, appears in vein-like and stratoid shapes. The 40Ar/39 Ar plateau age of sericites from pyrite-sericite-quartz altered granites shows that the gold mineralization occurred at 120±1Ma.2. In the Xincheng gold deposit, the host rocks are mainly fine grain monzogranite. LA-ICP-MS zircon U-Pb dating reveals that the magmatism started from ca. 132 Ma, and last to 123 Ma. The geochemical characteristics indicates that the fine grain monzogranite belongs to the high Ba-Sr granite. The comprehensive analysis of Sr-Nd-Pb-Hf isotopes and inherited zircon shows the Xincheng granitoids were formed by the mixing of mafic and felsic magma, derived from mantle and partial melting of the basement rocks of the Jiaobei terrane, respectively, with minor assimilation of upper crust materials. The subduction and roll-back of the paleo-Pacific slab beneath the North China Craton as well as associated asthenosphere upwelling may be the most likely mechanisms to form the Guojialing-type granites.3. Different stages of alteration assemblages(chloritization, reddish and/or K-feldspar alteration,(Pyrite)-sericite-quartz alteration-silicification and carbonation) were the products of the same hydrothermal event. The ore-forming fluids were modified progressively during the water-rock reaction. The red clouding in granitic rocks is considered to be attributed to the presence of ferric iron oxides in plagioclase, together with magmatic K-feldspar. The sericitization of plagioclase was much easier than that of K-feldspar.4. Elements migration during the process of alteration indicates that gold mineralization is closely associated with the pyrite-sericite-quartz alteration. Granitoids may provide partial Na+ and Cl- for gold mineralization. The breakdown of dark minerals may provide a certain amount of Fe. Stable isotope studies suggest that the ore-forming materials mainly came from the Jiaodong Group and the ore-forming fluid is mainly composed of metamorphic fluid, with minor contribution from granite.5. The spatial and temporal distribution of the orebodies, alteration mineral assemblages, ore elements, and regional tectonic environment suggest the Xincheng gold deposit is distinct from typical orogenic gold deposits and intrusion-related gold deposits. It is a typical representative of “Jiaodong type” gold deposit.