Tectonic Evolution And Gold Mineralization In The Jiaodong Peninsula

Zircon U-Pb ages reveal that the Mesozoic felsic magmatism in the Jiaodong Peninsula peaked at ?160 Ma,?130 Ma and ?115 Ma. The character of ?d(t) values, ?Hf(t) values and Hf TDM2 ages indicate that the magma were derived from the ancient lower crust. The elements and isotopes data show that the melting depths for three episodes of granitoids from ?160 Ma to ?115 Ma became shallow gradually in an extensional tectonic environment, which was controlled by the subduction of the Pacifia Plate. The lower crust of the Yangtze Craton (YC) was detached from the upper crust and thrust into the North China Craton (NCC) crust further than the YC upper crust during the Triassic westward subduction and collision between the YC and NCC. As a result, the lower crust of the YC intruded into the NCC crust at depths>40 km in the Jiaobei terrane. This explains why the parental magmas for the Late Mesozoic granitoids in both regions share a common source.Based on integrated analysis of detrital zircon U-Pb ages of sedimentary rocks in the Jiaolai Basin, it is obvious there are five peaks at ?2500 Ma,?1850 Ma,?770 Ma,-230 Ma and ?120 Ma, representing five tectonic-thermal events. The peak age of ?120 Ma is coeval to the gold mineralization in the Jiaodong Peninsula. The U-Pb ages confirm that the provenance of upper and lower layer in this basin are distinct. In the Jiaolai Basin, from the Early Cretaceous Laiyang and Qingshan Groups to Late Cretaceous Wangshi Group, the Cretaceous age populations of detrital zircons tend to be younger and more dominant, and ages older than 145 Ma are less common. The similarities between the Neoproterozoic detrital zircon age populations sourced from the Sulu Belt and the magmatic event ages in the western and northern margin of YC indicate that they shared a similar history in the Neoproterozoic, and both experienced subduction activity along the western and northern margins of the YC. The metasomatic mantle forming in the Triassic or before and the subducted Yangtze crust provided fluids, volatiles and mineralization materials.The fission track (FT) data of zircon and apatite suggest that the Jiaodong gold province experienced extensive crust exhumation during ?30-80 Ma. This period coincides with the timing of maximal sedimentation developed in the neighboring basins as well as voluminous basaltic eruptions in the eastern NCC, suggesting the eastern NCC has experienced crust uplift in Late Cretaceous and Paleogene as a consequence of the continuing lithosphere thinning, forming the horst-graben system in the eastern NCC. The Late Mesozoic granitoids are the main wall rocks for the major gold deposits in Jiaodong, and thus the crust denudation history as gathered from the FT data suggest that the gold mineralization formed at depths of ?6-11 km.The disseminated-veinlet type (also called Jiaojia-type) gold deposits in the Jiaodong gold province are characterized by diverse alterations and dispersive mineralization, including K-feldspar alteration, sericite alteration and carbonate alteration. There are genetic link between the different alterations. The various alteration and mineralization were suggested to be a domino chain, which was launched as a K+, H+, and gold-charged fluid encountered the plagioclase-bearing granite. The samples from different stages have diverse mineral assemblages and geochemistry characteristics due to various processes of alteration and mineralization. Samples from different locations of the measured profiles show unlike geochemistry characteristics, which are controlled by structures and the types and intensity of alteration. The contents of metallogenic elements are high near the major fault and have a decreased trend away from the major fault. The micro-fractures and joints are relatively developed near faults facilitating fluids permeation, with high intensity of alteration and mineralization, which also control the distribution of gold orebodies.