Abstract:

The quartz vein-type Fanjiabu gold deposit is economically the most important gold deposit in the Sulu ultrahigh pressure metamorphic terrane, Jiaodong Peninsula. Cathodoluminescence images and trace element and U-Pb isotope compositions by laser ICP-MS indicate that zircons extracted from auriferous quartz veins are exclusively inherited magmatic zircons from wall rocks of gold veins, namely the Neoproterozoic biotite monzogranite. A total of 22 analyses on 22 zircon grains were made, and a concordant homogeneous group was formed by 15 analyses, and a weighted mean 206Pb/238U age of (726 ± 14 ) Ma (MSWD = 4.6) was yielded. The remaining 7 analyses are plotted in a discordia, with an upper intercept U-Pb age of (758 ± 25) Ma and lower intercept age of (123 ± 11) Ma (MSWD = 0.45). The lower intercept age is consistent with 40Ar/39Ar hornblende ages of (118.8±1.6) Ma to (117.5±1.5) Ma of lamprophyre dikes closely associated with gold veins, and is therefore interpreted as the timing of mineralization at the Fanjiabu gold deposit. Results of this investigation and previous studies suggest that gold mineralization in the Sulu and Jiaobei terranes were contemporaneous, taking place at ca. 120 Ma and coincident temporarily with lithospheric thinning of the North China craton. This indicates that the Fanjiabu gold deposit and, by inference, other coeval deposits in the Sulu terrane were formed as a result of thinning of lithosphere mantle beneath the North China craton. Fluid inclusion and stable isotope data show that ore-forming fluids in the Fanjiabu gold deposit were characterized by middle to high temperature, high salinity (15.5％ to 23.2％) (NaCleq) and low δ18OH2O (-3.84‰ to -4.05‰) and δDH2O (-82.5‰ to -80.8‰) values. Gold-bearing pyrite has negative δ34S values ranging from -5.5‰ to -9.1‰. These are similar to hydrothermal fluids from hydrocarbon-bearing basins and imply that ore-forming fluids of the Fanjiabu gold deposit may have been mainly derived from the Mesozoic Jiaolai basin. The basinal fluids were channeled along the Wulian-Mishan fault and its secondary structures, when gold was leached from the basement rocks and precipitated in favorable structural sites under reduced conditions. In contrast, a large number of gold deposits in the Jiaobei terrane have low salinity, high δ18OH2O and δDH2O values, and positive δ34S values. Metallogenic differences of Early Cretaceous gold deposits between the Jiaobei and Sulu terranes were caused by distinct basement rocks, fluid sources, and ore-forming environments.

Key words: Fanjiabu gold deposit, mineralization age, ore-forming fluids, Jiaolai basin, Sulu terrane.