Hydrothermal alteration associated with Mesozoic granite-hosted gold mineralization at the Sanshandao deposit, Jiaodong Gold Province, China

摘要

The Sanshandao gold deposit (reserves of more than 200 t Au and average grade of 3.96 g/t), located at northwestern edge of the Jiaodong Peninsula, eastern North China Craton, is one of the largest gold deposits in the Jiaodong gold province. In this deposit, disseminated- and stockwork-style ores are hosted in Mesozoic granitoids; mineralization and alteration are largely controlled by the regional Sanshandao-Cangshang fault. Host granitic rocks for the deposit display a complex paragenetic sequence of alteration and mineralization. Activities of the Sanshandao-Cangshang fault created structurally controlled permeability allowing for infil tration of hydrothermal fluids, leading to diffusive K-feldspar alteration on the two fault planes. Later, large scale diffusive sericitization symmetrically developed across the main fault and partially overprinted the earlier K-feldspar alteration. Following the sericitization, relatively small scale silicification occurred, but now it is only retained in the hanging wall of the main fault. Subsequently, the fault gouge formed as a "barrier layer", which is impermeable for later fluids to move upward. After that, strong pyrite-sericite-quartz alteration occurred only in the footwall of the main fault, and was accompanied by gold precipitation. The last stage carbonation and quartz-carbonate veins marked the waning of gold-related hydrothermal ac tivity. Mass-balance calculations indicate complex behaviors of different types of elements during fluid-rock interaction. Most major elements were affected by intensive mineral replacement reactions. As expected, the fluid-mobile elements, L1LE and LREE, generally show moderate to high mobility. It is notable that even the commonly assumed fluid-immobile elements, such as HREE and HFSE, tend to be changed to various degrees. In addition, Y-Ho, Zr-Hf and Nb-Ta fractionations are observed in altered domains. Studies on alteration as semblages and fluid inclusions suggest that the ore-forming fluids were characterized by low salinity (≤8.4 wt.% NaCl equiv.), moderate temperature (300-400 °C), weakly acidic (pH: 3-5), and relatively reduc ing (log/02: -28) characteristics. In this type of fluids, gold was most likely transported as Au(HS)_2~- complex. With alteration going on, log (a_K~+/a_H~+) of fluids generally decreased due to significant formation of secondary K-bearing minerals. In addition, there might be a decrease of f_(o2) from pre-gold alteration stage to the main gold mineralization stage, and decrease of f_(o2) was probably one of the factors controlling gold precipitation. The Sr and Nd isotopic compositions of hydrothermal minerals, combined with previous H-0 and He-Ar isotopic studies, indicate that the hydrothermal fluids were mainly derived from crustal sources (e.g., degassing of felsic magmas and meteoric water), but with involvement of mantle derived components. The gold mineralization event just coincided with reactivation of the North China Craton, as marked by asthenosphere upwelling, volu minous igneous rocks, and high crustal heat flow, which may have provided sufficient heat energy and fluid input required for the formation of the gold deposits.