Gold metallogenetic model in Jiaodong coastal areas: Evidence from the Sanshandao metallogenic belt

The Sanshandao giant gold belt is located in the northwest of the Jiaodong Peninsula, with more than 1400 tons proven gold metal reserves. It consists of the Cangshang, Xinli, Sanshandao, Xiling, and Haiyu large or superlarge gold deposits from south to north. Some of these deposits and orebodies are covered by 0~20m deep seawater, making it a unique coastal gold belt in China. The particularity of metallogenic conditions in coastal areas, the complexity of preservation and evolution of the metallogenic system, and the new scientific issues brought by recent exploration breakthroughs have made the Sanshandao gold belt a research focus. Based on extensive exploration practices and research data, we summarize the regional metallogenic regularity, compare the structural-alteration-mineralization characteristics of marine and continental deposits/orebodies, and discuss the metallogenic process as well as the post-ore modifications and preservation. (1) The occurrence of gold orebodies is strictly controlled by the NE-trending Sanshandao fault, which is mainly hosted in the pyritic sericitized (granitic) cataclastic rocks in the footwall of the main fault zone. Gold mineralization is dominated by the altered-rock type in the fractured zones. (2) The gold deposits and orebodies beneath the sea and on land have similar geological characteristics, occurring approximately equidistant along the Sanshandao fault, and showing a NE-trending lateral pattern and spatial continuity. (3) Hydrothermal gold mineralization is closely related to sericitization and pyrite sericitization, and the scale of the alteration zone is positively correlated with the deposit scale. A new mineralization type of gold-bearing sulfide-carbonate vein was identified in the high-grade gold-rich ore section of the Haiyu deposit. In addition, some later polymetallic mineralization superimposed on shallow gold orebodies in Sanshandao and other deposits. (4) The ore-forming fluid belongs to a medium temperature, medium-low salinity, carbon-rich H2O-CO2-NaCl system, which was mainly derived from the deep metasomatized mantle supported by the He-Ar and S isotopic evidence. Gold and other ore-forming materials are mainly sourced from the sulfide cumulates in the lower crust, undergoing activation, transformation, and gold re-enrichment. Fluid-rock interaction was the key mechanism for forming altered-rock type ores. (5) In the Sanshandao belt, the gold mineralization mainly occurred at ~120Ma, followed by relatively slow uplift and denudation. Although some shallow gold orebodies have been partly eroded to the ground surface, mostly deep orebodies are still well preserved. Since the Neogene, regional crustal subsidence and marine transgression have caused part of the gold orebodies to be covered by seawater. Comparing the transformation of shallow wall rocks or altered rocks caused by the weathering under seawater with that on the surface, the migration amounts of elements (such as Na, K, Ca, Bi, Cu, and Zn) are different, but their overall trends (decrease or increase) are generally consistent. By comprehensively integrating the multiple factors of "source, migration, storage, transformation, and preservation" of the gold metallogenic system in the Sanshandao ore belt, a preliminary metallogenic model for coastal gold deposits has been proposed. It is speculated that the superposition of brittle deformation and hydrothermal alteration in the footwall of the fault zone was beneficial for mineralization enrichment. The deep part and offshore areas, where the NE-dipping orebodies along the Sanshandao fault zone extend northeastward beneath seawater, still have good prospecting potential.