Genesis of the White Mountain Gold Deposit, Jilin Province, China, and Exploration Implications for Sedimentary Rock-Hosted Gold

Abstract White Mountain is a sedimentary rock-hosted gold deposit located in the Hunjiang basin along the northeastern margin of the North China block in Jilin Province, China. With a premining endowment of 1.86 million ounces (Moz) Au at an average grade of 3.24 g/t Au, White Mountain is the largest gold deposit in the mineral-rich basin. The stratabound ore zones occur within a distinct <50-m-thick hematite-rich unit between footwall dolostone of the Proterozoic Zhenzhumen Formation and hanging-wall conglomerate and sandstone of the Diaoyutai Formation. The hematite-rich unit can be traced over 17 km along the northeastern margin of theHunjiang basin and represents a vanished evaporite that provided a chemical trap for ore-forming hydrothermal fluids. The deposit is located where the hematite-rich unit is intersected by northwest-trending normal faults that are interpreted to have provided the structural controls on mineralization. The hypogene ores at White Mountain comprise native gold occurring as small (<5-µm) grains in silicified breccia, as solid solution gold in marcasite, or as gold in late-stage barite. Ore zones show a distinct enrichment in the pathfinder elements Ag, As, Ba, Hg, and Sb. The ores are associated with silicification of the host breccia. Argillic alteration resulted in kaolinite and dickite formation within siliciclastic rocks, although fine-grained illite is also an abundant alteration phase. Carbonate-bearing rocks of the stratigraphic footwall have been affected by decarbonatization and the development of paleokarst enhancing permeability. A late supergene overprint has affected much of the ore zones, resulting in oxidation of the marcasite and liberation of gold originally contained in solid solution in this sulfide mineral. Sulfur isotope data for Au-bearing marcasite yielded δ34S values of –1.1 to +1.8‰, consistent with sulfur derived from a magmatic source with a δ34S signature of –3 to 0‰. Primary fluid inclusions hosted by hydrothermal quartz indicate that the ore-forming fluids were low temperature (≲220°C) with salinities of 1 to 4 wt % NaCl equiv. The Hunjiang basin is host to several porphyry Cu and Cu-Mo porphyry deposits, as well as skarn and carbonate-replacement deposits. White Mountain is, therefore, best interpreted to be a distal disseminated sedimentary rock-hosted gold deposit formed above or distal to a magmatic center. Gold transported by low-temperature hydrothermal fluids was deposited within the sedimentary rocks due to fluid cooling and progressive sulfidation of the hematite-rich host unit. The regional geologic context suggests that mineralization at White Mountain occurred during a Middle Jurassic to Early Cretaceous phase of crustal extension that accompanied lithospheric delamination under the eastern North China block.