Ore-forming physicochemical conditions of the Jijiawa gold deposit in Henan Province, southern North China Craton

Mineral system understanding

Mr Shaohao Zou

Guangzhou Institute of Geochemistry

China has been the world’s largest gold producer since 2007, most of which are from gold (Au) ore deposits in North China Craton (NCC). Gold mineralization in the NCC has been commonly
described as the result of the Late Mesozoic lithospheric thinning and associated hydrothermal processes. The Jijiawa Au deposit, located within the Xiaoqinling-Xiong’ershan gold district, a
second larger gold producer area in NCC, is a fault-controlled quartz vein-style deposit with the proven Au reserves of 6.7 t. This deposit, formed at 118.2 ± 2.4 Ma, shares the similar geological
and mineralogical conditions with the majority of gold deposits in NCC, and thus gives us a good opportunity to decipher the physicochemical conditions in the ore forming progresses.

Our studies, using representative samples from Jijiawa deposit confirm four different mineralization stages and found that the information of ore-forming fluid was recorded in the secondary fluid inclusion trails. The microthermometric results suggest a large pressure change during gold precipitation. Based on the mineral assemblage and textures as well as the estimate P-T conditions, we constraint the pH and fO2 were decreased from early to late mineralization stages while the fTe2 was increased and then almost invariable. More importantly, the solubility of Au in the ore-forming fluids was increased first and then decreased. Combined with the lines of evidence from the fluid inclusion assemblage features, we suggest the continuously mixing between the carbonic fluid and the hot tellurium-bearing fluid leading to the precipitation of gold during the pressure fluctuation. Considering the contemporary tectonic regime in NCC, the pressure fluctuations may affect the fluid boiling, fluid mixing, the rate of water-rock interaction and condensation, and which, in turn, control the transportation and deposition of gold as well as the gold deposit distribution.

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