The C-H-O isotope systematics of the Yangshan gold deposit, Gansu and its implication for the ore-fluid origin

Abstract

The Yangshan gold deposit, Wenxian county, Gansu province, containing 308 t Au with average grade of 4.74g/t, is now ranked as the China' s largest gold deposit. Locating in western Qinling Orogen, central China, it is a syn-collisionally formed Carlin-like gold deposit. Its orebodies are controlled by an east-trending shear-zone and hosted in the Devonian carbonaceous carbonate-phyllite-slate sequence or the granite-porphyry dikes intruding into the Devonian strata. The δ13 CCO2(PDB) and δ13 CCH4(PDB) ratios of fluid inclusions within quartz separates range -2.5‰ ∼ -5.6‰ and -23.1‰ ∼ -32.6‰, respectively, suggesting that the ore-fluids have been mainly sourced, through metamorphism and/or reworking, from the Devonian strata or/and similar lithologies which comprise carbonaceous phyllite, slate, chert and carbonate. This conclusion is strongly supported by the δ18 O values of the early- and main-stage ore fluids, which range from 9.5‰ to 15.3‰, with corresponding δD values between -86‰ ∼ -73‰. Two δ18 O values of late ore-forming stage ore-fluids are 2.7‰ and 6.8‰, implying a significant input of meteoric water. In general, the ore-forming fluid-system varies from early, deep, metamorphic fluid to late, shallow, meteoric water. It is worthy of stating that the above results and understandings are consistent with conclusions drawn from our studies of fluid inclusions and ore geology.