Age and fluid source constraints of the Haoyaoerhudong orogenic gold deposit,North China:Evidence from geochronology and noble gas isotopes

The Haoyaoerhudong gold deposit,located in the northwestern part of the North China craton(NCC),has produced over 120 metric tonnes(t)of gold since 2007.It has a total reserve of>240 t at average gold grade of 0.62 g/t,making it one of the largest open pit gold mines in China.The steeply dipping,large-tonnage,low-grade,vein-or veinlet-type gold orebodies are hosted in strongly-deformed Mesoproterozoic carbonaceous schist of the Bayan Obo Group.The laminated/boudinaged veins/veinlets in the sinistral ductile-brittle shear zones are dominated by quartz,biotite,gold-bearing löllingite,pyr-rhotite,(arseno)pyrite,with minor native gold,titanite and xenotime.In this paper,we present new in situ U-Pb geochronological data on magmatic zircon from the preore dikes,on metamorphic and hydrothermal xenotime,and on hydrothermal titanite from the hydrothermally altered carbonaceous schist and auriferous quartzsulfides veins/veinlets,as well as He-Ar isotopic analysis on gold-bearing(arseno)pyrite in the syn-ore stage.The metamorphic xenotime U-Pb age of 426±6.0 Ma(2σ)records a regional metamorphic event,possibly related to the accretion of the Bainaimiao arc onto the NCC.Two pre-ore andesitic dikes yielded similar emplacement ages at~278 Ma constrained by laser ablation-inductively coupled plasma-mass spectrometry(LA-ICP-MS)U-Pb zircon data.Hydrothermal xenotime grains from the altered carbonaceous schist and auriferous quartz-sulfides veins yielded U-Pb ages of 256.0±4.1 Ma(2σ)and 254.4±2.1 Ma(2σ),respectively,overlapping with that of the hydrothermal titanite at 255.4±0.8 Ma(2σ)from the laminated quartz-sulfides veinlets.This indicates that the gold mineralization occurred at ca.255 Ma.The~255 Ma gold mineralization age is much younger than the previously reported Early-Middle Permian regional magmatic activity(ca.291 Ma to 268 Ma),and may be associated with the regional sinistral strike-slip event in the late orogenic cycle related to the collision between the Siberian craton and the NCC.The 3He/4He(R/Ra)and 40Ar/36Ar values of the gold-bearing(arseno)pyrite are 0.04 to 0.09(average=0.07)and 375.8 to 2023(average=1045),which reveal the ore-forming fluids dominantly originated from the crustal rocks,with limited involve-ment from the mantle.Collectively,our new geochronological data,noble gas isotopic analyses,and geo-logical evidence support a typical orogenic gold deposit model at Haoyaoerhudong.