Geological-Geochemical Characteristics And Genesis Of Haoyaoerhudong Gold Deposit In Inner Mongolia

The Haoyaoerhudong gold deposit,also named Changshanhao gold deposit,is located 10 km northeast of Xinhure town of Urad Middle Banner,Inner Mongolia and tectonically belonging to the Langshan-Bayan Obo continental rift on the North China Craton.Hosted in the Bilute Formation of Bayan Obo Group,the Haoyaoerhudong gold deposit is the biggest dump leaching gold mine in northern China,with total proven reserves up to 148.5t,average grade of 0.82g/t and characters of large scale,shallow burial and easy mining.Scale 1:200 geological and geochemical section survey conducted in the mining area found that the grade of Au is positively correlated with the developmental of pyrite vein and quartz-pyrite vein.The intrusion itself may not be the minerals source because of its low gold content,but gold concentrations increase in varying degrees in the contact position between the surrounding rocks and the intrusions,indicating that the enrichment of gold mineralization may result from the late magmatic event on some level.The Au-bearing pyrites show strong LREE/HREE fractionation.Their chondrite-normalized REE patterns slope downwards to the right with negative Eu anomalies and weak Ce anomalies,which is similar to the distribution patterns and eigenvalues of the carbonaceous strata and the outer strata,but different from those of the magmatic rocks.This indicates that the mineralization should mainly derived from the formation.The ?34S values of the pyrites range from 11.2‰~14.7‰,with an average of 13.15‰,suggesting that the Haoyaoerhudong gold deposit has a single sulfur source.The Co/Ni ratios(<1)of these pyrites,in conjunction with the similar Y/Ho ratios to the carbonaceous strata,indicating that the ore-forming fluids were mainly metamorphic fluid,and the mineralization was derived from the carbonaceous strata.The strata has Sr/Ba ratios ranging from 0.036~0.702(average 0.198)which implys that carbonaceous strata is composed of mainly terrigenous clastics.Organic geochemistry analyses of the Haoyaoerhudong gold deposit show the organic carbon content of the strata is negatively correlated with the Au concentrations,which hints that the precipitation of Au should be related to the pyrolysis of organic matter.The n-paraffin carbons distribution ranges from C11 to C38,and mainly distributes at C16-C27 with major peak at C16 and C18,these mean that the organic matters were mainly from bacteria and algae.The(nC21+nC22)/(nC28+nC29)ranges from 0.74 to 3.10,with an average of 1.81,suggesting the participation of marine facies organic matter;Pr/Ph ranges from 0.51~0.84,with an average of 0.67,indicating that the source layer is likely to be formed in a hypoxia and normal salinity reductive deposit environment.The Ir contents are highest in the platinum group(average 3.19×10-9),almost close to the Ir contents(3.2×10-9)in the primitive mantle,hinting that the Ir in the carbonaceous strata may come from the primitive mantle.The contents of Pd is positively related to the content of gold elements(r=0.7970),but no significant correlation was observed between Ir and Au(r=-0.3221).The metallogenic model of Haoyaoerhudong gold deposit is constructed:,(1)the Bayan Obo Group deposited in the Mesoproterozoic,forming the initial carbonaceous strata which is rich in gold,sulfur,iron and organic matter in the anoxic reduction environment.Supporting evidences includes extensively developed star-like,acicular pyrite and carbonaceous components;(2)During Neoproterozoic to Late Carboniferous,the quartz-pyrite veins and pyrite veins deformed at the same time as regional tectonics,indicating that the initial enrichment of the mineral construction and a large number of low-grade mineralization.(3)The Hercynian magma emplacements results in the activation and migration of preconcentrated gold,enhancing the abundant of Au in the favorable location of the carbonaceous strata.