Study On Element Geochemistry Of The Yangshan Gold Deposit In Western Qinling Mountains

The Yangshan gold deposit in Wenxian County,Gansu Province is occurred in intercontinental collision orogen.The orebodies are controlled by E-W trending ductile-brittle shear zone,and hosted by Devonian carbonaceous carbonate-phyllite-slate and granitic porphyry dyke which intruded into the former strata.Petrography and ore microscopy studies on relative rocks and ores indicated that the Yangshan gold deposit is a fine-grained disseminated gold deposit.Pyrite,arsenic pyrite and arsenopyrite are the most important ore minerals.Ores are predominantly strongly altered phyllite,granite porphyry or occurred as quartz-sulfide veins.In this paper,we analyzed the element constitutes of strata,igneous rocks,ores,pyrite,as well as the in situ element composition of pyrite,and discussed the similarities and differences,trends,and their geological significance.Furthermore,we investigated the source of the ore-forming components and possible mineralization mechanism.Afterall,we draw the following conclusions.The whole-rock trace elements characteristics showed that the average content of As is 986 × 10-6 in phyllite while 203 × 10-6 in igneous rocks.The correlation between As and Au only existed in phyllite and ores,indicating that As may come from strata but Au were mainly provided by phyllite.Trace elements characteristics in pyrite powder indicated that a large-scale trace elements exchange between the fluid and host strata occurred through fluid migration.Moreover,there is a positive correlation between the trace elements and ore-forming elements such as Au,As,Ag,suggesting that ore-forming elements such as Au,As,Ag were also absorbed into fluid with trace elements.LA-ICP-MS study of pyrite suggested that diagenetic pyrite and hydrothermal pyrite have the same Au/As ratio,while the Au/Ag ratio of the hydrothermal pyrite is higher than diagenetic pyrite.Elements in the early-stage pyrite showed a regular change,i.e.Au and Ag content from pyrite core to margin varied from 1.18×10-6,0.17×10-6,0.12×10-5 and 0.06×10-6,0.04×10-6,0.03×10-6,respectively.Our study implied that Au and Ag are derived from the Sanhekou group(phyllite).In conclusion,the ore-forming components of the Yangshan gold deposit are derived from the Sanhekou group(phyllite).This doposit has the geological and geochemical characteristics similar to both the orogenic-and Carlin-type deposits.Thus we suggested that it belonged to the Carlin-like type gold deposit.