A Study On Pyrite Mineralogy And Geochemistry Of The Hongshi Gold Deposit In Tianshui Area,Gansu Province

The Hongshi deposit is a newly discovered cryptoexplosion breccia-type gold deposit in Gansu Province.Its geological features are representative in the West Qinling metallogenic belt.A detailed study on the Hongshi deposit would give a clue to better understand the gold mineralization mechanism of the West Qinling metallogenic belt.Pyrite is the most abundant sulfide and the most important gold-bearing mineral in the Hongshi gold deposit.Therefore,this paper presents pyrite mineralogical and geochemical data,and then discusses the sources of ore-forming materials and fluids,and ore genesis.Based on the cross-cutting relationship,mineral association and metasomatic relationship,the ore-forming process of the Hongshi gold deposit can be divided into four stages: barren ore,quartz pyrite,quartz polymetallic and carbonate stages.Corresponding to the four metallogenic stages,four generations of pyrite(Py-I,-II,-III and-IV)are identified in the Hongshi deposit.Py-I occurs as subhedral to anhedral fine grain and distributes in thick quartz veins;Py-? is euhedral medium-coarse grained pyrite,and intergrown with arsenopyrite;Py-III occurs as euhedral to subhedral coarse to fine grain and intergrown with sulfides such as chalcopyrite,galena,and sphalerite;Py-IV is euhedral coarse-grained pyrite,which is distributed in the calcite veins.LA-ICP-MS analysis results show that the elemental composition of different generations of pyrite is obviously different;Py-I is characterized by enrichment of Co,Ni,Cu,but poor in As,with high contents of Zn,Pb,Bi;Py-II is rich in As but poor in Co and Ni;Py-III is characterized by enrichment of Co,Ni,but poor in As,with low contents of Cu,Pb and Zn;Py-IV is rich in Co,Ni and As.From early to late,the good positive correlation between Au and Cu,Pb,Bi,As,Ag,combined with the Co-Ni-As element relationship,indicates that the pyrite in the Hongshi gold deposit is magmatic hydrothermal pyrite.Comprehensive analysis infers that(1)gold in the Hongshi gold deposit was transported mainly as Au-S complex,and fluid boiling may be the main mechanism that causes Au to precipitate in the deposit;(2)ore-forming materials and fluids in the Hongshi gold deposit mainly come from magmatic system,and the Hongshi deposit is a typical magmatic hydrothermal deposit.