Ore-forming Material Sources And Genesis Of The Manaoke Gold Deposit,Northwestern Sichuan

The Carlin-type gold deposit was first discovered in Carlin Town,Nevada,in1860s.Its main feature is that gold is hosted in gold bearing sulfides in the form of invisible gold.The Carlin-type gold deposits in China are mainly distributed in Dian-Qian-Gui and Chuan-Gan-Shan Golden Triangle.The Manaoke gold deposit is a typical Carlin-type gold deposit,which has great scientific and economic significance.Previous studies have made a detailed discussion on the ore-hosting strata,the occurrence state of gold.However,due to the limitation of analytical technique for the gold-bearing fine pyrite and its zoned texture,it is impossible to analyze in situ the sulfur isotopes of pyrite.Based on the field survey,and microscopic observation,This study analysed the compositions of Au-carrying pyrite using electron probe microanalysis?EPMA?,and determined the S isotopes of pyrites usnging Nano-SIMS.Arsenian pyrite and arsenopyrite are the two main Au-bearing sulfides in the Manaoke gold deposit.The pyrites can be divided into framboidal pyrite and euhedral pyrite.The euhedral pyrites generally have zoned textures with one core and one or two overgrowth rims.The element mapping shows that the cores and the outermost zones have lower As,whereas.the middle zones have higher As.There is a positive correlation between Au and As.This result is consistent with the results by line scanning analysis using Nano-SIMS.The EPMA analysis for Au in pyrites shows that the gold content of pyrite is0.011wt%in average.Besides the Fe?43.009-46.417wt%?and S?47.344-53.254wt%?elements,pyrite also contains As,Au,Co,Cu,Cr and other trace elements.Elemental correlation confirmed that As-S was negative correlation,indicating that As substitutes for S into pyrite.Most of the measurement points on the lgAu-lgAs diagram fall below the gold solubility curve,suggesting that gold may be mainly in the form of Au⁺in the arsenian pyrite.The gold concentration.in the arsenopyrite is0.052wt%,higher than that in the pyrite.In addition to the Fe,S and As elements,the arsenopyrite also contains trace elements such as Au,Co,Cu and Ag.The correlation analysis shows that S and As,Fe and As are negative correlattion.The pyrites used for the in-situ sulfur isotope analyse are from sericite and quartz sandstone.The results show that the?³⁴S values of pyrites are highly variable ranging-29.63‰to 29.77‰,this phenomenon indicates that the sulfur source of pyrite has"multi source characteristics"..According to the characteristics of the S isotopes,pyrites are classified as the following:1.Pre-ore pyrite:the?³⁴S values is from 18.8‰to 24.2‰,suggesting that the sulfur is mainly derived from bacterium reducation of sea-water sulfate in a closed or half-closed environment.This should be synsedimentary pyrite without zoned texture.2.Ore-stage pyrite:the?³⁴S values are high in the cores?21.18‰?,and the?³⁴S values are low?-1.88‰?in the overgrowth zones.The cores of the pyrites have sedimentary sulfur.The?³⁴S of the pyrite overgrowth rim was-1.88‰,indicating that the sulfur likely comes from magma sulfur.NanoSIMS elemental mapping shows that invisible gold occurs in the pyrite overgrowth rim.3.Post-ore pyrite:the?³⁴S values are significantly negative?-29.63‰?,suggesting that the sulfur in the pyrite may be derived from seawater sulfate reducation by bacteria in open environment.Based on the analysis above,we suggest that ore formation of the Mnaoke gold deposit in Northwest Sichuan is probably related to the early stage of magma activity in the ate Indosinian to early Yanshannian.The ore-forming fluid is mainly derived from deep-sourced magma,but it does not exclude the possibility of mixing formation water,metamorphic water and meteoric water.The genesis of the deposit should belong to the middle-low temperature mixed hydrothermal gold deposit.