Characteristics and metallogenic significance of carbonaceous materials in Zhenyuan super-large orogenic gold deposit in Ailaoshan gold belt,Yunnan Province,China
The Ailaoshan gold belt is located in the southeastern part of the Sanjiang Tethys orogenic belt.It consists of a series of large and super-large orogenic gold deposits.Moreover,carbonaceous materials(CM)are widely rec-ognized in those deposits.However,the role of carbonaceous materials in the deposit formation is still unclear.In order to determine the role of carbonaceous materials in the gold mineralization,different kinds of carbonaceous materials and symbiotic pyrites found in Zhenyuan super-large orogenic gold deposit in Ailaoshan gold belt were analyzed by petrographic observation,Raman spectroscopy,Fourier Transform infrared spectroscopy,carbon iso-tope and laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS)analysis.The results show that there are three types of carbonaceous materials in Zhenyuan gold deposit,namely CM1,CM2 and CM3.CM1 has a dark gray irregular shape.Raman spectroscopic analysis shows that CM1 has a low-intensity,wide D1 band and a high-intensity,narrow G peak,and D2 peak is more obvious.Calculation shows that its formation tem-perature is 172～200℃,lower than the metallogenic temperature(250～330℃).In addition,the FTIR shows that CM1 has a low degree of graphitization,contains less CH and C=O groups,indicating that it has not undergone hydrothermal alteration.The δ13C values of CM1 samples range from-26.46‰ to-26.89‰.Framboidal pyrite Py1 and CM1 are spatially associated and distributed in carbonaceous slate.The LA-ICP-MS results show that Py1 has high content of Co,Ni,Zn,Mo and Te.CM2 is gray slender asphalt.Raman spectroscopy shows that the D1 and G peaks of CM2 were sharper than CM1.The area of D1 peak and G peak of CM2 are similar,and D2 peak has a low degree of spectral separation.The calculation shows that its formation temperature is 358～463℃,which is higher than the metallogenic temperature.The Fourier analysis shows that CM2 is highly graphitized,with obvious spectral peaks at 2925 cm-1 and 1705 cm-1,containing more CH2+CH3,C=O and C=C groups.CM3 is gray and fine granular.Raman spectroscopic analysis shows that the spectral characteristics of CM3 are similar to those of CM1,but the D2 peak of CM3 has an independent peak segment.The calculation shows that the for-mation temperature of CM3 is 258～322℃,which is consistent with the metallogenic temperature.The δ13C val-ues of CM3 range from-9.09‰to-14.12‰.Euhedral Pyrite(Py2),symbiosis with CM3,is distributed in the au-riferous quartz veins,and LA-ICP-MS results show that Py2 has high content of As and Au.Based on the analyti-cal results mentioned-above,we proposed that CM1 is derived from organic matter and formed in the early diage-netic stage of carbonaceous slate,which belongs to metamorphic origin.The symbiotic Py1 is formed in the dia-genetic stage,belonging to sedimentary pyrite.The formation temperature of CM2 is higher than that of mineral-ization,which is also of metamorphic origin.CM3 comes from ore-forming fluid,which is due to the reaction be-tween iron-bearing minerals in wall rock and ore-forming fluid,belonging to the hydrothermal origin,while Py2 and CM3 co-precipitate from the fluid.CM1 and CM3 play important roles in gold mineralization.CM1 acts as a reductant and effectively reacts with the sulfur hydrogen complex of gold in the ore-forming fluids to promote gold precipitation.While CM3 co-precipitates with Py2 in the sulfide process,resulting in a large amount of H2S consumption in ore-forming fluids,which further destroys the stability of the sulfur hydrogen complex of gold,re-sulting in gold precipitation and reenrichment.
orogenic gold depositscarbonaceous materialsRaman spectroscopyZhenyuan gold depositAilaoshan gold belt
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