藏北商旭金矿床S、Pb同位素组成:对成矿物质来源的指示

Sulfur and Lead Isotope Compositions of the Shangxu Orogenic Gold Deposit in Northern Tibet:Implication for the Source of Ore-forming Material

商旭造山型金矿床处于班公湖—怒江缝合带中段南侧,其热液成矿作用可划分为四个阶段:石英阶段(S1)、石英—黄铁矿阶段(S2)、石英—多金属硫化物阶段(S3)和碳酸盐阶段(S4),金主要赋存于S2和S3阶段。该矿床的赋矿围岩为中—下侏罗统木嘎岗日群(J_(1-2)M)的深水复理石碎屑沉积岩。商旭金矿床S3阶段硫化物的硫同位素较为均一(δ^(34)S值介于-4.5‰^-1.0‰之间,均值为-3.1‰),与围岩中硫化物的硫同位素δ^(34)S值一致,表明硫可能来自于矿区木嘎岗日群的深水复理石碎屑沉积。同时,该阶段δ^(34)S值满足δ^(34)S_(Gn)<δ^(34)S_(Sp),说明不同硫化物间硫同位素分馏基本平衡;闪锌矿—方铅矿硫同位素热力学平衡温度为197℃。S3阶段硫化物的铅同位素^(206)Pb/^(204)Pb=18.35~18.69、^(207)Pb/^(204)Pb=15.64~15.70、^(208)Pb/^(204)Pb=38.57~38.98,μ值介于9.55~9.63之间,ω值介于37.75~38.15之间,表明其铀铅富集、钍铅亏损且铅源物质成熟度高的特点,暗示其铅来自于上地壳物质,可能有造山带中混杂岩的贡献。

The Shangxu orogenic gold deposit locates to the south of the middle part of the Bangong-Nujiang suture zone in northern Tibet. Four stages of mineralization were identified, which are quartz stage （S1）, quartz-pyrite stage （S2）, quartz-polysulfide stage （S3） and carbonate stage （S4）. Furthermore, the precipitation of gold mainly occurs in S2 and S3. The wall rocks of the deposit are the Middle-Lower Jurassic metamorphic flysh Mugagangri Group. The sulfides in S3 has a narrow δ34S range of -4.5‰ to -1.0‰ with an average of -3.1‰, which is consistent with the δ34S of pyrite from the wall rocks, indicating sulfur mainly derived from the metamorphic flysh Mugagangri Group. The δ34 S values of s3 satisfy δ34 SGn 〈δ34 SSp , representing the outcome of sulfur isotopic thermodynamic equilibrium fraetionation; meantime, the temperature is 197 ℃ according to sphalerite-galena sulfur isotopic thermodynamic equilibrium. Three samples from S3 yield 206Pb/204 Pb values of 18.35- 18.69 and 207Ph/204 Pb values of 15.64-15.70, and 208Pb/204pb values of 38.57-38.98; theμvalues vary from 9.55 to 9.63, while the co values vary from 37.75 to 38.15, indicating that the lead source was highly mature and derived from the upper crust, maybe including the contribution of melange in orogenic belts.