如何解释陈家杖子金矿的铅同位素资料——与佘宏全等商榷

How to explain Pb isotope data:Comments on a paper by She Hongquan et al.in 《Mineral Deposits》

文章对赤峰地区陈家杖子、安家营子金矿床的铅同位素资料进行了综合分析,结果表明:①两个矿区硫化物的铅均为异常铅,其模式年龄无任何意义;②在常规Pb/Pb图解上,陈家杖子硫化物数据点趋势线是204Pb误差线,而不是来自两个不同源区的成矿物质混合作用所形成的混合线;③在常规Pb/Pb图解上,陈家杖子_安家营子硫化物数据点构成一条等时线,用此等时线计算,矿化物源区源岩的年龄(tr)为2·587Ga,与建平杂岩变质高峰期的年龄基本一致;④建平杂岩经历高级变质_分异作用之后,其μ值显著降低(μ3=6·03),变成了一套具代表性的下地壳岩石;⑤陈家杖子、安家营子金矿的成矿物质直接来自下地壳岩石———建平群变质岩,但它们至少经历了3个阶段的演化史。

She Hongquan et al. have published a paper entitled ＂Geological and geochemical characteristics and genesis of Chenjiazhangzi cryptoexplosive breccia pipe gold doposit, Chifeng, Inner Mongolia＂ in 《Mineral Deposits》 （She et al., 2005）. In view of the fact that there are some wrong explanations about the Pb isotopic data in She＇s paper, the author decides to reexplain She＇s Pb isotopic data and make the following comments：①Lead in sulfides of Chenjiazhangzi gold ores is exclusively anomalous lead, so its model ages are meaningless.②In the ^207Pb/^204pb vursus ^206pb/^204pb diagram, the data points of Chenjiazhangzi sulfides exhibit a linear array, which is a ^204Pb error line rather than a ＂mixing line＂.③In the conventional Pb/Pb diagram, the data points of sulfides from the Chenjiazhangzi-Anjiayingzi gold orefield define an isochron, the slope of which is （0.181± 0.040） （2δ, 95% confidence limit）. Using the known ore-forming age （ts= 0.191 Ga）, the age（tr） of the source region of ore-forming material calculated from the slope of isochron is （2.587 ±0.334） Ga （2δ）. This age corresponds to the peak of granulite-facies metamorphism of the host rocks Jianping Complex. Sm-Nd whole-rock isochron date of （2 846 ±68） Ma has been reported and interpreted as the time of separation of igneous protoliths from the mantle. During a period of 300 Ma which separates this date from the peak metamorphism of Jianping Complex, the whole process of ＂crustal accretion-differentiation superevent＂ was completed, and the newly- formed segment of the crust accomplished stabilization.④Generally, the ore-forming material has experienced three-stage evolutionary history： at time t1（2.85 Ga ago）, the components differentiated from the mantle reservoir and evolved for a period in the higher U/Pb new accretive segment of the crust （μ2 = 14.81 ） ; at time t2 （2.5 Ga ago）, this segment underwent high-grade metamorphism-differentiation which lowered the U/Pb of the new crust （μ3 = 6.03）; at time t3（0.191 Ga ago）, Pb was extracted from the underlying old metamorphic rocks by tectono-magmatic events, and then incorporated into sulfides of gold ores. ⑤According to the facts mentioned above, the ore-forming material of Chenjiazhangzi gold ores was formed during Yanshannian orogeny and derived mainly from the lower crust reservoir, so lead in their sulfides possessed less radiogenic isotopic composition of lead.