| Abstract: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:① Lead in sulfides of Chenjiazhangzi gold ores is exclusively anomalous lead, so its model ages are meaningless. ② In 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”. ③ In 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) (2σ, 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 (2σ). 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. ④ Generally, 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. ⑤ According 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. |
