湖南鲁塘石墨矿Re-Os同位素研究

石墨具有较高的Re、Os含量,可望成为理想的Re-Os同位素测年对象,但迄今国内外研究较少,尤其在煤层经变质作用形成石墨过程中,其中Re-Os同位素体系的变化还有待研究。湖南鲁塘石墨矿是我国典型的隐晶质石墨矿床之一,矿体产于二叠系龙潭组煤系地层中。本文采用Carius管逆王水溶解样品,直接蒸馏、微蒸馏分离纯化Os,丙酮萃取法分离富集Re,热表面电离质谱法对鲁塘矿区石墨样品以及外围原煤进行了Re-Os同位素分析。结果表明:鲁塘石墨的Re含量为0.901~9.794 ng/g,Os含量为7.3~189.5 pg/g,Re-Os同位素等时线年龄为155.6±3.6 Ma,该年龄与鲁塘石墨矿东侧骑田岭岩体第二阶段中粒黑云母花岗岩锆石U-Pb年龄153~157 Ma一致,表明了龙潭组煤层受到骑田岭岩体"烘烤"作用,发生热接触变质作用,使得靠近骑田岭岩体原煤变质为石墨,形成石墨矿床。通过对比石墨、原煤和骑田岭岩体Re、Os含量及比值,发现石墨中的Re、Os主要来源于原煤,并根据石墨Re-Os等时线初始(187Os/188Os)i值(0.686±0.032),推测骑田岭岩体在侵入煤系地层过程中,有少量具有较低187Os/188Os值的Os被碳质吸附。

Study on the Re-Os Isotope Composition of Graphite from the Lutang Graphite Deposit in Hunan Province

Lutang graphite deposit, Hunan is one of typical cryptocrystalline graphite deposits in China. The ore body occurs within Permian Longtan Formation. In this research, the Re-Os isotopic characteristics of the graphite samples from Lutang deposit and the raw coal samples from the peripheral area are analyzed by digesting sample in the Carius tube, direct distilling Os from the Carius tube, extracting Re by acetone and determining the isotopes of Re and Os by NTIMS. The results indicate that the Re and Os content is 5-10 ng/g and 100-150 ng/g respectively, and the Re-Os isotope isochron age is 155 3.6 Ma which identifies with the zircon U-Pb age of Stage II medium-grained biotite granite of Qitianling pluton on the east side of the deposit. The closer to the Qitianling pluton, the higher grade of the graphite mineralization is, which demonstrates that Lutang graphite deposit is thermal contact metamorphic origin. When Qitianling pluton intruded, the coal seams within Longtan Formation were baked, and the some of them close to the pluton metamorphosed into the graphite and formed the deposit. The Re-Os isotope analysis on the unmetamorphed raw coal samples far from the pluton indicates that their structure had not altered, but the Re-Os isotopic system had been disturbed. The comparison of Re-Os content and Os isotopic composition between the graphite, coal and Qitianlign pluton demonstrates that the Re and Os in the graphite are mainly sourced from the raw coal. During the thermal contact metamorphization, the Re-Os isotopic system had been reset and recorded the metamorphic age. The (¹⁸⁷Os/¹⁸⁸Os)i of the graphite isotope isochrone is 0.686 0.032 which identifies with the ¹⁸⁷Os/¹⁸⁸Os of the seawater when the coal seam deposited. If all the Os in the graphite came from the raw coal, the theoretical (¹⁸⁷Os/¹⁸⁸Os)i of the graphite should be greater than 0.686, therefore it is speculated that in the process of Qitianling pluton with lower ¹⁸⁷Os/¹⁸⁸Os intruding, a small amount of Os from the magma had been adsorbed by the carbonaceous materials in the coal seam.