西藏山南地区成矿带S、Pb同位素地球化学研究

Sulfur and lead isotope geochemistry of the Shannan metallogenic belt in Tibet,China

西藏山南地区位于冈底斯东段南缘的火山-岩浆构造带,是一条资源潜力巨大的成矿带。本文通过对该成矿带的典型矿床矿石硫化物S、Pb同位素进行系统性分析,并结合区域构造演化,从成矿系统中"源"的角度对其来源特征和成矿规律进行探讨。研究结果显示,各个矿床的岩石铅和矿石铅的206Pb/204Pb、207Pb/204Pb、208Pb/204Pb比值范围分别为18.34～19.03、15.54～15.86、38.31～39.66和18.38～19.58、15.54～15.86、38.25～39.66,均富含放射成因铅,且在Pb同位素构造演化图中表现出良好的相关性,反映了物质来源上的同一性。结合Pb构造图和Δγ-Δβ成因分类图,矿石铅样品均投影于冈底斯岩基区域,表明成矿物质发生壳幔相互作用,显示造山带铅的特征。在时空上矿石铅也显示明显的变化规律,矿床成矿物质从雅鲁藏布江北侧到南侧,地壳来源物质逐渐增多,同时成矿物质由早期偏向于地幔铅向晚期地壳铅演化,指示晚期成矿物质主要为地壳来源,而早期成矿物质来源可能混有较多的幔源物质。S同位素组成具有一致的深源岩浆硫特征,克鲁、双步结热矿床矿石的硫同位素的δ34S平均值明显小于努日、程巴、明则、冲木达等矿床,也表明晚期成矿物质来源中参与了较多的地壳物质。

The Shannan area is in a volcanic-magmatic tectonic belt on the southern margin of the eastern section of the Gangdise belt. It is a metallogenic belt with great mineral resource potential. This paper has presented the sulfur and lead isotopic data of sulfides from ores of typical deposits in the metallogenic belt, and discussed the source characteristics and metallogenic regularity of the Shannan belt from aspect of the sources of ore-forming materials with the combination of the regional tectonic evolution. The results show that the 206Pb/204 Pb, 207Pb/204 Pb and 208Pb/204 Pb ratios of rocks and ores in various deposits vary from 18.34 to 19.03, from 15.54 to 15.86, from 38.31 to 39.66, and from 18.38 to 19.58, from 15.54 to 15.86, from 38.25 to 39.66 respectively, with rich radiogenetic lead and good correlations in the illustration of tectonic environment evolution by lead isotopes, reflecting the consistency of their material sources. Combined with the illustration of tectonic environment evolution by lead isotopes and Δγ-Δβ genetic classification of lead isotopes, data of the ore samples are plotted into a field of the Gangdise granite batholith, indicating that the ore-forming materials were derived from the crust-mantle interaction, with the lead isotope characteristics of the orogenic belt. Lead isotopic compositions of ores show an obviously special and temporal variation regularity as the percentage of crustal materials in the ore-forming materials is gradually increased from the north side to south side of the Yarlung Zangbo River. Meanwhile, the ore-forming materials of early stage have dominant mantle lead while those of late stage have dominant crustal lead, indicating that the ore-forming materials of late hydrothermal stage are mainly derived from crustal sources with minor mantle materials, while the ore-forming sources of early stage may be mainly derived from mantle materials with minor crust materials. The sulfur isotopic compositions have consistent characteristics of the deep magmatic originated sulfur. Moreover, the average δ34S values of ores in the Kelu and Shuangbujiere deposits are obviously lower than those of ores in the Nuri, Chengba, Mingze, and Chongmuda deposits, indicating that the late ore-forming materials of the late stage contain relatively high percentage crustal materials.