西藏浦桑果矿区中新世中酸性侵入岩锆石微量元素特征及地质意义

Zircon trace elemental characteristics and its geological significance of the Miocene intermediate—acid magmatic rocks in the Pusangguo deposit in Xizang(Tibet)

浦桑果是近年来西藏冈底斯成矿带内发现的唯一一个富钴矽卡岩型铜多金属矿床,铜、铅、锌品位高,开发价值巨大。矿区主要出露黑云母花岗闪长岩和闪长玢岩中酸性侵入岩体,岩体形成时代均为中新世(13~14 Ma),但其岩体含矿性目前尚未查明,是否具有斑岩型铜矿成矿潜力值得进一步探讨和研究。本文主要采用LA-ICPMS原位成分分析方法分别对黑云母花岗闪长岩和闪长玢岩中锆石展开系统的原位微量元素分析。分析结果表明,黑云母花岗闪长岩和闪长玢岩中锆石均发育典型的振荡环带结构,两种岩体具有相似的REE配分模式、Ce正异常和Eu负异常特征,锆石成因类型属于典型的岩浆锆石类,锆石主要结晶形成于陆壳环境。锆石初始饱和温度和锆石Ti结晶温度计算结果表明,黑云母花岗闪长岩锆石初始饱和温度范围为633~645℃,锆石Ti结晶温度为658~817℃;闪长玢岩锆石初始饱和温度分布在549~626℃,锆石Ti结晶温度为640~772℃,锆石初始饱和温度均低于锆石Ti结晶温度,综合指示矿区中酸性岩体形成过程中存在多期次岩浆熔体的补给作用。锆石n(Ce^(4+))/n(Ce^(3+))、Eu_(N)/Eu_(N)^(∗)、微量元素Dy/Yb、(Ce/Nd)/Y值与氧逸度(ΔFMQ)特征表明,黑云母花岗闪长岩较闪长玢岩具有更高的氧逸度范围(ΔFMQ+0.44~ΔFMQ+3.86)和相对高的水含量,从而更有利于浦桑果矿区铜、硫等成矿元素在岩浆中进一步富集成矿,且矿区可能发育中—大型规模的金属矿床,然而进一步发育斑岩型铜矿床的可能性较低。

Objectives:Pusangguo is the only cobalt-rich skarn copper polymetallic deposit found in the Gangdese metallogenic belt in Xizang(Tibet)in recent years.It has high grade of copper,lead and zinc and great development value.Biotite granodiorite and diorite porphyrite intermediate acid intrusive rocks are exposed in this mining area.These intrusions are formed in Miocene,but its ore bearing property has not been identified.Whether there is porphyry copper mineralization potential is worthy of further discussion and research.Methods:In this paper,LA-ICP-MS analysis technology is mainly used for systematic in-situ trace element analysis of zircon in biotite granodiorite and diorite porphyrite.Results:The calculation results of zircon initial saturation temperature and zircon crystallization temperature show that the zircon initial saturation temperature range of biotite granodiorite is 633~645℃,and the zircon crystallization temperature is 658~817℃.The initial saturation temperature of zircon in diorite porphyrite is 549~626℃,the crystallization temperature of zircon Ti is 640~772℃,and the initial saturation temperature of zircon is lower than the crystallization temperature of t_(Zr-Ti),which comprehensively indicates that there is multi-stage magmatic melt recharge during the formation of medium-acid magma.The characteristics of zircon n(Ce^(4+))/n(Ce^(3+)),Eu_(N)/Eu_(N)^(∗),trace element Dy/Yb,(Ce/Nd)/Y ratio and oxygen fugacity(ΔFMQ)show that biotite diorite has a higher oxygen fugacity range(ΔFMQ+0.44~ΔFMQ+3.86)and relatively high water content than diorite porphyrite.Conclusions:The biotite granodiorite is more conducive to the further enrichment and mineralization of copper,sulfur and other ore-forming elements in the magma of pusangguo mining area,and medium-large-scale metal deposits may be developed in the mining area.However,the possibility of further development of porphyry copper deposits is low.