西藏班公湖-怒江成矿带西段拿厅斑岩Cu(Au)矿床的火成岩岩石成因与成矿物质来源

Petrogenesis of igneous rocks and ore-forming material source of the Nating porphyry Cu( Au) deposit in the western section of the Bangong Co-Nujiang metallogenic belt,Tibet

位于西藏班公湖-怒江成矿带西段多龙矿集区内的拿厅斑岩Cu(Au)矿床是近年来开展勘查取得重大找矿突破的矿床之一,控制Cu资源量已达到大型规模。但目前对该矿床的地质特征、成矿岩石成因及成矿物质来源还缺乏深入的了解和研究。本文基于系统的野外地质观察、岩相学研究,结合精细的锆石SIMS U-Pb定年、锆石Hf-O同位素、岩石地球化学和蚀变-矿石矿物的H-O-S同位素分析,认为该矿床是形成于121~118Ma的斑岩型Cu(Au)矿床,与多龙Cu(Au)矿集区内多不杂、波龙、拿顿、色那、拿若和铁格隆南(荣那)矿床为同一期岩浆-热液成矿事件,其成岩-成矿峰期为120~118Ma。拿厅斑岩铜(金)矿床成矿岩体显示轻稀土富集的分配型式和缺少Eu异常,且富集大离子亲石元素(如Rb、Ba等),而亏损高场强元素(如Nb、Ta等),显示弧岩浆的特征,形成于班公湖-怒江洋向北俯冲的大陆边缘弧背景。锆石Hf-O同位素模拟结果显示,矿区早期的安山-英安质次火山岩(εHf(t)=-2. 0~10. 3;δ18O=5. 07‰~6. 77‰)具有早期幔源岩浆和经历MASH过程混入壳源的岩浆(10%~20%壳源物质)的特征;而二长花岗斑岩和成矿花岗闪长斑岩(εHf(t)=1. 7~3. 3;δ18O=6. 40‰~6. 95‰)则是MASH过程逐渐演化(壳源-幔源混合,约20%~40%壳源物质)的产物。在形成拿厅斑岩铜金矿成矿岩浆的过程中,没有发生陆壳加厚的过程;岩浆演化过程中有较少斜长石分离结晶,主要为角闪石和钛铁氧化物结晶分异。另外,热液蚀变(石英、磁铁矿)和硫酸盐-硫化物(硬石膏、黄铜矿、黄铁矿)的H-O-S同位素结果显示,拿厅成矿流体和成矿物质主要来源于岩浆,与岩浆流体出溶过程密切有关。

The large Nating porphyry Cu(Au)deposit in the Duolong ore district is one of the most significant deposits newly discovered in the western section of the Bangong Co-Nujiang metallogenic belt(Tibet).However,there is still no deep understanding on geological characteristics of the deposit,the genesis of its ore-forming rocks,and ore-forming material sources.Based on systemic field geological survey and petrographic studying upon rocks of this deposit,combined with precise zircon SIMS U-Pb dating and Hf-O isotopic analysis,petrochemistry analysis,and H-O-S isotope study of the hydrothermal minerals of this deposit,it is concluded that the Nating deposit is a Cretaceous(ca.121~118 Ma)porphyry Cu(Au)deposit.The age is consistent with those of other deposits(e.g.,Bolong,Nadun,Sena,Naruo,and Teglongnan/Rongna deposits)in the Duolong Cu(Au)ore district,suggesting that they belong to the same period of magmatic-hydrothermal event with peak period of mineralization of 120~118 Ma.The magmatic rocks show LREE enrichment patterns with absent of Eu anomalies,and enrichment in large ion lithophile elements(e.g.,Ba,Rb)and depletion in high-field strength elements(e.g.,Nb,Ta),showing geochemical characteristics of arc-type magma.All these evidences indicate that this deposit was likely formed in a continental marginal arc during the Bangong Co-Nujiang Ocean subduction.Moreover,zircon Hf-O isotopic compositions indicate that the early-stage andesitic-dacitic volcanic rocks(εHf(t)=-2.0~10.29;δ18O=5.07‰~6.77‰)were likely derived from magma mixing(10%~20%crustal material)between mantle-derived mafic and crust-derived melts during the MASH process.Whereas the ore-forming granodiorite porphyry and the monzonitic granite porphyry(εHf(t)=1.72~3.26;δ18O=6.40‰~6.95‰)are possibly formed by evolution of the MASH process with 20%~40%crustal material addition.During the period of ore-forming magma formation,there is no thickening of the continental crust occurred in this area,but a few plagioclases fractional crystallization,mainly amphibole and Fe-Ti oxide,does happen in this region.In addition,the H-O-S isotope results of hydrothermal minerals(quartz,magnetite)and sulfate-sulfide(anhydrite,pyrite,and chalcopyrite)show that ore-forming fluids and materials are mainly derived from magma,which is closely related to magma degassing(fluid exsolution).