西藏多龙矿集区尕尔勤斑岩铜矿床年代学及地球化学--兼论硅帽的识别与可能的浅成低温热液矿床

Chronology,geochemical characteristics of the Gaerqin porphyry copper deposit in the Duolong ore concentration area in Tibet and discussion about the identification of the lithoscaps and the possible epithermal deposit

尕尔勤斑岩铜矿床是多龙超大型铜金矿集区内具有较大找矿潜力的矿床之一。本文以尕尔勤斑岩铜矿区内成矿花岗闪长斑岩及新发现的硅帽为研究对象,进行了矿床年代学、地球化学及找矿方向研究工作。尕尔勤矿床花岗闪长斑岩SiO_2含量在60.01%~62.81%之间,K_2O含量在1.86%~2.16%之间,Al_2O_3含量在15.12%~16.51%之间,属于钙碱性岩石系列。斑岩体锆石U-Pb年龄为124.4±0.4Ma(MSWD=0.42,n=25),属于早白垩世侵位;锆石εHf(t)值主要集中于1.15~9.71之间,tDMc模式年龄主要集中于559~1105Ma之间,表明岩浆岩区起源于具有幔源特征的深成熔体,而个别负εHf(t)值(-18.65,-19.75)的出现表明其受到了古老成熟地壳的轻微混染。斑岩体稀土元素显示为重稀土亏损的右倾分布型式,在原始地幔标准化图解中微量元素显示为Rb、Th、U、Sr等大离子亲石元素相对富集,而Nb、Ta、Ti、P、Zr等高场强元素相对亏损特征,体现出岛弧岩浆特有的地球化学特征,综合研究表明矿床形成与班公湖-怒江洋北向俯冲密切相关。硅帽中角砾状样品与层纹状(致密块状)样品地球化学特征具有明显区别,发育较少的层纹状硅帽样品可能为热水沉积与生物化学沉积共同作用形成的硅质岩,而大量存在的角砾状硅帽体现出明显热液充填交代成因特点,经地球化学剖面测量显示硅帽展布区域Cu、Au、Ag、Pb、Zn、As、Sb等成矿指示元素异常明显,结合高光谱矿物识别表明该区域存在浅成低温热液型矿床的巨大可能性,具有重要的勘查指示意义。

Gaerqin porphyry copper deposit is one of the most prospecting deposits in Duolong large copper-gold ore concentration area. This paper has a research for the chronology,geochemistry and prospecting direction of the deposit based on the ore-bearing porphyry-granodiorite porphyry and new discovered lithoscaps. The result shows that granodiorite porphyry（ SiO_2 of 60. 01% ~62. 81%,K_2O of 1. 86% ~ 2. 16%,Al_2O_3 of 15. 12% ~ 16. 51%） belong to the calc-alkaline series. Zircon U-Pb age of granodiorite porphyry is 124. 4 ± 0. 4Ma（ MSWD = 0. 42,n = 25）,which suggest Early Cretaceous emplacement. Zircons from ore-forming porphries main show positive εHf（ t） values of 1. 15 ~ 9. 71 and younger tDMcmodel ages of 559 ~ 1105 Ma,while few negative εHf（ t）values（-18. 65,-19. 75） have been found,it indicates that the granodiorite porphyry derived from mantle-derived magmas with minor ancient crustal contamination. The granodiorite porphyry show a right-sloping Rare earth elements（ REE） pattern,and enrichment of large-ion lithophile elements（ LILE： Rb,Th,U,Sr）,depletion of high field strength elements（ HFSE： Nb,Ta,Ti,P,Zr）,consistent with geochemical feature of arc magmas. Comprehensive studies show formation of the deposit is closely relate to northward subduction of the Bangong-Nujiang ocean. The geochemical characteristics of brecciated lithoscaps samples show obvious hydrothermal replacement and are different from that of lamellar lithoscaps（ dense block） samples which may be the products of the bio-genesis and hydrothermal sedimentation. Mineralization indicator elements Cu,Au,Ag,Pb,Zn,As,Sb are anomaly obvious according to geochemical profile,suggesting enormous possibilities of epithermal deposits in this deposit and important exploration significance.