西藏格玛铅多金属矿床的发现、地质特征、年代学与成矿地质背景

Discovery,Geological Characteristics,Chronology and Metallogenic Geological Background of Gema Pb Polymetallic Deposit in Tibet

格玛铅多金属矿床是2010年由笔者团队发现的矿床,也是目前在班公湖—怒江成矿带东段发现的唯一一处中型富铅锌银多金属矿床。该矿床大地构造位置处于昂龙岗日—班戈—腾冲岩浆弧带的东段,矿体均呈脉状产于早白垩世花岗闪长岩体外接触带围岩中,矿体产出的形态、位置严格受近东西向和北西向两组断裂构造控制,赋矿围岩为中侏罗统马里组(角岩化)砂岩、粉砂岩。本文通过对格玛铅多金属矿床地质特征的阐述和对矿床岩矿石地球化学、花岗闪长岩LA-ICP-MS锆石U-Pb年代学的研究,分析了矿床的成矿物质来源、成矿时代与成矿地质背景,探讨了矿床的成矿作用,并初步建立了班公湖—怒江成矿带东段铅锌银多金属矿床的成矿地质模型。结果表明:区内花岗闪长岩具有低硅、高铝、相对富钾主量元素特征,为板块碰撞前俯冲阶段岩浆作用的产物;花岗闪长岩LA-ICP-MS锆石U-Pb定年结果为(119.86±0.45)Ma,表示其成岩时代为早白垩世;花岗闪长岩与矿石、围岩的稀土元素特征均表现为轻稀土元素富集、重稀土元素亏损、Eu负异常,暗示三者在成因上具有一定联系;与中国陆壳丰度值相比,花岗闪长岩中Pb、Ag分别属较富集和强富集元素,Zn属亏损元素,马里组变质粉砂岩中Pb、Ag属极强富集元素,Zn属强富集元素。该矿床成矿物质不仅来源于花岗闪长岩,同时也有围岩马里组中成矿物质大量的加入,成矿物质来源具有明显的“双源性”特征,矿床形成于班公湖—怒江洋盆闭合前的岛弧环境,成矿时代为早白垩世,矿床成因类型为岩浆期后热液型矿床。

The Gema Pb polymetallic deposit was discovered by our team in 2010.It is currently the only medium-sized Pb-Zn-Ag polymetallic deposit in the eastern section of Bangong Lake-Nujiang metallogenic belt.The deposit is located in the eastern section of the Anglonggangri-Bange-Tengchong magmatic arc zone.The ore bodies are produced in the surrounding rocks of the outer contact zone of the Early Cretaceous granodiorite in the form of veins.The shapes and locations of the ore bodies are strictly controlled by two groups of faults in the nearly east-west and northwest directions.The ore-bearing surrounding rocks are sandstone and siltstone of the Mali Formation(hornfelization)of the Middle Jurassic.Based on the geological characteristics of the Gema Pb polymetallic deposit and the studies of the geochemistry of the surrounding rocks and ores,and the LA-ICP-MS zircon U-Pb chronology of granodiorite,this paper examined the source of ore-forming materials,the ore-forming age and the ore-forming geological background,also discussed the ore-forming process of the deposit,and established a preliminarily metallogenic geological model of the Pb-Zn-Ag polymetallic deposit in the eastern section of the Bangong Lake-Nujiang metallogenic belt.The results show that the granodiorite in the area has low silicon,high aluminum and relatively rich potassium,which is the product of magmatism in the subduction stage before plate collision.The LA-ICP-MS zircon U-Pb dating of granodiorite yields an age of(119.86±0.45)Ma,indicating an Early Cretaceous diagenetic age.The rare earth element(REE)of granodiorite,ore and surrounding rocks show enrichment in light REEs,depletion in heavy REEs,and a negative Eu anomaly,suggesting a common origin.Compared to the abundance values of Chinese continental crust,the granodiorite contains relatively enriched Pb and Ag,while Zn is deficient.In the metamorphic siltstone of Mali Formation,Pb and Ag are extremely strong enriched elements,and Zn is also strongly enriched.The ore-forming materials of this deposit are derived not only from granodiorite but also from the surrounding rocks of Mali Formation,indicating obvious“double source”feature.The deposit was formed in an island arc environment before the closure of the Bangong Lake-Nujiang Ocean basin.The ore forming age is Early Cretaceous,and the genetic type of the deposit is a post-magmatic hydrothermal deposit.