胶东夏甸金矿床金红石成因判别与U-Pb定年

Characteristics and in situ U-Pb dating of rutile in Xiadian,Jiaong gold provience,eastern China

热液金红石U-Pb测年是确定金成矿时代的有效方法,然而金红石的多种成因限制了其在金成矿过程和年代学方面的应用。胶东夏甸金矿床中广泛发育多种类型的金红石,是鉴别金红石成因和厘定金成矿时代的理想选择。该矿床中的金红石呈簇状分布,与银金矿、黄铁矿、黄铜矿、闪锌矿和辉钼矿等热液矿物共生,且发育交代结构,呈不规则环带状,表明其为热液成因。通过BSE和EPMA分析,识别出两种热液金红石:高W金红石(W含量1.672%~6.125%,平均3.48%)和低W金红石(W含量0.002%~0.788%,平均0.204%)。在金成矿期的早阶段,富钛或含钛矿物的分解或重新平衡导致了低W金红石的沉淀,而成矿主阶段热液流体携带难熔元素及部分高场强元素多次脉动式上涌,流体中的V^(3+)、W^(6+)、Fe^(3+)、Cr^(3+)和Nb^(5+)等离子以2M^(3+)+W^(6+)3Ti^(4+)为主要机制置换早阶段低W金红石中的Ti^(4+)形成高W金红石。夏甸金矿床中高W金红石通常与石英-硫化物阶段的热液矿物共生,而且其W含量超过7000×10^(-6),Zr含量范围为200×10^(-6)~600×10^(-6)。综上,本文认为高W金红石是热液成因,与金矿化关系更为密切。此外,低W金红石的LA-ICP-MS U-Pb定年结果为119.3±8.4Ma(2σ,n=17,MSWD=2.9),为夏甸金矿床的成矿时代提供了新的约束。

Hydrothermal rutile U-Pb dating is an effective method to determine the age of gold mineralization,however,the multiple genesis of rutile limits its application in gold mineralization process and chronology.The Xadian gold deposit in Jiaodong has a wide range of rutile types,which is ideal for identifying the rutile genesis and determining the age of gold mineralization.The rutile in this deposit is distributed in clusters,and commonly intergrown with hydrothermal minerals such as electrum,pyrite,chalcopyrite,sphalerite and molybdenite,and developed metasomatic texture and irregular zonation,indicating that it is of hydrothermal origin.Two types of hydrothermal rutile were identified by BSE and EPMA analysis,one is high W content rutile(W content range from 1.672%to 6.125%,mean 3.480%)and the other is low W content rutile(W content range from 0.002%to 0.788%,mean 0.204%).In the early stage of gold mineralization,the decomposition or rebalancing of Ti-rich or Ti-containing minerals leads to the precipitation of low-W rutile,and the hydrothermal fluid carries refractory elements and some high field-strength elements in the ore-forming stage.The ions in the hydrothermal fluid,such as V^(3+),W^(6+),Fe^(3+),Cr^(3+)and Nb^(5+),substitute the Ti^(4+)in the early low-W rutile to form the high-W rutile,mainly via 2M^(3+)+W^(6+)3Ti^(4+).The high W rutile in the Xiadian gold deposit is usually associated with hydrothermal minerals in quartz-metal sulfides stage,and the LA-ICP-MS trace element analysis results show that the W content exceeded 7000×10^(-6) and the Zr content ranged from 200×10^(-6) to 600×10^(-6).After comprehensive analysis,high W rutile is determined to be of hydrothermal origin,which is closely related to gold mineralization.In addition,the LA-ICP-MS U-Pb dating results of low-W rutile is 119.3±8.4Ma(2σ,n=17,MSWD=2.9),which provides a new constraint for the mineralization age of the Xiadian gold deposit.