黑龙江东安金矿床赋矿岩浆岩锆石U-Pb年代学及岩石地球化学特征

Zircon U-Pb geochronology and petrogeochemistry of host igneous rocks of the Dong'an gold deposit in Heilongjiang Province,NE China

东安金矿床是环太平洋成矿域的一处大型低硫型浅成低温热液金矿床,赋存于燕山期碱长花岗岩和中酸性火山岩中。本文通过LA-ICP-MS锆石U-Pb同位素定年,获得赋矿的碱长花岗岩和光华组流纹岩的加权平均年龄分别为183.2±1.3Ma和109.1±1.2Ma,表明碱长花岗岩的侵位年代为早侏罗世,光华组火山岩的喷出时代为早白垩世。在地球化学组成上,东安碱长花岗岩具高硅、高钾和低磷的特征,富集Rb、Th和K,亏损Nb、Ta、Sr、P和Ti,属于高分异的I型花岗岩,是太平洋板块俯冲作用的产物。光华组中酸性火山岩富集Rb、Th、U和K,亏损Nb、Ta、P和Ti,为太平洋板块俯冲方向发生改变后的岩石圈伸展减薄环境下,镁铁质下地壳部分熔融而形成的。东安金矿床成矿年龄（107～108Ma）与光华组火山岩的成岩年龄在误差范围内一致,表明成矿与成岩作用为同一地质事件,均形成于早白垩世太平洋板块俯冲背景下的拉张构造环境中。结合区内其他浅成低温热液型金矿床的赋矿围岩特征,认为早白垩世陆相火山岩是东北地区寻找浅成低温热液金矿床的有利场所。

The Dong＇an low-sulfidation epithermal gold deposit,located in the circum-Pacific metallogenic domain,is hosted by Mesozoic alkali-feldspar granites and intermediate to acid volcanic rocks. LA-ICP-MS zircon U-Pb dating of this study gives a weighted average age of 183. 2 ± 1. 3Ma for alkali-feldspar granites and 109. 1 ± 1. 2Ma for rhyolites of the Guanghua Formation,indicating that the alkali-feldspar granites were intruded in the Early Jurassic,while the volcanic rocks were erupted in the Early Cretaceous.Geochemically,alkali-feldspar granites from the Dong ＇an deposit are characterized by high contents of SiO2 and K2O and low concentrations of P2O5,with enrichment in Rb,Th,U and K and depletion in Nb,Ta,Sr,P and Ti,indicating that they belong to highly fractionated I-type granites related to subduction of the Pacific plate. Intermediate to acidic volcanic rocks of the Guanghua Formation are enriched in Rb,Th,U and K and depeleted in Nb,Ta,P and Ti,suggesting that they were generated by partial melting of the lower mafic crust under the conditions of lithospheric extension-thinning when subduction direction of the Pacific plate changed.The mineralization age（ 107 ～ 108Ma） of the Dong＇an deposit consists with the eruption age of the Guanghua Formation volcanic rocks,suggesting that both gold mineralization and host rock formation are the results of the same geologic event in an extensional tectonic environment related to subduction of the Pacific plate in Early Cretaceous. Combined with geological characteristics of host rocks from other epithermal gold deposits in this region,it is believed that Early Cretaceous subaerial volcanic rocks are favorable host rocks for epithermal gold deposits in Northeast China.