胶东焦家断裂带蚀变岩非镜像对称特征及其地质意义

焦家断裂蚀变带是胶东地区最重要的控矿构造之一。该断裂带控制的矿床是创立“焦家式”金矿理论的重要实例基础。目前,焦家断裂带累计探明Au资源储量超过1200 t,并且还在不断增加,展现了深部重要的勘查和研究价值。焦家断裂带控制的矿体主要赋存在主断面下盘,断裂带发育于花岗岩中时,上盘发育钾长石化花岗岩、绢英岩化花岗岩、黄铁绢英岩化花岗质碎裂岩、(黄铁)绢英岩质碎裂岩,下盘发育黄铁绢英岩质碎裂岩、黄铁绢英岩化花岗质碎裂岩、黄铁绢英岩化花岗岩和钾化花岗岩,蚀变类型在主断面两侧呈现对称分布特征。但是在岩性特征、结构构造、蚀变强度、化学成分等方面差异明显,表现出非镜像对称特征。上盘蚀变岩厚度大于下盘,下盘花岗岩的构造破碎程度比上盘花岗岩更严重;断裂带上盘黄铁矿含量低、一般无矿化显示,下盘黄铁矿含量高,出现金矿化;断裂带上盘的中生代花岗岩中韧性变形不发育,以脆性破裂为主,下盘发育明显的韧性变形;断裂带上下盘不同蚀变带的成矿元素Au,矿化剂元素S,成矿伴生元素Ag、Pb、Zn,亲石分散元素Ba、Sr以及主量元素Na2O、MgO含量具有差异性,指示焦家断裂带主断裂面两盘经历了不同的成矿作用过程,下盘花岗岩的热液蚀变作用与成矿作用的关系更为密切。依据焦家断裂带不同蚀变带元素的非镜像对称性特征,可利用上、下盘花岗岩和构造蚀变带的地球化学标志识别矿体或者不同蚀变带的位置,对认识“焦家式”金矿床的成因机制、预测深部成矿前景、指导深部找矿具有重要理论和实际意义。

Asymmetrical Alteration in the Jiaojia Fault Zone,Jiaodong Peninsula and its Geological Significance

The Jiaojia fault alteration belt is one of the most important ore-controlling structures in the Jiaodong Peninsula,eastern China.The Jiaojia-style gold deposit was proposed on the basis of geological features of gold deposits hosted in this belt.The Jiaojia fault zone has total Au reserves exceeds 1200 t,and shows a promising target of deep exploration.The ore bodies hosted in the Jiaojia fault zone mainly occur in the lower wall of the fault.From the top to the bottom,there symmetrically distributes potassium altered granite,sericite-quartz granite,sericite-quartz altered fractured granites,and(pyrite)-sericite-quartz altered tectonic fractured rocks in the hanging wall,and pyrite-sericite-quartz altered tectonic fractured rocks,pyrite-sericite-quartz altered fractured granites,sericite-quartz granite and potassium altered granite in the lower wall,respectively.The belt shows symmetric distribution of alteration and asymmetric distribution of structure,alteration strength,and chemical compositions.The thickness of the alteration in the upper wall is thicker than that of the lower wall,in spite of the latter has significantly higher degree of fragmentation.The content of pyrite is low in the upper but high in the lower wall,and gold mineralization occurs in the lower wall.The ductile deformation of the Mesozoic granite is less well developed in the upper wall of the fault,mainly brittle fracture.Au content,mineralizer element S,ore-forming related elements such as Ag,Pb,Zn,lithophile elements Ba,Sr and main elements Na2O,MgO also show asymmetric distribution in the alteration zone,indicating different ore-forming processes in the upper and lower walls of the main fault surface of the Jiaojia fault zone.The geochemical signatures of the lower wall granites and tectonic alteration zones can be used as indicators to locate ore bodies and different alteration zones.Hence,it has important theoretical and practical significance for the understanding of the genetic mechanism of the Jiaojia-type gold deposits and guiding deep prospecting.