冀北张家口地区汉诺坝玄武岩中麻粒岩捕虏体的硫化物相

捕虏体麻粒岩是了解下地壳形成和演化的重要样品。汉诺坝新生代玄武岩中的二辉麻粒岩捕虏体样品中富含各种硫化物相,主要类型有:①孤立产出的球状出溶硫化物;②矿物颗粒之间或颗粒内的粗晶硫化物;③次生硫化物包裹体群;④裂隙充填硫化物。电子探针分析表明,硫化物的矿物成分均为贫镍磁黄铁矿,(Ni+Co+Cu)/Fe(原子比)远小于0.2;(Fe+Cu+Co+Ni)/S(原子比)比地幔岩的磁黄铁矿小,多小于0.875,反映了一种S过饱和环境。各种产状的磁黄铁矿中Au、Ag都有一定的含量,其平均值分别为0.19%～0.22%(Au)、0.01%～0.02%(Ag),反映下地壳的麻粒岩化与金矿化的成因联系。磁黄铁矿的Ni、Co、Cu含量与S正相关,说明微量重金属元素与S具有同源的关系,由于地幔去气伴随S而进入下地壳。

Sulfide phases in granulite xenoliths in the Hannuoba basalt,Zhangjiakou,northern Hebei,China

Granulite xenoliths are important samples for understanding the formation and evolution of the lower crust.The two-pyroxene granulite xenoliths enclosed in the Cenozoic Hannuoba basalt,Hebei,contain four types of sulfide,which are:1 isolated,spheroidal exsolution sulfides,2 intergranular or intragranular sulfides,3 linearly arrayed secondary sulfide inclusions,and 4 fissure-filling sulfides.Electron microprobe analysis shows that the sulfides are compositionally Ni-poor pyrrhotite.Its (Ni+Co+Cu)/Fe (atomic ratio) is far lower than 0.2 and the (Fe+Cu+Co+Ni)/S (atomic ratio) is lower than that of pyrrhotite of pyrolite,mostly <0.875,reflecting a sulfur-oversaturated environment.Pyrrhotite in various modes of occurrence contains Au and Ag in some degrees,with averages of 0.19 to 0.22 wt% (Au) and 0.01 to 0.02wt% (Ag),showing that gold mineralization is genetically related to granulitization of the low crust.Ni,Co and Cu contents in pyrrhotite have a positive correlation with S,indicating that trace heavy metal elements are derived from the same source as sulfur because of degasification of the upper mantle and sulfur entrance into the lower crust.