北祁连镜铁山海底喷流沉积铁铜矿床

THE JINGTIESHAN SUBMARINE EXHALATIVE-SEDIMENTARY IRON-COPPER DEPOSIT IN NORTH QILIAN MOUNTAIN

镜铁山矿区以开采铁矿著称，最近又发现铜矿床，并正在进行勘探。该铁铜矿床沉积于震旦纪裂陷海盆之中，该海盆同沉积期构造活动明显。铁矿层及其中上部产有大量纹层状、层状碧玉岩和重晶石岩，底板伴随硅化等同生退色蚀变，产有细（网）脉浸染状工业铜矿体。地质和地球化学特征揭示出铁铜矿床主期成矿为海底喷流沉积方式。

In the Jingtieshan iron deposit with long mining history,a copper ore deposit was recently discovered and is now being explored. Located in the western part of the North Qilian Caledonian fold belt and serving as the component part of Jingtieshan Group,the iron copper deposit was formed in Sinian rift sea basin characterized by obvious synsedimentary tectonic activities. The iron ore bed is a rhythmite made up of iron oxide ore,jasper rock,barite rock,siderite ore,ankerite rock etc.,with a large size independent barite ore bed overlying its top and a veinlet disseminated industrial copper orebody associated with syngenetic decolorization on its bottom. Iron and copper ores are chemically evidently different from wall rocks in that they are obviously deficient in terrigenous constituents and enriched in indicator elements of exhalation sedimentation. In addition,their typical element association is Fe Cu Si Ba,and the relationships of Fe,Mn,Al,Ti,P and Y are quite comparable with those of modern submarine metalliferous sediments.S,C,O,Pb isotopic geochemical studies show that,instead of being provided by terrigenous weathering materials,the metallogenic substances were related to deep fluids;ore forming fluid was acid(pH=4 87～5 37)and reductive(Eh=-0 29Vm～-0 58Vm)hot(175℃)water;metallogenic chronologic studies demonstrate the syngenetic sedimentation of the ore bed,and the existence of the old lead further proves that the metallogenic substances were derived from the extraction of sedimentary or volcanic sedimentary column by deep fluids. It is therefore concluded that the main stage ore forming process of the Jingtieshan iron copper deposit was submarine exhalative sedimentation,with epigenetic transformation leading merely to local reenrichment of copper. With such a new train of thought,the prospecting for copper and iron might be carried out more effectively.