Dykes And Gold Mineralization In Zhaoping Fault Belt(Zhaoye Region)

Mesozoic dykes in Zhaoping fault belt spread widely. According to petrography, they can be classified into mafic rocks, neutral rocks and acid rocks. The study of geochemistry characteristics of dykes and igneous rocks showes that their initial source probably is the JiaoDong Group. In the first stage of partial melting of the JiaoDong Group, three kinds of granite, neutral dykes, acid dykes and volcanic rocks are formed, while in the second stage of partial melting of it, the Guojialing granite, dykes and volcanic rocks come out.The age of gold mineralization in ZhaoPing fault belt is mainly between 110 and 120Ma, but dykes are formed before, in and after the mineralization, like "pulsing". In the respect of space, a part of dykes are ore veins, but most of dykes just accompany ore bodies, and they are both controlled by faults. In the respect of cause, gold does not come from dykes in ZhaoPing fault belt directly, it is may originally from the Jiaodong Group. In addition, the study of fluid inclusions and the trace element characteristics of two types of principal gold ore shows that although mantle fluids participate in mineralization, the ore-forming fluids are mainly composed of crustal fluids, and they have close relation with neutral and mafic magma.The two-stage partial melting model of the Jiaodong Group also can explain dykes forming and gold mineralization in ZhaoPing fault belt very well. Early, in the tectonic setting of active continental margin, the Jiaodong Group enter its first stage of partial melting to generate a part of neutral and acid dykes (volcanic rocks). In this stage, the temperature is low, Au is inactive, and hardly migrate. As a result, it is enriched in the remnant of Jiaodong Group. Lately, to the west of Zhaoping fault belt, the Yanshan movement is stronger, which can provide enough heat to make the Jiaodong Group melt partially again and form neutral and mafic magma, then Au can be activated and carried by the magma which contains mantle fluid. In a few cases, this kind of magma intrude or erupt rapidly and it becomes ore veins. But more often, in some physical ,chemical and structral conditions, ore-forming fluids may separate from the magma, finally enter into various favorable rocks and structures to deposit, by the way of filling or metasomatism. The neutral and mafic magma are also prone to intrude into the weak structures near the ore vein, and then become dykes after differentiation and stallization.