Alteration Of Metamorphic Rocks Of The Taihua Group From The Xiaoqinling Area In West Henan For Gold Mineralization

As one of the important localities of the orogenic type gold deposit,the Xiaoqinling area is located in the southern margin of the North China craton.In this paper,petrographical,mineralogical and petrogeochemical studies have been carried out on the biotite-plagioclase and plagioclase-amphibolitic gneisses collected along two profiles in the Taihua Group of Xiaoqinling area.One profile is close to the gold deposits and the other is far away from the gold deposits.The aim of this research is to find out alteration of the metamorphic rocks and its relation to gold mineralization.Field investigation and microscopical observation indicate no obvious difference in major mineral assemblage,large increasing in strength of alteration?mainly chloritization,epidotization and sericitization?,and evident adding in content of ore mineral such as pyrite,chalcopyrite and magnetite,from the metamorphic rocks far from ore bodies to those close to ore bodies.Mineralogical analyses show great increasing in strength of biotite and hornblende alteration.This alteration would be respossible for the activation of Fe,Au and other ore-forming elements in the biotite and hornblende to provide material basis for the formation of magnetite and pyrite and the precipitation and enrichment of gold.Compared with the primary pyrite,the epigenetic pyrite contains more Ag,Cu and As and has closer relation to gold metallization.Petrogeochemical studies show that the metamorphic rocks in the Taihua Group of the Xiaoqinling area could be formed by metamorphism of magmatic rocks.The obvious increasing in abundance of ore-forming elements such as Au,Ag,Cu,Pb and Zn from the metamorphic rocks far from ore bodies to those close to ore bodies indicates obvious enrichment of ore-forming elements through the alterations of the metamorphic rocks.The REE distribution patterns of the biotite-plagioclase and plagioclase-amphibolitic gneisses are similar to those of the gold ores,which indicates possible relation of the gold enrichment and mineralization mainly to the activation,migration and enrichment of gold from these rocks.The obtained geological and geochemical data support the derivation of ore-forming material and heat power for the gold mineralization from the biotite-plagioclase and plagioclase-amphibolitic gneisses in the Taihua Group and Yanshanian granitic magma activities.Gold with K+,Na⁺ and Fe²⁺ in the metamorphic rocks might be activated by the alteration and transformed in the form of complex in the hydrothermal fluid towards some locations with low pressure.After a series of strong water-rock reactions,decomposition of the Au-bearing complexes would result in precipitation of gold in suitable locations for theformation of gold deposits.