The Alteration Age of Fracture Zone and Its Implication for the Formation of Gold Deposits in Jiaodong Area

The gold deposits in Jiaodong are the mesothermal hydrothermal alteration vein deposits formed in Mesozoic tectono-magmatic activation area. The research about the isotope age of magmatic rocks and strata has fruitful achievements in Jiaodong region, but the research of the age of altered rock needs to be strengthened. Former researcher defined the gold deposits in Jiaodong area are crust source-deep melting-hydrothermal quartz vein-altered rock type gold deposit. The age of altered rock obtained by former researchers cannot give us an over view about regional metallogenic regularity. To find the method that using time of hydrothermal alteration to explain the regional metallogenic regularity, the author developed field research in the main mineralized alteration belts: (1) choose the beresitization and potassic altered rock belt which are widely distributed in fracture alteration zones in Jiaodong area to be the sample position, and they are easily to be contrasted; (2) these two kinds of altered rock have less affected by the brittle fracture, which means they have stable chemical components of alteration process; (3) the altered minerals in these two kinds of rock belts have ductile deformation with the same period of the alteration process, which can indicate the structural stress and strain status. The plagioclase and potassium feldspar particles in rock samples are the objectives of Potassium-argon dating (K-Ar dating) test. The test results show that the period of hydrothermal alteration was from 148.3±4.3 Ma to 52.03±0.70 Ma, which is the late Yanshanian of Early Cretaceous Epoch; and that the altered rock isotope age decreases gradually from west to east in Jiaodong region, which proves the occurrence time of hydrothermal alteration in west earlier than that in east. Through the research about the structure, the component of mineral hydrotherm, and the physical chemical parameters of mineralization, this paper analyzes the regional regularity and the epoch of ore formation. The conclusion indicates that the regulation of hydrothermal alteration and mineralization in Jiaodong area are controlled by the tectonic additional hydrostatic pressure, the temperature and pressure conditions of ore-forming fluid, and the distribution and change of ore-forming elements. The above-mentioned geological characteristics indicate that the gold deposits in Jiaodong region are of the compound tectonic hydrothermal deposit type