The Geologic And Geochemical Research On The Ore-Bearing Formation (Related To Au/Cu Ore-Deposits) In Volcanic Area In North Of Dehua, Fujian Province

The continental volcanic rock area of Dehua, Fujian Province located in southestern margin of Eurasia, on the west of Pacific ocean, which is the part of volcanic belt in southeast China. It located on northwest of Zhejiang-Fujian volcanic fault depression zone, and west of Daiyun-Shiniu mountains’ circle tectonic depression. Besides, the volcanic rock area of north Dehua lies in middle of Zhenghe-Dabu NE fault depression zone and south of Shouning-Huaan volcanic basement uplift belt. There are Presinian, Paleozoic and Mesozoic sequences in the study area, especially volcanic strata of late Jurassic. The Presinian and Paleozoic formation appear sporadically in volcanic rock, which have been undergoing metamorphism.The ore-bearing formation in the study area is the intermediate-felsic volcanic rocks of Changlin and Nanyuan group producing in upper Jurassic. It combines with different intermediate-felsic volcanic rocks, such as volcanic breccias, andisite, decite, crystal stuff and rhyollite stuff. It has typical characteristics of intermediate-felsic volcanic rocks, forming in volcano-magmatic-deposition during the first stage of Yanshanian.According to the geochemical characteristics, the ore-bearing formation belongs to calc-alkaline series in general and has the typical traits of high-K calc-alkaline series and shoshonite series. It is similar to the Changlin and Nanyuan groups, however, the content of Al2O3、Na2O is lower, Rittmann index is smaller and the ratio of K/Na is much higher than average of Changlin and Nanyuan group.What’s more, the ore-bearing formation has characteristics of Si-rich, Al-rich, K-rich, but Na-lean, Ti-lean and Mg-lean.The ore-bearing formation’s total content of rare elements is abundant, the distribution of REE curves to right. LREE is more obvious than HREE on fractional distillation. It can be devided into two classes in terms of δEu negative anomaly. The characteristics of trace element indicate that the ore-bearing formation derived from crust-mantle mixed magma, because of LILE rich, like Rb、K、La、Ce、Nd、Zr、Hf, and lean on Ba、Ta、Nb、Sr、P、Ti, which is similar to island arc volcanic rocks. Above all, the ore-bearing formation forming under the background of subduction of Pacific and Eurasian plates is island arc volcanic rocks origin to curst-mantle material.The SHRIMP zircon U-Pb age of ore-bearing formation in the continental volcanic rock area of Dehua is 157.6±2.1Ma~158.0±2.4Ma, on behalf of rock and ore-forming ages which belongs to the first stage of Yanshanian. K-Ar Age shows that the mineralization age is later than diagenesis age slightly, which is 111.6±1.7Ma~156.9±2.4Ma. The ore-bearing formation has the law of medium, intermediate-felsic to acid volcanic rock. The S isotope of ore-bearing formation shows that it mainly comes from upper mantle and lower crust magma, but with a little upper crust materials. The Pb isotope characateristics of it indicate that the ore-forming materials derived from lower crust and in accordance with the characteristics of island arc volcanic rocks in subduction belt.On the comprehensive analysis of the metallgenic factors in the study area, the ore-bearing formation is the intermediate-felsic volcanic rocks of Changlin and Nanyuan group, which formed under the subduction of Pacific and Eureasian plates, drived by early Yanshanian lava tectonic activity, and then the magma intrude into crustal weak part and the structural fissure with amount of ore-forming material, formed the ore-deposits finally.