Two Periods Of Mesozoic Gold Mineralization In Aohan Area,Inner Mongolia

Chifeng-Chaoyang gold mineralization area is located at the junction of the eastern part of the northern margin of North China Craton and Hingan-Mongolian Orogenic Belt.The Mesozoic gold mineralization in this area can be roughly divided into Indosinian and Yanshanian periods.The Zhuanshanzi and Jinchanggouliang gold deposit are chosen as two representative objects,based on the comprehensive research on the geological background,isotopic geochemistry and metallogenic fluid geochemistry,the coupling relationship between gold mineralization and tectono-magmatic activity and the genetic type have been discussed,and also the metallogenic models have preliminarily established.Furthermore,the prospecting direction was pointed out in this area.The ore bodies of the Jinchanggouliang deposit are mainly hosted in the Xiaotazigou formation of Archean Jianping Group.Most of the orebodies strictly controlled by faults striking NW.Wall rock alterations are dominated by silicification,sericitization,pyritization,chloritization.The ore minerals are predominantly pyrite,chalcopyrite,galena,sphalerite and native gold.the mineralization processes of the Jinchanggouliang gold deposit can be divided into three stages:including milky or grey quartz(?),quartz-pyrite(?),quartz-polymetallic sulfides(?).while the ore bodies of the Zhuanshanzi gold deposit are hosted in the Upper Permian Ranfangdi Formation.Wall rock alterations and the ore minerals are the same as the Jinchanggouliang gold deposit.The mineralization processes of the Zhuanshanzi gold deposit can be divided into three stages:quartz-pyrite(?),quartz-polymetallic sulfide(?),quartz-fluorite-carbonate stage(stage ?).The results of element geochemistry show that the Duimiangou intrusion which is closely related to mineralization,belongs to high fractionated I-type granite with characteristic of high potassium calc-alkaline.It formed in the intracontinental extension environment in the late Jurassic.The rhyolite porphyry belongs to high potassium calc-alkaline series with the feature of metaluminous to weak peraluminous,and it may be formed by the separation and crystallization from the mixing magma of crust-mantle in an intracontinental extensional environment.The ore-bearing rhyolite was formed in the continental marginal arc to collisional tectonic environment of active continental margin.The ore-bearing granitic porphyry and post-mineralization diorite were formed in the extensional environment after the collision of the North China plate and the Siberian plate.The study of fluid inclusions on the Jinchanggouliang gold deposit indicated the ore-forming fluids of the main mineralizing stage belonged to the NaCl-H2O-CO2 system with medium-high temperature(245?382?),medium-low/medium-high salinity(1.1?10.7 wt.%NaCl eq/34.2?41.4 wt.%NaCl eq)and low density(0.73?0.88g/cm3),while the ore-forming fluids of the main mineralization stage from the Zhuanshanzi gold deposit were generally characterized by medium-high temperature(252?332?),medium-low/medium-high salinity(2.1?11.0 wt.%NaCl eq/33.7?39.8 wt.%NaCl eq)and low density(0.75?0.84g/cm3).The study of fluid inclusions indicates that fluid boiling was the main mineral precipitation mechanism.C-H-O isotope studies indicate that the ore-forming fluids of the two typical were dominated by magmatic water,perhaps some meteoric water was added during later mineralization stage.S-Pb isotope data suggest that the metallogenic material are dominantly of deep magmatic origin.Combined with the metallogenic geological background and previous studies,the metallogenic material of the Jinchanggouliang deposit may be originated from the DMG intrusion,while that of the Zhuanshanzi deposit may be originated from the Trissic granitic porphyry.On the basis of the above comprehensive research,it is considered that there are Indosinian and Yanshanian gold mineralization in Chifeng-chaoyang area,which is presented by the Zhuanshanzi and Jinchanggouliang gold deposit,respectively.The former is closely related to the magmatic activity caused by the slab detachment in the extensional environment after the collision of the North China plate and the Siberian plate,while the latter is related to that formed by Yanshanian large-scale lithospheric thinning in the transitional environment from extrusion to extension.