Tectonic And Magmatic Constraints On Mineralization And Gold Prospecting Of Yangshan Gold Belt, Gansu Province

The Yangshan Gold Belt locates in the mid-south part of south sub-belt in western Qinling orogenic belt; the current gold resources/reserves of Yangshan belt were more than 320 tons. In this paper, using a large number of field data and integrated test and analysis as the support, the regional diagenesis and mineralization background, environment and age of dike formation, ore-controlling structural system and structural-magmatic mineralization were studied, and the dynamics mechanism of structural control and exploration direction in Yangshan gold belt were discussed.Based the structral analysis for structral geological phenomenon in Yangshan gold belt and study of Western Qinling tectonic evolution history, we comformed that the ore-controlling structure system of Yangshan gold belt, and considered that basic structure pattern in this area were formed during the continent-continent collision of Indosinian period. The regional tectonic evolution sequences of Yangshan gold belt were as follows: 1) the extension of Hercynian intracontinental rift caused the sediment of Sanhekou Group, later on the uplift background, bedding ductile shear deformation occurred and closed oblique fold formed in this area which accompanied by strong structural replacement, then regional penetrative foliation(S1) formed; 2) a series of arc structural belts were formed under the background of SN strike compression caused by the continent-continent collision in the Indosinian period. The Yangshan gold belt was the tip-arc and folds-faults system in its east part; and associated with the homochronous or post-Indosinian mid-acidic dykes intruded; 3) the deformation were mainly brittle structure in early Yanshanian period, including succeed thrust fault and derived bedding fracture, etc.; the regional extension caused the dustpan-shaped rift basin and normal faults formed in the late Yanshanian period, accompanied by the plagio-granite intrusion of the Yanshanian period; the structual setting of the Yangshan gold belt were mainly extension in Himalayan period.The study of spatial structure of belt, structural styles and deformation fabrics showed that the Tangbogou-Guanyinba fault was the important ore-controlling structure in south sub-zone of West Qinling fault zone, and had undergone different levels of deformation. Early deformation mainly thrust fault, left-slip falut deformation in deep, later deformation were cataclastic rocks system which caused by brittle deformation superposed to ductile deformation. The later ductile deformation and brittle desormation were accompanied by strong magmatic activity and hydrothermal mineralization.To find out the restriction rule of magmagic activity to gold mineralization, the SHRIMP U-Pb zircon geochronology of dykes were carried out based on the study of petrology, petrochemistry and geochemistry for mid-acidic dykes. The results showed that: 1) the mid-acidic dykes occurred in Yangshan gold belt formed major in Indosinian and Yanshanian period; 2) Indosinian dykes intruduction started at the end of continent-continent collision (about 220Ma), activitied at uplift-extend environment (about 210Ma) and continue until about 190Ma; 3) the Yanshanian dikes were first discovered in this area(U-Pb zircon age of 115.8±1.6Ma), indicating that magmatic activity occurred in the intracontinental extension phase; 4) superposition of multi-period magmatic activity and mineralization restricted the formation of the Yangshan super-large gold deposit.The studies of magmatic activity and gold mineralization suggest that: the Yangshan gold belt was controlled mainly by near EW-NEE strike regional faults, the ore bodies occurred in the Tangbogou-Guanyinba regional fault and its derived sub-fault, and the ore occurrence controlled by faults strictly. The formation of large, super-large scale deposits were controlled by fault-fold-magmatic activity. This paper pinpointed the ore-controlling mechanism of structural and established the tectonic-magmatic mineralization model of Yangshan gold belt based on the study of geometry and kinematics analysis for ore-controlling faults, the dike intrusion and ore body position in Yangshan gold belt had close relationship with expanded space formed in mineralization period.The metallogenic prediction using weights-of-evidence method was carried from integrated imformation (include geological, geophysical, geochemical and remote sensing) based on structure-magma ore-controlling law. Two prospecting targets were delineated (deep in west Anbali and north area of Anba), some prospecting areas were submitted (including Tangbogou and Bolinping). A middle scale gold deposit was discovered by drilling in north area of Anba.This paper recognizing that main ore-bearing structure in Anba-Getiaowan ore section were trunk structure (F2, F3) of Tangbogou-Guanyinba fault, and its occurrence dip to south in shallow. This pointed out the direction for the gold exploration in this area and provided geological evidence for circle ore body and appraises report.