The Structure Analysis And Ore-controlling Factors Of Xiaoqinling Gold Ore Field In Shaanxi Area

Situated in the sourthern part of the North China Craton,Xiaoqinling gold deposits are one of the China's major gold sources as well as an important part of the world's precambrian gold provinces.Since 1960s,much work in terms of exploration and research has been conducted.However,some critical issues like the structure of ore field,the chronology of mineralization,ore geneis and mechanism of mineralization remains controversial.This thesis tries to address the issue by both field observation and systematic analysis.We have gained a clear picture of metallogenic patterns as well as the controlling factors related to the mineralization.The main conclusions are as follows:1.Duringlate Triassic epoch,North China plate collided with Qinling plate,producing a series of tectonic slice,which formed Qinling-Yinzhi foldbelts.Since Jurassic period,the stress within Qinling,had transformed from NNW oriented to SSE oriented.Xiaoqinling area has gone through intracontinental orogeny period since Mesozoic and Cenozoic period.,forming a series of extensional collapse structures during the T₃^J₁ and thrust-nappe structures with extensive magmatic activities during the J-K₁ period.The main charactors of the structures within the Xiaoqinling disrict are:?1?Within the Qinling terran,the thrust-nappe structures that mainly occured from north to south were superimposed on the main faults?2?During the same period,Qinling Mountains began to subduct from south to north underlain the Qinling Mountains,forming the southern edge of the thrust nappe structure from the north to the south?3?Intensive magmatic activities happened as the result of A type subduction,forming massive amount of granite bases and medium and small porphyry with an peak age between 160¹20Ma.The Xiaoqinling gold field structure framework is dominated by the Near-EW tectonic Line.In general,an anticline structure which tilted toward the west is formed and the northeast-southwest fault and the NW-SE fault are superimposed on the east-west tectonic line.2.The ore-controlling structures of the Xiaoqinling Gold Mine are:?1?The linear structure of Xiqiao-Artemisia-Maohe is mainly oriented in the northeast direction and gold-bearing quartz vein gold ore body is developed along the fault which is related to the thrust under the compressive stress from the northwest to the southeast in the early period of Yanshan?2?Taiyu-Shancheyu-dongtongyu-xitongyu-wenyu linear structure is NWW-oriented to the north of the Daoyping anticline core,NWW oriented to the sorth of the Daoyping anticline core,and turns on the Taiqian anticline.The linear structure near the tip is NNE,and the Da Yueping anticline structure controls the distribution of faults and gold veins.Da Yueping's slow anticline undergoes tectonic transformation during the Indosinian period,with the two wings becoming steeper,developing faults and secondary faults along the nucleus and the two wings.?3?The linear structure of the Xiaohe rock mass group is dominated by a southwest-inclined thrust fault near the east-west direction.The strike-slip fault level in the northeast direction,especially the scale of the faults in the near-east-westward direction,has formed Xiaohe rock mass group.3.The ore-controlling factors of gold deposits in the Xiaoqinling gold field are mainly the Yanshanian rock mass and the NE and west-westward development fractures.During the Yanshanian period,mid-acidic magmatic activities moved mainly along the nearly EW thrust faults and brought the fluid activities related gold mineralization..In the Yanshanian period,a series of northeast,northwest,westwest oriented faults,and gold precipitated along favorable folds under the intracontinental orogeny accumulate.Therefore,the Yanshanian magmatic activity is the guiding and ore-conducting structure.The NEE and NW-trending structures are the main metallogenic and ore-forming structures in the Xiaoqinling ore field.4.This study shows that the mineralization age measured by the Rb-Sr method of the pyrite of the Chener gold deposit,Xiyu gold deposit,and Mahao gold deposit in the Xiaoqinling area was 134.8±1.1 Ma.Which is consistent with he age of the Ar-Ar method from sericite in the ore-forming period and represents the mineralization age.This age is consistent with the emplacement age of Yanshanian granitic magmatism,and the ore-forming age of other deposits in the region,but it is far from the age of the high-grade metamorphic Yanhua Formation.At the same time,Given the fact that previous results of trace isotope system traces,estimated temperature,pressure and salinity obtained by fluid inclusions,the distance where the ore body and the rock mass are generally within the range of 2-5 km,it is considered that the Xiaoqinling area is intrusion-related gold deposit.