Research Of Structural Ore Controlling Regularity In Xiaoqinling Gold Field

Xiaoqinling area was located on the border between Shanxi and Henan. Its tectonic position were important part of Qinling orogenic belt. Qinling orogenic belt experienced multi-stage and multi-system tectonic evolution from early Precambrian. Since Indosinian period, Qinling orogeny was formed by the collision among Yangtze Plate, southern Qinling massif and northern China plate and caused a mulit-terrain tiled geotectonic framework. Especially in the Yanshannian, it was superposed evidently by the effect of the peri-Pacific tectonic domain. During the continental collision and later tectonic superposition, it had several tectonic-metallogenetic cycles and crust-mantle exchange and geological processes in the surface or deep were strongly, then it formed abundant and various metallogenic systems and series. The gold mineralization was closely related to the structure-magma-hydrothermal fluid effects in the orogenic movement from the end of Indosinian period to Yanshan period. Xiaoqingling gold area is an important one of these metallogenic series.Xiaoqingling area was the eastern part of the northern margin of Qinling orogenic belt and upliftied as a regional EW fold. It was adjacented to a Quaternary sedimentary basin named as Weihe basin at the north side with a fault boundary named as Taiyao fault, adjacented to another Quaternary sedimentary basin at the southeast side, and connected Xiong’er tectnic belt with a fault boundary named as Xiaohe fault.The regional stratum were mainly a mesometamorphic granitic-greenstone series of archeozoic and paleoproterzoic Taihua group which was the gold ore host rock and secondarily a clastic rock-littoral carbonate formations of neo-proterozoic Guandaokou group.Xiaoqinling area had complex fold and fault structures and its general pattern was summarized as a metamorphic core complex which had one core, two boundaries and three detachment. Its shape was a EW complex anticline whose centarl was Laoyacha main anticline and bilateral were Wulicun anticline, Qishuping syncline, Miaogou syncline and Shangyangzhai anticline separately. There were hundreds ductile-brittle faults whose tendency were mainly nearly EW and secondarily NW, NE and NS. The most major ore-controlling fault was the nearly EW compresso-shearing fault which had undulatory shape along their strikes and dips and the secondary ore-controlling fault was the NE fault. These ore-controlling faults developed multiply on the base of early ductile shear belts.The regional volcanic-magmatic activity was frequently and magmatic rock was widely distributed, such as the lager-area proterozoic and mesozoic yanshannian magmatic rocks.There were plenty of gold-bearing quartz veins and gold deposit in Xiaoqinling gold ore field which were characterized by concentration, zonation and nearly parallel alignment and were divided into the west ore belt, the south ore belt, the middle ore belt and the north ore belt.The major gold metallogenic type was quartz vein type and a few was structural altered rock type.Dongchuang gold deposit, Yangzhaiyu gold deposit in Lingbao county, Henan province and Dongtongyu gold deposit in Tongguan county, Shanxi province were three most typical quartz vein type gold deposits which distributed near the axis of Laoyacha main anticline. The ore-bearing rock was mainly metamorphic rock of Taihua group and gold-bearing quartz veins chiefly distributed in the ductile shear belts or brittle fractures near the axis of the fold.The largest ore vein of Dongchuang gold deposit is vein No.507whose strike is nearly EW. Beside it, there are other EW larger veins such as No.508, No.505and No.504. The main ore vein of Yangzhaiyu gold deposit is EW vein No.S6O. The typical veins of Dongtongyu gold deposit are vein No.Q8and vein No.Q10whose strikes are EW and vein No.Q12whose strike is NE.All these veins had complex-thin,thinning-out and recurrence shape. Orebodies distributed nearly isometrically in the veins and mineralized in several stages. The Au grade was positively correlated with thickness. The main ore minerals were native gold, electrum, quartz, pyrite, galenite, chalcopyrite and sphalerite or so. The major texture were granular and filling texture. The typical structures were banding, net-work and impregnated structure. Pyritephyllic alteration had a close relationship with mineralization.In the major mineralize period, gold mineralization could be divided into four stages. The first one was pyrite-quartz stage which grew ivory white thick quartz vein and mineralization weakly. The second one was quartz-pyrite stage which grew banding or thin vein pyrite veins strongly. The third one polymetallic sulfide stage which grew massive pyrite, galenite, chalcopyrite and sphalerite and mineralization strongly. The last one was quartz-carbonate stage which mineralized very weakly.The quartz vein type gold deposits in Xiaoqinling were controlled strongly by geological structure. Regional reticulate complex structure was mineralization basic. Complex fold and fault concentrated zones controlled the distribution of gold ore zone. Fault activity controlled orebody shape and multistage mineralization.According to the relationship with gold mineralization evolution, the tectonic activity could be divided into pre-mineralized, mineralized and later mineralized tectonic activities.Because of the continental collision before the mineralization, the EW complex anticline, ductile shear zones and brittle fractures which were developed from ductile shear zones formed in Xiaoqinling area in NS bearing stress in Indosinian. In the first two stages of main mineralization period, the area was also in the near NS extrusion stress. In the first stage, the faults which struck EW and dipped to south thrust from SW to NE obliquely with leftward contortion. The ivory white thick quartz veins filled at EW strike or gentle inclined parts, but the mineralization was weakly. Besides, NE-NNE and NNW-SN conjugate fractures developed. The former had contorted leftward and the later contorted rightward. In the second stage, fault activity inherited and developed from the first stage and grew gold bearing pyrite-quartz veins strongly. In the third stage, regional tectonic stress converted from extrusion to extension. The EW faults had rightward contortion. The early orebodies or veins crushed and were filled with plenty of polymetallic sulfide. In the last stage, fault extended very weakly.After mineralization, tectonic activity weakened continuely. Only small normal faults and reverse faults grew and leaped orebodies or veins slightly.The research of directional structure samples showed the stress conversion mechanism during the mineralized process. Regional tectonic stress was N-S extrusion with left contortion. It was NNE-SSW extrusion in the first and second stages but extension in the third stage.Xiaoqinling gold mineralization started in the tectonic mechanism converting period of Qinling collisional orogeny then mineralized greatly in the extension stage of Yanshannian. Before and in the Indosinian, Qinling orogeny caused Xiaoqinling area folding and upwarping and formed EW tectonic system including a complex fold and many ductile shear zones. From late Indosinian to early Yanshannian, Qinling collisional orogeny started converting from extrusion to extension. The early tectonic stress was also extrusion. The rudiment of Xiaoqinling metamorphic core complexes formed and produced synorogenic detachment-extension. Brittle fractures grew gradually and produced early mineralization. In the middle-late Yanshannian, affected by the subduction of Pacific plate beneath Eurasian plate, the NE tectonic system superimposed and reformed the EW tectonic system gradually. Xiaoqinling metamorphic core complexes formed and produced collapse extension. Granitic batholiths invaded and brittle fractures grew greatly. These caused large-scale mineralization.By trend surface analysis of ore body thickness, Au and Pb grades on vein No.507in Dongchuang gold deposit, vein No.S60in Yangzhaiyu gold deposit, veins No.Q8and No. Q12in Dongtongyu gold deposit, the graphics revealed the characteristics of fault ore-controlling and the mineralized enrichment regular of these deposits well. Those graphics showed general morphological similarities and microscopic difference among orebody thickness, Au grade and Pb grade. These reflected that vines formed primarily in the first stage, gold mineralized strongly in the second stage and Au-Pb superposed mineralization in the third stage. The main mineral veins with EW strike pitched to southwest and it showed that the ore-controlling faults thrust from SW to NE obliquely with leftward contortion. Controlled by orebody plunging direction and wavelike fault occurrence jointly, the intersection parts of two structure lines which distributed proximal parallel and equidistantly were mineralization enrichment areas. So, the major deep prediction direction should include the lateral trending direction of orebody, the intersection parts of two structure lines and the center aberrations between orebody thickness and grade.By the computer numerical simulation of tectonic stress on the base of finite elem ent method, this paper verified the ore-controlling effect of the regional N-S extrusion stress and leftward shearing contortion.Using the software of ABAQUS, this paper simulated the tectonic stress on No.3prospecting line section in Dongchuang gold deposit. One hand, the position where the fault occurrence changed from precipitous to placid was the favorable mineralized place where the shear stress concentrated strongly and broke the rocks easily. On the other hand, through the stress simulation from early extrusion to extension, the result graphics could be in accordance with the mineralized enrichment characteristics of known ore veins. It would verify the ore-controlling effect of the tectonic stress conversional mechanism from early extrusion to extension.This paper studied the structural ore-controlling rules of quartz gold deposit in the Xiaoqinling area systematically from macro to micro. It analyzed ore-controlling effect by the tectonic stress conversion in the mineralization process, discussed the relationship between the main tectonic activity and the gold mineralization evolution, summarized mineralized enrichment regular and pointed out the major deep prediction direction of several typical ore veins. All these results would be helpful for mineralization regular research and deep prediction in Xiaoqinling gold ore field.