Tectono-Magmatic Evolution And Gold Mineralization In Western North Qilian Orogen

The Yingzuishan area is located in the western part of the North Qilian accretionary orogenic belt（NQOB）,bordered by the Alashan Block to the north and cutted off by the Altyn Tagh Fault to the west,and is one of the important gold mineralization zones in western Gansu.There are several gold deposits in the study area,such as Yingzuishan,Hanshan and Dishuishan,and a large number of lithological units related to the accretionary orogenic belt are exposed.In this thesis,the typical gold deposits and different lithologic units in Yingzuishan area are studied.Based on the field geological survey,the lithologic units in Yingzuishan area in the western NQOB are determined by high-precision chronology,and the nature of source area,diagenetic process and tectonic background of each lithologic unit are discussed by combining rock geochemical composition,whole-rock Sr-Nd isotope and zircon in situ Hf isotope.The melt source,petrogenesis and tectonic setting of each lithological unit are discussed.The geological characteristics of typical gold deposits,in situ trace elements of sulfides and in situ sulfur isotope compositions of sulfides were studied,the nature of mineralizing fluids and the source of mineralizing materials were investigated to construct a gold mineralization model for the western NQOB.The main insights obtained are as follows.（1）The Yingzuishan gabbro intruted at 452 Ma and are cumulate gabbro.It has undergone some degree of crustal contamination and crystallization of olivine,clinopyroxene and Fe-Ti oxides.It may have formed by 1%～20%partial melting of spinel lherzolites.The original mantle source of gabbro was modified by subducted fluids and melts.The Yingzuishan gabbro belongs to SSZ-type ophiolite,indicating the existence of a Late Paleozoic back-arc basin system in the western NQOB.（2）The Yingzuishan volcanic rocks erupted at 435 Ma and belong to A-type potassic trachyte-rhyolite brecciated tuff lava formed by partial melting of ancient crust.It belongs to the FIIIa and FII series,which are consistent with the felsic volcanic rock series associated with the post-Archean VMS deposits,indicating that it has a good mineralization endowment.It is the product of southward subduction of the North Qilian Ocean（NQO）,and the roll-back of the subducting plate led to the intraplate extension of the continental arc,and the upwelling of the deep mantle led to the partial melting of the ancient crust,and the melt formed the felsic volcanic rocks after MASH.（3）The Qingshan granodiorite and Qingshan MMEs formed at 435 Ma.Both hosts and MMEs have consistent ages,Sr-Nd-Hf isotopic compositions,and consistent mineral assemblages,suggesting that they may be derived from same magmatic system.The hornblende trace elements indicate that the MMEs are early cumulate,while the host was formed by partial melting of the oceanic crust mixed with few crustal material,and then underwent crystallization of hornblende and plagioclase.（4）The Basalt formed at 428 Ma with juvenileε_Nd（t）values and belong to intraplate oceanic island-seamount basalts.The parental magma has undergone some crystallization of olivine,pyroxene,and Fe-Ti oxides.It may be formed by 4%～6%partial melting of the depleted mantle.The Yingzuishan basalt represent oceanic island-seamount remnants within the back-arc basin formed during the southward subduction of the NQO,suggesting that magmatic activity related to the plume in the Yingzuishan back-arc basin.（5）Most of Yingzuishan chert is biogenic with a few belong to hydrothermal origin.It has formed in a continental margin environment with incorporation of terrigenous clastic material.The southward subduction of the NQO led to massive volcanic magmatism,which provided nutrients to algae and bacteria.The abundant growth of radiolaria provides abundant materials for the formation of chert.（6）The hanshan gabbro-diorite was formed at 404 Ma and belongs to the aluminous medium-potassium-calcium-alkaline series.It has a relatively enriched Nd isotopic composition and is formed by the partial melting of the enriched mantle modified by the fluid from the subducted plate.（7）Re-Os age of metallogenic stage pyrite indicates that Yingzuishan gold deposit was formed at 413 Ma.Yingzuishan gold ore can be devided into hydrothermal sedimentary stage,metamorphic deformation stage and epithermal stage.The hydrothermal stage is divided into high-temperature stage,mid-temperature and low-temperature stage.The metamorphic-deformation stage can be divided into syn-metamorphic stage and post-metamorphic stage.The pyrite is divided into five generations.Py1 is belong to the high-temperature stage and was further divided into two sub-stages,Py1a is framboid aggregate bounded by Py1b with few inclusions.Py2is belong to the mid-temperature stage which is associated with other sulfides（arsenopyrite,chalcopyrite,sphalerite and galena）.Py3 is belong to low-temperature stage.within the siliceous rocks,Py3a is pyrite formed by stable crystallization with relatively clean particle interiors,whereas Py3b is colloform pyrite with cyclic band;Py4a is pyrite with ductile deformation,Py4b is pyrite with boudinage structure,Py4c is pyrite with fractures,and Py5a is pyrite with equilibrium annealed structure,Py5b is large pyrite porphyroblast,Py5c is microcrystalline pyrite aggregate formed by strong chemical reactivation of early formed pyrite,Py5d is microcrystalline pyrite aggregate formed by chemical reactivation,Py5e is microcrystalline pyrite aggregate formed by strong chemical reactivation to form microcrystalline pyrite with obvious mechanical displacement,Py5f is pyrite formed in the post-metamorphic stage by annealing of pyrrhotite formed in the progressive metamorphism.The gold in Yingzuishan gold ore is mainly visible gold and invisible gold,where visible gold appears in pyrite,chalcopyrite and quartz and fracture in pyrite,invisible gold mainly appears in pyrite,arsenopyrite,chalcopyrite in the form of inclusions or solid solution,and sphalerite contains a small amount of gold.Gold is mainly transported in Au Cl₂^-form in high-temperature stage and Au（HS）^2-form in mid-and low-temperature stage,and the mixing effect plays an important role in gold precipitation.The mineralized material of Yingzuishan gold ore comes from hydrothermal fluids leaching the metallic elements from the trachyte-rhyolitic breccia tuff lava at the base plate,and precipitation occurs after mixing with seawater to different degrees.（8）Hanshan gold deposit may have been formed at 404 Ma.Pyrite can be divided into three generations（Py1～Py3）.Py1 is pyrite occurring in the surrounding rock,Py1a1 is coarse-grained pyrite with many pores in it,and Py1a2 is mainly developed in the periphery of Py1a1with reduced pores,Py1b1 is a medium-fine-grained pyrite with pores and inclusions.Py1b2 develops in the periphery of Py1b1with reduced pores and inclusions.Py1b3 is a fine-grained pyrite formed by dissolution,migration and re-precipitation of Py1b1 and Py1b2.Py2 was formed by metamorphism and deformation of quartz vein pyrite stage.Py2a1 is the inclusion-rich pyrite in quartz veins,Py2a2 is the inclusion-poor pyrite in the periphery of Py2a1,Py2b is the cracked pyrite,Py2c1 is annealing pyrite,Py2c2 is the fine-grained pyrite formed by recrystallization.Py2c3 is pyrite formed by dissolution and reprecipitation.Py3 is formed by metamorphism and deformation of the polymetallic sulfide mineralization stage.Py3a1 is inclusion-rich pyrite associated with polymetallic sulfide,Py3a2 is inclusion-poor pyrite associated with polymetallic sulfide,and Py3b is a fragmented pyrite formed by intense crushing.The gold in Hanshan gold deposit are mainly visible gold and invisible gold,and the invisible gold exists in the form of solid solution or gold-bearing inclusion.The mixing of meteoric water and magmatic water may be an important mechanism of gold precipitation for Hanshan gold deposit.The ore-forming materials of Hanshan gold deposit mainly come from the deep enriched mantle and are mixed with crustal sedimentary rock materials to some extent during the rising process.（9）In the western NQOB,the slab roll-back leaded to extension of the continental arc of the Qilian Block and formed a back-arc basin with a series of lithologic units,including（a）cumulate gabbro（ca.452 Ma）,representing oceanic crustal units;（b）trachyte-rhyolitic breccia tuff lava with A-type granite features（ca.435 Ma）;（c）Yingzuishan arc granite and MMEs（ca.435 Ma）;（d）pillow-oceanic island basalts erupted at ca.428 Ma with widespread chert and limestone;and（e）the Yingzuishan gold deposit（ca.413 Ma）.After the hydrothermal fluids leached the metallic elements and sulfur and other elements within the trachyte-rhyolitic breccia tuff lava,the hydrothermal fluids mixed with seawater after possibly experiencing some degree of boiling to form gold colloidal agglomerates,leading to the precipitation of gold with amorphous silicon,forming the main ore body and host rock of the Yingzuishan gold deposit.The Yingzuishan area experienced the collision between the Alxa massif and the Central Qilian Block,which led to metamorphic deformation of the units,and then,the Altun fault dislocated the orebody.Hanshan gabbro-diorite was formed by partial melting of enriched mantle caused by slab break-off.The mixing of gold-rich fluid and meteoric water and the reaction of water-rock with surrounding rock resulted in the precipitation of gold and the formation of Hanshan gold deposit.The late metamorphism and deformation had a strong effect on Hanshan gold deposit.