Chronology And Geochemistry Of The Early Paleozoic Intermediate-acid Plutons And Its Implications On Gold Mineralization In Danghenanshan Mt., South Qilian

The Qilian Mountain is a significant Paleozoic orogenic belt in the west of China,which preserved gigantic mineral resource amount.Thus it was called the Chinese Ural Mountain.It is composed of three tectonic belts from north to south:North-,Centre-and South-Qilian belts,and was the one of the earliest places to study the tectonic evolution and mineralization in China in the last century.In comparison with the north and centre of Qilian belts as well as the tectonic belt of the northern margin of Qaidam,the research on the Paleozoic tectonic history and mineralization in South Qilian belt have not attracted much more attention.In the latest two decades,ten gold deposits,with the prospective resource amount of 100 tons,had been found and explored in the Danghenanshan Mt.in the west of the South Qilian.The Jiagongtai gold deposit,a typical one of these deposits,was characteristic of that the gold is the dominative element rather than the accompanied element in the most multi-metals deposits in the Qilian Mountain.This provides a new insight of the gold exploration in the Qilian Mountain.However,the uncertainty of our understanding for the tectonic settings and its evolution and the geologic factors which controlled the gold mineralization,and the formation and mechanism of the deposit,as well as the metallogenic regularity,have extremely hindered the determination of the target regions and the perspectives of the gold exploration.The Danghenanshan Mt.located in the west of the South Qilian,with length of ca.150 kilometers and width of ca.20 kilometers.It is connected with Centre Qilian belt at north,the HP/UHP metamorphic belt of North Qaidam at south,and cut by the Altun Tagh fault at west and the Lajishan belt at east.In this region the Paleozoic tectonic and magmanism were strongly developed and hence these were to the benefit of the formation of the gold deposit.Based on the research on the petrography,lithogeochemistry and single zircon U-Pb dating of the intermediate-acid intrusions which linked to the gold mineralization,and the geologic and geochemistrical features of the typical gold deposit,my doctoral dissertation discussed the tectonic evolution during the Late Proterozoic through the Early Paleozoic,geologic factors which controlled the gold mineralization,gold metallogenic mechanism and regularities as well as the metallogenic models.This will be helpful to the study and exploration of the gold deposits in the Danghenanshan Mt.,even through all the regions of the Qilian Mountains.The conclusions of this doctoral dissertation are as followings.1)In the Danghenanshan Mt.,the intermediate-acid intrusions?including the plutons and dykes?are complex and exposed as huge batholith in the west,but a little stock or apophysis in the east.The dykes exposed all over the region.The lithologies encompass the quartz diorite,granitic diorite,pyroxene diorite,amphibole quartz monzonite,plagiogranite,monzonitic granite and porphyritic quartz diorite,etc..The plutons or dykes intruded into the Sinozoic?Late Proterozoic?and Lower Ordovician strata.The geochronologic ages of the representative intrusions by the method of zircon LA-ICP-MS U-Pb dating are:476.2±6.17Ma of the pyroxene diorite and455.6±5.6Ma of the amphibole quartz monzonite of jijiaogou complex,450±12 Ma of Qingshuigou monzonitic granite,457.8±6.3Ma of Wuligou amphibole quartz monzonite,442.7±6.8Ma and 437.6±8.1Ma respectively of Jiagongtai and Zhenxing-liang plagiogranite,and 420±11Ma of the Shikuaidi porphyraceous monzonitic granite.2)Taking into accord the geologic ages of the wall rock,the contact relationships between or among these intrusions,lithogeochemistries and the regional magmanism,the intermediate-acid intrusions in the Danghenanshan Mt.can be sorted five phases of the magma activity.The first phase?⁵00Ma?encompasses the Zhazigou granitic diorite and Banjiegou plagiogranite in the west.The former was characteristic of the Adakitic rocks and it indicated the island arc settings.Meanwhile the latter had a feature of M-type granite which indicated the oceanic crust expanded at the large grade.The second phase?480Ma-465Ma?formed the Jijiaogou pyroxene diorite.The third phase of magmanism developed thorough Danghenanshan Mt.during 460Ma-450Ma,and formed the amphibole quartz monzonite and monzonitic granite.The rocks of these two phases were developed in the island arc,and represent that the new oceanic crust started to subduct in the Middle Paleozoic.During the fourth phase?445Ma-420Ma?,the plagiogranite intruded in the Jiagongtai and Zhenxingliang in the east,whose geochemistry is similar with the C-type Adakite and this suggest the collision of Centre-Qilian with South Qilian,but the porphyraceous monzonitic granite in the Shikuaidi in the west,which is subject to the A-type granite and indicate the extension post plate collision.The fifth phase of magma expressed by the little intrusions or dykes in the Danghenanshan Mt..The lithologies include the intermediate-acid granitoid rocks and porphyrites and lamprophyres,indicating a crust extension event since the detachment of the thickened lithospheric mantle.3)The first phase of rocks are with relatively high SiO₂?55.51-69.32%?,low?K₂O+Na₂O?and varying TFe and MgO.The rocks are subject to the calc-alkaline,sub-aluminous series.The rocks are with low value of total REE(36.99×10^-6-176.35×10^-6)and rich in LREE,with slightly negative Eu abnormal,and rich in large ion lithophile element of Rb,Ba,Th,Hf and poor in Nb,Ta,Ti,Y.Their Source rocks were metabasite and they formed in sea-floor spreading settings.The second phase of rocks are with relatively lower SiO₂ and richer TFe and MgO than the first stage,The rocks are subject to the alkaline,sub-aluminous series.LREE and HREE are well fractionated.The total REE are lower and LREE-rich,with slightly negative Eu abnormal,and rich in large ion lithophile element of Rb,Ba,Th,U and poor in Nb,Ta.This pluton might originate in the arc subduction.The third phase of intrusions show the varying SiO₂?52.18-70.57%?and MgO,with high?K₂O+Na₂O?and CaO.The rocks are subject to the alkaline,sub-aluminous series.The total REE are moderate and LREE and HREE are well fractionated,with slightly negative Eu abnormal and rich in LILE Rb,Ba,Th,U and poor in Nb,Ta.Source rocks of these plutons may be in the arc subduction.The fourth stage plutons are with relatively high SiO₂?63.02-67.01%?and moderate?K₂O+Na₂O?.The rocks are subject to the calc-alkaline,sub-aluminous to aluminous series.The feature of poor in low MgO,CaO and rich in Al₂O₃ is thought to be the same with adakite.Total REE is low-moderate and well fractionated in LREE and HREE.The rock is rich in LREE with slightly negative to moderate positive Eu abnormal,and relatively rich in LILE of Ba,Th,Hf,Rb,Pb and poor in Ce,Y,Yb,Zr,Nb,Ta.The origin of the pluton may be the crust or magma mixed with crust material.The fifth pultons were with moderate or relatively high SiO₂ and moderate?K₂O+Na₂O?.The rocks are subject to the calc-alkaline to alkaline,sub-aluminous to per-aluminous series.Total REE is moderate and well fractionated in LREE and HREE.The rocks are rich in LREE,with slightly-moderate negative Eu abnormal,and rich in LILE Rb,Ba,Th,Hf and poor in Ce,Y,Yb,Zr.These indicate that the source rock of these dykes and prophyreous rocks originated from island arc,even they emplace in the extension settings after collision.4)The tectonic evolution during the Late Proterozoic to the Early Paleozoic underwent two times of oceanic plate expanding and closure.The history can be classified as following stages:?a?the occurrence and expansion of the Paleo-Danghenanshan ocean during 850Ma-700Ma,?b?the subduction and closure of the Paleo-Danghenanshan ocean during 700Ma-510?490?Ma,?c?the formation and subduction of the Danghenanshan-Lajishan ocean during 490Ma-460Ma,?d?subduction and collision of the Danghenanshan-Lajishan ocean during 460-445Ma and?e?the collision of Centre-Qilian with South Qilian during 445 Ma-420Ma?The second to fifth phases induced the intrusion of the intermediate-acid intrusions in the Danghenanshan Mt..5)The ore genetic type of the gold deposits in the Danghenanshan Mt.is subject to the medium-low hydrothermal ore.Jiagongtai gold deposit,as the biggest and most typical one,is preserved in the low-grade metamorphic clastic rocks of the middle part of the Lower Ordovician Wuligou Group.The gold mineralization linked to the Jiagongtai Plagiogranite intrusion,which shows adakitic geochemistry.The alterations of the wall rocks and the ore-bodies largely occurred within the intrusion or the contact zone with the wall rocks,and controlled by the faults striking in the NWW direction.The alternations of the wall rocks encompass the silicification,pyritization,sericitization and chloritization,where silicification is closely related to the mineralization,which indicates the middle-Low temperature hydrothermal talternations.This alternation might succeed the high temperature hydrothermal alternation of the muscovite and quattz,a high temperature hydrothermal alternation.The metallogenetic mineral assemblages are pyrite+chalcopyrite+molybdenite+galena+arsenopyrite+telluride,which also are the middle-Low temperature hydrothermal metal minerals assembledge.The metallogentic materials were derived from the wall rock?sandstones?and the intrusion rocks,and the intrusions also provided the thermal resource for the mineralization.The ore are middle to fine grain-sized texture and veined-,net-veined and immersion structure.The?³⁴S of pyrite are-5.33‰^-1.42‰,and the?value of pyrite are 9.54-9.64,?value are 37.18-38.28,all these indicate that the metallogenetic materials were orignated from the crust and mixed the materials from the mantle.The metallogenetic process are charatered by the two phases of the high-and middle and low temperature hydrothermals.The measurements of the inclusions show that the fluids have the nature of the middle-low temperature and salty and relative high density of the fluids,wiht the mainly liquid H₂O and gaseous CO₂or CH₄?H₂S?N₂.The mean age of the model age of the Pb isotope[t?Ma?]are 444Ma,very close to the age of the intrusions emplacement.The genetic type of the Jiagongtai deposit is the magmatic hydrothermal medium-low temperature deposit,formed in the collision settings.The Wuligou deposit also produced in the low-grade metamorphic clastic rocks of the middle part of the Lower Ordovician Wuligou Group.The mineralization was linked to the second phase of the amphibole quartz monzonite and monzonitic granite.The alternations of the wall rocks and mineralization were constrained by the structural zones.The metallogenetic mineral assemblages of pyrite+chalcopyrite+sphalerite+gold+electrum,also are the middle-Low temperature hydrothermal metal minerals assembledge.This shows that the deposits were formed in the settings of the island arc,but it may be imprinted by the low temperature hydrothermal mineralization.6)According to the tectonic settings,the formations of bearing minerals and the deposit distribution,the gold deposits in Danghenanshan area are preserved in the three ore-concertrated areas:the Shikuaid,Jiagongtai and Dongdonggou ore-concertrated areas.The intermediate-acid intrusions provided the thermal and partially metallgenetic materials for the mineralization.The ages of the gold mineralization were closely to the emplacement of the intrusions.The mineralization are temporally classified into three stages:the mineralization in the island settings during 460 Ma-450 Ma,and in the collision settings during 450 Ma-420 Ma as well as the extension after detachment of the thickened crust post plate collision.7)based on the tectonic setting and its evolution,the metallogenetic series and the models of the gold mineralization are classified as islandarc-,collision-and crustal extensional type.