Paleozoic-Mesozoic Tectono-magmatic Evolution And Gold Mineralization In Gouli Area,East End Of East Kunlun Orogen

The East Kunlun Orogen?EKO?,located in the west of the Central Orogenic belt of China?COB?,experienced intense Paleozoic-Mesozoic tectono-magmatic activities that create good conditions for metallic mineralization.In the past decades,the EKO has witnessed gold resource increasing rapidly due to more exploration work deployed.The Gouli goldfield?>110t Au?,in the east end of EKO,is one of the most important goldfields in this belt.Gold deposits in this goldfield show different geological characteristics.The genetic relationships between different gold deposits,and between gold deposits and tectono-magmatic activities are unclear,which make the exploration deployment difficult.In this dissertation,two typical gold deposits and magmatic rocks with different ages in the Gouli goldfield or adjacent area are studied in an effort to cast light on the above problems.For the magmatic rocks,geological survey,zircon U-Pb dating,bulk rock major and trace element and Sr-Nd-Hf isotopes analysis are performed to detect their origin and the rock-forming tectonic settings,and so as the temporal framework of the Tethys-related tectono-magmatic evolution regionally.For the two typical gold deposits,methods such as sulfide Re-Os dating,mica Ar-Ar dating and H-O-S-Pb-Os tracing are selected to decipher the timings of gold mineralization,origin of the ore-forming fluids and materials.The key ore-control factors are also analyzed and the results are implied in the regional mineral resource exploration.The primary conclusions from the dissertation are as below:?1?The gold orebodies in the Guoluolongwa and Walega gold deposits,Gouli goldfield are dominated by quartz vein-type mineralization,with minor pervasively altered fracture-hosted mineralization.Gold orebodies in the two deposits have been shown to be hosted by rock units of different lithology?Ordovician-Silurian low-grade metamorphic rocks vs.Silurian granitoids?.In the Guoluolongwa gold deposit,the gold orebodies strike roughly east and three stages of mineralization corresponding to three types of veins are defined based on mineral assemblages and crosscutting relationships.The first stage veins are mainly composed of quartz with minor pyrite,the second stage veins comprise quartz and massive pyrite and the third stage veins are consisted of quartz,pyrite,chalcopyrite,galena and sphalerite.Wall-rock alteration in this deposit is dominated by silification,sericitization and carbonification.In the Walega deposit,the main orebodies strike northeast and three mineralization stages,i.e.,quartz with less pyrite stage,quartz with multi-sulfide stage and quartz druse-arsenopyrite stage,are defined.Pyrite,arsenopyrite,galena,sphalerite,chalcopyrite and pyrrhotite are found in the second mineralization stage and arsenopyrite and less pyrite occur in the third stage.Wall rock alteration in the Walega is different from that in the Guoluolongwa,with kaolinization occur in the Walega.In summary,the two deposits are different in ore-bearing wall-rocks,ore-control structure,mineral assemblage and wall-rack alteration.?2?The Ordovician-Devonian magmatic rocks,such as the Aowade granodiorite,Danshuigou monzogranite and syenogranite,and Niantang syenogranite in the Gouli and adjacent area are studied.LA-ICPMS zircon U-Pb dating indicate that the Aowade granodiorite formed in Ordovician?454±2 Ma?,the Danshuigou monzogranite and syenogranite formed in Early Devonian?418±3 Ma?and the Niantang syenogranite formed in Late Devonian?403±2 Ma?.Bulk rock major and trace element data suggest that the Ordovician Aowade granodiorite is calc-alkaline–high-K calc-alkaline and weak peraluminous,with Na₂O/K₂O=0.92².68 and Mg#=39⁴3.The rare earth elements?REE?of this intrusion show right-dipping patterns with negative Eu/Eu*anomaly and high?La/Yb?_N and the trace element patterns show large ion lithophile element enrichment?LILE;e.g.,Rb,Ba,K and Sr?and high field strength element depletion?HFSE;e.g.,Nb,Ta and Ti?with high Sr/Yb ratio,which are similar to adakitic rocks.The Devonian Danshuigou monzogranite and syenogranite,and the Niantang syenogranite show calc-alkaline–shoshonitic and metaluminous–weakly peraluminous characteristics with low CaO,Al₂O₃,MgO and Sr contents and high FeO_t/MgO,Ga/Al,Zr and Nb,which are similar to typical A-type granites.On the Sr-Nd-Hf isotopes,the Aowade granodiorite display homogenous(⁸⁷Sr/⁸⁶Sr)i?0.7059⁰.7063?,negative?Nd?t??-4.1^-1.9?and high?Hf?t??5.4⁹.2?;the Devonian Danshuigou and Niantang A-type granite show similar?Nd?t??-5.3^-0.6?and?Hf?t??-5.9⁶.4?.These geochemical together with the petrographic characteristics indicate the Ordovician Aowade granodiorite was derived from partial melting of subducting oceanic slab and the Devonian A-type granites was generated by partial melting of Ordovician-Silurian granitoids.Further considering the temporal-spatial distribution of regional magmatic rocks,metamorphism and sedimentary faces,the Prototethys Ocean was under subduction in Ordovician and this ocean closed in Early-Middle Silurian before a short period of collision?⁴30 Ma?,after which the EKO experienced a long term post-collisional extension?⁴30—?that led to the formation of a Late-Silurian-Devonian A-type granite belt.?3?The extensive Triassic granitoids?Xiangride granodiorite,Asiha quartz diorite and enclaves enclosed from the Gouli batholith?in the Gouli goldfield are also studied in detail.LA-ICPMS U-Pb dating together with petrographic characteristics reveals that these rocks formed in²42 Ma,similar to the adjacent Annage hornblende gabbro?242±2 Ma?.No inherited zircon was found in the granodiorite and diorite,with zircons from the diorite show distinct core-mantle structure.The identical ages between the mantles and cores indicate there is a sharp change in the physical/chemical conditions or composition of the diorite magma during its evolution.The major and trace element data suggest the granodiorite,quartz diorite and enclaves are high-K,calc-alkaline and metaluminous-weakly peraluminous.A notable differentiation between LREE and HREE is observed from these rocks and all rocks are enriched in LILE and depleted in HFSE,similar to arc magmatic rocks,with the granodiorite displaying adakitic signatures?high?La/Yb?_N and Sr/Yb?.The Sr-Nd isotopic values from all these rocks are roughly similar((⁸⁷Sr/⁸⁶Sr)i=0.708167⁰.713553;?Nd?t?=-6.8^-5.3),while Hf isotopes are distinguishable,with?Hf?t?of the granodiorite are 0.3⁵.1,comparing to that of the quartz diorite?-1.6⁰.7?.These petrographic and geochemical signatures imply that the granodiorite originated from partial melting of subducting slab?basaltic oceanic crust and overlying terrigenous sediment?and the quartz diorite and enclaves were formed via mixing of slab melt and enriched mantle-derived melt.Further comprehensive analysis of the spatial and temporal distribution of the regional magmatic rocks,metamorphism and sedimentary facies reveal that the Gouli batholith and most of the Permian-Triassic granitoids in the EKO formed at the end of the subduction of the Paleotethys Ocean.This ocean should begin subducting at Permian?²60 Ma?and closed in Middle Triassic?²37?,after which the region witnessed collision and post collision extension.?4?Sulfide Re-Os and mica Ar-Ar geochronology are applied to define the mineralization timing of the Guoluolongwa and Walega gold deposit.The muscovite Ar-Ar dating yield 409.06±1.42 Ma for the Guoluolongwa gold deposit,with the sericite Ar-Ar dating reveal possible 379³49 Ma hydrothermal events.Re-Os data from the quartz-pyrite vein and quartz-multi-sulfide vein of the Guoluolongwa generate dates of375±11 Ma and 354±7 Ma,respectively,which are identical to the Re-Os dates from the northern Annage gold deposit within uncertainty?383±6 Ma and 349±6?.Further considering the ore geology,the first,second and third stage veins should form in 409.06±1.42,383³75 Ma and 354±7 Ma,respectively.Ar-Ar dating do not offer meaningful age for the Walega gold deposit,but yield a sericite Ar-Ar date of 234.63±1.22 Ma for the adjacent Asiha gold deposit that is situated in the Gouli gold field.No significant Re-Os date was gained from the quartz with multi-sulfide veins in the Walega,while the Re-Os dates?230.0±2.2 Ma and 217.2±1.3 Ma?from similar gold-bearing veins in the Wulonggou goldfield could offer some information.The quartz druse-arsenopyrite veins in the Walega gold deposit yield arsenopyrite Re-Os dates of 24.0³1.9 Ma and similar age is gained from the Wulonggou gold field?Re-Os date of 31.0±2.3 Ma?.In summary,the quartz multi-sulfide veins and the quartz druse-arsenopyrite veins from the Walega gold deposit should form in 234²17 Ma and 24³2 Ma,respectively.Considering the timing of the gold mineralization and the framework of the tectono-magmatic activity,it is obvious that the Guoluolongwa gold deposit mainly formed in an extension setting related to the evolution of the Prototethys and gold mineralization is coupled with the formation of the A-type granites and related mafic rocks in time.However,the formation of the Walega gold deposit is related to Paleotethys-related and Cenozoic plateau upraise-related extension.?5?Investigation of the fluid inclusions from the two gold deposits reveal that the ore-forming fluids are H₂O-NaCl-CO₂ system.The H-O isotopes indicate that ore-forming fluids of the two deposits are dominated by magmatic water,with minor meteoric water.The?³²S values concentrate around zero,suggesting that magma offer the S necessary for gold mineralization.The cluster of the Pb isotopes of sulfide from the two gold deposits are different from the scatter Pb isotopes of the wall rocks,implying the Pb is likely from the magma.The Osi of the quartz-pyrite veins and quartz-multi-sulfide veins from the Guoluolongwa gold deposit are 0.12±0.01 and 0.13±0.11,respectively,indicating mantle origin of their ore-forming metals.The Re-Os isochron of the third stage vein from the Walega yield Osi of 0.24±0.83.The high common Os samples from the same stage yield Osi of 0.2¹.5.Considering the possible addition of the wall-rock Os to the ore fluid,the primary Osi should<0.2,similar to the Osi value of the mantle,suggesting mantle origin of the metals.?6?Taking into account of the above results,it can be concluded that mantle-derived magma related to the post-collisional extension of the Prototethys and Paleotethys are key factors that control the formation of the Guoluolongwa and Walega gold deposit,respectively.The active faults during the extension offer space for transportation/precipitation of gold ore-forming fluids and metals.Regionally,the western COB?EKO,Qilian and West Qinling?are controlled by common tectono-magmatic activities and thus may possess similar potential for gold mineralization.