Indosinian Gold Polymetallic Mineralization In The Middle-east Segment Of The East Kunlun Orogenic Belt,Qinghai

The East Kunlun Orogenic Belt（EKOB）is located in the north of Tibet Plateau,and has undergone the evolution of Proto-Tethys and Paleo-Tethys Ocean in Paleozoic and Mesozoic,resulting in strong tectonic-magma activity.EKOB is an important gold polymetallic metallogenic belt in western China,producing large to super-large deposits such as Xiarihamu Ni-Co deposit and Wulonggou gold deposit.It is also an important distribution area of metal minerals in Indosinian,producing a large number of mesothermal vein type gold deposits,skarn and porphyry type deposits.Previous researchers have conducted extensive research on the East Kunlun metallogenic belt,but there are still scientific issues as follows:（i）the closure time of the Paleo-Tethys Ocean in the East Kunlun region;（ii）there is controversy over the genetic model of ore-forming rock related to skarn deposit,and the contribution of enriched mantle melts to diagenesis and mineralization has not been fully recognized;（iii）there are few researches on the properties of ore-forming magmas,such as oxygen fugacity,water,S and Cl contents,which greatly limit the understanding of skarn mineralization in the East Kunlun area.（iv）the metallogenic epoch of the mesothermal vein type gold deposit in the East Kunlun metallogenic belt has multi-stage mineralization characteristics,and the genetic model is controversial,which needs to be further studied and summarized.The closure time of the Paleo-Tethys Ocean in the eastern Kunlun orogenic belt is controversial.In this paper,the Indosinian granodiorite and diorite in the Heishishan are studied.The LA-ICP-MS zircon U-Pb ages of the granodiorite and diorite are 243.1±0.9 Ma and 241.6±1.0 Ma,respectively,which belong to the Middle Triassic.The granodiorite is rich in silicon,alkali,and magnesium,and is an I-type granite with relatively rich Sr-Nd-Hf isotopes.It is a product of partial melting of the lower crust.Diorite is poor in silicon,slightly rich in alkali,rich in aluminum,calcium,and iron,with Mg^#values of 38 to 47.It is enriched in Sr-Nd-Hf isotopes,indicating a mixed genesis of crust and mantle,mainly composed of crustal materials,with a small amount of mantle material involved.Both types were formed in the subduction tectonic setting of the Middle Triassic Paleo-Tethys Ocean,indicating that the Paleo-Tethys Ocean was not closed in the early Middle Triassic.Combined with regional magmatic evolution and stratigraphic information,it is considered that the Paleo-Tethys Ocean in the East Kunlun area was closed at about 237 Ma.In order to solve the scientific problems of mineralization,four typical ore deposits in the middle-east segment of the East Kunlun are selected for study,they are Heishishan copper polymetallic deposit and Dacaigou gold deposit in Wulonggou ore concentration area,Guolongwa gold deposit and Valega gold deposit in Gouli gold concentration area,and the following understandings are obtained:（1）The ore-forming rock of the Heishishan copper polymetallic deposit is redefined as monzogranite,with a zircon U-Pb age of 241.7±1.0 Ma,which can be roughly regarded as the ore-forming age of the deposit.The monzogranite has the characteristics of adakitic rocks,and is a product of the mixing of crust and mantle magma,formed in the subduction tectonic setting.The magma has a high oxygen fugacity,high H₂O,S,and Cl contents,showing a strong ore-forming ability.Based on C-H-O-S-Pb isotopes,it is determined that the metallogenic materials and fluids are mainly derived from the monzogranite magma.This paper argues that the addition of enriched mantle melts leads to the high initial oxygen fugacity,metal,water,S and Cl contents of the Heishishan monzogranite magma.Under the compressive tectonic setting,the ore-bearing magma undergoes the fractional crystallization of amphibole,becoming more oxidized,and being able to carry more metals to the shallow crust for mineralization.The ore-forming fluid of the Heishishan deposit is characterized by high temperature and medium salinity magmatic fluid.Fluid boiling leads to a large amount of precipitation of minerals such as chalcopyrite,and the further reduction of temperature,pressure,and salinity of the late sulfide stage system leads to a large amount of precipitation of sphalerite and galena.（2）The ore-forming fluid of the Dachaigou gold deposit is determined to be a medium high temperature and low salinity H₂O-CO₂-Na Cl system.H-O isotopes indicate that the ore-forming fluid has the characteristics of mantle derived fluid.In situ trace elements and S-Pb isotopes of sulfides indicate that the ore-forming materials mainly originate from the upper mantle metasomatized by the subduction fluid,with some from the surrounding rocks.（3）A pre-mineralization mantle derived diorite porphyrite with an zircon U-Pb age of 202.7±4.3Ma,which is closely related to the spatial relationship of the gold orebodies,which limits the formation of the Guoluolongwa gold deposit in the Late Indosinian post-collision extensional tectonic setting.The diorite porphyrite is the product of partial melting of lithospheric mantle metasomatized by subduction fluid.Bulk-rock Pb isotopes are consistent with the in-situ Pb isotopes of Galena near the electrum,indicating that the source of ore-forming materials is mantle.The in-situ pyrite Mo/Ni ratios shows that the ore-forming materials are mantle-derived,the in-situ S isotopes also show the characteristics of mantle-derived sulfur,the H-O isotopes show the characteristics of mantle-derived primary water,and the ore Pb isotopes are similar to mantle lead.These evidences indicate that the ore-forming fluids and materials of Guoluolongwa mainly derived from the upper mantle metasomatized by subduction fluids.The ore-forming fluid is a H₂O-CO₂-Na Cl system with medium-high temperature and low salinity.The decrease of temperature,pressure and salinity and the phase separation of CO₂lead to the precipitation of electrum and polymetallic sulfide.（4）The zircon U-Pb age of the diorite cut by the ore body in the Walega gold deposit is 255±11Ma.Combined with the metallogenic age of the adjacent Asiha gold deposit,the metallogenic age of the Walega gold deposit is limited to the Late Indosinian,which is a post-collision extensional tectonic setting.The H-O isotopes show the characteristics of mantle-derived primary water,and the in-situ sulfur isotope of the main metallogenic stage is 3.14‰～3.69‰,which is slightly higher than that of mantle sulfur.The in-situ Pb isotope of galena has the characteristics of crust-mantle mixed lead.The in-situ trace elements of pyrite also show the characteristics of crust-mantle mixed.This suggests that the ore-forming fluids and ore-forming materials originated from the upper mantle metasomatized by subduction components,and does not exclude the contribution of wall rocks.The ore-forming fluid is a H₂O-CO₂-Na Cl system,with medium-high temperature and low salinity.The decrease of temperature,pressure and salinity resulting in massive precipitation of sulfides.（5）This paper integrates geochemical data of ore-forming related rocks in the skarn Fe,Cu,and Pb-Zn deposits in the East Kunlun metallogenic belt.Most of these intrusive rocks have the characteristics of high oxygen fugacity.It is believed that the rocks related to Cu deposits have more contributions of enriched mantle materials than that of Fe deposits.The phenomenon that there is a close spatial relationship between intermediate-basic dikes and gold orebodies of mesothermal vein-type gold deposits is common in the eastern Kunlun metallogenic belt.The ore-forming materials and some intermediate-basic vein rocks mainly come from the mantle.Intermediate-basic dikes may provide thermal power and hydrothermal fluid for mineralization,and may also provide some metallogenic materials.（6）This suggests that there are two gold mineralization peaks in the Gouli gold ore concentration area under the post-collision extensional tectonic setting of the Late Indosinian,about 227 Ma and 202.7 Ma.The Wulonggou gold ore concentration area developed two stages of gold mineralization in the Indosinian.The Early Indosinian was240～235 Ma,which was related to the subduction tectonic setting.The late Indosinian was 220～210 Ma,with a post collision extensional tectonic setting and strong gold mineralization.（7）The magnetic anomaly of Heishishan deposit indicates that the two ends of the largest V ore belt and the deep area in the middle of the ore belt have good prospecting potential.The NW direction coverage area of the IV ore belt of the Dachaigou gold deposit has good prospecting potential.This suggests that the structural ore-controlling style of Gouli gold ore concentration area determines the idea of finding large ore in large structure and finding rich ore in hanging wall secondary structure.Based on the lateral trending rule,lateral correspondence rule,and segmented enrichment rule of the ore body,the deep and peripheral areas of typical deposits are predicted.The next step of the Guoluolongwa gold deposit should be to the deep part of the main vein and along the strike,and the next mineralization enrichment section may appear in the deep part.The discovery of the Waleganan gold deposit shows that there is a good prospecting potential in the deep part of the V vein.