Study On Metallogenesis Of Cu Pb Zn Deposits In Duocai Region In The Northern Segment Of The Sanjiang Metallogenic Belt

The Duocai area is located in the northern Sanjiang Metallogenic Belt,within Qinghai Province,China.This belt is an important part of the global-scale Tethys–Himalayan tectonic zone that records a series of Gondwanan and Eurasian collisional events.The Sanjiang Metallogenic belt with frequent tectonic and magmatic activities has undergone the Paleo Tethys oceanic subduction and collisional orogenesis during the Paleozoic to Mesozoic and the continental collisional orogenesis of India plate and Eurasia plate during the Cenozoic.The complicated dynamical evolution in the area results in intense magmatic activities and favorable metallogenic conditions.This paper presents petrography,zircon U-Pb dating,and geochemistry of volcanic and intrusive rocks in Duocai,Yushu area,to constrain the tectonic evolution since Paleozoic.Studies show that the Permian basalt belongs to E-MORB formed from spinel lherzolite lithosphere mantle during the Paleo Tethys oceanic expansion period.Late Triassic andesitic-basaltic tuffs(~231Ma,~238Ma)are formed from depleted mantle which suffered from crustal contamination and fluidic metasomatism and generated during the subduction of the Paleo Tethys oceanic plate beneath the North Qiangtang terrane.The Triassic diorite(~230Ma)belongs to high-Mg diorite and is formed from mantle peridotites by reacting with Si-rich melts released from subducted oceanic slab.The Cenozoic gabbro(~33Ma)is formed from metasomatic enriched mantle and experiences fractional crystallization in a continental transform orogenic environment.This paper carries on systematic research on the field observations and laboratory investigations for five deposits,including Galonggema Cu polymetallic deposit,Sanalongwa Cu polymetallic deposit,Chayong Cu polymetallic deposit,Duorirong Pb-Zn deposit,and Mizhananeng Pb deposit.Studies suggest that Galonggema Cu polymetallic deposit and Sanalongwa Cu polymetallic deposit are sedimentary exhalative deposits.Chayong Cu polymetallic deposit is meso-hypothermal vein-type.Duorirong Pb-Zn deposit and Mizhananeng Pb deposit are epithermal vein-type.Studies on fluid inclusions(FIs)revealed that the fluid inclusions in Galonggema Cu polymetallic deposit are both aqueous and gas-rich.Laser Raman spectroscopic analysis of the gas phase components of FIs are dominated by H2 O,CO2 and minor amounts of N2 and the ore-forming fluid is mainly H2O-NaCl-CO2-N2 system.Fluid inclusions(FIs)in vein-stockwork mineralization homogenize at temperature from 190°C to 250°C.Fluid inclusions in barite in exhalative-chemical deposit system homogenize at temperature from 110°C to 150°C.Hydrogen and oxygen isotopic study(?18OH2O: 0.25‰~1.75‰,?D:-103.2‰~-65.3‰)shows that ore-forming fluid is a mixture of seawater and magmatic water.Connecting with the predecessors research results of sulfur isotope indicate that the sulfur provided by magma and bacteria reduction seawater sulfate or basement rock.Lead isotopic study suggests that the lead mainly sourced from crustal components and little from magma.The mixing of ore-forming fluid and seawater is the main metallogenic mechanism of Galonggema deposit.The fluid inclusions in Chayong Cu polymetallic deposit are mainly aqueous inclusions.Laser Raman spectroscopic analysis of the gas phase components of FIs are dominated by H2 O,CH4,N2 and minor amounts of CO2 and the ore-forming fluid is mainly H2O-NaCl-CH4-N2±CO2 system,indicating the characteristics of the moderate to high temperature,low salinity,and low density.Hydrogen and oxygen isotopic study suggest that ore-forming fluid is main magmatic water and in the late stage with little meteoric water mixed.The ?34S values show the sulfur comes from stratum and magmatic hydrothermal ore-forming fluid.The Pb isotopic systems indicate that the ore metals sourced from upper crust.The fluid inclusions in Duorirong and Mizhananeng deposits are mainly aqueous inclusions and the the ore-forming fluid is mainly H2O-NaCl system,indicating the characteristics of the moderate to low temperature,low salinity,and low density.Sulfur isotope indicates that the sulfur comes from bacteria reducing layer.The Pb isotopic systems indicate that the ore metals sourced from upper crust.Based on the research above,this paper establishes the metallogenic model,summarizes the temporal and spatial distribution and analyzes the preservation conditions of deposits in Duocai,Yushu.It can be divided into two metallogenic periods,the Late Indosinian and the Himalayan.Sedimentary exhalative deposit is the main type in the Late Indosinian and hydrothermal vein-type deposit is mainly in the Himalayan.Indosinian deposits are distributed in the northeastern margin of Qinghai-Tibet Plateau along the Jinshajiang suture zone with zoneal distribution.Sedimentary exhalative deposits in Duocai,Yushu,are hosted in NW striking Late Triassic volcanic.Cenozoic vein-type deposits in Duocai have obviously zonation on the space.Three mineralization zones are divided from north to south: meso-hypothermal vein-type Cu polymetallic ore belt in the north,mesothermal vein-type Pb-Zn polymetallic ore belt in the middle,epithermal vein-type polymetallic ore belt in the south.The south belt with lower degradation is favorable area for epithermal deposit,the middle belt with higher degradation is the area for mesothermal vein-type deposit and the north belt has the highest degradation and is the area prospecting for Cu-Mo meso-hypothermal vein-type deposit.