Tungsten Mineralization In The Middle-Lower Yangtze Metallogenic Belt,Eastern China

Tungsten is a strategic metal,known as "industrial salt",is one of the important insoluble rare metals.China has the world's largest reserves of tungsten,which accounts for 52 percent of global reserves(USGS 2018 data).Meanwhile,China is the world's largest producer and consumer of tungsten.As for tungsten deposit,previous studies have been researched and achieved results,such as ore-bearing intrusive rock,metallogenic conditions.But the connection between tungsten deposits and other metal deposits(such as copper,gold and iron ore deposits)is constrained.Rather,an increasing number of tungsten deposits are associated with copper,gold and iron,or tungsten mineralization is developed in copper,gold and iron ore deposits.Hence,it is very important to choose the area with both tungsten deposit and copper/iron deposit for detailed comparative study such as metallogenic properties,metallogenic conditions of tungsten deposit,copper deposit and iron deposit.The Middle-Lower Yangtze Metallogenic Belt(MLYB)is the ideal place to study this scientific issue.The MLYB is not only an important accumulation area of copper,iron and gold polymetallic deposits in China,but also a hot area for geologists to study.It is also the birthplace of"stratabound skarn deposit" and "porphyry-skarn composite metallogenic theory".Compared with the study of copper,gold and iron ore deposits in MLYB,the study of tungsten deposits in the metallogenic belt is obviously poor.In particular,the newly discovered tungsten deposit in the MLYB in recent years has raised a new topic for the study of this metallogenic belt.Large scale tungsten deposits including Donggushan deposit,Ruanjiawan deposit,Guilinzheng deposit and Gaojiabang deposit are located in MLYB.Scheelites are occurs in Tongshankou large Cu-Au deposit and Longqiao large Fe deposit.In order to clarify the diagenetic and geochronology of tungsten deposits in MLYB,the type and source area of the fertile intrusions,the tungsten metallogenic system and its deep process similarities and differences with the copper,iron and gold metallogenic system in this metallogenic belt,this dissertation taking the four tungsten deposits as the subject,based on the summarize of previous studies,by the means of detailed geological investigation and laboratory analysis,aims to achieve a comprehensive understanding of tungsten mineralization of MLYB.The main recognition and achieved are as follows:Donggushan deposit is new discovery largest skarn tungsten deposit in north sub-belt in MLYB and the key object of this dissertation.The ore-bearing strata are Ordovician carbonate strata,the ore-forming rocks are biotite granite,and the ore bodies are mainly stratiform and flat lenticular.The ore bodies occur in the contact zone between rock bodies and surrounding rocks,and the ore deposits have reached a large scale.The ore-forming fluid is high-medium temperature and medium-low salinity.According to the fluid inclusions and isotope compositions,the ore-forming fluid was formed by mixing of magma water and meteoric water which reached to 40%at quartz-sulfide stage.The ?34S values of sulfides vary from 4.39‰ to 6.00‰,which indicate that sulfur of the sulfide stem from the magmatic source with parts of contribution from the ore-bearing strata sulfide.The three types of garnet were identified in the Donggushan skarn are:(1)vein type jacinth garnets in marble:Gro71-90Adr8-28Pyr1-2,enrich HREE with weak negative Eu anomalies(Grt-1);(2)disseminated type reddish brown garnets in skarn:Adr56-100Gro0-50Pyr0-6,enrich LREE with weak positive Eu anomalies(Grt-2);(3)massive type brown garnets in skarn:Adr90-100Gro0-10Pyr0-2,enrich LREE with observably positive Eu anomalies(Grt-3).From Grt-1,to Grt-2,then to Grt-3,the acidity and oxygen fugacity of hydrothermal fluid increases gradually.Two main types of pyrite have been recognized based on the location and mineralization.Pyrite 1(Pyl)occurs in tungsten orebody(deeper),while pyrite 2(Py2)occurs in zinc-lead orebody(superficial).The subtype of these two pyrite is according to the petrography.Pyl a(high content of Co)precipitated followed by the overgrown Py1b(elevated Ni,Cu,Pb,Bi,Zn,As and reduced Co than Pyla).Py2a formed and enrich As,depleted Co and Ni elements,then subhedral Py2b surrounded and contained more As,Pb,Bi,Cu,Zn,Au and Ag than Py2a.Notably,the Pb isotopic compositions from the Donggushan deposit differ remarkably among two types of pyrite.The ore-forming material in Donggushan deposit were primarily derived from Yangtze upper crust(Dongling basement)materials and formed scheelite orebody.The scheelites from Donggushan deposit show a strong HREE depletion and the decoupling of LREEs and HREEs.These scheelites can precipitate in two stages,which are skarn stage(Skarn)and oxide stage(Oxide).From Skarn to Oxide,the Mo6+in scheelite is decrease.And negative Eu anomalies are also weak.These show that the ore fluid is gradually reducing.The Sr-Nd isotope of scheelites from Donggushan deposit is significantly different with south sub-belt in MLYB(Jitoushan deposit)and Dahutang super-large tungsten deposit.The ?Nd(t)data and 87Sr/86Sr data of scheelite in Donggushan deposit is in the range of-17.7?-16.4 and 0.70957?0.711138 respectively,indicating the ore-forming material of the deposit from the crust,and the Dongling basement probably provided some ore-forming materials.This is an important reason for tungsten deposits to appear north of the Yangtze river.Based on the contact relationship between the intrusive rock in the field and the dating of zircon U-Pb and molybdenite Re-Os,the diagenetic and ore-forming ages of the deposit are 99.9±1.7-99.7±1.5 Ma and 97.22±0.70 Ma,respectively,indicating a previously unrecognized epoch of mineralization in the MLYB.Therefore,the Ore-forming age of MLYB range form 145-97Ma.There are three stages of tungsten mineralization in the MLYB.Specifically,it can be divided into 146-143 Ma,127 Ma and 97 Ma,which are characterized by multiple periods of time.Whether before the main metallogenic period of copper(140Ma)or after the iron metallogenic period(130Ma),tungsten metallogenic events have occurred.Tungsten fertile intrusions from MLYB enrich LREE with negative Eu anomaly.These fertile intrusions are also enriched Rb,Th,U and other LILE,depleted Nb,Zr,Ti and other HFSE,weakly depleted Sr,P,Eu and Ti elements and shown high K/Rb and Zr/Hf ratio but low Sr/Y ratio,indicating the magmatic differentiation and evolution are low,the magmatic oxygen fugacity is high.Compared with Qin-Hang Metallogenic Belt(QHMB)and NanLing Metallogenic Belt(NLMB),the magmatic differentiation and evolution of tungsten fertile intrusive rocks are increasing,but the magmatic oxygen fugacity is decreasing from north to south(i.e.from MLYB to QHMB,than to NLMB).The ?Nd(t)and(87Sr/86Sr)i data of tungsten fertile intrusive rocks in MLYB shown discrete points indicating the different magma source of these ore-forming intrusions.It means that the middle stage fertile intrusion(i.e.,Guilinzheng intrusion,located in the south sub-belt)has more upper crust of Yangtze river(Jiangnan basement)material added,while the late stage fertile intrusion(i.e.,Donggushan intrusion,located in the north sub-belt)has more Dongling basement material added.In order to interpret the specialization of copper,gold and iron fertile intrusions in this metallogenic belt,we chosen the Edong orefield which endowed Cu-Fe-W mineralization.The zircons of Ruanjiawan W deposit enrich tungsten element(1.14 ppm),while Tongshankou Cu deposit and Tonglvshan Cu-Fe deposit enriched copper element(0.80 and 1.23 ppm).The magma oxygen fugacity gradually reduce from Tongshankou Cu deposit(Ce4+/Ce3+ ratio:207.5)and Tonglvshan Cu-Fe deposit(Ce4+/Ce3+ ratio:263.6),to Jinshandian Fe deposit(Ce4+/Ce3+ ratio:189.0),than to Ruanjiawan deposit(Ce4+/Ce3+ ratio:71.7).Based on the study of zircon trace elements in Cu,Fe and W fertile rocks in the Edong orefield and the previous work,it can be summarized that the proportion of mantle and crust material of fertile magma source determine the mineralization and magma oxygen fugacity.The difference of ore-bearing magma source area is the primary cause of the difference of different minerals(copper,iron,tungsten)in Edong orefield.The scheelites trace elements compositions of Donggushan,Ruanjiawan,Gaojiabang and Guilinzheng tungsten deposit enrich LREE and show negative Eu anomaly,as well as enrich HFSE and ratio of Nb/La range from 1.217-52.455,indicating that the four tungsten ore-forming hydrothermal fluid enrichment of fluorine volatiles while the copper ore-forming hydrothermal fluid from Tongshankou deposit enrichment of chlorine volatiles.The scheelites in skarn tungsten deposits enrich LREE and Mo elements with positive Eu anomaly.The scheelites in quartz vein type tungsten deposits are depleted LREE and Mo elements with varied Eu anomaly.The scheelites in porphyry tungsten deposits slightly enrich LREE and Mo elements with varied Eu anomaly.Therefore,the(La/Lu)N and Mo/Eu ratio of scheelite can be used as a proxy to determine the deposit type(skarn type,porphyry type,quartz vein type).Because of the obstruct of Dabie Orogenic Belt,the MLYB experienced intra-continental subduction in Yanshanian,resulting delamination of crust and thickened lithosphere.The Dongling and Jiangnan basement of MLYB enrich in tungsten components,which provides a primary material for the crustal magma to become fertile magma.Through crystallization,the magma accumulates on top of the chamber and forms tungsten fertile magma.The tungsten fertile magma emplace in carbonate strata in the shallow and forms skarn tungsten deposit system.Based on the above comprehensive study results,we discussed the genesis of the tungsten deposits,and propose a metallogenic model of the MLYB tungsten deposits.The shifting of prospecting center for tungsten deposits both northwards and westwards is a new trend,showing significance for further exploration and programming of tungsten industry.