| The Xiongcun district is situated in the Rongma Township,Xietongmen County,Xigaze City,Tibet,is about 350 km east of Lhasa City.It is located in the south margin of middle segment of the southern Lhasa subterrane,and it is adjacent to the Xigaze fore-arc basin to the south.The Xiongcun district is a super large porphyry Cu-Au?Ag?district in the southern Lhasa subterrane.At present,No.1,No.2,No.3,No.4 and No.5 porphyry Cu-Au deposits have been found in the Xiongcun district.The previous studies for the No.1 deposit in the Xiongcun district mainly focus on the geological setting,geological characteristics,rock-forming age,geochemical characteristics,ore-forming fluid characteristics,and genesis,but the research on the metal sulfide is relatively weak.Therefore,this study taking the pyrite of No.1 deposit in Xiongcun district as the research object,and is supported by the Deep Resources Exploration and Mining of the National Key R&D Program of China?2018YFC0604105-05?,the National Natural Science Foundation Project?41502079?,and the China Geological Survey Program?DD20160346?.The thesis based on the detailed field core logging and microscopic identification,and the analytical test of electron probe microanalysis,LA-ICP-MS,thermoelectricity and crystal cell parameters for pyrite is carried out.The purpose of this study is to reveal the genesis and ore prospecting significances of pyrite in the No.1 deposit.The thesis has following achievements and understandings:?1?The strata in the Xiongcun district is mainly Lower-Middle Jurassic Xiongcun Formation(J1-2x),the ore-bearing porphyry of No.1deposit is Middle Jurassic quartz diorite porphyry,and the alteration types are potassic alteration and phyllic alteration.The metal minerals are mainly pyrite,chalcopyrite and pyrrhotite,while the non-metal minerals are mainly quartz,andalusite,biotite,muscovite,sericite,etc.The ore structures are mainly vein-disseminated,and the ore structures are mainly crystallization,metasomatism and solid solution separation.The ore-forming period is divided into hydrothermal ore-forming period and epigenetic ore-forming period,in which the hydrothermal ore-forming period mainly includes quartz sulfide stage,sericite sulfide stage and polymetallic sulfide stage.?2?Pyrite is mainly vein-disseminated,widely distributed,developed in three mineralization stages,i.e.quartz sulfide stage?early stage?,sericite sulfide stage?middle stage?and polymetallic sulfide stage?late stage?,with similar characteristics.The size of pyrite is generally between 0.1?1.5mm,and the reflection color is mainly yellow white,not presenting polychromism and internal reflection,with high hardness.They are mainly euhedral,subhedral and anhedral grains.The pyrite shapes are mainly cube,with a small amount of pentagonal dodecahedral and octahedral.They mainly develop crystalline and metasomatic texture,and are often metasomatized by magnetite,chalcopyrite,pyrrhotite and other minerals.?3?Pyrites are mainly cubic euhedral crystal{100},rarely develops pentagonal dodecahedral{210}and octahedral{111}pyrite,indicating its formation conditions are high temperature?>300??,low supersaturation and sulfur fugacity,rapid cooling,and insufficient material supply.?4?The average content of major elements in pyrites?w?S?=52.89%,w?Fe?=46.31%?and the ratio of atomic number?S/Fe=1.99?are less than the theoretical values,indicating its belonging to the type of sulfur poor and iron poor pyrite with the characteristics of sulfur deficit,and low sulfur fugacity ore-forming environment.The trace element compositions of pyrites show that Co,Ni,As,and Se are mixed into pyrite in the way of isomorphism,while its Co/Ni ratio is concentrated around 10,the S/Se ratio is?0.2?3.56?×104,and the?S and?Fe are-4.39?0.30 and-3.18?1.66 respectively,indicating that pyrites are of magmatic hydrothermal origin.?5?The thermoelectric conductivity of pyrites are mainly N-type pyrite,and the thermoelectric coefficients are concentrated between-200?0?V·?-1.The formation temperature of N-type pyrites are calculated to be 203.86?386.58??average temperature is 321.43??,indicating that pyrites are formed in the environment of medium-high temperature and lower sulfur fugacity.?6?The variation range of cell parameters and volume of pyrites are5.41438?5.41966×10-10m(a0,average value is 5.41747×10-10m)and158.7190?159.1946×10-10m3(Vol,average value is 158.9896×10-10 m3).The most of them are smaller than the theoretical value(a0=5.41759×10-10m,Vol=159.3324×10-10m3),but a small number of cell parameters are larger than the theoretical value.These characteristics of pyrites are mainly related to sulfur deficit and the uneven distribution of Co and Ni elements in vertical space.?7?Pyrite is characterized by cubic crystal type,low cell parameter value,S/Fe ratio close to 2,low content of trace elements?such as Co,Ni,As,etc.?,N-type as the main type of thermoelectric conductivity.Combined with the extremely low gold content in pyrite and significant positive correlation between Cu and Au contents in the ore,it is considered that pyrites of No.1 deposit do not contain gold or are not gold carrying mineral,and gold is more closely related to chalcopyrite.?8?In vertical space,the thermoelectric coefficient of pyrite is from shallow to deep,N-type pyrites show the tendency of increasing?decreasing?increasing,while P-type pyrites show decreasing?increasing?decreasing.In combination with the geological characteristics of No.1 deposit,it indicates that the orebody probabaly extend to deeper than that of present orebody location,but the deeper orebody may have been destroyed by the late emplaced Eocene biotite granodiorite. |
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