The Geological,geochemical Characteristics And Genesis Of The Dashizhai Zn Polymetallic Deposit, Inner Mongolia

Dashizhai,administratively part of the Ke‘erqin youyi qianqi,Inner Mongolia,is located in the inner Mongolia Orogenic Belt?IMDOB?which is one of the most important lead-zinc-tungsten-tin-copper polymetallic deposits in northern China.The newly founded Mesozoic quartz monzodiorite porphyrite and rhyolite are ore-related and of great significance to the study the genesis of the deposit and further exploring.Through petrographic observation,petrochemistry,temperature measurement of inclusions,zircon U-Pb dating and so on,we get the following understandings:Based on hand specimens and microscopic observations,combined with petrochemistry and the QAP plot,the original“granite porphyry”in the Dashizhai area was renamed“quartz monzodiorite porphyrite”?hereinafter referred to as diorite porphyrite?.The diorite porphyrite is relatively riched in silicon?but Si O₂<65%?and exhibits strong enrichment in light rare earth element?LREEs?and positive U and Pb anomalies,but the negative Eu anomaly is not obvious.It shows slight depletion in large ion lithophile elements?LILEs?.The rhyolite is riched in silicon?SiO₂>75%?and exhibits slight enrichment in light rare earth element?LREEs?and positive U and Pb anomalies,the negative Eu anomaly is obvious,the slight depletion in large ion lithophile elements?LILEs?is the same as the diorite porphyrite,while the rhyolite exhibits a negative P anomaly.The Dashizhai zinc polymetallic ore is a typical magmatic hydrothermal zinc-lead-silver deposit associated with a terrestrial volcanic-subvolcanic magmatism.The formation of the deposit is closely related to the dioritic porphyrite and is obviously controlled by the northwest linear structure and the ring-shaped volcanic structure.Most of the ore bodies are vein-like in the various structural fractures or contact interfaces between and different rocks.The temperature measurement of fluid inclusions in quartz veins which are widely developed in the mining area shows that the fluids in Dashizhai have hydrothermal fluid properties of low salinities?1⁸wt%?and medium-low temperatures?190²37°C?.The zircon U-Pb date measured by LA-ICP-MS for the two possible ore-related Mesozoic rocks in the area shows that the porphyritic diorite‘s age is 147.31±0.65 Ma,while the age of rhyolite is 150.8±1.5Ma,they formed in a similar time,the late Jurassic.The combined characteristics of porphyritic diorite and rhyolite can be well explained by the mush model of silicic magmatism.The porphyritic diorite may be the corresponding crystalline phase of the rhyolite,the rhyolite first erupted during the volcanic activity,the formation of the porphyritic diorite may be caused by the decompression remelting or mafic magmas under the extended tectonic background,or induced by underplating of mafic magmas which heated the crystal phase of magma and caused its eruption,the source of the mafic magma may have been affected by subduction-related fluid.The chemical composition of main elements of Dashizhai rock mass is characterized by silicon-rich?SiO₂>75%?,obvious heavy rare earth enrichment has occurred in Dashizhai rock mass,showing a typical tetrad effect.The same as the characteristics of the A-type granite,but it may also be the result of the highly fractionated felsic I-type or S-type granite.High Pb values,along with low P and Ti values,showing the characteristics of the crust source.It has no obvious genetic link with the deposit.The diorite porphyrite rock mass is the focus of follow-up surveys.The vicinity of the porphyrite rock mass and its contacting surfaces with other rocks,the sections with developed middle-low temperature hydrothermal alterations,and the deep part of the coexistence site of the frontal and tail halo elements are all favorable mineralization locations.