Chronology,geochemistry and geological significance of syenogranite from Baiyinhan quartz vein wolframite deposit in Inner Mongolia
The Baiyinhan deposit is a quartz vein wolframite deposit situated in the southern segment of the Great Xing'an Range. This research presents an in-depth analysis of the syenogranite,encompassing zircon U-Pb chro-nology,a detailed examination of major and trace elements,and Hf isotope studies. Utilizing LA-ICP-MS zircon U-Pb dating,it has been determined that the Baiyinhan syenogranite dates to (139.1±2.5)Ma. This discovery is corroborated by the Re-Os isochron age of molybdenite,which is (139.6±7.6)Ma,indicating a likely close rela-tionship between the syenogranite and the mineralization events. Petrographic and geochemical analyses have characterized the syenogranite as a highly fractionated A-type granite. It is marked by significant enrichment in silica,with a content of SiO2 ranging from 75.70％ to 76.71％,and in alkali elements,where the sum of Na2O and K2O varies from 8.32％ to 8.92％. The syenogranite also exhibits a slightly peraluminous nature,with A/CNK val-ues fluctuating between 1.00 to 1.05. The chondrite-normalized rare earth element (REE) patterns show concen-trations varying from 214×10-6 to 347×10-6,with a pronounced enrichment in light rare earth elements (ΣLREE from 129× 10-6 to 219×10-6) and strong negative europium anomalies (δEu ranging from 0.03 to 0.06). Trace ele-ment analysis indicates an enrichment in Rb,Th,and K,while depletion is noted for Nb,Ta,P,and Ti. The zircon trace element signature is consistent with that of magmatic zircon,showing a substantial depletion in light rare earth elements (58×10-6 to 99×10-6) and a high enrichment in heavy rare earth elements (ΣHREE from 859×10-6 to 2457×10-6). The LREE/HREE ratios are notably low,ranging from 0.03 to 0.07,with an average of 0.05. Addi-tionally,the zircon displays significant positive cerium anomalies (δCe from 3.6 to 41) and weak negative europi-um anomalies (8Eu from 0.3 to 0.7). The eHf(t) values are tightly clustered between 2.4 and 5.0,all positive. The two-stage Hf isotope model age (TDM2) of the zircon is also relatively uniform,ranging from 871 Ma to 1042 Ma.Previous studies have predominantly attributed the formation of tungsten deposits to concealed granitoids. How-ever,this study posits that the metallogenetic granitoids associated with the Baiyinhan deposit are specifically the highly fractionated syenogranite. Both the Baiyinhan deposit and the syenogranite were coeval,forming during the Early Cretaceous period. This formation occurred in the backdrop of the retreat of the Paleo-Pacific plate and regional extensional tectonics,which is interpreted as a response to asthenospheric upwelling.
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