Geological Characteristics And Ore Genesis Of The Epithermal Gold Deposits In The Eastern Section Of Beihuaiyang Metallogenic Zone

The Early Cretaceous Xiaotian volcanic basin in the eastern section of the Beihuaiyang metallogenic zone hosts a group of epithermal gold deposits,including the Daijiahe,Longxing and Dongxi gold deposits from the close to the distant,with the Lingjiachong complex in the west margin of the volcanic basin.Since the gold mineral resources have been gradually exhausted after many years'of mining,it has been more and more difficult to prospect in this area.Therefore,it is beneficial to carry out the study on metallogenic mechanism to help the subsequent prospecting work.In this study,we focused on the epithermal gold deposits in the Xiaotian volcanic basin,to perform detailed observation of wall rock alteration,X-ray diffraction?XRD?mineral identification,LA-ICP-MS zircon U-Pb dating,whole-rock Sr-Nd isotope,zircon Hf isotope,fluid inclusion microthermometry,Raman spectroscopy and hydrogen-oxygen analysis.The associations of altered minerals in this area show obvious spatial relationships around the Lingjiachong complex.The Raman spectroscopy results confirm the existence of alunite in the Daijiahe gold deposit,which is one of the characteristic altered minerals in the high-sulfidation type epithermal gold deposits.In combination with the commonly developed propylitization,silicification and sericitization,we deduce that the gold deposit was developed in a acidic and oxidising environment close to the Lingjiachong complex.The XRD mineral identification confirms the existence of adularia in the Dongxi gold deposit,which is one of the characteristic altered minerals in the low-sulfidation type epithermal gold deposits.In combination with the commonly developed silicification and carbonatization,we deduce that the gold deposit was developed in a relatively alkaline and reduced environment which is far from the Lingjiachong complex.Regional Yanshanian igneous rocks are calc-alkaline diorite,granodiorite,monzogranite,quartz-monzonite,andesite and rhyolite,and alkaline keisantite.The LA-ICP-MS zircon U-Pb dating results indicate that the igneous rocks in the Xiaotian volcanic basin have formed during the Early Cretaceous.The zircon U-Pb age of the rhyolite in the Longxing gold deposit is 136Ma;the zircon U-Pb age of the andesite in the Dongxi gold deposit is 130Ma;and the zircon U-Pb ages of the Lingjiachong complex that composed of diorite,granodiorite,keisantite,monzogranite and quartz-monzonite show a narrow range of 134 to 127Ma.Major elements show that they mainly present high K calc-alkaline,and a few of them belong to shoshonite series?such as keisantite?.Trace elements show that the igneous rocks in this area generally show similar characteristics of element depletion,which are similar to those in the island arc area.They mainly present Nb,Ta and Ti depletion in the spider diagram,and all the rock samples display no obvious Eu abnormalities.The Sr-Nd-Hf isotopes display that the?_Hf?t?and?_Nd?t?compositions of these igneous rocks are similar to those in the North Dabie region,indicating similar magmatic sources of Paleoproterozoic basement.Based on the evidence from wall rock alteration,LA-ICP-MS zircon U-Pb dating and Sr-Nd-Hf isotopes,we suggest that the gold mineralization and associated magmatism was likely related to the collapse of the Dabie orogenic belt.Fluid inclusion petrographic study shows that liquid-rich aqueous fluid inclusions were ubiquitous presence in the Dongxi gold deposit.Fluid inclusion microthermometric results show homogenization temperatures?Th?of 216??254?and salinities of 0.2 wt.%?1.7 wt.%Na Cl equivalent,indicating that the ore-forming fluids are moderate-low temperature and low salinity.The homogenization temperature further decreased in the post-ore stage,and its salinity also decreased slightly,indicating the involvement of the meteoric water in the post-ore stage.Hydrogen and oxygen isotopic data are plotted nearby the line defined by meteoric water,but far from the range of magmatic water.Such hydrogen and oxygen isotopic data,combined with moderate-low temperature and low-salinity fluid characteristics,indicate that a large amount of circulated meteoric water was involved in mineralization.Based on the evidence from alteration mineralogy,fluid inclusion and H-O isotopes,we suggest that the Daijiahe and Longxing gold deposits are high-sulfidation type epithermal deposit,while the Dongxi gold depoist is a low-sulfidation type epithermal deposit.We proposed that the eastern section of Beihuaiyang metallogenic zone is a typical ore cluster hosting both high-sulfidation and low-sulfidation epithermal gold deposits.