Characteristics And Genesis Of The Huanxiangwa Gold Deposit In The Xiong'ershan District, Central China

Large to super-large scale mineral district(deposit)is of great economic value and strategic significance,and play an important role in the sustainable development of a country and the world economy and mining industry.Therefore,the mineralization and enrichment mechanism of the large to super-large scale mineral district(deposit)has become one of the important topics in metallogeny.The study of typical deposit offers the potential to better understand the metallogenic mechanism of mineral district.The Huanxiangwa deposit is one of the most representative gold deposits in the Xiong'ershan district,which is the third-largest gold-producing district in China.A detailed study on the Huanxiangwa deposit would give a clue to better understand the gold mineralization mechanism in the Xiong'ershan district and even the large to super-large scale mineral districts in the margin of the North China Craton.Therefore,a detailed geological and geochemical study has been carried on the Huanxiangwa gold deposit,with focus on an analysis of the sources of ore-forming fluids and ore-forming materials,the relationship between magmatic rocks and gold mineralization,and the ore genesis.The following are major achievements obtained in this dissertation.The Late Jurassic to Early Cretaceous magmatism in the Xiong'ershan area can be divided into two episodes: early(165-150 Ma)and late(138-113 Ma)episodes.LA-ICP-MS zircon U-Pb dating yields ages of 160.7 ± 0.6 Ma and 127.2 ± 1.0 Ma for the Wuzhangshan and Huashan plutons in the Xiong'ershan district,respectively,fingerprinting the two magmatic episodes.Detailed petrologic and geochemical data indicate that the Wuzhangshan pluton was derived directly from partial melting of the Neoarchaean to Palaeoproterozoic Taihua Group metamorphic basement rocks(especially the mafic amphibolites),whereas the Huashan pluton was probably generated by mixing of mantle-derived magmas and melts generated through the reworking of the Taihua Group metamorphic basement rocks.The petrologic and geochemical data further suggest that the crustal thickness in the Xiong'ershan area must have been at least 50 km when the Wuzhangshan pluton was formed,but less than 40 km when the Huashan pluton was formed.This suggests that the transformation of the lithosphere from thickening to thinning in the Xiong'ershan area probably occurred between ~160 Ma and ~127 Ma.The gold mineralization is coeval with the emplacement of the late episode magmas,implying that crustal-mantle mixed magma might be a better target for gold mineralization compared to the ancient metamorphic basement melt,and that the gold metallogenic and late episode magmatic activities during Late Jurassic to Early Cretaceous in the Xiong'ershan district were probably related to the lithospheric thinning.Two mineralization styles are found in the Huanxiangwa deposit: disseminated in altered rocks(type 1)and auriferous quartz veins(type 2).These two types of ore exhibit contrasting geological and geochemical features,indicating that they have different sources of ore-forming fluids and ore-forming materials.Field evidence and petrographic observations indicate that two stages of hydrothermal activity(early and late)contributed to the formation of the Huanxiangwa gold deposit.The early and late stages of hydrothermal activity saw the formation of the type 1 and type 2 ores,respectively.The ore-forming fluids were dominated by metamorphic water in the early stage and magmatic fluid in the late stage.The ore-forming materials of the type 1 ores may be derived from regional reducing rocks and the Taihua Group metamorphic basement rocks,but the ore-forming materials of the type 2 ores are mainly from Early Cretaceous magma system.Gold was transported mainly as Au-S complexes in the early metamorphic fluids and as Au-Cl complexes and Au-S complexes in the late magmatic fluids.The gold precipitation probably induced by fluid-rock interaction in the early stage,and mainly contributed to the combination of boiling and immiscibility in the late stage.Based on the geological and geochemical data,we conclude that the Huanxiangwa gold deposit was a result of two stages of hydrothermal mineralization associated with two distinct source reservoirs.This conclusion,together with regional geological data,lead to proposing a model for the Huanxiangwa gold deposit in the Xiong'ershan district.