Study On Geological Characteristics And Ore Genesis Of Da Fanshan Alunite Deposit In Luzong Basin, Anhui

The luzong Mesozoic volcanic basin is located in the Lower Yangtze fault Belt, the north margin of the Yangtze Block,it extends from nearby the east of the Tan-Lu fault zone to east.There are abundant of metal-rich mineral and non-metallic mineral resources in it.The reserves of alunite in Luzong Basin ranks the second in China,Luzong Basin is also the key propecting area for alunite resource in China.Dafanshan alunite deposit is the largest alunite deposit in this area.This paper takes Dafanshan alunite deposit as the main research objiect, discusses the Geochemical Characteristics,the physicochemical conditions,the Ore-forming Process and formation Process of alunite in Dafanshan deposit according to the study of petrographical,micro-thermal analysis of fluid inclusion, electon probe analysis of mineral,SEM studies on alunite samples,isotopic dating, and geologic features.The following conclusions are reached.Dafanshan alunite deposit was formed during the deposition of the Zhuanqiao formation and the Shuangmiao formation.The mineralization can be divided into two styles.One is the alunite-pyrite ore,which is formed in magmatic hydrothermal environment.The other is alunite-kaolinite style formed in steam-heated environment.The homogenous temperature of fluid inclusions for the first mineralization stage ranged from 260 to 290℃,with salinity 3.6%NaCl equivalent and ore-forming presssure from 36.4×10~5～-53.7×10~5Pa.Alunite was formed in an acidic and oxidation environment.Al and K in the deposit were from the volcanic formations.Due to strong acid sulfate alteration,K-feldspar and volcanic glass in the host rock were altered and K and Al were released,faciliating the formation of alunite.Sulfur isotope analysis reveals that the Sulfur in the deposits has a magmatic origin.The early ore-forming fluid was mainly of magmatic hydrothermal,from which deposited the alunite-pyrite ore body.The later magmatic hydrothermal fluid mixed with meteoritic water and formed the alunite-kaolinite ore body.The acidity of hydrothermal solution decreased from the upper to lower position in the ore body. The deposits exhibits alteration zoning,from an inner silicified to an outer silica-alunite,kaolinite,smectite-chlorite zones.They were formed as a result of the acid hydrothermal gradually reduced during the hydrothermal evolution.Based on the above characters,as well as compareing with typical high sufidation epithermal hydrothermal deposits in the world,the Dafanshan alunite ore deposit was regarded as a high sulfidation magmatic hydrothermal replacement deposit.The formation of the Da fanshan alunite deposit indicates that there existed a high sulfidation epithermal hydrothermal metallogenic system in Luzong basin in late Mesozoic,this discovery provides an important indication for the Cu-Au hydrothermal ore-forming mineralization and epithermal hydrothermal polymetallic deposits in this area.We also deduced that a bured pluton may be under the Dafanshan mining area.This expands the idea of exploring deposits formed in porphyry mineralization in the study area.