Origin and evolution of ore-forming fluids in the Hemushan magnetite–apatite deposit, Anhui Province, Eastern China, and their metallogenic significance

Abstract

The Middle–Lower Yangtze River Metallogenic Belt in the northern Yangtze Block is one of the most important economic mineral districts in China. The Hemushan deposit is a medium-class Fe deposit located in the southern part of the Ningwu iron ore district of the Middle–Lower Yangtze River Metallogenic Belt. The Fe–orebodies are mainly hosted in the contact zone between diorite and Triassic marble. The actinolite–phlogopite–apatite–magnetite ore shows metasomatic/filling textures and disseminated/mesh–vein structures. Based on evidences and petrographic observations, the ore-forming process can be divided into three distinct periods—the early metallogenic period (albite–diopside stage), the middle metallogenic period (magnetite stage and hematite stage), and the late metallogenic period (quartz–pyrite stage and carbonate stage). Fluid inclusion studies show four types of inclusions: type I daughter mineral-bearing three-phase inclusions (L+V+S), type II vapor-rich two-phase inclusions (L+V), type III liquid-rich two phase inclusions (L+V), and minor type IV liquid-phase inclusions (L). Apatites from the magnetite stage contain type I, type II and type III inclusions; anhydrites from the hematite stage mainly contain abundant type II inclusions and relatively less type I inclusions; quartz and calcite from the late metallogenic stage are mainly characterized by type III inclusions. Laser Raman spectroscopy and microthermometry of fluid inclusions show that the ore-forming fluids broadly correspond to unsaturated NaCl–H2O system. From the magnetite stage to the carbonate stage, the ore-forming fluids evolved from moderate-high temperature (average 414°C), moderate salinity (average 25.01 wt.% NaCl equiv.) conditions to low temperature (average 168°C), low salinity (average 6.18 wt.% NaCl equiv.) conditions. Hydrogen and oxygen isotopic studies indicate that the ore-forming fluid during the early stage of middle metallogenic period was mainly of magmatic water, and mixing of the ore fluids with meteoric water took place during the late phase. During this evolution, water–rock interaction, and boiling and mixing of the ore fluids with meteoric water occurred. The boiling of fluids was a potential mechanism for the formation of magnetite.