Fluid evolution in the formation of the Fenghuangshan Cu-Fe-Au deposit, Tongling, Anhui, China

The Fenghuangshan Cu-Fe-Au deposit is located in Tongling, Anhui province in central-east China, within the Middle-Lower Yangtze River polymetallic belt. The mineralization is related to Late Yanshanian magmatic intrusions emplaced in Lower Triassic limestones and is divided into two episodes. The first and more important, episode A, is related to granodiorite-granodiorite porphyry and characterized by massive, disseminated, veinlet- and breccia-style mineralization in skarn formed at the contact zone, whereas the second, episode B, is related to a quartz monzodiorite porphyry stock which crosscuts the previous skarn and is characterized by disseminated and veinlet-style mineralization within the porphyry. Based on mineral assemblages and their crosscutting relationships, the hydrotherinal evolution of episode A is further divided into five substages (A1-A5), and that of episode B into four (B1-B4), among which A4, A5, and B3 are the main copper mineralization stages. This paper focuses on microthermometric studies of fluid inclusions in the various stages, with an aim to characterize the thermal and compositional evolution of hydrothermal fluids and to discuss their implications for mineralization in terms of fluid sources and metal deposition mechanisms. Fluid inclusions were studied in garnet, quartz, and calcite from stages A1, A4, A5,B1, B2, B3, and B4. Four types and 1.3 subtypes of fluid inclusions were distinguished based on fluid composition and phase assemblages. Tape I fluid inclusions contain one or more salt daughter minerals (halite and sylvite) and homogenize into the liquid phase, whereas type II inclusions homogenize into the vapor phase, both commonly containing all opaque daughter mineral. Type III fluid inclusions are aqueous inclusions that do not contain daughter minerals and homogenize into the liquid phase, and type IV inclusions are CO(2) hearing, homogenize to the CO(2) or the aqueous phase, and commonly contain an opaque daughter mineral. Primary fluid inclusions in stage A1 are of type I and II, interpreted to represent two immiscible phases resulting from boiling, and those in stages A4 and A5 are of type III. Stages B1 and B2 are characterized by type I primary inclusions, and stages B3 and B4 by type III. Type IV inclusions occur as secondary inclusions in B1 quartz and are interpreted to be related locally to B3 stage mineralization. Microthermometric data indicate a trend of decreasing temperatures and salinities from early to late stages for both mineralization episodes, reflecting a change from a magmatic fluid-dominated system to one invaded by fluids from the country rocks and meteoric water. The fact that the main stages of mineralization (A4, A5, B3) coincided with the development of fluid inclusions without daughter minerals of salt suggests that sulfide precipitation did not take place until the system was diluted by cooler fluids with lower salinities. However, the observation that the fluid inclusions hosted in minerals before sulfide mineralization (i.e., stages A1, B1, and B2) commonly contain opaque daughter minerals inferred to be sulfides suggests that the ore-forming components were likely derived from the higher temperature magmatic fluids, and the fluids derived from the country rocks mainly contributed to the precipitation of sulfides through mixing and cooling.