Iron transport and deposition mechanisms in the Taochong iron-rich skarn deposit, Middle–Lower Yangtze Valley, Eastern China

Abstract

The Taochong skarn iron deposit is one of 38 iron prospects in the Fanchang iron ore district of the Middle–Lower Yangtze River Valley, one of the most important economic mineral districts in China. Orebodies are hosted mainly within the middle–upper Carboniferous Huanglong dolomitic limestone. In this deposit, two types of ore (types 1 and 2) are recognized based on the mineral assemblage. The type 1 ore is dominant throughout the deposit and consists mainly of hematite, magnetite, quartz, and calcite. This ore is characterized by euhedral and skeletal textures, and massive, brecciated, comb-like, and vein structures. The type 2 ore is composed of hematite, garnet, pyroxene, actinolite, chlorite, quartz, and calcite, and has a relict metasomatic texture and disseminated structure. Field evidence and petrographic observations indicate that three stages of hydrothermal activity (i.e., the skarn, iron oxide, and carbonate stages) contributed to the formation of the Taochong iron deposit. The iron oxide stage can be further divided into early and late oxide stages, which saw the formation of the type 2 and type 1 ores, respectively.Fluid inclusion and stable isotope data indicate that the skarn formed at depths of ~2km under lithostatic pressure of ~500bars, from high-temperature (~500°C), acidic, saline (up to ~50wt.% NaCl equiv.), and iron-rich fluids that had chemical and isotopic compositions (δ18Owater=4.5‰–11.7‰) similar to those of magmatic fluid replacing a certain amount of the Huanglong dolomitic limestone. The iron ores formed under hydrostatic conditions after fracturing of the early skarn. Fluid inclusions within quartz in equilibrium with hematite in the type 2 ores yields homogenization temperatures of 372–390°C and salinities of 31–39wt.% NaCl equiv., whereas fluid inclusions within quartz in equilibrium with hematite in the type 1 ores has lower homogenization temperatures (280–320°C) and variable salinities (0–40wt.% NaCl equiv.). During the formation of the type 1 ores, fluids contained moderate concentrations of ions, variable δ13C values (−5.3‰ to +5.8‰), and relatively convergent δ18O values (1.7‰–4.5‰). The temporal decrease in the concentration of ions and δ18O values in the fluid inclusions is matched by marked decreases in fluid salinity and temperature, suggesting that the fluids of the oxide stage were mixtures of magmatic brine and meteoric water. The coexistence of saline and vapor-rich fluid inclusions as internal trails or clusters within individual quartz grains of the late oxide stage, with contrasting homogenization characteristics (for the liquid and vapor) at similar temperatures, strongly suggests that fluid boiling occurred during the late oxide stage.Our estimates of the physico-chemical conditions during ore formation, together with fluid inclusion data and the results of previous experimental studies, indicate that iron in the Taochong deposit was transported mainly as FeCl4–, along with minor amounts of other chloride complexes (e.g., FeCl2+ and FeCl2(H2O)4+). The precipitation of hematite followed the breakdown of the iron chloride complexes, probably induced by a decrease in temperature and ligand (chloride) ion activity, and an increase in pH. Based on the results, we infer that deposition of the type 2 ore was caused by the replacement of garnet and pyroxene by a retrograde alteration assemblage, which was likely induced by fluid–rock interaction and the mixing of magmatic brine and meteoric water. The type 1 ore was deposited in open spaces, probably induced by fluid boiling.