Pyrite micro-scale trace element and Fe isotope signatures from the hydrothermal Pb-Zn deposit in Qingchengzi orefield, North China Craton: implications for detailed mineralization processes

Micro-scale analyses of Fe isotope and trace elements of pyrite， combined with microscopical observation and elemental mapping have been studied to reveal detailed ore-forming processes of the hydrothermal Pb-Zn deposit in the Qingchengzi orefield in this work. The disseminated metamorphic pyrite （Py0） exhibited high concentrations of Mn （3.42-303 ppm）， Co （2.94-1619 ppm）， Ni （4-776 ppm） and inherited the similar Fe isotopic compositions of Liaohe group (from -0.85‰ to +0.72‰， with the average δ⁵⁶Fe value of 0.00 ±0.39‰). The hydrothermal pyrite （Py I-a） which underwent strong fluid-rock interactions， remobilized metals from wall rocks and rapidly precipitated （with an intermediate phase of FeS） under kinetically favored hydrothermal conditions recorded by light Fe isotope fractionations of -0.52‰ ±0.55‰ （from -1.55‰ to +0.69‰）. Followed by severe fluid-rock interactions and rapid precipitation of Py I-a， the early ore-related pyrite （Py I） of coarse to medium grained and anhedral shaped with minor chalcopyrite was gradually precipitated from the reduced ore-forming fluids (Fe²⁺-bearing) in equilibrium and relatively closed system， characterized by heavy Fe isotopic compositions of +0.35‰ ±0.28‰ （from -0.30‰ to +1.04‰） with high concentrations of Cu （up to 3665 ppm） and Zn （up to 1338 ppm）. The later ore-related pyrite （Py II） occurring as discrete and euhedral-subhedral grains precipitated from the reduced ore-forming fluids in equilibrium system with heavy Fe isotopic compositions from +0.05‰ to +1.19‰ (with the average δ⁵⁶Fe value of +0.72‰ ±0.28‰) and disturbed by addition of magmatic hydrothermal along fracture zone with extremely high concentrations of As （up to 10426 ppm）. The last ore-related pyrite （Py III） coexisting with pyrrhotite， galena and sphalerite occurs as subhedral-anhedral and coarse grained， and precipitated from the reduced ore-forming fluids in equilibrium system with high concentrations of Pb and the heaviest Fe isotopic compositions ranging from +0.57‰ to +1.41‰ (with the average δ⁵⁶Fe value of +0.94‰ ±0.23‰). To conclude， in situ Fe isotopic analysis combined with other in situ analysis techniques， is a vibrant tool for re-constructing the fluid evolution history and revealing the detailed mineralization processes.