Pyrite–anhydrite–magnetite–pyroxene-type deposits and coexisting hydrothermal fluids in Mesozoic volcanic basins, Yangtze River Valley, China

Anhydrite–pyrite–magnetite–pyroxene–type deposits occur in the Mesozoic volcanic areas of the Middle–Lower Yangtze Valley in China. These deposits are hosted in alkaline basaltic rocks, and are generally accompanied by melanocratic and leucocratic alteration zones, both of which are characterized by a distinct vertical zonation pattern. Investigation of these zones indicates that the chemical compositions of solid solutions and polymorphs of various minerals vary spatially in the alteration profile, upwards from the lowest level, and outwards from the center.Here we report a case study on the Luohe deposit. In the melanocratic-alteration zone, the composition of magnetite (including trace elements Ti, V, Mg, Mn), pyroxene (Mg, Fe2+, Fe3+, Al2O3), plagioclase (AnxAb1 − x), pyrite (Co, Ni) and apatite (F, CeO2 + Y2O3 + La2O3) changes with depth. The isotherms of hydrothermal fluids determined from fluid inclusion data, including homogenization temperature and salinity, also vary with depth.Activity diagrams were constructed from mineral and isotherm analysis to estimate the chemical constraints on the alteration-mineral assemblages and the coexisting hydrothermal solutions for the Na2O–K2O–CaO–MgO–FeO–Fe2O3–A12O3–SiO2–H2SO4–H2S–HCI–H2O system at 350 to 600 °C and 500 bars (50 MPa), assuming that the major alteration mineral assemblages along the profile reflect the nature of the coexisting hydrothermal solutions. The activity diagrams adopted the major minerals as buffers to fix the activities of the aqueous species in the system, simulating the physicochemical conditions of the magnetite–anhydrite–pyroxene equilibrium and of solid solutions of diopside–hedenbergite, grossular–andradite and anorthite–albite found in the profile.This study provides an approach to modeling the chemical constraints of coexisting fluids in ore-alteration zones based on field observations.