Nature and origin of primary ore-forming fluids in the highly metamorphosed Archean Hongtoushan VMS deposit, North China: Insights from in situ S isotopes and trace elements of sulfides

Abstract

Many ancient volcanogenic massive sulfides (VMS) deposits underwent various degrees of deformation and metamorphism, which modified the primary orebody geometries and sulfide textures, resulting in the great challenge of revealing the source and physicochemical properties of primary ore-forming fluids and original mineralization processes. Here, we observed the textural evolution of sulfides in the highly metamorphosed Archean VMS Hongtoushan Cu-Zn deposit, China, to constrain the histories and P-T conditions of metamorphic remobilization and to evaluate the influence of metamorphism on geochemical characteristics of sulfide. Further, the in situ sulfur isotopic and trace elemental analyses were utilized to reveal the source and physicochemical conditions of initial fluids. During metamorphism, pyrite developed elongated texture, bulging recrystallization, coarse porphyroblasts, and foam texture. These textures all occurred at a temperature of 550 – 700 °C, at which sulfide remobilization and deformation occurred in an anhydrous environment. The geochemical exchange among sulfides did not happen during metamorphism. The sulfur isotope and trace elements of sulfides thus can be used to track the source and physicochemical properties of primary ore-forming fluids. In situ sulfur isotopic analyses of sulfides yielded values of δ34S approaching zero and Δ33S situating mass-dependent isotope fractionation (MDF) area, indicating that the sulfur of the original submarine hydrothermal system was mainly derived from the magmatic fluids. The content of Se in pyrite revealed the primary fluids were acidic and reductive and were used to constrain the temperature of ore-forming fluids, which was higher than 250 °C. The ratio of Co/Ni of pyrite was higher than 10, indicating primary fluids were of magmatic-hydrothermal origin. The high content of Se and As in pyrite and Cd and Hg in sphalerite documented that these elements were partially derived from the magmatic volatiles or magma degassing process. Overall, our study demonstrates that coupling textures and in situ geochemical features of sulfides could be a helpful tool for tracking the primary ore-forming processes of the metamorphosed massive sulfide deposits.