Osmium isotopic constraints on sulphide formation in the epithermal environment of magmatic-hydrothermal mineral deposits

Abstract

In the magmatic-hydrothermal environment, fluids with similar metal concentrations and sources may yield contrasting mineral assemblages in successive stages of sulphide mineralization. These differences are linked to the physico-chemical conditions of the mineralizing fluids (e.g., T, pH, fS2, fO2) acquired during their interaction with country rocks and/or by mixing with groundwater. Here, we integrate petrography and osmium (Os) isotope (187Os/188Os) sulphide geochemistry, and discuss novel constraints on magmatic fluid-rock interaction and magmatic fluid-groundwater mixing that are deemed to govern sulphide deposition in magmatic-hydrothermal systems. We studied pyrite (FeS2) and enargite (Cu3AsS4) from the porphyry-related polymetallic Cerro de Pasco (14.54–14.41 Ma) and Colquijirca (10.83–10.56 Ma) epithermal deposits in the Central Andes, Peru. Sulphide mineralization is genetically associated with Miocene magmatism and includes breccia and replacement bodies of carbonate country rocks, and veins cutting the magmatic and sedimentary country rocks. At both deposits, pyrite is followed by enargite in the paragenesis. Pyrite has a radiogenic initial 187Os/188Os isotopic composition (187Os/188Osi-pyrite or Osi-pyrite = 0.80 to 1.45). Enargite (I) enclosing pyrite or filling in cracks in pyrite also has a radiogenic initial 187Os/188Os isotopic composition (Osi-enargite I = 0.56 to 1.24). Conversely, enargite (II) that formed on irregular surfaces on earlier pyrite has an unradiogenic 187Os/188Os isotopic composition (Osi-enargite II = 0.13 to 0.17). Our data show that the paragenetic evolution from pyrite to enargite records a sharp change in the osmium isotope composition within these sulphides. Pyrite and enargite (I) record radiogenic initial 187Os/188Os isotopic compositions, indicating interaction of magmatic hydrothermal fluids with the sedimentary country rocks. However, the unradiogenic initial 187Os/188Os isotopic composition of enargite (II) suggests that magmatic fluids with a mantle-like 187Os/188Os signature ascended from parental magmatic chambers to the epithermal environment without incorporation of crustal Os via fluid-rock interaction or mixing with groundwater. This difference may be due to the country rocks being altered during previous stages, with the radiogenic crustal Os signature being flushed by earlier magmatic pulses. Our findings imply that ore metals (i.e., Cu, Au) are magma-derived, whereas the Os isotopic composition of pyrite and some enargite in epithermal deposits may capture the signature of the interaction of magmatic fluids with country rock lithologies (e.g., the Eifelian black shale in the study area) and/or groundwater. Thus, the isotopic composition of the siderophile and chalcophile trace element Os in sulphides may act as a tracer of metal source, and degree of wall-rock interaction.