Syn- and Postdepositional Controls on the Composition of Pyrite and Pyrrhotite in the Windy Craggy Cu-Co Volcanogenic Massive Sulfide Deposit, British Columbia, Canada

Abstract

Pyrite and pyrrhotite from the Windy Craggy volcanogenic massive sulfide (VMS) deposit, British Columbia, Canada, were investigated using combined in situ compositional mapping (by laser ablation-inductively coupled plasma-mass spectrometry [LA-ICP-MS]) and microstructural mapping (by electron backscatter diffraction [EBSD]) to elucidate how their compositions were affected by hydrothermal processes and subsequent deformation and metamorphism. Early pyrite was precipitated rapidly from cool (<250°C) hydrothermal fluids with a significant seawater component and incorporated substantial quantities of a wide variety of trace elements, including the low-temperature suite As-Ag-Sb-Au-Tl-Pb. As the hydrothermal system evolved, this early pyrite was overgrown during subseafloor zone refining and replaced by massive pyrrhotite-chalcopyrite-pyrite mineralization containing Co-Ni-Cu-Se-Mo-Bi, under high-temperature (~350°–380°C), reducing conditions. During deformation and metamorphism at greenschist facies conditions (≥370°C), pyrrhotite was mechanically remobilized by dislocation creep and dynamic recrystallization, expelling elements hosted in mineral inclusions (Cu, Mo, Ag, Sb, Pb, and Bi) but largely retaining direct, stoichiometric substitution elements (Co, Ni, and Se). Pyrite is more competent than pyrrhotite, but local dynamic recrystallization did occur and similarly expelled most elements incorporated by coupled substitution or in inclusions (Cu, Zn, Mo, Ag, Sn, Sb, some Te, Au, Tl, Pb, and Bi), while retaining direct substitution elements (Co, As, Se, and some Te). Synmetamorphic phases, like pyrite overgrowths and minor cobaltite, are distinctly Co and As rich. Within the Windy Craggy deposit, significant variations exist in pyrite and pyrrhotite trace element compositions, reflecting both syn- and postdepositional processes. In general, low-temperature elements are present in sulfide mineral inclusions or as coupled substitutions and may be positive indicators of proximity to hydrothermal mineralization, but they are readily remobilized during hydrothermal, metamorphic, or deformational sulfide modifications. Several high-temperature elements are incorporated tightly into the crystal lattice of pyrite and pyrrhotite and are thus better retained through such modifications.