Self-organizational origin of oscillatory zoning in pyrite: A coupled diffusion–reaction model and pH-dependent As adsorption feedback mechanism

Abstract

Pyrite from hydrothermal deposits often exhibits oscillatory zoning, controlling the distribution of trace elements, like As and Au. The origin of this zonal pattern has long been debated between episodic fluid changes and local chemical disequilibrium. Our study of an epithermal Pb-Zn deposit reveals arsenic oscillatory zonation in monocrystalline pyrite grains. Guided by the results of examination and the fact that As can be adsorbed onto pyrite under high pH conditions, inhibiting crystallization, we propose a quantitative diffusion–reaction model to numerically simulate the formation of oscillatory zoning in pyrite. The simulation reveals that in As-bearing supersaturated hydrothermal fluids, pyrite growth consumes HS− and releases H+, reducing As adsorption and accelerating crystallization. With Fe2+ on the crystal surface rapidly consumed, growth speed of pyrite is reduced to allow As to be adsorbed and thus to inhibit growth. This cycle restarts as Fe2+ is replenished from the enveloping bulk fluid via diffusion, leading to periodic growth and As adsorption. The formation of As-oscillatory zoning in pyrite is resultant from a fine balance between multiple parameters in environments, including diffusion coefficients (DFe2+ and DH+) and concentrations (CFe2+, CHS-, CH+) of ions, pH, temperature and hydrodynamic conditions. The simulation reveals that slightly acidic (e.g., pH = ∼4) As-bearing supersaturated epithermal environments (<∼180 °C) with CHS-(e.g., ∼30 mM) higher than CFe2+ (e.g., ∼2 mM) and ratios of DFe2+/DH+ lower than 0.3 are favorable for the formation of As-oscillatory zoned pyrite. Various As zonal patterns can be formed under different parameters. The increase of DFe2+/DH+ ratios, CFe2+ and CHS- and decrease of pH in hydrothermal fluids would lead to change of zonation patterns from regular oscillation to damped oscillation and to convergent oscillation. In addition, formation of oscillatory zonation requires a stagnant environment and would be significantly suppressed in turbulent environments. Our diffusion–reaction model coupled with pH-dependent As adsorption mechanism robustly interpret the formation of As oscillatory zonation in pyrite via a self-organizational process rather than episodic change of bulk composition of hydrothermal fluids. Our model thus provides new insights into the evolution of hydrothermal systems.