From formation to maturation: Trace element systematics of sulfide chimneys from the Niaochao vent field (East Pacific Rise)

Abstract

The identification of the maturity stages of collapsed sulfide chimneys remains challenging due to alterations in their original structural and mineralogical characteristics. The trace element composition of sulfides can provide valuable information about the mineralization processes in submarine hydrothermal systems. However, the link between trace element variations and the maturity stages of sulfide chimneys remains unclear. The present study employs in-situ trace element analysis and elemental mapping of sulfides in three maturity grades of sulfide chimneys at the Niaochao vent field (East Pacific Rise, 1–2°S) to elucidate the relationship between trace element variations, mineralogical changes, and fluid evolution.

The investigated sulfide chimneys are classified into seven mineralogical assemblages across five evolutionary stages, typically transitioning from sphalerite-pyrite-rich to chalcopyrite-pyrite-(high-Fe sphalerite)-rich and eventually reverting to pyrite-sphalerite-rich layers as chimneys mature. Initially, trace elements such as Cu, Zn, Pb, and Ag appear as micro- or nano-inclusions in anhedral pyrites, likely formed under rapid, disequilibrium, low-temperature conditions. Over time, these elements, along with Co, Ni, Se, and As, were incorporated into the lattices of euhedral and massive pyrites under high-temperature conditions. As chimneys grew inward, trace element enrichment transitioned from seawater-derived elements (e.g., Mg, V, U) and low-temperature-responsive elements (e.g., Zn, Pb, As, Tl) to high-temperature-responsive elements (e.g., Cu, Se, Co). This progression is marked by increasing concentrations of Co, As, Sb, Mo, and rising Co/Ni ratios in pyrites from chimney rims to cores, reflecting chimney maturity. In higher maturity grades of chimneys, elevated levels of Sb, Mg, and Se in anhedral pyrites, Se, Zn, and Cd in euhedral pyrites, and Se in chalcopyrite indicate a more advanced stage of hydrothermal evolution. These compositional trends suggest that trace elements can serve as indicators of chimney and hydrothermal system maturity, capturing the evolution of hydrothermal activity from initiation to maturation.