© 2022 American Chemical Society. All rights reserved.Extensive sampling of high-Temperature hydrothermal fluids and diffuse flows within <2 m of the vent orifices at the 9°50′N East Pacific Rise (EPR) hydrothermal vent field reveals formation of nanoparticulate phases and rapid precipitation/aggregation of metal sulfide minerals upon mixing of vent fluid with ambient seawater. Here, we characterize metal sulfide phases via scanning and transmission electron microscopy (SEM, TEM) in addition to quantifying the concentrations of major and trace metals in filtered and unfiltered fluid samples. Analyses demonstrate that, despite coprecipitation in phases such as chalcopyrite, iron speciation and transport is decoupled from that of copper and zinc. We observe the formation of â 10-500 nm diameter (nano)particulate Zn and Cu sulfide phases that are near-quantitatively removed by filtration. Iron sulfides, conversely, are typically present in SEM images as larger particles up to tens of microns in diameter. Few small nanoparticles (20-100 nm diameter) are captured on the filter, but determination of nitric-Acid-soluble iron in 0.2 μm filtered samples indicates the presence of pyrite nanoparticles. Physical mixing and temperature play a larger role in determining the extent of nanoparticulate pyrite formation than fluid chemistry. In diffuse flow environments, Fe and Cu more commonly co-occur as aggregates of very small crystallites, with the Zn sulfide phases occurring separately. Sample pH and the ZPC (zero point of charge) of metal sulfides exhibit chemical control on nanoparticle and large particle formation versus aggregation. The concentrations of the additional trace metals analyzed vary between vent sites, despite the short distances between the sites and likely similar magmatic sources. Measured trace metal concentrations highlight the importance of diffuse flow systems in hydrothermal metal emissions. These near-vent behavioral differences have implications for the long-distance transport of metals away from vent fields in buoyant and nonbuoyant plumes.