Fluid inclusion studies of the Pea Ridge iron-oxide-rare earth elements deposit, Missouri

The Pea Ridge iron oxide-REE deposit is hosted by the Mesoproterozoic St. Francois granite-rhyolite terrane in southeast Missouri and is considered to be an example of an Olympic Dam-type deposit. Primary aqueous, liquid-vapour (LV) inclusions in apatite from the early amphibole stage have salinities of 14 to 24 equiv. wt. % NaCl+CaCl₂ and Th (L-V) values of 130°C to 180°C. Primary fluid inclusions in quartz from the post-magnetite hematite stage comprise aqueous liquid-only and liquid-vapour inclusions. This association, and homogenization temperatures of around 100°C for many of the LV inclusions, indicates low depositional temperatures. The LV inclusions have salinities of 21 ± 3 equiv. wt. % NaCl+CaCl₂.; Primary inclusions in quartz from the silicification stage, which surrounds the magnetite and hematite zones, comprise liquid-vapour-halite (LVH) inclusions that have salinities between 34 and 48 equiv. wt. % NaCl.; Primary inclusions in quartz from late-stage REE-rich breccia pipes consist of LV inclusions that exhibit two salinity populations; one is moderately saline (∼20 wt. %) and one is dilute (0–5 wt. %).; Microthermometry data suggest that there are three types of fluids responsible for the formation of the Pea Ridge deposit. (Abstract shortened by UMI.)...