MICROTHERMOMETRIC AND ION RATIO ANALYSES OF FLUID INCLUSIONS FROM THE PEA RIDGE IRON OXIDE-APATITE (IOA) AND BOSS-BIXBY IRON OXIDE-COPPER-GOLD (IOCG) DEPOSITS SUPPORT A MAGMATIC-HYDROTHERMAL ORIGIN

Pea Ridge (136Mt Fe ore w/ 0.6Mt of 12% REE in breccia pipes) and Boss Bixby (36Mt w/ 0.8% Cu ± {Co, Au, REE}) are the largest IOA and IOCG deposits in the St. Francois Mountains of SE Missouri. Alkali and silica metasomatism at Pea Ridge is restricted to adjacent wall rocks. At Boss-Bixby, it is more widespread. They are essentially coeval with a 1.47±0.1 Ga A-type granite and rhyolite terrane with minor Fe-rich trachyte and basalt that have depleted mantle ε_Nd (2.6 to 6.2) signatures. The plethora of rhyolite ignimbrites suggests that volcanism was subaerial. Stromatolites at one locale are marine or lacustrine. Coeval Sn greisen and epithermal veins in the Silver Mine granite have H and O isotope signatures of magmatic and meteoric water. This terrane is overlain by lower Paleozoic marine ss and ls that were aquifers for the upper Paleozoic migration of warm basinal brines that produced regional hydrothermal dolomite and world class MVT Pb-Zn deposits.

At Pea Ridge, previous workers identified 320-530°C, 40-60 wt. % NaCl fluid inclusions that were trapped along the L+V+H curve at pressures of 80 to 380 bars, which correspond to hydrostatic depths of 700 to 3700 m. Fluid inclusions in REE breccias are cooler and more dilute. To constrain the source of salt in the hydrothermal systems, fluid inclusion extracts from 16 samples of magnetite, apatite, actinolite, quartz, hematite, pyrite, barite, fluorite, calcite, and epidote were analyzed by ion chromatography. Magnetite and pyrite with the highest Cl/Br have the highest K/Na, consistent with the observation of halite and sylvite in hypersaline inclusions in quartz. Na/Cl/Br systematics suggest salt was derived from 3 sources: magmatic (high Cl/Br in magnetite and pyrite); metamorphic (high Na/Cl in actinolite); and marine. Ca/Na and K/Na ratios are suggestive of mixtures between a marine and a high K+Ca end member.

Pea Ridge and Boss-Bixby have ion ratios that are similar to those from IOA and IOCG deposits in Kiruna, Sweden and Carajas, Brazil where magmatic and marine or basinal brines have been interpreted. Though the magmatic fluid signature is to be expected in this granite-rhyolite terrane, further work is needed to determine the origin of external fluids in each hydrothermal system (ie. meteoric or marine) and whether or not the marine signature is due to a Paleozoic overprint.