PAULINA LAKE, NEWBERRY VOLCANO, OR AS A MODERN ANALOG FOR PRECAMBRIAN IRON FORMATIONS

Iron Formations (IF) in cratonic rocks of ~ 3.7 Ga to 1.8 Ga in age are chemical precipitates of Fe and Si oxides from anoxic ocean waters with hydrothermal inflows. Paulina Lake (PL) is a volcanic lake in the Newberry Volcano caldera, OR, with hydrothermal inputs. PL lake water has ~400 ppm HCO₃^-, pH~8, with Ca, Mg, Na, K and Si at a few to several tens of ppm. The lake water is well oxygenated, and has very low dissolved Fe and other metals (few ppb). Subaqueous, dispersed, ephemeral hydrothermal seeps are covered by probable Fe-oxidizing bacteria in flocs and mats. Lake sediment samples from two long piston cores and grabs were analyzed by XRF and ICP-MS. The sediment consists of >90% SiO₂ and Fe-oxides, with variable amounts of C_org (2-8 %) and dispersed rhyolitic tephra. Fresh lake sediments are olive green to blue/black, with some samples rich in green vivianite, a ferrous phosphate. Silica occurs in geyserite and diatom skeletons. After correction for tephra contributions and normalizing on a volatile free basis, PL sediments have 4-19 % Fe, 30-40 % Si, 50-600 ppm As, 30-100 ppm Co, 100-250 ppm Ni, and 50-200 ppm V, with up to 1 % P and Mn, and thus are rich in these “hydrothermal trace elements”. The REE patterns are dominated by tephra, but low-tephra samples show a positive Eu anomaly and Y/Ho above crustal values. Sorption processes on hydrous ferric oxides (HFO) are commonly thought to influence Y/Ho. We propose that sediment accretion in PL dominantly occurs just below the sediment-water interface, where HFO precipitate through bacterial Fe²⁺ oxidation, with subsequent adsorption of hydrothermal elements; geyserite forms upon cooling. This assembly is buried by younger lake sediment that includes diatom silica. The HFO redissolve in the reducing sediment matrix, and the final diagenetic mineral phases include vivianite and the poorly constrained, mixed-valency hydrous Fe oxides. The PL sediment has lower Fe/Si than average IF and shows stronger enrichments in the hydrothermal elements. Its REE patterns are similar to those from Archean IF, including the +Eu anomaly, high Y/Ho, and low LREE/HREE. The comparable REE patterns suggest similar element pathways with HFO interactions, although PL is an oxygenated lake with a thriving ecosystem versus the anoxic archean oceans.