CHEMICAL EVIDENCE FOR THE MODERN DEPOSITION OF IRON FORMATIONS AT PAULINA LAKE, NEWBERRY VOLCANO (OR)

Paulina Lake (PL) at Newberry Volcano (OR, USA) is an 80 m deep, 2.5 km wide bowl-shaped lake with an outlet and subaqueous hot springs. PL water is slightly basic, rich in HCO₃- (~400 ppm) and dissolved silica (~20 ppm), suggestive of carbonate-rich hot spring inputs. The most unusual aspect of PL sediment is the high concentrations of Fe and Si, similar to ancient Banded Iron deposits. Sediment consists of hydrothermal silica, opal diatom frustules, and Fe-oxides, with elevated concentrations of Phosphorus, Arsenic, Manganese, and Vanadium.

Sediment cores PL1 (2.6 m) and PL2 (5 m), provide a >3000-year record of sediment chemistry, suggesting variable rates of hydrothermal input over time. Tephrochronology and 210Pb data provide sediment mass accumulation rates over the last 1300 years. WD-XRF and ICP-MS analyses show that the sediment contains up to 14% Fe, 65-70% SiO₂, along with spatially varying amounts of volcanic ash, organic matter, and minor carbonates. Sediments deposited at 2000-2600 BP have higher concentrations of Fe-P-Mn-As, which we interpret as a period of enhanced hydrothermal input.

The hydrothermal elements are separated into three groups: 1. Conservative elements that enter the lake and are flushed out through the outgoing river, 2. Elements such as Fe, As, P, Mn, and Si that are poorly soluble in the cold oxygenated lake water, and form chemical precipitates in the sediment, and 3. Elements that upon entering the lake are taken up by biota, then incorporated as biogenic components in the sediment (C, Si, P). Iron is brought in as Fe²⁺ in the hot spring fluids, and precipitates upon mixing with lake water as hydrous ferric oxides, scavenging P, As and V from the surrounding fluids. After burial, oxides are reduced with organic matter, and Fe re-precipitates in mixed Fe-oxides and vivianite. The lake-wide deposition of Fe is on the order of 40-70 tonnes per year, increasing to >100 tonnes per year during the peak hydrothermal period. Modern silica hydrothermal inputs are ~ 550 tonnes/yr, based upon ash corrected-mass accumulation rates for each core and dissolved Si in the outlet discharge. The high concentrations of Fe and Si in the sedimentary-volcanic complex of Paulina Lake indicates the modern formation of an iron ore body similar to Pre-Cambrian Banded Iron Formations.