CONSTRAINING IRON FORMATION PRIMARY MINERALOGY USING FERRUGINOUS LAKE SEDIMENTS

Iron formations (IFs) are distinctive iron-rich sediments that contain important records of Precambrian redox conditions, geochemical cycling, and microbial activity. A longstanding question in the study of IFs concerns the nature of the primary mineral phases that led to their development. Candidates for primary phases include Fe³⁺ oxyhydroxides such as ferrihydrite, and Fe²⁺ phases such as greenalite (silicate) and siderite (carbonate). The rarity of modern analog systems favorable for iron-rich (≥15% Fe) sediment genesis contributes to the challenge of identifying viable primary IF phases. However, recent work has shown that ferruginous (enriched in dissolved Fe²⁺) lakes host biogeochemical processes that mimic some aspects of IF environments. Here we present new data from two recently described ferruginous lakes: Canyon Lake, Michigan and Brownie Lake, Minnesota. Canyon Lake is in a wilderness setting in the Upper Peninsula of Michigan, while Brownie Lake located in Minneapolis is nutrient and chloride impacted. We examined particulate phases collected from sediment traps, surface sediment cores, and water filters from both lakes. We employed bulk analysis using both X-ray diffraction (XRD) and fluorescence (XRF), and microanalysis by scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDS). Neither lake contains surface sediments that are iron-rich by conventional standards. At Canyon Lake, sediment iron content gradually increased from 8.2% Fe₂O₃ at the surface to 14.1% Fe₂O₃ at ~50 cm depth. The iron-enriched portions of the Canyon Lake sediment core returned XRD patterns consistent with siderite (FeCO₃), and SEM-EDS analysis revealed ~5-µm sized siderite crystals in egg- to bowtie-shaped morphologies. At Brownie Lake, bulk sediments range from 7.45-9.19% Fe₂O₃, but there is a pronounced dilution of chemical precipitates from a high flux of siliciclastic materials. Under SEM, iron-rich Brownie Lake phases appear to be associated with carbonates and potentially phosphates, but identifying authigenic iron minerals in this system is more challenging. However, the evidence from Canyon Lake implies that carbonates may represent a major driver of iron burial in ferruginous systems without requiring the deposition of a precursor iron oxide phase.