Paul Bielski MSc Thesis Abstract
Differences between deeper water and shallow water iron-formation (IF) deposited during the Archean are poorly understood. Sedimentology, bulk rock geochemistry combined with laser ablation geochemistry, X-ray fluorescence false-colour mapping and 34S isotopes were utilized to compare a variety of IFs deposited at different water depths. These are the deeper water Morley sulfides, Deloro, Temagami, and Soudan IFs; the intermediate depth IF at Red Lake; and the shallow water Lake St. Joseph and Beardmore IFs. The Morley sulfides are massive to laminated sulfides associated with hydrothermal venting deposited at depth. The Deloro IF contains both oxide-facies IF (OIF) and sulfide-facies IF (SIF) and was deposited within a deeper water volcanic assemblage. The Temagami IF is entirely OIF sitting atop volcanics and banded chert with overlying siliciclastic rock. The Soudan IF is located within the Ely greenstone containing both volcanics and siliciclastics. The intermediate depth IF at Red Lake was deposited on the basinward slope of the oldest known carbonate platform on Earth. The shallow water IFs at Lake St. Joseph and Beardmore were deposited within deltaic successions, associated with the distributary channel mouth bars and proximal prodeltas.
The Morley massive and laminated sulfides have the same REE(PAAS) patterns as overlying mudstones, indicating that REE signatures within these sulfides are derived from what was scavenged on clay minerals. Small negative Y anomalies are present in the sulfides and mudstones, with the resulting Y/Ho ratios almost exclusively subchondritic (<28). 34S isotopes of the Morley and nearby sulfide deposits show heavier 34S values in laminated and mudstone associated sulfides compared with the lighter, hydrothermal-magmatic range of 34S within the massive sulfides, indicative of inorganic sulfate reduction deep in the Archean ocean.
The deeper water IF of the Deloro, Temagami, and Soudan are similar geochemically but vary greatly in jasper content and magnetite layer thickness. REE(PAAS) patterns are HREE enriched, containing large Eu and Y anomalies in each IF, with Y/Ho ratios from 28 to 55. Enrichments in V and low Th/U ratios are present in each deeper water IF, indicating scavenging of V and U from at least somewhat oxidizing sub-aerial weathering conditions and adsorption of these elements onto the iron-hydroxides in the deep ocean. P/Fe ratios range from 0.001 to 0.006, overlapping with shallow water IF but greater than the Morley sulfides by up to a factor of 30. Magnetite within these deeper water IFs appears to contain trace amounts of Ti (<0.1% TiO2) within its structure, which was most likely supplied by detrital material. δ34S values within the Deloro SIF are very light, suggesting biologic reduction of sulfate rather than a reduced hydrothermal source of the sulfur. However, SIF at Temagami shows 34S values similar to the Morley massive and laminated sulfides, indicating a dominantly hydrothermal source of S.
The IF present at Red Lake was deposited at intermediate depths of a few hundred meters yet resembles deeper water IF with HREE enriched REE(PAAS) patterns and similar ranges for Y anomalies. However, Eu anomalies are smaller by up to a factor of 2 and there are some samples with larger Y/Ho ratios (up to 75) than the highest deeper water values. V and Cr appear enriched within the Red Lake IF and Th/U ratios are lower than in both deep and shallow IFs. P/Fe ratios are generally much lower than the deep and shallow water IF, owing to P sequestration from the water into organisms on the carbonate platform.
The Beardmore and Lake St. Joseph IFs show flat REE(PAAS) patterns with very small Eu and few Y anomalies, due to significant amounts of siliciclastic contamination. Y/Ho ratios are near chondritic due to this as well. Laser ablation REE(PAAS) show Beardmore to have a HREE enriched signatures with small Eu anomalies. P/Fe ratios are comparable to deeper water.
The major difference between deep and shallow IF is chert content, which is much greater in deeper water IF. REE(PAAS) patterns are similarly HREE enriched at all depths but this is masked by siliciclastic contamination in the shallow water IF. Eu anomalies decrease from deep to shallow water, while Y/Ho ratios appear highest in intermediate depths. Redox elements such as V, Cr, and U appear enriched in deep and intermediate IFs, rather than in shallow IFs.