Genesis of iron ore in the Snelgrove Lake Area, Labrador Trough, Western Labrador

Lachance, Nicolas (2015) Genesis of iron ore in the Snelgrove Lake Area, Labrador Trough, Western Labrador. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Snelgrove Lake property, located in western Labrador, is underlain by the late Proterozoic Sokoman Formation, the same geological formation that hosts economically significant iron ore deposits elsewhere in Labrador and Quebec. This project aims to better understand the origin of the iron formation, the controls on grade of the iron ore including whether or not there is secondary iron enrichment, and the ambient marine environment in the late Paleoproterozoic, including ocean redox conditions present during the primary deposition of the iron formation. Iron formations in the Snelgrove Lake area consist of chemically precipitated jasper and Fe-rich sedimentary rocks that contain hard, metallic bluish-grey iron oxides, with total Fe2O3 ranging from 22 – 64 wt%. On the basis of petrography, iron minerals occur predominantly as: 1) syngenetically deposited (primary) iron oxides and iron silicates; 2) microplaty hematite (secondary and remobilized); and 3) euhedral magnetite (secondary). Despite textural features suggesting that microplaty hematite occurrences are due to secondary Fe-mobilization, the lack of coarse-grained textures and structurally controlled mineralization argues against hydrothermal enrichment as the cause. Furthermore, the general absence of hydrous ores, the lack of light rare earth element (LREE) enrichment within the iron formation, and the preservation of most primary sedimentary textures argues against pre- or post-metamorphic supergene enrichment. Evidence within suggests that iron was locally remobilized by post-depositional diagenetic fluids, which resulted in the alteration, modification and enrichment of original bedding with Fe-minerals. The current physical assemblage of iron-bearing minerals, and the local iron grades, were also significantly affected by the early diagenetic appearance of silica cements. Those samples that received silica cement during early diagenesis contain lower Fe grades, and show signs of having been less affected by compaction than those that had no silica cement. The geochemistry of the iron formation suggests various inputs from hydrothermal, detrital, and hydrogenous sources. Immobile element contents indicate the Snelgrove Lake iron formation has minimal detrital input, but that which is present was likely derived from a mafic detrital source. Flat to weakly positive Eu/Eu* anomalies are common and suggest precipitation of iron minerals in the ocean from low temperature fluids (< 250°C). The iron formations also have REE-Y signatures comparable to other late Paleoproterozoic iron formations and have flat post-Archean average shale (PAAS)- normalized patterns, and Y/Ho ratios ranging from 20-50. The Y/Ho ratios of late Paleoproterozoic iron formations have wider ranges and are generally lower in comparison to early Paleoproterozoic iron formations and is interpreted to reflect the appearance of oxygen in the oceans at that time, and more precisely, the appearance of a redox-stratified ocean. The paleoredox-conditions present during the deposition of iron formation in Snelgrove Lake, as deduced from Ce/Ce* anomalies and other redoxsensitive elements, are generally indicative of an anoxic environment, while its bounding units record signatures that reflect deposition in a more oxic environment, thus suggesting deposition in a redox-stratified basin with iron precipitation near a redox-boundary.

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