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

The Snelgrove Lake property, located in western Labrador, is underlain by the late
\nProterozoic Sokoman Formation, the same geological formation that hosts economically
\nsignificant iron ore deposits elsewhere in Labrador and Quebec. This project aims to
\nbetter understand the origin of the iron formation, the controls on grade of the iron ore
\nincluding whether or not there is secondary iron enrichment, and the ambient marine
\nenvironment in the late Paleoproterozoic, including ocean redox conditions present during
\nthe primary deposition of the iron formation.
\nIron formations in the Snelgrove Lake area consist of chemically precipitated
\njasper and Fe-rich sedimentary rocks that contain hard, metallic bluish-grey iron oxides,
\nwith total Fe2O3 ranging from 22 – 64 wt%. On the basis of petrography, iron minerals
\noccur predominantly as: 1) syngenetically deposited (primary) iron oxides and iron
\nsilicates; 2) microplaty hematite (secondary and remobilized); and 3) euhedral magnetite
\n(secondary). Despite textural features suggesting that microplaty hematite occurrences are
\ndue to secondary Fe-mobilization, the lack of coarse-grained textures and structurally
\ncontrolled mineralization argues against hydrothermal enrichment as the cause.
\nFurthermore, the general absence of hydrous ores, the lack of light rare earth element
\n(LREE) enrichment within the iron formation, and the preservation of most primary
\nsedimentary textures argues against pre- or post-metamorphic supergene enrichment.
\nEvidence within suggests that iron was locally remobilized by post-depositional
\ndiagenetic fluids, which resulted in the alteration, modification and enrichment of original
\nbedding 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
\nof silica cements. Those samples that received silica cement during early diagenesis
\ncontain lower Fe grades, and show signs of having been less affected by compaction than
\nthose that had no silica cement.
\nThe geochemistry of the iron formation suggests various inputs from
\nhydrothermal, detrital, and hydrogenous sources. Immobile element contents indicate the
\nSnelgrove Lake iron formation has minimal detrital input, but that which is present was
\nlikely derived from a mafic detrital source. Flat to weakly positive Eu/Eu* anomalies are
\ncommon and suggest precipitation of iron minerals in the ocean from low temperature
\nfluids (< 250°C). The iron formations also have REE-Y signatures comparable to other
\nlate Paleoproterozoic iron formations and have flat post-Archean average shale (PAAS)-
\nnormalized patterns, and Y/Ho ratios ranging from 20-50. The Y/Ho ratios of late
\nPaleoproterozoic iron formations have wider ranges and are generally lower in
\ncomparison to early Paleoproterozoic iron formations and is interpreted to reflect the
\nappearance of oxygen in the oceans at that time, and more precisely, the appearance of a
\nredox-stratified ocean. The paleoredox-conditions present during the deposition of iron
\nformation in Snelgrove Lake, as deduced from Ce/Ce* anomalies and other redoxsensitive
\nelements, are generally indicative of an anoxic environment, while its bounding
\nunits record signatures that reflect deposition in a more oxic environment, thus suggesting
\ndeposition in a redox-stratified basin with iron precipitation near a redox-boundary.

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