Genesis of iron ore in the Snelgrove Lake Area, Labrador Trough, Western Labrador (2015)

The Snelgrove Lake property, located in western Labrador, is underlain by the late&#13;\nProterozoic Sokoman Formation, the same geological formation that hosts economically&#13;\nsignificant iron ore deposits elsewhere in Labrador and Quebec. This project aims to&#13;\nbetter understand the origin of the iron formation, the controls on grade of the iron ore&#13;\nincluding whether or not there is secondary iron enrichment, and the ambient marine&#13;\nenvironment in the late Paleoproterozoic, including ocean redox conditions present during&#13;\nthe primary deposition of the iron formation.&#13;\nIron formations in the Snelgrove Lake area consist of chemically precipitated&#13;\njasper and Fe-rich sedimentary rocks that contain hard, metallic bluish-grey iron oxides,&#13;\nwith total Fe2O3 ranging from 22 – 64 wt%. On the basis of petrography, iron minerals&#13;\noccur predominantly as: 1) syngenetically deposited (primary) iron oxides and iron&#13;\nsilicates; 2) microplaty hematite (secondary and remobilized); and 3) euhedral magnetite&#13;\n(secondary). Despite textural features suggesting that microplaty hematite occurrences are&#13;\ndue to secondary Fe-mobilization, the lack of coarse-grained textures and structurally&#13;\ncontrolled mineralization argues against hydrothermal enrichment as the cause.&#13;\nFurthermore, the general absence of hydrous ores, the lack of light rare earth element&#13;\n(LREE) enrichment within the iron formation, and the preservation of most primary&#13;\nsedimentary textures argues against pre- or post-metamorphic supergene enrichment.&#13;\nEvidence within suggests that iron was locally remobilized by post-depositional&#13;\ndiagenetic fluids, which resulted in the alteration, modification and enrichment of original&#13;\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&#13;\nof silica cements. Those samples that received silica cement during early diagenesis&#13;\ncontain lower Fe grades, and show signs of having been less affected by compaction than&#13;\nthose that had no silica cement.&#13;\nThe geochemistry of the iron formation suggests various inputs from&#13;\nhydrothermal, detrital, and hydrogenous sources. Immobile element contents indicate the&#13;\nSnelgrove Lake iron formation has minimal detrital input, but that which is present was&#13;\nlikely derived from a mafic detrital source. Flat to weakly positive Eu/Eu* anomalies are&#13;\ncommon and suggest precipitation of iron minerals in the ocean from low temperature&#13;\nfluids (&lt; 250°C). The iron formations also have REE-Y signatures comparable to other&#13;\nlate Paleoproterozoic iron formations and have flat post-Archean average shale (PAAS)-&#13;\nnormalized patterns, and Y/Ho ratios ranging from 20-50. The Y/Ho ratios of late&#13;\nPaleoproterozoic iron formations have wider ranges and are generally lower in&#13;\ncomparison to early Paleoproterozoic iron formations and is interpreted to reflect the&#13;\nappearance of oxygen in the oceans at that time, and more precisely, the appearance of a&#13;\nredox-stratified ocean. The paleoredox-conditions present during the deposition of iron&#13;\nformation in Snelgrove Lake, as deduced from Ce/Ce* anomalies and other redoxsensitive&#13;\nelements, are generally indicative of an anoxic environment, while its bounding&#13;\nunits record signatures that reflect deposition in a more oxic environment, thus suggesting&#13;\ndeposition in a redox-stratified basin with iron precipitation near a redox-boundary.