OF THE SOKOMAN FORMATION IN THE GABBRO LAKE AREA , EASTERN LABRADOR TROUGH

The Gabbro Lake area is situated in the southeastern portion of the Labrador Trough (NTS map area 23H/11), close to the boundary of the Churchill and Grenville provinces. Previous geological mapping in the area has been hampered by poor outcrop, coupled with a complex structural and metamorphic history. This study presents data from detailed re-logging of exploration drillcore, combined with petrographic analysis (including SEM‒MLA) and whole-rock geochemical data, which provide new insights into the depositional history of the Sokoman Formation in the Gabbro Lake area. The Sokoman Formation in the study area has four main facies: magnetite-rich banded iron formation (MBIF); magnetite-bearing mudstones (MM); hematite-rich granular iron formation (HGIF); and non-magnetic iron-rich siltstones and shales at the base of the Sokoman Formation. The Sokoman Formation also contains numerous interbedded tuffaceous units, which have been subdivided into Fe-rich and Ca-rich units, based on their petrographic and geochemical characteristics. The MBIF and MM are the main oxide-bearing facies in the Sokoman Formation, with HGIF restricted to thin (<5 m) intervals near the top of the formation. In contrast to the Sokoman Formation on the western margin of the Labrador Trough, which is predominantly composed of granular iron formation interpreted to have been deposited in a nearshore environment, the MBIF and MM in the Gabbro Lake area were likely deposited as chemical muds in deeper waters below the storm wave base, in either an outer shelf or slope-break setting. The Sokoman Formation in the Gabbro Lake area is underlain by sandstones, conglomerates, breccias and graphitic mudstones, which are interpreted as deep-water equivalents of the Wishart Formation, and were previously unrecognized in the southeastern Labrador Trough. Geochemical analysis show that the MBIF and MM from the Gabbro Lake area have higher concentrations of Al2O3, K2O, TiO2, P2O5, LREE and selected trace elements compared with the HGIF and with data from the Sokoman Formation in the western part of the Labrador Trough. Petrographic studies and geochemical comparisons of tuffaceous units indicate that these higher concentrations are due to an input of volcaniclastic detritus during the deposition of the iron formation. There are possibly multiple sources of this volcaniclastic material, with the Fe-rich tuffaceous units likely being related to the Nimish Formation volcanic rocks to the north, and the Ca-rich tuffaceous units derived from an unknown volcanic centre located to the south or east.

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