Origins of stratiform and stratabound Fe–Cu–Zn horizons in the Lower Proterozoic Moran Lake Group, Labrador Central Mineral Belt

Abstract

Zn, Cu, and Fe are concentrated as stratiform and stratabound sulphide-rich beds in the Lower Proterozoic Warren Creek Formation, Moran Lake Group, central Labrador. Upper Member sedimentary rocks have a hydrothermal-like Fe enrichment but a dominantly hydrogenous signature as indicated by high Al2O3 relative to SiO2, and high Al and Fe relative to Mn. The Upper Member shales and sulphide-rich beds were deposited as Fe-rich pelagic sediments. The paucity of Mn and abundance of Fe in typical shale samples and lack of Cu, Pb, and Zn fractionation in stratiform massive sulphide beds that contain up to 4702 ppm Zn, 533 ppm Cu, and 15 ppm Pb suggest that deposition occurred in restricted brine pools (i.e., Cu and Zn were precipitated rapidly and were not fractionated). Stratabound sphalerite mineralization containing > 3.7% Zn and 121 ppm Cu (but no Pb) was deposited in a porous lithology at the top of the Warren Creek Formation and represents a unique style of metal concentration.The stratiform deposits probably formed by advection of low-temperature connate waters in a situation typical of sediment-hosted exhalative mineralization (SEDEX). The potential for ore-grade metal concentration is apparently low because metal associations (Fe,Cu,Pb,Zn,Ba) are unlike those of sediment-hosted massive sulphide deposits, the sediments have a dominantly hydrogenous rather than hydrothermal signature, and the absolute grades of known occurrences are very low. The stratabound Zn deposit was probably formed by converting Zn-rich brines (≤ 200 °C) trapped during development of a hydrothermal convection system during a period of increased geothermal gradient. The potential for this type of occurrence in the Warren Creek area to reach economic grade is limited because the convection cells were shallow, ephemeral, and without the metal associations of sediment-hosted massive sulphide deposits.