The geology, mineralization, geochemistry, and metallogeny of the Paleoproterzoic Ramah Group, northern Labrador

The Paleoproterozoic (ca. 2000 Ma) Ramah Group represents a relatively undeformed cover sequence developed on the Nain craton, northern Labrador. The group is 1.7 km thick, 65 km long and up to 16 km wide, and is most extensive between Nachvak and Saglek Fiords. Six formations make up the group, of which the two basal formations represent a shallow shelf sequence, while the upper four represent deep marine sequences. The dolomite unit capping the shelf sequence is metallogenically the most interesting as it hosts the majority of base metal mineralization present within the Ramah Group. -- Two types of metallic mineralization are represented within the Ramah Group, namely syngenetic and epigenetic. The syngenetic mineralization formed due to rapid basin deepening which resulted in the formation of anoxic bottom waters. These bottom waters were enriched in sulphur-reducing bacteria which led to the precipitation of a two metre thick pyrite bed, and to the consequential ³⁴S enrichment of the remaining seawater in the basin. Further sedimentation led to the trapping of this ³⁴S-enriched water as a pore fluid within shales of the Nullataktok Formation. Following the deposition of the sedimentary formations, the Ramah Group was intruded by numerous gabbro-diabase sills which resulted in the formation of Cu and Ni mineralization within or adjacent to the sills. -- The main metallogenic event to affect the Ramah Group resulted from the collision of the Nain and Rae Structural Provinces during the Torngat Orogeny (1860 Ma). The collision caused the early migration of hydrocarbons, followed by large volumes of evolved basin brines. These brines contained metal chloride complexes, and ³⁴S-enriched pore water (in the form of dissolved sulphates) which migrated from deeper parts of the group to the basin margins. Precipitation of sulphides occurred in areas of increased porosity, such as collapse breccia zones, within the Reddick Bight Dolomite Member when the expelled sulphate and metal rich fluids mixed with resident hydrocarbons. Further epigenetic mineralization was formed due to remobilization of sulphides within fault zones as the Torngat Orogen deformation front progressed eastwards. -- Mineralization within the Reddick Bight Dolomite Member is analogous to the Mississippi Valley-type (MVT) and as such represents the second oldest occurrence of this style of mineralization identified to date