Sediment and shear-hosted gold mineralization of the Tartoq Group supracrustals, southwest Greenland

Abstract

Abstract Gold mineralization occurs in the Archaean Tartoq Group supracrustals, which are dominated by greenschist-amphibolite facies metamorphosed mafic rocks. Although deformed, recognizable features of the stratigraphy (pillowed lavas, intercalated sills and interflow sediments), suggest shallow to deep sea-floor deposition. Large, intensely carbonatized structural zones cut through the greenstones, subparallel to regional layering, and postdating major folding. They are interpreted as major shear zones and are characterized by internal ductile deformation due to high fluid pressures with brittle deformation of peripheral zones. Erratic high-grade gold mineralization is associated with quartz-pyrite lenses and veins that occur subparallel to layering within the shear zones. Moderate, but more consistent values are found in more extensive pyrite-arsenopyrite-quartz layers, also subparallel to layering. These lenses and layers occur adjacent to, or within the major carbonatized shear zones. Microscopically, gold is found as small (10 μm) grains in both pyrite and arsenopyrite. Low, but consistently anomalous grades of gold are also found in strongly contorted banded, muscovite-rich, quart-zpyrite rocks. These are thought tobe thin, chemically deposited interflow sediments within the greenstone pile, analogous to oxide-facies “banded iron formation” which is found nearby. Metamorphism has obscured critical textures, but the gold was either chemically deposited along with exhalative silica and sulphides, or may have been introduced later, during deformation and alteration of primary BIF. The Tartoq Group rocks are similar in most respects to those of other Archaean greenstone belts, being distinguished only by their excellent outcrop exposure. Current models for the origin of Archaean gold deposits are adequate to explain the Tartoq gold mineralization. A multistage model of gold concentration is proposed. Gold is initially leached from mafic volcanics (and possibly underlying gneisses) and redeposited at anomalous levels in chemical sediments. These may be further enriched by sulphidation reactions. High-grade gold mineralization is typically epigenetic in style, where very large volumes of fluid are channeled through shear zones undergoing brittleductile deformation.