Mineralogical characterisation and process mineralogy of goldbearing rocks from the Nalunaq gold deposit, Greenland

Abstract

The Nalunaq deposit is a shear-zone hosted gold deposit, characterised by high grades (up to 5240 g/t Au over 0˙8 m). The host rocks are Palaeoproterozoic metamorphosed mafic rocks in the Nanortalik district in South Greenland. As a generic type, the Nalunaq gold mineralisation is a mesothermal vein-type gold deposit, hosted in amphibolitefacies metamorphosed mafic rocks. The gold is associated with sheeted quartz veins, hosted in a large-scale shear structure, which appears to relate to regional thrusts. They vary in width between 0˙05 m and 1˙8 m and form a relatively continuous structure. However, possibly due to extensive post-mineralisation deformation, there is no simple relationship between gold grade and thickness of the quartz veins. Eight samples were examined from amphibolites (host rock), quartz veins and amphibolites cut by quartz veins. The mineralisation is a low-S system, and consists mainly of pyrrhotite, löllingite, pyrite, arsenopyrite, chalcopyrite, gold and electrum, scarce Bi-sulphosalts, covellite and galena, with gangue minerals dominated mainly by amphibole, plagioclase feldspars, quartz and clinopyroxene. Most of the gold grains occur predominantly as inclusions in silicate minerals (72–99˙9%), interstitial to silicates (0–27˙5%), inclusions in sulphides that are locked in silicates (0–6˙7%), and inclusions in sulphides (0–2˙9%). Preliminary upgrading of gold by heavy media mineral separation in one quartz vein sample revealed that gold primarily occurs in free form (97%). Mineralogical information on gold provides significant information on gold-bearing grains, recognition of the different phases, grain size and associations, important in both exploration and gold recovery.