Summary:
Nalunaq is located in South Greenland and is a small high gold-grade deposit, which for the majority of its operational life was Greenland's only metalliferous mine. Gold is hosted in narrow quartz veins which are cross-cut by late-stage faults. Gold-quartz veins are hosted by fine-and - medium grained amphibolite of the Nanortalik Nappe. Detailed petrographic and geochronological studies have revealed a multi-stage hydrothermal alteration system, with alteration pre-and post-dating gold mineralisation. The hydrothermal alteration records a transition from upper-amphibolite facies to lower greenschist facies metamorphic conditions, over a period of 40 million years. The development of the hydrothermal alteration can be correlated to deformation events in the wider Palaeoproterozoic Ketilidian Orogen. Petrographic thin section analysis has shown that sulphidation, saussuritization and hydrolysis reactions are important in locally precipitating gold, which may suggest an Au(HS)2- transport complex for gold. A c. 20 cm wide biotite-arsenopyrite alteration zone surrounds the gold-quartz veins locally, and denotes areas of high-gold grades.
Implicit 3-Dimentional modelling has revealed structural controls on gold distribution, where the highest gold-grade sections of the deposit are located in steeper sections of the gold-quartz vein. Gold is concentrated into three ore shoots plunging 20-25° SW. The steep sections of the gold-quartz veins occur along boundaries between the fine- and-medium grained wall rock amphibolite. Late-stage faults have haloes of low gold-grades, which may indicate that gold remobilisation has occurred.
Stable oxygen, hydrogen and carbon isotope analysis suggests that the fluids which formed the early amphibolite alteration have a crustal source, the fluids which formed the gold-quartz veins have an orogenic source and the fluids which form the greenschist facies alteration have a meteoric source. The greenschist facies fluids are contemporaneous with late-stage faults, which may have acted as conduits for surface fluids. Sulphur isotope analyses indicate a crustal source for sulphur in the deposit.