PRECAMBRIAN BASEMENT OF GREENLAND
Geological mapping in the Nuuk region

Julie Hollis, Jeroen van Gool, Adam Garde, with Henrik Stendal and Peter Appel (Department of Economic Geology), and Allen Nutman (Australian National University), Simon Harley (University of Edinburgh, UK), and Ali Polat (University of Windsor, Canada).

GEUS currently has two major mapping projects in the Nuuk region:
1. Greenstone belts and structures in central Godhåbsfjord with relevance to gold mineralisation
2. The Kapisillit 1:100 000 geological map sheet (64 V.2 Syd)

Mapping in the Nuuk region will further our understanding of this geologically complex and important area of Archaean crust, specifically allowing us to characterise the formation and evolution of greenstone belts and major structures that may be important in the distribution of gold mineralisations.

The Nuuk region of West Greenland contains some of the world's oldest rocks. In large part because of its antiquity, advancements in our geological understanding of this area have been marked by controversy. However, these advancements have also stimulated the progress of our understanding of Archaean tectonic processes in general, and the origins of life on earth. This complex region is also the backyard to Greenland's capital and, in recent times, the focus of ongoing exploration for gold and other economic minerals.

The Nuuk region is located within the Archaean North Atlantic Craton. It comprises several tectono-stratigraphic terranes, each with their own magmatic, tectonic and thermal history. At present a total of six terranes are recognised, formed in the Early to Late Archaean (> 3600 Ma to c. 2800 Ma) and amalgamated in a series of events from the Mid to Late Archaean (c. 2950 to 2700 Ma), though the regional extent, geological histories, and amalgamation of these terranes is far from fully understood. In addition, there are large areas of the Nuuk region that remain largely un-mapped.

Greenstone belts and structures in central Godhåbsfjord with relevance to gold mineralisation

Greenstone belts throughout the Nuuk region have long been recognised as showing potential for economic mineralisations. A current exploration license is held by NunaMinerals A/S for a gold prospect on Storø. In addition, stratabound tungsten (scheelite) occurrences have been found in Mid to Late Archaean greenstone belts throughout the region. Furthermore, geochemical data outlining a c. 25 km wide, NNE-trending LILE enriched zone, coinciding with large regionally important structures and known gold prospects suggest the possibility of more extensive gold mineralisation throughout the region (Appel et al. 2003).

In July and August 2004 mapping was carried out in the central Godhåbsfjord region, spanning the largely Mid to Late Archaean Færingehavn and Akia terranes. Particular focus was placed on the boundary between these terranes - the Archaean Ivinguit fault, which may have been reactivated in the Palaeoproterozoic - and on other regionally important structures such as the Storø shear zone and the Palaeoproterozoic Kobbefjord fault. Regional geochemical data indicates these structures may be important in controlling the regional distribution of gold mineralisation. Investigation of the greenstone belts has revealed that they are markedly different in their lithostratigraphy, structure and metamorphic grade. A large sample set of representative lithologies from these belts have been collected for petrographic and geochemical studies. Major and trace element analysis of amphibolite and metasedimentary rock samples, currently underway, will be used to constrain the tectonic setting of formation of the different belts. A geochronological program has also been initiated to establish the timing of deposition of the sedimentary precursors to metasedimentary rocks and to constrain the timing of deformation and thermal history of important Archaean and Proterozoic structures.

GEUS will report initial findings to the Bureau of Minerals and Petroleum in Nuuk by the end of 2004, in a publication that will include detailed field maps for key areas, petrographic descriptions, geological interpretations, and an extensive preliminary analytical dataset.

The Kapisillit 1:100 000 geological map sheet (64 V.2 Syd)
In 2004 GEUS initiated the Kapisillit map sheet project, covering the area 49°00'W to 51°45'W and 64°00'N to 64°30'N. This is a three year project that will result in publication of the last 1:100 000 geological map sheet in the Nuuk region.

The Kapisillit map sheet area was earlier included in helicopter reconnaissance work for the Frederikshåb Isblink to Søndre Stromfjord 1:500 000 geological map sheet. Detailed work has been conducted in small parts of this region by Allen Nutman, Clark Friend, John Myers and co-workers over the past three decades. However, large parts of this region remain essentially unknown, e.g. the boundaries of tectonostratigraphic terranes in the Kapisillit area, and indeed it is possible that there are additional complexities not yet recognised. A major goal will be to address the nature of a major NW-trending aeromagnetic anomaly that cuts across the middle of the map sheet area. Another significant goal will be to map and characterise a large greenstone belt in the southern part of the map, which is poorly known but larger in area than the Ivisartoq belt. The mapping will build on continuing advancements in our understanding of the evolution of the North Atlantic Craton and Archaean crustal processes in general. In addition mapping will contribute to the understanding of the economic mineral prospectivity in this largely unexplored region.

In June and July 2004 boat-based reconnaissance mapping was undertaken along the Kapisillit and Ameralik fjords. Samples collected during the fieldwork will be used in petrographic studies of the tectonometamorphic history. A large number of samples have been collected for a planned geochronological program designed to elucidate the timing of magmatism and metamorphism, and deposition of precursors to metasedimentary rocks. Sample preparation for this analytical work is underway and reconnaissance LA-ICPMS and SHRIMP analytical work is planned for early 2005.

PhD students from the University of Edinburgh and the Australian National University have been recruited for involvement in work related to the mapping program. It is expected that the map sheet will be published in late 2007.