Until recently, in situ U-Pb zircon geochronology could be carried out only using ion microprobes, requiring lengthy analysis times of c. 20 minutes. However, new developments in laser ablation inductively coupled plasma mass spectrometer technologies have resulted in zircon geochronology techniques that are much faster, simpler, cheaper, and more precise than before (e.g. Frei et al. 2006, this volume). Analyses approaching the precision obtained via ion microprobe can now be undertaken in 2–4 minutes using instruments such as the 213 nm laser ablation (LA) system coupled with Element2 sector-field inductively coupled plasma mass spectrometer (SF-ICP-MS) housed at the Geological Survey of Denmark and Greenland (GEUS). The up to tenfold decrease in analytical time means that zircon geochronology can now be used in a much wider range of studies. The Godthåbsfjord region, southern West Greenland, contains some of the oldest rocks exposed on the Earth’s surface reflecting a very complex Archaean geological evolution (Figs 1, 2). Over recent years GEUS has undertaken a range of mapping projects at various scales within the Godthåbsfjord region (see also below). These include the mapping of the 1:100 000 scale Kapisillit geological map sheet (Fig. 1), and regional and local investigations of the environments of formation and geological evolution of supracrustal belts, hosting potentially economic mineral occurrences. Zircon geochronology is an important tool for investigating a range of geological problems in this region. By breaking down the complex geology into a series of simple problems that can be addressed using this tool, the geological evolution can be unlocked in a stepwise manner. Three examples are presented below: (1) the mapping of regional structures; (2) characterising and correlating supracrustal belts; and (3) dating metamorphism and mineralisation. Although focus is on the application of zircon geochronology to these problems, it is important to note that the resulting data must always be viewed within a wider context incorporating geological mapping and structural, geochemical and petrographic investigations.

A gold prospect on central Storø in the Nuuk region of southern West Greenland is hosted by a sequence of intensely deformed, amphibolite facies supracrustal rocks of late Mesoto Neoarchaean age. The prospect is at present being explored by the Greenlandic mining company NunaMinerals A/S. Amphibolites likely to be derived from basaltic volcanic rocks dominate, and ultrabasic to intermediate rocks are also interpreted to be derived from volcanic rocks. The sequence also contains metasedimentary rocks including quartzites and cordierite-, sillimanite-, garnet- and biotite-bearing aluminous gneisses. The metasediments contain detrital zircon from different sources indicating a maximum age of the mineralisation of c. 2.8 Ga. The original deposition of the various rock types is believed to have taken place in a back-arc setting. Gold is mainly hosted in garnet- and biotite-rich zones in amphibolites often associated with quartz veins. Gold has been found within garnets indicating that the mineralisation is pre-metamorphic, which points to a minimum age of the mineralisation of c. 2.6 Ga. The geochemistry of the goldbearing zones indicates that the initial gold mineralisation is tied to fluid-induced sericitisation of a basic volcanic protolith. The hosting rocks and the mineralisation are affected by several generations of folding.

This contribution presents a 3D assessment of metamorphosed and deformed, hydrothermally altered volcanic rocks, hosting the massive sulphide deposits of the Kristineberg area in the 1.9 Ga Skellefte mining district in northern Sweden, using six calculated alteration parameters: the Ishikawa alteration index, the chlorite-carbonate-pyrite index and calculated net mass changes in MgO, SiO 2 , Na 2 O and Ba. The results, which are also available as film clips in the Supplementary data, confirm inferences from geological mapping; namely that the sericite-and chlorite-rich alteration zones have complex and cross-cutting geometries and that most of these zones are semi-regional in extent and range continuously from surface to over a kilometre deep. The major known massive sulphide deposits occur proximal to zones characterised by coincidence of high values for the alteration index and chlorite-carbonate-pyrite index and large MgO gains, which corresponds to zones rich in magnesian silicates. These zones are interpreted as the original chlorite-rich, proximal parts the alteration systems, and form anomalies extending up to 400 m away from the sulphide lenses. In addition, the stratigraphically highest VHMS are hosted by rocks rich in tremolite, talc, chlorite and dolomite with lesser clinozoisite, which have high chlorite-carbonate-pyrite index and low-medium alteration index values, reflecting a greater importance of some chlorite-carbonate alteration at this stratigraphic level. Vectoring towards massive sulphide deposits in this area can be improved by combining the AI and CCPI indexes with calculated mass changes for key mobile elements. Of the ones modelled in this study, MgO and SiO 2 appear to be the most useful.

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