Developing a 3-D model for the Skaergaard intrusion in East Greenland: constraints on structure, mineralisation and petrogenetic models

ied layered gabbro intrusion in the world (Wager & Deer 1939; Wager & Brown 1968; McBirney 1996; Nielsen 2004). The intrusion is c. 54.5 Ma old and was formed during the Palaeogene opening of the North Atlantic Ocean, intruding into the base of the East Greenland flood basalts. The intrusion is relatively small with a volume of c. 300 km3 (Nielsen 2004). Spectacular magmatic layering and systematic evolution in the compositions of liquidus phases and estimated melt compositions (e.g. Wager & Brown 1968) have made the intrusion the most studied example of the development of the ‘Fenner trend’ of iron enrichment in basaltic liquids (e.g. Thy et al. in press; Veksler in press). The identification in the late 1980s of significant platinum-group elements (PGE) and gold (Au) occurrences in the intrusion (e.g. Bird et al. 1991; Nielsen et al. 2005) has led to continued investigation and exploration drilling. The Skaergaard intrusion is suggested to hold c. 33 million ounces (1000 tonnes) of PGE and c. 13 million ounces (400 tonnes) of Au (Nielsen et al. 2005). The mineralised zone is located in a c. 100 m thick zone of anomalous PGE and Au enrichment in the upper part of the Middle zone (Bird et al. 1991; Nielsen et al. 2005) of the Layered series. The mineralised zone consists of a succession of bowl-shaped, stratiform and very tightly controlled levels of palladium (Pd) enrichment referred to as Pd1 to Pd5 (Fig. 2; Nielsen et al. 2005). The bottom level, Pd5, is developed from margin to margin of the intrusion, whereas the overlying levels Pd4 to Pd1 are increasingly restricted in width, and the entire succession of Pd levels is only developed in the central part of the intrusion. The structure of the mineralised zone can be compared to a set of bowls with upward-decreasing diameters. Gold is always concentrated in the uppermost palladium levels or in a level above the top palladium level, irrespective of the number of developed Pd levels. More detailed descriptions are provided by Nielsen et al. (2005). The exploration drill cores provide material and structural information from previously inaccessible parts of the intrusion (Nielsen et al. 2005). The 3-D image presented in Fig. 4 is based on drill-core information (petrographical, petrophysical, geochemical etc.) and surface information. It allows an unprecedented insight into the internal structure of the upper part of the intrusion and offers a possibility of refinement of volume estimates and quantitative modelling of the Developing a 3-D model for the Skaergaard intrusion in East Greenland: constraints on structure, mineralisation and petrogenetic models