GEOPHYSICAL IDENTIFICATION OF MAFIC-ULTRAMAFIC INTRUSIONS IN PLUME CENTER REGIONS

The importance of Large Igneous Provinces (LIPs) has been increasingly recognized in recent years as studies suggest a link between LIPs and layered mafic-ultramafic intrusions that host valuable Ni-Cu-PGE deposits. Furthermore, it has been proposed that layered intrusions located within a few hundred kilometers of the plume center are the most prospective mining targets.

A protocol for identifying unexposed layered intrusions in the vicinity of plume center regions is proposed. Intrusions are often associated with prominent gravity and magnetic anomalies. Those in plume center regions are of particular interest in that they can be postulated to be linked to that plume center and the related LIP event. A large number of plume centers have been identified in recent years at the focus of giant radiating dyke swarms, which provides an opportunity to use geophysics to search numerous plume center regions for associated layered mafic-ultramafic intrusions.

The availability of the Earth Gravitational Model (EGM2008) from GOCE satellite data and the Earth Magnetic Anomaly Grid (EMAG2) from satellite, ship and airborne measurements has made a global survey possible. We assessed the plume center regions of 17 LIP events. The 1270 Ma Mackenzie LIP, with a plume center near Victoria Island in northern Canada, is associated with prominent sub-circular positive gravity (+40 to +70 mgal) and magnetic anomalies that circumscribe the plume center (at a distance of 100 – 200 km), one of which is linked to the Ni-Cu-PGE mineralized Muskox layered intrusion (Baragar et al., 1996, J. Petrol., 37: 317–359). Other plume center regions (e.g., 1.37 Ga Lake Victoria plume center, east Africa) exhibit a distribution of geophysical anomalies with potential similarities to that of the Mackenzie LIP. In addition, we recognize LIPs with anomalies distributed along associated rifts (e.g., 65 Ma Deccan Traps, India) and isolated sub-circular anomalies within a few hundred kilometers of the plume center (e.g., 130-90 Ma High Arctic LIP; 1890 Ma Southern Bastar-Cuddapah LIP, India).

These geophysical anomalies are being studied to determine their depth and composition based on joint modelling of gravity and aeromagnetic data. Those that are shallowly emplaced may represent new targets for mineral exploration.