Geophysical Interpretation of Mantle Magmatism in the Seiland Province: the Geometry of the Reinfjord Ultramafic Complex

Abstract:

The Seiland Igneous Province (SIP) is the largest complex of mafic and ultramafic intrusions in northern Fennoscandia, with an exposed area of more than 5500 km². The SIP (ca 560-570 Ma) is located within the Kalak Nappe Complex, a part of the Middle Allochthon of the North Norwegian Caledonides.

Reinfjord is one of the ultramafic complexes of the SIP. It was emplaced during three magmatic events in a time span of 4 Ma and consists in a cylindrically zoned complex with a dunite core (Central Series) surrounded by wehrlite, lherzolite and olivine-rich pyroxenite dominated series (Upper and Lower Layered Series). Olivine cumulates with various amounts of pyroxenes (mostly clinopyroxenes) compose the main lithological units of the intrusion.

Layered gabbros and sedimentary garnet gneiss hosting the intrusion are respectively found mainly on eastern/southern and western sides of the ultramafic complex.

Within the Reinfjord complex is a promising Ni-Cu-and a PGE reef deposit of high economic interest.

High-resolution aeromagnetic and ground magnetic surveys were completed in 2014 in Reinfjord.

Here, we developed a magnetic model for the Reinfjord complex integrating petrophysical data (density, magnetic) from samples collected in the field (2014/15) and 4 drill cores (up to 390m) with ground magnetic and helicopter data (SkyTEM survey). The magnetic data have been processed and both measured TMI and calculated vertical gradients data have been used for modelling.

The combination of different datasets and the petrophysical properties for surface and core samples allowed the estimation of the geometry, depth and magnetisation of the magnetic sources contributing to, and refining the geological interpretation of the Reinfjord ultramafic intrusion. A 3D model (Model Vision) of the geometry of the ultramafic intrusion is presented.