Platinum metal geochemistry of the Bruvann nickel-copper deposit, Råna, North Norway

The Bruvann Ni-Cu deposit is situated in the northwestem part of the 70 km2 Råna layered intrusion near Narvik in North Norway. The intrusion is an intraorogenic body emplaced during a tensional episode within the Caledonian orogeny, and its primary features have been disturbed by late- and postmagmatic deformation. Norite and gabbro-norite predominate, but there are ultramafic units as thick as 800 m near the periphery of the body. The body probably resulted from multiple magma pulses emanating from a chamber undergoing differentiation at a lower level in the crust. Low-grade disseminated Ni mineralization occurs interstitially to silicates in dunitic and harzburgitic units at two levels in the ultramafic zone in the northwestern part of the intrusion. The deposit contains 43.6 million tons with 0.33% sulphide-bound Ni, 0.08% Cu and 0.015% Co at a cut-off of 0. 15% Ni, or alternatively 26.4 million tons with 0.42% Ni., 0.1% Cu and 0.02% Co at a cut-off of 0.3% Ni,. Fifty samples from the deposit have been analyzed for platinum metals: they include lO bulk samples representative of whole drill-hole intersections with ore zones. The absolute concentrations in the disseminated mineralization are <1-5 ppb Pt, <1-7 ppb Pd, <0.5-0.8 ppb Rh, <0.01-Ø.55 ppb Ir and <1-75 ppb Au. Ru is below detection limit. Recalculated to 100% sulphides, the bulk samples yield 52 ppb Pt., 70 ppb Pd., 9.0% Ni, and 2.1% Cu, (above detection limit for Pt and Pd in all cases). Comparable figures for Ir and Au based on the average of individual samples of disseminated mineralization are approximately 6.2 ppb Ir, and 1010 ppb Au,. The Pt, and Pd, concentrations are 1 to 2 orders of magnitude lower than those in deposits with comparable Ni, contents (e.g. Katiniq, Vakkerlien). Conversely, other deposits with similar Pt, and Pd, contents (e.g. Montcalm, Pipe) have much lower Ni, and Cu,. The concentrations of Ni, Cu, Pt and Pd are explicable if the parent magma was depleted in Pt and Pd by the formation and removal of a small amount of immiscible sulphide magma prior to the emplacement of successive magma pulses in the magma chamber at the present level.