T159 Geology and Mineral Deposits of the Midcontinent Rift II

Tuesday, 11 October 2011
8:00 AM - 12:00 PM
Room 101H-J

THUNDER BAY NORTH PT-PD-NI-CU DEPOSIT

Paper number: T159-4
Program: Technical Sessions
Time: 8:45 AM – 9:00 AM

Authors

Abstract

The 1.1 Ga Midcontinent Rift (MCR) centred on Lake Superior hosts a number of recently discovered orthomagmatic nickel-copper-platinum group element deposits. The Thunder Bay North Deposit (TBN), discovered in 2006, is the most recent. An indicated open pit resource of 8.46 million tonnes (mT) grading 1.04 g/t Pt, 0.98g/t Pd, 0.18% Ni and 0.25% Cu, and a smaller, higher-grade, underground resource of 1.03 mT grading 1.63g/t Pt, 1.51 g/t Pd, 0.24% Ni and 0.39% Cu, have been defined in the TBN-system over a strike-length of 3.4 km.

The TBN-system has a number of unique features differentiating it from other MCR-related orthomagmatic deposits and occurrences. Mineralization identified to date is dominated by disseminated sulphide with lesser semi-massive to massive sulphide accumulations. Mineralization is chalcophile element enriched, with massive sulphide accumulations grading 60-100 g/t combined platinum and palladium, in a one to one ratio. Mineralization occurs as semi-continuous pods hosted by olivine cumulate in a magmatic conduit. The magmatic feeder exhibits rapid changes in morphology and orientation, but is dominated by visually homogenous peridotite to feldspathic peridotite. Olivine appears as a dominant cumulate phase with minor oxides. Feldspar occurs as an intercumulus phase, and pyroxenes occur as both minor cumulate phases, and ubiquitous oikocrysts. A thin carapace of a feldspar dominant rock with heterolithic xenoliths is commonly observed at the top of the magmatic feeder. Petrology and geochemistry indicates that the TBN-system is similar to other early mafic-ultramafic intrusions identified within Lake Nipigon embayment of the rift. However, in contrast the other intrusions to date host limited mineralization.

A dynamic ore forming model involving the transport of immiscible sulphide from an up stream source to the currently defined resource area is proposed. Immiscible sulphides accumulated in basal topographical lows, and areas of decreased magma flow velocity. Exploration continues following up on both the upstream and downstream potential of the system.