PGE-Enrichment in Magnetite-Bearing Olivine Gabbro: New Observations from the Midcontinent Rift-Related Echo Lake Intrusion in Northern Michigan, USA

Alexander Koerber,Joyashish Thakurta

doi:10.3390/min9010021

Abstract

The Echo Lake intrusion in the Upper Peninsula (UP) of Michigan, USA, was formed during the 1.1 Ga Midcontinent Rift event in North America. Troctolite is the predominant rock unit in the intrusion, with interlayered bands of peridotite, mafic pegmatitic rock, olivine gabbro, magnetite-bearing gabbro, and anorthosite. Exploratory drilling has revealed a platinum group element (PGE)-enriched zone within a 45 m thick magnetite-ilmenite-bearing olivine gabbro unit with grades up to 1.2 g/t Pt + Pd and 0.3 wt. % Cu. Fine, disseminated grains of sulfide minerals such as pyrrhotite and chalcopyrite occur in the mineralized interval. Formation of Cu-PGE-rich sulfide minerals might have been caused by sulfide melt saturation in a crystallizing magma, which was triggered by a sudden decrease in fO2 upon the crystallization and separation of titaniferous magnetite. This PGE-enriched zone is comparable to other well-known reef-like PGE deposits, such as the Sonju Lake deposit in northern Minnesota.

Highlights

Magmatism during the 1.1 Ga old Midcontinent Rift (MCR) event is associated with the origin of several metallic mineral deposits in the Great Lakes region of North America [1,2,3,4,5]
These mineral deposits have been classified into two groups: sulfide-poor, platinum group element (PGE) (Platinum Group Element)-rich, layered intrusions and sulfide-rich, conduit-type, high-grade Ni-Cu sulfide deposits (Figure 1) [6,7]
The PGE-enrichment reported from the Echo Lake intrusion is discussed with reference to the geochemical principles of sulfide-melt saturation in basaltic and picritic magmas

Summary

Introduction

Magmatism during the 1.1 Ga old Midcontinent Rift (MCR) event is associated with the origin of several metallic mineral deposits in the Great Lakes region of North America [1,2,3,4,5]. These mineral deposits have been classified into two groups: sulfide-poor, PGE (Platinum Group Element)-rich, layered intrusions and sulfide-rich, conduit-type, high-grade Ni-Cu sulfide deposits (Figure 1) [6,7]. The Eagle Ni-Cu sulfide deposit is hosted within two mafic-ultramafic intrusive bodies, called the Eagle and the Eagle

Methods

Results

Conclusion