Chapter 3.1 - Geology and Petrogenesis of Magmatic Ni-Cu-PGE-Cr-V Deposits: An Introduction and Overview

Abstract

Magmatic Ni-Cu-PGE-Cr-V Deposits formed in a variety of geologic environments and periods. PGE-Cr-V deposits predominantly occur in large layered intrusions emplaced during the late Archean and early Proterozoic into stabilized cratonic lithosphere. The magmas ascend through translithospheric sutures characterized by limited extension and rifting. The laterally extensive ore layers (so-called reefs) formed through hydrodynamic phase sorting when the central portions of large, incompletely solidified magma chambers subsided due to crustal loading. The bulk of global PGE-Cr-V resources occur in the largest layered intrusions, namely the Bushveld complex of South Africa, the Great Dyke of Zimbabwe, and the Stillwater complex of the USA, but significant deposits additionally occur in Finland, namely in the Kemi, Portimo, and Koillismaa intrusions. Due to the large size (tens of kilometers) and limited complexity of the deposits and their host intrusions, they are relatively easy to locate and delineate. As a result, the search space is relatively mature and few new discoveries have been made in the last few decades. The parental magmas to most intrusions are likely derived from the asthenosphere, followed by contamination with crust. Some intrusions, notably the Bushveld complex, may have crystallized from magma sourced in the subcontinental lithospheric mantle, as suggested, for example, by Os isotopic data.