Chapter 3 - Metallic Ore Deposits Associated With Mafic to Ultramafic Igneous Rocks

Abstract

Ore deposits that are associated with mafic to ultramafic igneous rocks include Ni-Cu-PGE deposits, chromite deposits, Fe-Ti-V deposits (magnetite/titanomagnetite), and Fe-Ti±P (ilmenite, titanomagnetite±apatite) deposits. Ni-Cu-PGE deposits may be sulfide-rich, with PGEs held as sulfides, alloys, arsenides, telluirdes, bismuthinides, or selenides. Deposits are normally relatively small and occur as part of conduit systems, including extrusive rocks in the case of komatiites. The world’s largest PGE deposits occur in large layered intrusions (e.g., Bushveld Complex, Great Dyke Stillwater Complex) where sulfide volume percentage is low, generally less than five. Chromite deposits occur as stratiform layers in large intrusions, and as irregular podiform deposits in ophiolite sequences. V-bearing magnetite may occur as massive layers in large layered intrusions like the Bushveld Complex, or may be disseminated in oxide-rich gabbros in layered intrusions of various sizes. Titanomagnetite and ilmenite occur in gabbroic rocks in the Emeishan Large Igneous Province as massive layers or as disseminations. Fe-Ti±P deposits often occur in gabbroic rocks that are associated with Proterozoic anorthosite suites and in layered intrusions. Ti-rich deposits also occur with ultramafic rocks in the Duluth Complex and within Ural-Alaskan intrusions. Genetic models for the origin of the various deposits must account for the concentration of metals via either chemical or physical processes. Immiscible sulfide liquids strongly sequester Ni, Cu, and PGEs due to the very chalcophile nature of these elements. Due to their density, gravitational settling may be important as a concentration mechanism, as well as accumulation in conduits where restrictions may cause decreases in magma velocity. For chromite and Fe-Ti-V-P deposits, processes such as fractional crystallization accompanied by gravitational crystal accumulation and removal of buoyant residual material are important concentration mechanisms. In some cases the separation of immiscible Si-rich and Fe-rich melts may be an important process, with the Fe-rich melts giving rise to Fe-Ti-P ores. The tectonic settings of the deposits are varied, ranging from intraplate settings where hydrous mantle plumes may have been important, to divergent zones of intracontinental rifting, to convergent zones where asthenospheric mantle-derived melts produced layered bodies and conduit-related intrusions where interaction with sulfide-bearing country rocks has occurred.