Metal endowment at craton edges controlled by amphibole-rich pyroxenites dykes and incipient melts in the mantle

As many critical metals are initially locked up in the volatile-bearing mantle, the first critical stage to any mineralizing process requires their liberation by partial melting of the mantle source, followed by the onset of an effective upward transport mechanism into the overlying crust. The initial melts of volatile-bearing mantle lithologies are incipient melts rich in volatile and incompatible components, which are effective transport agents at low mantle temperatures. This means that understanding the formation, composition, and migration of these melts is crucial to constraining metal transport in the mantle.

The incipient melts of peridotite usually solidify in the mantle to form dykes rich in hydrous minerals such as amphibole and mica. These assemblages commonly also contan abundant clinopyroxene and are known as “hydrous pyroxenites”, and may also contain several other accessory phases including apatite, ilmenite, and rutile.  We have recently gained abundant experimental information on the melting conditions and compositions of these hydrous pyroxenites, which can be viewed as second-stage melts. Although volumetrically minor in the lithosphere as a whole, these hydrous pyroxenites all produce melts at lower temperatures than peridotite, rapidly exhausting hydrous minerals such as amphibole, which have been discovered to host large concentrations of critical metals  [1].

Our results for trace element analyses of melts and residual minerals indicate that hydrous minerals such as phlogopite, amphiboles and apatite all have high partition coefficients for Ni (3-20) and other transition elements, meaning that the formation of hydrous pyroxenites during first-stage melting processes may lead to the formation of important repositories for Ni in the mantle sources of igneous rocks. The contribution of hydrous pyroxenites to the metal endowment of mantle melts may have been underestimated or overlooked in the past [1] due to the traditional association of magmatic Ni-sulfide ore deposits with basaltic to komatiitic rocks that originate by partial melting of uniform four-phase peridotite. The lower melting temperatures of hydrous pyroxenites (≈300˚C less than dry peridotite) also means that the generation of magmatic ore deposits may not require a major thermal perturbation such as a plume. Hydrous pyroxenites are commonly associated with continental regions, where their melting may be accentuated by erosion by edge-driven convection [2] or lateral advection of solids [3] at craton edges, thus explaining the association of both volatile-rich magmatism [4] and metal deposits [5] with craton edges. Thus, predictive exploration models should consider domains of the lithospheric mantle where hydrous pyroxenites may be localised and concentrated, as they may have been episodically melted throughout the long-lived geological evolution of cratonic blocks.

[1] Ezad IS et al. (2024) Mineralium Deposita. doi: 10.1007/s00126-023-01238.

[2] Davies DR and Rawlinson N (2014) Geology 42, 1031-1034.

[3] Muirhead JD et al. (2020) Nature 582, 67-72.

[4] Foley SF and Fischer TP (2017) Nature Geoscience 10, 897-902.

[5] Hoggard MJ et al. (2020) Nature Geoscience 13, 504-510.