Layered intrusions in the Precambrian: Observations and perspectives

• Published in: Precambrian research 2024-12, Vol.415, p.107615, Article 107615
• Main Authors: Smith, William D., Jenkins, M. Christopher, Augustin, Claudia T., Virtanen, Ville J., Vukmanovic, Zoja, O’Driscoll, Brian
• Format: Article
• Language: English
• Subjects: Critical metals; Electron backscatter diffraction; Igneous petrology; Layered intrusion; microXRF; Mineral system; Precambrian; Sciences of the Universe
• ISSN: 0301-9268
• DOI: 10.1016/j.precamres.2024.107615

•Spatiotemporal distribution of Precambrian layered intrusions.•Drivers and debates in layered intrusion research.•Mantle sources and primary-parent melts associated with layered intrusions.•Formation and exploration of critical ore deposits in layered intrusions.•Summary of exploration tools for delineating orebodies in layered intrusions.•Multiscale study on the formation of the snowball oiks outcrop in the Stillwater Complex. Layered intrusions are plutonic bodies of cumulates that form by the crystallization of mantle-derived melts. These intrusions are characterized by igneous layering distinguishable by shifts in mineralogy, texture, or composition. Layered intrusions have been fundamental to our understanding of igneous petrology; however, it is their status as important repositories of critical metals – such as platinum-group elements, chromium, and vanadium – that has predominantly driven associated research in recent decades. Many layered intrusions were emplaced during the Precambrian, predominantly at the margins of ancient cratons during intervals of supercontinent accretion and destruction. It appears that large, layered intrusions require rigid crust to ensure their preservation, and their geometry and layering is primarily controlled by the nature of melt emplacement. Layered intrusions are best investigated by integrating observations from various length-scales. At the macroscale, intrusion geometries can be discerned, and their presence understood in the context of the regional geology. At the mesoscale, the layering of an intrusion may be characterized, intrusion-host rock contact relationships studied, and the nature of stratiform mineral occurrences described. At the microscale, the mineralogy and texture of cumulate rocks and any mineralization are elucidated, particularly when novel microtextural and mineral chemical datasets are integrated. For example, here we demonstrate how mesoscale observations and microscale datasets can be combined to understand the petrogenesis of the perplexing snowball oiks outcrop located in the Upper Banded Series of the Stillwater Complex. Our data suggest that the orthopyroxene oikocrysts did not form in their present location, but rather formed in a dynamic magma chamber where crystals were transported either by convective currents or within crystal-rich slurries. Critical metals may be transported to the level of a nascent intrusion as dissolved components in the melt. Alternatively, ore minerals are