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Hydromagmatic Processes and Platinum-Group Element Deposits in Layered Intrusions

BoudreauThis is a welcome book with a case to make that volatiles have key roles in the development of PGE deposits in layered intrusions. Alan Boudreau has argued for the role of volatiles for a number of years and it reflects the tenor of this book that he addresses the concerns of skeptics head-on in a final chapter on ‘Some Objections Considered’.

Volatiles strongly influence the generation and evolution of magmas, melt migration and the concentration of metals. An introduction to layered intrusions is followed by a discussion of magmatic volatiles and fluids before considering the behavior of the PGE and evidence for the role of volatiles in layered intrusions. Evidence is summarised from different layered intrusions, in particular from the Bushveld, Skaergaard and Stillwater complexes, discussing melt and fluid inclusions and pegmatoids, and the effects of volatiles on mineral stability and volatile fluxing. There is little doubt that the host magmas contain significant volatile abundances, and the questions are mostly over the role of those volatiles in the development of mineralization.

The Bushveld, for example, is the world’s largest layered continental intrusion, and contains more than half of the known PGE reserves. Its distinctive features include a strong lithospheric signature, the marked changes in Sr isotope ratios with stratigraphy indicating open system behavior, the associated volcanic and intrusive rocks, and evidence for movement of magma and crystal mushes subsequent to the formation of the distinctive planar layered fabrics. The Bushveld has elevated volatile contents (Cl, and >1% H2O), and parental magmas with elevated PGE contents, and it is of interest both in terms of what happens sub-volcanically in mafic magmatic systems and for the processes involved in ore formation.

Models for sulphide saturation include crustal contamination, magma mixing and fractional crystallization, and whereas volatiles influence phase relations, their role in the formation of ore bodies is less clear. There is compelling evidence for volatiles in the PGE-bearing pipes and pegmatoids in the Bushveld, and more widely in the presence of hydrous minerals and fluid inclusions. Thus volatiles were present, and much of the debate is over the extent to which the reefs were the product of fluid/volatile saturation and sulphur and PGE were concentrated by vapour refining processes, as vapour is exsolved from crystallizing interstitial liquid.

The jury is still out, but this timely book brings together evidence for the presence and the possible roles of volatiles in continental ore-bearing magmatic systems. It is well written and it deserves to be widely read given the current interest in the roles of volatiles for continental magmatic systems, and in the formation of PGE-bearing ore bodies. 

By Chris Hawkesworth

Hydromagmatic Processes and Platinum-Group Element Deposits in Layered Intrusions, by Alan Boudreau. Cambridge University Press 2019, ISBN 978-1-108-41600-9 Hardback, 270pp.