Origins of layers in layered mafic intrusions

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Large igneous provinces represent events during which millions of cubic km of extremely hot magma passed through the Earth's crust and were either emplaced as collections of intrusions or erupted as voluminous lava flows. Intrusions within large igneous provinces are dominated by magnesium- and iron-rich (i.e., mafic) rocks and display layering that superficially resembles the layering in some sedimentary rocks, hence the general term "layered mafic intrusions" (LMI). LMI contain the world's most important reserves of important metals including chromium and platinum; they are also important sources of copper, nickel, palladium, phosphorus, iron, titanium, vanadium, and niobium. The origins of mineralized layers in LMI remain a matter of intense research activity and controversy. The LMI research community tends to favor the concept of magma chambers as large as ten km deep and hundreds of km wide within which crystals accumulate along the base to form an upward-aggrading series of layers. However, the study of layered rocks in granitic intrusions and of layered mafic intrusions at mid-ocean ridge settings has shown that in many cases layering results from the repeated intrusion of separate thin horizontal sheets of magma to produce a layered stack of sills. The difference is critical for guiding mineral exploration strategies in newly discovered deposit groups like the Ring of Fire deposits of northwestern Ontario. My group and a few others have recently suggested that the magma chamber hypothesis for very large sill-shaped LMI should be replaced by the idea of sequential emplacement of stacks of sills in many examples. The objective of the proposed research is to develop rigorous tests of the sill hypothesis for LMI and some of its competing hypotheses. Some key questions to be answered will include: "How can we recognize the margins of sills intruded into older sills that remain very hot themselves?"; "Can we detect evidence for short, intense periods of heating of rocks adjacent to sills within LMI?"; "What sequence of rocks is expected to be produced if a magma solidifies without having sills emplaced within it?" We will use four examples of LMI as natural field laboratories; the iconic Bushveld Complex of South Africa and Stillwater Complex of Montana, which are candidates for the sill hypothesis, the Sudbury Igneous Complex of Ontario, which is known to have been a classical magma chamber, and the Kyak Bay Intrusion of Quebec, whose geology is poorly known but exceptionally well-exposed. The work will involve mapping, sampling and measurement of rock and mineral composition and textures. The field observations will be compared with the predictions that can be made using thermodynamic and numerical models of igneous processes such as chemical diffusion, crystal nucleation and growth, maturation of igneous textures, magma flow and emplacement, and heat flow within magmas and their solid host rocks.