Halogen cycling and precious metal enrichment in sub-volcanic magmatic systems: Insights from the Rum layered intrusion, Scotland
Résumé
The cycling of the halogens in sub-volcanic systems may play an important role in magmatic ore deposit formation and the emission of climate-altering gases via volcanic outgassing. However, the mobile and reactive nature of the halogens, as well as their typically low abundances in most intrusive igneous rocks, means that their behaviour in shallow crustal level magmatic systems is difficult to constrain. We present data from the ∼60 Ma Rum Layered Suite, NW Scotland, utilising the neutron irradiation-noble gas mass spectrometry (NI-NGMS) technique, to provide insight into the provenance and behaviour of Cl, Br and I in magma chamber and precious metal enrichment processes. As a well-characterised and superbly exposed example of an open system sub-volcanic magmatic body, the Rum intrusion is an ideal locality to investigate halogen behaviour in this context. Our data reveal concentrations of <1 ppm to 124 ppm for Cl, <7 ppb to 484 ppb for Br and <1 ppb to 363 ppb for I in the Rum rocks. Halogen ratios span a range of 2.3 × 10-4 to 1.5 × 10-2 for Br/Cl and 4.0 × 10-5 to 7.4 × 10-2 for I/Cl, defining an array from MORB-like to sedimentary-like compositions. Chlorine and Br behave sympathetically in all lithologies, and in the rocks considered to be primary magmatic differentiates (i.e., peridotites, troctolites and gabbros), the variation in abundances is likely controlled by accessory quantities of primary hydrous magmatic phases crystallising from the mush interstitial liquid. Iodine behaviour is decoupled from that of Br and Cl, and when combined with noble gas isotope data, suggests variable but pervasive contamination by a sedimentary source or sources during crystallisation of the Rum intrusion. In PGE-rich chromitites, high I/Cl ratios relative to their putative picritic parental melts point to the involvement of a crustal I source and hydrous fluids during chromitite formation. Our combined halogen and noble gas isotope datasets therefore reveal a record of high temperature processes associated with magmatic-crustal interactions and precious metal mineralisation, and provide new insights into volatile evolution during solidification of the Rum intrusion. A key implication of this work is that magma-crust interactions in mafic magmatic systems may lead to I enrichment (i.e., from 12% to up to two orders of magnitude greater than MORB, in the Rum intrusion), before potential outgassing to the atmosphere.