| الوصف: |
Layered mafic-ultramafic intrusions provide the ideal opportunity to study magmatic processes operating at shallow-mid crustal levels. Layered intrusions may also contain substantial concentrations of sulfide-hosted precious metals, including the platinum group elements (PGE). Despite decades of study, the provenance and role of volatiles (e.g., halogens and sulfur) in the evolution of these magmatic systems and the relationship of volatiles to PGE ore formation remains a matter of significant debate. This project concerns the volatile geochemistry of two classic examples of layered intrusions; the Rum Layered Suite (NW Scotland) and the Stillwater Complex (USA), using the novel neutron-irradiated noble gas mass spectrometric technique for halogen abundance and noble gas isotope determination. In situ sulfur isotope analyses are also carried out on selected Rum samples. The intrinsic link between halogens and fluid activity, as well as their affinity for particular reservoirs in the Earth's crust, mean that the halogens can provide information on the provenance and subsequent processing of volatile-enriched fluids in layered intrusions. Sulfur isotopes may also reveal insights into volatile sources, as well as post-magmatic effects in the crystal mushes that solidify to form layered intrusion cumulates. For the Rum intrusion, analysis of picrite dykes, considered to be proxies for Rum parental magmas, reveal an SCLM-like halogen signature for the magma source region. Noble gas isotopes as well as evidence of pervasive iodine enrichment (< 363 ppb), which is decoupled from Br and Cl for Rum intrusion rocks, highlights the influence of magma-crust interaction on the evolution of the Rum system. This is particularly evident at PGE-enriched chromitite horizons, which exhibit high I/Cl ratios relative to their putative picritic parental melts (52.85 x 10⁻³ wt. compared to 0.04 x 10⁻³ wt. I/Cl respectively). This relative enrichment, in addition to evidence of a fluid phase, points towards the reprocessing of pegmatoidal footwall during chromitite petrogenesis in the Rum intrusion. The Rum cumulates display overall lower concentrations of Cl (e.g. majority fall in the range 0.7-20.7 ppm) relative to the bulk composition of the picrites (23.1-74.5 ppm). The bulk composition of Rum pegmatoids, which are taken as representative of late-stage volatile-rich magmas, likewise display low Cl concentrations (2.1-53.2 ppm); this apparent Cl depletion in Rum cumulates and pegmatoids implies the potential loss of Cl through degassing processes. Volatile loss in Rum intrusion cumulates is also highlighted by microscale variations in sulfur isotope compositions; a range of ~9.1‰ δ³⁴S is observed in the chromitite seams over an area of ~1-2 cm² . Degassing of sulfides during crystallisation of base metal sulfides (e.g. pentlandite and chalcopyrite) is the best explanation for this remarkable variation. In situ analysis of sulfur isotopes may therefore be a key tool in understanding the processing of volatiles during the solidification of crystal mushes. Halogen data from the Stillwater Complex, which is host to the PGE-rich J-M Reef, highlights a dominantly magmatic signature, ranging to unusually high Br/Cl ratios (from MORB-like to 35.3 x 10⁻³ wt.), indicating that Stillwater parental magmas may have originated from mantle enriched by subductionderived metasomatic fluids. The importance of volatile-enriched fluids in sulfidehosted ore formation is hinted at by the variations in volatile content between a sulfide-rich and sulfide poor sample of the J-M Reef. For both intrusions, hydrous minerals such as biotite mica and amphibole control the distribution of halogens, whilst apatite is a particularly important halogen-bearing phase in the Stillwater Complex. A significant feature of halogen data from both intrusions are elevated Br/Cl and I/Cl ratios of sulfide separates relative to their bulk rock counterparts, suggesting chalcophilic behaviour of Br and I over Cl. Such fractionations may thus have the potential to inform on the presence of hidden sulfide-hosted ore bodies in other intrusions. The new halogen data point towards an important relationship between the presence of sulfide liquid and halogen fractionations. The post-magmatic interaction of fluids and sulfides may exert a key influence on the grades and style of mineralisation in layered intrusions. |