Mr Jordan James Lindsay
Metallogenic controls of mantle plumes of platinum-group elements and precious metals
From Continent to Ocean - the platinum-group element geochemistry of the migrating Tristan plume
The Paraná-Etendeka Large Igneous Province (PELIP) is one of the largest areas of continental flood basalts (CFB) in the world, with an extrusion volume of 1.7 x 106 km3. The province is thought to be the surface expression of the deep mantle Tristan plume impinging under the continental landmass of Gondwana in the Early Cretaceous (135-128 Ma), on the boundary between the future African and South American continents. The South American (mainly Brazilian) portion of the PELIP, the Serra Geral Formation, covers 15 times more area than the African equivalent lava field (Etendeka) and hosts multiple units of chemically distinct yet synchronous flood lavas. Classic petrological and volcanological studies categorised the lavas into three major groupings, High-Ti, Low-Ti and Silicic[2,3]. Throughout the Cretaceous, potentially as a direct consequence of the insurgent plume, Africa and South America rifted apart, opening the Southern Atlantic Ocean in the process. Plume magmatism persisted through this rifting period, continually erupting lava from a continental setting to the plume’s current central oceanic position under Tristan da Cunha. The changing geodynamic environment is reflected in variations in major and trace geochemical data between the High-Ti, Low-Ti, and oceanic plume trail lavas on each side of the Atlantic. Studies suggest that the sub-continental lithospheric mantle (SCLM) may be an enriched reservoir of platinum-group elements (PGE), and the high temperature melting of the lithosphere during plume ascent could incorporate these metals into generated magmas. The similar continental-to-oceanic development of the Icelandic plume in the North Atlantic has been proven to systematically affect relative PGE abundances in Greenlandic, British and Icelandic lavas. This study aims to investigate the effect changing geodynamic settings have on magma PGE abundances by comparing onshore and offshore PELIP lava major, trace and PGE abundances, in addition to Os, W and He isotopic signatures. The suspected deep plume source for the Paraná lavas, comparable plume environment in Iceland, and ability of plumes to sample a wide variety of geochemical reservoirs are expected to play key roles in the investigation, and findings are will contribute to the understanding of global plume models and deep mantle geochemistry as a whole.
Distinct sulphur saturation histories within the Palaeogene Magilligan Sill, Northern Ireland: Implications for Ni-Cu-PGE mineralisation in the North Atlantic Igneous Province
The ~60 m thick Northern Irish Magilligan Sill is a dolerite and olivine gabbro intrusion, thought to be connected to the Irish dyke swarm plumbing system that is part of the British Palaeogene Igneous Province (BPIP) in the North Atlantic. The sill has received interest as an exploration target for Ni-Cu-PGE sulphide mineralisation due to its morphological similarity to Noril’sk-Talnakh (Russia). We present new petrological, geochemical and S-isotope data for the sill, to assess its prospectivity and detail the underlying magmatic plumbing system. Most sulphides in the dolerite portions of the sill contain negligible PGE. In the olivine gabbros, sulphides contain significant PGE, Cu, Ni, Co and Ag. Pyrite from the dolerites have δ34S ranging from -10.0 to +3.4 ‰ and olivine gabbro sulphides range from -2.5 to -1.1 ‰, suggesting widespread crustal contamination. The S/Se ratios of sulphides in the dolerites and olivine gabbros range from 3,500 to 19,500 and from 1,970 to 3,710, respectively, indicating that olivine gabbro sulphides may have come from upstream in the magma plumbing system. The Magilligan Sill records multiple injections of mafic magma into a single intrusive package, each with distinct mechanisms towards S-saturation. The divergence in S-saturation histories and metal contents suggest that a larger volume of olivine gabbro sulphides at depth may be prospective.