Fluxing of mantle carbon as a physical agent for metallogenic fertilization of the crust

Abstract

Magmatic systems play a crucial role in enriching the crust with volatiles and elements that reside primarily within the Earth's mantle, including economically important metals like nickel, copper and platinum-group elements. However, transport of these metals within silicate magmas primarily occurs within dense sulfide liquids, which tend to coalesce, settle and not be efficiently transported in ascending magmas. Here we show textural observations, backed up with carbon and oxygen isotope data, which indicate an intimate association between mantle-derived carbonates and sulfides in some mafic-ultramafic magmatic systems emplaced at the base of the continental crust. We propose that carbon, as a buoyant supercritical CO 2 fluid, might be a covert agent aiding and promoting the physical transport of sulfides across the mantle-crust transition. This may be a common but cryptic mechanism that facilitates cycling of volatiles and metals from the mantle to the lower-to-mid continental crust, which leaves little footprint behind by the time magmas reach the Earth's surface.

Emplacement of magmatic Cu-Au-Te(-Ni-PGE) sulfide blebs in alkaline mafic rocks of the Mordor Complex, Northern Territory, Australia

“…The intra-sulfide bleb textures imply that carbonate may have been included within the sulfide liquid droplets, or possibly even exsolved from the sulfide liquid on cooling. This is also seen in lower crustal alkaline ultramafic pipes at Valmaggia, NW Italy (Sessa et al 2017;Blanks et al 2020), and to a certain extent, in the lamprophyric Sron Garbh intrusion, Scotland (Graham et al 2017;Blanks et al 2020).…”

“…The magmatic sulfide mineralisation in the Braveheart area of the MAIC has a number of key characteristics that provide evidence for the processes and controls on magmatic sulfide mineralisation in the MAIC and alkaline igneous systems in general. These are: Here we address these features in relation to the wider subject of magmatic sulfides in unconventional settings, specifically alkaline magmatic systems, and how Mordor fits within recent models for this style of mineralisation (Holwell et al 2019;Hou and Wang 2019;Blanks et al 2020); and discuss the origin of the MAIC itself.…”

“…Further recent advances outlined in Blanks et al (2020) also show that in volatile-rich mafic-alkaline systems, carbonate appears to be intimately associated with sulfide, particularly in the lower and mid-crust, and may play a crucial role in fluxing metalrich sulfides from the mantle into the crust. The sulfide in these systems typically have a very Cu-Au-Te-rich metal character, hosted by rocks that possess a hydrous and carbonated mineralogy (as seen at Braveheart by the large amounts of phlogopite and carbonate).…”

“…A suite of Ni-Cu-PGE sulfide occurrences in post-collisional settings, defined by the alkaline nature of the host magmas, and a geochemical signature that is generally more enriched in Cu, Au and Te compared with most ultramafic-mafic hosted deposits, have been investigated by Holwell et al (2019) and Blanks et al (2020). These occurrences are typically hosted by rocks that are the product of low (< 10%)-degree partial melts that appear to be linked by an intimate association between sulfide and carbonate of mantle origin, and Blanks et al (2020) proposed that CO 2 can aid the physical transport of mantle sulfides up into the lower and mid-crust. The host intrusions for these occurrences span the lithosphere and are generally small (e.g.…”

“…This, along with a few other alkaline ultramafic intrusions in Scotland (Loch Borralan and Loch Ailsh; Gunn and Styles 2002;Styles et al 2004), contains magmatic Cu-Ni-PGE sulfides. In addition, Sron Garbh has been used as an example of a mid-crustal alkaline intrusion in the studies on the chalcophile element 'DNA' of postsubduction magmatism (Holwell et al 2019) and the link between carbonate and sulfide in magmatic systems (Blanks et al 2020).…”