The role of mantle melting and associated granitoid magmatism in the genesis of orogenic gold in the Laurentian Caledonides

Orogenic gold vein deposits are an economically important, well-documented mineralization class with limited consensus regarding the role of juvenile magmatism in their genesis. Here we use He isotopes to determine the contribution of mantle heat in driving the ore fluids responsible for major gold deposits in the Laurentian Caledonides of Britain and Ireland, including all active mines (Cononish, Curraghinalt, and Cavanacaw), many of which are tentatively classed as orogenic. The 3He/4He of fluids in Au-bearing sulfides (0.09−3.3 Ra) require a significant contribution from exsolved magmatic volatiles, implying that mantle heat is intrinsic to ore formation. The largest deposit, Curraghinalt, formed from the hottest ore fluids with the highest proportion of magmatic He during the Grampian Event. The smaller late-Caledonian deposits precipitated from cooler fluids with lower 3He/4He. Ore-fluid 3He/4He does not correlate with sulfide δ34S (−1.9−9.1‰) reflecting multiple crustal S sources. A positive correlation between the maximum 3He/4He and Au reserve implies that gold was sourced from mafic mantle melts generated by post-subduction processes during the Caledonian orogeny.