Magmatic cannibalisation of a Permo-Triassic Ni-Cu-PGE-(Au-Te) system during the breakup of Pangea – Implications for craton margin metal and volatile transfer in the lower crust

Abstract

The Ivrea Zone of NW Italy records the polyphased evolution of a magmatic sulfide mineral system, which occurred at multiple stages over a >80 Ma time interval. Between 290 and 250 Ma, a series of hydrated and carbonated ultramafic alkaline pipes containing Ni-Cu-PGE-(Te-Au) mineralisation was emplaced in the lower continental crust. At 200 Ma, a subsequent mineralising event occurred in association with the emplacement of the La Balma-Monte Capio (LBMC) intrusion. Modelling of the LBMC parental magma shows derivation from up to 60% partial melting of an anhydrous depleted juvenile mantle outside of garnet stability. The inferred composition of the parental melt is consistent with magmatism associated with the Central Atlantic Magmatic Province (CAMP). However, its tellurium-enriched composition together with the S-C-O isotope signature of the associated magmatic sulfide mineralisation (δ34S = +0.53 to +1.00‰; δ13C = −10.41 to −4.07‰; δ18O = +6.57 to +13.74‰) cannot be reconciled with the CAMP source. It is argued that the geochemical and isotopic signature of the LBMC intrusion reflects interaction and mixing of a primitive magma sourced from a juvenile source with localised domains enriched in carbonate and metal-rich sulfides located in the lower crust, consistent with the composition of the Permo-Triassic pipes. We propose that this interaction resulted in sulfide-supersaturation and enrichment in volatiles and metals of the LBMC magma. Upwards magma transport may have been facilitated by devolatisation of magmatic carbonate from the pipes and the generation of a CO2 supercritical fluid that acted as buoyancy aid for the dense sulfide liquid. Evidence of this magmatic interaction informs on the first-order processes that control enhanced metallogenic fertility along the margins of lithospheric blocks. The scenario depicted here is consistent with reactivation and enrichment of a Gondwana margin Ni-Cu-PGE-(Te-Au) mineral system during the breakup of Pangea.