The timing of sulfide segregation in a Variscan synorogenic gabbroic layered intrusion (Beja, Portugal): Implications for Ni-Cu-PGE exploration in orogenic settings

Abstract

A growing number of magmatic Ni-Cu sulfide deposits discovered in orogenic environments has renewed the interest on this category and reinforced that most orogenic mafic magmas are PGE depleted. The Beja Layered Gabbroic Sequence (LGS) was emplaced during the same Variscan tectono-magmatic event as the Aguablanca Ni-Cu-PGE deposit in Spain, however, no economic mineralization was yet found in LGS. Here we analyze three sulfide ore-showings developed within different environments of LGS that provide a unique frame to assess sulfide segregation in orogenic mafic magmas: the Internal Sulfides hosted at the core of the layered suite, the Marginal Sulfides located at a complex domain where the LGS is intruded by late magmatic anorthosite-tonalite-trondhjemite (ATT) suite and the Sulfide Veins, associated to late magmatic-hydrothermal activity. For this we performed detailed mineralogical (electron probe micro-analysis, enabling to assess ƒS2-T for sulfide equilibrium), whole-rock and isotopic (sulfur and lead) geochemistry for the sulfide occurrences which are further supported by a wide data-set available for LGS and related mesocratic suites.The Internal Sulfides developed under low sulfidation, non-dynamic magmatic conditions, similar to those of the accessory sulfides in LGS gabbros. They segregated from chalcophile depleted melts at the end of a crystal fractionation cycle, without contribution from external sulfur.The Marginal Sulfides have a pyrite-bearing, magmatic sulfide assemblage hosted in exceptionally coarse-grained clinopyroxenites at the LGS SW border. The metal-poor, Sulfide Veins occur within a metasomatic halo adjoining amphibole-albite pegmatoid dykes. The latter two sulfide types share geochemical affinities and represent late-magmatic systems developed during uplift and intrusion by evolved, lower-crustal contaminated magmas. The Marginal Sulfides are mafic precursors of the coeval magmatic-hydrothermal system to which pegmatoid dykes and Sulfide Veins are related. Although estimated primary PGE abundances for LGS magmas are within the range of MORBs, all sulfide types derived from PGE ± Ni depleted melts due to sulfide segregation during underplating. The presence of deep crustal faults that prevent magma stalling and sulfide saturation at depth is paramount to generate orogenic magmatic sulfide deposits of economic value.