Geology and geochemistry of the Ni deposits in the Umburana mafic-ultramafic system, Curaçá Valley, Bahia, Brazil: Constraints on crustal contamination

• New mineral, lithogeochemical, and isotopic data on new Ni deposits in Brazil. • Strong evidence of crustal contamination in mineralized mafic–ultramafic rocks. • Sulfur sourced from sulfide- and sulfate-bearing country rocks. • New genetic model for magmatic sulfide deposits in the Curaçá Valley. The Umburana mafic ultramafic system, part of the Cu-rich Curaçá Valley district within the Itabuna-Salvador-Curaçá belt, features intrusions that host the recently discovered LZ and VB Ni-sulfide deposits. These intrusions notably contain abundant xenoliths and intersect several types of metamorphosed and deformed country rocks containing variable proportions of sulfide and sulfate. In this study, we provide new field-based, mineralogical, lithogeochemical and S-isotopic data to constrain the formation of LZ and VB. The LZ deposit comprises an upper orthopyroxenite with minor disseminated sulfides and a lower hornblende-clinopyroxenite hosting abundant patchy pyrrhotite-pentlandite-pyrite (up to 1.5 wt% Ni). The VB deposit consists of interlayered websterite and gabbronorite, hosting varying proportions of disseminated and patchy sulfides, with xenolith-bearing massive pyrrhotite-pentlandite lenses (up to 8 wt% Ni) occurring in central zones and associated with hornblende-diorite layers. In LZ and VB, hybrid intrusions and mineralized zones containing xenoliths have variably high proportions of plagioclase, quartz, monazite, zircon, and phlogopite, along with diverse Al 2 O 3 /MgO and CaO/MgO values resembling country rocks. Both hybrid and country rocks are enriched in REEs and Zr, though some xenolith-free rocks also show this enrichment. These characteristics imply that this mafic–ultramafic system was affected by crustal contamination during magma transport and emplacement. The high S/Se (avg. = 10535) and S-isotope (avg. = +5.3 ‰) values within high grade zones of LZ, particularly within or near an anhydrite-bearing xenolith (δ 34 S = +12.49 ‰; S/Se = 38750), suggest that sulfate-bearing contaminants were an important S-source. In VB, high-grade zones yielded elevated S/Se (avg. = 7535) with S-isotope values (avg. = +3 ‰) that are above but close to mantle signatures. These signatures may reflect the assimilation of S-bearing wall rocks with different S/Se and δ 34 S values, such as regional country rocks (S/Se = 40 to 15958; δ 34 S = –14.3 to +10.4 ‰), combined with partial homogenization with magmatic S during magma transport and emplacement. Hence, our data indicates that crustal contamination pre- to syn -emplacement was crucial in the formation of the LZ and VB deposits. Both deposits exhibit an MSS-dominance with localized sulfide fractionation observed in minor disseminated Cu-sulfides near massive sulfides of VB. Low PGE contents and high Cu/Pd ratios in LZ and VB suggest magmas depleted in PGE, possibly due to early sulfide segregation influenced by contamination during magma ascent.