Abstract

The Caboclo dos Mangueiros deposit, the most recent discovery of Ni-Cu sulfide mineralization in Brazil, has estimated resources of 200 million tonnes (Mt) with grades of 0.20 wt % Ni and 0.13 wt % Cu. The deposit is located in the northwestern margin of the São Francisco craton, close to the contact with the Rio Preto Neoproterozoic fold belt. Due to regional covers of Tertiary-Quaternary sediments and ferruginous lateritic crusts, geologic features of the deposit and its ultramafic plutonic host rocks were mainly described using drill hole logs and geophysical surveys. Mineralization is hosted within an elongated (~2 km long and ~500 m wide) ultramafic sill emplaced into sedimentary country rocks. The absence of well-defined layering, together with abrupt changes in textures, suggests a magmatic structure broadly similar to those described in channelized sills. Except for minor discrete sheared zones and a thin (<4 m thick) fine-grained border zone, the ultramafic intrusion consists of massive cumulates with magmatic textures. Primary minerals are partially to extensively altered to hydrous minerals, as indicated by the pseudomorphic replacement of olivine by serpentine and magnetite and of clinopyroxene by tremolite-actinolite. Metamorphic assemblages described in the country rocks (i.e., albite + quartz + biotite + muscovite + epidote), cumulate pseudomorphs (i.e., tremolite-actinolite + serpentine + magnetite), and rocks from the border zone (i.e., actinolite + chlorite), are consistent with recrystallization under conditions of the greenschist facies of regional metamorphism. The dynamic magmatic system consists of dunite in the channelized facies and more fractionated wehrlite and clinopyroxenite toward the margins. Bulk-rock composition and the compositional range of Mg# of cumulus clinopyroxene from 0.78 to 0.94 support a moderate to primitive mafic composition for the parental magma. Sulfide Ni-Cu mineralization consists of disseminated sulfide blebs (1–5 vol %) with mineralogy and textures typical of those originated from immiscible sulfide liquids segregated from mafic-ultramafic magmas. Disseminated sulfides, homogeneously distributed in different cumulates, occur throughout the entire ultramafic intrusion. Positive correlations between S-Ni (0.81) and S-Cu (0.87) in the sulfide ore indicate that both metals are mainly contained in sulfides. The sulfide mineralization has moderate Ni (3.46 wt %) and Cu (2.93 wt %) tenors and Ni/Cu ratios close to 1. Positive correlations between S-Pd (0.90) and S-Pt (0.87) in the sulfide ore indicate that both metals are associated with sulfides. High Cu/Pd ratios and very low Pt and Pd tenors indicate a sulfide liquid composition depleted in platinum group elements (PGEs). Combined modeling of Cu, Pt, and Pd tenors suggests that metal contents of the parental magma resulted from previous sulfide segregation of the primitive magma, possibly during residence in lower staging chambers. The Caboclo dos Mangueiros deposit, the first significant Ni-Cu sulfide mineralization hosted in a dynamic magmatic system in the underexplored northern margin of the São Francisco craton, opens a new window for exploration for Ni-Cu-PGE sulfides in Brazil.

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