Summary:
Magmatic Ni-Cu-PGM sulfides form by the accumulation of immiscible sulphide liquid that scavenged chalcophile elements from a coexisting silicate magma (e.g., Naldrett, 2004, Barnes et al., 2016). Textural relationships between sulphides and their host silicates are key evidence for their origin as immiscible sulphide liquids (Barnes et al., 2017, 2018). The magmatic origin of the Luanga PGM + Au + Ni deposit is supported by textural and mineralogical features described in different styles of PGM mineralization, particularly the MSZ, Ni-Rh and SZ.
In these different PGM zones, sulphide blebs consisting of po+pn±cpy are interstitial to cumulus olivine and/or pyroxene. In addition, sulphide blebs enclosed in cumulate crystals, as well as their rounded/corroded faces, provide unequivocal evidence for a magmatic origin of sulphides and PGM. Variable lithochemical features in PGM zones located in distinct stratigraphic horizons of the Luanga Complex, including different metal tenors, as well Pt/Pd and Rh/Pd ratios, indicate that several events of mineralization occurred during the magmatic evolution of the Luanga Complex. The occurrence of several mineralized horizons in the Luanga Complex, including PGM mineralization hosted in chromitites, has remarkable similarity with reef-type productive deposits (e.g., Bushveld and Stillwater).
The widespread alteration of rocks from the Luanga Complex has partially disrupted their primary magmatic features. In the Luanga Complex, magmatic silicates are partially altered and commonly occur as pseudomorphs. The magmatic sulphides have also been partially altered during the widespread alteration. The most common alteration of primary sulphides (po-pn-cpy) consists of their replacement by magnetite and Fe-hydroxides. Because this alteration is heterogeneous at different scales (from mineral crystals, up to several hundred meters thick zones) and largely preserves primary textures and compositions of cumulate rocks and PGM mineralized zones, magmatic features can be recognized throughout the layered intrusion.
Mineralization
The Luanga PGM + Au + Ni mineralized envelope follows the arc-shaped structure of the Mafic-Ultramafic Complex along approximately 8.1 km. The deposit is subdivided into three separate mineralized sectors, named North, Central and Southwest.
The TZ of the Luanga Complex hosts several PGM mineralized units, including the Main Sulphide Zone (MSZ) which hosts the bulk of PGM resources of the Luanga Complex. Other mineralized layers are identified within the TZ and on the UZ.
In addition, several thin chromitites layers or lenses occur in the Luanga Complex either in the upper or in lower stratigraphic portions of the TZ, the latter occurring where they are hosted by ultramafic cumulates. The upper chromitite layers are developed on the immediate contact with the overlying MZ, where they are hosted by plagioclase-bearing norite cumulates.
Mineralized zones of the Luanga Complex are grouped into six different styles of PGM + Au + Ni mineralization. They are named as: (i) MSZ, (ii) Low Sulphide Zone (LSZ), (iii) Chromite-associated Zone (Chr-PGM) (iv) Nickel/Rhodium Sulphide Zone (Ni-Rh), (v) Sulphide Zone (SZ) and (vi) Massive Sulphide Zone (MASU).
Main Sulphide Zone (MSZ)
The MSZ hosts most of the PGM + Au + Ni resources of the Luanga Complex. It forms a 10–50 m stratabound interval of disseminated sulphides along the UZ–TZ contact. Mineralization occurs as sulphide blebs interstitial to cumulus Opx–Ol or pseudomorphs, composed mainly of Po and Pn with minor Ccp. Pn represents ~30–50% of the sulphide fraction, typically as fine-grained crystals (<1 mm). PGM assemblages are dominated by Pt-Pd-bismuth tellurides, with stannides, arsenides, and antimonides, hosted within sulphides or at sulphide-silicate contacts.
Low Sulphide Zone (LSZ)
The LSZ comprises PGM-mineralized rocks with little to no base metal sulphides and/or abundant chromite. At the Luanga Complex, it forms stratabound zones up to 30 m thick across the TZ, hosted mainly by harzburgite and orthopyroxenite. LSZ occurs as irregular stratabound lenses above the MSZ and throughout the TZ stratigraphy in the Central, Southwestern, and North sectors. Lenses dip steeply southeast in the Central and Southwestern sectors and west in the North sector. Mineralization consists mainly of Pt-arsenides (sperrylite, PtAs2), with subordinate Pt-Pd stannides, antimonides, and minor bismuth tellurides.
Chromite-associated Zone (Chr-PGM)
Chr-PGM mineralization occurs as chromitite pods (<60 cm) and disseminated chromite clusters within orthopyroxenite, harzburgite, and norite. It is located mainly at the upper TZ and lower MZ. Chromitite pods consist of fine-grained cumulus chromite, while disseminated chromite appears as euhedral crystal clusters.
Ni-Rh Sulphide Zone (Ni-Rh)
The Ni-Rh mineralized zone has been identified only within the UZ on the Central and on the Southwest sectors. It occurs as lenses of disseminated to net-textured sulphides (up to 25%) interstitial to cumulus Cpx and olivine or their pseudomorphs.
Ni-Rh zones have variable thickness (up to 40 m) and are commonly hosted by interlayered wehrlite and dunite, and minor clinopyroxenite.
Sulphides consist of Po (~60-70%), Pn (~30-40%) and minor Ccp. Pentlandite occurs mainly as medium-grained crystals (1-2 mm).
Sulphide Zone (SZ)
The SZ, limited to the Northern sector of the Transition Zone, consists of irregular N-S stratabound bands of disseminated sulphides hosted mainly in orthopyroxenite, closely associated with Chr-PGM and LSZ mineralization. It extends ~2 km with variable thickness up to 40 m and 1–5% fine-grained sulphides, mainly Po-Pn with minor Ccp. Chemically, it contains 2–3 ppm Pt+Pd over 30–40 m, low Pt/Pd ratios (<0.5, locally to 1.0), and variable Ni up to 0.20%.
Massive Sulphide Zone (MASU)
Identified in 2022 in the North sector, the MASU was first intersected in drill hole DDH22LU047, returning 11 m at 4.24 g/t 3EqPt+Au, 2.04% Ni and 1.23% Cu within a hydrothermal alteration zone at the eastern margin of the Luanga Complex.
Host rocks and footwall consist of amphibolitite, amphibole–garnet–biotite–magnetite assemblages, and banded iron formation, with sulphide-bearing orthopyroxenite and norite above and adjacent. Hydrothermal Fe-Ca-K alteration has pervasively modified cumulate rocks, forming MASU to semi-MASU breccias, suggesting Ni-Cu-PGM mineralization derived from remobilized primary sulphides, possibly linked to the regional IOCG system of Carajás.
Sulphides are mainly pyrrhotite (80–90%) and pentlandite (10–20%), with chalcopyrite-rich domains (locally 60–70% Ccp). Pentlandite occurs with pyrrhotite as fine (<0.5 mm) grains or exsolutions. Mineralization generally has Ni>Cu and Pd>Pt, with approximate tenors of Ni <6%, Pd <6 ppm, Pt <2.5 ppm and Rh 0.2 ppm.
Widespread alteration has overprinted magmatic silicates and sulphides, replacing them with magnetite and Fe-hydroxides. Despite this, primary cumulate textures and PGM mineralized features remain locally preserved across the intrusion.