Magmatic structure and petrology of the Vermelho Complex, Carajás Mineral Province, Brazil: Evidence for magmatic processes at the lower portion of a mafic-ultramafic intrusion

Abstract

The Vermelho Complex, located in the Carajás Mineral Province, is part of a cluster of Neoarchean (ca. 2.76 Ga) mafic-ultramafic layered intrusions best known for hosting world-class nickel laterite deposits (e.g., Onça-Puma, Jacaré, Vermelho). Resources of nickel laterite hosted by the Vermelho Complex (220 Mt at 1.23 wt% Ni) occur in two plateau-type hills (known as V1 and V2) where a thick (~30–40 m) weathered profile was preserved from erosion. This paper is the first detailed geological and petrological study of the layered rocks of the Vermelho Complex. Results reveal that the intrusive architecture of the Vermelho Complex includes a complex basal sequence of mafic-ultramafic rocks in contact with sialic host rocks, providing further evidence for the variability of marginal lower zones of layered complexes. In addition, our results add new insights into the potential of the Neoarchean mafic-ultramafic magmatism in Carajás to host magmatic deposits.The medium-size (~9.5 km long and ~1.5 km wide) Vermelho Complex is a NE-SW trending mafic-ultramafic intrusion emplaced into granitic-gneissic rocks of the Xingu Complex (ca. 3.0 Ga). The magmatic structure consists of a horizontally layered pile of ultramafic rocks (Upper Zone) exposed in the plateau-like hills, overlying a complex but broadly subhorizontal basal sequence of mafic-ultramafic rocks (Lower Zone).The Lower Zone consists of three subzones: (1) interlayered dunite, harzburgite and orthopyroxenite at the base; (2) interlayered orthopyroxenite and melanorite; and (3) gabbronorite and quartz-bearing leucogabbronorite in the upper subzone. The crystallization sequence of the Lower Zone consists of olivine + chromite, orthopyroxene + chromite, orthopyroxene, orthopyroxene + plagioclase, orthopyroxene + plagioclase + clinopyroxene, with abundant primitive olivine + chromite ultramafic cumulates in the lower subzone and fractionated quartz-bearing mafic cumulates in the upper subzone. The Lower Zone extends for more than 400 m below the V1 plateau where a conduit-type structure is suggested, and becomes progressively shallower toward the northeast and southwestern portions of the intrusion. Core samples from drill holes that intersect the contact of the ultramafic cumulates and country rocks consist of ~50 m of interlayered harzburgites and orthopyroxenites, with no fine-grained chilled margins. The most primitive mineral compositions of ultramafic cumulates occur dozens of meters away from the contact with country rocks, possibly due to fractionation of the parental magma in the feeding system when the first batches of magmas filled the magma chamber. Evidence for fully developed marginal reversals is not indicated by the sequence of cumulates and fractionation trends of the rocks located at the basal contact of the Vermelho Complex.The Upper Zone, just preserved in the V1 and V2 plateaus, consists of two subzones with a total thickness of about 175 m, broadly corresponding to the height of the V1 plateau. The lower subzone consists of up to 50 m thick orthopyroxenite with associated chromitite pods. The sharp concordant contact of orthopyroxenite with underlying gabbroic rocks indicates a major break in the fractionation trend. Orthopyroxenite in the Upper Zone is a coarse-to medium-grained orthopyroxene + chromite adcumulate. Chromite occurs mainly as disseminated fine-grained euhedral crystals in orthopyroxenite, but also occurs in few centimeters thick chromite-rich pods or discontinuous layers. The upper subzone (up to 125 m thick) consists of extensively weathered dunite and harzburgite. The sharp contact between lower and upper subzones is well exposed in the slopes of the V1 and V2 hills. Due to pervasive weathering or serpentinization, primary silicates are not preserved in rocks from the upper subzone. Ultramafic rocks are serpentinized dunite and harzburgite with commonly preserved adcumulate to mesocumulate textures indicated by olivine and orthopyroxene pseudomorphs. Chromite, a conspicuous accessory mineral (2–3 vol%) in the serpentinites, occurs as partially altered euhedral crystals.The compositional range of cumulus olivine in the Vermelho Complex (Fo85.6-90.5) is comparable with those reported for layered intrusions originated from primitive parental magmas (i.e., high Mg#). The composition of the most primitive cumulus olivine of the Vermelho Complex is also comparable with those reported for olivine in layered intrusions hosting large Ni-laterite deposits in Carajás (e.g., Fo92 for the Serra da Onça Complex; Fo89 for the Serra do Puma Complex). The early crystallization of orthopyroxene relative to clinopyroxene in the Vermelho Complex suggests a silica-saturated primitive parental magma. Mantle-normalized incompatible trace element patterns of melanorite and gabbroic rocks of the Vermelho Complex are fractionated, as indicated by relative enrichment in LREE and Th, with pronounced negative Nb and Ta anomalies. These lithogeochemical features, together with the early crystallization of orthopyroxene, may be interpreted as the product of a primitive mantle melt partially contaminated with continental crust. Assimilation of older sialic crust during emplacement and/or ascent of the parental magma of the Vermelho Complex is also supported by Nd model ages between 2.90 and 3.30 Ga and εNd (T=2.77 Ga) values from −7.3 to +0.1. These results, consistent with a primitive mantle melt variably contaminated with older continental crust, are supported by the crosscutting contact between mafic-ultramafic rocks and older country rocks of the Xingu Complex (ca. 3.0 Ga).The magmatic evolution of the Vermelho Complex is described as the result of two major events of magma emplacement, the first associated with the Lower Zone and the second with the Upper Zone. The sequence of cumulate rocks and orthopyroxene compositions resulting from both magmatic events is similar, suggesting that their parental magmas have close compositions. However, the presence of chromitites just associated with orthopyroxenites at the basal zone of the Upper Zone suggests that, different from the first event, the primitive magma of the second event was chromite-saturated. This scenario is consistent with chromitites resulting of chromite-saturated slurries during the second event of magma emplacement.Mafic-ultramafic intrusions in large igneous provinces are a primary host for magmatic mineralizations, including major Ni–Cu-(PGE) sulfide deposits. Several features commonly used as guidelines for regional exploration for Ni–Cu-(PGE) magmatic deposits, including abundant olivine-rich mafic-ulramafic rocks, dynamic magmatic systems and crustal scale structures, are contemplated in the mafic-ultramafic complexes in Carajás. Although no major magmatic deposits are known in Carajás, the potential for future discoveries in this underexplored region should not be disregarded.