Abstract The Vermelho Complex, located in the Carajas 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., Onca-Puma, Jacare, 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 Carajas 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 Carajas (e.g., Fo92 for the Serra da Onca 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 eNd (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 Carajas. Although no major magmatic deposits are known in Carajas, the potential for future discoveries in this underexplored region should not be disregarded.

中文翻译：

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巴西 Carajás 矿产省 Vermelho 杂岩的岩浆结构和岩石学：镁铁质-超镁铁质侵入体下部岩浆过程的证据

摘要 Vermelho Complex 位于 Carajas 矿产省，是新太古代（约 2.76 Ga）镁铁-超镁铁层状侵入体群的一部分，以拥有世界级红土镍矿床（例如 Onca-Puma、Jacare、Vermelho ）。Vermelho Complex（220 Mt，含 1.23 wt% Ni）的镍红土资源存在于两个高原型山丘（称为 V1 和 V2）中，其中厚厚的（~30-40 m）风化剖面没有受到侵蚀。这篇论文是对 Vermelho Complex 层状岩石的第一次详细的地质和岩石学研究。结果表明，Vermelho 杂岩的侵入结构包括与唾液质主岩接触的镁铁质-超镁铁质岩的复杂基底层序，为层状杂岩边缘下部带的变异性提供了进一步的证据。此外，我们的研究结果为卡拉哈斯新太古代基性-超镁铁质岩浆活动承载岩浆沉积的潜力增加了新的见解。中等大小（长约 9.5 公里，宽约 1.5 公里）的 Vermelho 杂岩是一个 NE-SW 走向的镁铁质-超镁铁质侵入体，侵入新谷杂岩（约 3.0 Ga）的花岗质-片麻岩中。岩浆构造由出露在高原状丘陵中的水平层状超基性岩堆（上区）组成，覆盖在复杂但广泛的亚水平基性岩-超基性岩（下区）基底层序之上。下带由三个亚带组成：（1）底部为夹层纯英岩、方辉石和斜方辉石；(2) 夹层的斜方辉石和黑斑；(3) 上亚带的辉长岩和含石英的白辉长岩。下带的结晶序列为橄榄石+铬铁矿、斜辉石+铬铁矿、斜方辉石、斜方辉石+斜长石、斜方辉石+斜长石+单斜辉石，下亚带丰富的原始橄榄石+铬铁矿超镁铁质堆积物和分馏含石英累积基镁铁质上亚区。下带在 V1 高原下方延伸超过 400 m，在那里建议一种管道型结构，并朝着侵入体的东北和西南部分逐渐变浅。来自与超镁铁质堆积物和围岩接触面相交的钻孔的岩心样品由约 50 m 的层状菱镁矿和正辉石岩组成，没有细粒冷硬边缘。超镁铁质堆积体最原始的矿物成分发生在距围岩接触几十米的地方，这可能是由于第一批岩浆充满岩浆房时母岩浆在进给系统中的分馏。位于 Vermelho 杂岩体基底接触处的岩石的堆积序列和分馏趋势没有表明完全发展的边缘反转的证据。上带仅保留在 V1 和 V2 高原，由两个亚带组成，总厚度约为 175 m，大致对应于 V1 高原的高度。下部亚带由厚达 50 m 的斜方辉石岩和伴生的铬铁矿荚组成。斜方辉石岩与下伏辉长岩的急剧一致接触表明分馏趋势发生了重大突破。上带的斜方辉石是粗至中粒的斜方辉石+铬铁矿堆积体。铬铁矿在斜方辉石岩中主要以播散的细粒自面体晶体的形式出现，但也出现在几厘米厚的富含铬铁矿的豆荚或不连续层中。上亚带（厚达 125 m）由广泛风化的纯晶岩和菱镁矿组成。V1 和 V2 山坡的下、上亚带之间的急剧接触很好地暴露。由于普遍的风化或蛇纹石化，原生硅酸盐无法保存在来自上亚带的岩石中。超镁铁质岩石是蛇纹石化的纯英岩和菱镁矿，通常保存有堆积至中堆积纹理，由橄榄石和斜方辉石假晶型指示。铬铁矿，蛇纹岩中显眼的副矿物（2-3 vol%），作为部分改变的自形晶体发生。Vermelho 杂岩体 (Fo85.6-90.5) 中积云橄榄石的成分范围与报道的源自原始母体岩浆（即高 Mg#）的层状侵入体的成分范围相当。Vermelho 复合体中最原始的积云橄榄石的成分也与在 Carajas 拥有大型镍红土矿床的层状侵入体中橄榄石的成分相当（例如，Serra da Onca 复合体的 Fo92；Serra do Puma 复合体的 Fo89） . Vermelho Complex 中斜辉石相对于单斜辉石的早期结晶表明存在二氧化硅饱和的原始母岩浆。Vermelho 杂岩的黑​​长石和辉长岩的地幔归一化不相容微量元素模式被分馏，如轻稀土和钍的相对富集所示，具有明显的负 Nb 和 Ta 异常。这些岩石地球化学特征，连同斜方辉石的早期结晶，可以解释为部分被大陆地壳污染的原始地幔熔体的产物。在 Vermelho 杂岩体的母岩浆就位和/或上升过程中，较旧的唾液腺地壳的同化也得到了 Nd 模型年龄介于 2.90 和 3.30 Ga 和 eNd (T=2.77 Ga) 值从 -7.3 到 +0.1 的支持。这些结果与被较旧大陆地壳不同程度地污染的原始地幔熔体一致，得到了基性-超基性岩与新谷杂岩（约 3.0 Ga）的较旧乡村岩石之间的横切接触的支持。Vermelho Complex 的岩浆演化被描述为两个主要岩浆侵位事件的结果，第一个与下区相关，第二个与上区相关。两次岩浆活动产生的堆积岩和斜方辉石成分的序列相似，表明它们的母岩浆成分相近。然而，在上带基底带刚好与正辉石岩伴生的铬铁矿的存在表明，与第一次事件不同，第二次事件的原始岩浆是铬铁矿饱和的。这种情况与第二次岩浆侵位事件中铬铁矿饱和泥浆产生的铬铁矿相一致。大型火成岩省中的镁铁质-超镁铁质侵入体是岩浆成矿的主要宿主，包括主要的 Ni-Cu-(PGE) 硫化物矿床。通常用作 Ni-Cu-(PGE) 岩浆矿床区域勘探指南的几个特征，包括丰富的富含橄榄石的镁铁质-超镁铁质岩石、动力岩浆系统和地壳尺度结构，都被考虑在 Carajas 的镁铁质-超镁铁质杂岩中。尽管在卡拉哈斯没有已知的主要岩浆矿床，但不应忽视在这个未充分勘探的地区未来发现的潜力。