DellaGiustina
21 Votorantim Metais Ltda, Praça Ramos de Azevedo, 254, São Paulo, SP. 01037-912. Brazil 2 Instituto de Geociências, Universidade de Brasília, Brasília, DF. 70910-900. Brazil
Abstract
The Limoeiro deposit, located in a high-grade mobile belt of the Brasiliano Orogenic Cycle (650 – 500 Ma), represents one of the few magmatic Ni-Cu(-PGE) sulfide discoveries in recent years. The deposit is hosted by an ultramafic chonolith consisting of a partly deformed and metamorphosed tube-like intrusion affected by brittle faults within high-grade gneiss and schist. The absolute age of the ultramafic intrusion that host the Limoeiro deposit is not yet known, but regional geological constraints suggest that both intrusion and hosts rocks belong to a terrain older than the orogenic cycle. Peak metamorphic parageneses of the ultramafic intrusion and country rocks indicates granulite facies metamorphism, interpreted to result from the Neoproterozoic Brasiliano Orogenic Cycle. The Limoeiro deposit is the first significant discovery of Ni-Cu sulfide in the Borborema Province, opening a new window for exploration in this large orogenic belt.
Despite the high-grade metamorphism and the tectonic deformation, the primary magmatic structure of the intrusion hosting the Limoeiro deposit is remarkably well preserved. This intrusion forms a tube-like conduit (chonolith) with cross sections a few hundreds of meters in diameter and with a considerable length of more than 4 km. The conduit consists of two distinct sequences of ultramafic rocks, denominated Upper and Lower sequences, each one consisting of a core of harzburgite enveloped by orthopyroxenite, with an irregular and
discontinuous outer shell of amphibolite. These ultramafic sequences, characterized by similar composition and structure, have distinctively different S contents resulting from the presence of disseminated Ni-Cu sulfide mineralization only in the Upper Sequence. The magmatic system of the Limoeiro deposit is interpreted to result from the input of two major pulses of magma with similar composition, but sulfide saturation and segregation is restricted to the second magma pulse (Upper Sequence). Nickel-Cu(-PGE) sulfide mineralization is broadly concordant with the Upper Sequence and different ore bodies resulted from subsequent tectonic slicing of the originally continuous tube-like structure. The bulk of the mineralization in all ore bodies consists of thick (up to 150 meters) and elongated (up to 1 km) masses of rock containing disseminated sulfides broadly concordant with the chonolithic structure. Thin layers of massive ore occur in the lower portions of some of the ore bodies, and represent about 3 vol% of the Limoeiro deposit. The intrusion hosting the Limoeiro deposit is probably the best-preserved chonolith that has been overprinted by granulite facies metamorphism within an orogenic terrain. This study indicates that reconstruction of small magma conduits, an important tool for exploration for Ni- Cu sulfide deposits, may be successfully achieved within high-grade orogenic terrains.
Introduction
The Limoeiro Ni-Cu(-PGE) sulfide deposit was discovered in northeastern Brazil by Votorantim Metais Ltda (VM) in 2009 (Votorantim Metais Ltda, 2012). The deposit represents a recent nickel sulfide discovery, probably resulting from additional investments in exploration by the mining industry following the increase of base and precious metals prices in 2004. After the discovery in 2009, VM started an advanced project in the area to evaluate the deposit, including 27,500 m of diamond drilling completed in September 2012. Unfortunately Ni-Cu(-PGE) resources are not available for publication at this moment.
The deposit is hosted within a complex, concentrically zoned, tube-like (chonolithic) subhorizontal orthopyroxenite-harzburgite intrusion in the Borborema Province, northeastern
Brazil (Fig. 1.1). The chonolithic architecture of the ultramafic intrusion has similarities with other mafic-ultramafic intrusions that host Ni-Cu magmatic sulfide deposits, such as Nebo-Babel in Australia (Seat et al., 2007; Seat et al., 2011), the Kabanga North in Tanzania (Evans et al., 2000; Maier et al., 2010), Uitkomst in South Africa (Gauert et al., 1995; De Waal and Maier, 2001; Li et al., 2002), Eagle in USA (Ripley and Li, 2011) and Thunder Bay North in Canada (Goodgame et al., 2010). Although chonolithic structures have discrete geochemical and geophysical fingerprints, especially when deformed and metamorphosed in mobile belts, they are critical features for Ni-Cu sulfide exploration.
A SC C WA BP CP NP RP K SA Phanerozoic orogen Brasiliano Pan-African orogen Paleoproterozoic to early Neoproterozoic orogen Craton
A
Phanerozoic coverRio Capibaribe Terrane Limoeiro Ni-Cu Deposit Cities PeSZ PaSZ Recife Aracajú Fortaleza 6º S 9º S 36º W 39º W
B
São Francisco Craton 200 kmFig. 1.1 (A) Pre-rift reconstruction of western Gondwana showing the main cratons and Brasiliano/Pan-African
provinces (modified from Neves and Alcantara, 2010). Abbreviations: A, Amazonia; BP, CP, NP: Borborema, Nigeria and Cameroon provinces; K, Kalahari; RP, Rio de La Plata; SA, Sahara Metacraton; SCC, São Francisco/Congo; WA, West Africa. (B) Sketch showing the main shear zones that limit the domain subdivision of the Borborema Province. Abbreviations: PaSZ, Patos Shear Zone; PeSZ, Pernambuco Shear Zone.
The Limoeiro deposit is the first discovery of a nickel sulfide deposit in the Borborema Province (Almeida et al., 1981), a high-grade mobile belt formed during the Neoproterozoic Brasiliano/Pan-African orogenic cycle. This geological setting was usually considered barren regarding Ni-Cu(-PGE) deposits, probably due to the dominance of gneiss, mylonite and migmatite rocks in a intensely deformed and faulted terrain.
This study is part of an assessment of the Limoeiro deposit supported by VM (Mota-e- Silva, in prep.). In this first paper, we present the results of a field and petrographic study of the Limoeiro deposit, host intrusion and country rocks. We discuss the intrusive architecture and localization of nickel sulfides, focusing on sections of the chonolith, which are best preserved from later metamorphism and deformation. Our findings provide new insights for the identification of Ni–Cu sulfide deposits in high-grade mobile-belt terrains worldwide.