Regional Geology

CHAPTER 2 GEOLOGICAL SETTING

The Serra do Navio deposit is hosted by Paleoproterozoic rocks forming part of the Guiana of the Amazonian (Fig. 2.1). This chapter describes the geologic terranes of the Amapa province, before providing a generalized and simplified geologic setting of the area around Serra do Navio in the northern portion of the . The evolution of the Amazonian craton with its lithostructural relationships, rock types, mineralization and geochronology is discussed first, to place the Serra do Navio in a regional geologic context. An appraisal of the stratigraphy and structure in the area immediately around the Serra do Navio is also given. This is meant to establish a possible relationship between the nature of the rock association and major mineral deposits with the stratigraphy and structures in the sub region as well as other domains of similar geographic and tectonic setting on other continents.

2.1 Amazonian Craton

The Amazonian Craton is one of the most extensive cratonic domains in the world. It comprises of two main that expose suites of and rocks; namely, the Guiana shield and the Guaporé shield (Central Brazilian Craton) (Fig. 2.1). The Sao Francisco Craton and the poorly exposed Rio de la Plata Craton (Zhao et al., 2002) are both located to the south of the Amazonian craton. A fourth, smaller craton, the São Luis craton is present to the east of the Guaporé shield.

The Guiana Shield, with a surface area of nearly 900,000km2, represents the northern most segment of the Amazonian Craton and is composed of both Archean and Paleoproterozoic crustal blocks. However, intense weathering and poor bedrock cover by younger Phanerozoic rocks obscure some cratonic domains (Fig. 2.2).

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Fig.2.1: The cratonic domains of , . All the cratons shown are part of the Amazonian Craton (After Zhao et al., 2002).

Dardenne and Schobbenhaus (2000) contend that the Guiana Shield may be divided into (a) a terrane of Archean rocks in the western region of ; (b) a Paleoproterozoic granite-greenstone terrane some 300km wide, lying along the Atlantic coast; (c) a non- differentiated terrain consisting of granite and and; (d) a central and western part with extensive cover of Paleoproterozoic felsic volcanic rocks and Mesoproterozoic continental sediments. Terrains underlain by rocks of defined Archean age are rare in the Guiana Shield. The only Archean terrane known in this shield is the c.a 3.4 Ga Imataca Complex in Venezuela (Fig. 2.3). The Archean consists of 3.4–2.9 Ga TTG / migmatites and voluminous 2.8 – 2.6 Ga plutons and greenstones (Barbosa and Fonteilles, 1989).

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Fig. 2. 2: The tectonic map of Brazil; younger Phanerozoic cover masks the cratonic domains (CPRM, 2001).

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The Paleoproterozoic fold belts consist mostly of 2.5 – 2.0 Ga supracrustals, thick turbidite deposits, volcanics and syntectonic granites (Zhao, et al., 2002). The Serra do Navio deposit is situated in one of these younger fold belts.

Most of the Guiana Shield formed during protracted periods of intense magmatism, metamorphism and deformation culminating in the Trans-Amazonian tectonothermal event, bracketed at 1.9 – 2.1 Ga (Voicu et al., 2001). The Trans-Amazonian Orogen is mostly exposed on the northeastern part of the Guiana Shield and consists of reworked Archean basement and Paleoproterozoic fold belts. The Serra do Navio manganese deposit outcrops on a Paleoproterozoic fold belt on the south eastern part of the Guiana shield (Fig.2.3). This Paleoproterozoic in the region of the Serra do Navio has similar ages (Zhao et al., 2002; Dantas et al., 2004) and geographic setting with the Birimian of West Africa (Nyame, 1998).

2.2 Mineralization

Mineralization in the Guiana Shield is associated with both sedimentary and volcanic rocks, and may be related to the tectonic evolution of the shield itself. Most prominent among the mineral resources relevant to this study are manganese ores. The manganese oxide ores at Serra do Navio have been well studied and are postulated to be lateritic in origin (Rodrigues et al., 1986; Costa, 1997). Numerous but economically insignificant manganese deposits are known on the western part of the Guiana shield.

Shear zone-hosted gold deposits are known from both Archean and Paleoproterozoic domains of the craton (Voicu et al., 2001). Most gold mineralization is closely associated with veins, stock works and breccias (Lafrance et, al, 1997; Voicu et al., 2001) along major and minor shear zones and in host rocks of variable composition. In rocks of the Paleoproterozoic domain, several porphyry copper deposits are reported, and are sometimes associated with gold. Cu-Au deposits are also exploited elsewhere on the

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Amazonian craton, mainly with three types of mineralization recognized: volcano- sedimentary, hydrothermal and supergene (Dardenne and Schobbenhaus, 2000).

Fig.2. 3: Regional geology of the northern Guiana shield, showing the geologic setting of the Serra do Navio deposit. Insert shows the areal extent of the Guiana shield as well as the southern Guaporé shield. Modified after Voicu et al. (2001).

Large chromite resources hosted by layered mafic complexes, as well as magmatic tin deposits are also mined (Dardenne and Schobbenhaus, 2000). Other ore deposits related to laterization are common in areas on the shield that have been dissected and undergone extensive erosion and weathering (Costa, 1997). These include lateritic nickel and copper deposits as well as some of the world’s largest lateritic iron ore and bauxite resources (Horbe and Costa, in press, 2004).

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2.3 Geology of the Amapa Province

The geology of the Amapa province in the vicinity of the Serra do Navio deposit is comprised predominantly of six tectonic units namely, an Archean granitic gneissic terrain, Paleoproterozoic supracrustals, a Paleoproterozoic greenstone belt, a Paleoproterozoic fold belt, Meso to Paleoproterozoic igneous intrusives and Cenozoic cover rocks (Fig. 2.4).

The Archean granitic-gneissic terrain forms the basement to the rocks in this area and is exposed to the northeast of the Serra do Navio deposit. These rocks underwent regional to granulite facies metamorphism and are in places associated with large scale NW directed faulting (Ledru et al., 1994). The granitic gneissic terrain is unconformably overlain by some Paleoproterozoic rocks that outcrop in the northern part of the Amapa province. The Serra do Navio lies to the south west of the Amapa province within such a metasedimentary succession of Paleoproterozoic age (Fig. 2.4). The NW-SW trending metasedimentary succession is thought to be similar in age to granite-greenstone terranes of low metamorphic grade that stretch in a similar direction and have been dated at 2.11 Ga (Voicu et al., 2001). The Paleoproterozoic metasedimentary succession and the greenstone belt have been intruded by Meso- to Paleoproterozoic orogenic igneous rocks. The igneous rocks are syn- to post-tectonic, and are correlated with the unmetamorphosed and undeformed, compositionally diversified intrusions of (Voicu et al., 2001). Younger Cenozoic rocks strike from N – S in the eastern part of Amapa and obscure large parts of the southeastern portion of the Paleoproterozoic fold belt (Fig. 2.4).

2.4 Geology of the Serra do Navio Deposit

2.4.1 Stratigraphy

The stratigraphy in the area surrounding the Serra do Navio deposit is subdivided into a basement of gneisses overlain by the Jornal and Serra do Navio formations of the Vila Nova Group (Fig. 2.5).

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Fig. 2.4: Geological map of the Amapa Province, showing all the geologic terranes that are exposed in the area.

The Vila Nova group is correlated with the Amapa Series that contains metasedimentary rocks of the Santa Maria Group, hosting iron formations and uneconomic manganese deposits that are located about 85 km west of the Serra do Navio deposit. The manganese ores of the Serra do Navio deposit are hosted within the Serra do Navio formation. Dardenne and Schobbenhaus (2000) note that the deposit has a strike length of 10km and N30ºW direction. The relative stratigraphic order and age of the Serra do Navio, the Jornal and Santa Maria groups is not certain; they could be contemporaneous, or the Santa Maria group could be older than the others (Nagell, 1962).

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Fig. 2.5: Geologic map of the Serra do Navio deposit. Note the location of the 3 drill holes described in this study at Mina F12 (modified after Scarpelli, 1973 and Rodrigues et al., 1986).

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2.4.1.1 The Basement

The basement in and around the Serra do Navio deposit is composed of granitic-gneisses of the Guianese Complex that are of poorly defined Paleoproterozoic to Archean age. The granitic-gneisses outcrop to the southwest and the northeast (Fig. 2.5). Leucocratic gneiss is composed of quartz, microcline ± oligoclase, and biotite, occasionally with some dark hornblende-rich zones parallel to the foliation (Scarpelli, 1973). The granitic-gneisses have been thrust onto the younger schists of the Serra do Navio formation to the northeast of the Serra do Navio deposit (Fig.2.5). In some places, quartz-rich gneisses form prominent ridges that are interpreted to have been derived from a silica-rich rock, possibly quartzite (Scarpelli, 1973).

2.4.1.2 The Jornal Formation

The Jornal Formation is essentially comprised of amphibolite. Scarpelli (1973) postulates that the amphibolites unconformably overlie the basement gneisses (Fig. 2.5). Amongst the lithological units of the Serra do Navio area, the Jornal Formation is the most widespread (Dorr, 1973). The amphibolites trend in a NW-SE direction with steep dips to the NE and SW (Fig. 2.5). They occur adjacent to belts of quartzites and biotite schists, parallel to foliation. The association may indicate that the original rock from which the amphibolite was derived was not homogenous (Scarpelli, 1973). Whether the amphibolite is igneous or sedimentary in origin is unresolved, as it can display characteristics typical of both (Dorr, 1973). No geochemical data is available to illustrate an igneous or sedimentary origin.

2.4.1.3 The Serra do Navio Formation

Overlying the Jornal Formation is the Serra do Navio Formation, host to the Serra do Navio deposit. It remains unclear whether the contact between the two formations is conformable or not. The Serra do Navio Formation is essentially a sequence of metasediments composed of quartzites, schists and carbonate-rich units. These metasedimentary lithologies alternate in a cyclic pattern (Scarpelli, 1973). The rocks

15 Regional Geology trend NW-SE and have shallow to steep dips in both the southwest and northeast directions (Fig.2.5). The rocks are also intensely folded (Fig.2.6), and it is in the exposed cores of these anticlines and synclines of these folds that open pits were established to mine manganese oxides (Fig.2.7). The Serra do Navio Formation is interbedded and interfolded with amphibolites and have been intruded by syntectonic granite (Fig.2.6). Additionally, basement granitic-gneisses have been thrust onto the Serra do Navio Formation. Locally, younger discontinuous cummingtonite schist and biotite quartzite are present in the NE of the Serra do Navio Formation (Fig.2.5). Even though schists dominate the Serra do Navio Formation, Scarpelli (1973) subdivided them into several distinct sedimentary “facies” based on the predominant mineralogy. The mineralogy of these different facies is, broadly speaking similar, but constituents occur in different proportions (Scarpelli, 1973).

Fig. 2.6: Cross-section along line A-B (see Fig. 2.6) at the Serra do Navio deposit showing the structural relationships of the different lithologies.

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Fig.2.7: Idealized sketch map of the outcrop of the carbonate protore and the lateritic oxide ore at the Serra do Navio deposit. Note that the vertical scale is extremely exaggerated.

Scarpelli (1968, 1973) separated the metasedimentary “facies” succession into a series of stratigraphic units in order to distinguish between the different lithologies. However, in this study, the nomenclature has been revised to account for the apparent effects of metamorphism (Table. 2.1). In particular, we avoid usage of the term “facies” in naming the metamorphosed lithologies, as this term has a very particular meaning in both sedimentology and metamorphic petrology, rendering its usage here vague and imprecise.

Table 2.1: The revised nomenclature of the rocks at Serra do Navio.

This Study Scarpelli (1968 and 1973)

Rhodochrosite marble Carbonatic protore

Mn-calcite marble Silicatic protore

Mn-carbonate schist Quartzous facies

Graphite schist Graphitous facies

Biotite schist Biotite facies

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2.4.2 Structure

The overall structure of the Serra do Navio deposit is only poorly known due to the scarcity of outcrop, lack of available data from mining operations for this study and the presence of similar lithologies at several levels of the stratigraphic column, rendering lateral correlation difficult (Scarpelli, 1973). It is, however, possible to decipher some general structure by considering surface geology (Fig.2.6) and a type drill core section (Fig.2.8). This drill core section is based on undated sections drawn by geologists during mining and that were available during this study (Fig. 2.8). Because it has several drill holes, the section seems very well constrained and is ideal to outline the general subsurface geology at the Serra do Navio deposit. The section illustrates the interbedding of carbonate protore and non-carbonate rocks. Additionally, it shows supergene-enriched manganese oxide ore capping carbonate protore. The section demonstrates that there is lateral continuity of the lithologies intersected by the drill cores. The non-carbonate rocks however, sometimes pinch out within the carbonate protore (Fig.2.8).

Fig 2.8: Profile across the Serra do Navio showing the inferred structure in the area occupied by drill core DH116.

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The metasedimentary sequence at the Serra do Navio is intensely folded, sheared and metamorphosed (Dorr, 1973). The folds vary from open to tight, plunging northeast and southeast at small to moderate angles (Fig 2.8). Their axial planes dip northeast. In some areas they turn into isoclinal folds, with parallel limbs dipping steeply northeast (Dorr, 1973). The carbonate-rich rocks attain their maximum thickness in synclines and anticlines (Fig. 2.8). Non-carbonate rocks may be occasionally deformed, like incompetent layers while the more quartz rich bands are more resistant to tectonic deformation (Scarpelli, 1973). Small-scale folds are pervasive, especially in places where schists are interbedded with amphibolites (Fig. 2.6). Trans-Amazonian foliation (Voicu et al., 2001), which is observed throughout the Guiana Shield trends in a WNW–ESE direction at Serra do Navio. In rocks where foliation is absent (carbonatic protore) recrystallization, twinning and undulose extinction show that the lithologies have experienced tectonic deformation. This deformation is associated with at least 3 stages of metamorphism, namely regional metamorphism, contact metamorphism and local dynamic metamorphism (Scarpelli, 1968). Late mesoscopic – microscopic fractures appear to reflect a late brittle deformation phase. Some fractures host sulphide mineralization, carbonates as well as quartz.

2.4.3 Drill Core logs

Due to scarcity of fresh drill core at the time of this study, only 3 drill cores from open pit F-12 (see section 1.2) were logged (Fig.2.5). These include drill cores DH-114, DH-116 and DH-140 (Fig. 2.9). The resultant logs compare very well with the sections constructed by mine geologists (e.g. Fig. 2.8). Even though only carbonate and non- carbonate rocks were described in the mine sections, the logs in this study outline 5 different lithological types (Table 2.1). The carbonate protore has been logged as Rhodochrosite marble, Mn-calcite marble and Mn-carbonate schist while the non- carbonate rocks were logged as graphite schist and biotite schist. These lithologies seem laterally continuous over the three drill cores even though it is possible that some of the layers pinch out.

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Fig.2.9: Drill core logs from the F-12 open pit in the northwestern section of the Serra do Navio deposit.

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