NORTHERN SECTOR OF CARAJÁS MINERAL PROVINCE, BRAZIL: CONSTRAINTS FROM WHOLE-ROCK GEOCHEMISTRY AND TRACE ELEMENTS IN APATITE
13.1 - INTRODUCTION
The origin and nature of the ore-forming fluids in the iron oxide-copper-gold deposits (or IOCG; Hitzman et al. 1992) have always been a matter of debate worldwide and distinct genetic models have been proposed in the last decades (e.g., Barton & Johnson 1996, Willians et al. 2005, Pollard 2006, Groves et al. 2010). The temporal and spatial association between granite magmatism and IOCG mineralization is recognized in some relevant IOCG provinces and world-class IOCG deposits (e.g., Olympic Dam, Candelaria, Manto Verde and Candelaria-Punta del Cobre; Marschik et al. 2000, Sillitoe 2003, Skirrow et al. 2007). This relationship, however, remains very contentious and the adhesion of a magmatic-hydrothermal model for the IOCG deposits has been extensively debated.
Some authors endorse the role of granitic magmatism as a source of fluids and metals for IOCG systems (e.g., Sillitoe 2003, Pollard 2006). On the contrary, other studies consider these intrusions only as heat engines, whereas the brines could be derived from evaporitic sources (e.g., Barton &
Johnson 1996, Xavier et al. 2008). The development of wide magmatic-hydrothermal systems was assigned as the main ore-forming processes for the IOCG formation (Courtney-Davies et al. 2020, Melo et al. 2021). However, translithosferic structures and the ascension of mantle-derived fluids could also be the source for the ore-forming fluids (Teixeira et al. 2021).
The archean Carajás Province, in the Amazonian Craton, Brazil, represents one of the best-endowed mineral provinces of the world and stands out for hosting a large number of world-class IOCG deposits. Together, these deposits correspond to more than 87% of Brazil's copper reserves
(Juliani et al. 2016). In the Carajás Domain, the IOCG deposits are clustered in its northern (e.g., Salobo, Igarapé Bahia, Grota Funda, GT-46, Furnas), and southern (e.g., Sossego, Cristalino, Alvo 118) portions (Figura 3-1).
The Carajás copper metallogeny has been assigned to the development and superposition of multiple hydrothermal-mineralizing events (Moreto et al. 2015a, b, Toledo et al. 2019, Melo et al.
2021). The deposits located in the southern record mineralizing events during the (i) Neoarchean (ca.
2.71-2.68 Ga; Sequeirinho and Pista bodies at the Sossego mine, and Bacaba and Bacuri deposits;
Moreto et al. 2015a, 2015b) and (ii) Paleoproterozoic (ca. 1.90-1.88 Ga; Sossego and Curral bodies at the Sossego mine, and Alvo 118 deposit; Moreto et al. 2015a, b). In the northen, on the other hand, the timing of the IOCG formation in the northern portion of the Carajás Domain is constrained at ca. 2.5 Ga (e.g., Salobo, Igarapé Bahia and Grota Funda; Réquia et al. 2003, Tallarico et al. 2005, Hunger et al. 2019). Mineralizing ages at ca. 2.71-2.68 Ga are also proposed for the northern sector (e.g., GT-46;
Silva et al. 2005, Toledo et al. 2019), although the significance of this age is still barely understood.
The contemporaneity between the IOCG mineralization and the 2.5 Ga granite magmatism led some authors to link magmatic fluids and IOCG formation in the Carajás Domain (Tallarico et al.
2005, Melo et al. 2019b). At the Salobo deposit, the genetic model considers the main mineralization in ca. 2.54-2.58 Ga and correlates its genesis to the Old Salobo granite (2.57-2.54 Ga; Machado et al.
1991, Réquia et al. 2003, Melo et al. 2016, 2019a). The GT-46 deposit also records a hydrothermal-ore-forming event at 2557 ± 8 Ma (Re-Os in molybdenite; Silva et al. 2005) likely associated with the emplacement of 2557 ± 26 Ma and 2588 ± 38 Ma pegmatitic and granite bodies (U-Pb in zircon;
Toledo et al. 2019). On the contrary, some authors have associated this age with an important tectonic-thermal event (Réquia et al. 2003, Tallarico et al. 2005, Toledo et al. 2019).
Granite whole-rock geochemistry coupled with trace elements in apatite has been used as a powerful tool to constrain magmatic and hydrothermal processes in ore deposits (Sun et al. 2019, Liu et al. 2019). The geochemical signatures of the granitic bodies of the area can provide important information on magmatic process, tectonic implications and the characteristics of magmatic events in the northern portion of the Carajás Domain. Likewise, apatite has been considered a key tool for revealing petrogenetic and metallogenetic processes (Belousova et al. 2002, Cao et al. 2012, Mao et al. 2016, Pan et al. 2016, Krneta et al. 2017, Chen & Zang 2018, Sun et al. 2019, Sun et al. 2021).
Apatite is a ubiquitous accessory phase in granitoids and also represents an important hydrothermal mineral within the IOCG ore zones. Due to extensive element replacement in apatite structure, this mineral phase may be useful to: i) elucidate evolution of magmas (Pan et al. 2016, Li et al. 2017, Zarfar et al. 2020); ii) fingerprint hydrothermal process (Krneta et al. 2017, Slobodnik et al. 2020); iii) assess oxygen fugacity (Mukherjee et al. 2017, Sun et al. 2019); iv) estimation of volatiles (Jiang et al.
2018, Sun et al. 2019); v) determination of hydrothermal fluid origin (Palma et al. 2019, Mercer et al.
Contribuições às Ciências da Terra, Série M, vol. 80, 106p.
25
& Watson 1991, Creaser & Gray 1992), apatite can be sensitive to metasomatic alteration (Harlov 2015) and may fingerprint superimposed hydrothermal events.
In this paper, we aim to shed more light to the understanding of the superposition of magmatic and hydrothermal processes related to the IOCG formation in the northern portion of the Carajás Domain.
Contrasting apatite geochemical signatures from granitic bodies and mineralized zones of the Salobo and GT-46 deposits are used to unravel hydrothermal processes associated with the emplacement of granitic magmas. The results of this study lead to the improvement of genetic models for the IOCG deposits of the Carajás Domain and the comprehension of the overlapping of magmatic and hydrothermal processes in the deposit and regional scale.
Figura 3-1. (A) Location of the Carajás Province, in Brazil. (B) Geologic map of the Carajás Domain (modified from Vasquez et al. 2008) displaying important mineral deposits from the northern sector and Southern Copper Belt with red rectangle indicating the location of the GT-46 and Salobo deposits. Abbreviations: BD = Bacajá Domain, CD = Carajás Domain, RMD = Rio Maria Domain.
3.2 - GEOLOGICAL SETTING
The Carajás Province comprises the oldest Archean crustal nucleus in the Amazonian Craton, northern Brazil (Macambira et al. 2020). The province is subdivided into two domains: the Rio Maria, in the south, and Carajás, in the north (Santos 2003, Vasquez et al. 2008). The Carajás Domain comprises a Mesoarchean basement overlaid by the Neoarchean to Paleoproterozoic metavolcanosedimentary rocks, crosscut by granitoids and mafic-ultramafic complexes (Vasquez et al.
2008).
Basement rocks in the Carajás Domain include gneisses and migmatites of the Xingu Complex (2859 ± 2 Ma, 3066 ± 6.6 Ma; Machado et al. 1991, Delinardo da Silva 2014) with tonalitic to trondhjemitic composition (Silva et al. 2021), and mafic orthogranulites of the Pium Complex (or Chicrim-Cateté; 3002 ± 14 Ma; Pidgeon et al. 2000). Undeformed granitoids are also part of the basement, including: (i) ca. 3.0 Ga: Bacaba Tonalite and Sequeirinho Granite, (ii) 2.96-2.93 Ga: Canaã dos Carajás Granite, and (iii) 2.87-2.83 Ga: Rio Verde Trondhjemite, Campina Verde Tonalite and Bom Jesus, Cruzadão and Serra Dourada granites (Moreto et al. 2011, Feio et al. 2013).
The basement rocks are overlain by the 2.76 to 2.73 Ga metavolcanosedimentary sequences of the Carajás Basin, including the Itacaiúnas Supergroup (DOCEGEO, 1988) and the Rio Novo Group (Hirata et al. 1982). The Rio Novo Group consists of quartzites, banded iron formations, metabasalts, gabbros, shales and metapelites (Araújo & Maia 1991, Oliveira et al. 1994). The Itacaiúnas Supergroup can be divided into (i) lower association, dominated by volcanic rocks; (ii) intermediate association, which comprises thick layers of banded iron formations interlayered with black shales;
and (iii) upper association, mainly composed of clastic metasedimentary rocks and partially of volcanic/volcanoclastic rocks (Tavares et al. 2018). The Itacaiúnas Supergroup was recently divided into Serra da Bocaina and Grão Pará groups (Costa et al. 2016). Other metavolcanosedimentary sequences include São Felix, São Sebastião, Liberdade and Aquiri groups (Costa et al. 2016).
The psamitic and pelitic successions of the Águas Claras Formation constitute a progressive marine platform (Nogueira et al. 1995) which partially cover the metavolcanosedimentary rocks of the Carajás Basin. Mafic dikes intruded in the sequence indicate a minimum age of 2.71 Ga (Mougeot et al. 1996), and detrital zircon pointed out a maximum age of 2.68 Ga (Trendall et al. 1998).
Layered mafic-ultramafic bodies of the Luanga Complex (2.76 Ga; Machado et al. 1991) and the Cateté Suite (2.76 Ga; Lafon et al. 2000) intrude the metavolcanosedimentary units and the basement rocks.
Three main episodes of granite magmatism are recognized in the Carajás Domain during the Neoarchean (2.76-2.73 and 2.57-2.56 Ga) and Paleoproterozoic (ca. 1.88 Ga). The 2.76-2.73 Ga magmatism, widespread in the Carajás Domain, include the Estrela Complex and the Plaquê, Planalto,
Contribuições às Ciências da Terra, Série M, vol. 80, 106p.
27
Serra do Rabo and Igarapé Gelado plutons (Avelar et al. 1999, Sardinha et al. 2006, Barros et al.
2009, Feio et al. 2012). This magmatism is syntectonic to the development of shear zones and constitutes intensely deformed bodies, generally elongated in the east-west direction (Avelar et al.
1999, Sardinha et al. 2006, Feio et al. 2012). The intrusive bodies are metaluminous to slightly peraluminous, alkaline to alkali-calcic and commonly have high values of Zr, Y, Nb, Ce, Ga and HREE, similar to A-type granites (Avelar et al. 1999; Sardinha et al. 2006, Barros et al. 2009, Feio et al. 2012).
The 2.57-2.55 Ga granite magmatism constitutes small and restricted plutons particularly at the northernmost part of the Carajás Domain, but its origin and tectonic significance remain controversial. It is represented by the Old Salobo and Itacaiúnas granites (Machado et al. 1991, Souza et al. 1996, Melo et al. 2016). Recent studies also dated granite bodies at ca. 2.55 Ga within the GT-46 IOCG deposit and the Buritirama Mn deposit (Salgado et al. 2019, Toledo et al. 2019).
Anorogenic magmatism extends across all the Carajás Province. This event is represented by the 1.88 Ga Serra dos Carajás Intrusive Suite, including the Central de Carajás, Cigano, Pojuca, Young Salobo and Breves granites (Machado et al. 1991, Dall'Agnol et al. 1994, Lindenmayer & Teixeira 1999, Tallarico 2003). These bodies constitute intraplate ferroan, metaluminous to weakly peraluminous A-type granites (Dall'Agnol et al. 2005).
The tectonic evolution of the Carajás Domain comprises a sequence of compression and extension cycles, with deposition of volcanosedimentary sequences and plutonism (Tavares et al.
2018, Trunfull et al. 2020). The collision between Rio Maria and Carajás blocks in ca. 2.86 Ga triggered the development of the Itacaiúnas Shear Zone and the emplacement of the 2.86-2.83 Ga granitoids (Trunfull et al. 2020). This compressional event promoted a stable crustal substrate, which upon rifting at 2.76-2.70 Ga and led to the deposition of the Itacaiúnas Supergroup volcanosedimentary sequence (Tavares et al. 2018). The 2.76 Ga mafic-ultramafic rocks point out mantle involvement and may represent mantle plume or post-orogenic collapse (Tavares et al. 2018).
This heat supply established a regional hydrothermal system and caused the emplacement of A-type like 2.76 Ga granites (i.e., Estrela and Igarapé Gelato granites; Trunfull et al. 2020). The intrusion of these 2.76-2.73 Ga granites indicate Carajás Basin inversion (Trunfull et al. 2020) or the continuity of rifting process (Tavares et al. 2018). The collision with the Bacajá Domain, in the north of Carajás Province, would have occurred at 2.09-2.06 Ga as part of the Transamazonian Orogeny, followed by another compressive event in 2.00-1.95 Ga (Tavares et al. 2018). A post-orogenic collapse resulted in the emplacement of 1.88 Ga A-type granitic bodies (Tavares et al. 2018).
3.3 - PREVIOUS STUDIES AT THE SALOBO AND GT-46 DEPOSITS: GEOLOGICAL ATTRIBUTES AND GENETIC MODELS
3.3.1 - Salobo
The Salobo deposit is located in the northernmost part of the Carajás Domain along the Cinzento Shear Zone (Figura 3-1) and is hosted by the basement rocks of the Xingu Complex (2950 ± 25 Ma, 2857 ± 6.7 Ma; Melo et al. 2016), neoarchean orthogneisses of the Igarapé Gelado Suite (2763
± 4.4 Ma; Melo et al. 2016; Tabela 3-1) and relicts of the metavolcanosedimentary sequences of the Itacaiúnas Supergroup (Melo et al. 2016; Figura 3-2). Intense ductile deformation due to shearing gave origin to NW-SE trending, steeply dipping mylonitic rocks with intense hydrothermal alteration (Réquia et al. 2003, Melo et al. 2016). Orebodies are, in general, structurally controlled along the ductile-brittle shear zones (Réquia et al. 2003).
The mineral paragenesis of the central zone of the Salobo deposit suggests a complex hydrothermal history and intense alteration process. The sequence of the main hydrothermal alteration encompasses: early (i) distal calcic-sodic alteration, characterized by hastingsite and actinolite ± scapolite ± titanite ± chalcopyrite ± allanite ± dravite and (ii) silicification, which includes multiple phases of hydrothermal quartz formation. An intense process of (iii) iron enrichment, the main hydrothermal alteration stage, consists of grunerite + almandine + schorlite + fayalite + magnetite in the central area of the deposit, and is followed by (iv) pervasive proximal potassic alteration with biotite, synchronous with mineralization (Melo et al. 2016). The copper-gold mineralization is often hosted within ferric and potassic alteration zones (Réquia et al. 2003). Late alteration, with Fe-rich hydrated silicates (i.e., stilpnomelane, chamosite and greenalite), and post-ore alteration, with hematite-bearing potassic feldspar, are also recognized within the Salobo deposit (Melo et al. 2016).
The copper-gold ore at the Salobo consists of abundant magnetite with disseminated/massive bornite and chalcocite, and subordinated chalcopyrite (Réquia et al. 2003, Lindenmayer 2003, Melo et al. 2016). Accessory molybdenite, covellite and other gangue minerals, including almandine, biotite, grunerite, fayalite, ilmenite, tourmaline and apatite are also present (Réquia et al. 2003, Lindenmayer 2003).
The Old Salobo and Young Salobo granites crosscut the hydrothermal alteration zones and the mylonitic rocks of the Salobo deposit (Lindenmayer 1990, Melo et al. 2016; Figura 3-2). The Old Salobo granite (2573 ± 2, 2547 ± 5.3 Ma; Machado et al. 1991, Melo et al. 2016; Tabela 3-1) is an alkaline, metaluminous, within-plate body (Réquia et al. 2003). It is a foliated, locally isotropic, equigranular rock, with composition ranging from granodioritic to tonalitic (Melo et al. 2016). The Young Salobo granite (1880 ± 80 Ma; whole-rock Rb-Sr; Cordani 1981; Tabela 3-1) is an alkaline,
Contribuições às Ciências da Terra, Série M, vol. 80, 106p.
29
metaluminous, anorogenic sill (Lindenmayer 1990) and comprises a isotropic, equigranular, fine-grained rock of granodioritic composition (Melo et al. 2016).
The Re-Os age at 2576 ± 8 Ma (Réquia et al. 2003; Tabela 3-1) for ore-related molybdenite is interpreted as the main mineralization stage of the Salobo deposit (Réquia et al. 2003, Lindenmayer 2003, Melo et al. 2016, 2019a). Other U-Pb ages of 2535 ± 8.4 Ma and 2452 ± 14 Ma from ore-related zircon and monazite (Melo et al. 2016; Tabela 3-1) suggest multistage hydrothermal events at the Salobo deposit. The main mineralization event and the Old Salobo granite emplacement (2573 ± 2 to 2547 ± 5.3 Ma; Machado et al. 1991, Melo et al. 2016; Tabela 3-1) were considered contemporaneous (Réquia et al. 2003) and related to the Cinzento Shear Zone reactivation (Melo et al. 2016).
Figura 3-2. Simplified geological map and cross-section of the Salobo deposit, showing the host rocks, hydrothermal alteration zones and the Old Salobo granite (Melo et al. 2016).
3.3.2 - GT-46
The GT-46 deposit is also located within the Cinzento Shear Zone, 40 km to the northwest of the Salobo deposit (Figura 3-1), along the subvertical flanks of a folded structure (Toledo et al. 2019).
The deposit is mainly hosted by metavolcanosedimentary sequences, including amphibolites (2774 ± 19 Ma; Toledo et al. 2019; Tabela 3-1), alternating layers of biotite schist, actinolite-biotite schist and almandine-biotite schist and banded iron formation (Toledo et al. 2019). Granitoids and diabase dikes intrude the sequence (Toledo et al. 2019; Figura 3-3).
Toledo et al. (2019) defined the GT-46 hydrothermal alteration and ore stages as: (i) early stage, which comprises zones of sodic-calcic and potassic alteration, iron metasomatism and disseminated mineralization I; (ii) Mineralization II, characterized by veins and breccias; and (iii) late alteration, dominated by chloritization. The sodic-calcic alteration is marked by hastingsite and albite, while the potassic alteration is biotite-dominated. Magnetite is the main mineral in iron metasomatism and replaces garnet in the almandine-biotite schist.
Figura 3-3. Simplified geological map and cross section of the GT-16 deposit, exposing the host rocks,
Contribuições às Ciências da Terra, Série M, vol. 80, 106p.
31
Mineralization I was considered the main ore stage dominated by chalcopyrite, bornite and magnetite in ductile structures (Toledo et al. 2019). Mineralization II was attributed to a brittle regime, associated with veins and breccias, and dominated by chlorite, calcite, magnetite, chalcopyrite, quartz, allanite and albite (Toledo et al. 2019). Late chloritization halos, with biotite, quartz and almandine, are spatially associated with pegmatites (Toledo et al. 2019).
Silva et al. (2005) differentiate the GT-46 granitoids into: (i) isotropic pink granite and alkali-feldspar granite; (ii) gray granitoid with quartz monzonitic to granitic composition; (iii) tourmaline-bearing gray granitoid; and (iv) mylonitic garnet-sillimanite granitoids. Whole-rock geochemistry diagrams indicate sin-collisional granitoids of volcanic arcs, and subalkaline nature, in a calcic-alkaline trend (Silva et al. 2005). REE and trace elements spider diagrams suggest a cogenetic relationship between the garnet-sillimanite granite and the others GT-46 granitoids. Geochronological data indicate minimum ages of 2612 ± 15 (U-Pb monazite), 2600 ± 8, 2557 ±8 and 2554 ± 8 Ma (Re-Os molybdenite) for these granitoids (Silva et al. 2005).
Alternatively, Toledo et al. (2019) grouped the GT-46 granitoids into: (i) foliated tonalite to granodiorite (2639 ± 16 to 2532 ± 26 Ma, U-Pb in zircon); (ii) pegmatite (2588 ± 38 Ma, U-Pb in zircon); and (iii) undeformed granite (2557 ± 26 Ma, U-Pb in zircon; Tabela 3-1). The pegmatite and isotropic granite crosscut the tonalite/granodiorite. Foliated tonalite/granodiorite is a light gray to brownish pink rock, medium- to fine-grained, composed of plagioclase, quartz, K-feldspar and biotite.
(Toledo et al. 2019). Pegmatite is a light pink rock, composed of albite, quartz, microcline, biotite, muscovite, epidote and tourmaline. Isotropic pink granite preserves the phaneritic inequigranular texture, and comprises microcline, quartz, plagioclase, chlorite, biotite and epidote.
According to Toledo et al. (2019), the geological evolution of the deposit constitutes: (i) deposition of a volcanosedimentary sequence prior to 2.77 Ga; (ii) regional tectonometamorphic event between 2.78 and 2.72 Ga; (iii) development of the Cinzento Shear Zone, together with the mineralizing hydrothermal system and Mineralization I, ca. 2.72 Ga; (iv) superimposed hydrothermal events responsible for Mineralization II, ca. 2.60 Ga; and (v) granitic bodies intrusions and late chloritization, at 2557 ± 8 Ma. Younger Re-Os ages (2449 ± 44 and 2503 ± 51 Ma; Toledo et al. 2019) were related to later hydrothermal stages (Tabela 3-1).
Tabela 3-1. Geochronological data compilation of the intrusive bodies and hydrothermal stages at the Salobo and GT-46 deposits.
Sample Method Age (Ma) Interpretation References
Salobo
Young Salobo
granite Rb-Sr whole rock 1880 ± 80 Igneous crystallization Cordani (1981) Magnetite leachates Pb-Pb magnetite 2112 ± 12 Later hydrothermal alteration
Tassinari et al. (2003) Chalcopyrite
leachates Pb-Pb chalcopyrite 2427 ± 130 Reactivation of the Itacaiunas Belt Cu-Au ore U-Pb monazite 2452 ± 14 Post hydrothermal alteration
Melo et al. (2016) U-Pb zircon 2535 ± 8.4 Main mineralization
Re-Os molybdenite 2562 ± 5 Development or reactivation of local
shear zones Réquia et al. (2003) 2576 ± 8 Main mineralization
Amphibole-rich rock U-Pb titanite 2497 ± 5 K-metasomatism
Machado et al. (1991) Magnetite-rich rock U-Pb monazite 2551 ± 2
Metamorphic event Amphibole-rich rock U-Pb zircon 2555 +4/-3
U-Pb titanite 2581 ± 5 Tectonic reactivation
Old Salobo granite U-Pb zircon 2547 ± 5.3 Igneous crystallization / resetting of
U-Pb isotopic system Melo et al. (2016) 2573 ± 2 Igneous crystallization Machado et al. (1991) Tourmaline leachates Pb-Pb tourmaline 2587 ± 150 Tectonic reactivation
Tassinari et al. (2003) Chalcocite leachates Pb-Pb chalcocite 2705 ± 42 Primary mineralization
Granitic vein
U-Pb zircon
ca. 2758
Igneous crystallization
Machado et al. (1991) Igarapé Gelado
gneiss 2763 ± 4.4 Melo et al. (2016)
GT-46
Pegmatite
Re-Os molybdenite
2449 ± 44
Later hydrothermal events Toledo et al. (2019) Intensely chloritized
rock 2503 ± 51
Granite pegmatoital Re-Os molybdenite
2554 ± 8 Late alteration (interpretation of Toledo et al. 2019) (Mineralization II?)
Silva et al. (2005) 2557 ± 8
Undeformed granite
U-Pb zircon 2557 ± 26
Igneous crystallization Toledo et al. (2019)
Pegmatite 2588 ± 38
Granite Re-Os molybdenite 2600 ± 8 Mineralization II (interpretation of
Toledo et al. 2019) Silva et al. (2005) Gray granitoid U-Pb monazite 2612 ± 1.5 Possible igneous crystallization
Foliated
tonalite/ganodiorite U-Pb zircon
2532 ± 26 Possible igneous crystallization
Toledo et al. (2019) 2639 ± 16
Xenocryst cores and inherited zircon grains / Disturbance of U-Pb isotope
system Amphibolite Re-Os molybdenite 2718 ± 56 Mineralization I
U-Pb zircon 2774 ± 19 Igneous protolith crystallization
3.4 - MATERIALS AND METHODS
3.4.1 - Drill core logging, sampling and petrography
Drill core logging was conducted at the Salobo and GT-46 deposits aiming for the recognition of granitic bodies and their relationship with hydrothermally-altered and ore zones. The sampling at the GT-46 and Salobo deposits was conducted mainly in the less hydrothermally altered granites.
Samples from the most representative mineralized zones in both deposits were also collected.
3.4.2 - Whole-rock geochemistry
Whole-rock geochemistry of the granitic samples collected at both deposits were conducted at the ALS Laboratory (Brazil). Major elements were measured by X-ray fluorescence (XRF) spectrometer. Trace elements and Rare Earth Elements (REE) were determined by Inductively
Contribuições às Ciências da Terra, Série M, vol. 80, 106p.
33
Coupled Plasma Mass Spectrometry (ICP-MS) and Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES), using two different techniques (ME-MS61 and ME-MS81; ALS Geochemistry 2014, 2015). Duplicate and blank samples were used to assure and control the quality of the data.
3.4.3 - Apatite mineral chemistry
Electron Probe MicroAnalysis (EPMA) analyses were performed using a JEOL JXA-8230 superprobe, equipped with five wavelength dispersive spectrometers (WDS), at the Microscopy and Microanalysis Laboratory (LMic) of Federal University of Ouro Preto, Brazil. The JEOL EMPA software Ver3.0.1.16 package was used to perform the calibration, overlap correction and quantification. Operating conditions were 20kV accelerating voltage, 40nA probe current and 5µm spot diameter. Common matrix ZAF corrections were applied. The standards and main elements analyzed are as follows: anorthoclase (Na), CaF2 (F), quartz (Si), anorthite (Al), olivine (Mg), BaSO4
(Ba), magnetite (Fe), scapolite (Cl), pyrite (S), fluor-apatite (Ca and P), microcline (K), rutile (Ti), strontianite (Sr), MnO (Mn), NdPO4 (Nd), Pr2O3 (Pr), Ce2O3 (Ce) and La2O3 (La). Counting time for S, Pr, Ce and La were set at 30s at peak and 15s at background while for Na, F, Si, Al, Mg, Ba, Fe, Cl, Ca, P, K, Ti, Sr, Mn and Nd was set at 10s at peak and 5s at background. The major spectral interferences were corrected during the standard analysis and during the quantification. Analytical errors are within 0.10% and 0.99%. The total iron content obtained by the microprobe is considered as FeO.
Trace element in situ analyses of apatite grains were conducted by LA-ICP-MS at the Isotope Geochemistry Laboratory of Federal University of Ouro Preto, using a single collector sector field (SF) ICP-MS of Thermo-Finnigan Element II, coupled to a CETAC UV NdYAG 213 nm laser ablation system. The laser was operated with an energy density of 4 J/cm2 at a repetition rate of 10 Hz, producing spots of 30 μm diameter in the sample. The apatite grains were measured in short runs bracketed by analyses of external (NIST SRM 610, BCR-2G) and internal (Ipira) standards. After each analysis, data reduction was carried out using GLITTER software.
3.5 - RESULTS
3.5.1 - Hydrothermal halos and ore zones at the Salobo and GT-46 deposits
The central zones of the Salobo and GT-46 deposits consists predominantly of hydrothermal mineral assemblages, with scarce or absence of relicts of the host rocks. These are comprised of extensive Fe-K alteration halos that envelop the copper-gold ore bodies. The Fe-K hydrothermal alteration is mainly represented by almandine, grunerite and biotite with large amounts of magnetite.
Almadine develop porphyritic crystals in the biotite + grunerite matrix (Figura 3-4A, C). Less abundant minerals are represented by fayalite, tourmaline, apatite, fluorine and quartz.
The Cu-Au ore zones from the Salobo deposit comprise elongated bodies of massive magnetite, chalcocite and bornite, commonly with relicts of biotite and grunerite from the Fe-K alteration (Figura 3-4B). Chalcocite and bornite can also occur as disseminated ore or filling fractures.
Apatite, schorl, fluorite and later Fe-rich minerals (i.e., stilpnomelane, greenalite and chamosite) are also common mineral phases in the ore.
At the GT-46 deposit, ore bodies are represented by disseminated chalcopyrite and bornite, associated with magnetite, in commonly foliated rocks (Figura 3-4D). In this deposit some relicts of the hosts (i.e., BIF, amphibolites and granitoids) are still recognized in the central zone. Mineralized veins and breccias also occur, usually comprising massive chalcopyrite and magnetite. Apatite, chlorite, calcite, quartz, albite and allanite constitute gange minerals.
Figura 3-4. Main aspects of the central zones of the Salobo and GT-46 deposits: (A) Fe-K alteration at the Salobo deposit, with biotite, grunerite and porphyroblasts of almandine. (B) Salobo Cu-Au ore. (C) Fe-K alteration at the GT-46 deposit, with porphyroblasts of almandine in a biotite and grunerite matrix. (D) GT-46 copper-gold ore bodies. Mineral abbreviations (Whitney & Evans, 2010): Alm = almandine, Bn = bornite, Bt = biotite, Ccp = chalcopyrite, Cct = chalcocite, Gru = grunerite, Mag = magnetite.
3.5.2 - Modes of occurrence and petrography of granites from the Salobo and GT-46 deposits
A diversity of granitic bodies is recognized within the Salobo and GT-46 deposits (Figura 3-2, Figura 3-3), but their link with hydrothermally altered and ore zones is commonly ambiguous. At the Salobo, the Old Salobo granite outcrops in the northeastern portion of the deposit but is also recognized as apophysis within the ore zones. At the GT-46, at least two granite bodies are identified.
Contribuições às Ciências da Terra, Série M, vol. 80, 106p.
35
by Toledo et al. (2019). These granitic bodies are generally sheared and strongly affected by hydrothermal alteration, although a few isotropic and less altered portions have been recognized.
Old Salobo granite
The Old Salobo granite crosscuts the host rocks (i.e., gneisses and mylonitic rocks), but its relation with hydrothermal alteration zones is dubious. The Old Salobo granite is a phaneritic isotropic (Figura 3-5A, B) to foliated rock (Figura 3-5C, D) with medium-grained texture. In least-altered zones, the granite is gray (Figura 3-5A) but usually becomes brownish pink in more hydrothermally altered zones (Figura 3-5B). The main mineralogy comprises quartz (35-50%), alkali feldspar (20-25%), plagioclase (10-20%), hornblende (5-10%) and magnetite (up to 5%). Apatite (<1%) and allanite (<1%) are accessory mineral phases. Secondary minerals are represented by carbonate (up to 2%), epidote (<1%), clinozoisite (<1%) and chlorite (<1%). Possible evidences of albitization process suggest that albite may comprise up to 5% of the rock.
Quartz crystals exhibit lobulated boundaries (Figura 3-5G), display undulose extinction, and are frequently stretched (Figura 3-5H). They commonly develop quartz subgrains that surrounds feldspar and quartz crystals, due to the bulging shearing process (Figura 3-5H, I). Plagioclase often exhibits deformed tapering twin planes. Oriented hornblende makeup the mylonitic foliation (Figura 3-5J). Magnetite crystals display intersertal texture and are often associated with hornblende (Figura 3-5E, F, K). Epidote and clinozoisite partially replace hornblende, while carbonate replaces plagioclase.
In hydrothermally altered portions, the Old Salobo commonly displays inequigranular texture.
Whithin these portions, quartz subgrains with bulging texture are more developed, while feldspars form albitized coarse grains (Figura 3-5L). Hornblende and magnetite are not common in these portions.
Figura 3-5. Aspects of the Old Salobo granite: (A) Isotropic gray rock in least-altered zone. (B) Brownish pink rock in more hydrothermal altered zone. (C) Foliated rock, with oriented hornblende and stretched quartz. (D) Unequigranular foliated rock, with medium-sized quartz and feldspar crystals and fine-grained recrystallized matrix. (E, F) Magnetite associated to hornblende. (G) Interlobate boundaries and undulose extinction quartz.
(H) Foliation developed by stretched undulose quartz. (I) Quartz subgrains surrounding feldspar and quartz crystals, in a bulging shearing texture (J) Hornblende preferred orientation defining rock foliation. (K) Magnetite associated to hornblende in intersetal texture. (L) Albite alteration over microcline. Mineral abbreviations (Whitney & Evans, 2010): Ab = albite, Afs = alkali feldspar, Hbl = hornblende, Mag = magnetite, Mc = microcline, Plg = plagioclase, Qz = quartz.
G1 granite
The G1 granitic body occurs interspersed with hydrothermally altered lithotypes (i.e., biotite-, garnet-biotite and (actinolite)-grunerite-biotite-rich rocks) and host rocks (amphibolite and BIF) at the GT-46 deposit. The G1 is a medium- to fine-grained foliated gray rock (Figura 3-6A, B). The bulk of mineralogy is comprised of quartz (40-55%), microcline (15-25%), alkali feldspar (10-15%), plagioclase (5-10%) and biotite (5-10%). Secondary minerals encompass chlorite (up to 3%),
Contribuições às Ciências da Terra, Série M, vol. 80, 106p.
37
muscovite (up to 1%) and epidote (up to 1%). Albite may comprise up to 5% of the rock, indicated by possible albitization feature.
Quartz crystals commonly form subgrains with undulose extinction in a core-and-mantle texture (Figura 3-6G). In the less deformed zones, oriented biotite and stretched quartz and feldspar crystals define the mylonitic foliation (Figura 3-6E, F). In more sheared zones, submilimetric layers of subgrains enclose ribbons of medium-grained quartz and feldspars developing a well-defined foliation (Figura 3-6H). Fine-grained biotite is abundant in the subgrain layers, while medium-grained biotite is dominant on their boundaries (Figura 3-6I). Feldspars crystals display interlobate boundaries with quartz.
Where hydrothermal alteration is more intense, G1 displays prominent shearing features and higher biotite content, particularly along with the shearing bands. Biotite content commonly increases and forms, in some parts, biotite-rich rocks. In the central portion of the deposit, G1 granite is usually altered to hydrothermal biotite, garnet and chalcopyrite (Figura 3-6C, D). The albitization process is often recognized, and hydrothermal assemblages (i.e., chlorite, muscovite and epidote) partially replace biotite in this body.
Figura 3-6. Aspects of G1 granite: (A, B) Foliated light gray granite in the least-altered zone. (C) Alteration to the hydrothermal mineral phases – biotite, garnet and chalcopyrite – along foliation planes. (D) Alteration to biotite and garnet close to contact with Fe-K alteration halos. (E, F) Oriented biotite and stretched quartz and feldspar defining the least-deformed rock foliation. (G) Undulose extinction quartz grains constituting a core-and-mantle texture. (H) Subgrains enclosing ribbons of medium-grained quartz and feldspars, developing a
well-defined foliation. (I) Fine-grained biotite in subgrains domains and medium-grained biotite in the borders.
Mineral abbreviations (Whitney & Evans, 2010): Afs = alkali feldspar, Bt = biotite, Ccp = chalcopyrite, Fps = Feldspar, Grt = garnet, Mc = microcline, Qz = quartz.
G2 granite
The G2 body at the GT-46 deposit occurs as centimetric to metric apophysis interspersed with the host rocks, G1 granite and hydrothermally altered lithotypes (i.e., biotite, garnet-biotite and (actinolite)-grunerite-biotite-rich rocks). The G2 granite is phaneritic, isotropic (Figura 3-7A, B) to locally foliated (Figura 3-7C), light pink, with fine- to very coarse-grained texture (Figura 3-7A, E).
Pegmatitic textures are commonly identified in the coarser-grained portions (Figura 3-7D, E, I). G2 granite is composed of alkali feldspar (20-35%), plagioclase (15-35%), quartz (25-35%) and biotite (0-10%). Hornblende is locally recognized, and apatite (<1%) and allanite (<1%) are accessory minerals.
Secondary minerals are represented by chlorite (up to 5%), carbonate (up to 3%), epidote (up to 3%), garnet (up to 1%) and sericite (up to 1%). Evidences of the albitization process indicate that albite may comprise up to 10% of the rock assemblage.
The G2 apophyses seems to crosscut G1 granite, although the contact between both lithotypes is not easily recognized due to hydrothermal alteration overprinting (Figura 3-7F). The G2 granite seems to crosscut the K-Fe alteration halos (Figura 3-7A) and the main mineralization. However, in other portions, the G2 body displays Fe-K alteration to garnet and biotite (Figura 3-7E, H) and crosscutting mineralized veins (Figura 3-7G). Sometimes, G2 appears as pockets in the hydrothermal rocks, close to the contact between both lithotypes (Figura 3-7D). It also exhibits magnetite pockets and intense chloritization (Figura 3-7I). The Fe-K alteration and chloritization are more recurrent in coarse to very coarse-grained rocks (Figura 3-7E, H, I).
In more deformed portions, quartz subgrains commonly surround the boundaries of feldspar and quartz crystals, with a bulging dynamic recrystallization texture (Figura 3-7K). Stretched quartz and elongated biotite mark the mylonitic foliation. Undulose extinction occurs both in quartz and feldspar crystals. Plagioclase exhibits deformed twin planes (Figura 3-7K). Sparse hornblende occurs in the biotite-bearing rocks. Potassic alteration, with perthitic alkali-feldspar replacing plagioclase is commonly observed in G2 (Figura 3-7L). Albitization is often recognized and biotite is frequently altered to secondary chlorite and magnetite.
Contribuições às Ciências da Terra, Série M, vol. 80, 106p.
39
Figura 3-7. Aspects of G2 granite: (A, B) Isotropic fine-grained rock, in least-altered zones. (C) Foliated medium- to coarse-grained G2 granite showing biotite and stretched quartz crystals along foliation. (D) Medium-grained G2 granite in contact with (actinolite)-grunerite-biotite-bearing hydrothermal rock. Pegmatitic feldspathic pockets form in the hydrothermal rock, close to the contact. (E) Medium- to coarse-grained granite, with garnet and aggregates of biotite close to contact with Fe-K alteration halo (F) G2 apophyses crosscutting G1 granite. (G) Mineralized veins of magnetite and chalcopyrite crosscutting G2 granite. (H) Hydrothermal alteration to garnet. (I) G2 granite in pegmatitic texture, hydrothermally altered to magnetite and chlorite. (J) Preserved granitic texture in less deformed rock. (K) Bulging dynamic recrystallization texture with quartz subgrains in the contacts of medium-sized quartz and feldspar crystals. Plagioclase displays deformed tapering twin planes. (L) Plagioclase alteration to alkali-feldspar. Mineral abbreviations (Whitney & Evans, 2010): Ab = albite, Afs = alkali feldspar, Bt = biotite, Ccp = chalcopyrite, Chl = chlorite, Fsp = feldspar, Gru = grunerite, Kfs
= K-feldspar, Grt = garnet, Mag = magnetite, Plg = plagioclase, Qz = quartz.
3.5.3 - Whole-rock geochemistry
Whole-rock geochemistry analyses were carefully conducted in the Old Salobo granite, at the Salobo deposit, and G1 and G2 granites, at the GT-46 deposit. The geochemical results are listed Supplementary Table 1.
Most of the samples plot in the granite field on the K2O + Na2O versus SiO2 diagram (Middlemost, 1994), except for three samples (two of the Old Salobo granite and one of the G2 granite) that plot in the granodiorite and quartz monzonite fields (Figura 3-8A). The samples with granitic composition have SiO2 contents ranging from 71.1 to 76.7 wt.%, whereas the others have lower contents varying from 65.7 to 69.4 wt.%.
The Old Salobo, G1 and G2 granites are metaluminous to weakly peraluminous (Figura 3-8B).
The Old Salobo and G1 granites plot in the calc-alkalic to alkalic fields, while the G2 granite indicates the most variable compositions, ranging from alkalic to calcic (Figura 3-8C). The granites are mostly ferroan, although one Old Salobo sample plot in the magnesian field (Figura 3-8D). The tectonic diagrams show G2 granite plots in the volcanic arc field (Figura 3-8E, F). The Old Salobo and G1 granites establish a trend from volcanic arc to within-plate granites in the diagram of Pearce et al.
(1984; Figura 3-8F), although they plot predominantly in the volcanic arc field in the diagram of Harris et al. (1986; Figura 3-8E). In the A-type discrimination of Whalen et al. 1987, the Old Salobo and G1 plot in the A-type granites field while G2 plot outside this field (Figura 3-8G).
The main difference between these granitoids lie in their REE contents. The Old Salobo and G1 granites display a very similar REE pattern. These lithotypes are moderately fractionated in REE, with LaN/YbN ratios of 4.03-10.29 and slightly to strongly negative Eu anomaly (Eu/Eu* = 0.23-0.82;
Figura 3-9A). Only one sample of the Old Salobo granite displays a strongly positive Y anomaly, likely related to the presence of allanite. The total REE content in the Old Salobo and G1 granites is much higher than in G2, though Eu concentrations are similar in the three granitic bodies (Figura 3-9A).
The G2 body commonly reveals a strongly positive Eu anomaly (Eu/Eu* = 3.02-8.98), except for two samples. The G2 granite shows relative enrichment in light REE (LREE) with moderate to high LaN/YbN ratios of 4.09-27.65 (Figura 3-9A), except for one sample that shows a very high ratio of 76.51. This granite is highly fractionated in LREE to middle REE (MREE) with LaN/SmN ratios of 3.36-24.99. Fractionation from MREE to heavy REE (HREE) establishes a listric shape in the primitive mantle normalized diagram (Figura 3-9A), marked by depletion in Y, Ho and Dy, and relative enrichment in Yb and Lu. Some samples, however, display a negative Lu anomaly.
Contribuições às Ciências da Terra, Série M, vol. 80, 106p.
41
Figura 3-8. Lithogeochemical classification diagrams for the Old Salobo, G1 and G2 granites, including other data from the literature for comparison (Lindenmayer 1990, Salgado et al. 2019). (A) Total álcalis-silica (TAS) diagram (Middlemost, 1994). (B) Aluminosity index. (C) Modified álcali-lime index (MALI) vs. SiO2 diagram (Frost et al. 2001). (D) Ferroan/magnesian classification. (E) Tectonic classification of granitic rocks (Pearce et al. 1984). (F) Tectonic classification of granitic rocks (Harris et al. 1986). (G) Discrimination diagrams of A-type granites (Whalen et al. 1987).
The trace elements contents of the Old Salobo and G1 granites display quite similar patterns in the multi-elementary diagrams (Figura 3-9B). On the contrary, the G2 has lower concentrations of trace elements and a more dispersed pattern than Old Salobo and G1 (Figura 3-9C). Nevertheless, this body does not exhibit a distinct pattern of G1 and Old Salobo granites. The three lithotypes indicate enrichment in LILE (Large Ion Lithophile Elements; e.g., Rb, Ba, K, Pb), except for Sr, and some HFSE (High Field Strength Elements) depletion (Ti, Nb and Ta; Figura 3-9B, C). Other HFSE (Th, Zr and Hf) display high concentrations in G1 and Old Salobo and very distinct behavior in G2 granite (Figura 3-9B, C). However, one sample of the Old Salobo granite displays a highly negative Zr anomaly, possibly related to zircon fractionation.
The three granites are characterized by strongly negative anomalies of Sr (Figura 3-9B, C).
The G2 granite displays low Sr (57.8-110.5 ppm) and very low Y (1.0-6.6 ppm) contents, resulting in moderately high Sr/Y ratios (12.4-93.9; Figura 3-9D). On the other hand, the Old Salobo and G1 granites show low Sr (21.1-201 ppm) and low to high Y (9.4-98.8) contents, resulting in moderate to low Sr/Y ratios (Figura 3-9D).
Figura 3-9. Trace elements plots of the old Salobo, G1 and G2 granites, including other data from the literature for comparison (Lindenmayer 1990, Salgado et al. 2019). (A) Primitive mantle-normalized REE patterns. (B, C) Primitive mantle-normalized trace element patterns. (D) Sr/Y vs. Y (Defant & Drummond, 1993). Normalization
Contribuições às Ciências da Terra, Série M, vol. 80, 106p.
43
3.5.4 - Apatite characterization and mineral chemistry
Petrographic aspects of apatite
The modes of occurrence of apatite in the Old Salobo (OS apatite) and G1 (G1 apatite) granites are quite similar. They primarily occur (i) as disseminated subdiomorphic grains, up to 100 µm, with interstitial texture among quartz and feldspars crystals (Figura 3-10A); or (ii) as idiomorphic, grains with size up to 300 µm associated with igneous hornblende and magnetite (Figura 3-10B, C).
Apatite from the G2 granite (G2 apatite) appears (i) as disseminated grains within the quartz-feldspathic matrix; or (ii) as agglomerates. The disseminated grains are mostly idiomorphic to xenomorphic, up to 400 µm, interstitial to quartz and feldspars grains (Figura 3-10D). In some portions, they occur in contact with secondary chlorite and magnetite (Figura 3-10E). Apatite agglomerates encompass two or more subdiomorphic to xenomorphic grains, up to 400 µm. The agglomerates seem to occur associated with secondary minerals (e.g., chlorite; Figura 3-10F) or close to fractures.
Apatite grains from the Salobo ore zone (OZS apatite) are xenomorphic to idiomorphic, with variable sizes, up to 600 µm. These grains occur primarily within Fe-K hydrothermal halos and also as inclusions in tourmaline or garnet grains (Figura 3-10G, H). Apatite grains from the GT-46 ore zone (OZGT) are idiomorphic to xenomorphic, with variable sizes, up to 800 µm. They occur interstitial to Fe-K hydrothermal halos (i.e., garnet, magnetite, biotite) and quartz subgrain aggregates, or included in some mineral phases (i.e., magnetite and garnet grains; Figura 3-10I).
Figura 3-10. Photomicrographs and backscattered electron microscopy images of the apatite occurrences in granites bodies and ore zone from the Salobo and GT-46 deposits: (A) OS apatite interstitial to quartz and feldspars. (B) OS apatite associated to hornblende. (C) OS apatite inclusion in magnetite and hornblende. (D) G2 apatite interstitial to quartz and feldspars. (E) G2 apatite in contact with secondary chlorite and magnetite. (F) G2 apatite agglomerate with associated secondary chlorite, which displays different compositions on core and rim.
(G) OZS apatite interstitial to biotite and grunerite. (H) OZS apatite associated to biotite, magnetite and tourmaline. (J) OZGT apatite associated to garnet, magnetite and quartz. Mineral abbreviations: Afs = alkali feldspar, Ap = apatite, Bt = biotite, Cb = carbonate, Chl = chlorite, Fsp = feldspar, Grt = garnet, GRU = grunerite, Hbl = hornblende, Mag = magnetite, Plg = plagioclase, Qz = quartz, Tur = tourmaline.
Apatite mineral chemistry
EPMA and LA-ICP-MS analyses were performed in apatite from the Old Salobo (OS apatite) and G2 (G2 apatite) granites, and in the mineralized rocks from the central zone of the GT-46 (OZGT
apatite) and Salobo (OZS apatite) deposits. The central zone of the GT-46 deposit is represented only by Mineralization II samples (i.e. 2.5 Ga IOCG mineralization). The analytical results of apatite grains are listed in supplementary files (Tables 2 and 3).
Collectively, the analyzed apatite grains are fluorapatite and contain 3.80 to 7.24 F (wt%) and 0.00 to 0.08 Cl (wt%). All the results seem to have overestimated F concentrations (i.e., >3.767 wt%), probably related to uncertainties in the EPMA analyses and excess F bound to CO3
-2 (Piccoli and Candela, 2002). Nevertheless, we used F and Cl contents in some plots, as the halogens use to kept the