Evandro Luiz Klein

Paleoproterozoic Crustal Evolution of the São Luís Craton, Brazil: Evidence from Zircon Geochronology and Sm-Nd Isotopes

Abstract The Sao Luis Craton, northern Brazil, is composed of a few granitoid suites and a metavolcano-sedimentary succession. New single zircon Pb evaporation ages and Nd isotope data, combined with other available information, show that the metavolcano-sedimentary succession developed from 2240 Ma to approximately 2200-2180 Ma from juvenile protoliths. The subduction-related calc-alkaline suites of granitoids, spatially associated with the metavolcano-sedimentary sequence, formed in an oceanic island arc setting between 2168-2147 Ma. Most of these granitoids are tonalitic and formed from juvenile, mantle- or oceanic plate-derived protoliths, whereas minor true granites are the product of the reworking of the juvenile island arc material. These arc-related successions represent an accretionary event around 2.20±0.05 Ga, which is coincident with one of the main periods of crustal growth in the South American Platform. This accretionary orogen has subsequently been involved in a collision episode, at ca. 2100-2080 Ma, which is mainly recorded in the nearby Gurupi Belt. The rock associations, inferred geological settings, and the crustal evolution detected in the Sao Luis Craton are similar to what is described in Paleoproterozoic domains of major geotectonic units of the South American Platform, such as part of the Sao Francisco Craton, southeastern Guyana Shield, and of the West African Craton.

São Luís Craton and Gurupi Belt (Brazil): possible links with the West African Craton and surrounding Pan-African belts

Abstract The São Luís Craton and the Palaeoproterozoic basement rocks of the Neoproterozoic Gurupi Belt in northern Brazil are part of an orogen having an early accretionary phase at 2240–2150 Ma and a late collisional phase at 2080±20 Ma. Geological, geochronological and isotopic evidence, along with palaeogeographic reconstructions, strongly suggest that these Brazilian terrains were contiguous with the West African Craton in Palaeoproterozoic times, and that this landmass apparently survived subsequent continental break-up until its incorporation in Rodinia. The Gurupi Belt is an orogen developed in the southern margin of the West African–São Luís Craton at c. 750–550 Ma, after the break up of Rodinia. Factors such as present-day and possible past geographical positions, the timing of a few well-characterized events, the structural polarity and internal structure of the belt, in addition to other indirect evidence, all favour correlation between the Gurupi Belt and other Brasiliano/Pan-African belts, especially the Médio Coreaú domain of the Borborema Province and the Trans-Saharan Belt of Africa, despite the lack of proven physical links between them. These Neoproterozoic belts are part of the branched system of orogens associated with amalgamation of the Amazonian, West Africa–São Luís, São Francisco and other cratons and minor continental blocks into the West Gondwana supercontinent.

Age Constraints on Granitoids and Metavolcanic Rocks of the São Luís Craton and Gurupi Belt, Northern Brazil: Implications for Lithostratigraphy and Geological Evolution

Single-zircon Pb evaporation dating was undertaken on granitoids and metavolcanic rocks of different lithostratigraphic units of the São Luís craton and the bordering Gurupi belt in northern Brazil, allowing the determination of the crystallization ages of these rocks and a partial reassessment of the regional lithostratigraphy. In the Sao Luis craton, zircons from granitoids of the Tromai Suite (dominantly tonalitic) yielded 207Pb/206Pb ages between 2149 ± 5 Ma and 2165 ± 2 Ma, and a metapyroclastic rock of the metavolcanosedimentary Aurizona Group was dated at 2240 ± 5 Ma. In the Gurupi belt, the Itamoari Tonalite (the deformed correspondent of the Tromai Suite) gave an age of 2148 ± 4 Ma, similar to the younger ages of the Tromai Suite. Two felsic metavolcanics of the metavolcanosedimentary Gurupi Group showed crystallization ages between 2148 ± 1 Ma and 2160 ± 3 Ma, which are similar to those of the granitoids of the cratonic domain. The Th/U ratios of the whole set of analyzed samples, calculated from the 208Pb/206Pb ratio, ranged from 0.23 to 0.51, which is typical of magmatic zircons. The determined ages are in good agreement with those of the correlative Eburnean granitoids and Birimian sequences of the West African craton. A Rb-Sr internal isochron was calculated for a collisional-type granitoid (Maria Suprema Granite) in the Gurupi belt, yielding an age of 1710 ± 32 Ma, interpreted as a partial resetting of the isotopic system. Considered as a minimum age, this dating places the granitoid in the Paleoproterozoic. The age of the rocks and of the establishment of the Gurupi belt have been controversial, owing to the widespread Neoproterozoic (Brasiliano/Pan-African) Rb-Sr and K-Ar signature of its rocks. Our data, combined with other recent geological and geochronological information, suggest that rocks of the present Gurupi belt have likely participated in collisional/accretionary processes occurring in the final stages of the Transamazonian orogenic cycle in the Paleoproterozoic (2.0-2.1 Ga). This was part of the major process of assembly of the Atlantica supercontinent. The belt was tectonically reactivated in the Neoproterozoic, with widespread reworking of older rocks and only localized rock generation, leading to its present configuration.

Síntese geológica e geocronológica do Cráton São Luís e do Cinturão Gurupi na região do Rio Gurupi (NE-Pará / NW-Maranhão)

The Gurupi region in northern Brazil has been divided into Paleoproterozoic (Sao Luis Craton) and Neoproterozoic (Gurupi Belt) geochronological domains based on Rb-Sr and K-Ar. Recent zircon (Pb evaporation) dating of most of the lithostratigraphic and lithodemic units, in addition to limited whole-rock Sm-Nd determinations, shows virtually all rock units (juvenile or reworked) formed between ~2.0 Ga and 2.2 Ga. A single granitoid shows a Neoproterozoic crystallization age (~0,55 Ga), and the Archean has been recorded only in inherited zircons and in protolith Sm-Nd model ages. The compositional, metamorphic, structural, geophysical and geochronological characteristics of the geological units supports the previous subdivision into two domains. However, they also reveal a common Paleoproterozoic evolution of both the Gurupi and Sao Luis domains. The Gurupi domain is characterized as a collisional orogen, whereas the Sao Luis domain shows accretionary characteristics, with both participating in the widespread Paleoproterozoic collage at 2.2 - 2.0 Ga. During the Neoproterozoic, the southern border of this region was affected by strong strike-slip shearing and very limited lithogenesis, reflecting the Brasiliano orogenic collage elsewhere and defining the presently observed relationship between the Sao Luis Craton and the Gurupi Belt.

Geology of Paleoproterozoic Gneiss-and Granitoid-Hosted Gold Mineralization in Southern Tapajós Gold Province, Amazonian Craton, Brazil

Vein-quartz gold mineralization in Southern Tapajós Province is hosted by arc-related, calc-alkaline tonalitic orthogneisses (Cuiú-Cuiú Complex, 2033-2005 Ma) and post-collisional, calc-alkaline, K-rich granitoids (Creporizão Intrusive Suite, 1997-1957 Ma). The deposits are structurally controlled and form typically tabular bodies that parallel the hosting structures, and are characterized by quartz veins surrounded by halos of strongly altered wall rock, which are usually narrow and show weak to prominent ductile fabric. Steeply dipping fault-fill veins and shear veins account for 80% of the structural style, followed by breccia veins and lesser stockworks and veins hosted in low-angle reverse-oblique faults. Hosting structures vary from ductile-brittle to brittle in nature, and together with structural and textural evidence provided by the veins, indicate a wide range of depth of emplacement for the mineralization, from shallow to mid-crustal. Quartz and sericite are the main alteration minerals and pyrite is ubiquitous. We modify current structural models for the Tapajós Province, proposing the NW-SE-trending strike-slip event that affected the Creporizão Suite, and transposed the gneissic banding of the Cuiú-Cuiú Complex as the second (D2) major structural event in the province, occurring broadly coeval with the emplacement of the late-stage plutons of the Creporizão Suite (~1.97-1.95 Ga). Emplacement of low-angle fault-hosted veins, shear veins, and fault-fill veins that show evidence of plastic deformation (i.e., most of the deposits in Southern Tapajós Province) are related to this structural phase.

Ore fluids of orogenic gold deposits of the Gurupi Belt, Brazil: a review of the physico-chemical properties, sources, and mechanisms of Au transport and deposition

Abstract The Neoproterozoic Gurupi Belt in northern Brazil developed at the southwestern margin of the Palaeoproterozoic São Luís-West Africa Craton. Orogenic gold deposits of this belt are hosted in Palaeoproterozoic (2160–2147 Ma) metavolcano-sedimentary and calc-alkaline granitoid rocks formed in arc and/or back-arc settings during a protracted Rhyacian orogeny (2240–2080 Ma). These host rock assemblages were tectonically and isotopically reworked during the Neoproterozoic and represent the reworked margin of the craton, that is, the external domain of the Neoproterozoic (Brasiliano-Pan African) orogen. The location of the gold deposits is controlled by the Tentugal shear zone, which represents the tectonic boundary between craton and the Gurupi Belt, and its subsidiary structures. Gold occurs in veins and in association with pyrite, and subordinately arsenopyrite and chalcopyrite, in strongly altered and variable deformed host rocks. Geological characteristics, petrographic, fluid inclusion, and isotopic evidence indicate near-neutral, reduced aqueous-carbonic metamorphic fluids, with local contributions from host rocks at the deposit site. Ore deposition occurred at about 300–370 °C and up to 3 kbars in response to fluid immiscibility and fluid-rock reactions (sulphidation, desulphidation, carbonatization, CO2 removal) and local fluid mixing and oxidation.

U-Pb SHRIMP and 40Ar/39Ar constraints on the timing of mineralization in the Paleoproterozoic Caxias orogenic gold deposit, São Luís cratonic fragment, Brazil

Caxias is an orogenic gold deposit in the Sao Luis cratonic fragment, which is correlated with the Rhyacian terranes of the West-African Craton. The deposit postdates peak metamorphism (estimated at 2100 ± 15 Ma) and is hosted in a shear zone that cuts across schists of the Aurizona Group (2240 ± 5 Ma) and the Caxias Microtonalite. The emplacement age of the microtonalite, as determined in this work by SHRIMP U-Pb zircon dating, is 2009 ± 11 Ma and represents a late-stage magmatic event in the Sao Luis cratonic fragment. Older zircon age of 2139 ± 10 Ma is interpreted as due to inheritance from the older granitoid or volcanic suites (magmatic sources?) or to contamination during emplacement. Lead isotope compositions indicate that the Pb incorporated in ore-related pyrite was probably sourced from regional, orogenic calc-alkaline granitoids of ca. 2160 Ma. Hydrothermal sericite from Caxias yielded a 40Ar/39Ar plateau age of 1990 ± 30 Ma, which combined with the emplacement age of the Caxias Microtonalite brackets the age of gold mineralization between 2009 ± 11 and 1990 ± 30 Ma.

Predictive Mapping of Prospectivity in the Gurupi Orogenic Gold Belt, North–Northeast Brazil: An Example of District-Scale Mineral System Approach to Exploration Targeting

Abstract The Gurupi Belt hosts a Paleoproterozoic gold province located in north–northeastern Brazil, at the borders of Pará and Maranhão states. It is considered to be an extension of the prolific West African Craton’s Birimian gold province into South America. Additionally, the belt has been the object of recent mineral exploration programs with significant resource discoveries. This study presents the results of predictive mapping using up-to-date mineral system concepts and recently finished regional-scale geological mapping, stream sediment and airborne geophysical surveys conducted by the Geological Survey of Brazil. We relate gold mineralization to an initially enriched crust, metamorphism, deep fluid pathways, structurally controlled damage zones and hydrothermal alteration. Prospective targets were generated using only regional public datasets and knowledge-driven targeting technique. This work did not incorporate any known gold deposits, yet it predicted the largest known deposits and their satellite targets. Besides, high prospective targets mapped almost 40% of known primary gold occurrences within 7% of the project area. This work allowed considerable search area reduction and identification of new target areas, thus collaborating on reducing costs, time and risk of mineral exploration. Results indicate that we achieved an efficient understanding of the geological processes related to the Gurupi Belt mineral system.

The mineralizing fluid in the Piaba gold deposit, São Luís cratonic fragment (NW-Maranhão, Brazil) based on fluid inclusion studies in quartz veins

Piaba is the first gold mine to operate in the Sao Luis cratonic fragment, NW-Maranhao, northern Brazil. The geological setting comprises chiefly metavolcano-sedimentary sequences (Au- rizona Group) and subduction-related granitoids (Tromai Intrusi- ve Suite), formed in island arc between 2240 and 2150 Ma. Gold mineralization is hosted in a fine-grained granophyric granodiorite (Piaba Granophyre) and in a subvolcanic andesite of the Aurizona Group. The mineralized zone is confined within the limits of the Piaba fault (ENE-WSW-trending brittle-ductile shear zone) and con- sists of quartz veins and veinlets and accompanying hydrothermal haloes (chlorite + muscovite + carbonate + pyrite + chalcopyrite + gold) disposed in stockwork geometry. Petrographic, microthermo- metric and microRaman spectroscopic studies of quartz have defined two- and three-phase aqueous-carbonic fluid inclusions produced by heterogeneous trapping during phase separation, in addition to late aqueous fluids. The mineralizing solution is an aqueous-carbonic fluid composed of CO 2 (5 – 24 mol%), H 2 O (74 – 93 mol%), N 2 (≤ 1 mol%) and CH4 (≤ 1mol%). It presents low salinity (5.5 wt.% NaCl equivalent) and density of 0.96 – 0.99 g/cm 3 ). Ore deposition occurred at 267 – 302 oC and 1.25 – 2.08 kbars, corresponding to 4 – 7 km in depth, in agreement with the structural information. The P-T-X and reduced characteristics (log ƒO 2 -31.3 to -34.3) of the fluid, combined with host rock sulfidation, altogether indicate that gold has been transported as a sulfur complex and that ore deposition occurred in response to phase separation and lowering of the sulfur activity and ƒO 2 during fluid-rock interaction.

Reconnaissance Stable Isotope (C, O, H, S) Study of Paleoproterozoic Gold Deposits of the São Luis Craton and Country Rocks, Northern Brazil: Implications for Gold Metallogeny

Caxias, Areal, and Pedra de Fogo are structurally controlled syn- to late tectonic and post-metamorphic gold deposits of Paleoproterozoic age located in the São Luis craton, northern Brazil. Previous fluid inclusion studies indicated reduced, low-salinity CO2-H2O (±CH4-N2) fluids trapped at 260°-300°C (Caxias, Areal) to 330°-400°C (Pedra de Fogo), and at 2 kbar as responsible for the mineralization. Oxygen isotope ratios of quartz (+10.4 to +16.2‰), chlorite (+7.6‰), and sericite (+5.1‰), and hydrogen isotope ratios of chlorite (-46‰), sericite (-58‰), and fluid inclusion water (-32 to -70‰) indicate fluid Δ18O values of +2.6 to +5.6 per mil (Caxias), +0.6 to +3.5 per mil (Areal), and +10.3 to +12.1 per mil (Pedra de Fogo) and fluid ΔD composition of -24 to -53 per mil (Caxias), -49 to -62 per mil (Areal), and -70 per mil (Pedra de Fogo) at the assumed temperatures. These estimated fluid compositions are consistent with metamorphic sources. Calcite and fluid inclusion CO2 Δ13C values between -3.1 and -10.9 per mil are not diagnostic of a particular origin, but a more negative value (-20.2‰) found at Caxias indicates a greater organic component, at least locally. Sulfide Δ34S values of -2.8‰ to -11.0‰ reflect possible achievement of more oxidized conditions. Hydrogen and oxygen isotope analyses of the country rocks suggest minor isotope disequilibrium and resetting of oxygen isotope geothermometers. This might indicate subsolidus post-crystallization isotopic exchange, linked with metamorphism and/or hydrothermal alteration.