Lena Virgínia Soares Monteiro

The Batalha Au–granite system—Tapajós Gold Province, Amazonian craton, Brazil: hydrothermal alteration and regional implications

Abstract The Tapajos Gold Province (TGP) is part of the ∼2.10–1.87 Ga Ventuari–Tapajos (or Tapajos–Parima) tectonic province of the central Amazonian craton. The Ventuari–Tapajos Province resulted from an ocean–continent orogeny and includes two calc-alkaline magmatic arcs that are represented by the Cuiu-Cuiu complex (∼2.1–1.90 Ga) and Parauari intrusive suite (∼1.92–1.88 Ga). The Parauari magmatic event was accompanied by sedimentation and intrusion of post-tectonic ∼1.88 Ga calc-alkaline granites and was followed by subalkaline and alkaline A-type magmatism related to the ∼1.87 Ga Uatuma event. The ∼1880 Ma Batalha granite consists of metaluminous to peraluminous, calc-alkaline to sub-alkaline, hornblende-biotite syeno- to monzo-granites, and shows a late- to post-collisional affinity. Intense pervasive and fissural post-magmatic hydrothermal alteration, linked to gold mineralization, affected the Batalha granite. The early alteration stage involved Na metasomatism represented by crystallization of interstitial albite and replacement of plagioclase and K-feldspar by albite. Precipitation of albite caused Na impoverishment and K enrichment in the hydrothermal fluid, leading to microcline and biotite crystallization and pervasive replacement of older feldspars by K-feldspar. Pervasive and fissural propylitization overprints magmatic and early hydrothermal mineral associations, and is characterized mainly by epidote, clinozoisite, chlorite, albite, carbonate and sulphide. Fissural to pervasive sericitization was the last hydrothermal stage and led to crystallization of sericite, quartz and sulphide. Gold mineralization was related to the last two alteration stages, most importantly sericitization. The hydrothermal alterations were related to exsolved magmatic aqueous fluids 500–290xa0°C and 2.6–1.2 kbar. The Batalha granite shares certain characteristics with both (1) porphyry-related systems (biotite halogen content, large alteration haloes and base metals associated with gold) and (2) intrusion-related gold systems (reduced environment, depth of emplacement and gold mineralization associated with sericitic alteration). Classification of the Batalha gold mineralization is thus not straightforward. However, calc-alkaline to sub-alkaline subvolcanic granites similar to Batalha (i.e., with identical hydrothermal alteration patterns) are associated with Paleoproterozoic quartz–alunite high-sulphidation volcanic-hosted epithermal gold mineralization in the TGP, implying coeval evolution of the hydrothermal systems. Thus, the shallower and more oxidized Batalha granite may have generated a porphyry Cu–Au type deposit. This has new implications for gold exploration in the TGP—apart from the known gold deposits within shear zones there is high potential for low-grade, large-volume gold and base metal deposits.

Timing of multiple hydrothermal events in the iron oxide–copper–gold deposits of the Southern Copper Belt, Carajás Province, Brazil

The Southern Copper Belt, Carajás Province, Brazil, hosts several iron oxide–copper–gold (IOCG) deposits, including Sossego, Cristalino, Alvo 118, Bacuri, Bacaba, Castanha, and Visconde. Mapping and U–Pb sensitive high-resolution ionxa0microprobe (SHRIMP) IIe zircon geochronology allowed the characterization of the host rocks, situated within regional WNW–ESE shear zones. They encompass Mesoarchean (3.08–2.85xa0Ga) TTG orthogneiss, granites, and remains of greenstone belts, Neoarchean (ca. 2.74xa0Ga) granite, shallow-emplaced porphyries, and granophyric granite coeval with gabbro, and Paleoproterozoic (1.88xa0Ga) porphyry dykes. Extensive hydrothermal zones include albite–scapolite, biotite–scapolite–tourmaline–magnetite alteration, and proximal potassium feldspar, chlorite–epidote and chalcopyrite formation. U–Pb laser ablation multicollector inductively coupled mass spectrometry (LA-MC-ICP-MS) analysis of ore-related monazite and Re–Os NTIMS analysis of molybdenite suggest multiple Neoarchean (2.76 and 2.72–2.68xa0Ga) and Paleoproterozoic (2.06xa0Ga) hydrothermal events at the Bacaba and Bacuri deposits. These results, combined with available geochronological data from the literature, indicate recurrence of hydrothermal systems in the Southern Copper Belt, including 1.90–1.88-Ga ore formation in the Sossego–Curral ore bodies and the Alvo 118 deposit. Although early hydrothermal evolution at 2.76xa0Ga points to fluid migration coeval with the Carajás Basin formation, the main episode of IOCG genesis (2.72–2.68xa0Ga) is related to basin inversion coupled with Neoarchean (ca. 2.7xa0Ga) felsic magmatism. The data suggest that the IOCG deposits in the Southern Copper Belt and those in the Northern Copper Belt (2.57-Ga Salobo and Igarapé Bahia–Alemão deposits) do not share a common metallogenic evolution. Therefore, the association of all IOCG deposits of the Carajás Province with a single extensive hydrothermal system is precluded.

Geologia da Faixa Eclogítica de Forquilha, Domínio Ceará Central, noroeste da Província Borborema

A partir de mapeamento geologico em escala de detalhe, 1:10.000, e estudo petrografico sistematico foram caracterizados o modo de ocorrencia, a distribuicao e as associacoes minerais do retroeclogito de Forquilha, que situa-se em uma faixa N-S ao longo de mais de 16 km na regiao NW da Provincia Borborema. Tais rochas ocorrem como boudins em gnaisses orto- e paraderivados e, raramente, associados a rochas calcio-silicaticas. Esta associacao compoe a Faixa Eclogitica de Forquilha, que estruturalmente esta em contato tectonico de empurrao com metapelitos e corpos subordinados de rochas metacarbonaticas e metamaficas metamorfizadas entre condicoes facies granulito de alta pressao e facies anfibolito superior pertencentes ao Grupo Ceara. O retroeclogito mostra tres principais estagios de retrometamorfismo representados pelas seguintes reacoes: 1o) [Grt + Cpx + Qtz + H 2 O → Pl + Amp] e [Grt + Cpx + Rt + Qtz + H 2 O → Amp + Ilm + Pl]; 2°) [Omp → Di + Pl]; 3o) [Grt + Di + Pl + H 2 O → Amp] e [Grt + Pl + Qtz + H 2 O → Grt 2 + Pl (An) + Amp], alem de incipiente formacao tardia de Fe-clorita e actinolita. O retrometamorfismo ocorreu de forma heterogenea nestes corpos, sendo mais intenso nas bordas devido a interacao com fluidos metamorficos canalizados ao longo de zonas de cisalhamento durante o processo de exumacao progressiva. A ocorrencia de retroeclogito em Forquilha e importante para a contextualizacao da evolucao tectonica da Provincia Borborema durante a formacao do Gondwana Ocidental, uma vez que este litotipo pode representar uma zona de sutura neoproterozoica.

Paragenesis and evolution of the hydrothermal Bacuri iron oxide-copper-gold deposit, Carajás Province (PA)

The Bacuri copper deposit is located about 9 km east of the world-class Sossego iron oxide-copper-gold deposit in the Carajas Province. It is hosted by the ca. 2.84 Ga Serra Dourada granite, the Bacuri Porphyry, and subordinated gabbro. The Bacuri deposit is situated within the regional-scale, WNW-ESE-trending ductile Canaa shear zone, characterized by early albite, scapolite-magnetite, potassium feldspar and biotite-scapolite-magnetite hydrothermal alteration zones. Copper mineralization was controlled and largely simultaneous to the development of a NE-SW-trending transcurrent fault zone. Within this zone, intense chlorite alteration, silicification and quartz veining overlap previous hydrothermal stages. Copper ore xad(chalcopyrite-pyrite-magnetite) is disseminated and related to veins and breccias. Chalcopyrite occurs along the mylonitic foliation in chlorite alteration zones, in brecciated quartz-(muscovite-fluorite) and undeformed milky quartz veins and in late potassium feldspar-epidote veinlets with open space textures. The relative temporal history of the Bacuri deposit reveals significant changes in physico-chemical parameters during the system evolution. Contribution of hot hypersaline metalliferous fluids was important during early sodic and potassic alteration stages and related to a regional hydrothermal system developed at relatively deep crustal levels. Influx of externally derived diluted fluids within the NE-SW-trending fault zone favoured the late shallow-emplaced chlorite alteration and silicification, and may have triggered the ore deposition due to decrease of temperature and salinity, accompanied by ƒO2 increase and pH decrease. Such significant changes could indicate a hybrid system that evolved in multiple pulses during progressive exhumation.

Temporal evolution of the giant Salobo IOCG deposit, Carajás Province (Brazil): constraints from paragenesis of hydrothermal alteration and U-Pb geochronology

The giant Salobo copper-gold deposit is located in the Carajás Province, Amazon Craton. Detailed drill core description, petrographical studies, and U-Pb SHRIMP IIe and LA-ICP-MS geochronology unravel its evolution regarding the host rocks, hydrothermal alteration and mineralization. Within the Cinzento Shear Zone, the deposit is hosted by orthogneisses of the Mesoarchean Xingu Complex (2950xa0±xa025 and 2857xa0±xa06.7xa0Ma) and of the Neoarchean Igarapé Gelado suite (2763xa0±xa04.4xa0Ma), which are crosscut by the Old Salobo granite. Remnants of the Igarapé Salobo metavolcanic-sedimentary sequence are represented by a quartz mylonite with detrital zircon populations (ca. 3.1–3.0, 2.95, 2.86, and 2.74xa0Ga). High-temperature calcic-sodic hydrothermal alteration (hastingsite-actinolite) was followed by silicification, iron-enrichment (almandine-grunerite-magnetite), tourmaline formation, potassic alteration with biotite, copper-gold ore formation, and later Fe-rich hydrated silicate alteration. Myrmekitic bornite-chalcocite and magnetite comprise the bulk of copper-gold ore. All these alteration assemblages have been overprinted by post-ore hematite-bearing potassic and propylitic alteration, which is also recognized in the Old Salobo granite. In the central zone of the deposit the mylonitized Igarapé Gelado suite rocks yield an age of 2701xa0±xa030xa0Ma. Zircon ages of 2547xa0±xa05.3 and 2535xa0±xa08.4xa0Ma were obtained for the Old Salobo granite and for the high-grade copper ore, respectively. A U-Pb LA-ICP-MS monazite age (2452xa0±xa014xa0Ma) from the copper-gold ore indicates hydrothermal activity and overprinting in the Siderian. Therefore, a protracted tectono-thermal event due to the reactivation of the Cinzento Shear Zone is proposed for the evolution of the Salobo deposit.

Superfícies estriadas no embasamento granítico e vestígio de pavimento de clastos neopaleozóicos na região de Salto, SP

Near Guarau Ceramic, localized southwest of Salto city in the State of Sao Paulo, two granite outcrops, distant some tens of meters from each other, display Neopaleozoic striated surfaces. These surfaces are in contact with diamictites from the Itarare Subgroup. The striae correspond to sub parallel grooves with millimetric spacing and depth, oriented about N48E and dipping 12° to 42° towards SE. Observed features and association with diamictites indicate an origin by glacial abrasion due to ice movement from southeast towards northwest. About 1.8 km east of Salto, unconsolidated material containing flat-iron-shaped and striated clasts was found on top of granite outcrops, interpreted as clast pavement remains.

Geology of the Riacho do Pontal iron oxide copper-gold (IOCG) prospect, Bahia, Brazil: hydrothermal alteration approached via hierarchical cluster analysis

The Riacho do Pontal prospect is situated on the border between the Borborema Province and the Sao Francisco Craton, in Bahia state. It comprises rocks polydeformed during the Neoproterozoic. The prospect area includes migmatites and gneissic rocks intruded by several sin- to post-tectonic granites. Structural analysis indicates a strong relationship between the development of ductile to brittle-ductile shear zones and associated hydrothermalism. The main tracts of high-strain rate are represented by the Riacho do Pontal (north) and Macurure (south) shear zones. Several copper occurrences have been mapped within the Riacho do Pontal prospect along secondary shear zones. In these areas, the gneissic rocks were affected by intense hydrothermal alteration. Hierarchical cluster analysis permitted the identification of the main hydrothermal mineral associations present in these rocks, which resulted from potassic (biotite) and sodic-calcic (amphibole-albite) alteration, in addition to silicification and iron alteration (hematite). These hydrothermal alteration types are similar to those typically found in iron oxide copper-gold deposits developed at intermediate crustal levels. Hematite-quartz-albite-chalcopyrite-pyrite hydrothermal breccias host the highest-grade copper ore (chalcopyrite-pyrite-chalcocite) zones. The spatial relationship between copper deposits and shear zones improves the metallogenic potential for copper of the Borborema Province and has important implications for mineral exploration in the region.