Magmatic evolution, igneous petrogenesis and metallogenetic implications of the Orosirian magmatism at the Tapajós Mineral Province, Amazonian Craton

• Autor(a) principal: Cassini, Lucas Villela
• Data de Publicação: 2021
• Tipo de documento: Tese
• Idioma: eng
• Título da fonte: Biblioteca Digital de Teses e Dissertações da USP
• Texto Completo: https://www.teses.usp.br/teses/disponiveis/44/44137/tde-04042022-095105/
• topic: Amazonian Craton; Arco magmático; Cráton Amazônico; Depósitos magmático hidrotermais; Geochemistry; Geochronology; Geocronologia; Geoquímica; Magmatic arc; Magmatic-hydrothermal mineral deposits; Província Mineral do Tapajós; Tapajós Mineral Province; Zircão; Zircon

Continental crust growth ratio has been challenging researchers significantly over the past decades, and the existing models vary from episodic to continuous growth ratios, the latter with a more significant role of the Archean Eon. In the South American Platform, the Amazonian Craton (AC) provides an excellent window into Precambrian continental crust, with many exposure of Archean and Paleoproterozoic continental crust. Interestingly, on a global perspective, Archean tectonics and rockassociations has been targeted more frequently and the number of papers overcomes significantly Paleoproterozoic-related publications. Looking closely to south-central AC, the Tapajós Mineral Province (TMP) consists on an excellent area of study as it comprises multiple Orosirian igneous suites within the 2.0 1.87 Ga interval with associated mineral deposits and occurrences of both base and precious metals. Within this aspect, this study provides new petrogenetic constraints on the TMPs igneous suites based on whole-rock and zircon-geochemistry, with the respective tectonic and metallogenetic implications. The results allow the separation of two main age groups within the TMP: the older- (2.0 1.95 Ga) and younger magmatic sequences (1.89 1 .86 Ga). The older magmatic sequence (OMS) is divided into groups I, II and III granitoids that according to the results acquired evolved through fractional crystallization and represents the first batches and evolution of the magmatic arc. Group I marks the onset of OMS and encompasses ferroan, calc-alkalic and peraluminous granites and granodiorites dated in 2011 Ma, characterized by evolved Nd(T) and high zircon saturation temperature (798°C), all compatible with a strong crustal component on magmatic arc magmas. Geochemical modelling shows a dry and oxidized evolution towards high La/Yb and Sr/Y ratios through extraction of plagioclase, pyroxene, biotite, magnetite and titanite. In addition, zircon-based fO2 estimates below FMQ combined with their pyroxene-bearing evolution renders group I magmas a low metallogenetic favorability for the formation of magmatic hydrothermal (Cu Au) mineral deposits. Group II rocks are compositionally more variable, represent the evolution of the arc magmatism, and comprises magnesian to ferroan, alkali-calcic to calc-alkalic, metaluminous to felsic-peraluminous granodiorites, monzo- and syenogranites of ca. 1986 Ma, characterized by moderately negative Nd(T) (from -1 to -3). Geochemical modelling shows an evolution compatible with amphibole and magnetite extraction, which is also corroborated by low Dy/Yb zircon rims and fO2 between FMQ +0 and +4. These set of attributes allow group II granites (and its volcanic equivalents) to be considered as metallogenetic fertile for the formation of magmatic-hydrothermal Cu Au mineral systems. In fact, most rocks from the Tocantinzinho, Chapéu do Sol, São Jorge, Coringa and Patrocínio deposits belong to group II rocks, where mineralization is defined by disseminated gold-bearing sulfides or by pyrite-quartz ±gold ±chalcopyrite veins and veinlets most often found in the sericitic and, less frequently, in the potassic and propylitic alteration zones. Group III rocks are rarer in the TMP and are defined by high-K, ferroan, alkalic, metaluminous to slightly peraluminous quartz-monzonites and quartz-syenites with ages that vary from 1993 and 1974 Ma. Whole-rock geochemistry shows an evolution coherent with K-feldspar, plagioclase, biotite, apatite, clinopyroxene and ilmenite fractionation, which is accompanied by stronger negative Eu anomalies, Sr and Ba depletion, and low Sr/Y and La/Yb ratios. Its anhydrous and reduced petrologic trend is supported by fO2 values below FMQ +1. Despite its synchronicity with group II, group IIIs petrogenesis is harder to be constrained. In view of their reducing and anhydrous evolution, decompression melting of metasomatized mantle is proposed as the main mechanism for generating group III magmas. Moreover, these rocks should be considered as metallogenetically unfertile (pyroxene- and ilmenite-bearing evolution), yet, on the field syenites and monzonites are often affected by hydrothermal alteration and show disseminated and gold-bearing pyrite and pyrrhotite crysts on the sericitic, potassic and/or carbonatic alteration halos. As a consequence, group III rocks are mineralized as the result of the interaction with the more hydrous, oxidized and fertile group II magmas. The younger magmatic sequence (YMS) is represented by the Iriri Group volcanics and by the Parauari (PAR) and Maloquinha (MLQ) Intrusive Suites (described on the V3, Palito and Batalha deposits). Despite their spread on TMPs geologic maps, these rocks are more often spotted on the eastern part of the province. The first pulses of YMS are represented by the PAR granitoids and coeval Iriri volcanics. Both comprise magnesian to ferroan, calc-alkalic to alkali-calcic and alkalic, metaluminous to felsic-peraluminous rocks. Intermediate Iriri rocks evolved towards higher Dy/Yb, Sr/Y and La/Yb ratios through K-feldspar, amphibole, anorthite-rich plagioclase and biotite extraction. Acid Iriri and PAR granitoids evolved on a similar petrologic trend compatible with albiticplagioclase, K-feldspar, pyroxene, titanite and apatite fractionation, that drives the evolving magmas towards lower Sr/Y, La/Yb and Eu/Eu* ratios and suggest anhydrous and reduced mafic sources. PAR granitoids show more strongly negative Nd(t) (average of -4.85) when compared with Iriri rocks, indicating a longer period of residence in crustal hot zones. Its geochemistry signature is interpreted as the result of metasomatized mantle melting on a late- or post-orogenic tectonic setting. Despite the anhydrous and reduced characteristic of the magmas, the geologic framework of YMS is adequate for the formation of gold-rich (or gold only) magmatic-hydrothermal mineral deposits, as these magmas efficiently mobilize previously formed, chalcophileand siderophile-rich sulfides. MLQ marks the last pulses of the Orosirian magmatism in the TMP and is characterized by ferroan, alkali-calcic and moderately-peraluminous alkali-granites that evolved on a pyroxene- and ilmenite-bearing trend. These characteristics and the close to 0 or slightly negative Nd(T) (between -2.64 and -0.28) indicates metasomatized mantle melting, likely involving the directly underthrusted SCLM. From a metallogenetic perspective, MLQ magmatism show a low favorability for the formation of magmatic-hydrothermal mineral systems.