Apatite cathodoluminescence and trace element geochemistry of Neoarchean and Paleoproterozoic iron oxide-copper-gold (IOCG) deposits of the Carajás domain, Brazil: implications for fluid evolution and metallogenesis

Abstract

Apatite is a common accessory mineral observed in alteration zones or sulfide-rich mineralized bodies of Neoarchean (ca. 2.7 and 2.5 Ga) and Paleoproterozoic (ca. 1.88 Ga) iron-oxide copper-gold (IOCG) deposits from the Carajás Province. Based on in-situ LA-ICP-MS and EPMA chemical analyses, combined with cathodoluminescence imaging, we investigate morphological and compositional variations among apatite samples from six IOCG deposits recognized in the province, including the Neoarchean Sequeirinho, GT-46, Grota Funda, and Igarapé Bahia deposits, as well as the Paleoproterozoic Sossego orebody and Alvo 118 deposit. The results of this study demonstrate that apatite in these deposits exhibit complex textural domains marked by distinct trace element compositions and REE patterns. Most of the investigated apatite varieties exhibit a hydrothermal signature linked to coupled dissolution-reprecipitation processes, which were the main responsible for REE internal remobilization within grains. This remobilization led to the precipitation of secondary REE-bearing phases (e.g., monazite, allanite) as inclusions or crystal overgrowths associated with altered domains in the analyzed grains. The compositional and morphological variations found in apatite involved distinct fluid regimes and are, therefore, interpreted to reflect the history of fluid-apatite interaction at a deposit scale. Redox conditions under which apatite crystalized vary from relatively reduced (i.e., Sequeirinho, GT-46, Grota Funda, and Sossego) to more oxidized states (i.e., Alvo 118 and Igarapé Bahia). The interaction of primary apatite domains with highly oxidizing fluids possibly suggests a Paleoproterozoic hydrothermal overprint in some of the Neoarchean studied deposits (i.e., Sequeirinho, Grota Funda, and Igarapé Bahia).