Geochemistry and isotopic geology of the Lagoa Seca gold deposit in the Andorinhas greenstone-belt, Carajás Province, Brazil

Abstract

The Lagoa Seca is a gold deposit located in the Mesoarchean Andorinhas greenstone belt in the eastern portion of the Amazonian Craton, northern Brazil. It consists of clastic metasedimentary (metagreywacke and metasiltstone) and metavolcanic (metadacite and metaultramafic) rocks. It was operated by Troy Resources Ltd. as an open pit mine from 2008 to 2014, and it produced approximately 150 koz of gold. The hydrothermal alteration halo is directly associated with the internal zonation of the Lagoa Seca shear zone, and it includes i) a distal zone composed of chlorite-epidote-carbonate, ii) an intermediate zone with minerals from the distal alteration plus moderate silicification, iii) a proximal zone composed of biotite-magnetite (carbonate-amphibole-sulfide-gold), and iv) a gold-rich ore zone that has been affected by sulfidation and potassification. Whole-rock geochemistry and mineral chemistry studies on the hydrothermally altered host rocks show that Al2O5 and Na2O increase with silica content, while Fe2O3, MnO, MgO, CaO and the LOI decrease. The minor and trace elements are characterized by very good correlations among Zr, Nb, Ti and LREEs. The REE patterns of hydrothermal rocks show high to moderate fractionation with the La/Lu(cn) of the LREEs and HREEs towards the center of the hydrothermal conduit and are accompanied by a weak negative europium anomaly. The trace-element geochemistry is consistent with that of the subduction related calc-alkaline magmas in continental arcs. In addition, the tectonic discriminant diagrams show a calc-alkaline affinity and classify the host rocks as being derived from subduction processes. The hydrothermal chlorites are classified as brungsvite; mica is phlogopite and amphiboles range from actinolite, tremolite to Mg-hornblende. The main sulfide is pyrite, which is accompanied by minor amounts of chalcopyrite, galena, pyrrhotite and traces of millerite. The EPMA analysis of the pyrite shows that As, Co and Cu increase towards the center of the hydrothermal zone, as well as Zn, Ni and Au decrease in the opposite direction. Gold is present predominantly as inclusions in pyrite/chalcopyrite, with a high Au content. A chlorite geothermometer indicates that the temperatures ranged from 275 °C to 351 °C in the hydrothermal zones. The oxygen and carbon stable isotope data from the calcite of the ore zone return values of 8.12‰ SMOW and −6.36‰ PDB, respectively, which are compatible with a metamorphic fluid source. However, sulfur stable isotope displays 1.92‰ V-CDT, which may suggest a magmatic sulfur source. The composition of the fine-grained pyrite indicates that the hydrothermal fluid presents a deep crustal source of metamorphic origin, and it is accompanied by a mixed magmatic contribution, which was probably generated during the dominant TTG evolution of the area. The whole-rock Sm-Nd isotopic analyses produced a TDM of 2.92–3.04 Ga and an εNd (2.9) of +0.39 to +1.67 for metaultramafic rocks, a TDM of 2.78–2.92 Ga and an εNd of (2.9) +1.61 to +2.92 for metasedimentary rocks, and a TDM of 2.79–2.83 Ga and an ƐNd of (2.9) +2.83 to +3.02 for metadacite rocks. An Sm–Nd isochron age of 2867 ± 40 Ma indicates a Mesoarchean age for the host rocks.