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

Abstract

The Tapajós Gold Province (TGP) is part of the ∼2.10–1.87 Ga Ventuari–Tapajós (or Tapajós–Parima) tectonic province of the central Amazonian craton. The Ventuari–Tapajós Province resulted from an ocean–continent orogeny and includes two calc-alkaline magmatic arcs that are represented by the Cuiú-Cuiú 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 Uatumã 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 500–290 °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.