Hydrothermal alteration and mineralization in the Faina greenstone belt: evidence from the Cascavel and Sertão orogenic gold deposits

Linking Gold Systems to the Crust-Mantle Evolution of Archean Crust in Central Brazil

“…Gold mineralization is associated with D2 thrust faults and, to a minor extent, D3 shear zones. The Au-related hydrothermal assemblages in all deposits overprint greenschist facies metamorphic assemblages and therefore post-date peak metamorphism [12,106]. Contrasting structural evolution is recorded in the northern and southern greenstone belts for the Paleoproterozoic D1, D2 and D3 events (Table 3).…”

“…In greenstone belts of the northern GAB, structures developed throughout these events include: (i) subtlyW -dipping S1 foliation (S0 transposed into S1), subtly W-dipping, NS-trending thrust faults, greenschist faces metamorphism and stratigraphic inversion; (ii) subtly S-dipping S2 foliation, planar axial to isoclinal F2 folds, local mm-to cm-wide shear zones, subtly N-dipping, EW-trending thrust faults and stacking of the stratigraphy; (iii) subtly E-dipping S3 foliation, subtly E-dipping, NS-trending F3 folds and lateral thrust ramps [99,103,107,108]. In the southern GAB, Paleoproterozoic deformation is largely described based on observations in the Faina greenstone belt [106]. The D1 event is expressed by moderately N-dipping S1 foliation (S0 transposed into S1), subtly W-dipping, NNW-trending F1 folds, stratigraphic inversion and greenschist facies metamorphism.…”

“…Gold (≤50 µm) is in equilibrium with pyrrhotite (e.g., Maria Lázara) and arsenopyrite (e.g., Sertão, Jordino, and Ogó), or as inclusions and fracture-fills within these sulphides, with rare free gold (≤30 µm) in both quartz veins and proximal alteration assemblages. The approximated gold fineness associated with this style of mineralization is 920 [106,107]. At the Crixás deposits, estimated production of 3 Mt of ore is reported with an average of 6 g/t Au [12], whereas at the Sertão deposit, oxidized sulfide-rich ore resulted in 256 Koz at 24.95 g/t [115].…”

“…In the Faina greenstone belt, minor stratabound-like, massive sulfide mineralization is associated with chemically reactive host rocks such as BIF, e.g., at Sertão. In this deposit, pervasive replacement of Fe-rich carbonates (Fe-dolomite, ankerite, and siderite) by Fe-sulphides is observed [106]. [118] and (B) massive sulfide [119] mineralization in carbonaceous schist of the Mina III orebody; (C) disseminated sulphides in the Maria Lázara deposit.…”

“…At the Crixás deposits (e.g., Mina III), typical white mica-carbonate-chlorite-pyrrhotite-arsenopyrite assemblages enveloping D2-related veins contain free gold (0.1-2 mm) with a fineness of 910 [12]. At Cascavel, proximal white mica-quartz ± pyrite ± chalcopyrite alteration zones developed around V2 veins display free gold (≤250 mm) with a fineness of 992 [106]. [106]; (E) textural contrast of fine-grained, deformed Py1 and coarse-grained, poikiloblastic Py2 after replacement of ankerite/siderite in proximal BIF at the Sertão deposit [106]; (F) siderite overprinting Py1 and being replaced by Py2 in BIF at the Sertão deposit [106]; (G) gold (≤50 µm) in equilibrium with Asp shows microfractures and dissolution features in proximal carbonaceous schist at the Sertão deposit; (H) BSE image of elongated Au (ca.…”