The structural setting of the Canaã dos Carajás region and Sossego-Sequeirinho deposits, Carajás – Brazil

Abstract

The Carajás Terrane in the Amazon Craton hosts Precambrian Cu-Au deposits with resources larger than 100 million tonnes of ore (e.g. Igarapé Bahia-Alemão, Cristalino, Sossego, 118 and Salobo). This thesis examines at a local scale, structural aspects of the Sossego and Sequeirinho deposits, located in the Archaean granite-gneiss basement of the Canaã dos Carajás region, the boundary between the Carajás and Rio Maria Granite Greenstone Terranes.
The study focuses on the understanding of the processes, controls and styles of two regionally representative Archaean IOCG examples and also investigates the tectonic framework and structural evolution of the Canaã dos Carajás region. Primary data comprises traditional methods of field mapping and structural analysis, microscopic investigation, combined with modern digital mapping, lineament and particles size analysis. The principal aims of the study include: (1) identification of crustal scale regional lineaments and their relationship with mineral deposits; (2) a new tectonic framework and structural model for the Canaã dos Carajás region; and (3) description and interpretation of the structural framework of the Sossego and Sequeirinho mines.
The most prominent crustal scale lineaments in the Carajás Terrane comprise early WNW-ESE sets that configure the regional structural trend and mark the basement-cover assemblage contact, and relatively late NE-SW lineaments. Higher lineament frequencies and density were observed in the cover assemblage domain and coincide with numerous lineament intersections. Mineral occurrences and deposits are clustered and spatially associated with domains of higher lineaments density and areas where major lineaments intersect. Then, there is a spatial relationship between major lineaments and the occurrence of mineral deposits.
The Canaã dos Carajás region comprises Archaean TTG gneisses, lens shaped amphibolite bodies, 2.7 Ga. syn-tectonic alkali granitoids and 1.88 Ga. isotropic granites. These rocks display widespread heterogeneous, anastomosing WNW-ESE and NE-SW foliation sets, related steeply dipping ductile shear zones associated with steep-to-moderately plunging mineral lineations. The nature and geometry of the ductile fabrics are compatible with a bulk pure-shear dominated transpression with partitioning of strain intensity with shortening and extensional directions oriented at approximately near horizontal (~020° Az) and near vertical respectively. Microstructures in quartz and feldspar indicate deformation at metamorphic conditions compatible with middle to upper amphibolite facies (~650-700°C), overprinted by deformation at middle to upper greenschist facies conditions (~400-500°C). The Canaã dos Carajás region represents part of an original granite-greenstone terrane that has undergone substantial reworking during a late sinistral transpressional deformation. The reworking took place at c.a. 2.7 Ga, coeval with syn-tectonic sub-alkaline magmatism. The basement comprises intensely deformed rocks uplifted from the lower-to-middle crust, deformed under high amphibolite facies conditions and later affected by localised deformation at greenschist facies conditions.
The structural framework of the Sossego and Sequeirinho deposits comprises regional WNW-ESE structures (foliations and shear zones) offset by NE-SW sinistral faults. Sequeirinho is hosted along a NE-SW sinistral fault, associated with a positive magnetic anomaly. It comprises an “S” shaped tabular orebody whose tips are hosted by sub-vertical WNW-ESE sheared and foliated granitoids and schists. These are linked by a NE-SW sinistral fault zone containing mineralized breccias. Sossego comprises a sub-circular, vertical, pipe-like orebody with a central breccia body surrounded by a stockwork array of sulphide veins, faults and shear zones. Tensile and shear veins show single or composite mineral fillings consistent with episodic vein opening, with a progressive change in hydrothermal fluid composition during time. The Sossego breccias show high clast angularity, characteristic of immature explosion breccias, whilst the Sequeirinho breccias display rounded fragments with low angularity, typical of mature breccias whose particle fragmentation was dominated by wear and attrition during subsequent slip along a fault zone. Microstructures in quartz and feldspar indicate that the deformation at Sossego and Sequeirinho initially took place under low-to-middle (300-400°C) and middle-to-upper (400-500°C) greenschist facies, respectively. Latter overprinted by brittle-ductile structures and veins containing lower-temperature minerals formed between 170-250°C. The rocks in the area of the mines record deformational processes that initially took place under the viscous regime (>15km), represented by mylonites and ultramylonites. Progressive exhumation, possibly synchronous with regional transpressional thickening led to conditions compatible with the frictional-viscous transition with intense fluid activity, with mineralization.