The Archean BIF-hosted Lamego gold deposit, Rio das Velhas greenstone belt, Quadrilatero Ferrifero: Evidence for Cambrian structural modification of an Archean orogenic gold deposit

摘要

The Lamego orogenic gold deposit (1.3 Mt measured resources at 7.26 g/t Au) is located at the south-western end of the 5 km-long Cuiaba-Lamego trend, Quadrilatero Ferrifero region, Brazil. Both Archean orogenic gold deposits are situated in the Rio das Velhas greenstone belt with the lithological succession at Lamego consisting of metamorphosed (greenschist facies) mafic volcanic rock, chert and banded iron formation (BIF), and carbonaceous and micaceous pelites. The Lamego fold, which controls the Lamego deposit, is the most visible structure related to the D-1-D-2 event, with a perimeter of about 4.8 km and a maximum width of 450 m. The fold is defined by the layering of the mafic unit with minor BIF and large exposures of carbonaceous and micaceous pelites. It is a rootless, reclined, isoclinal, cylindrical fold with an axial trace striking northwest-southeast dipping 20 degrees to 30 degrees. The hinge zone is thickened and the limbs are thinned, with the limbs dipping 20 degrees to 30 degrees to the SE. Orebodies consist of the Lamego BIF, where gold-mineralized zones are related to iron-rich bands, and associated silicification zones. Replacement-style mineralization is associated with sulfide bands; mainly pyrite, As-rich pyrite, and arsenopyrite. Two structural generations, G(1) and G(2), are recognized and encompass a set of structural elements. The G(1) structural generation developed in a progressive deformation event and resulted in structures oriented from NE to SW and dipping to the SE. Structures pertaining to the G(2) structural generation are oriented N-S dipping to the W. The four major high-grade gold orebodies are Carruagem, Queimada, Arco da Velha, and Cabeca de Pedra. Their gold grade shows a spheroidal pattern and a distribution that varies along the S1-2 foliation. These lenses represent the hinge zone of F-2 reclined folds with the plunge of the orebodies controlled by the F-2 fold axes. The lower-grade gold lenses are controlled by pinch and swell, and locally expressed quartz boudins developed during D-1-D-2. They have two orthogonal directions, one to the NW-SE and the other to the NE-SW, thereby defining chocolate-tablet style boudinage. Hydrothermal monazite grains in a mineralized mafic volcanic rock indicate that mineralization formed at 2730 +/- 42 Ma (U-Pb SHRIMP). Younger monazite dated at 2387 +/- 46 Ma, and xenotime dated at 518.5 +/- 9 Ma suggest Siderian and Cambrian imprint in the Lamego deposit area. The Cambrian age reflects the late stages of the Brasiliano orogenic cycle, which is expressed in the Lamego deposit by the S-3 crenulation cleavage (trend NS and dip steeply to the E). Importantly, the Cambrian structural modification is responsible for the present geometry of the Lamego orebodies, but is not associated with hydrothermal alteration; it has not introduced any new gold and also has not caused remobilization of the orebodies or ore minerals. (C) 2015 Elsevier B.V. All rights reserved.