Shear-zone-related gold mineralization in quartz-carbonate veins from metamafic rocks of the BIF-hosted world-class Cuiaba deposit, Rio das Velhas greenstone belt, Quadrilatero Ferrifero, Brazil: Vein classification and structural control

摘要

The Cuiaba deposit, located in the Quadrilatero Ferriferro district (QF) in the southeastern part of the Minas Gerais state, is the largest underground mine in Brazil (5.78 Moz resources of 10.26 g/t Au - 1.18 Moz of ore reserves and 6 Moz of overall production). This world-class orogenic gold deposit is part of the Rio das Velhas greenstone belt, a typical association of mafic volcanic rocks, banded iron formation (BIF), carbonaceous and micaceous phyllites, metamorphosed to the greenschist facies conditions. Historically, economic gold production at Cuiaba derives from BIF-hosted, replacement-style sulfide (pyrite and pyrrhotite) ores. More recently, economically significant shear-zone-related gold mineralization in quartz-carbonate veins systems, hosted in metamafic rocks, were identified. They are associated with distal (chlorite-), intermediate (carbonate-) and proximal (sericite-dominant) hydrothermal alteration zones. Structures related to the gold-bearing quartz-carbonate veins were generated during a single, progressive ductile to brittle deformational event (D1), with the compositional banding S-b and the flexural foliation S-f created in the early stages of deformation. As deformation progressed, shear zones, isoclinal folds, the mylonitic foliation S-m, the mineral lineation L-min and the intersection lineation L-int were formed simultaneously to the main stage of hydrothermal fluid input. Flanking folds and flanking shear bands represent the late stage of deformation and the final hydrothermal fluid pulse. Four quartz-carbonate vein systems are identified: V1, V2, V3 and V4. The V1 mineralized system is an S-m-controlled fault-fill type, associated with the principal, gold-bearing fluid input stage. The V2 veins are also mineralized, controlled by the S-f foliation and subdivided in three sub-types, according to internal crystal orientation: V2a extensional, V2b oblique-extensional and V2c, a fault-fill vein. The extensional array V3 veins are also controlled by the S-m foliation, whereas the extensional, breccia-style V4 system has veins associated with flanking structures. The structural analysis of veins indicates a shortening vector rotation during D1 accompanied by hydrothermal fluid input. Since all vein systems are related to the D1 event, each one defining an incremental deformational stage, they provide information about the hydrothermal fluid evolution at Cuiaba. A proposed evolutionary model for the vein systems suggest that the ore-forming fluid responsible for gold mineralization was channeled along shear zones and dispersed out into the S-b and S-f structures. The internal vein features and the trapping temperatures of quartz from each vein system indicate a progressively decrease in tectonic activity.