Chapter 5: The Low-Grade, Neoproterozoic, Vein-Style, Carbonaceous Phyllite-Hosted Paracatu Gold Deposit, Minas Gerais, Brazil

Abstract

The Paracatu deposit in Brazil is a shallowly dipping, bulk-tonnage, low-grade, vein-style orogenic Au orebody hosted in very strongly deformed Neoproterozoic carbonaceous phyllite of the southern Brasília fold belt. At regional to district scales, the gold orebody lies along the eastern, hanging-wall edge of a major thrust of the ~630 Ma Brasiliano orogeny. This thrust cuts through a facies transition between clastic-dominated rocks of the Canastra Group and carbonate-dominant rocks of the Vazante Group, deposited at ~1000 Ma in a rift to passive-margin environment on the flank of the São Francisco craton. At the same scales, the footwall of this major thrust system hosts numerous structurally controlled zinc deposits including Vazante and Morro Agudo. At Paracatu, ore genesis occurred primarily by the formation of early tectonic quartz sulfide-carbonate veins, prior to substantial ductile deformation (boudinage), local physico-chemical reworking of these veins, and redistribution of some gold. Structural, geochemical, and isotopic data indicate a strong influence of the local rocks (cm to 100-m scales) on many ore ingredients, and the quartz and carbonate in ore veins were most likely derived locally (cm to m scales). However, the coassociation of gold and arsenic with the boudinaged veins and a major thrust, and the absence of metal enrichments normally associated with syngenetic metalliferous black shales, supports a model of far-field derivation of gold within this metasedimentary package (km to 10-km scales). Transport of metal-bearing fluids toward a favorable structural and chemical site during thrusting and orogenesis was possibly focused, during precipitation to ore grades, by the position of transverse structures in the basement, which also influenced deposition of the adjacent zinc deposits. Successful mining of the low-grade resource was initially favored by the subhorizontal orebody geometry and weathering characteristics, and subsequently by high production rates from the 100-m-thick mineralized zone.