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.

ABSTRACT The analysis of the pre-Andean history of the Central Andes shows a complex tectonic evolution. The basement of the Andean continental margin was formed by the accretion of Precambrian blocks during the formation of Rodinia in late Mesoproterozoic times. There are two magmatic arcs of Grenvillian age, one developed on the margin of the craton, known as the Sunsas belt, and another on the accreted terranes. The suture between these blocks with the Amazonian craton has been continuously reactivated by tectonic and magmatic processes. The terranes of Paracas and Arequipa, both of Grenvillian age, have a contrasting Paleozoic evolution. The Arequipa terrane amalgamated to the craton by the end of the Mesoproterozoic, and during the Paleozoic its suture acted as a crustal weakness zone. This zone concentrated the extension and the formation of a large platform in the retro-arc basin, where the Eopaleozoic sediments accumulated. The Famatinian magmatic arc of Ordovician age (475–460 Ma) is preserved in this segment along the continental margin. The Eopaleozoic extension that affected the Paracas terrane reopened the old suture and formed oceanic crust between Amazonia and Paracas. The subduction of this oceanic crust developed a magmatic arc over the cratonic margin, which is preserved in the Eastern Cordillera of Peru as orthogneisses associated with metamorphic rocks of Famatinian age. There are ophiolitic assemblages, paired metamorphic belts, and intense deformation associated with the Paracas collision (~460 Ma)against the Amazonian craton. In northern Eastern Cordillera of Peru the late Paleozoic orogen has within-plate granitic belts and was far away from the active margin. The orogen was deformed and uplifted in two phases (336–285 Ma and 280–235 Ma) known as the early and late Gondwanide orogenies. They are preserved as medium grade metamorphic belts developed along the Paracas segment. Further south along the Arequipa segment in southern Peru and Bolivia, the late Paleozoic–Triassic rocks are represented by granites and acidic volcanic rocks, which are not metamorphosed and are associated with sedimentary rocks. Relics of a magmatic arc are exposed as tonalites and metamorphic rocks (~260 Ma) along the northern continental margin of Peru and in the near offshore platform. The extensional regime that dominated most of the Mesozoic developed rift basins in the hanging-wall of the terrane sutures, which controlled the structural highs and basin margins. The Peruvian Late Cretaceous orogeny produced the emplacement of the Coastal batholith, the beginning of deformation along the coast, and the first foreland basins. The giant Ayabacas submarine syn-tectonic collapse is also controlled by previous sutures. The Cenozoic Andean evolution was dominated by a wave of shallowing of the subducted slab, the migration of the magmatism to the foreland, the steepening of the oceanic plate, and the consequent “inner arc” magmatism. The “inner arc” plutonic and volcanic rocks are the expression of deep crustal melts, associated with crustal delamination and lithospheric mantle removal. The flattening of the oceanic slab is related to ablative subduction and shortening in the Altiplano and Eastern Cordillera. The steepening is associated with rapid removal of mantle lithosphere and crustal delamination, expressed at surface by the “inner arc” magmatism. The suture crustal weakness zones between different terranes partially controlled the location of the delaminated blocks and the “inner arc” magmatism. Both processes triggered the lower crust ductile shortening and subsequent upper crustal brittle development of the sub-Andean fold-and-thrust belt.

The Granjeno Schist of northeastern México is the oldest component of the Sierra Madre terrane and comprises polydeformed, pelitic metasedimentary and metavolcaniclastic rocks that enclose lenses of serpentinite-metagabbro. This low-grade Paleozoic assemblage is exposed in the core of a NNW-trending frontal anticline of the Laramide fold-thrust belt where it is tectonically juxtaposed against ca. 1 Ga granulites of the Novillo Gneiss. Silurian strata that unconformably overlie the Novillo Gneiss are unmetamorphosed and contain fauna of Gondwanan affinity. LA-ICPMS (laser ablation inductively coupled plasma mass spectroscopy)U-Pb ages for detrital zircons from a Granjeno phyllite yield age populations that cluster in the ranges ca. 1375–880 Ma, ca. 650–525 Ma, and ca. 460–435 Ma and slightly discordant grains with individual ages of ca. 1435 Ma, ca. 1640 Ma, ca. 2105 Ma, and ca. 2730 Ma. The youngest detrital zircon indicates a maximum depositional age for the Granjeno Schist of ca. 435 Ma (Lower Silurian). Detrital zircons of Neoproterozoic–Cambrian age suggest a provenance in the Maya terrane beneath the Yucatan Peninsula or the Brasiliano orogens of South America, and a source for the detrital zircons of Ordovician–Silurian age is present in the Acatlán Complex of southern México. Provenance of the Mesoproterozoic detrital zircons is likely to have been the adjacent Novillo Gneiss, which has yielded ages of ca. 990–980 Ma, ca. 1035–1010 Ma, and ca. 1235–1115 Ma. These detrital ages closely match those recorded from the Cosoltepec Formation of the Acatlán Complex and support correlation of the two units, which are both interpreted to be vestiges of the southern margin of the Rheic Ocean.