Abstract The Quadrilátero Ferrífero district, located on the southern portion of the San Francisco craton in Minas Gerais, Brazil, comprises Archean greenstone terranes of the Nova Lima Supergroup and the Paleoproterozoic cratonic cover sequences of the Minas Supergroup that consist of quartzites, metaconglomerates, phyllites, dolomites, and banded iron formations. The Minas Supergroup was affected by two orogenic events—the Paleoproterozoic Transamazonian-Mineiro (2.1–2.0 Ga) orogeny and the Neoproterozoic to Early Paleozoic Brasiliano-Araçuaí (0.65–0.50 Ga) orogeny, resulting in complex deformation and metamorphic grades that increase from greenschist facies in the West to amphibolite facies in the East. Metamorphosed iron formations, referred to as itabirites, are found in three compositionally distinct lithofacies, namely quartz itabirite, dolomitic itabirite, and amphibolitic itabirite; these lithofacies are host to a large number of economically important high-grade iron ore deposits that give rise to the name Quadrilátero Ferrífero, or "Iron Quadrangle." High-grade iron ores replace itabirites in tectonically favorable, low-strain sites. faults acted as conduits while large fold hinges were sinks for mineralizing fluids. Hard and fine-grained hematite and/or magnetite orebodies are in the western low-strain domain of the Quadrilátero Ferrífero. Subsequent deformation led to recrystallization and development of distinctly schistose high-grade hematite ores characteristic of the eastern high-strain domain. A combination of hypogene and geologically recent supergene processes is thus invoked to explain the formation of the high-grade iron ores of the Quadrilátero Ferrífero. Three stages of hypogene ore formation are distinguished. The first two of these stages took place early during the Transamazonian orogeny (2.1–2.0 Ga) and are well preserved in the western low-strain domain. During the first stage metamorphic fluids leached SiO 2 and carbonates and mobilized iron, which resulted in the formation of massive magnetite bodies, Fe oxide veins, and Fe-rich itabirite bodies; during the second stage, low-temperature, low-salinity fluids caused oxidation of magnetite and Fe-rich dolomite to hematite. The resulting ore is porous to massive and has a granoblastic fabric. The third and final hypogene stage of ore formation is related to thrusts of uncertain age (Transamazonian or Brasiliano orogeny), which dominate the tectonic structure of the eastern high-strain domain of the Quadrilátero Ferrífero. Crystallization of tabular hematite and large platy specularite crystals that overprint the preexisting granular fabric in the presence of high-salinity hydrothermal fluids are characteristic of this stage. During the Neogene, supergene residual enrichment processes gave rise to the formation of soft to friable hematite orebodies. The larger soft orebodies that surround some smaller hard high-grade orebodies are typically associated with dolomitic itabirite. Together, both ore types comprise the giant high-grade iron deposits typical for the Quadrilátero Ferrífero, resulting from the superposition of both hypogene and supergene processes. Pure supergene deposits are considerably smaller and do not extend to deeper levels below the erosion surface.

Abstract The Chuquicamata district of northern Chile contains > 130 million metric tons (Mt) Cu in resources and past production. The mineralization occurs in various types of Eocene to early Oligocene porphyry Cu systems hosted by Paleozoic and Triassic volcanic and Triassic granodiorite rocks. Emplacement of the deposits occurred during E-W-directed contraction, crustal shortening, and uplift related to the Incaic orogeny between 38 and 31 Ma. The N-S-oriented West Fissure is the most important structural feature of the district, with significant postmineral, left-lateral displacement of 30 to 35 km estimated by regional mapping, although it remains to be confirmed by displaced alteration-mineralization features at the individual deposit scale. The West Fissure, across which both the geology and mineralization differ at any given point, divides the district into two domains. The Eastern Block comprises Paleozoic metamorphic and intrusive complexes overlain by Permian and Triassic volcanic and sedimentary rocks and intruded by Triassic granodiorite. These are overlain in the north by Cretaceous continental sedimentary and volcanic rocks and in the south by Eocene to Miocene sedimentary rocks of the Calama Basin, the lowermost units of which show evidence of syntectonic deposition. The Eastern Block contains the Chuquicamata and Radomiro Tomic deposits, both hosted by the Chuqui Porphyry Complex, a N-NE-oriented, 14- × 1.5-km megadike intruded into Triassic volcanic and intrusive rocks, and with a SHRIMP U-Pb age of ∼ 36 Ma. Hypogene mineralization at Chuquicamata occurs mainly in the East porphyry, the dominant phase of the porphyry complex and does not show a close relationship to smaller, later porphyry bodies with SHRIMP U-Pb ages of ∼ 34 Ma. Much of the Cu was introduced early (∼ 34–35 Ma; 40 Ar/ 39 Ar) during potassic alteration, which comprises a large low-grade body containing biotitized hornblende. Within this large low-grade body higher Cu grades occur as bornite and other Cu-bear-ing sulfides, without pyrite, in intense potassic alteration halos (K-feldspar-sericite) along early fractures. This was followed by introduction of quartz-molybdenite veins, with an Re-Os age of ∼ 35 Ma. Sericite-quartz-pyrite alteration, with advanced argillic alteration near veins, is later, as shown by crosscutting relationships, and returns 32 to 31 Ma 40 Ar/ 39 Ar ages. Veins with high sulfidation Cu-bearing sulfide-pyrite assemblages are part of this later stage, which overprinted and sulfidized the earlier Cu-bearing sulfides. Similar early and late mineralization occurs at Radomiro Tomic, but there the late-stage veins are of lesser importance. The Western Block comprises Paleozoic metamorphic complexes, Permian and Triassic volcanic and sedimentary rocks, and Triassic granodiorite in the south, overlain by Jurassic carbonates and continental sedimentary rocks. In the north, Cretaceous and Lower Tertiary volcanic and continental sedimentary rocks overlie the Jurassic unit. Jurassic to Eocene strata in the central to northern parts of the Western Block hosts the Eocene Los Picos and Fortuna batholiths. The Mina Ministro Hales deposit, at the eastern edge of the Western Block, is associated with Eocene porphyries (∼ 39–35 Ma SHRIMP and LA-ICP-MS U-Pb ages) that intrude wall rocks similar to those at Chuquicamata. Mina Ministro Hales is characterized by mineralization similar to that at Chuquicamata, but late-stage sericitic and advanced argillic alteration (∼ 32 Ma 40 Ar/ 39 Ar ages) are of greater importance, especially at the shallower levels. The Toki Cluster deposits, farther to the west, are associated with swarms of small porphyries thought to be late-stage phases of the ∼ 38 Ma Fortuna granodiorite batholith. The mineralization consists of bornite and/or chalcopyrite, commonly with magnetite but little or no pyrite, in A-type veins accompanying potassic alteration. The strongest mineralization is in and near the earliest porphyries and is truncated by the next younger porphyries, which in turn are cut by similar but less intense mineralization and still younger, less-mineralized porphyries. Late pyrite-sericite alteration occurs mostly on the peripheries of these deposits and carries little Cu. A favorable combination of desert climate and morphotectonic evolution resulted in the formation and preservation of significant supergene enrichment and/or oxidation. The potassic zones at Chuquicamata, Radomiro Tomic, and the Toki Cluster underwent in situ oxidation. Supergene enrichment blankets formed in quartz-sericite-pyrite alteration at Chuquicamata and Mina Ministro Hales. Supergene processes also resulted in lateral migration of Cu and formation of the Mina Sur exotic deposit. From an exploration perspective, the history of the district demonstrates how geologic observations and interpretation have played a key role in development of the resource base. Only the Chuquicamata oxide Cu mineralization cropped out, and original open-pit development in 1912 was followed by exploration and evaluation of the giant, high-grade enrichment blanket from the 1930s onward. District-scale exploration resulted in discovery of Radomiro Tomic in the 1950s, Mina Sur in the 1960s, Mina Ministro Hales in the 1990s and the Toki Cluster at the beginning of this century, none of which were exposed at surface. The Chuquicamata district, now producing ∼ 900,000 t Cu/yr, has been operating for 100 years and retains a substantial resource base that will enable it to continue for many years to come.

Abstract Exposed near Galeana, Nuevo Leon, are sedimentary deposits and contemporaneous structures that record the rifting and opening of the Gulf of Mexico, passive-margin development, and Laramide compression in the region. A mapping, sedimentologic, and structural study utilizing thin sections, measured sections, aerial photos, and kinematic models has produced a detailed stratigraphic section that records the transition from terrestrial to open-marine deposition and the orientation and timing of deformational events. Triassic to Early Jurassic red beds in the Huizachal Group are composed of fluvial and marine sands. The La Boca and La Joya formations are separated by a polymictic cobble conglomerate. The Callovian Minas Viejas represents the initial stages of marine transgression and the overlying interbedded carbonates and evaporites of the Zuloaga and Olvido Formations were deposited in response to cyclic eustatic sea-level fluctuations. The increase of biodiversity through the upper Jurassic strata reflects a change from restricted to open-marine conditions. Rift-related tectonics and gravity driven brittle extensional features within the carbonates have been overprinted by structures that formed during the Laramide orogeny. The red beds have been folded and faulted into a broad Laramide anticlinorium, in which smaller intrusion cored folds have amplitudes on the order of tens of meters. The decollement for Laramide thin-skinned deformation occurs within an evaporate unit. There is a high degree of structural complexity in the overlying carbonates as indicated by tight folding patterns. By integrating concepts of depositional systems with structural deformation the hypothesis, “There is an exhumed preraft block in the Zuloaga Limestone around San Pablo de Tranquitas.” will be tested.

Abstract The Late Palaeozoic evolution of the St Marys Basin, mainland Nova Scotia, preserves evidence of protracted dextral shear along an intracontinental fault zone during collisional orogenesis and the assembly of Pangaea. The St Marys Basin formed within the E-W-trending Minas Fault Zone (MFZ) along the boundary between the Avalon and Meguma terranes and contains latest Devonian-Tournaisian continental clastic rocks that are 3000–4000 m in thickness. The origin and evolution of the basin is attributed to either discrete or progressive dextral strike-slip tectonics along the MFZ between the Late Devonian and Late Carboniferous. Evidence for the Late Devonian origin of the basin is recorded along its southern flank by the fabrics of the deformed c. 370 Ma granites, the overall sedimentary facies distribution, and some syndepositional features within the clastic rocks. The most intense deformation within the basin is concentrated in a relatively narrow ENE-trending zone, in which predominantly fine-grained clastic rocks are deformed into periclinal folds and related reverse faults. The orientation of this zone relative to the MFZ is consistent with dextral shear. At least some of this deformation occurred after the deposition of the overlying Visean Windsor Group. The style of deformation along the present northern margin of the basin (the Chedabucto Fault) is also consistent with regional dextral shear. The St Marys Basin is an example of basin development and evolution adjacent to an intracontinental fault zone associated with oblique convergence during orogenesis. Its evolution provides constraints on the potential relationship between the termination of the mid-Palaeozoic Acadian orogeny, subsequent basin development, and the ongoing interactions between the Avalon and Meguma terranes, and between Laurentia and Gondwana during the assembly of Pangaea. More generally, because the relationship between fabric development and motion along intracontinental strike-slip faults in continental zones is difficult to interpret, the sedimentology and structural geology in basins developed along these fault zones may preserve a less ambiguous record of the main tectonic events.