Geologic mapping, age determinations, and geochemistry of rocks exposed in the Abiquiu area of the Abiquiu embayment of the Rio Grande rift, north-central New Mexico, provide data to determine fault-slip and incision rates. Vertical-slip rates for faults in the area range from 16 m/m.y. to 42 m/m.y., and generally appear to decrease from the eastern edge of the Colorado Plateau to the Abiquiu embayment. Incision rates calculated for the period ca. 10 to ca. 3 Ma indicate rapid incision with rates that range from 139 m/m.y. on the eastern edge of the Colorado Plateau to 41 m/m.y. on the western part of the Abiquiu embayment. The Abiquiu area is located along the margin of the Colorado Plateau–Rio Grande rift and lies within the Abiquiu embayment, a shallow, early extensional basin of the Rio Grande rift. Cenozoic rocks include the Eocene El Rito Formation, Oligocene Ritito Conglomerate, Oligocene–Miocene Abiquiu Formation, and Miocene Chama–El Rito and Ojo Caliente Sandstone Members of the Tesuque Formation (Santa Fe Group). Volcanic rocks include the Lobato Basalt (Miocene; ca. 15–8 Ma), El Alto Basalt (Pliocene; ca. 3 Ma), and dacite of the Tschicoma Formation (Pliocene; ca. 2 Ma). Quaternary deposits consist of inset axial and side-stream deposits of the ancestral Rio Chama (Pleistocene in age), landslide and pediment alluvium and colluvium, and Holocene main and side-stream channel and floodplain deposits of the modern Rio Chama. The predominant faults are Tertiary normal high-angle faults that displace rocks basinward. A low-angle fault, referred to as the Abiquiu fault, locally separates an upper plate composed of the transitional zone of the Ojo Caliente Sandstone and Chama–El Rito Members from a lower plate consisting of the Abiquiu Formation or the Ritito Conglomerate. The upper plate is distended into blocks that range from about 0.1 km to 3.5 km long that may represent a larger sheet that has been broken up and partly eroded. Geochronology ( 40 Ar/ 39 Ar) from fifteen volcanic and intrusive rocks resolves discrete volcanic episodes in the Abiquiu area: (1) emplacement of Early and Late Miocene basaltic dikes at 20 Ma and ca. 10 Ma; (2) extensive Late Miocene–age lava flows at 9.5 Ma, 7.9 Ma, and 5.6 Ma; and (3) extensive basaltic eruptions during the early Pliocene at 2.9 Ma and 2.4 Ma. Clasts of biotite- and hornblende-rich trachyandesites and trachydacites from the base of the Abiquiu Formation are dated at ca. 27 Ma, possibly derived from the Latir volcanic field. The most-mafic magmas are interpreted to be generated from a similar lithospheric mantle during rifting, but variations in composition are correlated with partial melting at different depths, which is correlated with thinning of the crust due to extensional processes.

Abstract The Carajás iron ore deposits located in the southern part of the state of Pará in Brazil were discovered in 1967 and have produced about 70 million metric tons (Mt) of iron ore annually. The deposits are hosted by the Neoarchean metavolcano-sedimentary sequence of the Grão Pará Group, Itacaiúnas Supergroup. The protoliths to high-grade iron ore in the Serra Norte deposits are jaspilites, which are under- and overlain by basalts. The major Serra Norte N1, N4E, N4W, N5E, and N5S iron ore deposits of the Carajás mineral province are distributed along, and structurally controlled by, the northern flank of the Carajás fold. High-grade iron mineralization (>65% Fe) is made up of hard and soft ores. The hard ores can be banded, massive and/or brecciated, and are characterized by hematite-martite and hematite types. The soft ores are very porous, discontinuous and tabular, friable and banded. The basal contact of high-grade iron ore is defined by a hydrothermally altered basaltic rock mainly composed of chlorite and microplaty hematite. Varying degrees of hydrothermal alteration have affected jaspilites to form iron ores. The study of variably altered jaspilites and hard ores indicates that the distal alteration zone represents an early alteration stage. It is mainly characterized by the recrystallization of jasper and the removal of its iron, and the formation of magnetite (commonly martitized), overgrowing original microcrystalline hematite and associated with quartz and calcite veins. Two vein breccia types characterize the distal alteration zone: V1a (quartz ± sulfide breccias) and V1b carbonate ± sulfide breccia veins). Sulfides are pyrite and chalcopyrite. The intermediate alteration zone, synchronous with the main iron ore-forming event, is characterized by (1) progressive leaching of chert and quartz, leaving oxides and vugs; (2) presence of martite as the dominant oxide along altered jaspilite layers; and (3) partial filling of open spaces with microplaty and/or platy hematite. The intermediate alteration zone also contains the V2a (quartz ± hematite bedding-discordant veins), V2b (vug-textured quartz + hematite discordant vertical veins), and V3 (hematite ± quartz veins crosscutting and/or parallel to the jaspilite bedding). The proximal alteration zone, also synchronous with the iron ore-forming event, represents an advanced alteration stage (i.e., the high-grade iron ore) and is characterized by progressive martitization, forming anhedral hematite, continued space filling by comb-textured euhedral and tabular hematite in veinlets and along banding. The proximal alteration zone contains intense carbonate alteration associated with the high-grade ores, resulting in the formation of ore breccias cemented by dolomite. Vein breccias are classified as V4 (carbonate (iron cloud)-quartz breccia), and V5 (quartz ± microplaty hematite breccia), both located in high-grade ore. The distribution of the rare earth elements in variably altered jaspilites and hard ores follows two main distinctive patterns. Jaspilites from the N4W, N5E, and N5S deposits, and hard ores from N1 and N4E have a low ΣREE content, are enriched in light REE, and exhibit positive europium anomalies (Eu/Eu* >1), which is typical of Archean banded iron formations. The REE pattern defined by N5E ores is nearly flat and displays an increase in ΣREE and absence of the positive Eu anomaly. The increase in LREE was accentuated during the formation of magnetite and microplaty hematite and the advance of martitization to form anhedral hematite. This may have favored the relative increase of HREE in the residual fluid, resulting in an increase in HREE in advanced-stage precipitates and almost flat REE patterns associated with the advanced stage of mineralization. It is during this hydrothermal stage that euhedral and tabular hematite are dominant. The REE increase in N4E and N5E ore samples further suggests the presence of significant amounts of Fe in the mineralizing fluid. The first evidence for hydrothermal fluids that infiltrated the jaspilites is the vein breccia type 1, which contains Ca-Fe-rich, high-salinity (up to 29.3 wt % CaCl 2 equiv) fluid inclusions in quartz and carbonate with T tapping of 209° to 285°C. The next stage of hydrothermal fluid infiltration is characterized by vein type 2, which contains medium-to high-salinity Na-Fe-Mg-rich (13.6–21.2 wt % CaCl 2 equiv) and Ca-rich fluid inclusions (6.8–18.4 wt % CaCl 2 equiv) with T trapping of 225° to 275°C and 190° to 295°C, respectively. Vein type V3 is characterized by low- and medium-salinity Ca-(Mg)-Fe-Na-rich inclusions (1.2–19.2 wt % CaCl 2 equiv) with T trapping of 195° to 255°C and medium-salinity Na-Mg-rich fluid inclusions (8.9–14.4 wt % CaCl 2 equiv) with T trapping of 240° to 277°C. Brecciated vein types V4 and V5 have Ca-rich, medium- to high-salinity fluid inclusions in quartz and high-salinity inclusions in carbonate (9.7–24.5 and 19. 2–30.1 wt % CaCl 2 equiv, respectively), both trapped at 237° to 314°C, and low-salinity Na-K-Mg fluid inclusions (0.2–7.3 wt % NaCl equiv) trapped at 245° to 316°C. Oxygen isotope analyses on quartz from V1 to V3 veins range from +10 to +18 per mil, respectively, and –1.0 per mil on martite to –10.0 per mil for the paragenetically latest euhedral-tabular hematite in the high-grade ores. This shift in δ 18 O values of oxides may reflect influx of meteoric water during the advanced hydrothermal alteration stage and/or represents a result of intense fluid fluxes (i.e., high fluid/rock ratios). Sulfur isotope analyses of pyrite within distal V1 veins display a range of δ 34 S from +2.5 to +10.8 per mil, with lighter δ 34 S values (-5 to +5‰) in sulfides from the intermediate alteration zone of the wall-rock basalts; these latter values are compatible with juvenile magmatic fluids. Carbon and oxygen isotopes on carbonates (i.e., calcite, kutnahorite, and dolomite) from V1 and V4 vein types revealed a restricted range of δ 13 C from –6.0 to –2.0 per mil and a wider range of δ 18 O from +8.0 to +20.0 per mil, suggesting variable oxygen sources due to interaction with more than one fluid type, or significant changes in fluid-rock ratios during interaction with a heavy δ 18 O fluid, possibly magmatic. Strontium isotope ( 87 Sr/ 86 Sr) ratios of calcite ± kutnahorite (V1 vein type) in equilibrium with magnetite, and kutnahorite-dolomite (V4 vein type) range from 0.712 to 0.750. The extremely radiogenic 87 Sr/ 86 Sr values from V1 vein-type carbonates are probably only compatible with a granitic source. The mineralogical, geochemical, and isotopic changes from jaspilites to high-grade iron ores suggests a hydrothermal origin for hard ore via interaction with an early-stage medium- to high-salinity Ca- and Ca-Fe-rich, relatively reduced magmatic fluid, which leached silica and formed magnetite. This fluid evolved to more oxidizing conditions, with the advance of martitization, increase in the REE concentration, and microplaty hematite precipitation in veins and martite borders. Low δ 18 O values of oxides suggest mixing with meteoric water during this intermediate hydrothermal alteration stage. The predominance of oxidized phases such as anhedral and euhedral and/or tabular hematites, low-salinity Na-rich fluid inclusions, and decreasing oxygen isotope values toward late hematite types, indicate that the advanced alteration stage is dominated by the meteoric fluids. The proposed magmatic-meteoric hydrothermal mineralization model for the Carajás hard ores is substantially different from models for the Hamersley or Iron Quadrangle iron ores but may have a genetic link to the numerous Proterozoic magmatic hydrothermal deposits in the Carajás mineral province. The new hydrothermal model has also significant implications for iron ore exploration under cover sequences and/or the exploration for deep extensions of existing shallow orebodies. New exploration parameters include the distinct structural control of ore zones by faults and folds, widespread hydrothermal alteration zones, and related pathfinder minerals and chemical pathfinder elements such as REE, Ca, Na, Fe, and S.