Metalliferous sediments over the Nazca plate constitute an enormous potential resource because of their widespread surficial distribution. The total value of contained metals is nearly $67.50 per metric ton based on current prices for iron, manganese, cobalt, copper, molybdenum, nickel, zinc, and silver. However, economic comparisons are unfavorable with alternative or existing sources of metals such as the manganese nodules and ores of the Cuajone and Cerro de Pasco mines. The principal obstacles are the relatively low concentrations for all metals and the high proportion of total value in the metallurgically complex association of iron and manganese. Other largely negative uncertainties notwithstanding, we conclude that these metalliferous sediments will remain an untapped resource into the foreseeable future.

The best evidence for a direct relation between plate convergence and ore genesis is the close spatial occurrence of many ore deposits and active or relict convergent plate boundaries. Speculations abound on the source of the metals and the role of metalliferous sediment in the formation of these mineral deposits, but both remain intriguing and enigmatic problems.

Abstract The Nazca Plate Project is a detailed and systematic geological, geochemical, and geophysical investigation of a distinct and tectonically active lithospheric plate of the southeast Pacific Ocean basin. Major tectonic boundaries of the Nazca plate are defined by the East Pacific Rise on the west, along which new oceanic crust of basalt is being generated; the Peru-Chile Trench on the east, where older oceanic crust is being consumed by subduction beneath the overriding South American plate; and the Galapagos spreading center and the Chile Rise on the north and south, respectively. The motion of this plate relative to the Pacific plate is estimated to be eastward at more than 16 cm/yr, as documented by stratigraphic, structural, paleontologic, magnetic, and seismic evidence. Normal sediments of the deep-sea floor cover basaltic basement rocks over much of the plate, but are locally dominated by a metal-rich component on and near the East Pacific Rise and within the Bauer Deep. These metalliferous sediments, which are composed of crystalline ferromanganese hydroxyoxides (goethite, psilomelane, etc) and Fe-montmorillonite, contain anomalously high concentrations of Fe, Mn, Cu, Ni, Zn, and other transition metals. Elemental abundances and ratios, extraordinarily high rates of sedimentation, lead-isotope ratios, and statistical analysis of the data suggest that Fe, Mn, Cu, and possibly other metals were deposited by hydrothermal fluids emanating from sources of basaltic magmatism along the East Pacific Rise. However, isotopic abundances of O, Sr, S, and C and distribution patterns of the rare-earth elements imply that deposition of the metalliferous sediments occurred at normally low temperatures of the ocean floor and that the nonmetallic components were derived from and/or equilibrated with seawater.

Volcanic and granitic rocks of the Jack River igneous field were erupted and emplaced in the suture zone between the accreted Wrangellia composite terrane and the former margin of southern Alaska. The volcanic rocks unconformably overlie Jurassic-Cretaceous shale and sandstone of the Kahiltna assemblage and include 100–300 m of basalt, basaltic andesite, and andesite lava flows overlain by a rhyolite unit that consists of over 900 m of lava flows and pyroclastic deposits. Seven basaltic and rhyolite lava samples yield 40 Ar/ 39 Ar ages ranging from 56.0 ± 0.3 to 49.5 ± 0.3 Ma. Two granitic samples yield 40 Ar/ 39 Ar ages of 54.6 ± 0.4 and 62.7 ± 0.4 Ma. These age dates indicate that the onset of Jack River magmatism at ca. 62.7 Ma coincided with the terminal phase of terrane accretion and continued after accretion to at least 49.5 Ma. The volcanic rocks range between tholeiitic and high-K calc-alkaline series and show a bimodal distribution with respect to silica (dacite is absent). The Jack River basalts are tholeiitic, have rare earth element and high field strength element ratios that are in the range between Pacific enriched mid-ocean-ridge basalts and Hawaiian ocean-island basalts (e.g., La/Yb = 5.0–8.4; Nb/Zr = 0.07–0.11), and have a within-plate geochemistry (e.g., Ti/V >50; high Zr/Y). All of the Jack River volcanic rocks exhibit some degree of enrichment in large ion lithophile and/or fluid mobile elements (e.g., Cs, Ba, Th, U, K, and Pb), although the basalts have low ratios between large ion lithophile and high field strength elements (e.g., Ba/Nb as low as 32.7 and Pb/Nb of 0.28–0.35). The granitic rocks (granites to granodiorites) are strongly depleted in the heavy rare earth elements, and most samples exhibit characteristics of adakites (e.g., Al 2 O 3 >15 weight %, Yb = 0.6–1.2 ppm, Y = 5.5–12.5 ppm, and Sr/Y = 20.4–66.2). The Jack River basalts were derived from partial melts of a mantle source that was more enriched than depleted mid-ocean-ridge basalt mantle and that ranged toward an enriched mantle (EM-I-type) composition.The basalts then evolved by assimilation and fractional crystallization to form intermediate magmas. Rhyolite magmas were formed later as anatectic melts of upper crustal argillaceous rocks (Kahiltna assemblage), resulting in the bimodal volcanism. The granitic adakite magmas may have formed by melting of garnet-bearing metamorphosed sedimentary rocks (meta-Kahiltna assemblage) that formed lower crustal rocks in the suture zone. Although the Jack River igneous rocks do exhibit some arc-like geochemical characteristics (e.g., elevated large ion lithophile elements), they differ from calc-alkaline arc rocks in that (1) they are a bimodal volcanic suite; (2) the rhyolites are not comagmatic with the basaltic and intermediate rocks; (3) the basalts and andesites have higher TiO 2 (>1.5 weight %) than is typical for arc basalts and andesites; (4) the basalts do not exhibit depletion of high field strength elements (e.g., Ta and Nb) with respect to large ion lithophile elements; (5) the basalts have an intraplate geochemical affinity; and (6) adakites are present. These characteristics show that the geochemistry of postcollisional suture zone magmatism can be transitional between calc-alkaline arc and intraplate magmatism. The Jack River volcanic field is deformed into a broad, northeast-trending syncline, which is crosscut by small-scale brittle faults that include northwest- and west-trending normal-slip and oblique-slip faults, and a southeast-dipping reverse fault that places Kahiltna assemblage rocks over the Jack River volcanic rocks. The pattern of Jack River deformation is consistent with right-lateral simple shear along the Denali fault system and indicates an episode of post-49.5 Ma strike-slip along the McKinley strand of the Denali fault. The Jack River rocks, therefore, record the magmatic response to terrane accretion and the kinematics of margin-parallel transport of an accreted terrane assemblage after it was sutured to the continental margin.

Abstract There is extensive stratigraphic accumulation of natural gas in the numerous lenticular standstone and conglomerate bodies of Early Pennsylvanian age in Jack, Parker, and Wise Counties, Texas. These sandstone and conglomerate lenses, locally known as “Bend Conglomerates,” are characterized by extreme variability in lateral extent. Conglomerate bodies present in one well commonly are not present in the offset well. Some wells contain as many as eight separate lenses with a combined pay thickness of more than 100 ft. The Boonsville (Bend Conglomerate) gas field and the Toto (lower Bend Conglomerate) gas field cover an area of approximately 450 sq mi in Jack, Parker, and Wise Counties and contain more than 875 gas wells producing from the Bend Conglomerate lenses. Monthly gas production from these wells is 5.75 billion cu ft, and estimated recoverable reserve is more than 1 trillion cu ft, making it the largest gas-producing area of North Texas.