S, C, Sr, and Pb sources in the Pliocene Silver Creek porphyry, Mo system Rio, Colorado
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The Pliocene Silver Creek porphyry Mo deposit at Rico, southwest Colorado, is the youngest known porphyry Mo deposit in the Colorado mineral belt. Associated sulfate replacement deposits, epithermal Pb-Zn-Ag vein and replacement deposits, and Recent hot springs also occur. We investigated the S, C, Sr, and Pb sources at the various levels of this well-preserved hydrothermal system by isotopic studies of the mineralization, and of sedimentary and igneous rocks in the area.Hydrothermal sulfide delta 34 S values are tightly grouped at ca. 0 per mil (x = 0.3 + or - 2.0ppm, 1Sigma ; n = 107) and are substantially lower than those of the hydrothermal sulfates (x = 15.2 + or - 1.6ppm, 1Sigma ; n = 14). Pliocene hydrothermal carbonate delta 13 C values (-7.9 to +1.1; x = -4.9 + or - 2.7ppm, 1Sigma ; n = 16) are similar to those for the Pennsylvanian Hermosa Formation carbonate (delta 13 C = -6.7 to +2.6ppm; x = -3.0 + or - 3.2, 1Sigma ; n = 5), which hosts most of the base and precious metal mineralization, whereas Recent hot spring tufas have somewhat higher values (delta 13 C = 1.8-5.5ppm; x = 3.7 + or - 1.3ppm, 1Sigma ; n = 5). Pliocene hydrothermal carbonates have delta 18 O values between -5.1 and 12.3 per mil (x = 5.0 + or - 6.3ppm, 1Sigma ; n = 16). The tufas have higher delta 18 O values (x = 15.4 + or - 2.6ppm, 1Sigma ; n = 5).Hydrothermal carbonates (including tufa) and sulfate have initial 87 Sr/ 86 Sr (sub (i)) values between 0.70700 and 0.71168. Pennsylvanian sedimentary rocks in the Rico district had 87 Sr/ 86 Sr ratios between 0.70877 and 0.71694 at the time of mineralization. Fresh and altered Pliocene monzonite intrusions at Rico have initial 87 Sr/ 86 Sr (sub (4)) values of 0.70581 and 0.70563, respectively, whereas variably altered rholite dikes have initial 87 Sr/ 86 Sr (sub (4)) values of 0.70987, 0.71574, and 0.71716. The 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, and 206 Pb/ 204 Pb ratios of molybdenite, pyrite, and galena taken together are 18.27 to 19.65, 15.55 to 15.70, and 37.75 to 38.62. Pliocene lamprophyre and rhyolite dikes at Rico have equivalent Pb isotope ratios of 18.26, 15.48, and 37.81 and 18.79, 15.65, and 38.02, respectively. These Pb isotope data are typical of Tertiary volcanic rocks and associated sulfides in southwest Colorado.Our delta 34 S data indicate a magmatic S source for the porphyry Mo and epithermal base metal sulfide mineralization. Most of the sulfide Pb is also of magmatic origin. In contrast, hydrothermal sulfate replacement deposits peripheral to the porphyry Mo deposit, carbonate gangue from the base metal deposits, and the Recent hot spring tufa derived their respective S, C, and Sr from local sedimentary rocks. Our delta 18 O data indicate that the hydrothermal carbonates were deposited from 18 O-shifted meteoric waters. Our comprehensive isotopic data set presents a similar genetic picture to that of other molybdenite deposits in the mineral belt, in that the porphyry Mo mineralization was dominated by magmatic components, whereas the peripheral base metal mineralization contains both magmatic and upper crustal components. These important genetic similarities are independent of fundamental differences in tectonic setting.Accretionary wedge
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Fluvial fans represent one of the dominant sedimentary systems at the active margins of non-marine foreland basins. The Puig-reig anticline at the north-eastern margin of the Ebro Foreland Basin (SE Pyrenees, Spain) exposes continuous outcrops of Late Eocene-Early Oligocene fluvial deposits, from proximal to medial fluvial fan environments. The proximal deposits are found in the north limb of the anticline, especially in the northwest zone. These deposits are characterised by conglomerates with minor interbedded sandstones, with thick and wide sheet-like geometries with unscoured or variably scoured basal surfaces. These are interpreted to be the deposits of unconfined flash floods and wide-shallow channel streams. The medial deposits, covering the rest of the anticline, consist of interbedded conglomerates, sandstones and claystones. These are interpreted to have been deposited from braided to meandering channel streams and overbank areas. Distal deposits are found towards the south, beyond the anticline, and are characterised by sandstone and clay deposits of terminal lobes and lacustrine deltas. This study assesses the impact of the primary depositional characteristics, diagenesis and deformation of the most heterolithic portion of the system, with implications for increasing our understanding of folded fluvial reservoirs. Diagenetic processes, mainly mechanical compaction and calcite cementation, resulted in overall low intergranular porosity, with limited relatively high porosity developed in sandstone lithofacies in the medial deposits. Deformation associated with thrusting and fold growth resulted in the formation of abundant fractures, with relatively high fracture intensities observed in sandstone lithofacies in the anticline crest. This study shows that post-depositional processes can both improve and diminish the reservoir potential of basin proximal fluvial deposits, through the development of fracture networks and by compaction-cementation. The comparison of the Puig-reig anticline with other similar settings worldwide indicates that foreland basin margin locations may be potential areas for effective reservoirs, even in the case of low intergranular porosity.
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Pennsylvanian foreland deformation associated with the Ouachita orogene reactivated a west-northwest-east-southeast Cambrian basement trend, the southern Oklahoma aulacogen, to form the Wichita uplift, southwest Oklahoma. The 30-km-wide subsurface Frontal fault zone separates the uplift from the Anadarko basin to the north. Horizontal shortening across this fault zone is estimated at 7-15 km (20-40%), vertical displacement totals 9-10 km from the uplift to the basin. Folds are mapped on an interformational scale within the Frontal fault zone, and on an intraformational scale (Cambro-Ordovician Arbuckle Group) in the Slick Hills, southwest Oklahoma. Additional shortening occurred along southwest dipping mountain flank thrusts and on bedding plane thrusts, respectively. Hanging wall blocks of major faults contain the shallow dipping limb and anticlinal hinge zone of the interformational scale folds. Oil and gas production is generally restricted to these anticlinal crests within Paleozoic rocks. Deep wells (> 6000 m) that have penetrated footwall imbricates of the mountain flank thrusts have drilled through steep-overturned beds and tight recumbent folds before passing through faults into a normal stratigraphic sequence. Basement thrust loading of the southern margin of the Anadarko basin controlled the trend (west-northwest-east-southeast) of the axis of maximum deposition within the basin during the Pennsylvanian.
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Sediments of Ouachita facies extend subsurface from the Ouachita Mountains of Oklahoma and Arkansas to the Marathon and Solitario uplifts of extreme southwest Texas. Although these sediments have been subjected to strong dynamic metamorphism, the metamorphic grade attained is uniformly low. Sediments of Ouachita facies are essentially unaltered in some areas; elsewhere they range in degree of metamorphism as high as the biotite zone. Mineralogical change and cataclastic effects are key criteria for discerning the stages of progressive metamorphism in sediments of Ouachita facies. Metamorphic changes are most easily observed in shales and argillaceous sandstones--limestones and cherts react less conspicuously to metamorphic stress. Mineralogical change resulting from metamorphism reduces the porosity and non-fracture permeability of sandstones to such an extent that they are not good petroleum reservoirs. The petrography of the so-called schists of Luling field, Caldwell County, Texas, and of some other deep wells along the Luling-Mexia-Talco fault system is discussed briefly. It is shown that they are generally comparable in mineralogy and degree of metamorphism with sediments exposed in the Ouachita Mountains of McCurtain County, Oklahoma. End_of_Article - Last_Page 2626------------
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