3 results on '"Twelker, Evan"'
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2. New Data and Insight on the Late Triassic Wrangellia Magmatic Ni-Cu-Co-PGE System in the Talkeetna Mountains, Central Alaska.
- Author
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Twelker, Evan, Wypych, Alicja, and Newberry, Rainer J.
- Subjects
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MINERALIZATION , *METALLOGENIC provinces , *IGNEOUS intrusions , *MAGMAS , *PLATINUM group , *GEOPHYSICAL surveys - Abstract
Ni-Cu-Co-PGE mineralization occurs as magmatic sulfides in Late Triassic mafic-ultramafic intrusions of the Wrangellia terrane in Alaska and the Yukon. Parts of this metallogenic system have seen detailed study and exploration, notably at the Quill Creek Complex and Wellgreen deposit in the Yukon Territory, and at several prospects in the Eastern Alaska Range, including the Alpha Complex. Recent mapping in the Talkeetna Mountains showed that these mafic-ultramafic intrusions and their extrusive equivalent, the Nikolai Greenstone Large Igneous Province (LIP), continue at least 60 km to the southwest of the previously recognized extent. This opens up geologic potential for magmatic Ni-Cu-Co-PGE deposits to occur in the Talkeetna Mountains of central Alaska. As part of its ongoing Airborne Geophysical/Geological Mineral Inventory and Strategic and Critical Minerals Assessment programs, the Alaska Division of Geological and Geophysical Surveys (DGGS) initiated a multi-year project to improve the understanding of mineral potential in western Wrangellia. The project began in 2013 with acquisition of magnetic and electromagnetic (EM) geophysical data over unsurveyed portions of the terrane in the Clearwater and Talkeetna Mountains. DGGS geologists completed a three week field program of geologic mapping, rock, and stream sediment sampling in 2013, and followed up with 1:50,000-scale mapping of the Talkeetna Mountains C-4 Quadrangle during the 2014 field season. Gabbroic and ultramafic rocks of probable Late Triassic age were sampled across the project area. Previous work divides the Nikolai Greenstone into a stratigraphically lower, depleted mantle-derived, low TiO2 group and an upper, enriched mantle-derived, high TiO2 group. Both intrusive and extrusive members of this earlier petrogenetic suite occur throughout western Wrangellia. DGGS investigations indicate that PGE enrichment is associated only with the earlier, low TiO2 suite. Outside of the Mount Hayes Quadrangle, the most prominent low-TiO2 type mafic-ultramafic complex is the 16 km long, up to 700m-thick sill-form body in the northeastern Talkeetna Mountains south of Butte Creek. Mapping and analytical work indicate that this is a multiphase intrusive complex dominated by gabbro to olivine gabbro, with texture varying from fine-grained to pegmatitic. The thickest, most mafic section shows large-scale gradational variation in mineralogy from olivine-rich troctolite at the center to olivine gabbro above and below, apparent differentiation which is similar to that seen at the Quill Creek Complex. Microprobe results show that olivine grains near the base of this intrusive complex have Ni concentrations around 2900 ppm, whereas olivine in the center of the complex average 1100 ppm Ni. This apparent Ni depletion is comparable to that observed at Wellgreen. Sulfide grains, including intergrown pyrrhotite and pentlandite, were found to occur at grain interstices and as rare inclusions in Ni-depleted olivine. Analyses from the northeastern half of the study area yield values of PGE per 100 percent sulfide that are on par with values for the Wellgreen deposit. These data, along with the olivine Ni depletion, imply an early separation of a sulfide melt and a high degree of sulfide-silicate melt interaction, indicating that this part of Wrangellia has completed important prerequisites for the formation of Ni-Cu-Co-PGE deposits. [ABSTRACT FROM AUTHOR]
- Published
- 2014
3. 40AR/39AR DATA FROM THE TALKEETNA MOUNTAINS C-4 QUADRANGLE AND ADJOINING AREAS, CENTRAL ALASKA.
- Author
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Benowitz, Jeff, Layer, Paul, Wypych, Alicja, and Twelker, Evan
- Subjects
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ARGON isotopes , *GEOLOGICAL time scales , *MAGMATISM , *MINERALIZATION , *SILLS (Geology) , *EOCENE stratigraphic geology - Abstract
This report presents 40AR/39AR step-heating geochronology results for igneous and metamorphic rocks from the Alaska Division of Geological & Geophysical Surveys' (DGGS) Talkeetna Mountains C-4 Quadrangle and Adjoining Areas geologic mapping project. The U.S. Geological Survey (USGS) National Cooperative Geologic Mapping Program partially funded work by DGGS to map approximately 450 mi2 of the Talkeetna Mountains region of central Alaska at 1:50,000 scale. This area contains significant exposures of Late Triassic mafic volcanics and gabbro sills that have been the focus of region-wide exploration for the Strategic and Critical platinum-group elements (PGEs). The area also exposes numerous inactive and possibly active faults that project through the area of the proposed Susitna-Watana hydroelectric dam and reservoir. Because this area was poorly mapped at a reconnaissance scale of 1:250,000, a team of eight DGGS geologists spent 42 days in the field to produce a new geologic map and a greatly improved understanding of the geology, structural history, and mineral potential of the area. Hornblende-bearing gabbro returned a Late Triassic crystallization age consistent with regional results for Nikolai Greenstone-related magmatism. We determined Early Jurassic metamorphic ages for hornblende and sericite from two samples of upper greenschist to amphibolite grade metavolcanic rocks. Our results indicate that a granodiorite pluton intruding Wrangellia in the study area has a Middle Jurassic age consistent with the Talkeetna Arc, indicating that the Wrangellia and Peninsular terranes were joined at that time. Cretaceous plutons and porphyry intrusions in the map area can be divided into two types: reduced granitic to felsic porphyry intrusions with latest Cretaceous ages, and an earlier, oxidized type with early Late Cretaceous ages. This observation corresponds to similar patterns in magmatism in the western Alaska Range, and may indicate geological potential for similar style intrusive-related gold, copper, and molybdenum mineralization in the study area. Finally, we obtained Eocene ages for andesite and rhyolite; these ages constrain the timing of displacement on some of the faults in the map area. [ABSTRACT FROM AUTHOR]
- Published
- 2015
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