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At least 721 records · Page 40Linked to original sources
USGS1969

Age and chemistry of mesozoic and tertiary plutonic rocks in south-central Alaska

On the basis of potassium-argon mineral ages, plutonic rocks in an area of approximately 22,000 square miles in the southern Alaska Range and the Aleutian Range can be assigned to age groups that show differences in chemical characteristics and geographic distribution. The plutonic groups are Early and Middle Jurassic, Late Cretaceous and early Tertiary, and middle Tertiary in age. Most of the plutonic rocks in the Aleutian Range south of Iliamna Lake appear to be Jurassic, but north of Iliamna Lake, Jurassic plutonic rocks seem to be restricted to a belt on the southeast side of the Chigmit Mountains—Alaska Range. In the western or northwestern part of the Alaska Range north of Iliamna Lake, only Cretaceous and Tertiary plutonic rocks have been found. Rocks rich in K-feldspar are predominant in the Cretaceous and Tertiary plutons, but subordinate in the Jurassic plutons. Most of the mineralization in the region is associated with the Cretaceous and Tertiary plutons.

GSA Bulletin↗
USGS1962

Thorium and rare earths in the United States, exclusive of Alaska and Hawaii

The accompanying map shows the location of the principal deposits of thorium and rare-earth minerals in the United States (excluding Alaska and Hawaii). Symbols of different shapes are used to depict deposits of different geologic types, and sizes of symbols denote the relative importance of the deposits. Because of scale limitations a symbol may represent groups of deposits too closely spaced to permit them to be distinguished separately. Some districts of considerable extent arc shown by a shaded pattern.

Conterminous United States↗
USGS2002

Absolute timing of sulfide and gold mineralization: A comparison of Re-Os molybdenite and Ar-Ar mica methods from the Tintina Gold Belt, Alaska

New Re-Os molybdenite dates from two lode gold deposits of the Tintina Gold Belt, Alaska, provide direct timing constraints for sulfide and gold mineralization. At Fort Knox, the Re-Os molybdenite date is identical to the U-Pb zircon age for the host intrusion, supporting an intrusive-related origin for the deposit. However, 40Ar/39Ar dates from hydrothermal and igneous mica are considerably younger. At the Pogo deposit, Re-Os molybdenite dates are also much older than 40Ar/39Ar dates from hydrothermal mica, but dissimilar to the age of local granites. These age relationships indicate that the Re-Os molybdenite method records the timing of sulfide and gold mineralization, whereas much younger 40Ar/39Ar dates are affected by post-ore thermal events, slow cooling, and/or systemic analytical effects. The results of this study complement a growing body of evidence to indicate that the Re-Os chronometer in molybdenite can be an accurate and robust tool for establishing timing relations in ore systems.

Geology↗
USGS1984

BOWIEITE: A NEW RHODIUM-IRIDIUM-PLATINUM SULFIDE IN PLATINUM-ALLOY NUGGETS, GOODNEWS BAY, ALASKA.

Bowieite (Rh,Ir,Pt)//2S//3, a new mineral species, is found in three nuggets of platinum from Goodnews Bay, Alaska. In linearly polarized reflected light, and compared to the host, higher reflecting white platinum-iridium alloy, bowieite is pale gray to pale gray-brown; neither bireflectance nor reflectance pleochroism is apparent. With polars crossed, its anisotropic rotation tints vary from gray to dark brown. Luminance values (relative to the CIE illuminant C) for R//1 and R//2, computed from full spectral data for the most bireflectant grain, are 45. 8% and 48. 2% in air, and 30. 5% and 33. 0% in oil, respectively. VHN//1//0//0 1288 (858 to 1635). Bowieite is orthorhombic, space group Pnca, with a 8. 454(7) -8. 473(8), b 5. 995(1)-6. 002(7), c 6. 143(1)-6. 121(8) A, Z equals 4. Some grains that are 2. 6 to 3. 8 atomic % metal-deficient occur as an optically coherent rim on bowieite; the rim and the bowieite grain are not optically continuous.

Canadian Mineralogist↗
USGS2000

Environmental geochemical studies of selected mineral deposits in Wrangell-St. Elias National Park and Preserve, Alaska

Environmental geochemical investigations at Wrangell-St. Elias National Park and Preserve, Alaska, between 1994 and 1997 included studies of the Kennecott stratabound copper mines and mill area; historic mines and mill in the Bremner District, gold placer mines at Gold Hill; the undisturbed porphyry, Cu-Mo deposits at Orange Hill and Bond Creek, and the historic mines and mill at Nabesna, The study was in cooperation with the National Park Service and focused on sample media including surface water, bedload sediment, rock, mine waste, and mill tailings samples. Results demonstrate that bedrock geology and mineral deposit type must be considered when environmental geochemical effects of historic or active mine areas are evaluated.

Alaska↗