File:Gold-quartz-sulfide hydrothermal vein (O'Dea Vein or Irishman Vein, Late Cretaceous; Grant Mine, Fairbanks Mining District, Alaska, USA) 1 (16945681997).jpg

Gold-quartz-sulfide hydrothermal vein from the Cretaceous of Alaska, USA. (cut & polished slab; public display, Leadville Mining Museum, Leadville, Colorado, USA)

This is hydrothermal vein rock from the Grant Mine, northwest of the city of Fairbanks, Alaska. The mine produced gold, silver, antimony, lead, and tungsten. Two hydrothermal veins were the principal targets of mining at this locality - the O’Dea Vein and the Irishman Vein. This specimen may be from the O’Dea Vein, which varies in width from 6 inches to over 6 feet. Both veins are developed in fault zones in Fairbanks Schist host rocks of probable Proterozoic age. The rock is dominated by whitish quartz (this has been described as a “quartz breccia”), which has been mostly stained pale yellowish to reddish-brown by iron oxide. Native gold occurs throughout the rock, plus a gray to dark gray sulfide mineral. Minerals reported from the O’Dea Vein include quartz (SiO2 - silicon dioxide), gold (Au), arsenopyrite (FeAsS - iron arsenic sulfide), galena (PbS - lead sulfide), pyrite (FeS2 - iron sulfide), stibnite (Sb2S3 - antimony sulfide), muscovite mica (KAl2(AlSi3O10)(OH)2 - potassium hydroxy-aluminosilicate), goethite (FeO(OH) - iron hydroxy-oxide), and scheelite (CaWO4 - calcium tungstate). The O’Dea Vein also includes fragments of Fairbanks Schist host rock and fault mylonite. Isotopic dates on vein-fault minerals from the nearby Ryan Lode Mine, which is probably developed on the same fault zone as the Grant Mine, are 88 to 89 Ma (Late Cretaceous). Dates from other mines in the district are also Late Cretaceous (for example, 92 Ma at the Tolovana Mine and 103 Ma at the Hi-Yu Mine).

Locality: Grant Mine (SE1/4 section 28, T1N, R2W, Fairbanks D-2 topographic quadrangle), ~0.75 air-miles south of the Ester Dome Road-St. Patrick Road intersection, eastern side of Ester Dome, southwestern Fairbanks Mining District, Tintina Gold Province, Fairbanks North Star Borough, northwest of the city of Fairbanks, central Alaska, USA

Grant Mine geology mostly synthesized from:

Freeman & Schaefer (2001) - Alaska resource data file, Fairbanks quadrangle. United States Geological Survey Open-File Report 2001-426. 355 pp.

Goldfarb et al. (2007) - Geology and origin of epigenetic lode gold deposits, Tintina Gold Province, Alaska and Yukon. United States Geological Survey Scientific Investigations Report 2007-5289-A. 18 pp.