Gold-quartz-sulfide hydrothermal vein at the old Grant Mine, Fairbanks Mining District
English: Gold-quartz-sulfide hydrothermal vein from the Cretaceous of Alaska, USA. (cut & polished slab; public display, Leadville Mining Museum, Leadville, Colorado, USA) A mineral is a naturally-occurring, solid, inorganic, crystalline substrance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 4900 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates. Elements are fundamental substances of matter - matter that is composed of the same types of atoms. At present, 118 elements are known (four of them are still unnamed). Of these, 98 occur naturally on Earth (hydrogen to californium). Most of these occur in rocks & minerals, although some occur in very small, trace amounts. Only some elements occur in their native elemental state as minerals. To find a native element in nature, it must be relatively non-reactive and there must be some concentration process. Metallic, semimetallic (metalloid), and nonmetallic elements are known in their native state as minerals. Gold (Au) is the most prestigious metal known, but it's not the most valuable. Gold is the only metal that has a deep, rich, metallic yellow color. Almost all other metals are silvery-colored. Gold is very rare in crustal rocks - it averages about 5 ppb (parts per billion). Where gold has been concentrated, it occurs as wires, dendritic crystals, twisted sheets, octahedral crystals, and variably-shaped nuggets. It most commonly occurs in hydrothermal quartz veins, disseminated in some contact- & hydrothermal-metamorphic rocks, and in placer deposits. Placers are concentrations of heavy minerals in stream gravels or in cracks on bedrock-floored streams. Gold has a high specific gravity (about 19), so it easily accumulates in placer deposits. Its high density allows prospectors to readily collect placer gold by panning. In addition to its high density, gold has a high melting point (over 1000º C). Gold is also relatively soft - about 2.5 to 3 on the Mohs Hardness Scale. The use of pure gold or high-purity gold in jewelry is not desirable as it easily gets scratched. The addition of other metals to gold to increase the hardness also alters the unique color of gold. Gold jewelry made & sold in America doesn’t have the gorgeous rich color of high-purity gold. The sample shown above 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. Swainbank & Clautice (1998) - Alaska’s mineral industry 1997: a summary. Alaska Division of Geological & Geophysical Surveys Information Circular 43. 12 pp.