Gold (Farncomb Hill, near Breckenridge, Summit County, Colorado, USA) 4 : 無料・フリー素材/写真
Gold (Farncomb Hill, near Breckenridge, Summit County, Colorado, USA) 4 / James St. John
| ライセンス | クリエイティブ・コモンズ 表示 2.1 |
|---|---|
| 説明 | Gold from Colorado, USA. (public display, Denver Museum of Nature & Science, Denver, Colorado)A mineral is a naturally-occurring, solid, inorganic, crystalline substance 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. 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 specimen shown above is "leaf gold" from the famous Farncomb Hill locality in Colorado. The bedrock at Farncomb Hill consists of tilted Pierre Shale (Upper Cretaceous) with early Tertiary intrusions of pophryritic quartz monzonite that form sills. Adjacent to the intrusions, the shales have been contact metamorphosed, brecciated, silicified, and mineralized with small amounts of finely disseminated (invisible) gold. Hydrothermal veins cut across both altered Pierre Shale and the quartz monzonite sills. In proximity to the veins, the shales have finely disseminated pyrite and chalcopyrite. Low-angle normal faults have offset the hydrothermal veins in places - observed offsets are usually less than 10 feet, but can reach up to 35 feet. These faults occur within the Pierre Shale & also occur at and parallel to shale-sill contacts.Native gold was mined from Farncomb Hill’s hydrothermal veins, which are usually about 0.5 inches wide. Unaltered, non-oxidized veins consist of calcite that hosts metallic minerals such as pyrite (FeS2 - iron sulfide), chalcopyrite (CuFeS2 - copper iron sulfide), sphalerite (ZnS - zinc sulfide), galena (PbS - lead sulfide), and native gold (Au). Most of the veins are partially to entirely oxidized, which has resulted in the formation of limonite (FeO·OH·nH2O - hydrous iron hydroxy-oxide), in which native gold occurs. Significant gold-bearing veins all occur within the Pierre Shale immediately above (adjacent to) quartz monzonite sills or above (adjacent to) within-shale fault zones. The famous “gold pockets” of Farncomb Hill are very rich, native gold-bearing hydrothermal veins that are two to three feet in diameter and up to one inch wide. In these pockets, gold forms abundant criss-crossing veinlets in orangish-brown limonite. Criss-crossing veins in matrix are often referred to as “stockwork” or “boxwork”. Leaf gold specimens like that shown above are produced by chemical removal of the limonite matrix.The occurrence of gold pockets directly above fault zones at Farncomb Hill suggests that the pockets contain secondary gold, and that gravity has played a role in their formation. Downward-percolating fluids have apparently leached the microscopic disseminated gold from the Pierre Shale and concentrated it just above faults, where the veins are offset. A gold deposit having this origin is said to have been formed by supergene enrichment. The native gold itself occurs as wires, leaves, and plates. Small, scale-like gold crystals often cover portions of leaf gold and plate gold.Locality: Farncomb Hill, ~4 miles east of the town of Breckenridge, Breckenridge Mining District, Summit County, Colorado, USA-----------------Photo gallery of gold:www.mindat.org/gallery.php?min=1720 |
| 撮影日 | 2012-10-25 12:34:01 |
| 撮影者 | James St. John |
| タグ | |
| 撮影地 |