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THE HISTORY OF GOLD
Gold USGS Part3
Article # : gold141
Continued From: Gold USGS Part2
The term "gold-filled" is used to describe articles of jewelry made of base metal which are covered on one or more surfaces with a layer of gold alloy. A quality mark may be used to show the quantity and fineness of the gold alloy. In the United States no article having a gold alloy coating of less than 10-karat fineness may have any quality mark affixed. Lower limits are permitted in some countries.
No article having a gold alloy portion of less than one-twentieth by weight may be marked "gold-filled," but articles may be marked "rolled gold plate" provided the proportional fraction and fineness designations are also shown. Electroplated jewelry items carrying at least 7 millionths of an inch (0.18 micrometers) of gold on significant surfaces may be labeled "electroplate." Plated thicknesses less than this may be marked "gold flashed" or "gold washed."
Gold is relatively scarce in the earth, but it occurs in many different kinds of rocks and in many different geological environments. Though scarce, gold is concentrated by geologic processes to form commercial deposits of two principal types: lode (primary) deposits and placer (secondary) deposits.
Lode deposits are the targets for the "hardrock" prospector seeking gold at the site of its deposition from mineralizing solutions. Geologists have proposed various hypotheses to explain the source of solutions from which mineral constituents are precipitated in lode deposits.
One widely accepted hypothesis proposes that many gold deposits, especially those found in volcanic and sedimentary rocks, formed from circulating ground waters driven by heat from bodies of magma (molten rock) intruded into the Earth's crust within about 2 to 5 miles of the surface. Active geothermal systems, which are exploited in parts of the United States for natural hot water and steam, provide a modern analog for these gold-depositing systems. Most of the water in geothermal systems originates as rainfall, which moves downward through fractures and permeable beds in cooler parts of the crust and is drawn laterally into areas heated by magma, where it is driven upward through fractures. As the water is heated, it dissolves metals from the surrounding rocks. When the heated waters reach cooler rocks at shallower depths, metallic minerals precipitate to form veins or blanket-like ore bodies.
Another hypothesis suggests that gold-bearing solutions may be expelled from magma as it cools, precipitating ore materials as they move into cooler surrounding rocks. This hypothesis is applied particularly to gold deposits located in or near masses of granitic rock, which represent solidified magma.
A third hypothesis is applied mainly to gold-bearing veins in metamorphic rocks that occur in mountain belts at continental margins. In the mountain-building process, sedimentary and volcanic rocks may be deeply buried or thrust under the edge of the continent, where they are subjected to high temperatures and pressures resulting in chemical reactions that change the rocks to new mineral assemblages (metamorphism). This hypothesis suggests that water is expelled from the rocks and migrates upwards, precipitating ore materials as pressures and temperatures decrease. The ore metals are thought to originate from the rocks undergoing active metamorphism.
The primary concerns of the prospector or miner interested in a lode deposit of gold are to determine the average gold content (tenor) per ton of mineralized rock and the size of the deposit. From these data, estimates can be made of the deposit's value. One of the most commonly used methods for determining the gold and silver content of mineralized rocks is the fire assay. The results are reported as troy ounces of gold or silver or both per short avoirdupois ton of ore or as grams per metric ton of ore.
Source: USGS.gov