A gold nugget is a naturally occurring piece of native gold. Watercourses often concentrate nuggets and finer gold in placers. Nuggets are recovered by placer mining, but they are also found in residual deposits where the gold-bearing veins or lodes are weathered. Nuggets are also found in the tailings piles of previous mining operations, especially those left by gold mining dredges.
Formation
Nuggets are gold fragments weathered out of an original lode. They often show signs of abrasive polishing by stream action, and sometimes still contain inclusions of quartz or other lode matrix material. A 2007 study on Australian nuggets ruled out speculative theories of supergene formation via in-situ precipitation, cold welding of smaller particles, or bacterial concentration, since crystal structures of all of the nuggets examined proved they were originally formed at high temperature deep underground. Other precious metals such as platinum form nuggets in the same way. A later study of native gold from Arizona, US, based on lead isotopes indicates that a significant part of the mass in alluvial gold nuggets in this area formed within the placer environment.
Composition
Nuggets are usually 8 K to 22K purity. Gold nuggets in Australia often are 23K or slightly higher, while Alaskan nuggets are usually at the lower end of the spectrum. Purity can be roughly assessed by the nugget color, the richer and deeper the orange-yellow the higher the gold content. Nuggets are also referred to by their fineness, for example "865 fine" means the nugget is 865 parts per thousand in gold by mass. The common impurities are silver and copper. Nuggets high in silver content constitute the alloy electrum. The chemical composition of supergene gold nuggets can reveal the characteristics of the primary ore.
Largest nuggets
Two gold nuggets are claimed as the largest in the world: the Welcome Stranger and the Canaã nugget, the latter being the largest surviving natural nugget. Considered by most authorities to be the biggest gold nugget ever found, the Welcome Stranger was found at Moliagul, Victoria, Australia in 1869 by John Deason and Richard Oates. It weighed gross, over and returned over net. The Welcome Stranger is sometimes confused with the similarly named Welcome Nugget, which was found in June 1858 at Bakery Hill, Ballarat, Australia by the Red Hill Mining Company. The Welcome weighed. It was melted down in London in November 1859. The Canaã nugget, also known as the Pepita Canaa, was found on September 13, 1983 by miners at the Serra Pelada Mine in the State of Para, Brazil. Weighing gross, and containing of gold, it is among the largest gold nuggets ever found, and is, today, the largest in existence. The main controversy regarding this nugget is that the excavation reports suggest that the existing nugget was originally part of a nugget weighing that broke during excavations. The Canaã nugget is displayed at the Banco Central Museum in Brazil along with the second and third largest nuggets remaining in existence, weighing respectively and, which were also found at the Serra Pelada region. The largest gold nugget found using a metal detector is the Hand of Faith, weighing, found in Kingower, Victoria, Australia in 1980. Historic large specimens include the crystalline "Fricot Nugget", weighing – the largest one found during the California Gold Rush. It is on display at the California State Mining and Mineral Museum. The largest gold nugget ever found in California weighed. It was found in August 1869 in Sierra Buttes by five partners – W.A. Farish, A. Wood, J. Winstead, F.N.L. Clevering and Harry Warner. The Victoria, Australia gold rush of the early 1850s produced a number of large nuggets. They include the Welcome Nugget which weighed which is considered to be the second largest gold nugget ever found. Another find, the Lady Hotham, which weighed, was found by a group of nine miners on September 8, 1854 in Canadian Gully, Ballarat at a depth of 135 feet. The Lady Hotham was named after the wife of the Governor, Sir Charles Hotham who happened to be visiting the area when the nugget was found. Eighteen months earlier, in January and early February 1853, three other large nuggets weighing,, and were also found in Canadian Gully at a depth of. Another nugget, the Heron, was found in 1855 in Golden Gully in the Mount Alexander goldfield. It weighed and was found by a group of inexperienced miners who had received a supposedly empty claim. The miners found the nugget on their second day of digging; the nugget was named after one of the gold commissioners, a Mr. Heron. On 16 January 2013, a large gold nugget was found near the city of Ballarat in Victoria, Australia by an amateur gold prospector. The Y-shaped nugget weighed slightly more than, measured around 22 cm high by 15 cm wide, and has a market value slightly below 300,000 Australian dollars, though opinions have been expressed that it could be sold for much more due to its rarity. The discovery has cast doubt on the common rumour that Victoria's goldfields were exhausted in the 19th century.
