The common impurities are silver and copper. Nuggets are also referred to by their fineness, for example "865 fine" means the nugget is 865 parts per thousand in gold by mass. Purity can be roughly assessed by the nugget color, the richer and deeper the orange-yellow the higher the gold content. Gold nuggets in Australia often are 23K or slightly higher, while Alaskan nuggets are usually at the lower end of the spectrum. Nuggets are usually 20.5K to 22K purity (83% to 92% by mass). 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 (i.e., they were of hypogene origin). They often show signs of abrasive polishing by stream action, and sometimes still contain inclusions of quartz or other lode matrix material. Nuggets are gold fragments weathered out of an original lode. Nuggets are also found in the tailings piles of previous mining operations, especially those left by gold mining dredges. Nuggets are recovered by placer mining, but they are also found in residual deposits where the gold-bearing veins or lodes are weathered. Watercourses often concentrate nuggets and finer gold in placers.
Most of the large nuggets were melted down into ingot form and so only have historical records of their size and mass.Ī gold nugget is a naturally occurring piece of native gold. Three of the biggest nuggets come from the Brazilian Serra Pelada Mine. The Welcome Stranger is the largest alluvial gold nugget found, which had a calculated refined weight of 97.14 kilograms (3,123 ozt). Historically, they are melted down and formed into new objects. Gold nuggets of various sizes have been found throughout the world. The scale bar across the bottom represents 12 inches (30 cm). A wood-engraving of the Welcome Stranger published in The Illustrated Australian News for Home Reader on 1 March 1869.
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