Gold Placers -Colors, Forms, Size
The external form color, luster and other features of placer gold visible to the naked eye are commonly characteristic. The miners and bankers in a placer district can generally tell from which creek the gold originated simply by visual inspection.
Placer gold usually has an entirely different appearance from that found in veins and other deposits. A subdued luster in the placer replaces the high luster of the vein gold; due, it appears, to incipient crystallization on the surface. Some varieties of gold are colored due to a number of reasons. Black and deep brown gold are commonly due to the presence of coatings of manganese and iron oxides or manganese and iron humates. Alternatively some black and brownish gold is due to a coating of very finely divided colloidal or microcrystalline gold. Whitish and dull grayish varieties of gold owe their color generally to thin coatings of calcium carbonate, colloidal silica or fine-grained (colloidal) clay. In most placers the gold has the characteristic golden color, but in others foreign coatings are quite common. 'Black gold' (ouro preto) is very characteristic of a number of placer districts in Brazil and other tropical areas.
Practically every form of gold exists in placers and the closely associated oxidized zo
nes of gold deposits. The most common forms are scales, plates and nuggets. In addition some placers contain crystals of gold, hopper-shaped crystallized particles, masses having filiform, reticulated and dendritic shapes and films, wire and mossy gold. The growth forms of large nuggets are commonly fairly regular, many looking like nuts or potatoes; other nuggets are highly irregular and gnarled in shape. Some partly enclose vein quartz fragments or rounded quartz and other types of pebbles and mineral fragments.
Some nuggets have unusual growth protuberances that may appear as distorted crystals. A few nuggets have been found that have secondary crystalline outlines.
Various terms are used in the following descriptions to give a semi-quantitative estimation of the size of the gold particles in economic gold placers. A 'color' has no exact meaning; it is used by placer miners to refer to a small piece of gold that usually varies from l.5mm upward. Estimation of the value of gold colors in a pan and hence of the value of placer grounds can only be gained by experience since the value varies with the size of the colors, their thickness, and their purity. A good estimate can be made by picking out the colors from a number of pans, weighting them on a small pocket microbalance, and calculating the value per yard.
There are about 50 pans (18 in. diameter) per yard. Coarse gold (nuggets) is usually considered to be that which remains on a 10-mesh screen (>l.5mm); medium gold will pass a 10-mesh screen and be held on a 20-mesh screen; fine gold passes a 20-mesh screen and is held on a 40-mesh screen (value I color per cent); and very fine gold passes a 40-mesh screen. Flour gold is very much finer than the last; in some places 500 to 1000 colors or more are required to equal the value of a cent!