Origins of Gold Deposits

There are some hydrothermal gold alunite deposits that are confined to an intricate process of very strongly altered rocks. In some cases the bulk is thought to be of ignimbritic origin, and in some cases has a large horizontal extension.

A penecontemporaneous syngenetic origin of gold ore and ignimbrites can be seen when: there is an obvious absence of gold veins in the underlying dacite, when there is wedging out of gold veins in the route of the underlying dacite, when gold veins cannot be seen in the rocks that are overlying, when there is a concentration of gold deposits under a number of levels and that most likely represent the top surfaces of various ash flows, and finally when there is a usual happening of a gypsum rich alteration area that can be seen along the border of the complex of ore bearing ignimbrites. 

Gold bearing volatiles are for that reason believed to have originated from the ignimbrite material and to have deposited their gold content along the walls of fumaroles similar to the ones that are in the Valley of Ten Thousand Smokes and where some deposition of ore minerals has also been seen. The gold veins are as a result thought to be a symbol of fossil fumaroles.

Types of Placer Deposits
In history placer deposits provided man with the earliest samples of gold and since that moment in time have accounted for a massive production of this metal. If we take account of the Witwatersrand and other quartz pebble conglomerates like fossil placers or modified fossil placers, then the placer type of deposit has given more than two thirds of man's store of gold.

Before going on more, certain terms as far as placers is concerned should be defined.

The word placer unmistakably comes from the Spanish language and was utilized by the early Spanish miners in both South America and North America as a name for gold deposits that are in the sands and gravels of streams. In the beginning, it appears to have signified “sand bank'” or “an area in a stream where gold was deposited”. Despite the fact that many other terms have been coined for deposits in weathered residuum and alluvium, none of them is quite as to the point and significant as the word placer. 

The terms of the area or section that has a good concentration of gold in eluvial and alluvial placers can be seen in various forms. We will utilize the miner's term 'pay streak', which is frequently used in the United States as it is in Canada. There are additionally some other terms in English that are used such as 'pay gravel', 'pay sand', 'pay dirt', 'pay wash', 'pay channel', 'pay lead', 'run of gold', 'gutter' and 'wash dirt'.

The tenor of pay streaks or of placer gold gravels and sands, on the whole, is said according to the value which can be in ounces, grams, pennyweights, or in any unit of currency, per cubic yard or meter, per running length – such as foot or meter of a channel, per surface unite of cross section, or per unit of surface – such as square foot or meter). In some cases also every once in awhile in bonanzas by dollars or some other unit of currency per pan.

The pay streaks of placer deposits might be placed on or close to bedrock or on some stratum on top of the bedrock. The bedrock in placer deposits is frequently referred to as the “true bottom”, even though the term is not used that much these days anymore. Once the streaks rest on a well-defined stratum of sand, gravel, or clay on top of the bedrock, they are then referred to as “false bottom”. 

Placers have been diversely classified, however here we will use an uncomplicated categorization based on whether the placers are formed by concentration of gold in situ over or in the direct area of primary deposits, that is to say residual or eluvial placers, or by agencies that have concentrated the gold in the close by area or at some distance from the main starting place. There are also alluvial, beach and aeolian placers. The expressions saprolite or saprolitic placer in the past used to be used for some kinds of eluvial placers, for the most part in the eastern part of the United States.