The particles of gold found in placers originally came from veins and other mineralized zones in bedrock, from which they were released through surface weathering and disintegration of the rock matrix. The original source of gold may not in every case have been a deposit which could today be mined at a profit; the richer placers usually indicate a comparatively rich source. A long period of deep weathering, resulting in separation and release of large quantities of gold from the bedrock, followed by a more active period of erosion, usually due to uplifts is an ideal condition for gold to be swept into stream channels and there to be concentrated into rich placers.
Richer gold placer deposits may be formed through re-concentration from older gold-bearing gravels. Basically, these are the most important geologic conditions which have been found to exist in the various gold belts of the world and especially in the California. For the most part, the original source is not far from the place where it was first deposited after being carried by running water. This is certainly true in some places of California such as Sierra Nevada. The streams flowing through regions of metamorphic and intrusive igneous rocks threaded throughout by gold bearing veins were found by the early miners to contain auriferous gravels. Nevertheless, the most recent streams which have had only lavas containing low gold contents to pass over as in the volcanic covered area have confirmed their low gold grade.
The great majority of gold placers have been derived from the weathering and disintegration of auriferous veins, lodes, shear zones, or more irregular replacement deposits. It is important to indicate that in many regions the rocks contain abundant joints, seams or small veins in which gold has been deposited with quartz. In this way, it is usually considered that gold is distributed as fine particles in schists and massive rocks and that placer gold in some locations is derived from this source. Most of these considerations are derived from prospecting activities on field and the variability of gold sources is not a surprise.
Other aspect that has influence in the form that gold is found is related to the modification of the placer deposits, which may be greatly altered in form and structure by earth deformation. The gravel content may also become firmly cemented by interstitial deposition of mineral matter such as lime or silica, through the action of infiltrating solutions. The older placer, the more apt it is to have been modified in these ways from its original form and attitude.
Gold from Bedrock | Associated Minerals to Gold Placers | Deposition of Gold in Placers | Concentration Process of Gold in Placers | Geological Age of a Gold Placer Deposit | Physiographic Criteria and Gold Placer Age | Lithological Aspects and Gold Placer Age | Life History of Gold Placers Streams | Terms Related to Stream Placers | Shallow Gold Placers | Gold Veins and Placers | Sections of Strata in Gold Placers | Importance of Water in Placer Mining |