Types of Placer Gold Deposits
There are four main types of placer gold deposits that you, as a recreational or small-scale miner, should be aware of. These are the residual, elluvial, stream (alluvial), and bench placers. In some of my earlier posts we have discussed the latter two briefly, but a more developed explanation of all four types is warranted here:
RESIDUAL: This type of placer gold deposit will occur at the surface of the ground where a gold-bearing host vein crops out either on or near the surface. Residual deposits are formed when the original host rock (quartz in most instances) erodes away, leaving varying amounts of gold in place directly at (or very near) the quartz outcropping. In climates with reasonable amounts of precipitation (rain, snow, and so on) and when exposed to extreme weathering by other natural elements, these gold-bearing veins will degrade, erode, and sometimes disintegrate fairly quickly. Residual gold placers will often be fairly extensive in terms of depth and gold "richness." The old timers searched high and wide for residual placers (what they termed "gold seams") and wasted no time claiming them, once located.
ELLUVIAL: These placer deposits are formed after gold has traveled downward a short distance (sometimes only a few feet) from the point of its residual outcrop. The gold contained in an elluvial deposit has NOT yet traveled down far enough to reach an arroyo, drywash, or streambed. Elluvial deposits, like residual deposits, can be very rich diggings for the prospective miner, although the elluvial placer gold will be spread out over a greater area than the gold contained in a residual outcrop.
ALLUVIAL: Not to be confused with the previous placer deposit type despite the similarity in sound and spelling, alluvial placers are formed when the gold eroding out from a residual outcrop has made the downhill journey past the elluvial stage and actually enters a streambed, wash, or arroyo. Alluvial or stream placer deposits are the type of deposit most often encountered and worked by recreational or small-scale placer miners.
BENCH: Bench placers are essentially stream placers left high and dry by a stream as it "cuts" its way down through its original bed, eventually forming a downslope watercourse or canyon. Bench placers can also be formed when massive earth movements such as pressure uplifts, earthquakes, vulcanism, and the like occur over millenia. A classic example of uplift and erosion forming extensive (and rich) bench placers can be found in the "Great Blue Lead" tertiary gravels of California's Northern Motherlode region. Bench placers are always worth more than a cursory examination by miners in the field because they can contain good gold values.
You can now see that most gold placers are usually formed by the downward movement of eroding veins, outcrops, or ancient gold-bearing bench gravels. It's always worthwhile to check any "interesting" areas located above the stream or wash you are currently working. I've found some good gold this way in the past.
(c) Jim Rocha (J.R.) 2008