As seen in the September/October 2020 Gold Prospectors Magazine
By Kevin Hoagland
Let’s do something a bit different for this edition of “Where’s the Gold.” How about identifying the many different types of deposits?
Over the years at gold show seminars, field trainings and just general prospecting talk, I have started off the general discussion of Geology 101 with this statement, “There are only two types of gold deposits, one is lode the other is placer. After that, it’s all subsets of placer.”
Lode is everything that is still within its original matrix and placer is everything else.
In the rather horrible cut-away drawing, you see the lode (yellow contiguous lines) and then you see the placer deposits in its many different forms. For you Desert Diggers, I’m planning a separate horrible cutaway of a desert lode and associated placers in the near future.
Your task: using the numbered deposits assign a number to the deposit you think is appropriate before looking at the answers elsewhere in the publication. And no, I am not putting them in order. Test yourself on what you know of naming the deposits.
Then for you personally, pick the placer deposit in your order of how you would work them. This part is for you and there are no right or wrong answers. It’s more of an opportunity to know you are heading in the right direction or to rethink your dollars-per-yard recovery.
There is one part in here that may trick you up a bit. It is not a deposit, but rather a geological feature that causes a lot of prospectors to miss out on some very good gold. Do you see it?
Lode – Mineral deposits locked within their original matrix.
Residual – From weather, geologic events that expose the lode to the surface where the gold breaks from the lode, becomes the first placer deposit.
Eluvial – As the residual deposit begins to migrate, eluvial deposits are formed when the gold travels only a short distance from the source and are locked in place by geologic features
Alluvial – These deposits are far more traveled past the eluvial stage. Many are carried by erosional events caused by weather, geo events. Many of these will travel during a major moving event creating fans.
Bench - Bench deposits are formed mostly by geological events that change water courses abruptly, leaving gold-bearing gravels stranded in place when the water course shifts and in many cases changes direction.
Flood - During high-water events such as winter snow melts and floods, flood deposits begin as subsurface or stranded deposits close to the surface and are easily worked.
Stream - These deposits have had the ability to work their way based on water events, specific gravity and time to settle into deeper, more concentrated deposits.
Crevice - Gold’s final resting place, sometimes. Once trapped within a crevice there are very few events that will disrupt this gold and cause it to move back into a water channel to be redeposited.
Now did you see in the photo the area that many miss or misinterpret? It is the layer between the stream and the bedrock. In many areas during flood events and geological shift, material can be moved over the actual bedrock creating a false layer that many miners have found, worked and walked away from not realizing that the actual bedrock may be just a few inches away.
Frankly, these are rare in most cases, but they are there. Always be aware that if and when you are desert prospecting, what you think is bedrock may not be.
Kevin Hoagland is the Executive Director of Development at the Gold Prospectors Association of America and the Lost Dutchman's Association of America. He can be reached at firstname.lastname@example.org