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Learning about Desert Placer Deposits

Last week I went up to the mud hills north of Barstow to explore some of the placer deposits at Coolgardie Camp.  I am studying known claims to learn more about various types of deposits to learn how to identify the environmental / geo characteristics associated with them.  I do get more satisfaction prospecting and the discovery of mining potential rather in less developed areas than working an established club claim, but it affords a great opportunity to gain experience and keep the muscles in shape. The weather was quite pleasant although probably hotter than most would put up with, I found the breeze sufficient to keep me cool enough to shovel uninterrupted.  I spent some time here last month as we passed through on our way to AZ, and sampled several of the GPAA claim areas so I was able to concentrate in the more productive areas.

This area has caliche and bedrock mounds that are covered by a light sand overburden that is 1-6 feet deep. The gold is settling through the overburden to the caliche.  Breaking up the caliche itself was not productive when panned in my test samples. I also didnít break through, although that may be possible as there are some old shafts nearby.  Being solo, I didnít explore any shafts. I concentrated a 10x12 ft area from an average overburden of 2-3 feet through the dry wash. I wanted to pace myself so I panned my concentrates after several hours work.  Basically I concentrated for 2 hours, panned for one.  This comfortable cycle would go from dawn to well into the night. The regular planningís kept me going and the consistent results gave me some clues to the secrets of this type of placer deposit.  1. The gold extracted is coarse to small nugget with most weight in the picker range. No flour and only a very small amounts of fines are found. 2. The gold is hammered, smoothed not delicate or jagged. No host material was found on the nuggets.  3. Gold is found in the loose overburden, and although light the overburden does have some stability as there are gravel pockets and layers.  4. Sweeping and vacuuming the caliche and bedrock yielded no significant concentrations even in pockets and crevices. 5. The caliche is contains much lime. At the big picture, this area is a basin between low, eroded metamorphic hills that appear full of Fe, Cu, Pb and Ag. Metal detecting is tough as iron laden rocks cause a lot of noise in the most promising banks. 

Therefore from these clues I suspect the area was once undersea and the withdrawal of the water eroded the hills in addition to much geothermal activity causing the alkali based caliche to forming the valleys. The distant hills probably leached gold particles and gold sulfides from epithermal deposits. Alluvial erosion of the hills released the gold and seasonal floods beat up the particles and spread them through the basin floor, to concentrate towards the bedrock. Noting the wide expanse of valley separating the hills in this area probably accounts for the even distribution of the particles.  Knowing that the area has been well worked in the past, and the fact that I didnít find an significant concentration on the caliche it is possible that the area I cleared may have been already worked and that I am picked up the crumbs left behind and gold missed from prior tailings. That said, I did not see any tool scrapings in the caliche, ash or other trash beyond the surface layers so it seemed virgin but there were competing diggings all around. So my conclusion here is my best guess. I think it would be interesting to find out whatís down some of those shafts around here, especially if they follow along the bedrock beneath the caliche. I donít have any experience in shafts and deep incline mines so thatís something I will need to try in the company of experienced mine explorers. Perhaps the sugget shooting master,  Bob Schwartz is willing to take this step with me?  Peter Martin.



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