More on Gold Deposition
(Note: I've written this post as a direct response to a reader's questions concerning various aspects of gold deposition. Since I answered at length, I thought it might be beneficial to share this with all of you out there. J.R.)
Mother Nature Loves Monkey Wrenches
One thing all placer miners should understand is that 75%-90+% of the time the standard or general rules of stream hydraulics and the physics of gold deposition remain consistent no matter where you are. That said, it's also good to realize that Mother Nature loves to throw monkey wrenches into the works now and again.
In my 30+ years of working both wet and dry placers there have been a number of instances where I've recovered good gold values from areas or locations that should not have been productive if you followed the basic tenets of gold deposition. I've accidentally pulled small nuggets from shallow "blow" sand spots with no heavy backs sands anywhere in sight and I've also recovered consistent gold in the form of fines and flakes from high-pressure stretches that should not have contained any gold at all.
Exceptions Rather Than the Rule
An important point to remember here is that these were anomalies however, and exceptions rather than the rule. As miners and prospectors we have to go with what we know and what has worked in the past both for ourselves and those who came before us.
Most of the time this mining knowledge serves us well...sometimes it stymies us or surprises us. That's why it's good not to become too "fixated" on hard and fast rules when you are out in the field. You should always remain open to other possibilities and alternate scenarios that may even defy mining logic at certain times.
Highly Fractured versus Smooth or Rounded Bedrock
Another consideration to ponder is the fact that highly fractured or "bookshelf" type bedrock is always going to be a better gold trap (even in poor deposition stretches) than smooth or rounded surfaced bedrock. As my reader rightly pointed out, here in the Western U.S. quite a lot of the bedrock fits the description of being "smooth or rounded."
Obviously this sort of bedrock is not going to grab the gold as well as highly fractured bedrock. By the same token, I've worked many areas here in the West where highly fractured bedrock was readily available or constituted the most consistent form of bedrock in the immediate area.
In his book on underwater gold sniping Sam Radding gives a number of instances and examples of working this sort of bedrock to good effect. He also recovered excellent gold values from this sort of bedrock, especially when compared to the smooth or rounded "serpentine" or granitic bedrock found in many areas of California's Motherlode region.
"Drop Offs" and Other Factors
You should also consider the "drop off" factor when trying to determine gold deposition patterns in a given locale. I've only scraped the surface on the importance of drop offs in "Bedrock Dreams" but this aspect of gold deposition carries much more weight than most recreational or small-scale miners fully realize.
In the greatest sense what I am saying here is that there are innumerable factors that can have an impact on all aspects of gold deposition. These include (but are not limited to):
low pressure areas
water direction and flow
availability of large obstructions
type of underlying bedrock
flooding frequency and intensity
gold replenishment amounts
Again, the factors that govern gold deposition can be very large or significant events or what I term "macro" factors like floods, flash floods, etc. Alternately these deposition factors can be minute or seemingly insignificant. I call the latter "micro" factors and they can be anything from a seeming inconsequential drop off sitting in just the "right place" to a small, periodic water "jet" that forces material into a tiny crevice.
That's it for this round. I hope this little treatise on gold deposition helped.
If you liked this post, you may want to read: "Gold Mining Questions and Answers: Part 17"
(c) J.R. 2010
Questions? E-mail me at firstname.lastname@example.org