("Nuggets is nuggets," despite their shape or size.)
Here's the fifth installment of this series. Read on if you want to learn more about finding placer gold nuggets.
POINT 5: Gold deposition physics is your best friend.
I've written about gold deposition physics in Bedrock Dreams too many times to count and most of you know exactly what I'm talking about here. For those of you who may be unsure of what deposition physics means, I'm not speaking about chemical or large-scale geological processes, but the impact of the following factors on where nuggets (and gold in general) gets deposited in streams or washes:
- Water flow (a.k.a stream hydraulics)
- Stream or wash configuration
- Stream gradient
- Frequency of stream obstructions/obstacles
- Depth to bedrock or false bedrock
- Bedrock configuration
- Frequency of flooding or flash-flooding occurrences
Let's take a closer look at these deposition factors:
Water flow is probably the single-most important factor in placer gold and nugget deposition. Stream hydraulics influence deposition in many ways, not the least of which is providing gold transport and a mechanism for allowing heavier gold to sink deeper while being shifted and covered by overburden rock and gravels. A constant water flow also helps create high and low-pressure areas in streams where nuggets will "stop and drop" or continue downstream. That's why wet placers can be relatively "predictable" in terms of gold deposition. Even in dry placers where water flow is only intermittent or occasional, given enough time the influence of water flow on nugget deposition will "even out" eventually.
(Never underestimate the importance of water flow to gold and nugget deposition.)
Stream or wash configuration is also a highly important deposition factor. For example, imagine a gold-bearing stream or desert wash that's configured in nearly a straight line or "chute." Where are the nuggets likely to congregate? Sure, a few will be trapped by occasional obstructions or bedrock anomalies, but most of the gold in this sort of hypothetical context will head its merry way downstream as far as it can go or perhaps beyond. Why? There is nothing in that stream or wash's configuration to dictate otherwise. That's why the twists and turns we are all so familiar with in streams and washes are so important to nugget deposition. They provide the high and low-pressure points that either send gold on its way in suspension or allow it to drop and work its way downward toward bedrock or false bedrock.
Stream gradients can vary greatly depending on their altitude and general geographical location. A simple illustration for understanding the importance of stream gradient to gold and nugget deposition is to imagine a gold-bearing stream or wash as a gigantic sluice box. If the box is tilted at too steep an angle (gradient) most of the gold and nuggets inside it will be driven far, far downstream ("out the other end") and lost to recovery for the most part. Conversely, if a stream or wash's gradient is too flat or "even," low-pressure hydraulics will dominate and most gold and nugget deposition will occur in its upper reaches accompanied by countless tons of overburden...that is, if any deposition takes place at all since little gold will be "washed" in. Again, I ask you to imagine that stream or wash as a huge sluice box. Streams with gradual drops in gradient over specific distances tend to be the best gold and nugget producers. Like "Little Bear's" porridge in the children's tale, Goldilocks and the Three Bears, these latter stream gradients are "just right" when it comes to getting a line on nugget deposition.
We'll talk more about this stuff in my next post. Hang tough till then.
(c) Jim Rocha (J.R.) 2014
Questions? E-mail me at firstname.lastname@example.org