Friday, May 18, 2012

Basic Tips for "Reading" a Stream (Part 1)



 (Good gold country...section of the North Yuba River a few miles from Downieville, California.)

Most Important Gold-Mining Skill

As a placer miner, the ability to "read" a stream is the single most important gold-mining skill you'll employ out in the field. Without this knowledge and a practiced eye, you are essentially blind when it comes to knowing where to search for gold in a river, stream, or dry wash.

As you read my posts over time you'll notice the terms "the physics of gold deposition" and "stream hydraulics" keep popping up. Knowledge and experience in these two areas are fundamental to successful placer mining, whatever the scale or level of effort involved.

Don't allow these two terms to intimidate you, however. The physics of gold deposition simply refers to the physical laws governing gold's movement and eventual resting place in a given watercourse. Stream hydraulics refers to the physical laws governing how water moves in a stream, including its force and speed, and its ability to move gold through that same watercourse.

Gold Concentrators
Metal Detectors

At its most basic level, the most common and likely areas for streambed gold deposition are the following:

1) LOW PRESSURE AREAS: These are locations in stream courses where significant slowing of the water occurs. Since gold is extremely dense and heavy, a very strong current or flow (i.e., rapids or flooding) is required to transport it. When and where this high-pressure flow drops off significantly, low-pressure deposition will occur.

2) INSIDE BENDS: Most streams and washes meander or perform snake-like loops at various points over their length. Gravel bars or bedrock locations on the inside bends of these loops are good spots to sample because they are formed by low pressure hydraulics and anytime water slows, gold will be deposited.

3) BEHIND LARGE OBSTRUCTIONS: Obstructions such as larger boulders in low-pressure portions of the stream or wash can be good gold traps as well and should be sampled on the downstream side where eddy currents form and deposit gold.

(The downstream sides of large boulders or obstructions can be good gold deposition points.)

4) BENCH GRAVELS: As a stream cuts its way through gold-bearing gravels it can leave areas of loosely consolidated, or conversely, tightly packed gravels some distance above the current level of the stream. These so-called bench gravels are always worth examining and often carry flakes and small nuggets, as well as fine gold.

5) MOSS and PLANT ROOTS: The root systems of plants in or near the stream course can trap fine gold and sometimes flakes as well. However, I have to tell you this...in my 30+ years experience as a small-scale placer miner I have rarely found much gold in moss or plant roots.)

Carhartt Wear
Wolverine Boots

6) BEDROCK: Always search any areas of exposed bedrock for gold. The same holds true for false bedrocks such as clay layers or caliche. Process all gravels sitting just above or atop bedrock, and clean out individual potholes and cracks and crevices thoroughly. Why? Bedrock is the final resting place for gold in a streambed since the yellow metal cannot penetrate down any deeper. Bedrock is where you will find not only coarse flakes and nuggets, but potential multi-ounce pockets or paystreaks.

Once again, these are very basic tips. Being able to read a stream or wash (under non-flood conditions) accurately and with confidence is an ability achieved over years of trial-and-error experience. Coincidentally, the capacity to visualize a streambed in flood stage and then also "see" where gold is being deposited after the flood waters recede is sometimes more of an art than a science.

A note of caution is warranted here though. Mother Nature does not always conform to strict rules in this regard, so keep your eyes open and be ready to think outside the box.

 (NOTE: Here's a bit more on this topic from Australian miner and businessman Adrian Keighran, designer/developer and purveyor of the CreVac Mini-Gold Dredge: )

Vortex or "Dunny Flush"

"Hi Jim...just saw your latest and thought I'd add 'me 2 bob worth' regarding inside bends. I've found that the secondary current of the flow (lowest) creates a vortex (dunny flush) and pulls the heavier items UP the bench (sort of anyway)"

"I'm not sure of its overall value, but I've found in the mining and sampling I've done in this regard that the paystreak can meander up and down, as well as slightly left to right. For example, on one occasion I found that the paystreak went from start to finish on the bench... it went from about water level then 50 cm above, and then back to water level, etc., to the other end. I guess this was caused by the secondary flow pulling the paystreak up the bench as well as the normal flow drop outs." 
  
My thanks to Adrian for providing his perspective here. Good on ya Mate and best of luck with the Mini-Gold Dredge!

If you liked this post, you may want to read: "Gold Mining Questions and Answers: Part 1"

(c)   Jim Rocha (J.R.)  2012

Questions? E-mail me at jr872vt90@yahoo.com

No comments:

Post a Comment