(These bench gravels show a number of "classic" signs or traits.)
It's time to get back on track and talk a bit more gold-bearing bench gravels. In this post I'll cover some additional things to look for and also give you some ideas on how to work benches.
In the past here in Bedrock Dreams I've mentioned the significance of colors like red, reddish-orange, yellow-orange, pink (all connected to iron sulfides) as well as blue, bluish-grey, and even greenish-grey as they relate to clay layers specifically. As far as benches are concerned, at times you'll run into clays bearing similar coloration and this sort of visual clue should not be ignored.
However, some of the very best gold-bearing benches I've ever worked were those with a very subtle, almost difficult to discern pinkish-beige to yellow-orange coloration and where the clay itself was not in layers or levels, but acting instead as a cement or bond holding the entire bench or sections of bench in place. If you see something like this while you're out and about slow down, take a good look at the overall situation, and start sampling carefully and thoroughly. You may be into some good stuff.
Here's an example of what I'm talking about. Many years ago after spending way too many days suction dredging a well-known gold-bearing river in the northern half of the California Motherlode, I decided to take a break from dredging and a couple of pain-in the ass pards who were really starting to get on my nerves...we three eventually parted ways in case you're wondering. The moral here is that even the best of partnerships can go sour.
(Even the closest of "pards" will part ways eventually.)
Anyway, frustrated and angry I grabbed up my gold pan, miner's pick, a small shovel, 5-gallon bucket, and a few odds and ends and headed up a nearby feeder creek. Fighting clouds of mosquitoes (this creek is still notorious for the little blood-sucking bastards) I headed upstream, prospecting and sampling as I went. The results of all this activity were not impressive at all...in fact, I was barely turning up any color and was about to throw in the towel and rejoin my bickering comrades-in-arms for better or worse.
That's when I came to a slight inside bend in a ravine with fairly steep walls on both sides. The right side of the creek (i.e., looking upstream) at that spot was in a high-pressure zone with little gold appeal to it. On the left side of the creek and slightly upstream I could see a large shaft had been cut into solid rock some distance above a high gravel bench. Interesting in and of itself, but not as interesting as the 8-10' high bench wall that was exposed to my front about 15 feet away from that inside bend.
This bench had all the classic signs:
1) left "high and dry;"
2) bound together by a very subtle pinkish to yellowish-orange colored clay;
3) composed of tightly consolidated gravels and rock;
4) contained varying sizes, shapes, and smoothness of rock and gravel; and
5) sitting in a good gold deposition area in terms of stream hydrology.
Oh one last thing...on closer examination I could see decent amounts of coarse black sands mixed in with the clay, gravel, and rock.
More to Come
That bench was hell to sample and nearly pick and shovel "resistant," not to mention the fact I had to soak or "puddle" the clay coated rock and gravel in water in my 5-gallon bucket before panning it. But let's just say what showed up in pan was encouraging, to say the least! Dumping my gear on the spot I quickly headed back to camp, grabbed my battered old Keene sluice box, and rushed back to that bench without so much as a whisper to my pards who were still down on the river arguing about whose turn it was underwater.
I'll leave things at that for now, but there's more to come so stay tuned.
(c) Jim Rocha (J.R.) 2014
Questions? E-mail me at firstname.lastname@example.org