Help needed: Hot rocks or what?

[UncleVinnys]

Scanning an old tailings pile I've noticed a few things and I need help to
interpret.

1. Is gold found mostly in quartz or what?
I have found several darker gray rocks (looks like basalt, possibly shale)
that set off the detector. A couple of these have rust-like stains that seem to
be the trigger. Others are just even-gray color that set off the detector.
A couple appear to be in sedimentary rock.
So, what does that mean? Are they just minerals, or is it worth
breaking them up to see if anything is inside?

2. Spread among the tailing pile are quartz rock, mostly rounded by
river currents, but some broken up and cracked. I suspect another
detector has been there before me, breaking up the quartz rocks in
their searching. Is that a valid conclusion, or do dredgings also contain
unweathered, sharp-cornered rock?

Thanks for your response.

 

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[UncleVinnys]

Here is a photo.
The one in the center, you can see the rust stain.
Upper right is slate or shale, you may be able to make out the layers.
Large lower left is pitted and soft, like sandstone.
Top center includes a black, mica-like inclusion.
All of them are "hot rocks."
Any clues??

 

[Jim Hemmingway]

Howdy...river-run rocks should show wear ie rounded...anything sharply defined is the result of recent activity, as you've surmised.
In these parts, precious metals are generally associated with quartz (esp.gold), calcite, porphyries and so forth...but not always. In dredgings you would expect to see free gold as well.
From the photos, I suspect your hot rocks are just that; non-conductive iron mineralized rocks with a different ground balance setting from your detector setting. If you were using the XT-30, then ground balance is preset, to cover most commonly encountered ground mineral conditions. But it is preset at some given point.

Hot rocks repond either positive with a metallic "zip zip" or negative with a "boing" sound. The positive rocks require reducing the ground balance (decreasing the sound as you lower your coil to the ground) to silence them...which leaves you underbalanced for the general matrix. In an area where there are just too many to ignore, sometimes you are obliged to quiet them down with just enough discrimination to eliminate them.
The negative hot rocks require an increase in ground balance (increasing the sound as you lower your coil to the ground) to silence them...which leaves you over ground balanced for the general ground matrix you're hunting in. Since negative rock sounds are readily identifiable, you can easily ignore them for what they are. Also, without auto retuning (SAT, Autotune...) none of these -ve rocks would give any kind of response in wrt increased sound. In fact they would cause your detector to go quiet as the coil passes over them. With autotune, the "boing" you hear is the result of autotune trying to readjust the threshold, but "overshooting" into an audible sound briefly.

So with your preset ground balance, what you hear just depends on whether the rock has a significantly more (or less) ground balance setting requirement than your current preset setting. With this unit you're better of using sufficient discrimination, but with time, it's better to learn to recognize and ignore them. One more thing, hot rocks have no real power in their signal compared to metal; you find that as you raise your coil they tend to disappear quickly. It's all about where your ground balance is set in any given area, that decides which rocks will be "hot rocks". Hope this helps...Jim.

 

[UncleVinnys]

That was a BIG, BIG help, Bill.
Thanks for explaining.

 

[rockhead]

Ahh all I hear is booooing! Some of those rocks are on the thing I hate most list. One more thing to add is soil matrix generally will be unreliable if you lift you coils up a little a good target will be there. However if it's a faint signal dig or kick some soil off the top try again.

 

[Hoser John]

The absolute worst detector to use is the BEST detector to use in your situation. An old manual controlled TR detector has the ability to also tell you ferrous or non ferrous. You simple adjust it with a iron sample to accept or reject iron for ID purposes. Instant answer without lugging around rocks--aka leaverite--as in leave it right there-Tons a au 2 u 2-John

 

[dave wiseman]

Vinny,the broken shale or slate is from the dredge digging in the bedrock in the ponds and the waste shot back behind it filling in,that is one reason you can can find species in the quartz boulders and cobbles as the oversized from the trommels was discarded.Lot's of fellows speciaize in placer tailings,whether from doodlebugs,hydraulicor tertiary river channels......Dave

 

[Eu_citzen]

I see some greenish rocks (upper right corner) in my area I'd associate those with Ilmenite and other Iron minerals.(they often contain that)
I always look into those 'suspect' hot rocks...curiosity. (killed the cat?)
And then I'm an amature geologist.

 

[Red_desert]

My Fisher Gold Bug-2, to tell whether a rock contains metal or is just hot, I turn the coarse balance to 3 1/2. Then set the detector across something to keep the coil away from the ground. Use a long handle plastic spoon or plastic scoop to bring the rock toward the coil. That would give a metal or positive signal. If no sound, bring it back away from the coil. A hot rock will make it scream...

The rock in the lower left of your photo looks like ones we have, except ours are hard. We have a lot of gravel glacial drift deposits in northern Indiana here. There are plenty of even grapefruit sized hot rocks. Those have dark black colored spots which are iron. I find Milk Quartz nearly as large sometimes also.

 

[nero_design]

Here's another type of "Hot Rock" I photographed a couple of months ago that might be worth noting. It's a type of 'Slag" from smelting in the early days. I have no doubt it looks much the same like this all over the world. Feels light... like volcanic pumice. Triggers the detectors like crazy. I bought back some samples for further testing and it's fairly common around a lot of the old gold diggings when they used to melt tine, silver or gold and scrape off the impurities.

 

[Red_desert]

Oh yes, I found those around an old smelter out back from Los Cerrillos ghost town in New Mexico. Some of the pieces still had enough metal to set off a coin detector.

 

[B1u3Dr4g0n1]

Some times the black sand in the creeks around here sets off my ace 250